Parylenes N and C have been employed for coating a range of ever smaller implantable and external devices. The recent availability of Parylene HT now offers additional advantages for medical device applications. The properties of these materials are reviewed.
Protective packaging
Although the demand for reliability and safety has always been at the forefront in any medical device design, the challenges of minimally invasive, micro and nano medical devices require new and different packaging solutions. Many devices used for diagnoses or analyses are in intimate contact with body tissue or fluids and must not be the source of undesired effects on the human body. It is imperative that these devices are resistant to corrosive body fluids and tissue. Implantable devices often incorporate microelectronic components and systems intended to sense or facilitate a physiological response. They require isolation from moisture, chemicals, electric potentials and other substances. It may also be useful to immobilise microscopic particles found in the materials from which the device is fabricated, or to impart surface lubricity.
For some time, medical device manufacturers have been challenging the limits of current technology and have often adopted technologies proven in other markets. New packaging techniques, driven by the desire for less invasive procedures, have dramatically reduced the size of devices and component space requirements. Device components such as magnetic and other sensors, capacitors, power sources, active and passive devices are sized into smaller packages and achieve enhanced performance and reduced weight. The safety and efficacy of these devices depend on the quality of their components, and the design, assembly and the integrity of their packaging.
Efforts to protect devices from their operating environments and to protect patients from the devices have led manufacturers into the world of exotic materials and polymer coatings. Microminiaturisation of devices demands thinner coatings without loss of their protective properties.
Parylenes have been used in the medical device industry for decades. Parylene coatings, applied via vapour phase deposition, have found application in numerous implantable devices, including pacemakers, implantable cardiac defibrillators, coronary stents, cochlear and ocular implants and neurostimulators. External devices such as transdermal drug delivery devices, digital dental imaging systems, surgical and endoscopic devices, to name a few, have also benefitted from Parylene’s lubricity
and barrier properties. Its combination of physical and chemical properties and biocompatibility encourage designers to explore new roles for the material in micro and nano dimension medical devices. Recently, researchers have created a variety of devices utilising Parylene and microelectromechanical systems technology. Parylenes have also been employed as structural materials for microfluidic devices. Examples of these developments include a mass flow controller (consisting of an electrostatically actuated microvalve), electrophoresis channels and electrostatic actuators.
Coating process
Parylene is the generic name for members of a unique polymer series. The Parylenes (xylylene polymers) have been classified as thermoplastic polymers that are formed on substrate surfaces using vacuum deposition polymerisation. They are polycrystalline and linear in nature and possess useful dielectric and barrier properties per unit thickness. They are also chemically inert and form thin layer coatings without pinholes.
Parylenes are applied to substrates in a vacuum chamber and have certain similarities with vacuum metallising. Unlike vacuum metallisation, however, which is conducted at pressures of 10-5 torr or below, Parylene coatings are formed at approximately 0.1 torr. Under these conditions, the mean free path of the gas molecules in the deposition chamber is in the order of 0.1cm. As a result, all sides of an object to be coated are uniformly covered by the gaseous monomer, which provides a high degree of conformability. Although research and development efforts during the past four decades have resulted in several Parylene types, only two of them, Parylenes N and C, have found wide commercial application in medical fields to date. Properties of Parylenes N and C are described extensively in the literature. The recent availability of Parylene HT now offers additional advantages for medical device applications.
Characteristics of reliability
The most valued attributes that affect medical applications include biostability; biocompatibility; controlled thickness down to 500 Angstroms; chemical inertness; sterilisation stability; microencapsulation capabilities; chemical, electrical, moisture barrier and corrosion resistance; and dry-film lubricity. Each property has its own significance for specific applications.
Parylenes N, C and HT have met biological evaluation requirements according to ISO 10993, Biological Evaluation of
Medical Devices. Certification to ISO 10993 and United States Pharmacopeia Class VI biological studies can be referenced via the Food and Drug Administration Master File that is maintained by Parylene coating service providers.
Barrier properties
[5]The material’s moisture and chemical barrier attributes are well suited to medical devices. Experimental results demonstrated (Figure 1) that Parylene C coating decreased extraction of metals from coated rubber samples. A coating one micron in thickness on a rubber sample decreased the zinc concentration from 50 ppm to 0 ppm and aluminum concentration from 4 ppm to 0 ppm.
To protect cardiac pacemaker electronic modules, several coatings were evaluated, including Parylene C. The coated modules were immersed in 0.9 % saline solution at 37 °C. On the first day of testing, units were removed every four hours from the saline bath, washed in de-ionised water and dried in air for 30 min at 55 °C. The test parameters such as pulse width, current drain and pulse interval were then measured and modules were returned to immersion for continued testing. After the first 24 hours, parameters were measured daily until units failed completely. The test unit coated with Parylene C performed well over a period of 30 days compared with silicone, the second best coating, which lasted for 58 hours. All other coatings, which included epoxies, polyurethanes, silicones and polytetrafluoroethylenes, failed within 8 hours.
Parylene C has been found to be suitable for protecting nonhermetically packaged integrated circuits for use inside the human body. Evaluation of Parylene C has shown that it can withstand immersion in saline solution for more than 320 days under continuous bias of 3V and still protect the current carrying conductors on the surface of the integrated circuit device it coats.
Parylene coatings demonstrate resistance to organic solvents, inorganic reagents and acids. At temperatures below 150 °C,
Parylenes resist attempts to dissolve them, and exposure to organic solvents causes a slight swelling (< 3%). Water vapour transmission rate (WVTR) is one of the important attributes of any protective coating and these materials offer low WVTR.
Glass panels coated with Parylenes N, C and HT showed no fungal growth when subjected to fungus resistance test in accordance with ASTM G-21, Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi, which indicates good antifungal properties. This is beneficial for nonimplantable medical electronic devices that are used in fungal prone environments.
Bulk electrical properties
The dielectric constant and dielectric losses of Parylenes are low and unaffected by moisture absorption. The bulk resistivities are advantageously high because of their low moisture absorption and, in particular, their freedom from trace ionic impurities. The dielectric strengths of Parylenes range from 5400–7000 volts/mil. Parylene HT is particularly suitable for high frequency devices applications because of its low dielectric constant.
Sterilisation
Implantable devices and substrates that use Parylene coating for protection may be required to be sterilised once or repeatedly. Parylenes N and C were subjected to sterilisation testing, including steam autoclave, gamma and electron beam irradiation, and processing by H2O2 plasma and ethylene oxide (EtO). Post-sterilisation analysis evaluated the impact of these sterilising agents on Parylenes against unsterilised control samples. Electron beam and gamma sterilisation have no impact on the coating properties of Parylenes N and C. However, steam sterilisation affected the WVTR and tensile modulus of Parylenes N and C, and the coefficient of friction of Parylene N. EtO sterilisation had an effect on the WVTR of Parylene N and C, and on the coefficient of friction of Parylene N. H2O2 plasma altered the coefficient of friction of Parylenes N and C and the dielectric strength of Parylene C.
Films of Parylenes N, C and HT
demonstrate a high degree of resistance to degradation by gamma rays in vacuum. Tensile and electrical properties were unchanged after 1000 kGy dosage at a dose rate of 16 kGy/hr.
Thermal oxidation
Parylenes exhibit changes in mechanical properties with changes in temperature much as do other materials. In oxygen-free atmospheres or in the vacuum of space, the continuous service temperature projections exceed 200 °C for Parylenes N and C. Parylene HT has the ability to resist thermal oxidation up to 450 °C in oxygen and oxygen-free atmospheres.
Dry film lubricity
The dry film lubricity properties of a coating are generally indicated by its coefficient of friction. A lower coefficient of friction indicates that surfaces are “slicker,” that is, there is less resistance to sliding motion. The coefficient of friction values (static) for Parylenes N, C and HT per ASTM D 1894, Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, are 0.25, 0.29 and 0.15, respectively. Parylenes N and C are suitable for many medical applications, but Parylene HT offers the advantage of an even lower coefficient of friction.
Expanding options
With their valuable inherent physical and chemical properties, Parylenes N, C and HT offer interesting options for various medical device applications, particularly for protection and enhancing overall reliability.
Dr Rakesh Kumar is Vice President of Technology at Specialty Coating Systems Inc., 7645 Woodland Drive, Indianapolis, Indiana 46278, USA, tel. + 1 317 244 1200, e-mail: rkumar@scscoatings.com [6].
Address in Europe: Forsythe Road, Sheerwater, Woking GU21 5RZ, UK, tel. +44 1483 541 000, www.scscoatings.com [7]