Mechanical characterization of a novel cell stimulating system (CSS) to apply dynamic, uniform and isotropic biaxial strains to cells in vitro

Mechanical loading alters cellular responses. While in vitro mechanical stimulation is a powerful tool for exploration of mechanotransduction, very little has been published documenting techniques for validation of such devices. We have developed an in vitro experimental system that imposes well-defined temporal and spatial strain profiles using a pressure-actuated, tethered diaphragm substrate (Bioflex dishes). More importantly we have accurately characterized the strain and strain rate performance of this system and herein describe that methodology. The prototype CSS deflected cell substrates over cylindrical platens, producing dynamic biaxial strains. Dynamic studies at 1 Hz were conducted at 8.0, 9.0, 10.0 and 13.0 kPa peak transmural pressures for a total of 1000 loading cycles. To study the effects of frequency, experiments were also run at 0.5 and 1.5 Hz at 8 and 13 kPa. A series of 33 dots were placed collinearly in rings on the membrane. Dot motions were monitored via a CCD video camera and acquisition was performed using an 8-bit gray-scale video board and NIH Image software. Strain fields and rates were subsequently calculated using Mathematica software. Results confirmed that the strains were biaxially uniform over the frequencies and pressures examined: e.g., at 9.0 KPa, max radial & circumferential strain = 0.009 +/- 0.001. It was also shown that, as transmural pressure was increased, both membrane strains and strain rates increased; however biaxial strain isotropy was preserved. While we cannot measure out-of-plane deflections, video-based image analysis is a very useful technique for validation of dynamic planar biaxial strains in cell stimulation systems.