Universal biomechanical energy harvesting from joint movements using a direction-switchable triboelectric nanogenerator

One of the abundant energy sources around us to be harvested is a human body movement, which entirely depends on joint movements with exclusive characteristics of limited displacement with a high degree of directional freedom. In this study, we firstly propose a novel type of the direction-switchable triboelectric nanogenerator, called a STRING, which incorporates an appropriate power transmission unit and the rotational triboelectric nanogenerator. A flexible string plays the critical role in universally transferring power regardless of direction and displacement of the applied force. It is shown that the impulse as well as the speed of the human body movement are the key parameters determining the output performance of the STRING. The lightweight STRING (total weight less than 160 g) is applied to more than 5 joints and the ability to easily generate output current more than 10 μA from natural body movements is shown. The generated electricity is directly utilized to continuously generate the DC 5 V in real time, which is favorable for practical operation of electronic devices. Furthermore, simple adoption of voltage doubling circuit significantly boosts the open circuit voltage (more than 2 kV). The present STRING with universal biomechanical energy harvesting would not only power electronic devices effectively, but also could be used in a wide range of applications by enabling portability and self-powered operation.