DOI: 10.1002/adem.201500018 Flexible, Stretchable and Weavable Piezoelectric Fiber By Hyeon Jun Sim and Changsoon ChoiChang Jun Lee, Youn Tae Kim,* Geoffrey M. Spinks, Marcio D. Lima, Ray H. Baughman and Seon Jeong Kim* 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.Piezoelectricmodulesareattractiveforconverting mechanical energy to electrical energy and various flexible systemshavebeendevelopedtocaptureenergyfrombending 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. Thefiber-basedpiezoelectricsystemsdescribedtodatehave 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.Thesefiberscouldbewovenintoatextilestructure [7] anddeformedbybendingtoasmallstrainof %0.1% [8] However, smallradiusbendingasoccursinknittingandhighstrainelastic stretchingwerenotyetdemonstrated.Kim etal. [9] haverecently 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- tricfibers(FPFs)canbestretchedtoatensilestrainof5%andcan generate over 50 mW/cm 3 . The FPFs are sufficiently robust for knotting, sewing, and weaving and can also be converted to piezoelectriccoilsbytwistinsertion.Thesespring-likecoilscan be reversibly stretched to 50% strain without failure. FPF energy generators were fabricated in a four stage process, as illustrated in Figure 1a, b. Firstly, electrospun mats were prepared from polyvinylidene fluoride-co- trifluoroethylene (PVDF–TrFE) as randomly oriented nano- fibers of 750nm average diameter (Figure 1c) and to a mat thicknessof %30 mm.Theelectrospunmatswerenextmanually wrapped around a multifilament silver coated nylon yarn that actedastheinnerelectrode.Theouterelectrodewasformedby wrapping with carbon nanotube (CNT) sheets drawn from a forest of chemical vapordeposited multiwalled CNTs [10] The highly conductive CNTs adhered strongly to the PVDF–TrFE and their helical orientation (Figure 1d) accommodated large fiber axis strains without failure, as has been demonstrated previously for stretchable supercapacitor [11] and photovoltaic fibers [12] The piezoelectric fiber generators were completed by dip coating in a mechanically protective and electrically insulating layer of elastomeric styrene-ethylene-butylene- styrene(SEBS). The final fabricated FPF shows a uniform diameter of 320 mm (Figure 1e) and can be produced in long lengths above 1m (Figure S1). The cross section of the FPF shows that it mostly consists of silver coated nylon multifila- ment inner core (diameter of 180 mm) with the electrospun PVDF–TrFE mats having an average thickness of 30 mm and CNT outer electrode of 400nm and protective SEBS layer of 40 mm (Figure S2). Electrospinningwaschosenasthemeansforproducingthe piezoelectric PVDF–TrFE mats since fiber formation and piezoelectric poling occur simultaneously, unlike in processes such as fiber melt extrusion where a separate poling stage is needed [7,13] Thehighelectricfieldstrengthsinelectrospinning are known to induce the phase transition from non polarized a-phase to polarized b-phase in PVDF–TrFE [14,15] (Figure S3). *[*] Prof. S. J. Kim, H. J. Sim, C. ChoiC. J. Lee Center for Bio-Artificial Muscle and Department of Biomedical Engineering, Hanyang University, Seoul 133-791, Korea E-mail: sjk@hanyang.ac.kr C. ChoiC. J. Lee, Prof. Y. Kim IT Fusion Technology Research Center and Department of IT Fusion Technology, Chosun University, Gwangju 501-759, Korea E-mail: petruskim@chosun.ac.kr Prof. G. M. Spinks Intelligent Polymer Research Institute, ARC Center of Excellence for Electromaterials Science, University of Wollon- gong, Wollongong, NSW 2522, Australia Dr. M. D. Lima, Prof. R. H. Baughman The Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083, USA DOI: 10.1002/adem.201500018 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1 ADVANCED ENGINEERING MATERIALS 2015, COMMUNICATION