Mechanics and physics of hydrogels

Hydrogels are a special class of polymeric gels that usually contain water as the solvent, namely the dispersion medium for the polymer network as well as other solutes. The polymer network of a hydrogel is crosslinked either chemically by covalent bonds or physically by hydrogen bonds, electrostatic interactions, van der Waals forces, or physical entanglements. Due to the hydrophilicity of the networks, hydrogels are often highly absorbent, capable of absorbing up to several hundred to a thousand times the dry weight of water.¹ The swelling caused by the absorption of water can be triggered by various types of environmental stimuli, such as the changes in temperature, humidity, pH, salt concentration, and even specific molecules.² The stimuli sensitivity of hydrogels makes them ideal candidates for applications in bioseparation,³ drug delivery,⁴ microfluidics,⁵ and artificial muscles,^6,7 and thus the name “smart gels” or “intelligent gels” has been gained for hydrogels. The high water content and biocompatibility also makes hydrogels perfect materials for tissue scaffolding⁸ as well as contact lenses. On the other hand, some biological tissues and other natural materials can also be regarded as hydrogels.⁹ These applications aside, the intriguing phenomena of hydrogel systems and the complex mechanisms behind them have attracted even more interest among scientific communities. This themed issue focuses on bringing together recent advances in experimental observation and characterization, theoretical analyses, and numerical modeling of hydrogel materials and structures, as well as new applications that utilize the unique mechanical properties of hydrogels. Despite the limited scope as it appears in the title, most of the research presented in this issue is multidisciplinary and involve coupling between thermal, mechanical, and chemical fields.

Standing in a state between the conventional regimes of fluid and solid, a hydrogel shares some similarities with solids and some others with fluids. The solid-like features of a crosslinked polymer network enable it to retain its shape, and to respond to stress or strain through a change in shape and volume. While simple free-swelling experiments are often performed on gels, the swelling/deswelling of a gel under mechanical constraints or stress exhibits the uniqueness of a gel in contrast to a non-crosslinked polymer solution, as reviewed in this themed issue (DOI: 10.1039/c2sm25359c). On the other hand, the fluid-like features of a gel enable models originating from fluid-dynamics to be applied to hydrogel systems, such as the study on the dynamic electromechanical response of a hydrogel-loaded microchannel (DOI: 10.1039/c2sm07467b). The peculiar low-friction property of hydrogel surfaces has recently attracted great interest.¹⁰ A model that captures the effects of multi-lengthscale roughness on fluid lubrication between two hydrogels is presented in this issue (DOI: 10.1039/c2sm25414j).

While common synthetic hydrogels are often soft and brittle, making them less useful as structural materials, some tough hydrogels have recently been developed by engineering their molecular conformation and microstructures.^11–14 It is believed that the macroscopic mechanical properties of hydrogels, weak or strong, are correlated to their microscopic structures, just as common solids. In this issue, experimental evidence through small-angle neutron scattering has been reviewed to elucidate the microstructural origin of the strength of tetra-PEG gels (DOI: 10.1039/c2sm25325a). A new material model that better describes the behavior of tetra-PEG gels is proposed (DOI: 10.1039/c2sm25340b). By varying the preparation temperature, the structure of the microcrystalite-crosslinked network of a poly(vinyl alcohol) gel is tuned, and the mechanical testing results demonstrated a strong correlation between the macroscopic properties and the microstructure (DOI: 10.1039/c2sm25513h). The mechanical performance of a tri-block copolymer gel is investigated through specially designed cavity growth measurements (DOI: 10.1039/c2sm25458a). While most theories describe a hydrogel by presuming affine deformation, non-affine deformation of hydrogels are reviewed in this themed issue (DOI: 10.1039/c2sm25364j), on both experimental and theoretical aspects, to emphasize the effects of microscopic heterogeneity on macroscopic properties.

Besides improved fracture resistance, some hydrogels are made to be capable of self-healing.¹⁵ A constitutive model is developed to capture some mechanical properties of a type of self-healing gel (DOI: 10.1039/c2sm25367d). It is suggested through numerical simulations that the concurrent solvent migration and stress redistribution at the crack tip of a gel may improve its mechanical integrity via self-healing (DOI: 10.1039/c2sm25399b), as well as delay the fracture of a gel under some circumstances (DOI: 10.1039/c2sm25553g). A gap often exists between experiments and theories of hydrogels: nonlinear theories have been developed to have high fidelity, while some linear approximations are usually adopted to interpret experimental data for simplicity. Through a numerical approach, the predictions of linear and nonlinear theories of swelling kinetics of polymeric gels are compared and a new procedure is suggested to fit the experimental data to the nonlinear theory (DOI: 10.1039/c2sm25467k).

Modifying the microstructures could not only improve the mechanical properties of hydrogels, but also help in developing hydrogels with multifunctionality. Some examples are highlighted in this themed issue. By introducing a uniaxially aligned lamellar structure, a highly anisotropic hydrogel with enhanced strength and toughness can be used as a photonic crystal (DOI: 10.1039/c2sm25670c). A hydrogel synthesized with interpenetrating poly(acrylic acid) and poly(N-isopropylacrylamide) networks is reported to have sensitivity to both pH and temperature, and the results indicate further hydrogen-bonded complexation (DOI: 10.1039/c2sm25389e). The synthesis of a novel thermosensitive triblock-copolymer–ionic-liquid gel that exhibits a peculiar low-temperature-sol–high-temperature-gel transition is also reported in this themed issue (DOI: 10.1039/c2sm25375e). Furthermore, recent advances in photodeformable gels are reviewed together with crosslinked liquid-crystalline polymers (DOI: 10.1039/c2sm25474c).

The large deformation associated with the swelling or drying processes of hydrogels may also cause surface instabilities. It is well known that the swelling-induced compression may cause creases and wrinkles on a hydrogel,¹⁶ and the resulting structure may be used in surface patterning.¹⁷ In a polyelectrolyte gel with semi-rigid molecules, it has been found that the mechanical creasing during swelling could induce a long-range periodic birefringence pattern through a non-equilibrium coupled process (DOI: 10.1039/c2sm25814e). It has also been observed that wrinkles may also form during the drying process of a hydrogel, possibly due to the glass-transition of the surface layer, as reported in this issue (DOI: 10.1039/c2sm25480h). A capillary-force induced instability that changes the shape of the periodically distributed holes, and modifies the surface morphology of a porous hydrogel membrane is also reported (DOI: 10.1039/c2sm25393c).

On the application side, the actuating performance of a single hydrogel nanofibre was investigated using AFM, with the results compared to that of skeletal muscle (DOI: 10.1039/c2sm25387a). Porous hydrogels have long been used in cell biology. A tough double network gel was found to induce spontaneous cartilage regeneration in vivo.¹⁸ By using a hydrogel with tunable stiffness as a host, the influence of mechanical signals on the biological behavior of stem cells could be systematically studied, as reported in this themed issue (DOI: 10.1039/c2sm25501d). Cornea gel is a representative example of mechanically tough hydrogels in biological tissues. The mechanical properties of cornea gel were tested during dehydration, and the results, which are clearly correlated to its microstructure are reported in this themed issue (DOI: 10.1039/c2sm25370d).

Jian Ping Gong, Hokkaido University, Sapporo, Japan.
Wei Hong, Iowa State University, USA.

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