Fully 3D-Printed Analytical Device Based on a Novel Floating Electrode Mechanism for Sweat Rate Acquisition

We present herein a unique analytical methodology based on a one-step, three-dimensional printed sensing device for measuring sweat rate noninvasively, which allows for the assessment of hydration levels. It comprises a multilayered construction, and diverse designs are investigated, with the printing components consisting of conductive (CB-PLA) and nonconductive polylactic acid (PLA) printable materials. A prominent aspect is the integration of what is termed here as "floating electrodes", which advantageously modify the overall impedance of the device during sweat flow measurements. To rationalize the mechanism behind this, multiple sensor configurations, including two-, four-, six-, eight-, or multilayer structures, were thoroughly examined. Although multilayer configurations demonstrated the potential to enhance the overall capacity and sensing area, multiplying the number of electrodes in the device effectively resulted in a higher frequency of impedance variations while providing additional sweat rate data points from a single device. In off-body testing, the optimal system (based on four electrodes) exhibits a calibration range of 1-10 μL min^-1 with a total capacity of 37 μL and a correlation coefficient of r = 0.927 between the sensor and timer-based flow validation. With variations oscillating between 0 and 15% as well as a strong correlation (r = 0.79-0.93) with the cotton patch gravimetric method, on-body validation utilizing iontophoresis and cycling to generate sweat in the tested subjects demonstrated strong correlation with standard methods. Our findings illustrate the potential of the "floating electrode" concept for scaling up to larger and higher-capacity sweat sensing devices, as it functions effectively in both single- and double-layer designs. In addition, 3D printing technology will allow for on-demand customization of the corresponding analytical devices.