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NatureInterface > No.03 > P081-082 [Japanese]

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Energy Transmission for Implantable Devices

KENJI SHIBA

Former Research Fellow of the Japan Society for the Promotion of Science

(Currently an assistant at the Graduate School of Frontier Sciences, the University of Tokyo)

KOHJI KOSHIJI

Professor, Faculty of Science and Technology, Tokyo University of Science


Pacemakers, implantable cardioverter defibrillators and artificial hearts are some of the implantable devices that are currently commercialized or in the research & development stage. Most pacemakers and implantable cardioverter defibrillators, which use little energy, utilize lithium batteries, which must be replaced every several years. Artificial hearts, on the other hand, consume far more energy; electricity therefore is wired into the body from an external power source. Considering that the wires create the possibility of infection through the pierced skin, and that they cause discomfort and a lower quality of life, this method leaves much to be desired. Thus, transcutaneous energy transmission, a wireless method to transmit energy through the skin using electromagnetic induction, has been regarded as the most effective method and has been researched since the 1960s. However, maintaining a constant supply of electricity and achieving high transmission efficiency were difficult because the transcutaneous transformer was easily dislocated by body movements and so forth. Here, we propose an externally coupled type of transcutaneous transformer for energy transmission. We have analyzed, designed and developed the externally coupled transcutaneous energy transmission system (ECTETS) using this transformer. This system enables us to finally realize high transmission stability and efficiency.

People naturally worry about energy transmitted through the body, even at levels as low as those transmitted by cellular phones--a mere several hundred mW. On the other hand, several W of energy is transmitted through the skin to an artificial heart. Is that really safe for the human body?

A thermal effect is one of the electromagnetic influences on biological tissue. The power of electromagnetic wave absorbed by tissue per unit mass of 1 kg (specific absorption rate (SAR): W/kg) is used to evaluate the thermal effect. In this research, we transmitted energy transcutaneously and with electromagnetic simulation, and then calculated the SAR in the tissue around the externally coupled type of transcutaneous transformer. The results showed that the SAR around the transcutaneous transformer was ca. 0.3 W/kg at maximum even when 40 W of energy was transmitted. This value was below 8 W/kg, the safety level established by the Ministry of Public Management, Home Affairs, Posts and Telecommunications of Japan. This indicated that the transcutaneous transformer is not harmful to the human body. Furthermore, a transcutaneous transformer was installed in the abdominal hypodermis of a goat and 20-40 W of energy was transmitted continuously for four months. We confirmed that there were no negative effects on the goat tissue.

As the technology to miniaturize and lighten electronic devices improves, not only artificial organs or medical devices but also communication and information devices will become wearable or, moreover, implantable. We hope the transmission technology that we developed will be adopted for such wearable or implantable devices that will be invented in the near future.

(Presented at the regular meeting on Jan. 25, 2001.)

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