A whitening temperature indicator based on polyvinyl acetal-based polymers

Abstract

The present application relates to temperature sensitive indicating material, cold chain monitoring and biomedical packaging technical field, specifically relates to a kind of temperature indicator based on polyvinyl acetal polymer white type.It is specifically technical scheme that the high temperature indicator includes temperature sensitive indicating layer, the temperature sensitive indicating layer includes modified polyvinyl acetal polymer, the modified polyvinyl acetal polymer is prepared by acetalization reaction of polyvinyl alcohol and aliphatic aldehyde.This application solves the problems of high cost, not intuitive, difficult to characterize cumulative heat exposure history and difficult to grade severity in the existing vaccine temperature monitoring mode.

Description

Technical Field

[0001] This invention relates to the fields of temperature-sensitive indicator materials, cold chain monitoring and functional polymer materials, specifically to a whitened temperature indicator based on polyvinyl acetal polymers. Background Technology

[0002] Vaccines are typical temperature-sensitive biological products, and their storage, transportation, and use typically require cold chain management. For most routinely refrigerated vaccines, the recommended storage and transportation temperature is +2°C to +8°C. When vaccines are exposed outside this recommended temperature range, especially under conditions of high or low temperature exposure, freezing, or repeated temperature fluctuations, it may lead to decreased vaccine potency, weakened immune protection, increased risk of revaccination, and vaccine waste. Improper storage and handling can reduce vaccine efficacy and result in significant economic losses and administrative burdens.

[0003] Besides high-temperature exposure, freezing is also a significant risk factor in the vaccine cold chain. Some liquid vaccines, especially those containing adjuvants, are highly sensitive to freezing; improper exposure to low temperatures can lead to irreversible quality loss. Related documents indicate that vaccines can be sensitive to heat, cold, and light, and some frozen-sensitive vaccines may have already suffered quality damage even if no obvious abnormalities are visible. Therefore, relying solely on visual observation is usually insufficient to accurately determine the actual temperature history a vaccine has experienced.

[0004] Current vaccine temperature monitoring methods mainly include electronic temperature recording devices, cold chain temperature recording sheets, freezing indicators, and vaccine vial monitors (VVMs). Electronic temperature recording devices can continuously monitor temperature changes, but they typically suffer from high costs, require power supplies or integrated reading systems, and are complex to use and maintain on-site. Freezing indicators are primarily used to identify low-temperature freezing events, and their applicability is relatively limited. VVMs, on the other hand, respond to cumulative temperature exposure over time using thermosensitive materials and achieve irreversible recording through gradually deepening color. Therefore, they offer advantages such as no need for power, lower cost, and ease of on-site interpretation, and have been widely used in the vaccine supply chain. However, existing technologies still have certain shortcomings. Documents regarding the application of controlled temperature chains (CTCs) and VVMs indicate that VVMs are mainly used to reflect the cumulative heat exposure experienced by vaccines, but cannot characterize freezing exposure, and are difficult to identify sudden temperature peaks above 37°C in a timely manner. For severe overheating events exceeding 40°C, peak temperature threshold indicators are still needed for auxiliary identification in certain application scenarios. In other words, the existing indicator system still has shortcomings in the rapid identification of high-temperature anomalies, severity classification, characterization of sudden thermal shocks, and more intuitive material-level visualization. In the actual cold chain distribution of vaccines, overcooling exposure events occur frequently, especially in low-resource areas with insufficient power supply and rudimentary temperature control equipment. Refrigerated trucks used for cold chain transportation and cold storage facilities are prone to temperature control failures. Coupled with the lack of effective real-time monitoring methods, sub-zero overcooling exposures often go undetected. Furthermore, traditional cold chain temperature monitoring relies heavily on electronic temperature recorders, which have drawbacks such as requiring power, being easily damaged, requiring specialized equipment to read data, and lacking intuitive interpretation. These devices are poorly suited for low-resource areas, making it difficult to achieve rapid traceability of cold chain anomalies and on-site screening of vaccine quality.

[0005] Furthermore, the vaccine cold chain management process is long, involves many links, and has significantly different equipment conditions. Any oversight in any intermediate link can affect vaccine quality. Related studies have shown that if vaccines are not stored under suitable conditions, their effectiveness may decrease significantly. Therefore, based on practical application needs, there is an urgent need to develop a novel temperature indicator material that is low-cost, easy to interpret on-site, requires no complex equipment, can record abnormal heat exposure history, and is easily integrated with vaccine packaging systems.

[0006] Based on this, developing a temperature-sensitive indicator that exhibits significant whitening after exposure to abnormally high or low temperatures, with the degree of whitening cumulatively increasing over exposure time, has significant application value. This type of material is expected to enable rapid identification of abnormal high or low temperature events and further visualize and classify the degree of abnormality, thus providing a new technical means for risk monitoring throughout the entire vaccine cold chain process. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a whitening temperature indicator based on polyvinyl acetal polymers. This overcomes the problems of insufficient intuitiveness in temperature anomaly identification, inadequate characterization of sudden heat exposure, limited severity grading capabilities, and difficulty in achieving low-cost, rapid on-site interpretation in existing technologies. The temperature indicator can write and visually read out abnormal temperature history, making it suitable for abnormal temperature monitoring in vaccine storage, transportation, turnover, and vaccination scenarios. The abnormal temperatures include high-temperature exposure and supercooling exposure.

[0008] To achieve the above objectives, the present invention provides the following technical solution: This invention utilizes the adaptive swelling response of polyvinyl acetal polymer networks to temperature history, incorporating anomalous temperature information into the material's equilibrium moisture content, chain segment association state, and network elasticity. Subsequently, through microphase structure reconstruction and light scattering enhancement, the anomalous temperature history is visualized and read out. This readout manifests macroscopically as changes in transparency, turbidity, opacity, or whitening.

[0009] This invention discloses a whitened temperature indicator based on polyvinyl alcohol acetal polymers. The temperature indicator includes a temperature-sensitive indicator layer, which comprises a modified polyvinyl alcohol acetal polymer. The modified polyvinyl alcohol acetal polymer is prepared by an acetalization reaction between polyvinyl alcohol and aliphatic aldehydes.

[0010] Preferably, the degree of polymerization of the polyvinyl alcohol is 300-2600 and the degree of hydrolysis is 88-99%.

[0011] Preferably, the fatty aldehyde is one or more of propionaldehyde, butyraldehyde, pentanaldehyde, hexanal, heptanaldehyde, and octanaldehyde.

[0012] Accordingly, a method for preparing a whitening temperature indicator based on polyvinyl acetal polymers includes the following steps: (1) Preparation of modified polyvinyl alcohol acetal polymers Polyvinyl alcohol is dissolved in water or an organic reagent to obtain a transparent solution. After activation with an acidic catalyst, aliphatic aldehydes are added dropwise to carry out an acetalization reaction. After the reaction is completed, the reaction is terminated and the obtained product is washed and dried to obtain a modified polyvinyl alcohol acetal polymer. (2) Preparation of modified polyvinyl alcohol thermosensitive hydrogel The modified polyvinyl alcohol acetal polymer was dissolved in an organic solvent and cast into a mold. After drying and swelling treatment, the modified polyvinyl alcohol thermosensitive hydrogel was obtained.

[0013] Preferably, in step (1), the organic reagent is one of ethanol, DMF, and DMSO, and the acidic catalyst is one or more of p-toluenesulfonic acid monohydrate, hydrochloric acid, methanesulfonic acid, and sulfuric acid.

[0014] Preferably, in step (1), the concentration of polyvinyl alcohol is 2-10 wt%, and the amount of acidic catalyst added is 8-12% of the mass of polyvinyl alcohol; in step (2), the concentration of the modified polyvinyl alcohol acetal polymer in the organic solvent is 50-100 mg / mL.

[0015] Preferably, in step (2), the organic solvent is at least one of dimethylformamide, dimethyl sulfoxide, and dimethylacetamide.

[0016] Preferably, in step (2), the dry film obtained after drying is swollen in deionized water at a preset temperature. After swelling equilibrium is reached, a modified polyvinyl alcohol thermosensitive hydrogel is obtained.

[0017] Accordingly, a method for using a whitened temperature indicator based on polyvinyl acetal polymers to indicate the failure of cold chain vaccines due to overcooling exposure includes the following steps: 1) Store the prepared temperature indicator in deionized water at the same storage temperature as the expiration indicator label; 2) Place the expiration indicator label together with the vaccine in a cold chain transportation storage device, and let it complete the entire cold chain circulation with the vaccine; 3) During vaccine storage and use, under a safe temperature environment of 2-8℃, directly observe the appearance of the temperature indicator on the label: if the gel remains transparent, it indicates that the vaccine has not been exposed to sub-zero temperatures and the cold chain is normal; if the gel becomes opaque, it indicates that the vaccine has been exposed to sub-zero temperatures and is at risk of denaturation, flocculation, or decreased efficacy.

[0018] The present invention has the following beneficial effects: 1. The temperature indicator prepared by the present invention can passively write and read abnormal temperature history without relying on power supply and complex reading devices.

[0019] 2. The temperature indicator prepared by this invention can be used for both high temperature anomaly monitoring and supercooled exposure monitoring, and has a unified material basis and mechanism basis. 3. In high-temperature applications, the temperature indicator prepared by the present invention records the history of thermal exposure through irreversible whitening; in low-temperature applications, the temperature indicator records the history of supercooling exposure through changes in transparency / opaqueness.

[0020] 4. The polyvinyl acetal polymers used in this invention are widely available and have relatively simple processes. They can be made into films, labels, or patches, which facilitates integration with vaccine packaging systems.

[0021] 5. The temperature-sensitive hydrogel of the present invention has low-temperature adaptive properties. During the low-temperature reswelling process, the critical phase transition temperature can be stably reset and written, and the supercooling temperature and time information during the cold chain transportation and storage of vaccines can be passively recorded in the gel medium. Moreover, the hydrogel can read out the supercooling exposure information through the transparent / opaque appearance of the phase transition within the safe storage temperature range of 2-8°C for vaccines. It accurately indicates whether the vaccine has experienced sub-zero (≤0°C) supercooling exposure and related information, solving the technical problem of difficulty in timely detection and traceability of supercooling exposure events in the vaccine cold chain. It provides an intuitive, passive, and convenient indication solution for vaccine cold chain quality monitoring, and is especially suitable for low-resource areas with insufficient power and temperature control refrigeration equipment.

[0022] 7. The thermosensitive hydrogel of the present invention is used in the indication of cold chain vaccine overcooling exposure failure. As a passive indicator medium, it is used to record and indicate sub-zero (Te≤0℃) overcooling exposure events during the cold chain transportation and storage of vaccines. Within the safe storage temperature range of 2-8℃ for vaccines, the overcooling exposure information can be read intuitively through the phase change of the transparent / opaque appearance of the gel, so as to realize the traceability of cold chain abnormalities. Attached Figure Description

[0023] Figure 1 A schematic diagram of the preparation process for albino vaccine temperature indicators; Figure 2 The NMR spectrum of PVA-C4; Figure 3 The transmittance of PVA-C4-gel based on the 550nm wavelength ultraviolet-visible (UV) spectrum; Figure 4 Real images of PVA-C4-gel at different pre-equilibrium temperatures and different overheating durations; Figure 5 A true image of PVA-C4-gel as temperature increases; Figure 6 The transmittance of PVA-C4-gel at 550 nm wavelength as temperature increases is shown in the UV-Vis spectrum. Figure 7 This is a scanning electron microscope image of PVA-C4 thermosensitive hydrogel in its transparent state. Figure 8 The image is a scanning electron microscope image of PVA-C4 thermosensitive hydrogel after high-temperature treatment and irreversible whitening. Figure 9 This is a schematic diagram of the preparation process of PVA-C4-gel; Figure 10 A schematic diagram illustrating how heat and time drive the rewriting of gel phase transition behavior in PVA-C4-gel; Figure 11 Appearance of PVA-C4-gel under different exposure temperatures and times (non-cooled: transparent gel; supercooled exposure: opaque gel). Figure 12 The graph shows the changes in gel transparency ΔT550nm caused by the time effect (t=0-75min) and exposure temperature effect (Te=0℃, -5℃, -10℃) of the PVA-C4-gel indicator label. Figure 13 The graph shows the changes in the equilibrium water content of PVA-C4-gel due to the temperature effect of PVA-C4-gel exposure (Te=0℃, -5℃, -10℃); Figure 14 The MRI image of PVA-C6; Figure 15 The transmittance of PVA-C6-gel based on the 550nm wavelength ultraviolet-visible (UV) spectrum; Figure 16 Real images of PVA-C6-gel at different pre-equilibrium temperatures and different overheating durations; Figure 17 A true image of PVA-C6-gel as temperature increases; Figure 18 The transmittance of PVA-C6-gel at 550 nm wavelength as temperature increases is shown in the UV-Vis spectrum. Figure 19 Appearance of PVA-C6-gel under different exposure temperatures and times (non-cooled: transparent gel; overcooled exposure: opaque gel). Figure 20 The graph shows the changes in gel transparency ΔT550nm caused by the time effect (t=0-75min) and exposure temperature effect (Te=0℃, -5℃, -10℃) of the PVA-C6-gel indicator label. Figure 21 The graph shows the changes in the equilibrium water content of PVA-C6-gel due to the temperature effect of PVA-C6-gel exposure (Te=0℃, -5℃, -10℃). Detailed Implementation

[0024] 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. 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.

[0025] Unless otherwise specified, the technical means used in the implementation examples are conventional means well known to those skilled in the art.

[0026] This invention provides a whitening temperature indicator based on polyvinyl acetal polymers, comprising a temperature-sensitive indicator layer; the temperature-sensitive indicator layer comprises a modified polyvinyl acetal polymer, preferably a short-chain alkyl-modified polyvinyl acetal polymer hydrogel material. As another embodiment, the temperature-sensitive indicator layer can be disposed on a substrate layer, wherein the substrate layer is one of PET, PP, PE, composite aluminum-plastic film, paper-based labels, medical nonwoven fabrics, cold chain hang tags, and medicine box window materials. Of course, the substrate layer can also be omitted, and the temperature-sensitive indicator layer used alone, if necessary.

[0027] The modified polyvinyl alcohol acetal polymer is formed by acetalization of polyvinyl alcohol (PVA) with aliphatic aldehydes to create a PVA-Cn material with a certain degree of hydrophobic modification. PVA-Cn represents a polyvinyl alcohol acetal polymer obtained by acetalizing PVA with an aliphatic aldehyde having n carbon atoms. PVA-Cn-gel represents a gel film further prepared from the modified polyvinyl alcohol acetal polymer. Here, n is 3, 4, 5, 6, 7, or 8. The degree of polymerization of the polyvinyl alcohol is 300-2600, and the degree of alcoholysis is 88-99%.

[0028] The amount of aliphatic aldehyde added can be 30-70 mol% of the molar amount of hydroxyl groups on the PVA molecular chain. In this invention, the modification rate of the modified polyvinyl acetal polymer is mainly controlled by the amount of aliphatic aldehyde added. The above-mentioned amount is not considered a limitation of this invention, but rather one option, and the amount of aliphatic aldehyde added can be selected as needed. The aliphatic aldehyde is one or more of propionaldehyde, butyraldehyde, pentanaldehyde, hexanal, heptanaldehyde, and octanaldehyde.

[0029] The temperature-sensitive indicator layer is formed by solvent-non-solvent induced phase separation and then undergoes subsequent heat treatment and swelling treatment to obtain an initial transparent or semi-transparent state. When the temperature-sensitive indicator layer is exposed to heat above a preset threshold temperature, its internal microphase structure undergoes irreversible evolution, resulting in enhanced light scattering and whitening. The degree of whitening increases with the heat exposure time.

[0030] The temperature indicator can be set with one or more temperature thresholds, preferably: a first warning temperature zone: 8-20℃; a second abnormal temperature zone: 20-30℃; and a third severe heat exposure temperature zone: 30-40℃. 15℃, 25℃, and 35℃ can be used as grading decision points.

[0031] The temperature indicator exhibits an irreversible or substantially irreversible whitening response after thermal exposure, enabling the cumulative recording of abnormal thermal history. The degree of whitening can be qualitatively determined by visual observation or quantitatively evaluated by changes in transmittance at wavelengths of 550 nm and / or 650 nm.

[0032] After experiencing low-temperature exposure, the temperature indicator records the history of supercooling exposure through changes in transparency / opaqueness.

[0033] This invention provides a method for preparing a whitening temperature indicator based on polyvinyl acetal polymers, comprising the following steps: (1) Preparation of modified polyvinyl alcohol acetal polymers Polyvinyl alcohol (PVA) was dissolved in deionized water or an organic reagent and heated and stirred at 80-95°C until completely dissolved, yielding a transparent PVA solution. Then, an acidic catalyst was added for activation, followed by the dropwise addition of aliphatic aldehydes to initiate an acetalization reaction. After the reaction was complete, the solution was slowly poured into a saturated sodium bicarbonate solution at room temperature to terminate the reaction, at which point a white solid precipitated. The resulting precipitate was washed (with deionized water) and dried to obtain a white granular modified polyvinyl alcohol acetal polymer.

[0034] The PVA can be selected from polyvinyl alcohol raw materials with different degrees of polymerization, molecular weights, and degrees of alcoholysis. The PVA concentration is controlled at 2-10 wt%, and the amount of acidic catalyst added is 8-12% of the mass of polyvinyl alcohol.

[0035] The organic reagent is one of ethanol, DMF, and DMSO, and the acidic catalyst is one or more of p-toluenesulfonic acid monohydrate, hydrochloric acid, methanesulfonic acid, and sulfuric acid.

[0036] (2) Preparation of modified polyvinyl alcohol thermosensitive hydrogel The modified polyvinyl acetal polymer is added to an organic solvent at a concentration of 50-100 mg / mL. The solution is heated and stirred at 90-105°C for 6-15 hours until the polymer is completely dissolved, yielding a transparent solution. The resulting transparent solution is poured into a mold for casting. The mold is then vacuum-dried for 12-24 hours to obtain a modified polyvinyl alcohol dry film. The organic solvent is at least one of DMF (dimethylformamide), DMSO (dimethyl sulfoxide), and DMAc (dimethylacetamide).

[0037] After the dry film is formed, the resulting film material is immersed in deionized water at a preset temperature for swelling treatment, allowing it to fully absorb water and form a thermosensitive hydrogel (i.e., modified polyvinyl alcohol thermosensitive hydrogel). The swelling temperature (i.e., the preset temperature) can be set to different temperatures as needed (e.g., 2-8℃) to regulate the structural state and thermosensitive response behavior of the resulting hydrogel; for example, after adjusting to an appropriate temperature in a temperature-controlled water bath, the swelling temperature can be set to the storage temperature required for the vaccine, and the color change of the hydrogel within this temperature range can be observed after subsequent heating.

[0038] This invention provides a method for monitoring temperature anomalies. A temperature indicator is placed on the vaccine vial, outer packaging, turnover box, refrigerated bag, or cold chain record card. By observing whether the temperature indicator whitens and the degree of whitening, the method determines whether the vaccine has experienced abnormal heat exposure and its severity. The temperature indicator remains whitened after abnormal heat exposure, thus irreversibly recording the thermal history. After low-temperature exposure, the temperature indicator records the supercooling exposure history through changes in transparency / opaqueness.

[0039] This invention provides a method for using a whitened temperature indicator based on polyvinyl acetal polymers to indicate the failure of cold chain vaccines due to overcooling exposure, comprising the following steps: 1) The prepared thermosensitive hydrogel was stored in deionized water at the same storage temperature as an expiration indicator label; 2) Place the expiration indicator label together with the vaccine in a cold chain transportation storage device, and let it complete the entire cold chain circulation with the vaccine; 3) During vaccine storage and use, under a safe temperature environment of 2-8℃, directly observe the appearance of the temperature indicator on the label: if the gel remains transparent, it indicates that the vaccine has not been exposed to sub-zero temperatures and the cold chain is normal; if the gel becomes opaque, it indicates that the vaccine has been exposed to sub-zero temperatures, and the vaccine is at risk of denaturation, flocculation, or decreased efficacy. The vaccines mentioned are those requiring safe storage at 2-8℃, including one or more of insulin vaccines, diphtheria vaccines, tetanus vaccines, and aluminum adjuvant-adsorbed vaccines.

[0040] The high temperature indicator and low temperature indicator of this invention correspond to different abnormal temperature scenarios and belong to the "temperature history writing-microphase structure reconstruction-light scattering readout" mechanism.

[0041] The polyvinyl acetal polymer network contains both highly hydrophilic residual hydroxyl regions and highly hydrophobic acetal segment regions, thus naturally giving the dry film or gel a certain degree of heterogeneity. This can be summarized as a hierarchical network composed of relatively dense associative regions, relatively loose network regions, and larger-scale microgel-like structures; wherein, the residual hydroxyl groups provide water absorption capacity and hydrogen bonding, while the hydrophobic acetal segments provide the association driving force and the basis for physical cross-linking.

[0042] When the material (dry film) swells under specific temperature conditions, the equilibrium water content of the system is not a fixed value, but is determined by both polymer-water compatibility and network elasticity. Changes in compatibility determine how much water the material can absorb, while network elasticity limits further expansion. Therefore, different temperature histories correspond to different equilibrium water contents, different chain segment arrangements, and different microphase structure distributions. In other words, the external temperature history is first incorporated into the material's internal equilibrium state.

[0043] In high-temperature applications, when the material (i.e., the temperature indicator) is in its initial transparent state, its internal microphase structure is relatively homogeneous, and its refractive index fluctuates little, resulting in weak visible light scattering and macroscopically manifesting as transparency or high transmittance. If the sample undergoes thermal exposure above a preset threshold, the polymer-water compatibility decreases, hydrophobic association microregions further aggregate, local networks become denser, and the microphase scale increases, thus forming more stable scattering centers. Since this structural change cannot be completely reversed after returning to a lower temperature, the material exhibits irreversible whitening. After high-temperature whitening, the sample shows more pronounced microregion aggregation, local density, and an increase in non-homogeneous structures, indicating that the whitening originates not from simple reversible molecular chain expansion and contraction, but from irreversible reconstruction of the microphase structure.

[0044] In cryogenic applications, materials (i.e., temperature indicators) can exist in a transparent, single-phase state after pre-equilibrium at safe storage temperatures. If they undergo further sub-zero supercooling exposure, the gel re-swells at low temperatures, reaching a new equilibrium water content, while its critical phase transition temperature is reset and written into the material's interior. The key to this process is that the swelling stage writes the history into the equilibrium water content, and subsequent readout relies on the mechanical constraint of network elasticity on phase separation nucleation. In other words, network elasticity does not allow immediate phase separation under arbitrary conditions, but rather makes the phase separation threshold adjustable.

[0045] When a sample that has undergone cold exposure returns to its safe storage temperature range of 2-8°C, if its critical phase transition temperature has been lowered to near or below the current ambient temperature, the system enters or approaches a two-phase region, forming a heterogeneous structure of polymer-enriched and water-enriched phases. Due to the increased difference in refractive index between the two phases, the material macroscopically appears turbid or opaque. This state can be maintained after returning to the safe temperature range, thus enabling passive recording of supercooling exposure events. (See reference...) Figure 20 The results showed that "ΔT550nm increased after supercooling and the equilibrium water content changed".

[0046] Therefore, this invention can explain the monitoring of two types of abnormal temperatures, high temperature and low temperature, using the same overall mechanism: abnormal temperature first changes the equilibrium water content, chain segment association state and network elasticity of the temperature indicator, then induces microphase structure reconstruction or phase separation, and finally manifests as transparent-turbid-whitening changes through enhanced refractive index inhomogeneity and increased light scattering, thereby realizing the visualization, passive and traceable recording of the history of abnormal temperatures.

[0047] The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. The thermosensitive hydrogel based on polyvinyl acetal polymers used in the present invention can be written as PVA-Cn-gel, where n represents the number of carbon atoms in the acetalized aliphatic aldehyde. The thermosensitive hydrogel can be used as an irreversible whitening temperature indicator in high-temperature anomaly monitoring, and also as a transparent / opaque temperature indicating medium in low-temperature supercooling exposure monitoring. Although the two applications correspond to different abnormal temperature scenarios, they are both based on the adaptive swelling response of the thermosensitive hydrogel to temperature history and the resulting microphase structure reconstruction and optical readout.

[0048] In practice, it is preferable to first prepare a dry film from the modified polyvinyl acetal polymer, and then perform a swelling treatment at a preset temperature to form a thermosensitive hydrogel. In high-temperature applications, samples with higher modification rates are preferred to obtain a more significant irreversible whitening response; in low-temperature applications, samples with lower modification rates are preferred to obtain a more obvious ability to record supercooled exposures.

[0049] The preferred PVA raw material is a commercially available PVA specification in the range of 0.3-2.6, or a product with the corresponding equivalent molecular weight / degree of polymerization.

[0050] Example 1: Preparation of a whitened temperature indicator (1) Preparation of modified polyvinyl alcohol acetal polymers, including the following steps: 1) Polyvinyl alcohol (PVA) is added to deionized water and heated and stirred at 95°C until completely dissolved to obtain a transparent PVA solution. The PVA is 1799PVA, with a degree of polymerization of 1700, a molecular weight of 1700×44.05, and a degree of hydrolysis of 99%. The concentration of PVA added is controlled at 5 wt%.

[0051] 2) After cooling the above PVA solution to 60°C, add p-toluenesulfonic acid monohydrate, the amount added being 8% of the mass of PVA; continue stirring for 0.5 h to activate the system. Subsequently, add aliphatic aldehydes dropwise to the reaction system, preferably propionaldehyde, butyraldehyde, pentanaldehyde, hexanal, heptanaldehyde, or octanaldehyde, i.e., aliphatic aldehydes with C3-C8 carbon atoms. The amount of aliphatic aldehyde added can be 65 mol% of the molar amount of PVA hydroxyl groups. After the addition is complete, continue the reaction for 4 h to achieve acetalization modification of some hydroxyl groups on the PVA molecular chain.

[0052] 3) After the reaction is complete, the reaction solution is slowly poured into a saturated sodium bicarbonate solution at room temperature to terminate the reaction. At this time, a white solid precipitates out. The precipitate is then washed three times with deionized water to remove residual acid and soluble impurities until the washing solution is foam-free and the sample has no obvious odor. Finally, after drying, a white granular modified polyvinyl acetal polymer is obtained.

[0053] Depending on the length of the carbon chain of the aliphatic aldehyde used, the resulting products are denoted as PVA-C3, PVA-C4, PVA-C5, PVA-C6, PVA-C7, or PVA-C8.

[0054] (2) Preparation of whitening temperature indicator, including the following steps: 1) Preparation of PVA-C4 solution Add 10g of PVA-C4 particles prepared in step (1) to 100mL of N,N-dimethylformamide (DMF), heat and stir at 90℃ for 6h to prepare a transparent homogeneous solution with a concentration of 100mg / mL; centrifuge the solution at 8000r / min for 10min and take the supernatant for later use.

[0055] 2) Preparation of PVB dry film The supernatant after centrifugation was poured into a quartz glass mold with a depth of 4 mm. The mold was then placed in a vacuum drying oven with a vacuum degree of -100 kPa and a temperature of 70 °C for 2 days to dry. After the solvent was completely evaporated, a transparent PVB dry film was obtained by peeling it off.

[0056] After the dry film is formed, the resulting membrane material is immersed in deionized water at 5°C for swelling treatment, allowing it to fully absorb water and form a temperature-sensitive hydrogel, thus obtaining the modified polyvinyl alcohol temperature-sensitive hydrogel, denoted as PVA-C4-gel. The preparation process is as follows: Figure 9 As shown.

[0057] (3) Application of vaccine temperature indicator PVA-C4-gel The PVA-C4-gel can be used as a stand-alone temperature-sensitive film, or it can be attached, laminated, or partially embedded in a packaging substrate to form a temperature indicator label.

[0058] During vaccine use, the label can be placed on the vaccine vial, outer packaging box, turnover box, refrigerated bag or cold chain record card to visually monitor abnormal heat exposure during transportation and storage.

[0059] (4) Performance characterization of vaccine temperature indicator PVA-C4-gel (4.1) Nuclear magnetic resonance The PVA-C4 prepared in step (1) above was characterized using a Bruker 400MHz NMR spectrometer to confirm the completion of the post-modification of PVA, i.e., the grafting of short alkanes onto polyvinyl alcohol. The specific operation is as follows: d Using 6-DMSO as a solvent, 6 mg of PVA-C4 was added, and the solution was completely dissolved at 70-80°C. 1 1H NMR spectroscopy analysis was performed, using PVA-C4 (PVB) as an example. The preparation flowchart is as follows: Figure 1 As shown, the NMR spectrum is as follows Figure 2 As shown.

[0060] from Figure 2 It can be seen that the modification rate of PVA-C4 is 62% (the acetalization rate AD of polyvinyl alcohol acetal, in mol%). According to the formula, AD = [2y / (x+2y+z)] × 100%.

[0061] Where x represents the number of moles of vinyl alcohol structural units that did not participate in the acetalization reaction; y represents the number of moles of vinyl alcohol units that participated in the acetalization reaction and formed an acetal structure; and z represents the number of moles of unhydrolyzed units, which will not participate in the acetalization reaction. The relative molar ratios of x, y, and z can be quantitatively calculated by the proportional relationship between the peak areas (integral values) in the 1H NMR spectrum, and then substituted into the above formula to calculate the acetalization rate.

[0062] (4.2) Image and optical characterization results The UV-Vis spectrum of PVA-C4-gel was measured using a UV-Vis spectrophotometer (PerkinElmer Lambda 950), with the spectral range set to 250-800 nm. Transmittance (T) was measured, and optical curves were plotted as follows: Figures 3-6 As shown. The tested gel samples had a diameter of 25 mm and a uniform thickness of 400 μm.

[0063] like Figure 3 , Figure 4 As shown, the samples gradually changed from transparent to white as the overheating duration increased under different pre-equilibrium temperatures; and under the same exposure time, the higher the pre-equilibrium temperature, the more significant the whitening degree, indicating that the system can simultaneously reflect the differences in heat exposure history and initial storage state.

[0064] At an initial swelling temperature of 5°C, the hydrogel underwent color changes as the temperature was continuously increased to 15°C, 25°C, and 35°C, as shown below. Figure 5 , Figure 6 As shown, the whitening severity parameter, defined based on the change in transmittance at 550 nm, increases monotonically with increasing temperature, according to real images, further demonstrating the feasibility of constructing a quantitative criterion for this system. Moreover, it allows for both qualitative judgment through visual observation and quantitative evaluation through changes in transmittance at 550 nm wavelength.

[0065] (4.3) SEM Sample Preparation and Characterization Methods (4.3.1) Scanning electron microscopy sample preparation method PVA-C4 thermosensitive hydrogel samples in a transparent state and PVA-C4 thermosensitive hydrogel samples that showed irreversible whitening after high-temperature treatment were selected as comparisons.

[0066] The transparent sample is a transparent hydrogel obtained by immersing a modified PVA dry film in deionized water at a preset swelling temperature until swelling equilibrium is reached.

[0067] The whitened sample is prepared by transferring the above-mentioned transparent hydrogel to a higher temperature for treatment until the sample shows obvious whitening and remains whitened even after returning to a lower temperature.

[0068] To preserve the internal microstructure of the sample, free water on the sample surface was first gently absorbed, followed by low-temperature fixation. Specifically, the sample was pre-frozen at low temperature and then freeze-dried to remove internal moisture and preserve the original micronetwork morphology as much as possible. Subsequently, the freeze-dried sample was fractured to obtain the cross-section, and the sample surface was sputtered with gold. Finally, scanning electron microscopy was used to observe the microstructure of the sample cross-section or surface.

[0069] (4.3.2) Scanning electron microscopy test conditions The scanning electron microscope (SEM) uses an accelerating voltage of 1-10 kV, a working distance of 3-10 mm, and a magnification of 500-10000x. The results are as follows: Figure 7As shown in the SEM image, the transparent PVA-C4 hydrogel exhibits a relatively continuous internal network, a relatively uniform distribution of microphase structure, and small local pore and phase separation scales. Due to the low refractive index differences and characteristic scales between different microregions, the scattering effect on visible light is weak, thus the sample appears macroscopically transparent or highly transparent.

[0070] The SEM image of the sample after heating is shown below. Figure 8 As shown, more pronounced micro-region aggregation, locally dense structures, and an increase in heterogeneous phase regions can be observed. This indicates that under high temperature, the originally dispersed hydrophobic associated micro-regions further aggregate, and the microphase structure within the network transforms from a relatively homogeneous state to a coarser, denser heterogeneous state. This change cannot be completely reversed even after returning to a lower temperature, indicating that a relatively stable irreversible aggregated structure and / or a stronger physical cross-linking network have formed within the system.

[0071] The final results show that PVA-C4 hydrogel has a relatively uniform microphase structure in the transparent state; however, after high-temperature induced whitening, more obvious phase region aggregation, local densification, and non-uniform structure enlargement phenomena appear inside.

[0072] The above results demonstrate that the irreversible whitening of the material of the present invention originates from the irreversible reconstruction of the microphase structure and the enhancement of physical cross-linking induced by high temperature, rather than the reversible molecular chain stretching-contraction process in the prior art. This reveals the technical essence of the temperature indication behavior of the present invention at the microscopic level.

[0073] Example 2: Preparation of vaccine cold chain over-cold exposure failure indicator label Includes the following steps: (1) Preparation of PVA-C4 solution 10g of modified polyvinyl acetal polymer PVA-C4 particles (modification rate 30%) were added to 100mL of N,N-dimethylformamide (DMF) and heated and stirred at 90℃ for 6h to prepare a transparent homogeneous solution with a concentration of 100mg / mL. The solution was centrifuged at 8000r / min for 10min and the supernatant was collected for later use.

[0074] (2) Preparation of PVB dry film The supernatant after centrifugation was poured into a quartz glass mold with a depth of 4 mm. The mold was then placed in a vacuum drying oven with a vacuum degree of -100 kPa and a temperature of 70 °C for 2 days to dry. After the solvent was completely evaporated, a transparent PVB dry film was obtained by peeling it off.

[0075] (3) Cut the PVB dry film into a circle with a diameter of 10 mm and store it in deionized water at the same storage temperature to obtain a vaccine cold chain over-cold exposure failure indicator label.

[0076] Application of vaccine cold chain over-cold exposure failure indicator labels 1: 1) The indicator label prepared in Example 2 was affixed to the surface of the cold chain refrigerated box of the conventional vaccine. The refrigerated box was stored in a cold storage at 2-8°C and then transported to the primary health care center in a low-resource area by refrigerated truck. The cold chain temperature was kept stable at 2-8°C throughout the process, and no sub-zero (Te≤0°C) exposure events occurred.

[0077] 2) During the warehousing and acceptance process, visually inspect the gel indicator label at 5℃: PVA-C4-gel should remain completely transparent (corresponding to...). Figure 11 (not undercooled state), combined with Figure 12 ΔT at t=0min 550nm A result of ≈0 indicates that the vaccine's cold chain status is normal, there is no risk of overcooling and failure, and it can be stored and used normally.

[0078] Application of vaccine cold chain over-cold exposure failure indicator labels 2: 1) The indicator label prepared in Example 2 is affixed to the vaccine vial. If the vaccine is exposed to -10°C due to the failure of the refrigerated truck temperature control during cold chain transportation, the exposure time is 1 hour.

[0079] 2) During the warehousing and acceptance process, visually observe the gel indicator label at 5°C: PVA-C4-gel changes from completely transparent to opaque milky white. If this indicates that the vaccine has been exposed to sub-zero cold and is at risk of failure, immediately isolate and discontinue use of this batch of vaccines, and trace the cause of the cold chain abnormality.

[0080] Validation of the correlation between the opacity of PVA-C4-gel and the degree of supercooling exposure: Multiple sets of PVA-C4-gel indicator tags from Example 2 were placed together with diphtheria vaccines at different sub-zero temperatures (Te=0℃, -5℃, -10℃) for gradient time exposure tests (t=0-75min). Subsequently, the change in transmittance ΔT of the gel at 550nm was measured at 5℃. 550nm And observe the appearance. Taking an exposure time of 45 minutes as an example (corresponding to...) Figure 12 ): When Te=0℃ and t=45min: ΔT 550nm ≈18.4%, the gel is slightly cloudy (slightly opaque); Te=-5℃, t=45min: ΔT 550nm ≈43.9%, the gel is slightly to moderately emulsified; Te=-10℃, t=45min: ΔT 550nm ≈56.5%, the gel is moderately milky white; The experimental results show that the opacity of PVA-C4-gel is significantly positively correlated with the temperature and time of vaccine supercooling exposure, and the severity of supercooling exposure can be semi-quantitatively determined by appearance gradient.

[0081] Validation of the correlation between PVA-C4-gel equilibrium water content and supercooled exposure temperature: Multiple groups of PVA-C4-gels from Example 2 were exposed at Te=0, -5, and -10℃ for 75 min, respectively, and then the equilibrium water content of the gels was tested at 2-8℃ (corresponding to...). Figure 13 ): When Te=0℃: the equilibrium water content increases by only about 8% compared to the initial value; When Te = -5℃, the equilibrium water content increases by approximately 21% compared to the initial value. When Te = -10℃: the equilibrium water content increases by approximately 24% compared to the initial value; The results showed that the lower the supercooling exposure temperature, the more significant the decrease in the equilibrium water content of PVA-C4-gel, further verifying the mechanism of the critical phase transition temperature being reset during low-temperature reswelling, and providing support for the compositional changes in transparency.

[0082] It is important to note that the PVA-C4-gel indicator label of this invention must be stored in a sealed container at 5°C. When interpreting the appearance of the PVA-C4-gel, it must be done within the vaccine's safe temperature range of 5°C to avoid interpretation errors caused by temperature deviations. The transparency / appearance phase change induced by supercooling in the PVA-C4-gel is irreversible, allowing for a permanent record of supercooling exposure events, facilitating subsequent cold chain traceability (corresponding to...). Figure 10 (Schematic diagram of phase transition behavior rewritten by "heat + time").

[0083] Based on Figure 12 A graph showing the severity of low-temperature exposure was constructed, and different ΔT values ​​were established. 550nm The threshold corresponds to the failure risk level, enabling more accurate cold chain traceability.

[0084] Example 3 Preparation of whitened temperature indicator The preparation process is the same as step (2) of Example 1, and the selected modified polyvinyl acetal polymer is PVA-C6.

[0085] (1) Application of PVA-C6-gel, a high-temperature indicator for vaccines Same as step (3) of Example 1, the modification rate of PVA-C6 is 60%.

[0086] (2) Performance characterization of vaccine high-temperature indicator PVA-C6-gel (2.1) Nuclear magnetic resonance The prepared PVA-C6 was characterized using a Bruker 400MHz NMR spectrometer to confirm the completion of the post-modification of PVA, specifically the grafting of short alkanes onto polyvinyl alcohol. The specific procedures were as follows: Using... d Using 6-DMSO as a solvent, 6 mg of PVA-C6 was added and dissolved at 70-80°C. 1 For H NMR spectroscopy analysis, PVA-C6 (PVB) was selected as an example. The NMR spectrum is as follows: Figure 14 As shown.

[0087] (2.2) Image and optical characterization results The UV-Vis spectrum of PVA-C6-gel was measured using a UV-Vis spectrophotometer (PerkinElmer Lambda 950), with the spectral range set to 250-800 nm. Transmittance (T) was measured, and optical curves were plotted as follows: Figures 15-18 As shown. The tested gel samples had a diameter of 25 mm and a uniform thickness of 400 μm.

[0088] like Figure 15 , Figure 16 As shown, the samples gradually changed from transparent to white as the overheating duration increased under different pre-equilibrium temperatures; and under the same exposure time, the higher the pre-equilibrium temperature, the more significant the whitening degree, indicating that the system can simultaneously reflect the differences in heat exposure history and initial storage state.

[0089] At an initial swelling temperature of 5°C, the hydrogel underwent color changes as the temperature was continuously increased to 15°C, 25°C, and 35°C, as shown below. Figure 17 , Figure 18 As shown, the whitening severity parameter, defined based on the change in transmittance at 550 nm, increases monotonically with increasing temperature, according to real images, further demonstrating the feasibility of constructing a quantitative criterion for this system. Moreover, it allows for both qualitative judgment through visual observation and quantitative evaluation through changes in transmittance at 550 nm wavelength.

[0090] Example 4: Preparation of vaccine cold chain over-cold exposure failure indicator label The preparation process is the same as in Example 2, except that a modified polyvinyl alcohol acetal polymer, PVA-C6 (modification rate of 30%), was selected to prepare the vaccine cold chain overcooling exposure failure indicator label. The application of this vaccine cold chain overcooling exposure failure indicator label is the same as in Example 2, and the final results are as follows: Figures 19-21 The conclusion is the same as that of Example 2.

[0091] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A whitening type temperature indicator based on polyvinyl acetal polymers, characterized in that: The temperature indicator includes a temperature-sensitive indicator layer, which comprises a modified polyvinyl alcohol acetal polymer, which is prepared by acetalization of polyvinyl alcohol with aliphatic aldehydes.

2. The whitening temperature indicator based on polyvinyl acetal polymers according to claim 1, characterized in that: The degree of polymerization of the polyvinyl alcohol is 300-2600, and the degree of hydrolysis is 88-99%.

3. The whitening temperature indicator based on polyvinyl acetal polymers according to claim 1, characterized in that: The fatty aldehyde is one or more of propionaldehyde, butyraldehyde, pentanaldehyde, hexanal, heptanaldehyde, and octanaldehyde.

4. A method for preparing a whitening temperature indicator based on polyvinyl acetal polymers as described in any one of claims 1-3, characterized in that: Includes the following steps: (1) Preparation of modified polyvinyl alcohol acetal polymers Polyvinyl alcohol is dissolved in water or an organic reagent to obtain a transparent solution. After activation with an acidic catalyst, aliphatic aldehydes are added dropwise to carry out an acetalization reaction. After the reaction is completed, the reaction is terminated and the obtained product is washed and dried to obtain a modified polyvinyl alcohol acetal polymer. (2) Preparation of modified polyvinyl alcohol thermosensitive hydrogel The modified polyvinyl alcohol acetal polymer was dissolved in an organic solvent and cast into a mold. After drying and swelling treatment, the modified polyvinyl alcohol thermosensitive hydrogel was obtained.

5. The preparation method according to claim 4, characterized in that: In step (1), the organic reagent is one of ethanol, DMF, and DMSO, and the acid catalyst is one or more of p-toluenesulfonic acid monohydrate, hydrochloric acid, methanesulfonic acid, and sulfuric acid.

6. The preparation method according to claim 4, characterized in that: In step (1), the concentration of polyvinyl alcohol is 2-10 wt%, and the amount of acidic catalyst added is 8-12% of the mass of polyvinyl alcohol; in step (2), the concentration of the modified polyvinyl alcohol acetal polymer in the organic solvent is 50-100 mg / mL.

7. The preparation method according to claim 4, characterized in that: In step (2), the organic solvent is at least one of dimethylformamide, dimethyl sulfoxide, and dimethylacetamide.

8. The preparation method according to claim 4, characterized in that: In step (2), the dry film obtained after drying is swollen in deionized water at a preset temperature. After swelling equilibrium is reached, modified polyvinyl alcohol thermosensitive hydrogel is obtained.

9. A method for using a whitened temperature indicator based on polyvinyl acetal polymers prepared by the preparation method according to any one of claims 4-8 for indicating the failure of cold chain vaccines due to overcooling exposure, characterized in that: Includes the following steps: 1) Store the prepared temperature indicator in deionized water at the same storage temperature as the expiration indicator label; 2) Place the expiration indicator label together with the vaccine in a cold chain transportation storage device, and let it complete the entire cold chain circulation with the vaccine; 3) During vaccine storage and use, under a safe temperature environment of 2-8℃, directly observe the appearance of the temperature indicator on the label: if the gel remains transparent, it indicates that the vaccine has not been exposed to sub-zero temperatures and the cold chain is normal. If the gel becomes opaque, it indicates that the vaccine has been exposed to sub-zero cold, suggesting a risk of denaturation, flocculation, or decreased efficacy.

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