Swelling and mechanical properties of hydrogels composed of binary blends of inter-linked pH-responsive microgel particles

Preparation of poly(acrylamide‐co‐2‐acrylamido‐2‐methylpropan sulfonic acid)‐g‐Carboxymethyl cellulose/Titanium dioxide hydrogels and modeling of their swelling capacity and mechanic strength behaviors by response surface method technique

It is very important that new generation, unique, high mechanical strength, and biocompatible hydrogel composites are developed due to their potential to be used as biomaterials in the biomedical field. Modeling of the swelling capacity and mechanical strength behavior of hydrogels is a domain of steadily increasing academic and industrial importance. These behaviors are difficult to model accurately due to hydrogels show very complex behavior depending on the content. In this study, a series of poly(acrylamide‐co‐2‐acrylamido‐2‐methylpropan sulfonic acid)‐g‐carboxymethyl cellulose/TiO2 (poly(AAm‐co‐AMPS)‐g‐CMC/TiO2) superabsorbent hydrogel composites were prepared by free‐radical graft copolymerization in aqueous solution. Structural and surface morphology characterizations were conducted by using Fourier‐transform infrared spectroscopy and scanning electron microscope analysis techniques. For modeling the equilibrium swelling capacity and fracture strength behaviors of hydrogels, the composition parameters (such as mole ratio of AMPS/AAm, wt% of CMC, and wt% of TiO2) was proposed by response surface method (RSM) Design Expert‐10 software. Statistical parameters showed that the RSM model has good performance in modeling the swelling capacity and mechanic fracture strength behaviors of poly(AAm‐co‐AMPS)‐g‐CMC/TiO2 hydrogel composites. According to the RSM model results, the maximum swelling capacity and fracture strength values were calculated as 270.39 g water/g polymer and 159.23 kPa, respectively.

Highly compressive and stretchable poly(ethylene glycol) based hydrogels synthesised using pH-responsive nanogels without free-radical chemistry

Poly(ethylene glycol) (PEG) based hydrogels are amongst the most studied synthetic hydrogels. However, reports on PEG-based hydrogels with high mechanical strength are limited. Herein, a class of novel, well-defined PEG-based nanocomposite hydrogels with tunable mechanical strength are synthesised via ring-opening reactions of diglycidyl ethers with carboxylate ions. The pH responsive crosslinked polyacid nanogels (NG) in the dispersed phase act as high functionality crosslinkers which covalently bond to the poly(ethylene glycol) diglycidyl ethers (PEGDGE) as the continuous matrix. A series of NG-x-PEG-y-z gels are prepared where x, y and z are concentrations of NGs, PEGDGE and the PEGDGE molecular weight, respectively. The hydrogel compositions and nano-structural homogeneity of the NGs have strong impact on the enhancement of mechanical properties which enables property tuning. Based on this design, a highly compressive PEG-based nanocomposite hydrogel (NG-13-PEG-20-6000) exhibits a compressive stress of 24.2 MPa, compressive fracture strain greater than 98% and a fracture energy density as high as 1.88 MJ m-3. The tensile fracture strain is 230%. This is amongst one of the most compressive PEG-based hydrogels reported to-date. Our chemically crosslinked gels are resilient and show highly recoverable dissipative energy. The cytotoxicity test shows that human nucleus pulposus (NP) cells remained viable after 8 days of culture time. The overall results highlight their potential for applications as replacements for intervertebral discs or articular cartilages.

Do the properties of gels constructed by interlinking triply-responsive microgels follow from those of the building blocks?

Microgels (MGs) are swellable crosslinked polymer colloids. They can also be used as the only building block to construct nanostructured hydrogels which are denoted as doubly crosslinked microgels (DX MGs). Here, new triply responsive DX MGs comprised of interlinked MGs of oligo(ethylene glycol)methacrylate (OEGMA), 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA), methacrylic acid (MAA) and a o-nitrobenzyl-based UV photocleavable crosslinker are investigated. The MGs swelled or collapsed in response to temperature and pH changes. These behaviours were rationalised with a generic model using Monte Carlo simulations. The MGs also degraded when UV irradiated due to photocleavage of nPh. DX MGs were assembled from the MGs to give injectable gels that were not cytotoxic to nucleus pulposus cells. Comparison of the responsive properties of the DX MGs and MGs showed that the temperature and pH responses of the former were mostly governed by the latter. However, two key differences were found. Firstly, whilst increasing the crosslinker mol% in the MG building blocks (x) did not change MG particle swelling, the compression modulus (E) and swelling of the DX MG gels were strongly affected by x. The E value for the gels was tuneable using x which is a potentially useful new observation for DX MGs. Secondly, UV irradiation of the DX MGs enhanced gel mechanical photostability in contrast to the behaviour of the MGs. We find that the properties of the DX MGs do not simply follow those of the parent MGs and propose mechanisms to account for the differences. The new family of multi-responsive DX MGs presented in this study have potential application for soft tissue repair as injectable gels or as gel implants which report sterilisation.

Plasmonic and colloidal stability behaviours of Au-acrylic core-shell nanoparticles with thin pH-responsive shells

The localised surface plasmon resonance (LSPR) of Au nanoparticles (NPs) as well as its interaction with nearby entities provides a wealth of fundamental and practical information at the nanometre scale. A number of studies have investigated core-shell NPs with Au cores and polymer shells that are temperature-responsive. However, there are very few studies of pH-responsive Au-polymer NP shells. Precipitation polymerisation is a scalable method and here we establish such a method to synthesise pH-responsive Au-poly(methyl methacrylate) copolymer core-shell NPs without the need for pre-functionalisation. The comonomers used were methacrylic acid (MAA) or 2-carboxyethyl acrylate (CEA) and the shells were crosslinked with ethylene glycol dimethacrylate. A series of five core-shell systems with collapsed shell thicknesses less than 30 nm are studied. The shell-thicknesses for the CEA-based core-shell NPs are relatively thin (≤5 nm) compared to related Au-polymer core-shell NPs prepared using precipitation polymerisation. The LSPR properties of the core-shell NPs were dependent on the shell thickness and were successfully simulated using finite difference time domain (FDTD) calculations. Two systems are considered further as exemplars. The MAA-based core-shell system with the thickest shell exhibited enhanced colloidal stability to added electrolyte. The CEA-based core-shell dispersion with the thinnest shells displayed reversible pH-triggered aggregation and was cytocompatible for HeLa cells. Proof-of-concept data are presented that demonstrate intracellular pH reporting.

Highly deformable hydrogels constructed by pH-triggered polyacid nanoparticle disassembly in aqueous dispersions

Most hydrogels are prepared using small-molecule monomers but unfortunately this approach may not be feasible for certain biomaterial applications. Consequently, alternative gel construction strategies have been established, which include using covalent inter-linking of preformed gel particles, or microgels (MGs). For example, covalently interlinking pH-responsive MGs can produce hydrogels comprising doubly crosslinked microgels (DX MGs). We hypothesised that the deformability of such DX MGs was limited by the presence of intra-MG crosslinking. Thus, in this study we designed new nanoparticle (NP)-based gels based on pH-swellable NPs that are not internally crosslinked. Two polyacid NPs were synthesised containing methacrylic acid (MAA) and either ethyl acrylate (EA) or methyl methacrylate (MMA). The PMAA-EA and PMAA-MMA NPs were subsequently vinyl-functionalised using glycidyl methacrylate (GMA) prior to gel formation via free-radical crosslinking. The NPs mostly disassembled on raising the solution pH but some self-crosslinking was nevertheless evident. The gels constructed from the EA- and MMA-based NPs had greater breaking strains than a control DX MG. The effect of varying the solution pH during curing on the morphology and mechanical properties of gels prepared using PMAA-MMA-GMA NPs was studied and both remarkable deformability and excellent recovery were observed. The gels were strongly pH-responsive and had tensile breaking strains of up to 420% with a compressive strain-at-break of more than 93%. An optimised formulation produced the most deformable and stretchable gel yet constructed using NPs or MGs as the only building block.

Synthesis of polyacid nanogels: pH-responsive sub-100 nm particles for functionalisation and fluorescent hydrogel assembly

Nanogels are crosslinked polymer particles with a swollen size between 1 and 100 nm. They are of major interest for advanced surface coatings, drug delivery, diagnostics and biomaterials. Synthesising polyacid nanogels that show triggered swelling using a scalable approach is a key objective of polymer colloid chemistry. Inspired by the ability of polar surfaces to enhance nanoparticle stabilisation, we report the first examples of pH-responsive polyacid nanogels containing high -COOH contents prepared by a simple, scalable, aqueous method. To demonstrate their functionalisation potential, glycidyl methacrylate was reacted with the -COOH chemical handles and the nanogels were converted to macro-crosslinkers. The concentrated (functionalised) nanogel dispersions retained their pH-responsiveness, were shear-thinning and formed physical gels at pH 7.4. The nanogels were covalently interlinked via free-radical coupling at 37 °C to form transparent, ductile, hydrogels. Mixing of the functionalised nanogels with polymer dots enabled covalent assembly of fluorescent hydrogels.

Using intra-microgel crosslinking to control the mechanical properties of doubly crosslinked microgels

Microgels (MGs) are crosslinked polymer particles that swell when the pH approaches the pKa of the constituent polymer. Our earlier work showed that concentrated MG dispersions can be covalently interlinked to form macroscopic hydrogels, which are termed doubly crosslinked microgels (DX MGs). Here, we study for the first time the effects of intra-MG crosslinking on the swelling of the MGs and the mechanical properties of the DX MGs. The MGs were synthesised by emulsion copolymerisation of ethyl acrylate (EA) or methacrylic acid (MAA) and divinylbenzene (DVB). The latter was a crosslinking monomer. For comparison, MGs were prepared where DVB was replaced by either 1,4-butanediol diacrylate (BDDA) or a 1 : 1 mixture of both DVB and BDDA. The MG swelling behaviours were studied by dynamic light scattering; whereas, the DX MG mechanical properties were studied by dynamic rheology and uniaxial compression measurements. Inclusion of DVB within the MGs resulted in both highly swelling MGs and highly ductile DX MGs. The average strain-at-break value for the DVB-containing DX MGs was 76% which represents the highest value yet reported for a DX MG prepared using commercially available monomers. It was also shown that good tuneability of the DX MG properties could be obtained simply by controlling the DVB and BDDA contents within the MG particles. Analysis of the swelling and compression data enabled relationships between the volume-swelling ratio of the MGs and either the modulus or strain-at-break values for the DX MGs. These relationships also applied to a DVB-free system prepared with a low BDDA content. An interesting conclusion from this study is that the DX MGs can be thought of mechanically as macroscopic MG particles. The results of this study provide design tools for improving DX MG ductility and hence increasing the range of potential applications for this new class of hydrogel.

Click reactive microgels as a strategy towards chemically injectable hydrogels

Doubly crosslinked microgels (DX microgels) are hydrogels constructed by covalently interlinked microgel particles, offering two levels of hierarchy within the network, the first one being the microgel and the second being the interlinked microgel network.