Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.     

Preparation and characterization of sodium carboxymethylcellulose/poly(N-isopropylacrylamide)/clay semi-IPN nanocomposite hydrogels

A new kind of pH-/temperature-responsive semi-interpenetrating polymer network hydrogels based on linear sodium carboxymethylcellulose (CMC) and poly(N-isopropylacrylamide) (PNIPA) cross-linked by inorganic clay (CMC/PNIPA/Clay hydrogel) was prepared. The temperature- and pH-responsive behaviors, the mechanical properties of these hydrogels were investigated. The CMC/PNIPA/Clay hydrogels exhibited a volume phase transition temperature around 32 °C with no significant deviation from the conventional PNIPA hydrogels. The swelling ratio of the CMC/PNIPA/Clay hydrogels gradually decreased with increasing the contents of clay. The influence of pH value on swelling behaviors showed that there is a maximum swelling ratio at pH 5.9. Moreover, the CMC/PNIPA/Clay hydrogels exhibited excellent mechanical properties with high tensile stress and elongation at break in excess of 1200%.     

Evaluation of the degree of cross-linking of cellulose-based superabsorbent hydrogels: a comparison between different techniques

Three different techniques have been applied to the evaluation of the degree of cross-linking of superabsorbent cellulose-based hydrogels obtained from water solutions of carboxymethylcellulose sodium salt (CMCNa) and hydroxyethylcellulose (HEC), chemically cross-linked with divinyl sulfone. These polyelectrolyte hydrogels are biodegradable and have the same sorption capacity as acrylate-based superabsorbents on the market. A ¹³C solid state NMR analysis was carried out on dry samples of hydrogel to obtain the degree of cross-linking, an important parameter that affects the swelling and mechanical properties of a hydrogel. Dynamic mechanical analysis was performed during the hydrogel cross-linking using a parallel plate rheometer under oscillatory deformations in order to monitor the evolution of the hydrogel viscoelastic properties during the synthesis. The value of |G*| and the slope of the stress-deformation ratio plots from uniaxial compression tests were used to evaluate the elastically effective degree of cross-linking according to classical rubber elasticity theory. Moreover, a dynamic mechanical analysis was carried out on cross-linked hydrogels at different degrees of swelling in order to investigate the influence of the swelling on the mechanical properties and the application of rubber elasticity theory to swollen hydrogels.     

Preparation and characterization of pH- and temperature-responsive semi-IPN hydrogels of carboxymethyl chitosan with poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide) crosslinked by clay

A new kind of pH- and temperature-responsive semi-interpenetrating polymer network hydrogel based on linear carboxymethylchitosan (CMCS) and poly (N-isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The pH-and temperature-responsive behaviors, the deswelling kinetics, and the mechanical properties of the hydrogel were investigated. The hydrogels exhibited a volume phase transition temperature around 33 °C with no significant deviation from the conventional PNIPA hydrogels. The results of the influence of pH value on the swelling behaviors showed that the minimum swelling ratios of the hydrogels appeared near the isoelectric point (IEP) of CMCS, and when pH deviated from the IEP, the hydrogels behaved as polycations or polyanions. The novel hydrogels had much higher response rate than the conventional CMCS/PNIPA hydrogels. Moreover, the semi-IPN hydrogels crosslinked by clay could be elongated to more than 800% and the elongation could be recovered almost completely and instantaneously.     

Supramolecular Peptide/Polymer Hybrid Hydrogels for Biomedical Applications

Peptides and polymers are the “elite” building blocks in hydrogel fabrication where the typical approach consists of coupling specific peptide sequences (cell adhesive and/or enzymatically cleavable) to polymer chains aiming to obtain controlled cell responses (adhesion, migration, differentiation). However, the use of polymers and peptides as structural components for fabricating supramolecular hydrogels is less well established. Here, the literature on the design of peptide/polymer systems for self‐assembly into hybrid hydrogels, as either peptide‐polymer conjugates or combining both components individually, is reviewed. The properties (stiffness, mesh structure, responsiveness, and biocompatibility) of the hydrogels are then discussed from the viewpoint of their potential biomedical applications.     

Mechanical and transport properties of the poly(ethylene oxide)-poly(acrylic acid) double network hydrogel from molecular dynamic simulations

We used atomistic molecular dynamics (MD) simulations to investigate the mechanical and transport properties of the PEO-PAA double network (DN) hydrogel with 76 wt % water content. By analyzing the pair correlation functions for polymer-water pairs and for ion-water pairs and the solvent accessible surface area, we found that the solvation of polymer and ion in the DN hydrogel is enhanced in comparison with both PEO and PAA single network (SN) hydrogels. The effective mesh size of this DN hydrogel is smaller than that of the SN hydrogels with the same water content and the same molecular weight between the cross-linking points (Mc). Applying uniaxial extensions, we obtained the stress-strain curves for the hydrogels. This shows that the DN hydrogel has a sudden increase of stress above approximately 100% strain, much higher than the sum of the stresses of the two SN hydrogels at the same strain. This arises because PEO has a smaller Mc value than PAA, so that the PEO in the DN reaches fully stretched out at 100% strain that corresponds to 260% strain in the PEO SN (beyond this point, the bond stretching and the angle bending increase dramatically). We also calculated the diffusion coefficients of solutes such as D-glucose and ascorbic acid in the hydrogels, where we find that the diffusion coefficients of those solutes in the DN hydrogel are 60% of that in the PEO SN and 40% of that in the PAA SN due to its smaller effective mesh size.     

Preparation and characterization of a novel stimuli-responsive nanocomposite hydrogel with improved mechanical properties

A novel stimuli-responsive organic/inorganic nanocomposite hydrogel (NC hydrogel) with excellent mechanical properties was synthesized by in situ polymerization of 2-(2-methoxyethoxy) ethyl methacrylate (MEO(2)MA), oligo (ethylene glycol) methacrylate (OEGMA) and acrylic acid (AAc), as the polymeric matrix (PMOA), and fibrillar attpulgite (AT), as the reinforcer and cross-linker. The effect of the AT content on the mechanical properties for the swollen and dried NC hydrogels was determined by tensile testing and dynamic mechanical analysis (DMA), respectively. The tensile testing results showed that the incorporation of AT nanoparticles significantly enhanced the mechanical properties of NC hydrogels. As the content of AT increased, the tensile strength, tensile modulus and effective cross-linked chain density increased. The DMA results showed that the storage modulus of AT/PMOA NC hydrogels was increased and the glass transition temperatures shifted to higher temperature compared to the pure PMOA hydrogel, which further indicated that the enhancement of mechanical property depended upon the presence and content of AT. In addition, the faster swelling rates of the NC hydrogels were observed in comparison with the corresponding physically cross-linked PMOA hydrogel, except for 1% AT/PMOA sample. However, the deswelling kinetics of NC hydrogels was obviously retarded.     

Synthesis and characterization of a multi-sensitive crosslinked injectable hydrogel based on Pluronic

This paper reports the construction of a novel multi-sensitive chemically crosslinked injectable hydrogel with strong mechanical strength by modifying Pluronic F127 responsive against temperature, pH and redox potential. Crosslinked polymer between benzaldehyde grafted Pluronic (P-A) and amine end capped Pluronic having disulfide linkage (P-B) have been synthesized and characterized with 1H NMR spectroscopy and GPC. The hydrogel under physiological conditions significantly altered sol-gel transition behaviors with much lower critical gelation concentrations and temperatures, compared to Pluronic hydrogels. The rheological characterization demonstrated that the moduli of the hydrogels were able to be tuned depending on molecular weight as well as pH, redox and temperature conditions.     

Porous alginate-Ca hydrogels interpenetrated with PNIPAAm networks: Interrelationship between compressive stress and pore morphology

Thermo-sensitive porous hydrogels composed of interpenetrated networks (IPN) of alginate-Ca²⁺ and PNIPAAm have been obtained. The hydrogels were prepared by cross-linking alginate-Na⁺ with Ca²⁺ ions inside PNIPAAm networks. Compressive tests and scanning electron microscopy were used to evaluate gel strength and pore morphology, respectively. IPN hydrogels displayed two distinct pore morphologies under thermal stimuli. Below 30-35 °C, the LCST of PNIPAAm in water, IPN hydrogels were highly porous. The pore size of hydrogel heated above LCST became progressively smaller. Alginate-Ca²⁺ and PNIPAAm hydrogels, used as references, did not present such behaviour, indicating that the porous effect is due to IPN hydrogel. It was verified that higher strength is achieved when the hydrogel presents small pore size and the temperature is increased. It is suggested that at temperatures above LCST, the PNIPAAm chains shrink and pull the alginate-Ca²⁺ networks back. During shrinking, the polymer chains occupy the open spaces (pores from which water is expelled), and therefore, the hydrogel becomes less deformable when subjected to compressive stress. The results presented in this work indicate that the mechanical properties as well as the pore morphologies of these IPN hydrogels can be tailored by thermal stimulus.     

Investigation of the swelling behavior of hydrogels derived from high-molecular-weight poly(2-ethyl-2-oxazoline)

Thermoresponsive hydrogels are of great importance as smart materials. They are usually composed of cross-linked polymers with a lower critical solution temperature (LCST). Although much is known about networks of poly(N-isopropylacrylamide), all other polymers are somewhat neglected. In this work, the temperature-dependent swelling behavior of differently cross-linked thermoresponsive poly(2-ethyl-2-oxazoline) (PEtOx) hydrogels were investigated with regard to varying parameters of the network composition. It was found that the degrees of swelling of the hydrogels converge for a certain polymer/solvent system at a distinct temperature independent of its degree of cross-linking. Furthermore, this temperature correlates with the LCST of the respective starting PEtOx. Its net chain molecular weight M_c only affects the maximum degree of swelling and thus, the swelling–deswelling rate of the hydrogel. The fundamental structure/property relations found in this study could be useful to predict the behavior of other thermoresponsive hydrogels.     

A bioinspired 4D printed hydrogel capsule for smart controlled drug release

With the ever-increasing demands for personalized drugs, disease-specific and condition-dependent drug delivery systems, four-dimensional (4D) printing can be used as a new approach to develop drug capsules that display unique advantages of self-changing drug release behavior according to the actual physiological circumstances. Herein, a plant stomata-inspired smart hydrogel capsule was developed using an extrusion-based 4D printing method, which featured with UV cross-linked poly(N-isopropylacrylamide) (PNIPAM) hydrogel as the capsule shell. The lower critical solution temperature (LCST) of the PNIPAM hydrogels was approximately 34.9 °C and macroporous PNIPAM hydrogels were prepared with higher molecular weight polyethylene glycols (PEGs) as the pore-forming agents. Owing to the LCST-induced shrinking/swelling properties, the prepared PNIPAM hydrogel capsules exhibited temperature-responsive drug release along with the microstructure changes in the PNIPAM hydrogels. The in vitro drug release test confirmed that the PNIPAM hydrogel capsules can autonomously control their drug release behaviors on the basis of ambient temperature changes. Moreover, the increased PEG molecular weights in the macroporous PNIPAM hydrogel capsules caused an obvious improvement of drug release rate, distinctly indicating that the drug release profiles can be well programmed by adjusting the internal pore size of the hydrogel capsules. In vitro biocompatibility studies confirmed that the PNIPAM hydrogel capsules have great potential for biomedical applications. The bioinspired 4D printed hydrogel capsules pioneer the paradigm of smart controlled drug release.     

Stimuli-responsive nanocomposite gels

A new class of polymer hydrogels, nanocomposite hydrogels (NC gels), consisting of a unique organic (polymer)/inorganic (clay) network structure, was synthesized by in situ free-radical polymerization in the presence of exfoliated clay nanoparticles in an aqueous system. The resulting NC gels overcame most of the disadvantages associated with chemically cross-linked hydrogels, such as mechanical fragility, structural heterogeneity, and slow de-swelling rate. By using thermo-sensitive poly(N-isopropylacrylamide) (PNIPA) as a constituent polymer, NC gels with remarkable mechanical, optical, and swelling properties as well as thermo-sensitivity were obtained. The various properties of NC gels, such as transparency, gel volume, cell culturing, and surface friction changed significantly in response to the temperature and surrounding conditions. All the excellent properties and new stimuli-responsive characteristics of NC gels are attributed to the unique PNIPA/clay network structure. The thermo-sensitivities and the transition temperature can largely be controlled by varying the clay content and by the addition of solutes.     

Brillouin scattering study of a polymer hydrogel

Brillouin light-scattering measurements of H₂O imbibed in hydrogels of poly(2-hydroxyethyl methacrylate) of two cross-linking densities have been made at 294 K. Increase in the amount of water imbibed in 19-30 Å size pores of the cross-linked network causes the velocity of sound to decrease monotonically from a value, which differs for the two pure polymers, to a limiting value for pure water. The absorption coefficient reaches a maximum at about 30% water content and then decreases toward that for pure water. The velocity and absorption coefficient of both pure polymer and hydrogel containing 32% water were measured from 110 to 300 K. The former decreases and the latter increases with increasing temperature, and both show a change in the slope at about 160 K for the hydrogel, which agrees with the calorimetric glass transition temperature of the hydrogel. The effect of the water on the velocity and absorption coefficient of the polymer increases with temperature. © 1992 John Wiley & Sons, Inc.     

Self-healing mineralized hydrogel scaffolds for stretch-triggered dye release

Hydrogels, with self-healing properties that can self-repair spontaneously when subjected to mechanical stress, are gaining popularity in the biomedical field. Numerous attempts have been made to create distinctive hydrogels with self-healing properties, along with stimuli-responsiveness and biocompatibility. Several techniques exist for fabricating hydrogels, including physical and chemical crosslinking via the creation of covalent bonds, and so on. Here, we prepared self-healing, stimuli-responsive, mineralized hydrogel by simply dissolving Kollidon 90-F, sodium chloride (NaCl), and potassium carbonate (K₂CO₃) in an aqueous solution. The dissociated CO₃²⁻ replaces the water molecules from the Kollidon 90-F polymer backbone and facilitates the cross-linking of the polymer chain, resulting in hydrogel formation. In addition, the in-situ produced sodium carbonate (Na₂CO₃) strengthens the hydrogel network. We optimized the mineralized hydrogels by taking various metal salts and different concentrations of K₂CO₃. The optimized hydrogel showed good stability over a period of time, was able to maintain viscoelastic properties, possessed good self-healing ability, and showed a shape retention ability. The shear-thinning property demonstrated by the optimized hydrogel could open a ray of hope in the bioprinting or 3D printing industry. Further, the stretch-responsive release of dye from the Self-healing mineralized hydrogel (SHMH) matrix confirms the mechanoresponsive behavior of the hydrogel. Overall, the findings could be utilized in the future to fabricate a stable drug delivery system that can autonomously release the drug molecules when stretched by daily processes such as joint movements.     

Thermoresponsive behavior of poly(n-isopropylacrylamide) hydrogels containing gold nanostructures

We report the changes in the structure and thermoresponsive behavior of poly(N-isopropylacrylamide) (PNIPAm) hydrogels when gold nanostructures are synthesized in situ within the hydrogel matrix. Cross-linked PNIPAm hydrogels were synthesized using NIPAm and 0.00-3.50% (w/w versus NIPAm) of N,N'-methylenebisacrylamide (MBAm) and/or N,N'-cystaminebisacrylamide (CBAm) as cross-linking agents. The hydrogels were soaked in potassium tetrachloroaurate to introduce gold ions. The hydrogels containing Au3+ were then immersed in a sodium borohydride solution to reduce the gold ions. Infrared spectroscopy, UV-visible spectroscopy, and equilibrium swelling were used to examine the structural/physical differences between gels of different compositions; UV-visible spectroscopy and mass measurements were used to observe the kinetics and thermodynamics of the hydrogel volume phase transition. These studies revealed several differences in the physical characteristics and thermoresponsive behavior of hydrogels based on cross-linker identity and the presence or absence of gold nanostructures. Hydrogels with gold nanostructures and high CBAm and low MBAm content have equilibrium swelling masses 3-20 times their native analogues. In comparison, gold-containing hydrogels with high MBAm and low CBAm content have swelling masses that are equal to their native analogues. Additionally, the gold-containing PNIPAm hydrogels cross-linked with only CBAm have a deswelling temperature of approximately 40 degrees C, approximately 8 degrees C above the samples cross-linked with only MBAm. Varying the CBAm content and introducing gold enables tuning of the deswelling temperature.     

A new polymer/clay nano-composite hydrogel with improved response rate and tensile mechanical properties

A new type of stimuli-responsive organic/inorganic nano-composite hydrogel was prepared by introducing fibrillar attapulgite into poly(2-hydroxyethyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate-co-methacrylic acid) network, in which the nanosized attapulgite fibril worked as the cross-linker instead of conventional chemical cross-linker. In the preparation process, a prepolymerization route was adopted to effectively stabilize the dispersion of attapulgite. The structure and morphology of the nano-composite hydrogels were characterized by SEM, FTIR and DSC. The swelling/deswelling behaviors and tensile mechanical properties of the nano-composite hydrogels were compared with that of the corresponding chemically cross-linked hydrogel. The results showed that the nano-composite hydrogels had much greater equilibrium-swelling ratio, much faster response rate to pH and significantly improved tensile mechanical properties. As the content of AT increased, the tensile strength, effective cross-link chain density and glass transition temperature increased, while the equilibrium swelling ratio, deswelling rate and elongation at break decreased.     

基于Pickering乳液的磁性高强水凝胶的制备及性能表征

合成了一种磁性Fe3O4纳米颗粒稳定的水包油(O/W)Pickering乳液并以其作为交联剂,在适宜条件下引发单体丙烯酰胺聚合来制备了一种新型的磁性高强复合水凝胶.采用X射线衍射(XRD)及场发射扫描电子显微镜(SEM)分别对磁性Fe3O4纳米颗粒和复合水凝胶的结构进行了表征,结果表明Pickering乳胶粒子较均匀地分布在复合凝胶网络中.溶胀性能测试及溶胀动力学分析表明复合水凝胶具有良好的溶胀性能,能够吸收自身干重100倍左右的水,其溶胀过程不遵循Fickian扩散模型;拉伸测试表明该水凝胶具有优异的力学性能,其拉伸强度能够达到150 kPa左右,断裂伸长率能够达到300%左右,并且当其承受的应力释放后能快速地恢复到初始形态.磁性能测试的结果显示该水凝胶具有良好的磁性.     

Rapid deswelling of sodium alginate/poly(N-isopropylacrylamide) semi-interpenetrating polymer network hydrogels in response to temperature and pH changes

In this study, temperature-/pH-responsive semi-interpenetrating polymer network (semi-IPN) hydrogels based on linear sodium alginate (SA) and cross-linked poly(N-isopropylacrylamide) (PNIPAAm) were prepared. The semi-IPN hydrogels reached an equilibrium deswelling state within 6 h in response to temperature or pH stimuli. Compared with the conventional PNIPAAm hydrogel, their dewelling rate in response to temperature was improved significantly, owing to the formation of a porous structure within the hydrogels in the presence of ionized SA during the polymerization process. Moreover, the deswelling process could be well described with a first-order kinetics equation and it is possible to design any hydrogel with the desired deswelling behavior through the control of the SA content in the semi-IPN hydrogels.     

Thermo-responsive chitosan-graft-poly(N-isopropylacrylamide) injectable hydrogel for cultivation of chondrocytes and meniscus cells

A thermo-responsive comb-like polymer with chitosan as the backbone and pendant poly(N-isopropylacrylamide) (PNIPAM) groups has been synthesized by grafting PNIPAM-COOH with a single carboxy end group onto chitosan through amide bond linkages. The copolymer exhibits reversible temperature-responsive soluble-insoluble characteristics with the lower critical solution temperature (LCST) being at around 30 degrees C. Results from SEM observations confirm a porous 3D hydrogel structure with interconnected pores ranging from 10 to 40 microm at physiological temperature. A preliminary in vitro cell culture study has demonstrated the usefulness of this hydrogel as an injectable cell-carrier material for entrapping chondrocytes and meniscus cells. The hydrogel not only preserves the viability and phenotypic morphology of the entrapped cells but also stimulates the initial cell-cell interactions.     

Water and polymer dynamics in chemically cross-linked hydrogels of poly(vinyl alcohol): a molecular dynamics simulation study

A topologically extended model of a chemically cross-linked hydrogel of poly(vinyl alcohol) (PVA) at high hydration degree has been developed for a molecular dynamics simulation with atomic detail at 323 K. The analysis of the 5 ns trajectory discloses structural and dynamic aspects of polymer solvation and elucidates the water hydrogen bonding and diffusion in the network. The features of local polymer dynamics indicate that PVA mobility is not affected by structural constraints of chemical junctions at the investigated cross-linking density, with a prevailing dumping effect due to water interaction. Simulation results are validated by a favorable comparison with findings of an incoherent quasi-elastic neutron scattering study of the same hydrogel system.