Feature Article

By: D. Schulz, Korntal, Germany

Cleanliness is nonnegotiable in a medical manufacturing context, and that is especially true when it comes to the production of orthopaedic implants. Effective cleaning technologies enable manufacturers not only to fulfil strict cleanliness requirements but they also may help to optimise manufacturing costs and product quality and to support new product development efforts.
 

Image courtesy of Amsonic

There is no one-size-fits-all solution for compliance with cleaning requirements: each orthopaedics-related industrial cleaning operation needs an individualised solution. Essential criteria that should be taken into account when considering a cleaning technology include the part’s material or combination of materials, the type of contaminants that will need to be removed, the component’s shape, cleanliness requirements with regard to particulates and film-like contamination and the desired production throughput. Careful analysis of these factors will lead manufacturers to the most suitable process, which can be validated by cleaning tests conducted by equipment or cleaning agent manufacturers.

 

Orthopaedic components are usually cleaned in batches by means of a wet chemical process. The most commonly used media are water-based cleaning agents and solvents. This solution was chosen by an implant manufacturing company when it enhanced its cleaning capacity. The company settled on two solvent cleaning machines for preliminary and intermediate cleaning of parts and an aqueous system for final cleaning.
 

Solvent is used to clean the parts after each machining cycle. This ensures complete removal of particulates, oil or grease, which may lead to tolerance deviations during further processing. Intermediate cleaning operations also prevent lubricants and processing oil from mixing together, which often causes cleaning problems. Isoparaffin, a so-called A3 solvent, is used in this step. It’s a nonhalogenated hydrocarbon that removes almost all lubricants and cutting oils used during chip-removal machining processes. Because the solvent’s flash point is above 56°C, the cleaning process is conducted under vacuum conditions to eliminate the risk of explosion. The use of vacuum technology offers an additional advantage: contaminants can be dissolved from drill holes that measure approximately 200 mm long and from undercuts.
 

Both solvent-filled cleaning machines are equipped with an ultrasonic cleaning module. The cleaning agent is continuously distilled and the separated oil is immediately removed from the system. The cleaning agent washes over the parts and then passes through a filtration system, which captures particulates. This flow path ensures that the solvent maintains consistent cleaning efficacy over a long time period.
 

Preliminary cleaning is done in a solvent-filled machine with a 15-L basket. Various cleaning programs corresponding to specific part requirements are stored in the system. These include ultrasonic immersion cleaning, injection of solvent under pressure, spray rinsing, vapour degreasing and drying under vacuum. Depending on the selected program, cycle time is within the 6- to 12-minute range.
 

The second system has a 33-L capacity and is equipped with two tanks, one for precleaning and the other for final cleaning with a distilled solvent. Different programs matched to the various cleaning tasks and parts are also stored in the cleaning system. The cleaning cycles are identical to the first machine.
 

Following solvent-based cleaning (that has removed all oil and grease), the surface of the parts will be covered with a protective hydrocarbon film that is approximately 2 to 10 nm thick. To remove this layer, a final validated aqueous process step is initiated. Aqueous cleaning agents are based on an organic or inorganic builder and surfactants. The latter are able to “push” themselves between the contaminants and the material that needs to be cleaned. The surfactants remove both nonpolar contaminants such as hydrocarbons, oil and grease and polar contaminants such as emulsions, salts and particles. A multistage rinsing process with deionised water removes any residue left by the cleaning agent or surface spots. This last cleaning step ensures the biocompatibility of the parts, which is monitored periodically.
 

Because oil and grease have been completely removed from the parts during the solvent-based cleaning cycles, pollution of the aqueous detergent is dramatically reduced, resulting in a longer lifetime for the cleaning bath and lower operating costs.

 

The cleanliness and aesthetic appearance of the implant components do not depend solely on the cleaning process, chemicals used and duration of treatment—the cleaning tank’s architecture also plays a role.

The layout of the cleaning tank and parts basket impacts the effectiveness of the technology, treatment time, cleaning temperature and media. Making sure the parts in the basket are uniformly exposed to the cleaning agent is a prerequisite to the quick, reliable removal of contaminants. This ensures that the mechanical washing process is effective in removing the film-like contaminants and particulates. Accessibility is also indispensable for drying the parts with compressed or hot air. To ensure accessibility, baskets should not have large, contiguous surfaces; only round wire should be used in their construction. Compared with closed containers or baskets made of perforated sheet metal, cleaning baskets made of round wire are also distinguished by significantly better draining characteristics. Consequently, less contamination and cleaning agent is carried over to downstream processes. This results in a longer service life for the cleaning bath, and, thus, improved cleaning system availability and efficiency.

 

Cleanability is an essential criterion in the development of new or improved orthopaedic implants. This is borne out by the experience of one manufacturer who designed new implant parts made of stainless steel and a titanium alloy. The components have extremely small drill holes that need to be cleaned in an absolutely reliable manner. Even the tiniest residue of processing media would cause problems and result in scrap during the assembly process.
 

The defined cleanliness requirement could not be fulfilled by conventional wet chemical cleaning methods. The company found an alternative solution by using supercritical carbon dioxide.
 

In this innovative cleaning technology, the CO₂ is in an aggregation condition, where its physical characteristics lie between the liquid and gaseous states. While in this state, the CO₂ demonstrates only minimal viscosity and surface tension. In this way, nonpolar contaminants such as oil and grease can be removed from the finest cracks and very porous surfaces.
 

Cleaning with supercritical carbon dioxide usually takes place within a temperature range between 20° and 40°C and a pressure range between 200 and 300 bar. During the cleaning process, the controlled rotation of the cleaning basket ensures that all parts are equally exposed to the CO₂. Carbon dioxide is neither toxic nor flammable, and presents no health hazard. The technology complies with demands for environmentally sound, dry and residual-free cleaning processes. Because CO₂ is immediately sublimated at atmospheric pressure, the parts are dry as soon as cleaning has been completed.
 

Laboratory-scale tests as well as trials conducted at the machine builder’s lab showed that supercritical carbon dioxide based processing is capable of fulfilling all cleanliness requirements. After the cleaning process, the implant components met the specified chemical, physical and mechanical characteristics as well as biological parameters such as bioburden and cytotoxicity. Additionally the cleaning agent has a bacteriostatic effect. The CO₂ cleaning process was validated according to CGMP guidelines.
 

The CO₂ process takes place after the parts have gone through wet chemical cleaning and prior to their assembly, which is carried out in a cleanroom. The manufacturer integrated the CO₂ cleaning system into his facility in such a way that the operating panel and door of the cleaning chamber are accessible from the cleanroom. 

 

is Journalist at Schulz. Presse. Text., Martin-Luther-Strasse 39, D-70825 Korntal, Germany

tel. +49 711 854 085

e-mail: ds@pressetextschulz.de
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