Tissue engineering of heart valves: in vitro experiences

Polyhydroxyalkanoates: Medical Applications and Potential for Use in Dentistry

Polyhydroxyalkanoates (PHAs) are promising biopolymers as an alternative to traditional synthetic polymers due to their biodegradability and biocompatibility. The PHA market is blooming in response to the growing demand for biodegradable and environmentally friendly plastics. These biopolyesters are produced and degraded by a variety of microorganisms, making them environmentally friendly, while offering benefits such as biocompatibility (when adequately processed) and biodegradability. Their versatility extends to various areas, from biomedicine to agriculture and composite materials, where they pave the way for significative innovations. In the field of regenerative medicine, some PHAs have key applications, namely in vascular grafts, oral tissue regeneration, and development of self-healing polymers. In addition, PHAs have the potential to be used in the creation of dental implant materials and dental medical devices. PHAs can also be used to encapsulate hydrophobic drugs, providing an approach for more targeted and effective treatments. To summarize, PHAs open new perspectives in the field of medicine by improving drug delivery and offering ecologically biocompatible solutions for medical devices. The aim of this review is to present the medical and dental applications of PHA, their advantages, disadvantages, and indications.

Nonlinear Elasticity of Blood Vessels and Vascular Grafts

The transplantation of vascular grafts has emerged as a prevailing approach to address vascular disorders. However, the development of small-diameter vascular grafts is still in progress, as they serve in a more complicated mechanical environment than their counterparts with larger diameters. The biocompatibility and functional characteristics of small-diameter vascular grafts have been well developed; however, mismatch in mechanical properties between the vascular grafts and native arteries has not been accomplished, which might facilitate the long-term patency of small-diameter vascular grafts. From a point of view in mechanics, mimicking the nonlinear elastic mechanical behavior exhibited by natural blood vessels might be the state-of-the-art in designing vascular grafts. This review centers on elucidating the nonlinear elastic behavior of natural blood vessels and vascular grafts. The biological functionality and limitations associated with as-reported vascular grafts are meticulously reviewed and the future trajectory for fabricating biomimetic small-diameter grafts is discussed. This review might provide a different insight from the traditional design and fabrication of artificial vascular grafts.

Strategies for Development of Synthetic Heart Valve Tissue Engineering Scaffolds

The current clinical solutions, including mechanical and bioprosthetic valves for valvular heart diseases, are plagued by coagulation, calcification, nondurability, and the inability to grow with patients. The tissue engineering approach attempts to resolve these shortcomings by producing heart valve scaffolds that may deliver patients a life-long solution. Heart valve scaffolds serve as a three-dimensional support structure made of biocompatible materials that provide adequate porosity for cell infiltration, and nutrient and waste transport, sponsor cell adhesion, proliferation, and differentiation, and allow for extracellular matrix production that together contributes to the generation of functional neotissue. The foundation of successful heart valve tissue engineering is replicating native heart valve architecture, mechanics, and cellular attributes through appropriate biomaterials and scaffold designs. This article reviews biomaterials, the fabrication of heart valve scaffolds, and their in-vitro and in-vivo evaluations applied for heart valve tissue engineering.

Polyhydroxyalkanoates: the natural biopolyester for future medical innovations

Polyhydroxyalkanoates (PHAs) are a family of natural microbial biopolyesters with the same basic chemical structure and diverse side chain groups. Based on their excellent biodegradability, biocompatibility, thermoplastic properties and diversity, PHAs are highly promising medical biomaterials and elements of medical devices for applications in tissue engineering and drug delivery. However, due to the high cost of biotechnological production, most PHAs have yet to be applied in the clinic and have only been studied at laboratory scale. This review focuses on the biosynthesis, diversity, physical properties, biodegradability and biosafety of PHAs. We also discuss optimization strategies for improved microbial production of commercial PHAs via novel synthetic biology tools. Moreover, we also systematically summarize various medical devices based on PHAs and related design approaches for medical applications, including tissue repair and drug delivery. The main degradation product of PHAs, 3-hydroxybutyrate (3HB), is recognized as a new functional molecule for cancer therapy and immune regulation. Although PHAs still account for only a small percentage of medical polymers, up-and-coming novel medical PHA devices will enter the clinical translation stage in the next few years.

Computer Model-Driven Design in Cardiovascular Regenerative Medicine

Continuing advances in genomics, molecular and cellular mechanobiology and immunobiology, including transcriptomics and proteomics, and biomechanics increasingly reveal the complexity underlying native tissue and organ structure and function. Identifying methods to repair, regenerate, or replace vital tissues and organs remains one of the greatest challenges of modern biomedical engineering, one that deserves our very best effort. Notwithstanding the continuing need for improving standard methods of investigation, including cell, organoid, and tissue culture, biomaterials development and fabrication, animal models, and clinical research, it is increasingly evident that modern computational methods should play increasingly greater roles in advancing the basic science, bioengineering, and clinical application of regenerative medicine. This brief review focuses on the development and application of computational models of tissue and organ mechanobiology and mechanics for purposes of designing tissue engineered constructs and understanding their development in vitro and in situ. Although the basic approaches are general, for illustrative purposes we describe two recent examples from cardiovascular medicine-tissue engineered heart valves (TEHVs) and tissue engineered vascular grafts (TEVGs)-to highlight current methods of approach as well as continuing needs.

Mechano-regulated cell-cell signaling in the context of cardiovascular tissue engineering

Cardiovascular tissue engineering (CVTE) aims to create living tissues, with the ability to grow and remodel, as replacements for diseased blood vessels and heart valves. Despite promising results, the (long-term) functionality of these engineered tissues still needs improvement to reach broad clinical application. The functionality of native tissues is ensured by their specific mechanical properties directly arising from tissue organization. We therefore hypothesize that establishing a native-like tissue organization is vital to overcome the limitations of current CVTE approaches. To achieve this aim, a better understanding of the growth and remodeling (G&R) mechanisms of cardiovascular tissues is necessary. Cells are the main mediators of tissue G&R, and their behavior is strongly influenced by both mechanical stimuli and cell-cell signaling. An increasing number of signaling pathways has also been identified as mechanosensitive. As such, they may have a key underlying role in regulating the G&R of tissues in response to mechanical stimuli. A more detailed understanding of mechano-regulated cell-cell signaling may thus be crucial to advance CVTE, as it could inspire new methods to control tissue G&R and improve the organization and functionality of engineered tissues, thereby accelerating clinical translation. In this review, we discuss the organization and biomechanics of native cardiovascular tissues; recent CVTE studies emphasizing the obtained engineered tissue organization; and the interplay between mechanical stimuli, cell behavior, and cell-cell signaling. In addition, we review past contributions of computational models in understanding and predicting mechano-regulated tissue G&R and cell-cell signaling to highlight their potential role in future CVTE strategies.

Grand Challenges for Industrializing Polyhydroxyalkanoates (PHAs)

Polyhydroxyalkanoates (PHAs) are a diverse family of sustainable bioplastics synthesized by various bacteria, but their high production cost and unstable material properties make them challenging to use in commercial applications. Current industrial biotechnology (CIB) employs conventional microbial chassis, leading to high production costs. However, next-generation industrial biotechnology (NGIB) approaches, based on fast-growing and contamination-resistant extremophilic Halomonas spp., allow stable continuous processing and thus economical production of PHAs with stable properties. Halomonas spp. designed and constructed using synthetic biology not only produce low-cost intracellular PHAs but also secrete extracellular soluble products for improved process economics. Next-generation industrial biotechnology is expected to reduce the bioproduction cost and process complexity, leading to successful commercial production of PHAs.

Porcine Umbilical Cord Perivascular Cells for Preclinical Testing of Tissue-Engineered Heart Valves

Many children born with congenital heart disease need a heart valve repair or replacement. Currently available repair materials and valve replacements are incapable of growth, repair, and adaptation, rendering them inadequate for growing children. Heart valve tissue engineering (HVTE) aims to develop living replacement valves that can meet these needs. Among numerous cell sources for in vitro HVTE, umbilical cord perivascular cells (UCPVCs) are particularly attractive because they are autologous, readily available, and have excellent regenerative capacity. As an essential step toward preclinical testing of heart valves engineered from UCPVCs, the goal of this study was to establish methods to isolate, expand, and promote extracellular matrix (ECM) synthesis by UCPVCs from pigs (porcine umbilical cord perivascular cells [pUCPVCs]), as a relevant preclinical model. We determined that Dulbecco's modified Eagle's medium with 20% fetal bovine serum supported isolation and substantial expansion of pUCPVCs, whereas media designed for human mesenchymal stromal cell (MSC) expansion did not. We further demonstrated the capacity of pUCPVCs to synthesize the main ECM components of heart valves (collagen type I, elastin, and glycosaminoglycans), with maximal collagen and elastin per-cell production occurring in serum-free culture conditions using StemMACS™ MSC Expansion Media. Altogether, these results establish protocols that enable the use of pUCPVCs as a viable cell source for preclinical testing of engineered heart valves. Impact statement This study establishes methods to successfully isolate, expand, and promote the synthesis of the main extracellular matrix components of heart valves (collagen type I, elastin, and glycosaminoglycans) by porcine umbilical cord perivascular cells (pUCPVCs). These protocols enable further evaluation of pUCPVCs as an autologous, readily available, and clinically relevant cell source for preclinical testing of pediatric tissue-engineered heart valves.

Biomaterials in Valvular Heart Diseases

Valvular heart disease (VHD) occurs as the result of valvular malfunction, which can greatly reduce patient's quality of life and if left untreated may lead to death. Different treatment regiments are available for management of this defect, which can be helpful in reducing the symptoms. The global commitment to reduce VHD-related mortality rates has enhanced the need for new therapeutic approaches. During the past decade, development of innovative pharmacological and surgical approaches have dramatically improved the quality of life for VHD patients, yet the search for low cost, more effective, and less invasive approaches is ongoing. The gold standard approach for VHD management is to replace or repair the injured valvular tissue with natural or synthetic biomaterials. Application of these biomaterials for cardiac valve regeneration and repair holds a great promise for treatment of this type of heart disease. The focus of the present review is the current use of different types of biomaterials in treatment of valvular heart diseases.

Polyhydroxyalkanoates: Next generation natural biomolecules and a solution for the world's future economy

Petrochemical plastics have become a cause of pollution for decades and finding alternative plastics that are environmental friendly. Polyhydroxyalkanoate (PHA), a biopolyester produced by microbial cells, has characteristics (biocompatible, biodegradable, non-toxic) that make it appropriate as a biodegradable plastic substance. The different forms of PHA make it suitable to a wide choice of products, from packaging materials to biomedical applications. The major challenge in commercialization of PHA is the cost of manufacturing. There are a lot of factors that could affect the efficiency of a development method. The development of new strategic parameters for better synthesis, including consumption of low cost carbon substrates, genetic modification of PHA-producing strains, and fermentational strategies are discussed. Recently, many efforts have been made to develop a method for the cost-effective production of PHAs. The isolation, analysis as well as characterization of PHAs are significant factors for any developmental process. Due to the biodegradable and biocompatible properties of PHAs, they are majorly used in biomedical applications such as vascular grafting, heart tissue engineering, skin tissue repairing, liver tissue engineering, nerve tissue engineering, bone tissue engineering, cartilage tissue engineering and therapeutic carrier. The emerging and interesting area of research is the development of self-healing biopolymer that could significantly broaden the operational life and protection of the polymeric materials for a broad range of uses. Biodegradable and biocompatible polymers are considered as the green materials in place of petroleum-based plastics in the future.

Next-generation tissue-engineered heart valves with repair, remodelling and regeneration capacity

Valvular heart disease is a major cause of morbidity and mortality worldwide. Surgical valve repair or replacement has been the standard of care for patients with valvular heart disease for many decades, but transcatheter heart valve therapy has revolutionized the field in the past 15 years. However, despite the tremendous technical evolution of transcatheter heart valves, to date, the clinically available heart valve prostheses for surgical and transcatheter replacement have considerable limitations. The design of next-generation tissue-engineered heart valves (TEHVs) with repair, remodelling and regenerative capacity can address these limitations, and TEHVs could become a promising therapeutic alternative for patients with valvular disease. In this Review, we present a comprehensive overview of current clinically adopted heart valve replacement options, with a focus on transcatheter prostheses. We discuss the various concepts of heart valve tissue engineering underlying the design of next-generation TEHVs, focusing on off-the-shelf technologies. We also summarize the latest preclinical and clinical evidence for the use of these TEHVs and describe the current scientific, regulatory and clinical challenges associated with the safe and broad clinical translation of this technology.

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Poly‐3‐hydroxybutyrate‐based constructs with novel characteristics for drug delivery and tissue engineering applications—A review

Herein, we reviewed polymeric constructs of polyhydroxyalkanoates (PHAs) at large and poly‐3‐hydroxybutyrate (P3HB), in particular, for drug delivery and tissue engineering applications. Polymeric constructs that can efficiently respond to numerous variations in their surroundings have gained notable attention from different industrial sectors such as biomedical, clinical, pharmaceutical, and cosmeceutical. Among them, considerable importance is given to their drug delivery and tissue engineering applications. PHAs with peculiar reference to P3HB are gaining prominence attention as candidate materials with such requisite potentialities. The unique structural and functional characteristics of PHAs and P3HB are of supreme interest and being used to engineer novel constructs for efficient drug delivery and tissue regeneration purposes. So far, an array of methodological approaches, such as in vitro, in vivo, and ex vivo techniques have been exploited though using different materials with different geometries for a said purpose. However, a low‐level production majorly limits their proper exploitation. Various physiochemical characteristics and production strategies have been introduced in this review. The data have been summarized on PHAs production by several microorganisms aiming to cover the scope of the last 10 years. The present review highlights the recent applications of PHAs and P3HB‐based constructs, such as micro/nanoparticles, biocomposite, nanofibers, and hydrogels as novel drug carries for regenerative medicine and tissue engineering. In summary, drug delivery and tissue engineering potentialities of PHAs and P3HB‐based constructs are discussed with suitable examples and envisioned directions of future developments.

Current challenges and future trends in manufacturing small diameter artificial vascular grafts in bioreactors

Cardiovascular diseases are a leading cause of death. Vascular surgery is mainly used to solve this problem. However, the generation of a functional and suitable substitute for small diameter (< 6 mm) displacement is challengeable. Moreover, synthetic prostheses, made of polyethylene terephthalate and extended polytetrafluoroethylene show have shown insufficient performance. Therefore, the challenges dominating the use of autografts have prevented their efficient use. Tissue engineering is highlighted in regenerative medicine perhaps in aiming to address the issue of end-stage organ failure. While organs and complex tissues require the vascular supply to support the graft survival and render the bioartificial organ role, vascular tissue engineering has shown to be a hopeful method for cell implantation by the production of tissues in vitro. Bioreactors are a salient point in vascular tissue engineering due to the capability for reproducible and controlled variations showing a new horizon in blood vessel substitution. This review strives to display the overview of current concepts in the development of small-diameter by using bioreactors. In this work, we show a critical look at different factors for developing small-diameter and give suggestions for future studies.

The Potential Impact and Timeline of Engineering on Congenital Interventions

Congenital interventional cardiology has seen rapid growth in recent decades due to the expansion of available medical devices. Percutaneous interventions have become standard of care for many common congenital conditions. Unfortunately, patients with congenital heart disease often require multiple interventions throughout their lifespan. The availability of transcatheter devices that are biodegradable, biocompatible, durable, scalable, and can be delivered in the smallest sized patients will rely on continued advances in engineering. The development pipeline for these devices will require contributions of many individuals in academia and industry including experts in material science and tissue engineering. Advances in tissue engineering, bioresorbable technology, and even new nanotechnologies and nitinol fabrication techniques which may have an impact on the field of transcatheter congenital device in the next decade are summarized in this review. This review highlights recent advances in the engineering of transcatheter-based therapies and discusses future opportunities for engineering of transcatheter devices.

Polyhydroxyalkanoate (PHA): applications in drug delivery and tissue engineering

INTRODUCTION

The applications of naturally obtained polymers are tremendously increased due to them being biocompatible, biodegradable, environmentally friendly and renewable in nature. Among them, polyhydroxyalkanoates are widely studied and they can be utilized in many areas of human life research such as drug delivery, tissue engineering, and other medical applications.

AREAS COVERED

This review provides an overview of the polyhydroxyalkanoates biosynthesis and their possible applications in drug delivery in the range of micro- and nano-size. Moreover, the possible applications in tissue engineering are covered considering macro- and microporous scaffolds and extracellular matrix analogs.

EXPERT COMMENTARY

The majority of synthetic plastics are non-biodegradable so, in the last years, a renewed interest is growing to develop alternative processes to produce biologically derived polymers. Among them, PHAs present good properties such as high immunotolerance, low toxicity, biodegradability, so, they are promisingly using as biomaterials in biomedical applications.

P34HB electrospun fibres promote bone regeneration in vivo

OBJECTIVE

Bone tissue engineering was introduced in 1995 and provides a new way to reconstruct bone and repair bone defects. However, the design and fabrication of suitable bionic bone scaffolds are still challenging, and the ideal scaffolds in bone tissue engineering should have a three-dimensional porous network, good biocompatibility, excellent biodegradability and so on. The purpose of our research was to investigate whether a bioplasticpoly3-hydroxybutyrate4-hydroxybutyrate (P34HB) electrospun fibre scaffold is conducive to the repair of bone defects, and whether it is a potential scaffold for bone tissue engineering.

MATERIALS AND METHODS

The P34HB electrospun fibre scaffolds were prepared by electrospinning technology, and the surface morphology, hydrophilicity, mechanical properties and cytological behaviour of the scaffolds were tested. Furthermore, a calvarial defect model was created in rats, and through layer-by-layer paper-stacking technology, the P34HB electrospun fibre scaffolds were implanted into the calvarial defect area and their effect on bone repair was evaluated.

RESULTS

The results showed that the P34HB electrospun fibre scaffolds are interwoven with several fibres and have good porosity, physical properties and chemical properties and can promote cell adhesion and proliferation with no cytotoxicity in vitro. In addition, the P34HB electrospun fibre scaffolds can promote the repair of calvarial defects in vivo.

CONCLUSIONS

These results demonstrated that the P34HB electrospun fibre scaffold has a three-dimensional porous network with good biocompatibility, excellent biosafety and ability for bone regeneration and repair; thus, the P34HB electrospun fibre scaffold is a potential scaffold for bone tissue engineering.

Elastic materials for tissue engineering applications: Natural, synthetic, and hybrid polymers

Elastin and collagen are the two main components of elastic tissues and provide the tissue with elasticity and mechanical strength, respectively. Whereas collagen is adequately produced in vitro, production of elastin in tissue-engineered constructs is often inadequate when engineering elastic tissues. Therefore, elasticity has to be artificially introduced into tissue-engineered scaffolds. The elasticity of scaffold materials can be attributed to either natural sources, when native elastin or recombinant techniques are used to provide natural polymers, or synthetic sources, when polymers are synthesized. While synthetic elastomers often lack the biocompatibility needed for tissue engineering applications, the production of natural materials in adequate amounts or with proper mechanical strength remains a challenge. However, combining natural and synthetic materials to create hybrid components could overcome these issues. This review explains the synthesis, mechanical properties, and structure of native elastin as well as the theories on how this extracellular matrix component provides elasticity in vivo. Furthermore, current methods, ranging from proteins and synthetic polymers to hybrid structures that are being investigated for providing elasticity to tissue engineering constructs, are comprehensively discussed.

STATEMENT OF SIGNIFICANCE

Tissue engineered scaffolds are being developed as treatment options for malfunctioning tissues throughout the body. It is essential that the scaffold is a close mimic of the native tissue with regards to both mechanical and biological functionalities. Therefore, the production of elastic scaffolds is of key importance to fabricate tissue engineered scaffolds of the elastic tissues such as heart valves and blood vessels. Combining naturally derived and synthetic materials to reach this goal proves to be an interesting area where a highly tunable material that unites mechanical and biological functionalities can be obtained.

Animal Models for Heart Valve Research and Development

Valvular heart disease is the third-most common cause of heart problems in the United States. Malfunction of the valves can be acquired or congenital and each may lead either to stenosis or regurgitation, or even both in some cases. Heart valve disease is a progressive disease, which is irreversible and may be fatal if left untreated. Pharmacological agents cannot currently prevent valvular calcification or help repair damaged valves, as valve tissue is unable to regenerate spontaneously. Thus, heart valve replacement/repair is the only current available treatment. Heart valve research and development is currently focused on two parallel paths; first, research that aims to understand the underlying mechanisms for heart valve disease to emerge with an ultimate goal to devise medical treatment; and second, efforts to develop repair and replacement options for a diseased valve. Studies that focus on developmental malformation, genetic and disease epigenetics usually employ small animal models that are easy to access for in vivo imaging that minimally disturbs their environment during early stages of development. Alternatively, studies that aim to develop novel device for replacement and repair of diseased valves often employ large animals whose heart size and anatomy closely replicate human's. This paper aims to briefly review the current state-of-the-art animal models, and justification to use an animal model for a particular heart valve related project.

Pre-clinical evaluation of advanced nerve guide conduits using a novel 3D in vitro testing model

Autografts are the current gold standard for large peripheral nerve defects in clinics despite the frequently occurring side effects like donor site morbidity. Hollow nerve guidance conduits (NGC) are proposed alternatives to autografts, but failed to bridge gaps exceeding 3 cm in humans. Internal NGC guidance cues like microfibres are believed to enhance hollow NGCs by giving additional physical support for directed regeneration of Schwann cells and axons. In this study, we report a new 3D in vitro model that allows the evaluation of different intraluminal fibre scaffolds inside a complete NGC. The performance of electrospun polycaprolactone (PCL) microfibres inside 5 mm long polyethylene glycol (PEG) conduits were investigated in neuronal cell and dorsal root ganglion (DRG) cultures in vitro. Z-stack confocal microscopy revealed the aligned orientation of neuronal cells along the fibres throughout the whole NGC length and depth. The number of living cells in the centre of the scaffold was not significantly different to the tissue culture plastic (TCP) control. For ex vivo analysis, DRGs were placed on top of fibre-filled NGCs to simulate the proximal nerve stump. In 21 days of culture, Schwann cells and axons infiltrated the conduits along the microfibres with 2.2 ± 0.37 mm and 2.1 ± 0.33 mm, respectively. We conclude that this in vitro model can help define internal NGC scaffolds in the future by comparing different fibre materials, composites and dimensions in one setup prior to animal testing.

The future of heart valve replacement: recent developments and translational challenges for heart valve tissue engineering

Heart valve replacement is often the only solution for patients suffering from valvular heart disease. However, currently available valve replacements require either life-long anticoagulation or are associated with valve degeneration and calcification. Moreover, they are suboptimal for young patients, because they do not adapt to the somatic growth. Tissue-engineering has been proposed as a promising approach to fulfil the urgent need for heart valve replacements with regenerative and growth capacity. This review will start with an overview on the currently available valve substitutes and the techniques for heart valve replacement. The main focus will be on the evolution of and different approaches for heart valve tissue engineering, namely the in vitro, in vivo and in situ approaches. More specifically, several heart valve tissue-engineering studies will be discussed with regard to their shortcomings or successes and their possible suitability for novel minimally invasive implantation techniques. As in situ heart valve tissue engineering based on cell-free functionalized starter materials is considered to be a promising approach for clinical translation, this review will also analyse the techniques used to tune the inflammatory response and cell recruitment upon implantation in order to stir a favourable outcome: controlling the blood-material interface, regulating the cytokine release, and influencing cell adhesion and differentiation. In the last section, the authors provide their opinion about the future developments and the challenges towards clinical translation and adaptation of heart valve tissue engineering for valve replacement. Copyright © 2016 John Wiley & Sons, Ltd.

Three-dimensional imaging technologies: a priority for the advancement of tissue engineering and a challenge for the imaging community

Tissue engineering/regenerative medicine (TERM) is an interdisciplinary field that applies the principle of engineering and life sciences to restore/replace damaged tissues/organs with in vitro artificially-created ones. Research on TERM quickly moves forward. Today newest technologies and discoveries, such as 3D-/bio-printing, allow in vitro fabrication of ex-novo made tissues/organs, opening the door to wide and probably never-ending application possibilities, from organ transplant to drug discovery, high content screening and replacement of laboratory animals. Imaging techniques are fundamental tools for the characterization of tissue engineering (TE) products at any stage, from biomaterial/scaffold to construct/organ analysis. Indeed, tissue engineers need versatile imaging methods capable of monitoring not only morphological but also functional and molecular features, allowing three-dimensional (3D) and time-lapse in vivo analysis, in a non-destructive, quantitative, multidimensional analysis of TE constructs, to analyze their pre-implantation quality assessment and their fate after implantation. This review focuses on the newest developments in imaging technologies and applications in the context of requirements of the different steps of the TERM field, describing strengths and weaknesses of the current imaging approaches.

Heart Valve Replacements with Regenerative Capacity

The incidence of severe valvular dysfunctions (e.g., stenosis and insufficiency) is increasing, leading to over 300,000 valves implanted worldwide yearly. Clinically used heart valve replacements lack the capacity to grow, inherently requiring repetitive and high-risk surgical interventions during childhood. The aim of this review is to present how different tissue engineering strategies can overcome these limitations, providing innovative valve replacements that proved to be able to integrate and remodel in pre-clinical experiments and to have promising results in clinical studies. Upon description of the different types of heart valve tissue engineering (e.g., in vitro, in situ, in vivo, and the pre-seeding approach) we focus on the clinical translation of this technology. In particular, we will deepen the many technical, clinical, and regulatory aspects that need to be solved to endure the clinical adaptation and the commercialization of these promising regenerative valves.

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.

Bioengineered living cardiac and venous valve replacements: current status and future prospects

Valvular heart disease remains to be a major cause of death worldwide with increasing prevalence, mortality, and morbidity. Current heart valve replacements are associated with several limitations due to their nonviable nature. In this regard, heart valve tissue engineering has shown to represent a promising concept in order to overcome these limitations and replace diseased cardiac valves with living, autologous constructs. These bioengineered valves hold potential for in situ remodeling, growth, and repair throughout the patient's lifetime without the risk of thromboembolic complications and adverse immune responses. For the fabrication of tissue-engineered heart valves, several concepts have been established, the "classical" in vitro tissue engineering approach, the in situ tissue engineering approach, and alternative approaches including three-dimensional printing and electrospinning. Besides first attempts have been conducted in order to produce a tissue-engineered venous valve for the treatment of deep venous valve insufficiency. Here we review basic principals and current scientific status of valvular tissue engineering, including a critical discussion and outlook for the future.

Relative Effects of Fluid Oscillations and Nutrient Transport in the In Vitro Growth of Valvular Tissues

Engineered valvular tissues are cultured dynamically, and involve specimen movement. We previously demonstrated that oscillatory shear stresses (OSS) under combined steady flow and specimen cyclic flexure (flex-flow) promote tissue formation. However, localized efficiency of specimen mass transport is also important in the context of cell viability within the growing tissues. Here, we investigated the delivery of two essential species for cell survival, glucose and oxygen, to 3-dimensional (3D) engineered valvular tissues. We applied a convective-diffusive model to characterize glucose and oxygen mass transport with and without valve-like specimen flexural movement. We found the mass transport effects for glucose and oxygen to be negligible for scaffold porosities typically present during in vitro experiments and non-essential unless the porosity was unusually low (<40%). For more typical scaffold porosities (75%) however, we found negligible variation in the specimen mass fraction of glucose and oxygen in both non-moving and moving constructs (p > 0.05). Based on this result, we conducted an experiment using bone marrow stem cell (BMSC)-seeded scaffolds under Pulsatile flow-alone states to permit OSS without any specimen movement. BMSC-seeded specimen collagen from the pulsatile flow and flex-flow environments were subsequently found to be comparable (p > 0.05) and exhibited some gene expression similarities. We conclude that a critical magnitude of fluid-induced, OSS created by either pulsatile flow or flex-flow conditions, particularly when the oscillations are physiologically-relevant, is the direct, principal stimulus that promotes engineered valvular tissues and its phenotype, whereas mass transport benefits derived from specimen movement are minimal.

Biomechanical conditioning of tissue engineered heart valves: Too much of a good thing?

Surgical replacement of dysfunctional valves is the primary option for the treatment of valvular disease and congenital defects. Existing mechanical and bioprosthetic replacement valves are far from ideal, requiring concomitant anticoagulation therapy or having limited durability, thus necessitating further surgical intervention. Heart valve tissue engineering (HVTE) is a promising alternative to existing replacement options, with the potential to synthesize mechanically robust tissue capable of growth, repair, and remodeling. The clinical realization of a bioengineered valve relies on the appropriate combination of cells, biomaterials, and/or bioreactor conditioning. Biomechanical conditioning of valves in vitro promotes differentiation of progenitor cells to tissue-synthesizing myofibroblasts and prepares the construct to withstand the complex hemodynamic environment of the native valve. While this is a crucial step in most HVTE strategies, it also may contribute to fibrosis, the primary limitation of engineered valves, through sustained myofibrogenesis. In this review, we examine the progress of HVTE and the role of mechanical conditioning in the synthesis of mechanically robust tissue, and suggest approaches to achieve myofibroblast quiescence and prevent fibrosis.

Generation and Assessment of Functional Biomaterial Scaffolds for Applications in Cardiovascular Tissue Engineering and Regenerative Medicine

Current clinically applicable tissue and organ replacement therapies are limited in the field of cardiovascular regenerative medicine. The available options do not regenerate damaged tissues and organs, and, in the majority of the cases, show insufficient restoration of tissue function. To date, anticoagulant drug-free heart valve replacements or growing valves for pediatric patients, hemocompatible and thrombus-free vascular substitutes that are smaller than 6 mm, and stem cell-recruiting delivery systems that induce myocardial regeneration are still only visions of researchers and medical professionals worldwide and far from being the standard of clinical treatment. The design of functional off-the-shelf biomaterials as well as automatable and up-scalable biomaterial processing methods are the focus of current research endeavors and of great interest for fields of tissue engineering and regenerative medicine. Here, various approaches that aim to overcome the current limitations are reviewed, focusing on biomaterials design and generation methods for myocardium, heart valves, and blood vessels. Furthermore, novel contact- and marker-free biomaterial and extracellular matrix assessment methods are highlighted.

A novel bioreactor for mechanobiological studies of engineered heart valve tissue formation under pulmonary arterial physiological flow conditions

The ability to replicate physiological hemodynamic conditions during in vitro tissue development has been recognized as an important aspect in the development and in vitro assessment of engineered heart valve tissues. Moreover, we have demonstrated that studies aiming to understand mechanical conditioning require separation of the major heart valve deformation loading modes: flow, stretch, and flexure (FSF) (Sacks et al., 2009, "Bioengineering Challenges for Heart Valve Tissue Engineering," Annu. Rev. Biomed. Eng., 11(1), pp. 289-313). To achieve these goals in a novel bioreactor design, we utilized a cylindrical conduit configuration for the conditioning chamber to allow for higher fluid velocities, translating to higher shear stresses on the in situ tissue specimens while retaining laminar flow conditions. Moving boundary computational fluid dynamic (CFD) simulations were performed to predict the flow field under combined cyclic flexure and steady flow (cyclic-flex-flow) states using various combinations of flow rate, and media viscosity. The device was successfully constructed and tested for incubator housing, gas exchange, and sterility. In addition, we performed a pilot experiment using biodegradable polymer scaffolds seeded with bone marrow derived stem cells (BMSCs) at a seeding density of 5 × 10(6) cells/cm(2). The constructs were subjected to combined cyclic flexure (1 Hz frequency) and steady flow (Re = 1376; flow rate of 1.06 l/min (LPM); shear stress in the range of 0-9 dynes/cm(2) for 2 weeks to permit physiological shear stress conditions. Assays revealed significantly (P < 0.05) higher amounts of collagen (2051 ± 256 μg/g) at the end of 2 weeks in comparison to similar experiments previously conducted in our laboratory but performed at subphysiological levels of shear stress (<2 dynes/cm(2); Engelmayr et al., 2006, "Cyclic Flexure and Laminar Flow Synergistically Accelerate Mesenchymal Stem Cell-Mediated Engineered Tissue Formation: Implications for Engineered Heart Valve Tissues," Biomaterials, 27(36), pp. 6083-6095). The implications of this novel design are that fully coupled or decoupled physiological flow, flexure, and stretch modes of engineered tissue conditioning investigations can be readily accomplished with the inclusion of this device in experimental protocols on engineered heart valve tissue formation.

Current progress in tissue engineering of heart valves: multiscale problems, multiscale solutions

INTRODUCTION

Heart valve disease is an increasingly prevalent and clinically serious condition. There are no clinically effective biological diagnostics or treatment strategies. The only recourse available is replacement with a prosthetic valve, but the inability of these devices to grow or respond biologically to their environments necessitates multiple resizing surgeries and life-long coagulation treatment, especially in children. Tissue engineering has a unique opportunity to impact heart valve disease by providing a living valve conduit, capable of growth and biological integration.

AREAS COVERED

This review will cover current tissue engineering strategies in fabricating heart valves and their progress towards the clinic, including molded scaffolds using naturally derived or synthetic polymers, decellularization, electrospinning, 3D bioprinting, hybrid techniques, and in vivo engineering.

EXPERT OPINION

Whereas much progress has been made to create functional living heart valves, a clinically viable product is not yet realized. The next leap in engineered living heart valves will require a deeper understanding of how the natural multi-scale structural and biological heterogeneity of the tissue ensures its efficient function. Related, improved fabrication strategies must be developed that can replicate this de novo complexity, which is likely instructive for appropriate cell differentiation and remodeling whether seeded with autologous stem cells in vitro or endogenously recruited cells.

P34HB film promotes cell adhesion, in vitro proliferation, and in vivo cartilage repair

The management of chondral defects is a challenging topic of current interest for scientists and surgeons, which has a crucial impact on human cost.

Crosslinking strategies for preparation of extracellular matrix-derived cardiovascular scaffolds

Heart valve and blood vessel replacement using artificial prostheses is an effective strategy for the treatment of cardiovascular disease at terminal stage. Natural extracellular matrix (ECM)-derived materials (decellularized allogeneic or xenogenic tissues) have received extensive attention as the cardiovascular scaffold. However, the bioprosthetic grafts usually far less durable and undergo calcification and progressive structural deterioration. Glutaraldehyde (GA) is a commonly used crosslinking agent for improving biocompatibility and durability of the natural scaffold materials. However, the nature ECM and GA-crosslinked materials may result in calcification and eventually lead to the transplant failure. Therefore, studies have been conducted to explore new crosslinking agents. In this review, we mainly focused on research progress of ECM-derived cardiovascular scaffolds and their crosslinking strategies.

Anisotropic poly(ethylene glycol)/polycaprolactone hydrogel-fiber composites for heart valve tissue engineering

The recapitulation of the material properties and structure of the native aortic valve leaflet, specifically its anisotropy and laminate structure, is a major design goal for scaffolds for heart valve tissue engineering. Poly(ethylene glycol) (PEG) hydrogels are attractive scaffolds for this purpose as they are biocompatible, can be modified for their mechanical and biofunctional properties, and can be laminated. This study investigated augmenting PEG hydrogels with polycaprolactone (PCL) as an analog to the fibrosa to improve strength and introduce anisotropic mechanical behavior. However, due to its hydrophobicity, PCL must be modified prior to embedding within PEG hydrogels. In this study, PCL was electrospun (ePCL) and modified in three different ways, by protein adsorption (pPCL), alkali digestion (hPCL), and acrylation (aPCL). Modified PCL of all types maintained the anisotropic elastic moduli and yield strain of unmodified anisotropic ePCL. Composites of PEG and PCL (PPCs) maintained anisotropic elastic moduli, but aPCL and pPCL had isotropic yield strains. Overall, PPCs of all modifications had elastic moduli of 3.79±0.90 MPa and 0.46±0.21 MPa in the parallel and perpendicular directions, respectively. Valvular interstitial cells seeded atop anisotropic aPCL displayed an actin distribution aligned in the direction of the underlying fibers. The resulting scaffold combines the biocompatibility and tunable fabrication of PEG with the strength and anisotropy of ePCL to form a foundation for future engineered valve scaffolds.

In vivo application of poly-3-hydroxyoctanoate as peripheral nerve graft

OBJECTIVE

This study aims to investigate the degree of biocompatibility and neuroregeneration of a polymer tube, poly-3-hydroxyoctanoate (PHO) in nerve gap repair.

METHODS

Forty Wistar Albino male rats were randomized into two groups: autologous nerve gap repair group and PHO tube repair group. In each group, a 10-mm right sciatic nerve defect was created and reconstructed accordingly. Neuroregeneration was studied by sciatic function index (SFI), electromyography, and immunohistochemical studies on Days 7, 21, 45 and 60 of implantation. Biocompatibility was analyzed by the capsule formation around the conduit. Biodegradation was analyzed by the molecular weight loss in vivo.

RESULTS

Electrophysiological and histomorphometric assessments demonstrated neuroregeneration in both groups over time. In the experimental group, a straight alignment of the Schwann cells parallel to the axons was detected. However, autologous nerve graft seems to have a superior neuroregeneration compared to PHO grafts. Minor biodegradation was observed in PHO conduit at the end of 60 d.

CONCLUSIONS

Although neuroregeneration is detected in PHO grafts with minor degradation in 60 d, autologous nerve graft is found to be superior in axonal regeneration compared to PHO nerve tube grafts. PHO conduits were found to create minor inflammatory reaction in vivo, resulting in good soft tissue response.

Progress in developing a living human tissue-engineered tri-leaflet heart valve assembled from tissue produced by the self-assembly approach

The aortic heart valve is constantly subjected to pulsatile flow and pressure gradients which, associated with cardiovascular risk factors and abnormal hemodynamics (i.e. altered wall shear stress), can cause stenosis and calcification of the leaflets and result in valve malfunction and impaired circulation. Available options for valve replacement include homograft, allogenic or xenogenic graft as well as the implantation of a mechanical valve. A tissue-engineered heart valve containing living autologous cells would represent an alternative option, particularly for pediatric patients, but still needs to be developed. The present study was designed to demonstrate the feasibility of using a living tissue sheet produced by the self-assembly method, to replace the bovine pericardium currently used for the reconstruction of a stented human heart valve. In this study, human fibroblasts were cultured in the presence of sodium ascorbate to produce tissue sheets. These sheets were superimposed to create a thick construct. Tissue pieces were cut from these constructs and assembled together on a stent, based on techniques used for commercially available replacement valves. Histology and transmission electron microscopy analysis showed that the fibroblasts were embedded in a dense extracellular matrix produced in vitro. The mechanical properties measured were consistent with the fact that the engineered tissue was resistant and could be cut, sutured and assembled on a wire frame typically used in bioprosthetic valve assembly. After a culture period in vitro, the construct was cohesive and did not disrupt or disassemble. The tissue engineered heart valve was stimulated in a pulsatile flow bioreactor and was able to sustain multiple duty cycles. This prototype of a tissue-engineered heart valve containing cells embedded in their own extracellular matrix and sewn on a wire frame has the potential to be strong enough to support physiological stress. The next step will be to test this valve extensively in a bioreactor and at a later date, in a large animal model in order to assess in vivo patency of the graft.

Transplantation of pulmonary valve using a mouse model of heterotopic heart transplantation

Tissue engineered heart valves, especially decellularized valves, are starting to gain momentum in clinical use of reconstructive surgery with mixed results. However, the cellular and molecular mechanisms of the neotissue development, valve thickening, and stenosis development are not researched extensively. To answer the above questions, we developed a murine heterotopic heart valve transplantation model. A heart valve was harvested from a valve donor mouse and transplanted to a heart donor mouse. The heart with a new valve was transplanted heterotopically to a recipient mouse. The transplanted heart showed its own heartbeat, independent of the recipient's heartbeat. The blood flow was quantified using a high frequency ultrasound system with a pulsed wave Doppler. The flow through the implanted pulmonary valve showed forward flow with minimal regurgitation and the peak flow was close to 100 mm/sec. This murine model of heart valve transplantation is highly versatile, so it can be modified and adapted to provide different hemodynamic environments and/or can be used with various transgenic mice to study neotissue development in a tissue engineered heart valve.

Scaffolds for tissue engineering of cardiac valves

Tissue engineered heart valves offer a promising alternative for the replacement of diseased heart valves avoiding the limitations faced with currently available bioprosthetic and mechanical heart valves. In the paradigm of tissue engineering, a three-dimensional platform - the so-called scaffold - is essential for cell proliferation, growth and differentiation, as well as the ultimate generation of a functional tissue. A foundation for success in heart valve tissue engineering is a recapitulation of the complex design and diverse mechanical properties of a native valve. This article reviews technological details of the scaffolds that have been applied to date in heart valve tissue engineering research.

Polyhydroxyalkanoates - what are the uses? Current challenges and perspectives

Over the past few decades, a considerable attention has been focused on the microbial polyhydroxyalkanoates (PHAs) owing to its multifaceted properties, i.e. biodegradability, biocompatibility, non-toxicity and thermo-plasticity. This article presents a critical review of the foregoing research, current trends and future perspectives on the value added applications of PHAs in the biomedical, environmental and industrial domains of life.

A new construction technique for tissue-engineered heart valves using the self-assembly method

Tissue engineering appears as a promising option to create new heart valve substitutes able to overcome the serious drawbacks encountered with mechanical substitutes or tissue valves. The objective of this article is to present the construction method of a new entirely biological stentless aortic valve using the self-assembly method and also a first assessment of its behavior in a bioreactor when exposed to a pulsatile flow. A thick tissue was created by stacking several fibroblast sheets produced with the self-assembly technique. Different sets of custom-made templates were designed to confer to the thick tissue a three-dimensional (3D) shape similar to that of a native aortic valve. The construction of the valve was divided in two sequential steps. The first step was the installation of the thick tissue in a flat preshaping template followed by a 4-week maturation period. The second step was the actual cylindrical 3D forming of the valve. The microscopic tissue structure was assessed using histological cross sections stained with Masson's Trichrome and Picrosirius Red. The thick tissue remained uniformly populated with cells throughout the construction steps and the dense extracellular matrix presented corrugated fibers of collagen. This first prototype of tissue-engineered heart valve was installed in a bioreactor to assess its capacity to sustain a light pulsatile flow at a frequency of 0.5 Hz. Under the light pulsed flow, it was observed that the leaflets opened and closed according to the flow variations. This study demonstrates that the self-assembly method is a viable option for the construction of complex 3D shapes, such as heart valves, with an entirely biological material.

Regenerative implants for cardiovascular tissue engineering

A fundamental problem that affects the field of cardiovascular surgery is the paucity of autologous tissue available for surgical reconstructive procedures. Although the best results are obtained when an individual's own tissues are used for surgical repair, this is often not possible as a result of pathology of autologous tissues or lack of a compatible replacement source from the body. The use of prosthetics is a popular solution to overcome shortage of autologous tissue, but implantation of these devices comes with an array of additional problems and complications related to biocompatibility. Transplantation offers another option that is widely used but complicated by problems related to rejection and donor organ scarcity. The field of tissue engineering represents a promising new option for replacement surgical procedures. Throughout the years, intensive interdisciplinary, translational research into cardiovascular regenerative implants has been undertaken in an effort to improve surgical outcome and better quality of life for patients with cardiovascular defects. Vascular, valvular, and heart tissue repair are the focus of these efforts. Implants for these neotissues can be divided into 2 groups: biologic and synthetic. These materials are used to facilitate the delivery of cells or drugs to diseased, damaged, or absent tissue. Furthermore, they can function as a tissue-forming device used to enhance the body's own repair mechanisms. Various preclinical studies and clinical trials using these advances have shown that tissue-engineered materials are a viable option for surgical repair, but require refinement if they are going to reach their clinical potential. With the growth and accomplishments this field has already achieved, meeting those goals in the future should be attainable.

Immune response to allograft implantation in children with congenital heart defects

Cryopreserved valved allografts are frequently used in the repair of congenital heart defects in children. Although the longevity of these grafts is generally good in most patients, there continue to be ongoing problems with allograft dysfunction and subsequent failure, particularly in infants and young children. The aim of this review is to discuss the immunogenicity of cryopreserved allograft tissue and measures that may minimize the deleterious effect of the immune system on allograft function and durability.