Double-layer composite membrane based on bletilla striata polysaccharide and chitosan as well as preparation method and application of double-layer composite membrane

Abstract

The invention provides a double-layer composite membrane based on bletilla striata polysaccharide and chitosan as well as a preparation method and application of the double-layer composite membrane. The preparation method is simple in process and low in cost. Layers and filaments in the prepared bletilla striata base dressing form a mutually perpendicular cross arrangement structure, and the bletilla striata base dressing has good mechanical properties and is suitable for being applied to the fields of functional dressings and tissue engineering.

Description

Technical Field

[0001] This invention belongs to the field of biomedical materials technology, and more specifically, relates to a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan, its preparation method and application. Technical Background Bletilla striata polysaccharide is a natural high-molecular-weight polysaccharide extracted from the stems and tubers of Bletilla striata. It possesses excellent biocompatibility, biodegradability, and hemostatic properties. Its molecular structure contains numerous hydroxyl and uronic acid groups, enabling it to bind tightly to wound tissue through hydrogen bonding, forming a physical barrier and effectively promoting platelet aggregation and thrombin generation, thereby accelerating the hemostasis process. Furthermore, Bletilla striata polysaccharide also exhibits anti-inflammatory and cell proliferation-promoting effects, providing a favorable microenvironment for wound healing. In recent years, the application of Bletilla striata polysaccharide in medical dressings has attracted considerable attention, but its potential in radiation-cooling functional dressings has not yet been fully explored.

[0002] Radiative cooling is a technology that achieves passive cooling by utilizing the high infrared emissivity and high solar reflectivity of materials. Introducing radiative cooling into wound dressings can effectively reduce the surface temperature of the wound, decrease inflammatory responses and bacterial growth caused by high temperatures, thereby accelerating wound healing.

[0003] Chitosan is a natural cationic polysaccharide with broad-spectrum antibacterial properties, biocompatibility, and film-forming properties, making it an ideal material for preparing nanofiber membranes. Nano-zinc oxide not only possesses excellent antibacterial properties but also enhances radiative cooling effects by reflecting solar radiation and increasing infrared emissivity. Therefore, combining chitosan with nano-zinc oxide can endow dressings with dual functions: antibacterial and radiative cooling.

[0004] However, combining these materials to leverage their respective advantages in the preparation of medical dressings remains a major challenge. This is because most dressings currently on the market are single-function materials, making it difficult to simultaneously meet the multiple needs of hemostasis, antibacterial properties, cooling, and promoting healing. While some existing dressings attempt to combine multiple functions, shortcomings remain in structural design, mechanical properties, and manufacturing processes. For example, traditional porous materials have randomly distributed pore structures and poor mechanical properties; while electrospun membranes often lack sufficient support when used alone.

[0005] Therefore, it is of great significance to develop a multilayer dressing that combines directional porous structure, excellent mechanical properties, efficient hemostasis, antibacterial properties, and radiation cooling functions. Summary of the Invention

[0006] In view of the above-mentioned defects and improvement needs of the existing technology, the purpose of this invention is to provide a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan, its preparation method and application, which provides a new idea for achieving radiation cooling to promote wound healing.

[0007] To achieve the above objectives, a first aspect of the present invention provides a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan, comprising: a Bletilla striata polysaccharide oriented porous sponge lower layer and a chitosan and polyvinyl alcohol composite nanofiber membrane loaded with nano zinc oxide upper layer.

[0008] As a further preferred embodiment of the present invention, the lower layer of the Bletilla striata polysaccharide directional porous sponge is a porous sponge-like structure with horizontally layered channels, formed by directional cryogenic casting using an ice template.

[0009] As a further preferred embodiment of the present invention, the upper layer of the nanofiber membrane loaded with nano-zinc oxide is a composite electrospun membrane of chitosan and polyvinyl alcohol loaded with nano-zinc oxide.

[0010] As a further preferred embodiment of the present invention, the nano zinc oxide is obtained by reacting zinc chloride in the presence of an alkaline catalyst, wherein the zinc chloride used accounts for 0.05-10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol raw materials used, preferably 0.1-5%, and even more preferably 0.5-4%, for example 1-3% or 0.5-1%.

[0011] As a further preferred embodiment of the present invention, the thickness of the lower layer of Bletilla striata polysaccharide oriented porous sponge in the double-layer composite membrane is 1 mm to 10 mm, preferably 2 mm to 8 mm, and even more preferably 3 to 7 mm.

[0012] As a further preferred embodiment of the present invention, the thickness of the upper layer of the chitosan and polyvinyl alcohol composite nanofiber membrane loaded with nano-zinc oxide is 1 mm to 10 mm, preferably 2 mm to 8 mm, and even more preferably 3 to 7 mm.

[0013] Another aspect of the present invention provides a method for preparing a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan as described above, using an ice template method, comprising the following steps: (S1) Preparation of the lower layer of Bletilla striata polysaccharide oriented porous sponge: Bletilla striata polysaccharide solution is poured into a mold, and the Bletilla striata polysaccharide solution in the mold is orientedly frozen to form a sponge. The sponge structure is freeze-dried, and after demolding, the Bletilla striata polysaccharide oriented porous sponge structure is obtained. (S2) Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: S2-1. Preparation of spinning solution: Dissolve a chitosan solution with a mass percentage concentration of 1% to 10% in a mixed solution of deionized water and acetic acid at a mass ratio of (100 to 1): 1, and then prepare a polyvinyl alcohol aqueous solution with a mass percentage concentration of 1% to 15%. Mix the chitosan solution and the polyvinyl alcohol solution at a volume ratio of 1 to 10: 1 to 10. Add 0.5% to 10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol as a plasticizer to the mixed solution and mix. Finally, add 0.05% to 10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol and mix to obtain the spinning solution. S2-2. Preparation of a chitosan nanofiber bilayer composite membrane loaded with zinc chloride: Electrospinning was performed on the directional porous sponge structure of Bletilla striata polysaccharide prepared in step S2-1 using the spinning solution prepared in step S2-1, thereby obtaining a chitosan and polyvinyl alcohol composite nanofiber membrane layer loaded with zinc chloride bonded to the directional porous sponge structure of Bletilla striata polysaccharide. S2-3. The bilayer composite membrane prepared in step S2-2 is reacted in the presence of an alkaline catalyst to generate a bilayer composite membrane in which nano-zinc oxide particles are loaded on chitosan and polyvinyl alcohol composite nanofibers.

[0014] As a further preferred embodiment of the present invention, the method further includes a post-processing step: washing the bilayer composite membrane obtained in steps S2-3 multiple times with anhydrous ethanol and drying it in a vacuum drying oven.

[0015] As a further preferred embodiment of the present invention, the electrospinning process parameters in step S2-2 include: the electrospinning parameters are set as follows: positive voltage 30 kV~50 kV, negative voltage -30 kV~0 kV, injection pump propulsion rate: 0.01 mm / min~0.1 mm / min, and the distance between the spinneret and the receiver is 10 cm~20 cm.

[0016] As a further preferred embodiment of the present invention, the electrospinning time in step S2-2 is 0.5~10h, for example 1~5h.

[0017] As a further preferred embodiment of the present invention, in steps S2-3, the alkaline catalyst includes metal hydroxide, metal oxide, ammonia, and organic amine. For example, the alkaline catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, and ethanolamine.

[0018] As a further preferred embodiment of the present invention, the method for preparing the bilayer composite membrane based on Bletilla striata polysaccharide and chitosan includes the following steps: (1) Preparation of the lower layer of the directional porous sponge of Bletilla striata polysaccharide: The prepared Bletilla striata polysaccharide solution was poured into a polytetrafluoroethylene rectangular groove mold. A cooling channel was established by using a metal conductor with the function of transferring temperature. One end of the metal conductor was immersed in the interface between the solution and the mold, and the other end was immersed in liquid nitrogen until the Bletilla striata polysaccharide solution in the mold was completely directionally frozen to form a sponge. The sponge structure and the mold were placed together in a freeze dryer and freeze-dried for 48~72 h. After demolding, the directional porous sponge structure of Bletilla striata polysaccharide was obtained. (2) Preparation of a bilayer composite membrane of chitosan nanofibers loaded with nano-zinc oxide: a. Preparation of spinning solution: Dissolve a chitosan solution with a mass percentage concentration of 2% to 5% in a mixed solution of deionized water and acetic acid at a mass ratio of (98 to 2): 1, and then prepare a polyvinyl alcohol aqueous solution with a mass percentage concentration of 5% to 10%. Mix the chitosan solution and the polyvinyl alcohol solution evenly at a ratio of 2 to 8: 2 to 8. Add 1% to 5% of the total mass of the polymer chitosan and polymer polyvinyl alcohol as a plasticizer to the mixed solution and mix evenly. Finally, add 0.1% to 5% (preferably 0.5% to 4%) of zinc chloride powder, which accounts for the total mass of the polymer chitosan and polymer polyvinyl alcohol, and mix evenly to obtain the spinning solution. b. Preparation of a bilayer composite membrane of chitosan and polyvinyl alcohol loaded with zinc chloride: Electrospinning was performed using an electrospinning device. A directional porous sponge of Bletilla striata polysaccharide was fixed onto tin foil of a spinning receiver as a substrate. A certain amount of the spinning solution prepared in step a was drawn up using a syringe and fixed onto a precision injection pump. The electrospinning parameters were set as follows: positive voltage 30 kV~50 kV, negative voltage -30 kV~0 kV, injection pump feed rate 0.01 mm / min~0.1 mm / min, and distance between the spinneret and the receiver 10 cm~20 cm. The nanofibers were spun onto the directional porous sponge of Bletilla striata polysaccharide for 2~5 h to obtain a chitosan and polyvinyl alcohol composite nanofiber membrane layer loaded with zinc chloride bonded to the directional porous sponge of Bletilla striata polysaccharide. c. Preparation of a bilayer composite membrane of chitosan and polyvinyl alcohol nanofibers loaded with nano-zinc oxide: The composite membrane of Bletilla striata polysaccharide oriented porous sponge and chitosan and polyvinyl alcohol nanofiber membrane loaded with zinc chloride was immersed in a 0.2 mg / L~1 mg / L sodium hydroxide ethanol solution and stirred for 1 h~4 h, so that the originally loaded zinc chloride reacted under alkaline conditions to generate nano-zinc oxide particles loaded on chitosan and polyvinyl alcohol composite nanofibers; the composite membrane was washed several times with anhydrous ethanol and dried in a vacuum drying oven to obtain a bilayer composite membrane of chitosan nanofiber membrane loaded with nano-zinc oxide and Bletilla striata polysaccharide oriented porous sponge.

[0019] As a further preferred embodiment of the present invention, the mass percentage of the Bletilla striata polysaccharide solution is 1% to 20%, preferably 2% to 15%, and even more preferably 2% to 12%.

[0020] As a further preferred embodiment of the present invention, the depth of the mold groove is 2 mm to 8 mm; As a further preferred embodiment of the present invention, the thickness of the directional porous sponge-like structure of Bletilla striata polysaccharide is 1 mm to 10 mm, preferably 2 mm to 8 mm.

[0021] The present invention also provides, as described above, a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan as a functional dressing (e.g., a wound dressing) and its use in the field of tissue engineering.

[0022] Compared with the prior art, the bilayer composite membrane based on Bletilla striata polysaccharide and chitosan of the present invention can achieve the following beneficial effects: 1. Excellent hemostatic properties: The lower layer of the bilayer composite membrane, which is oriented porous sponge of Bletilla striata polysaccharide, has high porosity and a horizontal layered pore structure (similar to a sponge). It can quickly absorb exudate and promote platelet aggregation and coagulation through the natural hemostatic properties of Bletilla striata polysaccharide, significantly shortening the hemostasis time.

[0023] 2. Highly efficient radiative cooling function: The upper layer of the bilayer composite membrane, composed of chitosan and polyvinyl alcohol nanofibers loaded with nano-zinc oxide structures, reflects solar radiation and enhances infrared emissivity, achieving passive cooling. Its radiative cooling effect lowers the temperature of the wound surface, thereby reducing inflammation and bacterial growth, creating a more suitable microenvironment for wound healing.

[0024] 3. Dual antibacterial effect: Chitosan in the double-layer composite membrane has broad-spectrum antibacterial properties, while nano zinc oxide further enhances its antibacterial effect, effectively inhibiting wound infection, especially showing a significant inhibitory effect on drug-resistant strains.

[0025] 4. Excellent mechanical properties: The layered structure of the oriented porous sponge in the double-layer composite membrane is combined with the nanofiber membrane to form a double-layer biomimetic structure, which can endow the double-layer composite membrane with excellent mechanical strength and flexibility, and can adapt to the dynamic mechanical stress of the wound site, avoiding easy breakage or fall-off when used as a dressing.

[0026] 5. Biocompatibility and biodegradability: Both Bletilla striata polysaccharide and chitosan in the double-layer composite membrane are natural polymer materials with good biocompatibility and can be gradually degraded in the body, avoiding the need for secondary surgery to remove them and reducing patient suffering.

[0027] 6. Multifunctional integrated design: When used as a wound dressing, this double-layer composite film can combine hemostasis, antibacterial, cooling and healing promotion, solving the problem of single function of traditional dressings and providing an innovative solution for the treatment of complex wounds.

[0028] Furthermore, this patent describes the process of treating a zinc chloride-loaded fiber membrane under alkaline conditions to transform it into a zinc oxide-loaded fiber membrane. The advantages of this method are: By directly adding the zinc chloride precursor to the spinning solution, the zinc oxide nanoparticles generated in situ during subsequent alkali treatment can achieve a uniform distribution at the molecular level. Compared with the agglomeration phenomenon that easily occurs when directly adding zinc oxide nanoparticles, this method ensures the monodispersity of nanoparticles in the fiber matrix. Electron microscopy results show that the zinc oxide particles converted by the method of this invention have a more uniform size distribution and exhibit a gradient distribution along the fiber axis. This unique gradient dispersion characteristic enables the composite material to exhibit a broad-spectrum response behavior in the solar radiation band (250-2500 nm), and its radiation cooling efficiency is better than that of samples prepared by traditional blending methods.

[0029] In this invention, the zinc oxide nanoparticles generated in situ in the bilayer composite film are chemically bonded to the chitosan molecular chains through coordination bonds and hydrogen bonds. This bonding energy is higher than that of physical mixing, thus reducing the shedding rate of the zinc oxide nanoparticles. This strong bonding ensures that the dressing's functionality is not diminished during use due to the loss of zinc oxide nanoparticles.

[0030] The zinc oxide crystals generated in situ in the bilayer composite film of this invention are more complete, with fewer crystal defects and optimized lattice vibration modes. Its infrared emissivity in the 8–13 μm atmospheric window band is significantly higher than that of physically mixed samples. Ultraviolet-visible-near-infrared (200–2500 nm) spectroscopy shows a flat reflectance curve with a high overall value, effectively reducing solar radiation absorption. Combined with its high emissivity in the mid-infrared band, this material achieves balanced control of broadband radiation, thereby significantly improving radiative cooling performance. Attached Figure Description

[0031] Figure 1 A cross-sectional scanning electron microscope image of a directional porous sponge made from Bletilla striata polysaccharide.

[0032] Figure 2 Scanning electron microscope image of a bilayer composite membrane of chitosan and polyvinyl alcohol nanofibers loaded with zinc chloride.

[0033] Figure 3 This is a scanning electron microscope image of the surface of the bilayer composite membrane.

[0034] Figure 4 This is a cross-sectional scanning electron microscope image of the bilayer composite membrane.

[0035] Figure 5The graph shows the temperature change of the bilayer composite membrane under sunlight. (Where Ambient refers to ambient temperature, 0%-ZnCl2 is the sample of Comparative Example 1, 0.5%-ZnCl2 is the sample prepared in Example 3, and 1%-ZnCl2 is the sample prepared in Example 1). Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0037] A bilayer composite membrane for preparing Bletilla striata polysaccharide / chitosan based on an ice template method includes: a lower layer of Bletilla striata polysaccharide oriented porous sponge and an upper layer of chitosan and polyvinyl alcohol nanofiber membrane loaded with nano-zinc oxide. The lower layer of Bletilla striata polysaccharide oriented porous sponge is a porous sponge with horizontally layered pores, oriented by ice template freeze casting; the upper layer of chitosan nanofiber membrane is a composite electrospun membrane of chitosan and polyvinyl alcohol loaded with nano-zinc oxide.

[0038] In the following examples and comparative examples, the temperature inside the composite membrane (referring to one side of the chitosan and polyvinyl alcohol composite nanofiber membrane loaded with nano zinc oxide) was measured as follows: The double-layer composite membrane was cut into squares of appropriate size and then attached to a foam board wrapped with tin foil. A temperature sensor probe was placed under each membrane, and another probe was attached directly to the foam board without being covered. Then, the entire surface of the composite membrane was completely covered with plastic wrap so that the interior of the composite membrane was almost unaffected by air convection. The prepared sample was placed in direct sunlight, and the temperature change curve of the temperature sensor was recorded.

[0039] Example 1 (1) Preparation of the lower layer of Bletilla striata polysaccharide oriented porous sponge: Pour the prepared Bletilla striata polysaccharide solution with a mass percentage of 10% into a polytetrafluoroethylene rectangular groove mold. Use a metal conductor with the function of transferring temperature to establish a cooling channel. Immerse one end of the metal conductor into the interface between the solution and the mold, and immerse the other end in liquid nitrogen until the Bletilla striata polysaccharide solution in the mold is completely oriented and frozen to form a sponge. Place the sponge and the mold together in a freeze dryer and freeze dry for 48~72 h. After demolding, the Bletilla striata polysaccharide oriented porous sponge is obtained. Figure 1 This is a scanning electron microscope image of the oriented porous sponge containing Bletilla striata polysaccharide. It can be seen that the obtained oriented porous sponge containing Bletilla striata polysaccharide has a lamellar oriented porous structure.

[0040] (2) Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: a. Preparation of spinning solution: A 4% (w / w) chitosan solution was dissolved in a mixture of deionized water and acetic acid at a mass ratio of 45:1, and a 7% (w / w) polyvinyl alcohol aqueous solution was prepared. The chitosan solution and polyvinyl alcohol solution were mixed evenly at a volume ratio of 1:1. Glycerin, accounting for 3% of the total mass of the chitosan and polyvinyl alcohol polymers, was added as a plasticizer and mixed evenly. Finally, zinc chloride powder, accounting for 1% of the total mass of the chitosan and polyvinyl alcohol polymers, was added and mixed evenly to obtain the spinning solution.

[0041] b. Preparation of a zinc chloride-loaded chitosan nanofiber bilayer composite membrane: Electrospinning was performed using an electrospinning device. A directional porous sponge of *Bletilla striata* polysaccharide was fixed onto tin foil of the spinneret as a substrate. A certain amount of the spinning solution prepared in step a was drawn up using a syringe and fixed onto a precision injection pump. The electrospinning parameters were set as follows: positive voltage 40 kV, negative voltage -20 kV, injection pump feed rate 0.05 mm / min, and distance between the spinneret and the receiver 15 cm. The nanofibers were spun onto the directional porous sponge of *Bletilla striata* polysaccharide and spun continuously for 3 h, thus obtaining a zinc chloride-loaded chitosan and polyvinyl alcohol composite nanofiber membrane bonded to the directional porous sponge of *Bletilla striata* polysaccharide. Figure 2 The image shows a scanning electron microscope (SEM) image of a chitosan and polyvinyl alcohol composite nanofiber membrane loaded with zinc chloride. It can be seen that the obtained bilayer composite membrane of chitosan and polyvinyl alcohol loaded with zinc chloride has uniform fiber diameter and smooth surface morphology.

[0042] c. Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: The bilayer composite membrane of chitosan and polyvinyl alcohol nanofiber loaded with zinc chloride prepared in step b was immersed in a 0.5 mg / L sodium hydroxide ethanol solution and stirred for 2 h, allowing zinc chloride to react with hydroxide ions under alkaline conditions to generate nano-zinc oxide particles loaded on chitosan nanofibers. The resulting bilayer composite membrane was washed several times with anhydrous ethanol and dried in a vacuum drying oven to obtain the bilayer composite membrane based on Bletilla striata polysaccharide and chitosan.

[0043] Figure 3 This is a scanning electron microscope (SEM) image of the surface of a bilayer composite membrane, consisting of a chitosan nanofiber membrane with zinc oxide loaded on the upper layer and a polyvinyl alcohol composite. Figure 3 It can be seen that the fiber surface is rough after sodium hydroxide treatment, and nanoparticles are attached to the surface. Figure 4 This is a scanning electron microscope image of the cross-section of a double-layer composite film, from... Figure 4 It can be seen that the chitosan fiber membrane layer in the bilayer composite membrane has a dense fibrous structure, while the Bletilla striata polysaccharide oriented porous sponge layer exhibits a lamellar porous structure. Figure 5 This describes the temperature control performance of the double-layer composite membrane under sunlight. Figure 5It can be seen that samples with different amounts of ZnCl2 added can reduce the temperature inside the composite membrane to a certain extent after treatment with sodium hydroxide, creating a temperature difference between the inside of the composite membrane and the ambient temperature. Moreover, the temperature reduction increases with the increase of the mass of ZnCl2 used as the raw material. The temperature difference between the 1%-ZnCl2 sample and the ambient temperature is as large as 10.7℃ and as small as 5.7℃. This indicates that the radiative cooling effect of the sample is better as the ZnO content increases.

[0044] Example 2 (1) Preparation of the lower layer of Bletilla striata polysaccharide oriented porous sponge: The prepared Bletilla striata polysaccharide solution with a mass fraction of 8% was poured into a polytetrafluoroethylene rectangular groove mold. A cooling channel was established by using a metal conductor with the function of transferring temperature. One end of the metal conductor was immersed in the interface between the solution and the mold, and the other end was immersed in liquid nitrogen until the Bletilla striata polysaccharide solution in the mold was completely oriented and frozen to form a sponge. The sponge structure together with the mold was placed in a freeze dryer and freeze-dried for 48~72 h. After demolding, the Bletilla striata polysaccharide oriented porous sponge was obtained.

[0045] (2) Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: a. Preparation of spinning solution: A 2% (w / w) chitosan solution was dissolved in a mixture of deionized water and acetic acid at a mass ratio of 98:1, and a 5% (w / w) polyvinyl alcohol aqueous solution was prepared. The chitosan solution and polyvinyl alcohol solution were mixed evenly at a volume ratio of 1:5. Then, 1% (w / w) of glycerol (by mass of the total chitosan and polyvinyl alcohol) was added to the mixed solution as a plasticizer and mixed evenly. Finally, 2% (w / w) of zinc chloride powder (by mass of the total chitosan and polyvinyl alcohol) was added and mixed evenly to obtain the spinning solution.

[0046] b. Preparation of a zinc chloride-loaded chitosan nanofiber bilayer composite membrane: Electrospinning was performed using an electrospinning device. A directional porous sponge of *Bletilla striata* polysaccharide was fixed onto tin foil of the spinneret as a substrate. A certain amount of the spinning solution prepared in step a was drawn up using a syringe and fixed onto a precision injection pump. The electrospinning parameters were set as follows: positive voltage 30 kV, negative voltage -30 kV, injection pump feed rate 0.01 mm / min, and distance between the spinneret and the receiver 10 cm. The nanofibers were spun onto the directional porous sponge of *Bletilla striata* polysaccharide and spun continuously for 2 hours, thus obtaining a zinc chloride-loaded chitosan and polyvinyl alcohol composite nanofiber membrane bonded to the directional porous sponge of *Bletilla striata* polysaccharide.

[0047] c. Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: The composite membrane of chitosan and polyvinyl alcohol nanofibers loaded with zinc chloride prepared in step b was immersed in a 0.2 mg / L sodium hydroxide ethanol solution and stirred for 1 h, allowing zinc chloride to react with hydroxide ions under alkaline conditions to generate nano-zinc oxide particles. The composite membrane was washed several times with anhydrous ethanol and dried in a vacuum drying oven to obtain a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan.

[0048] Example 3 (1) Preparation of the lower layer of Bletilla striata polysaccharide oriented porous sponge: The prepared Bletilla striata polysaccharide solution with a mass fraction of 12% was poured into a polytetrafluoroethylene rectangular groove mold. A cooling channel was established by using a metal conductor with the function of transferring temperature. One end of the metal conductor was immersed in the interface between the solution and the mold, and the other end was immersed in liquid nitrogen until the Bletilla striata polysaccharide solution in the mold was completely oriented and frozen to form a sponge. The sponge structure together with the mold was placed in a freeze dryer and freeze-dried for 48~72 h. After demolding, the Bletilla striata polysaccharide oriented porous sponge was obtained.

[0049] (2) Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: a. Preparation of spinning solution: A 5% (w / w) chitosan solution was dissolved in a 2:1 mixture of deionized water and acetic acid. A 10% (w / w) polyvinyl alcohol aqueous solution was then prepared. The chitosan solution and polyvinyl alcohol solution were mixed thoroughly at a 1:10 volume ratio. Glycerin (5% of the total mass of the chitosan and polyvinyl alcohol polymers) was added as a plasticizer and mixed thoroughly. Finally, zinc chloride powder (0.5% of the total mass of the chitosan and polyvinyl alcohol polymers) was added and mixed thoroughly to obtain the spinning solution.

[0050] b. Preparation of a zinc chloride-loaded chitosan nanofiber bilayer composite membrane: Electrospinning was performed using an electrospinning device. A directional porous sponge of *Bletilla striata* polysaccharide was fixed onto tin foil of the spinneret as a substrate. A certain amount of spinning solution was drawn up using a syringe and fixed onto a precision injection pump. The electrospinning parameters were set as follows: positive voltage 50 kV, negative voltage -5 kV, injection pump feed rate 0.1 mm / min, and distance between the spinneret and the receiver 20 cm. The nanofibers were spun onto the directional porous sponge of *Bletilla striata* polysaccharide and spun continuously for 5 h, thus obtaining a zinc chloride-loaded chitosan nanofiber membrane layer bonded to the directional porous sponge of *Bletilla striata* polysaccharide. c. Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: A composite membrane consisting of a oriented porous sponge of Bletilla striata polysaccharide and a chitosan and polyvinyl alcohol composite nanofiber membrane loaded with zinc chloride was immersed in a 1 mg / L sodium hydroxide ethanol solution and stirred for 4 h. Under alkaline conditions, zinc chloride reacted with hydroxide ions to generate nano-zinc oxide particles, which were then loaded onto the chitosan and polyvinyl alcohol composite nanofibers. The composite membrane was washed multiple times with anhydrous ethanol and dried in a vacuum drying oven to obtain the bilayer composite membrane based on Bletilla striata polysaccharide and chitosan. Figure 5 It can be seen that the composite membrane obtained after treatment with sodium hydroxide on the sample using 0.5% ZnCl2 can reduce the temperature to a certain extent, creating a temperature difference with the ambient temperature; the temperature difference between the 0.5% ZnCl2 sample and the ambient temperature is the highest at 8.2℃. This indicates that the sample prepared with 0.5% ZnCl2 has a certain radiative cooling effect.

[0051] Comparative Example 1 A composite membrane without zinc oxide loading was prepared according to the method in Example 1, denoted as composite membrane 0%-ZnCl. The temperature difference test results inside the obtained composite membrane are as follows... Figure 5 As shown. By Figure 5 It can be seen that the temperature of the composite membrane without zinc oxide loading is lower than the ambient temperature, but higher than that of the composite membrane with zinc oxide loading. The results indicate that the bilayer composite membrane of Bletilla striata polysaccharide and chitosan without zinc oxide loading also has a certain radiative cooling effect, but its radiative cooling effect is worse than that of the bilayer composite membrane with zinc oxide loading.

[0052] The exemplary embodiments of the present invention have been described above. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A bilayer composite membrane based on Bletilla striata polysaccharide and chitosan, characterized in that, include: The lower layer is a directional porous sponge made from Bletilla striata polysaccharide, and the upper layer is a composite nanofiber membrane of chitosan and polyvinyl alcohol loaded with nano-zinc oxide.

2. The double-layer composite membrane according to claim 1, characterized in that, The lower layer of the Bletilla striata polysaccharide directional porous sponge is a porous sponge-like structure with horizontally layered channels, formed by directional cryogenic casting using an ice template.

3. The double-layer composite membrane according to claim 1 or 2, characterized in that, The nano zinc oxide is obtained by reacting zinc chloride in the presence of an alkaline catalyst. The zinc chloride used accounts for 0.05-10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol raw materials used, preferably 0.1-5%.

4. The bilayer composite membrane according to any one of claims 1-3, characterized in that, The thickness of the lower layer of Bletilla striata polysaccharide oriented porous sponge in the double-layer composite membrane is 1 mm to 10 mm.

5. The bilayer composite membrane according to any one of claims 1-4, characterized in that, The thickness of the upper layer of the chitosan and polyvinyl alcohol composite nanofiber membrane loaded with nano-zinc oxide is 1 mm to 10 mm.

6. The method for preparing the bilayer composite membrane according to any one of claims 1-5, characterized in that, The ice template method includes the following steps: (S1) Preparation of the lower layer of Bletilla striata polysaccharide oriented porous sponge: Bletilla striata polysaccharide solution is poured into a mold, and the Bletilla striata polysaccharide solution in the mold is orientedly frozen to form a sponge. The sponge structure is freeze-dried, and after demolding, the Bletilla striata polysaccharide oriented porous sponge structure is obtained. (S2) Preparation of a bilayer composite membrane based on Bletilla striata polysaccharide and chitosan: S2-1. Preparation of spinning solution: Dissolve a chitosan solution with a mass percentage concentration of 1% to 10% in a mixed solution of deionized water and acetic acid at a mass ratio of (100 to 1): 1, and then prepare a polyvinyl alcohol aqueous solution with a mass percentage concentration of 1% to 15%. Mix the chitosan solution and the polyvinyl alcohol solution at a volume ratio of 1 to 10: 1 to 10. Add 0.5% to 10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol as a plasticizer to the mixed solution and mix. Finally, add 0.05% to 10% of the total mass of the polymer chitosan and polymer polyvinyl alcohol and mix to obtain the spinning solution. S2-2. Preparation of a chitosan nanofiber bilayer composite membrane loaded with zinc chloride: Electrospinning was performed on the directional porous sponge structure of Bletilla striata polysaccharide prepared in step S2-1 using the spinning solution prepared in step S2-1, thereby obtaining a chitosan and polyvinyl alcohol composite nanofiber membrane layer loaded with zinc chloride bonded to the directional porous sponge structure of Bletilla striata polysaccharide. S2-3. The bilayer composite membrane prepared in step S2-2 is reacted in the presence of an alkaline catalyst to generate a bilayer composite membrane in which nano-zinc oxide particles are loaded on chitosan and polyvinyl alcohol composite nanofibers.

7. The preparation method according to claim 6, characterized in that, The electrospinning process parameters in step S2-2 include: positive voltage 30 kV~50 kV, negative voltage -30 kV~0 kV, injection pump feed rate 0.01 mm / min~0.1 mm / min, and distance between the spinneret and the receiver 10 cm~20 cm.

8. The preparation method according to claim 6 or 7, characterized in that, In steps S2-3, the alkaline catalyst includes metal hydroxides, metal oxides, ammonia, and organic amines. For example, the alkaline catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, and ethanolamine.

9. The preparation method according to any one of claims 6-8, characterized in that, The mass percentage of the Bletilla striata polysaccharide solution is 1% to 20%, preferably 2% to 15%, and even more preferably 2% to 12%.

10. The use of the bilayer composite membrane according to any one of claims 1-5 as a functional dressing (e.g., a wound dressing) and in the field of tissue engineering.

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