Wearable medical devices range from diagnostic and assistive devices that monitor vital signs, glucose, and sleep to therapeutic devices for pain management, rehabilitation, and respiratory therapy.
Many devices are designed to adhere to the user’s skin with robust, skin-friendly adhesives. These medical-grade adhesives aren’t typical, off-the-shelf products. To succeed in demanding wearable applications, they must be intricately formulated with specific characteristics and capabilities. For wearable medical device manufacturers, designers, and converters, evaluating and choosing the best adhesive for the specific needs of the wearable application is vital.
Most medical wearable adhesives must meet two goals – functionality and user experience – performing as intended per the requirements of the technology while delivering a comfortable user experience.
This requires adhesives to be precisely formulated for user age, gender, ethnicity, skin type, environmental conditions, and size of skin-contact area.
Other considerations include intended length of wear, removability/reusability, and durability. In certain applications, adhesives may also need to address resilience-related variables related to users’ activity levels – from how often they shower, bathe, exercise or sweat, to changes in body heat and temperature.
To successfully meet stringent demands, most wearable medical device manufacturers, designers, and converters prefer medical-grade pressure-sensitive adhesives (PSAs).
PSAs form a bond when pressure is applied – adhesives don’t need heat, water, or solvent to activate. PSAs encompass three primary layers: carrier, adhesive, and liner. They’re available in rubber or acrylic constructions, offer varying levels of adhesion from low or soft to high or extra strength, and can be customized to meet specific applications.
When choosing a wearable PSA, manufacturers, designers, and converters should begin by considering the following:
- What adhesive type is more suitable for the application – rubber or acrylic?
- How was the adhesive manufactured?
- What adhesive carrier best aligns with the chosen adhesive and the application?
Rubber versus acrylic PSAs
Rubber and acrylic PSAs have pros and cons, and both can deliver varying performance for customized functionality. Some are designed for less holding power and feature a lighter adhesive coat weight for shorter-term applications; others are more advanced, delivering strong bonds at higher coat weights for longer-term applications.
Rubber-based PSAs combine natural or synthetic rubber with tackifying resins, antioxidants, oils, or other ingredients. They create excellent bonds and deliver high adhesion to various substrates, including low-surface energy (LSE) substrates. Best suited for short-term wearable applications, used one day or less, they are generally more cost-effective than acrylic adhesives and are more aggressive for multiple skin types. Rubber adhesives tend to be heat-sensitive and less effective in applications that require exposure to high temperatures. Prolonged exposure to UV rays or certain chemicals can lead to oxidation, affecting performance and causing rubber-based adhesives to darken and lose tack.
Acrylic-based PSAs are formulated by reacting and crosslinking monomers with the properties desired for an application to create strong, customized polymers. Acrylic adhesives are highly durable, possess color stability and clarity, and bond well to polar surfaces such as metal or glass. Long-term endurance and environmental resistance properties, not common to rubber PSAs, include resistance to temperatures, UV rays, oxidation, and chemicals. Acrylic adhesives tend to be more breathable and stretchable than rubber ones and are often designed for longer-wear applications used multiple days at a time.
The way in which a PSA is manufactured is also important. Device manufacturers, designers, and converters must confirm the adhesive has been formulated for sustained, low-irritability skin contact and has been assessed per ISO-10993 guidelines for skin contact properties.
Work with a manufacturer that pays close attention to the quality of each adhesive component before and after mixing, including before delivering mixed components to the adhesive coater, throughout the coating process, and during the manufacturing slitting stage.
Various coating processes can deliver significant benefits as well. Many medical wearable PSAs use 100% solids adhesive technology for speed, easy customization, low cost, and low energy consumption.
With the 100% solids process, also known as hot melt, chemists mix adhesives to reduce viscosity then melt them into a coated, molten form. Unlike the alternate solvent or emulsion technologies, 100% solids coating doesn’t use solvents, water, or large conventional drying ovens. Adhesives that are 100% solids-coated feature large temperature ranges and resist high temperatures, solvents, chemicals, and plasticizers.
They also offer precise deposition control and clean removability without irritating the skin. Long-term adhesion, in many instances, lasts up to 14 days. Acrylic adhesives that are 100% solids-coated combine the best properties of emulsion- and solvent-coated adhesives – delivering equal or better performance, but avoid high cost of raw materials, excessive energy consumption, environmental concerns, outgassing, and material shortages.
The carrier plays an imperative role in determining end-product requirements, such as breathability, conformability, thickness, strength, skin compatibility, latex-free and hypoallergenic properties, moisture vapor transmission rates (MVTR), fluid barrier abilities, and removability. Three primary material types pair well with medical-grade PSAs for wearable medical devices: transparent films or film tapes, foam tapes, and non-woven tapes.
Depending on the construction, films can offer performance characteristics such as conformability, breathability, good oxygen and moisture vapor exchange, waterproof and fluid barrier properties, and impermeability to bacteria and viruses.
However, films generally aren’t as comfortable and don’t offer much breathability or stretchability.
Common film materials include polyurethane, polyethylene, and polyester. Films are generally more versatile than other materials and can be coated with varying adhesive coat-weights depending on the level of adhesion and MVTR necessary for the application. Films typically have paper casting sheets and liners to accommodate different converting methods.
Foam tapes, while not breathable, offer softness and conformability, delivering a more comfortable user experience than films. Designed to hold tubes and wires in place comfortably while providing enhanced stability, foam tapes are optimal for wearable medical devices that require sustained skin contact.
They can be die-cut easily into various shapes and sizes; are available in various thicknesses, colors, and types; and can be printed on, if needed.
Non-woven tapes feature a soft, comfortable texture and are breathable and lightweight with good oxygen and moisture vapor rate exchange and removability. They typically include a soft backing and are often constructed of materials such as spun-laced polyester or elastic fabric, enhancing performance and comfort. They are also water-resistant.
Non-woven tapes are easily die-cut into various shapes and sizes. They can be used for longer periods of time and are typically recommended for electro-medical and bandage-type wearable applications. Many lightweight non-woven rubber adhesives, such as non-woven polyesters, feature lower adhesion levels, which offer greater flexibility and range of motion.
Upon assessing the adhesive type, how the adhesive was manufactured, and what adhesive carrier best aligns with the chosen adhesive and application, wearable medical device manufacturers, designers, and converters will be well prepared to choose an adhesive for their wearables technology. However, for best results, always consult the adhesive manufacturer before making a final decision.