Fabrication of degradable polymer microspheres via pH-responsive chitosan-based Pickering emulsion photopolymerization

Abstract

Pickering emulsion stabilized by pH-reversible chitosan is developed to prepare degradable polymer microspheres by emulsion photopolymerization, where chitosan acts as a green and recyclable particulate emulsifier. The thiol–ene photopolymerization of trimethylolpropane tris(3-mercaptopropionate) and trimethylolpropane triacrylate is initiated by UV irradiation. Chitosan adsorbed at the surface of the microspheres can be recycled for polymerization at least three times. Moreover, ibuprofen (IBU) is loaded into the microspheres. The higher the release temperature or pH value, the faster release rate and higher release extent of IBU from the microspheres are found. Meanwhile, the resulting microspheres exhibit a good degradability in 1 M NaOH aqueous solution, with a weight loss of about 90 wt% after 35 days. This study demonstrates a potential green and recyclable application of chitosan in fabrication of degradable polymer microspheres.

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Introduction

Chitosan, which stems from the deacetylated derivative of chitin, is the second most abundant natural biopolymer found in nature only after cellulose. It is a natural linear polysaccharide with a molecular structure of β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine units, where free amino and hydroxyl groups are along its backbone.^1,2 When pH is low, these amines are protonated, resulting in the dissolution of chitosan in acid solution. On the contrary, deprotonated amines cause chitosan to become insoluble at high pH values. Therefore, chitosan can easily undergo a pH-tunable sol–gel transition.³ So far, most of the applications of chitosan still focus on the preparation of chitosan-based membranes, microcapsules, or other materials by grafting, functionalization, self-assembly, direct chemical or physical modification.^1,2,4–6 Scant literature has been reported to study the role of chitosan without any hydrophobic modification in the stabilization of Pickering emulsion, which is the emulsion system stabilized by colloidal particles instead of surfactants.^7,8

Pickering emulsions have aroused particular attention in recent years due to their significant advantages in contrast to traditional systems, such as good stabilization and low toxicity.^7–12 Pickering emulsions, especially pH-responsive systems, are of increasing interest for study in biomedical, pharmaceutical, cosmetic and coating fields. A large amount of literature has been reported to design pH-responsive Pickering emulsions based on pH-sensitive inorganic particles, organic polymer latexes.^13–15

Stöver et al. designed a pH-responsive Pickering emulsion based on silica/potassium hydrogen phthalate, where stable emulsions were maintained only at pH values between 3.5 and 5.5.¹⁶ Fujii et al. had adopted nanocomposite microgels of poly(4-vinylpyridine)/silica to stabilize pH-responsive oil-in-water (O/W) Pickering emulsions, which could maintain stability at pH 8.0 and break quickly at pH 2.0.^17,18 Armes et al. prepared pH-responsive Pickering emulsions based on the copolymerization polymer of poly(2-(diethylamino)ethyl methacrylate) latexes, where stable Pickering emulsions were prepared at pH 8.0 but demulsification occurred within seconds when the pH value decreased from 10 to 3.^19,20 Poly(N-isopropylacrylamide)-based microgels are also important thermo-responsive or pH-responsive emulsifiers to prepare responsive Pickering emulsions; these emulsifiers required the precise control of appropriate comonomers.^21–23

Unfortunately, the pH-responsive particulate emulsifiers mentioned above usually demanded significant synthesis effort or presented a certain degree of biological toxicity, which restricted the application of these emulsion systems in biotechnology, food science, and environmental protection. Therefore, green and renewable bio-emulsifiers are particularly important in these fields.^24–27 Though the preparation of pH-responsive emulsifiers or emulsion systems has been well documented, few studies concentrated on the practical application of the pH-responsive emulsion system.

We have employed chitosan without any hydrophobic modification as a new and effective particulate emulsifier to prepare pH-reversible low and high internal phase Pickering emulsions for the first time.^3,28 In addition, we recently have successfully prepared pH-responsive O/W Pickering emulsions stabilized by natural polymer of lignin and further recycled this emulsifier to prepare polystyrene microspheres by Pickering emulsion polymerization.²⁹ In the present work, pH-responsive chitosan-based Pickering emulsion was developed to synthesize degradable microspheres, and the cyclic utilization of chitosan in stabilization of Pickering emulsions and Pickering emulsion photopolymerization were studied in detail. The emulsion polymerization was carried out by UV irradiation of trimethylolpropane tris(3-mercaptopropionate) (trithiol) and trimethylolpropane triacrylate (TMPTA).

A schematic drawing of the preparation of degradable microspheres is presented in Fig. 1. The advantage of the microspheres or this method is (i) the good controlled release behavior for the loaded drug, ibuprofen (IBU), and a certain extent of degradability of microspheres, (ii) a new route for green and renewable chitosan in the preparation of functional materials, and (iii) the versatile method for other reversible emulsifiers or emulsion systems. The major goal of the present study work is to develop a recycling route for photopolymerization base on pH-reversible Pickering emulsions. Through this method, we can prepare degradable microspheres more easily, economically and in an environmentally friendly manner. Drug delivery and degradation are investigated just as examples of properties for the obtained microspheres.

Fig. 1 Schematic representation of the recycling preparation of chitosan-coated microspheres and pure microspheres based on Pickering emulsion photopolymerization.

Experimental

Materials

Chitosan (degree of deacetylation ≥90%, M_v = 60 000), trimethylolpropane tris(3-mercaptopropionate) (trithiol), trimethylolpropane triacrylate (TMPTA), the photoinitiator, a blend of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO)/2-hydroxy-2-methylpropiophenone (Darocur 1173), ibuprofen (IBU), and fluorescein isothiocyanate (FITC) were purchased from Sigma-Aldrich and used without further purification. Chloroform, glacial acetic acid, dimethyl sulfoxide, sodium hydroxide (solid), and hydrochloric acid (Guangzhou Chemical Reagent Factory, China) were of analytical grade. Water used in all experiments was purified by deionization and filtration with a Millipore purification apparatus to a resistivity higher than 18.0 MΩ cm.

Preparation of Pickering emulsions

A typical preparation procedure was based on our previous work.³ Briefly, chitosan aqueous solutions (0.1, 0.5 or 1.0 wt%) were obtained by dissolving an appropriate amount of chitosan powder in glacial acetic acid solution, where pH values were adjusted to about 4.1 by adding 1 M NaOH or HCl aqueous solution. FITC-labelled chitosan was obtained by the reaction of chitosan solution and FITC solution (1 mg L⁻¹ in DMSO) at 40 °C for 4 h, followed by precipitating at pH 9.5 to remove excess FITC.

Appropriate amounts of trithiol, TMPTA, and photoinitiator were dissolved in chloroform, and chitosan solution at pH 6.7 was obtained by adding NaOH or HCl solution. Then, 4 mL chitosan solution (0.1, 0.5 or 1.0 wt%) and 2 mL chloroform were homogenized to obtain chitosan-based Pickering emulsions with an IKA Ultra Turrax T25 basic instrument at 12 000 rpm for 2 min in an ice bath.

Preparation of microspheres

The resulting chitosan-based Pickering emulsions were used for photopolymerization to obtain chitosan-coated microspheres under a UV radiator apparatus (Intelli-ray 400, 400 W, Uvitron International, Inc. USA) for 10 min. The resulting microspheres were immersed into 1 M HCl solution for 2 h under mechanical vibration and then purified by water washing to prepare pure microspheres. The supernatants produced during the dissolving and washing processes were adjusted to pH value of 9.5 to precipitate chitosan, followed by three centrifugation/redispersion processes to obtain clean chitosan solution. Then, Pickering emulsions stabilized by these recycling chitosan and corresponding microspheres were prepared by the same above-mentioned process.

Loading and release experiments

IBU-loaded chitosan-coated or pure microspheres were prepared by the procedure described above, except that a certain amount of IBU was added into chloroform before homogenizing. All the supernatants produced during the washing process were collected, and the loading amount of IBU inside the microspheres was estimated through the difference between the input amount and the amount in the supernatants produced during the washing process.

The IBU-loaded microspheres was dispersed into 20 mL PBS buffer solution (pH = 7.4 or 2.0), followed by transfer into a dialysis bag (M_w cut-off of 3500). Then, the dialysis bag was immersed into 380 mL PBS buffer solution under magnetic stirring, which was kept at different temperatures (20, 37 and 50 °C) using a thermostatic bath under continuous stirring of 200 rpm. After desired time intervals, 2.0 mL solution was taken out to analyze the IBU concentration. Next, this 2.0 mL solution was poured back into the PBS buffer solution. The release proceeded until the concentration of IBU in the PBS buffer solution remained unchanged. The concentration of IBU could be analyzed with UV-Vis spectrophotometer through the calibration curve, which was established from standard solutions of IBU at pH = 7.4 or 2.0. The release profiles were averaged from three experiments.

Degradation studies

Microspheres of 0.2 g were added to NaOH solutions (0.1, 0.5 and 1 M). The solutions were kept at different temperatures. After the desired time, the remaining microspheres were collected by three centrifugation/redispersion processes, followed by freeze drying. This process was carried out in triplicate and the percentage of weight loss was calculated from [(original mass − final mass)/original mass] × 100%.

Characterization

Pickering emulsion droplets were observed with an optical microscope (Carl Zeiss, German), and the average diameter was determined by a laser scattering particle size distribution analyser (Malvern Mastersizer 2000). Confocal micrographs were taken with a confocal laser scanning microscope (CLSM, Leica TCHITOSAN-SP2) with a 40× objective. In addition, chitosan was visualized by FITC-labelled polymer at excitation wavelength of 488 nm. Thermogravimetric analysis (TGA) was carried out with a NETZSCH TG 209F3 instrument. Samples were heated from 30 °C to 850 °C at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere. Scanning electron microscopy (SEM) was carried out with a Zeiss EVO 18 electron microscope equipped with a field emission electron gun. The samples were sputter-coated with gold prior to measurement. The quantification of IBU concentrations was evaluated by a Hitachi U-3010 UV-Vis spectrophotometer at 223 nm.

Results and discussion

Preparation of chitosan-based Pickering emulsions

It's well known that chitosan's amines are protonated at low pH value. However, the amines are deprotonated at high pH value. Chitosan nanoparticles can be formed to act as a particulate emulsifier for Pickering emulsions because of the deprotonation. Actually, for the various oils with the different polarity and viscosity, O/W Pickering emulsions stabilized by chitosan exhibited good long-term stability for more than three months.³ Herein, chloroform was selected as the model oil for Pickering emulsions.

The morphologies of Pickering emulsions stabilized by chitosan at pH 6.7 are shown in Fig. 2. The emulsion was O/W type, which was confirmed by droplet test.³ Spherical emulsion droplets with good dispersibility could be observed. The mean droplet diameter (about 20 μm) of Pickering emulsion in Fig. 2a was obviously bigger than the diameters (about 10 μm) in Fig. 2b and c. The concentration of particulate emulsifier plays an important role in the stability and morphology of Pickering emulsions. At low chitosan concentration, there were not enough emulsifiers to stabilize small emulsion droplets, which had high specific surface area and required more emulsifiers. Thus, big emulsion droplets appeared. However, there was enough emulsifier to ensure small emulsion droplets at the chitosan concentration of 0.5 wt%. Excess chitosan (1.0 wt%) had no obvious effect on the size of emulsion droplets, which is in accordance with a previous investigation.³ Therefore, the mean droplet diameter had almost no noticeable change at chitosan concentrations from 0.5 wt% to 1.0 wt% but became smaller than the diameter at the concentration of 0.1 wt%.

Fig. 2 Optical micrographs of chloroform-in-water Pickering emulsions prepared at chitosan concentrations of (a) 0.1 wt%, (b) 0.5 wt% and (c) 1.0 wt%. The oil-to-water volume ratio is 1 : 2. All scale bars represent 50 μm.

Preparation of microspheres

Colloidal particles adsorbed at oil–water interfaces need high energy to desorb from the interfaces, which imparts a unique advantage in the stabilization of Pickering emulsions over traditional systems.^8,9,30 Thus, Pickering emulsion droplet can be used as a reaction vessel or robust template to fabricate microspheres, microcapsules, or other supracolloidal structures.^10,31–35 In this work, degradable microspheres were prepared by chitosan-based Pickering emulsion polymerization via UV irradiation of monomer (trithiol) and multifunctional acrylates (TMPTA). Compared to traditional thermopolymerization, photopolymerization has obvious advantages in fast curing speed (within several minutes, even a few seconds), easy control, insensitivity to oxygen.^36,37 Furthermore, thiol–ene photopolymerization is very common and convenient, and various materials have been obtained by thiol–ene photocuring under ambient temperature conditions.^38–40 Different amounts of trithiol and acrylate (1 and 2 in Fig. S1,† respectively) were employed to produce the microspheres (Table 1). The morphology was observed by optical micrograph as shown in Fig. S2.†

Table 1 Mass loss of chitosan-coated microspheres at different formulations

	S1	S2	S3	S4	S5	S6
a Degradation was carried out at 37 °C for 7 days.
Trithiol (mL)	0.4	0.3	0.2	0.1	0.05	0.025
TMPTA (mL)	0.4	0.5	0.6	0.7	0.75	0.775
Mass loss^a (%)	79.6	80.5	79.8	71.4	81.2	81.1

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It is surprising that there were many microspheres binding together in Fig. S2a and b for entries S1 and S2 in Table 1,† which suggests that the flocculation among emulsion droplets occurred in the polymerization process. On the contrary, well-defined and discrete microspheres were found in Fig. S2c–f (entries S3–S6 in Table 1,† respectively). In all cases, photopolymerization proceeded steadily and no obvious phase separation was observed during the polymerization process. It could be supposed that excess trithiol (Fig. S2a and b†) had weak solubility in water and acted as a cross-linker to crosslink adjacent microspheres. However, all trithiol was responsible for the crosslinking and polymerization in individual emulsion droplets instead in Fig. S2b–d.† If there is no special declaration, the microspheres refer to entry S4 in Table 1† with 0.1 mL trithiol and 0.7 mL TMPTA.

Then, an attempt at removing the chitosan that had already adsorbed on the surface of the microspheres was conducted. Pure microspheres were obtained by dissolving the chitosan-coated microspheres in HCl solution to remove chitosan. Fig. 3 shows the morphologies of chitosan-coated microspheres and pure microspheres for entry S4 (Table 1†). The diameters of the two microspheres were roughly identical to the sizes of the corresponding emulsion droplets before polymerization. Comparable diameters, combining the phenomenon of no phase separation during the polymerization process, indicated that chitosan-based Pickering emulsion provided a very stable reaction vessel for producing microspheres.

Fig. 3 SEM images of chitosan-coated microspheres (a₁ and a₂) and pure microspheres (b₁ and b₂). EDS spectra at (a₃) surface area 1 of the microsphere in a₂ and (b₃) surface area 2 of the microsphere in b₂.

Rough surfaces of chitosan-coated microspheres are presented in Fig. 3a (more obvious at a high magnification image in Fig. 3a₂), but the surface of the pure microsphere in Fig. 3b₂ was clean and smooth after chitosan dissolved in HCl solution. Therefore, the aggregated small particles that appeared on the surface of the microsphere in Fig. 3a₂ should be the particulate emulsifier of chitosan. The presence of N element in EDS analysis (Fig. 3a₃) and absence of this element in Fig. 3b₃ demonstrated that chitosan existed on the surface of chitosan-coated microspheres, and chitosan was successfully removed after HCl washing. This phenomenon further suggested that the chitosan adsorbed at the oil–water interfaces prevented droplet coalescence during the photopolymerization process, and chitosan could be easily removed by dissolving in acidic condition.

To further investigate the surface structure of chitosan-coated microspheres, FITC-labelled chitosan acted as emulsifiers for preparing Pickering emulsions and microspheres. The microspheres before and after HCl washing are shown in Fig. 4a and b, respectively. It is noted that green fluorescence almost only existed at the surface of microspheres in Fig. 4a, and the fluorescence appeared in the surrounding medium in Fig. 4b. This observation is well in accordance with the SEM experiments (Fig. 3). However, irregular microspheres appeared in Fig. 4a, which was a little different from the morphologies in Fig. 3a. This difference may be due to the shorter photopolymerization time of Pickering emulsion stabilized by FITC-labelled chitosan, where the shorter time was employed in order to avoid the quenching of FITC under UV irradiation. Meanwhile, chitosan cannot be labelled by FITC uniformly, which accounts for the fluorescence with polydispersity in the confocal microscopy images.

Fig. 4 Confocal microscopy of the microspheres before (a) and after (b) HCl washing. Chitosan was labelled by FITC. All scale bars represent 25 μm.

The content of chitosan in chitosan-coated microspheres was roughly estimated by TGA analysis (Fig. 5). The residues of chitosan, chitosan-coated microspheres and pure microspheres above 800 °C were about 39.4, 11.1, and 6.6 wt%, respectively. Thus, the calculated content of chitosan in chitosan-coated microspheres was about 13.7 wt%.

Fig. 5 TGA curves of chitosan, pure microspheres and chitosan-coated microspheres.

Recycling route for photopolymerization

The role of pH-responsive chitosan in the preparation of microspheres was investigated. Chitosan was recycled by immersing the chitosan-coated microspheres in HCl solution, followed by precipitating at pH 9.5 and three centrifugation/redispersion processes, as shown in Fig. 1. Repeatability of chitosan-based emulsion photopolymerization was conducted at different chitosan concentrations of 0.1, 0.5 and 1.0 wt% (Table 2). SEM morphologies of three recycle emulsion polymerization processes are depicted in Fig. 6. When chitosan concentration was 0.1 wt%, irregular microspheres or microsphere aggregations were found after two recycles of emulsion polymerization (Fig. 6a₂ and a₃). However, well-defined microspheres were clearly presented in Fig. 6b and c under all three cycles. The mean diameters of microspheres are presented in Table 2. Diameter obviously increased with increasing recycle times at the chitosan concentration of 0.1 wt%, but the size of the final microspheres marginally increased at concentrations of 0.5 and 1.0 wt%. Chitosan concentration is crucial for the stability and droplet diameter of Pickering emulsion; as a result, it plays an important part in controlling the morphologies and diameters of microspheres. During the recycling of chitosan, a slight loss of chitosan appeared during the dissolving, precipitating, and centrifugation/redispersion processes. As discussed above, low chitosan concentration would form relatively big emulsion droplets and correspondingly result in big microspheres. At a chitosan concentration of 0.1 wt% there was not enough chitosan that could effectively stabilize Pickering emulsions after one recycle. Consequently, the emulsion droplets easily tended to break up, and it was not rare for polymerization among droplets to form microsphere aggregations. At chitosan concentration of 0.5 wt%, there were still sufficient emulsifiers to form stable Pickering emulsions, although there was a little loss of chitosan after three recycles. Further increasing chitosan concentration actually generated no additional effect on morphologies and sizes of final microspheres. Therefore, chitosan concentration was fixed at 0.5 wt% for the following microsphere preparation processes.

Table 2 Mean diameters of chitosan-coated microspheres at different cycling numbers

	Chitosan concentration (wt%)	Mean diameters (μm)
		1	2	3
S7	0.1	21.2	37.3	40.5
S4	0.5	9.7	10.8	19.3
S8	1.0	9.5	15.4	13.8

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Fig. 6 Optical micrographs of microspheres prepared at chitosan concentrations of (a) 0.1 wt%, (b) 0.5 wt% and (c) 1.0 wt%. The numbers, 1–3 represent the cycle times. All scale bars are 50 μm.

The technological process of the photopolymerization of chitosan-based Pickering emulsions is a simple and effective approach for preparing various functional materials. Pickering emulsion systems stabilized by chitosan could also be applied in other fields of industrialization, for example polystyrene synthesis, not shown in this work. Compared to lignin used in our previous work,²⁹ chitosan has unique advantages in some fields, especially in the biomedical field due to the biocompatibility and biodegradability of chitosan.

Controlled release of IBU from microspheres

Chitosan-coated and pure microspheres were used to study the drug release behavior. IBU delivery and release have been widely studied.^41,42 In this study, IBU was also selected as a model drug to load into the microspheres. The encapsulation efficiency of IBU in the microspheres was determined by the difference between the total amount and the amount in the supernatants during the washing process. The encapsulation efficiency was almost 100% for the chitosan-coated microspheres. Furthermore, for the pure microspheres, the encapsulation efficiency showed a very slight decrease due to the washing process with IBU-saturated HCl–water. The encapsulation efficiency is higher than that of the microspheres prepared in our previous work.⁴³

Fig. 7 shows the IBU release from the chitosan-coated and pure microspheres as a function of time at pH 7.4 and 2.0 under 37 °C. All microspheres followed a similar release tendency: the initial stage of fast release in about 65 min, and the second stage of slow release to a platform (65–130 min). In contrast to the IBU release at pH 2.0 (a maximum release of 33.9%), the release was much faster at pH 7.4 and the maximum release amount reached 58.1%. It is well known that the IBU molecule exhibits a better solubility at pH 7.4 than at pH 2.0, which results in the significant difference of IBU release at different pH values. To our surprise, IBU release from the chitosan-coated microspheres and pure microspheres displayed almost the same controlled release behavior. Presumably, the big gap among the chitosan aggregations, which adsorbed on the surface of the chitosan-coated microspheres, provided almost no obstacle to the channel of IBU release.

Fig. 7 IBU release from microspheres at pH 7.4 and 2.0.

Then, the effect of temperature (20, 37 and 50 °C) on IBU release at pH 7.4 was investigated as shown in Fig. 8. IBU release rates at 37 and 50 °C were obviously faster than that at 20 °C, and the release rate at 50 °C was a little faster than that at 37 °C. Besides, the total release amount gradually increased slightly with rising temperature, and the release amounts were 57.8, 58.1 and 58.9% at 20, 37 and 50 °C, respectively. The increment of release rate and amount may be due to the slight increase of IBU solubility in aqueous solutions at higher temperature. IBU release suggested that the release rate can be controlled by tuning the release temperature or ambient medium such as pH value.

Fig. 8 IBU release from chitosan-coated microspheres at pH 7.4 at different temperatures.

Degradation studies

Degradation is important to polymeric spheres for drug delivery. Because of the many ester linkages (R–COO–R′) in microspheres introduced by trithiol and TMPTA, resulting microspheres should degrade in alkaline condition. Degradation studies were initially performed at 1.0 M NaOH aqueous solution under 37 °C (Table 1). In this case, degradations were carried out for 7 days and degradation extent displayed as similar to each other in the range of 71–82% mass loss, regardless of the compositions of the microspheres. Actually, the degradation extent was mainly determined by the amount of ester linkages in the microspheres, where the total amount of ester linkages was almost unchanged in different entries in Table 1.

Next, degradation studies were performed at different NaOH aqueous solutions and temperature, as shown in Table 3. With increasing both the concentration of NaOH solutions and the degradation temperature, the degradation rate greatly increased as expected. However, in contrast to the degradation extent at 37 °C, the extent at 50 °C in 1 M NaOH solution showed little increase, indicating that temperature plays a weak role in speeding up the degradation rate at high concentration of NaOH solution.

Table 3 Mass loss of pure microspheres at different NaOH concentrations for 7 days

Temperature (°C)	Mass loss (%)
	0.1 M	0.5 M	1 M
20	11.6	23.8	40.0
37	15.0	27.3	71.4
50	27.8	70.4	72.4

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Fig. 9 shows the mass loss of pure microspheres in 1 M NaOH at 37 °C as a function of time. The degradation rate was initially very fast and then gradually decreased until a mass loss plateau of about 90% was achieved. The degradation extent of the microspheres in this work was smaller than that of the scaffold materials consisting of the same monomers,³⁷ due to the high porosity of scaffolds where NaOH solution more easily permeated into the interior section.

Fig. 9 Mass loss of pure microspheres in 1 M NaOH at 37 °C as a function of time. Error bars represent standard deviation of the mean (n = 3).

Morphologies of pure microspheres degraded for 7 days in 0.1 or 1 M NaOH solution are shown in Fig. 10. In contrast to Fig. 3, the images in Fig. 10 display a completely different morphology. A part of the polymer microspheres of thiol–ene was corroded off by the NaOH solution; there even appeared polymer fibers in Fig. 10b. The degradation extent in 1 M NaOH solution was higher than that in 0.1 M NaOH solution. Moreover, the higher contents of Na and O and lower content of C for the former from Fig. 10b₃ and a₃ also suggests the higher degradation extent in 1 M NaOH solution. All the results demonstrated that we had successfully prepared degradable microspheres through Pickering emulsion photopolymerization, and the degradation extent and rate could be adjusted by the concentration of NaOH solutions and degradation temperature. However, degradation at harsh conditions of 0.1 to 1 M NaOH will not occur in the human body. Microspheres have a certain extent of degradability, instead of biodegradability. May be this degradable behavior can lead to other applications, but in the body.

Fig. 10 SEM images of pure microspheres incubated in 0.1 M NaOH (a₁ and a₂) and 1 M NaOH (b₁ and b₂) at 37 °C for 7 days. EDS spectra at (a₃) surface area 1 of the microsphere in a₂ and (b₃) surface area 2 of the microsphere in b₂.

Conclusions

In summary, pH-reversible chitosan-based Pickering emulsion was demonstrated in the recycling preparation of degradable microspheres under UV thiol–ene photopolymerization. The pH-reversibility of chitosan endowed it with a recyclability of at least three times in emulsion polymerization. Pure microspheres were obtained by dissolving the chitosan-coated microspheres in acidic solution to remove chitosan. Both microspheres showed a controlled release behavior of IBU and a certain extent of degradability in alkaline solution, which could be tuned by the pH value and temperature of the surrounding medium. The proposed approach for the preparation of degradable microspheres based on Pickering emulsion stabilized by pH-responsive chitosan expands the application of a wide range of reversible emulsifiers or emulsion systems in the green economy or in environmental protection.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program, 2012CB821500), the National Natural Science Foundation of China (21274046) and the Natural Science Foundation of Guangdong Province (S20120011057).

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Footnote

† Electronic supplementary information (ESI) available: Structural formula, optical micrographs. See DOI: 10.1039/c4ra01660b

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