Wireless and noninvasive epidermal electronics

A device to measure a subdermal fluid flow parameter comprising:

• a substrate;
• at least one upstream temperature sensor supported by the substrate;
• at least one downstream temperature sensor supported by the substrate;
• and
• a thermal actuator supported by the substrate and positioned between said upstream temperature sensor and said downstream temperature sensor;
• wherein the thermal actuator comprises an array of resistors having a cumulative resistivity sufficient to provide for thermal actuation such that the upstream temperature sensor measures an upstream temperature and said downstream temperature sensor measures a downstream temperature to allow determination of said subdermal fluid flow parameter;
• and wherein the array of resistors of the thermal actuator provides a fill factor selected from the range of 10% to 60% of an area between said upstream temperature sensor and said downstream temperature sensor.