Polymeric materials for tissue engineering of arterial substitutes

Development of tissue-engineered vascular grafts from decellularized parsley stems

Cardiovascular diseases are mostly associated with narrowing or blockage of blood vessels, and it is the most common cause of death worldwide. The use of vascular grafts is a promising approach to bypass or replace the blocked vessels for long-term treatment. Although autologous arteries or veins are the most preferred tissue sources for vascular bypass, the limited presence and poor quality of autologous vessels necessitate seeking alternative biomaterials. Recently, synthetic grafts have gained attention as an alternative to autologous grafts. However, the high failure rate of synthetic grafts has been reported primarily due to thrombosis, atherosclerosis, intimal hyperplasia, or infection. Thrombosis, the main reason for failure upon implantation, is associated with damage or absence of endothelial cell lining in the vascular graft's luminal surface. To overcome this, tissue-engineered vascular grafts (TEVGs) have come into prominence. Alongside the well-established scaffold manufacturing techniques, decellularized plant-based constructs have recently gained significant importance and are an emerging field in tissue engineering and regenerative medicine. Accordingly, in this study, we demonstrated the fabrication of tubular scaffolds from decellularized parsley stems and recellularized them with human endothelial cells to be used as a potential TEVG. Our results suggested that the native plant DNA was successfully removed, and soft tubular biomaterials were successfully manufactured via the chemical decellularization of the parsley stems. The decellularized parsley stems showed suitable mechanical and biological properties to be used as a TEVG material, and they provided a suitable environment for the culture of human endothelial cells to attach and create a pseudo endothelium prior to implantation. This study is the first one to demonstrate the potential of the parsley stems to be used as a potential TEVG biomaterial.

Three-dimensional printing of cell-laden bioink for blood vessel tissue engineering: influence of process parameters and components on cell viability

Three-dimensional (3D) bioprinting is a potential therapeutic method for tissue engineering owing to its ability to prepare cell-laden tissue constructs. The properties of bioink are crucial to accurately control the printing structure. Meanwhile, the effect of process parameters on the precise structure is not nonsignificant. We investigated the correlation between process parameters of 3D bioprinting and the structural response of κ-carrageenan-based hydrogels to explore the controllable structure, printing resolution, and cell survival rate. Small-diameter (<6 mm) gel filaments with different structures were printed by varying the shear stress of the extrusion bioprinter to simulate the natural blood vessel structure. The cell viability of the scaffold was evaluated. The in vitro culture of human umbilical vein endothelium cells (HUVECs) on the κ-carrageenan (kc) and composite gels (carrageenan/carbon nanotube and carrageenan/sodium alginate) demonstrated that the cell attachment and proliferation on composite gels were better than those on pure kc. Our results revealed that the carrageenan-based composite bioinks offer better printability, sufficient mechanical stiffness, interconnectivity, and biocompatibility. This process can facilitate precise adjustment of the pore size, porosity, and pore distribution of the hydrogel structure by optimising the printing parameters as well as realise the precise preparation of the internal structure of the 3D hydrogel-based tissue engineering scaffold. Moreover, we obtained perfused tubular filament by 3D printing at optimal process parameters.

Resorbable Biomaterials Used for 3D Scaffolds in Tissue Engineering: A Review

This article provides a thorough overview of the available resorbable biomaterials appropriate for producing replacements for damaged tissues. In addition, their various properties and application possibilities are discussed as well. Biomaterials are fundamental components in tissue engineering (TE) of scaffolds and play a critical role. They need to exhibit biocompatibility, bioactivity, biodegradability, and non-toxicity, to ensure their ability to function effectively with an appropriate host response. With ongoing research and advancements in biomaterials for medical implants, the objective of this review is to explore recently developed implantable scaffold materials for various tissues. The categorization of biomaterials in this paper includes fossil-based materials (e.g., PCL, PVA, PU, PEG, and PPF), natural or bio-based materials (e.g., HA, PLA, PHB, PHBV, chitosan, fibrin, collagen, starch, and hydrogels), and hybrid biomaterials (e.g., PCL/PLA, PCL/PEG, PLA/PEG, PLA/PHB PCL/collagen, PCL/chitosan, PCL/starch, and PLA/bioceramics). The application of these biomaterials in both hard and soft TE is considered, with a particular focus on their physicochemical, mechanical, and biological properties. Furthermore, the interactions between scaffolds and the host immune system in the context of scaffold-driven tissue regeneration are discussed. Additionally, the article briefly mentions the concept of in situ TE, which leverages the self-renewal capacities of affected tissues and highlights the crucial role played by biopolymer-based scaffolds in this strategy.

Crimped fiber composites: mechanics of a finite-length crimped fiber embedded in a soft matrix

Composites comprising crimped fibers of finite length embedded in a soft matrix have the potential to mimic the strain-hardening behavior of tissues containing fibrous collagen. Unlike continuous fiber composites, such chopped fiber composites would be flow-processable. Here, we study the fundamental mechanics of stress transfer between a single crimped fiber and the embedding matrix subjected to tensile strain. Finite element simulations show that fibers with large crimp amplitude and large relative modulus straighten significantly at small strain without bearing significant load. At large strain, they become taut and hence bear increasing load. Analogous to straight fiber composites, there is a region near the ends of each fiber which bears much lower stress than the midsection. We show that the stress-transfer mechanics can be captured by a shear lag model where the crimped fiber can be replaced with an equivalent straight fiber whose effective modulus is lower than that of the crimped fiber, but increases with applied strain. This allows estimating the modulus of a composite at low fiber fraction. The degree of strain hardening and the strain needed for strain hardening can be tuned by changing relative modulus of the fibers and the crimp geometry.

Non-destructive 3D characterization of the blood vessel wall microstructure in different species and blood vessel types using contrast-enhanced microCT and comparison with synthetic vascular grafts

To improve the current treatment for vascular diseases, such as vascular grafts, intravascular stents, and balloon angioplasty intervention, the evaluation of the native blood vessel microstructure in full 3D could be beneficial. For this purpose, we used contrast-enhanced X-ray microfocus computed tomography (CECT): a combination of X-ray microfocus computed tomography (microCT) and contrast-enhancing staining agents (CESAs) containing high atomic number elements. In this work, we performed a comparative study based on staining time and contrast-enhancement of 2 CESAs: Monolacunary and 1:2 Hafnium-substituted Wells-Dawson polyoxometalate (Mono-WD POM and Hf-WD POM, respectively) for imaging of the porcine aorta. After showing the advantages of Hf-WD POM in terms of contrast enhancement, we expanded our imaging to other species (rat, porcine, and human) and other types of blood vessels (porcine aorta, femoral artery, and vena cava), clearly indicating microstructural differences between different types of blood vessels and different species. We then showed the possibility to extract useful 3D quantitative information from the rat and porcine aortic wall, potentially to be used for computational modeling or for future design optimization of graft materials. Finally, a structural comparison with existing synthetic vascular grafts was made. This information will allow to better understand the in vivo functioning of native blood vessels and to improve the current disease treatments. STATEMENT OF SIGNIFICANCE: Synthetic vascular grafts, used as treatment for some cardiovascular diseases, still often fail clinically, potentially because of a mismatch in mechanical behaviour between the native blood vessel and the graft. To better understand the causes of this mismatch, we studied the full 3D microstructure of blood vessels. For this, we identified Hafnium-substituted Wells-Dawson polyoxometalate as contrast-enhancing staining agent to perform contrast-enhanced X-ray microfocus computed tomography. This technique allowed to show important differences in the microstructure of different types of blood vessels and in different species, as well as with that of synthetic grafts. This information can lead to a better understanding of the functioning of blood vessels and will allow to improve current disease treatments, such as vascular grafts.

Atypical presentation of forearm compartment syndrome in a case of vascular type Ehlers-Danlos syndrome

A 30-year-old Chinese man with vascular type Ehlers-Danlos Syndrome presents with spontaneous right forearm compartment syndrome due to pseudoaneurysms along the radial artery. Emergency fasciotomy and reconstruction of the radial artery with a saphenous vein graft were performed. Genetic test showed a heterozygous DNA change c. 1852 G > C in COL3A1 gene.

Implants with Sensing Capabilities

Because of the aging human population and increased numbers of surgical procedures being performed, there is a growing number of biomedical devices being implanted each year. Although the benefits of implants are significant, there are risks to having foreign materials in the body that may lead to complications that may remain undetectable until a time at which the damage done becomes irreversible. To address this challenge, advances in implantable sensors may enable early detection of even minor changes in the implants or the surrounding tissues and provide early cues for intervention. Therefore, integrating sensors with implants will enable real-time monitoring and lead to improvements in implant function. Sensor integration has been mostly applied to cardiovascular, neural, and orthopedic implants, and advances in combined implant-sensor devices have been significant, yet there are needs still to be addressed. Sensor-integrating implants are still in their infancy; however, some have already made it to the clinic. With an interdisciplinary approach, these sensor-integrating devices will become more efficient, providing clear paths to clinical translation in the future.

Controllable synthesis of biomimetic nano/submicro-fibrous tubes for potential small-diameter vascular grafts

Mimicking the morphological structure of native blood vessels is critical for the development of vascular grafts. Herein, small-diameter composite vascular grafts that integrate the nanofibrous bacterial cellulose (BC) and submicrofibrous cellulose acetate (CA) were fabricated via a combined electrospinning and step-by-step in situ biosynthesis. Scanning electron microscopy (SEM) observation shows the nano/submicro-fibrous morphology and well-interconnected porous structure of the BC/CA grafts. It is found that the BC/CA graft with a suitable BC content demonstrates lower potential of thrombus formation and enhanced endothelialization as compared to the BC and CA counterparts. Western blotting and RT-qPCR results suggest that the BC/CA-2 graft promotes endothelialization by improving expressions of genes vWF-1 and CD31 and protein CD31. The in vivo tests demonstrate much lower inflammatory response to the BC/CA graft. These results suggest that the BC/CA graft shows a great potential as an artificial graft for rapid formation of an endothelial cell monolayer.

Strategies for re-vascularization and promotion of angiogenesis in trauma and disease

The maintenance of a healthy vascular system is essential to ensure the proper function of all organs of the human body. While macrovessels have the main role of blood transportation from the heart to all tissues, microvessels, in particular capillaries, are responsible for maintaining tissues' functionality by providing oxygen, nutrients and waste exchanges. Occlusion of blood vessels due to atherosclerotic plaque accumulation remains the leading cause of mortality across the world. Autologous vein and artery grafts bypassing are the current gold standard surgical procedures to substitute primarily obstructed vascular structures. Ischemic scenarios that condition blood supply in downstream tissues may arise from blockage phenomena, as well as from other disease or events leading to trauma. The (i) great demand for new vascular substitutes, arising from both the limited availability of healthy autologous vessels, as well as the shortcomings associated with small-diameter synthetic vascular grafts, and (ii) the challenging induction of the formation of adequate and stable microvasculature are current driving forces for the growing interest in the development of bioinspired strategies to ensure the proper function of vasculature in all its dimensional scales. Here, a critical review of well-established technologies and recent biotechnological advances to substitute or regenerate the vascular system is provided.

Future Perspectives in Small-Diameter Vascular Graft Engineering

The increased demands of small-diameter vascular grafts (SDVGs) globally has forced the scientific society to explore alternative strategies utilizing the tissue engineering approaches. Cardiovascular disease (CVD) comprises one of the most lethal groups of non-communicable disorders worldwide. It has been estimated that in Europe, the healthcare cost for the administration of CVD is more than 169 billion €. Common manifestations involve the narrowing or occlusion of blood vessels. The replacement of damaged vessels with autologous grafts represents one of the applied therapeutic approaches in CVD. However, significant drawbacks are accompanying the above procedure; therefore, the exploration of alternative vessel sources must be performed. Engineered SDVGs can be produced through the utilization of non-degradable/degradable and naturally derived materials. Decellularized vessels represent also an alternative valuable source for the development of SDVGs. In this review, a great number of SDVG engineering approaches will be highlighted. Importantly, the state-of-the-art methodologies, which are currently employed, will be comprehensively presented. A discussion summarizing the key marks and the future perspectives of SDVG engineering will be included in this review. Taking into consideration the increased number of patients with CVD, SDVG engineering may assist significantly in cardiovascular reconstructive surgery and, therefore, the overall improvement of patients' life.

Dynamic Luminal Topography: A Potential Strategy to Prevent Vascular Graft Thrombosis

AIM

Biologic interfaces play important roles in tissue function. The vascular lumen-blood interface represents a surface where dynamic interactions between the endothelium and circulating blood cells are critical in preventing thrombosis. The arterial lumen possesses a uniform wrinkled surface determined by the underlying internal elastic lamina. The function of this structure is not known, but computational analyses of artificial surfaces with dynamic topography, oscillating between smooth and wrinkled configurations, support the ability of this surface structure to shed adherent material (Genzer and Groenewold, 2006; Bixler and Bhushan, 2012; Li et al., 2014). We hypothesized that incorporating a luminal surface capable of cyclical wrinkling/flattening during the cardiac cycle into vascular graft technology may represent a novel mechanism of resisting platelet adhesion and thrombosis.

METHODS AND RESULTS

Bilayer silicone grafts possessing luminal corrugations that cyclically wrinkle and flatten during pulsatile flow were fabricated based on material strain mismatch. When placed into a pulsatile flow circuit with activated platelets, these grafts exhibited significantly reduced platelet deposition compared to grafts with smooth luminal surfaces. Constrained wrinkled grafts with static topography during pulsatile flow were more susceptible to platelet accumulation than dynamic wrinkled grafts and behaved similar to the smooth grafts under pulsatile flow. Wrinkled grafts under continuous flow conditions also exhibited marked increases in platelet accumulation.

CONCLUSION

These findings provide evidence that grafts with dynamic luminal topography resist platelet accumulation and support the application of this structure in vascular graft technology to improve the performance of prosthetic grafts. They also suggest that this corrugated structure in arteries may represent an inherent, self-cleaning mechanism in the vasculature.

Fibronectin Adsorption on Electrospun Synthetic Vascular Grafts Attracts Endothelial Progenitor Cells and Promotes Endothelialization in Dynamic In Vitro Culture

Appropriate mechanical properties and fast endothelialization of synthetic grafts are key to ensure long-term functionality of implants. We used a newly developed biostable polyurethane elastomer (TPCU) to engineer electrospun vascular scaffolds with promising mechanical properties (E-modulus: 4.8 ± 0.6 MPa, burst pressure: 3326 ± 78 mmHg), which were biofunctionalized with fibronectin (FN) and decorin (DCN). Neither uncoated nor biofunctionalized TPCU scaffolds induced major adverse immune responses except for minor signs of polymorph nuclear cell activation. The in vivo endothelial progenitor cell homing potential of the biofunctionalized scaffolds was simulated in vitro by attracting endothelial colony-forming cells (ECFCs). Although DCN coating did attract ECFCs in combination with FN (FN + DCN), DCN-coated TPCU scaffolds showed a cell-repellent effect in the absence of FN. In a tissue-engineering approach, the electrospun and biofunctionalized tubular grafts were cultured with primary-isolated vascular endothelial cells in a custom-made bioreactor under dynamic conditions with the aim to engineer an advanced therapy medicinal product. Both FN and FN + DCN functionalization supported the formation of a confluent and functional endothelial layer.

Major Histocompatibility Complex-Matched Arteries Have Similar Patency to Autologous Arteries in a Mauritian Cynomolgus Macaque Major Histocompatibility Complex-Defined Transplant Model

Background

Arterial bypass and interposition grafts are used routinely across multiple surgical subspecialties. Current options include both autologous and synthetic materials; however, each graft presents specific limitations. Engineering artificial small‐diameter arteries with vascular cells derived from induced pluripotent stem cells could provide a useful therapeutic solution. Banking induced pluripotent stem cells from rare individuals who are homozygous for human leukocyte antigen alleles has been proposed as a strategy to facilitate economy of scale while reducing the potential for rejection of induced pluripotent stem cell–derived transplanted tissues. Currently, there is no standardized model to study transplantation of small‐diameter arteries in major histocompatibility complex–defined backgrounds.

Methods and Results

In this study, we developed a limb‐sparing nonhuman primate model to study arterial allotransplantation in the absence of immunosuppression. Our model was used to compare degrees of major histocompatibility complex matching between arterial grafts and recipient animals with long‐term maintenance of patency and function. Unexpectedly, we (1) found that major histocompatibility complex partial haplomatched allografts perform as well as autologous control grafts; (2) detected little long‐term immune response in even completely major histocompatibility complex mismatched allografts; and (3) observed that arterial grafts become almost completely replaced over time with recipient cells.

Conclusions

Given these findings, induced pluripotent stem cell–derived tissue‐engineered blood vessels may prove to be promising and customizable grafts for future use by cardiac, vascular, and plastic surgeons.

Fabrications of small diameter compliance bypass conduit using electrospinning of clinical grade polyurethane

Objective

Compliance and viscoelastic mismatches of small diameter vascular conduits and host arteries have been the cause of conduit’s failure.

Methods

To reduce these mismatches, the aim of this study was to develop and characterize a polyurethane conduit, which mimics the viscoelastic behaviors of human arteries. Electrospinning technique was used to fabricate tubular polyurethane conduits with similar properties of the human common carotid artery. This was achieved by manipulating the fiber diameter by altering the syringe flow rate of the solution. The mechanical and viscoelastic properties of the fabricated electrospun polyurethane conduits were, then, compared with commercially available vascular conduits, expanded polytetrafluoroethylene, polyethylene terephthalate (Dacron®) and the healthy human common carotid arteries. In addition, a comprehensive constitutive model was proposed to capture the visco-hyperelastic behavior of the synthetic electrospun polyurethanes, commercial conduits and human common carotid arteries.

Results

Results showed that increasing the fiber diameter of electrospun polyurethanes from 114 to 190 nm reduced Young’s modulus from 8 to 2 MPa. Also, thicker fiber diameter yielded in higher conduits’ viscosity. Furthermore, the results revealed that proposed visco-hyperelastic model is strongly able to fit the experimental data with great precision which proofs the reliability of the proposed model to address both nonlinear elasticity and viscoelasticity of the electrospun polyurethanes, commercial conduits and human common carotid arteries.

Conclusions

In conclusion, statistical analysis revealed that the elastic and viscous properties of 190 nm fiber diameter conduit are very similar to that of human common carotid artery in comparison to the commercial expanded polytetrafluoroethylene and Dacron® that are up to nine and seven times stiffer than natural vessels. Therefore, based on our findings, from the mechanical point of view, by considering the amount of Young’s modulus, compliance, distensibility and viscoelastic behavior, the fabricated electrospun polyurethane with fiber diameter of 189.6 ± 52.89 nm is an optimum conduit with promising potential for substituting natural human vessels.

Application of adult mesenchymal stem cells in bone and vascular tissue engineering

Tissue engineering is a very promising field of regenerative medicine. Life expectancy has been increasing, and tissue replacement is increasingly needed in patients suffering from various degenerative disorders of the organs. The use of adult mesenchymal stem cells (e.g. from adipose tissue or from bone marrow) in tissue engineering seems to be a promising approach for tissue replacements. Clinical applications can make direct use of the large secretome of these cells, which can have a positive influence on other cells around. Another advantage of adult mesenchymal stem cells is the possibility to differentiate them into various mature cells via appropriate culture conditions (i.e. medium composition, biomaterial properties, and dynamic conditions). This review is focused on current and future ways to carry out tissue replacement of damaged bones and blood vessels, especially with the use of suitable adult mesenchymal stem cells as a potential source of differentiated mature cells that can later be used for tissue replacement. The advantages and disadvantages of different stem cell sources are discussed, with a main focus on adipose-derived stem cells. Patient factors that can influence later clinical applications are taken into account.

Recombinant Spider Silk Functionalized with a Motif from Fibronectin Mediates Cell Adhesion and Growth on Polymeric Substrates by Entrapping Cells During Self-Assembly

In vitro endothelialization of synthetic grafts or engineered vascular constructs is considered a promising alternative to overcome shortcomings in the availability of autologous vessels and in-graft complications with synthetics. A number of cell-seeding techniques have been implemented to render vascular grafts accessible for cells to attach, proliferate, and spread over the surface area. Nonetheless, seeding efficiency and the time needed for cells to adhere varies dramatically. Herein, we investigated a novel cell-seeding approach (denoted co-seeding) that enables cells to bind to a motif from fibronectin included in a recombinant spider silk protein. Entrapment of cells occurs at the same time as the silk assembles into a nanofibrillar coating on various substrates. Cell adhesion analysis showed that the technique can markedly improve cell-seeding efficiency to nonfunctionalized polystyrene surfaces, as well as establish cell attachment and growth of human dermal microvascular endothelial cells on bare polyethylene terephthalate and polytetrafluoroethylene (PTFE) substrates. Scanning electron microscopy images revealed a uniform endothelial cell layer and cell-substratum compliance with the functionalized silk protein to PTFE surfaces. The co-seeding technique holds a great promise as a method to reliably and quickly cellularize engineered vascular constructs as well as to in vitro endothelialize commercially available cardiovascular grafts.

Vascular Tissue Engineering: Progress, Challenges, and Clinical Promise

Although the clinical demand for bioengineered blood vessels continues to rise, current options for vascular conduits remain limited. The synergistic combination of emerging advances in tissue fabrication and stem cell engineering promises new strategies for engineering autologous blood vessels that recapitulate not only the mechanical properties of native vessels but also their biological function. Here we explore recent bioengineering advances in creating functional blood macro and microvessels, particularly featuring stem cells as a seed source. We also highlight progress in integrating engineered vascular tissues with the host after implantation as well as the exciting pre-clinical and clinical applications of this technology.

Tissue-Engineered Vascular Graft of Small Diameter Based on Electrospun Polylactide Microfibers

Tubular vascular grafts 1.1 mm in diameter based on poly(L-lactide) microfibers were obtained by electrospinning. X-ray diffraction and scanning electron microscopy data demonstrated that the samples treated at T = 70°C for 1 h in the fixed state on a cylindrical mandrel possessed dense fibrous structure; their degree of crystallinity was approximately 44%. Strength and deformation stability of these samples were higher than those of the native blood vessels; thus, it was possible to use them in tissue engineering as bioresorbable vascular grafts. The experiments on including implantation into rat abdominal aorta demonstrated that the obtained vascular grafts did not cause pathological reactions in the rats; in four weeks, inner side of the grafts became completely covered with endothelial cells, and fibroblasts grew throughout the wall. After exposure for 12 weeks, resorption of PLLA fibers started, and this process was completed in 64 weeks. Resorbed synthetic fibers were replaced by collagen and fibroblasts. At that time, the blood vessel was formed; its neointima and neoadventitia were close to those of the native vessel in structure and composition.

Fabrication of Polycaprolactone/Polyurethane Loading Conjugated Linoleic Acid and Its Antiplatelet Adhesion

Polycaprolactone/polyurethane (PCL/PU) fibrous scaffold was loaded with conjugated linoleic acid (CLA) by electrospinning method to improve the hemocompatibility of the polymeric surface. Fourier Transform Infrared Spectroscopy (FT-IR) analysis and Scanning Electron Microscopy (SEM) observation were employed to characterize the chemical structure and the changing morphology of electrospun PCL/PU and PCL/PU loaded with CLA (PCL/PU-CLA) scaffolds. Platelet adhesion and whole blood clot formation tests were used to evaluate the effect of CLA on antithrombotic property of PCL/PU-CLA scaffold. Endothelial cells (EC) were also seeded on the scaffold to examine the difference in the morphology of EC layer and platelet attachment with and without the presence of CLA. SEM results showed that CLA supported the spreading and proliferation of EC and PCL/PU-CLA surface induced lower platelet adhesion as well as attachment of other blood cells compared to the PCL/PU one. These results suggest that electrospinning method can successfully combine the antiplatelet effects of CLA to improve hemocompatibility of PCL/PU scaffolds for applications in artificial blood vessels.

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.

Translational Challenges in Cardiovascular Tissue Engineering

Valvular heart disease and congenital heart defects represent a major cause of death around the globe. Although current therapy strategies have rapidly evolved over the decades and are nowadays safe, effective, and applicable to many affected patients, the currently used artificial prostheses are still suboptimal. They do not promote regeneration, physiological remodeling, or growth (particularly important aspects for children) as their native counterparts. This results in the continuous degeneration and subsequent failure of these prostheses which is often associated with an increased morbidity and mortality as well as the need for multiple re-interventions. To overcome this problem, the concept of tissue engineering (TE) has been repeatedly suggested as a potential technology to enable native-like cardiovascular replacements with regenerative and growth capacities, suitable for young adults and children. However, despite promising data from pre-clinical and first clinical pilot trials, the translation and clinical relevance of such TE technologies is still very limited. The reasons that currently limit broad clinical adoption are multifaceted and comprise of scientific, clinical, logistical, technical, and regulatory challenges which need to be overcome. The aim of this review is to provide an overview about the translational problems and challenges in current TE approaches. It further suggests directions and potential solutions on how these issues may be efficiently addressed in the future to accelerate clinical translation. In addition, a particular focus is put on the current regulatory guidelines and the associated challenges for these promising TE technologies.

Decellularized extracellular matrices for tissue engineering applications

Decellularization removes cellular antigens while preserving the ultrastructure and composition of extracellular matrix (ECM). Decellularized ECM (DECM) scaffolds have been widely used in various tissue engineering applications with varying levels of success. The mechanical, architectural and bioactive properties of a DECM scaffold depend largely on the method of decellularization and dictate its clinical efficacy. This article highlights the advantages and challenges associated with the clinical use of DECM scaffolds. Poor mechanical strength is a significant disadvantage of some DECM scaffolds in the repair of load-bearing tissues as well as critical-size defects, where long-term mechanical support is required for the regenerating tissue. Combining DECM scaffolds with synthetic biocompatible polymers could provide a useful strategy to circumvent the issues of poor mechanical stability. This article reviews studies that have combined DECM scaffolds from various tissues with synthetic polymers to create hybrid scaffolds using electrospinning. These hybrid scaffolds provide a mechanical backbone while retaining the bioactive properties of DECM, thus offering a significant advantage for tissue engineering and regenerative medicine applications.

A composite chitosan-gelatin bi-layered, biomimetic macroporous scaffold for blood vessel tissue engineering

A composite chitosan-gelatin macroporous hydrogel-based scaffold with bi-layered tubular architecture was engineered by solvent casting-co-particulate leaching. The scaffold constituted an inner macroporous layer concealed by a non-porous outer layer mimicking the 3D matrix of blood vessels with cellular adhesion and proliferation. The scaffold was evaluated for its morphological, physicochemical, physicomechanical and biodurability properties employing SEM, FTIR, DSC, XRD, porositometry, rheology and texture analysis. The fluid uptake and biodegradation in the presence of lysozymes was also investigated. Cellular attachment and proliferation was analysed using human dermal fibroblasts (HDF-a) seeded onto the scaffold and evaluated by MTT assay, SEM, and confocal microscopy. Results demonstrated that the scaffold had a desirable tensile strength=95.81±11kPa, elongation at break 112.5±13%, porosity 82% and pores between 100 and 230μm, 50% in vitro biodegradation at day 16 and proliferated fibroblasts over 20 days. These results demonstrate that scaffold may be an excellent tubular archetype for blood vessel tissue engineering.

Materials and surface modification for tissue engineered vascular scaffolds

Although vascular implantation has been used as an effective treatment for cardiovascular disease for many years, off-the-shelf and regenerable vascular scaffolds are still not available. Tissue engineers have tested various materials and methods of surface modification in the attempt to develop a scaffold that is more suitable for implantation. Extracellular matrix-based natural materials and biodegradable polymers, which are the focus of this review, are considered to be suitable materials for production of tissue-engineered vascular grafts. Various methods of surface modification that have been developed will also be introduced, their impacts will be summarized and assessed, and challenges for further research will briefly be discussed.

Collagen and Elastin Biomaterials for the Fabrication of Engineered Living Tissues

Collagen and elastin represent the two most predominant proteins in the body and are responsible for modulating important biological and mechanical properties. Thus, the focus of this review is the use of collagen and elastin as biomaterials for the fabrication of living tissues. Considering the importance of both biomaterials, we first propose the notion that many tissues in the human body represent a reinforced composite of collagen and elastin. In the rest of the review, collagen and elastin biosynthesis and biophysics, as well as molecular sources and biomaterial fabrication methodologies, including casting, fiber spinning, and bioprinting, are discussed. Finally, we summarize the current attempts to fabricate a subset of living tissues and, based on biochemical and biomechanical considerations, suggest that future tissue-engineering efforts consider direct incorporation of collagen and elastin biomaterials.

A Closer Look at Schlemm's Canal Cell Physiology: Implications for Biomimetics

Among ocular pathologies, glaucoma is the second leading cause of progressive vision loss, expected to affect 80 million people worldwide by 2020. A primary cause of glaucoma appears to be damage to the conventional outflow tract. Conventional outflow tissues, a composite of the trabecular meshwork and the Schlemm's canal, regulate and maintain homeostatic responses to intraocular pressure. In glaucoma, filtration of aqueous humor into the Schlemm's canal is hindered, leading to an increase in intraocular pressure and subsequent damage to the optic nerve, with progressive vision loss. The Schlemm's canal encompasses a unique endothelium. Recent advances in culturing and manipulating Schlemm's canal cells have elucidated several aspects of their physiology, including ultrastructure, cell-specific marker expression, and biomechanical properties. This review highlights these advances and discusses implications for engineering a 3D, biomimetic, in vitro model of the Schlemm's canal endothelium to further advance glaucoma research, including drug testing and gene therapy screening.

Evaluation of autologous tissue sources for the isolation of endothelial cells and adipose tissue-derived mesenchymal stem cells to pre-vascularize tissue-engineered vascular grafts

Currently used synthetic vascular grafts bear a high infection risk due to insufficient microvascularization of the graft wall disabling the infiltration of immune cells. Tissue-engineered grafts with a functional pre-vascularization thus would be desirable. However, autologous tissue sources for capillary forming cells need to be evaluated. Here, peripheral blood outgrowth endothelial cells (PB-OEC) from 17 healthy donors and pericyte-like mesenchymal stem cells derived from adipose tissue (ASC) of 17 patients scheduled for visceral surgery were characterized and investigated regarding their ability to form capillary-like networks in plasma-derived fibrin gels. To obtain proliferating PB-OEC with endothelial cell-specific properties (CD31-, VE-cadherin-expression, ac-LDL uptake and three-dimensional (3D)-tube formation in fibrin gels) both enrichment of CD34

Endothelialization of electrospun polycaprolactone (PCL) small caliber vascular grafts spun from different polymer blends

Small caliber vascular grafts represent a challenge to material scientists. In contrast to large caliber grafts, prostheses with diameter <6 mm, lead to increased hemodynamic disturbances and thrombogenic complications. Thus, endothelialization of small caliber grafts should create a compatible interface for hemodynamic processes. The purpose of our study was to compare different compositions of electrospun scaffolds with conventional ePTFE grafts with an inner diameter of 4 mm as well as different pre-coatings to create an optimized physiological interface for endothelialization. Polycaprolactone, polylactide, and polyethylenglycol (PCL/PLA and PCL/PLA/PEG) electrospun grafts and ePTFE grafts were pre-coated with blood, gelatine or fibronectin and seeded with endothelial cells from the human term placenta. Best results were obtained with fibronectin-coated PCL/PLA/PEG grafts. Here, the number of attached viable cells was 78-81% higher than on fibronectin pre-treated ePTFE grafts. Cells attached to PCL/PLA/PEG grafts appeared in physiological cobblestone morphology. Viability analysis showed a high cell viability of more than 98%. Fibronectin-coated PCL/PLA/PEG grafts may be a promising improvement to conventionally used ePTFE grafts.

Using Polymeric Scaffolds for Vascular Tissue Engineering

With the high occurrence of cardiovascular disease and increasing numbers of patients requiring vascular access, there is a significant need for small-diameter (<6 mm inner diameter) vascular graft that can provide long-term patency. Despite the technological improvements, restenosis and graft thrombosis continue to hamper the success of the implants. Vascular tissue engineering is a new field that has undergone enormous growth over the last decade and has proposed valid solutions for blood vessels repair. The goal of vascular tissue engineering is to produce neovessels and neoorgan tissue from autologous cells using a biodegradable polymer as a scaffold. The most important advantage of tissue-engineered implants is that these tissues can grow, remodel, rebuild, and respond to injury. This review describes the development of polymeric materials over the years and current tissue engineering strategies for the improvement of vascular conduits.

Regeneration of the limb: opinions on the reality

Whenever the topic of re-growing human limbs is posed for discussion, it is often argued that ‘if a newt can do it, then so can we’. This notion, albeit promising, is somewhat like watching a science-fiction film; the individual components are currently available but we are far from realizing the complete picture. Today’s reality is that if we are faced with a limb-severing injury, any regenerative attempt would endeavour to accelerate the pace at which the tissue heals to a clinically relevant/functional state. The science of limb regeneration can be approached from three different angles, developmental biology; regenerative medicine; and tissue engineering. This opinion piece describes how each approach can be used to understand the concepts behind regeneration, how far each approach has advanced and the hurdles faced by each of the approaches.

Elastic, double-layered poly (l-lactide-co-ϵ-caprolactone) scaffold for long-term vascular reconstruction

Synthetic vessel grafts have been used as vascular substitutes for cardiovascular bypass procedures. In this study, we developed a novel tubular double-layered poly(l-lactide-co-ϵ-caprolactone) scaffold that did not require pretreatment with cell seeding by promoting autologous tissue regeneration by inducing the proliferation and differentiation of endothelial and smooth muscle progenitor cells after implantation. The patency and mechanical properties were maintained for one year after implantation, although 95% of the poly(l-lactide-co-ϵ-caprolactone) scaffolds had degraded. After this period, there was a lining of endothelial cells, an accumulation of collagen and elastin, and the development of neovascularization inside the poly(l-lactide-co-ϵ-caprolactone).

Small-diameter silk vascular grafts (3 mm diameter) with a double-raschel knitted silk tube coated with silk fibroin sponge

Small-diameter (less than 6 mm in diameter) vascular grafts are highly desirable due to the large demand for surgical revascularization; however, there are no available artificial grafts. Vascular grafts of 1.5 mm diameter prepared by our group with silk fibroin fiber have been proved to be excellent grafts with remarkably high patency and remodeling, based on rat implantation experiment (Enomoto et al., 2010). In this study, a silk fibroin vascular graft with 3 mm diameter which can be used for the coronary arteries or lower extremity arteries is prepared with a double-raschel knitted Bombyx mori silk fiber tube coated with B. mori silk fibroin sponge. Here the silk sponge is prepared from an aqueous solution of the silk fibroin and poly(ethylene) glycol diglycidyl ether as porogen. Sufficient strength, proper elasticity, and protection from loose ends in the implantation process are obtained for the silk fibroin graft; low water permeability and relatively large compliance are also attained. These excellent physical properties make silk fibroin grafts suitable to be implanted in a canine model.

Electrospun tubular scaffolds: on the effectiveness of blending poly(ε-caprolactone) with poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Tissue engineering can effectively contribute to the development of novel vascular prostheses aimed to overcome the well-known drawbacks of small-diameter grafts. To date, poly(ε-caprolactone) (PCL), a bioresorbable synthetic poly(α-hydroxyester), is considered one of the most promising materials for vascular tissue engineering. In this work, the potential advantage of intimate blending soft PCL and hard poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a polymer of microbial origin, has been evaluated. Nonwoven mats and small-diameter tubular scaffolds of PCL, PHBV, and PCL/PHBV were fabricated by means of electrospinning technique. Mechanical properties and suture retention strength were investigated according to the international standard for cardiovascular implants. Biological tests demonstrated that both PCL-based scaffolds supported survival and growth of rat cerebral endothelial cells in a short time. The fiber alignment of the electrospun tubular scaffolds contributed to a more rapid and homogeneous cell colonization of the luminal surface.

Influence of mechanical stimulation in the development of a medial equivalent tissue-engineered vascular construct using a gelatin-g-vinyl acetate co-polymer scaffold

Vascular regeneration in the area of small diameter (<6 mm) vessels via the tissue-engineering approach has been in focus for some time now. In this study, we report the development and evaluation of a tissue-engineered medial equivalent using gelatin-g-vinyl acetate co-polymer (GeVAc) as the scaffold material. GeVAc was synthesized by co-polymerizing gelatin and vinyl acetate monomer in the presence of AIBN as the initiator and subjected to physico-chemical characterization. A porous 3-D scaffold with open interconnected pores was then produced from GeVAc. The scaffold is non-cytotoxic with good smooth muscle cell proliferative capacity and high cell viability. Influence of smooth muscle cell phenotype in response to these scaffolds has been studied under mechanical stimulation. It was found that the cell-seeded tubular GeVAc constructs under mechanical stimulation preferentially supported the contractile phenotype of smooth muscle cells, as evidenced by the elevated expression of contractile protein markers such as alpha-SMA, calponin and SM22α. The mechanical properties and the ECM secretion were also increased on applying the mechanical stimulation. Hence, the results showed the promising potential of the GeVAc scaffolds in the regeneration of the medial equivalent tissue-engineered vascular construct.

Synthesis and evaluation of poly(hexamethylene-urethane) and PEG-poly(hexamethylene-urethane) and their cholesteryl oleyl carbonate composites for human blood biocompatibility

Two new urethane-based acrylates (UAA and PEG-UAA) were synthesized as polymer blocks. The chemical composition of the two monomers was confirmed by IR and NMR. After cross-linking these blockers by radical polymerization, “hexamethylene PU” [poly(hexamethylene-urethane)] and “PEG-hexamethylene PU” [PEG-poly(hexa-methylene-urethane)] were obtained. The platelet adhesion and platelet activation of these polymers were evaluated in the presence of Platelet Rich Plasma (PRP) blood. The relative blood clotting indexes of the polymers were determined to measure their capability of reducing thrombogenicity. The hemolysis of red blood cells was also assessed to examine the haemocompatibility of the polymers. The hexamethylene PU and PEG-hexamethylene PU showed less platelet adhesion, platelet activation, blood clotting and hemolysis than a commercial PU (Tecoflex). The liquid crystal molecule, cholesteryl oleyl carbonate (COC), showed further improved biocompatibility to human blood, after COC was embedded in the PU polymers. PEG-hexamethylene PU + 10% COC demonstrated the best activity in reducing thrombogenicity and the best haemocompatibility. The inclusion of PEG segments into the PEG-UAA structure increased its hydrophilicity. The methylene bis(cyclohexyl) segments in Tecoflex PU decreased haemocompatibility. These observations are in good agreement with performed contact angle measurements. The PEG-hexamethylene PU loaded with COC might be a promising material for applications in bioengineering.

Experimental results of using autologous rectus fascia sheath for venous patch grafts in dogs

Autologous vascular patch grafts developed from the internal rectus sheath were implanted onto the bilateral common iliac vein and jugular vein of 4 experimental beagle dogs. During the development and implanting of the grafts no technical difficulties or perioperative complications were encountered. The follow-up lasted 6 months and 3 months in the case of the common iliac vein grafts and the jugular grafts, respectively. In the postoperative period, the morphological and functional characteristics of the implanted venous sections were examined by Doppler ultrasonography and CT angiography. Normal patency was detected, and none of these check-ups showed obturation or stenosis. The histological survey showed no mesothelial cell layer, but the insides of the grafts showed total restructuring and were covered by a normal endothelial layer. No difference could be detected between samples harvested 3 and 6 months after implanting. The immunohistochemical examinations using anti-claudin-5 and anti-CD31 antibodies confirmed the preliminary results of the histological examinations that the luminal surfaces of the implanted grafts developed a differentiated monolayer endothelium which was free of degenerative and inflammatory signs. The control examinations show the suitability of the internal rectus sheath as a venous wall donor.

A new strategy of promoting vascularization of skin substitutes by capturing endothelial progenitor cells automatically

How to promote vascularization of a skin substitute is the key to successful skin transplantation. Current methods are mainly through releasing angiogenesis-related factors (ARF) or seeding angiogenesis-related cells (ARC), but the efficacy of these methods is not satisfactory, because angiogenesis needs participation of multiple factors, extracellular matrix and related cells. The latest research has demonstrated that endothelial progenitor cells (EPCs) originating from bone marrow and existing in peripheral blood are the key element participating in revascularization of adult tissues. They directly participate in both stem cell vasculogenesis of ischemic tissues and local angiogenesis. We therefore hypothesize whether it is possible to construct a new skin substitute and use it to mobilize EPCs in bone marrow to peripheral circulation and capture EPCs automatically as a simple and effective method of promoting vascularization of the skin substitute for the sake of improving its post-transplant survival.

Tissue Engineering of Blood Vessels: Functional Requirements, Progress, and Future Challenges

Vascular disease results in the decreased utility and decreased availability of autologus vascular tissue for small diameter (< 6 mm) vessel replacements. While synthetic polymer alternatives to date have failed to meet the performance of autogenous conduits, tissue-engineered replacement vessels represent an ideal solution to this clinical problem. Ongoing progress requires combined approaches from biomaterials science, cell biology, and translational medicine to develop feasible solutions with the requisite mechanical support, a non-fouling surface for blood flow, and tissue regeneration. Over the past two decades interest in blood vessel tissue engineering has soared on a global scale, resulting in the first clinical implants of multiple technologies, steady progress with several other systems, and critical lessons-learned. This review will highlight the current inadequacies of autologus and synthetic grafts, the engineering requirements for implantation of tissue-engineered grafts, and the current status of tissue-engineered blood vessel research.

Strategies in functional poly(ester amide) syntheses to study human coronary artery smooth muscle cell interactions

The design of new generation cardiovascular biomaterials focuses on biomimetic properties that are capable of eliciting specific cellular responses and directing new tissue formation. Synthetic poly(ester amide)s (PEAs) containing α-amino acid residues have the potential to elicit favorable cellular responses. Furthermore, they are biodegradable owing to the incorporation of naturally occurring amino acids. In this study, a family of PEAs was synthesized from selected α-amino acids using both solution and interfacial polymerization approaches to optimize their properties for vascular tissue engineering applications. By careful selection of the monomers and the polymerization approach, high-molecular-weight PEAs with low glass-transition temperatures were obtained. Human coronary artery smooth muscle cells (HCASMCs) cultured directly on bare PEA films attached and spread well up to 7 days of culture. Moreover, cell viability was significantly enhanced on all nonfunctional PEAs compared with tissue culture polystyrene controls. The trifluoroacetic acid salt of the lysine-containing functional PEAs was found to retard cell growth but still supported cell viability up to 5 days of culture. Immunostaining of HCASMCs revealed strong vinculin expression, suggesting that the HCASMCs initiated cellular processes for focal adhesion contacts with all PEA surfaces. Conversely, smooth muscle α-actin expression was not abundant on the PEA surfaces, suggesting a proliferative smooth muscle cell phenotype. Altogether, our results indicate that these PEAs are promising materials for vascular tissue engineering scaffolds.