Amir H. Milani

Injectable doubly cross-linked microgels for improving the mechanical properties of degenerated intervertebral discs.

The use of injectable pH-responsive doubly cross-linked microgels (DX microgels) to improve the mechanical properties of degenerated intervertebral discs is demonstrated for the first time. The microgel comprised methyl methacrylate (MMA), methacrylic acid (MAA), ethyleneglycol dimethacrylate (EGD) and glycidyl methacrylate (GM) and was poly(MMA/MAA/EGD)-GM. The GM facilitated covalent interparticle cross-linking. The DX microgels are shown to have tunable mechanical properties. Degeneration of model bovine intervertebral discs (IVDs) was induced using collagenase. When injected into degenerated IVDs the DX microgels were shown to improve the strain, modulus, toughness and resilience. The extent of mechanical property improvement was an increasing function of DX microgel concentration, suggesting tunability. Cytotoxicity studies showed that the DX microgel was biocompatible under the conditions investigated. The results of this study imply that injectable DX microgels have good potential as a future regenerative medicine strategy for restoring the mechanical properties of degenerated load-bearing soft tissue, such as IVDs.

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

We show that a new type of hydrogel can be prepared by covalently inter-linking binary blends of microgel (MG) particles and that the swelling ratio and modulus of the gels can be predicted from their composition. In previous work we established that physical gels of glycidyl methacrylate (GMA) functionalised poly(methyl methacrylate-co-methacrylic acid-co-ethyleneglycol dimethacrylate) microgel particles (GMA-MG) could be covalently inter-linked to give hydrogels, termed doubly crosslinked microgels, DX MGs. We build on this concept here by investigating the properties of DX MGs containing binary blends of GMA-MG particles and glycidyl oligo(ether ester) acrylate-functionalised microgel particles (GOE-MG). These new hydrogels were assembled by inter-linking nanoscale MG building blocks in the absence of small molecule monomers or crosslinkers. The volume fraction of GMA-MG particles used to prepare the GOE-GMA DX MGs was systematically varied. Rheology data showed that inclusion of GMA-MG and GOE-MG within the GOE-GMA DX MGs increased the modulus and yield strain, respectively, compared to the values measured for the respective physical gels. The data for the covalent GOE-GMA DX MG gels showed that the ductility increased with increasing GOE-MG content. GOE provided covalent inter-linking of the MG particles and also acted as a lubricant between particles due to its low Tg. By demonstrating compositionally determined swelling and mechanical properties for DX MG gels prepared using binary blends of MG particles, this study introduces a new, widely applicable, hydrogel construction assembly concept that is not available for conventional hydrogels.

Pickering Emulsions Stabilized by pH-Responsive Microgels and Their Scalable Transformation to Robust Submicrometer Colloidoisomes with Selective Permeability

Colloidosomes are micrometer-sized hollow particles that have shells consisting of coagulated or fused colloid particles. While many large colloidosomes with sizes well above 1.0 μm have been prepared, there are fewer examples of submicrometer colloidosomes. Here, we establish a simple emulsion templating-based method for the preparation of robust submicrometer pH-responsive microgel colloidosomes. The colloidosomes are constructed from microgel particles based on ethyl acrylate and methacrylic acid with peripheral vinyl groups. The pH-responsive microgels acted as both a Pickering emulsion stabilizer and macro-cross-linker. The emulsion formation studies showed that the minimum droplet diameter was reached when the microgel particles were partially swollen. Microgel colloidosomes were prepared by covalently interlinking the microgels adsorbed at the oil-water interface using thermal free-radical coupling. The colloidosomes were prepared using a standard high-shear mixer with two different rotor sizes that corresponded to high shear (HS) and very high shear (VHS) mixing conditions. The latter enabled the construction of submicrometer pH-responsive microgel-colloidosomes on the gram scale. The colloidosomes swelled strongly when the pH increased to above 6.0. The colloidosomes were robust and showed no evidence of colloidosome breakup at high pH. The effect of solute size on shell permeation was studied using a range of FITC-dextran polymers, and size-selective permeation occurred. The average pore size of the VHS microgel-colloidosomes was estimated to be between 6.6 and 9.0 nm at pH 6.2. The microgel-colloidosome properties suggest that they have the potential for future applications in cosmetics, photonics, and delivery.

Tuning the modulus of nanostructured ionomer films of core–shell nanoparticles based on poly(n-butyl acrylate)

In this study we investigate the structure-mechanical property relationships for nanostructured ionomer films containing ionically crosslinked core-shell polymer nanoparticles based on poly(n-butyl acrylate) (PBA). Whilst nanostructured ionomer films of core-shell nanoparticles have been previously shown to have good ductility [Soft Matter, 2014, 10, 4725], the modulus values were modest. Here, we used BA as the primary monomer to construct core-shell nanoparticles that provided films containing nanostructured polymers with much higher glass transition temperature (Tg) values. The core-shell nanoparticles were synthesised using BA, acrylonitrile (AN), methacrylic acid (MAA) and 1,4-butanediol diacrylate (BDDA). Nanostructured ionomer films were prepared by casting aqueous core-shell nanoparticle dispersions in which the shell -COOH groups were neutralised with KOH and ZnO. The film mechanical properties were studied using dynamic mechanical analysis and tensile stress-strain measurements. The use of BA-based nanoparticles increased the Tg values to close to room temperature which caused a strong dependence of the film mechanical properties on the AN content and extent of neutralisation of the -COOH groups. The Youngs modulus values for the films ranged from 1.0 to 86.0 MPa. The latter is the highest modulus reported for cast films of nanostructured ionomer films prepared from core-shell nanoparticles. The films had good ductility with strain-at-break values of at least 200%. The mechanical properties of the films were successfully modelled using the isostrain model. From comparison with an earlier butadiene-based system this study demonstrates that the nature of the primary monomer used to construct the nanoparticles can profoundly change the film mechanical properties. The aqueous nanoparticle dispersion approach used here provides a simple and versatile method to prepare high modulus elastomer films with tuneable mechanical properties.