Flow and Temperature Fields Generated by a Thermally Activated Interventional Vascular Device

Concern for the nonphysiologic energy required to actuate medical devices utilizing ?smart material? properties of shape memory polymer (SMP) compels a rigorous investigation into the flow and temperature fields surrounding a thermally activated catheter device. Multiple analyses include the theoretical approaches of exact analytical solutions and finite difference modeling combined with the experimental techniques of particle image velocimetry (PIV) and laser-induced fluorescence (LIF). The attained velocities and temperatures related to the convective heat transfer impact the potential for blood or tissue damage caused by intravascular heating. The clinical scenario involving a catheter device receiving heat within an artery is modeled in its simplest form as a cylindrical metal cap on the tip of a hollow glass rod placed inside of a long straight tube of constant cross-sectional area. Using a working fluid with properties comparable to blood, flow rates and energy input is then varied to determine their effects on velocity fields and temperature gradients. Analytical solutions for both the straight tube and concentric annulus demonstrate the two velocity distributions involved, as flow moves past the gap between the catheter and artery wall and then converges downstream to the Poiseuille solution for steady pipe flow of an incompressible fluid. To solve for the transition between the velocity profiles, computational fluid dynamics software simulates a finite volume model identical to the experimental setup used for intravascular heating experiments. PIV and LIF, both experimental techniques making use of similar hardware, determine velocity fields and temperature distributions, respectively, by imaging fluid seeding agents and their particular interaction with the light sheet. The velocity and temperature fields obtained experimentally are matched with the analytical and finite volume analysis through fluid properties, flow rates, and heating rates. Velocities determined during device heating show a small increase in local velocity, due to temperature dependent viscosity effects. When the device is centered in the model, flow patterns constrain the heat flow near the center axis and away from the channel walls. Increasing flow rate consequently decreases temperature rise, as the heat is carried more quickly downstream and away from the heat source. Using multiple analyses, fluid velocity and temperature distributions are first theorized with analytical and finite element methods and then validated through experimental imaging in a physical model.