Traction force microscopy is the most widely used technique for studying the forces exerted by cells on deformable substrates. However, the method is computationally intense and cells have to be detached from the substrate prior to measuring the displacement map. We have developed a new method, referred to as Hilbert phase dynamometry (HPD), which yields real-time force fields and, simultaneously, cell dry mass and growth information. HPD operates by imaging cells on a deformable substrate that is patterned with a grid of fluorescent proteins. A Hilbert transform is used to extract the phase map associated with the grid deformation, which provides the displacement field. By combining this information with substrate stiffness, an elasticity model was developed to measure forces exerted by cells with high spatial resolution. In our study, we prepared 10kPa gels and them with a 2-D grid of FITC-conjugated fibrinogen/fibronectin mixture, an extracellular matrix protein to which cells adhere. We cultured undifferentiated mesenchymal stem cells (MSC), and MSCs that were in the process of undergoing adipogenesis and osteogenesis. The cells were measured over the course of 24 hours using Spatial Light Interference Microscopy (SLIM) and wide-field epi-fluorescence microscopy allowing us to simultaneously measure cell growth and the forces exerted by the cells on the substrate.

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Shamira Sridharan, Yanfen Li, Basanta Bhaduri, Hassaan Majeed, Paul Dupenloup, Alex Levine, Kristopher A. Kilian, and Gabriel Popescu, "Hilbert phase dynamometry (HPD) for real-time measurement of cell generated forces (Conference Presentation)," Proc. SPIE 9718, Quantitative Phase Imaging II, 97182F (Presented at SPIE BiOS: February 17, 2016; Published: 27 April 2016); https://doi.org/10.1117/12.2216261.4848767848001.