Mechanical properties of biomaterials are difficult to characterize experimentally because many relevant biomaterials such as hydrogels are very pliable and viscoelastic. Furthermore, test specimens such as blood clots retrieved from patients tend to be small in size, requiring fine positioning and sensitive force measurement. Mechanobiological studies require fast data recording, preferably under simultaneous microscope imaging, in order to monitor events such as structural remodeling or localized rupture while strain is being applied. A low-profile tensile tester that applies prescribed displacement up to several millimeters and measures forces with resolution on the order of micronewtons has been designed and tested, using alginate as a representative soft biomaterial. 1.5% alginate (cross-linked with 0.1 M and 0.2 M calcium chloride) has been chosen as a reference material because of its extensive use in tissue engineering and other biomedical applications. Prescribed displacement control with rates between 20 μm/s and 60 μm/s were applied using a commercial low-noise nanopositioner. Force data were recorded using data acquisition and signal conditioning hardware with sampling rates as high as 1 kHz. Elongation up to approximately 10 mm and force in the range of 250 mN were measured. The data were used to extract elastic and viscoelastic parameters for alginate specimens. Another biomaterial, 2% agarose, was also tested to show versatility of the apparatus for slightly stiffer materials. The apparatus is modular such that different load cells ranging in capacity from hundreds of millinewtons to tens of newtons can be used. The apparatus furthermore is compatible with real-time microscope imaging, particle tracing, and programmable positioning sequences.

Understanding clot biomechanics is critical for the treatment of cardiovascular diseases. Based on our recent observation that that the structural configuration of the clot network correlates well with the mechanical properties such as stiffness, we hypothesized that the heterogeneity in the mechanical response of the microstructure dictates clot micromechanics and hence the macroscopic behavior. To test this hypothesis, we have custom-developed a microextensometer device coupled to a microscope to probe and image microstructural changes and micromechanical behavior of fibrin and blood clots. 20 μL clots were pulled at a prescribed strain rate of 60 μm/s using a programmable nano-positioner, and the force was measured using a 10 g load cell and acquired at 500 Hz. From the stress-strain measurements, we observed that both FFP and blood clots showed non-linear and abrupt changes in resistive tensile force in response to constant strain rate (Fig. 1A). Using fiduciary markers, we observed that cross-linked, but not uncrosslinked, fibrin clots showed a microscopically non-uniform deformation in response to macroscopically constant strain rate. Further, computational analysis of the mechanical response of clot microstructure to an applied stress revealed heterogeneity in strain energy distribution dictated by the network properties (Fig. 1B). Together, our results suggest that the heterogeneity in microscale translates to the non-linear response at the macroscale, and will ultimately dictate the pathophysiology of thrombosis.