External ultrasound can generate microampere direct currents in vivo from implanted piezoelectric materials

Under development is an internal fixation plate that incorporates a piezoelectric element to generate current when excited mechanically by either weight bearing or external application of ultrasound. The intent is to deliver this current to electrodes at a fracture or osteotomy site to aid in prevention or treatment of nonunion. The present study examines quantitatively the ability of external ultrasound to generate current from small piezoelectric ceramic elements implanted in tissue. An ultrasonic transducer (2.25 MHz, 10-20 V input, less than 10 mW/cm2 output) was employed to excite small test coupons of a piezoelectric ceramic in vitro and in vivo with various materials, including water, PVC gel, cortical bone, and living soft tissues, interposed. In all instances, it was possible to generate currents of up to 20 microA after rectification; currents up to 1 mA were achieved in some cases. The work indicates that external ultrasonic energy could effectively power small internal devices designed to stimulate bone healing, without the need for implanted batteries or percutaneous leads.