The passive sensor consists of 80μm thin ‘S’-shaped microfluidics connecting a central circular pocket, 5mm in diameter, with two side circular regions (2.5mm in diameter), all filled with the eutectic fluid metal GaIn (eGaIn). Molded out of soft silicone rubber bonded to a PET film with two strips of screen-printed silver electrodes running through the side circular regions, the sensor is both rugged and flexible. The use of an electrically conductive liquid metal means the sensor won't suffer from cracking or material fatigue.
When one applies a load on the central region, the rubber deforms and the liquid metal flows away into the side pockets, gradually changing the overall resistance of the assembly as the load increases. Readout circuitry is simple, resistance changes can be measured using a standard Analog to Digital Converter (ADC), without requiring any signal amplifications.
In their report "Triple-State Liquid-Based Microfluidic Tactile Sensor with High Flexibility, Durability, and Sensitivity" published in ACS Sensors, the researchers justify the S-shape with better fluid dynamics within the device and also observe a higher and sharper deformation profile as compared to the straight microfluidic structure, indicating a higher localized load sensitivity of the ‘S’-shaped microfluidic structure.
The scientists then submitted their sensor to various mechanical loads, from a light finger touch to foot stomping with and without shoes, or with high heel shoes.
They even drove a car over it. Making the most of the device's flexibility, they also carried out a battery of bending tests, showing that one could easily distinguish from -90°, -45°, +45°, and +90° bends (the fold being localised to the central region).