Interest is growing in the creation of wearable sweat sensors for continuous, lowcost, 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 Xray 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.

No abstract

This article presents the development of small sized piezoelectric micromachined ultrasound transducer (PMUT) with cost effective fabrication process. Improved ultrasound imaging quality requires small sized, high frequency and high performance PMUT cell and further high density arrays. Fabrication with currently well developed and cost effective sacrificial process benefits their mass production and application. To determine small sized AlN PMUT cell, commonly reported elastic layer materials including Si, Si 3 N 4 , SiO 2 are compared by mathematic derivation under the same stack thickness and device frequency. SiO 2 has the smallest Young modulus and it serving as elastic layer can yield the expected one. Arranging SiO 2 layer at the top of the device is compatible with phosphor silicate glass (PSG) sacrificial manufacturing process for cost effective fabrication. This fabrication strategy further downsizes cell size since extra device protection layer is omissible. We make 12MHz PMUT linear arrays with cell diameter, thickness and element pitch to be 40µm, 1.9µm and 120µm. Their electrical and ultrasound results are quite uniform at both device and wafer levels. One PMUT line with 20 parallelly connected units can produce pulseecho signal with over 25dB SNR at 10mm distance. Lateral and axial ultrasound imaging resolutions of ∼1 mm and ∼0.3 mm are obtained in practical verification. These indicate the good ultrasound performance of our developed PMUT array. Plus the advantages of small size and cost effective fabrication, using SiO 2 elastic layer and arranging it at the top of the device is a competitive strategy to develop phased array PMUTs. [2022-0184]

Developing a simple technique for in situ monitoring of sweating rate can provide real time feedback of individual's dehydration status and further assist in human health management. This work demonstrates the visible darkening of laser‐induced graphene (LIG) after wetting and its wearable practicability in monitoring sweating rate. LIGs with various porous morphologies are generated and their color differences are quantitatively assessed with hue‐saturation‐value model, correlated with the sheet resistance and the porous architecture. Embedding LIG pattern within microchannel enables the direct readout of water‐filled region and arranging length scale marks along microchannel allows direct readout of fluid volume with human eye for more simple measurements. Flexible design of length scale resolution can balance the detection frequency (i.e., real time ability) and its accuracy. The in situ and real time flow rate monitoring performance is successfully adopted on‐body for sweating rate analysis with reasonable accuracy. Apart from the simplicity and low cost, compared to other flow rate sensing methods, the proposed platform is reusable with no special cleaning needed for the microchannel but just drying, and no visible decline of the performance occurs. LIG's darkening strategy endows an avenue for the development of powerful in situ and real time sweating analytical platform.