Proteins at aqueous interfaces: Adsorption, cohesion and microfluidic applications

The adsorption of proteins at aqueous interfaces and the cohesion of protein films are important for many biomedical applications. Problems associated with the adsorption and cohesion of proteins include non-specific protein adsorption in microfluidic devices or in microarrays, and difficulties in controlling and assessing the cohesion of adhesive proteins. Surface properties play an important role in the adsorption and cohesion of proteins at many biomaterial surfaces. The introductory chapter reviews what is known about the behavior of proteins on materials commonly used in microfluidic devices, and how protein adsorption can be minimized or controlled. Structural changes in proteins upon their adsorption to surfaces make them lose their activity and are difficult to desorb. The surface coverage and the extent of structural change in proteins adsorbed on particulate surfaces have been characterized with a stirred cell and breakthrough curves analyzed by frontal analysis and numerical integration. These results of protein adsorption helped establish a method to prevent biomolecular adsorption in electrowetting-based microfluidic actuation, which utilizes electrical modification of the surface hydrophobicity for moving liquids in microfluidic chips. Biomolecular adsorption can be prevented by minimizing hydrophobic interactions and utilizing electrostatic repulsion between proteins and microfluidic surfaces. This is accomplished through choice of buffer and electrode polarity. These results are extended to a system in which the two microfluidic surfaces are in contact with the droplet (two-plate closed channel system). Protein adsorption could be further minimized by grid-patterning the electrode to minimize the surface area that is actively charged. Finally, the cohesion of a model adhesive protein, Mytilus edulis foot protein 1 (Mefp-1), has been investigated for potential use as an adhesive for dental, surgical or tissue engineering. Crosslinking and fragmentation of Mefp-1 thin films have been monitored through their changes in complex shear modulus using a thickness shear mode (TSM) resonator. Exposure to air, UV irradiation, hydrogen peroxide or bleach solution can induce crosslinking of Mefp-1. Ozone is found to fragment only pre-crosslinked Mefp-1 dry films. The shear modulus of Mefp-1 films was calculated as 6 MPa for non-crosslinked and ∼47 MPa for crosslinked films. The results extend an understanding of protein adsorption and cohesion, and suggest mechanisms for controlling adsorptions in microfluidics and biomaterial applications.