Non-specific adsorption of protein to microfluidic materials

Non-specific adsorption of proteins to the surfaces of microfluidic channels poses a serious problem in lab-on-a-chip devices involving complex biological fluids. Materials commonly used in the formation of microfluidic channels include CYTOP, silica and SU-8. CYTOP is a transparent fluoropolymer (Poly[perfluoro(4-vinyloxy-1-butene)]) with a low refractive index that approximately matches the refractive index of biologically compatible fluids, and is useful in optical biosensors. Using a microfluidic and fluorescence microscopy set-up, the non-specific adsorption of bovine serum albumin (BSA) labeled with fluorescein isothiocyanate (FITC) to three grades of CYTOP (S, M and A), silica, and SU-8 is investigated. Surface properties such as roughness and wettability are also characterized via an atomic force microscope and a contact angle measurement system. The non-specific adsorption of protein occurs with a highly variable load across these materials. Surprisingly, significantly lower adsorption occurred on SU-8 compared to the other materials, likely due to its hydrophilicity (post-cleaning). Among the 3 grades of CYTOP considered, the lowest adsorption occurred on S-grade. BSA adsorption to silica was higher than on S-grade CYTOP and significantly higher than on SU-8 despite being hydrophilic, due to a fixed positive charge formed within the layer during fabrication, which attracts negatively-charged BSA in buffer.