Mechanical Behavior Of Hydrogels Research Articles

A visco-hyperelastic model for hydrogels with tunable water content

Hydrogels without special treatment would lose water dramatically, which significantly alters their properties. No physically-based constitutive models have been developed so far to quantify the evolution of visco-hyperelastic responses with water content of hydrogels. In this work, systematical experiments are performed on polyacrylamide (PAAm) hydrogels with varied water content. The results reveal that the increase of modulus caused by deswelling is much more pronounced than the decrease of modulus caused by swelling. A more significant strain-softening phenomenon and a transition from almost pure hyperelastic behaviors to apparent viscoelastic behaviors are observed as the water content decreases. To fully capture the experimental observations, a micromechanical model is developed. In this model, the rate-independent hyperelastic response comes from the contributions of both cross-linked networks and entanglements, while the rate-dependent viscoelastic response arises from the reptation of free chains. The relations between model parameters (e.g., cross-linked shear modulus, entangled shear modulus, and relaxation time) and water content are further derived using the scaling law in polymer physics. The developed visco-hyperelastic model exhibits remarkable prediction ability for PAAm hydrogels with a wide water content distribution. The finite element analysis also verifies that the model can describe the mechanical responses of hydrogels in complex loading conditions. The current work deepens our fundamental understanding on the effect of water content on mechanical behaviors of hydrogels. It also provides an efficient theoretical framework to predict the performance of hydrogels in practical applications.

Torsion of hydrogel cylinder with a chemo-mechanical coupled nonlinear elastic theory

In this paper, a chemo-mechanically coupled elastic model is proposed for hydrogel based on the classical physical constitutive theory. This model includes a hydrostatic pressure dependent constant, two linear Lamé constants and three second-order elastic constants, where all the constants are coupled with the chemical field. The influences of key chemical and physical parameters are investigated on the elastic constants, and the deformation of a cylinder are then analytically studied subject to torsion in solvent through linear and nonlinear approaches. Both methods may reproduce the conventional relation between the torque and twist angle of classical mechanics, which incorporate the effect of the chemically coupled shear modulus. The results reveal that a negative Poynting effect is demonstrated that the cylinder tends to shorten on twisting, and that the chemical potential has significant effect on the elastic constants and subsequently on the deformation and mechanical behavior of hydrogels. Further studies show that the Flory parameter and the degree of crosslinking also have important impact on the torsion of hydrogels.

Modulating the Properties of Poly(glycerol sebacate)-Based Polyurethane Hydrogels Using an Organoclay.

Elastomeric hydrogels are promising in soft tissue applications due to their biomimetic mechanical and physical behaviors. In this study, we design and synthesize a poly(glycerol sebacate)-based polyurethane-clay nanocomposite hydrogel system with controllable mechanical, swelling, drug release, and biodegradation behaviors. The polymer-clay nanocomposites are synthesized by in situ polymerization in the presence of a solvent, which facilitates the dispersion of clay within the polymer matrix and their bonding. The nanocomposite hydrogels exhibit higher water swelling ratios in comparison to the neat polymer. The fully swollen hydrogels are capable of enduring complex mechanical deformations such as stretching and knotting, while the tensile moduli of the hydrogels mimic various soft tissues in human body. The mechanical behavior of hydrogels is significantly enhanced by the addition of no more than 3 phr clay, showing higher stiffness, strength, ductility, and toughness. The drug loading and release behavior of the hydrogels is investigated with three model drugs, showing selective drug loading capacity and sustained release, based on the Coulombic interaction between the clay and drug molecules. Biodegradation tests under a simulated body condition reveal a highly tunable degradation rate by the clay content in the nanocomposite hydrogels. Good cytocompatibility by the cell metabolic assay with mouse fibroblasts in vitro is also demonstrated. Finally, three-dimensional microporous foam is manufactured as a proof-of-concept study.

Uniaxial compression constitutive equations for saturated hydrogel combined water-expelled behavior with environmental factors and the size effect

For soft materials with high water content under compression loading, their water-expelled behavior leads to volume changes and should be considered in constitutive equations. In this study, uniaxial compression experiments are conducted for saturated hydrogels of different sizes in different environments. Furthermore, the constitutive equations of uniaxial compression combined water-expelled behavior considering environmental factors and the size effect were derived, which may describe the experimental results well and reveal the mechanism of the difference in mechanical behavior of hydrogels in different environments and different sizes. Our work has important reference value for characterizing the mechanical behavior of biological soft matter.

Recent advances of hydrogel network models for studies on mechanical behaviors

Current constitutive theories face challenges when predicting the extremely large deformation and fracture of hydrogels, which calls for the demands to reveal the fundamental mechanism of the various mechanical behaviors of hydrogels from bottom up. Proper hydrogel network model provides a better approach to bridge the gap between the micro-structure and the macroscopic mechanical responses. This work summarizes the theoretical and numerical researches on the hydrogel network models, aiming to provide new insights into the effect of microstructure on the swelling-deswelling process, hyperelasticity, viscoelasticity and fracture of hydrogels. Hydrogel network models are divided into full-atom network models, realistic network models and abstract network models. Full-atom network models have detailed atomic structure but small size. Realistic network models with different coarse-graining degree have large model size to explain the swelling-deswelling process, hyperelasticity and viscoelasticity. Abstract network models abstract polymer chains into analytical interactions, leading to the great leap of model size. It shows advantages to reproduce the crack initiation and propagation in hydrogels by simulating chain scission. Further research directions on the network modeling are suggested. We hope this work can help integrate the merits of network modeling methods and continuum mechanics to capture the various mechanical behaviors of hydrogels. The random polymer network structure determines the macroscopic mechanical behaviors of hydrogels. This work summarizes the theoretical and numerical researches on the hydrogel network models. Full-atom network models depict the fundamental configurations of hydrogel network in atomic scale. Realistic network models based on different coarse-grain strategies have large model size. Abstract network models with much larger size are capable to not only bridge the underlying mechanism in microscale or mesoscale with the mechanical response in macroscale, but also integrate the merits of discrete methods and continuum mechanics.

Static and dynamic experiments on hydrogels: Effects of the chemical composition of the fluid

In this paper, the effects of the chemical composition of the fluid are studied on the mechanical behaviors of hydrogels via static and dynamic experiment. In particular, the dynamic stress-strain curves of hydrogels are obtained via the split Hopkinson pressure bar (SHPB), and static compressive experiments are carried out. After that, the static compressive stress-strain curves are compared with the dynamic ones, and the effects of the strain rate and the water content of the hydrogels on their structural response are then discussed in details. Finally, the hydrogels are immersed in a FeCl3 solution to study the effect of the FeCl3 solution on the static and dynamic strength of the hydrogels. It is concluded that the strength of the hydrogels increases for hydrogels immersed in the FeCl3 solution.

Evaluation of the effects of cross-linking and swelling on the mechanical behaviors of hydrogels using the digital image correlation method

Experimental evaluation and modeling are important steps in the investigation of the mechanical behaviors of hydrogels in the small- to large-strain range. In this study, the effects of cross-linking and swelling on the true stress-strain response of a specific type of hydrogel (polyacrylamide) were evaluated using a uniaxial tensile test. The development of true strain on the surface of the hydrogel was measured using the digital image correlation method. The specimens with higher cross-link density exhibited a higher initial elastic modulus and earlier orientation hardening. The initial elastic modulus was reduced by the swelling, whereas the orientation hardening occurred in an earlier strain range in the swollen hydrogel. The mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents were fitted using a non-Gaussian statistical model. The conventional model underestimated the decrease in the elasticity owing to the swelling effect and overestimated the increase in the stress in the large-strain range. The mechanical model was suitably modified to yield an accurate reproduction of the mechanical responses. The proposed model, which was characterized by five material parameters, was found to reproduce the characteristics of the mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents.

Dually Heterogeneous Hydrogels via Dynamic and Supramolecular Cross-Links Tuning Discontinuous Spatial Ruptures

Biological tissues are often highly and multiply heterogeneous in both structure and composition, but the integrity of multi-heterogeneity in artificial materials is still a big challenge. Herein, dually heterogeneous hydrogels were constructed with two distinct strategies via dynamic bonds and supramolecular crosslinks. The hydrogels showed discontinuous spatial ruptures and the mechanical behaviors of hydrogels could be tuned. The primary heterogeneity resulted from non-uniform distribution of dynamic and/or static crosslinks. The presence of only primary heterogeneity within hydrogels led to uneven mechanical properties that were represented by discontinuous spatial ruptures during the stretching the hydrogel, and therefore caused the necking deformation. Further introduction of the secondary heterogeneity by incorporating anisotropic cellulose nanocrystals (CNC) into the hydrogels allowed the adjustment of the necking phenomenon. Moreover, distinct CNC with diverse surface functionalities exhibited di...

Effects of entanglements and finite extensibility of polymer chains on the mechanical behavior of hydrogels

Polymer networks usually have many entanglements as well as a finite extensibility due to the uncrossability and the full stretching of the network chains. In this paper, the nonaffine model proposed by Davidson and Goulbourne is adopted to characterize the influence of entanglements and finite extensibility of the polymer chains on the mechanical behavior of hydrogels. The Davidson–Goulbourne model only contains three material parameters and has a simpler mathematical form than the Edwards–Vilgis slip-link model, which is the only model that has been adopted to characterize the influence of entanglements on the swelling deformation of polymeric gels so far. Some benchmark problems such as free swelling, constrained swelling, uniaxial tension, equibiaxial tension in the immersed state and uniaxial compression in the isolated state are analyzed, respectively, based on the new constitutive equations. The results show that entanglements as well as chain extensibility have important influences on the mechanical response of hydrogels.

Amino Acid Based Hydrogels with Dual Responsiveness for Oral Drug Delivery.

This study reports a series of novel amino acid based dual-responsive hydrogels. Prepared by a facile one-pot 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) coupling reaction, the solid content, structure, and mechanical behavior of hydrogels could be easily adjusted by changing the concentrations of the polymers and the crosslinkers. With pH-responsive anionic pseudo-peptides as backbones and disulfide-containing l-cystine dimethyl ester as crosslinkers, these hydrogels are able to collapse and form relatively compact structure at an acidic pH, while swelled and partly dissociated at a neutral pH. Further addition of dithiothreitol (DTT) facilitated complete degradation of hydrogels. The high loading efficiency, rapid but complete triggered-release, and good biocompatibility make these hydrogels promising candidates for oral delivery.

Advances in Mechanics of Soft Materials: A Review of Large Deformation Behavior of Hydrogels

Hydrogels possess magnificent properties which may be harnessed for novel applications. However, this is not achievable if the mechanical behaviors of hydrogels are not well understood. This paper aims to provide the reader with a bird's eye view of the mechanics of hydrogels, in particular the theories associated with deformation of hydrogels, the phenomena that are commonly observed, and recent developments in applications of hydrogels. Besides theoretical analyses and experimental observations, another feature of this paper is to provide an overview of how mechanics can be applied.

Phase transition in swollen gels: 26. Effect of charge concentration on temperature dependence of swelling and mechanical behaviour of poly( N-vinylcaprolactam) gels

The swelling and mechanical behaviour of ionized networks of copolymers of N-vinylcaprolactam with an ionic comonomer, N, N-diallyl- N, N-dimethylammonium chloride (molar fraction x s=0−0.1), in the presence of a crosslinker, 3,3′-(ethane-1,1-diyl)bis(1-vinyl-2-pyrrolidone), was investigated in water and aqueous NaCl solutions ( c NaCl=10 −5–1 M) as a function of temperature. On heating, a continuous decrease in the swelling degree in water, Q, was observed; increasing x s shifts the volume transition temperature, T tr, from the swollen to collapsed state to higher temperatures. The expected decrease in the swelling degree, Q*, of charged hydrogels in aqueous NaCl solutions was observed and two shrinking regions in ionic poly( N-vinylcaprolactam) gels were found at 20°C. While the transition at low NaCl concentrations ( c NaCl<0.1) is due to Donnan-equilibrium effect, the transition at c NaCl⩾0.1 M is due to suppression of the transition temperature T tr by NaCl. The observed decrease in Q in water with increasing temperature and the decrease in Q* in aqueous NaCl solutions at 20°C with increasing c NaCl are accompanied by an increase in equilibrium modulus of gels, so that mechanical behavior of hydrogels is predominantly determined by their degree of swelling. The swelling behavior can be in the first approximation described by the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered.