The sterilisation of articles is of great concern today, in order to reduce the spread of disease throughout the world. One environment where sterilisation is of major importance is the medical environment, in order to reduce the risk of infection transmission between patients and/or staff.
The supply of sterile equipment in hospitals is usually under the responsibility of the Central Sterile Supply Department (CSSD). This department receives the medical and surgical instruments and supplies for cleaning and sterilisation. Typically the process involves automated washers followed by autoclaving at a temperature of 121° C. or above. It will be appreciated however that due to the heat present in an autoclave, some articles are not suitable for the autoclave procedure, such as for example endoscopes.
An alternative to the autoclave is a hydrogen peroxide vapour and plasma sterilizer system, such as the one developed by Johnson & Johnson Medical Inc., and described in U.S. Pat. No. 5,876,666. Other systems using hydrogen peroxide vapour as a precursor to plasma treatment for sterilisation purposes are described in U.S. Pat. No. 4,643,876 and No. 4,756,882, both of which are patents in the name of Surgikos Inc.
U.S. Pat. No. 5,084,239 in the name of Abtox Inc. describes a process of, first, exposing a medical instrument to a antimicrobial agent such as hydrogen peroxide and/or peracids and, second, exposing the instrument to a low pressure plasma discharge with gases such as argon, oxygen, helium, nitrogen, hydrogen or a mixture of same.
The Johnson & Johnson system described in U.S. Pat. No. 5,876,666 has been commercialized by ASP (Advanced Sterilization Products) as the STERRAD system [Validation of a low-temperature, low pressure, vaporized aqueous hydrogen peroxide-based, plasma sterilisation system—STERRAD 100S sterilizer, EFHSS Conference 2004, Izmir/Cesme, Turkey]. This system operates at pressures below one atmosphere. It operates by exposing medical instruments placed in a vacuum chamber to hydrogen peroxide vapour and then it runs low temperature plasma. The vapour exposure and the plasma process are run in sequence taking approximately 30 minutes for each step. This hydrogen peroxide—plasma sequence is typically run 2 to 3 times per load. The sterilisation cycle is then validated by cultivating a series of biological indicators for 24 to 72 hours, with the biological indicators containing spores of Geobacillus stearothermophilus ATCC 7953 (American Type Culture Collection). Multiple biological indicators are processed during the sterilisation of the load and are cultivated together with unprocessed indicators after the sterilisation cycle. It will be appreciated that processed indicators should not develop a bacterial population in order to validate the sterilisation cycle.
Another plasma sterilizer apparatus is described in International patent publication NO. WO 03/090796 by Human Meditek Co. Ltd. Similar to the STERRAD system, this system also uses hydrogen peroxide vapour, but is distinguished from the STERRAD process by the fact that the plasma discharge is generated remotely. This allows the plasma by-product radicals to fill the treatment chamber, while at the same time not exposing the medical instruments directly to the plasma.
Another apparatus with a remote plasma chamber connected to a sterilizing chamber is described in Abtox Inc.'s U.S. Pat. No. 5,413,758. Again, the medical instruments in this apparatus are exposed to the plasma by product radicals. International patent publication No. WO 2009/078361 of Saga University and University of the Ryukyus meanwhile describes an oxygen sterilizer, with an upstream ICP plasma source generating a high density of radicals downstream for sterilisation.
Another low pressure sterilizer, STERIZONE®, was developed by TSO3 Inc., and is described in their U.S. Pat. No. 7,128,872. It uses a mixture of water vapour and ozone at low pressure inside a chamber to perform the sterilisation. The process is divided into four main steps: vacuum (1 Torr), humidification, injection (of ozone) and exposure (to ozone and water vapour). The system is pre-conditioned by heating the chamber to prevent water vapour condensation, by setting the system temperature above water boiling point for the working pressure. The mix of the water vapour and ozone form hydroxyl radicals, these perform the sterilisation through their oxidizing action. The cycle takes about four and a half hours. Similar to the STERRAD® system, this system uses biological indicators to validate the performance of the sterilisation.
Although the above plasma sterilizing systems are suitable to non-autoclavable medical instruments, they are limited as to what articles they can sterilise. This is due to the fact that they are operated in a low pressure environment.
One alternative to low pressure sterilizing systems is described in US Patent No. 2004/0022673. This system operates a sterilisation chamber at atmospheric pressure. It includes four process stages, namely: (1) the introduction of oxidising agent chemicals such as peracetic acid, (2) the application of high energy gas plasma field, (3) the application of further plasma in combination with agent/biocides such as hydrogen peroxide, chlorine/hypochlorate, iodine and other aldehydes and, (4) the purging of the sterilisation chamber. The total sterilisation processing time of this system can be up to two hours. However, some of the drawbacks of this system include the use of toxic gases, which may not be suitable for use with some articles, as well as the limitation as to what articles can be sterilised, due to the fact that the sterilisation chamber is a fixed volume chamber.
A second alternative to low pressure sterilizing systems is described in US Patent No. 220/0037736. This system sterilizes an article by means of a plasma, and in the presence of moisture at a relative humidity of more than 50% using non-biocidal gas containing oxygen and nitrogen, such as for example air. The article is placed inside a sealed enclosure inside which a plasma discharge is sustained, where said discharge is not in contact with the article. The system includes three process stages, namely: (1) the introduction of humidified non-biocidal gas and first plasma discharge, (2) a second plasma discharge sustained by a second plasma source, or alternatively run with different parameters than the previous plasma discharge and, (3) rinsing with a non-humidified gas. A drawback to this system is however the use of humidified oxygen/nitrogen gas, which can lead to the formation of nitric acid on the article, which is a toxic and corrosive compound.
A third alternative to low pressure sterilizing systems is described in US Patent No. 2004/0161361. This system generates an oxygen plasma discharge inside a non-oxidizing metallic chamber, effectively generating ozone. In other embodiments of the system, oxygen gas may be mixed with moisture and/or hydrogen peroxide, or indeed run with air. The use of oxygen is recommended due to the formation of nitric acid, when operating the system with humidified air, which may have harmful effects on human body. Some disadvantages to this system include the requirement to use oxygen gas, and the limitation to those articles which can be sterilised in a fixed volume metallic chamber.
Although the above plasma sterilizing systems are operated at atmospheric pressures, their processes are complicated by the need to introduce, monitor and control chemicals and/or moisture. Other disadvantages of these described systems include the use of consumables, such as chemicals and/or gases, and the fact that hazardous compounds can form on the articles, such as nitric acid.
Furthermore, in hospitals, even after effective sterilisation of most medical instruments, it will be appreciated that there are still a series of articles and places where infectious diseases may be transmitted. Studies suggest that bacteria can be transmitted via contact with contaminated surfaces and by inhaling contaminated particles (airborne). Some of the highest profile bacteria are antibiotic resistant bacteria found in hospitals. These include some bacteria, known as super-bugs, such as Meticillin-Resistant Staphylococcus aureus (MRSA), Clostridium difficile (C. diff) and Vancomycin-resistant enterococci (VRE). These are some of the most common Health Care Associated Infection (HCAI) diseases, i.e. of an infection that is acquired as a result of contact with the healthcare system.
An environmental study at Beaumont Hospital in Dublin in Ireland found more than half of surface samples tested positive for MRSA in isolation rooms, while 28% of air samples were also found positive for MRSA [T. Sexton et al., J. Hosp. Infect. 62, 187 (2006)]. In this study it was also found that more than half of the beds and mattresses sampled were positive for MRSA. The potential risk of infection transmission through bed mattresses and pillows was also highlighted [E. Creamer and H. Humphreys, J. Hosp. Infection, 69, 8 (2008)]. A lack of an effective and practical solution for sterilisation of beds in general was also noted. It will be appreciated therefore that hospital and nursing home beds are some of the places where bacteria colonies may build up in large quantities.
To prevent patient to patient transmission, beds have to be cleaned, disinfected and sterilised. The sterilisation method has to be able to kill all types of bacteria and viruses. In particular, the removal of spores is of great importance, being reproductive structures adapted for dispersal and survival for extended periods of time in unfavourable conditions. It is found that bacteria resistance to bactericidals is significantly higher in spore state than in the vegetative state.
Another potential target for plasma sterilisation is prions. Prions are infectious agents composed primarily of protein. All known prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and fatal. An example of prion disease in humans is Creutzfeldt-Jakob disease (CJD), the human variation of the bovine disease known as “mad cow disease”. Prion diseases are resistant to ultraviolet radiation and heat. Sterilizing prions involves the denaturation of the protein, a process by which proteins or nucleic acids lose their tertiary structure and secondary structure by the application of some external stress or compound.
It will be appreciated therefore that there exists a need to provide a sterilisation method and apparatus suitable for use with all shapes and sizes of articles and which can be used in most environments.