How deeply cells feel: methods for thin gels

Abstract

Tissue cells lack the ability to see or hear but have evolved mechanisms to feelinto their surroundings and sense a collective stiffness. A cell can even sense theeffective stiffness of rigid objects that are not in direct cellular contact—like theproverbial princess who feels a pea placed beneath soft mattresses. How deeply a cellfeels into a matrix can be measured by assessing cell responses on a controlledseries of thin and elastic gels that are affixed to a rigid substrate. Gel elasticityE is readily varied with polymer concentrations of now-standard polyacrylamide hydrogels, butto eliminate wrinkling and detachment of thin gels from an underlying glass coverslip, vinylgroups are bonded to the glass before polymerization. Gel thickness is nominally specified usingmicron-scale beads that act as spacers, but gels swell after polymerization as measured byz-section, confocal microscopy of fluorescent gels. Atomic force microscopy is used to measureE atgel surfaces, employing stresses and strains that are typically generated by cells and yielding valuesfor E that span a broad range of tissue microenvironments. To illustrate cell sensitivitiesto a series of thin-to-thick gels, the adhesive spreading of mesenchymalstem 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 therigidity of an underlying ‘hidden’ surface starting at about 10–20 µm gel thickness with a characteristic tactile length of less than about 5 µm.