Tailoring force sensitivity and selectivity by microstructure engineering of multidirectional electronic skins

Electronic skins (e-skins) with high sensitivity to multidirectional mechanical stimuli are crucial for healthcare monitoring devices, robotics, and wearable sensors. In this study, we present piezoresistive e-skins with tunable force sensitivity and selectivity to multidirectional forces through the engineered microstructure geometries (i.e., dome, pyramid, and pillar). Depending on the microstructure geometry, distinct variations in contact area and localized stress distribution are observed under different mechanical forces (i.e., normal, shear, stretching, and bending), which critically affect the force sensitivity, selectivity, response/relaxation time, and mechanical stability of e-skins. Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. In particular, microdome structures exhibit extremely high pressure sensitivities over broad pressure ranges (47,062 kPa−1 in the range of <1 kPa, 90,657 kPa−1 in the range of 1–10 kPa, and 30,214 kPa−1 in the range of 10–26 kPa). On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept applications in healthcare monitoring devices, we show that our e-skins can precisely monitor acoustic waves, breathing, and human artery/carotid pulse pressures. Unveiling the relationship between the microstructure geometry of e-skins and their sensing capability would provide a platform for future development of high-performance microstructured e-skins. Customizing piezoresistive sensors with different microscale geometries makes it easier for ‘electronic skin’ devices to sense forces in different directions. Hyunhyub Ko and colleagues from South Korea’s Ulsan National Institute of Science and Technology report that carbon nanotube/silicone elastomer composites fabricated into three shapes—micro-domes, pyramids, and pillars—have unique responses to mechanical stress and deformations. Experiments with interlocked pairs of micropatterned films revealed hemispherical shapes were best at sensing tensile and bending stresses, as well as minute changes to pressure. Micropillars, on the other hand, exhibited strong sensitivity to shear stress. With help from computer simulations, the researchers identified changes in contact area and localized stress as the critical factors needed to guide design of multidirectional force sensitivity. Prototype e-skin devices containing the interlocked microshapes successfully monitored bio-signals including breath patterns, spoken words, and arterial blood pressure. We present piezoresistive electronic skins with tunable force sensitivity and selectivity in response to multidirectional forces (normal, shear, tensile, bending) by engineering microstructure geometries (dome, pyramid, pillar). Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept demonstrations, the e-skins are used for wearable healthcare devices to precisely monitor various bio-signals including sound, human breath, and artery/carotid pulse pressures.