INVITED: Challenges of interfacing vacuum coatings with biological environments in a wearable sweat sensor platform.

Sweat is becoming widely recognized as an untapped resource for non-invasive monitoring of physiological states. Eccrine Systems, Inc. is developing truly non-invasive, wearable devices for continuous monitoring of human physiology through sweat biomarker sensing, overcoming many challenges presented by this promising medium. For example, sweat has wider ranges of salinity and pH compared to those of blood, saliva, and urine. Creating a wearable device capable of providing valuable feedback requires understanding, planning for, and compensating for material-biofluid interactions, as biofluids provide a complex and challenging environment for coatings. The variety of molecules and ions present in solution opens many chemical pathways for surface modification and degradation, which must be considered when designing coatings for biomedical applications. We will examine some pathways in commonly used biofluids and how they interact with representative coating materials. These challenges take on greater importance when dealing with multiple materials in a less often studied biofluid such as sweat. In addition to the technical challenges facing vacuum coatings in wearable applications are economic considerations as vacuum coaters must operate with continually tightening tolerances and lower margins at larger scale. We will examine these competing factors using a wearable non-invasive sweat sensor platform as a case study.