Uncovering the Sweat Biofouling Components and Distributions in Electrochemical Sensors.

Interest is growing in the creation of wearable sweat sensors for continuous, low-cost, and noninvasive health diagnosis at the molecular level. The biofouling phenomenon leads to degradation of sweat sensors' performance over time, further limiting the successive monitoring of human health status. However, to date, the mechanism of sweat fouling is still unclear, with the inability to provide effective guidance on antifouling strategies. This study clarifies chemical compositions in sweat fouling and fouling distributions on the surface of sensors. Gold film electrodes were prepared on glass and poly(ethylene terephthalate) (PET) substrates and contaminated by human facial sweat (from eccrine sweat glands and apocrine sweat glands) and palm sweat (only from eccrine sweat glands). A scanning electron microscope (SEM), an optical microscope (OM), and an atomic force microscope (AFM) were employed to study the surface morphology of biofouling electrodes. The existence of sweat fouling was characterized by AFM adhesion force, a Fourier transform infrared spectrometer (FTIR), and X-ray photoelectron spectra (XPS). FTIR along with XPS was adopted to analyze the biofouling components, and differential reflectance spectroscopy (DRS) was undertaken to observe the distribution of biofouling on the surface of the electrodes. As a result, we found that neither skin cell pieces nor recognized protein adsorption is the dominant source of biofouling, but the lipids in sweat form an inhomogeneous fouling layer on the electrode surface to reduce the electrochemical reactivity of sensors. This study provides deeper insights into sweat biofouling components and distributions and points out the right direction for resolving the problem of limited continuity in wearable sweat sensors.