For many medical device manufacturers, the application of proprietary coatings and surface treatments can play a critical role in differentiating their products as they develop new devices and reimagine the next generation of their existing products. For products such as stainless steel guide wires, catheters, and vascular surgical tools, plasma-applied coatings offer properties that can distinguish them from their competitors. Coatings can provide everything from biocompatibility or lubricity to antimicrobial and anticorrosion properties. These modified surfaces can improve the functionality of devices such as intraluminal stents, occlusion balloons, and polymer coils.
Plasma-enhanced chemical vapor deposition (PECVD) can elevate a product by addressing surface reaction issues such as biocompatibility or lubricity. It is a unique and eloquent way of depositing and enhancing coatings because it allows OEMs to tailor the device's surface while retaining the properties of the bulk material.
PECVD of Organic Silicones
Silicon dioxide, or silica, is one of the most fundamental elements on earth. Most commonly found in nature as quartz, it is also the major constituent of sand and a primary component in silicone and glass. Now, this basic chemical compound is being applied using PECVD techniques as an antimicrobial barrier, a primer to promote adhesion between stainless steel and proprietary coatings, or to create hydrophobic or hydrophilic surfaces.
PECVD is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. To deposit silicon dioxide using PECVD, organic silicones are often required as the feedstock. Within this family, the best known are hexamethyldisiloxane (HMDSO) and tetramethyldisiloxane (TMDSO).
HMDSO, in particular, is an affordable and flexible reagent that is commercially available in a high purity, liquid form. The volatile, colorless liquid can be plasma-polymerized to create a variety of surface coatings that are safe for medical use. Depending on the composition of oxygen to HMDSO, the property of the surface can be hydrophobic or hydrophilic. In fact, it is this flexibility that makes HMDSO and other siloxanes the ideal choice for PECVD. By adjusting the parameters and other gasses added, chemists can tightly control the material to address a wide range of applications.
For the medical device industry, the primary uses of organic silicones fall into three primary categories: as protective barriers (antimicrobial, antifungal, anticorrosion), as a primer between stainless steel and exotic metals and proprietary surface coatings, or to modify the surface to make it hydrophobic (water repellent) or hydrophilic (affinity to water).
When the substrate is metallic, like stainless steel or exotic alloys, it can be difficult to adhere a coating to the surface. When this is the case, HMDSO can be used as an intermediate layer to improve the adhesion between a coating and the substrate. Guidewires are a good example. To ease insertion, stainless steel guidewires are often coated with proprietary surface coatings to make them more lubricious. By applying a thin layer of silicon dioxide, the lubricious coating grafts very nicely to a stainless surface.
Organic silicones can also be applied as a linking chemistry between surfaces that are difficult to adhere to, such as ceramics and polytetrafluoroethylene (PTFE). Drug-delivery devices that use ceramic nozzles with micronsized openings can become clogged, and so these are often coated with PTFE to prevent clogging. Depositing a 100–150 nm layer of HMDSO promotes the bond between the two substances.
Anticorrosion is becoming increasingly important in medical devices, particularly to protect the small microelectronic circuit boards in products such as hearing aids, intraocular devices, implantable sensors, and pacemakers.
To protect electronics against corrosion, HMDSO coatings are applied in a relatively thick layer of a micron or more. HMDSO is water and gas repellent — properties that are required to prevent corrosion. A thin layer (100 nm or so) of PTFE can also be applied if the HMDSO will be exposed to harsh chemical acids or bases.
Hydrophobic and Hydrophilic Surfaces
For vascular surgical tools and instruments that may become fouled with tissue debris or blood, a plasma enhanced chemical deposition technique can provide a coating that keeps the surgeon's tools cleaner for longer periods.
This protective barrier is typically accomplished by applying a hydrophobic coating that repels water or biological fluids like blood. When such a coating is used on vascular surgical devices, blood and tissue flows off the coated tool in sheets so that the surgeon can see more easily when cutting, for example.
At the other end of the spectrum are hydrophilic devices. Depending on what is required, organic silicones can be used to create such surfaces with either polar or dispersive surface energy. Potential applications include coating polypropylene or polystyrene plates with alcohol or coating a device to facilitate protein bonding to the surface.
Many strategies can be implemented to achieve an antimicrobial surface, including cell harpoons, amphipathic surfaces, antiseptics bound to the surface, and nonstick coatings. In a unique application, chemical vapor deposition is being used to embed nanosilver particles in a thin layer of organic silicone to prevent microbial adhesion and protect against corrosion. Nanosilver, or colloidal silver, has been known for its antimicrobial effects from the earliest days of its use. Using a PECVD process, the tiny silver ions can be embedded in a thin layer of silicon dioxide to kill any bacteria present.
Fine-Tuning the PECVD-Applied Chemistry
Despite the flexibility of PECVD-applied organic silicones, developing the precise chemistries, added gases, and even plasma equipment design requires a close, collaborative relationship between medical device designers and equipment manufacturers. MicroVention, for example, already had an established relationship with PVA TePla. Several of PVA TePla's plasma chambers were being used to aid in coating adhesion, so MicroVention began consulting with them on a project to determine the benefits of coatings for stents.
Plasma equipment manufacturers fall into two categories: those that produce commodity, off-the-shelf products and those that design and engineer systems to fit the needs of a specific application or to resolve unique surface energy challenges. In many ways, the application of plasma to meet unique surface requirements is the domain of chemists and other scientists. This is reflected in the level of expertise at the company, which includes chemists that specialize in surface, polymer, physical, bio, and organic sciences, as well as engineers, plasma physicists, and metallurgists.
When companies present PVA TePla with a challenging surface chemistry problem, they are encouraged to visit the lab in Corona, CA, to brainstorm with their technical team and run experiments together. It is during these technical meetings that many of the best experimental matrices and ideas are produced. In addition to designing and manufacturing plasma systems, the company also serves as a contract manufacturer and so it has in-house equipment to run parts and conduct experiments, with full customer involvement.
“When we start on something new, instead of poking around in the dark it is better to get expertise involved and [PVA TePla] is very willing to do experimentations — often free of charge — to get the project moving and improve the characteristics of the system and chemistries involved,” says Aaron Baldwin, R&D project group leader at MicroVention. “We were able to go to their facility and work on their plasma machines to determine our parameters and evaluate the equipment.”
For companies such as PVA TePla, every system is designed to meet the specific requirements of the application, which can include unique fixtures, possibly unique electrodes, and chamber modification to accommodate throughput and coating uniformity. With organic silicones, the ability to thoroughly clean the chamber after each application is a major consideration because the silicone coats the entire interior of the chamber (including the electrodes) in addition to the products targeted for coating. To address this issue, PVA TePla modifies the chamber to make it easier to clean by the end user after every coating application.
The application of proprietary coatings and surface treatments can often be the differentiating characteristic that sets a medical device apart from its competitors. Coatings can improve function by making changes ranging from providing an antimicrobial barrier to promoting adhesion between disparate materials. Using PECVD addresses surface reaction issues from biocompatibility to lubricity and offers OEMs a unique method for modifying a surface because of its ability to retain the bulk material's properties as it deposits the coating.
This article was written by Ray Chen, Sales Manager for PVA TePla, Corona, CA. For more information, Click Here .