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

In recent articles, our research group explored the use of crosslinked Poly (methylmethacrylate-acrylic acid) and composites based on this copolymer fo r bone implant applications such as suture anchors. The swelling response o f this system was studied first in, vitro, using a 0.85 g/100-mL saline sol ution (chosen because it simulates well the in, vivo environment), and late r in vivo by using samples implanted for various time periods in the latera l femoral condyles of New Zealand white rabbits. It was found that the swel ling response of the crosslinked copolymer in vivo was much greater than th at in the saline solution. The present investigation was conducted to deter mine the mechanism of excessive swelling in the in vivo tests. The approach used was to establish the changes occurring in the chemical structure of t he copolymer due to immersion in serum. A number of hypotheses that can pot entially 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 -CO OH groups in the copolymer to produce salts of acrylic acid, which are know n to result in greater swell due to their higher degree of dissociation com pared to free acid. It was also found that, for compositions containing the acrylic salts (produced by preswelling in high pH solutions and drying), t he swelling behavior in serum was similar to that in saline solution, and m ore importantly, equilibrium swelling was reached in a relatively shorter t ime period, which has several practical advantages for bioimplant applicati ons. (C) 2000 John Wiley & Sons, Inc.