Professor Yongtaek Hong’s Group Developed a Skin-Patch Piezoelectric Device Producing Electric Power from Body Heat
Thomas Seebeck, born in a rich German merchant’s family in 1770, graduated a medical school and opened a hospital according to the family’s wish, but he was not satisfied with his life as a physician. Wanting to be a scientist, particularly a physicist, he spent his free time doing various experiments as an amateur physicist. One day in 1821 when he was in his 50’s, Seebeck discovered an interesting phenomenon. When two metals, copper and bismuth, were connected and the temperature of the two metals was differentiated, an electromagnetic field was generated with a current! In the article that he published two years later, Seebeck called it a ‘thermoelectric effect.’
Written by Seokgi Kang (science columnist)
Later, it was found that a greater voltage can be produced when two types of semiconductors are used instead of two metals, and devices based on the semiconductor thermoelectric effect were manufactured. For example, a spacecraft is equipped with a thermoelectric generator that utilizes the heat from the radioactive decay. When the waste heat from the automotive engines is converted to electric power, the fuel consumption can be reduced by 3%. Nevertheless, there are few products based on the thermoelectric effect, because the cost-effectiveness is not so high. Recently, however, studies are actively conducted to utilize the thermoelectric effect as a power source for wearable devices attached to human body. The ‘energy harvesting’ technology is to convert the thermal energy into electric energy by using a thermoelectric element based on the temperature difference between the human body and the surrounding. The battery, called the semiconductor of the 21st century, has been developed remarkably. However, a battery attached to a skin-patch wearable device is like a burden, and it becomes useless if it is discharged for not being charged on time. How good it would be if a wearable device may be operated by the electric power produced from the temperature difference between the skin and the surrounding! The realization of the dream is dependent upon the manufacturing of a thermoelectric element that can give a sufficiently large thermoelectric effect from the small temperature difference.
Professor Yongtaek Hong’s group from the Department of Electrical and Computer Engineering and Doctor Seungjun Park’s group from the Center for Soft Convergent Material Research of the Korea Institute of Science and Technology (KIST) successfully developed a thermoelectric element that is one step closer to the realization of the dream, and published a relevant article in Nature Communications in November last year. The researchers developed a ‘stretchable thermoelectric element’ that has a high thermoelectric efficiency and is applicable to a mass production system. The thermoelectric materials that the researchers employed was two types of bismuth-telluride (Bi2Te3) alloy-based semiconductors (n-type with moving electrons and p-type with moving holes), which are extensively used these days. The core of this study was that the thermoelectric material was combined with flexible electrodes and high-efficiency heat conductors to fabricate a thermoelectric element that can tightly attached to a curved surface, such as the human skin, in order to maximize the thermoelectric effect. The temperature difference between the human skin and the air at room temperature is about 10 °C. When the conventional thermoelectric element is used to fabricate a wearable device, the actual temperature difference applied to the thermoelectric material is significantly decreased, because a considerable heat loss occurs while the heat is transferred from the skin or the air to the electrode. The researchers replaced the existing copper electrodes with the flexible silver-nanowire electrodes that can be stretched by 20%. In addition, instead of simply filling the gap between the electrodes and the skin and the air with the thin and soft silicon (PDMS), they put silver-nickel particles under the electrodes and applied an electromagnetic field to arrange soft heat conductors that are spontaneously structured. The PDMS is flexible like a rubber, but it cannot transfer heat efficiently. On the contrary, the structured silver-nickel particles have excellent heat conductivity. Therefore, when the temperature difference between the skin and the air was 10 °C, the actual temperature difference applied to the thermoelectric material was just 5.1 °C when only PDMS was used, but it was 8.6 °C when the thin film containing the soft heat conductors was used. As a result, the acquired voltage was increased by 56%. In addition, the silver-nanowire electrodes and the silver-nickel particle/PDMS films are flexible enough to let the device be attached to a surface skin surface. If there is a space between the skin and a wearable device, the thermoelectric effect is significantly decreased due to the heat loss in the thermal transfer process.
Kitchen gloves with the flexible thermoelectric element, developed by Professor Hong’s group and a sensor connected with an LED. When holding an object with the gloves, if the temperature difference is out of a predetermined range, the LED is turned on (red ‘H’ letter) to warn the hot temperature.
The researchers developed an automated fabrication system that can plant 440 thermoelectric units (thermoelectric materials with a pair of an n-type semiconductor and a p-type semiconductor) in an area of 3.9×4.4 cm2. The flexible and highly efficient thermoelectric element may be applied to not only wearable devices that can be attached to the skin to monitor the body conditions but also other devices that are attached to an object with a temperature difference from the surrounding to acquire various kinds of information. The published article introduces such an example, which is ‘hot surface warning gloves’ equipped with a flexible thermoelectric element and a sensor connected with an LED. The gloves give a warning by switching on the LED when the temperature difference from the connected object is out of a predetermined range, that is, when the voltage is over a certain limit. The researchers presented the future goals of decreasing the electric resistance and increasing the heat conductivity of the contact surfaces further to manufacture a thermoelectric element of a maximum thermoelectric effect and apply it to commercial wearable devices and IoT devices. We are looking forward to seeing the dream-come-true in this year, the 200th year since the discovery of the thermoelectric effect.