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 superabsorbent polymer (SAP) and a fiber that 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 min 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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Fabricating thermoelectric generators (TEGs) using the screen-printing process has advantages, including mass production, device scalability, and system applicability. However, the thick film formed through the process typically has low film density, and reduced performance, because of the presence of pores in the film created by the vaporization of the resin during hightemperature annealing. During the soldering process used for thermoelectric module fabrication, the printed solder infiltrates into the screen-printed electrodes through the micropores in the electrodes, causing cracks of the electrode film and an increase in resistivity. In this paper, an ultraviolet radiation (UV)-curable process for screen-printed electrodes is reported. The paste for the electrodes is synthesized by mixing Ag flakes that can be cured at low temperature with a UV resin. Scanning electron microscope images show that the UV-curing process significantly reduces pores and thereby results in a smooth-surfaced electrode layer. The film density after crystallization is also enhanced. TEGs composed of 72 couples with UV-curable Ag electrodes generate a high power density of ≈6.69 mW cm −2 at a temperature difference of 25 °C; the device resistance is ≈0.75 Ω, and the figure of merit of the device is recorded to be 0.57, which is the highest among the printed TEGs.

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TEGs can be classified into two types according to their size. One is the bulktype TEG, which usually has a thickness of more than 1 mm, and the other is the micro-TEG (m-TEG), which has a height of tens to hundreds of micrometer. [17] Most commercialized thermoelectric devices developed to date are bulk-type TEGs, and are used as power sources for satellites [18] or for recycling waste heat from automobiles. [19] Another category of currently emerging TEG applications are energy harvesters for wearable devices or bio-sensors. [20,21] However, before TEGs can be used as a semipermanent energy source for such applications, the device must be sufficiently thin, in the range of tens to hundreds micrometer. For this reason, research groups are increasingly carrying out work on m-TEGs based on various manufacturing methods such as flash evaporation, [22] sputtering, [23] and electrochemical deposition. [24-26]