Cardiac Valve Bioreactor for Physiological Conditioning and Hydrodynamic Performance Assessment

Towards technically controlled bioreactor maturation of tissue-engineered heart valves

Bioreactors are important tools for the pre-conditioning of tissue-engineered heart valves. The current state of the art mostly provides for timed, physical and biochemical stimulation in the bioreactor systems according to standard protocols (SOP). However, this does not meet to the individual biological variability of living tissue-engineered constructs. To achieve this, it is necessary to implement (i) sensory systems that detect the actual status of the implant and (ii) controllable bioreactor systems that allow patient-individualized pre-conditioning. During the maturation process, a pulsatile transvalvular flow of culture medium is generated within the bioreactor. For the improvement of this conditioning procedure, the relationship between the mechanical and biochemical stimuli and the corresponding tissue response has to be analyzed by performing reproducible and comparable experiments. In this work, a technological framework for maturation experiments of tissue-engineered heart valves in a pulsating bioreactor is introduced. The aim is the development of a bioreactor system that allows for continuous control and documentation of the conditioning process to increase reproducibility and comparability of experiments. This includes hardware components, a communication structure and software including online user communication and supervision. Preliminary experiments were performed with a tissue-engineered heart valve to evaluate the function of the new system. The results of the experiment proof the adequacy of the setup. Consequently, the concept is an important step for further research towards controlled maturation of tissue-engineered heart valves. The integration of molecular and histological sensor systems will be the next important step towards a fully automated, self-controlled preconditioning system.

Experimental and computational models for tissue-engineered heart valves: a narrative review

Valvular heart disease is currently a common problem which causes high morbidity and mortality worldwide. Prosthetic valve replacements are widely needed to correct narrowing or backflow through the valvular orifice. Compared to mechanical valves and biological valves, tissue-engineered heart valves can be an ideal substitute because they have a low risk of thromboembolism and calcification, and the potential for remodelling, regeneration, and growth. In order to test the performance of these heart valves, various animal models and other models are needed to optimise the structure and function of tissue-engineered heart valves, which may provide a potential mechanism responsible for substantial enhancement in tissue-engineered heart valves. Choosing the appropriate model for evaluating the performance of the tissue-engineered valve is important, as different models have their own advantages and disadvantages. In this review, we summarise the current state-of-the-art animal models, bioreactors, and computational simulation models with the aim of creating more strategies for better development of tissue-engineered heart valves. This review provides an overview of major factors that influence the selection and design of a model for tissue-engineered heart valve. Continued efforts in improving and testing models for valve regeneration remain crucial in basic science and translational researches. Future research should focus on finding the right animal model and developing better in vitro testing systems for tissue-engineered heart valve.

A design-based model of the aortic valve for fluid-structure interaction

This paper presents a new method for modeling the mechanics of the aortic valve and simulates its interaction with blood. As much as possible, the model construction is based on first principles, but such that the model is consistent with experimental observations. We require that tension in the leaflets must support a pressure, then derive a system of partial differential equations governing its mechanical equilibrium. The solution to these differential equations is referred to as the predicted loaded configuration; it includes the loaded leaflet geometry, fiber orientations and tensions needed to support the prescribed load. From this configuration, we derive a reference configuration and constitutive law. In fluid-structure interaction simulations with the immersed boundary method, the model seals reliably under physiological pressures and opens freely over multiple cardiac cycles. Further, model closure is robust to extreme hypo- and hypertensive pressures. Then, exploiting the unique features of this model construction, we conduct experiments on reference configurations, constitutive laws and gross morphology. These experiments suggest the following conclusions: (1) The loaded geometry, tensions and tangent moduli primarily determine model function. (2) Alterations to the reference configuration have little effect if the predicted loaded configuration is identical. (3) The leaflets must have sufficiently nonlinear material response to function over a variety of pressures. (4) Valve performance is highly sensitive to free edge length and leaflet height. These conclusions suggest appropriate gross morphology and material properties for the design of prosthetic aortic valves. In future studies, our aortic valve modeling framework can be used with patient-specific models of vascular or cardiac flow.

Evaluation of the role of peripheral artery plaque geometry and composition on stent performance

Peripheral stent fracture is a major precursor to restenosis of femoral artery atherosclerosis that has been treated with stent implantation. In this work, we validate a workflow for performing in silico stenting on a patient specific peripheral artery with heterogeneous plaque structure. Six human cadaveric femoral arteries were imaged ex vivo using intravascular ultrasound virtual histology (IVUS-VH) to obtain baseline vessel geometry and plaque structure. The vessels were then stented and the imaging repeated to obtain the stented vessel lumen area. Finite element (FE) models were then constructed using the IVUS-VH images, where the material property constants for each finite element were calculated using the proportions of each plaque component in the element, as identified by the IVUS-VH images. A virtual stent was deployed in each FE model, and the model lumen area was calculated and compared to the experimental lumen area to validate the modeling approach. The model was then used to compare stent performance for heterogeneous and homogeneous artery models, to determine whether plaque geometry or composition had added effects on stent performance. We found that the simulated lumen areas were similar to the corresponding experimental values, despite using generic material constants. Additionally, the heterogeneous and homogeneous lumen areas were also similar, implying that plaque geometry is a stronger predictor of stent expansion performance than plaque composition. Comparing stent stress and strain for heterogeneous and homogeneous models, it was found that stress from these two models had a strong linear correlation, while the strain correlation was weaker but still present. This implies that stent performance may be predicted with a simple homogeneous material models accounting for overall geometry of the plaque, providing that stent fatigue is calculated using stress criteria.

Clinical performance of decellularized heart valves versus standard tissue conduits: a systematic review and meta-analysis

Background

Valve replacement surgery is the definitive management strategy for patients with severe valvular disease. However, valvular conduits currently in clinical use are associated with significant limitations. Tissue-engineered (decellularized) heart valves are alternative prostheses that have demonstrated promising early results. The purpose of this systematic review and meta-analysis is to perform robust evaluation of the clinical performance of decellularized heart valves implanted in either outflow tract position, in comparison with standard tissue conduits.

Methods

Systematic searches were conducted in the PubMed, Scopus, and Web of Science databases for articles in which outcomes between decellularized heart valves surgically implanted within either outflow tract position of human subjects and standard tissue conduits were compared. Primary endpoints included postoperative mortality and reoperation rates. Meta-analysis was performed using a random-effects model via the Mantel-Haenszel method.

Results

Seventeen articles were identified, of which 16 were included in the meta-analysis. In total, 1418 patients underwent outflow tract reconstructions with decellularized heart valves and 2725 patients received standard tissue conduits. Decellularized heart valves were produced from human pulmonary valves and implanted within the right ventricular outflow tract in all cases. Lower postoperative mortality (4.7% vs. 6.1%; RR 0.94, 95% CI: 0.60–1.47; P = 0.77) and reoperation rates (4.8% vs. 7.4%; RR 0.55, 95% CI: 0.36–0.84; P = 0.0057) were observed in patients with decellularized heart valves, although only reoperation rates were statistically significant. There was no statistically significant heterogeneity between the analyzed articles (I² = 31%, P = 0.13 and I² = 33%, P = 0.10 respectively).

Conclusions

Decellularized heart valves implanted within the right ventricular outflow tract have demonstrated significantly lower reoperation rates when compared to standard tissue conduits. However, in order to allow for more accurate conclusions about the clinical performance of decellularized heart valves to be made, there need to be more high-quality studies with greater consistency in the reporting of clinical outcomes.

Ex Vivo Evaluation of IVUS-VH Imaging and the Role of Plaque Structure on Peripheral Artery Disease

Peripheral artery disease (PAD) results from the buildup of atherosclerotic plaque in the arterial wall, can progress to severe ischemia and lead to tissue necrosis and limb amputation. We evaluated a means of assessing PAD mechanics ex vivo using ten human peripheral arteries with PAD. Pressure-inflation testing was performed at six physiological pressure intervals ranging from 10-200 mmHg. These vessels were imaged with IVUS-VH to determine plaque composition and change in vessel structure with pressure. Statistical analysis was performed to determine which plaque structures and distributions of these structures had the greatest influence on wall deformation. We found that fibrous plaque, necrotic core, and calcification had a statistically significant effect on all variables (p<0.05). The presence of large concentrations of fibrous plaque was linked to reduced vessel compliance and ellipticity, which could lead to stent fractures and restenosis. For the plaque distribution we found that clustered necrotic core increased overall compliance while clustered calcification decreased overall compliance. The effect of plaque distribution on vessel wall deformation must be considered equally important to plaque concentration.

The Implementation of an Adjustable Afterload Module for Ex Situ Heart Perfusion

PURPOSE

Windkessel impedance analysis has proven to be an effective technique for instituting artificial afterload on ex situ hearts. Traditional fixed parameter afterload modules, however, are unable to handle the changing contractile conditions associated with prolonged ex situ heart perfusion. In this paper, an adjustable afterload module is described comprising of three fully adjustable sub-components: a systemic resistor, a proximal resistor and a compliance chamber.

METHODS

Using a centrifugal pump, the systemic resistor and compliance chamber were subjected to testing across their operating ranges, whereby the predictability of resistance and compliance values was evaluated. The components were then assembled, and the full module tested on three separate porcine hearts perfused for 6 h with success defined by the ability to maintain physiological systolic and diastolic aortic pressures across flow rate variability.

RESULTS

For both the systemic resistor and compliance chamber, experimental measurements agreed with their theoretical equivalents, with coefficients of determination of 0.99 and 0.97 for the systemic resistor and compliance chamber, respectively. During ex situ perfusion, overall 95% confidence intervals demonstrate that physiological systolic (95-96.21 mmHg) and diastolic (26.8-28.8 mmHg) pressures were successfully maintained, despite large variability in aortic flow. Left ventricular contractile parameters, were found to be in line with those in previous studies, suggesting the afterload module has no detrimental impact on functional preservation.

CONCLUSIONS

We conclude that due to the demonstrable control of our afterload module, we can maintain physiological aortic pressures in a passive afterload working mode across prolonged perfusion periods, enabling effective perfusion regardless of contractile performance.

Mechanical and finite element evaluation of a bioprinted scaffold following recellularization in a rat subcutaneous model

Tissue engineered heart valves (TEHV) provide several advantages over currently available aortic heart valve replacements. Bioprinting provides a patient-specific means of developing a TEHV scaffold from imaging data, and the capability to embed the patient's own cells within the scaffold. In this work we investigated the remodeling capacity of a collagen-based bio-ink by implanting bioprinted disks in a rat subcutaneous model for 2, 4 and 12 weeks and evaluating the mechanical response using biaxial testing and subsequent finite element (FE) modeling. Samples explanted after 2 and 4 weeks showed inferior mechanical properties compared to native tissues while 12 week explants showed a mechanical response of similar magnitude but did not demonstrate the anisotropy present in native tissues. In the FE analysis, the model utilizing mechanical properties from samples explanted after 12 weeks showed the closest mechanical behavior to the native tissues. However, in diastole native tissues showed higher stress in the leaflet belly and lower strain at the commissures compared to 12 week explants, likely due to the anisotropy present in the native tissues. Thus, either further remodeling is required in situ in the aortic valve position or by in vitro preconditioning in an environment such as a bioreactor. Regardless, these results demonstrate the utility of FE analysis to optimize bioprinting process parameters for the most favorable in vivo mechanical performance.

Patient specific characterization of artery and plaque material properties in peripheral artery disease

Patient-specific finite element (FE) modeling of atherosclerotic plaque is challenging, as there is limited information available clinically to characterize plaque components. This study proposes that for the limited data available in vivo, material properties of plaque and artery can be identified using inverse FE analysis and either a simple neo-Hookean constitutive model or assuming linear elasticity provides sufficient accuracy to capture the changes in vessel deformation, which is the available clinical metric. To test this, 10 human cadaveric femoral arteries were each pressurized ex vivo at 6 pressure levels, while intravascular ultrasound (IVUS) and virtual histology (VH) imaging were performed during controlled pull-back to determine vessel geometry and plaque structure. The VH images were then utilized to construct FE models with heterogeneous material properties corresponding to the vessel plaque components. The constitutive models were then fit to each plaque component by minimizing the difference between the experimental and the simulated geometry using the inverse FE method. Additionally, we further simplified the analysis by assuming the vessel wall had a homogeneous structure, i.e. lumping artery and plaque as one tissue. We found that for the heterogeneous wall structure, the simulated and experimental vessel geometries compared well when the fitted neo-Hookean parameters or elastic modulus, in the case of linear elasticity, were utilized. Furthermore, taking the median of these fitted parameters then inputting these as plaque component mechanical properties in the finite element simulation yielded differences between simulated and experimental geometries that were on average around 2% greater (1.30-5.55% error range to 2.33-11.71% error range). For the homogeneous wall structure the simulated and experimental wall geometries had an average difference of around 4% although when the difference was calculated using the median fitted value this difference was larger than for the heterogeneous fits. Finally, comparison to uniaxial tension data and to literature constitutive models also gave confidence to the suitability of this simplified approach for patient-specific arterial simulation based on data that may be acquired in the clinic.