Study of pH-Responsive Microgels Containing Methacrylic Acid:  Effects of Particle Composition and Added Calcium

Abstract

pH-responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the ionic monomer incorporated within the particles. In recent work from our group, it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of pH-responsive microgel dispersions (Saunders, J. M.; Tong, T.; LeMaitre, C.; Freemont, A. J.; Saunders, B. R. Soft Matter 2007, 3, 486). These dispersions change from a fluid to a gel with increasing pH. The present work investigates the pH-dependent properties of dispersions of microgel particles containing MAA (methacrylic acid) and also the effects of added Ca2+. Two microgels are discussed: microgel A is poly(EA/MAA/AM) (EA and AM are ethyl acrylate and allyl methacrylate), and microgel B is poly(EA/MAA/BDDA) (butanediol diacrylate). The pH-dependent particle properties investigated include hydrodynamic diameters and electrophoretic mobilities. The critical coagulation concentrations (CCC) of dilute dispersions and the elastic modulus (G') of concentrated, gelled microgel dispersions were also investigated. In the absence of added Ca2+, the particle swelling and G' were smallest and largest, respectively, for microgel A. The changes in hydrodynamic diameter and mobility with pH were explained in terms of a core-shell swelling mechanism. Added Ca2+ was found to significantly decrease the CCCs, extents of particle swelling, and magnitude of the electrophoretic mobility. This was attributed to the ionic cross-linking of neighboring RCOO- groups by Ca2+. It is suggested that the formation of ionic cross-links is inefficient within the microgel particles because of the presence of covalent cross-links that oppose the large-scale conformational rearrangement of neighboring RCOO- groups. The effect of Ca2+ on the properties of the gelled dispersions is important from the viewpoint of potential application in vivo. Rheological studies of the gelled microgel dispersions showed that added Ca2+ did not have a specific influence on G'. The differences observed in the presence of Ca2+ were attributed to ionic strength effects (screening). The key parameter that controls G' of the gelled microgel dispersions is pH. The results from this work suggest that the elasticity of the gels would be slightly reduced in vivo as a consequence of the high ionic strength present.

A study of poly(butadiene/methacrylic acid) dispersions: From pH-responsive behaviour to the effects of added Ca2+

“…Previous work at Manchester has investigated the structureproperty relationships of MAA-containing microgel particles [8,9,14]. These include poly(EA/MAA/BDDA) (ethyl acrylate and butanediol diacrylate).…”

Factors Affecting Peptide Interactions with Surface-Bound Microgels

“…These microgels have previously been well characterized for various applications [22][23][24] and, importantly, the co-monomer composition can be easily varied during synthesis to alter microgel swelling properties, allowing for tunable performance of the system. 25 In the present study we employed this microgel platform in combination with the model cationic peptide poly-L-lysine (pLys, pI≈9 (10.5 for isolated lysine monomers 12 )).…”

Spatially dependent diffusion coefficient as a model for pH sensitive microgel particles in microchannels

“…The three experimental examples given in the figure show typical pH sensitive behavior. [39][40][41] There are two examples of a MG which increases in diameter as the solution becomes more acidic, and one example (shown as triangles) where there is an decrease. In both cases, the change in size can range by a factor of 2 À 5 over the accessible pH range.…”

“…PEGMA/PDPA, 39 poly(2-vinylpyridine-co-styrene), 40 and poly(EA/MAA/BDDA). 41 The PEGMA/ PDPA MG is a combined monomethoxy-capped poly-(ethylene glycol) methacrylate macromonomer (PEGMA) and a poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) microgel. The poly(EA/MAA/AM) microgel particle contains MAA (methacrylic acid), where EA and AM are ethyl acrylate and allyl methacrylate.…”

“…Causes include hydrodynamic interactions mediated by boundary walls, a spatial temperature variation, 38 and a pH spatial variation. [39][40][41] We focus here on the latter type as the variation of MG size with pH can be well controlled. This is an important stimulus for MG as they are used as biosensors [5][6][7] and as drug delivery agents whose function depends on pH.…”