Energy Harvesting from Bite Force Using Electret Sheet

Extending healthy life expectancies is an issue that must be addressed in aging societies. One solution is to use wearable sensors that continuously measure physiological data to predict diseases in an unaffected state. In particular, the measurement of saliva and bite force as biomarkers for disease prediction and diagnosis is promising; however, chemical batteries cannot be placed in the oral cavity. Therefore, the key technology for the realization of these sensors is supplying power to the sensors in the oral cavity. In this chapter, an energy-harvester sheet using bite force is described, and its application to a mouthguard harvester and bite-force sensor is introduced. The principle of an electrostatic-induced energy harvester in which an electret and an elastomer are sandwiched between the upper and lower electrodes to generate electricity by bite force is proposed. A mathematical model is derived by combining a hyperelastic model that describes the compressive deformation characteristics of the elastomer and an equivalent electric circuit model that describes the electrical dynamic characteristics of the laminated sheet. Based on the mathematical model, a mouthguard-type harvester is designed and fabricated. Then, the prototype harvester was installed in a bite-force simulator. Experiments evaluating the amount of power generated showed that it produced 120 times more power than a piezoelectric element with the same dimensions. Furthermore, an inverse model of the mouthguard-type harvester was derived and its application to a wearable mouthguard-type bite-force sensor was presented.