Characterization and Modification of Adhesion in Dry and Wet Environments in Thin-Film Parylene Systems

Parylene C is a thin-film polymer used as a biocompatible barrier layer in biomedical implants and implantable MEMS; free-film Parylene C may serve as both the substrate and insulation in polymer-based microdevices, a growing branch of biomedical technology. The adhesion of vapor deposited Parylene C, particularly when exposed to wet, in vivo environments, is a critical determinant of device lifetime for such polymer-based implants. This paper explores several novel strategies for improving the adhesion of multi-layer Parylene structures, including thermal annealing and the use of several chemical interposer layers. Interfacial adhesion of Parylene-Parylene and Parylene-platinum-Parylene films was examined using a standard T-peel test to quantify adhesion and measure film integrity under chronic exposure to saline up to two years. Improved adhesion and barrier properties in Parylene-Parylene films resulted from the inclusion of diamond-like carbon and ethylene glycol diacrylate layers. Thermal annealing improved Parylene film integrity in wet environments but was insufficient for improving the integrity of Parylene-platinum interfaces. A 100-fold increase in adhesive strength at such interfaces was achieved using a commercially available adhesion promoter, and the corresponding improvements in resistance to moisture driven delamination were observed. X-ray diffraction and X-ray photoelectron spectroscopy results are provided to highlight the role of film morphology and surface composition in adhesion integrity.