The overall goal ofthis research is to investigate the interstitial fluid flow in ligament and tendon. This project is divided into three subsections. The first subsection is to develop a fiber matrix model to compute the permeabilities and interstitial fluid flows across or along collagen fibers. By assuming fibers to be a periodic array of cylinders with parabolic influx, hydraulic pressure is computed by the parameters of rheological properties and interfiber space using finite element softwares (FIDAP or FLUCODE). Results show that permeability is affected by material porosity and can be analytically expressed as a function of porosity.

The second subsection is to incorporate the permeability into a poroelastic model to calculate the strain generated pressure and potential under loading. A poroelastic model is introduced to define the solid-fluid interaction under tension. A Fourier series is utilized to solve the diffusion equation for pore pressure under haversine tensile stress and to satisfy the initial and boundary conditions similar to that of normal ligament and tendon. The pressure solution then couples with electrokinetic phenomenological equations to compute the electrokinetic transduction. Results show that using the proper Poisson’s ratio and porosity is essential for the poroelastic modeling ofligament and tendon.

The final subsection is to validate analytic models with electrokinetic experiments on rabbit patellar tendon. Electrical potentials are measured at axial locations 1/4 and 1/2 from patella to tibia under haversine tension. Results show that potential is pH and frequency dependent (p<0.05) with an isoelectric point at pH 4.32 which suggests an electrokinetic rather than a piezoelectric effect. A transient electrical potential, superimposed with first (exponential) and second (cyclic) order responses, is consistently observed with phase shifts (24-45°) from applied strain. Upon termination of cyclic tension, the second order potential ends, and the first order potential exponentially decays to its original potential. The pressure measured in the mid-substance center is positive under tension and slightly negative after removal loading. Experimental results are compared to poroelastic and electrokinetic predictions and suggest that strain induces an interstitial fluid flow which is important for nutrient delivery and cellular regulation in ligaments and tendons.