Designing Cutting-Edge Wearable Medical Devices


FEBRUARY 2012: As healthcare moves out of hospitals and becomes more integrated with peoples’ lives, medical devices are evolving from portable equipment to wearable devices that are meant to be used continuously for extended periods of time. These new devices present designers with many new challenges.

Infusion drug therapy
Medical devices that are worn on the body are not new. Most people are familiar with wearable products, such as nicotine patches and motion sickness patches. These devices laid the groundwork for a new generation of electronic products. One member of this new generation is the iontophoresis patch.

Iontophoresis uses electrical current to enable 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. This specially formulated ionised compound can then be moved through the skin via direct current (DC) (refer Fig. 1). Most patches used today can be worn for anywhere from a few minutes to a few hours, depending on the drug and the condition being treated.

Fig. 1: Typical iontophoresis operation
Fig. 1: Typical iontophoresis operation

There are several advantages of iontophoresis. The medicine can be locally administered at very high levels rather than being distributed throughout the body, which happens with syringe injections. This local administration can result in improved efficacy and reduced side effects. Advances in electronics technology, such as switched-mode power sup ply design, along with cost-effective high-performance microcontrollers have made the production of low-cost single-use dispensers for these drugs possible. Self-applied iontophoresis has already been used by many consumers to deliver medicines for many conditions including headaches, cold sores and wrinkles.

One of the biggest challenges that designers face while creating devices such as iontophoresis patches is that critical electronics is in the wearable portion of the device, which is meant to be used once and then thrown away. This creates intense pressure for the patch electronics to be small and inexpensive. Also, since this is a small disposable item, battery cost and energy capacity impose further constraints on the design. Finally, the design needs to be easily modified for additional features such as changes in the medication dose and duration.

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To infuse the drug through the skin, the device must produce sufficient voltages to drive the current level needed for the specific infusion dose rate, and for the required duration period. A working design for a small cost-sensitive iontophoresis device can be as simple as a DC-DC boost converter to drive a controlled current through the skin, along with a microcontroller to control the converter.

The boost regulator steps up the voltage from a low-voltage battery to sufficient levels for passing the required current through the skin. Inexpensive lithium coin or alkaline cell batteries can be used to provide power to patch electronics.

Meeting the requirements for both cost and functionality calls for a microcontroller that is small yet highly integrated. 8-pin, 8-bit microcontrollers are available for use in these devices. These meet the design integration requirement with an internal 10-bit analogue-to-digital converter, fixed voltage reference, comparator, pulse-width modulation, hardware timers and electrically erasable programmable read-only memory (EEPROM). The fixed voltage reference eliminates the need for a regulator or an external reference, and keeps the design to an 8-pin microcontroller in order to lower the cost and reduce board size.

Long-term monitoring
Long-term use of wearable medical devices is improving with innovations in electronics. Continuous glucose monitors and wearable cardiac-event recorders are examples of such devices.

A unique example of devices that take long-term use to a whole new level is the ovulation prediction system. These devices are used by women who want to maximise their chances of conceiving. One such wearable device is DuoFertility’s monitor (refer Fig. 2). This device made by Cambridge Temperature Concepts embodies a number of attributes that are essential to long-term monitoring systems in general.

Fig. 2: Fertility monitor’s reader and sensor
Fig. 2: Fertility monitor’s reader and sensor

The ovulation process in a woman’s body correlates to minute changes in her basal body temperature. Accurately measuring those temperature changes over multiple monthly cycles can help to estimate the day of ovulation.

While a continuous glucose monitor may be designed to operate for up to a week, the sensor on this fertility device continuously measures body’s basal temperature for up to six months. The device uses this information to predict when ovulation will occur, up to six days in advance. Constantly monitoring minute temperature changes eliminates the many variations that occur in taking the temperature manually.

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One challenge that designers face is creating a physical form factor that can be comfortably attached to the body for months at a time. In this case, the solution was to make a two-part system. The coin-sized sensor unit attaches to the user’s body with a biocompatible adhesive patch. The hand-held reader unit analyses the data and allows the user to transfer that data to medical professionals for further analysis. This functional partitioning enables the body-worn sensor to be as small and light as possible. A block diagram of the monitor’s sensor and reader is shown in Fig. 3.

Another challenge is to anticipate all of the environments that the user will be in, and the activities in which she might engage. With a usage time frame of months, a wearable device must accommodate a wide range of conditions including sleeping, exercising and showering. In this case, the design of the sensor and its packaging had to enable precise temperature measurements regardless of whether the sensor is open on one side or covered by the user’s arm.

The designers solved this problem by using a pair of matched thermistors. These measure the temperature and the heat flow from one side of the sensor to the other, making the sensor accurate to a few thousandths of a degree. In addition, the user’s movement is taken into account by incorporating an accelerometer in the sensor design.

Body-worn electronics has to be very small, which means the volume available for batteries is limited. So another challenge of the sensor design is to keep the power consumption extremely low. The designers of this sensor used an 8-bit microcontroller in order to minimise the sensor’s cur-rent consumption. Minimal current consumption was achieved by using the microcontroller’s ultra-low-power wake-up feature.

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When it’s time to take a reading, the sensor powers up, takes a measurement and then returns to sleep mode—all in less than one millisecond. This short wake-up time enabled the device designers to achieve average power consumption of less than 1 μA and a battery life of six months using a small CR1216 lithium coin cell battery.

Another challenge is to transfer the measured data. This sensor module sends data to the reader using a modified radio frequency identification (RFID) protocol, wherein communication is initiated by holding the reader near the sensor. This data transfer requires higher power consumption than that required for measuring the data. So the designers minimised the current drawn by holding the sensor’s temperature readings in 16-megabytes of standalone Flash. This allows reader data uploads to be spaced a few days apart.

Since the data collected by a long-term sensor may need to be analysed by a trained person, creating a straightforward and cost-effective way to transfer the measured data to a PC and communicate over the Internet is yet another important design consideration. The second part of this device—the handheld reader—is utilised for this purpose.

Fig. 3: Block diagram of sensor and reader
Fig. 3: Block diagram of sensor and reader

The reader transfers the data to a PC via the on-chip USB peripheral inside a 16-bit microcontroller with nanoWatt technology. The user can enter additional data via front-panel buttons that are implemented using the microcontroller’s internal charge time measurement unit and mTouch capacitive-touch technology.

Communication from the device manufacturer to the reader allows for refinement of the ovulation pre-diction. The same capability can allow remote reconfiguration of the microcontroller. With this flexibility, the manufacturer can run diagnostics and send software updates to the monitoring system.

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

The author is a staff engineer in medical products group of Microchip Technology Inc.



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