Fig. 1.
Soft, skin-interfaced platforms for automatic, wireless sensing of thermal transport properties of the skin. (A) Picture of a thin, flexible thermal actuator/sensor (TAS) module integrated with electronics to provide Bluetooth Low Energy (BLE) communication capabilities, resting on the tip of an index finger. The Inset features an image of the device bent between the thumb and index finger. (B) Circuit and block diagrams of the design. The TAS module consists of a thermal actuator (Joule heater; RH × 2), and Wheatstone bridge circuits that include two thermistors (NTC+, NTC−) with a known resistor (R) on each bridge. A digital on/off switch on the user interface activates a general-purpose input/output (GPIO) pin to source a predetermined periodic current (6.8 mA for 10 s, and 0 mA for 50 s in a 1-min cycle) into the resistive heater. A differential amplifier (AMP) in a BLE system-on-a-chip (SoC) amplifies the difference of the bridge voltages (V+, V−). The subsequent analog-to-digital converter (ADC) samples the AMP output voltages for transmission to a smartphone via BLE radio communication. (C) Exploded-view illustration of the constituent layers and components: silicone encapsulation layers, battery, and a flexible copper-clad polyimide (Cu/PI/Cu) sheet with circuit traces that interconnect the thermal actuator (skin side), NTCs (air side), and the BLE SoC. The Inset highlights an air pocket structure defined by the top silicone encapsulation layer as thermal insulation around the TAS module. (D) Picture of an encapsulated device adhered to the thenar eminence.