5 minute read
MT: Microvasculatureon-a-chip platform
from openME 52.2
by Simon Stevin
Master Thesis:
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WRITTEN BY ESTHER VAN DER AA
During my master thesis I designed and fabricated a Microvasculature-on-a-Chip Platform, which was a project in the Microsystems group. This means that we’ve made a model capable of mimicking the blood vessels, which can be used to investigate flow and distribution of particles inside the vasculature. The vascular structures are made using sugar and a 3D printing machine and combining this with a flow through the channels and a cell lining on the channel wall, this is a simple representation of a blood vessel. This subject might not be the first thing you think about when you think about Mechanical Engineering, so I will try to tell you something more about this interesting application of Mechanical Engineering!
The vasculature plays a crucial role for humans, as blood vessels deliver oxygen and nutrients to every part of the body. However, the blood vessels are also used by tumors to spread cancer cells throughout the body which sustains or even worsens the disease. Furthermore, blood vessels exist in and near tumors which are different from the blood vessels found in healthy tissue, mostly characterized by abnormal vessel growth. This means that the vessels form a disorganized network and the number of vessels is much higher. These blood vessels provide the tumor with oxygen and nutrients, which helps the tumor to grow even further. As cancer is still the second leading cause of death in the world, it is essential to further expand our knowledge of blood vessel growth and behaviour associated with cancer. This can lead to a better understanding of the disease and even to earlier or better detection of tumors. To achieve this, we need a model that mimics the microvasculature, so that this platform can for example be used to investigate flow and distribution of particles inside the vasculature.
For this, we’ve designed a perfusion system that you can see below. The perfusion system allows for a flow of fluid through the channel. The gray parts (2, 3 and 5) are made from polycarbonate using a milling machine. To give you an idea of the scaling, the device is 25x75 mm with respectively 6, 2 and 3 mm polycarbonate slides. The top layer (2) has inlets to which the tubing for perfusion can be connected, and inlets which can later be used to fill the chambers in the bottom layer (5) with hydrogel. The middle layer (3) is used as a substrate for printing the sugar networks, such a sugar fiber is shown in green. The pink layer (4) is an adhesive sheet, which ensures a leak-tight sealing between the middle (3) and bottom (5) layer of the device. The adhesive sheet is cut to size using a laser cutter. The bottom consists of glass slides, because they are optically better for imaging the channel with a microscope.
After fabrication of the device, multiple steps need to be taken which are shown schematically in the timeline below. These steps can be done multiple times with the same device. The first step is 3D printing using sugar, using a special 3D printer designed by Andreas Pollet (Pollet, et al., 3D Sugar Printing of Networks Mimicking the Vasculature, 2019). Sugar is stiff enough to be printed without support, which enables us to print a wide range of structures. A video of this can be seen by scanning the QR code on the next page, the video is shown at accelerated speed. The actual printing speed while printing the fiber is 50 mm/min. The diameter of the printed fibers depends on the temperature of the barrel which holds the sugar and the nozzle translation speed. The travel moves of the nozzle can be programmed using G-code, which you might be familiar with if you’ve used a 3D printer before.
Sugar can be dissolved in water, which is ideal for us as we are able to create channels after casting the sugar fibers in hydrogel. However, this also means that the sugar fiber will be dissolved in the hydrogel solution. Step 2 is thus to coat the sugar fibers with a PDLA in chloroform coating solution, which prevents the sugar fiber from dissolving immediately when it comes in contact with the hydrogel solution.
The third step is to prepare the hydrogel which will be used as an extracellular matrix (ECM). In my project I’ve used a few different hydrogels, which have different properties. For example, the type of material was different, either synthetic, hybrid or natural. Also the stiffness of the final hydrogel differs, as the stiffness of the ECM is different throughout the body as well. Depending on the research question, the device can thus be used with a hydrogel that is interesting for that subject. After preparation of the hydrogel, the device is sterilized and assembled in step 4.
(a) Structure printed with a 0.15 mm nozzle at a temperature of 110 °C. The resulting fiber diameter is 240 μm (b) Structure printed with a 0.25 mm nozzle at a temperature of 100 °C. The resulting fiber diameter is 430 μm
400 μm channel perfused ink
We can then inject the hydrogel solution into the chambers, step 5, and polymerize the hydrogel in step 6. Depending on the type of hydrogel, polymerization can for example be induced by UV light, so that the hydrogel forms a casting around the sugar fiber. The sugar fiber can then be dissolved, so that perfusable channels remain. In the figures below the channels are perfused by ink, to show that only the channels are perfused and there is no leaking into the surrounding matrix.
The next step is to inject a cell suspension into the channel, so that the cells can form a tight cell layer on the wall lining. Once the lining is formed, the channel is perfused with medium. This can be seen as a simple representation of a blood vessel in the human body. This simple representation can then be further improved in the future, for example by incorporating cells in the surrounding matrix. It would also be very interesting to ‘grow’ blood vessels from a bigger channel, which is called guided angiogenesis. This way, we can create even smaller blood vessels than is possible with this technique, as now we can go down to 50 µm channels. A schematic representation of this can be seen below.
Hopefully I’ve been able to show you something interesting about this maybe less known application of Mechanical Engineering!
Image by: Andreas Pollet
