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 synthesis, fractionation, 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 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 dense CO2 for modifying UHMWPE to sterilize it and to impregnate hydroxyapatite (HAP) nano-particles. Dense CO2 has been shown to work as a sterilization agent to kill bacteria and fungus. It will be shown quantitatively how well UHMWPE seeded with living bacteria can be sterilized using dense CO2 along with small amounts of ethanol and hydrogen peroxide. The variables of interest are CO2 pressure and temperature, and the amount of ethanol and hydrogen peroxide added to the system. The bacteria and fungus were captured off of contaminated ginseng and were placed on the UHMWPE. A high pressure syringe pump (ISCO 500D) is 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 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 could play two roles simultaneously, with the first being sterilization, and the second being impregnation of HAP. Impregnated particles of hydroxyapatite (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.