Four wearable design challenges and how to overcome them

What do medication patches, diabetic monitors, infusion sets, over-the-counter bandages, and EKG monitors have in common?

They’re all medical devices that stick to skin. And while they can provide life-sustaining and life-changing benefits, successfully adhering these devices to skin can be quite challenging.

There are unique characteristics of skin that must be accommodated for proper stickiness:

• Hair growth
• Oil production
• Sweat
• Temperature changes
• Bacteria
• Cells completely replacing themselves every two weeks

The above, in conjunction with age, environment, diet, culture, and overall health, create a unique combination for each person. Other daily routines, grooming habits, and hobbies – such as the use of lotions or powders and moisture from bathing or swimming – add additional wrinkles into the design mix.

Even with these shifting nuances, adhering to skin isn’t impossible. The key to success is understanding the science of skin and how its characteristics interact with adhesive properties.

1. Skin isn’t like fine wine

Unlike a nice bottle of wine, our skin doesn’t get better with age. Each stage of life presents new advantages and/or challenges. As babies, our skin starts out fragile with fewer cell layers. Adolescence brings an increase in oil production and potentially the amount we sweat. The skin of healthy young adults is the most durable, but as we continue to age, our skin becomes drier, thinner, less elastic, and more fragile. Around the age of 55, our epidermis thins and skin loses hyaluronic acid, making it stiffer. These characteristics make it unrealistic to use the same adhesive and expect the same results across the age spectrum. This affects the type of adhesive a project requires. 2 Don’t overstay your welcome Skin can be fickle with how long it will let an object stick to it, especially if that object doesn’t allow skin to function normally.

That’s why it’s important to select the right adhesive and backing to secure the device, along with the device’s housing material. All will help ensure the skin can breathe, flex, and move as needed for as long as the device is required to stay attached.

It’s important to consider the desired wear time when selecting a stick-to-skin adhesive because over-designing could destroy a device’s success. For instance, don’t use an adhesive meant to stay on for two weeks if a device will be removed after five days. The adhesion will be at its maximum value at that time, making removal painful and possibly leading to a medical adhesive related skin injury, or MARSI (see sidebar, pg. 32).

Even aggressive adhesives intended for long-term wear can experience loss of adhesion, edge lift, and eventual failure. Including a skirt – a border of tape extending beyond the footprint of the wearable device – can have a significant impact on the survivability of a device. In one case, a device exhibited 60% to 70% survival throughout 14 days without a skirt and 95% survival when the stick-to-skin tape included a 0.25" extension.

Silicone adhesives, known for their gentleness and consistent adhesive strength, are typically suitable for wear up to five days and can be repositioned. Acrylics, on the other hand, intensify in adhesion after the first day or two of wear and can be optimized for longer term wear.

3. Location, location, location

Just as not all skin is the same from person to person, not all skin is the same from location to location on the body, so don’t assume adhesives will perform the same way on different parts of the body. An adhesive securing a device to a healthy adult’s chest will have different requirements than one used on a baby’s face. If the location is fragile, using a silicone adhesive might be best. They’re a favorite for being gentle and causing less pain upon removal, as they don’t pull the skin or hair as much. If the location is more robust, acrylic tapes are often the way to go.

The device’s location on the body is not the only important location to note. Keep in mind any special environmental conditions of the end user, such as a baby in a moist incubator, which might impact the needed adhesion requirements.

4. Skin needs to breathe too

Ever have a bandage fall off a cut less than a day after applying it?

That’s your skin letting you know that the adhesive you stuck on it isn’t sufficiently breathable. The same thing will happen with wearable medical devices.

The longevity of an adhesive adhering to skin depends on its breathability. Skin, if unable to breathe or release moisture for a length of time, will feel suffocated.

If your project requires a device to stick to skin for more than a few hours, make sure both the adhesive layer and tape backing will allow the wearer’s skin to function as needed. Moisture vapor transmission rate (MVTR) is a function of adhesive and backing that lets engineers know how well the sweat generated underneath will escape. If it isn’t able to do so, sweat will accelerate the failure of the adhesive’s bond to skin. However, MVTRs typically reported for medical tapes aren’t applicable for tapes used beneath most wearables because these devices block moisture from passing directly through the tape in the Z-direction. Nonwoven backings allow some X-Y transmission of moisture, however, and tapes using them may perform better as they can help the moisture escape around the device.

Breathability of the adhesive becomes particularly important in instances when the end user’s skin might become sweaty or wet.

People who wear medical devices stay active and don’t want their device to impede those activities.

These end-use applications make design engineers cringe, but adhesive manufacturers are working toward creating adhesives that maintain breathability and durability when wet (see sidebar).

Engage with experts

It’s a challenge, but sticking to skin doesn’t have to feel insurmountable. Engineers should engage adhesives experts with experience designing, testing, researching, and innovating adhesive technology and do so early to have the right support throughout the entire design and development process. Adhesives experts can counsel on the nuances of working with skin, discuss the best medical adhesive options, and provide critical analysis of a project’s needs.

Better understanding the science of skin, and how new adhesive technologies can help make adhering devices to the skin more effective than before, will help produce a more successful product, a happier customer, and a more satisfied user.

3M
www.mmm.com

About the authors: Diana Eitzman, Ph.D., is director of agile commercialization for 3M’s Critical & Chronic Care Solutions Division and can be reached at dmeitzman1@mmm.com. Kris Godbey is an applications development specialist in the same 3M division and can be reached at kjgodbey1@mmm.com.