The average adult human has about the same amount of energy stored within their body as a 1 ton battery.
Current designs for implants in knees, both through transplant surgeries and braces can harvest kinetic energy to power small devices. Other devices harnessing the power of thermodynamics and kinetic energy have been used in constantly changing/moving parts of the human body such as the heart, lungs, legs, etc.
Another source of energy is the endocochlear potential – a potential generated in the space in the ear across the membrane. This potential created from vibrations in the eardrum has enough power to theoretically power a small hearing aid/device. This could result in the first piece of biotech which is fully powered by the body, and is one step closer to actual integration.