Swelling of crosslinked poly(methylmethacrylate-acrylic acid) copolymers in serum and saline solutions

Abstract

In recent articles, our research group explored the use of crosslinked Poly(methylmethacrylate-acrylic acid) and composites based on this copolymer for bone implant applications such as suture anchors. The swelling response of this system was studied first in vitro, using a 0.85 g/100-mL saline solution (chosen because it simulates well the in vivo environment), and later in vivo by using samples implanted for various time periods in the lateral femoral condyles of New Zealand white rabbits. It was found that the swelling response of the crosslinked copolymer in vivo was much greater than that in the saline solution. The present investigation was conducted to determine the mechanism of excessive swelling in the in vivo tests. The approach used was to establish the changes occurring in the chemical structure of the copolymer due to immersion in serum. A number of hypotheses that can potentially explain the observed excessive swelling in serum were investigated and are discussed in this article. The results of this study indicate that the mechanism of excessive swelling in serum was the neutralization of COOH groups in the copolymer to produce salts of acrylic acid, which are known to result in greater swell due to their higher degree of dissociation compared to free acid. It was also found that, for compositions containing the acrylic salts (produced by preswelling in high pH solutions and drying), the swelling behavior in serum was similar to that in saline solution, and more importantly, equilibrium swelling was reached in a relatively shorter time period, which has several practical advantages for bioimplant applications. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1653–1664, 2001