Continuation of previous post Disinfectants
The use and knowledge of gamma radiation has grown throughout the medical device industry. There
is great confidence in its effectiveness as a sterilant and its many advantages:
1.Gamma radiation is a penetrating sterilant. No area of the device or container is left with uncertain sterility. This includes prefilled containers.
2.There is no need for specialized packaging. Since there is no requirement for pressure or vacuum, seals are not stressed.
3.Gamma radiation is highly reliable due to its single variable to control-exposure time.
4.Gamma processing has demonstrated lower overall costs. Both large and small product volumes can be accommodated in a cost-effective manner.
I have picked this excerpt from a web site :
Gamma radiation sterilization is not without its drawbacks. Recently, tests have shown that the gamma radiation provides an environment conducive to the oxidation of the UHMWPE (Wright Medical Technology, 1995 and Naidu et al., 1997). Many researchers have concluded that this oxidation process explains the diminished wear properties of the UHMWPE in the human body by changing the percent crystallinity of the UHMWPE (Naidu et al., 1997).
UHMWPE is comprised of three types of structures: crystalline, amorphous, and the interfacial region between the first two structures. It is the percentages of these three types of structures that distinguish the mechanical and chemical properties between the different types of UHMWPE (BE 210 Lab Manual, 1997). Research has shown that the oxidation in the amorphous regions results in the growth of crystalline regions. It is this effect of the oxidation that has brought about the diminished mechanical properties of the UHMWPE (Naidu et al., 1997).
It would seem that best way to reduce the oxidative effect brought about by gamma radiation treatment would be to treat the UHMWPE without the presence of any strong oxidizing agents. One
possibility is the use of argon or nitrogen gas as a medium for gamma radiation instead of air because they are both gamma inert. Even though gamma radiation makes the UHMWPE more susceptible to
oxidation, the stable nature of the argon and molecular nitrogen may counter this deleterious effect of the gamma radiation (Wright Medical Technology, 1995). A further discussion of the possible oxidation mechanism of UHMWPE is key to the understanding of the studies which may determine whether argon gas as a medium for gamma radiation can prevent the oxidation of UHMWPE.
The proposed oxidation mechanism of UHMWPE is through free radicals. Here the gamma energy is responsible for the splitting of a hydrogen-carbon bond from a UHMWPE chain to form the radical.
Oxygen then attacks the radical carbon in the chain and forms a single bond with the carbon. Here,two reactions can occur. In one mechanism, one of the oxygen atoms in the diatomic molecule, may become a carbonyl oxygen bonded to the carbon atom adjacent to the former carbon radical atom.
At this point, the carbonyl carbon will have had lost two hydrogen atoms. The other oxygen atom remains attached to the former carbon radical and, with the addition of hydrogen, forms an alcohol
group. Here, the former carbon radical likewise receives a hydrogen atom to form a stable species while the carbon atom on the other side of the carbonyl carbon becomes a radical species. In the
other mechanism, the chain breaks apart at the initial radical carbon. Each of the oxygen atoms becomes the carbonyl atom in the aldehyde group of each of the chains. Also, the carbon atom next to the carbonyl carbon on one of the broken chains becomes a radical species. The radical species in both of the mechanisms are susceptible to an attack by an oxygen molecule at its radical carbon, where the attack may result in chain excision. It is this shortening of where the attack may result in chain excision. It is this shortening of the UHMWPE chains that changes the its properties (Naidu et al., 1997).
The short chains resulting from the free radical breaking mechanism are better able to align themselves in an orderly structure, thus forming a more crystalline structure. Such separation of UHMWPE chains in the amorphous region leads to the growth of crystalline regions. Because of the low energy associated with the bonds in the crystalline regions, greater energy is needed to overcome these bonds in order to produce a phase change. Hence, the melting point and the heat of fusion of the UHMWPE increases (BE 210 Lab Manual, 1997). Also the bonds in the crystalline regions are more apt to break under a high load instead of stretching. This characteristic of the crystalline regions is indicative of the increase in the brittleness of the UHMWPE. Because of this increased inclination to failure, the UHMWPE has decreased toughness and the ultimate elongation (Wright Medical Technology, 1996). But is there any way an inert environment can protect UHMWPE from oxidation in its role as a medium during gamma radiation treatment?
The purpose of sterilizing in an inert atmosphere would be to isolate the UHMWPE implant fromoxygen. However tests have confirmed that the oxygen diffusion in UHMWPE increases with the dosage of gamma radiation (Naidu et al., 1997). So not only does the radiation drive the free radical oxidation forward but also brings the important oxidizing agent closer to the UHMWPE. Packaging
the UHMWPE "airtight" will not prevent contact with oxygen because the oxygen will diffuse through packaging (Wright Medical Technology, 1996). Also, dissolved oxygen in vivo would find UHMWPE implant from oxygen. However tests have confirmed that the oxygen diffusion in
UHMWPE increases with the dosage of gamma radiation (Naidu et al., 1997). So not only does the radiation drive the free radical oxidation forward but also brings the important oxidizing agent closer
to the UHMWPE. Packaging the UHMWPE "airtight" will not prevent contact with oxygen because the oxygen will diffuse through packaging (Wright Medical Technology, 1996). Also, dissolved oxygen in vivo would find its way to the implanted UHMWPE.
Free radicals are not necessarily detrimental to the mechanical properties of UHMWPE. Sun et al. have shown that irradiation in an inert environment followed by time in an elevated temperature will
crosslink all of the reactive free radicals in a process called stabilization. Oxidation does not occur in UHMWPE when the component is re-exposed to oxygen or other oxidative agents. It also leads to
improvements in wear resistance, creep resistance, and other mechanical properties.
The Nature of Ethylene Oxide
As the use of EtO grew throughout the industry, its characteristics became better known and defined. Although it proved to be a useful sterilant, several limiting factors have since emerged:
1.EtO is only useful as a surface sterilant. It cannot be depended upon to reach blocked-off surfaces, such as those found in stopcock chambers and hypodermic plunger/barrel interfaces.
2.EtO requires the careful and simultaneous control of six variable, but interdependent parameters:
Gas concentration Vacuum Pressure Temperature Relative humididty Time of exposure A single deviation in any of these parameters can mean compromising the desired result.
3.Some of these parameters, notably vacuum and pressure, can work to lessen the effectiveness of the packaging seal which in turn jeopardizes the product's sterility.
Most important is the effect of EtO on workers, device users and patients. Due to its volatility, EtO was initially considered incapable of leaving a residue on treated products. As time and testing progressed, it was found that under conditions for effective fumigation, ethylene oxide reacts with moisture and chloride ions to form ethylene glycol and 2-chlorethanol (referred to as ethylene chlorohydrin), a non-volatile toxic residue. Studies performed with human subjects and experimental animals verified EtO's potential for toxicity, carcinogenicity and mutagenicity beginning in 1968.
This is some information from a website in sweden:
MEDICAL-EQUIPMENT-MANUFACTURING
CANCER INCIDENCE IN SWEDISH STERILANT WORKERS
EXPOSED TO ETHYLENE OXIDE
A study of cancer incidence in a cohort of 2,170 Swedish sterilant workers exposed to ethylene-oxide (75218) (EtO) was updated through 1990. The workers were employed for at least 1 year prior to 1986 at two facilities that produce disposable medical equipment sterilized with EtO. There were 40 malignant tumors, compared with 46.3 expected. There were nonsignificant increases in brain tumors, pancreatic cancer, prostate cancer, cervical cancer, and colon cancer. The risk estimate for leukemia increased, but not significantly, with the time since the start of exposure, and with cumulative exposures to EtO above the median value.
The evidence of EtO's hazards and concern about its potential effects continued to accumulate. In 1984, regulations were passed to limit worker exposure. Subsequently, the permissible residual levels have been continually decreased as new findings have warranted increased concern. For environmental protection, many states now severly restrict the amounts of EtO that can be emitted into the atmosphere. These concerns and protective measures have brought increased costs to EtO processing. Industry usage of EtO has been declining since 1978 when it comprised 95% of the steriliaztion market for medical devices. It is expected that EtO usage will level out at about 30-40%. See Chart: Comparison of Sterilization Methods.
A Matter of Choice
As restrictions on the use of EtO continue to mount, each manufacturer should be aware of the advantages afforded by gamma sterilization. In the final analysis, the manufacturer bears the responsibility for the sterility of the device. Therefore, selection of the appropriate sterilization method is a critical decision.
This is an excellent web site for general info on sterilization:
ahc.umn.edu
Another good site with great info on sterilization:
anpro.com
We are also pursuing the animal cage disinfection industry, Here is a good website
inforamp.net
Another good application for disinfection is in the brewing world where a lot of yeast and fungi are
acleaned before operations.
A site that throws a little insight on Gluteraldehyde which is widely used as a sterilant in the industry
bec.it
More on Gluteraldehyde:
allette.com.au
Sterilants are harmful to the environment ?
pwc.bc.doe.ca
FDA involvement in sterilants ???:
fda.gov
Top 10 FAQ's is Sterilization:
aorn.org
More stuff/definitions to clear the air:
anex.med.tokushima-u.ac.jp
Another Sterilant and perhaps the most powerful chemical sterilant currently being used is
H2O2 , Visit this Johnson and Johnson Website more more information:
sterrad.com
Here is one more good web site:
seas.upenn.edu
The medical disinfection industry in the whole world is in ecss of 200 billion dollars annually with US alone accounting for over 10% worldwide. We should be able to make a dent there with the Laudry systems, Sterox and Sterozone.
Hopefully this stuff is informative,
Regards
Sri. |
