Designing Cutting-Edge Wearable Medical Devices
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
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.
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
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.
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.
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
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.
Fig. 1: Typical iontophoresis operation
Fig. 2: Fertility monitor’s reader and sensor
Fig. 3: Block diagram of sensor and reader