한양대학교 Nano Bio Tech Lab.

Various ‘wearable technologies’ for personal activity monitoring have recently been released to the consumer market with great success. The desire of developers of such devices to increase functionality while reducing size and weight is currently constrained by the energy demands of the on-board electronics. The ideal solution would be to harness electrical energy from the wearer, such as through heat or movement. Piezoelectric modules are attractive for converting mechanical energy to electrical energy and various flexible systems have been developed to capture energy from bending motions associated with heartbeat,[1,2] respiration,[3] muscle stretching,[4,5] and eye blinking.[6] Ideally, energy harvesting garments should be elastically stretchable as well as bendable to ensure a close fit, enhance wearer comfort, and increase the range of human motions accessible for energy recovery. Stretch fabrics gain their high strain elasticity from a combination of knitted fiber bending and stretching of highly elastic fibers, such as elastane, which provides a restoring force. What are needed are robust piezoelectric fibers that demonstrate both high flexibility and high stretchability for incorporation into smart garments.
The fiber-based piezoelectric systems described to date have demonstrated only limited flexibility and very small extensi- bility. Kechiche et al.[7] and Lee et al.[8] have both developed coaxial fibers consisting of a layer of piezoelectric materials sandwiched between a conducting core and an outer sheath electrode. These fibers could be woven into a textile structure[7] and deformed by bending to a small strain of 0.1%[8] However, small radius bending as occurs in knitting and high strain elastic stretching were not yet demonstrated. Kim et al.[9] have recently described highly stretchable piezoelectric films formed by laminating polyvinylidene fluoride (PVDF) between a con- ducting, corrugated elastomeric substrate and a thin graphene layer. The laminate could be stretched to 30% without damage but fibers have not yet been prepared.
Here we introduce a novel composite material system and a method for constructing flexible, stretchable and weavable piezoelectric energy-generating fibers. The flexible piezoelec- tric fibers (FPFs) can be stretched to a tensile strain of 5% and can generate over 50 mW/cm3. The FPFs are sufficiently robust for knotting, sewing, and weaving and can also be converted to piezoelectric coils by twist insertion. These spring-like coils can be reversibly stretched to 50% strain without failure.