Chemical surface modification of polyimide film for enhanced collagen immobilization and cellular interactions

Different natural and synthetic polymers have been studied as an insulator and carrier for retinal prosthesis or as a scaffold for cell transplantation. The use of synthetic polymers outmatches natural polymers in some aspects including degradation, processability and strength. The synthetic polymers can be surface modified by proteins to promote their hydrophilicity, cell adhesion, and biocompatibility. Chemical surface modification is one of the stable means of protein immobilization in which proteins can be grafted covalently onto the surface. The generated covalent coupling protects the protein from shear stresses applied on the biomaterial and changes in pH of environment. The aim of this thesis was to modify the surface of polyimide (PI) membrane by covalent coupling of adhesive molecule collagen IV to improve the retinal cell interaction with PI substrates. Therefore, acrylic acid graft polymerization was carried out on the plasma treated membrane and the number of carboxyl groups on the membranes was determined using Toluidine Blue O (TBO) method. Lastly, a peptide bond was produced between collagen and carboxyl groups by means of carbodiimides and N-hydroxysuccinimide crosslinkers. The surface morphology and hydrophilicity of membranes were obtained by atomic force microscopy (AFM) and water contact angle measurements. The fluorescent labelling was applied to compare the surface density of immobilized collagen and its stability on the membranes. The modified PI substrate was further evaluated by in vitro study of ARPE-19 cell interactions. The results showed that the 25 % acrylic acid (AAc) monomer concentration provides more carboxyl groups on the membrane surface compared to 35 % AAc monomer concentration. The presence of grafted poly(acrylic acid) chains at the acrylic acid grafted membrane surface was also determined by Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Collagen-modified PI was found to be more hydrophilic in comparison to control membranes. Considering the AFM results, the surface modification protocol did not significantly affect the final surface roughness of membranes. High ranges of variation in the intensities were observed between the parallel samples in both collagen density determination and stability tests. The protein ZO-1 was observed more on surface modified membrane and ARPE-19 cells acquired more hexagonal cell morphology on them. As a conclusion, the concentration of grafted carboxyl groups on the membrane was strongly dependent on the concentration of AAc monomer solution. The surface modified membrane tested in this study show good potential as ARPE-19 cell substrate. However, means to prevent the aberrant cell division are suggested. The autofluorescence property of the membrane was the main issue in determination of collagen surface density. In addition, a more surface sensitive method like XPS is suggested to detect the presence of different functional groups on surface after each step in the protocol