We here describe the fabrication of superelastic conducting sheath-core fi bers that are as thin as 40 µm, characterize the properties of these downsized sheath-core fi bers, and show that they could be deployed as capacitors and sensors whose realized performance is either usefully highly sensitive or usefully insensitive to applied strains that can exceed 300%. Even when smaller in diameter than a human hair, these sheath-core fi bers, which comprise a carbon nanotube sheath on a rubber core, can be reversibly stretched by 800%, while undergoing a 72-fold increase in fi ber conductivity and a desirably low resistance change of 12%. The present downsizing has reduced the cross-sectional area of such strain sensors by a factor of 400, and correspondingly decreased the applied forces needed for their practical deployment by about the same amount. We show that Pt-containing carbonnanotube-sheath, rubber-core fi bers provide amperometric biosensors for glucose, whose response is insensitive to a 45% stretch, as well as supercapacitor electrodes that can be stretched 300% without signifi cantly changing capacitance. Variants of these sheath-core fi bers were woven as wires to transport current in highly uniaxially or biaxially stretched textiles, and deployed as high sensitivity, capacitance-based sensors for measuring the contraction of giant stroke artifi cial muscles.While many exciting previous advances have provided highly stretchable structures for energy harvesting, energy storage, sensing, and external transmission of sensor responses, [4][5][6][7][8][9][10][11][12][13][14][15][16][17] most of these strategies work by embedding relatively rigid microdevices having these functionalities into an elastomeric structure. [ 4,5 ] Important targeted applications are for such needs as wearable devices that monitor human body fl uids, like sweat, and sensor systems for morphing mechanical structures, like aircraft wings and robots. [18][19][20][21][22] For stretchable electronics applications, micrometer-scale elastic conducting fi bers with high electrical conductivity, large reversible elastic strain, and high quality factor ( Q R = percent strain/percent resistance change) are desired, [23][24][25][26] where the percent resistance change for a given strain range is defi ned using the difference between maximum and minimum resistances divided by the resistance at lowest strain. Conducting elastomers have been fabricated by methods such as incorporating conducting particles in rubbers [27][28][29] or attaching sheets of conducting nanofi bers, [6][7][8][9] graphene, [ 30,31 ] or other conductors to the surface of a rubber sheet. [ 32 ] Although strains exceeding 700% have been achieved in reversible elastic fi bers, their diameters are in the range of Downsized diameter superelastic conducting fi bers are needed for electronic interconnects having a strain-independent conductance, sensitive strain sensors enabling giant stroke ranges, artifi cial muscles, and energy storage and chemical sensing fi bers whose perform...
