A Novel Soft Metal‐Polymer Composite for Multidirectional Pressure Energy Harvesting

Abstract

A cost-effective 3D sponge composed of rubber and conductive wires is introduced, which generates electric energy by periodic compression. It can be fabricated in a symmetric configuration, hence there is not a preferred surface or a specific direction for compression to obtain the energy harvesting or pressure sensing. The energy conversion occurs due to both triboelectric and electrostatic effects, which benefit from the air gap between polymer and wires. Although the triboelectric effect has been known since 1913, little progress has been achieved toward generating electric energy using this effect.1, 2 Recently, work was undertaken by the nanoscience research group in Georgia Tech to use this for energy harvesting and sensing purposes. During the last year, they used this effect as a competitive method for electric generation at micropower scales.3 Amazing results were published by the same group, who developed efficient triboelectric power sources by implementing micropatterns on flexible surfaces;4 a transparent pressure sensor;5 nanoscale patterns for powering portable devices;6 nanoparticles enhanced generators;7 and hybrid energy harvesters.8 All the mentioned works share the same benefit, i.e., micro/nanoscale patterns on the employed surfaces. Moreover, the above systems are based on two semiparallel electrodes with an intermediate material between them. In the resulting structures, at least two components move relative to each other for generating charges. Therefore, a matrix of layers containing a free space between them is necessary in the fabrication process. Providing a constant space between the layers is one of the challenges that has been dealt with by using several approaches, such as including a spacer,4 arc shape surfaces,6 springs,7 etc. Although these approaches can increase the efficiency of the system, they bring restrictions for implementing and aligning those systems in the integrated structures, especially where a soft body is requested, such as in the emerging field of soft robotics.9 Here, we address the above limitations by introducing a very simple and cost effective approach to build a composite consisting of a rubber matrix embedding in a disordered fashion, conducting materials, and air gaps. In the proposed technology all parts can be developed together on a single surface or in a 3D structure. The fabrication process does not require bonding steps for assembling the system and creating air gaps, thus the overall fabrication complexity is dramatically reduced. Air bubbles remain trapped inside the sponge by means of solving sacrificing particles. The triboelectric phenomenon is promoted by periodic contact between miniature pieces of copper microwires encapsulated in a polydimethylsiloxane (PDMS) sponge. The strong advantages of this work are its simplicity and the cost effectiveness of the process, which makes it suitable for mass manufacturing and fabrication with easy-access facilities. Moreover, because the prototypes are made out of a mixture, the building process can be integrated with other fabrication methods in a broad range of scales. The possibility of trapping conductive wires in rubber opens the way to a new generation of energy harvesting devices, targeting all application fields where a flexible and soft material is used and/or needed. Moreover the fabrication process is not constrained to planar technology but fully 3D shapes can be built. The wired-sponge polymer could be tailored for consumer goods used in everyday life, for which energy harvesting mechanisms have not been considered to date, such as some parts of wheels, mattresses, toys and playgrounds, dance floors, etc. It is noteworthy that this approach could be addressed in advanced robotic systems where the body of the robot is built with soft materials via a bioinspired design paradigm.10 On the other hand there are some aspects that must be considered, including that the amount of the charges generated on the wir