As the complexity of device design increases, so does the process of assuring that microbial contamination is controlled during manufacturing.
Fig.1: Sources of Microbial Contamination
A common goal of medical device manufacturers is to produce safe products. Sterility, essential to ensure safety and to prove that a medical device is free of viable microorganisms, can be achieved through different processes.
Most single use devices are terminally sterilized by ethylene oxide gas (EtO), or gamma or electron beam radiation. Every sterilization process must be validated for each product to verify that it effectively and reliably kills any organisms that may be present on the pre-sterilized product.
The sterility of any product is defined by the probability of a viable microorganism on the product after it has been sterilized. This probability is referred to as a sterility assurance level (SAL).
When a product is manufactured a number of microorganisms are introduced (see Fig.1: Sources of Microbial Contamination)1. Depending on the circumstances, such as whether or not a clean room was used, the level or degree of contamination varies. Microbial detection methods include Bioburden and Sterility Testing.
Bioburden testing
Bioburden is the population of microorganisms on a raw material, product component or finished medical device just before sterilization, and is the sum of the microbial contributions from a number of sources, including raw materials, manufacturing of components, assembly processes, manufacturing environment, assembly/manufacturing aids (e.g. compressed gases, water systems, lubricants), cleaning processes, and packaging of finished product. To control bioburden, attention should be paid to the microbiological status of these sources.
For finished medical devices, bioburden results are used to establish parameters for an effective sterilization process. To insure the ongoing safety of the sterilization process, it is necessary to verify that the bioburden level remains consistent over time. Part of a good quality control program is the tracking of bioburden counts for a given product. Fluctuations in bioburden counts are usually a reliable indicator that something has changed in regards to how the particular product is manufactured or handled.
Bioburden testing of medical devices is an important part of the initial process validation and ongoing process control as set out in various ISO 11737 standards2,3,4. Establishing a good bioburden program and controlled manufacturing process is the first step in demonstrating that the products are meeting a required sterility assurance level.
After initial data is generated, bioburden testing should be conducted regularly, depending on frequency and volume of production. It is also important to check bioburden levels whenever any changes are made in packaging location, manufacturing processes, raw material vendors, or personnel involved with production. If the bioburden data shows extreme variability, the manufacturing process should be investigated and corrective measures implemented.
Bioburden determination is also useful to monitor microorganism levels on materials that could affect the bioburden of the finished device, such as product components, manufacturing fluids and product packaging.
Conducting the bioburden test
To perform a test, a sample is aseptically transferred to an appropriate volume of extraction fluid and then mechanically agitated to remove microorganisms. Traditional analytical method uses membrane filtration of the extracted fluid and deposit on a Petri dish for culture and microbial enumeration5.
For medical devices, the most commonly requested data are Aerobic Total Count, Aerobic Total and Spore Count, Mold and Fungi Total Count, Anaerobic Total Count, and Anaerobic Total and Spore Count.
Also, the extracted fluid can be used for endotoxin testing. It is an essential lipopolysaccharide component of the cell wall of gram-negative bacteria which elicits a pyrogenic response when injected into the human bloodstream. The detection of bacterial endotoxins that may be present in an injectable component of finished product is a vital function in many pharmaceutical companies.
For products or environmental samples that are found positive, i.e., confirming the presence of microorganisms, a microbial identification is recommended.
Sterility testing
Sterility testing of medical devices is required during the sterilization validation process as well as for routine quality control. The need to provide adequate and reliable sterility test data is an important quality control issue.
The method of choice for EtO sterilized products is the official USP <71> procedure6. Direct transfer (product immersion) is the method of choice for medical devices6 because the device is in direct contact with test media throughout the incubation period. Viable microorganisms that may be in or on a product after faulty or inadequate sterilization have an ideal environment within which to grow and proliferate. Hence, the test article should be completely immersed in the test media.
The method requires that the product be transferred to separate containers of both fluid thioglycollate medium (FTM) and soybean casein digest medium (SCDM). FTM supports the growth of anaerobic and aerobic microorganisms, and SCDM supports a wide range of aerobic bacteria and fungi (i.e. yeasts and molds). After transferring, the samples are incubated for 14 days.
Environmental monitoring
Medical device manufacturers are constantly assessing the environmental impact on the product during the manufacturing process, and hence use microbial monitoring programs to evaluate the effectiveness of cleaning and disinfection procedures and to assess the overall microbial cleanliness of their manufacturing environment.
An effective program to control microorganism levels in the manufacturing environment is essential to minimize the bioburden on the medical device being manufactured and reduce potential for bioburden spikes. Spikes in the bioburden of finished medical devices can cause a reduction in the sterility assurance level for the product.
Air and surface samples are taken during routine production operations. A potential source of contamination is the compressed air system that is used in various applications such as injection molding, operation of conveyor belts, and/or aseptic cleaning.
Air samples can be collected and evaluated using different methods: passive air collection using settling plates, or active air collection using air samplers. Settling plates are Petri dishes containing nutrient growth medium exposed to the environment. Microorganisms which are airborne will eventually move down and settle on the surface of the medium. Air samplers are devices that actively aspire air volumes and the microorganisms will be collected on a medium for growth. Different types of devices with different collection principles can be used.
Surface monitoring can be evaluated using different methods such as contact plates or swabs. The method needs to be suitable for the type of surface to be tested.
Different methods can provide either qualitative or quantitative information: For instance, contact plates, such as Replicate Organism Detection and Counting or RODAC, give quantitative results.
Swabs can be used for sampling equipment and irregular surfaces that are not suitable for sampling with contact plates. A new type, flocked swabs (nylon fiber coating), brings better recovery and release compared to traditional rayon swabs.7
Controlling microbial contamination is essential to medical device performance and reliability. As the complexity of device design increases, so does the process of assuring that microbial contamination is controlled during manufacturing. Manufacturing process control is essential to achieve the appropriate surface quality, optimal functionality, and maintain sterility.
References
1. J. Broad, B. Kanesberg, Minimizing viable and non-viable contamination: standards and guidance for medical device manufacturers, Controlled Environments, September 2007.
2. ANSI/AAMI/ISO 11737-1, Sterilization of medical devices—Microbiological methods - Part 1: Estimation of the population of microorganisms on product.
3. ANSI/AAMI/ISO 11737-2, Sterilization of medical devices—Microbiological methods - Part 2: Test of sterility performed in the validation of a sterilization process.
4. ANSI/AAMI/ISO 11737-3, Sterilization of medical devices—Microbiological methods - Part 3: Guidance on the evaluation and interpretation of bioburden data.
5. Sterility Assurance compliance—SGS, 2007.
6. S. Richter, Sterility testing: essential things you must know, White Paper.
7. G. Dalmaso, M. Bini, R. Paroni, M. Ferrari, Qualification of high-recovery, flocked swabs as compared to traditional rayon swabs for microbiological environment monitoring of surfaces, PDA Journal, June 2008.
About the Author:
Pascal Yvon holds a Doctorate in Pharmacy and an MBA and has over 20 years of experience working with international diagnostics, pharmaceutical, biotechnology, and cosmetics companies. An expert in implementation of new technologies, Pascal speaks about microbiology at international conferences, and has authored magazine articles and book chapters in industry reference publications, such as “The Encyclopedia of Rapid Microbiological Methods.” A member of the Biotechnology Council of New Jersey, Pascal is the founder and president of BioSciences Expansion, which specializes in working with small- to mid-sized companies in the life sciences industry, in sectors such as industrial and clinical lab diagnostics, medical devices, scientific instruments, lab equipment, and biotech and pharmaceuticals. He can be contacted at Pascal@BioSciencesExpansion.com or www.BioSciencesExpansion.com.