Fig. 1. (Color online) Implantable, biocompatible Au resistive temperature sensors. (a) A flexible implantable neural probe on a polyimide (PI) substrate, attached to printed circuit boards (PCBs) with connectors. (b) A detailed view of the probe tip, containing electrodes and temperature sensors. (c) Temperature sensor made of a serpentine gold wire 5 μm wide and 1800 μm long. (d) The temperature and voltage responses of the temperature sensor when a 1 mA current is applied. (e) The resistive temperature sensor exhibits a linear relationship between resistance and voltage under a 1 mA current, with a temperature coefficient of 0.32%. (f) Optical image of the neural probe implanted in the retina, monitoring temperature changes during insertion. (g) Temperature changes during probe implantation in the retina. (h) Optical image of the neural probe implanted in the deep brain region. (i) Temperature changes during probe implantation were monitored in the deep brain. Reproduced with permission from Ref. [25]. Copyright (2017) IEEE.

Fig. 2. (Color online) Implantable, biocompatible soft multilayer electronic arrays for the functional temperature sensor. (a) Schematic illustration of the multimodal, multiplexed soft sensors in a multilayer configuration, including 8 × 8 electrodes for radiofrequency ablation (RFA) and irreversible electroporation (IRE), temperature sensors for precision thermography, and pressure sensors for measuring forces associated with soft-tissue contact. (b) Detailed view of the temperature sensor array in a planar format. (c) Temperature sensor made of a thin Au metal wire with a thickness of 100 nm and a width of 4 µm, configured as a resistance element. (d) Histogram and Gaussian lineshape fitting of the temperature coefficient of resistance (TCR) values for the temperature sensors. (e) Resistance measurements from the sixty-four temperature sensors at temperatures ranging from 30 to 80 °C. (f) Comparison of results from the infrared camera and the stretchable temperature sensor array, with the circuit properly grounded to eliminate crosstalk. (g) Fractional change of resistance of a temperature sensor under cyclic 20% uniaxial stretching (orange points) and different biaxial strains (blue points). (h) Spatiotemporal temperature mapping measured by the temperature sensor array during bipolar RFA on a rabbit heart. Reproduced with permission from Ref. [30]. Copyright (2020) Nature Publishing Group.

Fig. 3. (Color online) Implantable, biocompatible, transparent temperature sensor based on a Mn–Co–Ni–O nanofilm. (a) Optical image of the Mn–Co–Ni–O nanofilm temperature sensors on a Si substrate. (b) Scanning electron microscopy (SEM) image of a sandwich-structured thin-film temperature sensor on a Si substrate with a total thickness of approximately 4.1 µm. (c) Resistance variation with temperature for the Mn–Co–Ni–O (MCN) transparent temperature sensor. (d) Resolution test for the freestanding MCN transparent temperature sensor, demonstrating the ability to detect a small temperature change of 0.03 °C. (e) Response time of the MCN transparent temperature sensor is 224 ms when transitioning from 25 to 70 °C. (f) Schematic illustrations of the microscale LED probe integrated with the MCN transparent temperature sensor. (g) Optical image of the microscale LED probe integrated with the MCN transparent temperature sensor, showing blue light from the LED penetrating through the sensor. (h) Luminous power of LEDs with and without integrated temperature sensors as a function of input current. (i) Measurement of the increase in temperature of the LED surface under different injection currents by the MCN transparent temperature sensor and an infrared camera. (j) Hematoxylin and eosin (H&E)-stained histological section image of the control group, showing the back region of a rat without implantation. (k) Histological section image of the tissue surrounding the sensor after 21 days of implantation in the back region of a rat. Reproduced with permission from Ref. [38]. Copyright (2024) Nature Publishing Group.

Fig. 4. (Color online) Biocompatible, battery-less wireless implantable temperature sensor based on silicon nanomembrane (Si-NM). (a) Optical image of the soft, flexible wireless electronics module interconnected with the biosensing module via insulated, flexible fine wires (blue dotted box). (b) Optical images of biosensing modules with pressure, temperature, and flow sensors. (c) Silicon serpentine strain gauges on a flat substrate for temperature monitoring. (d) Measured resistance change (∆R/R₀) response of approximately 0.10% °C^–1 in the temperature sensor when exposed to temperature changes from 30 to 50 °C. (e) Optical image of the artificial heart system, including two cylinders with mechanical pumps simulating the right ventricle (RV) and right atrium (RA), two prosthetic heart valves mimicking the pulmonary valve (PV) and tricuspid valve (TV), a temperature control module, and commercial sensors. (f) Optical image of the sensing module inserted into the pulmonary arteries (PA) extracted from a pig. (g) Simultaneous measurements of flow sensor resistance (Rf, black) and normalized resistance change of the temperature sensor (ΔR_T, blue) using the artificial heart system. Reproduced with permission from Ref. [41]. Copyright (2023) Nature Publishing Group.

Fig. 5. (Color online) Implantable optical wireless integrated circuit temperature sensor based on silicon and Ⅲ−Ⅴ semiconductors. (a) Optical micrograph of a voltage-sensing optical wireless integrated circuit (OWIC) on the back of a penny. (b) Schematic of the OWIC in operation, illustrating the integration of optical communication, power, and electronics into a fully integrated microscopic sensor package. (c) Optical image of the setup used for temperature measurement of the OWIC sensor. (d) Graph depicting the characteristics of optical power output as a function of temperature. (e) Measurement of small, rapid temperature changes induced by a resistive element adjacent to the OWIC temperature sensor. (f) 3D reconstruction of the implanted OWIC and vasculature in a mouse brain, with blood vessels labeled using fluorescein and imaged via three-photon microscopy (1320 nm excitation, red), while the implanted OWIC is visualized through label-free third harmonic generation (yellow). (g) In vivo optical recording of temperature from the OWIC sensor, as input power heats the device and surrounding tissue. Reproduced with permission from Ref. [46]. Copyright (2020) National Academy of Sciences.

Fig. 6. (Color online) Implantable optoelectronic upconversion temperature sensor based on Ⅲ−Ⅴ semiconductors. (a) Circuit schematic diagram of the optoelectronic temperature sensor, featuring an InGaP red LED and a GaAs double junction photodiode connected in series. (b) Scanning electron microscopy (SEM) image detailing the design of the optoelectronic upconversion device. (c) Optical image of upconversion devices transferred onto the tips of fibers (diameter approximately 600 µm), with insets providing a zoomed-in view of the fiber tips with a red-emitting device excited by coupled near-infrared light. (d) Microscopic top view of an upconversion device for temperature sensing under infrared excitation, along with the photoluminescence (PL) emissions of the device as temperature varies from 26 to 92 °C. (e) Relationship between PL intensity and temperature, measured by counting the average number of captured photons with an imaging sensor. (f) Optical image of a mouse with an implanted fiber sensor and thermocouple in the brain for temperature sensing. (g) Dynamic temperature signals obtained in the mouse brain compared to results recorded simultaneously with thermocouples, with the shaded gray region indicating when the mouse was placed in a hot environment (~40 °C). Reproduced with permission from [51]. Copyright (2022) Nature Publishing Group.

Fig. 7. (Color online) Implantable physiological temperature sensor for organic semiconductor-based materials. (a) Image of a flexible 12 × 12 temperature sensor sheet (scale bar: 1 cm). (b) Cross-sectional illustration of a flexible large-area active-matrix sensor array with 12 × 12 temperature pixels. (c) Structure of the organic transistor, featuring a gate dielectric made of an anodized aluminum oxide layer and a phosphonic acid self-assembled monolayer (SAM); a 30 nm thick layer of dinaphtho[2, 3-b:2'9, 3'-f]thieno[3, 2-b]thiophene (DNTT) serves as the air-stable organic semiconductor forming the channel layer. (d) Optical image of the organic transistor. (e) Temperature dependence of the electrical characteristics after integrating the temperature sensor with the organic transistor. (f) Temperature dependence of the resistivity of the temperature sensor and the on-current of the organic transistor with the integrated temperature sensor. (g) Temperature measurement of respiratory lung tissue, illustrating heat exchange between the lung tissue and air. (h) Time course of the living lung's temperature (black: temperature output from the sensor, green: displacement of the lung surface). (i) Temperature mapping measurement of a rat lung using a 5 × 5 array of temperature sensors. Reproduced with permission from Ref. [55]. Copyright (2015) National Academy of Sciences.