Downsized Sheath–Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors

Abstract

Hair-like-diameter superelastic conducting fibers, comprising a buckled carbon nanotube sheath on a rubber core, are fabricated, characterized, and deployed as weavable wires, biosensors, supercapacitors, and strain sensors. These downsized sheath–core fibers provide the demonstrated basis for glucose sensors, supercapacitors, and electrical interconnects whose performance is undegraded by giant strain, as well as ultrafast strain sensors that exploit strain-dependent capacitance changes. Downsized diameter superelastic conducting fibers are needed for electronic interconnects having a strain-independent conductance, sensitive strain sensors enabling giant stroke ranges, artificial muscles, and energy storage and chemical sensing fibers whose performance is largely independent of the applied strain.1 Such fibers, when used alone or inserted into elastomeric textiles, could potentially be used for such diverse applications as monitoring health during daily activities and characterizing the performance of morphing structures.2, 3 We here describe the fabrication of superelastic conducting sheath–core fibers that are as thin as 40 μm, characterize the properties of these downsized sheath–core fibers, 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 fibers, which comprise a carbon nanotube sheath on a rubber core, can be reversibly stretched by 800%, while undergoing a 72-fold increase in fiber 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 carbon-nanotube-sheath, rubber-core fibers provide amperometric biosensors for glucose, whose response is insensitive to a 45% stretch, as well as supercapacitor electrodes that can be stretched 300% without significantly changing capacitance. Variants of these sheath–core fibers 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 artificial muscles. While many exciting previous advances have provided highly stretchable structures for energy harvesting, energy storage, sensing, and external transmission of sensor responses,4-17 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 fluids, like sweat, and sensor systems for morphing mechanical structures, like aircraft wings and robots.18-22 For stretchable electronics applications, micrometer-scale elastic conducting fibers with high electrical conductivity, large reversible elastic strain, and high quality factor (QR = percent strain/percent resistance change) are desired,23-26 where the percent resistance change for a given strain range is defined 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 rubbers27-29 or attaching sheets of conducting nanofibers,6-9 graphene,30, 31 or other conductors to the surface of a rubber sheet.32 Although strains exceeding 700% have been achieved in reversible elastic fibers, their diameters are in the range of 400–1300 μm with quality factors below 3 when strains over 500% are applied.9, 10, 30, 33-36 Other approaches include filling a hollow rubber tube with liquid metal alloys, which can be stretched up to 1000% strains with large resistance change (QR ≈ 0.14).37 We recently demon