Living artificial heart valve alternatives: a review

A Biomimetic Leaflet Scaffold for Aortic Valve Remodeling

Heart valve disease poses a significant clinical challenge, especially in pediatric populations, due to the inability of existing valve replacements to grow or respond biologically to their microenvironment. Tissue-engineered heart valves (TEHVs) provide a solution by facilitating patient-specific models for self-repair and remodeling. In this study, a 3D-bioprinted TEHV is designed to emulate the trilayer leaflet structure of an aortic valve. A cell-laden hydrogel scaffold made from gelatin methacrylate and polyethylene glycol diacrylate (GelMA/PEGDA) incorporates valvular interstitial-like (VIC-like) cells, being reinforced with a layer of polycaprolactone (PCL). The composition of the hydrogel scaffold remains stable over 7 days, having increased mechanical strength compared to pure GelMA. The scaffold maintains VIC-like cell function and promotes extracellular matrix (ECM) protein expression up to 14 days under two dynamic culture conditions: shear stress and stretching; replicating heart valve behavior within a more physiological-like setting and suggesting remodeling potential via ECM synthesis. This TEHV offers a promising avenue for valve replacements, closely replicating the structural and functional attributes of a native aortic valve, leading to mechanical and biological integration through biomaterial-cellular interactions.

Clinical Impact of Computational Heart Valve Models

This paper provides a review of engineering applications and computational methods used to analyze the dynamics of heart valve closures in healthy and diseased states. Computational methods are a cost-effective tool that can be used to evaluate the flow parameters of heart valves. Valve repair and replacement have long-term stability and biocompatibility issues, highlighting the need for a more robust method for resolving valvular disease. For example, while fluid-structure interaction analyses are still scarcely utilized to study aortic valves, computational fluid dynamics is used to assess the effect of different aortic valve morphologies on velocity profiles, flow patterns, helicity, wall shear stress, and oscillatory shear index in the thoracic aorta. It has been analyzed that computational flow dynamic analyses can be integrated with other methods to create a superior, more compatible method of understanding risk and compatibility.

A computational framework for biomaterials containing three-dimensional random fiber networks based on the affine kinematics

Understanding the structure-function relationship of biomaterials can provide insights into different diseases and advance numerous biomedical applications. This paper presents a finite element-based computational framework to model biomaterials containing a three-dimensional fiber network at the microscopic scale. The fiber network is synthetically generated by a random walk algorithm, which uses several random variables to control the fiber network topology such as fiber orientations and tortuosity. The geometric information of the generated fiber network is stored in an array-like data structure and incorporated into the nonlinear finite element formulation. The proposed computational framework adopts the affine fiber kinematics, based on which the fiber deformation can be expressed by the nodal displacement and the finite element interpolation functions using the isoparametric relationship. A variational approach is developed to linearize the total strain energy function and derive the nodal force residual and the stiffness matrix required by the finite element procedure. Four numerical examples are provided to demonstrate the capabilities of the proposed computational framework, including a numerical investigation about the relationship between the proposed method and a class of anisotropic material models, a set of synthetic examples to explore the influence of fiber locations on material local and global responses, a thorough mesh-sensitivity analysis about the impact of mesh size on various numerical results, and a detailed case study about the influence of material structures on the performance of eggshell-membrane-hydrogel composites. The proposed computational framework provides an efficient approach to investigate the structure-function relationship for biomaterials that follow the affine fiber kinematics.

Toward Biomimetic Scaffolds for Tissue Engineering: 3D Printing Techniques in Regenerative Medicine

Three-dimensional (3D) printing technology allows fabricating complex and precise structures by stacking materials layer by layer. The fabrication method has a strong potential in the regenerative medicine field to produce customizable and defect-fillable scaffolds for tissue regeneration. Plus, biocompatible materials, bioactive molecules, and cells can be printed together or separately to enhance scaffolds, which can save patients who suffer from shortage of transplantable organs. There are various 3D printing techniques that depend on the types of materials, or inks, used. Here, different types of organs (bone, cartilage, heart valve, liver, and skin) that are aided by 3D printed scaffolds and printing methods that are applied in the biomedical fields are reviewed.

Stem cells and new intervention measures as emerging therapy in cardiac surgery

Cardiovascular disease (CVD) presents a great burden for elderly patients, their caregivers, and health systems. Structural and functional alterations of vessels accumulate throughout life, culminating in increased risk of developing CVD. Several inflammatory pathway are involved in vascular ageing. The growing elderly population worldwide highlights the need to understand how aging promotes CVD in order to develop new strategies to confront this challenge. In this review we analyzed the role of stem cells and new intervention measures as emerging drugs for vascular aging.

Biomechanical Cues Direct Valvulogenesis

The vertebrate embryonic heart initially forms with two chambers, a ventricle and an atrium, separated by the atrioventricular junction. Localized genetic and biomechanical information guides the development of valves, which function to ensure unidirectional blood flow. If the valve development process goes awry, pathology associated with congenital valve defects can ensue. Congenital valve defects (CVD) are estimated to affect 1–2% of the population and can often require a lifetime of treatment. Despite significant clinical interest, molecular genetic mechanisms that direct valve development remain incompletely elucidated. Cells in the developing valve must contend with a dynamic hemodynamic environment. A growing body of research supports the idea that cells in the valve are highly sensitive to biomechanical forces, which cue changes in gene expression required for normal development or for maintenance of the adult valve. This review will focus on mechanotransductive pathways involved in valve development across model species. We highlight current knowledge regarding how cells sense physical forces associated with blood flow and pressure in the forming heart, and summarize how these changes are transduced into genetic and developmental responses. Lastly, we provide perspectives on how altered biomechanical cues may lead to CVD pathogenesis.

Serum-Free Manufacturing of Mesenchymal Stem Cell Tissue Rings Using Human-Induced Pluripotent Stem Cells

Combination of stem cell technology and 3D biofabrication approaches provides physiological similarity to in vivo tissues and the capability of repairing and regenerating damaged human tissues. Mesenchymal stem cells (MSCs) have been widely used for regenerative medicine applications because of their immunosuppressive properties and multipotent potentials. To obtain large amount of high-quality MSCs without patient donation and invasive procedures, we differentiated MSCs from human-induced pluripotent stem cells (hiPSC-MSCs) using serum-free E6 media supplemented with only one growth factor (bFGF) and two small molecules (SB431542 and CHIR99021). The differentiated cells showed a high expression of common MSC-specific surface markers (CD90, CD73, CD105, CD106, CD146, and CD166) and a high potency for osteogenic and chondrogenic differentiation. With these cells, we have been able to manufacture MSC tissue rings with high consistency and robustness in pluronic-coated reusable PDMS devices. The MSC tissue rings were characterized based on inner diameter and outer ring diameter and observed cell-type-dependent tissue contraction induced by cell-matrix interaction. Our approach of simplified hiPSC-MSC differentiation, modular fabrication procedure, and serum-free culture conditions has a great potential for scalable manufacturing of MSC tissue rings for different regenerative medicine applications.

The Heart and Great Vessels

Cardiovascular disease is the leading cause of mortality worldwide. We have made large strides over the past few decades in management, but definitive therapeutic options to address this health-care burden are still limited. Given the ever-increasing need, much effort has been spent creating engineered tissue to replaced diseased tissue. This article gives a general overview of this work as it pertains to the development of great vessels, myocardium, and heart valves. In each area, we focus on currently studied methods, limitations, and areas for future study.

Tissue-Engineered Heart Valves: A Call for Mechanistic Studies

Heart valve disease carries a substantial risk of morbidity and mortality. Outcomes are significantly improved by valve replacement, but currently available mechanical and biological replacement valves are associated with complications of their own. Mechanical valves have a high rate of thromboembolism and require lifelong anticoagulation. Biological prosthetic valves have a much shorter lifespan, and they are prone to tearing and degradation. Both types of valves lack the capacity for growth, making them particularly problematic in pediatric patients. Tissue engineering has the potential to overcome these challenges by creating a neovalve composed of native tissue that is capable of growth and remodeling. The first tissue-engineered heart valve (TEHV) was created more than 20 years ago in an ovine model, and the technology has been advanced to clinical trials in the intervening decades. Some TEHVs have had clinical success, whereas others have failed, with structural degeneration resulting in patient deaths. The etiologies of these complications are poorly understood because much of the research in this field has been performed in large animals and humans, and, therefore, there are few studies of the mechanisms of neotissue formation. This review examines the need for a TEHV to treat pediatric patients with valve disease, the history of TEHVs, and a future that would benefit from extension of the reverse translational trend in this field to include small animal studies.

Pressure-induced microstructural changes in porcine tricuspid valve leaflets

Quantifying mechanically-induced changes in the tricuspid valve extracellular matrix (ECM) structural components, e.g. collagen fiber spread and distribution, is important as it determines the overall macro-scale tissue responses and subsequently its function/malfunction in physiological/pathophysiological states. For example, functional tricuspid regurgitation, a common tricuspid valve disorder, could be caused by elevated right ventricular pressure due to pulmonary hypertension. In such patients, the geometry and the normal function of valve leaflets alter due to chronic pressure overload, which could cause remodeling responses in the ECM and change its structural components. To understand such a relation, we developed an experimental setup and measured alteration of leaflet microstructure in response to pressure increase in porcine tricuspid valves using the small angle light scattering technique. The anisotropy index, a measure of the fiber spread and distribution, was obtained and averaged for each region of the anterior, posterior, and septal leaflet using four averaging methods. The average anisotropy indices (mean ± standard error) in the belly region of the anterior, posterior, and septal leaflets of non-pressurized valves were found to be 12 ± 2%, 21 ± 3% and 12 ± 1%, respectively. For the pressurized valve, the average values of the anisotropy index in the belly region of the anterior, posterior, and septal leaflets were 56 ± 5%, 39 ± 7% and 32 ± 5%, respectively. Overall, the average anisotropy index was found to be higher for all leaflets in the pressurized valves as compared to the non-pressurized valves, indicating that the ECM fibers became more aligned in response to an increased ventricular pressure.

STATEMENT OF SIGNIFICANCE

Mechanics plays a critical role in development, regeneration, and remodeling of tissues. In the current study, we have conducted experiments to examine how increasing the ventricular pressure leads to realignment of protein fibers comprising the extracellular matrix (ECM) of the tricuspid valve leaflets. Like many other tissues, in cardiac valves, cell-matrix interactions and gene expressions are heavily influenced by changes in the mechanical microenvironment at the ECM/cellular level. We believe that our study will help us better understand how abnormal increases in the right ventricular pressure (due to pulmonary hypertension) could change the structural architecture of tricuspid valve leaflets and subsequently the mechanical microenvironment at the ECM/cellular level.

Developing a Clinically Relevant Tissue Engineered Heart Valve-A Review of Current Approaches

Tissue engineered heart valves (TEHVs) have the potential to address the shortcomings of current implants through the combination of cells and bioactive biomaterials that promote growth and proper mechanical function in physiological conditions. The ideal TEHV should be anti-thrombogenic, biocompatible, durable, and resistant to calcification, and should exhibit a physiological hemodynamic profile. In addition, TEHVs may possess the capability to integrate and grow with somatic growth, eliminating the need for multiple surgeries children must undergo. Thus, this review assesses clinically available heart valve prostheses, outlines the design criteria for developing a heart valve, and evaluates three types of biomaterials (decellularized, natural, and synthetic) for tissue engineering heart valves. While significant progress has been made in biomaterials and fabrication techniques, a viable tissue engineered heart valve has yet to be translated into a clinical product. Thus, current strategies and future perspectives are also discussed to facilitate the development of new approaches and considerations for heart valve tissue engineering.

The combination of stem cells and tissue engineering: an advanced strategy for blood vessels regeneration and vascular disease treatment

Over the past years, vascular diseases have continued to threaten human health and increase financial burdens worldwide. Transplantation of allogeneic and autologous blood vessels is the most convenient treatment. However, it could not be applied generally due to the scarcity of donors and the patient’s condition. Developments in tissue engineering are contributing greatly with regard to this urgent need for blood vessels. Tissue engineering-derived blood vessels are promising alternatives for patients with aortic dissection/aneurysm. The aim of this review is to show the importance of advances in biomaterials development for the treatment of vascular disease. We also provide a comprehensive overview of the current status of tissue reconstruction from stem cells and transplantable cellular scaffold constructs, focusing on the combination of stem cells and tissue engineering for blood vessel regeneration and vascular disease treatment.

Replacing the Anterior Mitral Valve Leaflet With Autologous Jugular Vein in a Sheep Model

BACKGROUND

This study evaluated the use of an autologous vein graft, supported by expanded polytetrafluoroethylene (ePTFE) chordae tendineae, to replace an anterior mitral valve leaflet.

METHODS

A double-layered autologous jugular vein graft, supported by ePTFE chords, was used to create an anterior mitral valve leaflet in 21 sheep. Mitral valve function was monitored with echocardiography for up to 10 months. Surviving sheep were euthanized between 6 and 10 months later, and vein implants were examined histologically.

RESULTS

One sheep died intraoperatively. Fourteen sheep had trace to mild mitral regurgitation (MR), 5 had mild to moderate MR, and 1 had moderate to severe MR. Ten sheep died between 2 days and 6.2 months. Echocardiography at 6 months showed MR progression in 8 of 11 sheep. The vein leaflet developed intimal fibroplasia and fibrous proliferation in response to the increased stress on the tissue, but the vein remained flexible without shortening or contracture. The 6- to 10-month vein implants showed viability with intact endothelium, myofibroblasts, collagen, and elastin. A normal healing pattern was seen at the suture lines, and no calcification was observed in the vein leaflet apart from the ePTFE sutures.

CONCLUSIONS

Autologous vein has the potential to function as a mitral valve leaflet substitute because it adapted morphologically and remained viable in the intracardiac position. Technical refinement in creating and implanting the leaflet is needed to improve the progression of MR.

Numerical investigation of the influence of pattern topology on the mechanical behavior of PEGDA hydrogels

Poly(ethylene glycol) diacrylate (PEGDA) hydrogels can be potentially used as scaffold material for tissue engineered heart valves (TEHVs) due to their good biocompatibility and biomechanical tunability. The photolithographic patterning technique is an effective approach to pattern PEGDA hydrogels to mimic the mechanical behavior of native biological tissues that are intrinsically anisotropic. The material properties of patterned PEGDA hydrogels largely depend on the pattern topology. In this paper, we adopt a newly proposed computational framework for fibrous biomaterials to numerically investigate the influence of pattern topology, including pattern ratio, orientation and waviness, on the mechanical behavior of patterned PEGDA hydrogels. The material parameters for the base hydrogel and the pattern stripes are directly calibrated from published experimental data. Several experimental observations reported in the literature are captured in the simulation, including the nonlinear relationship between pattern ratio and material linear modulus, and the decrease of material anisotropy when pattern ratio increases. We further numerically demonstrate that a three-region (toe-heel-linear) stress-strain relationship typically exhibited by biological tissues can be obtained by tuning the pattern waviness and the relative stiffness between the base hydrogel and pattern stripes. The numerical strategy and simulation results presented here can provide helpful guidance to optimize pattern design of PEGDA hydrogels toward the targeted material mechanical properties, therefore advance the development of TEHVs.

STATEMENT OF SIGNIFICANCE

Poly(ethylene glycol) diacrylate (PEGDA) hydrogels can be used as scaffold material for tissue engineered heart values (TEHVs) providing a promising alternative to generate suitable heart valve replacement method. The patterning of PEGDA hydrogels using photolithographic techniques creates materials that mimic the mechanical behavior of native heart valve tissues. However, targeted material properties are obtained via a trial-and-error process. Depending on experiments alone to explore the influence of pattern topology is expensive and time-consuming. We combine a newly proposed computational framework with published experimental data to numerically investigate the influence of pattern geometry on the mechanical behavior of patterned PEGDA hydrogels. The numerical strategy and simulation results presented here can provide guidance to optimize the design of PEGDA hydrogels with targeted material properties, therefore advance the development of TEHVs.

Application of hydrogels in heart valve tissue engineering

With an increasing number of patients requiring valve replacements, there is heightened interest in advancing heart valve tissue engineering (HVTE) to provide solutions to the many limitations of current surgical treatments. A variety of materials have been developed as scaffolds for HVTE including natural polymers, synthetic polymers, and decellularized valvular matrices. Among them, biocompatible hydrogels are generating growing interest. Natural hydrogels, such as collagen and fibrin, generally show good bioactivity but poor mechanical durability. Synthetic hydrogels, on the other hand, have tunable mechanical properties; however, appropriate cell-matrix interactions are difficult to obtain. Moreover, hydrogels can be used as cell carriers when the cellular component is seeded into the polymer meshes or decellularized valve scaffolds. In this review, we discuss current research strategies for HVTE with an emphasis on hydrogel applications. The physicochemical properties and fabrication methods of these hydrogels, as well as their mechanical properties and bioactivities are described. Performance of some hydrogels including in vitro evaluation using bioreactors and in vivo tests in different animal models are also discussed. For future HVTE, it will be compelling to examine how hydrogels can be constructed from composite materials to replicate mechanical properties and mimic biological functions of the native heart valve.

Biological and mechanical evaluation of a Bio-Hybrid scaffold for autologous valve tissue engineering

Major challenge in heart valve tissue engineering for paediatric patients is the development of an autologous valve with regenerative capacity. Hybrid tissue engineering approach is recently gaining popularity to design scaffolds with desired biological and mechanical properties that can remodel post implantation. In this study, we fabricated aligned nanofibrous Bio-Hybrid scaffold made of decellularized bovine pericardium: polycaprolactone-chitosan with optimized polymer thickness to yield the desired biological and mechanical properties. CD44⁺, αSMA⁺, Vimentin⁺ and CD105^- human valve interstitial cells were isolated and seeded on these Bio-Hybrid scaffolds. Subsequent biological evaluation revealed interstitial cell proliferation with dense extra cellular matrix deposition that indicated the viability for growth and proliferation of seeded cells on the scaffolds. Uniaxial mechanical tests along axial direction showed that the Bio-Hybrid scaffolds has at least 20 times the strength of the native valves and its stiffness is nearly 3 times more than that of native valves. Biaxial and uniaxial mechanical studies on valve interstitial cells cultured Bio-Hybrid scaffolds revealed that the response along the axial and circumferential direction was different, similar to native valves. Overall, our findings suggest that Bio-Hybrid scaffold is a promising material for future development of regenerative heart valve constructs in children.

Micro and nanotechnologies in heart valve tissue engineering

Due to the increased morbidity and mortality resulting from heart valve diseases, there is a growing demand for off-the-shelf implantable tissue engineered heart valves (TEHVs). Despite the significant progress in recent years in improving the design and performance of TEHV constructs, viable and functional human implantable TEHV constructs have remained elusive. The recent advances in micro and nanoscale technologies including the microfabrication, nano-microfiber based scaffolds preparation, 3D cell encapsulated hydrogels preparation, microfluidic, micro-bioreactors, nano-microscale biosensors as well as the computational methods and models for simulation of biological tissues have increased the potential for realizing viable, functional and implantable TEHV constructs. In this review, we aim to present an overview of the importance and recent advances in micro and nano-scale technologies for the development of TEHV constructs.

Iranian homograft tissue processing

Tissue transplantation is a life-enhancing therapeutic modality for damaged or non-functioning tissues. In most cases, there is no alternative other than human tissue as replacement, and taking into account the ever-increasing demand for tissue grafts, it makes sense to set up an establishment in charge of human tissue procurement to meet local needs. A quality assurance system, clearly defined standards, and regular audits complement the infrastructure which make this activity feasible. The process of tissue procurement consists of donor identification, consent, tissue recovery, donor screening and testing, tissue processing, preservation, packaging, labeling, terminal sterilization, storage and distribution. The transplantation of homograft heart valves remains controversial, due to the availability of prosthetic and bioprosthetic alternatives. The limited durability of homografts has not yet outweighed the advantages which this graft offers. Adherence to regulations and regularly revised guidelines improve long-term efficacy and minimizes complications or malfunction. Furthermore, the lower price of homograft heart valves and the removal of the need for a lifetime of anticoagulation therapy are noteworthy advantages of this replacement. In our practice, the proportion of homograft heart valves meeting release criteria and successfully implanted grafts were 83% and 95%, respectively.

Engineering natural heart valves: possibilities and challenges

Heart valve replacement is considered to be the most prevalent treatment approach for cardiac valve-related diseases. Among current solutions for heart valve replacement, e.g. mechanical and bioprosthetic valves, the main shortcoming is the lack of growth capability, repair and remodelling of the substitute valve. During the past three decades, tissue engineering-based approaches have shown tremendous potential to overcome these limitations by the development of a biodegradable scaffold, which provides biomechanical and biochemical properties of the native tissue. Among various scaffolds employed for tissue engineering, the decellularized heart valve (DHV) has attracted much attention, due to its native structure as well as comparable haemodynamic characteristics. Although the human DHV has shown optimal properties for valve replacement, the limitation of valve donors in terms of time and size is their main clinical issue. In this regard, xenogenic DHV can be a promising candidate for heart valve replacement. Xenogenic DHVs have similar composition to human valves, which will overcome the need for human DHVs. The main concern regarding xenogeneic DHV replacement is the immunological reaction and calcification following implantation, weak mechanical properties and insufficient recellularization capacity. In this review, we describe the essential steps required to address these impediments through novel engineering approaches. Copyright © 2016 John Wiley & Sons, Ltd.

High-throughput investigation of endothelial-to-mesenchymal transformation (EndMT) with combinatorial cellular microarrays

In the developing heart, a specific subset of endocardium undergoes an endothelial-to-mesenchymal transformation (EndMT) thus forming nascent valve leaflets. Extracellular matrix (ECM) proteins and growth factors (GFs) play important roles in regulating EndMT but the combinatorial effect of GFs with ECM proteins is less well understood. Here we use microscale engineering techniques to create single, binary, and tertiary component microenvironments to investigate the combinatorial effects of ECM proteins and GFs on the attachment and transformation of adult ovine mitral valve endothelial cells to a mesenchymal phenotype. With the combinatorial microenvironment microarrays, we utilized 60 different combinations of ECM proteins (Fibronectin, Collagen I, II, IV, Laminin) and GFs (TGF-β1, bFGF, VEGF) and were able to identify new microenvironmental conditions capable of modulating EndMT in MVECs. Experimental results indicated that TGF-β1 significantly upregulated the EndMT while either bFGF or VEGF downregulated EndMT process markedly. Also, ECM proteins could influence both the attachment of MVECs and the response of MVECs to GFs. In terms of attachment, fibronectin is significantly better for the adhesion of MVECs among the five tested proteins. Overall collagen IV and fibronectin appeared to play important roles in promoting EndMT process. Great consistency between macroscale and microarrayed experiments and present studies demonstrates that high-throughput cellular microarrays are a promising approach to study the regulation of EndMT in valvular endothelium. Biotechnol. Bioeng. 2016;113: 1403-1412. © 2015 Wiley Periodicals, Inc.

Differentiation and Distribution of Marrow Stem Cells in Flex-Flow Environments Demonstrate Support of the Valvular Phenotype

For treatment of critical heart valve diseases, prosthetic valves perform fairly well in most adults; however, for pediatric patients, there is the added requirement that the replacement valve grows with the child, thus extremely limiting current treatment options. Tissue engineered heart valves (TEHV), such as those derived from autologous bone marrow stem cells (BMSCs), have the potential to recapitulate native valve architecture and accommodate somatic growth. However, a fundamental pre-cursor in promoting directed integration with native tissues rather than random, uncontrolled growth requires an understanding of BMSC mechanobiological responses to valve-relevant mechanical environments. Here, we report on the responses of human BMSC-seeded polymer constructs to the valve-relevant stress states of: (i) steady flow alone, (ii) cyclic flexure alone, and (iii) the combination of cyclic flexure and steady flow (flex-flow). BMSCs were seeded onto a PGA: PLLA polymer scaffold and cultured in static culture for 8 days. Subsequently, the aforementioned mechanical conditions, (groups consisting of steady flow alone-850ml/min, cyclic flexure alone-1 Hz, and flex-flow-850ml/min and 1 Hz) were applied for an additional two weeks. We found samples from the flex-flow group exhibited a valve-like distribution of cells that expressed endothelial (preference to the surfaces) and myofibroblast (preference to the intermediate region) phenotypes. We interpret that this was likely due to the presence of both appreciable fluid-induced shear stress magnitudes and oscillatory shear stresses, which were concomitantly imparted onto the samples. These results indicate that flex-flow mechanical environments support directed in vitro differentiation of BMSCs uniquely towards a heart valve phenotype, as evident by cellular distribution and expression of specific gene markers. A priori guidance of BMSC-derived, engineered tissue growth under flex-flow conditions may serve to subsequently promote controlled, engineered to native tissue integration processes in vivo necessary for successful long-term valve remodeling.

Expression of COLLAGEN 1 and ELASTIN Genes in Mitral Valvular Interstitial Cells within Microfiber Reinforced Hydrogel

OBJECTIVE

The incidence of heart valve disease is increasing worldwide and the number of heart valve replacements is expected to increase in the future. By mimicking the main tissue structures and properties of heart valve, tissue engineering offers new options for the replacements. Applying an appropriate scaffold in fabricating tissue-engineered heart valves (TEHVs) is of importance since it affects the secretion of the main extracellular matrix (ECM) components, collagen 1 and elastin, which are crucial in providing the proper mechanical properties of TEHVs.

MATERIALS AND METHODS

Using real-time polymerase chain reaction (PCR) in this experi- mental study, the relative expression levels of COLLAGEN 1 and ELASTIN were obtained for three samples of each examined sheep mitral valvular interstitial cells (MVICs)-seeded onto electrospun poly (glycerol sebacate) (PGS)-poly (ε-caprolactone) (PCL) microfibrous, gelatin and hyaluronic acid based hydrogel-only and composite (PGS-PCL/hydrogel) scaffolds. This composite has been shown to create a synthetic three-dimensional (3D) microenvironment with appropriate mechanical and biological properties for MVICs.

RESULTS

Cell viability and metabolic activity were similar among all scaffold types. Our results showed that the level of relative expression of COLLAGEN 1 and ELASTIN genes was higher in the encapsulated composite scaffolds compared to PGS-PCL-only and hydrogel-only scaffolds with the difference being statistically significant (P<0.05).

CONCLUSION

The encapsulated composite scaffolds are more conducive to ECM secretion over the PGS-PCL-only and hydrogel-only scaffolds. This composite scaffold can serve as a model scaffold for heart valve tissue engineering.

Development of a heart valve model surface for optimization of surface modifications

Current bioprosthetic valve replacements (BPVs) are susceptible to myriad complications, including calcification and thrombosis; however, recent research has explored surface modifications to encourage re-endothelialization of the tissue, preventing unwanted blood-tissue interactions. A bioprosthetic valve surface model (BVSM) was developed to facilitate rapid in vitro optimization of surface modification techniques for BPVs. The BVSM was manufactured by photopolymerization of PEGDA and collagen type I and subsequent addition of amine-rich peptide to provide reactive sites for surface modification. This BVSM mimics surface mechanical properties of bioprosthetic valve tissue, as measured by micropipette aspiration. The BVSM successfully mimics the latent toxic effects of glutaraldehyde fixation, as shown through MTT assay results. Amine content, assessed by XPS, was shown to be significantly lower in the BVSM than unfixed tissue. However, incubation of the surface with amine-reactive NHS-PEG-Cy5 revealed even coverage of the BVSM surface, suggesting that there exists sufficient surface reactive groups to anchor surface modifications, and that translation of the modification process to tissue will yield more complete modification of the BPV surface. These results indicate successful construction of a BVSM that mimics essential properties of bioprosthetic valve tissue and its usefulness for rapid in vitro optimization of surface modification methods for endothelialization.

STATEMENT OF SIGNIFICANCE

Current bioprosthetic valve replacements are susceptible to many complications, including calcification and thrombosis; however, recent research has explored surface modifications to encourage the integration of the replacement with the native tissue, which would prevent unwanted blood-tissue interactions. However, methods to analyze and optimize such modifications are limited by the complex surface topography, individual variability, and opacity of native tissue. Thus, we have developed a novel bioprosthetic valve tissue model (BVM) which mimics the important features of the bioprosthetic valve tissue and serves as a platform for rapid optimization and testing of surface modification strategies for tissue valves. Thus, the BVM will provide a needed platform to support rapid improvement of clinically available cardiovascular implants.

Study on the Antimicrobial Properties of Citrate-Based Biodegradable Polymers

Citrate-based polymers possess unique advantages for various biomedical applications since citric acid is a natural metabolism product, which is biocompatible and antimicrobial. In polymer synthesis, citric acid also provides multiple functional groups to control the crosslinking of polymers and active binding sites for further conjugation of biomolecules. Our group recently developed a number of citrate-based polymers for various biomedical applications by taking advantage of their controllable chemical, mechanical, and biological characteristics. In this study, various citric acid derived biodegradable polymers were synthesized and investigated for their physicochemical and antimicrobial properties. Results indicate that citric acid derived polymers reduced bacterial proliferation to different degrees based on their chemical composition. Among the studied polymers, poly(octamethylene citrate) showed ~70-80% suppression to microbe proliferation, owing to its relatively higher ratio of citric acid contents. Crosslinked urethane-doped polyester elastomers and biodegradable photoluminescent polymers also exhibited significant bacteria reduction of ~20 and ~50% for Staphylococcus aureus and Escherichia coli, respectively. Thus, the intrinsic antibacterial properties in citrate-based polymers enable them to inhibit bacteria growth without incorporation of antibiotics, silver nanoparticles, and other traditional bacteria-killing agents suggesting that the citrate-based polymers are unique beneficial materials for wound dressing, tissue engineering, and other potential medical applications where antimicrobial property is desired.

The Functional Study of Artificial Heart Valve Based on HHT

The functional study of artificial heart valve has the positive significance on the postoperative care of the patients with valvular heart disease and the precaution of postoperative complications. The analysis of the heart sounds is the most direct way to estimate the function normally of heart. Because of the heart sound is nonlinear and nonstationary. Compared with the normal method of time-frequency analysis, Hilbert-Huang transform can analyze the nonstationary and nonlinear signals more accurate and more effective. Hilbert-Huang transform is introduced to the functional study of artificial heart valve. It used for extracting the inherent characteristics. Using the empirical mode decomposition (EMD), the heart sound was decomposed into a series of intrinsic mode functions (IMF). The Hilbert spectrum was established by the calculating results of these IMFs. The Hilbert spectrum has the characters of time-frequency-energy, and then the marginal spectrum was structured. Comparison with the characters of pre-operation and post-operation was used for reveal the function of artificial heart valve. At last, we use the short-time average energy and the short-time average range to verify the credibility of Hilbert-Huang transforms method. The result of simulation show that this method was well analyzed the function of artificial heart valve.

Fiber-reinforced hydrogel scaffolds for heart valve tissue engineering

Heart valve-related disorders are among the major causes of death worldwide. Although prosthetic valves are widely used to treat this pathology, current prosthetic grafts cannot grow with the patient while maintaining normal valve mechanical and hemodynamic properties. Tissue engineering may provide a possible solution to this issue through using biodegradable scaffolds and patients' own cells. Despite their similarity to heart valve tissue, most hydrogel scaffolds are not mechanically suitable for the dynamic stresses of the heart valve microenvironment. In this study, we integrated electrospun poly(glycerol sebacate) (PGS)-poly(ɛ-caprolactone) (PCL) microfiber scaffolds, which possess enhanced mechanical properties for heart valve engineering, within a hybrid hydrogel made from methacrylated hyaluronic acid and methacrylated gelatin. Sheep mitral valvular interstitial cells were encapsulated in the hydrogel and evaluated in hydrogel-only, PGS-PCL scaffold-only, and composite scaffold conditions. Although the cellular viability and metabolic activity were similar among all scaffold types, the presence of the hydrogel improved the three-dimensional distribution of mitral valvular interstitial cells. As seen by similar values in both the Young's modulus and the ultimate tensile strength between the PGS-PCL scaffolds and the composites, microfibrous scaffolds preserved their mechanical properties in the presence of the hydrogels. Compared to electrospun or hydrogel scaffolds alone, this combined system may provide a more suitable three-dimensional structure for generating scaffolds for heart valve tissue engineering.

The aortic valve: structure, complications and implications for transcatheter aortic valve replacement

The aortic valve operates in a complex haemodynamic environment, opening and closing over 100,000 times a day. When complications arise, such as aortic stenosis, prognosis can be very poor, leading to death within the first few years. Surgical valve replacement is currently the standard treatment for aortic stenosis. A thorough understanding of the anatomy and function of the native valve is imperative when developing a prosthetic replacement that can withstand the complex demands of the heart. This review focuses on the anatomy, structure and disease of the aortic valve and the implications for a transcatheter aortic valve replacement (TAVR). Current complications with TAVR, such as major vascular bleeding, conduction disturbances and patient-prosthesis mismatch (PPM), can be overcome by reducing the delivery profile and through the use of more accurate imaging technologies to work towards a fully functional and durable prosthesis.

Trends in tissue engineering for blood vessels

Over the years, cardiovascular diseases continue to increase and affect not only human health but also the economic stability worldwide. The advancement in tissue engineering is contributing a lot in dealing with this immediate need of alleviating human health. Blood vessel diseases are considered as major cardiovascular health problems. Although blood vessel transplantation is the most convenient treatment, it has been delimited due to scarcity of donors and the patient's conditions. However, tissue-engineered blood vessels are promising alternatives as mode of treatment for blood vessel defects. The purpose of this paper is to show the importance of the advancement on biofabrication technology for treatment of soft tissue defects particularly for vascular tissues. This will also provide an overview and update on the current status of tissue reconstruction especially from autologous stem cells, scaffolds, and scaffold-free cellular transplantable constructs. The discussion of this paper will be focused on the historical view of cardiovascular tissue engineering and stem cell biology. The representative studies featured in this paper are limited within the last decade in order to trace the trend and evolution of techniques for blood vessel tissue engineering.

Fine structure of glycosaminoglycans from fresh and decellularized porcine cardiac valves and pericardium

Cardiac valves are dynamic structures, exhibiting a highly specialized architecture consisting of cells and extracellular matrix with a relevant proteoglycan and glycosaminoglycan content, collagen and elastic fibers. Biological valve substitutes are obtained from xenogenic cardiac and pericardial tissues. To overcome the limits of such non viable substitutes, tissue engineering approaches emerged to create cell repopulated decellularized scaffolds. This study was performed to determine the glycosaminoglycans content, distribution, and disaccharides composition in porcine aortic and pulmonary valves and in pericardium before and after a detergent-based decellularization procedure. The fine structural characteristics of galactosaminoglycans chondroitin sulfate and dermatan sulfate were examined by FACE. Furthermore, the mechanical properties of decellularized pericardium and its propensity to be repopulated by in vitro seeded fibroblasts were investigated. Results show that galactosaminoglycans and hyaluronan are differently distributed between pericardium and valves and within heart valves themselves before and after decellularization. The distribution of glycosaminoglycans is also dependent from the vascular district and topographic localization. The decellularization protocol adopted resulted in a relevant but not selective depletion of galactosaminoglycans. As a whole, data suggest that both decellularized porcine heart valves and bovine pericardium represent promising materials bearing the potential for future development of tissue engineered heart valve scaffolds.

Anisotropic strain transfer through the aortic valve and its relevance to the cellular mechanical environment

Aortic valve interstitial cells are responsible for maintaining the valve in response to their local mechanical environment. However, the complex organization of the extracellular matrix means cell strains cannot be directly derived from gross strains, and knowledge of tissue structure-function correlations is fundamental towards understanding mechanotransduction. This study investigates strain transfer through the valve, hypothesizing that organization of the valve matrix leads to non-homogenous local strains. Radial and circumferential samples were cut from aortic valve leaflets and subjected to quasi-static mechanical characterization. Further samples were imaged using confocal microscopy, to determine local strains in the matrix. Mechanical data demonstrated that the valve was significantly stronger and stiffer when loaded circumferentially, comparable with previous studies. Micromechanical studies demonstrated that strain transfer through the matrix is anisotropic and indirect, with local strains consistently smaller than applied strains in both orientations. Under radial loading, strains were transferred linearly to cells. However, under circumferential loading, strains were only one-third of applied values, with a less direct relationship between applied and local strains. This may result from matrix reorganization, and be important for preventing cellular damage during normal valve function. These findings should be taken into account when investigating interstitial cell behaviours, such as cell metabolism and mechanotransduction.

[Tissue engineering of heart valves]

With the introduction of heart valve prostheses cardiac valvular disease has become much more accessible to therapeutic options. However, currently available prostheses display significant limitations, such as limited long-term durability (biological prostheses) and a long-term necessity for anticoagulation therapy. Hence, alternative prosthesis types have been extensively explored in recent years particularly aiming at the development of vital and regenerative prostheses by means of tissue engineering. In the scientific field, different competing concepts have been introduced, including biological or synthetic scaffolds which can be further enhanced by cellular or extracellular components to promote further in vivo development of the prosthesis after implantation. Nowadays, decellularized donor heart valves are among the most advanced prosthesis types experiencing growing clinical attention and widespread use.

Influence of platelet-derived growth factor-AB on tissue development in autologous platelet-rich plasma gels

Fibrin-based scaffolds are widely used in tissue engineering. We postulated that the use of platelet-rich plasma (PRP) in contrast to platelet-poor plasma and pure fibrinogen as the basic material leads to an increased release of autologous platelet-derived growth factor (PDGF)-AB, which may have a consequent positive effect on tissue development. Therefore, we evaluated the release of PDGF-AB during the production process and the course of PDGF release during cultivation of plasma gels with and w/o platelets. The influence of PDGF-AB on the proliferation rate of human umbilical cord artery smooth muscle cells (HUASMCs) was studied using XTT assay. The synthesis of extracellular matrix by HUASMCs in plasma- and fibrin gels was measured using hydroxyproline assay. The use of PRP led to an increase in autologous PDGF-AB release. Further, the platelet-containing plasma gels showed a prolonged release of growth factor during cultivation. Both PRP and platelet-poor plasma gels had a positive effect on the production of collagen. However, PDGF-AB as a supplement in medium and in pure fibrin gel had neither an effect on cell proliferation nor on the collagen synthesis rate. This observation may be due to an absence of PDGF receptors in HUASMCs as determined by flow cytometry. In conclusion, although the prolonged autologous production of PDGF-AB in PRP gels is possible, the enhanced tissue development by HUASMCs within such gels is not PDGF related.

Decellularization of pericardial tissue and its impact on tensile viscoelasticity and glycosaminoglycan content

Bovine pericardium is a collagenous tissue commonly used as a natural biomaterial in the fabrication of cardiovascular devices. For tissue engineering purposes, this xenogeneic biomaterial must be decellularized to remove cellular antigens. With this in mind, three decellularization protocols were compared in terms of their effectiveness to extract cellular materials, their effect on glycosaminoglycan (GAG) content and, finally, their effect on tensile biomechanical behavior. The tissue decellularization was achieved by treatment with t-octyl phenoxy polyethoxy ethanol (Triton X-100), tridecyl polyethoxy ethanol (ATE) and alkaline treatment and subsequent treatment with nucleases (DNase/RNase). The quantified residual DNA content (3.0±0.4%, 4.4±0.6% and 5.6±0.7% for Triton X-100, ATE and alkaline treatment, respectively) and the absence of nuclear structures (hematoxylin and eosin staining) were indicators of effective cell removal. In the same way, it was found that the native tissue GAG content decreased to 61.6±0.6%, 62.7±1.1% and 88.6±0.2% for Triton X-100, ATE and alkaline treatment, respectively. In addition, an alteration in the tissue stress relaxation characteristics was observed after alkaline treatment. We can conclude that the three decellularization agents preserved the collagen structural network, anisotropy and the tensile modulus, tensile strength and maximum strain at failure of native tissue.

Application of decellularized scaffold combined with loaded nanoparticles for heart valve tissue engineering in vitro

The purpose of this study was to fabricate decelluarized valve scaffold modified with polyethylene glycol nanoparticles loaded with transforming growth factor-β1 (TGF-β1), by which to improve the extracellular matrix microenvironment for heart valve tissue engineering in vitro. Polyethylene glycol nanoparticles were obtained by an emulsion-crosslinking method, and their morphology was observed under a scanning electron microscope. Decelluarized valve scaffolds, prepared by using trypsinase and TritonX-100, were modified with nanoparticles by carbodiimide, and then TGF-β1 was loaded into them by adsorption. The TGF-β1 delivery of the fabricated scaffold was measured by asing enzyme-linked immunosorbent assay. Whether unseeded or reseeded with myofibroblast from rats, the morphologic, biochemical and biomechanical characteristics of hybrid scaffolds were tested and compared with decelluarized scaffolds under the same conditions. The enzyme-linked immunosorbent assay revealed a typical delivery of nanoparticles. The morphologic observations and biological data analysis indicated that fabricated scaffolds possessed advantageous biocompatibility and biomechanical property beyond decelluarized scaffolds. Altogether this study proved that it was feasible to fabricate the hybrid scaffold and effective to improve extracellular matrix microenvironment, which is beneficial for an application in heart valve tissue engineering.

Vascular smooth muscle cells as a valvular interstitial cell surrogate in heart valve tissue engineering

BACKGROUND

Vascular smooth muscle cells (VSMCs) are a potential autologous cell source for aortic valve tissue engineering, but have a phenotype that differs from that of valvular interstitial cells in vivo. We hypothesized that combining basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), or platelet-derived growth factor (PDGF) with transforming growth factor beta-1 (TGF-beta1) would achieve a valvular interstitial cell-like phenotype of VSMCs.

METHODS

VSMC phenotype was assessed by immunofluorescence, proliferation was measured by the tetrazolium reduction (MTT) assay, and extracellular matrix gene expression was determined by real-time polymerase chain reaction.

RESULTS

Combinations of growth factors that included PDGF showed the greatest increases in proliferation. Immunofluorescence for alpha-smooth muscle actin demonstrated an inverse correlation between proliferation and a myofibroblast-like phenotype, while combinations of TGF-beta1+ EGF+bFGF (TEF) and TGF-beta1+EGF+PDGF (TEP) induced the greatest change of alpha-smooth muscle actin expression compared to untreated controls. Finally, TEP treatment showed an increase in versican, fibronectin, and type I collagen mRNA expression, while decreasing matrix metalloproteinase 1 expression.

CONCLUSIONS

Combination of TGF-beta1 with EGF and PDGF induces VSMC proliferation and expression of extracellular matrix constituents found in the aortic valve. In vitro preconditioning of VSMCs provides a potentially viable surrogate cell source for developing a valve graft.