Deformation Of Hydrogels Research Articles

A theory of finite tensile deformation of double‐network hydrogels

AbstractA continuum damage model was developed to describe the finite tensile deformation of tough double‐network (DN) hydrogels synthesized by polymerization of a water‐soluble monomer inside a highly crosslinked rigid polyelectrolyte network. Damage evolution in DN hydrogels was characterized by performing loading‐unloading tensile tests and oscillatory shear rheometry on DN hydrogels synthesized from 3‐sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm). The model can explain all the mechanical features of finite tensile deformation of DN hydrogels, including idealized Mullins effect and permanent set observed after unloading, qualitatively and quantitatively. The constitutive equation can describe the finite elasto‐plastic tensile behavior of DN hydrogels without resorting to a yield function. It was showed that tensile mechanics of DN hydrogels in the model is controlled by two material parameters which are related to the elastic moduli of first and second networks. In effect, the ratio of these two parameters is a dimensionless number that controls the behavior of material. The model can capture the stable branch of material response during neck propagation where engineering stress becomes constant. Consistent with experimental data, by increasing the elastic modulus of the second network the finite tensile behavior of the DN hydrogel changes from necking to strain hardening.

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.

Grayscale Stereolithography of Gradient Hydrogel with Site‐Selective Shape Deformation

AbstractHydrogels can convert external stimuli into shape deformation and mechanical motion, leading to the potential applications in intelligent devices and soft robotics. However, it is difficult to control the site‐selective shape deformation in a single hydrogel system in comparison with the traditional bilayer actuators with difference in structure or composition across the thickness. Here, a strategy to achieve the programmable shape deformation of hydrogel from a single precursor by grayscale stereolithography is proposed. The designed grayscale patterns can govern the spatially gradient crosslinking density of hydrogels, which leads to the heterogeneity in physicochemical performance, especially swelling ability. As a result, the swelling mismatch inducing the nonuniform internal stresses can drive the printed hydrogels to deform as the expected shape in three dimensions although there is no apparent asymmetric features and bilayer configuration. Grayscale light intensity and exposure time, as the digital printing codes can parametrically regulate the predictable shape deformation. More importantly, hydration and dehydration characterizations can endow the reversible feature of shape deformation hydrogel, benefiting to design the hydrogel actuators and sensitive devices. It is believed that integration of the advanced grayscale stereolithography together with the functional hydrogels uncovers a facile strategy to broad opportunities for sensors, soft robots, actuators, and so on.

Thermoresponsive, magnetic, adhesive and conductive nanocomposite hydrogels for wireless and non-contact flexible sensors

Hydrogel sensors are increasingly investigated and applied in emerging flexible electronics, but the exploration of hydrogel sensors with multiple functionalities remains a challenge. In this work, thermoresponsive, magnetic, adhesive and conductive nanocomposite hydrogels are developed and further applied in fabricating multi-functional flexible sensors. In such nanocomposite hydrogels, poly(N-isopropyl acrylamide-co-acrylamide) (P(NIPAAm-co-Am)) and Fe3O4 nanoparticles are designed as the thermoresponsive and magnetic component respectively, poly(vinyl alcohol) (PVA) is introduced in a semi-interpenetrating way to enhance the adhesiveness and mechanical property, and potassium chloride (KCl) works as the conductive component. Interestingly, the adhesion property is switchable by temperature based on the thermoresponsiveness of the nanocomposite hydrogels. Resistive strain sensors assembled from the nanocomposite hydrogels achieve a gauge factor (GF) of 4.21, a response time of 140 ms, stability of 2000 sensing cycles and excellent reversibility. Such nanocomposite hydrogel sensors are used as wearable sensors for monitoring diverse human motions and further integrated with Bluetooth signal transmission and reception system to fabricate wireless wearable sensors. Because of the deformation of nanocomposite hydrogels in switching magnetic field, the nanocomposite hydrogel sensors also have the ability to respond in a non-contact way to switching magnetic fields.

The role of reduced graphene oxide as a “2D flexible crosslinking point” in composite hydrogels

AbstractThe composite hydrogels are widely used in various research directions. How to improve the mechanical properties is one of the concerns of researchers. In this work, the mechanical properties of composite hydrogels with reduced graphene oxide (rGO), kaolin (KL), montmorillonite (MMT), and attapulgite (APT) as additives were compared. The morphology of polyacrylamide (PAM) composite hydrogel was observed by electron microscope, and the hydrogen bonding between inorganic additives and PAM chain in the hydrated samples were detected by IR. The rheological data (G′, G", and tan δ) confirmed the beneficial effect of rGO as a two‐dimension (2D) material on the mechanical properties of hydrogels. In the tensile‐fracture experiments, the maximum tensile strain of PAM‐rGO was 568% and the maximum tensile force was 1.18 N, both of which were larger than that of the clay‐based composite hydrogel with the same mass concentration. Based on these experimental data, the response behavior of rGO in the process of hydrogel deformation was elucidated by the concept of “2D flexible crosslinking point,” and the reason why PAM‐rGO composite hydrogel had better mechanical strength was explained in detail.

Light‐Addressable Nanocomposite Hydrogels Allow Plasmonic Actuation and In Situ Temperature Monitoring in 3D Cell Matrices

AbstractThis paper reports a multifunctional platform based on a nanocomposite hydrogel combining poly(ethylene glycol), with rhodamine B‐containing silica nanoparticles (RhB@SiO2), as temperature sensors, and gold nanorods (AuNRs) as plasmonic heaters. This composite material acts as a light‐addressable cellular matrix able to induce 3D temperature gradients locally and dynamically using the localized surface plasmon resonance (LSPR) of AuNRs under near‐infrared (NIR) laser illumination. At the same time, the temperature changes are probed locally by monitoring changes of the RhB@SiO2 NPs fluorescence. As a result of plasmonic heating, and, depending on the preparation protocol, the light‐addressable hydrogel also deforms controllably and reversibly, allowing mechanical and thermal cellular stimulation in a 3D matrix. The hydrogel deformation is quantified by means of inline holographic microscopy. This approach makes it possible to accurately and locally control and simultaneously measure temperature gradients and deformation in soft, 3D deformable materials and will enable novel platforms for studying cellular thermo‐ and mechanobiology.

Wound-contractible hydrogel for skin regeneration, a new insight from mechanobiology

Wound-contractible hydrogel for skin regeneration, a new insight from mechanobiology

Constitutive modeling of pH-sensitive hydrogel: Multi-physics coupling of electromagnetics with mechanics and thermodynamics

The field theory of pH-sensitive hydrogel deformation is presented coupling electro-magnetic (EM) field (external or internal) and diffusion (Poisson–Nernst–Planck and Maxwell equations) of mobile ionic species through irreversible thermodynamics. The evolution of fixed-charge in the hydrogel is modeled by Langmuir monolayer adsorption theory. The important contribution of present work is a derivation of Cauchy stress tensor constitutive relation with EM field and species diffusion through Maxwell’s stress and spherical tensors. The EM field work done is computed in the frame-invariant settings employing Poynting vector concept, which is another novel contribution of the present work. The hydrogel deformation is finally computed by mechanical equilibrium equation coupled with the diffusion equations through traction boundary conditions. The applicability of the presented framework is successfully demonstrated by several pH-sensitive hydrogel simulations (one- and two-dimensional). The constitutive relations derived here are also verified with the other published results.

Liquid Stratification and Diffusion‐Induced Anisotropic Hydrogel Actuators with Excellent Thermosensitivity and Programmable Functionality

Smart hydrogel actuators with programmable anisotropic structures present fascinating prospects considering their distinctive shape transformation and controllable environmental responsiveness under external stimuli. However, the design of anisotropic hydrogels with simple and universal fabrication and programmable functionality is challenging for their valuable applications in smart actuators and soft robots. Herein, a simple, green, and devisable strategy is proposed to construct a heterogeneous porous hydrogel system by the different liquid diffusion (such as amyl alcohol, water, and ethanol) into a monomeric precursor solution of thermosensitive hydrogels. The well‐defined micro/nanoporous gradient and patterned structures related to selective liquid stratification and interfacial diffusion favor the fast response and accurate programmable deformation of hydrogels under temperature stimuli. Inspiringly, this simple diffusion‐driven tactic can be perfectly applicable for different responsive hydrogels with programmable multifunctionality by adding functional nanomaterials into the diffusible liquid. This green, general, and facile diffusion‐driven strategy provides significant guidance for fabricating environmentally responsive hydrogels with tailorable functionality for their multipurpose applications in drug delivery, bioengineering, smart actuators, and soft robots.

Investigation of transient mass transport induced deformation of PEGDA hydrogel in photocurable solution

Hydrogels have attractive characteristics such as notable biocompatibility, outstanding hydrophilia and efficient mass transport (MT), which makes hydrogel suitable for tissue engineering scaffold materials. Stereolithography is a prevalent method to prepare hydrogel scaffold with complex structure. However, it is difficult to ensure dimension accuracy in printing complex hydrogel scaffold since solvents can migrate in gel and render MT behavior, which consequently induce deformation and dimensional inaccuracy of printed hydrogel. Little investigation of MT of gel in mixture like solution is conducted. Aiming to depict multiple solvents MT in gel, a model is established by mainly modifying the representation of free energy of mixing to the multi-component one. This model is feasible to describe concurrent multiple solvents migration and mechanical deformation for hydrogel in liquid mixtures, which fulfills understanding of multi-component MT. In this work, the transient MT behaviors of poly(ethylene glycol) diacrylate gel in its photocurable solution are investigated by the model. The numerical model is validated by experimental results and then applied to simulate free and confined MT process of the system. It is found that the deformation trend is not monotonous and total gel deformation can be regulated by adjusting printing time. The correspondence between numerical and experimental results proved that this model can promisingly illustrate multi-component MT induced deformation and provide instructions on improving dimensional accuracy for printing hydrogels.

Preparation and characterization of polyvinyl alcohol/ sodium alginate/carboxymethyl cellulose composite hydrogels with oriented structure

ABSTRACT In order to control complex motion behavior in organisms, most biological tissues have highly oriented structures and anisotropic mechanical properties, such as articular cartilage, muscle, skin and blood vessel. In this study, inspired by the directional structure and excellent mechanical properties of biological soft tissues, polyvinyl alcohol/carboxymethyl cellulose hydrogels with different contents of sodium alginate were prepared by combining freeze-thaw and prestretching methods. Compared with SEM images without pre-stretch hydrogels, there are obvious ordered pore structures in the pre-stretch samples along the tensile direction, and the network structure becomes denser. XRD and FTI R results indicate that the introduction of SA and CMC increases the number of internal hydrogen bond and forms new covalent bonds. The tensile strength of the hydrogel without annealing and stretching treatments is only 0.41MPa. After annealing and stretching treatments, the tensile strength of the hydrogel is increased to 0.93MPa. Furthermore, the creep deformation of hydrogel was reduced from 1.08 mm to 0.27 mm by annealing and stretching treatments. To sum up, these results suggested that PVA/SA/CMC composite hydrogels with oriented structure might have potential applications in soft tissue repair and other biomaterials.

PVA hydrogel deformation in response to change in temperature or pH

PVA hydrogel deformation in response to change in temperature or pH

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.

Transient modeling of magneto-chemo-electro-mechanical behavior of magnetic polyelectrolyte hydrogel

The transient magneto-chemo-electro-mechanical behavior is investigated by a multiphysics model for a magnetic polyelectrolyte hydrogel submerged in ionic solution with mobile ions under an external magnetic field. The model characterizes four physicochemical reactive mechanisms, namely the hydrogel magnetization, the ion polarization, the transports of solvent and ions, and the large hydrogel deformation. Moreover, the computational domain includes the hydrogel and the bath solution, where the Maxwell stresses are considered at hydrogel-surrounding interface. The model can capture the kinetics of magnetic polyelectrolyte hydrogel subject to various environmental and geometric parameters, such as the external magnetic intensity, ambient pH, salt concentration, and hydrogel-magnet distance. The simulation results reveal that a step change is found for spatial distributions of the magnetic intensity, ion concentration, as well as electric potential, if approaching hydrogel-surrounding interface. Moreover, the stronger magnetic intensity and the shorter hydrogel-magnet distance induce the larger compression of the hydrogel, while the higher pH level contributes to larger hydrogel extension. These findings may provide the guidance in design and optimization of the magnetic polyelectrolyte hydrogel and its related applications.

Recent progress in the shape deformation of polymeric hydrogels from memory to actuation.

Shape deformation hydrogels, which are one of the most promising and essential classes of stimuli-responsive polymers, could provide large-scale and reversible deformation under external stimuli. Due to their wet and soft properties, shape deformation hydrogels are anticipated to be a candidate for the exploration of biomimetic materials, and have shown various potential applications in many fields. Here, an overview of the mechanisms of shape deformation hydrogels and methods for their preparation is presented. Some innovative and efficient strategies to fabricate programmable deformation hydrogels are then introduced. Moreover, successful explorations of their potential applications, including information encryption, soft robots and bionomic systems, are discussed. Finally, remaining great challenges including the achievement of multiple stable deformation states and the combination of shape deformation and sensing are highlighted.

Nonlinear frictional contact behavior of porous polymer hydrogels for soft robot

Focusing on the oblique frictional contact problem of hydrogel for soft robot, a numerical contact model is developed to analyze the nonlinear behaviors including local contact large deformation and friction effect of hydrogel soft material during oblique contact. Based on the constitutive relation of hyperelastic material, the updated free energy function of hydrogel is derived. The contact algorithm is given to compute the contact responses of both collinear contact example and oblique contact example. The applicability of classical Hertz contact theory is discussed. The influence of coefficient of friction on the stress distribution of contact zone and the contact states of contact surface are also investigated. The numerical results show that the material nonlinearity and the geometric nonlinearity (i.e. large deformation) of hydrogel lead to the invalidation of classical Hertz contact theory. For the oblique contact, the internal stress grads of hydrogel will be redistributed as the coefficient of friction increases. Meanwhile, the maximum stress position moves from underside to the contact surface. Two high stress zones occur inside and on the contact surface at the same time, respectively. Besides, when the coefficient of friction is smaller (i.e. <i>μ</i> < 0.05), for the collinear contact all contact points on the contact surface are in critical state from static friction to dynamic friction. But for the oblique contact, some contact points are always in stable static friction state.

A solid-shell finite element method for the anisotropic swelling of hydrogels with reinforced fibers

In this paper, a solid-shell finite element method for the inhomogeneous swelling of anisotropic thin-walled hydrogels with reinforced fibers is developed. In this numerical framework, the anisotropic mechanical deformation of fiber-reinforced hydrogels is driven by solvent diffusion. The solid-shell model including only displacement degree of freedom is developed for anisotropic hydrogels. The model can directly incorporate three-dimensional material laws based on the modified Flory-Rehner theory for anisotropic thin-walled hydrogels and bypass complex update algorithms for the normal vector in conventional shell models. The constitutive relation and the corresponding tangent moduli for anisotropic hydrogels, referring to convected coordinate system, are derived according to the modified material model. The weak form of equilibrium equation is derived based on the three-field variational principle. Moreover, enhanced assumed strain and assumed natural strain methods are adopted to avoid Poisson-thickness locking and shear locking encountered in simulations of thin-walled hydrogels with reinforced fibers, respectively. Finally, two representative examples considering various moduli and initial directions of fibers are presented to verify the validity, accuracy and efficiency of the proposed method. The applications to predict the swelling of flowers demonstrate that the solid-shell based method for anisotropic swelling of soft materials can assist design of bionic devices.

Enhancement of deformation of redox-active hydrogel as an actuator by increasing pendant viologens and adding filler or counter-charged polymer

Strategies for enhancement of redox-driven deformation of a hydrogel, viologen pendant poly-L-Lysine based glutaraldehyde cross-linked hydrogel (PLLV-GA-gel), were pursued. To surpass the previously reported fastest reductive contraction, a 93 % volume decrease within 100 s on dithionite reduction, and to accelerate expansion on oxidation, we examined the pendant rate dependence and the effect of the addition of conductive fillers or an anionic polymer, poly(styrene sulfonate) (PSS). In testing deformation of water-floated PLLV-GA-gels, the contraction of a 25 % pendant rate gel was greater and faster than others. The initial rate of reductive contraction of the gels with fillers was in the order of graphene nano-platelet (GNP) > gold nanoparticle > carbon nanotube (CNT) > no-filler. The initial rates of swelling of GNP and CNT gels on oxidation were faster than the no-filler gel. PSS incorporation enhanced the swelling rate and amplitude but slowed down the contraction. All the PLLV-based samples on a Au electrode showed largely diffusion-controlled redox reactions of viologen in voltammograms and radical cation dimer rich light-absorption features in electroreflectance spectra. The guide for the choice of conductive filler and coexistence of anionic polymer described herein should be applicable to enhance the deformation of many redox-driven hydrogel actuators.

A highly elastic, Room-temperature repairable and recyclable conductive hydrogel for stretchable electronics

Conductive hydrogels present great potential in bioelectronics, ionotronic devices, and electronic skin. However, the creeping and plastic deformation of hydrogel often lead to poor stability and low reliability in applications. Here, we report a highly elastic conductive hydrogel based on crosslinked carbon nanotubes (CNT) and poly(vinyl alcohol) (PVA). With the formation of double crosslinking interactions, i.e., strong interaction from covalent acetal bonds and weak interaction from hydrogen bonds, CNT-PVA networks exhibit good stretchability (fracture stain up to 500%), rapid recovery, zero-residual deformation, and excellent mechanical stability. As such, the electromechanical response of this dual-crosslinked conductive hydrogel is stable and repeatable for a wide range of loading rates. Benefiting from the abundant hydroxyl groups and reversible acetal linking bridges in hydrogel networks, the prepared conductive hydrogel is not only repairable at room temperature, but also recyclable.

Functionalized Bead Assay to Measure Three-dimensional Traction Forces during T-cell Activation.

When T-cells probe their environment for antigens, the bond between the T-cell receptor (TCR) and the peptide-loaded major histocompatibility complex (MHC) is put under tension, thereby influencing the antigen discrimination. Yet, the quantification of such forces in the context of T-cell signaling is technically challenging. Here, we developed a traction force microscopy platform which allows for quantifying the pulls and pushes exerted via T-cell microvilli, in both tangential and normal directions, during T-cell activation. We immobilized specific T-cell activating antibodies on the marker beads used to read out the hydrogel deformation. Microvilli targeted the functionalized beads, as confirmed by superresolution microscopy of the local actin organization. Moreover, we found that cellular components, such as actin, TCR, and CD45 reorganize upon interaction with the beads, such that actin forms a vortex-like ring structure around the beads and TCR is enriched at the bead surface, whereas CD45 is excluded from bead-microvilli contacts.