Energy harvesting is a promising solution to generate energy from the environment of the human body exploiting the movements or heat. Many researchers and companies are investigating in novel solution to make small size energy harvesting the new way to supply and store energy to achieve self-sustainable mobile and wearable devices.
Advances in low power processors, wireless communication, sensors, and machine learning are driving the pace of innovation in wearable devices. Due to the interest of companies developing wearable consumer electronics, wireless sensor devices attached in different ways to the human body have become successful products in the wellness, fashion, and sports domains, with many applications that usually are interfaced directly to the smartphones. According to Gartner, the number of commercial devices targeting wearable applications have will increase of 25% in 2019 with respect of 2018, reaching the volume of 225 Million with a market of 45 billion of dollars.
Wearable devices are a subcategory of the internet of things (IoT), which has as the main feature of consisting on miniaturized electronics that can be worn always-on with minimal invasiveness on a person. There are wearables devices that have a display to release information to the end user in real-time, on the other hand, many wearable devices store the data in memory and transmit them to other devices or to the cloud to offload computationally intensive data analysis.
On the current market it is possible to identify four main types of wearable device: 1) probably the biggest is on the fashion, fitness and traditional watches. 2) Smartwatches from consumer electronics brand such as Fitbit, Apple Watch, and Samsung Gear. 3) Gadgets for children, 4) Specific application wearable devices from start-ups that are targetting niche applications such as medical or others. However, all those types of systems are facing the same major problem, namely the limited available energy in their batteries due to the limited size of the devices.
To extend the lifetime of the device, and in that sense, maximize the possible use time of the wearable device, the entire design (hardware and software) has to be optimized. On the one hand, the power consumption can be reduced by optimizing the data acquisition and processing (i.e., duty-cycling, sleep-wake up, reducing radio activities, etc.). However, reducing the low power is not always possible and reduce the performance of the application. Thune, energy harvesting is becoming more and more attractive for wearable devices. Energy can be harvested from the environment or the body movements and heats to constantly charge batteries and therefore compensate for the power being consumed. Increasing the battery capacity would be the simplest strategy, but it is, in most cases, impossible to implement due to size constraints. Moreover, energy harvesting can provide an energy resource in theory infinite.
As wearable devices are required to operate on the human body for long periods (i.e., weeks or months), achieving self-sustaining systems by harvesting energy (EH) from environmental sources, movement or heat is particularly attractive. The most promising sources of energy for harvesting on and near the body include thermal, vibration, light and Radio Frequency (RF). They can provide continuously average power in the range of µW when deployed on the human body. Thermal energy harvesting has the advantage to provide all the time as the body is always warmer or colder of the environmental temperature, and it does not depend on the deployment (i.e., light need to be deployed on the clothes) and movements (i.e., energy acquired in the night ). However, all the energy sources can be exploited to generate and accumulate energy.
There are still challenges to overcome to make energy harvesting a mature solution for wearable devices, and researchers are investigating and finding novel solutions to power wearable devices. If they become successful as it happens for energy harvesting for bigger scale (i.e., with solar and wind turbine), the traditional batteries of today will become a thing of the past, and the self-suitability of wearable and mobile device will be achieved.
