Wearable medical devices are notoriously tricky to design. There are several different issues concerning components, materials, and functions that must be considered before the device will perform as intended. To mitigate challenges and ensure a painless design process, engineers often rely on innovative approaches, including adhesives. They may be used to hold a device to the patient’s skin for a day or more or to prevent components from shifting, among other critical functions. Though a seemingly small part of the overall design, adhesives can greatly affect how successful the device will be.
There is no single adhesive that will work well in all roles, so engineers must be informed when choosing them. Application considerations include skin characteristics and compatibility and manufacturing processes. For the best results, discuss the following with your adhesive supplier at the start of the design process:
What the device will be made of. The adhesive must be compatible with all other materials it will come in contact with for it to work. Popular materials, such as polycarbonates, acrylics, silicone, and polyolefin don’t always pair well with an adhesive. For example, silicone is flexible but tough to stick to, limiting adhesive options or resulting in other design compromises. By informing your adhesive supplier of a device’s other materials, you’ll help avoid costly redesigns later.
It’s also in your best interest to let your adhesive supplier know if you’re planning on using a plasticizer in an adjacent layer. While great for creating softer, more flexible features, they degrade an adhesive’s properties. Initial adhesion to these altered materials is typically good, delaying potential issues until it’s too late to make changes.
Knowledgeable suppliers should help you find the best-suited adhesive and material combinations.
Intended wear time. The strongest adhesive isn’t always the best option for longer wear times if it’s being used to stick a device to skin. Some devices only need to be on the patient for a few days, so using an adhesive that’s at its peak strength when it needs to be removed can strip off the top layers of skin. Adhesive strength needs to be balanced with gentleness and breathability to keep skin healthy and protect the patient from injury, especially for devices requiring longer-term wear.
The substrate the adhesive will stick to. Sticking a device to skin is vastly different from sticking two components made of metal or plastic together. Skin is a living, breathing organ that needs to move and expel moisture. For a stick-to-skin device, the device and its adhesive can’t interfere with the skin’s normal functions. The adhesive needs to be breathable (allowing moisture to evaporate through the material); gentle (not causing harm to the substrate or wearer); and conformable (adheres to substrate to allow for flexing and moving); and must account for other human factors, such as hair growth, the age of the wearer, and the wearer’s activity level.
On the flip side, if the adhesive will be holding two components together, the substrate will likely be metal or plastic. Therefore, characteristics such as breathability and gentleness do not apply. In these cases, it all comes down to friction and strength. The adhesive must withstand the friction of manufacturing and everyday wear and tear for it to securely hold parts in place.
Age of intended patients. A person’s skin characteristics change over time, so the age of the intended patients influences which adhesive is right for the application. If the device needs to adhere to a baby or an elderly person, it must accommodate fragile, thin skin. Whereas adhering to adolescent skin will need to accommodate oily skin. Not every adhesive works for every stage of life.
Where the device will be applied. Skin thickness depends on where it is on the patient. Skin is thickest on the bottom of the feet and the palms of our hands, and thinnest on eyelids. Skin thickness also affects its sensitivity to different stimuli.
When designing a wearable device, designers must know if the device will be stuck to thinner, more sensitive skin or thicker, more tolerant skin to select the adhesive’s proper sticking power.
Environmental conditions the device will be exposed to. Materials in dry, mild climates might perform differently than in wet, humid climates. Adhesives are no exception. Moisture levels on the skin, whether due to climate or the skin excreting it, can cause issues if not properly planned for. An adhesive’s moisture-vapor transmission rate (MVTR) can help select proper adhesives. MVTRs predict how an adhesive will perform in high-moisture environments. However, MTVRs commonly reported aren’t applicable to many adhesives used under most wearables because most wearables are occlusive or prohibit moisture from moving directly through them. They can prevent moisture from evaporating through the adhesive. An adhesive with a nonwoven backing rather than one that’s film-backed might allow moisture to move back-and-forth.
Manufacturing processes being used. Manufacturing processes can be hard on materials, and not every adhesive is compatible with every process. Some are too soft and can gum up equipment, while others won’t bond at the needed performance level. Conduct experiments on pilot or full-scale processing equipment with your adhesive and converting partners to avoid redesigns, cost overruns, and delays.
Sterilization methods to be used. There are three commonly used sterilization methods used on adhesives: ethylene oxide, gamma radiation, and e-beam. Each can alter a material’s properties. An adhesive supplier can help determine which tests will best predict how specific adhesives will react to your desired method. Be sure to ask your supplier before getting too far into the device’s development.
Tony Kaufman manages new business ventures in 3M’s Medical Materials & Technologies Div., and Del R. Lawson is R&D manager in 3M’s Medical Solutions Div. If you have questions about medical wearable and adhesives, visit www.3M.com/MedTech.