The global market for wearable medical devices, both diagnostic and therapeutic, is estimated to reach $27.8 billion by 2022, according to Grand View Research.
Technological advancements such as new wireless innovations, wearable biosensors, and 3D printing are keys to the expansion of this sector. Further, growth in wearable medical devices is being propelled by mega trends including population aging, a higher incidence of chronic diseases such as diabetes and hypertension, greater patient interest in self-management and efforts to control spiraling healthcare costs.
Skin-adhered medical devices – from diagnostic heart and sleep monitors to therapeutic insulin pumps – address all these trends, but present their own challenges. While many patients, in theory, would prefer home monitoring or treatment with a wearable device to repeated visits to a healthcare provider, in practice, they may fail to use the device correctly – or at all. Noncompliance may increase if the device is uncomfortable, irritating to the skin, or difficult or painful to remove.
Because comfort drives higher patient compliance, designers of new wearable medical devices need an appropriate adhesive material for each use case.
Choosing the Right Adhesive
Variables that device designers need to consider when selecting an adhesive include:
- Duration of use: A device designed for short-term wear, such as a fetal monitor (image right), can use an adhesive with high tack and lower peel adhesion, so it can adhere to the skin with light pressure and be removed with a low peel force. This means both the initial and subsequent applications are gentle for the patient, while the adhesion properties remain.
In contrast, wearable devices for extended use – such as ostomy bags and long-term ambulatory monitoring mechanisms – require strong and stable adhesion and high shear strength, while ensuring patient comfort. Water repellency of the adhesive is important to allow the patient to bathe, shower, or perspire without risk of adhesive separation. Permeability to oxygen and moisture also helps keep the underlying skin in good condition.
- Skin type and condition: The age and health of the target patient plays an important role in determining the best adhesive for a wearable device. Elderly people tend to have thinner, less-elastic skin that can be torn or damaged during device repositioning or removal, while infants and young children have delicate skin that can be sensitive to irritation. People with skin diseases or conditions are also at risk for damage from the wrong adhesive. Therefore, a gentle, non-sensitizing adhesive with low peel force can help to protect skin integrity in these patients.
- Device size and weight: Although device miniaturization is a major trend, some wearables must accommodate additional functionalities and therapies that expand their dimensions and weight. Obviously, larger and heavier medical devices require a different adhesive technology than miniaturized devices. For example, an external prosthesis calls for a stronger adhesive than a small monitoring device for guiding back therapy (below).
With these parameters in mind, designers can evaluate popular adhesive types: silicones, acrylics, and polyurethanes.
Silicone adhesives offer many advantages that make them uniquely suitable for wearable devices. Medical-grade silicones are biocompatible and have delivered proven performance in medical device applications for 70 years. They are non-cytotoxic, non-irritating and non-sensitizing to skin.
Their mechanism of action combines low interfacial bonding for reduced impact on the skin with high flowability for stable adhesion via a mechanical interlock with skin pores. Silicones also spread easily to form films over the skin.
Silicones are hydrophobic, so a device using a silicone adhesive could potentially be worn in the shower or for sports activities. They are also several hundred times more breathable than any other organic polymer, which enhances patient comfort. These materials conform well to body contours for improved fit and comfort.
Another key design benefit of silicone technology is its versatility. Properties such as adhesion level, conformity to the skin, peel strength, and permeability – as well as transparency and even processing parameters – can be tailored to meet a device’s specific requirements.
Acrylic adhesives, which provide a very strong, secure bond with the skin, are ideal for long-term device use. Most release cleanly off substrate surfaces without leaving a residue. However, they also carry significant drawbacks. Primarily, they cannot be repositioned, and removal can cause pain and skin trauma in elderly or neonatal patients. Acrylic adhesives are also known to cause skin sensitization.
The third option, polyurethane adhesives, deliver medium adhesion. Polyurethanes are higher in cost than acrylics and less expensive than silicones. Because they are more hydrophilic than silicones, polyurethanes provide better exudate management. However, moisture absorption can lead to a reduction in skin adhesion. Also, these adhesives have very low breathability compared to silicones, and they tend to leave a residue when removed. Unlike silicones, polyurethanes are gamma sterilizable.
PSA or SSA?
Within the silicone temporary skin adhesive category, there are two major sub-types available in the market: pressure-sensitive adhesives (PSA) and soft skin adhesives (SSA). Although both are applied using pressure, PSAs generally offer stronger adhesion than SSAs, making them suitable for devices that are worn longer or are larger and heavier – up to several grams. Typical PSA properties include high shear strength and strong, stable adhesion for up to two weeks.
Applications for PSAs include scar treatments, external catheters and sheaths, and wig attachments.
Soft skin adhesives exhibit significantly lower peel adhesion than acrylic and polyurethane, resulting in less skin trauma on removal. This property makes them suitable for devices worn by patients with compromised or delicate skin.
To bridge the gap between PSAs and SSAs, the industry has developed SSAs with high adhesion designed for longer-duration medical devices suitable for patients with sensitive or fragile skin. Applications include insulin pumps and glucose, fetal and cardiac monitors.
Comfort Encourages Compliance
The efficacy of a wearable diagnostic or therapeutic device depends directly on the patient’s compliant usage. Comfort during wear and removal of skin-adhered devices is an important compliance factor. That is why many designers of wearable medical devices are turning to silicone adhesives. In addition to their proven safety and design versatility, silicone PSAs and SSAs deliver a more comfortable patient experience than competitive adhesive technologies.
All images from Dow Corning
Dr. Vinita Pandit is a technical service and development specialist at Dow Corning, specializing in pressure sensitive adhesives, soft skin adhesives, and related silicone-based technologies for the healthcare industry. Her areas of expertise include protein/peptide synthesis and characterization, radiolabelled silicone synthesis, polymer synthesis and characterization, formulations of adhesives and testing. Pandit holds a Ph.D. in bioanalytical chemistry from Missouri University of Science and Technology.