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Wearable sensors have been implemented widely to provide comfortable and continuous long-term monitoring in many applications. Minimal requirements on maintenance is a main characteristic of wearable sensors, but unfortunately, many of them are still powered by battery with limited capacity which need to be charged or replaced regularly. Energy harvesting technologies are applied to provide a reliable solution to this issue. This paper presents several design considerations for self-powered wearable sensors. Suitable energy sources are discussed, such as ambient energy sources (solar, RF, and ultrasonic energy), human body energy (mechanical, piezoelectric, triboelectric, electromagnetic, electrostatic, and thermal energy), and the subcutaneous energy harvesting technique for implantable sensors. Moreover, power management integrated circuits, energy storage options, and the material selection and conditioning circuit of triboelectric nanogenerator (TENG) are discussed. Five case studies utilizing different energy harvesting techniques are discussed and evaluated in terms of their system implementation and performance to provide some deeper understandings of wearable sensors.

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