Human metabolism is strictly connected to the environment, and, on a large scale, to sustainability, pollution, and climate change. In fact, it is a well-established fact that “anthropic pressure” is the main determinant of environmental pollution, including the exploitation of resources, such as water and food. It is well known that ground consumption is also linked to human metabolism. The need to reduce carbon emissions worldwide is one of the biggest challenges science faces today and in the foreseeable future. In this definition, it refers to the amount of energy that humans use from the planet’s resources, which is the social sense of human consumption. The “green transition” and related increased public awareness about environmental problems motivated the search for alternative sources of energy. Historically, human beings have been studied as machines for energy production and consumption [1] and for their impact on the environment [2]. Energy harvesting from the human body is a rapidly developing field of research. Interest in harvesting energy from the human body is not new, and it is practically incorporated in some everyday use devices, such as automatic watches, which use energy from movement to charge. In this case, the oscillation of the arm (pendulum) during walking feeds the watch mechanism, which is used to store energy. Large movements with much greater applied force can generate higher energy, which can then be used to sustain the functioning of various devices (e.g., cell phones) and, in some cases, partially or totally, can even sustain the energy requirements of a gym or sports facility, particularly if harvested at the same time from many individuals [3]. The mechanical efficiency of the human body is in the range of 15–30%, which means that 70% of the energy provided by food is dissipated into heat [4].

The human body contains a great amount of energy. In fact, the average adult’s body fat deposits store as much energy as a one-ton battery [5], and it has been calculated that the monthly energy capacity of a person taking 7500 steps/day is equivalent to a 0.40 mAh battery rated at 1.2 V [6]. An average person could generate power comparable to a 1 m² solar panel on a sunny day and 10 m² of solar panels on an overcast day [7]. Robert Obrest, an athlete competing as a strongman, eats 15,000 to 20,000 calories per day, storing the capacity to produce a large amount of energy [8].

The heat from the body can be a source of continuous energy given that the core body temperature is maintained at 37 °C by the metabolic processes. It has been calculated that the whole human body can dissipate 60–180 W depending on the type of activity performed [9]. Thermoelectric devices have been proposed to harvest this energy, and it has been calculated that if this device has a conversion efficiency of ∼1%, the resulting power produced would be in the range of ~0.6–1.8 W, which is sufficient to supply energy to many wearable sensors [7]. This energy is generated from energy-dense sources (fat). Motion energy is particularly interesting as a source of energy because it has a power density as high as 200 μW/cm² and is available on demand [6], depending on fatigue. The average energy expenditure for one-person (energy used by the body) is 1.07 × 107 J per day [9], equivalent to 800 AA (2500 mAh) batteries, which would weigh about 20 kg. This amount of energy can be produced from 0.2 kg of body fat [10].