# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 4 In Vivo Application
### Flexible PCB (FPCB_ELEC10)  
Figure 7 shows a photograph of the FPCB_ELEC10 that will be attached to the sciatic nerve for signal acquisition and temperature measurement. FPCB_ELEC10 consists of a neural interface part, a thermistor part, a circuit interface part, latches and latch holes for fixation to the nerve. The neural interface part consists of four 0.5 mm \(\times\) 5 mm Au electrodes arranged at 1.5 mm intervals. The enlarged image in Figure 7a shows the thermistor attached to the FPCB. The thermistor has a length of 0.6 mm, a width of 0.3 mm, and a thickness of 0.15 mm. The thermistor was soldered to the gold-plated pads and then coated with nonconductive material for insulation. A thin polyimide film slightly larger than the thermistor size was used to cover the thermistor like a roof to ensure insulation on the top surface of the thermistor. For the insulation of the side of the thermistor, the liquid polyimide was permeated through the thermistor side between the top polyimide film and the bottom FPCB by the capillary force and then cured. Liquid polyimide (HD-4100, HDMicroSystems(tm), Parlin, NJ, USA) was used and cured at 280 \({}^{\circ}\)C for 1 h. In reference [18], [PERSON] used HD-4100 to make neural electrodes and implanted them into the transected sciatic nerve of a rat for measurements. In reference [19], [PERSON] used HD-4100 to re-record brain activity and fabricate electrocortical electrodes implanted into the dura mater of the brain. In both references, HD-4100 was used as the biocompatible material for the biocompatible electrodes to verify biocompatibility. Therefore, in this study, we used HD-4100 for passivation of devices used in vivo by referring to the literature. The insulation of the thermistor was confirmed through a leakage current test. The FPCB_ELEC10 with thermistor passivation was immersed in PBS to test the current leakage. A voltage of 0 V was applied to a separate electrode immersed in the PBS solution, and a voltage of 3.3 V was applied to the metal pad connected to the thermistor. The passivation effect was verified by measuring the current using a multimeter (34460A, Keysight(tm), Santa Rosa, CA, USA). If the measured current was less than 0.02 nA, the passivation was considered successful.  
Figure 7b shows how the FPCB_ELEC10 is attached to the nerve. A 2 mm diameter wire was used as a nerve phantom instead of a real nerve. Fixing FPCB_ELEC10 to the nerve is performed by two pairs of fastening parts. The fastening part consists of a latch (LT), a latch hole (LH), and a holding pad (HP). First, wrap the nerve with FPCB so that the nerve interface part touches the nerve surface. While holding the holding pad HP1 with forceps, pull the assembly head AH1 through the latch hole LH1. The outermost width of the latch is wider than the latch hole, and the inner width of the latch is narrower than the latch hole. The latch saw blade allows the latch to move well in the direction of fixing the nerve through the latch hole but prevents it from moving in the opposite direction. Since the latch saw blades are arranged at 0.5 mm intervals, the fastening resolution in the direction of the outer surface of the nerve is 0.5 mm, and the fastening resolution in  
the direction of the nerve diameter is 0.5/\(\pi\) mm. When the nerve is sufficiently fixed, the holding pad configured for the convenience of fixation can be removed by cutting the CC1 and CC2 parts in Figure 7b. Similarly, the unnecessary protruding long part of the assembly head can be cut for the convenience of fastening. This fastening method provides more convenience than the conventional method of tying with surgical thread.