M 2000 spectroscopic ellipsometer

Solutions of hydrophobin SC3 (SC3) were prepared by sonication of SC3 in DI water. With limited hydrophobin availability, only DI water was investigated as a solvent and low concentrations of SC3 (10–100 µg) were investigated. SC3 was adsorbed onto the surface of CarboSil®-SNAP wafers and the PDMS-SNAP samples by completely covering the samples with a 100 µL aliquot of SC3 solution for 24 hours at room temperature. The thickness of the adsorbed SC3 layers for the CarboSil®-SNAP wafers were measured using an M-2000 spectroscopic ellipsometer (J.A. Woollam Co., Inc.) with a white light source at three angles of incidence (65°, 70°, and 75°) to the silicon wafer normal. This was used as a validation to ensure adsorption of SC3 to the samples as PDMS-SNAP samples were too thick and undefined in the model to be used for ellipsometric measurements.

The thicknesses of spin coated films (prepared according to section 2.4.) were measured using a M-2000 spectroscopic ellipsometer (J.A. Woollam Co., Inc.) with a white light source at three angles of incidence (65, 70, and 75°) to the silicon wafer normal. Three replicates were used for each measurement. After thickness of each film was measured, a non-saline phosphate buffer solution was prepared from 1 M sodium phosphate dibasic and 1 M potassium phosphate monobasic. The solution was adjusted to a pH of 7.41 at room temperature (25°C). Samples were kept in the non-saline phosphate buffer for 30 minutes at 37°C. A solution of fibrinogen from bovine plasma and non-saline phosphate buffer was prepared to achieve a concentration of 1mg mL⁻¹ once added to the non-saline phosphate buffer the samples were first placed in. After the protein solution was added to the samples, they were allowed to incubate at 37°C for 90 minutes. After incubation each sample was washed with 5mL of non-saline PBS five consecutive times followed by 5mL of distilled water five consecutive times. The thickness of the wafers before and after submersion in the protein solution was measured by spectroscopic ellipsometry. Care was taken to measure the thickness on the same area of the films as measured before to avoid any inconsistency in data collection.

CarboSil®, a copolymer marketed by DSM Biomedical as a combination of silicone elastomers and thermoplastic polycarbonate-urethanes was chosen to be the NO-releasing base for the contact angle measurements and protein adhesion assessments. This is a copolymer that is easily spin-coated on silicon wafers and hence avoids the incorrect readings that the thinnest PDMS spin coated films (usually upwards of 200 nm thickness compared to 50 nm thickness of CarboSil® thin films) can produce on ellipsometry measurements. A thin layer of CarboSil®, with and without SNAP, was deposited on silicon wafers by spin coating. The spin coating solution was prepared by dissolving CarboSil® in THF to achieve a concentration of 1 mg mL^-1. After the CarboSil® was completely dissolved, 10 wt.% of SNAP was added to the CarboSil®-THF solution. This mixture was protected from light and stirred until the SNAP crystals were dissolved completely. Using a CHEMAT Technology KW-4A spin coater, films were spin coated at 2500 rpm for 30 seconds. The resulting films formed were highly uniform with a surface thickness of 40–50 nm. These thin films were used for contact angle measurements using a Krüss DSA100 Drop Shape Analyzer (sessile drop method with deionized water) and for studying protein adhesion using an M-2000 spectroscopic ellipsometer (J.A. Woollam Co., Inc.)