(97a) CO2 Assisted Sterilization and Modification of Uhmwpe for Artificial Human Joints

Every year, over 1.4 million human joint implants made of Ultra-High Molecular Weight Polyethylene (UHMWPE) components are used in patients suffering from injury or disease making it the most widely accepted implant material. UHMWPE is a unique polymer with desirable physical and mechanical properties such as a high tensile and impact strength and resistance to corrosion and abrasion. Despite the success and worldwide acceptance of total joint arthroplasty and restorative procedures, wear and concomitant debris generation is still a major obstacle limiting the longevity of implanted UHMWPE components. Once significant wear has occurred, particulate wear debris can be released inside the joint capsule and this debris can activate macrophages. This often leads to inflammation of the surrounding tissues, and subsequently to necrosis and failure of artificial joints.

There is evidence that conventional sterilization techniques such as γ-radiation, decrease

UHMWPE longevity. The ability to prevent wear and oxidation of UHMWPE components and enhance the biological fixation between a prosthesis and hard tissue will radically decrease health care costs, minimize hospitalization time by eliminating the need for reoperation due to implant failures, relieve suffering and prolong life.

Supercritical fluids occur at pressures and temperatures higher than their critical point. Supercritical technologies avoid the use of toxic chemicals and high processing temperatures, a priority for biomedical polymer processing. Supercritical technology has been used for synthesis, fractionation, impregnation, and fabrication of polymeric scaffolds as well as sterilization of biocompatible polymers. Supercritical CO₂ can swell and plasticize polymers, reducing the glass transition temperature, and as a consequence increase the free volume of the polymer. These effects are crucial to the impregnation and modification of polymeric materials.

A variety of coating techniques including plasma spraying, sol- gel, ion sputtering, laser

ablation, electrophoretic deposition, hydrothermal and biomimetic methods are used to fabricate bioactive coatings on a substrate. Plasma spray coating is the preferred technique, however, this method can be destructive for coating the polymer matrix due to its lower thermal resistance.

In this paper we will describe using supercritical CO₂ for modifying UHMWPE to sterilize it and to immobilize hydroxyapatite (HAP) nano-particles. Supercritical CO₂ has been shown to work as a sterilization agent to kill bacteria and fungus. It will be shown how well UHMWPE seeded with living bacteria can be sterilized using CO₂ along with small amounts of ethanol, water and hydrogen peroxide. The variables of interest are the amount of ethanol, water and hydrogen peroxide added to the system which contains CO₂ at 170 bar and 37^oC. The bacteria and fungus were captured off of contaminated ginseng and were placed on the UHMWPE. A high pressure syringe pump (ISCO 500D) was then connected to the pressure vessel, containing the contaminated UHMWPE, and it is pressurized with CO₂. Sterilization takes place due to CO₂ diffusing into the bacteria or fungi and reacting with the entrapped water to form carbonic acid, thus causing a decrease in the pH and resulting in death.

The CO₂ could play two roles simultaneously, with the first being sterilization, and the second being impregnation of HAP. Impregnated particles of HAP increase the artificial joint's bond strength when place next to bone in vivo. Also studied in this paper is how much the mechanical strength of the UHMWPE is compromised by this high pressure processing technique. The results of a tensile strength test will be shown for samples both unmodified and modified by this technique to conclude that the mechanical strength and durability of an artificial joint processed with dense CO₂ is not compromised.