Cells in multicellular organisms, such as humans, have evolved a mechanically-sensitive signaling pathway called mechanotransduction. [1] Interestingly, mechanotransduction has been shown to direct stem cell differentiation towards different cell types, such as muscle or bone cells, simply by changing the environmental elasticity.[2] Additionally, cell proliferation and differentiation have also been shown to be viscosity-sensitive, with a potential effect on mechanotransduction.[3] Despite its importance to cell culture and growth, the effect of environmental viscoelasicity on mechanotransduction is still not well understood. While complex, light-based techniques exist to tune environmental stiffness, they are incapable of tuning environmental viscosity, missing a potentially important effect on cells.

Here, we present a simple, cost-effective means of applying macromolecular crowding (MMC) to polymeric hydrogels used as cell scaffolds to tune their viscoelasticity. Crowding agents dissolved in solution can apply MMC conditions to polymer hydrogels by infiltrating their matrix. MMC both increases solution viscosity, and reduces the number of conformations available to the hydrogel polymers, restricting their thermally-induced structural fluctuations and causing the hydrogel to stiffen.[4] Alginate, a polymeric hydrogel derived from algae, is an ideal candidate for MMC-induced viscoelastic control: with a relatively large ~5 nm pore size, small crowding agents can passively diffuse into the hydrogel. In this work, we show rheometry data demonstrating how poly(ethylene glycol), or PEG, dissolved in solution is used to tune alginate viscoelasticity. We also demonstrate the clear effect on the morphology of cells grown on crowded alginate hydrogels, with cells displaying compelling differences when compared to controls both with and without crowding (see also poster by Villiou, et al.). This study will be of interest to the biophysical community, demonstrating a simple means of tuning the cellular environment.

[1] M., Raab Science, 2016, 352, 359-362.

[2] S. Jungbauer Biophysical Journal, 2008, 95, 3470-3478.

[3] O. Chaudhuri Nature Materials, 2016, 15, 326-334.

[4] G. Guigas Biophysical Journal, 2007, 93, 316-323.