Thrust III: Structure-based assessment of renal artery mechanics

Infrastructure for Biomechanical ExperimentsThe program for Biomedical Engineering has developed material experimentation, material characterization and advanced vision capabilities for our studies Facilities

Facilities include (a) Bose Biodynamic Test Systems (2) for combined tension-pressure loading of small/large arterial specimens, (b) Bose 3200 test system with additional pressure-control hardware for combined tension-pressure and torsional loading, (c) Nikon SMZ-U stereo-microscope system with fluorescence attachment for three-dimensional deformation measurements on small artery specimens undergoing multiaxial mechanical loading, (d) high speed cameras for rapid motion events and (e) bio-reactors/bio-chambers coupled with mechanical loading systems for live-cell studies.

Bose Bio-dynamic SystemWith Bio-chamber

High speed stereo imaging system

Bose 3200 System

Thrust III: Structure-based assessment of renal artery mechanicsGoalIntegrate biomechanical experiments and theory to develop a structure-based constitutive description of the primary renal artery ApproachExperiments/theory distinguish between the passive (elastin and collagen) and active (SMC) components that dictate overall mechanical behavior

Scientific Importance• Constitutive model of the renal artery• Provide insight into the primary factors that dictate renal artery function• Enables subsequent assessment of mechanically-induced growth and remodeling

Relation to RII• Provides a baseline for evaluation of synthesized constructs• Provides insights into the target composition (mass fractions of load bearing

elements) of synthesized constructs in order to achieve mechanical similarity

Axial Load vs. Axial Displacement

The first branch artery has a more linear relationship between • Hoop Strain and Axial Strain• Axial Load and Axial Displacement

up to the onset of permanent tissue deformation, indicating that (a) the range of pressure and axial loading experienced by the branched natural arteries must be

lower(b) the structure of the tissue in the primary and first branch arteries is changing considerably.

Thrust III: Structure-based assessment of renal artery mechanics

Right Renal Artery First Branch

Hoop Strain vs. Axial Strain

To date, we have acquired a full set of mechanical test data for three specimens of the main renal artery and one specimen at the first branch level.  These data were sent to our collaborators at ENSM-SE in France for parameter identification by fitting to a Holzapfel-type constitutive model. 

Preliminary analysis of the experimental data sent to our ENSM-SE colleagues demonstrates that

• the measured in-vivo circumferential strain differed between the main renal artery and the first branch by a factor of 2 (12% for main renal artery, 25% for first branch)

• the measured in vivo longitudinal strain differed between the main renal artery and the first branch by a factor of 2 (25% for main renal artery, 12% for first branch)

The measured differences in in-vivo conditions, as well as the clear differences shown for the Axial Load- Axial Displacement and Hoop Strain-Axial Strain measurements, suggest there are significant differences in arterial structure between the primary artery and the first branch artery.

On-going experimental studies will quantify the tissue-level structure for both the primary and first branch specimens, providing the necessary information for how arterial structure and mechanical response vary in natural, branched arterial specimens.

Thrust III: Structure-based assessment of renal artery mechanics

Main renal artery

First branch

Thrust III: Structure-based assessment of renal artery mechanics

Main renal artery First BranchPreliminary Holzapfel Parameters

Main renal artery First branch

k1 0.20 0.30

k2 40.00 42.00

Fiber angle 30.00o 23.00o

Preliminary theoretical analysis of the mechanical loading data sent to our ENSM-SE colleagues resulted in the following comparisons and our first estimates of the Holzapfel parameters for the primary and branch:

Thrust III: Structure-based assessment of renal artery mechanicsGoalDevelop mathematical models and a constitutive descriptions of renal artery mechanics. Use descriptions to develop models that predict the outcomes of pressure-, flow-, and structure-induced growth and remodeling

ApproachFormulate and solve (i) a direct problem in biomechanics that yields the local baseline mechanical environment of the renal artery and (ii) an inverse problem in biomechanics that predicts the remodeled zero-stress configuration required to restore the baseline state following various modes of perturbation. Scientific Importance• Provide validated mathematical model for predicting effects of pressure-, axial force- and

other forcing functions• Provide insight into the processes that govern adaptive and maladaptive remodeling of

the renal artery; below is a graph of recent model prediction of wall thickness vs vessel pressure

• Increase understanding of the genesis and progression of certain forms of renal vascular diseaseRelation to RII

Understanding mechanically-induced growth/remodeling of vascular tissue is central to the maturation of synthesized constructs

10 15 20 25 30 351

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Thrust III: Structure-based assessment of renal artery mechanics

GoalDesign and build a bioreactor that enables control of the local mechanical environment in the arterial wall while subjecting artery to a controllable range of mechanical loading conditions

ApproachModify a typical bioreactor with feedback control that (i) measures global parameters, (ii) calculates local parameters using the developed SEF, (iii) tunes global parameters to achieve a desired local state.

Scientific Importance• Novel (seeking patent)• Can allow delineation of stress and strain as signals for mechanotransduction• Can evaluate tissue-level response to compressive stress.

Relation to RIIProvides a way to impart high stresses at low strains – this will promote growth/remodeling of immature constructs without inducing damage.