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Process Cleaning Magazine
© 2012
AMT-The Association For Manufacturing Technology
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Cleaning has always been an important step in the manufacture of orthopedic implants, but as technology advances, more exotic materials and sophisticated geometries are being used, making the cleaning process more complicated and the results more critical. When implants are not properly cleaned, the costs can be significant. With this increased scrutiny of the cleaning process, it is imperative that suppliers not only meet the standards, but also that they can document the cleaning process.
Final Cleaning for Orthopedic Implants
It is no longer good enough for a supplier to produce clean orthopedic implants. The process and equipment must comply with FDA guidelines, while having adequate process and product data acquisition and documentation capabilities. Real-time verification and data logging of the implant cleaning process at either the lot or part level are now required.
Developing a medical-grade cleaning process has four basic steps:
- Select a cleaning process that provides adequately clean parts on a lab or pilot-line scale.
- Select a piece of production equipment that can effectively reproduce the process on a production scale.
- Test and verify that the process and equipment are robust and capable of providing acceptable results on a consistent basis.
- Provide for documentation of the equipment status and process parameters relative to each lot (or part) processed, both in real-time and as a retrievable historical record.
Selecting the Cleaning Process
Depending on the material of construction and the soil to be cleaned, a solvent-based or aqueous-based process may be appropriate, or, in some cases, a combination of the two methods. Many suppliers choose to incorporate an additional surface treatment process (such as passivation) into the final cleaning.
Selecting Production Equipment
Once an appropriate cleaning process has been identified, the cleaning system should be designed to consistently reproduce that process. In addition to duplicating the lab scale process, there are other variables to consider when scaling up to production levels. Production throughput requirements, fixturing, and management of all of the fluids used in the process are all considered, and because many production facilities operate more than one shift per day, consideration must be given to ease of routine maintenance and periodic service.
It’s important to manage the process fluids to ensure consistent, documentable cleaning results. The first tank in an aqueous cleaning system is generally an ultrasonic wash tank using an FDA-approved detergent.
Maintaining the liquid level keeps the detergent concentration consistent over time. There are several methods to accomplish this, the most common being the use of level sensors. When the liquid level drops to a predetermined low point, the float opens a valve to add more DI water, while a metering pump adds a measured amount of detergent. Some systems incorporate multiple dosing rates to accommodate evaporative losses or other special circumstances.
But an even bigger issue is monitoring and maintaining the cleaning efficacy of detergent systems. Several methods directly analyze the contamination level in the detergent and use that as a measure of the detergent’s usefulness. Turbidity and refractive indexes – the most common of these techniques – measure changes in the “look” of the fluid to determine contamination levels. Unfortunately, these methods can’t determine whether the cause of a change will impact the cleanliness of the part being cleaned. They often give false failures, resulting in premature dumping of chemistry.
Another method measures pH to determine the condition of the cleaning fluid. But this technique generally is not reliable because most alkaline detergents on the market contain buffers. These buffers are designed to maintain the pH of the cleaning solution until the detergent is spent, at which point the pH drops dramatically. This sudden drop often leads to processing of components that are not adequately clean.
The last component of maintaining the wash solution is managing particulate contamination in the wash tank. This is most often accomplished with depth filters of appropriate size in a recirculation loop. The optimum flow rate for the recirculation loop is 10 to 20 percent of the tank volume per minute. Filters have a finite life determined by the level of particulate contamination introduced into the system. Most often, differential pressure gages are used to determine when a filter is reaching the end of its useful life. This information can be easily included in the data acquisition system as well. Parallel filtration can be utilized when more flexibility is required for scheduled maintenance. In this case, the system can automatically switch from one filter to another when the differential pressure gage indicates that this is required. Filter housings should be located in an easily accessible area to facilitate change-over and maintenance.
Rinsing
The next step in the process is rinsing, and significant costs can sneak in here. Hot DI water can cost $0.10 per gallon or more. In many cleaning systems, the water cost is actually the biggest component of operating costs. Rinse systems should be designed to optimize cleaning effectiveness and water use. One common technique for this is to counterflow the water, meaning that water overflows from the cleanest rinse tank (next to the dryer) to the first rinse (next to the wash station). Using this type of process flow, only one source of water is required. For most orthopedic implant applications, a flow of 3 to 5 percent of the rinse tank volume per minute is adequate. It’s worth noting that the rinse tank design can also impact the performance of the system.
Using a four-sided overflow design can reduce the time and/or the flowrate required to achieve adequate rinsing, but there are some cases where this cascading arrangement may not be appropriate. For instance, if the process includes passivation, it would be better for the first rinse to have an independent water supply. This eliminates the chance of the implants being exposed to acid residues in the rinse step before they move to the passivation step.
Many final clean systems incorporate an acid passivation step. Nitric acid is commonly in use, although several chemical suppliers are working to qualify citric acid-based products as an alternative. Currently, the best method for safe handling of concentrated nitric acid is to manually actuate an acid-resistant diaphragm pump to add the acid to the tank after it has been partially filled with water. This eliminates the need for the operator to manually handle the acid. To maintain the liquid level and concentration, the same acid pump can be linked to liquid level controls very similar to those used in the wash tank. When the liquid level drops, water is automatically added to the tank along with a metered amount of acid. The acid tank should also utilize a pump for draining, further reducing the risk to the operator.
If these guidelines are followed the production cleaning system should be able to routinely reproduce the cleanliness levels achieved in the lab scale tests.
Product and Process Verification
Following the first three steps helps establish a production process capable of consistently cleaning implants to acceptable levels. But verification and documentation are required to show the customer and the FDA that the process was operating correctly or that a particular implant was cleaned using the process. And for that, the incorporation of a special control system using a PLC and an industrial PC is required. The control system has two functions – first, to capture all of the key operating parameters of the cleaning system and store them in a database, and second, to track each part or batch of parts so that they can be matched to the system operating parameters from the time they were processed.
Capturing all that data requires sensors wherever there is a process parameter that must be monitored and documented. The number, type and location of sensors can be designed into the system depending on customer and FDA requirements.
In addition to capturing the process data and storing the information in a database, the control system also analyzes the data against predetermined set points. Most of the variables have two tracking bands – one narrower band defined as a “warning” and a broader band defined as an “alarm.” If the data varies outside the narrower control band, the system alerts the operator to the change in condition so appropriate action can be taken. If the process varies outside the second, wider band, the system alerts the operator and will shut down, preventing additional baskets from being processed. An additional advantage of having a control system that captures this much process information is the ability to remotely troubleshoot the hardware and process. This capability can increase the system uptime by streamlining any troubleshooting and service.
At this point, the system has captured the cleaning system operating parameters and placed that information in a database. The remaining function is to add information about the implants being cleaned to this database. This data is generally captured with a barcode scanner. Each lot of implants, or in some cases individual implants, is presented to the cleaning system with a production control document that includes a barcode. The operator scans this information into the system before processing the basket through the cleaning system. The control system then marries the batch data to the operating parameters that existed during the cleaning of that particular part. If a process interruption occurs or if some component of the process is out of specification, it is noted in the database. This stored information can be accessed by the operator in several ways. The system can be programmed to alert the operator every time a problem basket exits the system so that it can be segregated. Another option is to print out the process information on each basket and add it to the production control package for each implant. This information would then be used to identify the baskets that were not processed according to the specification so they may be reviewed or reprocessed.
Step by Step
The liability associated with inappropriately cleaned parts makes cleaning critical to the success of an orthopedic implant manufacturer. Meticulously following these steps helps to ensure consistently clean parts as well as process and product information to document the effectiveness of the cleaning process and that it was used on specific parts. In addition, the significant amount of data that is accumulated about the process and products allows better monitoring and management of this critical final step in implant manufacturing. The management saying, “If it can be measured, it can be managed” is a significant side benefit of having a cleaning system with process verification. PC
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