Silvia Halacheva

This study focuses on the development of new biocompatible and biodegradable particle gel scaffolds based on PCL-HBPG/1SiHBPG triblock copolymers composed of a polycaprolactone (PCL) core and two outer blocks of trimethoxysilyl end-capped hyperbranched polyglycidol (HBPG/1SiHBPG) that have the potential to be used in soft tissue regeneration. The relationship between the gel’s composition, structure, mechanical properties, and performance has been investigated for the first time and the copolymer design parameters have been optimized. The particle gel scaffolds were formed from the concentrated dispersions of the most hydrophobic PCL-45HBPG/1SiHBPG at low temperatures, and were the result of the numerous hydrogen bonds formed from the HBPG/1SiHBPG moieties as well as the formation of siloxane crosslinks (i.e., Si–O–Si bonds). These gels were formed in the physiological temperature range. Gels with a mechanical strength that gradually increases were formed from the physically crosslinked PCL-45HBPG/1SiHBPG particles effectively and safely, in the absence of UV radiation. They feature high elasticity and undergo enzyme-triggered disassembly. The gels are biocompatible and have the potential to invoke cell attachment and differentiation in the absence of exogenous biological stimuli. A successful outcome of this study will be the prospect of a new approach for tissue regeneration that is currently not available.

Aqueous solutions from two types of PMMA–AA polymers (PMMA–25AA and PMMA–50AA) are investigated. The aim is to outline the effect of AA content on the bulk solution and air/solution interface properties. The experiments include dynamic light scattering, surface tension and interfacial rheology measurements. The drainage kinetics and stability of microscopic foam films are also investigated. It is established that at similar conditions (pH = 10, temperature 20 °C) the polymer molecules have almost the same mean molecular weight and size distribution of the bulk globules. Dynamic and equilibrium surface tension measurements reveal systematically higher values for PMMA–50AA as compared to PMMA–25AA. These outcomes are related to higher bulk electrophoretic mobility in the PMMA–50AA case and the overall more stretched configuration of the polymer at the air/solution interface. Surface dilational rheology characteristics are particularly sensitive to the polymer structural peculiarities: while no significant changes are registered in the case of PMMA–25AA, the solutions of PMMA–50AA exhibit a pronounced maximum in surface dilational elasticity for the concentration ∼1 × 10⁻⁴ mol L⁻¹. This observation is a clear sign of specific bulk and/or interfacial (structure) transition which has to be investigated in further studies. Microscopic foam films provide additional evidence for the effect of fine-tuning the AA content of the copolymer. All microscopic films are stable which is due predominantly to the overwhelming electrostatic repulsion effects. The obtained results add new knowledge to the structure–property relationships of the PMMA–AA based aqueous formulations. They give valuable hints for further fine-tuning opportunities of these systems, that have high innovative potential for various applications.

Comblike polyethylenimines with varying degrees of polymerization of both the main and side chains as well as different grafting densities were evaluated as gene delivery vectors. They were able to condense linear and plasmid DNA into nanosized polyplex particles with dimensions and surface potentials in the 130–330 nm and −30 to +15 mV ranges, respectively, depending on the amine/phosphate (N/P) ratio. The polyplexes remained stable in aqueous and buffer solutions from several hours up to several days. The moderate colloidal stability was also manifested in a relatively broad size distribution (PDI typically above 0.2) and structural polymorphism observed by transmission electron microscopy. Both the neat polymers and polyplexes displayed low cytotoxicity in WISH cells as the relative cell viability was more than 60%. Experiments with lysosomal fluorescence staining revealed that the internalization pathways and, in turn, transfection efficiency of the polyplex nanoparticles depended on the polymer chain topology. The vector systems based on the polymers of denser structure can be considered to be promising systems for gene transfection in eukaryotic cells.

Doubly crosslinked microgels (DX MGs) are hydrogels constructed by covalently interlinked vinyl-functionalised microgel particles. Until now it has not been possible to precisely control the extent of vinyl functionalisation of the microgel (MG) particles which act as the colloidal building blocks for hydrogel assembly. Furthermore, the range of DX MGs prepared to date has been modest. This study addresses both of these challenges by constructing a new class of DX MG using MG particles that were vinyl functionalised by copper catalysed azide–alkyne cycloaddition (CuAAC). Here, poly(2-vinylpyridine-co-propargyl acrylate) (PVP-PA) MG particles were prepared and vinyl functionalised by CuAAC using azidopropyl methacrylate (AZPMA) to give PVP-PA_x–PMA_y MGs. Values for the extent of vinyl functionalisation (y) were varied from 0 to 7.0 mol% in a precisely controlled manner. Concentrated dispersions were transformed from fluids to physical gels at pH values of less than or equal to 3.0 and covalently inter-linked to give PVP-PA_x–PMA_y DX MGs by free-radical coupling of the vinyl groups of neighbouring particles. The storage modulus of the DX MGs varied linearly with both MG concentration and the value for y. Not only did the new DX MGs studied here enable precise control of MG functionalisation and DX MG mechanical properties, they also showed evidence of colloidal crystallinity which may lead to future photonic gel applications. Our CuAAC-based approach should be versatile and is expected to enable a range of new DX MGs to be prepared.

The potential of various pH-responsive alkyl (meth)acrylate ester- and (meth)acrylic acid-based copolymers, including poly(methyl methacrylate-co-acrylic acid) (PMMA-AA) and poly(n-butyl acrylate-co-methacrylic acid) (PBA-MAA), to form pH-sensitive biocompatible and biodegradable hollow particle gel scaffolds for use in non-load-bearing soft tissue regeneration have been explored. The optimal copolymer design criteria for preparation of these materials have been established. Physical gels which are both pH- and redox-sensitive were formed only from PMMA-AA copolymers. MMA is the optimal hydrophobic monomer, whereas the use of various COOH-containing monomers, e.g., MAA and AA, will always induce a pH-triggered physical gelation. The PMMA-AA gels were prepared at physiological pH range from concentrated dispersions of swollen, hollow, polymer-based particles cross-linked with either cystamine (CYS) or 3,3′-dithiodipropionic acid dihydrazide (DTP). A linear relationship between particle swelling ratios, gel elasticity, and ductility was observed. The PMMA-AA gels with lower AA contents feature lower swelling ratios, mechanical strengths, and ductilities. Increasing the swelling ratio (e.g., through increasing AA content) decreased the intraparticle elasticity; however, intershell contact and gel elasticity were found to increase. The mechanical properties and performance of the gels were tuneable upon varying the copolymers’ compositions and the structure of the cross-linker. Compared to PMMA-AA/CYS, the PMMA-AA/DTP gels were more elastic and ductile. The biodegradability and cytotoxicity of the new hollow particle gels were tested for the first time and related to their composition, mechanical properties, and morphology. The new PMMA-AA/CYS and PMMA-AA/DTP gels have shown good biocompatibility, biodegradability, strength, and interconnected porosity and therefore have good potential as a tissue repair agent.

Gels that feature high internal porosity and have both high elasticity and ductility have potential to provide immediate load support and enable subsequent tissue regeneration of damaged soft tissue if combined with cells. Herein, we report results from a recent investigation of novel poly(methyl methacrylate-co-methacrylic acid), (PMMA-MAA) and poly(ethyl acrylate-co-methacrylic acid), (PEA-MAA) biodegradable, pH-sensitive particle gels which are with high porosity, elasticity and ductility. These gels formed at physiological pH range and are potentially injectable. The particles were prepared using solvent evaporation. They were functionalized by crosslinking the MAA groups of the particles via bis-amide formation with either cystamine (CYS) or 3,3′-dithiodipropionic acid dihydrazide (DTP) which simultaneously incorporated reversibility due to the presence of disulphide bonds within the crosslinker. The crosslinked particles were observed by dynamic light scattering to swell appreciably in size upon increasing the pH. Concentrated dispersions formed elastic and ductile physical gels within the physiological pH range. A key finding of this study was that for crosslinked particles of similar composition the formation of considerably more elastic and ductile gels was observed from the most lightly crosslinked particles. Furthermore, compared to the PMMA-MAA/CYS and PEA-MAA/CYS gels, those formed from DTP-crosslinked particles had higher elasticity, thicker pore walls and improved interconnectivity. For the PMMA-MAA/DTP gels an elastic modulus value as high as 100 kPa and a yield strain greater than 100% were observed for a gel containing only 5 wt% of particles. The improved mechanical properties of these new gel-forming dispersions imply that they now have good potential for future application as injectable gels for regenerative medicine.