Antioxidant stabilized crosslinked ultra-high molecular weight polyethylene for medical device applications

Abstract

An antioxidant combined with UHMWPE prior to subjecting the UHMWPE to crosslinking irradiation. In one exemplary embodiment, the antioxidant is tocopherol. After the antioxidant is combined with the UHMWPE, the resulting blend may be formed into slabs, bar stock, and/or incorporated into a substrate, such as a metal, for example. The resulting product may then be subjected to crosslinking irradiation. In one exemplary embodiment, the UHMWPE blend is preheated prior to subjecting the same to crosslinking irradiation. Once irradiated, the UHMWPE blended product may be machined, packaged, and sterilized in accordance with conventional techniques.

1 . A method for processing UHMWPE for use in medical applications, the method comprising the steps of:
 combining UHMWPE with an antioxidant to form a blend having 0.01 to 3.0 weight percent of the antioxidant;   processing the blend to consolidate, the consolidated blend having a melting point;   preheating the consolidated blend to a preheat temperature below the melting point of the consolidated blend; and   irradiating the consolidated blend while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.   
   
   
       2 . The method of  claim 1 , wherein said combining step further comprises combining UHMWPE with an antioxidant to form a substantially homogenous UHMWPE blend having 0.01 to 3.0 weight percent of the antioxidant. 
   
   
       3 . The method of  claim 1 , wherein said combining step further comprises mixing the UHMWPE powder with tocopherol by at least one of solvent blending, high shear mixing, precision coating, fluidized bed, atomization, emulsion polymerization, electrostatic precipitation, wetting or coating of particles, and master batch blending. 
   
   
       4 . The method of  claim 1 , wherein said combining step further comprises mixing the UHMWPE powder with tocopherol by high shear mixing. 
   
   
       5 . The method of  claim 1 , wherein said combining step further comprises combining the UHMWPE with an antioxidant selected from the group consisting of tocopherol, Vitamin C, lycopene, honey, phenolic antioxidants, amine antioxidants, hydroquinone, beta-carotene, ascorbic acid, CoQ-enzyme, and derivatives thereof. 
   
   
       6 . The method of  claim 1 , wherein said combining step further comprises combining the UHMWPE with d,1-α-tocopherol. 
   
   
       7 . The method of  claim 1 , wherein said processing step further comprises processing the blend to consolidate the blend by at least one of compression molding, net shape molding, injection molding, extrusion, monoblock formation, fiber, melt spinning, blow molding, solution spinning, hot isostatic pressing, high pressure crystallization, and films. 
   
   
       8 . The method of  claim 1 , wherein said processing step further comprises processing the blend to consolidate by compression molding the blend to extend at least partially into a porous substrate. 
   
   
       9 . The method of  claim 8 , wherein the substrate comprises a highly porous biomaterial useful as at least one of a bone substitute, a cell receptive material, a tissue receptive material, an osteoconductive material, and an osteoinductive material. 
   
   
       10 . The method of  claim 8 , wherein the substrate comprises a highly porous biomaterial useful as at least one of a bone substitute, a cell receptive material, and a tissue receptive material having a porosity of at least 55 percent. 
   
   
       11 . The method of  claim 1 , wherein said processing step further comprises processing the blend to consolidate by compression molding the blend into a roughened surface on the substrate having macroscopic mechanically interlocking features. 
   
   
       12 . The method of  claim 1 , wherein the melting point of the consolidated blend is determined by differential scanning calorimetry. 
   
   
       13 . The method of  claim 1 , wherein said preheating step further comprises preheating the consolidated blend to a preheat temperature substantially between 70 degrees Celsius and 130 degrees Celsius. 
   
   
       14 . The method of  claim 1 , further comprising the additional step of machining the consolidated blend to form a medical product. 
   
   
       15 . The method of  claim 1 , further comprising the additional steps of:
 identifying a total crosslinking irradiation dose to be received by the consolidated blend;   identifying a maximum individual irradiation dose to be received by the consolidated blend while maintaining the consolidated blend below the melting point of the consolidated blend; and   wherein the step of irradiating the consolidated blend further comprises irradiating the consolidated blend with a first irradiation dose that is one of equal to and less than the maximum individual irradiation dose.   
   
   
       16 . The method of  claim 15 , further comprising the additional step of irradiating the consolidated blend with at least one subsequent irradiation dose that is one of equal to and less than the maximum individual irradiation dose. 
   
   
       17 . The method of  claim 15 , wherein said step of irradiating the consolidated blend further comprises irradiating the consolidated blend with at least one of electron beam irradiation, gamma irradiation, and x-ray irradiation. 
   
   
       18 . The method of  claim 15 , wherein the total crosslinking irradiation dose is substantially between 50 kGy and 1000 kGy. 
   
   
       19 . The method of  claim 15 , wherein the total crosslinking irradiation dose is substantially between 100 kGy and 250 kGy. 
   
   
       20 . The method of  claim 15 , further comprising, after said first irradiating step, the additional steps of:
 equilibrating the temperature of the consolidated blend to the preheat temperature; and   irradiating the consolidated blend with a second irradiation dose that is one of equal to and less than the lesser of the difference between the total crosslinking irradiation dose and the first irradiation dose and the maximum individual irradiation dose for the consolidated blend.   
   
   
       21 . The method of  claim 1 , further comprising the additional step of sterilizing the UHMWPE blend by at least one of gas plasma sterilization, ethylene oxide sterilization, gamma sterilization, ionizing irradiation sterilization, autocalving, and supercritical fluid techniques. 
   
   
       22 . The method of  claim 1 , further comprising, after said irradiating step, the additional step of heat treating the consolidated blend. 
   
   
       23 . The method of  claim 1 , further comprising, before said irradiating step, the additional step of shielding at least a portion of the consolidated blend. 
   
   
       24 . A crosslinked UHMWPE blend for use in medical implants prepared by a process comprising the steps of:
 combining UHMWPE with an antioxidant to form a blend having 0.1 to 3.0 weight percent antioxidant;   processing the blend to consolidate the blend, the consolidated blend having a melting point;   preheating the consolidated blend to a preheat temperature below the melting point of the consolidated blend; and   irradiating the consolidated blend with a total irradiation dose of at least 100 kGy while maintaining the consolidated blend at a temperature below the melting point of the consolidated blend.   
   
   
       25 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an ultimate tensile elongation of at least 250 percent. 
   
   
       26 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has a tensile yield strength of at least 21 mega pascals. 
   
   
       27 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an ultimate tensile strength of at least 45 mega pascals. 
   
   
       28 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has a pin-on-disc wear rate of less than 2.75 mg/Mc. 
   
   
       29 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an izod impact strength of at least 53 kJ/m . 
   
   
       30 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend is molded into at least one of a substrate, a polymer, and a antioxidant stabilized polymer. 
   
   
       31 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index remaining below 0.1 after accelerated aging for two weeks in an oxygen atmosphere at a pressure of 72 pounds per square inch and a temperature of 70 degrees Celsius. 
   
   
       32 . The crosslinked UHMWPE blend of  claim 24 , wherein the antioxidant leached from 2 grams of the crosslinked UHMWPE in 100 milliliters of 37 degree Celsius water after 53 days results in an extraction solution absorbance at 297 nanometers that is no greater than 0.01 units from the reference water absorbance. 
   
   
       33 . The crosslinked UHMWPE blend of  claim 24 , wherein the antioxidant leached from 2 grams of the crosslinked UHMWPE in 100 milliliters of 70 degree Celsius water after 53 days results in an extraction solution absorbance at 297 nanometers that is no greater than 0.01 units from the reference water absorbance. 
   
   
       34 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index being less than 0.1 percent and the crosslinked UHMWPE blend has a free radical level measured by electron spin resonance that is less than 1×10 17  spins per gram. 
   
   
       35 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has an oxidation index, the oxidation index being less than 0.1 percent, and wherein the crosslinked UHMWPE exhibits a crosslink density of more than 0.200 moles/dm 3 . 
   
   
       36 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant and exhibits a pin-on-disc wear rate of less than 2.0 mg/Mc. 
   
   
       37 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE exhibits a crosslink density of more than 0.200 moles/dm 3 . 
   
   
       38 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant and exhibits a crosslink density of more than 0.190 moles/dm 3 . 
   
   
       39 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has at least 0.1 weight percent antioxidant, exhibits a crosslink density of more than 0.200 moles/dm 3 , and has a molecular weight between crosslinks of less than 11,200 daltons. 
   
   
       40 . The crosslinked UHMWPE blend of  claim 24 , wherein the UHMWPE blend has an L* value, an a* value, and a b* value as measured by a colormeter, wherein the L* value is greater than 90, the a* value is greater than negative 6, and the b* value is greater than 17. 
   
   
       41 . The crosslinked UHMWPE blend of  claim 24 , wherein the process further comprises the additional step of molding the consolidated blend into at least one of a porous substrate, a polymer, and a polymer containing an antioxidant. 
   
   
       42 . The crosslinked UHMWPE blend of  claim 24 , wherein the crosslinked UHMWPE blend has a free radical level measured by electron spin resonance that is less than 1×10 17  spins per gram.