1605630(1 of 8) temperature, and low viscosity. The latter property allows LM to flow in response to deformation, whereas solid metals are stiff and prone to fail at small strains. Embedding LM in elastomer decouples the electrical and mechanical properties; that is, these composites have the electrical properties of the metal and the mechanical properties of the elastomer. Incorporating the LM into the hollow core of an elastomeric fiber results in a useful form for sensors because fibers may be integrated into clothing and fabrics. [12][13][14][15] Furthermore, fibers are inherently flexible, compliant, and conformal due to their narrow cross-section. Thus, fibers can readily wrap onto and conform to surfaces with Gaussian curvature whereas 2D sheets cannot without significant deformation. Fibers can also be mass produced at high speeds with small diameters (hundreds of microns) and produced by hand in a laboratory environment at room temperature. [16] The fibers described in this work have the additional advantage of being built from stretchable and soft materials. Fibers with LM cores have previously been used to make light-emitting structures [17] and stretchable wires that retain metallic conductance up to ≈800% strain. [18] We reasoned that elastomeric fibers filled with LM could also be used for capacitive sensing of torsion, strain, and touch.Here, we intertwined two fibers into a double-helix to create sensors of both torsion and strain since twisting or stretching the fibers increases the contact area between them, and therefore changes the capacitance. The complexity of torsion, which causes both normal and shear strain, has previously precluded the development of a simple sensor capable of measuring a large range of torsion. Existing torsion sensors measure changes in normalized resistance, [2,8,19,20] pressure, [9] and optical properties, [21] or utilize surface acoustic waves [22] or the inverse magnetostrictive effect. [23] Some of these sensors can detect changes as small as 0.3 rad m −1 and can measure torsion up to 800 rad m −1 before failure. Most existing torsion sensors, however, are rigid, cumbersome, expensive, and complex. The soft and stretchable sensor developed here offers a simple mechanism to measure large changes in torsion which may be useful for unconventional robotics [24,25] or artificial muscles. [26] In addition to sensing torsion, intertwined fibers increase capacitance in response to strain due to the increase in contact Soft and stretchable sensors have the potential to be incorporated into soft robotics and conformal electronics. Liquid metals represent a promising class of materials for creating these sensors because they can undergo large deformations while retaining electrical continuity. Incorporating liquid metal into hollow elastomeric capillaries results in fibers that can integrate with textiles, comply with complex surfaces, and be mass produced at high speeds. Liquid metal is injected into the core of hollow and extremely stretchable elastomeric fibers and the re...
