| Summary: | 博士 === 逢甲大學 === 材料科學所 === 100 === Parylene (or poly-p-xylene) coating is known to be a thin, continuous, inert, transparent and conformal, and has been widely used in medical, aerospace, military and electronic industries. With these unique properties, such a coating has been let to be used for biomedical material surface treatment with respect to its excellent biocompatibility, non-cytotoxicity, effective barrier and lubricity. However, there are some drawbacks of the traditional parylene deposition process, including the usage of expensive starting material, high thermal energy consumption for monomer generation, relatively low deposition rate, low film hardness and higher vacuum degree requirement. In this study, low-cost p-xylene was used as the starting material and a pulsed-dc plasma polymerization process was introduced to deposit solid films with adjusting the process parameters: pulse frequency of the input power (ωp) and p-xylene monomer flow rate (fp). The deposited films were examined for their microstructural characteristics, protective performance and biocompatibility with in vitro and in vivo tests.
Experimental results reveal that plasma-polymerized p-xylene (PPX) films are amorphous and featureless in cross-sectional view structure with a deposition rate ranging from 0.05 to 0.48 μm/h. Regardless of ωp and fp, PPX films exhibit good film adhesion (graded at 4B–5B determined by the cross-cut test), high film hardness (graded at 8H determined by the pencil hardness test), hydrophobicity (water contact angle ranging from 98.5o to 121.1o) and smoother surface roughness (root mean square surface roughness, RMS, value ranging from 0.2 to 0.5μm). At low ωp and high fp, PPX films present higher film growth rate, more hydrophobicity, higher surface roughness and short-chain alkane feature. In comparison with the results of quadrupole mass spectrometry (QMS) analysis, the concentration of species in plasma space (including C8H10, C8H9, C7H7, C6H5, CH3, H2 and H) increases at an increased fp, which subsequently results in higher film growth rate. By increasing ωp (equivalent to the increased power density), significant fragmentations occur to the larger molecular species (C8H10, C8H9, C7H7 and C6H5), which contribute to the films obtained at high ωp and low fp exhibiting lower film growth rate, less water repelling, lower surface roughness and long-chain alkene feature. Moreover, the increased H?and H2 at an increased ωp detected by optical emission spectrometry (OES) in plasma space indicate that the recombination of active species (in plasma), dehydrogenation reaction (with the condensed film) and physical etching by argon ion bombardment (increased intensity both in QMS and OES) result in the decreased film growth rate as well as the decreased surface roughness of the deposited films.
The results of in vitro tests reveal that PPX films all exhibit good cell proliferation (especially at high ωp or high fp), less cell adhesion and a comparatively lower level of platelet adhesion because of their hydrophobicity and smoother surface. In the in vivo study, PPX-coated specimens result in similar local tissue responses at 3, 7 and 28 days (short-term) and 84 days (long-term) after subcutaneous implantation in the abdominal wall of rodents compared with respective responses to parylene-coated specimens and medical-grade silicone sheets. These results suggest that the PPX films may be essentially bio-tolerate and can thus be considered for application in the surface modification of biomedical devices where tissue fluids or blood will be encountered.
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