Designing wearables: How to make sense of your material options

The medical device industry’s knowledge base continually evolves regarding what works — and what doesn’t — for wearable applications. Gain insight into some material selection considerations for skin-worn wearable design and development.

Deepak Prakash, Vancive Medical Technologies

In the dynamic wearables sector, medical device makers are searching for ways to translate their expertise into mobile, body-worn applications. Skin-worn wearable devices are gaining traction, particularly for end uses that demand a high level of data accuracy. Because these wearables have such intimate contact with the body, they often can pick up very subtle signals from the organs and muscles, making them ideal for some cardiac applications and activity monitoring. Their direct contact with the skin also opens possibilities for delivery of medications and supplements to patients.

Early in the wearables design process, it’s important to think about material selection and specification. Adhesive materials are a foundational component of most skin-worn wearables, and so it’s wise to explore various performance characteristics, start evaluating biocompatibility and even do some preliminary wear testing during the initial stages of product development. As medical device manufacturers are all too aware, once material and component specifications are locked into regulatory documentation, it’s very difficult and time-consuming to make changes down the line.

A quick review: wearable adhesive characteristics

Even some seasoned medical product designers, technologists and product managers may not have much hands-on experience with skin-worn materials. For example, a design engineer accustomed to developing diagnostic equipment might be tasked with a wearables project. He or she may have extensive knowledge about accurate data collection and patient monitoring but not necessarily about how to miniaturize and adhere such solutions to the patient’s skin for extended time periods.

Here are a few wearable adhesive basics that can help familiarize device designers and managers with their adhesive material choices. There are two primary types of adhesive materials used in wearable devices:

• Skin-contact layer adhesives — These are the materials that ultimately hold a wearable device directly to the patient’s body. They must be very comfortable, with the ability to effectively manage different moisture and activity levels, depending on the wearable’s end use.
• Construction-layer adhesives — These materials are also known as tie-layer adhesives because they tie, or connect, the different components of the wearable device together. While construction layers usually do not directly touch the patient’s skin, they must be compatible with the skin-contact layer. For example, if the skin-contact layer adhesive material is breathable, the construction-layer material must also be breathable. If not, it will negate much of the skin-layer material’s ability to allow for the patient’s skin to stay dry, comfortable and irritation free.

There are four major material characteristics that play an important role in the performance of any wearable device adhesive. These include:

• Static shear — The ability to hold in position in the presence of shearing forces, such as bending and twisting movements. It is also known as cohesion.
• Peel — The ability to resist removal by peeling. This is also known as the level of peel adhesion.
• Tack — The ability to adhere quickly. For example, some pressure-sensitive adhesives may adhere almost instantly whereas others may need to be held in place with some light pressure for a short time to achieve optimal securement.
• Moisture management — The ability to move moisture, such as perspiration and other bodily fluids, away from the patient’s skin to avoid discomfort and irritation. Moisture typically is managed in one of two manners, either through fluid absorption (being absorbed and contained within the material) or moisture-vapor transmission (evaporating through tiny pores in the material). The latter is measured by the moisture vapor transmission rate (MVTR).

A major challenge: Achieving comfort and function

There are some obvious and subtle similarities and differences between designing a wearable device vs. other types of medical devices, even some products that at first blush may seem very like wearables. For example, some may ask, “Is there really that big of a difference between designing a body-worn wearable and developing an adhesive wound dressing or ostomy flange that must be worn for an extended time period?” Or others might wonder, “How does putting on a wearable device differ that much from securing an IV catheter or diagnostic monitor to a patient in the hospital?”

Here are just a few reasons why wearable devices present unique design requirements.

• Patient autonomy — Wearable medical devices often are designed to be attached and activated in the patient’s home, which may be many miles from the nearest healthcare provider or institution. One must be able to easily apply the device to oneself, perhaps single-handedly. Tack plays a key role here, especially for drug-delivery applications in which the patient needs a quick, secure attachment. Materials suppliers may be able to engineer adhesives for repositionability if the patient is likely to miss the ideal securement spot on first attempt.
• Patient mobility — Whereas a hospitalized patient may be confined to a bed or somewhat limited in his or her movements, most patients using wearables will be going about their usual routines in their personal lives at home and at work. The wearable needs to move with the patient through daily activities, such as exercising, showering, sleeping and dressing. An appropriate level of static shear and peel adhesion is necessary to ensure the wearable can stay secure during all of this motion. And unlike a wound dressing, the wearable must not only stick to the patient but also have the strength to hold and secure the weight of the device itself. While this weight may only be a few grams, it can make a big difference in terms of the demands it puts on the adhesive. All the while, the skin will be exuding sweat and regenerating cells, so device materials must also manage this fluid and exudate.
• Patient discretion — In general, a wearable medical device should be small and unobtrusive. Again, within the inpatient setting, there are procedures and protocols that are very familiar (e.g., taping a catheter to a patient’s arm or attaching electrodes with wires to the chest). However, this approach becomes less acceptable in a non-medical setting. For example, a patient may be willing to endure the inconvenience of having these monitors attached to his or her body for a surgical procedure and the immediate postoperative period or for a stress test that takes about an hour. But once back at home in the swirl of professional and personal life, a device usually works best when it disappears under the clothing and can be all but forgotten. For wearable device design, this means the adhesive material must offer long-term wear, which requires excellent moisture management, peel adhesion and static shear.

Of course, another major reason why wearable device design is so complex is the diversity of expertise required to bring these solutions to market. Consider that many wearables bear more of a resemblance to consumer electronics than medical devices. Depending on the device’s functional footprint, it may need to be Bluetooth enabled or be able to dock with a USB port. Specialized software algorithms will be needed to convert the device’s digital signals into meaningful clinical information. A cloud computing infrastructure may be required to move and store enormous amounts of data. And as they are in so many other facets of society today, from hailing a cab to ordering pizza, user-friendly mobile apps are practically essential. They are needed to deliver all of this data to healthcare providers and patients in a highly accessible, easy-to-digest way.

In conclusion, a strategy that nurtures multidisciplinary collaboration is the key to successful wearable design. From adhesive materials to the software code, a patient-centered approach will win the day.

Deepak Prakash is senior director of global marketing at Vancive Medical Technologies, an Avery Dennison business.

The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.

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