Trends in wearable technology follow those of the broader biomedical and electronics industries — devices are getting smaller, smarter, and easier to use. Specifically, wearables in healthcare have moved toward solutions that reduce the device profile, provide more integration with smartphone apps, and most importantly enable patients to receive their treatments at home, outside of a doctor’s visit. These wearable devices range from on-body drug-delivery systems for cancer treatment to electrical nerve stimulation patches or simply sensors to monitor vitals. All treatments increase patient autonomy and are rapidly increasing in popularity.

Instron’s 6800 Series universal testing system is used to perform a 90° peel test to ASTM D3330. This test establishes baseline properties for adhesives and can also be used to evaluate adhesives after they have undergone conditioning to simulate real-world stresses. (Credit: Instron)

As this trend continues, manufacturers are working to develop robust testing methods to mechanically evaluate all aspects of these devices and ensure that they are performing as expected. In addition to testing the injection device components, manufacturers face challenges in evaluating and selecting the adhesives for their products.

Challenges in Adhesive Selection

Innovation in the field of medical adhesives has been ongoing since Band-Aids® were invented in 1921. Medical adhesives have long been used in ECG sensors, medical tapes, and bandages. However, the rise of wearables and more specifically on-body drug-delivery systems has intensified the need for standardization and testing of adhesives that must stay on the body for extended periods of time.

Scientists have long understood that designing adhesives for use on skin is uniquely challenging. Skin regenerates approximately every 27 days, and its sensitivity to different adhesives is dependent on biocompatibility factors such as age, allergens, hair, moisture/sweat levels, and how the adhesive itself performs over time and under different environmental conditions. Adhesives used on skin must be able to withstand products like soap or lotion and not degrade or react to contact with these substances. Additionally, a wearable device designed for extended use needs to withstand the motions and forces of day-to-day life while ensuring patient comfort.

Most medical pressure sensitive adhesives (PSAs) are made of either rubber, acrylic, or silicone-based adhesives. Each type is best suited to meet different challenges, including length of adhesion time, possible allergic reactions, and patient comfort upon application/ removal.

Adhesives Testing for Wearables

ISO 11608 is the internationally recognized standard for testing needle-based injection systems (NIS) for medical use. In 2021, a new section of ISO 11608-6 focused on on-body delivery systems was released as a draft. While this new section establishes standard practices for testing many aspects of these novel drug-delivery devices, it is still vague in how it recommends testing the adhesives used for these devices.

ISO 11608 points toward established standards like ASTM D3330 for characterizing the fundamental adhesive properties of the material. However, it does not provide recommendations for how to test the more functional aspects such as the torsional, shear, and peel forces encountered by these devices in practice.

Instron’s 6800 Series universal testing system with a Torsion Add-On is used to apply both axial and torsional loads to adhesives and other materials. It is used to simulate loads that a wearable device may experience while being worn by a patient. (Credit: Instron)

ASTM D3330 Methods A and F measure the peel strength required to remove pressure-sensitive adhesives from a standard steel panel or from another surface of interest at 90° or 180°, respectively. The adhesive is carefully applied to the test panel using a mechanical roller of specified requirements. The specimen is then peeled away from the substrate at a set rate, and the average adhesion force is recorded over the course of the test. The peel adhesion value is reported in N/10 mm. This test mimics removing an adhesive patch from a patient’s skin and can also serve as a baseline test to compare back to after a specimen is conditioned.

Tack tests are another functional test used to demonstrate how quickly an adhesive bond is formed. A pressure-sensitive adhesive is put in contact with a known substrate at a given load for a set amount of time in order to determine the load required to separate the two. One way to achieve this is by using an automatic XY translation stage. The adhesive patch is secured to the stage, and an upper probe with a flat surface is used as the substrate surface. The stage automatically moves between test points and runs the same test procedure on each, providing the adhesive strength across the full contact area of the patch.

Additionally, preconditioning materials under temperature, moisture, or other conditions can also provide useful information about how an adhesive’s properties might change. Temperature or environmental chambers set to mimic body temperature, or slightly elevated or lower temperatures, provide other useful test circumstances under which to evaluate these materials.

Conclusion

As wearable devices become more prevalent, innovation and standardization of their mechanical testing continues to develop. Manufacturers of testing equipment must work with global biomedical companies to ensure that these developing testing needs can be met.

This article was written by Meredith Bernstein, Applications Engineer at Instron, Norwood, MA. For more information, visit here .



Magazine cover
Medical Design Briefs Magazine

This article first appeared in the July, 2022 issue of Medical Design Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.