Energy harvesting using thermoelectricity for IoT (Internet of Things) and E-skin sensors

Recently, there has been a great amount of interest in health care and medical applications; thus, sensors capable of monitoring the patient's health conditions for a long period of time are longed for. There are numerous kinds of medical sensors that measure the conditions of a patient. For example, blood pressure sensors, electromyography sensors, electrocardiography (ECG) sensors, and SpO₂ pulse oximetry sensors are widely used to monitor patients with health issues. The most important requirement of the aforementioned medical sensors is a stable and continuous power supply. To supply sustainable power for medical sensors, thermoelectric devices capable of generating power by harvesting heat from a human body are used. The human body can be used as a heat source as it generates heat continuously though metabolism; however, to do so, it is important to understand the human thermoregulatory system. The thermoregulatory system of the human body controls its skin temperature and therefore certain parts of the body are more suitable to operate certain devices than the rest as shown in figure 3. However, the conventional TED is hard to adapt on to the human skin due to its rigidity. To make thermoelectric devices more applicable to the human skin, researches on flexible TEG are being actively conducted. Majority of flexible TEG for body heat harvesting are based on inorganic bulk thermoelectric materials instead of printed materials. Since thermal energy from a human body is so called the 'low-grade heat', it is hard to yield a sufficient amount of temperature difference in a film-type thermoelectric device due to its small thickness for flexibility. Kim et al [59] demonstrates the body heat harvesting using thin-film based flexible thermoelectric device while generating only an output power of 3 μW. Most researches recently published on flexible thermoelectric devices for body heat harvesting adopt inorganic bulk thermoelectric materials which have sufficient height to retain enough temperature difference.

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Table 1 shows the experimental results of body heat harvesting using TEG. Researches referenced in table 1 have used inorganic bulk thermoelectric materials based on Bi-Te compounds. Suarez et al [69] use inorganic bulk BiTe compounds and EGaIn as the flexible metal liquid electrodes which are placed on both top and bottom sides of their FTED, as in figure 4(a). The liquid metal is encapsulated in polydimethylsiloxane (PDMS) to prevent leakage. The device was able to generate a voltage ranging from 1.47 to 2.96 mV and power from 1.48 to 6.0 μW under varying air velocity conditions. Eom et al [63] demonstrate a bracelet-shaped thermoelectric device based on rigid inorganic bulk thermoelectric materials which has a hinge structure, so it can be structurally flexible as in figure 4(b). The bracelet-shaped thermoelectric device attains an output power ranging from 40 to 80 μW with open circuit voltage from 6.7 to 9.1 mV. Park et al [26] propose a mat-shaped flexible thermoelectric device. It is specialized to be applied to a curved surface. Cubic-shaped inorganic bulk thermoelectric materials are inserted into holders that can be connected to each other with a wire. Considering that the radius of curvature of an adult human arm is more than 30 mm, the radius of curvature of the mat-shaped FTED is less than 12 mm; thus, it is suitable for human body heat harvesting. The flexible thermoelectric device harvests energy from body heat and was able to obtain an output power of 138.67 μW with an 8.8 mV open circuit voltage. Kim et al [25] demonstrate a flexible thermoelectric device in which inorganic bulk thermoelectric materials are embedded in a flexible polymer, as shown in figure 4(c). Body heat harvesting is conducted under various conditions: varying height of thermoelectric materials, fill factor, and air velocity, while the maximum output power and voltage are 1.46 mW and 38.24 mV, respectively.

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As mentioned above, many studies on body heat harvesting using a flexible thermoelectric generator are reported and are still ongoing. Further applications on self-powered WSNs driven by body heat harvesting are being carried out widely. Thielen et al [73] propose an application of TEG on a human's wrist integrated with a DC–DC converter as in figure 5(a). Two different types of TEG are demonstrated: μTEG which has a low thermal resistance with a high output voltage and mTEG which has a high thermal resistance with a low electrical resistance. Proposed TEGs show high output power densities, which range from 13 to 14 μW cm⁻², with a maximum of 24% converting efficiency. Magno et al [74] propose a multi-source harvesting circuit which consists of a solar harvester and a TEG for body heat harvesting. While harvesting an average power up to 550 μW by photovoltaic in indoor situations, the TEG harvests 98 μW with a 3 K temperature difference from a human body. The multi-source harvesting system successfully operates several devices such as a camera, microphone, accelerometer and temperature sensor. Myers et al [75] present a thermoelectric body heat generator integrated with components that transmit power management data and various sensors, as shown in figure 5(b). The proposed system demonstrates a successful operation of sensors under varying temperature and humidity conditions while the subject is either running, walking or resting. Necessity for consideration of wet bulb temperature while body heat harvesting is also demonstrated. Wang et al [71] demonstrate the operation of a miniaturized accelerometer powered by body heat harvesting on a human's wrist, as in figure 6(a). The proposed flexible TEG is made of inorganic bulk thermoelectric materials which are embedded in PDMS. It successfully generates an open circuit voltage of 6.6 mV from a human body under indoor conditions. To operate a miniaturized accelerometer which requires 2.4 μW of power with a 1.6 mV voltage, a DC–DC converter with an efficiency of 50% is adopted to boost the voltage up to 2.2 V. A demonstrated thermoelectric system successfully operates the miniaturized accelerometer powered from body heat, as in figure 6(a). Kim et al [72] represent a self-powered wearable ECG system using a DC–DC converter, as in figure 6(b). The system consists of a DC–DC converter, VDD shifter and an ECG module. The ECG sensor is fabricated on a flexible printed circuit board while the power is supplied through the flexible TEG using human body heat. The flexible TEG harvests 1 mW of power with 40–100 mV of open circuit voltage. To operate the ECG sensor, voltage is boosted up to 3.3 V using a commercial DC–DC converter (LTC3108) and dropped down to 1.0 V through a VDD shifter. Due to the existence of efficiency for voltage converting, 70 μW of power is supplied, in which it successfully operates the ECG sensor.

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In designing the thermoelectric system, the thermal contact resistance between the thermoelectric device and the heat source or sink should be also considered. Especially when applied to a human body in which the temperature difference between the top and bottom side of thermoelectric device is relatively small compared to industry applications, the reduction of thermal contact resistance is crucial in achieving high power generation.

For accurate prediction or analysis of body heat harvesting by a flexible TEG, understanding of the human thermoregulatory system must be preceded. The human body has an intricate built-in system to regulate its temperature to maintain a balance between metabolic heat production and heat loss to the environment. This balance between heat production and loss is important in maintaining a constant core temperature of the human body [78]. This thermoregulatory system is divided further into two parts consisting of active and passive systems [79, 80]. The active system directly controls the temperature of the body through means of sweating, shivering, vasoconstriction, and vasodilation [80]. This leads to the rise and fall of the skin temperatures to maintain a balance between heat production and heat loss. The passive system represents the geometry of the human body, metabolic heat production, and the thermal interaction of the human body with the environment (convection, radiation, and evaporation) [81]. Throughout the years, many models have been developed to simulate the thermoregulation of the human body. The thermoregulatory models can be divided into two major categories as in figure 7(a). The first simpler model is the two-node model. The key feature of the two-node model is the representation of the human body as cylinders (single-segment) consisting of its core and skin [82]. This simplification allows for a quick analysis of the human thermal system. The multi-node model is more complicated as it represents the human body as cylinders subdivided into usually the core, muscle, fat, and skin layers [78].

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The most well-known two-node model was developed by Gagge in 1971 [82]. The human body in Gagge's model is treated as two coaxial cylinders each representing the core and the skin. The metabolic heat generated from the core is transferred to the skin through conduction and blood flow. The boundary condition is set on the surface of the skin surface and given as evaporation, convection, and radiation heat losses. The active system here is modeled using vasodilation, vasoconstriction, and sweating functions.

The Stolwijk model is the basis of many multi-node thermoregulatory models [84]. The human body geometry consists of six segments (head, trunk, arms, hands, legs, and feet), each with four nodes (core, muscle, fat, and skin), and one central blood pool. Each node exchanges heat with each other through conduction and with the blood pool through convection. The active system in Stolwijk's model is divided into three parts. In first part, the thermoreceptors sense the hot or cold state. Then the second part sends this information to the third part which controls the action of the active system according to the information received to induce actions such as vasodilation and constriction [84]. This model was developed for NASA to simulate the thermoregulatory system of astronauts and is capable of predicting the skin temperature in low-activity conditions [85]. Another well-known multi-node thermoregulatory model is the Fiala model [79, 80, 86, 83]. The human body is constructed as 15 cylindrical or spherical segments each containing multiple layers of tissue accordingly (figure 7(b)). The skin layer in this model is divided into the inner and outer layer. The inner layer is responsible for blood perfusion while the outer layer is responsible for heat exchange with the environment through evaporation [86]. Penne's so called bio-heat equation [87], is expressed in equation (1)

$$\begin{eqnarray}&&k\left(\displaystyle \frac{{\partial }^{2}T}{\partial {x}^{2}}+\displaystyle \frac{{\partial }^{2}T}{\partial {y}^{2}}+\displaystyle \frac{{\partial }^{2}T}{\partial {z}^{2}}\right)+{Q}_{m}+{c}_{b}{m}_{bl}({T}_{bl}-T)=\rho c\displaystyle \frac{\partial T}{\partial t},\end{eqnarray} \tag{ 1 }$$

where k, ρ and c are the thermal conductivity, density and specific heat of each layer respectively. Also, Q_m is the metabolic heat generation and m_bl is the volumetric blood flow rate. Equation (1) was applied to each tissue layer with appropriate material properties of an average man. Heat is transferred between the tissue layers through conduction and at the surface of the skin through convection, radiation, and evaporation to the environment. The Fiala model also takes into account countercurrent heat exchange to simulate a more accurate arterial blood temperature [79, 86]. The active system was modeled based on a regression analysis of physiological responses from multiple subjects which resulted in a temperature-based system driven by the body core temperature and the mean skin temperature [80]. The Fiala model was later adapted by the UTCI-Fiala model which is the basis of the Universal Thermal Climate Index [86]. A simplified thermoregulatory model was proposed by Wijethunge et al [88] that focuses on the forearm segment. The forearm was the main focus of this model as the purpose was to provide a simplified model for designing wearable thermoelectric devices. The boundary conditions at the skin surface were given as convection, radiation, and evaporation heat fluxes. Assuming the body core temperature as constant, shivering was neglected as it occurs when the body core temperature is considerably low. In addition, this model used thermal resistance of the skin to express the temperature difference across the thermoelectric elements. This temperature difference was then used to calculate the voltage and in return the power generated by a TEG.

As widely known, the temperature difference across the length of a thermoelectric material is a dominant factor in determining the performance of a thermoelectric device. A larger temperature difference leads to a higher power output and thus, an appropriate heat sink is necessary to achieve this large temperature difference. In that point of view, integration of heat sinks with a thermoelectric device is critical in improving its performance but is very poorly investigated. In this section, some clever designs of heat sinks, not specifically designed for TEGs, but possible to be integrated with thermoelectric systems are demonstrated.

Among the various designs of heat sinks that can be integrated with thermoelectric devices, ones with PCMs have shown great premise in reducing the temperature of a heat source. PCMs are known to possess high latent heat and thus, such property has been researched widely for its potential use in heat sinks. The mechanism behind PCMs is utilizing its high latent heat when it changes phase from solid to liquid to absorb the heat from its heat source. To enhance the performance of the heat sink based on PCMs, researches to increase thermal conductivity have been widely conducted. Wang et al [89] investigate the effect of using a porous metal fiber sintered felt (PMFSF) to increase the thermal conductivity of PCMs. They use a high-power LED with a power consumption ranging from 1 to 5 W as the heat source and used red copper as the metal foam to prepare the paraffin/PMFSF composite. The thermal conductivity of the composite was enhanced from 0.2 W m⁻¹ K⁻¹ (pure paraffin) to 26.76 W m⁻¹ K⁻¹. Bayat et al [90] also attempt to increase the thermal conductivity of a PCM heat sink by adding a low percentage of nanoparticles. They only numerically simulate the effects of nanoparticles due to complexities in nanoparticle fabrication and were able to obtain enhanced thermal conductivities as shown in figure 8(a). Unlike their expectations, adding only 2% of nanoparticles shows the best performance and the performance gradually decreases with further addition of nanoparticles. Qu et al [91] also design a passive thermal management system by utilizing a metal foam saturated with PCM in a heat sink to reduce the temperature of a heat source. Similar to the work done by Wang et al [89], copper is used as the metal foam and compared to a pure PCM and pure copper basement, the foam-PCM composite was the most efficient at preventing the increase in temperature of the heat source (figure 8(b)). Despite all these efforts, the biggest problem of the PCM heat sink still stands even with an alteration- passive thermal management. Without doubt, as it changes phase, it can absorb more heat than in its solid form due to its high latent heat; however, when the PCM reaches its maximum heat capacity after some time, the surface temperature of the heat source starts to increase rapidly. Thus, thermally managing an electronic device with a PCM heat sink for a long period of time is not pragmatic and is only appropriate for a short duration of time.

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Another well-known type of heat sinks is a system composed of micro-channels with a circulating fluid. First proposed by Tuckerman and Pease [93], a liquid-cooled heat-exchanger design by using fluid channels in microscopic dimensions allows the operation of circuits with power densities higher than 1000 W cm⁻². The idea behind micro-channels is to use numerous separate channels, rather than a single channel to flow over the surface of the heat source and to increase the surface area of the heat sink that the fluid makes contact with. Rajabifar [92] adds an additional channel on top of the bottom channel, ultimately designing a double layered counter-flow micro-channel with nanofluids and PCM slurries as the coolants as shown in figure 8(c). With different types of fluid employed in the upper and the lower channel, the heat flux absorbed by the heat sink is the greatest when the nanofluids and the PCM slurries are deployed in the upper and lower channel, respectively.

Different from heat sinks with PCMs or micro-channels mentioned above, not much research has been done on flexible heat sinks as their demand has been low. There have been several works [26, 72, 94] which adopt a flexible heat sink on a thermoelectric generator for body heat harvesting. Although the performance of body heat harvesting has been enhanced with the existence of a flexible heat sink, those research have aimed more on the flexible thermoelectric system rather than flexible heat sink. The demand for flexible heat sinks with sustainability and high performance is expected to be increased for broader applications.

To operate an IoT system integrated with WSNs, management of the harvested energy is as important as energy harvesting. The management is mainly focused on the design of a DC–DC voltage converter which boosts up harvested voltage, normally ranging from a few mV in body heat harvesting to several hundred mV-level in industry application, to a usable level for operating with high efficiency. Conventional DC–DC converters have a two-stage circuit [66], boost converter and a buck converter. Chen et al [95] demonstrate a one-stage DC–DC converter in which the input voltage is 100 mV and the output voltage ranges from 500 to 600 mV. Owing to the capacitive bootstrapping technique, the proposed DC–DC converter shows a maximum conversion efficiency of 76.4% over various loads ranging from 1 to 500 μW. Luo et al [96] demonstrate a power converter for a thermoelectric generator with low input voltage integrated with a self-start circuit. Once the chip self-starts at 210 mV, only 7 mV input voltage is required to continuously operate the main converter. The proposed power converter shows a maximum output power of 229 μW with 73.5% end-to-end efficiency. V et al [97] also demonstrate a voltage converter with a single-stage regulator. Since the proposed converter is aimed to be driven by human body heat, it possesses low input voltage ranging from 25 to 210 mV. The converting system shows maximum efficiency of 65% with a peak delivering power of 1.03 mW.