Coronary stents are tubular medical implants that serve as a scaffold to open clogged or narrowed arteries in an effort to increase blood flow and reduce the potential for adverse cardiac events such as heart attacks. And providing critical support to these support structures is parylene conformal coating.
Parylene use in stents has revolutionized the efficacy and capabilities of these life-changing medical implants. The novel conformal coating not only facilitates a smooth delivery of the medical device to its destination, it also provides a valuable vehicle for drug delivery.
The Parylene Deposition Process
In contrast to other conformal coatings that are brushed, dipped, or sprayed onto a substrate, parylene is applied via a vapor-deposition process in a vacuum chamber. During this vapor deposition polymerization process, raw dimer in powder form is subjected to high heat and, in turn, transforms into a gaseous monomer without an intermediate liquid stage. This gas is then deposited one molecule at a time onto the desired substrate in the coating chamber at ambient temperatures. Finally, the cold trap phase cools temperatures down to a range of -90° to -120°C in order to remove residual parylene.
This unique vapor-deposition method allows parylene to adequately penetrate and coat extremely small stent features. In addition, this process enables the uniform and consistent coating of the complex geometries that characterize the life-saving devices. Deposited in a 0.5-micron-layer thickness, parylene is also pinhole free and can fully coat inside crevices and contours as small as 0.001 mm. Parylene is impermeable upon deposition to a thickness of 1.4 nanometers.
Unlike some other conformal coatings, parylene doesn't adhere chemically to surfaces—it adheres mechanically. This characteristic is beneficial for parylene use in stents, as it promotes the effective coating of a wide range of substrates. In fact, parylene has been documented to coat everything from silicon to paper, from steel to a bird feather. Thus, parylene's ability to coat common stent materials such as stainless steel and nitinol is well documented.
Is Parylene Safe for Stents?
Parylene is an ideal conformal coating choice for stents, which are implantable medical devices, because it is extremely inert when inside the human body. It meets the USP Class VI implantable plastic material standard and conforms to RoHS standards, as well as to the ISO 10993 biocompatibility standard.
In addition to being inert and hydrophobic, parylene offers dry-film lubricity, which is particularly advantageous during device delivery and implantation. It features a coefficient of friction nearing that of Teflon, although it has significant benefits relative to Teflon, or PTFE, including a lack of toxic outgassing. Parylene is also clear, while Teflon is either milky or grainy in appearance.
Parylene use in stents serves multiple functions, ultimately helping to facilitate a smooth delivery while reducing irritation and inflammation.
Parylene Use in Drug-Eluting Stents
Despite the overall success of early stents, a sizable number of patients experienced restenosis, or the re-narrowing of arteries. And while coating stents with therapeutic agents was identified as a solution to this dangerous phenomenon, the solution brought its own technological challenges in terms of bonding drugs to the stent structure and controlling drug release.
Parylene use in stents has been key to overcoming these challenges, however, and making today's highly effective drug-eluting stents possible. First, stents were coated with parylene to give them a surface onto which drugs such as Sirolimus could be applied. The next step was to cover the drug-impregnated layer with another coating of parylene. This second coat is designed to be thin and partially porous, allowing the drug to pass through at a controlled rate of release. Some stents even use a series of multiple layers of drugs and parylene barriers for better long-term effectiveness.
As in other medical devices, parylene use in stents improves their effectiveness and, more importantly, patient outcomes. The conformal coating can be accurately deposited on any commonly used stent material and can serve both as a carrier for drug delivery as well as a mechanism for controlled release. Its capabilities in stents point to opportunities to create additional parylene-based drug-eluting technologies in the future.