文档介绍：Molecular Principles of Biomaterials Spring 2003
Lecture 2: Molecular Design and Synthesis of Biomaterials I: Biodegradable
Solid Polymeric Materials (continued)
Last time: chemistry and physical chemistry of degrading polymeric solids for biomaterials
Today: Theory of polymer erosion
Enzymatic degradation of synthetic biomaterials
Designing degradable materials
Reading: A. Gopferich, “Mechanisms of polymer degradation and erosion,’ Biomaterials 17, 103
(1996)
Ratner p. 243-259
Supplementary Reading: . Young and . Lovell, “Introduction to Polymers,” ch. 4 Polymer Structure pp. 241-
309 (crystallization of polymers, Tm, glass transition, etc.)
Surface vs. Bulk Hydrolysis: Göpferich’s theory for polymer erosion1-4
Biodegradable solids may have differing modes of degradation:
Surface erosion – degradation from exterior only with little/no water ration into bulk
Bulk erosion – water rates entire structure and degrades entire device simultaneously
Polymers hydrolyzing by mechanisms II or III can be either surface or bulk -7
Assuming that a polymer is water insoluble (initially) and that hydrolysis is the only mechanism of breakdown, the factors
listed above all vary two rates of importance:
rate of water diffusion into polymer
rate of chain cleavage by water ions
The balance of these rates determines whether a polymer erodes from the surface in or by simultaneous degradation
throughout the material:
Comparing velocities of water diffusion and chain cleavage:
Lecture 2 – Biodegradable Solid Polymers1 of 12
Molecular Principles of Biomaterials Spring 2003
Accounting for rate of water diffusion: Time required for water to diffuse a mean distance <x> into the solid
polymer:
2
(1) tdiff = <x> π/4DH2O
DH2O = effective diffusivity of water in polymer
See Atkins Phys. Chem p. 770 for derivation
Random walk:
(Atkins8)
1/2
Mean distance from origin tr