Huiliang Wang

Self‐Healing in Tough Graphene Oxide Composite Hydrogels

Polymer hydrogels that are capable of spontaneously healing injury are being developed at a rapid pace because of their great potential in biomedical applications. Here, the self-healing property of tough graphene nanocomposite hydrogels fabricated by using graphene peroxide as polyfunctional initiating and cross-linking centers is reported. The hydrogels show excellent self-healing ability at ambient temperature or even lower temperatures for a short time and very high recovery degrees (up to 88% tensile strength) can be achieved at a prolonged healing time. The healed gels exhibit very high tensile strengths (up to 0.35 MPa) and extremely high elongations (up to 4900%). The strong interactions between the polyacrylamide chains and the graphene oxide sheets are essential to the mechanical strengths of the healed gels.

Anisotropic tough poly(vinyl alcohol) hydrogels

Anisotropic tough hydrogels are of great importance in biomedical fields. Tough poly(vinyl alcohol) (PVA) hydrogels with anisotropic porous structure and mechanical properties are obtained with a facile directional freezing–thawing (DFT) technique. The PVA gels have an aligned porous structure, with long aligned channels in the direction parallel to the freezing direction and pores with similar sizes in the perpendicular direction. The degree of crystallinity of the freeze-dried PVA hydrogels increases with number of DFT cycles, and it can reach 55.8%. The PVA hydrogels have excellent mechanical properties, as exhibited by the high tensile strengths (0.3–1.2 MPa), medium moduli (0.03–0.10 MPa) and high fracture energies (160–420 J m−2) of the gels with solid contents of 10–12%. More importantly, the gels exhibit significant anisotropy in the mechanical properties, and their tensile strengths, moduli and fracture energies are higher in the perpendicular direction than those in the parallel direction. Anisotropic mechanical behaviors can also be found in the cyclic tensile tests of the PVA hydrogels. The anisotropic mechanical properties of the DFT PVA hydrogels could be attributed to the oriented arrangement of crystalline regions along the direction perpendicular to the direction of freezing.

Jellyfish gel and its hybrid hydrogels with high mechanical strength

The fabrication of hydrogels with well-defined structure and high mechanical strength has become a challenging and fascinating topic. The aim of this study is to develop a new method for fabricating hydrogels with high mechanical strength by utilizing the well-developed structure of biological gels. We firstly studied the mechanical properties and microstructure of a biological gel—the mesogloea of edible jellyfish Rhopilema esculenta Kishinouye (JF gel). JF gel has much higher mechanical strength than normal synthetic hydrogels due to its layered porous structure with pore walls consisting of nano-structured layers and fibers. We have also synthesized hydrogels by radiation-induced polymerization and crosslinking and found that they are distinctly stronger than those produced by the classical thermal polymerization using a crosslinking agent. When a synthetic gel is incorporated into JF gel by the radiation-induced polymerization and crosslinking of a hydrophilic monomer, a novel type of hybrid hydrogel with very high mechanical strength results. The compressive and tensile strengths of the hybrid hydrogels are generally several times to more than ten times higher than those of JF gel and the corresponding component synthetic gels. The hybrid gels combine the well-developed structure of biological jellyfish gel and the unique microstructure of the synthetic gel produced by the radiation method, and strong interactions between the two networks are formed.

Nanoparticles, microgels and bulk hydrogels with very high mechanical strength starting from micelles

We report a novel method to prepare nanoparticles, microgels and bulk hydrogels starting from micelles formed by molecules of a nonionic surfactant, alkylphenol polyoxyethylene (10) ether (OP-10). The micelles can be stabilized under γ-ray irradiation to form nanoparticles which have a similar size to that of the micelles. Microgels and bulk hydrogels can easily be obtained by controlling the reaction conditions of the grafting of a hydrophilic monomer acrylic acid (AA) onto the radiation-peroxidized micelles. The bulk hydrogels initiated and crosslinked by peroxidized micelles (pMIC hydrogels) are highly transparent, and they have very high compressive strengths, several to several tens of MPa at a strain of 0.95. The synthesis conditions such as irradiation time, OP-10 concentration and monomer concentration are optimized by measuring the compressive strengths and elastic moduli of the obtained gels. The as-prepared and swollen hydrogel samples seldom break during the compression tests even at a strain of 0.98. The hydrogels have excellent shape recoverability, they can restore their initial shapes immediately after the release of the load. The gels have an interesting oriented microstructure. By investigating the morphologies of the samples reacted for different times with SEM, the link between the polymerization process and the gel morphology has been established.

A tough hydrogel–hydroxyapatite bone-like composite fabricated in situ by the electrophoresis approach

Mechanically strong hydrogel-HAp composites have been successfully fabricated through in situ formation of hydroxyapatite (HAp) in a tough polyacrylamide (PAAm) hydrogel with a modified electrophoretic mineralization method. The pre-swelling of the PAAm hydrogels in CaCl2 buffer solutions makes the electrophoresis method able to produce large area (10 × 8 cm2) hydrogel-HAp composites. At the same time the CaCl2 solution with different concentrations could control the HAp contents. The obtained hydrogel-HAp composites exhibit enhanced mechanical properties, namely higher extensibility (>2000%), tensile strength (0.1-1.0 MPa) and compressive strength (up to 35 MPa), in comparison to the as-synthesized PAAm hydrogels. FTIR and Raman characterizations indicate the formation of strong interactions between PAAm chains and HAp particles, which are thought to be the main reason for the enhanced mechanical properties. The hydrogel-HAp composite also shows excellent osteoblast cell adhesion properties. These composite materials may find more applications in biomedical areas, e.g. as a matrix for tissue repair especially for orthopedic applications and bone tissue engineering.

Anisotropic tough poly(2-hydroxyethyl methacrylate) hydrogels fabricated by directional freezing redox polymerization

This work reports a novel method for fabricating anisotropic hydrogels starting from monomers by combining directional freezing and redox polymerization (DFRP), and poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels with anisotropic porous structures and mechanical properties are obtained. Scanning electron microscopy (SEM) investigations show that the hydrogels have long and wide (up to several tens of micrometers) aligned channels in the direction parallel to the freezing direction, and pores in the perpendicular direction. The sizes of the channels and pores decrease with increasing freezing rate. Tensile, compressive and tearing tests show that the hydrogels (70 wt% water content) show very good mechanical properties, with tensile strength up to 0.44 MPa, compressive strength more than 20 MPa, and fracture energy up to 1000 J m-2. More importantly, the hydrogels exhibit significant anisotropy in their mechanical properties, which are better in the parallel direction. The hydrogels also show different swelling behaviour in comparison with conventional synthetic hydrogels. The anisotropic tough PHEMA hydrogels may find further application.

Interactions affecting the mechanical properties of macromolecular microsphere composite hydrogels.

Macromolecular microsphere composite (MMC) hydrogel is a kind of tough hydrogel fabricated by using peroxidized macromolecular microspheres as polyfunctional initiating and cross-linking centers (PFICC). The contribution of chemical cross-linking (covalent bonding) and physical cross-linking (chain entanglement and hydrogen bonding) to the mechanical properties are understood by testing the hydrogels, which were swollen in water or aqueous urea solutions to different water contents. The as-prepared MMC gels exhibited moderate moduli (60-270 kPa), high fracture tensile stresses (up to 0.54 MPa), high extensibilities (up to 2500%), and high fracture energies (270-770 J m(-2)). The moduli of the swollen gels decrease dramatically, but there are no significant changes in fracture tensile strength and fracture strain, even slight increases. More interestingly, the swollen gels show much-enhanced fracture energies, higher than 2000 J m(-2). A gradual decrease in the hysteresis ratio and residual strain is also found in the cyclic tensile testing of the hydrogels that were swollen to different water contents. The covalent bonding determines the tensile strength and fracture energy of the MMC gels, whereas the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels, and they are also the main reason for the presence of hysteresis in the loading-unloading cycles.

Tough and super-resilient hydrogels synthesized by using peroxidized polymer chains as polyfunctional initiating and cross-linking centers

Developing tough hydrogels with excellent resilience is important for their practical applications. We report a facile strategy for synthesizing tough and super-resilient hydrogels by using peroxidized macromolecules as polyfunctional initiating and cross-linking centers (PFICC). Peroxy groups are first introduced onto poly(N-vinylpyrrolidone) (PVP) chains by the irradiation method, and then the peroxidized PVP is used as PFICC to initiate in situ grafting polymerization of acrylamide (AAm) molecules. The resultant hydrogels made with different monomer concentrations exhibit very high compressive strengths (up to 20 MPa at 95% strain), high tensile strengths (up to 0.38 MPa), high elongations (up to 2800%), and a very good elastic recovery property. When a very small amount of a chemical cross-linker N,N′-methylenebisacrylamide (MBA) is added, although the tensile strengths and the elongations of the gels decrease, the compressive strengths and the moduli of the gels increase; more impressively, the gels exhibit an excellent elastic recovery property, as indicated by the extremely low hysteresis ratios (<0.03) and negligible stress relaxation.

Highly conductive and semitransparent free-standing polypyrrole films prepared by chemical interfacial polymerization

Chemical interfacial polymerization is a facile and effective way to synthesize large-area free-standing polypyrrole (PPy) films. Unfortunately, the conductivities of the PPy films prepared by ordinary chemical interfacial polymerization are generally very low. In this work, large-area, highly conductive and semitransparent free-standing PPy films are prepared by chemical interfacial polymerization at the interface of cyclohexane/water at a low monomer concentration and/or at a short reaction time. PPy films with a thickness ranging from nanometers to submicrons can be obtained by adjusting the monomer concentration and reaction time. The conductivity of the PPy films is up to 560 S cm−1. And the PPy films exhibit moderate transparency with a transmittance up to 31%. FTIR, Raman and XRD characterizations show that the PPy films have very similar polymer backbones and aggregated structures. XPS analyses indicate that the PPy films are doped with both p-toluenesulfonic acid (PTS) and Cl−. The high doping levels and the compact and smooth surface morphologies are the main reasons for the high conductivities of the PPy films.

Mechanical properties, anisotropic swelling behaviours and structures of jellyfish mesogloea.

Learning from nature is a promising way for designing and fabricating new materials with special properties. As the first step, we need to understand the structures and properties of the natural materials. In this work, we paid attention to the mesogloea of an edible jellyfish (Rhopilema esculenta Kishinouye) and mainly focused on its structure, mechanical and swelling properties. Scanning electron microscope (SEM) investigations show that jellyfish mesogloea has a well-developed anisotropic microstructure, which consists of nano-sized membranes connected with many fibres. The tensile and compressive properties of swollen and dried jellyfish mesogloea samples are measured. The jellyfish mesogloea displays very high tensile strength (0.17 MPa) and compressive strength (1.43 MPa) even with 99 wt % water. The mechanical properties of jellyfish mesogloea exceed most synthetic hydrogels with similar or even lower water contents. Swelling in acidic and basic buffer solutions weakens the mechanical properties of jellyfish mesogloea. The dried jellyfish mesogloea has very high tensile strength and modulus, which are very similar to those of synthetic plastics. The swelling properties of jellyfish mesogloea in solutions with different pH values were studied. The jellyfish mesogloea exhibits pH-sensitive and anisotropic swelling properties. The jellyfish mesogloea swells (expands) in height but deswells (shrinks) in length and width, without significant change in the volume. This phenomenon has never been reported for synthetic hydrogels. This study may provide gel scientists new ideas in designing and fabricating hydrogels with well-defined microstructures and unique mechanical and swelling properties.