Hyeon Jun Sim

Stretchable, weavable coiled carbon nanotube/MnO2/polymer fiber solid-state supercapacitors

Fiber and yarn supercapacitors that are elastomerically deformable without performance loss are sought for such applications as power sources for wearable electronics, micro-devices, and implantable medical devices. Previously reported yarn and fiber supercapacitors are expensive to fabricate, difficult to upscale, or non-stretchable, which limits possible use. The elastomeric electrodes of the present solid-state supercapacitors are made by using giant inserted twist to coil a nylon sewing thread that is helically wrapped with a carbon nanotube sheet, and then electrochemically depositing pseudocapacitive MnO2 nanofibers. These solid-state supercapacitors decrease capacitance by less than 15% when reversibly stretched by 150% in the fiber direction, and largely retain capacitance while being cyclically stretched during charge and discharge. The maximum linear and areal capacitances (based on active materials) and areal energy storage and power densities (based on overall supercapacitor dimensions) are high (5.4 mF/cm, 40.9 mF/cm2, 2.6 μWh/cm2 and 66.9 μW/cm2, respectively), despite the engineered superelasticity of the fiber supercapacitor. Retention of supercapacitor performance during large strain (50%) elastic deformation is demonstrated for supercapacitors incorporated into the wristband of a glove.

Woven-yarn thermoelectric textiles

The fabrication and characterization of highly flexible textiles are reported. These textiles can harvest thermal energy from temperature gradients in the desirable through-thickness direction. The tiger yarns containing n- and p-type segments are woven to provide textiles containing n-p junctions. A high power output of up to 8.6 W m(-2) is obtained for a temperature difference of 200 °C.

Harvesting temperature fluctuations as electrical energy using torsional and tensile polymer muscles

Diverse means have been deployed for harvesting electrical energy from mechanical actuation produced by low-grade waste heat, but cycle rate, energy-per-cycle, device size and weight, or cost have limited applications. We report the electromagnetic harvesting of thermal energy as electrical energy using thermally powered torsional and tensile artificial muscles made from inexpensive polymer fibers used for fishing line and sewing thread. We show that a coiled 27 μm-diameter nylon muscle fiber can be driven by 16.7 °C air temperature fluctuations to spin a magnetic rotor to a peak torsional rotation speed of 70 000 rpm for over 300 000 heating–cooling cycles without performance degradation. By employing resonant fluctuations in air temperature of 19.6 °C, an average output electrical power of 124 W per kg of muscle was realized. Using tensile actuation of polyethylene-based coiled muscles and alternating flows of hot and cold water, up to 1.4 J of electrical energy was produced per cycle. The corresponding per cycle electric energy and peak power output, per muscle weight, were 77 J kg−1 and 28 W kg−1, respectively.

Stretchable Triboelectric Fiber for Self-powered Kinematic Sensing Textile

Stretchable fiber and yarn triboelectric nanogenerator are sought for such applications as wearable sensing system such as cloth communication devices, electronic textiles, and robotic sensory skin. Unfortunately, previously reported triboelectric fiber and yarn are difficult to have stretchable property. We introduce here a new type of stretchable and weavable triboelectric fibers with microdiameter dimensions. The stretchable triboelectric fibers can be reversibly stretched up to 50% in tensile direction while generating voltage output proportional to the applied tensile strain. The reversible distance change induced by the Poissons ratio difference between the core fiber (silver-coated nylon/polyurethane) and the shell (wrinkled polyvinylidene fluoride-co-trifluoroethylene/carbon nanotube layer) during tensile deformation is the key working principle for electrical generation. Owing to exceptional structural stability, the stretchable triboelectric fibers show high performance retention after 10,000 times repeated stretching/releasing cycle. Furthermore, the stretchable triboelectric fibers are mechanically strong to be woven into a commercial textile for textile based sensors, which can detect magnitude as well as direction of the motion.

Triboelectric generator for wearable devices fabricated using a casting method

In this study, we fabricate an efficient triboelectric generator (TEG) using inexpensive materials that are readily available in our surroundings. By casting polydimethylsiloxane (PDMS), we perform micropatterning on the surface of sandpaper. We use aluminum foil as an electrode and electrified body. To improve the durability and resilience of the aluminum foil, we use a polyethylene terephthalate (PET) film. PET/Al electrodes may act on the bottom and top performing the role of an electrode, and at the same time as an electrified body. We applied an external force of 1 N using the pushing tester on the TEG created using the PDMS, and we then connected an external resistor to confirm the output power. Based on the patterning TEG, we confirmed that there was an increase in the output voltage by a factor of about 10 compared to the flat TEGs output voltage of 15 V. We turned on 79 LEDs by hand pushing, and produced an output voltage of more than 250 V. In addition, we turned on 39 LEDs by performing a bending test with an average output voltage of more than 100 V.

Conductive functional biscrolled polymer and carbon nanotube yarns

Biscrolling aligned electrospun fiber (AEF) sheets and carbon nanotube (CNT) sheets were fabricated for conductive, functional yarns by a versatile dry composite method. Our biscrolling (twist-based spinning) method is based on spinnable polymer fiber sheets and spinnable CNT sheets unlike the previous biscrolling technique using unspinnable nanopowders and spinnable CNT sheets. The CNT sheet in composite yarns acted as effective electrical wires forming dual Archimedean multilayer rolled-up nanostructures. The weight percent of the electrospun polymer fibers in the composite yarns was over 98%, and the electrical conductivity values of the composite yarns was 3 orders higher than those of other non-conducting polymer/CNT composite fibers which were electrospun from polymer solutions containing similar loading of CNTs. We also demonstrate that biscrolled yarns having various structures can be fabricated from spinnable AEF sheets and spinnable CNT sheets.

Stretchable Fiber Biofuel Cell by Rewrapping Multiwalled Carbon Nanotube Sheets

The fiber-type biofuel cell is attractive as an implantable energy source because the fiber can modify various structures and the wound can be stitched like a suture. In addition, in daily life, the biofuel cell is forced by human motion, and stretchability is a critical requirement for real applications. Therefore, we introduce a new type of highly stretchable, stable, soft fiber biofuel cell with microdiameter dimensions as an energy harvester. The completed biofuel cell operated well in fluids similar to human fluids, such as 20 mM phosphate-buffered 0.14 M NaCl solution (39.5 mW/cm2) and human serum (36.6 μW/cm2). The fiber-type biofuel cell can be reversibly stretched up to 100% in tensile direction while producing sustainable electrical power. In addition, the unique rewrapping structure, which traps the enzyme between multiwalled carbon nanotube sheets, enormously enhanced the stability of the biofuel cell when the biofuel cell was repeatedly stretched (the power density retention increased from 63 to 99%) and operated in human serum (the power density retention increased from 29 to 86%). The fiber can be easily woven into various structures, such as McKibben braid yarn, and scaled up by series and parallel connections.

Correction: Triboelectric generator for wearable devices fabricated using a casting method

Correction for ‘Triboelectric generator for wearable devices fabricated using a casting method’ by Chang Jun Lee et al., RSC Adv., 2016, 6, 10094–10098.

Residual Charges during Electrospinning Assist in Formation of Piezoelectricity in Poly(Vinylidene Fluoride-co-Trifluoroethylene) Nanofibers

We confirm piezoelectric performance of bottom electrospun PVDF-TrFE mat is higher than that of top mat and report the mechanism of additional poling process of electrospun nanofibers by local electric field which is originating from residual charges in far-field electrospinning process. Piezoelectric output measurement of poly (vinylidene fluoride-co-trifluoroethylene) electrospun nanofibers was performed by push test and output signals of bottom and top were compared. The local electric field strength calculated by simulation was higher than reported electric field strength of near-field electrospinning (10 MV/m). It can be concluded that the piezoelectric outputs of electrospun nanofibers tend to be improved by residual charge density and electrospinning condition.