# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 1 Introduction  
Research and development of electroeucticals has been ongoing for decades [1]. Electroceucticals is an electronic device that controls biological functions by electrically stimulating nerves. Compared to traditional chemical medicine, it has the advantages of low development costs, can be platformed, allows for personalized prescriptions, has local effects rather than whole body effects, and allows immediate cessation of medical function. The main purpose of electronic drugs is to electrically stimulate nerves, but since they are inserted into the body, they can be equipped with additional functions such as measuring core body temperature. Body temperature is the most essential and basic physiological indicator. Body temperature rise is a phenomenon that accompanies many inflammatory diseases and health disorders. Core body temperature measurements can also monitor the extent of inflammatory responses induced by implanted devices. Some reports have shown that inflammatory responses can even be induced by implanted devices years after they are implanted [2]. Even in normal conditions, the body temperature fluctuates periodically according to the ovulation cycle. In this way, body temperature measurements can be utilized to monitor the condition of diseases or health. The following previous studies are examples of attempts to measure local body temperature using implantable devices in the brain, kidney, lung, and hip joint. [PERSON] stated that intracranial pressure and temperature information are essential for treating traumatic brain injury, and used optical sensors to measure them [3]. [PERSON] stated that local internal body temperature measurement is important for diagnosing abnormalities related to immune response and accurately managing the effects of hyperthermia and hypothermia therapy [4]. [PERSON] also measured the intracranial temperature of rats and showed that it is different from the body core temperature, suggesting the need for local body temperature measurement. [PERSON] used local temperature measurement for early detection of renal allograft rejection [5]. Local temperature was measured as a change in the resistance of a thin gold pattern, and the early warning signs of renal allograft rejection were detected 3 days to 3 weeks earlier than those detected by serum measurements. [PERSON] developed a composite material whose resistance changes sensitively with temperature changes and applied it to measure the temperature of the lungs of rats [6]. He confirmed that the lungs of warm-blooded animals maintain a remarkable temperature stability despite the large difference between the core temperature and the inhaled air temperature. [PERSON] wanted to investigate the risk of heat-induced osteonecrosis and subsequent implant failure in artificial hip joints [7]. He built a temperature-measuring device into the artificial prosthesis and calibrated it in a water bath. Thus, it is necessary to measure local body temperature from devices inserted into the body for various purposes, such as infection caused by implants, early monitoring of graft rejection, frictional heat from prostheses, and research on physiological phenomena. The body temperature measuring function is an additional function of the electroeticals that operate on a battery in the body, so it requires extremely low power consumption. The power consumed in body temperature measurement can be divided into the temperature converter part and the circuit part, and each part should strive for low power. Considering the reactivity of local temperature measurement, it is desirable to mount the temperature transducer on the neural electrode rather than inside the main implantable device. Since the interface with the nerve is made of metal, a method that uses metal with the characteristic of changing resistance with temperature change as a temperature transducer can be a candidate.