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.

Dense gases (DGs) are fluids with their pressure and temperature close to the critical point. DG technologies avoid the use of toxic chemicals and high processing temperatures, a priority for biomedical polymer processing. DG technology has been used for synt hesis, fractionation, and impregnation, and fabrication of polymeric scaffolds as well as sterilization of biocompatible polymers. Dense gas 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, modification of polymeric materials and generation of porosity.

Porosity can be introduced into polymer matrices using various techniques such as electrospinning, solvent casting/salt leaching, phase inversion, laser excimer and thermally induced phase separation. Disadvantages of these techniques include use of toxic solvents, removal of solvent by evaporation (days-to-weeks), presence of monomer residues, labor intensive processing, thin structures and irregular shaped pores. In this project we will use dense gas techniques to fabricate micro sized pores on the surface of UHMWPE to minimize or eliminate the drawbacks of the above methods.

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 is not suitable for coating the polymer matrix due to its lower thermal resistance.

In this paper we will describe using dense CO₂ for modifying UHMWPE to make it sterile and increase its surface porosity. Dense CO₂ has been shown to work as a sterilization agent to kill bacteria. It will be shown quantitatively how well UHMWPE seeded with living bacteria can be sterilized using dense CO₂. The variables of interest are CO₂ pressure, temperature, time of incubation, and number of times pressure is cycled. The first part of this process is culturing the bacteria (e.g. bacillus cereus, legionella dunnifii). Next, the contaminated UHMWPE is then placed in a high pressure vessel and the bacteria are incubated at biological temperature for a period of time, to allow them to grow to an appreciable amount. A high pressure syringe pump (ISCO 500D) is then connected to the pressure vessel and it is pressurized with CO₂. Sterilization takes place due to CO₂ diffusing into the cell and reacting with the entrapped water to form carbonic acid, thus causing a decrease in the pH and resulting in cell death. Optimization of the sterilization process will be discussed, with large scale mass production in mind. So cycle times will be kept as short as possible and temperatures will be kept close to ambient. Characterization of the resulting sterilization technique will quantitatively show the fraction of bacteria sterilized by this technique.

The CO₂ plays two roles simultaneously, with the first being sterilization, and the second is creating surface porosity. This extra porosity will later be used to house particles of hydroxyapatite (HAP), thus increasing 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 three-point bend test, tensile strength test, and impact 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.