Crosslinked polymeric material with enhanced strength and process for manufacturing
View Patent ↗A radiation crosslinked (50 kGy), pressure-treated UHMWPE material has been developed by applying compressive force on a crosslinked polymer in a direction orthogonal to an axial direction. The deformed material is then cooled while held in a deformed state. The resulting material is anisotropic, with enhanced strength oriented along the axial direction. The directionally engineered material is oxidatively stable even after four weeks of accelerated aging in a pressure vessel containing five atmospheres of oxygen (ASTM F2003). Because of its oxidative stability, the deformation processed material is a suitable candidate for air-permeable packaging and gas sterilization, which has thus far been reserved for remelted highly crosslinked UHMWPEs.
1. A method for preparing a UHMWPE pre-form suitable for use in medical implants comprising
heating a gamma-crosslinked UHMWPE rod characterized by a crystalline melting point and a diameter d 1 to a compression deformable temperature;
applying compression force on the crosslinked UHMWPE to reduce the diameter to d 2 , wherein d 2 <d 1 ;
cooling the reduced diameter rod of UHMWPE to a solidification temperature while maintaining compression force to keep the diameter at a diameter of d 3 , wherein d 3 <d 1 ; and
stress relieving the cooled rod by heating to a temperature at which the rod expands to a diameter d 4 , wherein d 4 >d 3 , and wherein d 4 is approximately equal to d 1 .
2. A method according to claim 1 , comprising extruding the rod through a reducing die into a cooling chamber to reduce the diameter from d 1 to d 2 .
3. A method according to claim 1 , wherein the compression deformable temperature is from the melting point minus 50° C. to the melting point.
4. A method according to claim 1 , wherein the compression deformable temperature is less than the melting point.
5. A method according to claim 1 , wherein the compression deformable temperature is from about 100° C. to about 135° C.
6. A method according to claim 1 , wherein the compression deformable temperature is about 125° C. to 135° C.
7. A method according to claim 1 , wherein the UHMWPE rod is crosslinked by gamma-irradiation at a dose of 0.01-10 Mrad.
8. A method according to claim 1 , wherein the UHMWPE rod is crosslinked by gamma-irradiation at a dose of 1-6 Mrad.
9. A method according to claim 1 , wherein the UHMWPE rod is crosslinked by gamma-irradiation at a dose of 2-5 Mrad.
10. A method according to claim 1 , wherein d 3 is approximately equal to d 2 .
11. A method according to claim 1 , wherein the UHMWPE is gamma-irradiated at a dose of 1-5 Mrad, the compression deformable temperature is greater than 100° C. and less than the melting point, applying compression force comprises extruding the heated crosslinked UHMWPE through a reducing die, the rod is cooled to a temperature of 30° C. or less after extrusion, and stress relieving is carried out at a temperature of 100° C. to 130° C.
12. A method for making a UHMWPE implant bearing component, comprising machining the component from a pre-form made according to claim 1 .
13. A method for making a UHMWPE implant bearing component, comprising machining the component from a pre-form made according to claim 11 .
14. A method for preparing a UHMWPE pre-form suitable for use in medical implants comprising
heating a gamma-crosslinked UHMWPE rod characterized by a crystalline melting point and a diameter d 1 to a compression deformable temperature below the crystalline melting point;
applying compression force on the crosslinked UHMWPE to reduce the diameter to d 2 , wherein d 2 <d 1 ;
cooling the reduced diameter rod of UHMWPE to a solidification temperature while maintaining compression force to keep the diameter at a diameter of d 3 , wherein d 3 <d 1 ; and
stress relieving the cooled rod by heating to a temperature below the crystalline melting point at which the rod expands to a diameter d 4 , wherein d 4 >d 3 , and wherein d 4 is approximately equal to d 1 .
15. A method according to claim 14 , comprising extruding the rod through a reducing die using a sacrificial puck made of crosslinked UHMWPE.
16. A method according to claim 14 , wherein the UHMWPE rod is crosslinked by γ-irradiation at a dose of 0.01 to 10 MRad.
17. A method according to claim 14 , wherein the UHMWPE rod is crosslinked by γ-irradiation at a dose of 1 to 6 MRad.
18. A method according to claim 14 , wherein d 3 is approximately equal to d 2 .
19. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is from 1.1 to 3.0.
20. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is from 1.2 to 3.0.
21. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is from 1.2 to 1.8.
22. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is from 1.2 to about 1.5.
23. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is greater than 1 and less than or equal to 2.5.
24. A method according to claim 14 , wherein the draw ratio d 1 /d 2 is greater than 1 and less than or equal to about 2.0.
25. A method for preparing a UHMWPE preform suitable for use in medical implants, comprising
heating a γ-crosslinked UHMWPE rod characterized by a crystalline melting point and a diameter d 1 to a compression deformable temperature less than the melting point;
extruding the heated crosslinked UHMWPE through a reducing die to reduce the diameter to d 2 ,wherein d 2 less than d 1 ;
cooling the reduced diameter rod of UHMWPE to a solidification temperature while maintaining compression force to keep the diameter at a diameter of d 3 , wherein d 3 less than d 1 ; and
stress relieving the cooled rod by heating to a temperature at which the rod expands to a diameter d 4 , wherein d 4 is greater than d 3 , and where d 4 /d 1 is from 0.9 to 1.0.
26. A method according to claim 25 , comprising extruding the rod through a reducing die using a sacrificial puck made of crosslinked UHMWPE.
27. A method according to claim 25 , wherein the compression deformable temperature is from about 100° C. to 135° C.
28. A method according to claim 25 , wherein the compression deformable temperature is about 125° C. to 135° C.
29. A method according to claim 25 , wherein the UHMWPE rod is crosslinked by γ-irradiation at a dose of 0.01 to 10 MRad.
30. A method according to claim 25 , wherein the UHMWPE rod is crosslinked by γ-irradiation at a dose of 1 to 6 MRad.
31. A method according to claim 25 , wherein stress relieving is carried out at a temperature below the melting point.
32. A method according to claim 25 , wherein the draw ratio d 1 /d 2 is from 1.1 to 3.0.
33. A method according to claim 32 , wherein the draw ratio is less than or equal to 2.5.
34. A method according to claim 32 , wherein the draw ratio is less than or equal to 2.0.
35. A method for making a UHMWPE implant bearing component, comprising machining the component made from a preform according to claim 34 .
36. A method for making an implant bearing component preform according to the method of claim 34 and machining the preform to make the component.
37. A method according to claim 32 , wherein the draw ratio is from 1.2 to 1.8.
38. A method for making a UHMWPE implant bearing component comprising machining the component from a preform made according to claim 25 .
39. A method for making an implant bearing component, comprising making a preform according to the method of claim 25 and machining the preform to make the component.