JP7500964B2 - Core-shell type composite particle and method for producing the same - Google Patents

Description

The present invention relates to core-shell type composite particles and a method for producing the same.

In recent years, nanoparticles and microparticles have been the subject of active research in the medical field. For example, they are expected to be used as drug carriers in drug delivery systems (DDS). When used in DDS, stimuli-responsive polymers, whose volume or state changes (swells or shrinks) in response to external stimuli such as heat, light, electric current, electric field, and pH, have been attracting attention as materials.

For example, a temperature-responsive polymer is known as a stimuli-responsive polymer. Temperature-responsive polymers are generally classified into two types: one that exhibits a lower critical solution temperature (LCST), where a polymer in a hydrated state dehydrates above a certain temperature, causing a change in volume, shape, properties, etc., and one that exhibits an upper critical solution temperature (UCST), where a polymer hydrates above a certain temperature, causing a change in volume, shape, properties, etc.

For example, homopolymers or copolymers of N-isopropylacrylamide (NIPAM) (e.g., Patent Document 1) are known to exhibit LCST, while copolymers of N-acetylacrylamide and acrylamide (e.g., Patent Document 2) are known to exhibit UCST. In particular, poly-N-isopropylacrylamide (PNIPAM)-based compounds have attracted attention because they undergo a swelling-shrinking volume transition at around 32°C, which is close to body temperature. In addition, research using PNIPAM-based compounds as microparticles has also been widely conducted.

Japanese Patent Application Laid-Open No. 11-228850 JP 2000-86729 A

However, PNIPAM-based compounds and other temperature-responsive compounds are synthetic polymers, and when considering their use in the body, they have the problem of poor biocompatibility. Poor biocompatibility raises concerns that they may have adverse effects on the human body, such as causing inflammation due to nonspecific adsorption to tissues in the body or accumulating in the body.

The present invention was made in consideration of these circumstances, and aims to provide temperature-responsive nanoparticles and microparticles that are highly biocompatible.

As a result of extensive research, the inventors discovered that by using cellulose nanofibers as a coating material to create core-shell type composite particles, it is possible to impart biocompatibility to the particle surface.

Core-shell type composite particles are composite particles that have different components in one particle, and are composite particles whose interior and surface are made of different substances. In other words, a coating material is provided on the surface of the core particle to form an outer shell, and while the components of the coating material exert their functions, the particle's properties such as weight and flexibility and particle shape can be adjusted according to the application by selecting the material and shape of the core particle.

That is, the core-shell type composite particle of the present invention is a core-shell type composite particle comprising a temperature-responsive polymer as a core material and cellulose nanofiber as a coating material, the temperature-responsive polymer being a polymer of a water-soluble monomer .

The temperature-responsive polymer preferably has a lower critical solution temperature (LCST), and is more preferably a polyisopropylacrylamide-based material.

The cellulose nanofibers preferably have anionic functional groups on the crystal surface, and more preferably the anionic functional groups are carboxy groups.

In addition, the method for producing core-shell type composite particles of the present invention is characterized by including the steps of preparing a cellulose nanofiber dispersion, dissolving a water-soluble monomer and a polymerization initiator in the cellulose nanofiber dispersion, and polymerizing the water-soluble monomer at a temperature equal to or higher than the lower critical solution temperature or equal to or lower than the upper critical solution temperature of the temperature-responsive polymer.

The present invention provides highly biocompatible temperature-responsive nanoparticles and microparticles.

FIG. 2 is a schematic cross-sectional view of a core-shell type composite particle of the present invention.

The following describes an embodiment of the present invention with reference to the drawings. This embodiment is an example of a configuration for embodying the technical idea of the present invention, and does not specify the material, shape, structure, arrangement, dimensions, etc. of each part as described below. The technical idea of the present invention can be modified in various ways within the technical scope defined by the claims.

[Core-shell type composite particles]
1 is a schematic cross-sectional view of a core-shell type composite particle of the present invention. The core-shell type composite particle 10 of the present invention contains a temperature responsive polymer as a core material 2 and cellulose nanofibers (hereinafter, sometimes referred to as CNF) 1 as a coating material.

CNF is a material made by micronizing natural cellulose to the nanometer order, and is biocompatible. Therefore, the core-shell composite particles of the present invention have temperature responsiveness while also imparting biocompatibility to their surfaces.

<Temperature-responsive polymer>
The temperature-responsive polymer will be described. However, the temperature-responsive polymer of the present invention is not limited to the embodiments described below, and various modifications can be made within the technical scope defined by the claims.

Examples of the temperature-responsive polymer that can be used in the present invention include poly(N-isopropylacrylamide), poly(N-isopropylacrylamide-acrylic acid) copolymer, poly(N-isopropylacrylamide-methyl methacrylate) copolymer, poly(N-isopropylacrylamide-sodium acrylate) copolymer, poly(N-isopropylacrylamide-vinylferrocene) copolymer, poly(vinyl methyl ether), and other poly N-substituted acrylamide derivatives, poly N-substituted methacrylamide derivatives, polyamino acid derivatives, polyasparagine derivatives synthesized from α/β-asparagine derivatives, polypropylene oxide, copolymers of propylene oxide and other alkylene oxides, polyvinyl methyl ether, polyvinyl alcohol partial acetylation products, polyalkylene oxides, and the like, as well as poly(ethylene glycol-block-(L-lactic acid)); (PEG-PLLA) diblock copolymers, poly(ethylene glycol-block-(D-lactic acid)); (PEG-PD
LA) diblock copolymer, poly(ethylene glycol-block-(DL-lactic acid)); (PEG-PDLLA) diblock copolymer, (PEG-PLLA-PEG) triblock copolymer, (PEG-PDLA-PEG) triblock copolymer, (PEG-PDLLA-PEG) triblock copolymer, poly(ethylene glycol-block-DL-lactic acid-random-glycolic acid-block-ethylene glycol); (PEG-PLGA-PEG) triblock copolymer, (PLLA-PEG-PLLA) triblock copolymer, (PDLA-PEG-PDLA) triblock copolymer, (PDLLA-PEG-PDLLA) triblock copolymer Examples of such block copolymers include block copolymers, (PLGA-PEG-PLGA) triblock copolymers, (branched PEG-PLLA) block copolymers consisting of PEG having a branched structure and polylactic acid, (branched PEG-PDLA) block copolymers, (branched PEG-PDLLA) block copolymers, (branched PEG-PLGA) block copolymers, copolymers of lactide and polysaccharides, copolymers of polyether and polyester and derivatives thereof, block copolymers in which polyester is copolymerized with polyether, hydroxyalkyl chitosan, copolymers in which polyether side chains are introduced into hydroxy acid units and aspartic acid units, or derivatives or crosslinked polymers thereof. These may be used alone or in combination of two or more types.

The temperature-responsive polymer preferably has a lower critical solution temperature (LCST). The LCST is preferably 0 to 80°C, more preferably 10 to 70°C, and even more preferably 20 to 60°C. By having an LCST, the particles become swollen at temperatures below the LCST and can retain drugs, and at temperatures above the LCST, the particles shrink and can appropriately release drugs. Furthermore, the temperature-responsive polymer is more preferably a polyisopropylacrylamide-based material. Polyisopropylacrylamide has an LCST of 32°C, which is relatively close to body temperature. Furthermore, its derivatives or copolymers, polyisopropylacrylamide-based materials, are capable of controlling the LCST near body temperature, making them desirable for use in vivo.

<Cellulose nanofiber>
Although the cellulose nanofiber 1 is not particularly limited, it is preferable that the CNF raw material is chemically modified. More specifically, it is preferable that anionic functional groups are introduced to the crystal surfaces of the CNF raw material. This is because the introduction of anionic functional groups to the cellulose crystal surfaces makes it easier for the solvent to penetrate between the cellulose crystals due to the osmotic pressure effect, which makes it easier to refine the cellulose raw material. The content of the anionic functional groups is preferably 0.1 mmol or more and 5.0 mmol or less per 1 g of cellulose.

The type and method of the anionic functional group to be introduced onto the cellulose crystal surface are not particularly limited, but a carboxy group or a phosphate group is preferred, and a carboxy group is particularly preferred because of the ease of selective introduction onto the cellulose crystal surface.
Furthermore, the cellulose nanofiber 1 preferably has a fiber shape derived from a microfibril structure. Specifically, the cellulose nanofiber 1 is fibrous, and preferably has a number average minor axis diameter of 1 nm or more and 1000 nm or less, a number average major axis diameter of 50 nm or more, and the number average major axis diameter is 5 times or more the number average minor axis diameter. The crystal structure of the cellulose nanofiber 1 is preferably cellulose type I. The reasons for this will be described below in [Method for producing core-shell type composite particles].

[Method of producing core-shell type composite particles]
The method for producing core-shell type composite particles of the present invention includes (step 1) a step of preparing a CNF dispersion, (step 2) a step of dissolving a monomer and a polymerization initiator in the CNF dispersion, and (step 3) a step of polymerizing the monomer.

<(First step) Preparation of cellulose nanofiber dispersion>
The first step of the production method, which is the preparation process of a cellulose nanofiber dispersion, is described below. More specifically, it comprises a step of oxidizing a cellulose raw material and a step of defibrating and micronizing the oxidized cellulose raw material in a solvent to obtain a CNF dispersion.

There are no particular limitations on the type or crystal structure of cellulose that can be used as the raw material for cellulose nanofiber 1. Specifically, raw materials consisting of cellulose type I crystals can include, for example, natural wood cellulose, as well as non-wood natural celluloses such as cotton linters, bamboo, hemp, bagasse, kenaf, bacterial cellulose, sea squirt cellulose, and valonia cellulose.

Furthermore, regenerated cellulose such as rayon fiber and cupra fiber made of cellulose type II crystals can also be used. In view of the ease of material procurement, it is preferable to use wood-based natural cellulose as the raw material. There are no particular limitations on the wood-based natural cellulose, and it is possible to use those that are generally used in the manufacture of cellulose nanofibers, such as softwood pulp, hardwood pulp, and waste paper pulp. Softwood pulp is preferable because it is easy to purify and refine.

The method for introducing carboxyl groups to the fiber surface of cellulose is not particularly limited. Specifically, for example, carboxymethylation may be carried out by reacting cellulose with monochloroacetic acid or sodium monochloroacetate in a highly concentrated alkaline aqueous solution. Alternatively, carboxyl groups may be introduced by directly reacting cellulose with a carboxylic anhydride compound such as maleic acid or phthalic acid that has been gasified in an autoclave.

Furthermore, a method using a co-oxidant in the presence of an N-oxyl compound such as TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy radical), which has high selectivity for the oxidation of alcoholic primary carbons while maintaining the structure as much as possible under relatively mild conditions in an aqueous system, may be used. Oxidation using an N-oxyl compound is more preferable for selectivity of the site for carboxyl group introduction and for reducing the environmental load.

Here, examples of the N-oxyl compound include TEMPO, 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl, 4-ethoxy-2,2,6,6-tetramethylpiperidine-N-oxyl, and 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl. Of these, TEMPO is preferred because of its high reactivity. The amount of the N-oxyl compound used may be the amount used as a catalyst, and is not particularly limited. Usually, it is about 0.01 to 5.0 parts by mass based on the solid content of the wood-based natural cellulose to be oxidized.

An example of an oxidation method using an N-oxyl compound is a method in which wood-based natural cellulose is dispersed in water and oxidized in the presence of an N-oxyl compound. In this case, it is preferable to use a co-oxidizing agent together with the N-oxyl compound. In this case, the N-oxyl compound is sequentially oxidized by the co-oxidizing agent in the reaction system to generate an oxoammonium salt, and the cellulose is oxidized by the oxoammonium salt. This oxidation process allows the oxidation reaction to proceed smoothly even under mild conditions, improving the efficiency of introducing carboxyl groups. When the oxidation process is performed under mild conditions, the crystal structure of the cellulose is easily maintained.

As the co-oxidizing agent, any oxidizing agent can be used, such as halogens, hypohalous acids, hypohalous acids, perhalogen acids, or their salts, halogen oxides, nitrogen oxides, peroxides, etc., as long as it is capable of promoting the oxidation reaction. Sodium hypochlorite is preferred due to its ease of availability and reactivity. The amount of the co-oxidizing agent used is not particularly limited and may be an amount capable of promoting the oxidation reaction. It is usually about 1 to 200 parts by mass based on the solid content of the wood-based natural cellulose to be oxidized.

At least one compound selected from the group consisting of bromides and iodides may be used in combination with the N-oxyl compound and the co-oxidizing agent. This allows the oxidation reaction to proceed smoothly, improving the efficiency of introducing carboxy groups. As such a compound, sodium bromide or lithium bromide is preferred, with sodium bromide being more preferred in terms of cost and stability. The amount of the compound used is not particularly limited and may be an amount that can promote the oxidation reaction. It is usually about 1 to 50 parts by mass based on the solid content of the wood-based natural cellulose to be oxidized.

The reaction temperature for the oxidation reaction is preferably 4 to 80°C, more preferably 10 to 70°C. If it is below 4°C, the reactivity of the reagent decreases and the reaction time becomes longer. If it exceeds 80°C, side reactions are promoted, the sample becomes low molecular weight, and the highly crystalline rigid CNF fiber structure collapses, making it impossible to use it as a stabilizer for O/W emulsions.

The reaction time for the oxidation treatment can be set appropriately taking into consideration the reaction temperature, the desired amount of carboxyl groups, etc., and is not particularly limited, but is usually about 10 minutes to 5 hours.

The content of carboxyl groups introduced into CNF is preferably 0.1 mmol/g or more and 5.0 mmol/g or less, and more preferably 0.5 mmol/g or more and 2.0 mmol/g or less. If the amount of carboxyl groups is less than 0.1 mmol/g, the solvent does not penetrate between the cellulose microfibrils due to the osmotic pressure effect, making it difficult to finely disperse the cellulose uniformly. If the amount of carboxyl groups exceeds 5.0 mmol/g, the cellulose microfibrils are depolymerized due to side reactions accompanying the chemical treatment, making it impossible to obtain a highly crystalline and rigid CNF fiber structure, and the CNF cannot be used as a particle stabilizer.

The pH of the reaction system during the oxidation reaction is not particularly limited, but is preferably 9 to 11. If the pH is 9 or higher, the reaction can proceed efficiently. If the pH exceeds 11, side reactions may proceed, which may accelerate the decomposition of the sample. In addition, as the oxidation proceeds in the oxidation process, carboxyl groups are generated, which lowers the pH in the system, so it is preferable to maintain the pH of the reaction system at 9 to 11 during the oxidation process. One method for maintaining the pH of the reaction system at 9 to 11 is to add an alkaline aqueous solution as the pH drops.

Examples of the alkaline aqueous solution include organic alkalis such as an aqueous sodium hydroxide solution, an aqueous lithium hydroxide solution, an aqueous potassium hydroxide solution, an aqueous ammonia solution, an aqueous tetramethylammonium hydroxide solution, an aqueous tetraethylammonium hydroxide solution, an aqueous tetrabutylammonium hydroxide solution, and an aqueous benzyltrimethylammonium hydroxide solution. From the standpoint of cost, etc., an aqueous sodium hydroxide solution is preferred.

The oxidation reaction by the N-oxyl compound can be stopped by adding an alcohol to the reaction system. In this case, it is preferable to maintain the pH of the reaction system within the above range. As the alcohol to be added, low molecular weight alcohols such as methanol, ethanol, and propanol are preferable in order to quickly terminate the reaction, and ethanol is particularly preferable in view of the safety of by-products generated by the reaction.

The reaction solution after the oxidation treatment may be directly subjected to the micronization process, but it is preferable to recover the oxidized cellulose contained in the reaction solution and wash it with a washing solution in order to remove catalysts such as N-oxyl compounds, impurities, etc. The recovery of oxidized cellulose can be carried out by known methods such as filtration using a glass filter or a nylon mesh with a pore size of 20 μm. Pure water is preferable as the washing solution used to wash the oxidized cellulose.

Various cellulose raw materials oxidized as described above are dispersed in a solvent to form a suspension. The concentration of the cellulose raw material in the suspension is preferably 0.1% or more and less than 10%. If it is less than 0.1%, the amount of solvent becomes excessive, which is undesirable as it impairs productivity. If it is 10% or more, the suspension rapidly thickens as the cellulose raw material is defibrated, which is undesirable as it makes it difficult to perform a uniform defibration process. The solvent used to prepare the suspension preferably contains 50% or more of water. If the proportion of water in the suspension is 50% or less, the dispersion of the cellulose nanofibers 1 is hindered in the process of obtaining a cellulose nanofiber dispersion by defibrating the cellulose raw material in a solvent.

The solvent other than water is preferably a hydrophilic solvent. There are no particular limitations on the hydrophilic solvent, but alcohols such as methanol, ethanol, and isopropanol, and cyclic ethers such as tetrahydrofuran are preferred.

If necessary, the pH of the suspension may be adjusted to increase the dispersibility of the cellulose and the resulting cellulose nanofibers 1. Examples of alkaline aqueous solutions used for pH adjustment include organic alkalis such as sodium hydroxide aqueous solution, lithium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia aqueous solution, tetramethylammonium hydroxide aqueous solution, tetraethylammonium hydroxide aqueous solution, tetrabutylammonium hydroxide aqueous solution, and benzyltrimethylammonium hydroxide aqueous solution. Aqueous sodium hydroxide solution is preferred from the standpoint of cost, etc.

The suspension is then subjected to a physical defibration process to reduce the size of the cellulose raw material. Methods for physical defibration are not particularly limited, but examples include mechanical processes using a high-pressure homogenizer, ultra-high-pressure homogenizer, ball mill, roll mill, cutter mill, planetary mill, jet mill, attritor, grinder, juicer mixer, homomixer, ultrasonic homogenizer, nanogenizer, and underwater counter-collision.

By carrying out the above-mentioned physical defibration treatment, the cellulose in the suspension is pulverized to obtain a dispersion liquid of cellulose pulverized to the nanometer order of at least one side of the structure. In addition, the number average minor axis diameter and number average major axis diameter of the obtained cellulose nanofiber 1 can be adjusted by changing the time and number of times of the physical defibration treatment.

In this manner, a dispersion of cellulose (CNF dispersion) is obtained in which at least one side of the structure is micronized to the nanometer order. The resulting dispersion can be used as is or after dilution, concentration, etc., as a stabilizer for O/W emulsions, as described below.

Since cellulose nanofibers 1 usually have a fiber shape derived from a microfibril structure, the cellulose nanofibers 1 used in the manufacturing method of this embodiment preferably have a fiber shape within the range shown below.

The number average minor axis diameter of the fibrous cellulose nanofiber 1 may be 1 nm or more and 1000 nm or less, preferably 2 nm or more and 500 nm or less. Here, if the number average minor axis diameter is less than 1 nm, a highly crystalline rigid CNF fiber structure cannot be obtained, and emulsion stabilization and polymerization reaction using the emulsion as a template cannot be carried out. On the other hand, if it exceeds 1000 nm, the size becomes too large to stabilize the emulsion, making it difficult to control the size and shape of the obtained core-shell type composite particle 10.

The number average major axis diameter is not particularly limited, but is preferably at least 5 times the number average minor axis diameter. If the number average major axis diameter is less than 5 times the number average minor axis diameter, it is not preferable because the size and shape of the core-shell type composite particles 10 cannot be sufficiently controlled.

The number-average minor axis diameter of CNF fibers is calculated by measuring the minor axis diameters (minimum diameters) of 100 fibers using a transmission electron microscope and an atomic force microscope, and taking the average value. On the other hand, the number-average major axis diameter of CNF fibers is calculated by measuring the major axis diameters (maximum diameters) of 100 fibers using a transmission electron microscope and an atomic force microscope, and taking the average value.

<(Second step) Step of dissolving monomer and polymerization initiator in CNF dispersion>
In the second step, a monomer and a polymerization initiator appropriate for each temperature-responsive polymer are dissolved in the CNF dispersion prepared in the first step.

<(Third step) Step of polymerizing monomers to form composite particles>
In the third step, the polymerization step, the reaction is initiated by a known method such as heating or ultraviolet irradiation, and the monomer is polymerized to obtain composite particles.

Examples of polymerization initiators include organic peroxides such as hydroperoxides, peroxyesters, dialkyl peroxides, peroxyesters, diacyl peroxides, peroxydicarbonates, peroxyketals, and ketone peroxides; persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; and azo compounds such as azobisisobutyronitrile. The polymerization initiator is preferably water-soluble in order to efficiently carry out polymerization.

The polymerization of the monomer in the third step is desirably carried out at a temperature equal to or higher than the LCST or equal to or lower than the UCST of the temperature-responsive polymer to be produced. By carrying out the polymerization at a temperature equal to or higher than the LCST or equal to or lower than the UCST, the temperature-responsive polymer precipitates as the polymerization progresses, and the CNF acts as a dispersion stabilizer, so that core-shell type composite particles can be formed in which the temperature-responsive polymer is the core material and the CNF is the coating material.

The present invention will be described in more detail below based on Examples 1 and 2 and Comparative Examples 1 and 2. However, the scope of the present invention is not limited to the following examples as long as it does not deviate from the gist of the present invention.

[Example 1]
<(First Step) Preparation of Cellulose Nanofiber Dispersion>
(TEMPO oxidation of wood cellulose)
70 g of coniferous kraft pulp was suspended in 3500 g of distilled water, and a solution of 0.7 g of TEMPO and 7 g of sodium bromide dissolved in 350 g of distilled water was added, and the mixture was cooled to 20 ° C. 450 g of sodium hypochlorite aqueous solution with a concentration of 2 mol/L and a density of 1.15 g/mL was added dropwise to start the oxidation reaction. The temperature in the system was always kept at 20 ° C., and the pH drop during the reaction was maintained at pH 10 by adding a 0.5 N aqueous sodium hydroxide solution. When the total amount of sodium hydroxide added relative to the weight of cellulose reached 3.50 mmol/g, about 100 mL of ethanol was added to stop the reaction. After that, filtration and washing with distilled water were repeated using a glass filter to obtain oxidized pulp.

(Measurement of Carboxylic Group Amount in Oxidized Pulp)
The oxidized pulp and reoxidized pulp obtained by the TEMPO oxidation were weighed out in an amount of 0.1 g in terms of solid content, dispersed in water at a concentration of 1 part by mass, and hydrochloric acid was added to adjust the pH to 2.5. The amount of carboxyl groups (mmol/g) was then determined by conductometric titration using a 0.5 M aqueous solution of sodium hydroxide. The result was 1.6 mmol/g.

(Oxidized pulp defibration treatment)
1 g of the oxidized pulp obtained by the above TEMPO oxidation was dispersed in 99 g of distilled water and refined for 30 minutes using a juicer mixer to obtain a CNF aqueous dispersion of 1 part by mass of CNF. The CNF aqueous dispersion was placed in a quartz cell with an optical path length of 1 cm, and the spectral transmission spectrum was measured using a spectrophotometer (Shimadzu Corporation's "UV-3600"), and the CNF aqueous dispersion showed high transparency. In addition, the number average minor axis diameter of the CNF contained in the CNF aqueous dispersion was 3 nm, and the number average major axis diameter was 1110 nm. Furthermore, as a result of measuring the steady viscoelasticity using a rheometer, the CNF aqueous dispersion showed thixotropy.

<(Second step) Dissolution of monomer and polymerization initiator, (Third step) Polymerization of monomer and formation of particles>
To 45 parts by mass of the CNF aqueous dispersion of Production Example 1, 2.5 parts by mass of N-isopropylacrylamide (NIPAM, manufactured by KJ Chemical Co., Ltd.) and 0.025 parts by mass of N,N-methylenebisacrylamide (MBAAm, manufactured by Kanto Chemical Co., Ltd.) were added as monomers, and the mixture was stirred and dissolved. Nitrogen replacement was further performed at 70°C for 30 minutes. Thereafter, while maintaining the temperature at 70°C, 0.025 parts by mass of 2,2'-azobis(2-aminopropane) dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries, Ltd.) dissolved in 2.5 parts by mass of water was added to initiate polymerization. The polymerization was performed for 1 hour to obtain a cloudy dispersion. A drop of the dispersion was dropped onto a slide glass, sealed with a cover glass, and observed with an optical microscope, and it was confirmed that particles of a size of about several μm were dispersed.

The resulting dispersion was treated for 5 minutes at a relative centrifugal force of 75,000 g, yielding a sediment. The supernatant was removed by decantation to recover the sediment, which was then repeatedly washed with pure water using a PTFE membrane filter with a pore size of 0.1 μm. The purified and recovered material thus obtained was redispersed in 20 parts by mass of water to obtain a dispersion.

The resulting dispersion was adjusted to 1 part by mass with pure water, and the particle size was evaluated using a particle size distribution analyzer (NANOTRAC UPA-EX150, manufactured by Nikkiso Co., Ltd.). It was confirmed that the average particle size at 25°C was 1.5 μm and the average particle size at 40°C was 0.6 μm, and that the particle size changed in response to temperature. The LCST was also around 32°C.

[Example 2]
Particles and a dispersion thereof were obtained under the same conditions as in Example 1, except that the monomers were 2.25 parts by mass of N-isopropylacrylamide (NIPAM, manufactured by KJ Chemical Co., Ltd.), 0.25 parts by mass of N,N-dimethylacrylamide (DMAA, manufactured by KJ Chemical Co., Ltd.), and 0.025 parts by mass of N,N-methylenebisacrylamide (MBAAm, manufactured by Kanto Chemical Co., Ltd.).

The particle size was evaluated and found to be 1.4 μm on average at 25°C and 0.6 μm on average at 40°C, confirming that the particle size changes in response to temperature. The LCST was around 37°C.

<Comparative Example 1>
Particles and their dispersion were obtained under the same conditions as in Example 1, except that the CNF aqueous dispersion was changed to pure water.

The particle size was evaluated and found to be 2.1 μm on average at 25°C and 1.0 μm on average at 40°C, confirming that the particle size changes in response to temperature. The LCST was around 32°C.

<Comparative Example 2>
Particles and their dispersion were obtained under the same conditions as in Example 2, except that the CNF aqueous dispersion was changed to pure water.

The particle size was evaluated to find that the average particle size was 2.2 μm at 25° C. and 1.2 μm at 40° C., and it was confirmed that the particle size changed in response to temperature.
The ST was around 37°C.

[Evaluation of biocompatibility]
The obtained particles were introduced into mouse L929 fibroblast cells, and their biocompatibility was evaluated by confirming their cytotoxicity.

First, mouse L929 fibroblast cells were seeded in a medium and cultured until they became subconfluent (a state in which the cell density was about 4/5 of the saturation density). Particles were added to this medium and incubated overnight at 37°C in the presence of 5% _CO2 . After that, the cells were observed, and if cell proliferation was promoted, the biocompatibility was rated as "good" (good) because of low cytotoxicity, and if cell proliferation was inhibited, the biocompatibility was rated as "not good" (bad) because of high cytotoxicity.

The results are shown in Table 1.

The biocompatibility was rated "good" in Examples 1 and 2, and "poor" in Comparative Examples 1 and 2. It can be said that the cytotoxicity was low in Examples 1 and 2 because the particle surface was covered with CNF, whereas the cytotoxicity was high in Comparative Examples 1 and 2 because no CNF was used and the temperature-responsive polymer came into contact with the cells.
The invention as originally claimed is set forth below.
[1]
A core-shell type composite particle comprising a temperature-responsive polymer as a core material and a cellulose nanofiber as a coating material.
[2]
2. The core-shell type composite particle according to item 1, wherein the temperature-responsive polymer has a lower critical solution temperature.
[3]
3. The core-shell type composite particle according to item 1 or 2, wherein the temperature responsive polymer is a polyisopropylacrylamide-based material.
[4]
Item 4. The core-shell type composite particle according to any one of Items 1 to 3, wherein the cellulose nanofiber has an anionic functional group on a crystal surface.
[5]
5. The core-shell type composite particle according to item 4, wherein the anionic functional group is a carboxy group.
[6]
A method for producing core-shell type composite particles, comprising the steps of:
A step of preparing a cellulose nanofiber dispersion;
Dissolving a monomer and a polymerization initiator in the cellulose nanofiber dispersion;
polymerizing the monomer;
A method for producing core-shell type composite particles, comprising:

1 Cellulose nanofiber 2 Core material 10 Core-shell type composite particle

Claims (6)

It contains a temperature-responsive polymer as the core material and cellulose nanofiber as the coating material.
The core-shell type composite particle is characterized in that the temperature-responsive polymer is a polymer of a water-soluble monomer. The core-shell type composite particle according to claim 1, characterized in that the temperature-responsive polymer has a lower critical solution temperature. The core-shell composite particle according to claim 1 or 2, characterized in that the temperature-responsive polymer is a polyisopropylacrylamide-based material. The core-shell type composite particle according to any one of claims 1 to 3, characterized in that the cellulose nanofiber has anionic functional groups on the crystal surface. The core-shell type composite particle according to claim 4, characterized in that the anionic functional group is a carboxy group. A method for producing core-shell type composite particles, comprising the steps of:
A step of preparing a cellulose nanofiber dispersion;
A step of dissolving a water-soluble monomer and a polymerization initiator in the cellulose nanofiber dispersion;
polymerizing the water-soluble monomer at a temperature equal to or higher than a lower critical solution temperature of the temperature-responsive polymer or equal to or lower than an upper critical solution temperature of the temperature-responsive polymer;
A method for producing core-shell type composite particles, comprising:

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