C.Rodrigues, A.Gomes, A.Ghosh, A.Pereira, J.Ventura
Power-generating footwear based on a triboelectric-electromagnetic-piezoelectric hybrid nanogenerator

There is an increasing need to monitor in real-time health or wellbeing parameters such as heart rate, burned calories, walked steps, blood pressure, time spent exercising or athletic performance. To suit such demands, wearable and portable electronic devices are being developed and have attracted a large amount of attention in recent years. However, as the number of portable and wearable electronics increases, so does the need for new and independent power sources that allow a continuous operation of these small devices. In fact, present developments in electronics are making devices increasingly smaller and operating at ultra-low power consumption, opening the possibility to power them by energy harvested from our living environment.

Thus, new technologies in the field of nanoenergy and energy harvesting are being re-searched as sustainable self-sufficient micro/nano-power sources. Various approaches for harvesting mechanical energy have been demonstrated based, for example, on piezoelectric, electromagnetic  and electrostatic effects. A novel solution appeared in 2012 with the development of the first triboelectric nanogenerator (TENG). A TENG is an energy harvesting device that converts external mechanical energy into electricity by a conjunction of contact electrification and electrostatic induction. Contact electrification is a process that generates a charge distribution at the interface of materials that come into contact, and electrostatic induction is the main mechanism that converts mechanical energy into electricity. This novel type of nanogenerators brings together high performance, efficiency, versatility, scalability, applicability and environmental friendliness. In recent years, it was demonstrated that TENGs can be integrated in clothes and/or footwear and thus harvest energy from human body movements. Bai et al. integrated a flexible multilayered TENG onto a shoe pad, while a power-generating shoe insole TENG with a multilayered zigzag-shaped structure was developed by Zhu et al. Recently, Liu et al. developed a triboelectric-electromagnetic hybrid nanogenerator that charged a 1000 μF capacitor to 5.09 V after 100 cycles of vibration. However, the optimization of the TENG configuration and corresponding hybridization for footwear applications still remains to be performed.

Here, we developed a triboelectric-electromagnetic-piezoelectric hybrid nanogenerator that can harvest energy through human walking, and that was integrated in a shoe insole. Due to its flexibility, this device was optimized considering different tribo-structures and various configurations in order to increase the electrical outputs generated and consequently increase the energy storage efficiency.

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