Sweat-powered metabolic sensors wrap around the skin
The flexible and fully perspiration-powered integrated electronic skin (PPES), as the authors describe it in a paper titled “Biofuel-powered soft electronic skin with multiplexed and wireless sensing for human-machine interfaces”, harvests energy from human sweat through lactate biofuel cells (BFCs). The battery-free e-skin performs continuous monitoring of key metabolic biomarkers including glucose, urea, NH4+, and pH, before transmitting them wirelessly to a user interface via Bluetooth low energy. The device is built on an ultrasoft polymeric substrate which complies with the skin’s modulus of elasticity. Hence it can be laminated conformally on different body parts for accurate biosensing.
The PPES was designed in two main parts. First, the nanoengineered flexible electrochemical patch contains a biofuel cell array and a biosensor array for energy harvesting and molecular analysis in human sweat, all on serpentine-connected electrode arrays. The second piece is a flexible electronic patch that consolidates the rigid electronics on an ultrathin polyimide substrate through flexible interconnects for power management, signal processing, and wireless transmission.
Scale bars, 1 cm.
The researchers also integrated a skin-interfaced microfluidic module with independent inlet-outlet design into the PPES, to achieve efficient fresh sweat sampling for stable BFC operation and accurate sweat analysis. The electronic components and the interconnects of the PPES are then encapsulated with polydimethyl siloxane (PDMS) to avoid sweat/electronic contact.
As for the actual biofuel cell, it consists of lactate oxidase (LOx) immobilized bioanodes that catalyze the lactic acid to pyruvate and Pt alloy nanoparticle-decorated cathodes that reduce oxygen to water. These redox reactions on the BFC electrodes were proven to yield a stable current, up to 3.5 mW/cm2, to power the metabolite sensors for up to 60 hours of continuous operation. The researchers demonstrated the PPES could selectively monitor key metabolic analytes as well as the skin temperature during prolonged physical activities.
Pushing their experiments further, the authors integrated soft strain sensors to their perspiration-powered e-skin to monitor muscle contraction. The strain signals were then sent to a human-prosthesis, in effect, turning the PPES as a human-machine interface. Even under mechanical deformation, with a bending curvature of 1.5cm in radius, the PPES maintained consistent sensor readings, the researchers reported.
bioanodes and Pt alloy nanoparticle–modified cathodes.
Applying CNTs/PDMS elastomer-based strain sensors on the hand and the elbow, connected to the PPES, the e-skin was able to accurately monitor the bending of the finger and elbow (from resistance changes of the strain sensors). A robotic arm wirelessly fed with these signals could mimick the gestures of the wearer’s arm, approaching and grabbing a target object. Another practical use case envisaged by the authors is robotic assistance in the rehabilitation settings.
By extrapolation, they anticipate that the incorporation of more physical sensors for electroencephalogram and electromyography recording along with the continuous metabolic monitoring could make multimodal PPES useful for the design and optimization of novel prostheses that bring the human into the loop of prosthesis control, for real-time user-specific responses to human intent and behavior.
California Institute of Technology – www.caltech.edu
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