The leaps in the electronics sector has revolutionized our world with their inalienable presence in our everyday life. However, the rectangular shape of the batteries in these devices restricts the possibilities of their adaptation in textiles and wearable electronics. Thus, a major caveat in the research of flexible electronic devices like flexible batteries, supercapacitors and PV cells is the availability of flexible conductors with adequate conductivity to replace rigid foils and semi-flexible films.

Challenges

• Combining high conductivity with high flexibility

• Retention of conductivity after repeated bending cycles

• Achieving performance in flexible devices comparable to rigid and semi-flexible counterparts

Current Research

Wearable textiles with integrated multimodal sensors have numerous uses in the healthcare, entertainment, fitness, and fashion industries. However, the majority of reported sensors use different measurement methods to measure different stimuli, i.e., strain, pressure, and temperature. Further, they lack repeatability and stretchability for multimodal sensing. We have solved these issues by fabricating hybrid piezoelectric-capacitive sensors based on the PVDF−PU blend. Though PVDF, being a piezoelectric polymer, can be used as a dielectric layer in a capacitive sensor, it shows a poor piezoelectric coefficient and is mechanically unstable to cyclic deformations. To overcome this problem, we have used a specific blend of PU with PVDF, which has both high stretchability and piezoelectric coefficient. PVDF79PU21 (with 21% PU) nanofiber capacitive sensors showed a multimodal response with an excellent sensitivity of 0.3 kPa−1 for up to 8 kPa pressure stimuli, a good gauge factor ranging between 0.5 and 0.75 for 0−40% cyclic strain, and high sensitivities of 0.8 and 2% °C−1 for 30−60 and 60−100 °C, respectively. They could be used for measuring the human body temperature in the range of 37−40 °C with a sensitivity of 0.9% °C−1 . The prototypes of PVDF79PU21 nanofiber-based capacitive sensors were attached to different body parts to measure extension and flexion movements with high sensitivity, which showed its great potential as a wearable sensor. 

Poly(vinylidene fluoride) (PVDF)-based piezoelectric nanogenerators, though flexible, exhibit poor stretchability and mechanical stability. This limits their application for harvesting energy from repeated deformations arising from human articular motions. In this study, we propose a simple and cost-effective approach to overcome the above issues while simultaneously enhancing the piezoelectric effect of PVDF by mixing a small amount of polyurethane (PU) in PVDF−PU nanofibers. The presence of PU in PVDF could enhance electroactive phases by up to 46%, as measured by Fourier transform infrared (FTIR) spectroscopy. Interestingly, an addition of 21% of PU in PVDF exhibited both an increase in the d33 value from 3.02 to 7.064 pm/V and stretchability to 90%. For developing a stretchable piezoelectric nanogenerator (S-PENG) device, stretchable electrodes with a 4.5 gauge factor at 100% strain were fabricated by spin-coating of poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS):graphene nanoplates on a prestrained PU substrate. SPENG produced 3.8 V, 0.65 μA, and 0.48 μW/cm2 peak open-circuit voltage, short-circuit current, and power density during cyclic deformation, respectively, with electrical and mechanical stability for at least 2000 cycles. Its performance was demonstrated for various human articular motions related to the knee, elbow, and foot by integrating it with wearables. The generated energy from the S-PENG could readily charge capacitors up to ∼650 mV in just 100 s. The designed S-PENGs showed great potential in harvesting energy from simple human motions. 

The objective is to create flexible and stretchable conducting fibres which can be incorporated in textiles. The popular research in the field involves the incorporation of wires into textile structures which reduce the flexibility and resilience of the fabric.

• Kiran Yadav, Surabhi Jha, Manjeet Jassal, Ashwini K. Agrawal, Internally coated highly conductive and stretchable AgNW-PU hollow fibers, Polymer, 2019, 169, 46-51 (Impact factor 3.483).  View

Conducting Devices

The current focus of work in SMITA Research Lab is two-fold. The primary objective is to develop flexible devices such as supercapacitors and batteries by replacing all the rigid components such as current collectors and exploring different architectures such as nonwovens, cables, fabric etc. In parallel we are also making progress in achieving property retention after subjecting the device to extreme conditions of knotting, stretching or bending.