Elastomeric Tubes with Self-Regulated Distension
Creators: Jen, Nathan A
Publisher: name: University of Alberta Library
Year: 2022

<p>This thesis discusses the deformation behavior of compliant tubing consisting of one of two silicone elastomeric bases and a reinforcing knitted fabric jacket. The primary application of this tubing is the ex vivo heart perfusion (EVHP) device, a medical implement that keeps a donated heart alive by pumping a blood substitute through it at body temperature. Currently used plastic EVHP tubing runs the risk of causing tissue damage over time due to its excessive stiffness; the compliant tubing in this thesis is specifically engineered to exhibit several biomimetic behaviors found in human aorta (such as strain-stiffening, self-regulation, and the Windkessel effect) and therefore theorized to be beneficial to donor heart health in the contexts of an EVHP device. It may also potentially be used for a variety of other purposes, which are briefly discussed.</p>

<p>There are two main parts to the research conducted in this thesis. In the first part, a fundamental understanding of the tubing behavior under static loading was established. The compliant tubing was subject to varying levels of hydrostatic pressure and the distension response was measured. Strong self-regulating behavior in the tubes was induced by stiffening of the fabric jacket, similar to the characteristic strain-stiffening behavior of human aorta; maximum operating pressures of the jacketed tubes were roughly double those of their unjacketed counterparts. Finite element simulations were also created in ABAQUS to virtually model the physical experiments using numerical material coefficients obtained from separate uniaxial quasi-hysteresis tests. The simulations were robust; they were capable of withstanding greater hydrostatic pressures than those used in the physical experiments and smoothly continued the observed trends in distension behavior. They also displayed excellent agreement with the experimental data in terms of predicted radial distension and volumetric expansion (R^2 &gt; 0.9 for all tubes). Additional simulations were created to explore the potential of kirigami relief patterns that could be used to realize complex deformation modes in the tubes. </p>

<p>In the second part, the deformation behavior of the tubes in a mock-EVHP environment was explored, and the extent of their biomimetic properties was evaluated. The compliant tubes underwent hydrodynamic pressure tests in which they were connected to a mock-EVHP `flow loop&#39; and subject to quasi-physiological pulsatile fluid flow. Pressure transducers and image analysis were used to generate time-dependent waveforms of pressure, distension, and other related parameters. As with the hydrostatic tests, addition of a fabric jacket expands the working pressure and distension ranges of the bare tubes; the resultant pressure waveforms resembled physiological pressure waveforms far more closely than those produced from a rigid tube installed in the flow loop. All tested compliant tubes achieved a similar percentage of stroke volume storage (50%) as observed in human aorta, and an approximation equation was derived to quickly estimate the amount of radial distension needed to achieve an arbitrary amount of fluid storage in a tube of any length. Viscoelastic material parameters of the jacketed tubes, obtained from pressure-distension hysteresis loops, were comparable to previously reported values in biological aortic specimens. Finally, various improvements were proposed for both the flow loop setup and the design of the tube in order to more closely replicate the behavior of biological aorta, mainly related to lowering the operating pressures to be fully within the normal physiological range.</p>