Energy harvesting from from the human body depends on the existing  gradient between the human body and the ambient as noted by George. Arrays of thermocouples are used to generate as much power required to feed the wireless sensors on the body for example. The produced energy are stored in super capacitors rather than recharging batteries. Therefore they normally displace the electrochemical batteries.For more information please follow the paper in the link:https://www.researchgate.net/publication/258167029_Thermoelectric_Energy_Harvesting_of_Human_Body_Heat_for_Wearable_Sensors.

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Article Thermoelectric Energy Harvesting of Human Body Heat for Wear...

If you want to use body heat, you will need both hot and cold regions, you can only extract energy from a temperature difference. Thermocouples can do that but you would need a number in series as the output would be at a very low voltage.

Energy harvesting from from the human body depends on the existing  gradient between the human body and the ambient as noted by George. Arrays of thermocouples are used to generate as much power required to feed the wireless sensors on the body for example. The produced energy are stored in super capacitors rather than recharging batteries. Therefore they normally displace the electrochemical batteries.For more information please follow the paper in the link:https://www.researchgate.net/publication/258167029_Thermoelectric_Energy_Harvesting_of_Human_Body_Heat_for_Wearable_Sensors.

wish you success

Article Thermoelectric Energy Harvesting of Human Body Heat for Wear...

If you can harvest from movements, you may want to consider the use of inductive generators - merely a spring-loaded permanent magnet inside a coil. This is quite effective and - depending on the amount of energy required - rather small. Old-fashioned but extremely reliable.

Beyond that, what Dr. Abdelhalim Zekry wrote about the use of supercaps applies as well for inductive energy harvesting generators.