Size-Dependent Suppression of Molecular Diffusivity in Expandable Hydrogels: A Single-Molecule Study

By repurposing the recently popularized expansion microscopy to control the meshwork size of hydrogels, we examine the size-dependent suppression of molecular diffusivity in the resultant tuned hydrogel nanomatrices over a wide range of polymer fractions of similar to 0.14-7 wt %. With our recently developed single-molecule displacement/diffusivity mapping (SMdM) mi-croscopy methods, we thus show that with a fixed meshwork size, larger molecules exhibit more impeded diffusion and that, for the same molecule, diffusion is progressively more suppressed as the meshwork size is reduced; this effect is more prominent for the larger molecules. Moreover, we show that the meshwork-induced obstruction of diffusion is uncoupled from the suppression of diffusion due to increased solution viscosities. Thus, the two mechanisms, respectively, being diffuser-size-dependent and independent, may separately scale down molecular diffusivity to produce the final diffusion slowdown in complex systems like the cell.

Automated summary

By using expansion microscopy, we investigated the size-dependent suppression of molecular diffusivity in hydrogel nanomatrices with controlled meshwork size. Our research demonstrated that larger molecules experience more impeded diffusion within a fixed meshwork size, and diffusion is progressively suppressed as the meshwork size decreases, particularly for larger molecules. We also found that the obstruction of diffusion caused by the meshwork is independent of the suppression of diffusion due to increased solution viscosities. These mechanisms can scale down molecular diffusivity to produce diffusion slowdown in complex systems like cells.