# A Sub-50 nW Implantable Body Temperature Sensor and Its Application to a Rat
## 1 Introduction
The transducers that measure temperature by utilizing the change in resistance of such thin metal routing patterns are called RTDs (Resistance Temperature Detectors). [PERSON] used gold RTDs as a temperature transducer in the neural probe [8]. To increase the resistance, a serpentine Au resistor with a 5 \(\upmu\)m-wide wire length of 1800 \(\upmu\)m was made, and a resistance of about 195 \(\Omega\) was obtained at 35 \({}^{\circ}\)C. In order to measure the resistance change according to temperature at this low resistance level, a very large current of 1 mA was used. [PERSON] developed an implantable temperature sensor using a thin Pt pattern with a resistance value of about 400-500 \(\Omega\)[9]. The flexible implantable temperature sensor has a sensitivity of 1.485 \(\Omega/^{\circ}\)C, and a large current of 8 mA was used to enable sufficient measurement due to the small sensitivity of the resistance. [PERSON] fabricated a temperature sensor using Pt film with a resistance range of 1548.58 \(\Omega\) at 0 \({}^{\circ}\)C and 1787.55 \(\Omega\) at 100 \({}^{\circ}\)C [10]. To measure resistance variations with temperature at this resistance level, a current of 0.1 mA was applied, achieving a sensitivity of 2.4 \(\Omega/^{\circ}\)C within the temperature range of 35 \({}^{\circ}\)C to 40 \({}^{\circ}\)C. RTD sensors have the advantage of being thin and easy to integrate with neural interface components, but they have the disadvantage of having small resistance and high-power consumption. Since the current consumption is large when the resistance of the temperature transducer is low, it is advantageous to use a temperature transducer with a large resistance. Therefore, in this paper, a thermistor with a resistance of 100 k\(\Omega\) was used as a temperature transducer instead of an RTD of several hundred \(\Omega\) or less. The method of measuring the change in resistance mainly uses a Wheatstone bridge configuration to generate a differential signal. If the differential signal of the Wheatstone bridge is small, it is additionally analog-amplified, and the analog signal is converted to a digital signal. Typically, a measurement circuit can be configured and used using a commercial OP amplifier and ADC, or a commercial readout circuit that is integrated into a single chip can be used [11]. [PERSON] also developed an integrated circuit (IC) for temperature measurement [12]. The design incorporates a resistive sensor in the front-end configuration of a Wheatstone bridge, amplifies the voltage difference induced by temperature changes, and processes the output using an ADC. The circuit operates with a power consumption of 9 uW at 1.8 V.  
In this paper, we approach the power saving of the body temperature sensor from two perspectives. One is to reduce the driving power of the body temperature transducer, and the other is to reduce the power of the readout circuit that reads the transducer. Normally, two resistor bridges are used to construct a Wheatstone bridge, but only one resistor bridge was used to reduce the transducer driving power. In addition, rather than a static method that always drives the transducer, a dynamic method was applied that drives it only when necessary. That is, a dynamic virtual Wheatstone bridge technique is proposed, which generates a differential output like a Wheatstone bridge by sequentially driving a single bridge consisting of a thermistor and a trimmable resistor for a short time. Additionally, the thermistor bridge operates at 1.8 V, but all functional blocks after the thermistor bridge are designed with an operating voltage of 0.6 V for low power consumption. Section 2 presents the top-level system configuration and the designed body temperature measurement circuit. Section 3 shows the measured performance of the fabricated body temperature measurement circuit. Section 4 describes the fabrication of the implantable body temperature measurement system, the rat surgery and experiments, and the in vivo body temperature measurement results.