Artificial Aortic Valves: An Overview

Moving Beyond the Sarcomere to Explain Heterogeneity in Hypertrophic Cardiomyopathy: JACC Review Topic of the Week

Hypertrophic cardiomyopathy (HCM) has been considered a heterogeneous cardiac disease ascribed solely to single sarcomere gene mutations. However, limitations of this hypothesis suggest that sarcomere mutations alone do not adequately explain all HCM clinical and pathobiological features. Disease-causing sarcomere mutations are absent in ∼70% of patients with established disease, and sarcomere gene carriers can live to advanced ages without developing HCM. Some features of HCM are also inconsistent with the single sarcomere gene hypothesis, such as regional left ventricular hypertrophy and myocardial fibrosis, as well as structurally abnormal elongated mitral valve leaflets and remodeled intramural coronary arterioles, which involve tissue types that do not express cardiomyocyte sarcomere proteins. It is timely to expand the HCM research focus beyond a single molecular event toward more inclusive models to explain this disease in its entirety. The authors chart paths forward addressing this knowledge gap using novel analytical approaches, particularly network medicine, to unravel the pathobiological complexity of HCM.

Bioengineering Strategies for Polymeric Scaffold for Tissue Engineering an Aortic Heart Valve: An Update

The occurrence of dysfunctional aortic valves is increasing every year, and current replacement heart valves, although having been shown to be clinically successful, are only short-term solutions and suffer from many agonizing long-term drawbacks. The tissue engineering of heart valves is recognized as one of the most promising answers for aortic valve disease therapy, but overcoming current shortcomings will require multidisciplinary efforts. The use of a polymeric scaffold to guide the growth of the tissue is the most common approach to generate a new tissue for an aortic heart valve. However, optimizing the design of the scaffold, in terms of biocompatibility, surface morphology for cell attachments and the correct rate of degradation is critical in creating a viable tissue-engineered aortic heart valve. This paper highlights the bioengineering strategies that need to be followed to construct a polymeric scaffold of sufficient mechanical integrity, with superior surface morphologies, that is capable of mimicking the valve dynamics in vivo. The current challenges and future directions of research for creating tissue-engineered aortic heart valves are also discussed.

Tissue engineered scaffolds for an effective healing and regeneration: reviewing orthotopic studies

It is commonly stated that tissue engineering is the most promising approach to treat or replace failing tissues/organs. For this aim, a specific strategy should be planned including proper selection of biomaterials, fabrication techniques, cell lines, and signaling cues. A great effort has been pursued to develop suitable scaffolds for the restoration of a variety of tissues and a huge number of protocols ranging from in vitro to in vivo studies, the latter further differentiating into several procedures depending on the type of implantation (i.e., subcutaneous or orthotopic) and the model adopted (i.e., animal or human), have been developed. All together, the published reports demonstrate that the proposed tissue engineering approaches spread toward multiple directions. The critical review of this scenario might suggest, at the same time, that a limited number of studies gave a real improvement to the field, especially referring to in vivo investigations. In this regard, the present paper aims to review the results of in vivo tissue engineering experimentations, focusing on the role of the scaffold and its specificity with respect to the tissue to be regenerated, in order to verify whether an extracellular matrix-like device, as usually stated, could promote an expected positive outcome.

Innovative developments of the heart valves designed for use in ventricular assist devices

Prosthetic heart valves are routinely used for replacing diseased natural heart valves. Even today, after five decades of prosthetic heart valve development, in the authors opinion the main problem associated with these valves is the risk of thromboembolic complications caused by unnatural hemodynamics. Further growing application of prosthetic heart valves is their use in the pulsatile ventricular assist devices (VADs). VADs may provide life-saving solutions to patients with severe cardiovascular diseases and are superior to drug therapy for patients with severe heart failure. However, the clinical applications of VADs still suffer from thromboembolic complications due to thrombus formations in the vicinity of the valves. Wherever the flow is stagnant or flow separation occurs, a thrombus is likely to form. The design of the heart valves with flow avoiding the formation of stagnant zones is one of the main goals in the development of new valves. This article reviews some innovative design approaches of the valves specially designed for use in VADs. Three design concepts are presented; one is based on the tilting disk valve, the second on the natural valve geometry and the third on the ball valve principle. However, these three different concepts have one common basic idea; the use of the freedom of the valve-housing design. This additional design freedom, which is available in VADs, is a key factor in the development of new valves with optimal hemodynamic performance.

CURRENT DEVELOPMENTS AND FUTURE CHALLENGES FOR THE CREATION OF AORTIC HEART VALVE

The concept of tissue-engineered heart valves offers an alternative to current heart valve replacements that is capable of addressing shortcomings such as life-long administration of anticoagulants, inadequate durability, and inability to grow. Since tissue engineering is a multifaceted area, studies conducted have focused on different aspects such as hemodynamics, cellular interactions and mechanisms, scaffold designs, and mechanical characteristics in the form of both in vitro and in vivo investigations. This review concentrates on the advancements of scaffold materials and manufacturing processes, and on cell–scaffold interactions. Aside from the commonly used materials, polyglycolic acid and polylactic acid, novel polymers such as hydrogels and trimethylene carbonate-based polymers are being developed to simulate the natural mechanical characteristics of heart valves. Electrospinning has been examined as a new manufacturing technique that has the potential to facilitate tissue formation via increased surface area. The type of cells utilized for seeding onto the scaffolds is another factor to take into consideration; currently, stem cells are of great interest because of their potential to differentiate into various types of cells. Although extensive studies have been conducted, the creation of a fully functional heart valve that is clinically applicable still requires further investigation due to the complexity and intricacies of the heart valve.

Prosthetic aortic heart valves: modeling and design

Although heart valve replacement is among the most common cardiovascular surgical procedures, their outcome is often difficult to predict. One of the reasons is the design and choice of the materials used for the fabrication of the prostheses. This review paper describes the use of modeling techniques in prosthetic heart valve (HV) design and aims at the justification and development of a polymer based trileaflet mechanical heart valve (MHV). The closing/opening phase behavior of the bileaflet MHV was investigated. The potential problem of valve failure due to crack propagation in the brittle pyrolytic carbon leaflet was also discussed. These studies suggest that although bileaflet MHV performs satisfactorily, there are justifications for improvement. Since the native aortic HV is trileaflet and made of anisotropic and hyperelastic tissue, one possible approach to a better MHV design is based on our ability to closely mimic the natural geometry and biomaterial properties.

The effects of hammer pressure on cellular response in a porcine heart valve tissue

Our objective was to design, develop, characterize and validate a prototype device for testing the response of aortic valve tissue to impact forces. With each cardiac cycle, the aortic valve, on closure, is subjected to a substantial impact force and the ability of valvular interstitial cells to withstand such forces without apoptosis has not been examined. Our aim was to correlate impact force with apoptosis, identifying the latter using a terminal transferase dUTP nick end-labelling (Tunel) assay. With our drop tower design, we created reproducible impact forces on heart valve tissue resulting in cellular trauma. The reliability of the impact tester design were verified and results showed that normal tissue can withstand impact forces more than 30× greater than the physiological forces to which the tissue is normally exposed. This provides a wide safety margin and indicates that bioengineered aortic valve tissue should have similar properties if it is to withstand physiologic forces long term.

Electrospun bioresorbable heart valve scaffold for tissue engineering

Currently marketed mechanical or biological prosthetic heart valves are regarded as valid substitutes for native heart valves suffering from degenerative pathologies. These devices require strict follow-up due to dysfunctions or post-surgical complications. Potential drawbacks of these medical devices are calcification, tearing of the cusps, thromboembolism and hemolysis. In this context, a tissue engineering approach offers a promising alternative scenario. In this paper, a trileaflet poly(epsilon-caprolactone) (PCL) heart valve scaffold prototype has been manufactured by electrospinning technique using a custom-made rotating target. Process parameters were selected in order to achieve suitable microstructure and mechanical performance. The electrospun heart valve prototype was functionally characterized by means of a pulse duplicator in order to evaluate the mechanical/hydraulic response to the imposed testing conditions. Leaflets synchronously opened in the ejection phase and the proper apposition of the leaflets prevented high leakage volumes in the diastolic phase. This preliminary study suggests a successful perspective for the proposed approach in designing a novel tissue engineered bioresorbable heart valve.

Transient fluid–structure coupling for simulation of a trileaflet heart valve using weak coupling

In this article, a three-dimensional transient numerical approach coupled with fluid-structure interaction for the modeling of an aortic trileaflet heart valve at the initial opening stage is presented. An arbitrary Lagrangian-Eulerian kinematical description together with an appropriate fluid grid was used for the coupling strategy with the structural domain. The fluid dynamics and the structure aspects of the problem were analyzed for various Reynolds numbers and times. The fluid flow predictions indicated that at the initial leaflet opening stage a circulation zone was formed immediately downstream of the leaflet tip and propagated outward as time increased. Moreover, the maximum wall shear stress in the vertical direction of the leaflet was found to be located near the bottom of the leaflet, and its value decreased sharply toward the tip. In the horizontal cross section of the leaflet, the maximum wall shear stresses were found to be located near the sides of the leaflet.

Characterization of an Artificial Valve Flow Using the Numerical Dye Washout Visualization Technique: Application to the Monoleaflet Valve With Purged Flow

Until today, no ideal heart valve prosthesis for the replacement of a diseased natural valve or for use in ventricular assist devices exists. Valves still cause thromboembolic complications originating from thrombus formations in the valve's stagnant zones. Optimization of valve design involves avoiding stagnation zones and zones of high shear stresses. This requires detailed flow field investigations. Usually, the regions which are more prone to thrombus formation can be estimated using a dye washout experiment. The method allows an assessment of regions with a high or low residence time that may in turn predict regions with a corresponding thrombus risk. This successful experimental method was simulated using numerical methods with a combination of the computational fluid dynamics program FLUENT (Fluent Inc., Lebanon, NH, USA) and of the visualization tool AMIRA (TGS Inc., San Diego, CA, USA). The numerical dye washout visualization was applied to four monoleaflet valves with varying valve housing geometries. The results show a significant difference in the washout processes of the examined valves. The dye washout was characterized by a time course of the gray value averaged over a defined region of interest. Finally, these curves were quantified by a half dye time. The half dye time in the best optimized valve was only 0.2753 s. The same time in the original valve was 0.6834 s. This study shows that the proposed numerical method of dye washout visualization can be used as an additional tool of the flow characterization in artificial organs.

Recent developments in tissue engineering and regenerative medicine

PURPOSE OF REVIEW

Currently, patients suffering from diseased and injured organs are treated with transplanted organs or cells. There is, however, a severe shortage of donor tissues and organs that is worsening yearly given the aging population. This paper reviews recent advances that have occurred in regenerative medicine and describes applications of new technologies to treat diseased or damaged organs and tissues.

RECENT FINDINGS

Most current strategies for tissue engineering depend upon a sample of autologous cells from the diseased organ of the patient. Biopsies from patients with extensive end-stage organ failure, however, may not yield enough normal cells. In these situations, stem cells are envisioned as being an alternative source. Stem cells can be derived from discarded human embryos (human embryonic stem cells), from fetal tissue or from adult sources (bone marrow, fat, skin). Therapeutic cloning offers a potentially limitless source of cells for tissue engineering applications.

SUMMARY

Increasingly, scientists in the fields of regenerative medicine and tissue engineering have applied the principles of cell transplantation, material science and bioengineering to construct biological substitutes that will restore and maintain normal function in diseased and injured tissues.

Percutaneous and surgical treatment of aortic stenosis

Aortic stenosis due to calcific degeneration is the most common valvular disorder among the elderly. With the growing elderly population, the prevalence of this disease will continue to increase. Based on converging lines of evidence linking calcific aortic stenosis with atherosclerosis, there has been interest in drug therapy to slow the progression of aortic stenosis. Unfortunately, recently completed prospective trials have been disappointing. Mechanical measures remain the principal form of therapy. Among percutaneous techniques, aortic valvuloplasty provides only transient and modest benefit at a significant risk of stroke and vascular injury. However, aortic valvuloplasty can play a useful role in stabilizing patients who require additional attention prior to definitive surgery. Building on this foundation, a bold new technique of percutaneously implanting a balloon-mounted valve has been developed. Although promising, there have been relatively few patients treated in this fashion (at a single center) and with only limited follow-up. Surgical treatment, specifically valve replacement, is still the definitive treatment of choice for patients with symptomatic aortic stenosis. Surgeons and patients must choose between a variety of models of both tissue and mechanical valves and a variety of surgical approaches. Recent trends include the use of tissue valves in increasingly younger patients and continued interest in alternatives to full median sternotomy in approaching the valve.

Heart Valve Tissue Engineering

Tissue-engineered heart valves have been proposed by physicians and scientists alike to be the ultimate solution for treating valvular heart disease. Rather than replacing a diseased or defective native valve with a mechanical or animal tissue-derived artificial valve, a tissue-engineered valve would be a living organ, able to respond to growth and physiological forces in the same way that the native aortic valve does. Two main approaches have been attempted over the past 10 to 15 years: regeneration and repopulation. Regeneration involves the implantation of a resorbable matrix that is expected to remodel in vivo and yield a functional valve composed of the cells and connective tissue proteins of the patient. Repopulation involves implanting a whole porcine aortic valve that has been previously cleaned of all pig cells, leaving an intact, mechanically sound connective tissue matrix. The cells of the patients are expected to repopulate and revitalize the acellular matrix, creating living tissue that already has the complex microstructure necessary for proper function and durability. Regrettably, neither of the 2 approaches has fared well in animal experiments, and the only clinical experience with tissue-engineered valves resulted in a number of early failures and patient death. This article reviews the technological details of the 2 main approaches, their rationale, their strengths and weaknesses, and the likely mechanisms for their failure. Alternative approaches to valvular tissue engineering, as well as the role of industry in shaping this field in the future, are also reviewed.

Bioreactor design for tissue engineering

Bioreactor systems play an important role in tissue engineering, as they enable reproducible and controlled changes in specific environmental factors. They can provide technical means to perform controlled studies aimed at understanding specific biological, chemical or physical effects. Furthermore, bioreactors allow for a safe and reproducible production of tissue constructs. For later clinical applications, the bioreactor system should be an advantageous method in terms of low contamination risk, ease of handling and scalability. To date the goals and expectations of bioreactor development have been fulfilled only to some extent, as bioreactor design in tissue engineering is very complex and still at an early stage of development. In this review we summarize important aspects for bioreactor design and provide an overview on existing concepts. The generation of three dimensional cartilage-carrier constructs is described to demonstrate how the properties of engineered tissues can be improved significantly by combining biological and engineering knowledge. In the future, a very intimate collaboration between engineers and biologists will lead to an increased fundamental understanding of complex issues that can have an impact on tissue formation in bioreactors.

Tissue engineering a functional aortic heart valve: an appraisal

Valvular heart disease is an important cause of morbidity and mortality, and currently available substitutes for failing hearts have serious limitations. A new promising alternative that may overcome these shortcomings is provided by the relatively new field of tissue engineering (TE). TE techniques involve the growth of autologous cells on a 3D matrix that can be a biodegradable polymer scaffold, or an acellular tissue matrix. These approaches provide the potential to create living matrix valve structures with an ability to grow, repair and remodel within the recipient. This article provides an appraisal of artificial heart valves and an overview of developments in TE that includes the current limitations and challenges for creating a fully functional valve. Biomaterials and stem cell technologies are now providing the potential for new avenues of research and if combined with advances in the rapid prototyping of biomaterials, the engineering of personalized, fully functional, and autologous tissue valve replacements, may become a clinical alternative.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.