Exploring the coatings used for silicones and thermoplastics

^{Freudenberg Medical explores coatings and surface treatments for silicone and thermoplastic components.}

New materials and technological advances continue to proliferate the medtech industry at a rapid rate and suppliers strive to offer innovative solutions to meet the demands of increasingly complex components and devices. A range of specialized materials are used today and many of those materials are suitable for the intended finished use only after the application of a customized surface treatment. These treatments can be used to reduce friction, improve haptic properties, introduce chemical functionalities, and improve medium resistance of the materials. To meet this growing demand, Freudenberg Medical uses traditional coating methods like wet chemicals and parylene coatings, as well as methods such as plasma treatments in order to alter the surface properties of medical components.

Surface modifications allow a variety of customer needs to be addressed. Color coating, for example, is a surface modification which enables control of the base materials’ transparency and optical properties. Plasma treatment is another modification option which will reduce friction. Silicone itself has a high friction coefficient and a plasma treatment will modify its friction properties. This can improve silicones manufacturability, particularly in automated assembly lines, as well as its ability to be used in low friction applications where gliding is necessary – such as with endoscopic catheters and the internal coating of flexible tubes used in cardiac pacemakers.

Plasma coating

Surface modifications such as plasma coating directly influence the surface energy of a component; this allows control and adjustment of the hydrophilic properties. Plasma processes enable multiple functionalities to be implemented within a single process; for example, plasma enables a bioactive layer with good adhesion to be applied to a substrate in a single step. The surfaces are plasma coated, activated, or plasma etched using environmentally-friendly processes. A variety of component geometries can be altered with vacuum or atmospheric pressurized plasma, without solvent emissions or a time-consuming drying process. Small parts can be handled in bulk goods processes under a vacuum, while atmospheric pressure plasmas are well-suited for integration into existing process chains.

What is driving the need for coatings with silicone components?  At the most basic level, ease of handling, so parts don’t stick to each other. The application or design property alternation is dictating what kind of coating process to apply. In accordance with different applications and different needs, each has an impact on the coating process or coating chemistry that is ultimately used to identify and apply the best performing coating based on the friction partners in the application.

Parylene coating

Parylene coating is another popular, medically approved surface coating. Parylene coatings are hydrophobic, inert, transparent, nonporous, biocompatible, biostable, and have an outstanding barrier effect against moisture, chemicals, and gases. These coatings also have a high dielectric barrier effect. Four different types of parylenes are produced. The basic type is Parylene N (polypara-xylene), which is characterized by a good gap penetration capability and a low friction coefficient. Parylene C has high chemical resistance and provides an excellent barrier effect against gases and moisture. Parylene D has long been used as a high-temperature parylene due to its continuous use at a temperature of 100º C. A new, fluorinated parylene, Parylene HT, is used at even higher temperatures (up to 350º C) and has the highest gap penetration capability, a high UV resistance, and the lowest friction coefficient.

Lubricous & hydrophilic coatings

Lubricous coatings on medical components reduce the adverse effects of dynamic and static friction, increase the ability to repel water, and assist with ease of insertion or glide. The performance, accuracy and durability of components and products is increased. Freudenberg Medical addresses the growing demand for high-performance hydrophilic coating solutions by offering expertise in customized coating design, surface activation, hydrogel chemistry, and a process development customization platform. By applying a systematic customization process to both standard and non-standard substrates, we target maximum coating performance that typically will outperform standard solutions available off-the-shelf. Thanks to integration of proven feasibility study routines and smart iterative design principles, high project success rates and short development times can be achieved.

Microstructures

Surface texturing is the dimensional modification of an otherwise smooth surface with microstructures.  This technique creates an effect on technical parts by applying surface textures similar to the lotus effect. The lotus effect refers to the self-cleaning properties exhibited by the leaves of the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Silicone surfaces are very susceptible to dust and particle adhesion and when microstructures are applied to a silicone surface it further repels dirt and increases hydrophobicity. With microstructures the tackiness of a silicone surface is reduced and friction is reduced by 50-60%. This application works well on tubes as well as molded components.  

Wet chemicals

Wet chemical application methods include spraying, dip coating, and brushing.  Bar coding and printing methods utilizing ink could also be considered wet chemical coatings. Spray coating is the most uniform method but also the most costly. Spray coating on a tube requires at least two to four different offset spray nozzles to achieve full coverage and results in a good amount of overspray as well as application within a cleanroom environment which adds additional cost. Ask yourself, what does my application really require and what am I willing to pay for?  If the component does not need to look optically perfect then the most expensive solution is not required.  If a brushed application will be sufficient, why not use it.  With brushing you will see the fiber texture in the coated surface.  This is not as uniform as spray coating but also not as expensive.

Jeff Mohror, vice president and general manager of Freudenberg Medical’s Carpinteria Operation notes: “In many cases, our customers require specific material properties for their device and due to regulatory aspects we cannot modify the material chemistry. In these situations, a customized surface treatment allows us to provide customers with individualized solutions.”

The medtech field continues to expand to meet the demands of new treatments and the complexity of devices that are part of those solutions. Unfortunately, there are no universal answers, it is important to find a production partner with the experience and technical capabilities to provide the right solution for these specialized needs. Surface engineering is one of the many areas in which Freudenberg Medical is maximizing the flexibility of its processes and services to achieve continuous innovations that keep pace with the ever-growing challenges of the healthcare industry.