A simple route to interpenetrating network hydrogel with high mechanical strength

A simple two-step method was introduced to improve the hydrogel mechanical strength by forming an interpenetrating network (IPN). For this purpose, we synthesized polyacrylate/polyacrylate (PAC/PAC), polyacrylate/polyacrylamide (PAC/PAM), polyacrylamide/polyacrylamide (PAM/PAM) and polyacrylamide/poly(vinyl alcohol) (PAM/PVA) IPN hydrogels. The PAC/PAC IPN and PAC/PAM IPN hydrogels showed compressive strength of 70 and 160 kPa, respectively. For the PAM/PAM IPN and PAM/PVA IPN hydrogels, they exhibited excellent tensile strength of 1.2 and 2.8 MPa, and elongations at break of 1750% and 3300%, respectively. A strain relaxation was also observed in the case of PAM series IPN hydrogels. From FTIR, TGA and SEM measurements, we confirmed that physical entanglement, hydrogen bonds and chemical crosslinking played major roles in improving hydrogel strength and toughening. The two-step technique contributes to the understanding of ideal networks, provides a universal strategy for designing high mechanical strength hydrogels, and opening up the biomedical application of hydrogels.     

Studies of sodium humate/polyacrylamide/clay hybrid hydrogels. I. Swelling and rheological properties of hydrogels

A type of novel hybrid hydrogels from sodium humate (SH), polyacrylamide (PAM), and hydrophilic Laponite clay were prepared using potassium persulfate (KPS) as the initiator and N,N′-methylenebisacrylamide (MBA) as the cross-linker. The structures of the hydrogels were characterized by field emission scanning electron microscope and FTIR. Their swelling properties, swelling mechanism and rheological properties were also investigated. Experiments show that the composite is heterogeneous in the PAM/SH hydrogel system, while the clay collaborates with SH and improves the network structure of PAM/SH/clay hydrogel. High water-absorbing capability is shown for both hydrogel systems. Han plot proves that clay and SH are compatible with PAM for PAM/SH/clay hydrogels.     

Graphene Oxide/Polyacrylamide/Aluminum Ion Cross‐Linked Carboxymethyl Hemicellulose Nanocomposite Hydrogels with Very Tough and Elastic Properties

Development of high‐strength hydrogels has recently attracted ever‐increasing attention. In this work, a new design strategy has been proposed to prepare graphene oxide (GO)/polyacrylamide (PAM)/aluminum ion (Al³⁺)‐cross‐linked carboxymethyl hemicellulose (Al‐CMH) nanocomposite hydrogels with very tough and elastic properties. GO/PAM/Al‐CMH hydrogels were synthesized by introducing graphene oxide (GO) into PAM/CMH hydrogel, followed by ionic cross‐linking of Al³⁺. The nanocomposite hydrogels were characterized by means of FTIR, X‐ray diffraction (XRD), and scanning electron microscopy/energy‐dispersive X‐ray analysis (SEM‐EDX) along with their swelling and mechanical properties. The maximum compressive strength and the Young's modulus of GO_3.5/PAM/Al‐CMH_0.45 hydrogel achieved values of up to 1.12 and 13.27 MPa, increased by approximately 6488 and 18330 % relative to the PAM hydrogel (0.017 and 0.072 MPa). The as‐prepared GO/PAM/Al‐CMH nanocomposite hydrogels possess high strength and great elasticity giving them potential in bioengineering and drug‐delivery system applications.     

Toughening Mechanism of Nanocomposite Physical Hydrogels Fabricated by a Single Gel Network with Dual Crosslinking——The Roles of the Dual Crosslinking Points

A facile method to fabricate tough and highly stretchable polyacrylamide (PAM) nanocomposite physical hydrogel (NCP gel) was proposed. The hydrogels are dually crosslinked single network with the PAM grafted vinyl hybrid silica nanoparticles (VSNPs) as the analogous covalent crosslinking points and the reversible hydrogen bonds among the PAM chains as the physical crosslinking points. In order to further elucidate the toughening mechanism of the PAM NCP gel, especially to understand the role of the dual crosslinking points, the PAM hybrid hydrogels (H gels) and a series of poly(acrylamide-co-dimethylacrylamide) (P(AM-co-DMAA)) NCP gels were designed and fabricated. Their mechanical properties were compared with those of the PAM NCP gels. The PAM H gels are prepared by simply mixing the PAM chains with bare silica nanoparticles (SNPs). Relative to the poor mechanical properties of the PAM H gel, the PAM NCP gel is remarkably tough and stretchable and also generates large number of micro-cracks to stop notch propagation, indicating the important role of PAM grafted VSNPs in toughening the NCP gel. In the P(AM-co-DMAA) NCP gels, the P(AM-co-DMAA) chains are grafted on VSNPs and the polydimethylacrylamide (PDMAA) only forms very weak hydrogen bonds between themselves. It is found that mechanical properties of the PAM NCP gel, such as the tensile strength and the elongation at break, are enhanced significantly, but those of the P(AM-co-DMAA) NCP gels decreased rapidly with decreasing AM content. This result reveals the role of the hydrogen bonds among the grafted polymer chains as the physical crosslinking points in toughening the NCP gel.     

Injectable supramolecular hybrid hydrogels formed by MWNT‐grafted‐poly(ethylene glycol) and α‐cyclodextrin

Poly(ethylene glycol)‐grafted‐multiwalled carbon nanotube (MWNT‐g‐PEG) was synthesized by a coupling reaction and formed inclusion complexes (ICs) after selective threading of the PEG segment of the MWNT‐g‐PEG through the cavities of α‐cyclodextrins (α‐CDs) units. The polypseudorotaxane structures of the as‐obtained hydrogels were confirmed by ¹H NMR, X‐ray diffraction and DSC analyses. The complexation of the PEG segments with α‐CDs and the hydrophobic interaction between the MWNT resulted in the formation of supramolecular hybrid hydrogels with a strong network. Thermal analysis showed that the thermal stability of the hydrogel was substantially improved by up to 100 °C higher than that of native hydrogel. The resultant hybrid hydrogels were found to be thixotropic and reversible, and could be applied as a promising injectable drug delivery system. The mechanical strength of the hybrid hydrogels was greatly improved in comparison with that of the corresponding native hydrogels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3145–3151, 2010     

A novel polyacrylamide nanocomposite hydrogel reinforced with natural chitosan nanofibers

Polyacrylamide (PAM) was used as a matrix material for fabricating novel nanocomposite hydrogels reinforced with natural chitosan nanofibers (CNFs) via in situ free-radical polymerization. The nanocomposite's structure, strength, morphology and rheological properties were investigated. The results showed that the CNFs had a strong interaction with PAM through hydrogen and covalent bondings. The CNFs acted as a multifunctional cross-linker and a reinforcing agent in the hydrogel system. The compression strength and storage modulus of the nanocomposite hydrogels were significantly higher than those of the pure PAM hydrogels and the corresponding PAM/chitosan semi-interpenetrating polymer network (PAM-SIPN) hydrogels. The swelling ratio (SR) of the nanocomposite hydrogels was lower than that of the PAM hydrogel, but was similar to that of the PAM-SIPN hydrogel. Among the CNF contents used, the 1.5 wt% CNF loading level showed the best combined swelling and mechanical properties for the hydrogels.     

利用聚乙烯醇结晶辅助制备多重交联的高强度聚丙烯酰胺水凝胶

在表面带有C=C双键的乙烯基杂化二氧化硅纳米颗粒(vinyl hybrid silica nanoparticle,VSNP)上接枝丙烯酰胺(AM),所得到的纳米刷状凝胶因子通过聚丙烯酰胺(PAM)间的氢键形成物理交联点,则多官能化的VSNP可作为拟共价交联点构筑双重交联的单一网络纳米复合物理水凝胶(nanocomposite physical hydrogel,NCP gel),表现出较高的强度和超拉伸性.为了进一步提高凝胶的强度和韧性,将少量PVA和PAM/VSNP纳米刷混合制成凝胶,通过冷冻-融化处理,使与PAM分子链相互缠绕并形成氢键作用的PVA结晶,形成新的交联点进一步交联PAM NCP gel,得到多交联的PAM NCP gel体系.通过拉曼光谱和示差扫描量热分析,证明凝胶中的PVA通过氢键既可以与PAM相互作用,又形成微晶为新交联点,大大增强了NCP gel的力学性能,与PAM NCP gel相比,凝胶的拉伸强度和断裂能分别从313 k Pa和1.41×104 J/m~2提高到了557k Pa和4.65×104 J/m~2.     

Conductive Tough Hydrogel for Bioapplications

Biocompatible conductive tough hydrogels represent a new class of advanced materials combining the properties of tough hydrogels and biocompatible conductors. Here, a simple method, to achieve a self‐assembled tough elastomeric composite structure that is biocompatible, conductive, and with high flexibility, is reported. The hydrogel comprises polyether‐based liner polyurethane (PU), poly(3,4‐ethylenedioxythiophene) (PEDOT) doped with poly(4‐styrenesulfonate) (PSS), and liquid crystal graphene oxide (LCGO). The polyurethane hybrid composite (PUHC) containing the PEDOT:PSS, LCGO, and PU has a higher electrical conductivity (10 × ), tensile modulus (>1.6 × ), and yield strength (>1.56 × ) compared to respective control samples. Furthermore, the PUHC is biocompatible and can support human neural stem cell (NSC) growth and differentiation to neurons and supporting neuroglia. Moreover, the stimulation of PUHC enhances NSC differentiation with enhanced neuritogenesis compared to unstimulated cultures. A model describing the synergistic effects of the PUHC components and their influence on the uniformity, biocompatibility, and electromechanical properties of the hydrogel is presented.     

Thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) nanostructured hydrogels

Here we report the preparation and characterization of nanostructured thermo-responsive poly(acrylamide) (PAM)-based hydrogels. The addition of slightly crosslinked poly(N-isopropylacrylamide) (PNIPA) nanogels to AM reactive aqueous solution produces nanostructured hydrogels that exhibit a volume phase transition temperature (T_VPT). Their swelling kinetics, T_VPT's and mechanical properties at the equilibrium-swollen state (H_eq) are investigated as a function of the concentration of PNIPA nanogels in the nanostructured hydrogels. Nanostructured hydrogels with PNIPA nanogels/AM mass ratios of 20/80 and above exhibit higher H_eq and longer time to reach the equilibrium swelling than those of the conventional PAM hydrogels. However, the PNIPA nanogels possess thermo-responsive character missing in conventional PAM hydrogels. The T_VPT of nanostructured hydrogels depends on PNIPA nanogel content but their elastic and Young moduli are larger than those of conventional hydrogels at similar swelling ratios. Swelling kinetics, T_VPT, and mechanical properties are explained in terms of the controlled in-homogeneities introduced by the PNIPA nanogels during the polymerization.     

细菌纤维素/聚丙烯酰胺水凝胶的制备及性能表征

通过自由基聚合在细菌纤维素(BC)网络中引入聚丙烯酰胺(PAM),制备了细菌纤维素/聚丙烯酰胺(BC/PAM)复合水凝胶,并采用扫描电子显微镜(SEM)、红外光谱(IR)、热失重分析(TGA)、X射线衍射(XRD)和力学测试等手段对复合凝胶的结构和性能进行了研究.研究结果显示在复合水凝胶中,虽然PAM自身没有交联,但由...     

Modification of cellulose nanocrystal-reinforced composite hydrogels: effects of co-crosslinked and drying treatment

The synthesis platform of composite hydrogels containing rigid reinforcing filler cellulose nanocrystals (CNCs) and polymer matrix polyacrylamide (PAM) has been proposed (Yang et al. in Cellulose 20:227–237, 2013). The features of CNCs as multifunctional crosslinkers and flexible polymer chain entanglements contributed to the unique arrangement of CNC/PAM clusters with reversible network structures. In this article, the chemical crosslinking agent N,N′-methylene-bisacrylamide (BIS) was added to obtain the dual crosslinked networks, and the mechanical properties of the resulting co-crosslinked hydrogels were examined by tailoring the CNC and BIS concentrations. The results indicated that the homogeneous dispersion of CNCs throughout the polymer matrix was disturbed in the presence of BIS, and the covalent crosslinkers led to weakness and brittleness of the hydrogels. Some new entanglements within the networks were formed after a simple drying treatment, which was verified by the greater tensile strength compared with the as-prepared ones. The mechanism for the formation of these new entanglements was ascribed to the irreversible rearrangement of the CNC/PAM network structure, whereas for co-crosslinked hydrogels no strength increment was observed after the drying treatment.     

Curcumin-loaded PLGA-PEG-PLGA triblock copolymeric micelles: Preparation, pharmacokinetics and distribution in vivo

Rod-shaped cellulose nanocrystals (CNCs) were manufactured and used to reinforce polyacrylamide (PAM) hydrogels through in situ free-radical polymerization. The gelation process of the nanocomposite hydrogels was monitored on a rheometer using oscillatory shear. The chemical structure, morphology, swelling property, and compression strength of the formed gels were investigated. A possible mechanism for forming hydrogels was proposed. The results showed that CNCs accelerated the formation of hydrogels and increased the effective crosslink density of hydrogels. Thus CNCs were not only a reinforcing agent for hydrogel, but also acted as a multifunctional cross-linker for gelation. The shear storage modulus, compression strength and elastic modulus of the nanocomposite hydrogels were significantly improved because of good dispersion of CNCs in PAM as well as enhanced interfacial interaction between these two components. Among the CNC contents used, a loading of 6.7 w/w% led to the maximum mechanical properties for nanocomposite hydrogels.     

Hydrophobic association and ionic coordination dual crossed-linked conductive hydrogels with self-adhesive and self-healing virtues for conformal strain sensors

As a promising functional material, conductive hydrogel has attracted extensive attention, especially in flexible sensor field. Despite the recent developments, current hydrogels still experience several issues, such as limited stretchability, lack of self-recovery and self-healing capability, and insufficient self-adhesion. Herein, dual cross-linked (DC) poly (AA-co-LMA)_SDS/Fe³⁺ hydrogels are fabricated subtly on the basis of ionic coordination interactions and the poly (AA-co-LMA)_SDS hydrophobic association networks, which may provide one plausible routine to compensate the mentioned drawback of hydrogels. The hydrophobic association and ionic coordination networks work synergistically to endow the hydrogels remarkable stretchability (>1200%), high-fracture strength (≈ 820 kPa), and excellent self-healing capability. In addition, the DC hydrogel-based strain sensors displayed a broad sensing range (0 ∼ 900%), conspicuous sensitivity (strain 0% ∼ 200%, gauge factor = 0.53; strain 200% ∼ 500%, gauge factor = 1.23; strain 500% ∼ 900%, gauge factor = 2.09), and pronounced durability. What's more, the self-adhesive feature ensures the strain sensor always forming a good conformal contact with the skin during human movements and displaying remarkable bidirectional detection capability.     

Synthesis and characterization of novel biodegradable unsaturated poly(ester amide)/poly(ethylene glycol) diacrylate hydrogels

A series of novel biodegradable hydrogels were designed and synthesized from four types of unsaturated poly(ester amide) (UPEA) and poly(ethylene glycol) diacrylate (PEG‐DA) precursors by UV photocrosslinking. These newly synthesized biodegradable UPEA/PEG‐DA hydrogels were characterized by their gel fraction (G_f), equilibrium swelling ratio (Q_eq), compressive modulus, and interior morphology. The effect of the precursor feed ratio (UPEAs to PEG‐DA) on the properties of the hydrogels was also studied. The incorporation of UPEA polymers into the PEG‐DA hydrogels increased their hydrophobicity, crosslinking density (denser network), and mechanical strength (higher compressive modulus) but reduced Q_eq. When different types of UPEA precursors were coupled with PEG‐DA at the same feed ratio (20 wt %), the resulting hydrogels had similar Q_eq values and porous three‐dimensional interior morphologies but different G_f and compressive modulus values. These differences in the hydrogel properties were correlated to the chemical structures of the UPEA precursors; that is, the different locations of the >C?C< double bonds in individual UPEA segments resulted in their different reactivities toward PEG‐DA to form hydrogels. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3932–3944, 2005     

Fabrication of a high-strength hydrogel with an interpenetrating network structure

Hydrogels have potential applications in many fields, but the poor mechanical strength has limited their further development. In this article, we designed a high-strength hydrogel with an interpenetrating network (IPN) structure from polyacrylamide (PAM) and poly(vinyl alcohol) (PVA). Synthesis parameters, such as PVA/AM mass ratio, crosslinker dosage and elongation time were carried out for high tensile strength and elongation. The results showed that chemical crosslinking, physical entanglement and PVA precipitates were the dominant parameters for the improvement of mechanical properties. The PVA structure transferred from crystal to amorphous due to intermolecular and intramolecular interactions (such as hydrogen bond and self-crosslinking). PVA precipitates scatterred in the brittle PAM matrix homogeneously which dispersed the applied stress and improved the hydrogel toughness. The tensile strength and elongation were extremely high, they were 2.4 MPa and 3100%, respectively. The simple method is versatile in synthesizing high-strength IPN hydrogels using many kinds of polymer species.     

Interaction of several polymers with p-substituted phenols in aqueous solution

The interactions between several water-soluble polymers, i.e., polyethyleneimine (PEI), poly-(ethylene glycol) (PEG), polyacrylamide (PAM), and poly(methacrylic acid) (PMAA), and p-X-substituted phenols (X = CH₃O, CH₃ C₂H₅, H, Cl, Br and NO₂) in aqueous solution at 30°C were investigated by means of equilibrium dialysis. By applying the Klotz equation to the results obtained, the bonding constants between these polymers, including poly(vinylpyrrolidone) (PVP) which was reported previously, and phenol were determined; they decreased in the following order: PVP > PMAA > PEI > PAM > PEG. This order was in agreement with that reported from the solubility of naphthalene, biphenyl, and alkyl halides in aqueous solution of these polymers caused by a hydrophobic bonding interaction. In this case, therefore, a hydrophobic bonding seemed to be significant. The bonding constants of these polymers with p-substituted phenols were also determined, and they were found to be approximately correlated with Hamett σ constants of the para substituents in phenols. Therefore, it was concluded that the interactions due to electrostatic and hydrogen-bonding forces were also important.     

Preparation and Characterization of Carbon Fibers from Lyocell Precursors Grafted with Polyacrylamide via Electron-Beam Irradiation

Carbon fibers, which act as reinforcements in many applications, are often obtained from polyacrylonitrile (PAN). However, their production is expensive and results in waste problems. Therefore, we focused on producing carbon fibers from lyocell, a cellulose-based material, and analyzed the effects of the process parameters on their mechanical properties and carbon yields. Lyocell was initially grafted with polyacrylamide (PAM) via electron-beam irradiation (EBI) and was subsequently stabilized and carbonized. Thermal analysis showed that PAM grafting increased the carbon yields to 20% at 1000 °C when compared to that of raw lyocell, which degraded completely at about 600 °C. Stabilization further increased this yield to 55%. The morphology of the produced carbon fibers was highly dependent on PAM concentration, with fibers obtained at concentrations ≤0.5 wt.% exhibiting clear, rigid, and round cross-sections with smooth surfaces, whereas fibers obtained from 2 and 4 wt.% showed peeling surfaces and attachment between individual fibers due to high viscosity of PAM. These features affected the mechanical properties of the fibers. In this study, carbon fibers of the highest tensile strength (1.39 GPa) were produced with 0.5 wt.% PAM, thereby establishing the feasibility of using EBI-induced PAM grafting on lyocell fabrics to produce high-performance carbon fibers with good yields.     

One-step synthesis of self-healable hydrogels by the spontaneous phase separation of linear multi-block copolymers during the emulsion copolymerization

Self-healable polyacrylamide-based hydrogels were prepared at room temperature via a one-step emulsion copolymerization of acrylamide(AM),dodecyl 2-methacryIate(DM),and 5-acetylaminopentyl acrylate(AAPA) using sodium dodecyl sulfonate(SDS) as the emulsifier and ammonium persulfate(APS)as the initiator.The produced linear multi-block copolymer chains are composed of randomly-linked hydrophilic polyacrylamide segments(PAM) and hydrophobic segments constituted by DM and AAPA units(P(DM-co-AAPA)).The P(DM-co-AAPA) segments will self-aggregate into hydrophobic microdomains during the polymerization process driven by the hydrophobic interactions,and finally separate from water phase,acting as the crosslinks and leading to the formation of strong hydrogels with a storage modulus as high as 400 Pa.These hydrophobic microdomains will be dissolved in water when the temperature increases to 70℃,resulting in a temperature-responsive reversible sol-gel transition of the prepared hydrogels.Furthermore,the prepared hydrogels have excellent self-healing ability.The broken hydrogels can be automatically healed into a body with a same strength within 2-min's contact.This work provides a new simple way to prepare reversible physical crosslinked hydrogel with high strength and self-healing efficiency.     

Antibacterial Broad-Spectrum Dendritic/Gellan Gum Hybrid Hydrogels with Rapid Shape-Forming and Self-Healing for Wound Healing Application

Treating wound infections is a difficult task ever since pathogenic bacteria started to develop resistance to common antibiotics. The present study develops hybrid hydrogels based on the formation of a polyelectrolyte complex between the anionic charges of dopamine-functionalized Gellan Gum (GG-DA) and the cationic moieties of the TMP-G2-alanine dendrimer. The hydrogels thus obtained can be doubly crosslinked with CaCl₂, obtaining solid hydrogels. Or, by oxidizing dopamine to GG-DA, possibly causing further interactions such as Schiff Base and Michael addition to take place, hydrogels called injectables can be obtained. The latter have shear-thinning and self-healing properties (efficiency up to 100%). Human dermal fibroblasts (HDF), human epidermal keratinocytes (HaCaT), and mouse monocyte cells (RAW 264.7), after incubation with hydrogels, in most cases show cell viability up to 100%. Hydrogels exhibit adhesive behavior on various substrates, including porcine skin. At the same time, the dendrimer serves to crosslink the hydrogels and endows them with excellent broad-spectrum microbial eradication activity within four hours, evaluated using Staphylococcus aureus 2569 and Escherichia coli 178. Using the same GG-DA/TMP-G2-alanine ratios hybrid hydrogels with tunable properties and potential for wound dressing applications can be produced.     

Transparent h-BN/polyacrylamide nanocomposite hydrogels with enhanced mechanical properties

In this study, a facile way has been proposed to prepare transparent, tough and flexible polyacrylamide (PAM) hydrogels which is composed of a dually crosslinked single network by chemical crosslinking of N,N'-methylenebisacrylamide (BIS) and physical crosslinking of hydrophilic hexagonal boron nitride (hBN) nanosheets. The resulting h-BN/PAM nanocomposite hydrogels are highly transparent, and exhibit significantly enhanced mechanical properties compared to the dark (GO)/PAM nanocomposite hydrogels or chemical crosslinking PAM hydrogels. Thus it opens up new opportunities for developing nextgeneration transparent, tough and flexible hydrogels that hold great promise in such important applications as light responsive soft robot and liquid microlenses.