How deeply cells feel: methods for thin gels

TLDR

Cells can sense the stiffness of their surroundings and even detect the effective rigidity of distant rigid objects through mechanotransduction. The authors measured how deeply cells probe a matrix by culturing mesenchymal stem cells on a series of thin polyacrylamide gels of controlled thickness and elasticity, bonded to glass via vinyl groups, with gel thickness calibrated by bead spacers and swelling, and gel stiffness quantified by atomic force microscopy. Cells begin to respond to the rigidity of an underlying hidden surface when gel thickness reaches 10–20 µm, indicating a tactile length of less than about 5 µm.

Abstract

Tissue cells lack the ability to see or hear but have evolved mechanisms to feel into their surroundings and sense a collective stiffness. A cell can even sense the effective stiffness of rigid objects that are not in direct cellular contact—like the proverbial princess who feels a pea placed beneath soft mattresses. How deeply a cell feels into a matrix can be measured by assessing cell responses on a controlled series of thin and elastic gels that are affixed to a rigid substrate. Gel elasticity E is readily varied with polymer concentrations of now-standard polyacrylamide hydrogels, but to eliminate wrinkling and detachment of thin gels from an underlying glass coverslip, vinyl groups are bonded to the glass before polymerization. Gel thickness is nominally specified using micron-scale beads that act as spacers, but gels swell after polymerization as measured by z-section, confocal microscopy of fluorescent gels. Atomic force microscopy is used to measure E at gel surfaces, employing stresses and strains that are typically generated by cells and yielding values for E that span a broad range of tissue microenvironments. To illustrate cell sensitivities to a series of thin-to-thick gels, the adhesive spreading of mesenchymal stem cells was measured on gel mimics of a very soft tissue (e.g. brain, E ∼ 1 kPa). Initial results show that cells increasingly respond to the rigidity of an underlying 'hidden' surface starting at about 10–20 µm gel thickness with a characteristic tactile length of less than about 5 µm.