Designing Adhesives for Wearable Applications

Although most wearables used in the healthcare space today are not classified as medical devices, the market for medical device wearable technologies is expected to grow to more than $5 billion by 2018. But in order to work, medical device wearables still have to be affixed to the body, necessitating the design and development of new adhesives. However, developing adhesives for wearable applications is no easy feat. Designers can create effective products only by knowing the skin type and age of the target patient population and by understanding the physical properties of the wearable device itself.

The Devil’s in the Details

“In designing a wearable system, most companies use adhesives that are already familiar to the regulatory bodies,” comments Martha Sloboda, director, global business development at Windsor, CT–based Scapa Healthcare. “Thus, they begin with such standard materials as acrylics, silicone, or polyurethane adhesive chemistries. But then it comes down to determining whether the device will be used under the clothing or whether it will be fully exposed, whether it will be exposed to moisture, whether it is a long- or a short-wear application, whether it will be affixed to a joint or a flexible site, whether it will be attached to broken or unbroken skin.”

Adhesives are all about fixation, but fixation means adhesion to the skin as well as to the device, explains John Bobo, Scapa Healthcare’s associate director, R&D for healthcare, North America. “The construction of a suitable adhesive for wearable medical device applications isn’t always straightforward. You must make sure that all the components work together, including the skin adhesive, the substrate, and the device-hold adhesive. This means that an adhesive for these applications will have two different types of bondable surfaces.”

Some types of adhesives might be better at holding a device and some might be better at adhering to the skin, Bobo adds. Designers must consider this factor when they develop the whole adhesive system. “We create adhesive products by building variations and experimentation. Sometimes, we find that if we put the same adhesive on all layers, it doesn’t work as well as if we use different adhesives on different layers because of the interplay of the different substrates and the skin.”

Scapa produces its adhesive chemistries and constructions by building layer upon layer to achieve the necessary structural integrity required by the total product, Sloboda notes. Thus, a system might have an adhesive chemistry on one side, a binding layer, a different adhesive chemistry on the other side, and a removable liner.

However, depending on the geometry of the device and the body surface and how the liners will be removed, the design can be as important as the adhesive itself, Sloboda adds. “Thus, you have to look at the choice of adhesive holistically.” Ultimately, the final question that the design team must weigh is achieving performance benefits versus avoiding critical failures. “If a critical failure means that the product will fall off, it may be necessary to select a more tenacious adhesive, one that may cause the patient discomfort when the adhesive is removed. The tradeoff is that the product won’t fail.”

Hydrocolloids and Soft Adhesives

In developing an adhesive for wearable applications, designers must consider a range of factors, according to Ravi Ramjit, vice president, R&D at Orangeburg, NY–based EuroMed. How long will it remain on the body? On which body part will it be placed? What is the patient’s age? In what part of the world does the patient live? Does the patient have healthy or unhealthy skin? If an acrylic adhesive is used, can the patient endure discomfort possibly caused when the adhesive is removed?

Although a multitude of adhesives are suitable for wearable applications, the primary candidates are hydrocolloids and soft adhesives. “Hydrocolloids,” Ramjit notes, “are ideal for wearables mainly because of their long track record as skin-friendly, long-term-wear adhesives. Other suitable products include soft adhesives, which are compatible with the skin. We prefer soft adhesives primarily because they are able to flow into the creases and crevices of the skin, leading to longer-term wear capability. Such adhesives also minimize pain when they are removed from the skin.”

To develop its HydroSoft line of low-trauma soft adhesives, EuroMed incorporated several elements into the design. First, it studied physics, chemistry, and a specific branch of physical chemistry known as rheology—the study of the flow of matter. In doing so, it investigated the elastic and viscous modulus, or material flow, and studied how adhesives behave when subjected to different temperatures. “Stiff adhesives don’t react well to elevated temperatures, Ramjit explains. “In general, they cannot flow and decrease the surface contact with the skin. Adhesives such as HydroSoft flow up to a point, but at body temperature, we were able to fine-tune the material to where we needed the elastic modulus to reside.”

To absorb perspiration and thus prevent wearables from slipping off the patient, the company also incorporated components such as carboxymethyl cellulose, an absorbent found in traditional hydrocolloid wound-care dressings. In addition, for adhesives that will remain on the body for up to seven days, the company integrated stiffening agents into the adhesive to ensure that it will lift up in one piece upon removal. “In sum,” Ramjit says, “it was rheology, the ability to handle perspiration, the ability to minimize pain upon removal of the adhesive, and the incorporation of stiffening agents that enables our product to adhere.”

Adhesives that are going to reside on the body should also be sterile because of the potential for bacterial or fungal bioburden propagation associated with the skin, Ramjit remarks. And if the adhesive will be used in combination with other products, it must be sterile. In such cases, gamma sterilization is advantageous.

“What sets HydroSoft apart from silicone-based adhesives is its ability to undergo gamma sterilization,” Ramjit comments. “Silicone deteriorates under gamma sterilization. And while you can’t sterilize the adhesive once it’s on the skin, if you start with a low bioburden count, the total colonization will not be as high as if you started with a nonsterile product. Bacteria and fungi grow exponentially. Thus, if we start low, we’ll end lower than we would if we were to use a nonsterile material.”

Design Challenges

Adhesives selected for affixing wearable medical devices to skin should exhibit four major properties to ensure product efficacy, according to Gozde Karabiyik, senior product development chemist at Glen Rock, PA–based Adhesives Research. First, they should exhibit high cohesive strength, or shear resistance, to prevent device movement. Second, they should offer high levels of initial tack to quickly adhere the device to skin. Third, they should provide a high moisture vapor transmission rate (MVTR) to ensure breathability, prevent the accumulation of moisture under the adhesive, and maximize adhesive strength. Finally, depending on the application, the device and the adhesive should be able to withstand gamma, E-beam, or EtO sterilization.

Suitable for a variety of wearable sensors, infusion pumps, and patch pumps, Adhesives Research’s skin-friendly and gentle-removing adhesives are designed for repositionability and clean release from the skin and hair. Because they can be removed gently from the skin, they are suitable for pediatric and geriatric applications. And because they require lower coat weights than other adhesives, they also feature a thin profile, ensuring reduced edge lift.

“Available in silicone and nonsilicone formulations, our SoftWear adhesives remain tacky even when removed from the skin,” Karabiyik comments. “Therefore, they can be applied and removed multiple times, in contrast to traditional skin adhesives. This property allows users to reposition the device, if needed.”

Body-worn devices present unique skin-attachment challenges. Their performance depends on how reliably and reproducibly they can be attached to the body. “Skin is a living and changing substrate,” Karabiyik says. “Wear performance is based on numerous skin variables, including the patient’s age, gender, race, diet, activity, and perspiration levels. It is also based on surface energy, curvature of the body, and the stretchability of the skin at the fixation site. Thus, it is challenging to design a single adhesive that performs well in a variety of different contexts.”

The wear performance of an adhesive can also be impacted by the size and shape of the device and by how well it conforms to the body. For example, devices with taller profiles can become entangled in clothing, while the shear forces associated with heavier devices can result in movements that lead to device failure. “Thus, it is important to understand the wear duration, skin type, activity level of the targeted patient population, and the device parameters to select the most appropriate adhesive,” Karabiyik remarks. “The adhesive also needs to have adequate shear strength to support the device without creep. And breathability of the overall adhesive-device design is critical so that moisture can escape from the skin and ensure a secure bond.”

Bob Michaels is senior technical editor at UBM Canon.

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