Ultrafine and Microinvasive Temperature Probe for Real‐Time Monitoring Core Body Temperature

Anesthetics can affect temperature regulation when administered to an anesthetized individual, which can be tracked by measuring the core body temperature. This is useful in assessing the depth and consistency of the anesthesia, as well as its effect on temperature control. In this study, a temperature probe measurement system is created with a remarkable sensitivity of 685.3 Ω °C−1, a high resolution of 0.01 °C, and a fast response rate of 0.16 s °C−1. This ultrafine and biocompatible probe is composed of a single carbon fiber with a diameter of 5.4 µm, encased in a 0.7 mm needle. The system is reliable and provides faster temperature measurements than existing technologies. By inserting the probe beneath the scapula to measure core body temperature in real‐time, a linear correlation can be seen between the duration of anesthesia, the minimum core body temperature, and the respiratory rate when different doses of tribromoethanol anesthetic are administered. This can improve the effectiveness and safety of clinical practice, accelerate the progress and utilization of anesthetic drugs, and give a better comprehension of the physiological mechanisms of anesthesia.


Introduction
Body temperature is usually determined by the temperature of the body's surface, including the armpit and mouth.[3][4] Therefore, core body temperature is an important indicator of health and is commonly utilized in medical practice.[7] Consequently, when administering anesthesia, it is crucial to accurately monitor the patient's core body temperature in real-time and take the required steps to prevent any potential problems brought about by hypothermia.Traditionally, body temperature was measured using a mercury or an infrared thermometer; however, these methods do not provide an accurate reading of the core body temperature.10][11][12][13][14] On the one hand, an optical fiber temperature sensor has several advantages over the traditional resistive temperature sensor, such as a quick response and resistance to electromagnetic interference.However, the optical fiber sensor has some drawbacks, such as vulnerability to mechanical damage and environmental influences, like bending, stretching, pressure, and vibration. [8,9]This can cause internal damage to the optical fibers, light loss, and temperature measurement errors, thus affecting the accuracy of the temperature measurement.Additionally, the installation and use of optical fiber sensors require higher technical requirements and costs than those of traditional temperature sensors, including the use of an optical fiber interface, optical fiber lines, light sources, and detectors. [10]On the other hand, surgical implantations of temperature sensors are usually made of bioabsorbable materials, thus decreasing the possibility of rejection and other side effects.This material is slowly absorbed by the body, so there is no need for its removal.It can be in direct contact with the tissue, providing a more accurate measurement of the core body temperature than non-contact sensors, with higher accuracy, and can monitor temperature changes in real time continuously. [11,12]But implantable temperature sensors are quite large in size, measuring up to tens of millimeters, and must be surgically inserted, which carries risks such as infection and bleeding.Another approach is to implant a thermocouple temperature sensor into the rectum to measure core body temperature. [13,14]However, this method may have some limitations.First, the process of implanting a thermocouple temperature sensor may cause discomfort or pain to the body and may interfere with its normal physiological activities, which can lead to inaccurate measurement results, especially during long-term monitoring.Second, the accuracy of the thermocouple temperature sensor may be influenced by internal factors, such as dietary intake and gastrointestinal movements, that can potentially interfere with the measurement of the core temperature.Therefore, to ensure accurate core body temperature monitoring, it is essential to develop a temperature measurement system that is precise, biocompatible, compact, affordable, and portable.
Here, we present an ultrafine temperature probe measurement system featuring real-time monitoring, wireless transmission, and a graphical application interface.This temperature probe consists of a single carbon fiber of 5.4 μm, which is encased within a needle of 0.7 mm in diameter.It is sensitive (685.3Ω °C−1 ), accurate (0.01°C), and highly stable, and can be used for microinvasive measurement of the core body temperature when inserted into the skin or blood vessels.In addition, our study investigated the link between the dosage of tribromoethanol anesthetic, the duration of anesthesia, the core body temperature, and the respiratory rate of mice.By analyzing the relationship between the anesthetic dose and the core body temperature and the respiratory rate, we can improve the effectiveness of temperature control during and after anesthesia by optimizing the selection and use of anesthetic drugs.This could potentially reduce intraoperative and postoperative complications, accelerate postoperative rehabilitation and recovery, and enhance the success rate and patient satisfaction.

Preparation and Characterization of Temperature Probe
The temperature probe had a design similar to that of a medical injection needle, allowing it to be easily inserted into the subcutaneous tissue or blood vessels to measure the core body temperature (Figure 1a).The stainless-steel probe tube had a diameter of 0.7 mm and contained a single carbon fiber of 5.4 μm diameter, encapsulated within the tube and functioning as a core temperature sensing unit (Figure S1, Supporting Information).As shown in Figure 1b, begin by placing a bundle of 5 mm long carbon fibers into the water, allowing the water tension to disperse them.Using tweezers, pick some of the fibers onto a filter paper, then use a microscope to separate a single carbon fiber filament.Next, place a single carbon fiber wire on one end of two copper wires with a diameter of 60 μm, and then use a silver paste to establish an electrical connection, where the copper protective layer in contact with the carbon fiber wire was removed by laser etching.Third, a thin layer of silicone was brushed onto the carbon fiber to form a stable structure and act as insulation.Finally, the copper wires connected to the carbon fiber were inserted into a needle tube and the silicone was added into it gradually.Then, the temperature probe was baked in an oven at 80 °C for 2 h before being used.

Measurement System of Temperature Probe
As shown in Figure 1c, the temperature measurement system with a temperature probe consists of two parts: a signal acquisition circuit hardware module and the software module for displaying the temperatures.The signal acquisition circuit hardware module comprises a temperature probe that measures the core body temperature and generates an electrical output; the signal modulation module then amplifies, filters, and linearizes this output.The analog signal was then converted to a digital signal by a 24-bit analog-to-digital converter (ADC), thus allowing digital processing of the analog signal.The microcontroller unit (MCU STM32F030C8T6) was the control core of the embedded system, responsible for processing, controlling, and managing various external devices and functional modules.Signal transmission was achieved through a combination of scanning, connection, and the Bluetooth protocol stack, made possible by a Bluetooth module (BLE 5.0).On the other hand, the temperature display software module was programmed using LabVIEW, which communicates with the signal acquisition circuit module.It then applies the impurity scattering correction formula algorithm for temperature calculation and calibration, and reads and displays the temperature data.Diagrams depicting the signal acquisition circuit for both the hardware and software modules can be seen in Figure S2 (Supporting Information).

Thermal Conductivity Measurement of Single Carbon Fiber
To perform a thermal conductivity measurement of a single carbon fiber, a self-constructed system was employed, incorporating a 532 nm laser Raman test system, a temperature-controlled stage, and a digital multimeter.The carbon fiber was affixed to the copper pillars via silver paste, and two pillars were placed on the temperature stage to act as heat sinks.This heat transfer allowed the temperature of the carbon fiber on the copper pillars to be the same as the temperature of the stage (Figure S6, Supporting Information).

Thermodynamic Simulation of Temperature Probe
The thermal performance of the temperature probe was simulated using the Solid Heat Transfer Model physical field in COMSOL Multiphysics.The Finite Element Mesh was used to build the model, taking into account the existing thermal insulation boundary conditions, and the Transient Analysis technique was employed to calculate the temperature distribution and response time of the temperature probe.The temperature probe was placed in a cylindrical water environment with a diameter of 10 mm and a height of 30 mm, and thermal insulation boundary conditions were applied to ensure a constant ambient temperature.To simplify the computation of the model and take into account the effects of boundary conditions, the probe was initially set to 25 °C and the final environment temperature was maintained at 40.5 °C.

Animals and Preparation of Anesthetics
The Committee on the Ethics of Animal Experiments of the International Healthcare Innovation Institute (Jiangmen, China) gave ethical approval for the animal experiments (approval number: N2022018).All animal experiments were conducted in accordance with the institution's Guidelines for the Care and Use of Laboratory Animals.Briefly, 8-week-old male ICR mice, with a body weight of 30 ± 2 g, were acquired from the Guangdong Medical Animal Center in Foshan, China.The mice were kept in an environment with a temperature of 22 ± 1 °C and a humidity of 55 ± 5%, with a 12-h light/dark cycle, for a period of 1 week.They were supplied with mouse pellet chow and allowed to drink filtered tap water from bottles without any hindrance.After an adjustment period of 1 week, mice were intraperitoneally injected with 300, 400, 500, 600, and 700 mg kg −1 of tribromoethanol ac-cording to their body weights.Anesthesia was administered and a temperature probe was implanted under the scapula to measure the core body temperature of the mice in real time for a period of 180 min.For the preparation of a mixture solution of tribromoethanol, 300 mg of tribromoethanol powder (C 2 H 3 Br 3 O, Nanjing Abbey Biotechnology Co., Ltd.) was mixed with 1 mL of tert-amyl alcohol (C 5 H 12 O, Shanghai Roche Biological Engineering Co., Ltd.) and 10 mL of 0.9% sterile physiological saline (NaCl, Beijing Beyotime Biotechnology Co., Ltd).

Measurements and Characterizations
To characterize the cross-section of a single carbon fiber, a field-emission scanning electron microscope (Carl Zeiss Microscopy GmbH, Sigma 500, Germany) with an acceleration voltage of 15 kV was used.Raman spectroscopy (HORIBA Scientific, LabRAM HR800, France) was utilized to determine the Raman spectra, with a green laser (532 nm) being used to excite the Raman signal and a 50× LWD objective lens with a numerical aperture of 0.5 focusing the laser onto the sample.The power of the laser (532 nm) was evaluated using a power meter (Thorlabs Inc., PM100D, USA).The electrical resistance was determined by making use of two tungsten microprobes and a digital multimeter (Tektronix Inc., Keithley 2636B, USA).Utilizing a digital microscope (Leica Microsystems GmbH, DVM6, Germany), the diameter of the probe was determined and individual carbon fibers were observed and separated.The insulation layer of the copper wire was removed by a laser milling machine (LPKF Laser & Electronics AG, U4, Germany).Heating was used to cure the silica gel in an oven (Memmert GmbH + Co. KG, UN55Plus, Germany).The temperature performance of the probe was assessed using a temperature-controlled stage with an accuracy of 0.1 K (LinKam Enterprises Ltd., LTS420E-P, UK).A digital multimeter (Tektronix, Inc., Keithley 2636B, USA) was used to measure the electrical resistance of the temperature probe.To ensure accuracy, the temperature was kept constant for 5 min at each step and at least 20 points of resistance were measured to calculate the average data.An infrared thermal imager (Nippon Avionics Co., Ltd., InfReC R450 Series, Japan) was used to measure the mice's body temperatures.To measure the respiratory rate of the mouse, a respiratory monitoring band (TS-PSMB-1095) from Guangdong Flexwarm Advanced Materials & Technology Co., Ltd. in China was employed.

Stability and Resolution Measurements of Temperature Probe
Carbon fiber is a conductive material with imperfections and impurities.When voltage is applied, the charge carriers travel along the carbon fiber.The ionized impurities form a Coulomb potential field, which affects the movement of charge carriers because of the scattering process inside the carbon fiber.With a rise in temperature, the kinetic energy of the charge carriers increases, reducing the power of the Coulomb force and resulting in an increase in conductivity.By contrast, traditional metal temperature sensors are based on the ion migration effect which is typically a material with a positive temperature coefficient.As the temperature rises, the ion migration within the lattice increases, thus resulting in an increased resistance.Therefore, the carbon fiber temperature probes are more sensitive to temperature fluctuations than traditional metal temperature sensors.Figure 2a reveals the sensitivity measurement of a temperature probe which enables precise temperature monitoring by converting resistance into the respective temperature values.The resistance of the temperature probe was evaluated at 5 °C intervals ranging from −10 to 60 °C, and these results were employed to perform a linear fitting.By analyzing the data through linear fitting, it was determined that the sensitivity of the temperature probe was −685.3Ω °C−1 , a value significantly greater than that of previously reported single-beam carbon fibers (−5 Ω °C−1 ), ≈100 times more. [15]This demonstrates that a highly sensitive temperature probe can provide faster, more accurate and more reliable temperature detection.To enhance the precision of the temperature calibration, a temperature probe is utilized that is based on the impurity scattering model of carbon fiber, which has a linear correlation between its conductance value and the 3/2 power of the temperature (Figure S3, Supporting Information). [16]This calibration curve is then input into the temperature measurement system, thus enabling the temperature probe to directly measure and display the temperature in real-time.As depicted in Figure 2b, the temperature calibration probe is placed on the variable temperature stage for long-term stability detection at −10, 0, 40, and 50 °C.It can be seen that the temperature measured by the temperature probe is consistent with that of the variable temperature stage, thus displaying good stability.
High resolution and accuracy are two key criteria for evaluating temperature sensors.By utilizing a high-resolution temperature probe, researchers can measure and record temperature changes with greater accuracy and precision.This allows for a more precise measurement of the core body temperature, which can provide valuable insights into biological reactions and physiological processes.As demonstrated in Figure 2c, the resistance of the temperature probe decreased from 153.5 kΩ to 151.9 kΩ when the temperature shifted within the range of 36.5−37.0°C with an interval of 0.1 °C.This resolution of the temperature probe measurement system is in line with that of the commercial temperature-controlled stage, indicating its higher resolution to detect minor temperature changes of 0.1 °C.Calculations reveal that the temperature probe can achieve a resolution of 0.01 °C, as a 0.01 °C change in temperature corresponds to a 32 Ω shift in resistance.Despite the temperature probe having a resolution of 0.01 °C, the temperature-controlled stage only offers a maximum resolution of 0.1 °C, thus preventing further assessment of the probe's resolution in this experiment.Figure 2d shows that the corrosion resistance of the temperature probe was evaluated by immersing it in solutions of weakly acidic (pH 6), neutral (pH 7), and weakly alkaline (pH 8) at room temperature (25.0 °C).The results showed that the temperature probe had a stable performance in the solutions, indicating that the temperature probe could accurately detect temperature changes in different acidbase solutions.The temperature fluctuation was estimated to be ≈±0.07°C, which could be caused by either the measurement error of the temperature probe or the fluctuation of environmental conditions.To ensure more reliable core body temperature measurements, a high-resolution and high-accuracy temperature probe can be employed to minimize the influence of the environmental temperature.

Thermodynamic Simulation and Dynamic Responses of Temperature Probe
In order to accurately detect and record temperature changes, it is essential to measure the response time of the temperature probe.A shorter response time would indicate a quicker response; therefore, the response speed of the temperature probe should be evaluated, taking into account the thermodynamic simulation and dynamic responses.To carry out the thermodynamic simulation, the COMSOL method is employed, which necessitates knowledge of the specific heat capacity, density, and thermal conductivity of the material used for the temperature probe.Aside from other material parameters that can be sourced from reference, the thermal conductivity of a single carbon fiber can be ascertained through the Raman spectroscopy method proposed by Liu et al. [17][18][19][20] Based on the physical model of the suspended single carbon fiber depicted in Figure S4 (Supporting Information), an Equation (1) that pertains to thermal conductivity was derived when the carbon fiber was exposed to both electric current and laser radiation simultaneously.It is noteworthy that the size of the laser spot can be disregarded in comparison to the length of the carbon fiber, as the point heat source is assumed to be heated, and the thermal resistance of the contact between the carbon fiber and the copper pillar is disregarded. (1) The temperature difference at the center of the carbon fiber, as measured by Raman spectroscopy, is represented by ∆T L/2 and is determined by the variation in electrical heating power, ∆S e , which is the difference between U 1 I 1 and U 2 I 2 .L and A signify the length and cross-sectional area of a single carbon fiber, respectively.
The images of a single carbon fiber with a diameter of 5.4 μm and a length of 6107.69 μm are shown in Figure S5a,b (Support-ing Information).Research results have demonstrated a linear relationship between the G band frequency and temperature.The first step is to identify the connection between the Raman G peak of a single carbon fiber and the laser heating power (Figure S6, Supporting Information).Specifically, the temperature control stage is initially set to 298.15 K, and then a digital source meter is used to apply a voltage source of 0.001 V to both ends of the carbon fiber.When the carbon fiber was heated at the midpoint by laser irradiation, the electrical signal was observed until the carbon fiber laser heating reached a steady state.Upon the output electrical signal remaining stable, the Raman spectra of the carbon fiber were tested under various laser powers, and the alteration of the Raman G peak value of a single carbon fiber was determined when the laser powers were 0.514, 1.001, 1.601, 2.230, and 2.780 mW, respectively.As shown in Figure 3a, the G peak positions of the five laser heating powers that can be obtained by Gaussian Lorentz fitting are corresponding to 1595.03, 1591.59,1587.91,1584.54, and 1581.08 cm −1 .The peak position of the Raman G peak changes almost linearly with the laser heating power (Figure 3b).Through linear fitting, it can be obtained that when the laser power is 0 mW, i.e., the temperature of the carbon fiber is equal to the initial temperature of 298.15 K, the Raman G peak position of the carbon fiber is 1597.87cm −1 .The second step is to ascertain the relationship between the Raman G peak and the different temperatures of a single carbon fiber.To record the Raman spectrum in a steady-state process, a micro-laser power of 0.514 mW was set to continuously heat the midpoint of the carbon fiber, and the temperature of the single carbon fiber was regulated by a temperature-control stage.The G-peak position of the carbon fiber changed as the temperature shifted from 133.15 to 493.15 K, with an interval of 90 K. Specifically, the G-peak positions were 1593.25,1595.10,1597.01,1598.77, and 1600.54cm −1 at temperatures of 493.15, 403.15,313.15, 223.15, and 133.15 K, respectively (Figure 3c).It is evident from Figure 3d that the linearity of the Raman G peak and the temperature of the carbon fiber have a strong linear relationship, with a correlation coefficient of 0.997.According to Figure 3d, the temperature dependency coefficient of the G band frequency is 0.02028 cm −1 K −1 ; in other words, a 1 cm −1 G-peak shift is associated with a temperature change of ≈49.31 K. Substituting the initial temperature of 298.15K and the Raman G-peak of carbon fiber of 1597.87 cm −1 into the formula provided in Figure 3d, and transforming the variables, a relationship between the carbon fiber temperature and Raman G-peak can be expressed as Equation ( 2).T = −49.31G+ 79089.12  (2) Finally, the temperature was maintained at 298.15 K, and a Raman spectrum was obtained at the midpoint of the carbon fiber with a laser of 0.514 mW power, using voltages of 0.001 and 3.000 V DC, respectively, as illustrated in Figure 3e.The peak of Raman G was observed to be 1597.62and 1595.61cm −1 when the power was 0.001 and 3.000 V, respectively, as shown in Figure 3e.Moreover, Equation ( 2) was used to calculate the temperature difference (∆T L/2 ) of the midpoint of the carbon fiber to be 99.11K.When 0.001 and 3.000 V were applied to the carbon fiber at a constant temperature, the current remained stable, as illustrated in Figure 3f, and the change in electrical heating power (∆S e ) was calculated to be 0.1519 mW.Thus, the thermal conductivity of a single carbon fiber could be determined by Equation ( 1) to be 51.

W/(m•K).
[23][24] Represented in Figure S7 (Supporting Information) is a model of the temperature probe, which is similarly proportioned to the actual sample.The material parameters of the model, consisting of a single carbon fiber wire, silicone, copper wire, and an aluminum needle, are provided in Table 1. Figure 4a illustrates the time evolution of the temperature distribution images in a 2D cross-section at different times of 0, 0.1, 0.   2, the temperature probe has a faster temperature response rate of 0.16 s °C−1 , demonstrating its superior thermal response speed.Commercial temperature sensors are rigid, large, and not very biocompatible, making it hard to implant them into animals to monitor core body temperature.Additionally, Table 2 reveals that the temperature probe in this study has the same resolution   as commercial temperature sensors (LM35CA Chip), and a faster temperature response time.

Relationship Between Core Body Temperature, Respiration Rate, and Anesthetic Dose in Mice
Infrared thermometry is advantageous due to its speed and noncontact capability; however, it is limited to measuring the temperature of the surface of the target object and cannot penetrate deeper for measurement.As depicted in Figure 5a, infrared cameras were used to measure the average surface skin temperatures on the scapula, the anus, and the tail, which were respectively 32.1, 31.8, and 28.5 °C.However, in clinical settings, a rectal temperature is often used to monitor the core body temperature, as the rectum is situated within the body and is directly linked to the core body's temperature.In contrast, an implanted temperature sensor can be inserted directly into the core area of the body, thus providing more accurate readings of the true core temperature.As illustrated in Figure 5b, core body temperatures measured by anorectal and subscapular thermometry are similar, being ≈2 °C higher than the infrared temperature measurement.Unlike anorectal thermometry, subcutaneous scapular insertion thermometry is not impacted by factors such as food intake or defecation and does not cause any discomfort or hinder one's daily activities.Additionally, it enables constant monitoring of body temperature over an extended period, for instance, in intensive care settings.In order to build the connection between core body temperature and an anesthetic, the technique of temperature measurement by inserting a temperature probe into the scapula was utilized in the study.Moreover, the temperature probe was small enough to measure the temperature of the blood vessels in the tail of mice, indicating that it could be employed to measure the temperature of human blood vessels in a similar fashion to intravenous injection (Figure S8, Supporting Information).In comparison to anorectal and subscapular ther-mometry, the temperature of the blood vessels in the tail of mice was observed to be lower (29.2 °C), which is similar to the infrared temperature measurement.This is due to the fact that the tail blood vessels are situated far away from the body tissues, thus being closer to the skin temperature.It can be observed from Figure S9 (Supporting Information) that a temperature probe was implanted beneath the skin to measure the core body temperature, akin to an intramuscular injection.Following the removal of the temperature probe, the wound healed rapidly, thus making the measurement of core temperature with the temperature probe a largely noninvasive procedure.By further investigating the correlation between an anesthetic dose and the core body temperature in mice, anesthesiologists and researchers can more accurately determine the appropriate dose of anesthetics.In the experiment, the anesthetic state of the mice was determined by stimulating their tails, with the onset of anesthesia occurring ≈1 min after injection.As illustrated in Figure 5c, the core body temperature of the mice experienced a parabolic change during the injection of tribromoethanol anesthetic.The anesthetic process is divided into three stages of body temperatures.The initial stage begins with the injection of an anesthetic, which suppresses the central nervous system of the mice, resulting in a decrease in their body temperature.The second stage is the lowest point, at which the effect of the anesthetic drug further inhibits metabolic, cardiac, and respiratory functions, leading to an even lower body temperature.[34][35] Repeated measurements of five mice were used to obtain the duration of anesthesia in our study.Figure 5e demonstrated a linear increase in the duration of anesthesia as the injection dose increased.This finding is of great importance, as it can be used to predict the depth of anesthesia.This is especially pertinent in scenarios where anesthesia needs to be closely monitored, such as surgeries or experiments, as inadequate depth can lead to consciousness and pain in patients or laboratory animals.Furthermore, understanding the linear relationship between the anesthetic dose and the duration of anesthesia can aid medical professionals or researchers in adjusting the dose to achieve the desired depth.If the duration of anesthesia is too long, it may increase the risk of adverse reactions; however, if the anesthetic time is too short, it may not be sufficient to meet the requirements of surgery or experiments.To gain insight into the influence of narcotic drugs on the central nervous system and the neuroscientific mechanisms of anesthesia, our study monitored and analyzed the respiratory rates of mice under different doses of an anesthetic simultaneously (Figure S10, Supporting Information).Figure 5d demonstrates that the respiratory rate and temperature curves have a similar changing pattern, indicating that anesthetics impede the central nervous system, thus influencing body temperature and respiration.Furthermore, the anesthetic process also suppresses the body's metabolic processes, resulting in a decrease in oxygen consumption, which reduces the respiratory rate and decreases the body temperature.Figure 5f reveals a linear correlation between the minimal body temperature and respiratory rate and the anesthetic doses.As a rule, when the anesthetic dose is increased, the body temperature and respiratory rate decrease to a minimum.Analyzing these minimums can help doctors or researchers determine the state of anesthesia and then adjust accordingly.Initially, the mice had a body temperature of 37.5 °C and a respiration rate of 221 bpm; however, when the dose of anesthesia was raised to 700 mg kg −1 , the body temperature dropped to 30.5 °C and the respiration rate decreased to 136 bpm.In addition, the mice perished when the dose was increased to 1000 mg kg −1 or the body temperature decreased to 30 °C.It is imperative to take into consideration that the association between the minimum temperature and respiration rate and the anesthetic dose may be influenced by multiple factors, including individual differences, the qualities of the anesthetic drug, and so forth.As such, in practical application, it should be integrated with other pertinent indicators and monitoring data for a comprehensive analysis and judgement.Figure 5g reveals the rate of decline of different anesthetic doses, indicating that a higher anesthetic dose leads to a more rapid temperature drop.This is because higher doses of narcotic drugs inhibit metabolism and thermoregulation mechanisms to a greater extent, thus causing the temperature to decrease at a faster rate.As the amount of anesthetic is increased, its inhibitory effect on the respiratory center is augmented, resulting in a decrease in the rate of respiration.Additionally, high doses of anesthetic drugs can obstruct the respiratory pathways, such as the back of the tongue collapsing or the airway being blocked, thus decreasing the flow of air and causing the breathing rate to decline. [36]herefore, investigating the correlation between different doses of anesthetics and core body temperature and respiratory rate can enhance the efficacy and safety of clinical practice, facilitate the advancement and utilization of anesthetic drugs, and gain a better understanding of the physiological mechanisms of anesthesia.

Conclusion
In this work, this temperature probe system is equipped with real-time monitoring, wireless transmission, and a visual application interface.The temperature probe is composed of a single carbon fiber, encased in a needle of 0.7 mm diameter, which has remarkable sensitivity of 685.3 Ω °C−1 and accuracy of 0.01°C.It is also very stable, having a faster response rate of 0.16 s °C−1 , and can be utilized for microinvasive measurements of core body temperature by inserting the probe into subcutaneous or blood vessels.In addition, this study investigated the correlation between the dosage of tribromoethanol anesthetic, the core body temperature, and the respiratory rate of mice by implanting a temperature probe beneath the scapula.Results indicated a linear relationship between the duration of anesthesia, the minimum core body temperature, and the respiratory rates when different doses of tribromoethanol anesthetic were administered.This research can help to enhance the efficacy and safety of clinical practice, promote the development and utilization of anes-thetic drugs, and gain a better understanding of the physiological mechanism of anesthesia.

Figure 1 .
Figure 1.Schematic illustration of the temperature probe measurement system.a) Application scenario of a temperature probe implanted into a subcutaneous blood vessel to measure body temperature.b) The preparation flowchart of the temperature probe.c) Block diagram showing the overall system operation.

Figure 2 .
Figure 2. Stability and resolution measurements of the temperature probe.a) The relationship between temperature and resistance of the temperature probe with an interval of 10 °C.The red line is a linear fitting curve.b) The temperature probe's stability measurement at different temperatures.c) Resolution measurement of the temperature probe with an interval of 0.1 °C.The red line is a linear fitting curve.d) The corrosion resistance measurement of the temperature probe in different acid-base solutions at room temperature.

Figure 3 .
Figure 3.The thermal conductivity measurement of a single carbon fiber.a) Raman spectra of the midpoint of the carbon fiber under different laser heating powers.b) G peak position versus laser power.c) Raman spectra of the carbon fiber versus different temperatures.d) G peak position versus temperature.e,f) Raman spectra and currents of the carbon fiber in voltages of 0.001 and 3.000 V DC, respectively.Raman spectra are processed by Gaussian fitting and normalization.
3, 1.0, 2.0, and 3.0 s from 25.0 to 40.0 °C.It is clear that the temperature of the probe quickly increases to reach 40.0 °C within 2.0 s.The corresponding temperature variation with time curve is depicted in Figure 4b, as the probe's temperature rises from 25.0 to 40.0 °C until equilibrium is attained, taking 2.5 s to reach and remain at 40.0 °C.In order to corroborate the

Table 1 .
Parameters of thermodynamic simulation for temperature probe.Material Specific Heat Capacity [J•kg −1 •K −1 ] Density [kg•m −3 ] Thermal Conductivity [W•m −1 •K −1 an experiment was conducted using a temperature probe with the same parameters.The repeatability analysis of the temperature probe for eight cycles from 25.0 to 40.0 °C in a water environment is shown in Figure 4c.It is noted that the temperature probe has a higher sensitivity than existing temperature sensors, making it able to respond to temperature changes faster and more accurately.The fluctuation in repeated measurements is due to the control of the measurement environment.The response time corresponding to the enlargement of the dashed frame in Figure 4c is shown in Figure 4d.The response time is calculated by measuring the time it takes for the output signal to transition from 10% of the initial temperature to 90% of the final temperature.The results show that the temperature probe has a response time of ≈2.4 s.The experimental results are in good agreement with the simulation results.When compared to the other implantable temperature sensors listed in Table

Figure 4 .
Figure 4. Thermodynamic simulation and dynamic responses of a temperature probe.a) The time evolution of the temperature distribution images from 25 to 40 °C using COMSOL Multiphysics.b) The calculation result of the temperature variation with time curve corresponding to a). c) The repeatability analysis of the temperature probe for eight cycles from 25 to 40 °C in a water environment.d) The response time corresponding to the enlargement of the dashed frame in c).

Figure 5 .
Figure 5. Relationship between core body temperature, respiration rate, and anesthetic dose in mice.a) Infrared thermogram of mice and average temperature of the scapula, anus, and tail.b) The body temperature measurements of subcutaneous scapulae, anus, and tail blood vessels.c,d) The changes in subcutaneous temperatures of the scapulae and respiration rates in mice over time, respectively, after injection of different doses of tribromoethanol.e) Relationship between different doses of tribromoethanol and the duration of anesthesia.f) Relationship between the minimum core temperature and the minimum respiration rate under different doses of anesthetics.g) Relationship between the rate of temperature decline (RTD) and the rate of respiration rate decline (RRRD) under different doses of anesthetics.

Table 2 .
Summary of implantable temperature sensors and their performances.