Skin attachable flexible sensor array for respiratory monitoring

Flexible electronic systems became increasingly popular in recent years. They usually can be bent or even stretched while fully maintaining their functionality opening up a wide field of various new applications. In this paper a novel 6 × 6 sensor array for curvature sensing in the format of a thin flexible polyimide foil is introduced. The sensor foil is to be used for respiratory monitoring of premature infants by directly attaching it to the skin for measuring the body deformations caused by breathing. Sensor signals shall in future be used not only to trigger the respiration device but also to provide time dependent body surface reconstruction as a diagnostic tool. One single sensor element consists of four gold strain gauges in a Wheatstone bridge configuration. For suppressing sensor response to foil stretching and for increasing bending sensitivity we introduced a double-sided sensor design with strain gauges on both surfaces of the thin foil leading to a 170% higher sensitivity than a one-sided sensor design. The complete sensor array foil of less than 20 μm in thickness can be fabricated without flipping the substrate. Single sensor elements can reliably measure in a bending radius range of 1 mm ≤ r ≤ 380 mm and provide a signal of Vout = κ*S with the curvature κ = 1/r and a sensitivity of S = 49.3 mV*mm. The response time of 25 ms is currently defined by the signal processing circuitry. Double sided sensor elements with different orientations are arranged in an alternating pattern across the array allowing to fully and unambiguously determine the bending vector utilizing plausibility considerations on basis of signals from neighboring elements. A small initial bending resulting from fabrication induced stresses was observed but could easily be compensated in digital sensor signal analysis. In summary, first tests of this novel sensor array together with the scalability of implemented fabrication processes are very promising. The fundamental criteria to be used as sensor array for respiratory monitoring of infants are met and typical problems associated with existing systems for respiration triggering can be avoided with skin attachable flexible systems.