M. Tabrizian

Low-temperature sterilization using gas plasmas: a review of the experiments and an analysis of the inactivation mechanisms

Utilizing an ionized gas (plasma) to achieve sterilization is an alternative to conventional sterilization means as far as sterilization of heat-sensitive materials and innocuity of sterilizing agents are concerned. The literature on plasma sterilization is reviewed. A major issue of plasma sterilization is the respective roles of UV photons and reactive species such as atomic and radicals. Insight into this matter is obtained by analyzing the survival curves of microorganisms. In contrast to classical sterilization where such plots show a unique straight line, plasma sterilization yields survival diagrams with two or three different linear segments. Three basic mechanisms are involved in the plasma inactivation of microorganisms: (A) direct destruction by UV irradiation of the genetic material of microorganisms; (B) erosion of the microorganisms atom by atom, through intrinsic photodesorption by UV irradiation to form volatile compounds combining atoms intrinsic to the microorganisms; (C) erosion of the microorganisms, atom by atom, through etching to form volatile compounds as a result of slow combustion using oxygen atoms or radicals emanating from the plasma. In some cases, etching is further activated by UV photons, increasing the elimination rate of microorganisms. These mechanisms make plasma sterilization totally different from classical sterilization techniques and suggest its use to inactivate nonconventional infectious agents such as the abnormal prions.

Using the flowing afterglow of a plasma to inactivate Bacillus subtilis spores: Influence of the operating conditions

The flowing afterglow of a microwave discharge can be used to efficiently inactivate bacterial spores. We have conducted a parametric study of the operating conditions of such a system, which shows that the species participating in the killing of spores are oxygen atoms and ultraviolet (UV) photons. The oxygen atoms and the excited atoms and molecules emitting the photons being carried by the flowing afterglow can be made available throughout the sterilization chamber. Typical operating conditions are: gas mixture 2%O2/98%N2, pressure range 1–7 Torr and gas flow 0.5–3 slm. Total inactivation of 106 B. subtilis spores is achieved within 40 min with 100 W absorbed microwave power, at afterglow gas temperatures not exceeding 50 °C, a feature of interest for heat sensitive medical devices. The present scheme depends on the gas flow reaching all parts of the objects to be sterilized and on the short-lived active species being transported there sufficiently rapid. Under our operating conditions, it is the UV em...

Effect of surface treatment and sterilization processes on the corrosion behavior of NiTi shape memory alloy

Nickel-titanium (NiTi) alloy derives its biocompatibility and good corrosion resistance from a homogeneous oxide layer mainly composed of TiO(2), with a very low concentration of nickel. In this article, we described the corrosion behavior of NiTi alloys after mechanical polishing, electropolishing, and sterilization processes using cyclic polarization and atomic absorption. As a preparative surface treatment, electropolishing decreased the amount of nickel on the surface and remarkably improved the corrosion behavior of the alloy by increasing the mean breakdown potential value and the reproducibility of the results (0.99 +/- 0.05 V/SCE vs. 0.53 +/- 0. 42). Ethylene oxide and Sterrad(R) sterilization techniques did not modify the corrosion resistance of electropolished NiTi, whereas a steam autoclave and, to a lesser extent, peracetic acid sterilization produced scattered breakdown potential. In comparing the corrosion resistance of common biomaterials, NiTi ranked between 316L stainless steel and Ti6A14V even after sterilization. Electropolished NiTi and 316L stainless-steel alloys released similar amounts of nickel after a few days of immersion in Hanks solution. Measurements by atomic absorption have shown that the amount of released nickel from passive dissolution was below the expected toxic level in the human body. Auger electron spectroscopy analyses indicated surface contamination by Ca and P on NiTi during immersion, but no significant modification in oxide thickness was observed.

Effects of sterilization processes on NiTi alloy: Surface characterization

Sterilization is required for using any device in contact with the human body. Numerous authors have studied device properties after sterilization and reported on bulk and surface modifications of many materials after processing. These surface modifications may in turn influence device biocompatibility. Still, data are missing on the effect of sterilization procedures on new biomaterials such as nickel-titanium (NiTi). Herein we report on the effect of dry heat, steam autoclaving, ethylene oxide, peracetic acid, and plasma-based sterilization techniques on the surface properties of NiTi. After processing electropolished NiTi disks with these techniques, surface analyses were performed by Auger electron spectroscopy (AES), atomic force microscopy (AFM), and contact angle measurements. AES analyses revealed a higher Ni concentration (6-7 vs. 1%) and a slightly thicker oxide layer on the surface for heat and ethylene oxide processed materials. Studies of surface topography by AFM showed up to a threefold increase of the surface roughness when disks were dry heat sterilized. An increase of the surface energy of up to 100% was calculated for plasma treated surfaces. Our results point out that some surface modifications are induced by sterilization procedures. Further work is required to assess the effect of these modifications on biocompatibility, and to determine the most appropriate methods to sterilize NiTi.

Study of biodegradation behavior of chitosan-xanthan microspheres in simulated physiological media.

Microspheres of a polyelectrolyte complex hydrogel were prepared from chitosan and xanthan after interaction between the two polyionic polymers. Their biodegradation was studied vs. chitosan. Simulated gastric fluid (SGF, pH 1.2) and intestinal fluid (SIF, pH 7.5) both as biodegradation media and phosphate buffered saline (PBS, pH 7.4) as a negative control were used. The degradation studies were performed at 37 degrees C at 240 rpm permanent stirring to mimic the physiologic conditions. High performance liquid chromatography (HPLC) was carried out to quantify the chitosan degradation products using glucosamine (GA) and N-acetyl-D-glucosamine (N-Ac-GA) as references. The peaks area integration method was used to determine the amount of each degradation product as a function of incubation time in the media. The effect of the media on the morphological structure of microspheres was assessed by scanning electron microscopy. From HPLC studies, it appeared that in SGF and SIF the major degradation products were glucosamine (GA) and N-acetyl-D-glucosamine (NAc-GA). In the first 15 days, oligochitosan fractions were released from the complex, whereas N-acetyl-D-glucosamine was detected in the media after this period. The degradation kinetics were assessed by the measurement of the cumulative degradation products, which showed faster degradation of chitosan than the complex in SGF and SIF. SEM micrographs showed an enhancement of microsphere porosity as a function of incubation time in the simulated physiological media. Our results suggest a better control of the degradation kinetics when chitosan is complexed to xanthan.

Excimer laser treatment of NiTi shape memory alloy biomaterials

Abstract NiTi Shape Memory Alloys (SMA) are potential biomaterial candidates. However, due to its possible corrosion in physiological solution, dissolution of toxic Ni might be happening, rendering this material nonbiocompatible. We have used excimer laser surface treatment to improve corrosion resistance of NiTi SMA plates. Potentiodynamic tests in physiological Hanks solution show that the laser treatment performed in air improved all corrosion parameters. The surface is homogenized and a Scanning Electron Microscopy (SEM) observation indicates a decrease of corrosion pit size and numbers. Laser treatment improvement resistance is explained by a combination of the homogenization of the surface by melting, the hardening due to N incorporation and the thickening of the oxide layer.

Plasma-based sterilization: effect on surface and bulk properties, and hydrolytic stability of reprocessed polyurethane electrophysiology catheters

Plasma-based sterilization is a promising alternative to ethylene oxide (EO) for reprocessing of electrophysiology catheters. To assess its safety in terms of material damage, modifications of surface and bulk properties as well as hydrolytic stability of sterilized catheters were evaluated. Polyurethane (PU) single-use electrophysiology catheters were subjected to one, five, and ten sterilization cycles by Sterrad-100S and Plazlyte, as well as by pure EO for comparison. Surface analysis techniques (ATR-FTIR, XPS, DCA) showed oxidation limited to the near-surface layer induced by both plasma-based sterilizers, whereas EO induced slight but deeper alkylation. Using bulk analysis techniques (RP-HPLC, SEC), oligomer alteration was observed after all three sterilization techniques, without modification of molecular weights. Hydrolytic stability of catheters was slightly changed by plasma-based sterilization, with a small increase in released oligomers. Finally, although Plazlyte and Sterrad are both plasma-based techniques, they induced different impacts on catheters, such as the degradation of an additive with Sterrad, and a clear difference in coloration with Plazlyte.

Improvement of the Corrosion Resistance of NiTi Stents by Surface Treatments

Because of its optimal radiopacity, superelasticity and shape memory properties Nickel-Titanium (NiTi) is an ideal material for the fabrication of stents. Indeed, these properties can facilitate the implantation and precise positioning of those devices. However, in vitro studies on NiTi report the dependency of the alloy biocompatibility and corrosion behavior to surface treatments. Oxidation of the surface seems to be very promising to improve both the corrosion resistance and the biocompatibility of NiTi. The present study investigate the effect of electropolishing, heat treatment (in air and in a salt bath) and nitric acid passivation to modify the oxide layer on NiTi stents. Techniques such as potentiodynamic polarization tests, Scanning Electron Microscopy (SEM) and Auger Electron Spectroscopy (AES) have been used to develop relationships between corrosion behavior, surface characteristics and surface treatment. Results show that all surface treatments improve the corrosion behavior of the alloy. SEM results indicate that treated stents which exhibit a smooth and uniform surface show a higher corrosion resistance than non treated stents which possess a very porous oxide layer. AES results, indicate that the best corrosion behavior was observed for the stents which exhibit the thinnest oxide layer (electropolished and passivated samples).

Surface characteristics of sterilized electropolished NiTi shape memory alloy as biomaterials

As a potential biomaterial for many medical applications, NiTi alloy derives its good biocompatibility and corrosion resistance from a homogeneous and protective oxide layer, mainly composed of TiO2, with little concentration of nickel. However, during corrosion testing at high potential, NiTi is susceptible to pitting corrosion, which may affect the amount of ions (nickel and titanium) released by the alloy and thus, may affect its biocompatibility. As a passivating treatment, electropolishing (EP) was demonstrated to decrease the amount of nickel on the surface and to remarkably improve the corrosion behavior of the alloy. After sterilization by ethylene oxide (EO), no modification of the promising corrosion behavior of electropolished NiTi were observed, although some surface modifications were reported. The corrosion resistance of ethylene oxide sterilized and electropolished samples ranked between that of the commonly used Ti6A14V and 316L (0.4 less than 1 less than 1.4 mV/SCE) implant alloys.

Plasma sterilization: spore destruction by microwave plasmas

Low-pressure plasmas are now being used for sterilization at ambient temperature. In this work, we have studied the possible mechanism of spore destruction by plasma, and compare it with etching of synthetic polymers. Bacillus subtilis spores were inoculated at the bottom of special glass vials and subjected to different plasma gas compositions, all known to etch polymers. The mortality rate of spores was found to vary with gas composition, between less than a 2 log decrease in 8 minutes for a pure oxygen plasma, to a 6 log decrease with 0 2 CF 4 mixture. Examination by scanning electron microscopy (SEM) showed that spores were significantly etched after 30 minutes of plasma exposure, but not completely. We speculate about their etch resistance, compared with that of synthetic polymers, on the basis of their morphology and their complex coating structure.