Found a press release from 2013 that research is being performed with ultrasound charging of cardiac/in body devices
http://www.newswise.com/articles/tune-in-turn-on-power-up
The Arizona research company is piezo energy technologies. http://gopiezo14.com
2011 patent: https://www.google.com/patents/US8082041
2014 patent: https://www.google.com/patents/US8974366
But I haven't seen anything commercially available in the medical industry