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Power (μW); Number of thermocouples . 1500 1000 500 0 n/2 l /t 0.001 0.01 0.1 1 10 100 Length of thermocouple legs (mm) interface area of 10 × 10 μm 2 is assumed (fixed) while t decreases to 2 μm at l = 1 μm. (On practice, this is done by using microelectronic technologies [2].) At l = 1 μm, the resistance still increases by a factor of 4 as compared with 1.5 cm-long thermopile. If the contact resistance could be decreased to 10 Ω μm 2 , it would allow reaching over 200 μW at l = 10 μm (i.e., about 64% of the power obtainable at l = 15 mm), and 180 μW at l = 3 μm. Calculations show that the thermal resistance of the thermopile and the temperature drop on it stay almost the same over the whole range for a thermocouple length, Fig. 5, unless the enlargement of electrical contacts is performed at l < 10 μm. This enlargement also adversely affects the parasitic thermal resistance R empty at l < 10 μm, Fig. 5, and demands reoptimizing the thermal resistance of a thermopile using the equation of thermal matching. To compensate for some decrease of ΔT at l < 10 μm due to enlargement of electrical contacts, a number of thermocouples, equal to n/2, is increased, Fig. 6. The ratio l / t shown in Fig. 6 demonstrates that the length of thermocouple legs in the range of 5-10 μm allows reaching relatively high power using microelectronic technologies. The related research is ongoing and 6 μm-tall structures have been already fabricated, Fig. 7 [3]. However, the structures are based on poly-SiGe that shows far worse thermoelectric properties than BiTe used in the modeling above, i.e., its factor Z is about 0.05-0.1, at least by a factor of 10 less. Therefore, one more step is required for reaching the calculated targets, i.e., development of the similar technological process, but based on materials with higher Z. P 7-9 October 2009, Leuven, Belgium 100 VI. APPLICATION OF THERMAL MATCHING IN WEARABLE MEDICAL DEVICES 10 1 0.1 Fig. 6. Optimum number of thermocouples, l / t ratio and maximum power in a wearable TEG of 3 cm × 3 cm × 1.7 cm size at a contact resistance of 100 Ω μm 2 between semiconducting legs and metal interconnects. Al p-SiGe n-SiGe 3 μm Fig. 7. Arcade thermopile: the design and fabricated thermopile-like test structure with 6 μm-tall released air bridges [3]. l / t ratio Small-size thermopiles available on the market do not fit the requirements for the thermocouple leg length coming out of the modeling of an optimal thermopile because their l / t ratio is much smaller than needed. An appropriate aspect ratio then can be obtained by stacking thermopiles on top of each other [4, 5]. This increases R TEG to the required R TEG,opt and hence allows reaching the maximum of output power. Before fabricating prototypes of body-powered medical devices, the principles of thermal matching have been verified on watch-size wrist TEGs [4, 5]. Then, one of such TEGs has been used for powering a wireless pulse oximeter (SpO 2 sensor) [6], Fig. 8a. The device non-invasively measures oxygen content in arterial blood. A TEG used in this device provides a power of about 200 µW on average with usual variations within the 100-600 µW range. Typically, battery-powered pulse oximeters existing on the market consume above 10 mW. Therefore, before making it powered from the human body, a power consumption of electronics has been reduced by a factor of 10 3 . As a result, the electronics module and 2.4 GHz wireless link together consume 62 µW. The device is battery-less and operational up to 25-26°C. At higher ambient temperatures, the power shortages are expected. If this happens, the device switches into a sleeping regime for a while. During sleep, its power consumption is extremely low and the device wakes up again upon collecting enough charge in the supercapacitor. Another example of body-powered battery-less devices is an electroencephalography (EEG) system-in-a headband [7] consuming 0.8 mW. The main challenges in creation such complex system powered by the wearer’s heat are (i) lowering power consumption of biopotential readout while maintaining the signal quality and (ii) real-time data transmission. In the above pulse oximeter, the signal processing is performed onboard, so that power consumed by the transceiver is minimal. In EEG, on the contrary, realtime brain waveform should be transmitted, so the radio consumes large power, and a large-size TEG is required. Therefore, it is designed as 10 sections of 2×4 cm 2 size on a stretchable headband, Fig. 8b. Radiators on the outer side of thermopiles ensure effective thermal matching of the TEG with the environment. The TEG is designed for indoor use at typical ambient temperatures maintained in hospital wards. At 22°C, it produces about 30 µW/cm 2 , i.e., close to the limit of power on people at this temperature. There is however a drawback of such high power generation: at lower ambient temperatures, the heat flow rapidly exceeds the sensation of discomfort and the device turns into uncomfortably cold object. (For example, at 19°C, the TEG already produces 3.7 mW, but sensation of cold becomes too annoying.) To avoid sensation of cold, another version of the EEG system, a headphone-like EEG diadem, Fig. 8c, has been provided with a hybrid thermoelectric-photovoltaic energy harvester, [7]. This device is comfortable down to 7°C. The electrocardiography (ECG) system-in-a-shirt, Fig. 8c, unlike above devices, is powered from a secondary battery [8]. The battery is constantly recharged using the wearer’s body heat. The power consumption is 0.5 mW, so about 0.7 mW are required from the TEG. Fourteen 6.5 mm-thin ©EDA Publishing/THERMINIC 2009 99 ISBN: 978-2-35500-010-2
Fig. 8. Low-power wireless medical sensors powered by thermally matched thermoelectric generators: (a) a pulse oximeter, (b) EEG headband with 2.5 mW-TEG, (c) a person wearing an EEG diadem with hybrid power supply, an SpO 2 sensor, ECG system-in-a-shirt, and a TEG with micromachined poly-SiGe thermopile (still produces very small power). TEG modules of 3 cm × 4 cm size have been integrated into the front side of shirt. The radiators of TEG modules have been painted like chameleon into the shirt colors, except one module that is to show the module size. The wiring and the other modules of ECG system are located on the inner side of the shirt. In the office, the TEG typically generates the power of 0.8-1 mW. Because of high thermal resistance of thermally matched TEG modules, they are never cold. In cold weather, the other pieces of clothing are worn on top of shirts. However, as measured at about 10°C outdoors on a person wearing a thick jacket, the power typically does not decrease. The power management module contains a fully integrated DC/DC upconverter that charges a 2.4 V NiMH battery. The converter contains a charge pump with variable number of stages and switching rate, and therefore operates with near-maximum efficiency. In parallel to the TEG power circuit there is a secondary parallel circuit that allows charging the battery directly from solar cells. Two amorphous silicon solar cells of 2.5 cm × 4 cm size each have been integrated into the shirt on its shoulders. Solar cells are added to the system because if the shirt is not worn for months, the battery can be discharged. Therefore, when the shirt is taken off and not used for a long time, it must be stored in an environment where light is available periodically, e.g., in a wardrobe with windows. The small power provided by solar cells is enough to compensate for the self-discharge of the battery and for the standby power. In this way, even after months of non-use, the electronics is maintained in the ready-to-start state, waiting for the moment the shirt is used again. The system components, i.e., a TEG, solar cells and electronics in a flex circuit, have waterproof encapsulation and sustain machine washing with drying cycle at 1000 rpm. If the voltage from the TEG drops to near-zero, which happens when the shirt is taken off, the system switches into a standby regime with 1 μW power consumption. The self-start of the system takes place within a few seconds while the shirt is being put on again. 7-9 October 2009, Leuven, Belgium VII. CONCLUSION The thermoelectric theory does not describe how to perform design optimization of a thermopile in energy harvesters. Therefore, all the works on micromachined thermopiles for harvesting low-grade heat waste have resulted in thermopile samples producing power insufficient for a majority of practical applications. The reasons for that are relatively high thermal resistance of the environment, and variable both heat flow and temperature difference on the thermopile under optimization. However, according to the literature, the optimizations were always conducted at a constant ΔT, i.e., in a quite different regime. As discussed in this work, a new approach based on electro-thermal analogy helps to find design optimum. This optimum is described by the thermal matching of a thermopile to the environment. The optimum is located just between the two regimes discussed in the thermoelectric theory, i.e., the regime of constant heat flow and the regime of constant temperature difference. The latter allows highest thermoelectric efficiency however as has been shown in this work, the power maximum takes place at about a half of efficiency. The method of thermal matching discussed in this paper has been successfully used in wearable wireless medical sensors: a pulse oximeter, EEG systems and the ECG system integrated into a shirt. ACKNOWLEDGMENT The work has been performed in 2005-2009 within the internal Human++ program at IMEC and Holst Centre on wearable wireless sensor networks. REFERENCES [1] V. Leonov, “Thermal shunts in thermoelectric energy scavengers,” Journal of Electronic Materials, vol. 38, no. 7, pp. 1483-1490, 2009. [2] Z. Wang, V. Leonov, P. Fiorini, and C. Van Hoof, “Realization of a wearable miniaturized thermoelectric generator for human body applications,” Sensors and Actuators A: Physical, 2009, (in press), DOI: 10.1016/j.sna.2009.02.028. [3] J. Su, R. J. M. Vullers, M. Goedbloed, Y. van Andel, R. Pellens, C. Gui, V. Leonov, and Z. Wang, “Process development on largetopography microstructures for thermoelectric energy harvesters,” Proc. 8th PowerMEMS + microEMS Workshop, Sendai, Japan, November 9-12, 2008, pp. 365-368. [4] V. Leonov, T. Torfs, N. Kukhar, C. Van Hoof, and R. Vullers, “Small-size BiTe thermopiles and a thermoelectric generator for wearable sensor nodes,” Proc. 5 th Eur. Conf. on Thermoelectrics, Odessa, Ukraine, September 10-12, 2007, pp. 76-79. [5] V. Leonov, T. Torfs, P. Fiorini, and C. Van Hoof, “Thermoelectric converters of human warmth for self-powered wireless sensor nodes,” IEEE Sensors J., vol.7, no.5, pp. 650-657, 2007. [6] T. Torfs, V. Leonov, and R. Vullers, “Pulse oximeter fully powered by human body heat,” Sensors and Transducers Journal, vol. 80, no. 6, pp. 1230-1238, 2007; http://www.sensorsportal.com/ HTML/DIGEST/P_151.htm. [7] M. Van Bavel, V. Leonov, R. F. Yazicioglu, T. Torfs, C. Van Hoof, N. E. Posthuma, R. J. M. Vullers, “Wearable battery-free wireless 2-channel EEG systems powered by energy scavengers,” Sensors & Transducers Journal, vol. 94, no. 7, pp. 103-115, 2008; http://www.sensorsportal.com/HTML/DIGEST/P_300.htm. [8] V. Leonov, T. Torfs, I. Doms, R. F. Yazicioglu, Z. Wang, C. Van Hoof, and R. J. M. Vullers, “Wireless body-powered electrocardiography shirt,” Proc. 3 rd European Conf. Smart Systems Integration, Brussels, Belgium, March 10-11, 2009, VDE VERLAG GMBH: Berlin, T. Gessner, Ed., pp. 307-314, 2009. ©EDA Publishing/THERMINIC 2009 100 ISBN: 978-2-35500-010-2
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