Jesse Grant1 Kejia Yang1 Jonathan Reeder2 Daphne Carrier1 Walter Voit1

1, The University of Texas at Dallas, Richardson, Texas, United States
2, Northwestern University, Evanston, Illinois, United States

E-skins are thin, stretchable bioelectronics that imbue surfaces with sensory capabilities. They are expected to revolutionize medical diagnostic and monitoring capabilities as wearable electronics, with highly intuitive biosensors that are adapted especially for the human anatomy. The primary function of skin is mechanical force sensing, which is likewise the most developed modality in e-skins. Further refinement of e-skins, however, means that the other modalities, notably temperature sensing, have become active areas of research. Temperature had previously been a confounding factor for pressure sensing by affecting the strain behavior of existing tactile sensors, which has been mitigated with complex circuitry and clever designs. The work to discriminate tactile and temperature stimuli, however, has since grown into the dual modality of e-skins to sense both temperature and touch.
In this presentation, we will discuss our approach to overcoming the disadvantages of acrylate polymers by implementing thiolene click chemistry, which is insensitive to oxygen, yielding more easily controllable thermal properties. In addition, whereas the making of an acrylate-based PPTC device consists of three discrete steps—the reaction, blending of filler, and screen printing—thiolene-based PPTC devices enable all three to be done simultaneously by selective photopatterning via radical-mediated step growth polymerization. Therefore, they are well suited to for use as thermal sensors in e-skin, under three conditions: if their melting temperature is centered on the skin-surface temperature, if they can be made stretchable, and if they can stick to a flexible substrate (Yokota, et al.).
Differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and out-of-plane X-ray diffraction (XRD) are carried out to characterize the melting points, thermal volume expansion, and crystal structure of the polymer composites, respectively. Temperature sensing performance of the polymer composites is demonstrated by their resistivity versus temperature curves. The flexible fabricated devices possess a sensitivity of an order of magnitude change of resistance per degree Celsius at the critical temperature. Therefore, the polymer composites are good candidates for the fabrication of electronic skins, which deliver diagnostic and monitoring capabilities, or alternatively imbue artificial surfaces, whether on prostheses or robots, with sensing capabilities.
Yokota T, et al. (2015) Ultraflexible, large-area, physiological temperature sensors for multipoint measurements. PNAS 112(47): 14533-14538