Employing energy harvesting in wearable medical device designs

June 11, 2019

Maurizio Di Paolo Emilio-June 11, 2019

One of the main challenges with battery-operated connected devices is autonomy. An increasing number will have requirements for greater battery power or longer battery life, but will not support bigger batteries. Some wearables, for example, might be medical implants; it’s not likely a bulky battery will be an option for such devices. Battery technology is keeping up by making use of energy harvesting.

Energy harvesting can be an incredible advantage for devices with a small form factor, such as those that proliferate in the internet of Things (IoT). These small devices often only require tiny amounts of current, and energy harvesting from various sources can be a valuable design element.

Wearable Medical Solutions

Wearable devices for medical use are wearable devices that can detect, store, and transmit vital parameters (such as heart rate, oxygen saturation, respiratory rate) measured in real time to report the overcoming of certain critical thresholds. According to the analysis by Frost & Sullivan – "Wearable Technologies in Clinical and Consumer Health" – the global market for wearable devices in the medical field will reach $18.9 billion in 2020.

Energy harvesting solutions have been designed as an auxiliary power source for batteries or as an independent source for the permanent use of wearable devices without restrictions associated with energy consumption. Energy harvesting is considered an unreliable source, in the sense that the availability of energy can vary considerably over time depending on the environmental conditions. It is possible to combine a source of energy harvesting such as vibration, heat, or solar with a rechargeable battery.

Triboelectric effect

Contact electrification is a process that produces surface charges on two dissimilar materials when they are contacted and separated. During this contact, each material develops a charge of opposite polarity. In recent years, there has been progress in developing systems for the triboelectric energy harvesting, called triboelectric nanogenerators (TENG). These systems require a minimum of essential components: at least two layers of triboelectric material, physical separation between them, and electrodes for collecting electricity, in addition to a circuit of conditioning to maximize collection efficiency (figure 1).

Figure 1: Conditioning circuit for the TENG; t1 is when the switch is closed (energy stored by the LC unit), t2 when the switch is open (energy released by the LC unit).

The classic DC-DC buck converter is coupled with an AC-DC buck conversion circuit for the TENG, as shown in figure 1. Between the switch and the load resistance R, a diode D1, a serial inductor L, and a capacitor C are added in sequence. The switch is used not only to maximize energy transfer but also to convert the incoming buck to the circuit. The switch can be realized by a micro-power voltage comparator with MOSFET to integrate a self-management mechanism

Thermal energy

The thermal energy harvesting is the process of capturing the heat that is freely available in the environment or represents the waste energy emitted by engines, human body, and other sources and puts it into use. The direct conversion of thermal energy into electrical energy can be achieved through the Seebeck effect, in which a heat flow induced through an appropriately designed thermoelectric device produces a voltage and an electric current. The pair PN, which is the fundamental component of a thermoelectric generator (TEG), comprises a single structure of thermoelectric material of type P and N, each electrically connected in a series.

By placing many PN pairs electrically and thermally in parallel, it is possible to build a typical TEG module that generates a voltage proportional to the thermal gradient. Thermoelectric or TEG power generation modules are already used in many applications, such as space ships, in which they collect the heat emitted by the decay of a supply of radioactive material.

The emerging field of medical wearable electronics also offers the potential for thermoelectric energy collection by fueling devices with body heat.

>> Continue reading page two of this article originally published on our sister site, EE Times: "Energy Harvesting for Utra-Low Power Wearable Medical Devices."

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