Inspired by biofuel cell designs, the patch operates battery-free, with a clever microfluidic channel design that leverages sweat pressure to reach different reagents and electrochemical sensors in a mode where target analytes spontaneously generate electrical signals proportional to their concentration. Described in a paper titled “Battery-free, skin-interfaced microfluidic/electronic systems for simultaneous electrochemical, colorimetric, and volumetric analysis of sweat” published in Science Advances, the soft disposable microfluidic network was built from a silicone elastomer.
Using soft lithographic techniques, the authors created isolated chambers for colorimetric and electrochemical sensing, a ratcheted channel for quantifying sweat rate and total sweat loss and a collection of interconnecting microchannels with passive, capillary bursting valves (CBVs) for routing sweat through the device. For easy re-use, the electronics readout part, which consists of a thin NFC electronic module, magnetically mounts on top of the disposable microfluidic systems via thin neodymium magnets, after the soft microfluidic system has been applied to the wearer via a skin-compatible adhesive. This ensures the disposable part would be very efficient and cheap to manufacture, with no electronics on board.
The 1.5mm thick and 32mm diameter flexible patch was designed to monitor the concentration of chloride, lactate, and glucose, simultaneously with pH, sweat rate, and total sweat loss.
It provides visual readout that can be analysed via a smartphone app collecting digital images for the colorimetric quantification of chloride, pH, and sweat rate/loss. Data from the biofuel cell–based lactate and glucose sensors is read-out wirelessly via NFC, using the smartphone as a reader.
The authors argue that the collective physiological relevance of all these measured parameters allows for comprehensive health status tracking. They also note that the sweat glands themselves provide enough pressure for routing the sweat through the network of microfluidic channels and valves designed to interface with separately located sensors, naturally eliminating contamination and cross-talk.
What’s more, the careful design and dimensioning of capillary bursting valves and microfluidic channels and reservoirs enables time-sequential sweat sampling. This means that even though the colorimetric assays exhibit an irreversible response, they can be distributed and connected so as to capture time-dependent changes in sweat composition. The prototype discussed in the paper was able to collect and store sweat in time sequence in separate chambers, with the left and right sides of the device providing chrono-sampling analysis of pH and chloride, respectively. The lactate and glucose electrochemical sensors being reversible, they only required a single-chamber design with a single channel.