setting bioburden limits

This article explains the process for setting and validating bioburden limits, and you will learn when investigations are needed. 

Last week, I was in Europe reviewing product specifications with a potential contract manufacturer for a medical device implant. Due to the raw materials that the contract manufacturer currently is using for a similar product, bioburden levels are higher than we can accept. The company uses an ISO Class 7 cleanroom for assembly and packaging, which is clean enough for these implants, but the molded components used for the assembly are not clean enough.

The average bioburden is 220 CFU/device (i.e., colony-forming units/device), and the maximum observed bioburden exceeded 500 CFU/device. We want to use a lower dose range of gamma radiation to prevent the deterioration of bioabsorbable plastics, but a lower dose range requires that the average bioburden never exceed 100 CFU/device.

There are quite a few Clauses in ISO 13485 that differ from ISO 9001. One example is Clause 6.4–Work Environment. Subsection 6.4(b) states, “If work environment conditions can have an adverse effect on product quality, the organization shall establish documented requirements for the work environment conditions and documented procedures or work instructions to monitor and control these work environment conditions.” This is the applicable clause of ISO 13485 related to setting bioburden limits. Unfortunately, this vague requirement does not explain how to establish or validate bioburden limits.

Rule of Thumb for Setting Limits

One of my microbiologist friends recommends using the following “rule of thumb”: +2 sigma for alert limits and +3 sigma for action limits. This rule of thumb assumes that you are performing data analysis of bioburden and that you have calculated a “sigma” value for the standard deviation. There are a few problems with the “rule of thumb” approach.

First, this method assumes a normal distribution and a controlled process–which bioburden seldom is. Second, the cleanliness you need for your product and the cleanliness your controlled environment is capable of are not always appropriately matched. In my example, we need the finished device to have a bioburden of <100 CFU/device before gamma sterilization. Molded parts are essentially bioburden free due to the hot temperatures of the parts ejected from the mold. Unfortunately, molded components attract dust like a magnet. Therefore, how you handle and store molded parts is important to the bioburden of the molded parts.

Which factors affect bioburden?

For this example, we have three aspects critical to the final bioburden limit of the finished medical devices.

1. How are the molded parts handled and stored?
2. Are molded parts cleaned before assembly?
3. What is the cleanliness of the work environment where the device is assembled?

The cleanliness of the molding environment matters, but parts can fall into a container that keeps the parts clean. It also matters how molding machine operators handle the parts. Gloves should be used, and typically the container the parts are in will be placed in an outer bag for storage. It is possible to clean molded parts with ultrasonic cleaning before assembly, but if the parts are kept clean after molding, this is unnecessary.

For your assembly operation, you need an environment with suitable cleanliness. Sometimes a controlled environment is sufficient. Other times a certified cleanroom is more appropriate. In either case, it is important to control the bioburden in the assembly area to a level that meets the needs of the most critical product assembled in that area. Cleanroom procedures, the design of the cleanroom, and your cleaning/sterilization processes should match the needs of the product. Fortunately, cleanroom procedures and bioburden limits for cleanrooms are well established in ISO 14644-1 (e.g., for an ISO Class 7 cleanroom, particles ≥ 0.5 microns must be fewer than 352,000). If you have devices of different types in the same manufacturing area, you must plan according to the most critical needs.

Validating Bioburden Limits

After establishing your bioburden limits, you need to validate these limits. Once again, cleanroom validation has established ISO Standards to follow. The more challenging validation is a validation of the bioburden of parts and the final assembly. It’s important to validate the component levels first to reduce the variability of inputs to the final assembly process. Typically the first step is to perform data analysis of other molded parts produced in the same molding area by the same operators. If this data meets your needs for cleanliness, then further measures for controlling bioburden may not be needed. However, if you need to reduce bioburden (i.e., bioburden failure), you might consider measuring parts at critical control points. The goal is to identify where the bioburden is being introduced. This analysis is typical of the type of root cause investigation performed when bioburden increases for unexplained reasons.

Once the sources of bioburden are identified and quantified, process controls should be implemented to reduce bioburden. Gloves, double-bagging of product, and keeping containers covered during the molding operation are typical risk controls that may be implemented. To validate the effectiveness of these measures, you should write a bioburden validation protocol that evaluates each of the following aspects:

1. lot variability of component bioburden
2. operator variability for assembly
3. variability in the cleanliness of the assembly area
4. number of operators in the assembly area
5. duration of the manufacturing lots

After you have validated the bioburden limits for the components, then the same process should be conducted for the final assembly of the product. A sampling of bioburden after transfer to the assembly area and before assembly begins should be done. This is important because often, improper storage of components and/or failure to remove and clean outer packaging will contaminate the parts and your assembly area.

If you are interested in learning more about process validation, please download the process validation webinar. We also published a blog on sterilization and shelf-life validation for 510k submissions.