Coatings: make a clean sweep
Cleaning and coating methods can significantly affect the consistency and quality of medical device performance. However, many design engineers and manufacturers are unaware of this fact. It’s important to keep in mind device cleaning and coating considerations at every stage of design and manufacturing for optimal performance once the device is in a doctor’s use.
Weighing options
Cleaning and coating systems help achieve consistently clean products and ensure consistent performance from a device. A well-engineered process is easy to validate and will reduce costs associated with device sterilisation by removing sources of bioburden from the manufacturing process.
During device manufacturing, engineers need to decide whether they must clean component parts prior to assembly, clean the final assembled product, or both. Virtually all devices will require cleaning to some extent to improve cosmetics by removing particulate, oil or inorganic contamination that results from the manufacturing process. The challenge is to specify a cleaning process that is suitable for a variety of materials and geometries that may include delicate plastic injection-moulded parts, stainless-steel micro-tubing or sophisticated mechanical assemblies.
Generally, application of a lubricant coating is dictated by the desired performance of the medical device once assembled; not all devices require a coating, or lubrication. A lubricant coating is typically applied so a device will function better with reduced friction. Any device that moves side-to-side, slides, or rotates may be a candidate for a lubricant coating. Devices such as a syringe needle or cannula for injecting medicine or fluids may be cleaned and then coated with a thin film of medical grade silicone fluid to reduce friction when the needle pierces the skin. Similarly, mechanical assemblies that consist of multiple component parts, such as a surgical stapler, often need an engineered dry lubricant coating to reduce friction and minimize stacked tolerance issues.
After the engineer whether or not a device must be cleaned or coated, the process is tailored to production volumes. In low-volume production environments, basic cleaning devices – including aerosols, dry wipes, presaturated wipes with water-based cleaners, solvents, or solvent- and water-based cleaners – may be specified. For high-volume production the engineer will typically employ more automated cleaning systems to improve cleaning consistency and reduce costs. Those systems may be either solvent or water based and use machines engineered for the application.
Lubricant coatings applied in-house will most likely be either silicone fluid or dry polytetrafluoroethylene (PTFE) based. Typically, sophisticated hydrophilic treatments are applied off the manufacturing premises because they require proprietary materials and application methods to impart surface properties that become lubricious when in contact with body fluids. In general, surfaces must be perfectly clean and dry prior to any coating application.
Key factors in choosing a cleaning or coating system include worker safety, equipment costs, cost per part treated, required floor space, reduced bioburden, materials compatibility, and ease of use. Each application will have its own requirements and design and manufacturing engineers should meet with cleaning and coating providers to discuss specific concerns.
In the cleaning process, the largest consideration is typically cost-effectiveness, materials compatibility, regulatory compliance, ease of use, and safety and environmental concerns.
Product performance is usually the number one consideration. When the device has been manufactured and is in physicians’ hands, will it perform the way in which it was designed? Following this matter (as with the cleaning process), manufacturers are most concerned with materials’ compatibility, cost, regulatory compliance and product cosmetics as well as safety and environmental issues.
Key challenges
1. Cosmetics. Medical devices must be pristine on both clinical and cosmetic levels, as well as function flawlessly. A doctor, nurse, or patient will not accept anything less than perfection on the cleanliness and cosmetic appearance of a device. They must see a device with surfaces that are smooth and spotlessly clean. Also, a device must be pristine-clean to be properly sterilised. The optimal surface is obtained through cleaning and coating processes that quickly eliminate defects such as fingerprints, oils or stray particles that may remain from the manufacturing process.
2. Bioburden. Many factors can cause bioburden in a manufacturing process but fundamentally water is a primary growth medium for bacteria. Therefore, removing water from the manufacturing process removes a major source of bioburden issues. Solvents are often preferable to aqueous (water-based) cleaners or coatings — solvents present an environment that is hostile to bacteria growth, and as a result greatly simplify process control requirements for eliminating bioburden.
If bioburden is not properly addressed, it can result in increased difficulty in the validation of subsequent product sterilisation processes. A solvent-based cleaning process with submicron filtration can run at very high production volumes while significantly reducing bioburden issues with a minimal footprint on the cleanroom floor, as well as a minimal capital outlay compared with a water system. Also, ] compared with a water-based cleaning system, solvent cleaning will reduce utility costs associated with water consumption and treatment, heating water, parts drying, and maintaining proper cleanroom air conditioning.
3. Stacked tolerances. One common challenge regarding design and assembly is stacked tolerances in mechanical assemblies, which can create noticeable production variances in device actuation forces. This is a particularly common challenge with complex, single-use mechanical assemblies such as staplers and arthroscopic devices.
In design and engineering, a dimensional tolerance refers to the permissible limit of variation in a physical dimension. Tolerances are specified by the design engineer to allow for reasonable leeway for imperfections and variability but without compromising performance. However, tolerances often become a challenge for design engineers and manufacturers when they begin to stack up on each other. For example, when a mechanical assembly such as a medical stapler is assembled, the tolerances of each metal stamping, spring, or plastic part may begin to combine in such a way that the assembled device requires excess force to actuate or execute. This issue is most commonly found in high-volume production, when tooling used to manufacture metal stampings, springs, and plastic parts begins to wear.
Design engineers and manufacturers address stacked tolerances in several ways. Engineers may choose to design components with tighter tolerances to gain high precision. However, increased precision requires frequent inspection and maintenance of tooling and fixtures throughout the manufacturing process, which drives up finished device costs. An alternative method of dealing with stacked tolerances is to apply a lubricant coating such as PTFE or silicone on the finished assembly to reduce operating friction.
Dry lubricants using PTFE particles are a low-cost way for the design engineer and manufacturer to reduce the effects of stacked tolerances. Many single-use medical devices on the market would not be commercially viable without this coating.
Dry lubricants reduce the force needed to actuate or execute a device by 25 to 30% and provide a silky, almost effortless actuation for the medical professional performing the procedure. In comparison to an untreated stainless steel surface that has a coefficient of friction of 0.80, a dry PTFE lubricant can produce a remarkably low coefficient of friction of about 0.06 to the same surface. As an added benefit, PTFE lubricants are nonmigrating, so they will not degrade product cosmetics by transferring to packaging or work surfaces.
4. Maintaining calibration. For dry lubricants in particular, maintaining calibration is a challenge. Maintaining calibration of lubricant dispersions and fluids is important to the consistency and quality of the coating and the consistent performance of the device.
The first step in maintaining calibration is controlling the evaporation of carrier fluid. Many PTFE dry lubricants are mixed with a carrier fluid that evaporates very quickly. This speeds up the production process as the carrier dries quickly and leaves a consistent coating on the device. It also means that the fluid can evaporate quickly out of the vessel during the coating process. In some cases, manufacturers will add an inexact amount of carrier fluid to maintain approximate percentage saturation but this is not precise and can affect the quality of the coating.
To control evaporation and keep fluids calibrated for maximum consistency and quality, use of process-specific equipment for the cleaning and coating process is recommended. This may include hermetically sealed equipment, controlled temperature baths, specialised solvent recovery systems, engineered parts feeding systems such as hoists or conveyers, or engineered application systems such as spray or brush applicators. Also, test equipment will soon be introduced to the market that allows for instant and easy real-time measurement of PTFE content in the carrier. All are relatively simple and low cost ways to maintain lubricant calibration for coating consistency.
Many coatings with PTFE micropowders require constant agitation because the particles have low hang time in the liquid carrier. This means that as the fluid sits in the vessel and parts are dipped, the PTFE particles in the fluid will sink to the bottom of the vessel. Many manufacturers address this issue by constantly agitating the fluid. However, if done improperly, this practice can have inconsistent results and lead to streaky coatings.
A key solution to the short hang time of PTFE particles in a carrier fluid is to specify dispersions that use micropowders that are matched to the carrier fluid by a knowledgeable supplier. These lubricants use a premixed and supplier-calibrated formula that maintains the ratio of carrier fluid to PTFE particles to improve hang time and coating quality.
It’s vital for medical device manufacturers to consider the role the cleaning and coating process can play in the performance, quality, and consistency of the finished device. There are many challenges to address in the process, but consulting with an expert cleaning or coating provider who can answer questions and concerns in an educated and timely manner is a helpful best practice.