Self-powered Wearable Electrocardiography Using a Wearable

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Self-powered Wearable Electrocardiography Using a Wearable Thermoelectric Power Generator Choong Sun Kim, Hyeong Man Yang, Jinseok Lee, Gyu Soup Lee, Hyeongdo Choi, Yongjun Kim, Se Hwan Lim, Seong Hwan Cho, and Byung Jin Cho ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01237 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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ACS Energy Letters

Self-powered Wearable Electrocardiography Using a Wearable Thermoelectric Power Generator

Choong Sun Kim1,§, Hyeong Man Yang2,§, Jinseok Lee1, Gyu Soup Lee1, Hyeongdo Choi1, Yong Jun Kim1, Se Hwan Lim3, Seong Hwan Cho1 and Byung Jin Cho1,2* 1

School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea 2

Engineering Research Center (ERC) for Flexible Thermoelectric Device Technology, Daejeon 34141, Republic of Korea

3

Tegway Co. Ltd., #711 National Nano Fab., 291 Daehak-ro, Yuseong, Daejeon 34141, Republic of Korea

§: Both authors contributed equally to this work

Corresponding Author:

Prof. Byung Jin Cho, Department of Electrical Engineering, KAIST, 291 Daehak-Ro, Yuseong, Daejeon, Republic of Korea Tel.: +82 42 350 3485 Fax: +82 42 350 8565 Email: [email protected]

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Abstract A self-powered wearable electrocardiography (ECG) system is demonstrated. The ECG sensing circuit was fabricated on a flexible PCB and powered by a wearable thermoelectric generator (w-TEG) using body heat as the energy source. To allow the TEG to obtain a large temperature difference for high power generation and also be wearable, a polymer-based flexible heat sink (PHS) comprised of a super-absorbent polymer (SAP) and a fiber which promotes liquid evaporation was devised. Parametric studies on the PHS were conducted, and the structure of the w-TEG was also optimized for the PHS. The output power density from the w-TEG with the PHS was over 38 µW/cm2 for the first 10 minutes, and over 13 µW/cm2, even after 22 consecutive hours of driving the circuits. This power level is high enough to continuously drive the wearable ECG system, including the sensors and the power management circuits.

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With the emergence of the Internet of Things (IOT) era, wearable applications using flexible electronic devices have already had an impact on lives in a variety of ways, especially on the use of various types of wearable medical sensors.1-5 For instance, electrocardiography (ECG), which provides useful information about the cardiovascular system, is an important wearable application.6-8 By allowing continuous monitoring of an ECG signal over a period of time, the wearable device can impact human health, because the acquired statistical data can notify a pathological condition. For this kind of application, a self-powered wearable ECG system using a semi-permanent energy harvester is very useful, as it significantly reduces inconvenience to the user. Several types of power sources have been proposed for the semi-permanent energy harvester, including solar cells9,10 and vibration-based energy harvesters11,12. A wearable thermoelectric generator (w-TEG) has also been considered a particularly attractive energy-harvesting device, due to its durability and ease of use13-19. For a self-powered ECG system to use a w-TEG, the w-TEG must be able to generate sufficient electric power from body heat to drive not only the wearable ECG sensors but also a voltage booster and regulator circuits (often called PMIC – power management integrated circuits). In addition, in order to minimize the power loss by the PMIC, input voltage to the PMIC, which is the output voltage of TEG, must be at least of several tens of mV or higher. One of the most critical factors that must be considered to achieve such high and stable wTEG output is the temperature difference across the w-TEG. For human body applications, the obtainable temperature difference across the device is severely limited by the very large thermal resistance of the surroundings, and the small difference between body temperature and the typical room temperature.20-22 The most common way to reduce thermal resistance is to use a metal heatsink, in which several metal pins are fixed to a thick base metal. In use, the 3



metal heatsink is attached to the cold side of the thermoelectric generator. However, for the metal heatsink to accomplish sufficient heat exchange with air, it must have a large enough surface area, which inevitably makes it heavy and bulky. Furthermore, the inflexible nature of a metal heatsink makes it unsuitable for human body applications. Instead, a completely new type of heatsink is required for wearable devices. In this work, we developed such a device, a polymer-based flexible heatsink (PHS), where the polymer particles contain water and the water slowly evaporates over time. The proposed w-TEG fitted with the PHS generated a high output power density of above 38 µW/cm2 for the first 10 minutes of operation, as compared to a w-TEG with a metal heatsink, which generated about 8 µW/cm2. The power density of the device with PHS decreased with time due to the loss of water by evaporation. However, even after 22 hours of continuous operation, the output power of the w-TEG with PHS was still 65% higher than the one with a metal heatsink. We also optimized the structure of the w-TEG to ensure maximum power generation while using the PHS. Finally, a wearable ECG was fabricated on a flexible printed circuit board (f-PCB) to verify that the w-TEG with PHS could actually drive the wearable ECG system. The wearable ECG sensors and peripheral electronic devices were stably operated by the power supplied by the w-TEG, and provided clear ECG signals. Figure 1a illustrates the overall schematic diagram of the self-powered wearable ECG system. As shown in the figure, the main components of the wearable ECG system are a wearable ECG module that can be wrapped around a wrist, a w-TEG for real-time power generation, and a flexible heatsink to create a suitable temperature difference across the wTEG. In this experiment, a commercially available PMIC was used, since the design of a new PMIC was outside the scope of this work. The PMIC adjusts the output voltage of the w-TEG to a level required to drive the ECG sensor module, and in this experiment it was connected 4


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externally to a separate board. However, it can also be integrated into the f-PCB with the ECG module, when a dedicated PMIC is designed for commercialization. Figure 1b shows a photograph of the actual wearable ECG sensor. To measure an ECG signal using the ECG sensor, at least two electrodes should be attached to two parts of the body, on opposite sides of the heart. To obtain signals from opposite sides of the heart, the ECG sensor was manufactured in the shape of a bracelet using an f-PCB. The first electrode is located on the top side of the f-PCB and makes contact with a right hand finger, and the second electrode is connect to the left wrist, so that the wearable ECG sensor can simultaneously obtain signals from opposite sides of the human body. The other electrode, named the body driver, is also located on the bottom side of the f-PCB to reduce the large common-mode interference of 50/60 Hz noise from body. Figure 1c shows a photograph of the w-TEG. As illustrated in the inset in the Figure 1a, the fabricated w-TEGs were made of thermoelectric legs connected electrically in series and thermally in parallel, similar to a conventional thermoelectric device. Details of device fabrication are described in Supporting Information. To make the device flexible, the rigid ceramic plates were removed and the empty space between the top and bottom sides was filled with a proprietary polymer material, which has very low thermal conductivity (0.03 W m-1K-1)23. For fabrication, commercially available thermoelectric legs with an area of 2.0 mm2 and heights of 2.5 mm were used. All of the fabricated w-TEGs had a square shape with an area of 40 cm2. The thermoelectric properties of the TE legs, the device ZT and the resistance of the entire fabricated device are shown in Tables S1 and S2 in the Supporting Information. To enhance the power generation efficiency of the w-TEG, a polymer-based flexible heatsink (PHS) shown in Figure 1d was developed. The PHS consists of an inner part containing a super-absorbent polymer (SAP), which can store a large amount of liquid, like 5



water, and an outer fabric (Ventex Co. Ltd.) which dissipates heat by evaporating the liquid. For the SAP, the commercially available cross-linked sodium polyacrylate (LG Chemical Co., CAS No.: 9003-04-7) was used. The details about fabrication process of the PHS is described in Figure S1. Sodium polyacrylate, the base material for the SAP has a long, chained molecule with many repeating units and it has a portion that holds an electrical charge.24 When the PHS is dipped in water, the water drawn into the polymer due to the osmotic pressure, the electric charges on the polymer attract water molecules and bind them to the polymer. Therefore, it can contain a lot of water and it is possible to keep the moisture even under a light pressure. When the PHS is attached to the w-TEG on skin, its temperature slowly reaches steadystate, due to the large heat mass of the water contained in the SAP. This behaviour is different from a metal heatsink which quickly reaches steady-state. During this transient period, a large temperature difference can be obtained across the w-TEG if the initial temperature of the PHS is lower than the core temperature of the human body. After it reaches steady-state, the evaporation of the liquid from the outer fabric allows absorbed heat to be continuously released to the ambient, which in this design causes a temperature difference across the wTEG that is sufficient to operate the wearable electronics. Figure 2a shows the open-circuit voltage of the w-TEG with the PHS under natural convection. For this test, the w-TEG with the PHS was attached to an artificial arm which mimicked the skin temperature and the thermal resistance of a human arm. Details of the measurement set-up can be found in the ‘Experimental details’ section in Supporting Information. As shown in the results, the voltage output decreased continuously as the temperature of the PHS gradually increased. Despite the decrease in voltage during this period, over 80 mV and 53 mV of voltage output was generated for the first 10 minutes, and 1 hour of operation, respectively. The output power 6


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density as a function of the operating time is represented in Figure 2b. The w-TEG generated a power of above 38 µW/cm2 for the first 10 minutes and maintained power above 15 µW/cm2 until an hour later. That means the w-TEG with an area of 40 cm2 can generate above 1.5 mW for the first 10 minutes after operation. A metal heatsink was also fabricated for comparison. To make the metal heatsink flexible, a very thin (0.25 mm) base panel was used. As illustrated in Figure S5 in the Supporting Information, unlike the PHS, the voltage output of the w-TEG reached the saturation voltage (≈ 37 mV) in about 3 minutes after being attached to the artificial arm. The w-TEG with the metal heat sink generated an output power of 8 µW/cm2 in the steady-state. Therefore, the w-TEG with the PHS was able to generate about 5 times (at 10 min) and two times (at 60 min) more power than the metal heat sink. Figure 2c shows the voltage output measurement results for the w-TEG with the PHS after 24 hours. It reached the steady-state after about 5 hours and maintained a high output level until about 22 hours of continuous operation, which is long enough for practical usage. Resupplying water to the PHS will reset the situation, and the high power initial stage generation starts again. In the transient period, the output characteristics of the w-TEG depends on the amount of super-absorbent polymer (SAP) in the PHS, as shown in Figure 2d. As the amount of SAP decreases, the total heat mass of the PHS becomes smaller, which means that the PHS reaches the steady-state more quickly. In other words, the time to steady-state increases with the amount of SAP. Figure 2e shows the voltage output for different initial liquid temperatures in the PHS. The initial temperature of the liquid determines the initial output level of the transient state. A lower initial temperature results in a higher initial voltage output, as it provides a greater cooling effect, but the difference made by the liquid temperature is not too 7



significant, as can be seen in Figure 2e. Furthermore, when it approaches steady-state, the difference becomes negligible. Because people can move around while wearing a wearable electrocardiograph device, the w-TEG will experience forced convection by airflow. Human walking can cause forced convection with an air velocity of 1 m/s ~ 3 m/s. The level of power generation under that range of forced convection is also an important consideration. Figure 2f shows the voltage output of the w-TEG when the air velocity was changed from 0 m/s to 3 m/s. As shown in the figure, in the initial stage, the output voltage levels of the w-TEG are not much different. This is because at this stage, the cooling of the w-TEG is primarily due to the temperature of the liquid itself. However, as the system approaches steady-state, and the w-TEG experiences forced convection, the higher air velocity generates higher voltage outputs. The heat dissipated by the evaporation can be represented as q = hwe [(25 + 19v ) A( χ s − χ ) / 3600] , where hws is the evaporation heat of water, v is the velocity of air above the surface, A is the

area of the surface, χ s is the humidity ratio of saturated air, and χ is the humidity ratio of air. In this case, as the velocity of the air increases, the evaporation on the PHS surface dissipates more heat from the PHS, and as a result the temperature of the cold side of the wTEG in steady-state decreases. The lower temperature on the cold side of the w-TEG increases the output of the w-TEG in steady-state. In order to generate enough power to drive self-powered wearable electronics, the structure of the w-TEG must be also carefully designed. According to the steady-state model for the power output of a thermoelectric generator using body heat, the output performance is determined by many factors, including the temperatures of the ambient and body core, the thermal resistance of skin, thermal resistance of the ambient-device interface, thermal contact 8


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between the device and skin, the thermal resistance of the TEG and so on.20-22 Among those components, the thermal resistance of the w-TEG can be easily modified by adjusting the device fill factor of the device, which is the ratio of the total area of the TE elements to the area of the whole device. When using the PHS, the thermal resistance of the ambient-device interface is greatly reduced, and then the thermal resistance of the w-TEG becomes one of the critical factors determining the temperature difference across the w-TEG. Therefore, the fill factor of the device will have a significant effect on the output power. Figure 3a shows the experimental results of the output voltage of w-TEGs with different fill factors while using the PHS. The voltage values here are the open-circuit voltage monitored 10 min after being attached to the human body. The output voltage is 142.6 mV when the fill factor is 38%, but it decreases to 80.2 mV when the fill factor becomes 10%, because the number of couples is reduced from 200 to 50. However, as shown in Figure 3b, the power density increases with the decrease in fill factor because the thermal resistance of the device is reduced and the voltage from each of the TE legs increased, due to the larger temperature difference across the legs. For instance, at 10 min after being attached to the human body, the w-TEG with a 10% fill factor produced a power of 38.6 µW/cm2, which is about 25% higher than that of the wTEG with 38% fill factor (30.8 µW/cm2). Another important structural parameter determining the output power of the w-TEG is the thickness of the copper electrode. Unlike a conventional TEG, the thickness of the wearable TEG copper electrode must be limited, because a thick copper electrode would make the device inflexible. The use of a thin copper electrode results in an increase in the electrical resistance of the electrode, which can be non-negligible compared to the total electrical resistance of the device. As represented in Figure 3c, when the electrode thickness was changed from 70 µm to 35 µm, the ratio of the copper electrode to the total increased from 9



9.8% to 17.7% (where the fill factor = 10%, the electrical contact resistance between the thermoelectric legs and electrodes = 10-6 Ω·cm2, and the width of the electrodes is equal to twice of the thermoelectric leg width). Figure 3d shows the calculated results for the normalized power density of the w-TEG using body heat under natural convection (heat transfer coefficient = 10.5 W/m2K) as a function of the thickness of the copper electrode. The calculation was based on a 1-dimensional heat transfer model for the w-TEG; the formulas are specified in the Supporting Information. As shown in the results, the power density decreases about 7% when the copper thickness is reduced from 70 µm to 35 µm. Figures 3e and 3f show the experimental results for the voltage and power output from the fabricated w-TEGs with the two different copper electrode thicknesses of 35 µm and 70 µm. The w-TEG with 70 µm thickness copper film generated about 4~6% higher voltage than the 35 µm thick electrode. In case of power density, similarly, a higher power density of about 7~11% was obtained when the 70 µm thickness copper film was used. Although the use of a thicker electrode improves the output performance of the w-TEG, it also has an adverse effect on the flexibility of the w-FEG. Therefore, the thickness of the electrode must be determined considering the minimum required bending radius for the particular application. In our experiments, the w-TEGs with 70 µm thick electrodes exhibited an enough bending radius to use the w-TEGs on a human body. Therefore, all the measurement results in this work were made with 70 µm thick electrodes, unless otherwise specified. On the other hand, it should be noted that the output voltage generated from the w-TEG using body heat is only several mV to tens of mV. This means that power management circuits including a voltage boost converter are indispensable to drive an electronic system like the ECG.25-30 And typical voltage boost circuits require more than tens of mV of cold10


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start voltage. The conversion efficiency of the power management circuit also becomes low when the input voltage is low.31, 32 Therefore, w-TEG must be able to generate not only high power output but also sufficiently large voltage output to stably drive wearable electronics. Figure 4 shows a comparison of the performances of wearable thermoelectric devices using body heat. All output values of the thermoelectric generators shown in Figure 4 were measured at room temperature, and the measurement conditions for each experiment are given in Table S4. As seen in the figure, our w-TEG with PHS provides outstanding performance both in terms of output voltage and output power. The high power efficiency and high voltage output of our w-TEG allows both a PMIC and a wearable ECG to be driven stably. Figure 5 shows a block diagram of the overall ECG operating system. In the first stage, a voltage of about 40 mV-100 mV is supplied from the w-TEG attached to the left arm. After that, through a boost converter (LTC3108), the output voltage of the w-TEG is boosted to 3.3 V, which is one of the selectable output levels from the boost converter. Since the ECG module and data acquisition (DAQ) buffer operate at 1.0 V, the 3.3 V voltage should be dropped to 1.0 V through the voltage level shifter. If the w-TEG supplied about 1 mW of power, since the efficiency of the LTC 3108 is very low, the boost converter consumes about 80% of the power. The remaining power, excluding the power consumed by the boost converter, is consumed again through the VDD shifter. As a result, approximately 70 µW of the power remains to drive the ECG module. The power consumed by the ECG module and DAQ buffer is less than 15 µW, so the proposed system can be driven with just the power supplied by the w-TEG. At present, the PMIC power consumption is the largest in the system. If the PMIC is customized for a TEG, the PMIC efficiency will increase to 60%~70% and the VDD shifter becomes unnecessary.32 If the customized PMIC is used, hundreds of µW of 11



residual power will be remain. On the other hand, recently developed low-power Bluetooth modules consumes only a few µW to tens of µW of power.33 Then, the required electronics for a wearable ECG including PMIC, ECG, and Bluetooth module can be sufficiently driven by harvested power from the w-TEG. In addition, if a high-efficiency PMIC is developed, the size of the w-TEG can be further reduced, which would make it easier to wear the w-TEG. Figure 6a shows a photo of the actual experimental set up used to drive the wearable ECG module. The w-TEG with the PHS wraps around the forearm and generates electricity, which drives the flexible ECG via the boost converter and VDD shifter, as shown in the photo. Figure 6b shows the voltage of the charge capacitor in the power management circuit as the wearing time of the w-TEG increases. The capacitor was fully charged after 200 seconds. Because the power supplied from the w-TEG was sufficient, a voltage level of 3.3 V from the boost converter was maintained after the starting operation of the ECG module. When the charge capacitor is fully charged, the ECG sensor is ready for operation, and the ECG signal is generated by placing a finger of the right hand on the ECG electrode. In this demonstration, a clear ECG signal was obtained, as represented in Figure 6c. The work presented here confirms that the w-TEG with the PHS is practically usable as a power source for wearable electronics. We demonstrated a self-powered wearable ECG system consisting of a wearable ECG, wearable TEG and a novel polymer heat sink (PHS). To the best of our knowledge, this work is the first demonstration of a wearable TEG successfully driving a wearable ECG with sufficient power and voltage output. The results in this work can advance the commercialization of self-powered wearable electronics, especially for self-powered medical applications using a wearable thermoelectric power generator.

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Supporting information. Fabrication process of the devices, experimental details and development of the formulas of the w-TEG on human body. Additional images of SAP and output fabric, analysis data, properties of the used thermoelectric materials, and measurement condition in Figures S1-S5 and Tables S1-S4.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133).

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ACS Energy Letters

Figure 1. (a) Overall schematic diagram of a self-powered wearable ECG system (b) Photograph of the flexible ECG module. (c) Photograph of the w-TEG. (d) Photograph of the PHS.

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Figure 2. Output performance of the w-TEG (a) Open circuit voltage output vs. operating time in the transient period. (b) Power density vs. operating time in the transient period. (c) Power density of the w-TEG during 24 hours of operation. (d) Open circuit voltage vs. operating time with different amounts of SAP. (e) Open circuit voltage vs. operating time with different initial temperatures. (f) Open circuit voltage vs. operating time with different air velocities.

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ACS Energy Letters

Figure 3. Output performance according to the w-TEG design. (a) Open circuit voltage vs. the fill factor of the w-TEG. (b) Power density vs. the fill factor of the w-TEG. (c) Resistance ratio according to the copper electrode thickness. (d) Normalized power density vs. copper electrode thickness of the w-TEG. (e) Open circuit voltage vs. operating time. (f) Power density vs. operating time.

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Figure 4. Benchmarks of output performance of wearable w-TEGs.

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ACS Energy Letters

Figure 5. Block diagram of ECG operating system.

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Figure 6. ECG in operation. (a) A photograph of the ECG system including the w-TEG, PHS, wearable ECG module, boost converter, VDD shifter and DAQ. (b) The voltage of the power capacitor according to the wearing time of the w-TEG. (c) The displayed ECG signal.

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Self-powered Wearable Electrocardiography Using a Wearable

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