Deformation Of Hydrogels Research Articles

Poroelastic fracture of polyacrylamide hydrogels: Enhanced crack tip swelling driven by chain scission

The deformation of hydrogels is accompanied by water migration, a process that plays a crucial role in their fracture behaviors. Previous investigations primarily focus on how the water migration between the environment and hydrogel affects the fracture of hydrogels. Herein, a novel mechanism of the rate-dependent fracture of hydrogels induced by interior water migration is uncovered. Notched polyacrylamide (PAAm) hydrogels are stretched at various stretch rates in both oil and deionized (DI) water environments. Notably, the critical stretches to crack propagation are positively correlated with the stretch rates in both the two environments. This rate-dependent fracture is attributed to the crack tip swelling of PAAm hydrogels. Delayed fracture tests conducted in oil further verify the co-existence of delayed fracture and rate-dependent fracture resulted from interior water migration in PAAm hydrogels. The experimental findings are interpreted by considering the imperfection of a real polymer network, in which the scission of short chains in the region neighboring the crack tip reduces the average crosslinking density locally, thereby greatly amplifying the degree of crack tip swelling and its influence on the fracture of hydrogels. A constitutive model coupling the evolution of polymer network and the diffusion of water molecules is proposed, which can predict the crack tip swelling of notched PAAm hydrogels through the finite element method. Assuming that the decrease in fracture toughness is positively related to the swelling along the crack propagation surface, the predicted normalized fracture toughness matches the experimental results of PAAm hydrogels stretched in water well, and satisfies those in oil environment qualitatively. This work highlights the significant influence of interior water migration on the fracture of hydrogels and provides insights that may guide the design of hydrogels with enhanced fracture resistance.

Cell tumbling enhances stem cell differentiation in hydrogels via nuclear mechanotransduction.

Cells can deform their local niche in three dimensions via whole-cell movements such as spreading, migration or volume expansion. These behaviours, occurring over hours to days, influence long-term cell fates including differentiation. Here we report a whole-cell movement that occurs in sliding hydrogels at the minutes timescale, termed cell tumbling, characterized by three-dimensional cell dynamics and hydrogel deformation elicited by heightened seconds-to-minutes-scale cytoskeletal and nuclear activity. Studies inhibiting or promoting the cell tumbling of mesenchymal stem cells show that this behaviour enhances differentiation into chondrocytes. Further, it is associated with a decrease in global chromatin accessibility, which is required for enhanced differentiation. Cell tumbling also occurs during differentiation into other lineages and its differentiation-enhancing effects are validated in various hydrogel platforms. Our results establish that cell tumbling is an additional regulator of stem cell differentiation, mediated by rapid niche deformation and nuclear mechanotransduction.

Modeling Mass Transport Dynamics in Deformable Hydrogels during Evaporation.

Hydrogels possess exceptional mechanical properties and biocompatibility, making them widely used in contemporary bioengineering. Specifically, in the development of wearable and implantable health monitoring devices as well as drug delivery systems, hydrogels are utilized to enable precise control over the transport of solutes. Nonetheless, predicting the distribution of substances within hydrogels still poses a significant challenge due to the complex interplay between the movement of water content, migration of solutes, and deformability of the hydrogel polymer network, which presents challenges to theoretical modeling. Our work introduces a numerical model that addresses the movement of water and solute within a flexible hydrogel, accounting for evaporation and/or moisture absorption at the boundary. The model solves for water saturation, solute concentration, and hydrogel deformation iteratively at each time step while computing the boundary movement velocity based on the transport process. By comparing the modeled results of geometry deformation and water and solute distributions during evaporation with our experiments, we demonstrate the accuracy and applicability of our proposed model. This capability to precisely analyze water and solute concentrations in deformable and nonuniform hydrogel environments paves the way for advancements in biosensing and drug delivery methods that rely on elastic porous materials.

Stimuli-Responsive Hydrogels Potentiating Photothermal Therapy against Cancer Stem Cell-Induced Breast Cancer Metastasis.

The self-renewal and differentiation properties of cancer stem cells (CSCs) result in chemoresistance in breast cancer. Even though numerous drugs have been developed to target CSCs, they have suffered from inefficient delivery and accumulation at the focal site. Here, a thermoresponsive hydrogel is developed by coencapsulating aggregation-induced emission (AIE)-active photothermal agent and thioridazine (THZ), demonstrating a controllable delivery system triggered by the AIE agent to augment THZ-mediated CSC ablation. Upon near-infrared laser stimuli, the photothermal effect from the AIE agent induces hydrogel deformation for burst drug release. The precise in situ tumor administration of the hydrogel accelerates drug diffusion and accumulation in deep breast cancer lesions. Thus, THZ can invade tumors and provoke massive CSC apoptosis via dopamine receptor blockage and oxidative stress induction. Consequently, effective CSC inhibition and significant suppression of tumor recurrence and metastasis are demonstrated in mice with breast cancer. We believe that this intelligent hydrogel-based delivery system represents a promising treatment strategy for metastatic breast cancer with clinical potential.

Mechanics of single-network hydrogels with network imperfection

Polymer network is a crucial component of hydrogels, and network imperfection is a prominent feature of polymer networks, significantly influencing the performance of hydrogels. Two essential features of network imperfection are unequal chain lengths and dangling chains, both of which have a significant impact on the mechanical properties of single-network (SN) hydrogels. However, a theoretical framework considering network imperfection in SN hydrogels is still lacking. Here, we propose a theoretical model for SN hydrogels with network imperfection to study the damage behavior during deformation, in which we adopt different chain length distributions to accurately depict the real physical characteristics of the polymer network and incorporate the normalized critical chain force for a more precise measurement of network damage. To verify our theory, we discuss the effects of model parameters on the stress-stretch responses of SN hydrogels and predict the results of uniaxial loading-unloading tests of SN hydrogels, which agree well with experimentally measured stress-stretch behaviors. Finally, we implement the constitutive model into ABAQUS as a user subroutine to study the inhomogeneous deformation of hydrogels.

A visco-hyperelastic model for hydrogels with different water content and its finite element implementation

The visco-hyperelastic properties of hydrogels, which are greatly influenced by water content, play a crucial role in the potential applications in cutting-edge fields. Therefore, quantifying the effect of water content on the visco-hyperelastic behavior of hydrogels is necessary. In this work, we have proposed a visco-hyperelastic constitutive model which considers the water content for large deformation of hydrogels by introducing internal variables that can characterize the viscous dissipation. In the model, the evolution equation of the internal variable is established to achieve the coupling of viscoelastic and hyperelastic responses. To further understand the influence of water content on the visco-hyperelastic behavior of hydrogels, we modified the power-law relationship between water content and initial modulus through experiments. The accuracy of our visco-hyperelastic model is validated by uniaxial tension and relaxation experiments on hydrogels with varying water content. To facilitate the application of the constitutive model, we implement the constitutive model into ABAQUS user subroutine UMAT through constructing linear interpolation function. The results of the finite element analysis further confirm the capability of proposed model to describe the viscoelastic response of hydrogels under complex loading conditions.

The effect of antimicrobial peptide HX‐12C on the properties of chitosan/polyacrylic acid hydrogel

AbstractAntimicrobial peptide (AMP) hydrogel is a novel biomaterial widely used in wound healing. However, there have been limited studies investigating the effect of AMP on hydrogel properties so far. Therefore, this study aimed to examine the influence of the AMP HX‐12C on the chitosan/polyacrylic acid (CS/PAA) double‐network (DN) hydrogel. The results showed that the mechanical properties of CS/PAA/HX‐12C hydrogel are significantly improved compared with those of CS/PAA hydrogel. The maximum tensile stress increased from 41.0 to 258.5 KPa, and the compression stress required for 80% hydrogel deformation increased from 3.7 to 6.7 MPa. Furthermore, the thermal stability of CS/PAA/HX‐12C showed a noticeable enhancement when compared with CS/PAA hydrogel. In addition, the CS/PAA/HX‐12C hydrogel exhibited improved porosity and swelling performance. The addition of HX‐12C significantly enhanced the antibacterial activity of the hydrogel against Escherichia coli and methicillin‐resistant Staphylococcus aureus. Cytotoxicity test showed that the viability of L929 cell remained above 90% after treatment with CS/PAA/HX‐12C hydrogel extract, indicating the good biocompatibility. In conclusion, AMP assuredly enhances the mechanical property, swelling performance and antimicrobial activity of hydrogel. The CS/PAA/HX‐12C hydrogel shows potential for use as anti‐infective medical material.

Photo-crosslinking speckle patterns for large deformation measurement of hydrogels using digital image correlation

Hydrogels often undergo large or inhomogeneous deformation when they are used in soft electronic devices, adhesives, or biological implants. To avoid the potential risk of damage and failure in service, the mechanical response of hydrogels, especially subjected to large deformation, requires meticulous evaluation. Digital image correlation (DIC) has been increasingly employed in the mechanical tests of hydrogels due to non-contact measuring the deformation field by tracking speckle patterns motion on the specimen. However, measuring large deformation of hydrogels using DIC is challenging because the speckle patterns painted on the wet surface suffer various issues, such as bleeding when water is squeezed out, fragmentation or debonding if the stress transferred from hydrogel exceeds the strength or adhesion of painting. In this work, we developed a UV lithography-based speckle pattern preparation method to overcome these difficulties. Speckle patterns are generated by curing a polymer on the surface of hydrogels through chemical-crosslinked bonds, making them an integral part of the hydrogel surface. Experiments indicate that the speckle patterns work as reliable information carrier for DIC to measure large deformation up to strain of 580% and highly concentrated localized strain field within specimen. The speckle patterns show good durability in cyclic loading tests with peak strain up to 150%, achieving low relative deviation (<6%) of the measured deformation field in different cycles. Furthermore, our method allows the optimization of speckle patterns by controlling the shape, size, and coverage of speckles through well designed masks, which guarantees the accuracy and robustness of DIC measurement.

Bio‐Inspired Stimuli‐Responsive Ti3C2Tx/PNIPAM Anisotropic Hydrogels for High‐Performance Actuators

AbstractNear‐infrared (NIR) light‐responsive hydrogels have the advantages of high precision, remote control and excellent biocompatibility, which are widely used in soft biomimetic actuators. The process by which water molecules diffuse can directly affect the deformation of hydrogel. Therefore, it remains a serious challenge to improve the response speed of hydrogel actuator. Herein, an anisotropic photo‐responsive conductive hydrogel is designed by a directional freezing method. Due to the anisotropy of the MXene‐based PNIPAM/MXene directional (PMD) hydrogel, its mechanical properties and conductivity are enhanced in a specific direction. At the same time, with the presence of the internal directional channels and the assistance of capillary force, the PMD hydrogel can achieve a volume deswelling of 70% in 2 s under light irradiation, further building a hydrogel actuator with a fast response performance. Additionally, the hydrogel actuator can lift an object 40 times its weight by a distance of 6 mm, realizing the advantages of both rapid responsiveness and high driving strength, which makes the hydrogel actuator have important application significance in remote control, microflow valve, and soft robot.

Large deformation and crack propagation analyses of hydrogel by peridynamics

In this paper, the large tensile deformation and Mode-I crack propagation of hydrogel are effectively simulated by the bond-associated (BA) non-ordinary state-based (NOSB) peridynamics (PD). Based on the nonlocal theory and meshless characteristic, the fracture involving large deformation of hydrogel is solved with simple implementations. In the analysis, the Gent model is first introduced into NOSB PD as the constitutive model to describe the stress-stretch response of hydrogel under a large tensile stretch. The bond-associated scheme is applied to overcome zero-energy modes and numerical oscillation in conventional NOSB PD. Effective stretch criterion is applied to capture the crack propagation. With the explicit dynamic solver, the tensile deformation of a hydrogel sheet with a hole and the crack propagation process of a hydrogel sheet with a pre-notch under pure shear are analyzed. The predicted stress-stretch responses are in good agreement with the experimental observations, which demonstrates the effectiveness and efficiency of the developed PD approach for predicting large deformation and crack propagation of hydrogel.

Heterogeneity Regulation of Bilayer Polysaccharide Hydrogels for Integrating pH- and Humidity-Responsive Actuators and Sensors

Bilayer hydrogel-based actuators have attracted much interest because inhomogeneous structures are easily constructed in hydrogels. We used three kinds of polysaccharides, including anionic carboxymethyl cellulose (CMC), cationic chitosan (CS), and amphoteric carboxymethyl chitosan (CMCS), as both structure-constructing units and actuation-controlling units in this work to fabricate physically crosslinked poly(vinyl alcohol) bilayer hydrogels. The spatial heterogeneity was tuned by changing the types and concentrations of polysaccharides in different layers, to regulate pH- and humidity-driven actions of bilayer hydrogels. Based on the distortion of the ionic channel during the humidity-motivated deformation of bilayer hydrogels, a two-in-one flexible device integrating a humidity-driven actuator and humidity-responsive sensor was then developed, which could detect the alterations of environmental humidity in real time. Moreover, good tensile toughness and interfacial bonding as well as the strain-resistance effect endowed the bilayer hydrogels with the capability of identifying human motion as a strain sensor, unlocking more application scenarios. This work provides an overall insight into the heterogeneity regulation of bilayer hydrogels using polysaccharides as stimulus-responsive units and also proposes an interesting strategy of manufacturing hydrogel-based flexible devices with both actuating and sensing capabilities.

Impact of interface stress on responsive deformation of magnetic hydrogel

A multiphysics model is presented to study the responsive deformation of the magnetic hydrogel subject to a magneto-chemo-mechanical coupled field. In this work, the influences of the surrounding solvent and the magnetic stress at the hydrogel-surrounding interface are particularly investigated on the performance of the magnetic hydrogel. The physicochemical processes of mass transfer, force balance, and magnetization are included. The constitutive relations are derived by the second law of thermodynamics, and the free energy density is proposed with nonlinear magnetization. The coupled partial differential equations are numerically solved by the finite element method. After that, the deformation patterns are compared with or without the interface stress, and then various parametric studies are performed to investigate the effects of the magnetic susceptibility, geometric shape, and surrounding size on the responsive deformation of the magnetic hydrogel. Results show that the interface stress contributes significantly to the final deformation pattern of the magnetic hydrogel, and the performance of the deformable magnetic hydrogel can be precisely controlled. The present findings offer predictive insights into the deformation mechanism of the magnetic hydrogel, and also provide guidance for developing the programmable magnetic hydrogel-based device.

Structuration and deformation of colloidal hydrogels.

The aim of the present paper is to determine the optimum conditions for the formation of homogeneous colloidal silica hydrogels by aggregation and drying processes, avoiding mechanical instabilities at the surface. Aggregation is controlled by adding monovalent salt to the silica nano-particle suspension while the drying of the sol is also modulated by changing the evaporation rate. A phase diagram reveals two regions in the parameter plane, ionic strength versus evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms the required homogeneous gel and a region where mechanical instabilities appear due to the formation of a solid skin at the gel surface. The frontier between these two regions can be determined by equating the following two characteristic times: the gelation time and the time for skin formation. Permeability measurements of the final gel provide an estimate of the drying stress which is compared to the yield stress of the material. In accordance with the determined phase diagram, our study shows that instabilities appear when the drying stress is larger than the yield stress.

Compact fiber sensor for pH measurement based on the composite effect of hydrogel deformation and LC refractive index variation.

A novel, to the best of our knowledge, type of compact pH fiber sensor combined with a hydrogel based on the whispering gallery mode (WGM) is proposed and integrates a liquid crystal (LC) microdroplet in a capillary in a compact structure as small as 180 µm. In the research, the hydrogel performs both as a supporting frame and a responsive material that causes morphological deformation of the LC microdroplet with pH variation. Moreover, a new phenomenon of pH-induced LC refractive index variation is observed and applied in the pH measurement, so that the acid itself can also lead the LC microdroplet structure transition. Thus, the WGM method is applied to detect the composite effect simultaneously to improve the sensing capability. The sensitivity of the sensor in the pH range from 4.55 to 6.86 reaches 3.19 nm/pH. The response time is short, within 60 s. The simple and compact structure of the sensor reduces the cost and enhances the stability, which is of great potential for biomedical pH measurement.

Frequency dependent sensitivity of hydrogel iontronic sensor

The hydrogel iontronic sensor (HIS) has attracted much attention in recent years due to its high sensitivity. The physical model to analyze the effects of various parameters on the sensitivity of the HIS is still lacking. In this work, we conduct experiments to study the effects of voltage frequency, sensor size, and ion concentration on the sensitivity of the HIS. The experimental results show that the sensitivity is highly dependent on frequency in the range of 20–1 MHz. We establish a theoretical model consisting of Possion–Nerust–Planck equations to describe the ion migration and incompressible Neo-Hookean constitutive equation to describe the hydrogel deformation. The theoretical results divide the sensitivity into three regions of frequency: the region dominated by fully formed electric double layers at the electrode-hydrogel interfaces, the region dominated by ionic relaxation of the hydrogel, and the region dominated by the dielectric property of hydrogel. The model agrees well with the experiments on the frequency dependence of sensitivity as well as the effect of size and ion concentration. This work may provide a guidance for the design of highly sensitive HIS.

3D printing of bioinspired hydrogel microstructures with programmable and complex shape deformations based on a digital micro-mirror device

4D printing, which enables programmable deformation of structures, has broadened the application of 3D printing. Nowadays, 4D printing is usually done by multi-material printing, in which different spatial locations are endowed with different properties. This approach, however, increases the difficulty of 3D printing. Here, we propose a programmable control of single-material hydrogel deformation stimulated by unilateral water absorption. The control scheme includes structure control, gray value control, droplet distribution control, and hydrogel concentration control. Various bionic deformation structures are fabricated. Furthermore, a water-absorbing and swelling powered micro-manipulator is fabricated to achieve controlled grasping and releasing of objects. The materials and stimulation modalities in the proposed programmable control scheme have good cytocompatibility and are easily accessible. This suggests that the control scheme holds great potential for applications in tissue engineering, bionic technology, and micro and nano fabrication.

Study on contact mechanical properties of reticulated hydrophilic polymer soft materials under vertical action of cylindrical indenter

Grid hydrophilic polymer soft materials show great potential in the field of soft robots [1]. Soft robots often need to clamp or contact objects, among which cylindrical objects are more common and representative. Therefore, it is of great significance to study the contact mechanical properties of this material under the vertical action of cylindrical indenter. Hydrogel is a typical grid hydrophilic polymer soft materials [2], therefore, this paper takes hydrogel as the research object, study on material nonlinearity of hydrogels and its contact mechanics under the vertical action of cylindrical indenter. Therefore, the uniaxial tension and compression experiments, the vertical contact test between hydrogel and cylindrical indenter, the measurement of water content and porosity of hydrogel and the compression experiment of anhydrous hydrogel are carried out. It is found that the variation curve of elastic modulus of hydrogel is like an exponential function related to strain. However, the elastic modulus of anhydrous hydrogel has no obvious change and shows brittleness in the compression test. At the same time, it is found that the traditional formula of circular head vertically pressing into the elastic half-space of solid contact mechanics is not applicable in the case of large deformation of hydrogel.

Rate-dependent fracture of hydrogels due to water migration

When a hydrogel deforms, the water molecules inside the hydrogel move together with the polymer network. At a particular time scale and length scale, the water migration within the hydrogel as well as the water exchange with the environment can greatly affect the deformation of the hydrogel. In this work, we study the rate-dependent fracture of hydrogels due to water migration both experimentally and theoretically. We build an experimental setup that can apply mechanical loads to hydrogel samples immersed in a liquid environment, water or oil. We apply monotonic load to hydrogel samples under different loading rates and find that the fracture stretch of hydrogel is highly dependent on the loading rate when the hydrogel is immersed in water, and is almost independent of the loading rate when the hydrogel is immersed in oil. We also apply step load to hydrogel samples and find that the hydrogel immersed in water is more susceptible to delayed fracture than that immersed in oil. We adopt the large deformation theory incorporating water migration developed by Hong et al. (2008) and use finite element software to calculate the deformation and fracture of hydrogels under the two types of environmental conditions. The numerical calculations show that at the time scale and length scale used in the experiment, the water migration satisfies the small-scale swelling condition of fracture. When the hydrogel is immersed in water, the water molecules can exchange with the environment under different loading rates. When the hydrogel is immersed in oil, the water molecules can hardly move in or out of the hydrogel. The theoretical predictions explain the experimental results.

Constitutive modeling for hydrogel with chain entanglements and application to adaptive hydrogel composite structures

Hydrogel is one of typical smart materials and has been used in many engineering applications, such as flexible electronics, flexible robots, and biomedical engineering. Modern industries with complex working environments require hydrogel to have higher strength without losing flexibility. Many published experimental studies focus on the preparation technique of hydrogel composite with high strength and toughness. However, the theoretical investigation into the influence of entanglements on the hydrogel deformation is rarely reported for the structure analysis and deformation mechanism of hydrogel composite, as a purpose to design of hydrogel devices. In this paper, the constitutive model is developed for thermal responsive hydrogel with chain entanglements, based on Flory-Rehner theory and slip-link model, and numerically solved by a user subroutine in ABAQUS. The uniaxial tensile and compressive simulations of standard specimens are carried out to validate the proposed model and demonstrate the applicability of the hydrogel composite, in which a great agreement is obtained by comparing the simulation results to the experimental data from published work. Subsequently, the hydrogel composites with three types of structure, such as hydrogel grippers, scaffolds and skins, are designed to improve the overall strength and maintain the flexibility of the devices, inspired by the deformation mismatch characteristics of multilayer hydrogels. The results show that the properties of composite hydrogels are closely related to the level of entanglements, cross-link and composite structures. This work may contribute to the preparation of reinforced hydrogels and provide insights for the design of hydrogel multifunctional devices.

Influence of initial permeability on seepage behavior of the lubricant stored in hydrogel surface

This paper studied the unsteady seepage characteristics of lubricant stored in hydrogel under different initial permeability. Results show that the distribution of the permeability is determined by the deformation of hydrogel. The normal velocity and normal pressure gradient on the hydrogel surface have diffusion, fluctuation and stabilization processes. The normal pressure gradient is the internal cause that drives the change of the velocity. With the increase of the initial permeability, the seepage velocity increases significantly. Higher initial permeability leads to a higher exudation rate of hydrogel at the initial stage of loading, but the exudation is unsustainable. Considering the instantaneous seepage rate of the lubricant and the long-term lubrication, it is necessary to reasonably design the initial permeability.