Plant Cell-Inspired Hydrogel Composites with High Mechanical Strength

Chapter 4

Downloaded by UNIV OF FLORIDA on October 26, 2017 | http://pubs.acs.org Publication Date (Web): October 25, 2017 | doi: 10.1021/bk-2017-1253.ch004

Plant Cell-Inspired Hydrogel Composites with High Mechanical Strength Naozumi Teramoto,* Keisuke Wakayama, Mitsuru Harima, Toshiaki Shimasaki, and Mitsuhiro Shibata Department of Applied Chemistry, Faculty of Engineering, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan *E-mail: [email protected]

We have proposed a novel strategy for reinforcing hydrogels with polymer foam based on the inspiration from the framework of plant cells, and we found that the mechanical property of hydrogels are significantly improved by the reinforcement with polyurethane foam. In the present study, we prepared poly(ethylene glycol) (PEG) hydrogels reinforced by open-cell type polyurethane foam. A cut piece of polyurethane foam was impregnated with an aqueous solutions of poly(ethylene glycol) diacrylate, followed by polymerization using 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50) radical initiator. Without polyurethane foam, the PEG hydrogel was broken at the compression stress of around 40 kPa. On the other hand, the PEG gel combined with polyurethane foam did not show the evident break point, and the hydrogel composites endured very high compression stress >2 MPa. We used two types of polyurethane foams with different cell sizes. The compression modulus was influenced by the cell size. When the polyurethane foam with a small cell size was used for preparation of the hydrogel composite, the compression modulus of the hydrogel composite was higher than that of the hydrogel composite with the polyurethane foam with a large cell size. We also observed the cyclic recovery from the 70% compression. The cyclic compression test revealed that large hysteresis exists at the first compression cycle of the hydrogel composite. The mechanical properties of our hydrogel

© 2017 American Chemical Society Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

composite in the compression test are comparable to tough hydrogels developed recently.


1. Introduction Hydrogel is one of promising materials as a biomaterial for regenerative medicine (1, 2). Hydrogels show solid properties, though they contain large amount of water. The distinguished advantages of hydrogels are biocompatibility and diffusibility of solutes in hydrogel due to its hydrophilicity and large content of water (2, 3). Though hydrogels are attractive for biomaterials, most traditional hydrogels are mechanically weak and brittle. Many trials to improve mechanical properties of hydrogels have been carried out, and some successful cases are being reported in this decade (4–13). Among these cases, double network gels (4), topological gels (5), nanocomposite gels (6) and tetra-PEG gels (7) are prominent examples of high-performance hydrogels. Other researches have been following them to produce high-performance hydrogels. Huang et al. (8) developed a composite hydrogel containing macromolecular microspheres as a novel type of composite hydrogel. The microspheres play a role as both an initiator and a crosslinker. The composite gel showed very high mechanical strength. Sun et al. (9) found that a double-network hydrogel prepared from ionically crosslinked alginate and covalently crosslinked polyacrylamide had extremely stretchable and tough mechanical properties. The hydrogel kept the stretchability even if it contains a notch. Zhang et al. (10) succeeded to enhance mechanical properties of a biodegradable hydrogel composed of poly(ethylene glycol) and oligo(trimethylene carbonate) by controlling the crosslinked density and hydrophilic-hydrophobic balance. A poly(vinyl alcohol) (PVA) hydrogel prepared by freezing-thawing cyclic processing has been known to show good mechanical properties (14). Tong et al. (11) prepared carbon nanotube (CNT)-reinforced PVA hydrogels and found that addition of CNT at 0.5% of weight of PVA improved mechanical strength by 94%. Zhang et al. (12) also improved the mechanical strength of PVA hydrogel using graphene. Li et al. (13) reported the very high mechanical strength of hybrid hydrogels prepared from PVA and poly(acrylamide) (PAAm) with long stability in water. The hybrid gel contains a physical network of PVA and chemical network of PAAm. It shows very high toughness compared to PVA hydrogels. We have proposed a novel strategy for reinforcing hydrogels using polymer foams based on the inspiration from the framework of plant cells, and we found that the mechanical property of poly(sodium acrylate) (Na-PAA) hydrogel was significantly improved by the reinforcement with polyurethane foam in the previous study (15). Plant cells are surrounded by the strong cell walls mainly composed of cellulose. Under water shortage conditions, green plants wilt and fall to the ground, which means that the water in cells also plays an important role to help maintain the mechanical strength of the plant body. This combination of cell walls and water with in the walls is important and gave good suggestion to us. Inspired from the similarity of the plant body framework and the cell structure of polymer foam, we proposed a new type of hydrogel composites (Figure 1). In our 80 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


hydrogel composite, polymer foam plays a role of cell walls, and hydrogel plays a role of water in a plant cell. From the different concepts, two research groups prepared and evaluated the composites from polyurethane foam and hydrogels (16, 17).

Figure 1. Schematic concept of plant cell-inspired hydrogel composites with high mechanical strength. In the previous study (15), we prepared the composite hydrogel from polyurethane foam and Na-PAA. Since Na-PAA has very high water absorbability, the close packing of the hydrogel in the polyurethane foam was expected. Our subsequent study, however, shows the additional water absorption of the hydrogel did not affect the mechanical properties largely (unpublished data). Therefore we intended to investigate the possibility of reinforcement of hydrogel with polyurethane foam using another polymer material having no ionic groups. In the present study, we prepared hydrogel composites from PEG and polyurethane foam (PUF). PEG is a nonionic water soluble polymer constructed from repeating units of oxyethylene. It is a non-toxic biocompatible polymer often used in biomedical applications (18–20). PEG is also soluble in organic solvents and ionic liquids, and is also one of promising candidates for the polymer gel electrolyte of ion-conductive materials, which is expected to apply to lithium-ion batteries (21, 22). Polyurethane is also non-toxic and often used for artificial organs (23, 24). For industrial applications, polyurethane was mainly used as elastomer and polymer foam. The mechanical properties of polyurethane are fascinating due to its flexibility and high toughness (25). PUF used here is an open-cell type foam so that it can be impregnated with aqueous solution of reactive PEG. Melamine foam (MF) with fibrous morphology was used to compare the effect of foam morphology on mechanical properties of the composite gel. Here we prepared the PEG hydrogel/PUF composites and found its extremely toughness in the compression test. 81 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


2. Sample Preparation Hydrogel composites were prepared by a simple method. The aqueous solutions of poly(ethylene glycol) diacrylate (PEGDA) (number average molecular weight ~700, Product No. 455008, Sigma-Aldrich, MO) were prepared at the concentration of 10%. To the solution was added 0.2wt% of a thermolabile radical initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50, Wako Pure Chemical Industries, Ltd., Osaka, Japan). A piece of polymer foam with a columnar shape (20 mm diameter × 10 mm height) is immersed in the PEGDA solution poured into a small Teflon® cylindrical container with 20 mm inner diameter equipped with a screw cap. The piece of polymer foam with a columnar shape was prepared by clipping using a steel punch cutter with a diameter of 20 mm. We used three types of polymer foams. Two are polyurethane foams (PUFs), CF-S with a small cell size (cell size: 68 cells/25 mm) and CFH-30 with a large cell size (cell size: 30 cells/25 mm) kindly provided from INOAC Corp. (Aichi, Japan). The other is a melamine foam (MF) (Basotect®) produced by BASF SE (Ludwigshafen am Rhein, Germany) purchased from Strider Co. Ltd. (Toyohashi, Japan). Figure 2 shows scanning electron microscope (SEM) photographs of these foams. MF did not show the cell structure but fibrous network. After degassing the polymer foam impregnated with the PEGDA solution for 30 min, the foam was heated at 70°C for 24 h in the screw-capped container to obtain the hydrogel composite. Calculated from the cell size, the averaged diameters of hydrogel in cells of PUFs are 370 μm (CF-S) and 830μm (CFH-30).

Figure 2. SEM photographs of foams used for hydrogel composites: (a) PUF CF-S, (b) PUF CFH-30, and (c) melamine foam (MF) SEM observation of hydrogels and their composites were carried out after lyophilization. The cross-sectional surface morphology of lyophilized samples was observed by a Hitachi S-4700 field emission scanning electron microscope (FE-SEM) (Hitachi High-Technologies Corp., Tokyo, Japan); the accelerating voltage was 1 kV or 5 kV, and the samples were coated with gold prior to the observation.

3. Compression Test of Samples Hydrogels and hydrogel composites with a columnar shape (20 mm diameter × 10 mm height) were prepared for the compression test. At least six samples 82 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

were prepared for one condition. The compression test was carried out by a Shimadzu EZ-S tabletop universal tester (Shimadzu Corp., Kyoto, Japan) with a 100 N load cell or a Shimadzu AG-I tabletop universal tester with a 5 kN load cell at a crosshead speed of 1 mm/min. The former tester was used for weaker samples such as non-reinforced hydrogels and polymer foams alone. The latter tester was used for stronger samples such as hydrogel composites.

4. Results and Discussion


4.1. Appearance of the Hydrogel Composites The hydrogel composites were easily prepared by our very simple method by which PEGDA was polymerized using thermolabile radical initiator in the aqueous solution in the presence of an impregnated polymer foam piece. The number average molecular weight of PEGDA was ~700. The length of PEG moiety influences hydrophilicity and crosslinking density. While the repeating units of oxyethylene are hydrophilic, the main chains which were formed by radical polymerization and composed of polymerized acrylate units are hydrophobic. Though the PEGDA aqueous solution was transparent, the hydrogel of PEGDA was white and turbid. Some poly(methyl methacrylate)s with short PEG side chains are known to show lower critical solution temperature (LCST) (26). We consider that the turbidity of PEGDA hydrogel was due to the phase separation of PEGDA during polymerization at a high temperature (70°C). Figure 3a shows the SEM photograph of lyophilized 10% PEGDA hydrogel (PEGDA-10). Our PEGDA hydrogel showed partial porous morphology without interconnected pores. Figure 3b and 3c shows SEM photographs of lyophilized 10% PEGDA hydrogel composites with PUF CF-S (PEGDA-10-CF-S) and PUF CFH-30 (PEGDA-10-CFH-30), respectively. We observed the cell structure of PUF and small lyophilized gel beads of PEGDA in each cell. We consider that the interconnection of hydrogel was broken during lyophilization, since the gel shrank during lyophilization. On the other hand, the fibrous melamine network of MF was almost covered by PEGDA gel (Figure 3d). The cleft observed in the SEM photograph may have appeared during lyophilization. Table 1 shows the hydrogel content of each composite. The hydrogel content was calculated from the weight of the hydrogel composite sample and the weight of the foam piece used for preparation of the hydrogel composite. The results revealed that the hydrogel composite was composed of at least 90% of hydrogel. This result suggests that the main component of the hydrogel composites is water, and it is expected that the composites keep many characteristics of hydrogels. The hydrogel content also related with the bulk density of the foam. When the bulk density of foam was higher, the hydrogel content became lower since the substantial volume of the foam polymer was larger. On the other hand, the apparent density of the hydrogel composites was 1.00-1.01 g cm-3 and close to that of the PEGDA hydrogel (1.02 g cm-3), when measured by Archimedes’ principle. The bulk density of foam did not influence the apparent density of the hydrogel composites, because the real density of the foam polymer is not so different from that of hydrogel. 83 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 3. SEM photographs of lyophilized hydrogel and hydrogel composites: (a) PEGDA-10, (b) PEGDA-10-CF-S, (c) PEGDA-10-CFH-30, and (d) PEGDA-10-MF

Table 1. The hydrogel content of each hydrogel composite.

a

Hydrogel composite sample

Hydrogel content (wt%)

Foam densitya (g/cm3)

PEGDA-10-CF-S

91.1

0.072±0.005

PEGDA-10-CFH-30

96.3

0.030±0.003

PEGDA-10-MF

98.8

0.010±0.003

Foam density is the value of bulk density published by the vender.

4.2. Mechanical Properties of Hydrogel Composites The mechanical properties of hydrogel composites were investigated comparing with those of the PEGDA hydrogel. Figure 4 shows the stress-strain curves of PEGDA hydrogel and the composite hydrogels of PEGDA with each foam in the compression tests. The strength of PEGDA hydrogel (PEGDA-10) was lower than 50 kPa and the gel was broken at the strain less than 60%. The slope of the curve was low, implying that the modulus of the PEGDA hydrogel was also low. On the other hand, the PEGDA hydrogel/PUF composites (PEGDA-10-CF-S and PEGDA-10-CFH-30) did not show an evident break point, and the composite gel material did not break after loading high stress >2 MPa and high strain ~95%. Secondly, the mechanical properties of hydrogel composites were compared with those of PUF. Figure 5 shows the stress-strain 84 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


curves of the hydrogel/PUF composites comparing with PUF. The slope of the stress-strain curve of PUF without hydrogel was very low. The stress on PUF kept very low throughout the test, though PUF was not broken even at high strain ~95%. These results reflect our concept of plant cell-inspired hydrogel composites. When there is no hydrogel, the foam does not bear the force of compression, as if a green plant body wilts and falls down from lack of water. The counteracting force of liquid surrounded by cell walls is important in the plant body. Furthermore hydrogel in each cell was protected by the cell wall of PUF. The compression force is converted to the force of expanding horizontally at the high compression strain and counteracting force (Figure 6). Though the strength of PUF is considered to influence the mechanical strength of the hydrogel composite at the breaking strain, we could not measure the stress and strain at break of hydrogel composites. The tensile strengths of CF-S PUF and CFH-30 PUF are reported as 196 kPa and 147 kPa by the manufacturer, respectively. On the other hand, MF did not show a significant reinforcing effect in the hydrogel composite (Figure 4). The modulus of the PEGDA hydrogel/MF composite (PEGDA-10-MF) increased in comparison with PEGDA-10, while its brittleness increased. This tendency is typical for fiber-reinforced materials (27, 28). The compression modulus of the hydrogel composite with the small cell-size PUF (PEGDA-10-CF-S) was higher than that of the hydrogel composite with the large cell-size PUF (PEGDA-10-CFH-30). These results imply that the reinforcing effect of foams depends on the morphology and cell size. It is noteworthy that the highest stress of the hydrogel composite with the small cell-size PUF was >5 MPa, and that the hydrogel composite did not broken after the test (Figure 7), though some small gel pieces have come out from the foam.

Figure 4. Stress-strain curves of hydrogel and hydrogel composites: PEGDA-10 (open triangle), PEGDA-10-CF-S (open diamond), PEGDA-10-CFH-30 (open square), and PEGDA-10-MF (open circle). The arrow shows the break point of PEGDA-10-MF. 85 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 5. Stress-strain curves of hydrogel composites comparing with PUF: PEGDA-10-CF-S (open diamond), PEGDA-10-CFH-30 (open square), CF-S (closed circle), and CFH-30 (closed triangle).

Figure 6. Schematic explanation of the counteracting force at the compression of foam and hydrogel/foam composite.

86 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 7. Photographs of hydrogel and its composites after the tensile test.

Figure 8. Reproducibility of the stress-strain curve of different PEGDA-10-CF-S samples.



We also checked the reproducibility of the stress-strain curve of the PEGDA hydrogel/PUF composite. Usually, hydrogels are very sensitive to the small cracks of their surface and inhomogeneity of crosslinking. So we must pay careful attention at handling hydrogels, and the stress-strain curves of conventional hydrogels are less reproducible. Figure 8 shows the stress-strain curves of PEGDA-10-CF-S in three compression tests using three different specimens. These curves overlapped each other with high reproducibility. This reproducibility is very important for the commercial use. The mechanical properties of PEGDA hydrogel and its composites with foams were summarized in Table 2. Standard deviations of PEGDA hydrogel/PUF composites were very low relative to their high stress values. The compression tests were stopped at the strain of 95% and the stress at 95% strain was described for PEGDA hydrogel/PUF composites, because the clearance of the upper compression plate and the lower compression plate was very small at 95% strain. The number of specimens was 6 for each test.

Table 2. The Mechanical properties of PEGDA hydrogel and hydrogel composites.

a

Hydrogel or composite sample

Stress at break or at 95% straina (MPa)

Strain at break (%)

PEGDA-10

0.034±0.013

52.5±5.4

PEGDA-10-CF-S

5.68±0.11a

N.D.a

PEGDA-10-CFH-30

2.67±0.16a

N.D.a

PEGDA-10-MF

0.189±0.015

46.8±3.8

The test was stopped at the strain of 95%. The strain at break was not determined.

4.3. Cyclic Compression Tests of Hydrogel Composites We investigated the hysteresis in the cyclic compression test of PEGDA-10CF-S and PEGDA-10-CFH-30. Each sample underwent three cycle compression and decompression at a crosshead speed of 1 mm/min without intervals. The maximum strain was set at 70%. Figure 9 and 10 shows the stress-strain curves of the cyclic compression tests for PEGDA-10-CF-S and PEGDA-10-CFH-30, respectively. These hydrogel composites showed large hysteresis especially at the first cycle and relatively small hysteresis at the second and third cycles. The curve of the third cycle was overlapped with that of the second cycle. This result indicates an interesting behavior of our hydrogel composites, suggesting that some irreversible event occurred in the first cycle. We consider that this behavior is caused by the breaking or weakening of the interconnection between each hydrogel particle in each cell of the foam at the first cycle. The deformation of the composite with the force toward the compression axis changes the shape of hydrogel particles in each cell to the disk shape. At the point where the hydrogel particle contacts the cell wall, the part of the hydrogel is protected from breaking as well as plant cells 88 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


are protected with cell walls. Therefore the stress for breaking is considered to be concentrated at the interconnection part. The similar behavior was observed also in the mechanical test of the double network hydrogels (29). Detailed analysis for this behavior will be required as a future study.

Figure 9. Stress-strain curves of PEGDA-10-CF-S at the cyclic compression test: The first cycle (open circle), the second cycle (open diamond), and the third cycle (open square)

Figure 10. Stress-strain curves of PEGDA-10-CFH-30 at the cyclic compression test: The first cycle (open circle), the second cycle (open diamond), and the third cycle (open square) 89 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


5. Conclusions Hydrogel composites of PEG and polymer foams were prepared and evaluated by compression tests. PEG hydrogel was synthesized by the radical polymerization of PEGDA using thermolabile radical initiator in the presence of polymer foam. The mechanical strength and toughness of the PEG hydrogel was significantly improved by combination with PUF. The hydrogel composites with PUF endured very high compression stress >2 MPa and high strain of 95%. When the polyurethane foam with a small cell size (68 cells/25 mm, 370 μm on average) was used, the compression stress went beyond 5 MPa at 95 % strain without an apparent break of the composites. Despite of this significant reinforcing effect, the content of the foam was only less than 10% by weight. In contrast, the PEG hydrogel was not reinforced with MF at such high level. In the cyclic test up to 70% strain, PEG hydrogel/PUF composites showed large hysteresis especially at the first cycle and relatively small hysteresis at the second and third cycles. Hydrogel composites with PUF also showed very good reproducibility in the compression properties, suggesting that they are suited for industrial and commercial uses in terms of quality stability. Since PEG hydrogel and polyurethane are both known as a good candidate for biomaterials, our composite is expected to be a new biomaterial. We expect that our concept of “plant cell-inspired hydrogel composites” can be applied not only to the combination of PEG and polyurethane but also to various combinations of hydrogel and foam, such as hydrogel from biopolymers and biodegradable foams. Future researches will seek detailed mechanical properties and biomaterial applications such as artificial cartilage, tendon, and ligament.

Acknowledgments The authors would like to acknowledge support on the presentation in the ACS fall meeting 2016 with Sasagawa Grants for Science Fellows by the Japan Science Society.

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Plant Cell-Inspired Hydrogel Composites with High Mechanical Strength

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