Hofstra Horizons Spring 2023 | Hofstra University

HORIZONS HOFSTRA

SPRING 2023

featuring Faculty Research from the Fred DeMatteis School of Engineering and Applied Science

Iam delighted to present the spring 2023 issue of Hofstra Horizons. This issue highlights the scholarly research of our faculty – and the shared interests of our students – from the DeMatteis School of Engineering and Applied Science.

A significant aspect of research at the DeMatteis School is for the faculty to inspire and encourage students’ curiosity and thirst for knowledge. The DeMatteis laboratories and research facilities provide our students with opportunities to work with faculty who are leaders in their fields.

In turn, these students consistently impress the faculty, the Hofstra community, and the community at large with their hard work and research acumen. We strive to support the Hofstra faculty and students, so that they can continue to pursue research that leads to new knowledge and positive change.

I congratulate all our research faculty and students on their accomplishments. I hope you enjoy this issue of Hofstra Horizons.

Sincerely,

contents

6

Tissue Engineering Vascular Grafts Using Decellularized Plants

10

Finding Forms for Spatial Structures: Hanging, Heating, and Flipping

16

Why Is My Virtual Machine in the Cloud so Slow?

Susan Poser, PhD President, Hofstra University

Susan Poser, PhD President

Charles G. Riordan, PhD Provost and Senior Vice President for Academic Affairs

Sofia Kakoulidis, MBA Vice Provost for Research and Sponsored Programs

Alice Diaz-Bonhomme, MSEd Assistant Provost for Research and Sponsored Programs

Research and Scholarship at Hofstra University

22

Balancing Protection against Pathogens with Exposure to Toxic Chemicals

28

Directing Waves Toward a Flexible Future

HOFSTRA HORIZONS is published annually by the Office for Research and Sponsored Programs, 144 Hofstra University, Hempstead, NY 11549-1440.

Each issue describes in lay language some of the many research and creative activities conducted at Hofstra. The conclusions and opinions expressed by the investigators and writers are their own and do not necessarily reflect University policy.

© 2023 by Hofstra University in the United States. All rights reserved. No part of this publication may be reproduced without the consent of Hofstra University. Inquiries and requests for permission to reprint material should be addressed to:

Editor, Hofstra Horizons

Office for Research and Sponsored Programs

144 Hofstra University Hempstead, NY 11549-1440

Telephone: 516-463-6810

As Hofstra’s provost, I am pleased to introduce this issue of Hofstra Horizons, which focuses on the work of several of our esteemed faculty in the Fred DeMatteis School of Engineering and Applied Science.

I am very proud of the research conducted by our faculty. It enriches the educational experiences for our students, and it creates new knowledge and new possibilities for our society.

In this issue of Hofstra Horizons, the research of five of our DeMatteis School faculty is highlighted. Dr. Nicholas J. Merna’s research focuses on the innovative use of plant-derived materials in vascular replacement and repair. This research holds promise for the treatment of patients with coronary and peripheral artery disease. Dr. Edward M. Segal describes his path to creating pavilion-scale projects upside down –hanging, heating, and flipping the physical models to result in an incredibly efficient and lightweight structure. Dr. Jianchen Shan explores cloud computing, specifically why the performance of applications hosted in the cloud is often poor and unreliable. Dr. Minjeong Suh discusses how irrigation and chemical disinfectants applied to fresh produce can produce toxic byproducts that pose a public health threat. Finally, Dr. D. Elliott Williams discusses the rapid technological progress in wireless communication and how increased control of electromagnetic radiation has enhanced detection and tracking of weather systems, automotive safety, and national security.

I congratulate the faculty featured in this issue. Their research has an immeasurable impact on our students, our community, and our world.

Sincerely,

dean’s COLUMN

As dean of Hofstra’s DeMatteis School of Engineering and Applied Science, I am delighted to share the scholarly work of our accomplished faculty as we celebrate our 10-year anniversary as an independent school.

In the last 10 years, we have experienced exceptional growth; over 850 students are currently enrolled in our undergraduate and graduate programs. The increase in the breadth of our programs and the accomplishments of our faculty have contributed to our climb in national rankings. We are ranked No. 29 by U.S. News & World Report (2023, Best Colleges), putting us in the top 13% of non-PhD-granting engineering schools in the United States. It is wonderful to see our unwavering commitment to excellence in teaching and student mentoring – which forms the backbone of the DeMatteis School – being recognized nationally. We are well positioned for continued growth as we expand our facilities into the new Science and Innovation Center.

The DeMatteis School comprises two strong departments. The Department of Engineering offers bachelor’s degree programs in bioengineering, civil engineering, electrical engineering, engineering science, industrial engineering, and mechanical engineering, all of which are ABET-accredited. It also offers a master’s program in engineering management. The Department of Computer Science offers bachelor’s degree programs in computer science, computer engineering (both ABET-accredited), computer science and cybersecurity, and computer science and mathematics, as well as master’s degree programs in computer science, cybersecurity, and data science.

At the same time, we are quite lean, and each and every faculty member plays a key role in ensuring the viability and relevance of their particular subject area. That is why we place such a premium on hiring faculty who are fully committed to teaching students, and who display creativity and breadth when they showcase their research interests, hence demonstrating the broad spectrum of careers awaiting students when they graduate.

Along those lines, we offer a thriving research environment for highly motivated undergraduates through our Advanced Summer Program in Research (ASPiRe), which prepares students for the rigors of graduate research. Participation in this program often leads to students presenting papers at professional conferences or co-authoring journal articles.

The DeMatteis Co-op Program offers students an integrated educational and professional working experience. This program is available to undergraduate engineering and computer science students after the first semester of their junior year. It is also available to students enrolled in our master’s programs. Co-op positions are paid, six- to eight-month internships in which students gain hands-on experience working in a field related to their major. The co-op experience gives students valuable insight into how companies work and what is expected of new employees. It is a tremendous advantage for graduating students seeking full-time employment.

This issue of Hofstra Horizons features the scholarly pursuits of some of our newest faculty members. They each have their own story to tell, and they each add immeasurably to the diversity of interests and the level of enthusiasm that permeates our hallways.

As we look to the future, I am confident that our faculty will continue to be inspiring teachers and impactful researchers, so that we will continue to send forth graduates who contribute to the betterment of human society.

Sina Y. Rabbany, PhD

Dean, Fred DeMatteis School of Engineering and Applied Science Jean Nerken Distinguished Professor in Engineering, Hofstra University

FACULTY RESEARCH INTERESTS

ENGINEERING

Lynn A. Albers • Engineering education; K-20 STEM outreach; statistics; renewable energy; thermal sciences

M. David Burghardt • Analysis of student learning in a ”flipped classroom” environment, where students learn by watching videos before class, taking notes, and working on problems to prepare for an in-class, interactive experience

Mauro J. Caputi • Enhancing the first-year student experience; introducing conceptual engineering design; promoting effective teamwork; establishing good communication skills

Kevin C. Craig • Application of model-based design in modern engineering practice, specifically mechatronic systems

Edward H. Currie • Development of autonomous AI systems capable or performing a variety of medical procedures, e.g., wound closure, biopsies, intra-abdominal procedures, and diagnostics/measurements

Roche C. de Guzman • Bionegineering research in hair protein biomaterials; bone tissue engineering; fiber capsule response to artificial pacemakers

Brian J. Galli • Diverse areas of engineering management

Sleiman R. Ghorayeb • Theoretical and experimental studies in therapeutic and diagnostic ultrasound as applied to translational medicine

Saryn R. Goldberg • Undergraduate engineering education to explore classroom practices that support and train students to ask questions and assess how asking high-quality questions supports student learning and metacognitive outcomes

Margaret A. Hunter • Fate and transport of contaminants in water and soil environments; watershed protection and management; chemical and biological system modeling; improving STEM education; promoting involvement of women in STEM careers

Wing C. Kwong • Innovative high-speed, large-capacity wireless networks that can support tens of thousands of mobile and Internetof-Things (IoT) devices using modern cognitive-radio (CR) technology

COMPUTER SCIENCE

Simona Doboli • Artificial intelligence; text mining; deep learning, creativity; neural cognitive models; computational neuroscience

Xiang Fu • Formal methods (model checking); static and dynamic program analysis; malware analysis; automated exploits generation; applied cryptography (zero knowledge proof systems)

Scott M. Jeffreys • Enterprise architecture; security software and protection techniques; mathematics in computer science

Gerda L. Kamberova • Decision making under uncertainty; computer vision; sensor-fusion

Chuck C. Liang • Mathematical logic and proof theory; type theory and the foundations of programming languages; compiler design

Tzer Hung Low • Use of natural language processing and machine learning transformers to extract information and sentiment from corporate SEC filings and meeting transcripts

Nicholas J. Merna • Recellularization strategies that incorporate multiple modes of cell delivery, followed by appropriate pre-conditioning of a vascular construct with the goal of successful long-term engraftment

Manuel J. Miranda • Probabilistic methods applied to structural engineering and structural mechanics

Richard J. Puerzer • Assessment of engineering education, especially at small schools; engineering management; healthcare management systems; and the application of engineering tools and techniques in baseball

Sina Y. Rabbany • Strategies for vascularization in tissue engineering and regenerative medicine

Salvador Rojas-Murillo • Understanding the visual learning processes required for hearing individuals when learning American Sign Language

David M. Rooney • Exploring the nature of fluid wake configurations, in particular vortex shedding patterns, that emerge behind structural shapes in subsonic flow

Edward M. Segal • Designing and building sustainable structures using nontraditional materials; suspended formwork for shell structures; rapidly deployable rope bridges; wood structures

Minjeong Suh • Water disinfection using low-cost solar technology; adsorption of pollutants using photocatalytic composite materials

John C. Vaccaro • Experimental fluid mechanics and aerodynamics with special interest in flow control of inlets, airfoils, and turbines

D. Elliott Williams • Dynamically re-configurable electromagnetic systems; phased-array geometry design; multi-modal antennas; deployable wireless networks for disaster relief; integrated circuits; circuit theory; engineering education

Gretchen Ostheimer • Group theory and theoretical computer science, specifically decidability questions, practical algorithms and relationships between group theory and formal language theory, with an emphasis in all cases on achieving a better understanding of infinite solvable groups

Krishnan Pillaipakkamnatt • Machine learning and data mining privacy

Oren Segal • Parallel heterogeneous computing systems, hardware/software co-design of heterogeneous computing systems; automated hardware/Neural Network Architecture (NNA) co-design of machine learning accelerators

Simon Shamoun • Theoretical problems in sensor and mobile ad hoc networks

Jianchen Shan • Cloud computing infrastructures; parallel and distributed systems; high-performance computing; mobile computing; operating systems; computer architecture

Tissue Engineering Vascular Grafts Using Decellularized Plants

Nicholas J. Merna, PhD, Assistant Professor of Engineering, Fred DeMatteis School of Engineering and Applied Science

Abstract

There is a critical need for vascular grafts for treatment of patients with atherosclerotic disease. However, the development of arterial replacements remains challenging due to the immune response, blood clotting, tissue mechanics, and time required to fabricate each graft. The processing of plant leaves to generate scaffolds for use in tissue engineering is a new approach that has presented exciting opportunities for the treatment of patients with skeletal, muscular, cardiac, and

vascular defects. This paper will present our research group’s novel use of plant leaves to engineer three-dimensional vascular grafts. We believe this approach can be used for the development of other tissue replacements as well.

Introduction

In the United States each year, an estimated 16.5 million Americans live with coronary heart disease. Surgical intervention may involve angioplasty, stent insertion, atherectomy, or arterial bypass. Each year, 370,000 coronary artery bypass procedures are performed to redirect

blood flow around blocked or partially blocked arteries [1]. Autologous saphenous veins are the current gold standard grafts for these small-diameter vessel bypasses. However, 20-30% of patients do not have a suitable saphenous vein due to inadequate diameter or blood clotting.

Synthetic graft alternatives, made from ePTFE or Dacron, have had limited success in small-diameter vessels due to long-term graft failure and blood clotting [2]. The success of these grafts has been limited by a lack of cellularity and a mismatch of mechanical properties. Natural graft

alternatives, using materials such as collagen, have faced similar challenges related to blood clotting and poor mechanical strength. Our research group sought to develop a robust small-caliber vascular graft using decellularized plant leaves.

The structural similarities between plant and animal tissues have led to the recent development of scaffolds derived from decellularized plants [3]. Decellularization is the process of removing cellular material from tissues and has been performed successfully on many tissues and organs in order to generate scaffolds. Removal of this cellular material allows for implantation of these cellulose-based scaffolds with minimal immune response [4]. Once these plant cells have been removed, new cells must be added to improve the performance of these grafts. Endothelial cells are normally found lining the cells of blood vessels and help prevent clots from forming. By incorporating endothelial cells into these grafts, they have a reduced chance of failure due to thrombosis and collapse of the vessel.

Once a scaffold has been recellularized, its function can be improved through application of fluid flow in a bioreactor system [5]. Other groups have reported that this mechanical stimulation helps endothelial cells seeded in these grafts survive the forces of blood flow after implantation. This study aims to determine if decellularized plant leaves are suitable for the development of vascular grafts and if bioreactor pre-conditioning can be used to improve their performance.

Methods

Several types of plant leaves (cabbage, black-eyed Susan, lettuce, and spinach) were evaluated for their decellularization potential using

detergents and enzymes as previously described by our research group [6]. Treatment in decellularization solutions was followed by a minimum of 6h of rinsing in water to ensure removal of residual chemicals. Non-decellularized and decellularized plant leaves were then finely ground and evaluated for DNA content to verify removal of all plant cell material.

The mechanical properties of these scaffolds were tested by cutting samples into dog-bone shapes and pulling them uniaxially until failure. The tensile stress and elastic modulus were then calculated based on the measured load, extension, and dimensions of the sample. Additional samples were prepared for imaging by fixing and drying the leaves, followed by application of a gold coating using a sputter coater. Imaging of the surface of the decellularized leaves was performed using scanning electron microscopy at 200x magnification.

Vascular grafts were constructed by wrapping decellularized plant leaves around an acrylic rod in combination with cross-linked gelatin to provide mechanical strength. The burst pressures of these vascular grafts were then evaluated by injecting water at a constant pressure.

The 2D scaffolds and 3D grafts were also seeded with endothelial cells to evaluate cell performance. The decellularized plant leaves were first sterilized in ethanol and then coated with proteins to promote cell adhesion. Cell density was evaluated at multiple time points to determine cell adhesion and viability.

A statistical analysis was performed for all samples to determine if data was normally distributed and to identify significant differences between conditions.

Figure 1: Representative photos of spinach before and after decellularization.

Results

Decellularized spinach was white in color, with a soft texture (Figure 1). A reduction in leaf thickness from 0.7 mm to 0.1 mm was observed. Decellularized cabbage, black-eyed Susan, and lettuce resulted in a similar appearance and texture. Removal of over 90% of plant cells from the decellularized spinach was confirmed by DNA quantification. However, only 50% of DNA was removed from cabbage, black-eyed Susan, and lettuce.

Spinach maintained its mechanical strength following decellularization, yet a reduction in the elastic modulus was found following decellularization of cabbage, black-eyed Susan, and lettuce. The maintenance of mechanical strength in spinach was accompanied by a conservation of plant structures on the leaf surfaces that could be observed through scanning electron microscopy (Figure 2). This included the presence of external trichome structures before and after decellularization.

Vascular grafts constructed with decellularized plant leaves (Figure 3) were found to be mechanically strong. Burst pressure testing revealed that these grafts consistently withstood pressures well beyond what is

experienced in the body. They also remained stable for extended periods of time with no observed degradation.

Endothelial cells seeded on these 2D scaffolds and 3D grafts adhered within hours and remained viable for several weeks. No cytotoxicity was observed; however, cell proliferation and migration were slightly reduced when compared to the control.

Discussion

Small-caliber tissue-engineered blood vessels hold tremendous potential for the treatment of patients with coronary and peripheral artery disease. These grafts must possess suitable mechanical properties and must be seeded with endothelial cells to prevent blood clotting. Here we present a new and exciting approach for the use of plant-derived materials in vascular replacement and repair.

Decellularized spinach showed adequate plant cell removal and mechanical properties, when compared to decellularized cabbage, black-eyed Susan, and lettuce. Perfusion

decellularization of spinach has previously been shown by others to provide a vascular network capable of recellularization with human cells [3, 7]. However, this vasculature is limited in size and unable to serve as small-diameter bypass grafts.

Recellularization of vascular grafts is a necessary step that ensures success of the scaffold prior to implantation. Endothelial cells seeded on our grafts remained viable for several weeks and continued proliferating. Other studies have also demonstrated that endothelial

Figure 3: Photo of vascular graft generated using decellularized leaves.

Figure 2: Representative scanning electron microscope images of leaves at 200x magnification.

Decellularization is the process of removing cellular material from tissues and has been performed successfully on many tissues and organs in order to generate scaffolds.

cells seeded in the vasculature of detergent-decellularized spinach remained viable for 48h after incubation [3, 8]. Our cell densities were found to be similar when evaluated by imaging and cell counting.

In summary, plant-derived scaffolds possess suitable mechanical strength and recellularization to serve as vascular grafts. In future studies, we’ll use a custom-built bioreactor system to improve the function of these vascular scaffolds and assess their performance in small animal models.

Undergraduate Student Involvement

Three bioengineering undergraduate students have been working on this research during the fall, spring, and summer. Gianna Rinaldi joined my research team in September 2020 and took the lead on this project. Preliminary data that she collected was crucial to the success of our application for NIH funding. In December 2021, Nicole Gorbenko and Amalia Sanchez joined the project. These three students have made incredible progress, which has resulted in presentations of their work at the BMES Annual Meeting, Hofstra’s Undergraduate Research Day,

and the DeMatteis ASPiRe (Advanced Summer Program in Research) Symposium. We submitted a manuscript describing this work that was recently accepted for publication in a peer-reviewed journal. Our group has plans to work with collaborators from the Department of Engineering and the Feinstein Institutes for Medical Research to validate the plant-derived scaffolds in small animal models.

Funding Information

Research reported here was supported by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under Award Number R15EB033168. The content is solely the responsibility of the author and does not necessarily represent the official views of the National Institutes of Health.

Endnotes

1. Benjamin, E.J., et al. Heart disease and stroke statistics-2019 update: A report from the American Heart Association. Circulation, 2019. 139 (10): pp. e56-e528.

2. Pashneh-Tala, S., S. MacNeil, and F. Claeyssens. The tissue-engineered vascular graft: Past, present, and future. Tissue Eng Part B Rev, 2016. 22 (1): pp. 68-100.

3. Gershlak, J.R., et al. Crossing kingdoms: Using decellularized plants as perfusable tissue engineering scaffolds. Biomaterials, 2017. 125: pp. 13-22.

4. Modulevsky, D.J., C.M. Cuerrier, and A.E. Pelling. Biocompatibility of subcutaneously implanted plant-derived cellulose biomaterials. PLOS One, 2016. 11(6): p. e0157894.

5. Niklason, L.E. and J.H. Lawson. Bioengineered human blood vessels. Science, 2020. 370 (6513): p. eaaw8682.

6. Wong, V., et al. Development of small-caliber vascular grafts using human umbilical artery: An evaluation of methods. Tissue Eng Part C Methods, 2022.

7. Dikici, S., F. Claeyssens, and S. MacNeil. Decellularised baby spinach leaves and their potential use in tissue engineering applications: Studying and promoting neovascularisation. Journal of Biomaterials Applications, 2019. 34 (4): pp. 546-559.

8. Robbins, E.R., et al. Creation of a contractile biomaterial from a decellularized spinach leaf without ECM protein coating: An in vitro study. Journal of Biomedical Materials Research Part A, 2020. 108 (10): pp. 2123-2132.

Nicholas J. Merna is an assistant professor of engineering in the Bioengineering Program at the Fred DeMatteis School of Engineering and Applied Science at Hofstra University. He holds a PhD in Biomedical Engineering from UC Irvine and a BS in Bioengineering from UCLA. Dr. Merna teaches undergraduate classes in the areas of bioengineering, cell and tissue engineering, biomechanics, and computer programming. His research interests include plant- and animal-based decellularization, the design of bioreactor systems, and the development of tissue-engineered vascular grafts. His research team consists of undergraduate bioengineering students, and collaborators from the Department of Engineering and the Feinstein Institutes for Medical Research.

Finding Forms for Spatial Structures: Hanging, Heating, and Flipping

Edward M. Segal, PhD, P.E., Associate Professor of Engineering, Fred DeMatteis School of Engineering and Applied Science

It’s not every day that you’re asked to build an entire structure upside down and then flip it over. Structures are built in a range of ways, but upside down and then right side up? Why would anyone build this way?

Fibrous Shell

It was early in 2011, and I was working as a staff engineer at Simpson Gumpertz & Heger, a structural engineering firm in New York City. Two architects, Daniel Affleck and Pablo Kohan, were finalists in the City of Dreams

design competition; my colleague, Joseph Schuster, and I were going to help them with their submission, Fibrous Shell. The competition was to build a pavilion on Governors Island. The architects’ idea was to create a thin, curved spatial structure (i.e., a “shell”) using a composite made from natural materials, jute fibers, and bio-resin. They wanted to construct the shell upside down and then flip it over. The architects were not suggesting this approach just for the sake of doing something different. A flip was untested at this scale, but not unprecedented at smaller scales and had the potential

to create a structure using minimal amounts of material.

When a vine, a string, or a chain hangs under its own weight, it takes on a form called a catenary. This form is in pure tension. Tension is a pulling force, and elements that carry pure tension are incredibly efficient and can be very lightweight. This is the reason the longest bridges in the world are suspension bridges, like the Verrazzano-Narrows Bridge. If you freeze that hanging form and flip it over, you arrive at an arch form that is pure compression. Compression is a pushing force, and

elements that carry pure compression are also very efficient.

This method of generating forms is not limited to individual strings. The Spanish architect and engineer Antonio Gaudí created physical models in which he combined a series of hanging elements to create a network. He would record the network’s geometry and then use that to create the geometry for the inverted form that would become the final structure (Huerta, 2006). The Swiss engineer Heinz Isler pin and hung small-scale membranes coated in a material that would harden and become rigid. Once rigid, he flipped the pieces over to create models of overhead shell structures that he could measure and test. Physical modeling of this kind informed a number of his concrete roof structures (Billington, 2003). Designers typically use variations of the hanging and flipping technique to make small-scale design models (Figure 1), but there are examples of individuals using this technique to create full-scale works. For example, engineers at the Central Building Research Institute in Roorkee, India, used this technique to form a series of 4 ft. by 4 ft. concrete shells (Ramaswamy and Chetty, 1960). Architects at the University of Michigan used the technique to create Glass Cast, a series of glass sculptures (McGee et al., 2012).

Here, Affleck and Kohan were proposing using the method to create a full-scale structure that was much larger, nearly 25 ft. by 40 ft. (when viewed from above). Jute-fiber composite materials were largely untested, so their idea of creating an efficient form with this hanging and flipping method held a lot of appeal. The full-scale flip had the potential to

merge the form generation and the construction. Schuster and I performed calculations and materials research that resulted in changes to the form and influenced the final design. In the end, we lost the competition, but since then, I have been thinking about how we can possibly use this flipping technique at a large scale.

Two Blue Shells

Seven years later, I found an opportunity to try again. The International Association for Shell and Spatial Structures had a call for a competition to build a pavilion in Barcelona for its 2019 symposium. The team I worked with, Lisa Ramsburg, Albert Chao, and Powell Draper, was interested in exploring how to reuse waste and we focused on reusing plastics, eventually settling on acrylic. Additionally, the competition required that the parts for the pavilion fit into six boxes. This constraint led us to develop a structure made from a series of smaller parts. Our concept was to connect flat acrylic tiles into larger sheets that we could then hang at the full scale, disassemble, and then

reassemble in the flipped position (Ramsburg et al., 2019; Segal et al., 2021). But acrylic does not behave like a membrane that is initially flexible before hardening and becoming rigid. Acrylic is already rigid. If you hang acrylic, it won’t take on a tension-only form. You need to transform the material.

A flip was untested at this scale, but not unprecedented at smaller scales and had the potential to create a structure using minimal amounts of material.

Figure 1. A series of small-scale hanging models.

When you heat acrylic, it eventually passes a point called the glass transition temperature, and the material’s rigidity decreases significantly. The material has not melted; it is just very stretchy. When it becomes stretchy, it deflects and takes on an efficient curved shape. When you cool the acrylic, you are able to lock in this shape. Others have experimented with this heat-based form generation at the small scale (Bellés et al., 2009), but we were interested in making it work at a large scale. Our proposed pavilion was one of the structures selected for inclusion in the exhibition, and we had the opportunity to build it.

Plexi-Craft, a furniture manufacturer in the Bronx, donated hot pink acrylic scrap material for the project. So, while we had initially planned to use blue acrylic (which led to the project name, Two Blue Shells), we ultimately used hot pink acrylic. To heat the structure, we used Advanced Surface Finishing’s walk-in oven. With the help of Hofstra students, staff, and an alumna in

New York and the assistance of other volunteers in Spain, we completed the fabrication and installation (Figure 2). Following the exhibition, the pavilion returned home, and we found another opportunity to display it. Our structure was selected for CultureHub’s Re-Fest in New York City. The festival was intended to feature works by artists, activists, and technologists in late March 2020. For this venue, we installed one shell in the hanging position. The opening of the event coincided with the beginning of the COVID-19 pandemic. Consequently, the event went virtual.

Acrylic Pixel

During the pandemic, acrylic and polycarbonate barriers like the ones that appeared in stores and classrooms proliferated. Now many are struggling to figure out what is going to happen with all this plastic, since recycling streams for these kinds of plastics are not as common. With so much material potentially available, the same team I worked with on Two Blue Shells started

thinking about different ways we could utilize the material.

Figure 3 shows one idea, which we named Acrylic Pixel (Chao, 2022). Here, we created a modular system consisting of small acrylic panels that are heat-formed in the same way we created Two Blue Shells, but with a smaller oven. For this system, the individual panels are in orientations that are not structurally ideal, but they are so small that we can accommodate this inefficiency in the spirit of generating a visually interesting public installation that distorts and reflects light. We designed this piece for an exhibition that took place in Aalborg, Denmark, in July 2022. While we had hoped to use COVID-19 barriers for the project, we found that many places, including Hofstra, were not yet ready to remove and donate them. We ended up receiving a donation of scrap material from Curbell Plastics Inc. to complete the project. As organizations discard more acrylic and polycarbonate produced during the pandemic, there

Figure 2. Two Blue Shells, Barcelona, Spain.

Figure 3. Acrylic Pixel, Aalborg, Denmark.

will be many opportunities to use it across scales. With Acrylic Pixel, we created a small-scale installation, and we have discussed creating installations of various sizes and configurations such as one that is taller and that encircles visitors. There is also potential to use this scrap material and forming method for building façade elements.

Resilient

Complementing this work with heating and hanging acrylic, I began looking at heating and hanging flat grids of 3D-printed bioplastic (polylactic acid or xPLA, a common 3D printer material that is theoretically biodegradable) with then senior engineering student Esther Zhang. In summer 2021, when she participated in the DeMatteis School’s Advanced Summer Program in Research (ASPiRe) with me, we began discussing what would become the focus of her honors thesis and one of her senior design projects. We brainstormed two potential options for the project. The first option was to make a series of small models and

methodically test a range of variables. The second option was to decide on the size of a larger-scale prototype and just have Zhang figure out how to make it. She opted for the second option. With the chosen approach, design leads the way. The experiments are not as systematic; they are in the service of making sure the structure stands.

For the larger scale prototype, Zhang settled on an arch-like structure that was over 10 ft. long by a few feet wide. To generate a structure this large, she designed a series of long but narrow grids. To create these long, narrow grids, she worked with Eugene Chang, one of the founders of the 3D printing company Tangible Creative, who printed the grids on his conveyor belt 3D printer for free. This kind of 3D printer allows you to make long objects by printing a portion of the object, moving the conveyor belt forward, printing more of the object, etc.

We then bolted the grids together and hung the piece in a walk-in oven at

Colorlife Powdercoating Concepts. We closed the oven and set the temperature to 150 F, but the temperature kept rising right past our set point. From our observations in the lab, we thought it would be all right to go a bit above that temperature and did not start to worry until we saw the temperature reach 175 F with no sign of stopping. At that point, we turned off the oven, and opened the door. We were stunned; the

When you heat acrylic, it eventually passes a point called the glass transition temperature, and the material’s rigidity decreases significantly.

Figure 4. 3D-printed structures hanging in an oven after heating.

Figure 5. Resilient, FORM Gallery, Hofstra University.

piece had deformed so much that it was in contact with the bottom of the oven and not in the proper form. We had spent six months troubleshooting and planning out different scenarios and now, over the course of a few minutes, everything changed.

After getting over our initial shock and letting the oven cool, we took a closer look and realized that nothing had broken. The whole structure was still intact. Zhang had previously conducted a series of experiments in which she explored the potential of reheating to re-form PLA structures. What if, for this piece, we just rearranged the supports and heated again in the walk-in oven?

We divided the larger structure into two smaller structures, and this time we cautiously heated the piece, paused heating, checked the deformation, and continued. The modification was a success. Figure 4 is a final image of the two structures hanging in the oven after heating. We temporarily displayed

the piece, Resilient, first, in Hofstra’s FORM Gallery (Figure 5) and then, in Hofstra’s Axinn Library.

Prototyping with 3D Printing Pens

Small-scale prototypes were critical in developing Resilient. While we made many of these prototypes with desktop and conveyor belt 3D printers, one prototype was made with a 3D printing pen. 3D printing pens operate much like hot glue guns, but deposit PLA instead of glue. Since 3D printing pens are relatively low in cost, don’t rely on modeling software, and don’t require specialized skills, it seems like an ideal tool for students and other designers to rapidly generate small-scale models before settling on a final geometry that can be scaled up for a larger structure.

As part of the Hofstra Rabinowitz Honors College Undergraduate Research Assistant Program, Lillian Moy began developing a design-tofabrication method using 3D printing pens and a small oven. She has been

working to bring down the time to draw, hang, heat, and flip the models to under one hour. While the method works well for regular grids, one of the method’s primary benefits may be freeing designers to explore more free form designs (Figure 6).

Conclusions

It has now been 12 years since I worked on Fibrous Shell. The project went unrealized, but one of its core concepts sent me down a path that ultimately led to my building a few pavilion-scale projects upside down. Now, these large-scale structures have led me toward creating small-scale design models with a twist on an old technique – which I would not have stumbled upon if I had not experienced everything else.

While computational modeling has replaced a lot of physical modeling, including to create hanging models to generate efficient structures, I will continue to make physical models with colleagues and students as a primary

Figure 6. Free form design drawn with a 3D printing pen (left, credit: Lillian Moy) and the same design after hanging, heating, and flipping (right).

way to explore ideas. Computational modeling is quicker and less messy, but physical making leads to serendipity.

Acknowledgments

All the projects that I described were collaborations with others, including architects, artists, fellow engineers, and students. Thank you to the following collaborators:

• Fibrous Shell: Daniel Affleck, Pablo Kohan, and Joseph Schuster

• Two Blue Shells and Acrylic Pixel: Albert Chao, Powell Draper, and Lisa Ramsburg

• Resilient : Esther Zhang

• Prototyping with 3D printing pens: Lillian Moy

I appreciate the support that the projects received from Advanced Surface Finishing, Colorlife Powdercoating Concepts, Curbell Plastics Inc., Plexi-Craft, Tangible Creative, DeMatteis School of Engineering and Applied Science Faculty Research and Development Grants, and Hofstra’s Rabinowitz Honors College Undergraduate Research Assistant Program.

Finally, thank you to Robert Cerro, Keith MacKenzie, Michael Herrick, Professor Jim Lee, Dr. J Bret Bennington, and the many others that helped realize these projects.

References

Bellés, P., N. Ortega, M. Rosales, and O. Andrés. 2009. Shell form-finding: Physical and numerical design tools, Engineering Structures 31(11), 2656-2666.

Billington, D. P. 2003. The Art of Structural Design: A Swiss Legacy. Princeton, NJ: Princeton University Art Museum.

Chao, A., E. M. Segal, L. Ramsburg, and P. Draper. 2022. Reuse and misuse with heat formed acrylic. In 5th International Conference on Structures & Architecture (ICSA 2022), Aalborg, Denmark, July 6-8.

Huerta, S. 2006. Structural design in the work of Gaudí. Architectural Science Review 49 (4): 324-339.

McGee, W., C. Newell, and A. Willette. 2012. Glass cast: A reconfigurable tooling system for free-form glass manufacturing. In ACADIA 2012, 287-294. San Francisco, October 18-21.

Ramaswamy, G. S., and S. M. K. Chetty. 1960. A new form of doubly-curved shells for roofs and floors. Bulletin of the International Association for Shell and Spatial Structures 1: 43-50.

Ramsburg, L., A. Chao, E. M. Segal, and P. Draper. 2019. Design of discretized acrylic shells with heat-induced form-finding. In International Association for Shell and Spatial Structures Symposium and Structural Membranes 2019, 482-489. Barcelona, Spain, October 7-10.

Segal, E. M., A. Chao, L. Ramsburg, and P. Draper. 2021. A pavilion made from scrap acrylic and physically form-found at full scale. Journal of the International Association for Shell and Spatial Structures 62 (3): 223-226.

Edward M. Segal is an associate professor of engineering at the Fred DeMatteis School of Engineering and Applied Science at Hofstra University and leads the Segal Structures Group. The group engages in material exploration, form generation, and historic analysis related to a range of engineering research, design, and teaching activities. Dr. Segal received an SOM Foundation Structural Engineering Travel Fellowship in 2008 and the ExCEEd (Excellence in Civil Engineering Education) Teaching Award from the American Society of Civil Engineers (ASCE) in 2017. In 2019, he was selected to be an ASCE ExCEEd Fellow and in 2020, he was named Teacher of the Year in the Fred DeMatteis School of Engineering and Applied Science. He holds a BS from Cornell University and an MSE and PhD from Princeton University. From 2008 to 2011, Dr. Segal worked at Simpson Gumpertz & Heger designing glass and metal enclosures. He is a licensed professional engineer in New York.

Why Is My Virtual Machine in the Cloud so Slow?

Jianchen Shan, PhD, Assistant Professor of Computer Science, Fred DeMatteis School of Engineering and Applied Science

Introduction

Cloud computing has become ubiquitous today. Most applications are hosted in the cloud for scalable and low-cost computing power. Behind the scenes, virtual machines are the building blocks created in the modern cloud infrastructure to provide resources, like CPU, memory, storage, network, etc. However, people often find that their virtual machines in the cloud are super slow, causing their applications to suffer from poor and unpredictable performances.

A famous analogy considers cloud computing as the new electricity, and people can simply plug in and get computing power on demand. However, the low quality of the cloud resources is preventing this vision from coming true.

In past years, my major research efforts focused on answering this question: Why is my virtual machine in the cloud so slow? It is a myth, since many efforts have been made to reduce the virtualization overhead with state-of-the-art hardware accelerations that promise to provide native-like performance to the virtual

machine. After investigation, we found that this is due to the nature of the cloud: multi-tenancy with resource sharing. The hypervisor, also called the virtual machine manager (VMM), is a software that creates and manages virtual machines, and would usually co-locate several virtual machines to share the same cloud server. The benefits are greater efficiency, cost savings, and robust security. However, extensive experiments show that the interference among the neighboring virtual machines due to resource sharing can significantly offset the benefits. In other words,

multi-tenancy is a double-edged sword in the modern clouds.

Overprovision can be an easy solution to make your virtual machine run faster by allocating more or dedicated resources. But it’s not an ideal answer, since the tenant will be paying for resources that are not being fully utilized. Thus, it is crucial to fully understand how resource sharing degrades the performance of the applications hosted in the virtual machine. Based on this understanding, solutions can be developed to turn the double-edged sword into one action that achieves both goals.

Through systematic experiments and analysis, we found that the multi-tenancy of the cloud would fundamentally change the characteristics of the virtualized resources. Compared to the physical resources that are static, virtualized resources are highly dynamic. A virtual machine may receive more resources when the neighbors are idle but may suffer from limited resources when there is high contention with resource-demanding neighbors. The operating system running inside the virtual machine is unaware of such dynamics and would make wrong decisions, resulting in poor, sometimes abnormal, virtual machine performance. The following sections detail several major findings published during my time at Hofstra University.

The Virtual CPU Is not Always Active

The physical CPU is always on and ready to process some tasks. Sometimes, for energy efficiency, the physical CPU may put itself into a deep sleep state if there is no job. But it

can quickly wake up to respond to new tasks. However, the virtual CPU of a virtual machine may constantly become inactive, not being able to process or respond to any tasks. This is because the hypervisor would schedule multiple virtual CPUs to time-share the same physical CPU. When one virtual CPU has depleted its time slice, it would get switched out and yield the physical CPU to other virtual CPUs to rotate. The inactivity of the virtual CPU breaks the assumption held by the virtual machine and introduces several performance problems.

One such issue is the excessive spinning when the spinlock is used for task synchronization within the virtual machine. Spinlock is a common method to coordinate tasks in which tasks will keep spinning (i.e., doing nothing but keeping the processor busy) while waiting for the lock holder to release the lock. This is a practical mechanism when used in the physical machine since the lock holder is expected to release the lock quickly to prevent other lock waiters from spinning for a long time. On the contrary, in the virtual machine, when a virtual CPU that holds the spinlock becomes inactive after getting

descheduled, other virtual CPUs that are waiting for the lock would have to spin excessively, wasting the CPU cycles.

To mitigate this issue, hardware facilities are provided on processors to preempt the virtual CPUs when they spin excessively, but experiments show that they don’t work for all workloads.

A virtual machine may receive more resources when the neighbors are idle but may suffer from limited resources when there is high contention with resource-demanding neighbors.

Figure 1. Virtual CPU (vCPU) inactivity leads to I/O inactivity.

There are two major challenges: It is difficult to determine the best time to preempt a spinning virtual CPU, and it is difficult to choose which virtual CPU to run after the spinning virtual CPU is descheduled due to the semantic gap between the virtual machine and physical machine. Even worse, the hardware facilities cannot detect the user-level spinning, which is commonly used by many applications.

The virtual CPU inactivity also leads to the I/O inactivity problem. In each virtual machine, when its virtual CPUs running I/O-bound tasks are descheduled, no I/O requests can be made until the virtual CPUs are rescheduled to be active again, as shown in Figure 1. These inactivity periods of I/O tasks cause severe underutilization of I/O resources by the virtual machine, since the inactive periods can be much longer than the latencies of storage devices. The underutilization becomes more serious with a higher consolidation rate (i.e., the number of virtual CPUs shared on

each physical CPU), because a virtual CPU may need to wait for multiple time slices before being rescheduled.

The I/O inactivity problem also causes the I/O scheduler running in the cloud server to work extremely ineffectively. To fully utilize the storage devices, based on the latencies of I/O devices, system designs would carefully control the factors affecting the latencies experienced by I/O workloads (e.g., wake-up latencies and priorities). Thus, I/O workloads running on bare metal can issue the next request after the previous request is finished. I/O inactive periods make these mechanisms ineffective.

Likewise, we found a similar issue in the GPU cloud. The virtual CPU inactivity can also lead to virtual GPU inactivity. For a virtual machine equipped with GPU, its virtual CPU becoming inactive would severely interfere with the CPU-GPU synchronization. During the inactive period, GPU tasks cannot be sent to the

GPU, and the GPU task completion cannot be detected and acknowledged, leaving the virtual GPU inactive and underutilized.

Careful fine-granular profiling with state-of-the-art tools proved this issue by comparing the timeline of a workload’s CPU and GPU activities in the virtual machine and in the physical machine side by side. In comparison, physical machines are able to immediately process GPU tasks without the risk of CPU being descheduled, as shown in Figure 2. Even worse, the GPU workloads with frequent task offloads and synchronizations, such as machine learning and gaming cloud, are more vulnerable to performance interference from virtual CPU inactivity.

The Virtual CPU Capacity Is Highly Dynamic

Due to the time-sharing, the capacity of a virtual CPU is determined by the co-running virtual CPUs. On one hand, a virtual CPU can reach its maximal capacity when there are no co-running virtual CPUs on the same physical CPU or all of the co-running virtual CPUs are idle. In this case, all the CPU cycles can be utilized by the virtual CPU due to the work-conserving principle, which may even allow a virtual CPU to consume more CPU time than it was assigned. On the other hand, a virtual CPU’s capacity can be severely limited when the physical CPU is highly contended or overcommitted by multiple busy virtual CPUs. The contention varies on different physical CPUs, which makes the virtual CPU capacities of a virtual machine dynamically asymmetric.

CPU GPU vCPU vGPU

Figure 2. vCPU inactivity leads to virtual GPU (vGPU) inactivity.

To experimentally prove the above observation, we developed a multithreaded tool that launches one thread on each virtual CPU running a CPU-bound task (i.e., incrementing a counter). Each thread will periodically collect the steal time, which is only available within the virtual machine to indicate the percentage of the time that the virtual CPU has to wait while other co-located virtual CPUs are running on the same physical CPU. For example, if the tool reports that the steal time of a virtual CPU over a certain period is 30%, then the vCPU capacity is 70% of the total CPU time in this period.

The tool is implemented inside the virtual machine to allow measurement in the public cloud where the user has no access to the hypervisor. We deployed our tool in an Amazon Web Service cloud instance, which has 8 vCPUs time-sharing a cloud server with other virtual machines. We ran the tool for 30 minutes. Figure 3 (left) is a snapshot of the vCPU capacity with min-max values during the test. Figure 3 (right) shows how the coefficient of variation (CV) of the vCPU capacity distribution and the total steal time of

the VM change over the course of the experiment. The results clearly demonstrate that the vCPU capacity can be significantly asymmetric. Moreover, such asymmetry changes notably over time.

However, most hypervisors expose the virtual CPU capacities to a virtual machine with the assumption that they have a similar architecture to the hosting cloud server, where the capacities of the physical CPUs are usually static and symmetric. As evidence, the virtual machines with multiple virtual CPUs on x86 architecture are called virtual symmetric multi-processing (SMP) systems. This mismatch would introduce performance issues by misleading some system components that make decisions based on the virtual CPU capacity.

For example, the Linux load balancer works well to evenly distribute the workload across multiple CPUs and succeeds in maintaining high CPU utilization. However, our experiment shows that the Linux load balancer performs poorly in virtualized

environments. Specifically, the CPU time available to a virtual machine cannot be fully utilized. The Linux load balancer with the false assumption in the virtual machine may make uninformed and wrong decisions, such as leaving a low-capacity virtual CPU overloaded or leaving a high-capacity virtual CPU under-loaded, which makes the virtual machine performance vulnerable to interference from the neighboring virtual machines. Also, the mismatch prevents the load balancer from taking advantage of the unique asymmetric capacity, such as matching the virtual CPU capacity and task loads or migrating critical tasks to the strong and stable virtual CPU to improve the application performance and increase resource utilization.

The Virtual Memory Is not Contiguous

In the physical machine, the memory is normally presented as a contiguous sequence of memory pages (i.e., memory blocks that are in the fixed size) that can be allocated to the user applications. The allocated pages are not disclosed to the application for security reasons. Address translation

Coefficient of Variation

Figure 3. vCPU capacity is dynamic and asymmetric in the cloud.

with a page table is performed by the operating system to retrieve the allocated pages for applications. However, in the virtual machine, the virtualized memory may be a sequence of discontiguous memory pages allocated from the physical machine.

This is due to the space sharing of the physical memory among the co-located virtual machines in the cloud server. For example, memory ballooning is used efficiently to manage memory sharing. The physical pages that are allocated to one virtual machine and haven’t been used recently can be temporarily reassigned to another virtual machine that has high memory demand. Later, the hypervisor may find other free pages to be returned to the previous virtual machine. As we can see, the physical pages allocated to a virtual machine can be dynamically changed with no guarantee that it is always contiguous. This broken assumption could introduce performance issues, especially for those

memory-intensive and big-memory workloads, such as in-memory databases and big-data analytics.

One issue we discovered is the huge page misalignment problem. Using huge pages has become a mainstream method to reduce address translation overhead. To create huge pages, system software usually uses page coalescing methods to dynamically combine contiguous base pages. Due to the broken assumption, their effectiveness is substantially undermined on virtualized platforms. A huge page created in the virtual machine can hardly help reduce address translation overhead if it is not backed by another huge page in the physical machine. As shown in Figure 4, VM1 created a misaligned huge page that is backed by scattered base pages in the physical machine. Existing page coalescing methods don’t consider dynamics in the virtualized memory and cannot effectively reduce the address translation overhead in the cloud.

Even worse, the discontiguous virtualized memory may increase the conflicts in the CPU cache in virtualized clouds, because memory page placement mechanisms become ineffective in reducing cache conflicts on these platforms. Implemented in system software, page placement mechanisms reduce cache conflicts by improving the allocation of memory pages to applications. They are important measures particularly when cache associativities are too low to effectively absorb cache conflicts. Leveraging the fixed mapping between pages (i.e., page physical addresses) and cache sets, they first identify conflicting pages (i.e., pages mapped to the same cache sets), and then allocate nonconflicting pages to hold the data to be accessed together.

Different mechanisms allocate nonconflicting pages with different page placement policies. For example, the popular page coloring method assumes sequential memory addresses and allocates nonconflicting pages that have different colors. The number of cache colors is determined by the number of cache sets. Each page is labeled with a color, which is the index of the corresponding cache color. When allocating pages, the page coloring mechanism tries to allocate pages in different colors, such that the data in these pages can be evenly mapped to different cache sets.

However, in the virtual machine, this page placement mechanism becomes ineffective. Figure 4 illustrates a case in which VM2 attempts to allocate four pages with different colors, but the selected four virtual pages are actually backed by the physical pages with the same color in the cloud server. Due to the semantic gap and security reasons, the virtual machine is unaware of such a mismatch, causing unexpected cache conflicts.

Figure 4. Issues caused by discontiguous virtualized physical memory.

Conclusion

In this article, we point out that multi-tenancy with resource sharing in modern clouds is the major culprit of slow virtual machines. Due to resource sharing, compared to static physical resources, virtualized resources are highly dynamic. Many assumptions that are true for physical resources do not hold true for virtualized resources. Many uninformed decisions are made because of the unawareness of such mismatches, degrading the application performance in the cloud virtual machines. To tackle these issues, we have developed several practical solutions [1-10, most recent listed first]. In future work, we will continue to explore the performance issues caused by the unique characteristics of the virtualized resources to improve the user experience of their virtual machines in the cloud.

Endnotes

[1] Weiwei Jia, Jiyuan Zhang, Jianchen Shan, and Xiaoning Ding. Making dynamic page coalescing effective on virtualized clouds. In Proceedings of the 18th European Conference on Computer Systems (EuroSys, 2023).

[2] Weiwei Jia, Jiyuan Zhang, Jianchen Shan, Jing Li, and Xiaoning Ding.

Achieving low latency in public edges by hiding workloads mutual interference. In Proceedings of the 13th Symposium on Cloud Computing (SoCC, 2022), pp. 477-492.

[3] Schildermans Stijn, Jianchen Shan, Kris Aerts, Jason Jackrel, and Xiaoning Ding. Virtualization overhead of multithreading in x86 state-of-the-art and remaining challenges. IEEE Transactions on Parallel and Distributed Systems 32, no. 10 (2021): pp. 2557-2570.

[4] Schildermans Stijn, Kris Aerts, Jianchen Shan, and Xiaoning Ding. Paratick: Reducing timer overhead in virtual machines. In 50th International Conference on Parallel Processing (2021), pp. 1-10.

[5] Shang Xiaowei, Weiwei Jia, Jianchen Shan, and Xiaoning Ding. CoPlace: Effectively mitigating cache conflicts in modern clouds. In 30th International Conference on Parallel Architectures and Compilation Techniques (PACT) (IEEE, 2021), pp. 274-288.

[6] Youssef Elmougy, Weiwei Jia, Xiaoning Ding, and Jianchen Shan. Diagnosing the interference on CPU-GPU synchronization caused by CPU sharing in multi-tenant GPU clouds. In IEEE International Performance, Computing, and Communications Conference (IPCCC) (IEEE, 2021), pp. 1-10.

[7] Weiwei Jia, Jianchen Shan, Tsz On Li, Xiaowei Shang, Heming Cui, and Xiaoning Ding. vSMT-IO: Improving I/O performance and efficiency on SMT processors in virtualized clouds. In USENIX Annual Technical Conference (USENIX ATC 20) (2020), pp. 449-463.

[8] Matthew Elbing and Jianchen Shan. The Linux load balance: Wasted vCPUs in clouds. In IEEE Cloud Summit (IEEE, 2020), pp. 174-175.

[9] Schildermans Stijn, Kris Aerts, Jianchen Shan, and Xiaoning Ding. Ptlbmalloc2: Reducing TLB shootdowns with high memory efficiency. In IEEE International Conference on Parallel and Distributed Processing with Applications, Big Data and Cloud Computing, Sustainable Computing and Communications, Social Computing and Networking (ISPA/ BDCloud/SocialCom/SustainCom) (IEEE, 2020), pp. 76-83.

[10] Jia, Weiwei, Cheng Wang, Xusheng Chen, Jianchen Shan, Xiaowei Shang, Heming Cui, and Xiaoning Ding. Effectively mitigating I/O inactivity in vCPU scheduling. In USENIX Annual Technical Conference (USENIX ATC 18) (2018), pp. 267-280.

Jianchen Shan is an assistant professor of computer science at the Fred DeMatteis School of Engineering and Applied Science at Hofstra University. He holds a PhD in Computer Science from New Jersey Institute of Technology, and an MS and BS in Computer Science from Shanghai University. His research interests and experience span the areas of cloud computing, parallel and distributed computing, high-performance computing, mobile computing, and operating systems. His work has been published in several high-impact journals and presented at numerous conferences, such as IEEE Transactions on Parallel and Distributed Systems, IEEE Transactions on Cloud Computing, USENIX Annual Technical Conference, The European Conference on Computer Systems, International Conference on Parallel Processing, International Conference on Parallel Architectures and Compilation Techniques, Symposium on Cloud Computing, and International Conference on Massive Storage Systems and Technology. He received Hofstra’s Lawrence A. Stessin Prize for Outstanding Scholarly Publication in 2021-2022.

Balancing Protection against Pathogens with Exposure to Toxic Chemicals

Minjeong Suh, PhD, Assistant Professor of Engineering, Fred DeMatteis School of Engineering and Applied Science

Abstract

With the increase in foodborne pathogen outbreaks, the application of chemical disinfectants to control pathogens in fresh produce at critical control points is rising. However, these chemical disinfectants can interact with fresh produce to form potentially toxic disinfection byproducts (DBPs). After a brief overview of food pathogenic outbreaks and chemical disinfection technologies, this article describes the risks related to DBP formation and accumulation within fresh produce. We first

discuss the formation and accumulation of chlorate (an inorganic DBP) inside fresh produce when irrigated with water containing two types of disinfectants, free chlorine and chloramine. Results indicate that sunlight increases the amount of chlorate formed and detected in produce, and that higher levels of chlorate are formed under both light and dark conditions in free chlorine solutions than in chloramine solutions. These results show that selection of the chemical disinfectant is critical in developing strategies to achieve effective

disinfection while minimizing the risks of DBP consumption for the public. The second project examines the initial transformation products of disinfectant reaction with biomolecules, a novel class of DBPs. We found that these products (chlorotyrosines and chlorohydrins) formed as amino acids and fatty acids, respectively, reacted with free chlorine, and present a risk for consumer exposure by remaining within the food. Cytotoxicity analyses revealed that these products were several times more toxic than DBPs that are currently regulated in U.S. drinking water, highlighting

the need for further research on the concentrations of these products in different types of fresh produce. Similar to drinking water disinfection, the food processing industry will need to optimize disinfectant exposures to balance pathogen inactivation against exposure to potentially toxic DBPs.

Foodborne Pathogen Outbreaks

The application of chemical disinfectants to inactivate pathogens in drinking water was one of the most important public health achievements of the past century. With this barrier against microbial contamination of drinking water, food has emerged as a critical pathogen exposure route in the U.S. An estimated 76 million illnesses are associated with foodborne pathogens per year in the U.S., compared with a conservative estimate of 19.5 million illnesses per year attributable to tap water.1 Examples include 199 illnesses across 26 states linked to Escherichia coli O157:H7 on spinach in 20062 and 19 illnesses across nine states associated with Listeria on lettuce in 2016.3 Consequentially, foodborne pathogens are estimated to cause nearly 50 million illnesses and 3,000 deaths each year in the U.S. Based on data from 2000 to 2008 provided from the U.S. Centers for Disease Control and Prevention (CDC), foodborne illnesses are estimated to cost $55 billion to $93 billion annually in the U.S., considering treatment costs and lost productivity.4 In addition to the illnesses and costs associated with foodborne pathogenic outbreaks, the decline in the public’s trust in fresh food supplies necessitates

interventions to ensure sufficient protection from foodborne pathogens.

Chemical Disinfection

The rise in foodborne pathogenic outbreaks culminated in the Food Safety Modernization Act of 2011.5 To control foodborne pathogenic outbreaks, irrigation and washing fresh produce with disinfected water represent critical control measures. Historically, the use of chemical disinfectants, most commonly free chlorine, has proved effective in suppressing pathogenic contamination in wastewater and drinking water treatment.

However, in the 1970s, research showed that reactions between chemical disinfectants and organic molecules in water led to the formation of potentially carcinogenic disinfection byproducts (DBPs).6 As a result, drinking water facilities have altered disinfection strategies to balance the acute risk posed by pathogens and the chronic risk posed by DBPs.7,8 As disinfection of waters used in the fresh

produce industry becomes increasingly prevalent, it will be equally important to balance these risks.

In our work, we identify two control points critical in preventing pathogenic contamination of fresh produce: irrigation and post-harvest food washing (Figure 1). We investigate the formation of different DBPs and their

The application of chemical disinfectants to inactivate pathogens in drinking water was one of the most important public health achievements of the past century.

Figure 1. Schematic providing a research overview.

accumulation inside vegetables when chemical disinfectants (specifically, chlorine-based chemical disinfectants) are applied to waters used in these control points to evaluate the risks associated with these DBPs when the produce is consumed by members of the public.

Irrigation

The increasing use of disinfectantcontaining irrigation waters to minimize foodborne pathogens is raising concerns about the formation and uptake of DBPs into irrigated produce. In addition to disinfection of irrigation water to prevent pathogenic contamination, the increasing use of reclaimed municipal wastewater for irrigation in the arid western U.S. means that the application of disinfected irrigation waters is expected to grow. For example, the Castroville Seawater Intrusion Project in California provides 4 billion gallons of reclaimed municipal wastewater annually to irrigate food crops.

After municipal wastewater treatment, the resulting water carries disinfectant residuals of up to ~3 mg/L as Cl 2 to prevent microbial contamination of water during its distribution. These disinfectants include both free chlorine and chloramine. Free chlorine is composed of hypochlorous acid (HOCl) and its conjugate base, hypochlorite (OCl-), and is a more effective disinfectant than chloramines. Chloramines (predominantly monochloramine (NH 2Cl)) are formed when free chlorine is added to ammonia-containing waters. Although less effective for disinfection, chloramines provide a longer-lasting residual. Moreover, municipal wastewater often contains ammonia, such that chloramines will form when chlorine is added for disinfection during reuse.

Among various DBPs, chlorate (ClO3 -) has received particular attention, because chlorate can behave as a competitive inhibitor of

iodide in the thyroid. As a result, the European Union has set temporary Maximum Residue Levels (MRLs) for various foods (e.g., 0.7 mg/kg for leafy vegetables). This is a concern, since previous research has shown that free chlorine decomposes via disproportionation reactions under dark conditions to form chlorate. These reactions can produce chlorate within the irrigation water prior to its application to plants. While previous research has demonstrated an increase in chlorate concentrations in vegetables when the plants take up the chlorate occurring in chlorine-containing irrigation waters from soil through their roots, our hypothesis was that chlorate can also be taken up directly via leaves in addition to the roots. The reactions of free chlorine and chloramine to form chlorate change under sunlit conditions, but the extent of this effect was previously unknown. We aimed to provide quantitative data to understand chlorate formation under conditions relevant to irrigation applications to help

Figure 2. Chlorate formation from 50 µM (3.6 mg/L as Cl2) (a) free chlorine and (b) monochloramine solution in a solar simulator at 21 degrees Celsius and pH 7 (2.5 mM phosphate buffer). Error bars represent the standard error of experimental triplicates for chlorate and the range of experimental duplicates for chlorine residual; many error bars are smaller than the symbols. Time (x-axis) refers to the time of irradiation by the solar simulator, designed to simulate natural sunlight.

practitioners select between disinfectant types (e.g., chlorine or chloramines) and time of irrigation water application (day/night) to minimize chlorate accumulation in produce.9

Our results demonstrated that chlorate formation can be significant after chlorine-containing irrigation droplets contact plant surfaces via two pathways. Using free chlorine solutions of concentrations representative of disinfectant residual concentrations encountered in municipal distributions, solar irradiation significantly enhanced the formation of chlorate. Sunlight photolysis of 50 µM (3.6 mg/L as Cl2) chlorine significantly enhanced the formation of chlorate, with a 7.2% molar yield relative to chlorine (Figure 2a). This enhancement can be attributed to the formation of reactive radical species that introduce new reaction pathways for chlorate formation. Previous research with the UV/chlorine systems has demonstrated that photolysis of chlorine or chloramines at 254 nm produces hydroxyl (•OH) and chlorine (•Cl) radicals, which can form chlorate under our reaction conditions. On the other hand, sunlight-driven chlorate formation was much less significant in monochloramine-containing solutions (Figure 2b). These results suggest that monochloramine, and likely other chloramines, would be the optimal choice in disinfectant over free chlorine in order to minimize chlorate-related risks in fresh produce.

We found that chlorate formation on irrigation droplets was similarly enhanced on leaf surfaces. When we repeated the same experiment on spinach leaf surfaces, chlorate levels inside spinach leaves were higher under light conditions than under dark conditions. In addition, we found that chlorate formed in water droplets during irrigation was directly absorbed

into spinach leaves, further proving the importance of understanding the risks of chlorate formation from irrigation water containing chlorine or chloramine. When live vegetables (broccoli, cabbage, chicory, and lettuce) were sprayed with chlorine-containing irrigation water in a sunlit field, sunlight promoted chlorate formation and uptake through the vegetable surfaces to concentrations above the regulatory limits (Maximum Residue Levels) in the European Union (Figure 3). This is significant, as produce with chlorate levels above these MRLs cannot be exported to countries in the European Union. Spraying with chloramine-containing waters in the dark minimized chlorate formation and uptake into the vegetables. Overall, this study indicated that application of chloramine-containing irrigation waters at night would minimize chlorate concentrations in vegetables treated with disinfected irrigation waters.

Post-Harvest Produce Washing

Another critical control point in minimizing pathogenic contamination of fresh produce is post-harvest produce washing facilities. In post-harvest washing facilities, produce is rinsed in a series of tanks, supplemented with hydraulic sprays over short contact periods (< 5 min), and depending on the produce, at low temperatures (~5 C). Chlorine is the most prevalent disinfectant applied to prevent the spread of pathogens within the wash waters, while other disinfectants (chloramine, chlorine dioxide) are also used. There are critical differences in conditions encountered in drinking water disinfection, in that these produce wash waters have high concentrations of organic matter and compounds, which means that much higher disinfectant concentrations (~75-200 mg/L as Cl2) are used than in drinking water treatment facilities (~5 mg/L as

Cl2). This, in turn, means that higher concentrations of DBPs may form under these conditions used in produce washing facilities.

We focused on a novel class of DBPs, which are the chlorinated products that form when free chlorine reacts with biomolecules inside fresh produce.10 A subset of the monomers constituting biopolymers are reactive with chlorine, including unsaturated fatty acids within lipids, and certain amino acids within proteins. We selected oleic acid and tyrosine as exemplar unsaturated fatty acid and amino acid, respectively, for this study. During chlorination of lettuce and spinach, chlorotyrosines formed at comparable or greater concentrations than the sum of all low molecular weight DBPs, even though low molecular weight DBPs could form from multiple precursors, while chlorotyrosines would form only from tyrosine.11 Similar results were observed for chlorhydrins, which form as chlorine adds across the double bond in fatty acids.

While chlorotyrosines and chlorohydrins have not been researched

The increasing use of disinfectant-containing irrigation waters to minimize foodborne pathogens is raising concerns about the formation and uptake of DBPs into irrigated produce.

Figure 3. Chlorate concentrations measured in broccoli, cabbage, chicory, and lettuce, after spraying the vegetable surfaces with deionized water containing 50 µM chlorine (Cl2) or monochloramine (NH2Cl) at pH 7 (2.5 mM phosphate buffer) with and without exposure to natural sunlight outdoors for 1 h. Controls indicate the levels of chlorate measured in samples collected without spraying irrigation waters. Only the controls and sunlit chlorine and monochloramine experiments were conducted with broccoli. Error bars represent the standard error of experimental replicates (n = 3 for spinach, and 9 for all other vegetables). The minimum detection limits for chlorate ranged from 40-100 µg/L in the vegetable matrices, which corresponded to ~0.04-0.1 µg/g vegetable.

extensively in environmental engineering, other DBPs (e.g., trihalomethanes (THMs) and haloacetonitriles (HANs)) have been widely investigated and are regulated in drinking water in the U.S. due to their toxicity. These are suspected to form and persist in our drinking water as organic matter naturally occurring in water goes through water treatment and distribution systems. For postharvest produce washing steps, these regulated, lower molecular weight DBPs were found primarily in the wash water, while chlorotyrosines and chlorohydrins were retained within the produce. Our hypothesis is that these chlorinated biomolecules are retained within the macromolecules (i.e., proteins and lipids), and will be liberated during digestion, enabling

uptake within the intestines and presenting a risk to consumers.

The health risks posed by these high levels of chlorotyrosines and chlorohydrins are even more concerning when we consider their cytotoxicity. The LC50 value from the CHO cell chronic cytotoxicity data was used to measure the toxicity of the chlorotyrosines and chlorohydrins, where a lower LC50 value corresponds to higher toxicity. The LC50 value of the chlorohydrin was 0.106 mM, which is significantly lower than the LC50 values for 3-chlorotyrosine (3.17 mM) and 3,5-dichlorotyrosine (0.71 mM) (the two different chlorotyrosines), indicating that the oleic acid 9,10-chlorohydrins are more cytotoxic by ~30-fold and ~7-fold, respectively.

To estimate the importance of exposure to DBPs in chlorinedisinfected vegetables relative to chlorinated drinking water, we considered a drinking water containing 80 µg/L chloroform and 60 µg/L trichloroacetic acid, the Maximum Contaminant Levels for regulated DBPs, trihalomethanes and haloacetic acids. The LC50 values for chloroform and trichloroacetic acid are 9.63 mM and 2.4 mM, respectively, indicating that they are less cytotoxic than oleic acid 9,10-chlorohydrins and 3,5-dichlorotyrosine.

To put this into perspective, consumption of 2 L of water per day would involve exposure to 1.34 µmol chloroform and 0.74 µmol trichloroacetic acid. These masses were normalized by their respective LC50 values, providing cytotoxic potency-weighted exposures of 1.4 × 10 - 4 for chloroform and 3.07 × 10 - 4 for trichloroacetic acid, and summing these cytotoxic potency-weighted exposures provides a total daily exposure of 4.47 × 10 - 4 from the water. We calculated how many grams of each vegetable (on a wet weight basis) would provide a total cytotoxic exposure equivalent to the 2 L of drinking water. The results indicated that 9-52 g of shredded or cubed vegetables washed with 100 mg/L as Cl 2 chlorine for 2 min at pH 7 and 7.5 C provided an equivalent cytotoxic exposure. Only 9-13 g of butterhead lettuce, broccoli, carrots, and kale were needed, which are low relative to the 85 g lettuce (~1.5 cups) and 148 g broccoli (~1 cup) recommended daily intakes; for perspective, 10 g broccoli is approximately one fork full.

Conclusion and Implication

These studies highlight the risks of disinfection byproducts in our attempts to reduce the risks of foodborne pathogens. The results underscore the critical need to optimize the

chemical disinfection practices for pathogen control for fresh produce in order to control the threats these toxic DBPs pose. Noteworthy is that oleic acid used in this research is one of many biomolecules featuring alkene functional groups, including other fatty acids and pigments (e.g., carotene in carrots). For instance, oleic acid (18:1) constitutes only 1.1% of the total fatty acids in spinach, relative to the 3.5% accounted for by palmitoleic acid (16:1), 4.6% by linoleic acid (18:2), and 69% by linolenic acid (18:3). Since all these alkenes are expected to form their respective chlorohydrins, the total cytotoxicity exerted by chlorohydrins within chlorine-disinfected vegetables may be far higher than calculated here.

One potential strategy to optimize our chemical disinfection practices is to compare the pathogen inactivation efficiencies and DBP formation potentials of different chemical disinfectants. For instance, chlorine dioxide is a more selective and highly oxidizing disinfectant than free chlorine. Ongoing research has demonstrated that chlorine dioxide yields significantly lower levels of DBPs (chlorate, chlorotyrosines, and chlorohydrin) than free chlorine when equivalent disinfectant concentration is used for fresh produce washing. Since proteins and lipids are present in our drinking water, chlorotyrosines and chlorohydrins are also anticipated to

form as the biomolecules pass through our water treatment systems, and disinfection in particular. Given the high cytotoxicity of the chlorinated biomolecules, it will be critical to measure the concentration of these compounds in our drinking water to assess their contributions to the overall toxicity of our drinking water.

Endnotes

1. Reynolds, K.A.; Mena, K.D.; Gerba, C.P. Risk of waterborne illness via drinking water in the United States. Rev. Environ. Contam. Toxicol., 2008, 192, 1170158.

2. Centers for Disease Control and Prevention. List of Selected Multistate Foodborne Outbreak Investigations. https://www.cdc.gov/foodsafety/ outbreaks/lists/outbreaks-list.html (accessed January 6, 2023).

3. Centers for Disease Control and Prevention. List of Selected Multistate Foodborne Outbreak Investigations. Multistate Outbreak of Listeriosis Linked to Packaged Salads Produced at Springfield, Ohio Dole Processing Facility (Final Update). https://www. cdc.gov/listeria/outbreaks/baggedsalads-01-16/index.html (accessed January 6, 2023).

4. Scharff, R. L. State estimates for the annual cost of foodborne illness. Journal of Food Protection, 2005, 78 (6), 1064-1071.

5. United States Food and Drug Administration. Full Text of the Food Safety Modernization Act (FSMA).

111th Congress Public Law 353.

6. Rook, J. J. Formation of haloforms during chlorination of natural water. Water Treat. Exam, 1974, 23, 234-243.

7. U.S. Environmental Protection Agency. Stage I and Stage 2 Disinfectants and Disinfection Byproduct Rules. https:// www.epa.gov/dwreginfo/stage-1-andstage-2-disinfectants-and-disinfectionbyproducts-rules (accessed January 6, 2023).

8. Richardson, S. D.; Plewa, M. J.; Wagner, E. D.; Schoeny, R.; DeMarini, D. M. Occurrence, genotoxicity and carcinogenicity of regulated and emerging disinfection by-products in drinking water: A review and roadmap for research. Mutation Research –Reviews in Mutation Research, 2007, 636, 178-242.

9. Suh, M.J.; Mitch, W.A. Sunlightdriven chlorate formation during produce irrigation with chlorineor chloramine-disinfected water. Environ. Sci. Technol., 2021, 55, 21, 14876-14885.

10. Komaki, Y.; Simpson, A. M.-A.; Choe, J.K.; Plewa, M. J.; Mitch, W. A. Chlorotyrosines versus volatile byproducts from chlorine disinfection during washing of spinach and lettuce. Environ. Sci. Technol., 2018, 52, 16, 9361-9369.

11. Simpson, A. M.-A.; Suh, M.-J.; Plewa, M.J.; Mitch, W.A. Formation of oleic acid chlorohydrins in vegetables during postharvest chlorine disinfection. Environ. Sci. Technol., 2021, 56, 2, 1233-1243.

Minjeong Suh is an assistant professor of engineering at the Fred DeMatteis School of Engineering and Applied Science at Hofstra University. She holds an MChem in Chemistry from the University of Oxford and a PhD in Chemical and Environmental Engineering from Yale University. Prior to joining Hofstra University, she worked at Stanford University as a postdoctoral researcher. Her research efforts span the intersection of environmental engineering, chemistry, and materials science, with a particular focus on light-driven water treatment technologies and the effect of sunlight on pollutant transformation in natural and engineered water systems.

Directing Waves Toward a Flexible Future

You may not know it, but over the last 50 years there has been a quiet, but profound advancement in wireless system technology that has fundamentally changed our world. Improvements in antenna technology have greatly enhanced our ability to control the direction of electromagnetic radiation precisely and rapidly. The impact of enhanced control of radiated signals is most apparent in the continuous growth of wireless network performance. The bandwidth of modern 5G networks is over 750,000 times larger than that of the first generation. Networks that

initially could support only audio calls can now stream HD videos and video conference calls. These improvements have been possible thanks to the precise steering of radiated signals.

Picking a Voice Out of a Crowd

Wireless communication is akin to communicating in a crowded room (Figure 1). Radio broadcast systems, like AM and FM radio, designate a handful of broadcasters that shout while everyone else listens. In a two-way communication network, users both listen and speak. Thus, it becomes more difficult to understand

each other as the crowd gets larger. Cellphone networks reduce this cacophony of voices by dividing the crowd into smaller rooms. However, these smaller rooms have the same fundamental problem as the crowd gets ever larger: Each voice in the room can be heard by everyone else.

One of the main advances of modern cellphone networks is the use of massive multiple-input, multiple-output (MIMO) techniques. In the crowded room analogy, MIMO is users directly whispering to each other instead of shouting for the whole room to hear. This addition of

D. Elliott Williams, Assistant Professor of Engineering, Fred DeMatteis School of Engineering and Applied Science

spatial selectivity to the transmission of sound limits the interference between users. In theory, a network with perfect spatial selectivity could support an arbitrarily large number of users. Since the introduction of MIMO in 3G networks, increasing the spatial selectivity of radio transmitters has been a key part of improving the bandwidth of each generation.

But the impact of the enhanced control of electromagnetic waves is far wider than simply improving network bandwidth. Satellites that can precisely image the surface of the earth have enabled high-fidelity measurements of our atmosphere and oceans, allowing us to carefully monitor the effects of climate change and make more accurate predictions of our future world.

Radar systems that can rapidly scan and accurately map targets have provided early detection and tracking of severe weather systems, have enhanced automotive safety, and have become a key component in national defense. Improvements in radiating systems have given birth to new applications that were previously infeasible, such as low-latency satellite internet, long distance wireless power transfer, and gesture-based human-machine interfaces.

Local Control Dictates Long Distance Behavior

To understand what drove this technological advancement, and thus what future improvements can be expected, it is necessary to have a basic understanding of how radiating structures work. Antennas are bidirectional transducers that convert electric signals in a device to radiating electromagnetic waves, and vice versa. The direction waves are transmitted or are picked up from depends on the construction of the antenna. Symmetric

antennas, such as AM and FM radio towers, radiate fields equally in all directions. Directional antennas, such as a satellite dish, focus the waves in a particular direction. This focusing ability is described by the radiation pattern: the angular pattern of both the sensitivity to incoming waves and the strength of transmitted waves.

The core physics of a radiating system is that of diffraction and interference. A wave spreads out as it propagates away from its origin, thus reducing the concentration of power at a given location. However, coherent waves originating from multiple locations will interfere; at some points they will cancel each other, while at other points they will combine. This interaction increases the concentration of radiated power in a particular direction. The interference pattern can be quite complex, depending on the waves’ relative amplitudes and phases and their points of origin.

The bandwidth of modern 5G networks is over 750,000 times larger than that of the first generation. Networks that initially could support only audio calls can now stream HD videos and video conference calls.

Meta-gap demonstration array from behind.

Figure 1: An analogy for cellphone networks. (A) In a crowded room it is difficult for people (black circles) to understand people talking to them (small colored circles) because they hear multiple voices at the same time (large colored circles). (B) By dividing the crowd into separate rooms, it becomes easier for people to listen to a particular speaker. However, there is still difficulty in rooms with multiple speakers. (C) Speakers can reduce the confusion by whispering only in the direction of their intended listener. The more control over a speaker’s voice, the more people can talk in the same room without difficulty.

Fundamentally, the radiation pattern of a radiator is related to the Fourier transform of the fields on its surface. This has two important consequences. First, the radiation pattern can be precisely controlled via the fields on the radiating structure’s surface. Second, the size of the radiator limits how much the waves can be concentrated in a particular direction. Thus, the quest for precise control of the radiation pattern has been the quest for increasingly large radiator sizes, or apertures, and for enhanced manipulation of the surface fields.

Mastering Control of Radiation Patterns

Both goals can be accomplished by putting multiple antennas into an array. The array aperture increases, and the surface fields can be manipulated by adjusting the amplitude and phase of the antenna input signals. By adjusting the phase of each element, the direction

of constructive interference can be shifted, thus electronically steering the direction the waves are radiated (Figure 2). Initially, these phased arrays were limited to large military applications, since each radiator was heavy, power hungry, and expensive. With the development of silicon radio-frequency integrated circuits (RFIC) over the last 20 years, it has become economically viable to construct arrays with numerous elements. This recent affordability of phased arrays has enabled the enhanced control of radio waves that have driven so much change.

Yet, our control of array radiation patterns is not complete. Current array and antenna designs have both pragmatic and theoretical limitations that prevent truly arbitrary control. To accomplish this goal, new degrees of freedom need to be introduced into radiating systems. Thanks to the

affordability of RFICs and advances in additive manufacturing, flexible substrates, and structural design, new degrees of freedom can be readily introduced. The challenge is in determining what degrees of freedom will enhance radiation pattern manipulation and then controlling those additional degrees of freedom.

Much like the affordability of RFICs drove the advances of the past, the affordability of computational power will drive the advances of the future. The rise of cloud computing over the last decade has greatly enhanced our ability to simulate and optimize electromagnetic systems. Advances in computation optimization and machine learning have enabled the design and control of massively complex systems previously too unwieldy to be pragmatic. This computational power allows simplifying assumptions about array and antenna design to be relaxed

and novel degrees of freedom to be explored. My research focuses on developing these novel degrees of freedom and leveraging them to explore the new applications enabled by the increased control of electromagnetic fields.

Shape-Changing Phased Arrays

One of the limiting factors of phased arrays is the fundamental tradeoff between the maximum focusing capability of the array and its sensitivity to steering. Due to diffraction, the amount of concentration is inversely proportional to the cross-sectional area of the radiating structure. When a planar phased array is steered at wide angles, its effective cross-sectional area decreases and thus the radiated power is less concentrated. Therefore, planar arrays are unable to effectively operate at wide angles.

Spherical arrays do not exhibit this problem since they are uniform in every direction. However, half of the elements in a spherical array are always pointed in the opposite direction and thus cannot contribute to the radiated power. In fact, any curvature in the array will reduce the maximum concentrated power while decreasing the sensitivity to steering angle.

To break the tradeoff between steering sensitivity and maximum concentration, an array must be able to change its geometry. Such a shape-changing array can assume the shape most suitable for its given task. For example, a receiving array can start in a spherical configuration to rapidly scan a wide field of view to locate a target. Once found, the array can morph into a planar configuration to observe the target with the highest possible sensitivity.

To demonstrate the opportunity presented by shape-changing phased

arrays, I collaborated with structural researchers in aerospace engineering to develop the first shape-changing phased array. The array, shown in Figure 3, consisted of 25 rigid tiles held together by a mechanical backbone capable of changing into spherical, cylindrical, and planar shapes. Using this array prototype, we demonstrated both the fundamental tradeoff between angular sensitivity and maximum concentration, and the ability of a shape-changing array to break this tradeoff.

Manipulation of Fields with Meta-Gaps

There is an important mathematical necessity for shape change that has major ramifications for shape-changing phased array performance. Carl Gauss’ Theorema Egregium states that for an object to morph between shapes with different Gaussian curvature (such as a plane and a sphere), the distances along its surface must change. As anyone who has tried to wrap a basketball as a gift knows, it is impossible to cover the surface of a sphere with a sheet of paper without tearing, folding, or crumpling it. For an array to undergo arbitrary shape change, the distances between elements in the array must change. This change in spacing alters the field distribution on the array surface in an undesirable way. In severe cases, the radiated power is split in two or more directions. In this case, the radiation pattern has multiple peaks called lobes. The lobe in the desired direction is called the main lobe, while the other peaks are referred to as side lobes or grating lobes. Grating lobes severely degrade array performance as power is radiated in and detected from undesired directions, increasing the interference between radiators.

Fortunately, the empty space between elements required for shape change can be used to mitigate this degradation in performance. We developed flexible,

metamaterial-inspired, sheets that can fill the gaps in a shape-changing array when it changes shape. These meta-gaps include reconfigurable metal patterns on their surface that interact with the local electromagnetic fields. These programmable metal patterns provide some degree of control over the surface fields and thus increase control of the radiation pattern. Due to the complex field environment and the dynamic requirements of phased-array systems, it is not clear which metal patterns will maximize performance. However, it is possible to use optimization techniques to identify patterns that improve performance.

To explore and characterize this optimization problem and demonstrate meta-gap capabilities, We developed a demonstration array with 25 radiators, 40 meta-gap sheets, and 960 switches. The demonstration array used suboptimal spacing, deliberately introducing grating lobes to model the performance degradation caused by shape-change. As the structure of the optimization problem was previously unexplored, we employed several

Advances in computation optimization and machine learning have enabled the design and control of massively complex systems previously too unwieldy to be pragmatic.

Figure 2: The interference of waves from multiple sources directs power in particular directions. (A) Each element in the array has equal phase and the fields constructively combine perpendicular to the array. (B) The associated pattern of radiated power at different angles in decibels. (C) Introducing a phase delay between elements causes the fields to constructively combine at a 45-degree angle. (D) The associated pattern of radiated power at different angles in decibels.

metaheuristic optimization algorithms that exploit different problem structures. The relative performance of the algorithms suggests that meta-gaps are well suited to machine-learning and other feature-based approaches. The optimal metal patterns identified could decrease the average side lobe level within the field of view by 2 decibels and increase the average maximum radiated power within the field of view by 0.46 decibels.

The Future Is Flexible

In addition to improving the capabilities of current communication,

imaging, sensor, and radar systems, increased control of radiation patterns will enable a host of new applications for radiating systems that were previously unattainable. With more control over surface fields, flexible arrays subject to shape-deformation and vibration can compensate for variations in element positions. Arrays can be embedded into textiles, creating smart fabrics for wearable sensors and on-body wireless communication networks that improve device connectivity while consuming less power. Lightweight flexible arrays can also be quickly deployed to augment

The revolution in wireless system capabilities is truly just beginning, and I look forward to my research group, my students, and Hofstra being a part of it.

damaged infrastructure in a disaster zone or launched into orbit to create affordable space-based solar power. New communication protocols can incorporate spatial information to prevent eavesdropping or compensate for multi-path reflections that degrade network performance. The revolution in wireless system capabilities is truly just beginning, and I look forward to my research group, my students, and Hofstra being a part of it.

References

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A. C. Fikes, M. Gal-Katziri, O. S. Mizrahi, D. E. Williams, and A. Hajimiri. Frontiers in flexible and shape-changing arrays, in IEEE Journal of Microwaves, doi: 10.1109/JMW.2022.3226125.

A. Hajimiri, B. Abiri, F. Bohn, M. Gal-Katziri, and M. H. Manohara. Dynamic focusing of large arrays for wireless power transfer and beyond, IEEE J. Solid-State Circuits, vol. 56, no. 7, pp. 2077-2101, Jul. 2021.

R. Han and E. Afshari. A CMOS high-power broadband 260-GHz radiator array for spectroscopy, IEEE J. Solid-State Circuits, vol. 48, no. 12, pp. 3090-3104, Dec. 2013.

X. He, Y. Cui, and M. M. Tentzeris. Tile-based massively scalable MIMO and

Figure 3: Shape-Changing Phased Array. (A) Planar Configuration. (B) Spherical Configuration. (C) Cylindrical Configuration. Note the gaps that are introduced between elements in the cylindrical and spherical configurations. (D) Maximum concentrated power at different angles for each geometry. Note that the planar array can focus the power the most but is also the most sensitive to steering angle.

phased arrays for 5G/B5G-enabled smart skins and reconfigurable intelligent surfaces, Sci. Rep., vol. 12, no. 1, Feb. 2022, Art. no. 2741. [Online]. Available: https://doi.org/10.1038/ s41598-022-06096-9

W. Hong et al. The role of millimeter-wave technologies in 5G/6G wireless communications, IEEE J. Microwaves, vol. 1, no. 1, pp. 101-122, Jan. 2021.

P.-S. Kildal, E. Martini, and S. Maci. Degrees of freedom and maximum directivity of antennas: A bound on maximum directivity of nonsuperreactive antennas, IEEE Antennas Propag. Mag., vol. 59, no. 4, pp. 16-25, Aug. 2017. [Online]. Available: https://ieeexplore. ieee.org/document/7954002/

B.-H. Ku, O. Inac, M. Chang, and G. M. Rebeiz. 75–85 GHz flip-chip phased array RFIC with simultaneous 8-transmit and 8-receive paths for automotive radar

applications, in Proc. IEEE Radio Freq. Integr. Circuits Symp., 2013, pp. 371-374.

J. Lavaei, A. Babakhani, A. Hajimiri, and J. C. Doyle. Solving large-scale hybrid circuit-antenna problems, in IEEE Transactions on Circuits and Systems I: Regular Papers, vol. 58, no. 2, pp. 374-387, Feb. 2011.

A. Pressley. Elementary Differential Geometry. London, U.K.: Springer, 2010.

L. Stark. Microwave theory of phased-array antennas – A review, Proc. IEEE, vol. 62, no. 12, pp. 1661-1701, Dec. 1974.

S. Venkatesh, D. Sturm, X. Lu, R. J. Lang, and K. Sengupta. Origami microwave imaging array: Metasurface tiles on a shape-morphing surface for reconfigurable computational imaging, Adv. Sci., vol. 9, no. 28, 2022, Art. no. 2105016. [Online]. Available: https:// onlinelibrary.wiley.com/doi/abs/10.1002/ advs.202105016

D. E. Williams, C. Dorn, S. Pellegrino, and A. Hajimiri. Origami-inspired shape-changing phased array, in Proc. IEEE 50th Eur. Microw. Conf., 2021, pp. 344-347.

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D. Elliott Williams is an assistant professor of engineering at the DeMatteis School of Engineering and Applied Science at Hofstra University. His research focuses on developing adaptive electromagnetic systems using novel degrees of freedom. His vision is to create devices with enhanced control of electromagnetic fields that can dynamically adapt to changing environments and/or needs. Working with colleagues in the aerospace department at Caltech, Dr. Williams invented the first shapechanging phased array and demonstrated that it can break the trade-off between maximum gain and steering range.

Dr. Williams holds a BS and an MEng in Electrical Engineering and Computer Science from MIT and a PhD in Electrical Engineering from Caltech. At MIT, he conducted research on generating THz noise using Schottky diodes in avalanche breakdown and developed a robotics platform for introductory engineering education. Dr. Williams has also worked at SpaceX, where he helped develop the phased arrays for the initial prototype Starlink satellites, and at Apple Inc., where he explored experimental technologies for new product features.

Dr. Williams received the Analog Devices Inc. Outstanding Student Designer Award in 2017, and he won the 2022 Young Engineer Prize at the European Microwave Conference for his work on flexible meta-gaps.

Hofstra at a Glance LOCATION

Hempstead, Long Island, 25 miles east of New York City Telephone: 516-463-6600

CHARACTER

A private, nonsectarian, coeducational university

PRESIDENT

Dr. Susan Poser

COLLEGES AND SCHOOLS

Academic Health Sciences Center (Donald and Barbara Zucker School of Medicine at Hofstra/Northwell; Hofstra Northwell School of Nursing and Physician Assistant Studies at Hofstra University; School of Health Professions and Human Services); Frank G. Zarb School of Business; Fred DeMatteis School of Engineering and Applied Science; Hofstra College of Liberal Arts and Sciences (Peter S. Kalikow School of Government, Public Policy and International Affairs; School of Education; School of Humanities, Fine and Performing Arts; School of Natural Sciences and Mathematics); The Lawrence Herbert School of Communication; Stuart and Nancy Rabinowitz Honors College; Maurice A. Deane School of Law; Hofstra University Continuing Education

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There are 1,214 faculty members, of whom 484 are full-time. Ninety-three percent of full-time faculty hold the highest degree in their fields.

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Undergraduate enrollment of 6,110. Total University enrollment, including graduate, School of Law, and School of Medicine, is 10,238. Undergraduate male-female ratio is 43-to-57.

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Bachelor’s degrees are offered in about 180 program options. Graduate degrees, including PhD, EdD, PsyD, AuD, JD, and MD, advanced certificates, and professional diplomas, are offered in about 190 program options.

THE HOFSTRA CAMPUS

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The Hofstra libraries contain 600,000+ volumes and provide 24/7 online access to more than 100,000 full-text journals and 800,000 electronic books.

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Hofstra is 100% program accessible to persons with disabilities.

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Hofstra offers a January session and three summer sessions between May and August.

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TRUSTEES OF HOFSTRA UNIVERSITY

As of February 2023

OFFICERS

Donald M. Schaeffer, Chair

Martha S. Pope, Vice Chair

Michael Roberge,* Vice Chair

David S. Mack,* Secretary

Susan Poser, President

Alan J. Bernon,* Immediate Past Chair

MEMBERS

Kenneth Brodlieb

Susan Catalano

Frederick E. Davis, Jr.*

Michael DeDomenico*

Michael P. Delaney*

Arno H. Fried

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Diana E. Lake*

Randy Levine*

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Samuel Ramos*

Robert Rosenthal*

Debra A. Sandler*

Jason Savarese*

Michael Seiman*

Leonard H. Shapiro

Joseph Sparacio*

DELEGATES

William Nirode, Speaker of the Faculty

William Caniano, Chair, University Senate Executive Committee

Kathleen Wallace, Chair, University Senate Planning and Budget Committee

Hillary Serota Needle,* President, Alumni Organization

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John J. Conefry, Jr., Chair Emeritus

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Walter B. Kissinger, Trustee Emeritus

Ann M. Mallouk,* Chair Emerita

Frank G. Zarb,* Chair Emeritus

*Hofstra alumni

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Finding Forms for Spatial Structures: Hanging, Heating, and Flipping

See page 10.