The permeation and translational diffusion of antibodies through the porous matrix of hydrogel materials is of fundamental relevance for many biological systems in living nature, but equally important in medical and technological applications, such as implanted drug release systems and biosensors. In this respect the diffusion of fluorophore-labeled protein immunoglobulin G (IgG) in micrometer thick, grafted hydrogel layers based on thermoresponsive poly(N-isopropylacrylamide) (pNiPAAm) is studied here by fluorescence correlation spectroscopy (FCS). The pore size of the gel gradually changes with its swelling state, which is controlled by the cross-link density of the network, temperature, and pH value of the surrounding medium. Notably, IgG permeation in these hydrogel layers exhibits a much more complex dependence on these factors. This rich variability of IgG permeation is attributed to the varying balance of protein interactions with the polymer network through electrostatics, controlled pH-dependent protein ionization, excluded volume repulsion, and hydrophobic attraction. A combined analysis of the fluorescence intensity profiles and the dynamics monitored by FCS allows us to quantify the thermodynamically controlled partitioning of IgG as well as the slowdown of its diffusion. Contrary to the complex behavior of the permeation, the diffusion slowdown seems to be a universal function of polymer volume fraction, which is rather robust with respect to temperature or pH changes. The presented findings suggest a model approach to explore the synergy between crowding and thermodynamics with respect to the controlled protein transport in pNiPAAm-based hydrogels.
- Biosensor Technologies