A non-contact biological tissue temperature measurement method based on sound-induced mechanical luminescence

Abstract

The application belongs to the technical field of temperature detection, and particularly relates to a non-contact biological tissue temperature measurement method based on sound-induced mechanical luminescence. 3+ Doped aluminum oxide is used as a mechanical luminescence temperature measurement material. 3+ The doped aluminum oxide is arranged on one side of a biological tissue, and ultrasonic excitation is applied to the other side of the biological tissue to induce sound-induced mechanical luminescence. 3+ The emission spectrum of the sound-induced mechanical luminescence is detected, spectral characteristic parameters of R-line emission are extracted from the emission spectrum, and a temperature value is determined according to a photoluminescence calibration corresponding relationship between the spectral characteristic parameters and temperature. The spectral characteristic parameters are the centroid positions of R-line emission.

Description

Technical Field

[0001] This invention belongs to the field of temperature detection technology, specifically relating to a non-contact biological tissue temperature measurement method based on acoustic-induced mechanoluminescence. Background Technology

[0002] Accurate, high-resolution temperature measurement is crucial for chemical, biological, and materials systems, where subtle temperature fluctuations can profoundly impact reaction kinetics, phase behavior, transport, and cellular function. In emerging applications, non-invasive temperature monitoring with spatial resolution is essential for monitoring the temperature of micro / nanoscale devices, soft matter, and biological tissues. Contact probes can interfere with localized fields or fail under harsh and constrained conditions, and they are susceptible to scratching soft workpieces and accuracy degradation due to probe wear. Currently, the primary non-invasive temperature measurement method is photoluminescence (PL)-based thermometry, which encodes temperature information through the emission of a luminescent probe, providing remote, sensitive, rapid, and microscope-compatible readings. Systems utilizing various methods, such as emission intensity ratios, spectral peak bandwidth, or excited-state fluorescence lifetime, have been applied across a wide range of spectral and temperature applications, from rare-earth-doped nanocrystals to organic dyes and semiconductors. Photoluminescence thermometry follows a common principle: optical excitation, multiple energy level transitions, and finally, radiation-induced light emission; the temperature is inferred by analyzing the photophysical properties of this process under temperature changes. Photophysical properties refer to thermal effects, which include population distribution among electronic energy levels, nonradiative relaxation, and electron-phonon coupling. These factors all affect the characteristics of photoluminescence (PL). Therefore, the temperature measurement results of photoluminescence are affected by excitation conditions, including light intensity, absorption, scattering, and the local environment.

[0003] The dependence of photoluminescence (PL) on optical excitation inherently limits its application in biological tissue temperature measurement. This is primarily because both luminescence generation and readout depend on the interaction between light and matter, making the measurement results highly sensitive to variations in excitation intensity, alignment, absorption, scattering, and photobleaching. In biological tissue, these effects lead to inhomogeneous excitation and distorted emission, affecting accuracy. The optical path fluctuates with changes in refractive index, particle position, and local density, resulting in significant temperature-independent luminescence variations. To address these issues, existing organic thermoluminescence sensors employ ratiometric or fluorescence lifetime-based sensing materials. While these effectively suppress intensity fluctuations, they cannot completely eliminate excitation-emission coupling, which is caused by thermal effects. Lattice distortion or defect activation can induce luminescence even without optical pumping, thus affecting the temperature measurement results. To address the aforementioned issues, existing technologies have pioneered a novel paradigm for thermal measurement by combining mechanoluminescence with temperature-sensitive luminescent centers. The emission spectrum of mechanoluminescence can reflect intrinsic thermal property changes without being affected by the absorption, scattering, or excitation inhomogeneities of biological tissues. In dynamic or acoustically active media, mechanoluminescence (ML) allows excitation and emission to occur at the same physical location, minimizing uncertainties associated with spatially separated optical excitations and facilitating temperature readings under limited optical observation conditions. Acoustic-induced mechanoluminescence (S-ML) is particularly attractive because sound waves can penetrate opaque or confined media, transferring energy long-range and uniformly while generating mechanical excitation and localized heating effects, providing an appealing platform for temperature measurement in environments where traditional optical excitation methods are difficult. However, the prerequisite for quantitative temperature sensing via S-ML is that the underlying thermal parameters must possess sufficient robustness, meaning that these parameters must maintain stable responses and remain unaffected by fluctuations in excitation light intensity during continuous changes in dynamic mechanical excitation conditions. Currently, quantitative temperature measurement using mechanoluminescence technology mainly focuses on changes in emission intensity for applications such as stress mapping, damage visualization, or qualitative mechanical sensing. However, intensity-based mechanoluminescence signals are inherently sensitive to changes in mechanical coupling, excitation distribution, and defect activation pathways, making them unsuitable for reliable temperature quantification of biological tissues. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence. The specific technical solution of this invention is as follows:

[0005] A non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence includes the following steps: With Cr 3+ Doped alumina is used as a mechanoluminescent thermometric material; the Cr 3+ Cr in doped alumina 3+ The doping concentration is 0.01 mol% to 10 mol%; the Cr 3+ The highest doping concentration yields the strongest S-ML signal, ensuring the most accurate temperature readings.

[0006] The Cr 3+ Doped alumina is disposed on one side of a biological tissue, and ultrasonic excitation is applied to the other side of the biological tissue to induce acoustic-induced mechanoluminescence; the emission spectrum of the acoustic-induced mechanoluminescence is detected. Extract Cr from the emission spectrum 3+ The spectral characteristic parameters of the R-line emission are determined; wherein, the spectral characteristic parameter is the centroid position of the R-line emission; the temperature value is determined based on the photoluminescence calibration correspondence between the spectral characteristic parameter and temperature. In a non-biological tissue environment, the intensity ratio of broadband emission to R-line emission was used as an auxiliary verification parameter for temperature calibration. This intensity ratio exhibits a monotonically increasing trend with increasing temperature and satisfies a Boltzmann-type distribution relationship, used to characterize Cr. 3+ of 2 E-state and 4 Mechanism of thermally driven particle number redistribution between T2 states.

[0007] This invention utilizes Cr 3+ The centroid position (spectral wavelength / frequency shift) of the R-line emission is used as a temperature measurement parameter. This is because the centroid position (spectral characteristic) is determined by Cr. 3+ The lattice energy level structure of the ions determines that the physical redshift / blueshift only occurs with temperature changes, and is almost independent of the ultrasonic pressure, the quality of material contact, and the defect concentration. This effectively solves the problem that the existing technology is inherently sensitive to changes in mechanical coupling, excitation distribution, and defect activation pathways due to the intensity-based mechanoluminescence signal. This makes it more suitable for reliable temperature quantification in dynamically changing environments, enabling robust and reliable quantitative temperature measurement.

[0008] In another preferred embodiment, the Cr 3+ Doped alumina refers to alumina in which Cr is added. 3+ A flexible mechanoluminescent thin film was prepared by dispersing doped alumina in a flexible matrix.

[0009] In another preferred embodiment, the emission spectrum includes the centroid of the R1 line and the centroid of the R2 line; The centroid position of the R-line emission is selected from one of the following: The centroid of line R1; The centroid of the R2 line; The centroid of the composite R-line emission band formed by merging R1 and R2 lines. Preferably, the centroid of the R-line emission band is selected from the centroid of the composite R-line emission band.

[0010] In another preferred embodiment, the photoluminescence calibration relationship is established in the following manner: Excite the Cr with laser 3+ Doped alumina is used to record photoluminescence emission spectra at different temperatures, establishing a linear correlation between the centroid position of the R-line and temperature, or a Boltzmann-type correlation between the broadband and the R-line intensity ratio and temperature. Preferably, the photoluminescence calibration relationship is established in the following manner:

[0011] Under controlled temperature conditions, the Cr is excited by laser. 3+ By recording the photoluminescence emission spectra of doped alumina at different temperatures, a linear correspondence between the centroid position of the R-line and temperature was established.

[0012] In another preferred embodiment, the wavelength of the laser excitation is 405 nm.

[0013] In another preferred embodiment, the biological tissue is at least one of soft tissue and bone tissue.

[0014] In another preferred embodiment, the thickness of the soft tissue is 2 mm ± 0.3 mm; The thickness of the bone tissue is 1.5 mm ± 0.3 mm.

[0015] In another preferred embodiment, the ultrasonic excitation frequency is 20kHz±500Hz.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes Cr 3+ Doped alumina, through the use of Cr 3+ Using the centroid position (spectral wavelength / frequency shift) of the R-line emission as a thermometric parameter, which is unaffected by ultrasonic magnitude, material contact quality, or defect concentration, combined with spectral centroid engineering and acoustic-induced mechanoluminescence (S-ML), reliable remote temperature measurement is achieved, suitable for dynamically changing environments, especially for measuring the temperature of biological tissues. Systematic photoluminescence (PL) calibration shows that sharp R-line emission exhibits stable temperature-dependent spectral evolution characteristics, thus providing reliable thermometric parameters based on centroid shift and thermally activated broadband-to-R-line intensity ratio (BIR), both of which are influenced by Cr. 3+The modulation of Boltzmann-type redistribution between excited states was employed. These calibrated optical responses established a quantitative temperature mapping framework with high spectral stability. Based on this, S-ML was used as a mechano-optical conversion path for non-contact thermometry under ultrasonic excitation. The emitted signal retains its inherent temperature dependence while remaining insensitive to changes in excitation intensity, allowing temperature values ​​to be reconstructed directly from the calibrated spectral response. The method was further validated in biologically relevant settings, encompassing whole-tissue measurements in soft biological media and bone-mediated configurations that introduce additional high-scattering biological barriers, enabling optical readout under increasingly demanding propagation conditions. Attached Figure Description

[0017] Figure 1 For Cr 3+ Performance analysis of doped alumina; (a) shows the performance of alumina based on Cr³⁺. + (a) Schematic diagram of the acoustic-induced emission thermometry principle; (b) Alumina:Cr³ at different magnifications + SEM image; (c) is alumina: Cr³ + (d) Emission spectra of force-induced mechanoluminescence (F-ML), acoustic-induced mechanoluminescence (S-ML), and photoluminescence (PL); (d) Emission spectra of F-ML under different applied forces; Inset shows the relationship between integral emission intensity and force; (e) Visual image of F-ML (top) and its corresponding three-dimensional intensity distribution map (bottom).

[0018] Figure 2 For Cr 3+ XRD pattern of doped alumina. Wherein, Cr... 3+ Doped alumina is simply referred to as Al2O3:Cr 3+ (The same applies below.)

[0019] Figure 3 For Cr 3+ Elemental analysis diagram of doped alumina; a represents Al2O3:Cr 3+ The EDX plot; b~d are the mapping plots of Al, O, and Cr respectively.

[0020] Figure 4 For Cr 3+ R-line emission analysis of doped alumina; (a) Al2O3:Cr 3+ (a) Temperature-dependent emission spectra; inset shows the normalized spectrum magnified in the R-line region used for centroid analysis; (b) shows the relationship between the centroid of the R1, R2, and R-line composite emission spectra and temperature, and the linear fitting results; (c) Cr³ + Simplified ion configuration coordinate diagram illustrating thermal coupling. 2 E and 4T2 excited state; (d) shows the relationship between broadband and R-line intensity ratio (BIR) and temperature (using Boltzmann fitting; inset shows the corresponding relative temperature sensitivity).

[0021] Figure 5 Figure 1 shows the analysis results of the emission of R1, R2, and R lines; figures a-c show the polynomial fitting results of the FWHM of the emission of R1, R2, and R lines as a function of temperature; figures d-f show the relative sensitivity (S) of the FWHM of the emission of R1, R2, and R lines, respectively. r (Results chart)

[0022] Figure 6 (a) Schematic diagram of acoustic-induced mechanoluminescence (S-ML); (b) Micrograph of S-ML under ultrasonic excitation; (c) Alumina:Cr³⁺ at different sample temperatures (297–393 K, controlled by a heating gun). + Emission spectra under ultrasonic excitation; the inset shows the redshift of the centroid of the composite R-line with temperature; (d) shows the temperature-dependent relationship of the centroid of the composite R-line extracted from (c); the right-hand axis represents the emission spectrum obtained from ultrasonic excitation. Figure 2 (b) shows the temperature value calculated by the centroid calibration of the composite R-line; (e) shows the temperature value obtained by the thermocouple, infrared thermal imager, and... during the heating and cooling process. Figure 2 (b) shows a comparison of temperatures measured by the composite R-line centroid calibration method; the inset shows representative thermal images at selected temperatures; (f) shows the trend of composite R-line centroid change over time under continuous ultrasonic excitation (20 seconds, three repeated cycles) and pulsed excitation (1 second on, 4 seconds off); the right axis represents the change in the centroid of the composite R-line under the conditions of continuous ultrasonic excitation (20 seconds, three repeated cycles) and pulsed excitation (1 second on, 4 seconds off); Figure 2 (b) is the corresponding temperature calculated from the calibration result; (g) is the infrared thermal image of the sample during the first continuous ultrasonic treatment cycle; (h) is the normalized emission spectrum recorded during the first continuous ultrasonic treatment cycle; the inset highlights the redshift phenomenon of the centroid of the composite R line; (i) is the emission spectrum at different ultrasonic power levels (10–50%) at a fixed temperature; (j) is the curve showing the relationship between the integrated emission intensity and the ultrasonic power.

[0023] Figure 7 Figure 1 shows the results of the temperature detection and analysis of chicken tissue; (a) is a schematic diagram of the acoustic-induced mechanoluminescence (S-ML) measurement device applied to chicken tissue temperature sensing; (b) is a photograph of the experimental setup, with the lower left inset showing the fiber optic detection view of the system under ambient light and ultrasonic excitation, respectively; the upper right inset shows the actual structure of the copper plate-sample-chicken tissue assembly; (c) when the temperature is raised from 296K to 320K using a heating gun, the alumina:Cr³ content recorded through the chicken tissue under ultrasonic excitation conditions. +Emission spectrum; the inset highlights the redshift phenomenon of the composite R-line centroid as a function of temperature; (d) shows the trend of the composite R-line centroid as a function of temperature extracted from Figure (c), with the right axis representing the corresponding temperature calculated according to the calibration relationship; the inset shows representative infrared thermograms at each temperature point.

[0024] Figure 8 The image shows the detection results of acoustic-induced mechanoluminescence (S-ML) on chicken bone tissue; (a) is a schematic diagram of the S-ML measurement device, in which ultrasonic excitation is directly applied to the surface of the chicken bone, and aluminum oxide and Cr are stacked below it. 3+ (a) Sample and chicken tissue; (b) Photographs of the experimental setup: the left image shows a magnified view of the ultrasonic excitation area; the right image shows the actual chicken bone-sample-chicken tissue assembly structure and the state of the sample attached to the chicken bone under sunlight and ultraviolet light; the lower image shows the system under ambient light and ultrasonic excitation as observed from the fiber optic detection end; (c) Alumina:Cr recorded through chicken tissue during continuous ultrasonic excitation as the temperature changed from 302K to 326K. 3+ Emission spectrum; the inset highlights the redshift of the centroid of the composite R-line as a function of temperature; (d) shows the temperature change over time under continuous ultrasonic excitation: the left axis shows the temperature measured by infrared thermal imaging (blue dot) and the temperature obtained by centroid calibration of the composite R-line (red dot), and the right axis represents the corresponding centroid offset extracted from (c). (e) shows representative infrared thermal images corresponding to each time point in (d). Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0026] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Cr 3+ Activated Al2O3 (ruby) provides a particularly favorable platform due to its spin confinement. 2 E→ 4 The R-line emission of the A2 transition exhibits well-defined characteristics, demonstrating an intrinsic temperature sensitivity through electron-phonon coupling and crystal field modulation. Importantly, these spectral features are fundamentally independent of the excitation mode, allowing thermal information to be encoded in the emission characteristics rather than the excitation conditions.

[0028] This invention utilizes alumina-based Cr 3+To address this challenge, a unified framework for photoluminescence calibration and scanning laser microscopy readout thermal measurement is established. First, the temperature-dependent spectral evolution of R-line emission is systematically calibrated under photoluminescence conditions, revealing a stable line centroid shift as a robust thermal measurement parameter. Simultaneously, a thermally activated broadband intensity-to-R-line ratio reflects… 2 E-state and 4 The population rearrangement between energy levels in the A2 states. Based on this calibration result, scanning laser microscopy was used as a non-optical excitation method to achieve temperature reconstruction under ultrasonic driving conditions without the need for an external light source. This design ensures complete separation between the excitation mechanism and the thermal measurement signal, while the readout signal is still generated by Cr. 3+ Dominated by inherent electronic transitions. This invention further validates the effectiveness of the method in biologically relevant environments, including soft tissue and bone-mediated layered structures—environments where optical detection is severely limited by scattering, absorption, and structural heterogeneity. By using spectrally stable Cr... 3+ By combining thermal measurement centers with mechanically driven luminescence phenomena, this study establishes a general strategy for quantitative temperature sensing of optically confined and mechanically active systems.

[0029] 1. Experimental materials: Cr 3+ Thin film preparation utilizes existing techniques to synthesize a film containing 1 mol% Cr in air via a high-temperature solid-state reaction. 3+ The Al2O3 phosphor used in this example was of analytical grade with a purity higher than 99.99%, and was used directly without further purification.

[0030] High-purity alumina and chromium oxide powders were used as starting materials. The alumina composition was based on the nominal composition (1 mol% Cr...). 3+ Weigh the raw materials, mix them thoroughly, and grind them in an agate mortar using ethanol as the grinding medium until homogeneous. Transfer the resulting powder mixture directly to an alumina crucible and calcine in air at 1650°C for 3 hours to form Cr. 3+ The doped α-Al₂O₃ phase is denoted as aluminum oxide:Cr 3+ The powder was then allowed to cool naturally to room temperature in the furnace and ground again for further characterization and optical measurements.

[0031] For ML measurements, the preparation steps for flexible ML films are as follows: First, 0.3 grams of the prepared alumina:Cr 3+ The powder was dispersed in a mixture containing 0.06 g of UV-curable adhesive (leaftop 9307) and 9 mL of anhydrous ethanol. After thorough stirring, the mixture was sonicated to ensure uniform dispersion. The resulting suspension was then poured into a square mold (3 × 3 cm²). After the ethanol had completely evaporated, a uniform powder layer was obtained.

[0032] The dried powder layer was sandwiched between two layers of PET-EVA encapsulation film (Deli No. 3817), and then subjected to ultraviolet irradiation to achieve curing and adhesion. Finally, the assembled film was heat-laminated to obtain a stable ML test film.

[0033] Characterization of basic properties: X-ray diffraction (XRD) patterns were recorded using a SmartLab multi-functional X-ray diffractometer (Rigaku, Japan) in conjunction with Cu Kα radiation (λ=1.5406Å) to confirm phase purity and crystal structure. The operating voltage and current were set to 45kV and 200mA, respectively. Diffraction data were recorded in 10-minute intervals within the 2θ value range of 10–90°. -1 The scanning rate was set at 0.02 steps. The morphology and elemental composition of the samples were characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray diffraction (EDX) analysis techniques on a FEI Quanta 250 FEG microscope equipped with an EDAX detector. Photoluminescence (PL) excitation and emission spectra were measured at room temperature using an FLS1000 fluorescence spectrometer (Edinburgh Instruments, UK) with a xenon lamp as the excitation source to characterize the Cr content in the alumina matrix. 3+ The optical transition characteristics.

[0034] 2. Experimental Methods Force-induced mechanoluminescence (F-ML) measurements were performed using a self-developed mechatronic coupling system, consisting of a digital force gauge, a single-axis controller, a stepper motor, a fiber optic spectrometer, a glass substrate, and a data acquisition computer. The emitted ML signal was transmitted via optical fiber and recorded using an Ocean Optics QE65Pro spectrometer. During measurement, the ML film was fixed to the glass substrate; a metal probe with a hemispherical probe (approximately 0.1 mm in diameter) contacted the sample surface, and a controlled force was applied by adjusting the position of the force gauge; the optical fiber was placed below the glass substrate to collect the emitted light signal generated by mechanical stimulation. The single-axis controller drove the stepper motor to achieve controllable horizontal movement, thereby enabling real-time acquisition of the ML signal; unless otherwise specified, the applied force was maintained at 30 N.

[0035] Acoustic-induced mechanoluminescence (S-ML) was generated using an ultrasonic homogenizer (Bandelin Sonopuls HD 2070.2, frequency 20kHz±500Hz, maximum output power 70W). S-ML spectra were recorded using an Andor Shamrock 500i spectrometer equipped with an iDus CCD detector, and all spectral data underwent instrument response correction. Cr was extracted from the S-ML spectra. 3+The spectral center position of the emission band is used as a temperature-dependent reading, while the emission intensity is monitored to assess the impact of acoustic excitation intensity.

[0036] Temperature calibration and validation: A standard photoluminescence (PL)-based method was employed, with powder samples used as a reference for temperature calibration. Emission spectra were acquired under controlled temperature conditions using an Andor Shamrock 500 spectrometer equipped with a silicon CCD camera, with a 405nm laser as the excitation source. Temperature-dependent PL spectra were recorded using a temperature-controlled heating-cooling system (Linkam THMS600) to ensure precise and stable temperature regulation. This process established a reliable correlation between spectral characteristics and temperature.

[0037] In S-ML measurements, the sample temperature is controlled by an external heating gun or allowed to vary naturally during continuous acoustic excitation. Infrared thermal images are recorded using a Sonel KT-200 thermal imaging camera to obtain independent temperature distribution maps and compare them with ML-derived temperature data induced by acoustic excitation. In this embodiment, continuous and intermittent acoustic excitation schemes were used to evaluate the repeatability of the measurement results.

[0038] Biological tissue validation: To evaluate the feasibility of the S-ML thermometry method in biologically relevant environments, the study used ex vivo chicken meat tissue and chicken bones purchased from the local market as tissue models.

[0039] In chicken tissue experiments, aluminum oxide:Cr 3+ The powdered sample was placed beneath a chicken tissue layer approximately 2 mm thick. Ultrasonic excitation was applied to a copper plate positioned above the sample assembly, allowing acoustic energy to be transferred to the phosphor via the copper plate. To minimize temperature fluctuations caused by ultrasonic heating, S-ML spectra were acquired intermittently after the sample temperature stabilized at various temperature points. The sample temperature was controlled by an external heating gun and simultaneously monitored by thermocouples, an infrared thermal imaging system, and an S-ML spectrometer. The emitted S-ML signals were acquired via an optical fiber positioned beneath the sample.

[0040] In the chicken bone experiment, chicken bone with a thickness of approximately 1.5 mm was used as the acoustic transmission medium. Ultrasonic excitation acted directly on the bone surface, while the alumina:Cr... 3+ The powder adheres to the lower surface of the bone. A layer of chicken tissue approximately 2 mm thick is then placed beneath the powder to construct a multilayer biological model. This embodiment utilizes continuous ultrasonic excitation to induce a gradual temperature increase through acoustic heating, eliminating the need for an external heating source. The emitted S-ML signal is acquired from the bottom using the same fiber optic detection configuration. Infrared thermal images are simultaneously recorded and compared with temperature values ​​calculated based on the R-line centroid.

[0041] 3. Results Environmental characteristics: The concept of the proposed S-ML temperature measurement strategy is as follows: Figure 1 As shown in a, firstly, through the Cr in alumina... 3+ The temperature-dependent photoluminescence (PL) response of ions establishes routine temperature calibration, where the characteristic R-line emission can serve as a sensitive temperature optical probe. Under ultrasonic excitation, the same Cr... 3+ The center generates an S-ML signal, thus enabling temperature information acquisition without external optical excitation. Non-contact temperature sensing under acoustic stimulation is achieved by correlating the spectral characteristics of the S-ML emission with a PL-derived calibration curve. Therefore, the alumina:Cr ratio was first investigated under ambient conditions. 3+ The structure and optical properties.

[0042] First, regarding aluminum oxide: Cr 3+ The structure and morphological characteristics of the crystal were characterized. For example... Figure 2 As shown, the XRD pattern is in high agreement with the standard diffraction pattern of α-alumina (corundum), confirming the formation of a trigonal crystal structure with space group R-3c. Sharp diffraction peaks and the absence of minor phases indicate high crystallinity and phase purity of the sample. Further observation of the sample morphology using scanning electron microscopy (SEM) is shown below. Figure 1 As shown in b, the sample exhibits an irregular polyhedral crystal morphology, with a particle size of approximately several micrometers (typically ~2 μm). The clear crystal surface features are consistent with the synthesized alumina:Cr 3+ The phosphor exhibits consistent high crystallinity. EDX elemental distribution map ( Figure 3 a) in the diagram confirms the presence of Al, O, and Cr; the corresponding elemental distribution diagrams for Al, O, and Cr are shown in [reference needed]. Figure 3 The bd in the sample further verified the homogeneity of its composition.

[0043] Alumina: Cr 3+ The optical properties were initially studied using photoluminescence (PL) measurements. As shown in Figure S3, the excitation spectrum monitored at 694 nm showed similarities to Cr. 3+ of 4 A2→ 4 T2 and 4 The characteristic absorption band corresponding to the A2→4T1 transition confirms that Cr 3+ Ions were successfully incorporated into the corundum lattice. Under photoexcitation, the emitted light mainly originated from Cr. 3+ Spin Forbidden State 2 E→ 4 The A2 transition is characterized by R-line transitions. This R-line emission contains closely adjacent R1 and R2 components generated by crystal field and spin-orbit splitting; due to the small energy level spacing between the two components, they exhibit a single merged characteristic at room temperature of 25°C.

[0044] To verify whether the same luminescent centers can be activated mechanically, the spectra of PL, fluorescence microscopy (F-ML), and scanning fluorescence microscopy (S-ML) were compared (see [link to study]). Figure 1 (c) All three excitation modes exhibit almost identical spectral profiles, all centered on Cr. 3+ The predominant emission of characteristic R-lines indicates that both direct mechanical loading and ultrasonic excitation can effectively activate the same radiative transition process that leads to photoluminescence.

[0045] To evaluate alumina:Cr 3+ The magneto-optical luminescence performance under different mechanical stimuli was investigated to determine its suitability for subsequent sensing applications. The relationship between F-ML emission intensity and external force was studied. Figure 1 As shown in d, the emission intensity gradually increases with increasing applied force, while the spectral profile remains essentially unchanged, indicating that mechanical stimulation mainly affects the emission intensity rather than the emission mechanism itself. The integrated emission intensity shows a clear trend of increasing force dependence.

[0046] To establish reliable optical temperature measurement parameters, the photoluminescence (PL) spectra of alumina as a function of temperature in the range of 193–493 K were recorded under 405 nm laser excitation. Figure 4 As shown in a, the emission spectrum is mainly composed of characteristic Cr 3+ The R-line emission peak dominates, exhibiting a gradual redshift with increasing temperature while maintaining the overall spectral profile. The magnified normalized spectrum in the inset more clearly shows the temperature-induced shift of the R-line region. To quantitatively assess the spectral evolution, the centroid positions of the emission peaks of the R1, R2, and composite R-line were extracted and plotted as a function of temperature. Figure 4 (b) All three parameters exhibit excellent linear temperature dependence: the slopes of the R1 centroid, R2 centroid, and composite R-line centroid are 0.0055, 0.0064, and 0.0059 nm·K, respectively. -1 The high temperature coefficient of the R2 emission peak is consistent with previous studies on rubies, which is attributed to... 2 The difference in electron-phonon coupling strength between the two Stark split components in the E excited state may lead to slightly different thermal shifts. The temperature coefficient of the composite R-line centroid lies between that of the individual R1 and R2 centroids, which is consistent with the expected characteristic that this parameter represents the intensity-weighted average position of the entire R-line spectral set. In addition to spectral centroid analysis, the evolution of emission linewidth with temperature was also studied as another thermal measurement parameter. Figure 5 As shown in ac, the full width at half maximum (FWHM) of the emissions from R1, R2, and the composite R-line all increase with increasing temperature, reflecting the enhanced electron-phonon interaction and the Cr 3+Thermal broadening of the emission bands. The three emission components exhibit different functional behaviors: the FWHM of R1 emission shows an approximately linear temperature dependence, while R2 and the combined R-line emission exhibit nonlinear characteristics, which can be accurately described by a cubic polynomial fitting function, with specific fitting parameters shown in Table 1. This difference indicates that the spectral broadening process is not uniform among the Stark splitting components of R-line emission. This is attributed to the different effects of electron-phonon interactions, crystal field perturbations, and spin-orbit coupling on the R1 and R2 transitions. To quantitatively compare their temperature response characteristics, the relative temperature sensitivity (S) is calculated using the following formula. r ): ;

[0047] Where λ represents the characteristic sensing parameter (i.e., the FWHM of the emission band). Based on this definition, the relative sensitivity calculated from the spectral centroid is first evaluated. As shown in df in Figure S5, the maximum S of the emission peaks of R1, R2, and the combined R line is... r The values ​​are 0.24% K. - ¹、0.52% K - ¹ and 0.34% K - ¹. These results indicate that Cr 3+ The linewidth broadening of the emission peak due to temperature can also produce a measurable thermal metric response.

[0048] Table 1 Fitting parameters of the thermodynamic model To further investigate the alumina:Cr 3+ The temperature-dependent luminescence behavior was analyzed, and the broadband emission to R-line intensity ratio (BIR) was investigated. This parameter is defined as the integrated intensity ratio of broadband emission to narrow-spectrum R-line emission. It is important to note that the broadband component represents an effective contribution to the spectral background rather than a strictly independent physical emission band; therefore, its value is obtained by calculating the integrated spectral area of ​​the broadband and R-line contributions: the broadband component is estimated based on the background spectral background below the R-line region, while the R-line contribution is the difference between the total integrated emission intensity and the broadband background area. Figure 4 As shown in d, the BIR increases monotonically with increasing temperature, indicating a rearrangement process of inter-level population caused by thermal activation among different emitting components. The physical mechanism of this phenomenon can be understood through… Figure 4 Explanation of the simplified configuration coordinate diagram shown in Figure c: At low temperatures, radiative relaxation mainly occurs through spin-forbidden transitions. 2 E→ 4 A2 emission occurs, producing sharp R-line emission; as temperature increases, the contribution of the background broadband emission gradually increases relative to the R-line emission. This behavior is typically associated with a thermally activated particle population redistribution involving high-energy excited states and enhanced electron-phonon interactions. In Cr... 3+ In the activation system, with4 The states associated with the T2 energy level group can enhance the contribution of broadband emission after thermal activation, thereby increasing the BIR value. Therefore, the observed relationship between the BIR value and temperature can be described by a Boltzmann-type model.

[0049] ; Where A is the offset correction parameter mainly related to spectral overlap, B is the material-dependent constant, kB is the Boltzmann constant, and ΔE represents the effective activation energy between two thermally coupled excited states. The high agreement between the fitted curve and the experimental data indicates that the observed intensity redistribution originates from the thermally coupled state. Figure 4 The fitting results shown in d indicate that the effective activation energy ΔE is 1986 cm⁻¹. - ¹ (≈0.17eV), detailed fitting parameters are summarized in Table 1. This value is within the range of Cr 3+ The typical energy range reported for thermally activated processes in doped corundum systems—in which narrow R-line emission ( 2 E→ 4 The rearrangement of the inter-level population between A2 and thermally accessible high-energy states is dominated by electron-phonon coupling, indicating that the observed intensity redistribution originates from intrinsic electronic processes rather than external effects. To evaluate the calorimetric performance of the BIR parameters, S was calculated according to equation (1). r Values, corresponding results as follows Figure 4 The illustration of d in the figure shows the maximum S measured at 438K. r The value is 1.04% K - ¹ indicates that the thermally activated spectral redistribution process has a significant temperature response. These results further confirm that the evolution characteristics of broadband emission and R-line emission with temperature can serve as effective indicators of temperature spectral performance.

[0050] Of the three thermal measurement parameters studied, the FWHM-based method was not further applied to subsequent S-ML thermal measurements. Although the linewidth exhibits measurable temperature dependence and moderate relative sensitivity, accurate determination of FWHM values ​​requires reliable fitting of the emission line profile, making it more susceptible to spectral noise, baseline fluctuations, and fitting uncertainties. Furthermore, the temperature dependence of linewidth for different R-line components is not linear; the R2 line and composite R-line emission exhibit significant nonlinear characteristics. These factors may be more pronounced under S-ML excitation conditions—where emission intensity is typically weaker, and spectral quality is easily affected by acoustic excitation conditions. Therefore, compared to centroid-based and BIR-based methods, the FWHM method has lower reliability in practical temperature measurements.

[0051] Acoustic-induced mechanoluminescence (S-ML) thermometry: To explore the potential of S-ML in non-contact temperature sensing, it is first necessary to determine whether the temperature-dependent spectral behavior observed under photoluminescence (PL) conditions can be maintained under ultrasonic excitation. Experimental setup as follows: Figure 6 As shown in a. Flexible alumina: Cr³ + The multilayer film was placed in an ultrasonic excitation environment, while the temperature of the external sample was controlled using a heating gun. To improve heat transfer and temperature uniformity during the measurement process, a copper sheet was attached to the back of the multilayer film. The sample temperature was monitored independently using both thermocouples and an infrared thermal imager. Figure 6 Figure b shows a representative image of S-ML emission during ultrasonic treatment, demonstrating that the film has a strong luminescence response under ultrasonic stimulation.

[0052] Figure 6 Figure c shows the temperature-dependent S-ML spectra recorded in the temperature range of 297 to 393 K. Unlike the PL spectra discussed in Section 3.2, the S-ML emission spectra exhibit broader characteristic peaks and a lower signal-to-noise ratio, making it difficult to reliably distinguish between the two components R1 and R2. Therefore, the centroid of the R-line (i.e., the weighted average position of the emission spectrum intensity across the entire R-line, exhibiting stable calorimetric performance in PL measurements) was chosen as the main parameter for subsequent S-ML analysis. As the temperature increases, although the emission spectrum maintains a basically unchanged spectral profile, a redshift gradually appears, indicating that the emission mechanism remains unchanged throughout the studied temperature range. Figure 6 The inset of 'c' in the diagram magnifies the centroid region of the synthesized R-line, clearly showing the temperature-induced emission band displacement. To verify whether the calorimetric relationship measured by PL remains valid under ultrasonic excitation conditions, Figure 6 The curve showing the relationship between the centroid of the composite R-line extracted from the S-ML spectrum and temperature is plotted: a significant monotonic redshift trend is observed throughout the temperature range, which is in high agreement with the PL measurement results. To facilitate direct temperature readings, a second vertical axis was added based on the centroid-temperature calibration relationship obtained from the PL measurement. The high agreement between the measured temperature and the temperature calculated based on the centroid position indicates that the thermometric characteristics established under optical excitation conditions are completely maintained under ultrasonic excitation. This result directly proves that S-ML can serve as a quantitative temperature probe, and not merely a mechanically induced luminescent signal.

[0053] To further evaluate the accuracy of temperature measurements based on the centroid method, researchers compared the temperature values ​​calculated using the R-line centroid with temperature values ​​measured independently during heating and cooling processes using thermocouples and infrared thermal imagers. Figure 6 As shown in 'e', ​​within the studied temperature range, all three temperature measurement methods exhibit highly consistent results. Figure 6The illustration in 'e' shows a representative infrared thermal image, clearly illustrating the corresponding temperature distribution of the sample at the selected temperature.

[0054] Biological tissue thermometry: To evaluate the applicability of S-ML thermometry in biologically relevant environments, its performance was first tested in an ex vivo soft tissue model. Biological tissues typically exhibit significant scattering and absorption effects, which can severely impact the transmission and detection of optical signals. Therefore, it is essential to assess whether reliable temperature readings can still be obtained under such conditions.

[0055] First, such as Figure 7 As shown in Figure a, a chicken tissue layer with a thickness of approximately 2 mm was introduced between the sample and the detection side to evaluate temperature sensing performance in soft biological tissue. Alumina:Cr 3+ The powder was placed below the tissue layer, and the copper plate-sample-chicken tissue stack was mechanically fixed using a compression mounting method to ensure stable contact between layers, thereby achieving effective transmission of ultrasonic vibration from the copper plate to the fluorescent layer. The experimental setup is as follows: Figure 7 As shown in b: The lower left image shows the fiber optic detection view from below the glass substrate under ambient light and ultrasonic excitation, respectively; the visible red emission confirms successful S-ML generation through the tissue layer; the upper right image shows the assembled copper plate-sample-chicken tissue stack structure used for measurement. Figure 7 As shown in c, the alumina:Cr content was recorded by ultrasonic excitation at different temperatures (296–320 K). 3+ S-ML spectra in chicken tissue layers. Sample temperature was increased using an external heating gun, while thermocouples were used to monitor the steady-state temperature at each measurement point. Clear Cr was observed throughout the entire studied temperature range. 3+ The emission characteristics indicate that the S-ML signal can be effectively detected by biological tissues. With increasing temperature, the centroid of the composite R-line exhibits a systematic redshift (e.g., Figure 7 (As shown in the illustration of c in the figure), it indicates that the temperature-dependent spectral response is still well preserved in the tissue environment.

[0056] To compare with the PL calibration results, a first-point correction was applied to compensate for minor shifts in the initial centroid position caused by differences in measurement configuration and surrounding materials. Figure 7 The relationship between the centroid of the composite R line extracted from the c-spectrum and temperature is as follows: Figure 7As shown in d, a clear monotonic trend in the centroid position with increasing temperature can be observed, consistent with the temperature-dependent behavior established by PL calibration measurements. To facilitate temperature readings, a second vertical coordinate axis (right axis) was introduced based on the centroid-temperature calibration relationship obtained from PL measurements. As shown by the dashed guide line of the second data point, the temperature determined by the centroid displacement closely matches the temperature measured by infrared thermal imaging. The infrared thermal imaging images corresponding to each temperature point are shown in the inset. The excellent consistency between the infrared temperature measurements and the centroid-derived temperature demonstrates that: Alumina:Cr 3+ The temperature-sensitive spectral response retains good properties even after penetrating biological tissue. More importantly, these results highlight the ability of S-ML thermometry to provide reliable temperature readings in the field of biology.

[0057] The above results demonstrate reliable temperature sensing through soft biological tissues, but many biomedical applications involve layered structures that simultaneously contain both hard and soft tissues. To further evaluate the applicability of S-ML thermometry in such environments, a tissue model mediated by bone (e.g., Figure 8 (as shown in a). In this configuration, ultrasonic excitation acts directly on the surface of the chicken bone, while alumina:Cr³ + The powder adheres to the lower surface of the bone and is covered by a layer of chicken tissue. This arrangement allows the ultrasonic energy to penetrate the bone before reaching the fluorophore, while the emitted luminescent signal can be collected by the overlying biological tissue. The corresponding experimental setup is as follows: Figure 8 As shown in b: the upper left image displays a magnified view of the ultrasonically excited area; the upper right image shows the actual chicken bone-sample-chicken tissue assembly under sunlight and ultraviolet light, respectively, as well as the close contact between the fluorescent powder and the bone surface; the lower image presents fiber optic detection images under ambient light and ultrasonic excitation, and the visible red emission signal confirms the successful generation and acquisition of S-ML signals in the bone tissue assembly. Figure 8 As shown in c, clear S-ML spectra were recorded throughout the entire study temperature range (302K–326K) under continuous ultrasonic excitation. Figure 7 Unlike the externally heated tissue experiment shown, the temperature rise in this embodiment directly stems from the deposition of acoustic energy during continuous ultrasonic treatment. As the temperature gradually increases, the centroid position of the R-line exhibits a systematic redshift phenomenon (e.g., Figure 8 (As shown in the inset of c in the figure), this demonstrates that the temperature-sensitive spectral response is well preserved even in the presence of both bone and soft tissue. To evaluate the dynamic thermometric performance of the system, the PL calibration relationship (using the same first-point correction method as in the aforementioned tissue studies) was applied. Figure 8 The centroid position extracted from 'c' is converted into a temperature value and compared with the temperature data measured by infrared thermal imaging. For example... Figure 8As shown in d, both methods exhibit similar temperature change trends during continuous ultrasonic excitation, indicating that the S-ML response can reliably track the ultrasonic heating process.

[0058] In summary, a method based on Cr³⁺ was established. + An activated alumina luminescence thermometry platform was developed, enabling quantitative temperature readings through both photoluminescence (PL) calibration and S-ML excitation. Temperature-dependent PL measurement results indicate that, within the studied temperature range, Cr³⁺… + The spectral center shift of the R-line emission is highly stable and nearly linear, providing a reliable calibration basis for subsequent S-ML temperature measurements. Meanwhile, BIR further captured... 2 E-state and 4 The rearrangement of interlevel populations driven by thermal effects between T2 states confirms the inherent thermal activation characteristics of this emission system. Based on this calibration method, S-ML signals can be generated using ultrasonic excitation without external optical pumping. Under ultrasonic conditions, the R-line center displacement-temperature relationship established under PL excitation is maintained, allowing direct reconstruction of temperature information from the ML spectrum. Among the evaluated parameters, the center displacement-based reading method exhibits superior robustness compared to intensity-dependent indices, which are more sensitive to changes in acoustic coupling strength and excitation intensity. The effectiveness of this method has been validated through direct comparison with thermocouple and infrared thermography measurements; highly consistent results were observed under both externally heated samples and dynamic heating conditions with continuous ultrasonic treatment, indicating that S-ML can accurately track temperature changes in real time. Furthermore, this method remains effective in biologically relevant environments, including soft tissue-covered samples where optical signals need to pass through scattering media, and bone-mediated layered structures where ultrasonic energy must first penetrate hard biological tissue to reach the fluorescent layer.

[0059] These results demonstrate that S-ML thermometry is not only applicable to simple mechanically excited environments, but can also be extended to optically demanding and structurally complex biological systems. By combining a precisely calibrated photoluminescence (PL) response with mechanically driven luminescence, this invention provides a practical method for achieving quantitative temperature sensing in acoustically active and optically confined environments.

[0060] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence, characterized in that, Includes the following steps: With Cr 3+ Doped alumina is used as a mechanoluminescent thermometric material; the Cr 3+ In doped alumina, Cr 3+ The doping concentration is 1 mol% ± 0.01 mol%; The Cr 3+ Doped alumina was deposited on one side of the biological tissue, and ultrasonic excitation was applied to the other side of the biological tissue to induce Cr. 3+ Doped alumina produces acoustic-induced mechanoluminescence; the emission spectrum of the acoustic-induced mechanoluminescence is detected; Extract Cr from the emission spectrum 3+ The spectral characteristic parameters of the R-line emission are: the centroid position of the R-line emission; and the temperature value is determined based on the photoluminescence calibration correspondence between the spectral characteristic parameters and temperature.

2. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 1, characterized in that, The Cr 3+ The doped alumina is Cr 3+ Doped alumina powder or Cr 3+ Doped alumina thin film; Cr 3+ Doped alumina thin films refer to films in which Cr is incorporated into the alumina film. 3+ A flexible mechanoluminescent thin film was prepared by dispersing doped alumina in a flexible matrix.

3. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 2, characterized in that, The emission spectrum includes the centroid of the R1 line and the centroid of the R2 line; The centroid position of the R-line emission is selected from one of the following: The centroid of line R1; The centroid of the R2 line; The centroid of the composite R-line emission band formed by merging R1 and R2 lines.

4. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 3, characterized in that, The photoluminescence calibration relationship is established in the following way: Excite the Cr with laser 3+ By recording the photoluminescence emission spectra of doped alumina at different temperatures, a linear correspondence between the centroid position of the R-line and temperature, or a Boltzmann-type correspondence between the broadband and the R-line intensity ratio and temperature, can be established.

5. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 4, characterized in that, The wavelength of the laser excitation is 405 nm.

6. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 1, characterized in that, The biological tissue is at least one of soft tissue and bone tissue.

7. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 6, characterized in that, The thickness of the soft tissue is 2 mm ± 0.3 mm; The thickness of the bone tissue is 1.5 mm ± 0.3 mm.

8. The non-contact biological tissue thermometry method based on acoustic-induced mechanoluminescence according to claim 1, characterized in that, The ultrasonic excitation frequency is 20kHz±500Hz.

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