Parylene C Deposition and Oxygen Penetration Measurements: Evaluation and Optimization of Coating Uniformity, Surface Adhesion, and Oxygen Barrier Properties for Organ-on-Chip Applications
Shakoor, Muhammad Umair (2025)
2025
Master's Programme in Biomedical Sciences and Engineering
Lääketieteen ja terveysteknologian tiedekunta - Faculty of Medicine and Health Technology
Hyväksymispäivämäärä
2025-12-11
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-2025121011476
https://urn.fi/URN:NBN:fi:tuni-2025121011476
Tiivistelmä
Organ-on-Chip (OoC) devices require precise oxygen control to mimic tissue specific oxygen environments. Polydimethylsiloxane (PDMS) is a widely used material in OoC fabrication, as it possesses many favourable properties, in including low thermal conductivity, biocompatibility, hydrophobicity, bioinertness, transparency and high gas permeability. While the high gas permeability can facilitate active localized oxygen control through thin membranes or channel walls, it can also create challenges in other parts of the device if they are allowed to interact with ambient conditions or to adjacent compartments maintained at different oxygen levels.
This study investigates the use of Parylene C, a low-permeability polymer, as a conformal coating to regulate oxygen diffusion in OoC devices made of PDMS. Parylene C was deposited via chemical vapor deposition (CVD) using two-stage system (upper and lower stages) by varying dimer amounts (3 g, 5 g, 10 g, and 15 g) and pre-surface treatments. Coatings were applied on glass, silicon, and PDMS-coated glass to identify optimal conditions for uniform, defect-free, and scalable films, and adhesion to PDMS was measured.
Deposition on the upper and lower stages of the chamber resulted in thickness yields of ~0.65 µm/g and ~0.55 µm/g of dimer, respectively. A key finding was the interaction between Parylene C and PDMS, producing ~50% thicker coatings (~0.95 µm/g) than on rigid substrates, likely due to monomer absorption or surface kinetics. All rigid substrates exhibited uniform coatings with low thickness % coefficient of variation (%CV ≤ 1.9), whereas higher values were observed for PDMS at higher dimer loads. The micrographs showed that the Parylene C coating remained continuous and free of defects at thicknesses of at least 6.5 µm.
Adhesion was assessed via ASTM D3359-17 cross-hatch and pull-off adhesion tests, achieving thickness-independent excellent adhesion (with 5B ratings), and adhesion strength of 0.53 MPa on untreated PDMS. This moderate adhesion strength is sufficient for static oxygen-barrier applications. Adhesion of Parylene C on silicon was also observed to be excellent with 5B rating even at thin coating of 2.2 µm. However, its adhesion on glass was thickness-dependent (0B at 3.1 µm, 2B at 6.5 µm, and 4B at 9.3 µm), while Parylene C on plasma-treated PDMS failed completely (0B rating). This failure may be related to hydrophobic recovery during the 40–60-minute delay between plasma treatment and deposition, and possibly to the poor intrinsic adhesion of Parylene C to the silica-like layer formed by plasma.
For adhesion evaluation, samples were immersed in deionized water for one and two months and then examined using microscopy and ASTM D3359-17 cross-hatch testing. The Parylene C/PDMS interface showed strong hydrolytic resistance, maintaining a 5B rating with a uniform surface. In contrast, Parylene C on glass exhibited complete adhesion failure (0B rating) regardless of thickness, while adhesion on silicon decreased to 1B–2B rating scale. Microscopy of glass and silicon revealed clear evidence of water ingress. Cell culture medium was applied to Parylene C coating and incubated for five days, salt crystallites formed on the surface; however, oxygen diffusion remained unchanged, and adhesion to PDMS persisted at an excellent 5B rating.
For oxygen diffusion, samples with integrated luminescence-based ratiometric pO2-sensing films were evaluated using step-response analysis. An individual Parylene C layer provided a time lag of only a few minutes, whereas integration on highly oxygen-soluble PDMS extended the lag to several hours. A thick PDMS substrate (~330 µm) further increased the time lag to over 18 hours. The coating sequence was critical, as placing Parylene C beneath the PDMS did not enhance oxygen regulation. These results indicate that, in the Parylene C-on-PDMS configuration, oxygen control is dominated by the thickness of the underlying PDMS rather than Parylene C. This study provides a practical strategy for sustaining long-term hypoxic conditions in OoC devices.
This study investigates the use of Parylene C, a low-permeability polymer, as a conformal coating to regulate oxygen diffusion in OoC devices made of PDMS. Parylene C was deposited via chemical vapor deposition (CVD) using two-stage system (upper and lower stages) by varying dimer amounts (3 g, 5 g, 10 g, and 15 g) and pre-surface treatments. Coatings were applied on glass, silicon, and PDMS-coated glass to identify optimal conditions for uniform, defect-free, and scalable films, and adhesion to PDMS was measured.
Deposition on the upper and lower stages of the chamber resulted in thickness yields of ~0.65 µm/g and ~0.55 µm/g of dimer, respectively. A key finding was the interaction between Parylene C and PDMS, producing ~50% thicker coatings (~0.95 µm/g) than on rigid substrates, likely due to monomer absorption or surface kinetics. All rigid substrates exhibited uniform coatings with low thickness % coefficient of variation (%CV ≤ 1.9), whereas higher values were observed for PDMS at higher dimer loads. The micrographs showed that the Parylene C coating remained continuous and free of defects at thicknesses of at least 6.5 µm.
Adhesion was assessed via ASTM D3359-17 cross-hatch and pull-off adhesion tests, achieving thickness-independent excellent adhesion (with 5B ratings), and adhesion strength of 0.53 MPa on untreated PDMS. This moderate adhesion strength is sufficient for static oxygen-barrier applications. Adhesion of Parylene C on silicon was also observed to be excellent with 5B rating even at thin coating of 2.2 µm. However, its adhesion on glass was thickness-dependent (0B at 3.1 µm, 2B at 6.5 µm, and 4B at 9.3 µm), while Parylene C on plasma-treated PDMS failed completely (0B rating). This failure may be related to hydrophobic recovery during the 40–60-minute delay between plasma treatment and deposition, and possibly to the poor intrinsic adhesion of Parylene C to the silica-like layer formed by plasma.
For adhesion evaluation, samples were immersed in deionized water for one and two months and then examined using microscopy and ASTM D3359-17 cross-hatch testing. The Parylene C/PDMS interface showed strong hydrolytic resistance, maintaining a 5B rating with a uniform surface. In contrast, Parylene C on glass exhibited complete adhesion failure (0B rating) regardless of thickness, while adhesion on silicon decreased to 1B–2B rating scale. Microscopy of glass and silicon revealed clear evidence of water ingress. Cell culture medium was applied to Parylene C coating and incubated for five days, salt crystallites formed on the surface; however, oxygen diffusion remained unchanged, and adhesion to PDMS persisted at an excellent 5B rating.
For oxygen diffusion, samples with integrated luminescence-based ratiometric pO2-sensing films were evaluated using step-response analysis. An individual Parylene C layer provided a time lag of only a few minutes, whereas integration on highly oxygen-soluble PDMS extended the lag to several hours. A thick PDMS substrate (~330 µm) further increased the time lag to over 18 hours. The coating sequence was critical, as placing Parylene C beneath the PDMS did not enhance oxygen regulation. These results indicate that, in the Parylene C-on-PDMS configuration, oxygen control is dominated by the thickness of the underlying PDMS rather than Parylene C. This study provides a practical strategy for sustaining long-term hypoxic conditions in OoC devices.