|Fig. 1: Yorick, mascot of the FDAs Mechanics and Materials Laboratory, was created by engineer Ed Mueller. The Skelton is outfitted in many different types of IMDs (Source: Wikimedia Commons)|
Implantable medical devices (IMDs) are critically requested for the survival of patients subject to certain serious diseases such as bradycardia, fibrillation, diabetes, and disability, etc. There are many types of IMDs as seen on Yorick in Fig. 1. However, implantable medical device (IMD) relies on a continuous supply of electricity. Therefore, a long-term, extremely safe power source is critical in the recharging of an IMD. Some of the power sources come from conventional batteries, such as lithium cell, nuclear cell and bio-fuel cell however new methods have been are being invested to recharge IMDs from outside of the human body. These emerging innovations to recharge the implanted batteries, include electromagnetic energy transmission, piezoelectric power generation, thermoelectric devices, ultrasonic power motors, radio frequency recharging and optical recharging methods to name a few. The article will touch on the typical batteries usage and discuss the investigation of emerging methods to power implantable medical devices.
The first implanted lithium cell used to power a pacemaker was designed by an Italian scientist in 1972.  With the advent of lithium batteries, the longevity of an implanted cardiac pacemaker had been extended to 10 years, and more than five million people have benefited from such advancement so far.  The lithium battery has been widely adopted as the power source in IMDs, besides the implanted cardiac pace-maker which has moved on to a safer Li/I2 battery that leak less than other liquid batteries. The three typical IMDs utilizing lithium battery are neuro-stimulators, drug delivery system, and defibrillators. 
The main reason why Li/I2 batteries take a dominant place in powering implantable cardiac pacemakers is their high discharge voltage and energy density. 
The thermoelectric power generator is a highly promising alternative to the typical batteries. The human body has inherent temperature differences. The maximum difference is between the body core and the skin surface at 8K which can be large enough to generate electricity. Additionally, the human body naturally creates heat energy which is typical released into the environment. Therefore, thermoelectric power generators are environment-friendly. Although having perfect self-powering capability by directly using the body heat, the thermoelectric generator has been far from being fully developed. 
As the healthcare industry grows, especially in the medical device sector, the functions of IMDs are continuously being explored. The paper outlines the fact that powering source for IMDs should be developed to be more effective, longevous, environment-friendly, and synthetic. It explores further advancements that would develop power sources outside the human body. Further advancements in this area are expected to come in the near future.
© Paige Voigt. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 F.-G. Zeng, "Trends in Cochlear Implants," Trends Amplif. 8, 1 (2004).
 P. Molina-Negro, "Role of Neurostimulators in the Treatment of Chronic Refractory Pain" Union Med. Can. 109, 41 (1980).
 X. Wei and J. Liu, "Power Sources and Electrical Recharging Strategies For Implantable Medical Devices," Front. Energy 2, 1 (2008).