Today’s medical and fitness wearables must be able to handle everyday life–a drop of liquid or a fall to the floor should not keep it from fulfilling its purpose.
Whether for everyday use by consumers or limited use for diagnostic testing, medical wearables must be durable and able to withstand cleaning, disinfecting, and general wear and tear–sometimes by multiple patients over the course of their lifetime. And, if they are to last, they must be inherently protected from liquids and other contaminants, impact from hard surfaces, and even radiation.
Many Shapes and Sizes Inform Medical Wearable Design and Materials
Medical and fitness wearables is a broad category. The global wearable medical device market size was valued at $21.3 billion in 2021 and is forecast grow at a compound annual growth rate of 28.1% from 2022 to 2030.
When most people think of a wearable, a FitBit or an Apple Watch may come to mind. But in addition to these popular consumer wearables, there is an entire host of small medical devices that are worn temporarily to evaluate patients, monitor them long-term, or even deliver a therapeutic intervention. One area of growth in recent years was in the diagnosis, treatment, and monitoring of patients with cardiac arrhythmias and other heart-related disorders.
Ground-dressing and electrode wearables such as electrocardiography (ECG/EKG), electroencephalography (EEG), electromyography (EMG), and electrotherapy devices use electrodes and grounding contacts along with stick-to-skin adhesives. For these biomedical and diagnostic devices that measure electrical impulses for diagnostics or transfer electronic pulses to the body, durability, biocompatibility, and patient comfort are key considerations.
Some wearable devices such as transdermal patches are for delivering extended-release medications. Usually worn for long periods of time, they must balance adhesive hold with breathability and be comfortable for the wearer. It’s also important that the materials used do not interact negatively with the medicines and pharmaceuticals that are being delivered to the wearer.
Blood glucose strips and diabetic testing are common examples of microfluidic diagnostic devices. Used to track biomarkers such as glucose and pH levels at a molecular level of blood, sweat, or other fluids, they are small and intricate with sensors that collect data from the wearer and must house printed flex circuits, PCB assemblies, electrodes, and batteries.
Wearable biometric monitoring devices is a broad category that tracks biometric markers and information including temperature, heart rate, respiration, and movement, among others. Examples include continuous glucose monitors, blood pressure monitors, and sleep trackers. Their functionality, including the ability to transmit information collected wirelessly, requires internal components such as flex circuits and battery assemblies, and they may need to stick to the user with adhesives.
Medical Wearable Internal Components and Users Must Be Protected
There are many elements that need to be kept out of a medical wearable if it’s to continue to function. Liquids such as water are the obvious ones, but dirt, dust, and even electromagnetic interference are all threats to their proper functioning. This creates several challenges for designers because the devices must be cost-effective, especially in the consumer market, while also being durable.
The materials used for medical wearables, particularly sealing gaskets, are critical to manufacturing medical wearables. They not only must keep out unwanted contaminants, but they must also be safe for human contact–depending upon where they’re located on the device. Medical sealing gaskets that come into contact with human tissue, bodily fluids, medical fluids, or drugs are made of different material than those inside medical electronics, such as the fireproof sealing gaskets inside the 3D printers used to manufacture prosthetics and orthotics.
The material used for sealing gaskets in medical devices and wearables needs to be more than cost-effective and reliable; it must also meet regulatory requirements, including those of the FDA and other standards depending upon their use–while still supporting high-volume, precision fabrication. All materials selected for use in medical wearables must balance functionality with user comfort.
All the parts, including sealing gaskets, may need to be flame-resistant, conduct electricity, and protect against electrostatic discharge . All these requirements guide the selection of materials, ranging from a wide spectrum of polymers and elastomers. Most of all, they must be durable.
Balancing Flexibility and Durability for Medical Wearables
Today’s medical wearables must consider how people live. Not only must they be able to accommodate the shape of the wearer, but they must also be able to do it for long periods of time.
Moisture is consistently a factor when considering the regular day-to-day living of medical wearable users, which is why sealing gaskets are so important. They must be able to handle sweat, bathing, and even a dunk in the pool. People have come to expect that electronics are water-resistant enough that they don’t have to worry about taking them off just to hop in the shower or go for a swim. They also expect their wearables to weather the occasional bump or fall on a hard surface. Whether it’s a consumer device with a sole owner, or a medical diagnostic device worn temporarily by multiple patients, they must work reliably over the course of a few days, weeks, months, and even years.
Because flexibility and durability are both key considerations, medical wearables need a combination of hard and soft materials. This is why polycarbonates and thermoplastic elastomers (TPEs) are typically used to build both the housings and interfaces to medical-grade standards, including those related to human tissue contact. TPEs are particularly well-suited for sealing gaskets and softer components that allow for wearables to be comfortable. Overall, durable devices that last a long time with regular use need to be composed of materials that offer chemical resistance and demonstrate strong adhesion.
Chemical resistance is important because wearable devices must endure regular cleaning but may also have contact with lotions and skin creams, which can degrade plastics over time. Also, a device applied to the skin by a healthcare worker may come into contact with an alcohol-based disinfectant. Of course, the electronics within these wearables must also be protected from cleaning solvents, and sometimes the best way to disinfect a medical wearable is radiation, which creates additional challenges.
Sterilization By Radiation Affects Medical Wearable Component Characteristics
One method of sterilizing medical devices is by exposing them to radiation. However, depending upon the complexity and components of the device, radiation may cause damage.
With smarter wearables that have memory for storing and processing data, radiation can be detrimental, particularly to flash memories. Not only are there limitations for use in medical devices subject to radiation, but they are difficult to use outside of the earth’s orbit for space applications.
There are other types of memory that are more tolerant of radiation, however. Conductive bridging random access memory (CBRAM), for example, is a non-volatile memory that uses typical cells to store digital ones and zeroes. To distinguish between them, a small electrical voltage is used to change the resistance of the memory cell between high and low resistance. Combined with a dielectric layer added into the manufacturing process, the fundamental physics of CBRAM is such that information is stored differently on the chip to make it tolerant to radiation. Other memories that can handle radiation well are SRAM and potentially MRAM.
Memories such as CBRAM are better able to handle the gamma radiation exposure used to sterilize medical devices–historically, electronics have not been able to handle sterilization through radiation. The alternative is to use steam, but that creates other challenges. Radiation works well for sterilization because it allows for a quick cleaning of devices so that they can be quicky deployed in a hospital environment.
Medical and fitness wearables are becoming increasingly complex. The more expensive the upfront cost, the greater the expectations are for longevity–whether it’s a consumer buying a fancy fitness tracker or a hospital deploying diagnostic trackers that need to be used by more than one patient. That means the conditions they must endure must be thought of during the design process, so materials and components are both flexible and durable.
