# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 4 In Vivo Application
### In Vivo Measurement Result  
Figure 9 shows the anesthesia and in vivo experiment photos. An isoflurane anesthesia machine from RWD Life Science(tm) (San Diego, CA, USA) was used for the anesthesia of mice, and the oxygen flow rate was set to 1 L/min during isoflurane anesthesia, and the isoflurane concentration was maintained at 3%. Figure 9a shows a photograph of an anesthetized rat. An anesthetic gas mask was placed over the rat's mouth to ensure continuous isoflurane inhalation. The charging terminal was exposed outside the skin to charge the implanted battery. In order to prevent foreign substances from entering the charging terminal, the charging terminal was temporarily coated with a flexible gum mask (Esthetic mask, DETAX, Ettlingen, Germany), a silicone material used for dental casts. When charging the battery, the dental silicone coating on the charging terminal is easily removed, and when recoating the charging terminal after charging is complete, it hardens easily at low temperatures. Figure 9b shows a photograph of real-time wireless body temperature measurement of a conscious rat that has recovered from anesthesia. The rat is in a cage covered with sawdust, and its red eyes are open. The rat's protruding back is visible due to the implanted MOD_IMP10. The rat's body temperature data are transmitted in real time from MOD_IMP10 to the laptop via BLE wireless communication. A graphical user interface (GUI) SW_IMP10 was developed to control MOD_IMP10 and process data received from MOD_IMP10. SW_IMP10 can control all registers for operating CH_T in NA10. For example, it can control trimming resistor value adjustment in BRG block, sample and hold time in SMP block, number of data buffering in digital block, etc. The received data are plotted in real time on the laptop screen and saved to a file at the same time. The GUI program SW_IMP10 was developed using.NET Framework 4.7.2, Aspose.Cells 24.5.2, and ZedGraph 5.1.7. Figure 8: Surgery photographs: (**a**) appearance of FFCB_ELEC10 attached to the sciatic nerve, (**b**) a rat implanted with module MOD_IMP10. Figure 10 shows the temperature changes in rats during anesthesia, anesthesia withdrawal, and awakening. Anesthesia started at point AA and lasted for 651 s to point BB. During isoflurane anesthesia, the oxygen flow rate was set at 1 L/min, and the isoflurane concentration was maintained at 3%. After 516 s of anesthesia termination at BB point, the rats woke up at CC point. Temperature was measured every 4 s and plotted using a moving average (window size = 4). When the anesthesia was initiated, the temperature began to decrease at a rate of approximately \(-0.18\)\({}^{\circ}\)C/min. The rate of decrease gradually slowed over time, and just before the anesthesia ended, the temperature decreased slowly at a rate of approximately \(-0.10\)\({}^{\circ}\)C/min. Experimental results showing temperature decrease during anesthesia have been reported previously [20; 21]. In reference [20], [PERSON] divided rats into preheated and unheated groups and monitored body temperature changes during anesthesia. The results showed that the rate of decrease in body temperature in the unheated group was approximately \(-0.2\)\({}^{\circ}\)C/min, and the rate of decrease gradually slowed over time. When the anesthesia was discontinued, the body temperature gradually increased and continued to rise to normal body temperature after awakening. During the 10 min intervals from 50 to 60 min when body temperature stabilized, the mean temperature was \(36.9\)\({}^{\circ}\)C and the standard deviation was \(0.02\)\({}^{\circ}\)C.  
Figure 10: In vivo body temperature response during the anesthesia experiment. Figure 9: In vivo measurement setup: (**a**) anesthetized rat, (**b**) real-time body temperature plot of conscious rat. Table 1 summarizes the performance of the temperature sensor and compares it with previous work. Previous works also have applications in body temperature measurement (34 \({}^{\circ}\)C to 43 \({}^{\circ}\)C).