Wearable Medical Devices: Design at the Cutting Edge

16 JULY 2013

Medical devices are evolving from portable equipment to wearable devices. These are meant to be used continuously for extended periods of time as health care moves out of hospitals and becomes more integrated with peoples’ lives.
Wearable medical devices are not a new concept; devices such as nicotine patches and patches for motion sickness are familiar and laid the groundwork for a new generation of electronic products, including the iontophoresis patch.

by Marten L. Smith, Staff Engineer, Medical Products Group, Microchip Technology Inc.

An electrical current facilitates iontophoresis, which enables the infusion of a drug through the skin. The transdermal drug is ionised, dissolved in an aqueous solution and applied to an electrode in the patch.

Figure 1: Typical Iontophoresis Operation.

Figure 1 shows how this specially formulated, ionised compound can then be moved through the skin via DC current. Depending on the drug and the condition being treated, modern patches can be worn for anything from a few minutes to a few hours.
Iontophoresis has several advantages. The medicine can be locally dosed at very high levels; rather than being distributed throughout the entire body, as occurs with syringe injections. Local administration can result in improved efficacy and reduced side effects.
The production of low-cost, single use dispensers for these drugs has been made possible by recent advances in electronics techno­logy, such as switched mode power-supply design – along with cost-effective, high performance MCUs.
Self-applied iontophoresis is already in common use by consumers for a range of conditions, including headaches and cold sores.
The fact that the critical electronics are in the wearable portion of devices such as iontophoresis patches poses a significant challenge to designers. This is because this is the part which is meant to be used once and then thrown away, which means patch electronics must be small and inexpensive. As a small, disposable item, battery cost and energy capacity impose further design constraints. Finally, the design needs to be easily modified for additional features, such as changes in the medication dose and duration.
The device must produce sufficient voltages to drive the current level needed to infuse the correct dose through the skin and for the required duration period.
A DC/DC boost converter can drive a controlled current through the skin, along with a microcontroller (MCU) to control the converter for the development of a small cost-sensitive iontophoresis device.
To reach the required current to penetrate the skin, a boost regulator can step up the voltage from a low-voltage battery such as inexpensive Lithium coin or alkaline cell batteries to provide power to the patch electronics. The MCU must be both small and highly integrated to meet the requirements for cost and functionality.

Figure 2: Fertility Monitor Reader and Sensor.

Microchip’s 8-pin, 8-bit PIC12F1822 MCU is used in these devices and meets the design integration requirement, with an internal 10-bit ADC, fixed voltage reference, comparator, PWM, hardware timers and EEPROM. The fixed voltage reference eliminates the need for a regulator or an external reference, and keeps the design to an 8-pin MCU in order to lower the cost and reduce board size.
In order to improve the quality of life and the quality of health care for patients, innovation in electronics technology now supports the development of medical devices that are intended to be worn on the body for long periods of time. Devices of this nature currently in use include continuous glucose monitors and wearable cardiac-event recorders.
An ovulation prediction system – used to maximise the opportunity for conception – is an interesting example of a device which long-term use. For example, the DuoFertility-brand fertility monitor, made by Cambridge Temperature Concepts (Figure 2) embodies a number of attributes that are essential to long-term monitoring systems.
The ovulation process correlates to minute changes in her basal body temperature; accurately measuring those changes over multiple cycles can help to estimate the day of ovulation.
Therefore the sensor on this fertility device continuously measures body basal temperature for up to six months, in contrast to a continuous glucose monitor may be designed to operate for up to a week. The device can predict when ovulation will occur up to six days in advance by using the data from constantly monitoring minute temperature changes. This approach eliminates the variations found from taking temperatures manually.
One design challenge is to create a device which can be comfortably attached to the body for months at a time. A two-part system comprised the solution in this instance; the coin-sized sensor unit attaches to the user’s body with a biocompatible adhesive patch while the handheld reader unit analyses the data and allows the user to transfer that data to medical professionals for further analysis. This ensures the body-worn sensor is as small and light as possible, as demonstrated in the functional partitioning diagram in Figure 3.
The environment of the user and their expected activities over a period of months also presents a challenge. A wearable device used for months at a time must accommodate a range of conditions including sleeping, exercising, showering or even skiing. Therefore, the design of the sensor and its packaging must deliver precise temperature measurements regardless of whether the sensor is open on one side or covered by the user’s arm.
A pair of matched thermistors measure temperature and heat flow from one side of the sensor to the other, making the sensor accurate to a few thousandths of a degree. Incorporating an accelerometer in the sensor design also ensures the user’s movement is taken into account.

Figure 3: Block Diagram of Sensor and Reader.

The volume available for batteries is very limited for small body-worn electronics. Therefore the power consumption must be extremely low. The designers of this sensor used an 8-bit PIC16F886 MCU; minimal current consumption was achieved by using the MCU’s ultra low power wake-up feature.
In less than 1mS, the sensor can power up, take a measurement and then return to sleep mode when it’s time to take a reading. This short wake-up time enabled the device’s designers to achieve average power consumption of less than 1µA, and a battery life of six months, using a small CR1216 Lithium coin-cell battery.
The sensor module sends data to the reader using a modified RFID protocol, wherein communication is initiated by holding the reader near the sensor.
This data transfer poses a challenge as it requires higher power consumption than measurement but holding the sensor’s temperature readings in 16 megabytes of stand-alone Flash minimised the current draw and allows reader data uploads to be spaced a few days apart.
The data collected by a long-term sensor may need to be analysed by a trained person, having transferred the measured data to a PC or via the Internet.
The handheld reader transfers the data to a PC via the on-chip USB peripheral inside Microchip’s 16-bit PIC24FJ256GB106 MCU with nanoWatt Technology. Front-panel buttons implemented using the MCU’s internal Charge Time Measurement Unit (CTMU) and mTouch™ Capacitive Touch technology allow the user to enter additional data.
Communication from the device manufacturer to the reader allows for refinement of the ovulation prediction; the same capability can allow remote reconfiguration of the MCU. The manufacturer can run diagnostics and send software updates to the monitoring system using this flexibility.

Wearable medical devices that are meant for long-term use will create new diagnostic and therapeutic options for even more illnesses and conditions as innovation continues in the fields of biology, physiology, chemistry and electronics.

Note: The Microchip name and logo, and PIC are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks mentioned herein are the property of their respective companies.)

www.microchip.com

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