Sterilization of bacterial spores by using supercritical carbon dioxide and hydrogen peroxide.

It was hypothesized that supercritical carbon dioxide (SC-CO(2)) treatment could serve as an alternative sterilization method at various temperatures (40-105 degrees C), CO(2) pressures (200-680 atm), and treatment times (25 min to 6 h), and with or without the use of a passive additive (distilled water, dH(2)O) or an active additive (hydrogen peroxide, H(2)O(2)). While previous researchers have shown that SC-CO(2) possesses antimicrobial properties, sterilization effectiveness has not been shown at sufficiently low treatment temperatures and cycle times, using resistant bacterial spores. Experiments were conducted using Geobacillus stearothermophilus and Bacillus atrophaeus spores. Spore strips were exposed to SC-CO(2) in commercially available supercritical fluid extraction and reaction systems, at varying temperatures, pressures, treatment times, and with or without the use of a passive additive, such as dH(2)O, or an active additive, such as H(2)O(2). Treatment parameters were varied from 40 to 105 degrees C, 200-680 atm, and from 25 min to 6 h. At 105 degrees C without H(2)O(2), both spore types were completely deactivated at 300 atm in 25 min, a shorter treatment cycle than is obtained with methods in use today. On the other hand, with added H(2)O(2) (<100 ppm), 6 log populations of both spore types were completely deactivated using SC-CO(2) in 1 h at 40 degrees C. It was concluded from the data that large populations of resistant bacterial spores can be deactivated with SC-CO(2) with added H(2)O(2)at lower temperatures and potentially shorter treatment cycles than in most sterilization methods in use today.

Sterilizing Bacillus pumilus spores using supercritical carbon dioxide.

Supercritical carbon dioxide (SC CO(2)) has been evaluated as a new sterilization technology. Results are presented on killing of B. pumilus spores using SC CO(2) containing trace levels of additives. Complete killing was achieved with 200 part per million (ppm) hydrogen peroxide in SC CO(2) at 60 degrees C, 27.5 MPa. Addition of water to SC CO(2) resulted in greater than three-log killing, but this is insufficient to claim sterilization. Neither ethanol nor isopropanol when added to SC CO(2) affected killing.

Experimental and numerical modeling of variable friction between nanoregions in coventional and crosslinked UHMWPE.

Recently, highly crosslinked UHMWPE components have been promoted for their high abrasive wear resistance over conventional UHMWPE (PE) in total joint replacement (TJR) prostheses to minimize osteolysis and consequent implant loosening. This study was aimed at investigating the role of friction gradients induced by localized coefficients of friction at both crystalline and amorphous nanoregions in PE, and crystalline and crosslinked nanoregions in crosslinked UHMWPE (XPE), in submicron wear debris generation. An abrasive wear study performed on both XPE and PE using atomic force microscopy (AFM) illustrated that the onset of plastic deformation for XPE occurred at a normal load that was approximately 3 times higher when compared to PE. Coefficients of friction (mu d) of 0.2, 0.35, and 0.61, experimentally derived using AFM, were used as representative mu d for crystalline, amorphous, and crosslinked nanoregions, respectively, in a numerical Hertzian model. An increase in mu (0.2 +/- 0.02, 0.35 +/- 0.01 and 0.6 +/- 0.04) was observed with a decrease in crystallinity and storage modulus at 22 degrees C. Using the Hertzian contact model, it was observed that variability in friction between nanoregions contributed to higher magnitude stresses for XPE (0.2 to 0.61; maximum sigma eff = 2.8) compared to PE (0.2 to 0.35; maximum sigma eff = 1.1) over a negligible thickness of the interfacial zone (IZ) between nanoregions. The experimentally observed increase in abrasive wear resistance of XPE could be attributed to an increase in the thickness of the interfacial zone between nanoregions with mu changing gradually from crystalline to crosslinked nanoregions, a situation that may not be observed with PE. This would cause a decrease in the friction gradient and resulting stresses thereby agreeing with the observed experimental higher abrasive wear resistance for XPE. However, in both PE and XPE, the presence of stress concentrations over a period of time could lead to irreversible damage of the material eventually generating submicron wear debris. Hence, semicrystalline, inhomogenous UHMWPE with several nanoregions (amorphous and crystalline) would be at a disadvantage for bearing application in terms of abrasive wear resistance compared to UHMWPE with relatively lower number of nanoregions and crosslinked nanoregions.

Utilization of moiré interferometry to study the strain distribution within multi-layer thermoplastic elastomers.

The use of soft elastomeric cushion form bearings as an alternative material to ultra-high molecular weight polyethylene (UHMWPE) has been proposed in the literature as providing enhanced lubrication and lower friction. However, the abrupt change in stiffness between the bearing's soft contact layer and its rigid support substrate results in high shear stresses and leads to the debonding of the soft layer from the substrate. The use of functionally modulus-graded material has been proposed as a solution to this problem. This paper investigates the use of moiré interferometry to study the strain distribution within and across the interfaces of multi-layer elastomeric samples, which were fabricated as models for functionally modulus-graded materials. While this technique has been widely used to study the strain distribution in rigid materials and composites, this paper represents the first report of its application to low-modulus polymers at temperatures where they exhibit significant viscoelastic behavior. The results presented clearly demonstrate that the moiré interferometry technique can be successfully applied in the field of low-modulus elastomeric materials. The analysis of the moiré patterns suggests that the soft elastomeric material under the contact point was subjected to a compressive epsilonx, and was pushed sideways. The analysis also showed that the maximum shear strain occurred where the deformation was constrained, which could possibly lead to a local fatigue failure in the sample.