WO2010064272A1

WO2010064272A1 - Sterilization of medical devices using expanded organic acids

Info

Publication number: WO2010064272A1
Application number: PCT/IT2009/000545
Authority: WO
Inventors: Ernesto Reverchon; Giovanna Della Porta; Renata Adami.
Original assignee: Universita degli Studi di Salerno
Current assignee: Universita degli Studi di Salerno
Priority date: 2008-12-03
Filing date: 2009-12-03
Publication date: 2010-06-10
Anticipated expiration: 2011-06-03
Also published as: WO2010064272A8; ITSA20080036A1

Abstract

The present invention concerns a process for sterilizing biomedical devices using organic acids expanded by compressed gases to form an "expanded liquid". The term "expanded liquid" means, in the technical literature, an organic solvent that is liquid at room temperature and pressure, modified by dissolution therein of a compressed gas to modify the diffusivity and the surface tension of the liquid compound, while maintaining part of its original In the proposed process the diffusivity and the surface tension of the liquid component is modified while maintaining part of its characteristics, such as the solvent power and the oxidizing ability. The invention proposes the use of expanded organic acids as sterilizing agents for complex biomedical devices, due to their particular effectiveness, both as a result of their favourable transfer properties (similar to the properties of a gas) and for the oxidizing properties, that can vary in accordance with the composition of the gas-liquid mixture.

Description

STERILIZATION OF MEDICAL DEVICES USING EXPANDED ORGANIC

ACIDS

SPECIFICATION

The present invention concerns procedures for the sterilization of medical devices using organic acids expanded by compressed gases to form an "expanded liquid". The term "expanded liquid" means, in the technical literature, an organic compound that is liquid at room temperature and pressure, modified by dissolution therein of a compressed gas to modify the diffusivity and the surface tension of the liquid compound, while maintaining part of its original liquid characteristics.

Background of the invention

Conventionally, medical devices (endoscopes, photogastroscopes, operation instruments, costly syringes, catheters, etc.) are easily contaminated with infectious bacteria and/or viruses or with secretions of human body, and therefore they require accurate cleaning and sterilization procedures after use. However, any risk of damage to their structure has to be avoided, since they have to be reused several times. Cleaning is the pre-treatment to be carried out before any sterilization; indeed, the efficiency of disinfectants is strongly affected by the presence of blood or bodily fluids. Sterilization represents the following step, and more properly consists of the eradication or the elimination of all microbes from an object. Sterilization is conventionally performed by different well known meth- ods, such as by means of pressurized steam, ethylene oxide, oxidizing agents (e.g. peroxides or peracids), gamma irradiation or ultrafiltration.

During their use, the endoscopes and similar devices come into contact with mucous membranes and other tissues of the patient under examination or treatment; therefore, they may be contaminated with several patho- genie microorganisms including HBV, HCV/HIV, spirochetes, tubercle bacilli, Pseudomonas aeruginosa, Helicobacter pylori and many others. These microorganisms can have high levels of pathogenicity, infectivity and resistance to disinfectants. Therefore, a high level sterilization is required, due to the risk of cross-infections with known and unknown bacteria and viruses.

In this connection, endoscopes are the most difficult devices to be sterilized due to their particular contamination conditions and complex micro- structure. However, repeated thermal stresses due to the sterilization procedures can produce distortions of their delicate internal structures as, for example, their telescopic lenses; whereas, gamma radiation showed to compromise the mechanical properties of some of the polymers involved in the fabrication of these medical devices. Ethylene oxide, on the other hand, is toxic, mutagenic and carcinogenic and it can also react with some polymers. Liquid organic acids, in turn, may not reach some internal parts of the concerned medical devices, and are frequently too reactive with some parts of these delicate devices. As a consequence, the adoption of improved sterilization technologies is particularly required for such kind of medical devices, in order to improve their useful life and assure their safe operation.

In the attempt to overcome the limitations of traditional thermal treatments and of the liquid oxidizing agents, several patents have recently proposed the use supercritical carbon dioxide (SC-CO₂) as a sterilizing agent.

Among these the international patent publication WO 99/66960 (Dillow et al., assigned to MIT) provides an exhaustive discussion about the use of SC-CO₂ as an alternative to the existing technologies for the sterilization of a wide range of products for the healthcare industry, with little adverse effects on the treated materials. Particularly, the authors disclosed the inactivation of a wide range of vegetative microbial cells using a pressure range of 100-250 bar and temperatures of 40-60⁰C with stirring and pressure cycling. However, only one spore-forming bacterium was investigated and reported in the patent document (particularly B. cereus), while no data about the efficiency of the proposed process on other accepted standard bio-indicators (used to verify the efficiency of the sterilization process) was reported. The patent application publ. No. US 04/0120852 (Kanno et al., assigned to Oliff & Berridge) also discussed the effects of several pressurization and depressurization cycles of SC-CO₂ on viruses and bacteria, indicating that the sudden expansion will destroy the bacteria cells in only 30 minutes of treatment.

The use of combinations of carbon dioxide in solid, liquid and gas state with small quantities of organic acids to be used for sterilization applica- tions has been proposed in the patent application publ. No. US 07/0073081 (Fisher, assigned to Air Liquid). This document proposes combinations of solid, liquid and possibly supercritical carbon dioxide with peracetic acid in concentrations between 0.05 to 15 mole %, to be used as an anhydrous sterilizing agent in view of their ability to treat solid surfaces. The patent mainly describes different ways to obtain the proposed combinations with the object of improving the sterilizing effect of carbon dioxide.

The patent application publ. No. US 07/0003432 (Christensen et al., assigned to Welsh & Flaxman) also proposed a sterilization method suitable to to achieve 6-log reduction in Colony Forming Units (CFU) of standard bacteria and spores using fluids near to or at supercritical pressure and temperature conditions. The proposed process uses SC-CO₂ with very small percentages of some additives such as peroxides and carboxylic acids in concentration percentages of 0.1 to 2 vol %.

Further, the international patent publication WO 07/008618 (Matthews et al., assigned to the University of South Carolina) recently proposed a similar high pressure sterilization process consisting in a treatment with a mixture of carbon dioxide with non-oxidant or oxidant sterilizing agents in concentrations up to about 1000 ppm. Again, several pressurization and depressuriza- tion cycles of CO₂ were proposed. In conclusion, a large emphasis has been given by the prior art to SC-

CO₂ and SC-CO₂ modified with small amounts of additives to obtain new and improved sterilizing processes.

Summary of the invention

In the present invention, a different approach is used and a new ster- ilization process is proposed, based on the improvement of the properties of the liquid oxidizing organic acids.

Thus, the present invention proposes an innovative sterilization proc- ess that uses "expanded" acetic acid or expanded mixtures of acetic acid and peracetic acid obtained by adding "dense gases" (where a "dense gas" is defined as a fluid used in the proximity of its critical point). The "expanded acids" are for the first time used as sterilizing agents with the advantage of achieving "gas-like" transport properties and large tunability of their oxidizing action.

Particularly, the invention is related to a sterilization process performed using "expanded' organic acids (peracetic acid or acetic acid or their mixtures in all proportions) adding a compressed gas to produce a solution (expanded liquid) that will have a reduced surface tension and, large diffusivi- ties that therefore, have improved mass transfer properties with respect to the disinfection of internal surfaces of medical devices, that can be very difficult to be reached. Indeed, the expanded solutions formed by an organic acid and a compressed gas (for example, carbon dioxide at operating conditions in the range between 50-150 bar and 35-80°C) show a reduced surface tension and high diffusivities; that allow their fast penetration in microscopic spaces inside complex devices, producing a fast and complete sterilization. Operating with these solutions, it is possible to overcome one of the major limitations in the use of the liquid sterilizing agent; i.e., the difficulties in penetrating into micro- spaces due to the presence of a high surface tension and to relatively small diffutivities. Moreover, the addition of a compressed gas to generate the "expanded liquid' is also useful to modulate the oxidant activity of the acid mixtures towards the several parts of medical devices that can suffer the oxidizing action of the pure liquid oxidizing medium. Surprisingly, the addition of even low amounts of compressed gases produces "expanded liquids" with large modification of diffusivity and surface tension properties, while maintaining most of their solvent power and their oxidizing action. Therefore, these "expanded acids" can be successfully used as sterilizing agents with the further advantage of a higher tunability of their oxidizing action varying the amount of the compressed gas added. Detailed description of the invention A particular embodiment of this invention uses expanded acids formed by an organic acid and carbon dioxide in percentages from 99 to 20% by weight, preferably from 90 to 50% by weight. Carbon dioxide is the preferred compressed gas for this application because it can further improve the sterilization process due to its non-oxidative bacteria killing properties; and because its strong action in modulating the oxidizing activity of the acid solutions is related to its operation near the critical conditions where small variations of the process parameters (mainly pressure and temperature) can induce large variations of its properties. However, other gases can also be used to generate the sterilizing "expanded liquid'. A non exhaustive list comprises, ethylene peroxide, nitrogen protoxide, trifluoromethane and propane. The disclosed process consists of the following steps: a) formation of an expanded liquid solution from one or more organic acids which are in the liquid state at room temperature and pressure and an expanding compound which is gaseous at room conditions, the lat- ter compound being included in the mixture in an amount of from 99% to 20% by weight, in a high pressure mixer; b) delivery of the expanded liquid solution in a pressurized autoclave, previously filled with the medical devices to be sterilized; c) sterilizing the devices at a fixed pressure and temperature, and for a fixed processing time; d) eliminating the organic acid residues in the autoclave by washing it with the same compressed gas; e) depressurizing the solution at the exit of the autoclave to recover the organic acids in one or more separators located downstream the auto- clave.

According to a preferred embodiment thereof, the proposed process also includes a further operation f) of precipitation of the extracted residues and , optionally, a further operation g) of depressurisation of the expanded solution to recover the organic acids. The compounds selected as the expanding component are gaseous at room conditions and can be chosen from the following: carbon dioxide, nitrogen protoxide, trifluoromethane, propane; but many other compounds can also be used. Carbon dioxide is the preferred expanding compound.

The organic acids used in the proposed process are preferably chosen in the group consisting of: acetic acid, peracetic acid and their mixtures in all proportions. The formed expanded solution contains from 99 to 20% by weight of the organic acids, preferably from 90 to 50% by weight, more preferably from 90 to 70%. The operating conditions in the mixer and in the autoclave (operations a) and b)) are performed in a pressure range between 80 and 200 bar (preferably between 80-100 bar) and at a temperature range between 25 and 80⁰C (preferably between 30 and 60⁰C). The residence time in the autoclave can vary between 5 and 190 minutes, and is preferably between 10 and 60 minutes.

The advantages of the claimed process are as follows:

^■ a fast and complete sterilization of medical devices due to the ex- panded solution characteristics, i.e., very limited surface tension and high diffusivity;

■ a reduction of the sterilization times from many hours to several minutes and

^■ a low environmental impact due to the fractional separation and the complete recovery of the acids used.

Some specific embodiments of the invention are described below for merely illustrative purposes, together with the results of some experimental studies carried out on the proposed sterilization process. Examples In the process claimed, the solution is obtained in a mixer and then, delivered to the sterilizing autoclave previously charged with the contaminated medical device. The microorganisms, bacteria and/or spores, present on the device surface are contacted with the expanded liquid of acetic acid or mixtures of acetic acids and peracetic acid with dense gases (for example CO₂) for a fixed period and at a given flow rate of time. The mixture at the exit of the sterilizing autoclave (organic solvent+CO₂) is, then, delivered to the separator, where, changing the operating pressure and temperature, the de-mixing of the organic solvent from CO₂ is obtained. Both CO₂ and the organic solvents can be recycled.

The degree of deactivation can be calculated from the measure of the number of microorganisms on an untreated device and the number of micro- organism on a treated device with the following equation (UNI EN 556-1):

τ _, j

Log Re duction =

^ number of surviving microorgan isms after treatment

All efforts have been made to ensure the accuracy of numbers (e.g. amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, tempera- ture is in ⁰C, and pressure is in bar.

EXAMPLE 1 Sterilization of tweezers and sea I pets using an expanded solution of peracetic adic/acetic acid and CO₂

Pieces of small medical devices, namely tweezers and scalpets, were placed with a sample of B. Stearothermophilus spores (1 ml_) of greater than 10⁶ CFU/mL in a stainless steel basket.

The autoclave was loaded with the contaminated devices on which the spores were previously spread. A solution of peracetic acid (PA) 20% in acetic acid (AA) and CO₂ in which the liquid and the gas were mixed at weight ratios between 90 and 20% of solution with CO₂ (preferably 80% of solution by weight) in a static mixer. The solution was used in a pressure range of 80-100 bar (preferably at 90 bar) and in a temperature range of 30-50⁰C (preferably at 35°C) to produce the expanded liquid. The solution was delivered to the sterilizing autoclave with a flow rate ranging between 0.8 and 5 kg/h (preferably 1 kg/h).

The expanded solution, because of its characteristics of surface tension, density and viscosity, covered all the surfaces reaching all the inner parts of the devices, also the most difficult to be reached. The process was performed for a period time ranging between 10 and 90 minutes (usually 30 minutes). Downstream the autoclave, the solution was sent to the separator operating at pressures between 30 and 10 bar (preferably 20 bar) and at temperatures between 0 and 25°C (preferably 10⁰C). In the separator, PA and AA were precipitated and recovered by decompression, whereas, CO₂ is separated as gas. The number of CFUs on each Petri dish was counted after 24 and 48 hours incubation at 30-35⁰C. The average number of CFUs was calculated from three Petri dishes at each dilution a converted to the corresponding number of surviving spores on the devices using the corresponding dilution factor. Finally, the log reduction of B. stearothermophilus spores was calcu- lated using the following equation:

_. , . , i number of untreated microorganisms \

Log Re auction = log - -

^ number of surviving microorganisms after treatment J

Operating at process conditions of 90 bar, 35°C with a flow rate of 1 kg/h for 30 min, with a PA/AA solution mass fraction of 80% with CO₂, the complete killing of the bioindicator was achieved over multiple experimental evaluations. These reductions correspond to a log reduction in CFUs between

6.2 and 6.6.

EXAMPLE 2

Sterilization of endoscopes using an expanded solution of peracetic adic/acetic acid and CO₂

In this experiment, the bacteria 8. atropheus and 8. anthracis, which were suspended in a liquid media, were swabbed onto the surface of two endoscopes (150 mm x 20 mm) and allowed to stand for about fifteen minutes. The same procedure as in Example 1 was used. Peracetic acid (PA)

10% in acetic acid (AA) and CO₂ were mixed at weight ratios between 90 and 20% (preferably 50% of PA solution by weight) in a static mixer. The solution was used in a pressure range of 80-1 10 bar (preferably at 90 bar) and in a temperature range of 30-50⁰C (preferably at 35°C) to obtain the formation of the expanded liquid. The solution was, then, delivered to the sterilizing auto- clave with a flow rate ranging between 0.8 and 5 kg/h (preferably 0.9 kg/h).

The high diffusivity and the near zero surface tension of the expanded solution allowed to reach all the inner parts of the endoscope. The process was performed for a period time ranging between 10 and 90 minutes (usually 25 minutes). Downstream the autoclave, the solution was sent to the separator operating at pressures between 30 and 10 bar (preferably 20 bar) and at temperatures between 0 and 25⁰C (preferably 20⁰C). In the separator, PA and AA are precipitated and recovered by decompression, whereas, CO₂ is separated as gas. The number of CFUs was calculated as in the Example 1. Log reductions of 6.3 and 6.1 were measured with B. atropheus and B. anthracis spores, respectively.

EXAMPLE 3

Sterilization of intubation fiberscopes using an expanded solution of peracetic acid/acetic acid and CO₂

The Example 1 was repeated, except that a sample of B. subtilis spores/vegetative preparations (1 mL) was used to contaminate the medical device, that was a piece of 80 mm length of intubation fiberscope.

The autoclave was loaded with the contaminated devices on which the spores were previously spread. A solution of peracetic acid (PA) 25% in acetic acid (AA) was mixed with CO₂ at weight ratios between 90 and 20% of solution with CO₂. The experiment was performed at a pressure of 60-100 bar and in a temperature range of 30-50°C, to obtain the formation of the expanded liquid. The solution was delivered to the sterilizing autoclave with a flow rate ranging between 0.8 and 5 kg/h (preferably 1 kg/h).

The process was performed for period of time of 30-40 minutes. Downstream the autoclave, the solution was sent to the separator operating at pressures between 30 and 10 bar (preferably 15 bar) and at temperatures between 0 and 25°C (preferably 18°C). In the separator, PA and AA are pre- cipitated and recovered by decompression, whereas, CO₂ is separated as gas.

The number of CFUs was calculated as in the Example 1. Log reduc- tion in CFUs of 6.3-6.5 were observed for multiple experimental evaluations.

The present invention has been disclosed with particular reference to some specific embodiments thereof, but it should be understood that modifications and changes may be made by the persons skilled in the art without de- parting from the scope of the invention as defined in the appended claims.

Claims

1. A process that uses expanded liquids formed by organic acids and dense gases to sterilize medical devices, which process comprises the follow- ing operations: a) formation of an expanded liquid solution from one or more organic acids which are in the liquid state at room temperature and pressure and an expanding compound which is gaseous at room conditions, the latter compound being included in the mixture in an amount of from 99% to 20% by weight, in a high pressure mixer; b) delivery of the expanded liquid solution in a pressurized autoclave, previously filled with the medical devices to be sterilized; c) sterilizing the devices at a fixed pressure and temperature, and for a fixed processing time; d) eliminating the organic acid residues in the autoclave by washing it with the same compressed gas; e) depressurizing the solution at the exit of the autoclave to recover the organic acids in one or more separators located downstream the autoclave.

2. A process according to claim 1 , comprising a further operation f) of precipitation of the extracted residues.

3. A process according to claims 1 or 2, comprising a further operation g) of depressurisation of the expanded solution to recover the organic acids.

4. A process according to any one of claims 1-3, wherein the said expanding compounds are chosen from the group consisting of: carbon dioxide, ethylene peroxide, nitrogen protoxide, trifluoromethane, propane.

5. A process according to any one of claims 1-4, wherein said one or more organic acids are chosen in the group consisting of: acetic acid, peracet- ic acid, and their mixtures in all proportions.

6. A process according to any one of claims 1-5, wherein the expanded liquids are obtained by mixing the said one or more organic acids with the said expanding compound in an amount of from 90 to 50% by weight of the expanding compound.

7. A process according to claim 6, wherein the said amount of the expanding compounds is from 90% to 70% by weight.

8. A process according to claim 7, wherein the said amount of the expanding compounds is from 85% to 75% by weight.

9. A process according any one of claims 1-8, wherein in the operations a) and b) the expanded liquid solution operates in a pressure range between 50 and 350 bar.

10. A process according to any one of claims from 1-9, wherein in the operations a) and b) the expanded liquid solution operates in a temperature range of from 20⁰C to 80⁰C.

11. A process according to any one of claims from 1-10, wherein the processing time ranges from 5 to 120 minutes.