Preparation of plasma-polymerized para-xylene as an alternative to parylene coating for biomedical surface modification

Parylene (or poly-para-xylene) coating is known to be a transparent, uniform and effective barrier. There are however some drawbacks, including an expensive starting material, high thermal energy consumption for monomer generation, high vacuum requirements and a low growth rate. In this study, low-cost para-xylene was used as the starting material to form plasma-polymerized films. A pulsed-dc plasma power supply was used and the deposits were examined for their microstructure, mechanical properties and fibroblast cytotoxicity.Experimental results reveal that the proposed coatings exhibit an amorphous structure with a deposition rate widely ranging from 50 to 480 nm/h, depending on the pulse frequency (ωp) of the input power and monomer flow rate (fp). At high fp and low ωp, the plasma-polymerized para-xylene (PPX) films exhibit alkane and alkene features, indicating a more organic character. In contrast, the films obtained at low fp and high ωp present inorganic features. For its mechanical properties, a pencil hardness of 7H–8H in the coated specimen is higher than that of the conventional parylene coating, and the film adhesion graded at 4B determined using the cross-cut test is similar to that of the conventional parylene coating. The water contact angle of PPX films in our study was measured and ranged from 60° to 85°, which is more hydrophilic than the conventional parylene coating. Cell cultures on PPX deposited specimens present a higher cell count than the parylene deposited film. The PPX films deposited at high ωp and low fp exhibit a lower water contact angle (hydrophilic), which accounts for its better cell compatibility. These quantitative indications imply that the PPX is a possible alternative to the conventional parylene coating for biomedical device surface modification.