By Joanne Moody, Zeta Scientific LLC, Livermore, CA
Wearable devices are skyrocketing based on sensor developments and remote monitoring. Device attachment can often be overlooked as a critical parameter in wearable devices. While many wearables use straps and clips, secure sensor attachment to the skin with adhesives improves measurement, comfort, and appearance. Based on the increasing trend for adhesives in wearables, it is important to consider adhesive selection early in the design. Below are key factors for wearable skin adhesives.
Wearable Device Factors
What are the wearable device requirements?
1. Application: Define the end application for the sensor, body location, battery, and human factors
2. Device Configuration: Develop the device’s size, shape, weight, and materials.
3. Regulatory: Determine medical device classification, biocompatibility, and sterilization.
4. Manufacturability: Address adhesive construction and processing in the design phase.
Skin Factors
What are the skin’s characteristics and the environment?
5. Body Location: Skin strength and elasticity varies based on body location. The surface could be smooth or contoured, with various types of exposure to ambient conditions.
6. Wear time: Identify the wear time. A long-term wearable adhesive is currently 14 days.
7. Activity and Environment: Predict exposure conditions such as sweat, temperature, friction, movement, and device repositioning. Activity level is another critical factor.
8. Age and Health: Infants have very delicate skin. Older adults have fragile skin and health complications with chronic disease affecting skin health. Special adhesives handle tender skin but typically with shorter wear time. Individuals with chronic diseases such as diabetes may be unable to use wearable with skin-contact adhesives.
Adhesive Factors
How will the adhesive construction be developed?
9. Skin-Contact Adhesives: Skin adhesives can attach wearable devices directly to the skin with pressure sensitive adhesives (PSAs). PSAs require just light pressure to stick. Several chemistries are available to match requirements.
10. Adhesive Properties: Skin adhesives must securely and comfortably bond the device to the skin. Also, the adhesive must handle movement, environment, and allow the skin to breathe.
11. Application Techniques: The device and adhesive design considers skin preparation (such as cleaning and hair removal) and attachment of the device to the skin. The device removal method should minimize skin injury and avoid residue left behind.
12. Repeated Applications: The device may need to be repositioned or repeatedly used in the same location. These considerations will impact adhesive selection.
13. Adhesive Selection: Adhesive selection must meet the device and user requirements. Adhesive construction may involve layers to attach to the device and skin.
14. Biocompatibility and Sterilization: Start with a PSA that already has passed biocompatibility testing, but plan to repeat tests with the device.
Summary
Wearables with skin adhesive are proliferating based on benefits of measurement, comfort, and appearance. Using the Adhesive Checklist for Wearable Devices will help move your project along.
About the Author
Joanne Moody is Principal Consultant at Zeta Scientific LLC. She specializes in solving complex, real world challenges involving adhesives, coatings, plastics, and materials science. Clients are startups to international corporations. Industry experience include medical devices, consumer products, electronics, transportation, food, chemical, and water filtration.
