Wearable Design Challenge: Stick-to-Skin Adhesion
Wearable device engineers are working miracles in wringing more performance out of ever-smaller devices. Advances in sensor technology, batteries, and materials have enabled innovations that were impossible only a few years ago. With these advances, one commonly overlooked layer in the early stages of device development is the stick-to-skin adhesive.
The Challenge of Sticking to Skin
Ask a pressure sensitive adhesive (PSA) developer to describe the ideal substrate and she’ll likely include words like clean and dry. Some would say “not low surface energy” (LSE). Two items that are taken for granted by most developers are rigid and consistent.
Now consider the list of challenges for stick-to-skin adhesives: Skin bends, stretches, and compresses; it has a lower surface energy than polyethylene; you have no control over how clean and dry it is; and all these criteria can vary significantly among individuals wearing your device. Add to the list that skin is a living, breathing substrate that doesn’t like to be covered and that traces of the wrong material might cause an allergic reaction. Finally, consider challenges from sweat and skin oils that can range somewhere between moderate and extreme.
It’s challenging enough for an adhesive to remain adhered for a few days under such conditions. Modern device adhesives are being asked to last a month in some applications, and then remove without damaging the skin and ideally without leaving any residue behind.
Primary Failures of Stick-to-Skin PSAs
The two most common causes of stick-to-skin PSA failure are:
1. Slow loss of adhesion through changes occurring in the adhesive or at the skin-adhesive interface due to sweat, skin oils and dead skin cells. Absorption of either sweat or oil by a PSA will make it softer and easier to deform. Softening due to sweat is typically a short-term issue peaking soon after a period of exertion. The adhesive properties are expected to recover as it dries. Conversely, oil absorption is likely to have a cumulative impact on the adhesive. The results of oil absorption can be negligible or catastrophic depending on location on the body, an individual’s skin type, demands placed on the PSA, and exactly how its rheology changes due to the oil. The nose and forehead are notoriously difficult areas to adhere to largely because the skin there is likely to be oily.
While oil and sweat are transient at the skin-PSA interface and can be absorbed by the adhesive, the most obvious thing to accumulate beneath a skin adhesive is dead skin cells. A layer of these cells make up the stratum corneum, the upper-most layer of skin. Over time, the number of dead skin cells beneath the adhesive increases and their cohesion with lower layers of the skin decreases. After enough time, the surface layer to which the PSA is attached may slough off, taking the adhesive with it.
2. The abrupt application of force (aka “It got knocked off”). The greatest threat to a wearable device’s bond to the skin is—depending on its position on the body—either being pulled off during the changing of clothing or being dislodged by a bump against furniture or a doorframe. This risk can be reduced through minimizing the device’s profile and/or by having the stick-to-skin patch extend beyond the footprint of the device. During that critical moment when a device is tugged away from the body, the adhesive in the extended border of the patch is put into a shear mode of failure. This relatively large area of adhesive can resist debonding much better than if the skin adhesive ends at the device’s edge.
Adhesives Are Keeping Up
The good news for wearable device developers is, we have decades of experience and entire families of stick-to-skin PSAs to draw on as we attempt to address the latest challenges presented by devices requiring wear times of one to four weeks. Current material sets are well tolerated by the vast majority of people, and PSAs appropriate for even the fragile skin of the very young or very old are available.
The search for long-wear adhesives started with a solid footing in materials, but the landscape of long-term wear was totally unmapped. When our team set out to make our first long-wear adhesive for this market, the longest stick-to-skin PSA clinical studies available lasted only seven days. In addition, participants in the studies had been instructed to limit their activity (no exercise) and to minimize the samples’ exposure to water while showering. Such limitations in clinicals were non-starters for the device market, whose customers won’t tolerate devices that limit their activities.
The big questions when pursuing wear times greater than a week were primarily related to skin health after prolonged occlusion and how to handle sweat and oil over extended periods. Six years ago, there was such a paucity of knowledge regarding the impact of sustained PSA wear on skin health that our first 21-day clinicals included removing half the samples on day 12 to ensure we weren’t damaging the skin of study participants.
These first studies eliminated a lot of good adhesives from consideration and revealed some of the attributes we’ve learned are critical to long-term use on skin. One of the earliest learnings: High peel values don’t equate to long duration. PSAs designed to have very high adhesion for critical applications such as tube securement were often found to have wear times of only a few days. This lack of correlation between peel value and wear time was a relief given that removing aggressive PSAs can irritate or even damage skin.
We’ve come a long way on the quest for longer-wearing PSAs and continue to reach new milestones. Recently launched stick-to-skin tapes have clinically demonstrated > 90% survival on back of arm skin for up to 28 days and the ability to maintain their tack for at least 12 months even after removal of their liner.
When to Start Talking to Adhesive Providers
The best time to begin the discussion of adhesive options is before you print the first prototype housing of your device. You’re going to want transfer adhesive and tape samples on hand to finish the mock-up and begin iterating. You’ll get expert advice on which types of adhesive will work best with your materials and may avoid going down some dead ends. Your medical adhesive provider can recommend proven solutions based on target location on the body and desired wear time. Consider a variety of tapes with different backings. Some backings have advantages for the end user, others may provide better handling through converting.
The challenges of adhering to skin are many and are multiplied when long wear times are desired, but with recent advancements in PSAs the skin adhesive should not be the component limiting your device lifetime.
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