Fibronectin (Fn) fibers are protein fibers, which are found in the extra cellular matrix of cells and connective tissue playing an important role in early wound repair. The mechanical characterization of Fn fibers gives insight into their structure and the mechanisms involved in their folding and unfolding.
During the micromechanical testing of fibers submerged in liquids, the fiber cannot be observed from above due to the diffraction of the air-liquid interface. The FT-RS1002 Microrobotic System is mounted on an inverted microscope or a confocal microscope. Fn fibers are deposited across the trenches of a microfabricated PDMS grid and rehydrated in PBS buffer. After aligning the Microforce Sensing Probe relative to the Fn fiber, the FT-W1002 Mechanical Testing and Handling Software is used to perform automated tensile tests of the fibers.
The tensile force applied to strain the fiber, the calculated stress (force per unit cross sectional area of the Fn fiber), and the stiffness (slope of the stress-versus-strain curve) are given as functions of the measured fiber extension for one representative fiber is shown in the figure. The stiffness of a single Fn fiber is not constant as expected for linearly elastic materials. Instead, the stress-versus-strain curves are highly nonlinear, being soft (compliant) first, and turning rigid at high extensions. Stiffness exhibited a small increase in the low strain regime, while increasing considerably once the fiber extension exceeded about 150% strain. The stiffness of a single fiber changes orders of magnitude, from less than 100 kPa to several MPa from relaxed to highly stretched fiber.