Marie Crane, Dow Food, Pharma & Medical, explains how silicone adhesives for wearable medical devices promote compliance and innovation.
Dow Microchip
The global wearable medical device market in 2016 was valued at just over $13.2 billion, according to Kalorama Information, a market research firm. By 2024, it is projected to reach $612 billion, as noted in a 2016 report by Grand View Research.1
Fueling this expected growth are mega-trends, which continue to drive the development and broader adoption of wearable devices for diagnosis, monitoring and treatment. These trends include increased prevalence of chronic diseases requiring ongoing monitoring; greater use of home-based and outpatient care for cost control; and technology advances such as miniaturisation, wireless innovations, wearable biosensors and 3D printing.
Unfortunately, because many wearable medical devices are used at home, instead of in a supervised inpatient or long-term care setting, some patients may fail to use them correctly or consistently. Patient behaviour directly affects the device’s efficacy, calling for solutions that can promote compliant usage. One important approach to optimise patient compliance is to ensure that skin-adhered devices are comfortable and non-irritating – both while wearing and when removing the device. Another way is to give patients the ability to use the device when taking part in everyday activities, such as showering, sports and recreation.
In both cases, choosing the right type of adhesive for a skin-adhered medical device can encourage patient compliance, which in turn improves efficacy and outcomes.
Dow - Sticking Point
Sticking point: It’s important to ensure that skin-adhered devices are comfortable and non-irritating
Adhesive considerations
Designers of wearable devices should consider these variables when selecting an adhesive:
Skin type and condition: The patient’s age and health are essential to determining the best adhesive for a wearable device. Elderly people can have thinner, less-elastic skin that can be susceptible to tearing or become damaged during removal or repositioning of the device. Alternatively, infants and young children tend to have delicate skin that can be sensitive and easily irritated. People with skin diseases or conditions are also at risk for damage from the wrong adhesive. A gentle, non-sensitising adhesive with low peel force can help to protect skin integrity in these situations.
Making matters even more complex, skin sensitivity, hair coverage and thickness and movement can vary by location on the body, so it’s important to consider where the device will be worn.
Device size and weight: While device miniaturisation is a major trend, some wearables must accommodate additional functionalities and therapies that expand their dimensions and weight. Larger and heavier medical devices require a different adhesive technology than miniaturized devices. A good example would be an external prosthesis which might call for a stronger adhesive than a significantly smaller monitoring device for guiding back therapy.
Duration of use: A device designed for short-term wear, such as a fetal monitor, can use an adhesive with high tack and lower peel adhesion, so it adheres to the skin with light pressure and can be removed with a low peel force. These properties ensure gentle removal to minimise discomfort for the patient.
On the other hand, 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 maintaining patient comfort. Water repellency is important for longer-term use as well, as it allows the patient to bathe, shower or perspire without risking adhesive separation. Permeability to oxygen and moisture helps keep the underlying skin in good condition over time.
Why silicone adhesives?
First, medical-grade silicones are biocompatible and have delivered proven performance in medical device applications for 70 years. Medical silicone adhesives offer key advantages that make them distinctively appropriate for wearable devices. They are non-cytotoxic, non-irritating and non-sensitising to skin. Silicones also spread easily to form films over the skin.
Because silicones are hydrophobic, a skin-adhered device could potentially be worn in the shower or for sports activities, helping to encourage consistent usage by the patient. They are also several hundred times more breathable than any other organic polymer, which enhances comfort. These materials also conform well to body contours for improved fit and comfort.
Silicone technology gives medical device designers a great deal of flexibility. Key properties such as adhesion level, conformity to the skin, peel strength and permeability – as well as transparency and even processing parameters – can be customized to meet specific requirements.
How do silicone adhesives stack up against acrylics and polyurethanes?
- Acrylic adhesives are widely used because of their cost advantages. They provide a very strong, secure bond with the skin, making them a good choice for long-term device use. Most release cleanly off substrate surfaces without leaving a residue. However, they cannot be repositioned, and removal can cause pain and skin trauma in the elderly or young children.
- Polyurethanes deliver medium adhesion. Cost-wise, they are more expensive than acrylics, but less expensive than silicones. Because they are more hydrophilic than silicones, polyurethanes also 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.
Which silicone adhesive?
The two main sub-categories of silicone temporary skin adhesive are pressure-sensitive adhesives (PSAs) and soft skin adhesives (SSAs). 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 – by up to several grams. Typical PSA properties include high shear strength and strong, stable adhesion for up to two weeks. They can be used in devices such as external catheters and sheaths.
SSAs exhibit significantly lower peel adhesion than acrylic and polyurethane. This property makes them suitable for devices worn by patients with compromised or delicate skin.
Growth in longer-duration wearable medical devices, such as wireless monitors, created a need for strong, durable adhesives that are gentle on sensitive skin – in effect, a middle ground between PSAs and traditional SSAs.
To meet that need, Dow Corning, developed Dow Corning MG 7-1010 Soft Skin Adhesive, a two-part, low-viscosity product that delivers the highest adhesion in the company’s SSA family. Applications include insulin pumps and glucose, fetal and cardiac monitors.
Dow Silicone
Flexible friend: Silicone technology gives medical device designers a great deal of flexibility.
Expanding the scope of wearables
Although diagnosis and monitoring continue to lead treatment as the most popular applications for wearable devices, manufacturers are adding therapeutic functionality to some of these products, in part because treatment devices command a higher price. For instance, a glucose monitoring device worn on the body may include a pump to inject insulin automatically, so the patient does not have to self-administer the drug. Taking this concept even further, microneedle patches could replace administration of insulin and other drugs using syringes.2
Other examples include smoking cessation devices that not only communicate with the smoker’s smartphone and transmit information to the device maker to track compliance, but also use transdermal patches to deliver nicotine at specific times of the day when smokers are likely to crave a cigarette. As the role of skin-adhered devices expands and diversifies, the need for next-generation adhesive solutions will only increase. Silicone adhesives offer unmatched design freedom, a long history of safety and biocompatibility, and desirable performance attributes – such as gentle removal and non-sensitisation/irritation. Silicone technology supports new innovations in wearables and promotes positive outcomes through improved patient compliance.