  Today, technology development has led humans to employ wearable and
implantable devices for biomedical applications. An important research issue in
this field is the wireless body area networks (WBANs), which focus on such
devices. In WBAN, using batteries as the only energy supply is a significant
challenge, especially in medical applications. Charging the batteries is a
problem for patients who use WBAN. Replacing the battery is not very difficult
for wearable devices, but implantable devices have different conditions. The
use of batteries in implantable devices has many problems, including pain and
costs due to surgery, mental stress, and lack of comfort. Batteries' life
depends on their type, operation, the patient's medical condition, and other
factors. This paper reviews recent energy harvesting methods for battery
recharge in WBAN's sensors. Moreover, we provide future research directions on
energy harvesting methods in WBANs. Therefore, active research fields such as
reinforcement learning (RL) and distributed optimization in WBAN applications
were investigated. We strongly believe that these insights will aid in studying
and developing a new generation of rechargeable sensors in WBANs for fellow
researchers.
