Chromic acid

An optimized in vitro test setup was used for the immersion tests [42 (link), 43 ]. Cell culture medium (CCM), Dulbecco's Modified Eagle's Medium (DMEM), and GlutaMAX +10% FBS (fetal bovine serum, PAA laboratories, Linz, Austria) were used for the immersion test and cell culture. The cell culture conditions were 5.0% CO₂, 20% O₂, 37.0°C, and 97.0% rH (relative humidity). Discs were weighed by a Scaltec SBA32 (Scaltec, Goettingen, Germany) and sterilized ultrasonically in 70% ethanol solution for 30 min. After drying, the discs were transferred to 12-well plates, which were filled with 2 mL CCM in each well, and then incubated in the cell culture conditions for 1 week. The old cell culture medium was replaced by a fresh one after 48 and 120 hours. After the immersion test was carried out for 168 hours, the degradation products were removed by chromic acid (180 g/L in a. dest., Sigma-Aldrich Chemie, Taufkirchen, Germany). The discs were dried in a vacuum box and weighed by the precision electronic balance. The mean degradation rate was calculated using the previously described weight loss method [44 (link)].

Phosphoric acid, acetic acid, chromic acid, nitric acid, perchloric acid, oxalic acid, ethanol, acetone, and terephthalic acid were purchased from Sigma-Aldrich and used without further purification. The photoresist AZ 1512 and the AZ developer CD30 were obtained from Clariant (Somerville, NJ), and high purity aluminum sheet was obtained from Sigma-Aldrich. Deionized water (18.3 MΩ cm) was obtained by using a reverse osmosis water system (Human Science, Korea).