# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 5 Discussion  
The ROIC for temperature sensors is fabricated using a commonly available 180 nm process. It does not include components such as analog amplifiers or ADCs. It is an architecture that can be easily converted between processes, consisting of switches, inverters, and basic digital components. Therefore, it can also be fabricated using more advanced processes. However, there is no reason for the body temperature sensor ROIC to perform high-speed signal processing, so there is no need to manufacture it using an expensive, high-tech process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **This Work** & **TECAS’19**[22] & **SSCI’20**[23] & **TCAS-I23**[24] & **TVLST’24**[25] \\  & & **[PERSON]** & **[PERSON]** & **[PERSON]** & **[PERSON]** \\ \hline Transducer type & Thermistor & Thermistor & On-chip resistor & Thermistor & On-chip resistor \\ \hline Transducer resistance [\(\Omega\)] & 100 k & 600 k & 600 k & 100 k & 66 k \\ \hline Transducer driving & Dynamic & Static & Dynamic & Static & Static \\ \hline Architecture & Virtual Wheatstone bridge, VCO, TDC & \begin{tabular}{c} Direct temperature to digital converter based on \(\Delta\Sigma\) modulation \\ \end{tabular} & \begin{tabular}{c} Wheatstone bridge, \(\Delta\Sigma\) \\ LIF, VCO, TDC \\ \end{tabular} & \begin{tabular}{c} Wheatstone bridge, \(\Delta\Sigma\) \\ modulator \\ \end{tabular} \\ \hline Fabrication technology [nm] & 180 & 130 & 180 & 180 & 180 \\ \hline Supply voltage [V] & 0.6, 1.8 & 0.9 & 1.6 & 1.2 & 1.5 \\ \hline Area [mm\({}^{2}\)] & 0.118 & N.A. & 0.12 & 0.5 & N.A. \\ \hline Temperature range [\({}^{\circ}\)C] & 34–43 & 32–42 & 27.5–47.5 & 0–75 & 32–42 \\ \hline Inaccuracy [\({}^{\circ}\)C] & \(\pm\)0.1\({}^{\star}\), \(\pm\)0.13 & \(\pm\)0.2/\(-\)0.1 & N.A. & \(\pm\)0.09 \\ \hline Power consumption [nW] & 40.9 & 1800 & 700 & 9200 & 35,000 \\ \hline Verification level & Rat-scitler nerve & External body & \
\begin{tabular}{c} External body \\ in vivo \\ \end{tabular} &
\begin{tabular}{c} External body \\ skin \\ \end{tabular} \\ \hline \hline \end{tabular}  
* Temperature range: 37 \({}^{\circ}\)C–39 \({}^{\circ}\)C. b Temperature range: 34 \({}^{\circ}\)C–37 \({}^{\circ}\)C, 39 \({}^{\circ}\)C–41 \({}^{\circ}\)C.  
\end{table} 
Table 1: Performance summary and comparison with previous works. Due to the nature of the circuit, it is designed to consume the least power near the normal body temperature, which is trimmed in the BRG block. As it becomes further away from the normal body temperature, the output frequency of the VCO increases and the power consumption increases, so the overall power consumption increases. Since most of the application targets stay at the normal body temperature for a long time, the power consumption at the normal body temperature can be considered the representative power consumption. At 43 \({}^{\circ}\)C, which is the furthest point from 37 \({}^{\circ}\)C in the body temperature measurement range, power consumption increases by at least 1.4 times compared to 37 \({}^{\circ}\)C.  
Additional methods for stable animal experiments need to be supplemented. Initially, only the electrodes equipped with thermistors were inserted into the rats, and the measurement and wireless main devices were attached to the rats' backs. After the surgery, the rats gnawed and cut off all the electrode connections exposed outside the body. To prevent the rat from cutting the electrode, we tried putting a collar around the neck and a jacket on the body, but both failed. To avoid exposing the electrode connectors outside the body and breaking them, the devices were made smaller and inserted entirely into the body. At least four rat experiments were performed to obtain the animal experimental data for this paper. In order to achieve success in animal testing, additional system miniaturization and experimental methodologies should be developed. In this paper, the battery takes up almost half the volume of the system used to implant it into the mouse body.