Over the past two years, Head has sold >50,000 actively controlled tennis
rackets and >10,000 smart skis. These markets are expanding, in part,
because recent research has indicated the potential for the ACI/Head rackets
to reduce, and in some cases eliminate, tennis elbow8–10 and that the skis
improve control by reducing ski chatter.10
The implications of a system that scavenges waste mechanical energy to
power an independent electronic system without the need for batteries are
enormous. Piezoelectric materials have always held this promise, but the
power output was so small that whatever power was produced was
consumed by the electronic control system.