Materials Science and Engineering C 47 (2015) 57–62
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Bacterial cellulose gels with high mechanical strength Yukari Numata a,⁎,1, Tadanori Sakata a, Hidemitsu Furukawa b, Kenji Tajima c a b c
Department of Materials Chemistry, Asahikawa National College of Technology, Asahikawa 071-8142, Japan Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
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Article history: Received 17 July 2014 Received in revised form 1 October 2014 Accepted 6 November 2014 Available online 8 November 2014 Keywords: Bacterial cellulose gels Thermo-responsiveness Composite gels
a b s t r a c t A composite structure was formed between polyethylene glycol diacrylate (PEGDA) and bacterial cellulose (BC) gels swollen in polyethylene glycol (PEG) as a solvent (BC/PEG gel) to improve the mechanical strength of the gels. The mechanical strength under compression and the rheostatic properties of the gels were evaluated. The compression test results indicated that the mechanical strength of the gels depended on the weight percent of cross-linked PEGDA in the gel, the chain length between the cross-linking points, and the cross-linking density of PEGDA polymers. The PEGDA polymers around the cellulose fibers were resistant to pressure; thus, the BC/ PEG-PEGDA gel was stronger than the BC/PEG gel under compression. The results of transmittance measurements and thermomechanical analysis showed that the rheostatic properties of the gels were retained even after composite structure formation. BC/PEG-PEGDA gels, which are expected to be biocompatible, may be useful for clinical applications as a soft material. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Bacterial cellulose (BC) is a unique type of biopolymer gel based on pure cellulose produced by Acetobacter [1]. BC gels have a threedimensional network structure of ultrafine fiber made from pure cellulose [1–3], and their fibers are several hundred times thinner than plant cellulose fibers. Thus, BC gels are especially fine polymeric materials. BC is a popular food in Japan in the form of “nata de coco,” which is a jellylike chewy food that is highly regarded for its abundant dietary fiber. BC gels have various unique properties such as softness, translucence, biocompatibility, and water-retention ability [4–6]. Therefore, BC gels have received much attention as potential soft materials for medical, cosmetic, and food applications. However, they have some drawbacks for such applications, as the large amount of water they contain is easily expelled under slight pressure, and the gels are easily dried. In our previous study, we established novel nonvolatile and thermoresponsive BC gels [7] that were swollen in polyethylene glycol (PEG) as a nonvolatile solvent rather than water; the resulting gels were termed BC/PEG gels. PEG has high water solubility and lacks volatility; moreover, its physical properties depend on its molecular weight. Novel BC/PEG gels are expected to be applied as novel gel materials with enhanced properties relative to typical BC gels. Additionally, BC/PEG gels are expected to be biocompatible because PEGs are biocompatible and ⁎ Corresponding author. E-mail address: [email protected] (Y. Numata). 1 Present address: Faculty of Commerce, Otaru University of Commerce, Otaru 047-8501, Japan.
http://dx.doi.org/10.1016/j.msec.2014.11.026 0928-4931/© 2014 Elsevier B.V. All rights reserved.
are extensively used in drug carriers [8–14], and BC is reported to have in vivo biocompatibility [6]. However, the PEG in BC/PEG gels is still easily expelled under slight pressure and BC/PEG gels thus show insufficient mechanical properties because the PEG does not bind to the BC by a strong bond such as a covalent bond. It is thus necessary to improve the mechanical properties of these gels for use in soft materials. Composite BC sheets, films, and gels have been reported as improvements over BC with regard to mechanical properties, biological activity, and biomedical applications [15]. BC hydrogels combined with collagen (gelatin) or polyacrylamide (PAAm) have higher mechanical strength than typical BC hydrogels. For example, at the degree of swelling (q) of 3.1, the fracture stress and elastic modulus of BC-gelatin hydrogels were 5.3 MPa and 3.9 MPa, respectively. The fracture stress of BCgelatin hydrogel was 550 times higher than the BC hydrogels [16]. A BC-PAAm gel in which the water content of the BC gel was decreased prior to formation of the composite structure had high mechanical properties for elongation where the fracture stress was 40 MPa (q = 1.9) [17] . We thus hypothesize that the mechanical strength of the BC/PEG gel may also be improved by forming a composite structure. In this study, we developed a BC/PEG gel with high mechanical strength by forming a composite gel with PEG diacrylate (PEGDA) polymer, which has repeat units of PEG and forms a network structure upon UV cross-linking. The compression test results indicated that it is possible to control the ability of the gel to withstand compression. In addition, the thermo-responsiveness properties were maintained in the gels. BC/PEG-PEGDA gels may thus be useful in the medical field for application as a soft material.
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2. Materials and methods 2.1. Chemicals PEG gels were purchased from Kishida Chemical Co., Ltd. (Osaka, Japan) with molecular weights (Mw) of 200 and 1000 (PEG 200 and PEG 1000, respectively). PEGDA was kindly provided by ShinNakamura Chemical Co., Ltd. (Wakayama, Japan). The degrees of polymerization were 9, 14, and 23 (9G, 14G, and 23G, respectively). 1-Hydroxycyclohexyl phenyl ketone as an initiator was purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). 2.2. Preparation of BC hydrogels BC gels were biosynthesized by Gluconacetobacter xylinus ATCC53582 in Hestrin and Schramm medium at 28 °C for 3 days under static conditions [18]. Subsequently, sample gels were purified in 1% (w/v) NaOH aqueous solution for deproteinization and washed with distilled water to neutralize the solution. 2.3. Preparation of BC gels swollen in PEG A mixture of PEG 200 and PEG 1000 (PEG 200–1000) was obtained by mixing equal weights of PEG 200 and PEG 1000 (1:1 weight ratio). The mixture was diluted 90% (v/v) in distilled water. BC gels swollen in PEG were prepared according to a previously described method [7]. Purified BC hydrogels with a diameter of 5 cm were added to the PEG solution and heated for 90 min to remove the water in the BC gels almost completely. The surfaces of the gels were washed with distilled water to remove excess PEG and dried at 65 °C for 1 day in air on a glass plate.
Fig. 1. Schematic representation of representative UV cross-linking network elements in PEGDA polymer.
measured. The weight of the BC fiber was negligible relative to that of PEG and cross-linked PEGDA because BC fibers comprise less than 1 wt.% of the BC/PEG gel [7]. The ratio of cross-linked PEGDA (Rcl PEGDA) in a BC/PEG-PEGDA gel was calculated as follows Eq. (1),
2.4. Preparation of BC composite gels
Rcl PEGDA ¼ W Residue =W 0
PEGDA 9G, 14G, and 23G were supplemented with 0.1% (w/w) 1hydroxycyclohexyl phenyl ketone as an initiator and diluted by 10–80 wt.% with the PEG 200–1000 described above. BC/PEG gels were immersed in these PEG solutions of PEGDA and incubated for 1 week at 35 °C in the dark. BC/PEG gels containing PEGDA were kept on the glass plate and irradiated by UV light (365 nm) for 90 s at 5 mW/cm2 at room temperature. The gels obtained were washed with distilled water to remove the PEG and PEGDA on the surface and dried on the glass plate.
where WResidue is the weight of the gel after PEG was removed and dried, and W 0 is the weight of the initial BC/PEG-PEGDA gel. The weight ratios of cross-linked PEGDA in the gels were used for evaluation in the compression test.
2.5. Fourier transform infrared spectroscopy Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was used to determine the conversion of the acrylate groups. The spectrometer was an FT-IR/350 (JASCO Co., Tokyo, Japan) device equipped with a ZnSe prism for ATR. For each measurement, 64 spectra were accumulated at a resolution of 4 cm−1. Fig. 1 presents a schematic representation of the network structures in cross-linked PEGDA. The acrylate groups polymerized each other and formed network structures. The disappearance of acrylate double bonds through polymerization led to a decrease in bands at 1410 cm−1 (deformation of the CH2 = CH bond) and 1190 cm−1 (acrylic C = O bond) [19]. 2.6. Determination of the ratio of cross-linked PEGDA in a BC/PEG-PEGDA gel The proportion of cross-linked PEGDA introduced in BC/PEG-PEGDA gels was established to remove PEG from the gels. First, the weight of the BC/PEG-PEGDA gels was measured. BC/PEG-PEGDA gels were then immersed in distilled water for 1 week at 35 °C to remove only PEG and dried completely for 3 days at 65 °C. Finally, the weight of the dried samples containing BC fibers and cross-linked PEGDA was
ð1Þ
2.7. Scanning electron microscopy The surface and cross-section of the BC/PEG-PEGDA and BC/PEG gels were observed using a JSM-6510LA (JEOL Co., Tokyo, Japan). The gel samples were immersed in distilled water for 1 week at 35 °C. The gels were completely depleted of PEG and then freeze-dried. The samples were coated with gold using a JEOL-1200 FINE COATER (JEOL Co., Tokyo, Japan) and observed at an accelerating voltage of 10 kV. 2.8. Compression test The mechanical properties of the gels were determined using a TENSILON RTG-1225 tensile-compressive tester (Orientec Co., Tokyo, Japan) at 30 °C. The samples were cut into a disk shape (diameter: 13 mm) and compressed perpendicular to the BC layers at a strain rate of 10% min−1 by 2 parallel metal plates connected to a load cell. The failure points of the compression tests were determined from the peak of the stress–strain curve. The elastic modulus was determined from the average slope of the stress–strain curve, with the strain ratio range from 0 to 0.1. 2.9. Determination of thermo-responsive characteristics For the transmittance measurement, the transmission of the gels was measured at 600 nm using a V-630iRM spectrometer (JASCO Co., Tokyo, Japan) equipped with an HMC-711 constant-temperature microcell holder (JASCO Co., Tokyo, Japan). The samples were fitted
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into the glass cell, which was held by a glass plate, and set in the microcell holder. The holder was set to temperatures between 15 °C and 35 °C, with intervals of 5 °C. 2.10. Thermomechanical analysis Thermomechanical analysis (TMA) was used to determine the change from the solid state to gel state. The measurements were performed using the compression load method with a Thermo plus EVO TMA 8310 (Rigaku Co., Tokyo, Japan) equipped with a compression probe in a nitrogen atmosphere. The sample gels were then heated from −15 °C to 50 °C at a rate of 2 °C/min under a 150 mN load perpendicular to the BC layers. 3. Results and discussion 3.1. Composite structure of BC/PEG-PEGDA To determine the introduction and polymerization of PEGDA, IR spectra of the sample before and after UV irradiation were measured. Fig. 2 demonstrates the IR spectra of a BC/PEG gel that was immersed in 60 wt.% PEGDA 14G (BC/PEG-60 wt.% PEGDA 14G) before and after UV irradiation. In the spectrum of the gel before UV irradiation, bonds derived from acrylate double bonds of PEGDA were observed at 1410 cm− 1 and 1190 cm− 1, suggesting that PEGDA was introduced into the gels. In the spectrum of the gel after irradiation, these bonds disappeared, which implies that the PEGDA introduced into the gel was polymerized in the gel [19]. These bonds showed the same tendency in all other spectra of gels with different concentrations of PEGDA (data not shown). Fig. 3 demonstrates the proportion of cross-linked PEGDA introduced into the sample, which was determined by removing PEG from the gels and then drying them. As mentioned above, the weight of the BC fibers is negligible relative to that of PEG and cross-linked PEGDA [7]. We thus assumed that the weight of cross-linked PEGDA was equal to that of dried BC/PEG-PEGDA without PEG. The filled circle, open circle, and open square in Fig. 3 show the weight ratios of PEGDA 9G, 14G, and 23G, respectively. In the PEGDA 9G and 14G samples, there was a good correlation between the initial concentration of PEGDA and the concentration of cross-linked PEGDA, and approximately 75–95% of the applied PEGDA was successfully integrated into the gel. At a PEGDA concentration of 80 wt.%, PEGDA 9G and 14G were present as 74.5 wt.% and 72.0 wt.% of the dried gel; with 93.1% and 90% yield, respectively. PEGDA 23G was obtained as 61.4 wt.% of the dried gel with 76.8% yield. The amount of PEGDA 23G introduced decreased as the PEGDA concentration increased, suggesting that PEGDA 23G, which has a high molecular weight and long chain length, was difficult to introduce into
Fig. 3. Ratios of cross-linked PEGDA in BC/PEG-PEGDA gels. The amount of cross-linked PEGDA includes the weight of BC fibers, which is negligible relative to that of cross-linked PEGDA. The BC/PEG-PEGDA gels were washed to remove PEG and then dried. Filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEGPEGDA 23G. The results are shown as the mean ± SD.
BC/PEG gels. In fact, the viscosity of PEGDA 23G (100 mPa·s/40 °C) is higher than that of PEGDA 9G and 14G (58 and 106 mPa∙s/25 °C), potentially influencing the amount of PEGDA introduced. Scanning electron microscopy images of BC/PEG-PEGDA and BC/PEG gels are shown in Fig. 4. Fig. 4(a) shows the surface of the BC/PEGPEGDA gel prepared from 80 wt.% 9G. BC/PEG has thin cellulose fibers, which form a porous three-dimensional network structure in the same manner observed for BC (Fig. 4(b)). In BC/PEG-PEGDA, cellulose fibers cannot be observed because the fibers are covered by the aggregates of PEGDA polymers. In fact, the aggregates covered the surface of BC/PEG and the surface became rough. Additionally the aggregates covered most of the pores of the gels and the pores became smaller than those of BC/PEG. The amount of PEGDA aggregates increased with the PEGDA concentration. Fig. 4(c) shows a cross-section of BC/ PEG-PEGDA prepared with 60 wt.% 9G. Aggregates of the PEGDA polymer were similarly formed within the gel. Cellulose fibers were also observed in the PEGDA polymers, which indicates that PEGDA polymers and cellulose fibers coexist in the gel. Therefore, PEGDA was completely introduced into the gel to form the composite structure. On the basis of these observations, we propose a composite structure for the BC/PEG-PEGDA gel, as shown in Fig. 5. We observed cellulose fibers comprising cellulose microfibrils that were in turn formed from cellulose molecular chains located in the PEGDA polymeric aggregates in the BC/PEG-PEGDA gel. Before UV irradiation, the PEGDA molecules
Fig. 2. IR spectra of BC/PEG-60 wt.% PEGDA 14G gel. Top: after UV irradiation; bottom: before UV irradiation.
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Fig. 4. Scanning electron microscopy images of BC/PEG and BC/PEG-PEGDA. (a) Surface of BC/PEG-PEGDA prepared from 80 wt.% 9G; (b) cross-section of BC/PEG; (c) cross-section of BC/ PEG-PEGDA prepared from 60 wt.% 9G.
introduced in the BC/PEG gel interacted with the cellulose fibers and not the cellulose molecular chains. The PEGDA polymerized under UV irradiation, and the PEGDA polymer chains formed a network structure around the cellulose fibers. 3.2. Mechanical properties of BC/PEG-PEGDA The effect of the cross-linked PEGDA in the gel on the mechanical properties of BC/PEG-PEGDA was investigated. Fig. 6 demonstrates the mechanical properties for compression at 30 °C. The amount of cross-linked PEGDA in the gel was determined from the weight of dried BC/PEG-PEGDA (Fig. 3). The measured values of cross-linked PEGDA contain the weight of BC fiber. The compressive fracture stress and strain of BC/PEG-PEGDA prepared from 0 wt.% cross-linked PEGDA, non-cross-linked BC/PEG, were not determined. The BC/PEG gel was very weak under compression, similar to the BC gel [16]. These were flattened without fracturing, so the fracture stress and strain values could not be obtained. This mechanical property is not suitable for soft materials. The compressive fracture stress was determined from the stress–strain curve shown in Fig. 6(a). The fracture stress increased with increased cross-linked PEGDA. The stress values for the
PEGDA 9G and 14G samples were similar. However, the stress values of the PEGDA 23G samples were lower than those of the PEGDA 9G and 14G samples. Because the cross-linked PEGDA was prepared according to the weight percentage of PEG and monomers with a low degree of polymerization have a larger acrylate group content relative to the rest of the PEG molecule, PEGDA 9G and 14G had more crosslinked points than PEGDA 23G, as observed from the IR spectra. Therefore, PEGDA 9G and 14G may tend to form tight network structures that resist compression. Fig. 6(b) demonstrates the compressive fracture strain determined from the stress–strain curve. In all samples, the strain in the samples decreased as the cross-linked PEGDA increased, which indicates that the gels became brittle as the cross-linked PEGDA increased. The strain was similar for PEGDA 9G and 14G; however, that of PEGDA 23G decreased gradually compared with PEGDA 9G and 14G, which suggests that the compressive fracture strain was related to the network structure of the PEGDA polymer at a low cross-linked PEGDA in the gel. We hypothesize that the effort of entanglement of PEGDA 23G molecules on fracture strain for cross-linked PEGDA in 20 wt.% PEGDA 23G is larger than that of tight network (more crosslinked points) structures on fracture strain for 20 wt.% PEGDA 9G and 14G. The viscosity of PEGDAs is different: 23G N 14G N 9G. Under our
Fig. 5. Schematic representation of the network structure for the BC/PEG-PEGDA gel. Cellulose fibers are located in the PEGDA polymeric aggregates and comprise cellulose microfibrils formed by assembled cellulose molecular chains. The PEGDA polymer chains form a network structure around the cellulose fibers.
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Fig. 6. Effects of the ratios of cross-linked PEGDA in gels on (a) compressive fracture stress, (b) compressive fracture strain, and (c) compressive elastic modulus for BC/PEG-PEGDA. The amount of cross-linked PEGDA contains the weight of BC fiber, which is negligible relative to that of cross-linked PEGDA. The values of fracture stress, fracture strain, and elastic modulus were determined from the stress–strain curve of the compression test performed at 30 °C. Filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEGPEGDA 23G. The results are shown as the mean ± SD.
conditions, PEGDA 23G molecules become more entangled than PEGDA 14G or 9G, which influences the degree of cross-linking. However, at a high degree of cross-linked PEGDA, the strain was not related to the network structure or to the amount of cross-linked PEGDA in the gel. Fig. 6(c) demonstrates the compressive elastic modulus determined from the average slope of the stress–strain curve, with the strain ratio range from 0 to 0.1. The elastic modulus of the gels increased as the cross-linked PEGDA increased, which indicates that the gels became harder as the PEGDA content increased. In general, the elastic modulus is proportional to the density of cross-linked points and inversely proportional to polymer chain length between cross-linking points on polymer network structure. However, 23G had the highest elastic modulus value, and 14G had the second highest value. We propose that the entanglement of PEGDA molecules affects the elastic modulus by forming cross-linked points with shorter intervening chain lengths. The formation of the composite structure with PEGDA polymers resulted in improved mechanical properties under compression for BC/PEG-PEGDA. The BC/PEG and BC gels were very weak under compression and were difficult to mold, which limited the potential application of the materials. The composite structure was shown to improve the mechanical characteristics of the gel. The aggregation of PEGDA polymers around the cellulose fibers enabled the gel to be resistant to pressure; thus, the BC/PEG-PEGDA gel was stronger under compression than the BC/PEG gel.
that light was diffused through the gel by the PEGDA network structure imparted by the cross-linking. The BC/PEG-PEGDA gel showed the same thermo-responsiveness as the BC/PEG gel; at a low temperature, it was opaque, whereas at a high temperature, it was transparent. However, the change between the solid state and the gel state was difficult to determine, as the change in transmission of BC/PEG-PEGDA 9G and 14G was small. We therefore evaluated the change in BC/PEG-PEGDA 9G and 14G by using TMA as shown in Fig. 8. In general, the volume of a substance increases as the temperature increases. At a low temperature, BC/PEG-PEGDA was in a solid state, and the thickness of the sample increased with heating. In contrast, the thickness decreased under the influence of loading as the sample shifted to the gel state. Fig. 8(a) demonstrates the TMA curves of BC/PEG-PEGDA 9G. BC/PEG-10 wt.% PEGDA (cross-linked PEGDA in the gel: 9.9 wt.%) contracted rapidly at temperatures higher than 20 °C. This temperature at which contraction occurred was the temperature of gelation and was found to be lower than that of samples with other PEGDA concentrations; the 20 wt.% and 30 wt.% PEGDA (crosslinked PEGDA in the gel: 18.9 wt.% and 28.6 wt.%, respectively) gels also contracted, but at a temperature of approximately 25 °C. The rate of contraction decreased with increased PEGDA concentration, which agrees with the change in compressive fracture strain (Fig. 6(b)). In contrast, 60 wt.% PEGDA (cross-linked PEGDA in the gel: 57.3 wt.%)
3.3. Thermo-responsiveness of BC/PEG-PEGDA To determine the thermo-responsiveness of the BC/PEG-PEGDA gels, the transmission was measured every 5 °C. Fig. 7 presents the transmission (λ = 600 nm) of the BC/PEG and BC/PEG-20 wt.% PEGDA gels with thickness of 2.6 mm (BC/PEG), 2.1 mm (BC/PEG-PEGDA 9G), 3.0 mm (14G), and 2.7 mm (23G). BC/PEG gels prepared using PEG 200–1000 showed reversible thermo-responsive characteristics [7]. They existed in a white, opaque solid state at low temperatures and in a transparent gel state at high temperatures because their physical changes depended on the melting point of PEG. The BC/PEG gel was opaque and more inelastic than the purified BC gel when swollen in water at temperatures below 20 °C. However, the BC/PEG gel became transparent and elastic as the temperature increased above 25 °C. BC/PEG-PEGDA gels also changed from opaque at low temperature to transparent at high temperature, and this property remained even after PEGDA was crosslinked. However, the temperature at which the gel changed from opaque to transparent shifted to 25–30 °C. In addition, the transmission in the gel decreased considerably; BC/PEG-PEGDA 9G and 14G exhibited approximately 10% and 20% transmission, respectively, which suggests
Fig. 7. Transmission (λ = 600 nm) of BC/PEG-20 wt.% PEGDA. Gel thicknesses were 2.6 mm (BC/PEG), 2.1 mm (9G), 3.0 mm (14G), and 2.7 mm (23G). Open triangles: BC/ PEG; filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEG-PEGDA 23G.
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Fig. 8. TMA curves of BC/PEG-PEGDA. The samples were heated from −15 °C to 50 °C at a rate of 2 °C/min under a 150 mN load perpendicular to the BC layers. (a) 9G, cross-linked PEGDA: 9.9 wt.% (PEGDA concentration 10 wt.%), 18.9 wt.% (20 wt.%), 28.6 wt.% (30 wt.%), and 57.3 wt.% (60 wt.%). (b) 14G, cross-linked PEGDA: 4.4 wt.% (10 wt.%), 14.0 wt.% (20 wt.%), 23.6 wt.% (30 wt.%), and 51.9 wt.% (60 wt.%).
showed a different trend. After the sample began to contract at approximately 25 °C, it expanded again, which indicates that a portion of the cross-linked PEGDA in the gels did not contract under the 150 mN load and expanded after part of the gel contracted. Fig. 8(b) demonstrates the TMA curves for BC/PEG-PEGDA 14G, which show the same tendency as the curves for 9G, except at concentrations of 10 wt.% and 20 wt.% (cross-linked PEGDA in the gel: 4.4 wt.% and 14.0 wt.%, respectively), at which the gel was strongly influenced by the cross-linking of PEGDA. The TMA results indicate that the elasticity of the sample in the gel state was decreased by cross-linked PEGDA and thus support the results of the compression test. PEG is not involved in the formation of network structure in this gel; however, it is necessary for the formation of the gel state and for providing nonvolatile and thermo-responsive properties, particularly, because the thermoresponsive properties depend on the melting of PEG. At a high level of cross-linked PEGDA, the PEG had a small effect on the thermoresponsiveness of the gel. The cross-linked PEGDA should thus be lower than 30 wt.% to obtain an elastic gel under a 150 mN load.
improved in further studies to provide suitable mechanical strength and hardness for the soft materials and to evaluate the inclusion and release of drugs. Acknowledgments We are grateful to Shin-Nakamura Chemical Co., Ltd. for providing the PEGDA samples. This study was supported in part by Grants-inAid for Regional R&D Proposal-Based Program from the Northern Advancement Center for Science & Technology of Hokkaido, Japan (No.: T-Asahikawa). References [1] [2] [3] [4] [5] [6]
4. Conclusions [7]
We succeeded in developing BC/PEG gels with high mechanical strength by forming a composite gel with a cross-linked PEGDA network structure. The compression test results indicated that it is possible to control the ability of the gel to withstand compression by altering the concentration and degree of polymerization of PEGDA. In addition, the transmission measurement and TMA results indicated that the thermo-responsiveness properties were maintained in the BC/PEG-PEGDA gels as the gels changed from opaque and solid to transparent and elastic. BC/PEG-PEGDA gels may thus be useful in the medical field as a healing material and drug delivery system. The low mass content of the composite in BC/PEG-PEGDA allows it to be a soft material at body temperatures when the Mw of PEG is carefully selected [7]. Additionally the gel is soft, biocompatible, and nonvolatile. The gel can be used as a healing material in surface materials of rehabilitation and welfare equipment. BC/PEG-PEGDA is also suitable for drug delivery because it retains drugs at low temperatures and releases them at high temperatures. The BC/PEG-PEGDA gels can be
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
M. Iguchi, S. Yamanaka, A. Budhiono, J. Mater. Sci. 35 (2000) 261–270. P. Ross, R. Mayer, M. Benziman, Microbiol. Rev. 55 (1991) 35–58. R. Malcolm Brown Jr., J. Macromol. Sci. A Pure Appl. Chem. 33 (1996) 1345–1373. A. Okiyama, M. Motoki, S. Yamanaka, Food Hydrocoll. 6 (1993) 493–501. D. Klemm, B. Heublein, H.P. Fink, A. Bohn, Angew. Chem. Int. Ed. 44 (2005) 2–37. G. Helenius, H. Backdahl, A. Bodin, U. Nannmark, P. Gatenholm, B. Risberg, J. Biomed. Mater. Res. A 76 (2006) 431–438. Y. Numata, K. Muromoto, H. Furukawa, J.P. Gong, K. Tajima, M. Munekata, Polym. J. 41 (2009) 524–525. M.K. Pratten, J.B. Lloyd, G. Horpel, H. Ringsdorf, Macromol. Chem. 186 (1985) 725–733. K. Kataoka, A. Harada, Y. Nagasaki, Adv. Drug Deliv. Rev. 47 (2001) 113–131. H.M. Aliabadi, A. Mahmud, A.D. Sharifabadi, A. Lavasanifar, J. Control. Release 104 (2005) 301–311. X.W. Wei, C.Y. Gong, M.L. Gou, S.Z. Fu, Q.F. Guo, S. Shi, F. Luo, G. Guo, L.Y. Qiu, Z.Y. Qian, Int. J. Pharm. 381 (2009) 1–18. L.R. Hirsch, R.J. Stafford, J.A. Bankson, S.R. Sershen, B. Rivera, R.E. Price, J.D. Hazle, N.J. Halas, Proc. Natl. Acad. Sci. 100 (2003) 13549–13554. T. Niidome, Y. Akiyama, M. Yamagata, T. Kawano, T. Mori, Y. Niidome, Y. Kayama, J. Biomater. Sci. Polym. Ed. 20 (2009) 1203–1215. T. Niidome, A. Ohga, Y. Akiyama, K. Watanabe, Y. Niidome, T. Mori, Y. Katayama, Bioorg. Med. Chem. 18 (2010) 4453–4458. N. Shah, M. UI-Islam, W.A. Khattak, J.K. Park, Carbohydr. Polym. 98 (2013) 1585–1598. A. Nakayama, A. Kakugo, J.P. Gong, Y. Osada, M. Takai, T. Erata, S. Kawano, Adv. Funct. Mater. 14 (2004) 1124–1128. Y. Hagiwara, A. Putra, A. Kakugo, H. Furukawa, J.P. Gong, Cellulose 17 (2010) 93–101. S. Hestrin, M. Schramm, Biochem. J. 58 (1954) 345–352. H. Lin, T. Kai, B.D. Freeman, S. Kalakkunnath, D.S. Kalika, Macromolecules 38 (2005) 8381–8393.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Bacterial cellulose gels with high mechanical strength Yukari Numata a,⁎,1, Tadanori Sakata a, Hidemitsu Furukawa b, Kenji Tajima c a b c
Department of Materials Chemistry, Asahikawa National College of Technology, Asahikawa 071-8142, Japan Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2014 Received in revised form 1 October 2014 Accepted 6 November 2014 Available online 8 November 2014 Keywords: Bacterial cellulose gels Thermo-responsiveness Composite gels
a b s t r a c t A composite structure was formed between polyethylene glycol diacrylate (PEGDA) and bacterial cellulose (BC) gels swollen in polyethylene glycol (PEG) as a solvent (BC/PEG gel) to improve the mechanical strength of the gels. The mechanical strength under compression and the rheostatic properties of the gels were evaluated. The compression test results indicated that the mechanical strength of the gels depended on the weight percent of cross-linked PEGDA in the gel, the chain length between the cross-linking points, and the cross-linking density of PEGDA polymers. The PEGDA polymers around the cellulose fibers were resistant to pressure; thus, the BC/ PEG-PEGDA gel was stronger than the BC/PEG gel under compression. The results of transmittance measurements and thermomechanical analysis showed that the rheostatic properties of the gels were retained even after composite structure formation. BC/PEG-PEGDA gels, which are expected to be biocompatible, may be useful for clinical applications as a soft material. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Bacterial cellulose (BC) is a unique type of biopolymer gel based on pure cellulose produced by Acetobacter [1]. BC gels have a threedimensional network structure of ultrafine fiber made from pure cellulose [1–3], and their fibers are several hundred times thinner than plant cellulose fibers. Thus, BC gels are especially fine polymeric materials. BC is a popular food in Japan in the form of “nata de coco,” which is a jellylike chewy food that is highly regarded for its abundant dietary fiber. BC gels have various unique properties such as softness, translucence, biocompatibility, and water-retention ability [4–6]. Therefore, BC gels have received much attention as potential soft materials for medical, cosmetic, and food applications. However, they have some drawbacks for such applications, as the large amount of water they contain is easily expelled under slight pressure, and the gels are easily dried. In our previous study, we established novel nonvolatile and thermoresponsive BC gels [7] that were swollen in polyethylene glycol (PEG) as a nonvolatile solvent rather than water; the resulting gels were termed BC/PEG gels. PEG has high water solubility and lacks volatility; moreover, its physical properties depend on its molecular weight. Novel BC/PEG gels are expected to be applied as novel gel materials with enhanced properties relative to typical BC gels. Additionally, BC/PEG gels are expected to be biocompatible because PEGs are biocompatible and ⁎ Corresponding author. E-mail address: [email protected] (Y. Numata). 1 Present address: Faculty of Commerce, Otaru University of Commerce, Otaru 047-8501, Japan.
http://dx.doi.org/10.1016/j.msec.2014.11.026 0928-4931/© 2014 Elsevier B.V. All rights reserved.
are extensively used in drug carriers [8–14], and BC is reported to have in vivo biocompatibility [6]. However, the PEG in BC/PEG gels is still easily expelled under slight pressure and BC/PEG gels thus show insufficient mechanical properties because the PEG does not bind to the BC by a strong bond such as a covalent bond. It is thus necessary to improve the mechanical properties of these gels for use in soft materials. Composite BC sheets, films, and gels have been reported as improvements over BC with regard to mechanical properties, biological activity, and biomedical applications [15]. BC hydrogels combined with collagen (gelatin) or polyacrylamide (PAAm) have higher mechanical strength than typical BC hydrogels. For example, at the degree of swelling (q) of 3.1, the fracture stress and elastic modulus of BC-gelatin hydrogels were 5.3 MPa and 3.9 MPa, respectively. The fracture stress of BCgelatin hydrogel was 550 times higher than the BC hydrogels [16]. A BC-PAAm gel in which the water content of the BC gel was decreased prior to formation of the composite structure had high mechanical properties for elongation where the fracture stress was 40 MPa (q = 1.9) [17] . We thus hypothesize that the mechanical strength of the BC/PEG gel may also be improved by forming a composite structure. In this study, we developed a BC/PEG gel with high mechanical strength by forming a composite gel with PEG diacrylate (PEGDA) polymer, which has repeat units of PEG and forms a network structure upon UV cross-linking. The compression test results indicated that it is possible to control the ability of the gel to withstand compression. In addition, the thermo-responsiveness properties were maintained in the gels. BC/PEG-PEGDA gels may thus be useful in the medical field for application as a soft material.
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2. Materials and methods 2.1. Chemicals PEG gels were purchased from Kishida Chemical Co., Ltd. (Osaka, Japan) with molecular weights (Mw) of 200 and 1000 (PEG 200 and PEG 1000, respectively). PEGDA was kindly provided by ShinNakamura Chemical Co., Ltd. (Wakayama, Japan). The degrees of polymerization were 9, 14, and 23 (9G, 14G, and 23G, respectively). 1-Hydroxycyclohexyl phenyl ketone as an initiator was purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). 2.2. Preparation of BC hydrogels BC gels were biosynthesized by Gluconacetobacter xylinus ATCC53582 in Hestrin and Schramm medium at 28 °C for 3 days under static conditions [18]. Subsequently, sample gels were purified in 1% (w/v) NaOH aqueous solution for deproteinization and washed with distilled water to neutralize the solution. 2.3. Preparation of BC gels swollen in PEG A mixture of PEG 200 and PEG 1000 (PEG 200–1000) was obtained by mixing equal weights of PEG 200 and PEG 1000 (1:1 weight ratio). The mixture was diluted 90% (v/v) in distilled water. BC gels swollen in PEG were prepared according to a previously described method [7]. Purified BC hydrogels with a diameter of 5 cm were added to the PEG solution and heated for 90 min to remove the water in the BC gels almost completely. The surfaces of the gels were washed with distilled water to remove excess PEG and dried at 65 °C for 1 day in air on a glass plate.
Fig. 1. Schematic representation of representative UV cross-linking network elements in PEGDA polymer.
measured. The weight of the BC fiber was negligible relative to that of PEG and cross-linked PEGDA because BC fibers comprise less than 1 wt.% of the BC/PEG gel [7]. The ratio of cross-linked PEGDA (Rcl PEGDA) in a BC/PEG-PEGDA gel was calculated as follows Eq. (1),
2.4. Preparation of BC composite gels
Rcl PEGDA ¼ W Residue =W 0
PEGDA 9G, 14G, and 23G were supplemented with 0.1% (w/w) 1hydroxycyclohexyl phenyl ketone as an initiator and diluted by 10–80 wt.% with the PEG 200–1000 described above. BC/PEG gels were immersed in these PEG solutions of PEGDA and incubated for 1 week at 35 °C in the dark. BC/PEG gels containing PEGDA were kept on the glass plate and irradiated by UV light (365 nm) for 90 s at 5 mW/cm2 at room temperature. The gels obtained were washed with distilled water to remove the PEG and PEGDA on the surface and dried on the glass plate.
where WResidue is the weight of the gel after PEG was removed and dried, and W 0 is the weight of the initial BC/PEG-PEGDA gel. The weight ratios of cross-linked PEGDA in the gels were used for evaluation in the compression test.
2.5. Fourier transform infrared spectroscopy Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was used to determine the conversion of the acrylate groups. The spectrometer was an FT-IR/350 (JASCO Co., Tokyo, Japan) device equipped with a ZnSe prism for ATR. For each measurement, 64 spectra were accumulated at a resolution of 4 cm−1. Fig. 1 presents a schematic representation of the network structures in cross-linked PEGDA. The acrylate groups polymerized each other and formed network structures. The disappearance of acrylate double bonds through polymerization led to a decrease in bands at 1410 cm−1 (deformation of the CH2 = CH bond) and 1190 cm−1 (acrylic C = O bond) [19]. 2.6. Determination of the ratio of cross-linked PEGDA in a BC/PEG-PEGDA gel The proportion of cross-linked PEGDA introduced in BC/PEG-PEGDA gels was established to remove PEG from the gels. First, the weight of the BC/PEG-PEGDA gels was measured. BC/PEG-PEGDA gels were then immersed in distilled water for 1 week at 35 °C to remove only PEG and dried completely for 3 days at 65 °C. Finally, the weight of the dried samples containing BC fibers and cross-linked PEGDA was
ð1Þ
2.7. Scanning electron microscopy The surface and cross-section of the BC/PEG-PEGDA and BC/PEG gels were observed using a JSM-6510LA (JEOL Co., Tokyo, Japan). The gel samples were immersed in distilled water for 1 week at 35 °C. The gels were completely depleted of PEG and then freeze-dried. The samples were coated with gold using a JEOL-1200 FINE COATER (JEOL Co., Tokyo, Japan) and observed at an accelerating voltage of 10 kV. 2.8. Compression test The mechanical properties of the gels were determined using a TENSILON RTG-1225 tensile-compressive tester (Orientec Co., Tokyo, Japan) at 30 °C. The samples were cut into a disk shape (diameter: 13 mm) and compressed perpendicular to the BC layers at a strain rate of 10% min−1 by 2 parallel metal plates connected to a load cell. The failure points of the compression tests were determined from the peak of the stress–strain curve. The elastic modulus was determined from the average slope of the stress–strain curve, with the strain ratio range from 0 to 0.1. 2.9. Determination of thermo-responsive characteristics For the transmittance measurement, the transmission of the gels was measured at 600 nm using a V-630iRM spectrometer (JASCO Co., Tokyo, Japan) equipped with an HMC-711 constant-temperature microcell holder (JASCO Co., Tokyo, Japan). The samples were fitted
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into the glass cell, which was held by a glass plate, and set in the microcell holder. The holder was set to temperatures between 15 °C and 35 °C, with intervals of 5 °C. 2.10. Thermomechanical analysis Thermomechanical analysis (TMA) was used to determine the change from the solid state to gel state. The measurements were performed using the compression load method with a Thermo plus EVO TMA 8310 (Rigaku Co., Tokyo, Japan) equipped with a compression probe in a nitrogen atmosphere. The sample gels were then heated from −15 °C to 50 °C at a rate of 2 °C/min under a 150 mN load perpendicular to the BC layers. 3. Results and discussion 3.1. Composite structure of BC/PEG-PEGDA To determine the introduction and polymerization of PEGDA, IR spectra of the sample before and after UV irradiation were measured. Fig. 2 demonstrates the IR spectra of a BC/PEG gel that was immersed in 60 wt.% PEGDA 14G (BC/PEG-60 wt.% PEGDA 14G) before and after UV irradiation. In the spectrum of the gel before UV irradiation, bonds derived from acrylate double bonds of PEGDA were observed at 1410 cm− 1 and 1190 cm− 1, suggesting that PEGDA was introduced into the gels. In the spectrum of the gel after irradiation, these bonds disappeared, which implies that the PEGDA introduced into the gel was polymerized in the gel [19]. These bonds showed the same tendency in all other spectra of gels with different concentrations of PEGDA (data not shown). Fig. 3 demonstrates the proportion of cross-linked PEGDA introduced into the sample, which was determined by removing PEG from the gels and then drying them. As mentioned above, the weight of the BC fibers is negligible relative to that of PEG and cross-linked PEGDA [7]. We thus assumed that the weight of cross-linked PEGDA was equal to that of dried BC/PEG-PEGDA without PEG. The filled circle, open circle, and open square in Fig. 3 show the weight ratios of PEGDA 9G, 14G, and 23G, respectively. In the PEGDA 9G and 14G samples, there was a good correlation between the initial concentration of PEGDA and the concentration of cross-linked PEGDA, and approximately 75–95% of the applied PEGDA was successfully integrated into the gel. At a PEGDA concentration of 80 wt.%, PEGDA 9G and 14G were present as 74.5 wt.% and 72.0 wt.% of the dried gel; with 93.1% and 90% yield, respectively. PEGDA 23G was obtained as 61.4 wt.% of the dried gel with 76.8% yield. The amount of PEGDA 23G introduced decreased as the PEGDA concentration increased, suggesting that PEGDA 23G, which has a high molecular weight and long chain length, was difficult to introduce into
Fig. 3. Ratios of cross-linked PEGDA in BC/PEG-PEGDA gels. The amount of cross-linked PEGDA includes the weight of BC fibers, which is negligible relative to that of cross-linked PEGDA. The BC/PEG-PEGDA gels were washed to remove PEG and then dried. Filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEGPEGDA 23G. The results are shown as the mean ± SD.
BC/PEG gels. In fact, the viscosity of PEGDA 23G (100 mPa·s/40 °C) is higher than that of PEGDA 9G and 14G (58 and 106 mPa∙s/25 °C), potentially influencing the amount of PEGDA introduced. Scanning electron microscopy images of BC/PEG-PEGDA and BC/PEG gels are shown in Fig. 4. Fig. 4(a) shows the surface of the BC/PEGPEGDA gel prepared from 80 wt.% 9G. BC/PEG has thin cellulose fibers, which form a porous three-dimensional network structure in the same manner observed for BC (Fig. 4(b)). In BC/PEG-PEGDA, cellulose fibers cannot be observed because the fibers are covered by the aggregates of PEGDA polymers. In fact, the aggregates covered the surface of BC/PEG and the surface became rough. Additionally the aggregates covered most of the pores of the gels and the pores became smaller than those of BC/PEG. The amount of PEGDA aggregates increased with the PEGDA concentration. Fig. 4(c) shows a cross-section of BC/ PEG-PEGDA prepared with 60 wt.% 9G. Aggregates of the PEGDA polymer were similarly formed within the gel. Cellulose fibers were also observed in the PEGDA polymers, which indicates that PEGDA polymers and cellulose fibers coexist in the gel. Therefore, PEGDA was completely introduced into the gel to form the composite structure. On the basis of these observations, we propose a composite structure for the BC/PEG-PEGDA gel, as shown in Fig. 5. We observed cellulose fibers comprising cellulose microfibrils that were in turn formed from cellulose molecular chains located in the PEGDA polymeric aggregates in the BC/PEG-PEGDA gel. Before UV irradiation, the PEGDA molecules
Fig. 2. IR spectra of BC/PEG-60 wt.% PEGDA 14G gel. Top: after UV irradiation; bottom: before UV irradiation.
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Fig. 4. Scanning electron microscopy images of BC/PEG and BC/PEG-PEGDA. (a) Surface of BC/PEG-PEGDA prepared from 80 wt.% 9G; (b) cross-section of BC/PEG; (c) cross-section of BC/ PEG-PEGDA prepared from 60 wt.% 9G.
introduced in the BC/PEG gel interacted with the cellulose fibers and not the cellulose molecular chains. The PEGDA polymerized under UV irradiation, and the PEGDA polymer chains formed a network structure around the cellulose fibers. 3.2. Mechanical properties of BC/PEG-PEGDA The effect of the cross-linked PEGDA in the gel on the mechanical properties of BC/PEG-PEGDA was investigated. Fig. 6 demonstrates the mechanical properties for compression at 30 °C. The amount of cross-linked PEGDA in the gel was determined from the weight of dried BC/PEG-PEGDA (Fig. 3). The measured values of cross-linked PEGDA contain the weight of BC fiber. The compressive fracture stress and strain of BC/PEG-PEGDA prepared from 0 wt.% cross-linked PEGDA, non-cross-linked BC/PEG, were not determined. The BC/PEG gel was very weak under compression, similar to the BC gel [16]. These were flattened without fracturing, so the fracture stress and strain values could not be obtained. This mechanical property is not suitable for soft materials. The compressive fracture stress was determined from the stress–strain curve shown in Fig. 6(a). The fracture stress increased with increased cross-linked PEGDA. The stress values for the
PEGDA 9G and 14G samples were similar. However, the stress values of the PEGDA 23G samples were lower than those of the PEGDA 9G and 14G samples. Because the cross-linked PEGDA was prepared according to the weight percentage of PEG and monomers with a low degree of polymerization have a larger acrylate group content relative to the rest of the PEG molecule, PEGDA 9G and 14G had more crosslinked points than PEGDA 23G, as observed from the IR spectra. Therefore, PEGDA 9G and 14G may tend to form tight network structures that resist compression. Fig. 6(b) demonstrates the compressive fracture strain determined from the stress–strain curve. In all samples, the strain in the samples decreased as the cross-linked PEGDA increased, which indicates that the gels became brittle as the cross-linked PEGDA increased. The strain was similar for PEGDA 9G and 14G; however, that of PEGDA 23G decreased gradually compared with PEGDA 9G and 14G, which suggests that the compressive fracture strain was related to the network structure of the PEGDA polymer at a low cross-linked PEGDA in the gel. We hypothesize that the effort of entanglement of PEGDA 23G molecules on fracture strain for cross-linked PEGDA in 20 wt.% PEGDA 23G is larger than that of tight network (more crosslinked points) structures on fracture strain for 20 wt.% PEGDA 9G and 14G. The viscosity of PEGDAs is different: 23G N 14G N 9G. Under our
Fig. 5. Schematic representation of the network structure for the BC/PEG-PEGDA gel. Cellulose fibers are located in the PEGDA polymeric aggregates and comprise cellulose microfibrils formed by assembled cellulose molecular chains. The PEGDA polymer chains form a network structure around the cellulose fibers.
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Fig. 6. Effects of the ratios of cross-linked PEGDA in gels on (a) compressive fracture stress, (b) compressive fracture strain, and (c) compressive elastic modulus for BC/PEG-PEGDA. The amount of cross-linked PEGDA contains the weight of BC fiber, which is negligible relative to that of cross-linked PEGDA. The values of fracture stress, fracture strain, and elastic modulus were determined from the stress–strain curve of the compression test performed at 30 °C. Filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEGPEGDA 23G. The results are shown as the mean ± SD.
conditions, PEGDA 23G molecules become more entangled than PEGDA 14G or 9G, which influences the degree of cross-linking. However, at a high degree of cross-linked PEGDA, the strain was not related to the network structure or to the amount of cross-linked PEGDA in the gel. Fig. 6(c) demonstrates the compressive elastic modulus determined from the average slope of the stress–strain curve, with the strain ratio range from 0 to 0.1. The elastic modulus of the gels increased as the cross-linked PEGDA increased, which indicates that the gels became harder as the PEGDA content increased. In general, the elastic modulus is proportional to the density of cross-linked points and inversely proportional to polymer chain length between cross-linking points on polymer network structure. However, 23G had the highest elastic modulus value, and 14G had the second highest value. We propose that the entanglement of PEGDA molecules affects the elastic modulus by forming cross-linked points with shorter intervening chain lengths. The formation of the composite structure with PEGDA polymers resulted in improved mechanical properties under compression for BC/PEG-PEGDA. The BC/PEG and BC gels were very weak under compression and were difficult to mold, which limited the potential application of the materials. The composite structure was shown to improve the mechanical characteristics of the gel. The aggregation of PEGDA polymers around the cellulose fibers enabled the gel to be resistant to pressure; thus, the BC/PEG-PEGDA gel was stronger under compression than the BC/PEG gel.
that light was diffused through the gel by the PEGDA network structure imparted by the cross-linking. The BC/PEG-PEGDA gel showed the same thermo-responsiveness as the BC/PEG gel; at a low temperature, it was opaque, whereas at a high temperature, it was transparent. However, the change between the solid state and the gel state was difficult to determine, as the change in transmission of BC/PEG-PEGDA 9G and 14G was small. We therefore evaluated the change in BC/PEG-PEGDA 9G and 14G by using TMA as shown in Fig. 8. In general, the volume of a substance increases as the temperature increases. At a low temperature, BC/PEG-PEGDA was in a solid state, and the thickness of the sample increased with heating. In contrast, the thickness decreased under the influence of loading as the sample shifted to the gel state. Fig. 8(a) demonstrates the TMA curves of BC/PEG-PEGDA 9G. BC/PEG-10 wt.% PEGDA (cross-linked PEGDA in the gel: 9.9 wt.%) contracted rapidly at temperatures higher than 20 °C. This temperature at which contraction occurred was the temperature of gelation and was found to be lower than that of samples with other PEGDA concentrations; the 20 wt.% and 30 wt.% PEGDA (crosslinked PEGDA in the gel: 18.9 wt.% and 28.6 wt.%, respectively) gels also contracted, but at a temperature of approximately 25 °C. The rate of contraction decreased with increased PEGDA concentration, which agrees with the change in compressive fracture strain (Fig. 6(b)). In contrast, 60 wt.% PEGDA (cross-linked PEGDA in the gel: 57.3 wt.%)
3.3. Thermo-responsiveness of BC/PEG-PEGDA To determine the thermo-responsiveness of the BC/PEG-PEGDA gels, the transmission was measured every 5 °C. Fig. 7 presents the transmission (λ = 600 nm) of the BC/PEG and BC/PEG-20 wt.% PEGDA gels with thickness of 2.6 mm (BC/PEG), 2.1 mm (BC/PEG-PEGDA 9G), 3.0 mm (14G), and 2.7 mm (23G). BC/PEG gels prepared using PEG 200–1000 showed reversible thermo-responsive characteristics [7]. They existed in a white, opaque solid state at low temperatures and in a transparent gel state at high temperatures because their physical changes depended on the melting point of PEG. The BC/PEG gel was opaque and more inelastic than the purified BC gel when swollen in water at temperatures below 20 °C. However, the BC/PEG gel became transparent and elastic as the temperature increased above 25 °C. BC/PEG-PEGDA gels also changed from opaque at low temperature to transparent at high temperature, and this property remained even after PEGDA was crosslinked. However, the temperature at which the gel changed from opaque to transparent shifted to 25–30 °C. In addition, the transmission in the gel decreased considerably; BC/PEG-PEGDA 9G and 14G exhibited approximately 10% and 20% transmission, respectively, which suggests
Fig. 7. Transmission (λ = 600 nm) of BC/PEG-20 wt.% PEGDA. Gel thicknesses were 2.6 mm (BC/PEG), 2.1 mm (9G), 3.0 mm (14G), and 2.7 mm (23G). Open triangles: BC/ PEG; filled circles: BC/PEG-PEGDA 9G; open circles: BC/PEG-PEGDA 14G; open squares: BC/PEG-PEGDA 23G.
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Fig. 8. TMA curves of BC/PEG-PEGDA. The samples were heated from −15 °C to 50 °C at a rate of 2 °C/min under a 150 mN load perpendicular to the BC layers. (a) 9G, cross-linked PEGDA: 9.9 wt.% (PEGDA concentration 10 wt.%), 18.9 wt.% (20 wt.%), 28.6 wt.% (30 wt.%), and 57.3 wt.% (60 wt.%). (b) 14G, cross-linked PEGDA: 4.4 wt.% (10 wt.%), 14.0 wt.% (20 wt.%), 23.6 wt.% (30 wt.%), and 51.9 wt.% (60 wt.%).
showed a different trend. After the sample began to contract at approximately 25 °C, it expanded again, which indicates that a portion of the cross-linked PEGDA in the gels did not contract under the 150 mN load and expanded after part of the gel contracted. Fig. 8(b) demonstrates the TMA curves for BC/PEG-PEGDA 14G, which show the same tendency as the curves for 9G, except at concentrations of 10 wt.% and 20 wt.% (cross-linked PEGDA in the gel: 4.4 wt.% and 14.0 wt.%, respectively), at which the gel was strongly influenced by the cross-linking of PEGDA. The TMA results indicate that the elasticity of the sample in the gel state was decreased by cross-linked PEGDA and thus support the results of the compression test. PEG is not involved in the formation of network structure in this gel; however, it is necessary for the formation of the gel state and for providing nonvolatile and thermo-responsive properties, particularly, because the thermoresponsive properties depend on the melting of PEG. At a high level of cross-linked PEGDA, the PEG had a small effect on the thermoresponsiveness of the gel. The cross-linked PEGDA should thus be lower than 30 wt.% to obtain an elastic gel under a 150 mN load.
improved in further studies to provide suitable mechanical strength and hardness for the soft materials and to evaluate the inclusion and release of drugs. Acknowledgments We are grateful to Shin-Nakamura Chemical Co., Ltd. for providing the PEGDA samples. This study was supported in part by Grants-inAid for Regional R&D Proposal-Based Program from the Northern Advancement Center for Science & Technology of Hokkaido, Japan (No.: T-Asahikawa). References [1] [2] [3] [4] [5] [6]
4. Conclusions [7]
We succeeded in developing BC/PEG gels with high mechanical strength by forming a composite gel with a cross-linked PEGDA network structure. The compression test results indicated that it is possible to control the ability of the gel to withstand compression by altering the concentration and degree of polymerization of PEGDA. In addition, the transmission measurement and TMA results indicated that the thermo-responsiveness properties were maintained in the BC/PEG-PEGDA gels as the gels changed from opaque and solid to transparent and elastic. BC/PEG-PEGDA gels may thus be useful in the medical field as a healing material and drug delivery system. The low mass content of the composite in BC/PEG-PEGDA allows it to be a soft material at body temperatures when the Mw of PEG is carefully selected [7]. Additionally the gel is soft, biocompatible, and nonvolatile. The gel can be used as a healing material in surface materials of rehabilitation and welfare equipment. BC/PEG-PEGDA is also suitable for drug delivery because it retains drugs at low temperatures and releases them at high temperatures. The BC/PEG-PEGDA gels can be
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
M. Iguchi, S. Yamanaka, A. Budhiono, J. Mater. Sci. 35 (2000) 261–270. P. Ross, R. Mayer, M. Benziman, Microbiol. Rev. 55 (1991) 35–58. R. Malcolm Brown Jr., J. Macromol. Sci. A Pure Appl. Chem. 33 (1996) 1345–1373. A. Okiyama, M. Motoki, S. Yamanaka, Food Hydrocoll. 6 (1993) 493–501. D. Klemm, B. Heublein, H.P. Fink, A. Bohn, Angew. Chem. Int. Ed. 44 (2005) 2–37. G. Helenius, H. Backdahl, A. Bodin, U. Nannmark, P. Gatenholm, B. Risberg, J. Biomed. Mater. Res. A 76 (2006) 431–438. Y. Numata, K. Muromoto, H. Furukawa, J.P. Gong, K. Tajima, M. Munekata, Polym. J. 41 (2009) 524–525. M.K. Pratten, J.B. Lloyd, G. Horpel, H. Ringsdorf, Macromol. Chem. 186 (1985) 725–733. K. Kataoka, A. Harada, Y. Nagasaki, Adv. Drug Deliv. Rev. 47 (2001) 113–131. H.M. Aliabadi, A. Mahmud, A.D. Sharifabadi, A. Lavasanifar, J. Control. Release 104 (2005) 301–311. X.W. Wei, C.Y. Gong, M.L. Gou, S.Z. Fu, Q.F. Guo, S. Shi, F. Luo, G. Guo, L.Y. Qiu, Z.Y. Qian, Int. J. Pharm. 381 (2009) 1–18. L.R. Hirsch, R.J. Stafford, J.A. Bankson, S.R. Sershen, B. Rivera, R.E. Price, J.D. Hazle, N.J. Halas, Proc. Natl. Acad. Sci. 100 (2003) 13549–13554. T. Niidome, Y. Akiyama, M. Yamagata, T. Kawano, T. Mori, Y. Niidome, Y. Kayama, J. Biomater. Sci. Polym. Ed. 20 (2009) 1203–1215. T. Niidome, A. Ohga, Y. Akiyama, K. Watanabe, Y. Niidome, T. Mori, Y. Katayama, Bioorg. Med. Chem. 18 (2010) 4453–4458. N. Shah, M. UI-Islam, W.A. Khattak, J.K. Park, Carbohydr. Polym. 98 (2013) 1585–1598. A. Nakayama, A. Kakugo, J.P. Gong, Y. Osada, M. Takai, T. Erata, S. Kawano, Adv. Funct. Mater. 14 (2004) 1124–1128. Y. Hagiwara, A. Putra, A. Kakugo, H. Furukawa, J.P. Gong, Cellulose 17 (2010) 93–101. S. Hestrin, M. Schramm, Biochem. J. 58 (1954) 345–352. H. Lin, T. Kai, B.D. Freeman, S. Kalakkunnath, D.S. Kalika, Macromolecules 38 (2005) 8381–8393.
