Processing and properties of two different poly (ortho esters).

Abstract

Processing and properties of two structurally different poly (ortho esters) (POE) and their in vitro behavior have been compared. One POE was a copolymer of diketene acetal 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5.5]undecane, trans-cyclohexanedimethanol and polyacetal. The other POE was a copolymer of diketene acetal 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5.5]undecane, trans-cyclohexanedimethanol and trans-cyclohexanedimethanol-dilactide. The polyacetal POE was also used as a matrix incorporating two different model drugs, caffeine and furosemide. Both polymers were ultrasonically molded into slabs. The lactide modified POE was also melt-extruded into rods and self-reinforced using a solid state die-drawing technique. The initial strength values of both ultrasonically molded and extruded samples were close to those of non-oriented poly lactic acids. Self-reinforcing increased the shear strength, bending strength and modulus of the rods from 41.7+/-1.7 MPa to 66.7+/-2.3 MPa, from 78.5+/-22.8 MPa to 183+/-23 MPa and from 2.5+/-0.3 GPa to 4.4+/-0.3 GPa, respectively. In vitro hydrolysis of samples was studied either in phosphate buffer saline (PBS) or in simulated body fluid (SBF) at 37 degrees C. Samples degraded faster in unchanged PBS than in PBS changed weekly. The total in vitro strength retention time was relatively short varying from 14 days to 42 days depending on the processing method and polymer type. Samples retained their wet weight at initial level for a longer time but dry weight started to decrease already after the first in vitro week. Polymers or degradation products crystallized in SBF but remained amorphous when hydrolyzed in PBS. Some of the devices showed evidence of surface erosion, and the processing method affected the erosion. The storage time of these polymers was found to be limited. However, both polymers were found to be of interest in bone fixation and tissue engineering.