Soft sensors have been playing a crucial role in detecting different types of physical stimuli to part or the entire body of a robot, analogous to mechanoreceptors or proprioceptors in biology. Most of the currently available soft sensors with compact form factors can detect only a single deformation mode at a time due to the limitation in combining multiple sensing mechanisms in a limited space. However, realizing multiple modalities in a soft sensor without increasing its original form factor is beneficial, because even a single input stimulus to a robot may induce a combination of multiple modes of deformation. Here, we report a multifunctional soft sensor capable of decoupling combined deformation modes of stretching, bending, and compression, as well as detecting individual deformation modes, in a compact form factor. The key enabling design feature of the proposed sensor is a combination of heterogeneous sensing mechanisms: optical, microfluidic, and piezoresistive sensing. We characterize the performance on both detection and decoupling of deformation modes, by implementing both a simple algorithm of threshold evaluation and a machine learning technique based on an artificial neural network. The proposed soft sensor is able to estimate eight different deformation modes with accuracies higher than 95%. We lastly demonstrate the potential of the proposed sensor as a method of human-robot interfaces with several application examples highlighting its multifunctionality.

been applied more closely to the human body. Owing to the soft and curved nature of human skin, the systems need to be flexible, robust, and transparent to ensure operational reliabilities and to provide comforts. [3,4] However, it has been challenging to ensure these desirable properties due to the complicated structures of conventional touch-sensing systems, which contain a network of individual electrodes and stacked multilayers. [4,5,8,10,12] More importantly, sensing systems inevitably rely on external power sources, which potentially sacrifice flexibility, add weight, and decrease the lifetime of the sensing systems. [4,5,8,[12][13][14] To address the issues originated from the use of external power sources, systems that rely on energy-harvesting technologies have been highlighted as an alternative to conventional touch-sensing systems. [3,[15][16][17] Recently, triboelectric nanogenerators (TENGs), which convert mechanical touch to electrical energy, have been developed to provide a platform for touch-sensing capability that does not rely on additional power sources. [16][17][18][19][20] The self-powered touch-sensing capability of TENGs can be realized with a couple of materials, i.e., a dielectric layer attached to a conducting layer, based on the combined effects of contact electrification and electrostatic induction. [3,17,18,21] To ensure that triboelectric touch sensors are skin-mountable, researchers have worked to make the components stretchable and transparent. Soft materials such as poly(dimethylsiloxane) (PDMS) and ionically conductive gels are suitable for use in skin-mountable triboelectric touch sensors thanks to their high transparency, stretchability, resilience, and easily tunable mechanical properties. [17,[22][23][24][25] Nonetheless, the development of triboelectric touch position sensors with reliable stretchability and transparency remains challenging due to the complicated structures of the sensors, which contain multiple stacked layers and arrays of individual electrodes. [16,17,19,21,26,27] Unfortunately, the stacking process deteriorates the outstanding mechanical/optical properties of the soft materials in TENGs since the sensors require an additional conducting layer of a material such as metal, ceramic, carbon materials, etc. [16,[28][29][30] The inclusion of transparent and stretchable materials like gels also results in easy delamination and blurring at the bonding interfaces. [17,19,31] Relying on a complex array of individual electrodes and corresponding wires Recent growing pursuit of skin-mountable devices has been impeded by the complicated structures of most sensing systems, containing electrode grids, stacked multilayers, and even external power sources. Here, a type of touch sensing, termed "triboresistive touch sensing", is introduced for gridless touch recognition based on monolayered ionic power generators. A homogeneous monolayer, i.e., ionic poly(dimethylsiloxane) (PDMS), generates electricity based on the electric field generated by touch. Voltages generated at ...

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