Flow visualization study of a novel respiratory assist catheter.

Respiratory assist using intravenous catheters may be a potential therapy for patients with acute and acute-on-chronic lung failure. An important design constraint is respiratory catheter size, and new strategies are needed that enable size reduction while maintaining adequate gas exchange. Our group is currently developing a percutaneous respiratory assist catheter (PRAC) that uses a rotating bundle of hollow fiber membranes to enhance CO(2) removal and O(2) supply with increasing bundle rotation rate. In this study, particle image velocimetry (PIV) was used to analyze the fluid flow patterns and velocity fields surrounding the rotating fiber bundle of the PRAC. The goal of the study was to assess the rotational flow patterns within the context of the gas exchange enhancement that occurs with increasing fiber bundle rotation. A PRAC prototype was placed in a 1-in. internal diameter test section of an in vitro flow loop designed specifically for PIV studies. The rotation rate of the PRAC was varied between 500 and 7000 rpm, and PIV was used to determine the velocity fields in the primary (r-theta) and secondary (r-z) flow planes. The secondary flow exhibited time-varying and incoherent vortices that were consistent with the classical Taylor vortices expected for Taylor numbers (Ta) corresponding to the rotation speeds studied (2200 < Ta < 31 000). In the primary flow, the tangential velocity exhibited boundary layers of less than (1/2) mm adjacent to the fiber bundle and vessel wall. The estimated shear stress associated with the Taylor vortices was approximately 11 dyne/cm(2) at 7000 rpm and was over 10 times smaller than the shear stress in the primary flow boundary layers.

A novel flex-stretch-flow bioreactor for the study of engineered heart valve tissue mechanobiology.

Tissue engineered heart valves (TEHV) have been observed to respond to mechanical conditioning in vitro by expression of activated myofibroblast phenotypes followed by improvements in tissue maturation. In separate studies, cyclic flexure, stretch, and flow (FSF) have been demonstrated to exhibit both independent and coupled stimulatory effects. Synthesis of these observations into a rational framework for TEHV mechanical conditioning has been limited, however, due to the functional complexity of tri-leaflet valves and the inherent differences of separate bioreactor systems. Toward quantifying the effects of individual mechanical stimuli similar to those that occur during normal valve function, a novel bioreactor was developed in which FSF mechanical stimuli can be applied to engineered heart valve tissues independently or in combination. The FSF bioreactor consists of two identically equipped chambers, each having the capacity to hold up to 12 rectangular tissue specimens (25 x 7.5 x 1 mm) via a novel "spiral-bound" technique. Specimens can be subjected to changes-in-curvature up to 50 mm(-1) and uniaxial tensile strains up to 75%. Steady laminar flow can be applied by a magnetically coupled paddlewheel system. Computational fluid dynamic (CFD) simulations were conducted and experimentally validated by particle image velocimetry (PIV). Tissue specimen wall shear stress profiles were predicted as a function of paddlewheel speed, culture medium viscosity, and the quasi-static state of specimen deformation (i.e., either undeformed or completely flexed). Velocity profiles predicted by 2D CFD simulations of the paddlewheel mechanism compared well with PIV measurements, and were used to determine boundary conditions in localized 3D simulations. For undeformed specimens, predicted inter-specimen variations in wall shear stress were on average +/-7%, with an average wall shear stress of 1.145 dyne/cm(2) predicted at a paddlewheel speed of 2000 rpm and standard culture conditions. In contrast, while the average wall shear stress predicted for specimens in the quasi-static flexed state was approximately 59% higher (1.821 dyne/cm(2)), flexed specimens exhibited a broad intra-specimen wall shear stress distribution between the convex and concave sides that correlated with specimen curvature, with peak wall shear stresses of approximately 10 dyne/cm(2). This result suggests that by utilizing simple flexed geometric configurations, the present system can also be used to study the effects of spatially varying shear stresses. We conclude that the present design provides a robust tool for the study of mechanical stimuli on in vitro engineered heart valve tissue formation.

Investigating the effects of random balloon pulsation on gas exchange in a respiratory assist catheter.

We are developing an intravenous respiratory assist catheter, which uses hollow-fiber membranes wrapped around a pulsating balloon that increases oxygenation and CO2 removal with increased balloon pulsation. Our current pulsation system operates with a constant rate of pulsation and delivered balloon volume. This study examined the hypothesis that random balloon pulsation would disrupt fluid entrainment within the fiber bundle and increase our overall gas exchange. We implemented two different modes for random (rates and delivered volume) versus constant pulsation. The impact on gas exchange was measured in a 3 l/min water flow loop at 37 degrees C. CO2 gas exchange for randomized beat rate mode was comparable to its corresponding average constant pulsation (e.g., constant 286 beats/min versus randomized 200-400 beats/min was 299.5+/-0.9 and 302.2+/-1.4 ml/min/m, respectively). Random volume mode CO2 exchange was also comparable to constant delivered balloon volume (100% inflation and deflation) (e.g., 294.3+/-0.6 and 301.1+/-1.7 ml/min/m, random 50-100% inflation and constant, respectively). Greater active mixing was seen with constant pulsation as compared with randomly changing the parameters of balloon pulsation.

Flow visualization study of a pulsating respiratory assist catheter.

Our group is currently developing an intravenous respiratory assist device that uses a centrally located pulsatile balloon within a hollow fiber bundle to enhance gas exchange rate via active mixing mechanism. We tested the hypothesis that the non-symmetric inflation and deflation of the balloon lead to both nonuniform balloon-generated secondary flow and nonuniform gas exchange rate in the fiber bundle. The respiratory catheter was placed in a 1-in. internal diameter rigid test section of an in vitro flow loop (3 L/min deionized water). Particle image velocimetry (PIV), which was used to map the velocity vector field in the lateral cross-section, showed that the balloon pulsation generated a nonuniform fluid flow surrounding the respiratory assist catheter. PIV was also used to characterize the fiber bundle movement, which was induced by the balloon pulsation. Gas permeability coefficient of the device was evaluated by using both the fluid velocity and the relative velocity between the fluid and the fiber bundle. The highest difference in the gas permeability coefficient predicted by using the relative velocity was about 17% to 23% (angular direction), which was more uniform than the 49% to 59% variation predicted by using the fluid velocity. The movement of the fiber bundle was responsible for reducing the variation in the fluid velocity passing through the bundle and for minimizing the nonuniformity of the gas permeability coefficient of the respiratory assist catheter.