The Effect of Hard Segment Chemistry on the In-Vivo Biostability of Polyurethanes
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THE EFFECT OF HARD SEGMENT CHEMISTRY ON THE IN-VIVO BIOSTABILITY OF POLYURETHANES ROBERT W. HERGENROTHER AND STUART L. COOPER University of Wisconsin-Madison, Dept. of Chemical Engineering, 1415 Johnson Dr., Madison, WI 53706
ABSTRACT Four polyurethanes that were synthesized with different hard segments as well as four commercial polyurethanes were investigated for in-vivo biostability. The four polyurethanes with the varying hard segments were based on a 3/2/1/ mole ratio of methylene diphenylene diisocyanate (MDI) or methylene dicyclohexylene diisocyanate (H12MDI); butanediol (BD) or ethylene diamine (ED); and polytetramethylene oxide (PTMO) (MW=I000). Two commercial polyurethanes were also used: Biomer (Ethicon, Inc.) and Pellethane (Dow). Films of the polymers were implanted subcutaneously in rats for up to three months to assess their biostability. Polymer films were implanted with either a 100% strain applied or in the unstressed state. Measurement of tensile properties and surface properties before and after implantation was used to assess the stability of each of the polymers. Surface cracking was observed with scanning electron microscopy and the extent and depth of cracking was determined. It was found that BD chain extended polymers maintained their tensile properties better than the ED chain extended polymers after implantation. H1 2MDI-based polyurethanes were more susceptible to surface cracking than MDI-based polyurethanes, possibly due to the lack of a crystallizable hard segment.
INTRODUCTION Polyurethanes were first suggested for use as biomaterials in 1967 by Boretos and Pierce [(] and have gained acceptance in the biomedical field due to their excellent physical properties and good biocompatibility [2]. Microphase separation of the polyurethane results in a hard phase which reinforces the elastomeric material. Under certain conditions, polyurethanes can undergo significant degradation. Degradation can result from enzymatic or oxidative attack [3,4] and be accelerated residual stress which can cause environmental stress cracking [5]. Previously, the effect of soft segment chemistry on the stability of polyurethanes has been investigated [6,7]. In this study, the effect of hard segment chemistry on the biostability of polyurethanes was investigated by systematically varying the diisocyanate group and chain extender. These polymers were studied along with four commercial polyurethanes. Polymer films were implanted subcutaneously in rats for up to 12 weeks, with and without prestrain.
Mat. Res. Soc. Symp. Proc. Vol. 252.
1992 Materials Research Society
258
EXPERIMENTAL Materials Four polyurethanes were synthesized in solution using a and a extender, diisocyanate, chain ratio of 3/2/1 The (MW=I000) soft segment. poly(tetramethylene oxide) (PTMO) synthesis was by a two-step reaction that has been previously used was either 4,4'The diisocyanate [8,9]. described hydrogenated analog, diphenylmethane diisocyanate (MDI) or its 1,4-butanediol 4,4'-dicyclohexylmethane diisocyanate (H12MDI). (BD) was u
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