An epifluidic electronic patch with spiking sweat clearance for event-driven perspiration monitoring

Sensory neurons generate spike patterns upon receiving external stimuli and encode key information to the spike patterns, enabling energy-efficient external information processing. Herein, we report an epifluidic electronic patch with spiking sweat clearance using a sensor containing a vertical sweat-collecting channel for event-driven, energy-efficient, long-term wireless monitoring of epidermal perspiration dynamics. Our sweat sensor contains nanomesh electrodes on its inner wall of the channel and unique sweat-clearing structures. During perspiration, repeated filling and abrupt emptying of the vertical sweat-collecting channel generate electrical spike patterns with the sweat rate and ionic conductivity proportional to the spike frequency and amplitude over a wide dynamic range and long time (> 8 h). With such ‘spiking’ sweat clearance and corresponding electronic spike patterns, the epifluidic wireless patch successfully decodes epidermal perspiration dynamics in an event-driven manner at different skin locations during exercise, consuming less than 0.6% of the energy required for continuous data transmission. Our patch could integrate various on-skin sensors and emerging edge computing technologies for energy-efficient, intelligent digital healthcare.

temperature at 33 °C. Thermal images of the bottom surface of the sensor being operated for 8 h were obtained every hour using a thermal image camera (Fluke Ti95) with the emissivity set as 0.95, the emissivity of polyimide, the substrate material of the FPCB 1-3 .

Supplementary Discussion
1. Theoretical analysis of the sweat VIA sensor

Correlation between sweat rate and spike frequency
The design parameters of the sweat VIA sensor are shown in Figure 1a. For the sweat VIA sensor, we estimated the time required to fill the VIA channel by using several equations, one of them being ′′ = 1 3 ℎ( )(3 0 2 + 3 0 ℎ( ) where Q'' is the sweat rate per unit area of the skin at the inlet, AInlet is the inlet area of the VIA channel, and h(t) id the sweat height at time t. r o is the inlet radius of the channel and β is the wall angle of the channel. When the sweat fills the whole channel, if the design of the sweat VIA sensor is determined, this equation can be reformulated as where T is the spike period and ho is the height of the sweat when the sweat is in contact with the top of the channel, and ro, ho, β and AInlet are in this case each constant. As a result, the higher the sweat rate, the higher the spike frequency f (= 1/T), with these data points able to be fit by the line defined by the equation = ′′ / (red line in Fig. 3e).
However, at a high flow rate, the sweat clearance time would not be negligible, and the sweat collection time would become too short. Therefore, in this case, the correlation equation must be adjusted by considering the clearance time. Simply, with the assumption of a constant clearance time ( ), the spike period T would be represented as the sum of the sweat collection and clearance times,

Change of the electric conductance during the sweat collection and clearance processes
The relationship between the measured electric conductance, G, and the area of contact between the sweat and nanomesh electrode, S(t), may be described using the equation where i is current, V is voltage, c is the ion concentration of species i (e.g., + , − ), λ is the ion mobility of species i and E is the electric field generated by electrodes. During the course of a sweat collection, S(t) would increase, and as a result so would the electric conductance -whereas during the course of a sweat clearance process, they would both decrease.

Correlation between admittance and sweat conductivity
The admittance of the filled VIA channel can provide information about sweat conductance. For sweat coming into contact with the top of the VIA channel, i.e., with the sweat height h(t) increased to ho, S(t) and G would show their maximum values. For a channel fully filled with sweat, from Gauss's law, the conductance G would be for a closed surface S containing charge q, where q/v is the capacitance C, and ε is the permittivity of the electrolyte. Here the capacitance would be a constant due to the fixed geometry and material of the nanomesh electrode in the sweat VIA sensor. Therefore, the admittance (S) of the fully filled VIA channel would also be proportional to the ionic conductivity of the sweat in the VIA channel. Figure S1. Fabrication of the sweat VIA sensor components. a. Fabrication of the sweat VIA sensor with nanomesh electrodes. Agarose hydrogel precursor was molded using a 3D-printed plastic mold engraved with a truncated cone. AgNW and CNT solutions were sequentially spray-coated through a pre-patterned shadow mask onto the cured agarose gel mold. Then, a PDMS-based sweat VIA channel was molded from the agarose gel mold, after which patterned AgNWs and CNTs were transferred to the PDMS. Then, an Au layer was electroplated onto the transferred AgNW/CNT nanomesh electrodes on the PDMS surface. b. Fabrication of the super-hydrophilic CNT-PDMS sponge. A sugar cube was immersed in a PDMS precursor under vacuum conditions. The sugar cube absorbed the PDMS precursor, and the resulting composite was cured, and then dissolved in warm water to dissolve the sugar component and produce a PDMS sponge. Finally, the sponge was treated with O2 plasma and then dipped into a CNT solution to produce a CNT-PDMS sponge.    Comparison of evaporation volumes of the four cases and the cumulative sweat loss from the on-body measurement (Figure 5f). The highest evaporation rate was observed for the larger sponge at the lower relative humidity. In all cases, the cumulative evaporation volumes were greater than the cumulative sweat loss from the on-body test; thus, the sweat VIA sensor can be operated for a long period of time.    ) is ~ 0.14 mL, which is calculated from 0.31 μL/min (1 μL/min (flow rate) -0.69 μL/min (evaporation rate, Figure S5) × 27,514 s. This value is about 38% of the sponge's absorption capacity (0.37 mL, Figure S5).