Enhanced diffusion by binding to a crosslinked polymer gel

Biological hydrogels play an important role as selective permeability barriers in regulating the diffusive transport of molecules in a wide variety of systems. In many cases, the mechanism for selective filtering is well understood. However, such a mechanism is not obvious when particles larger than the mesh size differ only in the presence of a binding sit. This is because non-binding particles are trapped by the gel, so binding must result in increased, not decreased, mobility. We present an equilibrium mechanism for this counter-intuitive filtering strategy that does not consume energy. We show that selective mobility can be achieved and controlled in a simple crosslinked polymer gel by controlling the crosslink dynamics. Our results lead to specific design rules for manufacturing complex selective gels and could help explain how the Nuclear Pore Complex attains selectivity.