NMR microscopy of polyacrylamide hydrogel

The use of polyacrylamide gel in soft-tissue augmentation: an experimental assessment

BACKGROUND

An increasing number of soft-tissue filler substances that lack experimental and clinical data have been introduced into plastic surgery practice outside the United States. One of these substances is polyacrylamide gel. It contains 2.5% polyacrylamide and 97.5% water. It is homogenous and stable, and has optimum viscosity and elasticity.

METHODS

One milliliter of polyacrylamide gel was injected into the subcutaneous layer of the right ear in 28 rabbits. The rabbits were divided into two groups, according to when the material was harvested and evaluated. Material was harvested at 4 months in 15 rabbits and 7 months in 13 rabbits. Each group underwent volumetric ultrasound evaluation, magnetic resonance imaging, and histological evaluation with hematoxylin and eosin and CD68 staining.

RESULTS

Results were easily observed because of the superficial position of the injected material. There were no systemic or local complications. The samples harvested showed a clear and jelly-like consistency similar to that of the initially injected material. The volume was constant after 6 weeks, after an initial period of acute stretching. Ultrasound volumetric analysis was also constant in all groups. At 7 months, a stable volume of 1.0 +/- 0.2 ml was observed. Magnetic resonance imaging scanning showed that the material was stable and that there was no inflammatory reaction. Histological analysis revealed a minimal foreign-body reaction, and the injected material was occasionally surrounded by a thin collagen membrane. The material remained in place.

CONCLUSIONS

Polyacrylamide gel has a long-lasting effect, with minimal volume variation. It remains soft to the touch and in place.

Proton Diffusion and T1 Relaxation in Polyacrylamide Gels: A Unified Approach Using Volume Averaging

The structure of polyacrylamide gels was studied using proton spin-lattice relaxation and PFG diffusion methods. Polyacrylamide gels, with total polymer concentrations ranging from 0.25 to 0.35 g/ml and crosslinker concentrations from 0 to 10% by weight, were studied. The data showed no effect of the crosslinker concentration on the diffusion of water molecules. The Ogston-Morris and Mackie-Meares models fit the general trends observed for water diffusion in gels. The diffusion coefficients from the volume averaging method also fit the data, and this theory was able to account for the effects of water-gel interactions that are not accounted for in the other two theories. The averaging theory also did not require the physically unrealistic assumption, required in the other two theories, that the acrylamide fibers are of similar size to water molecules. Contrary to the diffusion data, T1 relaxation measurements showed a significant effect of crosslinker concentration on the relaxation of water in gels. The model developed using the Bloch equations and the volume averaging method described the effects of water adsorption on the gel medium on both the diffusion coefficients and the relaxation measurements. In the proposed model the gel medium was assumed to consist of three phases (i.e., bulk water, uncrosslinked acrylamide fibers, and a bisacrylamide crosslinker phase). The effects of the crosslinker concentration were accounted for by introducing the proton partition coefficient, Keq, between the bulk water and crosslinker phase. The derived relaxation equations were successful in fitting the experimental data. The partition coefficient, Keq, decreased significantly as the crosslinker concentration increased from 5 to 10% by weight. This trend is consistent with the idea that bisacrylamide tends to form hydrophobic regions with increasing crosslinker concentration. Copyright 1998 Academic Press.