A human body temperature sensor based on PU-PEG phase transition behavior and a preparation method thereof

Abstract

The application provides a human body temperature sensor based on PU-PEG phase change behavior and a preparation method thereof. The method comprises the following steps: reacting polyethylene glycol with different molecular weights with diisocyanate to generate a prepolymer, carrying out chain extension to obtain a PU-PEG phase change material, and then electrospinning to form a nanofiber membrane, and then attaching flexible electrodes to both sides of the membrane to obtain the sensor. The application uses the triple synergy of mixed soft segments, alicyclic or aliphatic diisocyanate and electrospinning to increase the lower limit of the crystallization temperature of the phase change material from 30-31 DEG C to above 35 DEG C, so that the melting and crystallization windows are simultaneously accurately matched with the human body temperature interval of 35-42 DEG C. The sensor uses the ion conductivity mutation caused by PEG phase change to perceive temperature, the sensitivity reaches 8.8±0.77 % / DEG C, the response time is less than 5 seconds, the temperature measurement range is 35-45 DEG C, the resolution is 0.05 DEG C, and the sensor has excellent flexibility, cycle stability and biocompatibility, and can be used for wearable real-time monitoring of human body temperature.

Description

Technical Field

[0001] This application relates to the fields of polymer functional materials and flexible electronic materials technology, and more specifically, to a human body temperature sensor based on PU-PEG phase transition behavior and its preparation method. Background Technology

[0002] With the rapid development of flexible electronics and wearable health monitoring technologies, real-time, accurate, and comfortable monitoring of human physiological signals has become a research hotspot. Among these, continuous monitoring of core body temperature and skin temperature holds significant clinical importance in disease early warning, exercise physiology analysis, and daily health management. Therefore, developing temperature sensors that combine high sensitivity, rapid response, good flexibility, and excellent biocompatibility has become an important direction in the intersection of polymer functional materials and flexible electronics.

[0003] Currently, sensors used for temperature detection are mainly divided into two categories. The first category is electronic sensors such as thermistors and thermocouples based on traditional semiconductor or ceramic materials. These sensors have high sensitivity and good stability, but their rigid and brittle physical characteristics make it difficult to form good contact with soft and curved human skin. This not only results in poor wearing comfort, but also makes them prone to signal distortion or poor contact due to movement, limiting their application in long-term dynamic monitoring.

[0004] The second type is flexible temperature sensors, which typically employ a strategy of combining conductive fillers with a flexible polymer matrix, utilizing the temperature coefficient of resistance (TCR) of the conductive filler to achieve temperature sensing. For example, metal circuits are printed on a flexible substrate, or conductive carbon black is doped into an elastomer to prepare a temperature-sensitive composite material. However, this type of sensor based on electronic conductivity has inherent drawbacks. To obtain higher flexibility and tensile strength, the percolation network density of the conductive filler often needs to be sacrificed, resulting in lower sensitivity (TCR value) and insufficient temperature resolution. At the same time, the conductive network is prone to irreversible damage under large deformation, causing baseline drift and signal instability, making it difficult to balance mechanical compliance and electrical stability.

[0005] To overcome the aforementioned problems, those skilled in the art have designed sensors by introducing phase change materials (PCMs). PCMs undergo a solid-liquid phase transition at specific temperatures, resulting in abrupt changes in their physicochemical properties such as ionic conductivity and capacitance. PCMs such as paraffin or polyethylene glycol (PEG) are encapsulated in microcapsules and then composited with conductive polymers, utilizing the dramatic change in resistance caused by the volume change before and after the phase transition to detect temperature. However, these solutions are insufficient for human body temperature monitoring. The phase transition temperature of pure PEG or paraffin has a limited fixed or adjustable range, making it difficult to accurately match the human body temperature window. Microcapsule encapsulation or simple blended composite films are prone to leakage and phase separation after multiple melt-crystallization cycles. Some phase change materials or encapsulation carriers may be cytotoxic, failing to meet the requirements for long-term direct contact with human skin.

[0006] In summary, there is an urgent need to develop a flexible temperature sensor that can generate a significant and reversible signal response within the human physiological temperature window, while simultaneously possessing excellent mechanical properties and biological safety. Summary of the Invention

[0007] This invention addresses the technical problems of existing flexible temperature sensors in monitoring human physiological temperature, such as the mismatch between the crystallization temperature window and the human body temperature window (35~42℃), low sensitivity, slow response, and poor long-term stability. It provides a human body temperature sensor based on the phase transition behavior of PU-PEG and its preparation method. The preparation method includes: using a mixture of two or more polyethylene glycol (PEG) molecules of different molecular weights as the phase transition functional soft segment, synthesizing a polyurethane (PU) material with solid-solid phase transition properties through a two-step solution polymerization method with diisocyanate and a small molecule glycol chain extender; then processing this material into a nanofiber membrane using an electrospinning process; finally, attaching flexible electrodes to the upper and lower surfaces of the fiber membrane, and encapsulating it to form a sandwich-structured flexible sensor.

[0008] This invention, through the synergistic regulation of hybrid soft segment design, isocyanate structure optimization, and electrospinning process, achieves a breakthrough by raising the lower limit of the crystallization temperature of phase change materials from the conventional 30-31℃ to over 35℃, thus achieving precise matching of melting temperature and crystallization temperature within the human body temperature window. The sensor fabricated based on this material utilizes the abrupt change in ionic conductivity before and after the PEG phase transition for temperature sensing, achieving a sensitivity of 8.8±0.77% / ℃, more than an order of magnitude higher than traditional conductive filler-type sensors. It also exhibits a response time of less than 5 seconds and good linearity within the 35-45℃ temperature range (R0). 2 =0.992), with a resolution of 0.05℃. Furthermore, PEG is chemically bonded to the polyurethane main chain to form a solid-solid phase transition, fundamentally solving the leakage problem. It possesses excellent long-term cycling stability and biocompatibility, and has broad application prospects in the field of wearable human body temperature monitoring.

[0009] In a first aspect, the present invention provides a method for fabricating a human body temperature sensor based on the phase transition behavior of PU-PEG, comprising the following steps: S1: After mixing polyethylene glycol and an organic solvent, diisocyanate is slowly added dropwise, controlling the NCO / OH molar ratio to be 1.05~2.1:1. Then, dibutyltin dilaurate is added to generate a prepolymer solution with isocyanate groups at both ends. A chain extender is then added dropwise to the prepolymer solution to obtain a polyurethane prepolymer reaction mixture. The polyethylene glycol includes low molecular weight polyethylene glycol with a molecular weight of 2000~4000 g / mol and high molecular weight polyethylene glycol with a molecular weight of 8000~10000 g / mol. S2: Pour the polyurethane prepolymer reaction mixture into a mold, and after drying, obtain a PU-PEG phase change material block or film; S3: Dissolve the PU-PEG phase change material bulk or film in a mixed solvent to prepare a spinning solution, and electrospin the solution to obtain a PU-PEG phase change material nanofiber film. S4: Cut the PU-PEG phase change material nanofiber membrane to the required size, attach flexible electrodes to its upper and lower surfaces, and encapsulate it to obtain a human body temperature sensor based on the PU-PEG phase change behavior.

[0010] The core of this invention, which achieves a breakthrough in the lower limit of crystallization temperature to above 35°C, lies in the triple synergistic effect of mixed soft segments, optimized isocyanate structure, and electrospinning. The mixed soft segments utilize low-molecular-weight PEG (such as PEG2000) to disrupt the excessive crystallinity of high-molecular-weight PEG (such as PEG10000), adjusting the melting initiation temperature to the lower limit of human body temperature (35°C), providing a suitable starting point for phase transition. Furthermore, the preferred alicyclic or aliphatic diisocyanate, with its asymmetric structure, provides a moderate physical crosslinking density, promoting PEG soft segment nucleation without completely restricting chain segment movement, laying the structural foundation for precise control of crystallization behavior. Finally, the electrospinning process, through rapid stretching under a high-voltage electric field and rapid solvent evaporation, prevents the polymer chains from forming large, perfect grains, instead generating a large number of smaller, imperfect grains. Simultaneously, the ultra-high specific surface area of ​​the ultrafine fibers enhances the mobility of the surface molecular chains, thereby shifting the overall crystallization temperature range upward, significantly raising the lower limit of crystallization from 31°C to above 35°C. The synergy of these three factors enables the melting and crystallization windows of the phase change material to precisely match the human body temperature range (35~42℃), providing a material basis for highly sensitive and fast-response temperature sensing.

[0011] Preferably, the polyethylene glycol is vacuum dried at 80-100°C for 2-4 hours before use; the molar ratio of low molecular weight polyethylene glycol to high molecular weight polyethylene glycol in the polyethylene glycol is 3:7.

[0012] Preferably, the diisocyanate is selected from one or more of isophorone diisocyanate, toluene diisocyanate, trimethylhexane diisocyanate, and isophthalic diisocyanate, mixed in any proportion.

[0013] Preferably, the organic solvent is selected from one or two of N,N-dimethylformamide and tetrahydrofuran, mixed in any proportion; the organic solvent needs to be dried before use to control the water content to be less than 50 ppm.

[0014] Preferably, in step S1, polyethylene glycol and an organic solvent are mixed and heated to 50-80°C, diisocyanate is added dropwise, and the mixture is stirred at 60-80°C for 2-3 hours.

[0015] Preferably, the amount of dibutyltin dilaurate is 0.1% to 0.5% of the total mass of the reactants.

[0016] Preferably, the chain extender is a small molecule diol selected from one or more of 2,2-dimethylolpropionic acid, 1,4-butanediol, 4,4'-dihydroxybiphenyl, and 1,5-dihydroxynaphthalene, mixed in any proportion, and the molar ratio of the chain extender to the polyethylene glycol is 1:1.

[0017] Preferably, the specific steps of the electrospinning are as follows: the PU-PEG phase change material bulk or film and thermoplastic polyurethane are dissolved in a mixed solvent of N,N-dimethylformamide and tetrahydrofuran at a mass ratio of 1:1, and then 20% of lithium bis(trifluoromethanesulfonylimide) by solid mass fraction is added to prepare a spinning solution with a mass fraction of 15%~25%; electrospinning is performed using an electrospinning device, and the spinning process parameters are: voltage 8~12 kV, injection speed 0.8~1.2 mL / h, receiving distance 10~15 cm, and roller speed 200~400 rpm to obtain a PU-PEG phase change material nanofiber membrane.

[0018] Secondly, the present invention provides a human body temperature sensor.

[0019] Thirdly, the present invention provides an application of a human body temperature sensor in human body temperature monitoring.

[0020] In summary, the present invention has at least one of the following beneficial technical effects: 1. Addressing the long-standing technical bias in this field that "the crystallization temperature of a single PEG molecular weight system is always lower than the lower limit of human body temperature (≤30℃)," this invention, through the triple synergistic regulation of mixed soft segments, optimized alicyclic / aliphatic diisocyanate, and electrospinning process, has for the first time significantly increased the lower limit of the crystallization temperature of phase change materials from the conventional 30~31℃ to above 35℃. This achieves precise coverage of the human body temperature window (35~42℃) for both melting and crystallization temperatures, solving the core problem that existing phase change materials cannot achieve completely reversible crystallization within the normal human body temperature range.

[0021] 2. This invention utilizes the abrupt change in ionic conductivity before and after the PEG phase transition for temperature sensing, rather than the temperature coefficient of resistance (TCR) mechanism of traditional conductive filler-type sensors. The sensor provided by this invention achieves a sensitivity factor of 8.8±0.77% / ℃ in the range of 32~50℃, which is more than an order of magnitude higher than that of traditional conductive filler-type sensors such as CNT / PDMS; the thermal response time is shortened to less than 5 seconds, which is more than 6 times higher than that of traditional bulk materials; finger touch test shows that the resistance change rate reaches -82% within 0.2 seconds of contact, achieving millisecond-level transient response.

[0022] 3. This invention incorporates PEG into the polyurethane backbone via chemical bonds to form a solid-solid phase change material, fundamentally avoiding the leakage and phase separation problems that occur with traditional microencapsulation or physical blending phase change materials after multiple melt-crystallization cycles. Testing showed that the sensor's error was less than ±0.1℃ during continuous 72-hour simulated human body temperature monitoring, with good resistance recovery and no baseline drift after multiple cycles. Furthermore, the sensor's temperature measurement error was less than ±0.2℃ when bent, demonstrating good flexibility and fit. All materials used meet biocompatibility requirements, making it suitable for long-term direct contact with human skin and showing broad application prospects in the field of wearable body temperature monitoring. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 A schematic diagram of the synthesis mechanism of PU-PEG phase change material provided in the embodiments of this application is shown; Figure 2 A schematic diagram of the electrospinning apparatus and its spun products provided in the embodiments of this application is shown; Figure 3 This illustration shows a macroscopic diagram of the phase transition behavior of the PU-PEG sample provided in this application as a function of temperature. Figure 4 The following are DSC curves of PU synthesized from PEG and IPDI of different molecular weights provided in the embodiments of this application; Figure 5 The DSC curves of PU10k synthesized from different isocyanates provided in the embodiments of this application are shown. Figure 6 The following are DSC curves of PU synthesized by copolymerizing PEG2K and PEG10K in different proportions according to embodiments of this application. Figure 7The accompanying diagram shows a comparison of PU10k spinning and sliver samples provided in an embodiment of this application. Figure 8 The diagram shows the structure and performance test results of the PU-PEG phase change temperature sensor provided in this application embodiment; wherein... Figure 8 A is a schematic diagram of the sensor structure; Figure 8 B is the resistance-temperature response curve; Figure 8 C represents the cyclic stability test curve; Figure 8 D is the response curve for breathing (a) and touch (b). Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0026] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0027] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0028] In the description of this invention, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0029] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] To enable those skilled in the art to better understand this application, the following embodiments are provided to illustrate in detail a human body temperature sensor based on PU-PEG phase transition behavior and its preparation method.

[0031] Example Example 1: Fabrication of a human body temperature sensor based on PU-PEG phase transition behavior Polyethylene glycol (PEG) was dried in a vacuum drying oven at 90°C for 3 hours to remove trace amounts of moisture. In this example, the PEG used was a mixture of PEG2000 (number-average molecular weight 2000 g / mol) and PEG10000 (number-average molecular weight 10000 g / mol) in a molar ratio of 3:7. N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were dried using molecular sieves to control the moisture content to below 50 ppm.

[0032] Under a dry nitrogen atmosphere, the dried PEG was added to a four-necked flask, along with an appropriate volume of DMF solvent. The mixture was heated to 65°C to ensure complete dissolution. Isophorone diisocyanate (IPDI) was slowly added dropwise, maintaining an NCO / OH molar ratio of 1.1:1. Dibutyltin dilaurate (DBTDL) catalyst was added at 0.3% of the total monomer mass. The mixture was stirred at 70°C for 2.5 hours to generate a prepolymer with isocyanate groups at both ends.

[0033] The prepolymer solution was cooled to 55°C, and 2,2-dimethylolpropionic acid (DMPA), a chain extender dissolved in DMF, was slowly added dropwise at a rate of 1-2 drops / second to avoid excessively high local concentrations. The molar ratio of DMPA to PEG was 1:1. The reaction was continued at 65°C with stirring for 2.5 hours to form linear block polyurethane long chains.

[0034] like Figure 1 As shown, the polyurethane elastomer of this invention is prepared using a two-step method. First, under DBTDL conditions, the primary hydroxyl groups of PEG undergo nucleophilic addition with the highly reactive isocyanate groups in excess IPDI, forming urethane bonds and generating a prepolymer with isocyanate groups at both ends. Second, the isocyanate groups at the ends of the prepolymer react with the chain extender DMPA at a higher temperature to extend the molecular chain. Simultaneously, by utilizing the relative excess of isocyanate groups compared to hydroxyl groups in the system, some unreacted isocyanate groups participate in the reaction to form chemical crosslinking points, ultimately constructing a polyurethane elastomer with a three-dimensional network structure.

[0035] After the reaction is complete, the viscous product is poured into a polytetrafluoroethylene mold and dried in a vacuum drying oven at 70°C for 36 hours to remove the solvent, thus obtaining PU-PEG phase change material bulk or film.

[0036] The obtained PU-PEG phase change material and thermoplastic polyurethane (TPU) were dissolved in a DMF / THF mixed solvent (volume ratio 1:1) at a mass ratio of 1:1, and then 20% of lithium bis(trifluoromethanesulfonylimide) by solid mass fraction was added to prepare a spinning solution with a mass fraction of 20%.

[0037] like Figure 2 As shown, the prepared spinning precursor liquid was poured into a 10 ml syringe and allowed to stand for 10 min to cool to room temperature to prevent solvent evaporation. The electrospinning parameters were set as follows: high-voltage power supply voltage 15 kV; syringe pump advance speed 1 ml / min; roller collector rotation speed 3000 rpm; collector lateral movement speed 30 mm / s; and round-trip distance 60 mm. Environmental control was maintained at room temperature (25 ± 2 ℃). During the spinning process, the syringe was fixed to the spinning equipment, the distance between the needle and the collector was adjusted to 15 cm, the high-voltage power supply and syringe pump were turned on, and spinning was carried out for 5 h. The fiber morphology and solution residue were observed during spinning to prevent needle blockage and experimental interruptions due to unforeseen circumstances. After spinning, the spinning equipment was turned off, the fiber membrane was removed, and stored in a desiccator for later use. The resulting PU-PEG phase change material nanofiber membrane can be directly used as the sensitive layer of a temperature sensor.

[0038] The PU-PEG phase change material nanofiber membrane obtained above is cut into the required size (e.g., 1 cm × 1 cm), and flexible electrodes (conductive fabric is used in this embodiment) are attached to its upper and lower surfaces. After encapsulation, a human body temperature sensor based on PU-PEG phase change behavior is obtained.

[0039] like Figure 3 As shown, the TPU sample is opaque at 20 °C, with crystalline and amorphous regions coexisting in its molecular chains. At this point, due to the presence of crystalline regions, the chain movement of the polyurethane soft segments is severely restricted, and the transport of conductive ions via the complexation-decomplexation mechanism is highly dependent on the movement of polymer chain segments, resulting in a low ionic conductivity. When the temperature is raised to 40 °C, the crystalline regions melt, and the material transforms into a uniform amorphous state. The thermal motion of the molecular chains is significantly enhanced, and the released chain segment movement greatly promotes the dissociation and migration of internal conductive ions, leading to a significant increase in the material's conductivity. Subsequently, upon cooling back to 20 °C, the crystalline regions reform, chain segment movement is restricted again, and the conductivity decreases, returning to a near-initial low conductivity state.

[0040] During this process, changes in crystallinity directly affect the ionic conductivity of the material: heating leads to melting of the crystals, increasing the mobility of the chain segments and enhancing conductivity; cooling leads to re-crystallization, freezing of the chain segments, and weakening of conductivity. This indicates that the polyurethane sample is quite sensitive to temperature changes, and its crystallization-melting phase transition behavior directly regulates the transport efficiency of conductive ions in PU-PEG, resulting in significant differences in conductivity under different temperature conditions.

[0041] Based on the above principle, the sensor of this invention can achieve highly sensitive and rapid response detection of human body temperature by monitoring the changes in ionic conductivity or resistance caused by the crystallization-melting phase transition of PEG soft segments.

[0042] Example 2: Effect of PEG molecular weight on phase transition behavior PEG with different number-average molecular weights (2000, 4000, 6000, 8000, 10000 g / mol) were reacted with IPDI to synthesize PU-PEG phase change material bulk according to the preparation method described in this invention. Its thermal properties were tested by differential scanning calorimetry (DSC) at a heating rate of 5℃ / min and a test range of -20 to 80℃.

[0043] The results are as follows Figure 4 As shown, the thermal properties of five polyurethane samples with different molecular weights from PU2k to PU10k show that both Tm and Tc increase with the increase of PEG molecular weight.

[0044] The PEG2000 system (PU2k) has a melting temperature range of 22~39℃, and no obvious crystallization peak was observed. Its upper melting limit (39℃) just touches the lower limit of human body temperature (35℃), but it lacks reversible crystallization behavior and cannot meet the bidirectional regulation requirements of phase change materials.

[0045] PEG4000 system (PU4k): melting range 33~47℃, crystallization range 16~28℃. The melting temperature covers the human body temperature range, but the crystallization temperature is much lower than the human body temperature window (35~42℃).

[0046] The PEG6000 system (PU6k) has a melting range of 38~52℃ and a crystallization range of 25~37℃. The melting onset temperature (38℃) is relatively high, and the highest crystallization temperature is only 37℃, failing to fully cover the 35~42℃ range.

[0047] The PEG8000 system (PU8k) has a melting range of 40~55℃ and a crystallization range of 29~42℃. The upper limit of the crystallization temperature (42℃) matches the human body temperature window, but the lower limit of crystallization (29℃) is still significantly lower than 35℃, and the melting initiation temperature (40℃) is relatively high.

[0048] The PEG10000 system (PU10k) has a melting range of 40~60℃ and a crystallization range of 30~42℃. It also has the problem of a low lower crystallization limit (30℃) and a high melting initiation temperature (40℃).

[0049] The above results indicate that in the single molecular weight PEG system, only PU8k and PU10k have an upper limit of crystallization temperature of 42℃, but their lower limits of crystallization of 29℃ and 30℃ are still significantly lower than the human body temperature of 35℃, and their melting initiation temperature is ≥40℃.

[0050] Therefore, simply adjusting the molecular weight of PEG cannot precisely match the melting temperature and crystallization temperature to the human body temperature window (35~42℃). Other control methods must be introduced to solve the technical problem of low crystallization temperature.

[0051] Example 3: Effect of Isocyanate Type on Phase Transition Behavior Using PEG10000 as the fixed soft segment, it was reacted with three diisocyanates: toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and trimethylhexamethylene diisocyanate (TMDI) to synthesize PU-PEG phase change material bulk according to the preparation method described in this invention. Its thermal properties were tested using DSC at a heating rate of 5℃ / min, within a test range of -20 to 80℃. The results are as follows. Figure 5 As shown.

[0052] Diisocyanates with different structures have varying abilities to restrict the crystallization behavior of PEG soft segments due to differences in molecular symmetry, rigidity, and reactivity, which in turn affects the Tm and Tc of polyurethane materials.

[0053] The specific results are as follows: The TDI system (PEG10k-TDI) has a Tm of 50-62℃ and a Tc of 41-47℃. TDI is an aromatic diisocyanate, with its isocyanate groups directly linked to the benzene ring. Its high molecular rigidity and the coexistence of isomers result in poor symmetry, leading to irregular hard segment stacking. This steric hindrance primarily restricts PEG crystallization. The crystallization temperature range (41-47℃) falls entirely within the human body temperature window (35-42℃), and its sharp DSC peaks indicate well-formed and uniform crystals. However, the melting initiation temperature (50℃) is too high, far exceeding the upper limit of human body temperature, preventing the material from initiating a melt phase transition at normal body temperature.

[0054] IPDI system (PEG10k-IPDI): Tm=41~60℃, Tc=31~42℃. IPDI is an alicyclic diisocyanate with moderate reactivity, good regularity of the hard segments formed, and moderate physical crosslinking density. The upper limit of crystallization temperature (42℃) of this system is well matched, and the DSC peak is broadened, which is due to the limitation of hard segment stacking by its alicyclic asymmetric structure. However, the lower limit of crystallization (31℃) is still lower than the lower limit of human body temperature (35℃), and the melting onset temperature (41℃) is slightly higher than the upper limit of human body temperature.

[0055] The TMDI system (PEG10k-TMDI): Tm = 38~57℃, Tc = 27~42℃. TMDI is an aliphatic diisocyanate with a methyl side chain in its molecule, resulting in significant steric hindrance, low reactivity, and a weak constraint on PEG crystallization by the hard segments formed. The melting onset temperature (38℃) of this system is closest to the ideal human body temperature, and the broadened DSC peak reflects imperfect crystals and low regularity. However, the lower crystallization limit (27℃) is significantly low, failing to meet the requirements for reversible crystallization.

[0056] The above results indicate that no single isocyanate system can simultaneously achieve the ideal melt initiation temperature and crystallization lower limit. The TDI system has a perfect crystallization range but an excessively high melt temperature; the IPDI system has moderate overall performance but a high melt initiation temperature and a low crystallization lower limit; and the TMDI system has an ideal melt initiation temperature but an excessively low crystallization lower limit. Therefore, a single isocyanate cannot simultaneously meet the dual requirements of melt and crystallization temperatures, and further integration with other control methods such as mixed soft segments and electrospinning is necessary.

[0057] Example 4: Effect of blending PEGs of different molecular weights on phase transition behavior Since a single molecular weight PEG system cannot simultaneously meet the requirements for melting temperature and crystallization temperature, this invention further adopts a mixed soft segment strategy, that is, PEGs of different molecular weights are blended in a certain proportion as soft segments. Long-chain PEGs provide higher phase transition enthalpy, while short-chain PEGs disrupt the excessive crystal regularity of the soft segments, thereby widening or shifting the phase transition temperature window while retaining a high latent heat of phase transition.

[0058] This invention systematically studied the thermal properties of blends of PEG2000 and PEG10000 at different molar ratios. The test results are as follows: Figure 6 As shown, the four formulations share a common performance defect: the lower limit of the crystallization temperature is significantly lower than the lower limit of the human body temperature window, which prevents the material from completing the full exothermic crystallization process within the normal human body temperature range.

[0059] PEG2000:PEG10000=1:9: Tm is 39~59℃, Tc is 25~41℃. With this ratio, the melting initiation temperature is relatively high, the upper limit of the crystallization temperature reaches 41℃, but the lower limit of crystallization (25℃) is still too low.

[0060] PEG2000:PEG10000 = 2:8: Tm = 36~57℃, Tc = 20~37℃. The melting temperature range is suitable, but the upper limit of crystallization is only 37℃, which cannot completely cover the human body temperature window (35~42℃).

[0061] PEG2000:PEG10000=3:7:Tm=35~59℃,Tc=21~35℃. The melting onset temperature is closest to the lower limit of human body temperature (35℃), but the upper limit of crystallization is only 35℃, and the lower limit of crystallization (21℃) is still significantly low.

[0062] PEG2000:PEG10000 = 3:1: Tm = 39~56℃, Tc = 18~34℃. The overall crystallization temperature is too low and does not meet the requirements.

[0063] In addition, this invention also investigated the blending system of PEG2000 and PEG6000 in an equimolar ratio (1:1), with Tm=27~52℃ and Tc=12~30℃. The melting initiation temperature was too low, and the crystallization temperature range was completely below the human body temperature window.

[0064] Further analysis revealed that while introducing low molecular weight PEG2000 into the PEG10000 matrix could adjust the melt initiation temperature to some extent, shifting it towards human skin temperature (e.g., reducing the initial Tm to 35℃ in a 3:7 ratio), this optimization came at the cost of sacrificing crystallinity. The introduction of PEG2000 disrupted the regularity of the soft segment chains, reducing the material's crystallinity, manifested as a further downward shift in the lower limit of the crystallization temperature and a decrease in the enthalpy of crystallization. The trade-off between improved melt initiation temperature and deteriorated crystallinity indicates that a single mixed soft segment strategy cannot simultaneously meet the dual requirements of melting and crystallization temperatures for human body temperature monitoring.

[0065] In summary, the mixed soft-segment strategy can effectively regulate the melt initiation temperature, bringing it closer to the lower limit of human body temperature (e.g., Tm in a 3:7 ratio drops to 35℃). However, this strategy still does not solve the core problem of a low lower limit of crystallization temperature; the lower limit of Tc is ≤25℃ in all blend ratios, making it impossible for the material to complete reversible crystallization within the normal human body temperature range. Therefore, the mixed soft-segment strategy needs to be combined with other control methods such as isocyanate structure optimization and electrospinning to achieve a precise match between the crystallization temperature and the human body temperature window.

[0066] Example 5: Regulation of Phase Transition Behavior by Electrospinning Taking the PEG10000-IPDI system as an example, the PU-PEG phase change material block prepared according to the method of this invention, denoted as "strip", was compared with the nanofiber membrane obtained by electrospinning by DSC test. The results are as follows: Figure 7 As shown: PU10k spline: Tm=41~60℃, Tc=31~42℃.

[0067] PU10k-20% spun film: Tm=39~60℃, Tc=35~43℃ and above.

[0068] PU10k-18% spun film: Tm=37~60℃, Tc=35~44℃ and above.

[0069] like Figure 7 As shown, electrospinning reduces the melting initiation temperature from 41℃ to 37~39℃, and significantly increases the crystallization temperature range from 31~42℃ to above 35~44℃. The lower limit of crystallization falls exactly at the lower limit of human body temperature (35℃), achieving a precise match between the crystallization temperature and the human body temperature window.

[0070] During the spinning process, the molecular chains are rapidly stretched and oriented under a high-voltage electric field, and the solvent evaporates quickly. The polymer does not have time to form large and perfect grains, but instead generates a large number of small and imperfect grains. At the same time, the ultra-high specific surface area of ​​the ultrafine fiber enhances the mobility of the surface molecular chains, enabling them to initiate melting motion at a relatively low temperature.

[0071] The above results indicate that electrospinning is an effective means of controlling the crystallization behavior of PU-PEG phase change materials, which can shift the overall crystallization temperature range upward and achieve precise matching with the human body temperature window. However, when electrospinning is used alone, the melting initiation temperature is still too high (37~39℃) and fails to drop to the ideal range (35℃).

[0072] Further analysis of Examples 2-4 confirms that no single control method can simultaneously achieve a precise match between the melting and crystallization temperatures and the human body temperature window. All three methods must be used in combination to simultaneously achieve a melting initiation temperature ≤38℃ and a crystallization lower limit ≥35℃, thus ensuring a precise match between the phase transition window and the human body temperature range (35~42℃).

[0073] Example 6: Sensor Structure and Performance Testing like Figure 8As shown in Figure A, the human body temperature sensor prepared in Example 1 adopts a sandwich structure, consisting of a flexible substrate (PU-PEG composite film) and a temperature-sensitive layer in the middle, as well as positive and negative electrodes located on the upper and lower sides of the film, respectively. Its working principle utilizes the temperature-sensitive properties of PEG: when the temperature changes, the electrical properties of the film, such as ionic conductivity, change; the temperature response is achieved by measuring the change in the electrical signal between the two electrodes. This structural design gives the sensor both good flexibility and temperature response capability.

[0074] like Figure 8 As shown in Figure B, the resistivity change of the PU-PEG spun film was tested in the range of 32-50℃. The sensitivity factor was calculated using the formula ΔR / R0, and the results showed that GF = 8.8 ± 0.77 % / ℃, and the linear fit R0 was [value missing]. 2 =0.992. This indicates that the material is highly sensitive to temperature and exhibits excellent linearity; the resistance decreases by 8.8% for every 1°C increase in temperature, making it suitable for high-precision temperature detection. The resistance decreases monotonically with increasing temperature, reaching 82% at 50°C, which is consistent with the characteristics of thermistor materials.

[0075] like Figure 8 As shown in Figure C, the PU-PEG film exhibits good reversibility and stability during temperature cycling from RT to 40 / 45 / 50℃ and back to RT. The resistance decreases with increasing temperature at each level, and recovers to near the baseline after cooling (offset <5%), without irreversible structural damage, demonstrating long-term stability. The magnitude of the resistance decrease increases with the temperature gradient, stemming from a thermally activated conductivity mechanism that enhances carrier migration efficiency, making it suitable for detecting minute temperature differences without requiring frequent calibration.

[0076] like Figure 8 As shown in Figure D, during the breathing test, the PU-PEG spun film exhibits a large oscillatory response to periodic exhaled airflow, with a maximum change of 23% per cycle, influenced by both temperature and humidity physical fields. During exhalation, heat enhances molecular chain motion, moisture permeation releases protons, and the resistance decreases instantaneously; during inhalation, cooling and moisture evaporation allow the resistance to recover within 0.3 seconds. No baseline drift was observed for 10 consecutive cycles, demonstrating stable thermal sensing performance. In the finger touch test, ΔR / R0 reached -82% within 0.2 seconds of contact and returned to baseline after 0.3 seconds of removal. This superior performance is attributed to the nanoscale thermal conduction pathways formed by the film's high specific surface area and porosity, achieving millisecond-level transient response and extremely high sensitivity.

[0077] In summary, this invention provides a human body temperature sensor based on the phase change behavior of PU-PEG and its preparation method. A nanofiber membrane with solid-solid phase change characteristics was obtained using the preparation method in Example 1. Systematic comparative studies in Examples 2-5 show that simply adjusting the PEG molecular weight, changing the isocyanate type, using a mixed soft segment strategy, or performing electrospinning alone cannot simultaneously and accurately match the melting temperature and crystallization temperature of the phase change material to the human body temperature window (35-42℃).

[0078] This invention combines the use of mixed soft segments, alicyclic / aliphatic diisocyanates, and electrospinning processes to produce a synergistic effect far exceeding the linear superposition of individual methods: the lower limit of the crystallization temperature is significantly increased from 30~31℃ to above 35℃, and the melting initiation temperature is reduced to below 38℃, achieving a precise match between the melting and crystallization windows of the phase change material and the human body temperature range.

[0079] The sensor fabricated based on this synergistic strategy exhibits excellent temperature sensing performance: sensitivity reaches 8.8±0.77% / ℃, and linear fit R0 is high. 2 =0.992, response time less than 5 seconds, temperature measurement range 35~45℃, resolution 0.05℃, continuous 72-hour monitoring error less than ±0.1℃, error under bending condition less than ±0.2℃, and has millisecond-level transient response and good cycle stability.

[0080] This invention incorporates PEG into the polyurethane backbone through chemical bonding to form a solid-solid phase change structure, fundamentally eliminating the leakage problem of traditional microcapsule-encapsulated phase change materials. It also possesses excellent flexibility, long-term stability, and biocompatibility, and has broad application prospects in the field of wearable real-time human body temperature monitoring.

[0081] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0082] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0083] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element.

[0084] The above provides a detailed description of a human body temperature sensor based on PU-PEG phase transition behavior and its preparation method. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for fabricating a human body temperature sensor based on the phase transition behavior of PU-PEG, characterized in that, Includes the following steps: S1: After mixing polyethylene glycol and an organic solvent, diisocyanate is slowly added dropwise, controlling the NCO / OH molar ratio to be 1.05~2.1:

1. Then, dibutyltin dilaurate is added to generate a prepolymer solution with isocyanate groups at both ends. A chain extender is then added dropwise to the prepolymer solution to obtain a polyurethane prepolymer reaction mixture. The polyethylene glycol includes low molecular weight polyethylene glycol with a molecular weight of 2000~4000 g / mol and high molecular weight polyethylene glycol with a molecular weight of 8000~10000 g / mol. S2: Pour the polyurethane prepolymer reaction mixture into a mold, and after drying, obtain a PU-PEG phase change material block or film; S3: Dissolve the PU-PEG phase change material bulk or film in a mixed solvent to prepare a spinning solution, and electrospin the solution to obtain a PU-PEG phase change material nanofiber film. S4: Cut the PU-PEG phase change material nanofiber membrane to the required size, attach flexible electrodes to its upper and lower surfaces, and encapsulate it to obtain a human body temperature sensor based on the PU-PEG phase change behavior.

2. The preparation method according to claim 1, characterized in that, The polyethylene glycol needs to be vacuum dried at 80~100℃ for 2~4 hours before use; the molar ratio of low molecular weight polyethylene glycol to high molecular weight polyethylene glycol in the polyethylene glycol is 3:

7.

3. The preparation method according to claim 1, characterized in that, The diisocyanate is selected from one or more of isophorone diisocyanate, toluene diisocyanate, trimethylhexane diisocyanate, and isophthalic diisocyanate, and is composed of any proportion of these components.

4. The preparation method according to claim 1, characterized in that, The organic solvent is selected from one or two of N,N-dimethylformamide and tetrahydrofuran, mixed in any proportion; the organic solvent must be dried before use to control the water content to be less than 50 ppm.

5. The preparation method according to claim 1, characterized in that, In step S1, polyethylene glycol and an organic solvent are mixed and heated to 50-80°C. Diisocyanate is then added dropwise and the mixture is stirred at 60-80°C for 2-3 hours.

6. The preparation method according to claim 1, characterized in that, The amount of dibutyltin dilaurate used is 0.1% to 0.5% of the total mass of the reactants.

7. The preparation method according to claim 1, characterized in that, The chain extender is a small molecule diol selected from one or more of 2,2-dimethylolpropionic acid, 1,4-butanediol, 4,4'-dihydroxybiphenyl, and 1,5-dihydroxynaphthalene, mixed in any proportion, and the molar ratio of the chain extender to the polyethylene glycol is 1:

1.

8. The preparation method according to claim 1, characterized in that, The specific steps of the electrospinning are as follows: the PU-PEG phase change material bulk or film is dissolved in a mixed solvent of N,N-dimethylformamide and tetrahydrofuran at a mass ratio of 1:1 with thermoplastic polyurethane, and then 20% lithium bis(trifluoromethanesulfonylimide) by solid mass fraction is added to prepare a spinning solution with a mass fraction of 15%~25%; electrospinning is carried out using an electrospinning device, and the spinning process parameters are: voltage 8~12 kV, injection speed 0.8~1.2 mL / h, receiving distance 10~15 cm, and roller speed 200~400 rpm to obtain a PU-PEG phase change material nanofiber membrane.

9. A human body temperature sensor prepared by any one of the preparation methods described in claims 1 to 8.

10. The application of the human body temperature sensor according to claim 9 in human body temperature monitoring.

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