Mouser Electronics - High performance Medical devices design ebook

High performance Medical devices design Maurizio Di Paolo Emilio

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Abstract

Healthcare is a sector of great value, although highly controversial. Rising costs and complex insurance regulations drive the industry towards various solutions so that patients can interact with doctors and other health professionals in real time. The demand for services related to health and medicine in general is constantly growing worldwide. In this scenario, the technology maintains a role of primary importance, supporting the creation and introduction of increasingly advanced solutions. Current and future wireless applications, digital technologies, the need to power some devices with batteries only, as well as compliance with stringent safety standards, present challenges for testing and developing of these applications.

Introduction In the medical applications, as in most situations where electronic technologies have a primary role, many healthcare solutions such as remote patient monitoring, are emerging with an immediate positive impact.

The key element that unites all these solutions is the IoT infrastructure.

Additional value-added features are represented by the use of radio frequency communication systems, artificial intelligence and predictive data analysis. Another element to keep in mind is the battery, which must have maximum energy efficiency in order to ensure correct operation. The strategy to optimize of the battery energy efficiency must be based on the management of device downtimes.

The advent of wearable devices requires batteries with special technology both in form factor and in chemistry.

Energy harvesting could be a way to increase battery life, by means of various environmental sources such as solar, radio waves and kinetics by using the movement of the human body. Most wearable medical devices to date have been focused in various applications such as jogging, with the goal to measure the distance traveled, calories burned and heart rate.

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RF communication

Growing economic strength and innovations have led to many new home care applications. Today there are many medical devices such as hearing aids, heart rate monitor, devices for computerized tomography (CT), pacemakers, and magnetic resonance imaging equipment (MRI). The development and testing of these applications must be conducted keeping in mind the following fundamental criteria: 1..Avoid interference phenomena that may affect performance or even interrupt signal transmission or reception.

2. The devices must comply with strict safety standards, such as IEC 60601-1-2 or other equivalent.

Most medical wearable devices are equipped with one or more sensors, a microcontroller and wireless connectivity for data communication with a mobile device. There are various communication protocols available, such as Bluetooth, ZigBee and Wi-Fi, as well as proprietary interfaces developed by semiconductor suppliers. With the rapid growth of the wearable electronics and IoT technology market, Bluetooth Low Energy (BLE), also known as intelligent Bluetooth, has become the de-facto standard for all types of wearable devices (Figure 1).

3 2

BLE ZigBee

Years 1 0 0.25

Bluetooth

1 Data rate Mbit/s 2 3 Figure 1: comparing RF protocols according to battery life (years) and data rate.

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Bluetooth Low Energy (BLE) has been specifically designed to achieve the lowest possible power for short-range communication. BLE operates in the 2.4 GHz ISM band with a bandwidth of 1 Mbps. The protocol is optimized to transmit small blocks of data at regular intervals, allowing the host processor to maximize the time slot in a lowpower mode when no information is transmitted. The addition of Bluetooth to a wearable is now simplified by the availability of a good number of wireless modules and microcontrollers certified and compliant with international radio standards.

DA14580, represents a highly integrated Bluetooth chip that incorporates a 32-bit ARM® Cortex®-M0 processor core to handle the control operations, but also running the Bluetooth stack software, thus eliminating the need for a second microcontroller. Housed in a tiny 2.5mm package, the chip and surrounding components provide a complete Bluetooth communication subsystem that requires little space on the additional board (Figure 2).

ARM Cortex M0

XTAL 32.768 kHz

CORE

AES-128

ROM 84 KB

LDO RET

LDO RF

RCX

TIMER 2 3xPWM

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GPIO MULTIPLEXING

QUAD DECODER

GP ADC

I2C FIFO

SPI

UART2 FIFO

TIMER 0 1xPWM

UART FIFO

SW TIMER

KEYBOARD CTRL

OTPC

Radio Transceiver

Figure 2: block diagram of the DA14580

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RC 32 kHz POReset

LINK LAYER HARDWARE

WAKE UP TIMER

DMA

POWER/CLOCK Management (PMU)

Ret. RAM4 1 KB OTP 32 KB

LDO SYS

RC 16 MHz

APB bridge

Ret. RAM3 2 KB

Memory Controller

System/ Exchange RAM 42 KB

Ret. RAM2 3 KB

DCDC (BUCK/BOOST)

BLE Core

SWD (JTAG)

Ret. RAM 2 KB

XTAL 16 MHz

The further development of the Bluetooth protocol has directed the technology towards Bluetooth 5 which also includes a series of techniques to further minimize possible interference with other wireless technologies. One of the most important aspects for the new reference standard is the greater security of connections during the data transmission and reception phase. Texas Instruments Incorporated introduced new devices within the SimpleLink family of low-power wireless microcontrollers (Figure 3).

SimpleLink™ CC26xx wireless MCU RF core

cJTAG Main CPU

ARM® Cortex® -M3

ROM 128KB Flash 8KB cache 20KB SRAM

General peripherals / modules IC

4x 32-bit Timers

UART

2x SSI (SPI, μW, TI)

I2S

Watchdog timer

10 / 15 /31 GPIOs

TRNG

AES

Temp. / batt. monitor

32 ch. μDMA

RTC

2

ADC ADC Digital PLL DSP modem ARM® Cortex® -M0

4KB SRAM ROM

Sensor controller Sensor controller engine 12-bit ADC, 200 ks/s

DC-DC converter

2x comparator SPI-I2C digital sensor IF Constant current source Time-to-digital converter 2KB SRAM

Figure 3: Block diagram of SimpleLink CC2640 with Bluetooth 5

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These devices are compatible with Bluetooth 5 and continue the integration of advanced features within the same hardware based on ARM Cortex-M3 core, automatic power management and low-power sensor controllers. For example, the CC2640R2F-Q1 wireless MCU contains a 32-bit ARM® Cortex®-M3 processor that runs at 48 MHz. Its architecture improves power consumption performance of the overall system.

Medical devices must be built to high standards to meet the demanding expectations of patients and medical personnel. Data collection must be error-free and tamper-proof, so that data cannot be accessed in the event of device theft. If a data collection sensor is violated, this could be a diagnostic problem. Most medical devices are small, so it is likely that you need to transfer data frequently to a user’s phone or directly to the cloud via an RF protocol such as Bluetooth. Data transfer must be encrypted using various authentication methods.

Security features are largely based on some core elements, including stable cryptographic ciphers, such as the Advanced Encryption Standard (AES), Secure Hash Algorithm (SHA), and the RSA and ECC public key figures.

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Power Management

In addition to the importance (or rather, the need) of the medical devices to have an appropriate communication interface, the designers have also to face another difficult challenge imposed by the absorption of power: a key aspect for the battery-powered devices. This feature requires a power management strategy for the device to provide the required functions while remaining reliable. As a result, the main energy source for most portable medical devices remains the battery.

Battery life is of vital importance for portable medical devices. The microcontroller is the main active components that most influence the devices energy consumption. The convergence of features such as wireless connectivity, high-speed digital processing, and real-time monitoring requires an accurate measurement of the drain current of the battery.

One of the drawbacks in the use of wearable devices is to have malfunctions, despite the good state of charge of the battery. A circuit example for resolving the criticality of monitoring the charge status of a battery is offered by the “ModelGauge� technology, by which the open circuit battery voltage level is checked, allowing excellent accuracy in the detection of the power budget of a battery.

MAX1720x and MAX1721x implement the ModelGauge m5 algorithm without requiring host interaction for configuration. MAX17201 / MAX17211 monitor a single packet of cells. MAX17205 / MAX17215, instead, can monitor a multi-series cell package. For a better hardware security, these ICs integrate SHA-256 authentication with a 160-bit secret key to prevent cloning of the battery pack. Each IC incorporates a unique 64-bit ID (Figure 4).

High energy efficiency is therefore required for portable and therefore battery-operated systems. Less power consumption will increase the operating time of the device without recharging or replacing the battery. The voltage regulator is the element that supplies the regulated voltage to the active components of the device, therefore, as it is important to reduce as much as possible the voltages involved, it is equally important to choose the right dc-dc regulator or converter. The main requirement that almost all specifications for the appropriate converters in medical applications have in common is a secure galvanic separation between the input and the output, usually expressed as an isolation voltage. The ability of an isolated converter to withstand high voltages through its isolation barrier depends on the materials used to build the converter, the physical separation between the input and output traces on the internal PCB and the transformer’s insulation capacity to withstand stress electric between the inlet and outlet windings. The MOOPs (Means of operator protection) and the MOPP (Means of patient protection) are two specific parameters with the key requirements to reduce the risk of an accidental current that can cross the human body and therefore cause irreparable damage. In addition to insulation resistance, the other most critical specification for DC / DC converters is the operating temperature range. REM1 converters integrate a 1W grade medical DC / DC converter in a SIP7 package with 5.2 kVAC / 1 minute isolation. It is available with 3.3, 5, 12, 15 or 24V inputs and offers 3.3, 5 or 12V outputs with an efficiency up to 85%.

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PACK+

10Ω Vbatt 0.1μF

REG3

DQ/SDA

CELL1

OD/SCL

CELL2

COMM INTERFACE

CELLx

+

REG2 0.47μF

SINGLE-CELL EXAMPLE

MAX17201 PACK PROTECTOR CSP

EP

CSN

PACKRsense

Figure 4: Typical application with the Maxim Integrated ModelGauge

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Energy Harvesting

Implantable electronic medical devices with very low power are able to operate without batteries with small amounts of energy that can be derived from environmental sources such as the movement of the human body, while larger devices can use the energy collected to recharge the battery. Different types of energy can be used, such as solar, thermal as a temperature gradient between the skin and environment, and kinetics derived from the movement of human activities, such as running or walking. Lithium-ion (Li-ion) and lithium-polymer batteries are gaining popularity in the field of energy-harvesting for the medical sector. Lithium polymer batteries are mainly used in commercial applications, and are produced in thin and flexible sheets. Lithium-ion (Li-ion) batteries, on the other hand, are mainly used in industrial applications. Another trend is represented by supercapacitors able to replace rechargeable batteries, offering a new way of storage based on nanotechnology. Unlike batteries, supercapacitors can recharge within seconds, and withstand virtually unlimited charge cycles (Figure 5).

Au Plating

W: 20mm

(-) Negative terminal (bal 1) Balance terminal (bal 2) Balance terminal (+) Negative terminal

T: Max 0.4mm

2.25V 70mF 150Ί

(-) Negative terminal

2.25V 70mF 150Ί

(bal 2) Balance terminal

(bal 1) Balance terminal

(+) Negative terminal

L: 20mm

Au Plating

Figure 5: Apparatus and dimension of the Murata Supercapacitor

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Eliminating the battery in pacemaker devices would remove the weakest link in the device reliability chain. Once the pacemaker’s battery reaches a critical energy level, doctors must replace it with surgery, which increases costs and increase the patient’s complications. The goal for energy collection in this case is to eliminate the need for a battery by generating electricity derived from an external source. The heart is a promising source of external energy for harvesting energy flow in the body because its contractions are repetitive and occur 24 hours a day, 7 days a week all day. All this through piezoelectric sensors. An example of an integrated power circuit that integrates a low loss full wave bridge rectifier with a high efficiency voltage regulator is shown in Figure 6. This is a solution for collecting energy optimized for high energy sources output impedance, such as piezoelectric transducers.

MIDE V21BL PZ1 1μF 6V Cstorage 25V 4.7μF 6V

Vin

PZ1 LTC3588-1

SW

47μF 6V

Vout

CAP

PGOOD

Vin2

D0, D1

GND

10μH

2

OUTPUT VOLTAGE SELECT

35881 TA01

Figure 6: Energy collection for piezoelectric transducers with LTC3588-1

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Vout

Conclusion

Medical devices equipped with wireless communication such as Wi-Fi or Bluetooth enable machine-to-machine communication that is the basis of the next Internet of Medical Things (IoMT) that will change the way of the medical industry. IoMT devices connect to cloud platforms can provide health care; the acquired data can be archived and analyzed. Security solutions will offer the possibility to authenticate sensors and medical peripherals, making sure that they are authentic OEM and not counterfeit.

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About the Author

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. Working as a software/hardware developer in the data acquisition system, he participated as the designer of the thermal compensation system (TCS) for the optical system used in the Virgo/Ligo Experiment (an experiment for detection of the gravitational wave that achieved the 2017 Nobel Prize in Physics). Actually, he collaborates with University of L’Aquila and INFN to design devices for radiobiological and microscopy applications and new data acquisition and control systems for space applications. Moreover he works in the software/hardware engineering field as editor and technical writer. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.

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