New design of hydrogels with tuned electro-osmosis: a potential model system to understand electro-kinetic transport in biological tissues

Electro-osmosis is the bulk fluid flow of electrolyte solution generated by an electric field in contact with charged immobile objects, such as the surface of polymer gels. Electro-osmosis may be used to assist in analytical separation, and plays an important role in biological tissues. Despite its importance, the contribution of electro-osmosis to the overall solute transport has never been fully understood because of the complexity of the biological systems. To understand the electro-osmosis, a model system with controlled charge density, charge composition and stiffness is necessary. In this paper, we propose a new design of a gel-based system (tetra-PEG gel) with well controlled electro-osmotic properties. Tetra-PEG gels are formed by two four-armed poly(ethylene)glycol units with mutually reactive end groups, allowing the introduction of positive or negative charges by variation in the monomer stoichiometric ratio. Using this system, we for the first time succeeded in preparing a series of hydrogels with precisely and independently tuned charge densities and stiffness. Electro-osmotic flows of these gels were distinctly different from each other, and varied linearly with the total charge amount. Interestingly, a hydrogel with the same amount of positive and negative charges did not show any electro-osmotic flow. Our new hydrogel system provides an important basis for understanding the electro-osmotic flow as an additional mechanism in mass transport and will find use in biomaterials and analytical separations, and as a model system for biological tissues.