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Polymer Characterization

Guidance for mechanical loading, fluid flow or both. Loading type, magnitude or frequency are also covered.

ASTM F1635-11 (in vitro Degradation Testing of Hydrolytically Degradable Polymer Resins and Fabricated Forms for Surgical Implants) is designed to determine degradation rates of polymers and devices made from resorbable polymers.  The polymers and copolymers covered by this method are composed of l-lactide, d-lactide, d,l-lactide, glycolide, caprolactone, and p-dioxanone. Degradation rates can be greatly affected by exact composition, specimen size, polymer molecular weight and crystallinity.

Method Specifics

Mechanically loaded and unloaded hydrolytic evaluation is described in this ASTM standard.  Unloaded mechanical hydrolytic evaluation is often performed to obtain a baseline profile of the polymer.  Mechanical loading is considered if loading will be encountered in vivo.  Cyclic loading can be approximated by static loading at the maximum load in the cycle if it does alter the failure mode. 

Devices that are subjected to flow conditions while in use may have a different degradation rate than other devices. Flow conditions must be estimated for devices such as catheters and replicated in vitro.

Sterilization of resorbable material may cause changes in mechanical and molar mass properties. This could affect the base-line testing. For this reason, specimens must be sterilized and packaged as in actual use. Non-sterilized specimens may be tested for comparative results.

Phosphate buffered saline (PBS) shall be used as the soaking solution and maintained at a pH of 7.4 unless it has been documented that the pH should be different in the specific device and its use.  Bovine serum may also be used as the solution.  

PBS to HDP ratio shall be as high as is practical. 100 to 1 is described.  Multiple samples may be stored in the same container as long as suitable sample separation is maintained. A minimum of three samples shall be tested per time period. 

In Vitro Degradation is often used to test surgical implants in the medical industry

Testing Methods

Testing will include mass loss, molar mass, and mechanical testing.

For many HDP resins, inter-lot variations in molar mass and residual monomer can be significant. This can affect degradation rates. It is recommended that the residual monomer of the parts be analyzed.

Molar Mass shall be evaluated by solution viscosity or size exclusion chromatography (SEC).

pH of the PBS will be tested at least once per week

Weight loss shall be measured to a precision of 0.1% of the total sample weight.  Samples must be dried to a constant weight. 

Mechanical test methods will be chosen from the following (other methods may be included):

  • ASTM D638 Standard Test Method for Tensile Properties of Plastics
  • ASTM D695 Standard Test Method for Compressive Properties of Rigid Plastics
  • ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
  • ASTM D882 Standard Test Method for Tensile Properties of Thin Plastic Sheeting
  • ASTM D1708 Standard Test Method for Tensile Properties of Plastics by Use of Microtensile Specimens
  • ASTM D1822 Standard Test Method for Tensile-Impact Energy to Break Plastics and Electrical Insulating Materials

Testing Termination

Test termination will occur when one of more of the following have occurred:

  1. A predetermined time point has been reached
  2. Sample integrity has been compromised to the point that the data is no longer meaningful
  3. The solution has drifted outside the pH range of 7.2-7.6