Imagine patients with chronic conditions being monitored without having to be in a doctor’s office, or patients with more serious issues being monitored and treated outside the intensive care unit in a much less costly hospital room — or even at home. That’s the promise of wearable medical devices, a market that has been around for decades, but is being re-energized by new technology, including wireless communication.

An adaptable sensor platform with a flexible form factor leverages printed electronics comprised of a printed base circuit, printed batteries, a flexible electrophoretic display, and a traditional microcontroller. The system is capable of reading signals from an array of sensors and displaying the results. The sensor platform’s operational life is extended by energy harvesting that recharges the printed batteries. (Credit: Molex)

The latest generation of medical wearables, when fully developed, will not only offer more patient convenience, but also the capability to drastically reduce medical costs. By eliminating the need for expensive in-hospital monitoring or potentially eliminating the need for costly medical procedures by providing early detection and diagnosis, medical wearables may dramatically improve the delivery of medical services. Also, healthcare reforms are driving compliance to prescribed therapies to ensure a positive outcome, and miniature wireless devices can fill this gap.

Medical wearables also have the potential to provide better-quality data and improved monitoring compared to large, expensive machines located in hospitals and research units. For example, today, sleep apnea patients or research subjects must come into a clinic and be wired for sleep studies. However, people generally have trouble sleeping in rooms other than their own. A wearable medical device that could monitor sleep apnea metrics in a much less intrusive way could obtain much more reliable, higher-quality, and longer-term data to diagnose patients and study the effects of the disorder.

Also, consider a condition like epilepsy where known physiological changes that occur in people over time simply can’t be measured 24/7 with today’s equipment. With a wearable device, a patient could be monitored for an extended period of time, providing a much better understanding of how they interact with their environment and the medicines they take. That information could potentially be used to develop a predictive capability that warns a patient when they’re about to have a seizure, or if they are at a higher probability of having a seizure.

Types of Devices

There are four primary types of wearable medical devices, ranging from the simplest to the most complex: passive monitors, surveillance devices, diagnostic devices, and therapeutic devices. Of the latter three, surveillance is the simplest. Moving along the continuum from surveillance to therapeutics, barriers to commercialization increase. Surveillance devices simply report, while diagnostic devices make decisions, and therapeutic devices make decisions and treat the patient.

Diagnostic devices carry a higher level of legal risk than do surveillance devices because they find problems and medical decisions are being based on the data they produce. Therapeutic devices take that risk to an even higher level because they treat the condition. Most therapeutic wearable devices are still in the research phase, and would have to be approved as medical devices by the FDA, making the development cycle much longer — particularly if a company is developing a drug/device combination. Monitoring devices make sure that diagnostic and therapeutic decisions and procedures have been done correctly.

Development and Design

Two key factors in wearable medical device design are ease of use and patient comfort. They are typically determined by how the electronics are configured and what type of materials are used to fabricate the electronics. Use of rigid materials must be minimized so the device can conform to the body and survive bending during normal human motion.

Patient monitoring is an area of great interest for healthcare providers. Here, a patient is wearing a vital sign monitoring device in the form of a bandage.

As a result, designers and component suppliers must first understand the use scenario. For example, where would the wearable device be applied — wrist, chest, elbow, lower back, or elsewhere? How will it be used — once a day and then replaced, or for a week or more before being replaced? Understanding use scenarios includes asking questions such as:

  • How does the device need to function electronically?
  • Will the devices include wireless interfaces to support telehealth and mHealth applications?
  • At what intervals does it need to collect data for the different metrics being monitored?
  • How is that data stored? Does the device access a local storage site, or does it communicate with the cloud?

Because skin contact is involved in most wearable applications, special adhesives or other fastening means must be included in the design. The adhesives must not cause any skin irritation and should account for varying patient sensitivities. Fortunately, a wide range of biocompatible medical adhesives can be used, with the choice of adhesive dependent on a number of factors, including the length of time the device is going to be worn; for example, for 15 minutes or seven days. Another consideration is if the wearer will be exposed to water or if they will shower with the device on.

Types of Electronic Circuits

The choice of electronics for medical wearables typically includes traditional PCBs, copper flex circuits, printed electronics, antennas, fine wires, or combinations thereof. PCBs, being rigid, are typically appropriate for applications such as smart watches and wellness-type devices that tolerate the form factor and its rigidity.

Flexibility is a key advantage of etched copper flex circuits because it enables improvement in form factor relative to rigid PCBs. Complicated components such as microcontrollers with high I/O counts and other fine-pitch devices can be attached to copper flex substrates. While these packaged devices limit the flexibility and conformability of the overall circuit, they can meet many performance requirements.

Copper flex circuits are fabricated in a subtractive process by taking sheets of copper laminates, masking the desired conductive paths, and then chemically removing all unwanted copper, leaving the desired circuit patterns. This is followed by component attachment, as discussed above. While copper flex circuits remain a key part of many medical devices, in certain applications, they may have issues with biocompatibility as well as failure due to repeated bending cycles.

Additive manufacturing techniques are used to create the base circuit in printed electronics. Typically, silver, silver chloride, carbon, and dielectric materials are printed on a variety of substrates that can be chosen for biocompatibility. The materials and the manufacturing process contribute to making base circuits that are extremely flexible and able to withstand many bend cycles. In addition, certain substrates and printed materials can be stretchable, offering additional robustness for applications where patient motion is involved. The substrates and manufacturing approach for the base circuits can often provide cost benefits compared to copper flex circuits.