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“Nonswellable” Hydrogel Without Mechanical Hysteresis

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Science  21 Feb 2014:
Vol. 343, Issue 6173, pp. 873-875
DOI: 10.1126/science.1247811

Optimizing Injectable Hydrogels

Injectable hydrogels are showing promise as scaffolds in regenerative medicine because they can be injected in liquid form and transform in situ into the gel state. However, when exposed to ionic solutions, such as those found in the body, hydrogels can increase in volume by a factor of 2, which can weaken the material. Kamata et al. (p. 873) added a thermoresponsive component to a hydrogel so that the thermoresponsive component would tend to collapse in shape when heated and counteract the hydrogel's tendency to swell. Indeed, the resulting gel retained its unswollen volume following immersion in a physiological solution and retained its mechanical strength during repeated stretching or compression.

Abstract

Hydrogels are three-dimensional polymer networks that contain a large amount of water inside. Certain hydrogels can be injected in solution and transformed into the gel state with the required shape. Despite their potential biomedical applications, the use of hydrogels has been severely limited because all the conventional hydrogels inevitably “swell” under physiological conditions, which drastically degrades their mechanical properties. We report the synthesis of injectable “nonswellable” hydrogels from hydrophilic and thermoresponsive polymers, in which two independently occurring effects (swelling and shrinking) oppose each other. The hydrogels can endure a compressive stress up to 60 megapascals and can be stretched more than sevenfold without hysteresis. Our results demonstrate that the suppression of swelling helps retain the mechanical properties of hydrogels under physiological conditions.

Hydrogels are used as scaffolds for tissue engineering (1), temporary supports for cells (2), and vehicles for drug delivery systems (3). Although specially engineered hydrogels are known to exhibit excellent physical properties (47), they may have limited applicability because conventional hydrogels “swell” as a result of the difference in osmotic pressure. Swelling drastically weakens the mechanical toughness. Further, in the case of hydrogels that exhibit hysteresis while being deformed, the equilibrium between osmotic and elastic energies is inevitably lost when the polymer network is partly (or even temporarily) broken. This occurrence consequently leads to swelling (8), and a continual mechanical load eventually destroys the hydrogels. This phenomenon commonly occurs in engineered gels made of natural components (911); self-healing gels (1216), which are based on noncovalent cross-links; and exceptionally tough double-network gels (4, 8), which use sacrificial bonds.

Our strategy to achieve a robust hydrogel that can operate under physiological conditions is based on the control of swelling by means of the introduction of thermoresponsive segments—which collapse above a certain critical temperature (Tc) because of predomination of hydrophobic interactions (17)—into the polymer network. We designed and fabricated hydrogels composed of tetra-armed hydrophilic and thermoresponsive polymer units (Fig. 1). The cross-linking reaction is based on the mutually reactive functional end-groups (active ester and amino end-groups). Here, the molar amount of active ester end-groups always equals to that of amino end-groups. We can prepare hydrogels with the required shape simply by mixing aqueous solutions of the polymer units, and the resultant hydrogels contain a high amount of water (89.8 to 90.4%) in the as-prepared state. No organic solvent, catalyst, or ultraviolet radiation is involved in the preparation step. The gelation time after injection can be controlled from seconds to hours (fig. S1), which allows for a range of potential applications. The swelling behavior can be controlled through a selection of the proper ratio of polymer units without losing the ideal network structure (Fig. 1D). The homogeneous network structure contributes to the high deformability (7). Furthermore, because the polymer network is made of covalent cross-links and has no special energy dissipation mechanism, the hydrogels do not undergo hysteresis during the deformation process.

Fig. 1 Schematic of the hydrogel system.

(A) Tetra-armed poly(ethylene glycol) with active ester end-groups. (B) Tetra-armed poly(ethylene glycol) with amino end-groups. (C) Tetra-armed poly(ethyl glycidyl ether-co-methyl glycidyl ether) with amino end-groups. (D) Polymer network composed of hydrophilic (blue) and thermoresponsive (pink) polymer units, where r represents the thermoresponsive segment ratio. Solid square indicates junction amide bonds.

The swelling behavior is regulated by the thermoresponsive segment ratio (r); for example, when r = 0, the hydrogel is composed only of hydrophilic segments (Fig. 1D, left). An alternating structure is formed when r = 0.5 (Fig. 1D, right). The swelling ratio (Q) of hydrogels with different r values is shown in Fig. 2A. All the hydrogels, which were prepared at 10°C, swelled in an aqueous environment at 10°C (Q ≈ 300%). However, the hydrogels drastically changed their volume around their Tc (~25°C, irrespective of r). This control over swelling at a fixed Tc cannot be achieved with conventional thermoresponsive hydrogels that are fabricated by a random copolymerization of hydrophilic and thermoresponsive monomer; Tc increases with an increase in the amount of hydrophilic monomer and can exceed 37°C (18). Based on the volume change, the water content also decreased; the degree of water release depended on r (Fig. 2C). In the same manner as conventional hydrogels, the swelling ratio for r = 0 is greater than 100% over the whole temperature range, indicating that the hydrogel swells and alters its original shape in an aqueous environment. Although the hydrogel with r = 0.4 also swelled at 10°C, the hydrogel recovered its original shape at ~37°C (Q ≈ 100%) (Fig. 2A). The hydrogels with r = 0.4 still retain a high amount of water at 37°C [equilibrium water content (W) ≈ 90%] (Fig. 2C). Further, unlike other conventional hydrogels with similar compositions that often induce turbidity (1921), our hydrogels retain their transparency even above Tc, irrespective of r (Fig. 2B), which suggests that the hydrogels have a homogeneous network structure.

Fig. 2 Swelling behavior of the hydrogels in D-PBS.

(A) Q as a function of temperature. The symbols represent r; r = 0 (open square), 0.1 (open triangle), 0.2 (open circle), 0.3 (asterisk), 0.4 (solid circle), and 0.5 (solid triangle). Q = V/V0 × 100, where V is the volume of the samples in the equilibrium-swollen state at each temperature and V0 is the initial volume of the samples. The dotted line is a guide to the eye for 100%. (B) Photos of hydrogels that exhibit different swelling degrees depending on r; as-prepared samples at 10°C (top) and samples equilibrated in D-PBS at 37°C (bottom). The transparent hydrogels were colored only for visibility. (C) Equilibrium water content (W) of hydrogels at 10°C (solid squares) and 40°C (solid triangles). Initial water content (W0) of the as-prepared samples (open inverted triangles). W = (VVp)/V × 100, where Vp is the total volume of polymers. (D) The swelling ratios in the hydrophilic (Qh, solid circles) and thermoresponsieve (Qt, solid diamonds) segments at 40°C. Qh = Vh/Vh_0 × 100, where Vh is the volume of the hydrophilic segment in the equilibrium-swollen state at 40°C and Vh_0 is the initial volume of the hydrophilic segment. Qt = Vt/Vt_0 × 100, where Vt is the volume of the thermoresponsive segment in the equilibrium-swollen state at 40°C and Vt_0 is the initial volume of the thermoresponsive segment. The dotted line is a guide to the eye for 100%.

Because of the polymer network structure, the hydrogels exhibit an anomalous water allotment between hydrophilic and thermoresponsive segments. We estimated the swelling ratios of the thermoresponsive (Qt) and swollen phases (Qh), assuming that the water content of the thermoresponsive segments at 40°C (Wt) is an intrinsic value for each chemical compound (Wt ≈ 12%) (supplementary text). Although the hydrophilic segments are highly hydrated even at 40°C (Qh > 135%), the swelling ratio decreased with an increase in r (Fig. 2D). When the two segments independently hold water molecules, Qh should be constant irrespective of r. This r-dependent swelling ratio suggests that the water molecules in the hydrophilic segments are also expelled by the shrinkage of the thermoresponsive segments.

The mechanical properties of hydrogels are strongly affected by their degree of swelling. To examine the general effect of swelling, we performed elongation tests with different r values after the hydrogels reached their equilibrium-swollen state in Dulbecco’s phosphate-buffered saline (D-PBS) at 37°C. The representative stress-elongation curves showed that the maximum elongation ratio (λmax) diminished with a decrease in r (Fig. 3A). This decrease in λmax can be explained by the following definition: λmax of polymer gels is defined as the ratio of two lengths of network strands―the fully stretched and initial states (22). Because swelling prestretches the network strands of the initial state, λmax inevitably decreases.

Fig. 3 Mechanical properties of hydrogels equilibrated in D-PBS at 37°C.

(A) Representative stress–elongation curves. The colors represent r. Only for r = 0 (as-prepared), the samples were measured immediately after taken from the mold. (B) Photos of hydrogels (r = 0 and 0.4) during elongation tests. (C) Stress–elongation curves of hydrogel (r = 0.4). (D) The results of a compression test (r = 0.4). ε represents strain.

The hydrogel with r = 0.4, in which the swelling is suppressed, showed improved mechanical properties. Although a conventional hydrogel (r = 0) in the equilibrium-swollen state was easily torn off before the hydrogel was stretched threefold, the hydrogel with r = 0.4 did not rupture even after being stretched more than sevenfold (Fig. 3B). In addition, the “nonswellable” hydrogel showed practically no hysteresis, at least when stretched less than fourfold, which indicates that the elongation did not break the covalent bonds of the polymer network (Fig. 3C). This reversible feature is prominent when compared with the hydrogels that exhibit hysteresis during deformation, which essentially fail to tolerate a continual mechanical load. In contrast to such hydrogels, our hydrogels showed no swelling or weakening in aqueous media even after a repetitive mechanical overload. The nonswellable hydrogel endured a compressive stress of up to 60 MPa even though the hydrogel was in its equilibrium-swollen state (Fig. 3D), which is comparable with so-called “tough hydrogels” previously reported (47), whereas the swollen hydrogel (r = 0) fractured at a strain of ~80%, showing a maximum stress of ~0.4 MPa.

The properties of the hydrogels can thus be tuned through the selection of r, which will give specific values for extensibility, breakage strength, or elasticity. The hydrogels can be further adjusted via the introduction of other functional polymer units. For example, the hydrogels can be used as biodegradable materials for certain applications by introducing a cleavable polymer unit (fig. S2). The degradation profiles of the modified hydrogels are simply regulated by the amount of cleavable polymer units (fig. S3); hydrogels with a higher amount of cleavable linkages are subject to faster degradation. Our results demonstrate that the swelling suppression of hydrogels may help maintain their initial shape and retain their mechanical properties under physiological conditions.

Supplementary Materials

www.sciencemag.org/content/343/6173/873/suppl/DC1

Materials and Methods

Schemes S1 to S4

Figs. S1 to S3

References and Notes

  1. Acknowledgments: This work was supported by the Japan Society for the Promotion of Science (JSPS) through the Grants-in-Aid for Scientific Research, the Center for Medical System Innovation (CMSI), the Graduate Program for Leaders in Life Innovation (GPLLI), the International Core Research Center for Nanobio, Core-to-Core Program, A. Advanced Research Networks, and the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST program); the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT) through the Center for NanoBio Integration (CNBI) and Grants-in-Aid for Scientific Research from MEXT (23700555 to T.S. and 24240069 to U.C.); the Japan Science and Technology Agency (JST) through the S-innovation program and COI STREAM.
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