Abstract
Thin layers of gels with mechanical properties mimicking animal tissues are widely used to study the rigidity sensing of adherent animal cells and to measure forces applied by cells to their substrate with traction force microscopy. The gels are usually based on polyacrylamide and their elastic modulus is measured with an atomic force microscope (AFM). Here we present a simple microfluidic device that generates high shear stresses in a laminar flow above a gel-coated substrate and apply the device to gels with elastic moduli in a range from 0.4 to 300 kPa that are all prepared by mixing two components of a transparent commercial silicone Sylgard 184. The elastic modulus is measured by tracking beads on the gel surface under a wide-field fluorescence microscope without any other specialized equipment. The measurements have small and simple to estimate errors and their results are confirmed by conventional tensile tests. A master curve is obtained relating the mixing ratios of the two components of Sylgard 184 with the resulting elastic moduli of the gels. The rigidity of the silicone gels is less susceptible to effects from drying, swelling, and aging than polyacrylamide gels and can be easily coated with fluorescent tracer particles and with molecules promoting cellular adhesion. This work can lead to broader use of silicone gels in the cell biology laboratory and to improved repeatability and accuracy of cell traction force microscopy and rigidity sensing experiments.
Highlights
Animal tissues have a broad range of elastic moduli, E, from,1 kPa in brain to,10 GPa in bone
When cells are grown on soft substrates, the cellular traction forces produce substrate deformations that are substantially large to be measured with tracer particles under a microscope
The relative difference between the elastic modulus values obtained with the two methods, jE{E0j=E, had an average value of only,5% for the three gels, well within the estimated errors of the two techniques. (Whereas a similar tensile stress test has been successfully applied to PAA gels with E as small as,5 kPa [10], we found it difficult to measure the elastic moduli of silicone gels with E,100 kPa under tension.)
Summary
Animal tissues have a broad range of elastic moduli, E, from ,1 kPa in brain to ,10 GPa in bone. When animal cells are grown on a substrate, they sense its rigidity, especially in the physiological range of soft tissues, with E = 0.1–100 kPa [1]. The most commonly used soft substrates for TFM and cell rigidity sensing studies are polyacrylamide (PAA) gels [10]. Silicone gels, such as gels based on polydimethylsyloxane (PDMS), are less widely used in biological laboratories, in spite of a long history of applications [11,12,13,14,15] and several advantages over PAA gels. Silicone gels have higher refractive indices than PAA gels [16], and their surface can be patterned with a microrelief for better tracking of the substrate deformations [13,15]
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