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Biodegradation of polymers
The degradation profiles of biodegradable polymers are related to many factors: hydrophilicity promotes degradation, while high crystallinity, Tm, Tg, and molecular weight help to decelerate degradation.
Processing conditions have the capability of altering the degradation characteristics of a material. When injection molding a biodegradable polymer, higher shear rates and longer residence times (the period during which the material is in the barrel of the molding machine) promote degradation. Although high temperatures are sometimes needed to reduce melt viscosity so that a polymer will flow through the small orifice in a molding machine, high temperatures may also accelerate degradation. For example, increasing the residence temperature from 190º to 230ºC was shown to hasten the degradation of L-PLA [von Oepen1].
Also, it is extremely important to keep a biodegradable polymer dry before, during, and after processing. Otherwise the polymer will degrade too quickly due to humidity.

Degradation of PCL
Polycaprolactone belongs to a family of polymers called polyesters. Degradation of PCL occurs by random hydrolytic secession of its ester linkages. Ester linkages are very susceptible to hydrolysis. The rate of hydrolysis is modulated by the crystallinity and hydrophobicity of the monomer components of the polymer chain.

Chemical structure of PCL

It is desirable for a polymeric bone plate to degrade at a rate that will slowly transfer load to the healing bone. Since it takes up to ? years for PCL to completely reabsorb [?3], it is clear that an implant fabricated from this material will support a bone for its entire duration of healing.

Modeling degradation in vivo
In vitro experiments can simulate the degradation of polymeric implants within the body. Some researchers utilize phosphate-based saline solutions with pH of 7.0-7.4 to mimic the body's environment [Middleton4]. However, these experiments were performed in real time (i.e. they lasted up to two years).

Gel Permeation Chromatography (GPC)
We have performed a degradation experiment by placing single pellets of PCL in vials of distilled water. We removed the pellets at different time points, and analyzed their molecular weight using GPC. We used THF (tetrahydrofuran) as our GPC solvent.

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GPC Data

GPC curve, PCL sample removed at Day 22

(Please click for larger image)

Data from the GPC curve was inputed into a Fortran program. The results show no appreciable decrease in molecular weight, supporting the notion that PCL has a long degradation time.

Degradation Period (days) Molecular Weight (g/mol) Mn PDI
0 123292 81357 1.52
8 116892 79975 1.46
15 122046 80808 1.51
22 117097 79461 1.47

Bone Plates/Materials | Bone/Tibia | Injection Molding | Instron | DSC | GPC

Last Updated
May 13, 2001