In vivo assessment of a tissue-engineered vascular graft combining a biodegradable elastomeric scaffold and muscle-derived stem cells in a rat model

Tissue engineered vascular grafts: current state of the field

INTRODUCTION

Conventional synthetic vascular grafts are limited by the inability to remodel, as well as issues of patency at smaller diameters. Tissue-engineered vascular grafts (TEVGs), constructed from biologically active cells and biodegradable scaffolds have the potential to overcome these limitations, and provide growth capacity and self-repair. Areas covered: This article outlines the TEVG design, biodegradable scaffolds, TEVG fabrication methods, cell seeding, drug delivery, strategies to reduce wait times, clinical trials, as well as a 5-year view with expert commentary. Expert commentary: TEVG technology has progressed significantly with advances in scaffold material and design, graft design, cell seeding and drug delivery. Strategies have been put in place to reduce wait times and improve 'off-the-shelf' capability of TEVGs. More recently, clinical trials have been conducted to investigate the clinical applications of TEVGs.

Arterial graft with elastic layer structure grown from cells

Shortage of autologous blood vessel sources and disadvantages of synthetic grafts have increased interest in the development of tissue-engineered vascular grafts. However, tunica media, which comprises layered elastic laminae, largely determines arterial elasticity, and is difficult to synthesize. Here, we describe a method for fabrication of arterial grafts with elastic layer structure from cultured human vascular SMCs by periodic exposure to extremely high hydrostatic pressure (HP) during repeated cell seeding. Repeated slow cycles (0.002 Hz) between 110 and 180 kPa increased stress-fiber polymerization and fibronectin fibrillogenesis on SMCs, which is required for elastic fiber formation. To fabricate arterial grafts, seeding of rat vascular SMCs and exposure to the periodic HP were repeated alternatively ten times. The obtained medial grafts were highly elastic and tensile rupture strength was 1451 ± 159 mmHg, in which elastic fibers were abundantly formed. The patch medial grafts were sutured at the rat aorta and found to be completely patent and endothelialized after 2.5 months, although tubular medial constructs implanted in rats as interpositional aortic grafts withstood arterial blood pressure only in early acute phase. This novel organized self-assembly method would enable mass production of scaffold-free arterial grafts in vitro and have potential therapeutic applications for cardiovascular diseases.

In Vivo Functional Evaluation of Tissue-Engineered Vascular Grafts Fabricated Using Human Adipose-Derived Stem Cells from High Cardiovascular Risk Populations

Many preclinical evaluations of autologous small-diameter tissue-engineered vascular grafts (TEVGs) utilize cells from healthy humans or animals. However, these models hold minimal relevance for clinical translation, as the main targeted demographic is patients at high cardiovascular risk such as individuals with diabetes mellitus or the elderly. Stem cells such as adipose-derived mesenchymal stem cells (AD-MSCs) represent a clinically ideal cell type for TEVGs, as these can be easily and plentifully harvested and offer regenerative potential. To understand whether AD-MSCs sourced from diabetic and elderly donors are as effective as those from young nondiabetics (i.e., healthy) in the context of TEVG therapy, we implanted TEVGs constructed with human AD-MSCs from each donor type as an aortic interposition graft in a rat model. The key failure mechanism observed was thrombosis, and this was most prevalent in grafts using cells from diabetic patients. The remainder of the TEVGs was able to generate robust vascular-like tissue consisting of smooth muscle cells, endothelial cells, collagen, and elastin. We further investigated a potential mechanism for the thrombotic failure of AD-MSCs from diabetic donors; we found that these cells have a diminished potential to promote fibrinolysis compared to those from healthy donors. Together, this study served as proof of concept for the development of a TEVG based on human AD-MSCs, illustrated the importance of testing cells from realistic patient populations, and highlighted one possible mechanistic explanation as to the observed thrombotic failure of our diabetic AD-MSC-based TEVGs.

Synthesis and characterization of polycaprolactone urethane hollow fiber membranes as small diameter vascular grafts

The design of bioresorbable synthetic small diameter (<6mm) vascular grafts (SDVGs) capable of sustaining long-term patency and endothelialization is a daunting challenge in vascular tissue engineering. Here, we synthesized a family of biocompatible and biodegradable polycaprolactone (PCL) urethane macromers to fabricate hollow fiber membranes (HFMs) as SDVG candidates, and characterized their mechanical properties, degradability, hemocompatibility, and endothelial development. The HFMs had smooth surfaces and porous internal structures. Their tensile stiffness ranged from 0.09 to 0.11N/mm and their maximum tensile force from 0.86 to 1.03N, with minimum failure strains of approximately 130%. Permeability varied from 1 to 14×10(-6)cm/s, burst pressures from 1158 to 1468mmHg, and compliance from 0.52 to 1.48%/100mmHg. The suture retention forces ranged from 0.55 to 0.81N. HFMs had slow degradation profiles, with 15 to 30% degradation after 8weeks. Human endothelial cells proliferated well on the HFMs, creating stable cell layer coverage. Hemocompatibility studies demonstrated low hemolysis (<2%), platelet activation, and protein adsorption. There were no significant differences in the hemocompatibility of HFMs in the absence and presence of endothelial layers. These encouraging results suggest great promise of our newly developed materials and biodegradable elastomeric HFMs as SDVG candidates.

Superelastic, superabsorbent and 3D nanofiber-assembled scaffold for tissue engineering

Fabrication of 3D scaffold to mimic the nanofibrous structure of the nature extracellular matrix (ECM) with appropriate mechanical properties and excellent biocompatibility, remain an important technical challenge in tissue engineering. The present study reports the strategy to fabricate a 3D nanofibrous scaffold with similar structure to collagen in ECM by combining electrospinning and freeze-drying technique. With the technique reported here, a nanofibrous structure scaffold with hydrophilic and superabsorbent properties can be readily prepared by Gelatin and Polylactic acid (PLA). In wet state the scaffold also shows a super-elastic property, which could bear a compressive strain as high as 80% and recovers its original shape afterwards. Moreover, after 6 days of culture, L-929 cells grow, proliferate and infiltrated into the scaffold. The results suggest that this 3D nanofibrous scaffold would be promising for varied field of tissue engineering application.

Cell-based strategies for vascular regeneration

Vascular regeneration is known to play an essential role in the repair of injured tissues mainly through accelerating the repair of vascular injury caused by vascular diseases, as well as the recovery of ischemic tissues. However, the clinical vascular regeneration is still challenging. Cell-based therapy is thought to be a promising strategy for vascular regeneration, since various cells have been identified to exert important influences on the process of vascular regeneration such as the enhanced endothelium formation on the surface of vascular grafts, and the induction of vessel-like network formation in the ischemic tissues. Here are a vast number of diverse cell-based strategies that have been extensively studied in vascular regeneration. These strategies can be further classified into three main categories, including cell transplantation, construction of tissue-engineered grafts, and surface modification of scaffolds. Cells used in these strategies mainly refer to terminally differentiated vascular cells, pluripotent stem cells, multipotent stem cells, and unipotent stem cells. The aim of this review is to summarize the reported research advances on the application of various cells for vascular regeneration, yielding insights into future clinical treatment for injured tissue/organ.

Establishment of a rat and guinea pig aortic interposition graft model reveals model-specific patterns of intimal hyperplasia

OBJECTIVE

Although the aortic interposition bypass model has been widely used to evaluate biomaterials for bypass grafting, there is no comprehensive description of the procedure or of the distribution of intimal hyperplasia that results. The objectives of this study were to (1) review and summarize approaches of aortic interposition grafting in animal models, (2) determine the pertinent anatomy for this procedure, (3) validate this model in the rat and guinea pig, and (4) compare the distribution of intimal hyperplasia that develops in each species.

METHODS

A literature search was performed in PubMed from 1980 to the present to analyze the use of anesthesia, anticoagulation, antiplatelet agents, graft material, suture, and anastomotic techniques. Using 10-week-old male Sprague-Dawley rats and Hartley guinea pigs, we established pertinent aortic anatomy, developed comparable models, and assessed complications for each model. At 30 days, the graft and associated aorta were explanted, intimal formation was assessed morphometrically, and cellularity was assessed via nuclear counting.

RESULTS

We reviewed 30 articles and summarized the pertinent procedural findings. Upon establishing both animal models, key anatomic differences between the species that affect this model were noted. Guinea pigs have a much larger cecum, increased retroperitoneal fat, and lack the iliolumbar vessels compared with the rat. Surgical outcomes for the rat model included a 53% technical success rate and a 32% technical error rate. Surgical outcomes for the guinea pig model included a 69% technical success rate and a 31% technical error rate. These two species demonstrated unique distribution of intimal hyperplasia at 30 days. Intimal hyperplasia in the rat model was greatest at two areas, the proximal graft (5400 μm²; P < .001) and distal graft (2800 μm²; P < .04), whereas the guinea pig model developed similar intimal hyperplasia throughout the graft (4500-5100 μm²; P < .01).

CONCLUSIONS

In this report, we summarize the literature on the aortic interposition graft model, present a detailed description of the anatomy and aortic interposition graft procedure in the rat and guinea pig, and describe a unique distribution of intimal formation that results in both species. This information will be helpful when designing studies to evaluate novel graft materials in the future.

Three-dimensional culture of small-diameter vascular grafts

Analysis of efforts to engineer 3D small-diameter (<6 mm) vascular grafts, indicating the importance of stem cells, co-culture, and pulsatile flow.

The Tissue-Engineered Vascular Graft-Past, Present, and Future

Cardiovascular disease is the leading cause of death worldwide, with this trend predicted to continue for the foreseeable future. Common disorders are associated with the stenosis or occlusion of blood vessels. The preferred treatment for the long-term revascularization of occluded vessels is surgery utilizing vascular grafts, such as coronary artery bypass grafting and peripheral artery bypass grafting. Currently, autologous vessels such as the saphenous vein and internal thoracic artery represent the gold standard grafts for small-diameter vessels (<6 mm), outperforming synthetic alternatives. However, these vessels are of limited availability, require invasive harvest, and are often unsuitable for use. To address this, the development of a tissue-engineered vascular graft (TEVG) has been rigorously pursued. This article reviews the current state of the art of TEVGs. The various approaches being explored to generate TEVGs are described, including scaffold-based methods (using synthetic and natural polymers), the use of decellularized natural matrices, and tissue self-assembly processes, with the results of various in vivo studies, including clinical trials, highlighted. A discussion of the key areas for further investigation, including graft cell source, mechanical properties, hemodynamics, integration, and assessment in animal models, is then presented.

Potential of Newborn and Adult Stem Cells for the Production of Vascular Constructs Using the Living Tissue Sheet Approach

Bypass surgeries using native vessels rely on the availability of autologous veins and arteries. An alternative to those vessels could be tissue-engineered vascular constructs made by self-organized tissue sheets. This paper intends to evaluate the potential use of mesenchymal stem cells (MSCs) isolated from two different sources: (1) bone marrow-derived MSCs and (2) umbilical cord blood-derived MSCs. When cultured in vitro, a proportion of those cells differentiated into smooth muscle cell- (SMC-) like cells and expressed contraction associated proteins. Moreover, these cells assembled into manipulable tissue sheets when cultured in presence of ascorbic acid. Tubular vessels were then produced by rolling those tissue sheets on a mandrel. The architecture, contractility, and mechanical resistance of reconstructed vessels were compared with tissue-engineered media and adventitia produced from SMCs and dermal fibroblasts, respectively. Histology revealed a collagenous extracellular matrix and the contractile responses measured for these vessels were stronger than dermal fibroblasts derived constructs although weaker than SMCs-derived constructs. The burst pressure of bone marrow-derived vessels was higher than SMCs-derived ones. These results reinforce the versatility of the self-organization approach since they demonstrate that it is possible to recapitulate a contractile media layer from MSCs without the need of exogenous scaffolding material.

Patient-specific cardiovascular progenitor cells derived from integration-free induced pluripotent stem cells for vascular tissue regeneration

Tissue-engineered blood vessels (TEBVs) are promising in regenerating a live vascular replacement. However, the vascular cell source is limited, and it is crucial to develop a scaffold that accommodates new type of vascular progenitor cells and facilitates in vivo lineage specification of the cells into functional vascular smooth muscle cells (VSMCs) to regenerate vascular tissue. In the present study, integration-free human induced pluripotent stem cells (hiPSCs) were established from patient peripheral blood mononuclear cells through episomal vector nucleofection of reprogramming factors. The established hiPSCs were then induced into mesoderm-originated cardiovascular progenitor cells (CVPCs) with a highly efficient directed lineage specification method. The derived CVPCs were demonstrated to be able to differentiate into functional VSMCs. Subcutaneous implantation of CVPCs seeded on macroporous nanofibrous poly(l-lactide) scaffolds led to in vivo VSMC lineage specification and matrix deposition inside the scaffolds. In summary, we established integration-free patient-specific hiPSCs from peripheral blood mononuclear cells, derived CVPCs through directed lineage specification, and developed an advanced scaffold for these progenitor cells to further differentiate in vivo into VSMCs and regenerate vascular tissue in a subcutaneous implantation model. This study has established an efficient patient-specific approach towards in vivo regeneration of vascular tissue.

High pressure assessment of bilayered electrospun vascular grafts by means of an Electroforce Biodynamic System®

INTRODUCTION

Tissue engineering offers the possibility of developing a biological substitute material in vitro with the inherent properties required in vivo. However, the inadequate performance in vascular replacement of small diameter vascular grafts (VG) reduces considerably the current alternatives in this field. In this study, a bilayered tubular VG was produced, where its mechanical response was tested at high pressure ranges and compared to a native femoral artery.

MATERIALS AND METHOD

The VG was obtained using sequential electrospinning technique, by means of two blends of Poly(L-lactic acid) and segmented poly(ester urethane). Mechanical testing was performed in a biodynamic system and the pressure-strain relationship was used to determine the elastic modulus.

RESULTS

Elastic modulus assessed value of femoral artery at a high pressure range (33.02×106 dyn/cm(2)) was founded to be 36% the magnitude of VG modulus (91.47×106 dyn/cm(2)) at the same interval.

CONCLUSION

A new circulating mock in combination with scan laser micrometry have been employed for the mechanical evaluation of bioresorbable bilayered VGs. At same pressure levels, graft elasticity showed a purely "collagenic" behavior with respect to a femoral artery response.

Quality of healing: defining, quantifying, and enhancing skeletal muscle healing

Skeletal muscle injury is common in everyday physical activity and athletics, as well as in orthopedic trauma and disease. The overall functional disability resulting from muscle injury is directly related to the intrinsic healing properties of muscle and extrinsic treatment options designed to maximize repair and/or regeneration of muscle tissue all while minimizing pathologic healing pathways. It is important to understand the injury and repair pathways in order to improve the speed and quality of recovery. Recent military conflicts in Iraq and Afghanistan have highlighted the importance of successfully addressing muscular injury and showed the need for novel treatment options that will maximize functional regeneration of the damaged tissue. These severe, wartime injuries, when juxtaposed to peacetime, sports-related injuries, provide us with interesting case examples of the two extreme forms of muscular damage. Comparing and contrasting the differences in these healing pathways will likely provide helpful cues that will help physicians recapitulate the near complete repair and regeneration in less traumatic injuries in addition to more severe cases.

The innate immune system contributes to tissue-engineered vascular graft performance

The first clinical trial of tissue-engineered vascular grafts (TEVGs) identified stenosis as the primary cause of graft failure. In this study, we aimed to elucidate the role of the host immune response in the development of stenosis using a murine model of TEVG implantation. We found that the C.B-17 wild-type (WT) mouse (control) undergoes a dramatic stenotic response, which is nearly completely abolished in the immunodeficient SCID/beige (bg) variant. SCID mice, which lack an adaptive immune system due to the absence of T and B lymphocytes, experienced rates of stenosis comparable to WT controls (average luminal diameter, WT: 0.071 ± 0.035 mm, SCID: 0.137 ± 0.032 mm, SCID/bg: 0.804 ± 0.039 mm; P < 0.001). The bg mutation is characterized by NK cell and platelet dysfunction, and systemic treatment of WT mice with either NK cell-neutralizing (anti-NK 1.1 antibody) or antiplatelet (aspirin/Plavix [clopidogrel bisulfate]; Asp/Pla) therapy achieved nearly half the patency observed in the SCID/bg mouse (NK Ab: 0.356 ± 0.151 mm, Asp/Pla: 0.452 ± 0.130 mm). Scaffold implantation elicited a blunted immune response in SCID/bg mice, as demonstrated by macrophage number and mRNA expression of proinflammatory cytokines in TEVG explants. Implicating the initial innate immune response as a critical factor in graft stenosis may provide a strategy for prognosis and therapy of second-generation TEVGs.

Contrasting biofunctionalization strategies for the enhanced endothelialization of biodegradable vascular grafts

Surface modification of biodegradable vascular grafts is an important strategy to improve the in situ endothelialization of tissue engineered vascular grafts (TEVGs) and prevent major complications associated with current synthetic grafts. Important strategies for improving endothelialization include increasing endothelial cell mobilization and increased endothelial cell capture through biofunctionalization of TEVGs. The objective of this study was to assess two biofunctionalization strategies for improving endothelialization of biodegradable polyester vascular grafts. These techniques consisted of cross-linking heparin to graft surfaces to immobilize vascular endothelial growth factor (VEGF) or antibodies against CD34 (anti-CD34Ab). To this end, heparin, VEGF, and anti-CD34Ab attachment and quantification assays confirmed the efficacy of the modification strategy. Cell attachment and proliferation on these groups were compared to unmodified grafts in vitro and in vivo. To assess in vivo graft functionality, the grafts were implanted as inferior vena cava interpositional conduits in mice. Modified vascular grafts displayed increased endothelial cell attachment and activity in vivo, according to microscopy techniques, histological results, and eNOS expression. Inner lumen diameter of the modified grafts was also better maintained than controls. Overall, while both functionalized grafts outperformed the unmodified control, grafts modified with anti-CD34Ab appeared to yield the most improved results compared to VEGF-loaded grafts.

Development and evaluation of elastomeric hollow fiber membranes as small diameter vascular graft substitutes

Engineering of small diameter (<6mm) vascular grafts (SDVGs) for clinical use remains a significant challenge. Here, elastomeric polyester urethane (PEU)-based hollow fiber membranes (HFMs) are presented as an SDVG candidate to target the limitations of current technologies and improve tissue engineering designs. HFMs are fabricated by a simple phase inversion method. HFM dimensions are tailored through adjustments to fabrication parameters. The walls of HFMs are highly porous. The HFMs are very elastic, with moduli ranging from 1-4MPa, strengths from 1-5MPa, and max strains from 300-500%. Permeability of the HFMs varies from 0.5-3.5×10(-6)cm/s, while burst pressure varies from 25 to 35psi. The suture retention forces of HFMs are in the range of 0.8 to 1.2N. These properties match those of blood vessels. A slow degradation profile is observed for all HFMs, with 71 to 78% of the original mass remaining after 8weeks, providing a suitable profile for potential cellular incorporation and tissue replacement. Both human endothelial cells and human mesenchymal stem cells proliferate well in the presence of HFMs up to 7days. These results demonstrate a promising customizable PEU HFMs for small diameter vascular repair and tissue engineering applications.

A cautionary tale for autologous vascular tissue engineering: impact of human demographics on the ability of adipose-derived mesenchymal stem cells to recruit and differentiate into smooth muscle cells

Autologous tissue-engineered blood vessels (TEBVs) generated using adult stem cells have shown promising results, but many preclinical evaluations do not test the efficacy of stem cells from patient populations likely to need therapy (i.e., elderly and diabetic humans). Two critical functions of these cells will be (i) secreting factors that induce the migration of host cells into the graft and (ii) differentiating into functional vascular cells themselves. The purpose of this study was to analyze whether adipose-derived mesenchymal stem cells (AD-MSCs) sourced from diabetic and elderly patients have a reduced ability to promote human smooth muscle cell (SMC) migration and differentiation potential toward SMCs, two important processes in stem cell-based tissue engineering of vascular grafts. SMC monolayers were disrupted in vitro by a scratch wound and were induced to close the wound by exposure to media conditioned by AD-MSCs from healthy, elderly, and diabetic patients. Media conditioned by AD-MSCs from healthy patients promoted the migration of SMCs and did so in a dose-dependent manner; heating the media to 56°C eliminated the media's potency. AD-MSCs from diabetic and elderly patients had a decreased ability to differentiate into SMCs under angiotensin II stimulation; however, only AD-MSCs from elderly donors were unable to promote SMC migration. Gender and body-mass index of the patients showed no effect on either critical function of AD-MSCs. In conclusion, AD-MSCs from elderly patients may not be suitable for autologous TEBVs due to inadequate promotion of SMC migration and differentiation.

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

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

Electrospinning of biomimetic scaffolds for tissue-engineered vascular grafts: threading the path

Tissue-engineered vascular grafts (TEVGs) offer an alternative to synthetic grafts for the surgical treatment of atherosclerosis and congenital heart defects, and may improve graft patency and patient outcomes after implantation. Electrospinning is a versatile manufacturing process for the production of fibrous scaffolds. This review aims to investigate novel approaches undertaken to improve the design of electrospun scaffolds for TEVG development. The review describes how electrospinning can be adapted to produce aligned nanofibrous scaffolds used in vascular tissue engineering, while novel processes for improved performance of such scaffolds are examined and compared to evaluate their effectiveness and potential. By highlighting new drug delivery techniques and porogenic technologies, in addition to analyzing in vitro and in vivo testing of electrospun TEVGs, it is hoped that this review will provide guidance on how the next generation of electrospun vascular graft scaffolds will be designed and tested for the potential improvement of cardiovascular therapies.

Biomechanical and biocompatibility characteristics of electrospun polymeric tracheal scaffolds

The development of tracheal scaffolds fabricated based on electrospinning technique by applying different ratios of polyethylene terephthalate (PET) and polyurethane (PU) is introduced here. Prior to clinical implantation, evaluations of biomechanical and morphological properties, as well as biocompatibility and cell adhesion verifications are required and extensively performed on each scaffold type. However, the need for bioreactors and large cell numbers may delay the verification process during the early assessment phase. Hence, we investigated the feasibility of performing biocompatibility verification using static instead of dynamic culture. We performed bioreactor seeding on 3-dimensional (3-D) tracheal scaffolds (PET/PU and PET) and correlated the quantitative and qualitative results with 2-dimensional (2-D) sheets seeded under static conditions. We found that an 8-fold reduction for 2-D static seeding density can essentially provide validation on the qualitative and quantitative evaluations for 3-D scaffolds. In vitro studies revealed that there was notably better cell attachment on PET sheets/scaffolds than with the polyblend. However, the in vivo outcomes of cell seeded PET/PU and PET scaffolds in an orthotopic transplantation model in rodents were similar. They showed that both the scaffold types satisfied biocompatibility requirements and integrated well with the adjacent tissue without any observation of necrosis within 30 days of implantation.

Electrospun scaffolds for tissue engineering of vascular grafts

There is a growing demand for off-the-shelf tissue engineered vascular grafts (TEVGs) for the replacement or bypass of damaged arteries in various cardiovascular diseases. Scaffolds from the decellularized tissue skeletons to biopolymers and biodegradable synthetic polymers have been used for fabricating TEVGs. However, several issues have not yet been resolved, which include the inability to mimic the mechanical properties of native tissues, and the ability for long-term patency and growth required for in vivo function. Electrospinning is a popular technique for the production of scaffolds that has the potential to address these issues. However, its application to human TEVGs has not yet been achieved. This review provides an overview of tubular scaffolds that have been prepared by electrospinning with potential for TEVG applications.

Nerve regeneration and elastin formation within poly(glycerol sebacate)-based synthetic arterial grafts one-year post-implantation in a rat model

The objective of this study was to evaluate the long-term performance of cell-free vascular grafts made from a fast-degrading elastic polymer. We fabricated small arterial grafts from microporous tubes of poly(glycerol sebacate) (PGS) reinforced with polycaprolactone (PCL) nanofibers on the outer surface. Grafts were interpositioned in rat abdominal aortas and characterized at 1 year post-implant. Grafts remodeled into "neoarteries" (regenerated arteries) with similar gross appearance to native rat aortas. Neoarteries mimic arterial tissue architecture with a confluent endothelium and media and adventita-like layers. Patent vessels (80%) showed no significant stenosis, dilation, or calcification. Neoarteries contain nerves and have the same amount of mature elastin as native arteries. Despite some differences in matrix organization, regenerated arteries had similar dynamic mechanical compliance to native arteries in vivo. Neoarteries responded to vasomotor agents, albeit with different magnitude than native aortas. These data suggest that an elastic vascular graft that resorbs quickly has potential to improve the performance of vascular grafts used in small arteries. This design may also promote constructive remodeling in other soft tissues.

Phenotypic fitness of primary endothelial cells cultured from patients with high cardiovascular risk or chronic kidney disease for vascular tissue engineering

Vascular tissue engineering relies on the combination of patient-derived cells and biomaterials to create new vessels. For clinical application, data regarding the function and behavior of patient-derived cells are needed. We investigated cell growth and functional characteristics of human venous endothelial cells (HVECs) from coronary arterial bypass graft (CABG), chronic kidney disease (CKD), and control patients. HVECs were isolated from venous specimens that were obtained during elective surgical procedures by means of collagenase digestion. Gene expression, proliferation, migration, secretory functions, and thrombogenic characteristics were evaluated using high-throughput assays. A total of 48 cell batches (14 control, 19 CABG, and 15 CKD subjects) were assessed. Proliferation, population doubling times, and migration of HVECs derived from CABG and CKD patients did not differ from controls. Thrombomodulin expression was higher in CABG-HVECs compared with controls. HVEC-induced thrombin formation in plasma did not differ between groups, and the contact activation pathway was the major contributor to coagulation. Patient-derived HVECs were able to attach and survive on polycaprolactone scaffolds that were coated with fibrin. HVECs from cardiovascular-diseased and CKD patients showed comparable functional characteristics with HVECs derived from uncompromised patients. We, therefore, conclude that endothelial cells from aged patients with comorbidities can be safely used for isolation and in vitro expansion for vascular tissue engineering.

Development and in vivo evaluation of small-diameter vascular grafts engineered by outgrowth endothelial cells and electrospun chitosan/poly(ε-caprolactone) nanofibrous scaffolds

Successful engineering of a small-diameter vascular graft is still a challenge despite numerous attempts for decades. The present study aimed at developing a tissue-engineered vascular graft (TEVG) using autologous outgrowth endothelial cells (OECs) and a hybrid biodegradable polymer scaffold. OECs were harvested from canine peripheral blood and proliferated in vitro, as well as identified by immunofluorescent staining. Electrospun hybrid chitosan/poly(ε-caprolactone) (CS/PCL) nanofibers were fabricated and served as vascular scaffolds. TEVGs were constructed in vitro by seeding OECs onto CS/PCL scaffolds, and then implanted into carotid arteries of cell-donor dogs (n=6). After 3 months of implantation, 5 out of 6 of TEVGs remained patent as compared with 1 out of 6 of unseeded grafts kept patent. Histological and immunohistochemical analyses of the TEVGs retrieved at 3 months revealed the regeneration of endothelium, and the presence of collagen and elastin. OECs labeled with fluorescent dye before implantation were detected in the retrieved TEVGs, indicating that the OECs participated in the vascular tissue regeneration. Biomechanical testing of TEVGs showed good mechanical properties that were closer to native carotid arteries. RT-PCR and western blot analysis demonstrated that TEVGs had favorable biological functional properties resembling native arteries. Overall, this study provided a new strategy to develop small-diameter TEVGs with excellent biocompatibility and regeneration ability.

Vascular tissue engineering: from in vitro to in situ

Blood vessels transport blood to deliver oxygen and nutrients. Vascular diseases such as atherosclerosis may result in obstruction of blood vessels and tissue ischemia. These conditions require blood vessel replacement to restore blood flow at the macrocirculatory level, and angiogenesis is critical for tissue regeneration and remodeling at the microcirculatory level. Vascular tissue engineering has focused on addressing these two major challenges. We provide a systematic review on various approaches for vascular graft tissue engineering. To create blood vessel substitutes, bioengineers and clinicians have explored technologies in cell engineering, materials science, stem cell biology, and medicine. The scaffolds for vascular grafts can be made from native matrix, synthetic polymers, or other biological materials. Besides endothelial cells, smooth muscle cells, and fibroblasts, expandable cells types such as adult stem cells, pluripotent stem cells, and reprogrammed cells have also been used for vascular tissue engineering. Cell-seeded functional tissue-engineered vascular grafts can be constructed in bioreactors in vitro. Alternatively, an autologous vascular graft can be generated in vivo by harvesting the capsule layer formed around a rod implanted in soft tissues. To overcome the scalability issue and make the grafts available off-the-shelf, nonthrombogenic vascular grafts have been engineered that rely on the host cells to regenerate blood vessels in situ. The rapid progress in the field of vascular tissue engineering has led to exciting preclinical and clinical trials. The advancement of micro-/nanotechnology and stem cell engineering, together with in-depth understanding of vascular regeneration mechanisms, will enable the development of new strategies for innovative therapies.

Advancements in electrospinning of polymeric nanofibrous scaffolds for tissue engineering

Polymeric nanofibers have potential as tissue engineering scaffolds, as they mimic the nanoscale properties and structural characteristics of native extracellular matrix (ECM). Nanofibers composed of natural and synthetic polymers, biomimetic composites, ceramics, and metals have been fabricated by electrospinning for various tissue engineering applications. The inherent advantages of electrospinning nanofibers include the generation of substrata with high surface area-to-volume ratios, the capacity to precisely control material and mechanical properties, and a tendency for cellular in-growth due to interconnectivity within the pores. Furthermore, the electrospinning process affords the opportunity to engineer scaffolds with micro- to nanoscale topography similar to the natural ECM. This review describes the fundamental aspects of the electrospinning process when applied to spinnable natural and synthetic polymers; particularly, those parameters that influence fiber geometry, morphology, mesh porosity, and scaffold mechanical properties. We describe cellular responses to fiber morphology achieved by varying processing parameters and highlight successful applications of electrospun nanofibrous scaffolds when used to tissue engineer bone, skin, and vascular grafts.

Animal models for vascular tissue-engineering

Because of rise in cardiovascular disease throughout the world, there is increasing demand for small diameter blood vessels as replacement grafts. The present review focuses on the animal models that have been used to test small-diameter TEVs with emphasis on the attributes of each model. Small animal models are used to test short-term patency and address mechanistic hypotheses; and large, preclinical animal models are employed to test long-term patency, remodeling and function in an environment mimicking human physiology. We also discuss recent clinical trials that employed laboratory fabricated TEVs and showed very promising results. Ultimately, animal models provide a testing platform for optimizing vascular grafts before clinical use in patients without suitable autologous vessels.

Reconfigurable biodegradable shape-memory elastomers via Diels-Alder coupling

Synthetic biodegradable elastomers are a class of polymers that have demonstrated far-reaching utility as biomaterials for use in many medical applications. Biodegradable elastomers can be broadly classified into networks prepared by either step-growth or chain-growth polymerization. Each processing strategy affords distinct advantages in terms of capabilities and resulting properties of the network. This work describes the synthesis, processing, and characterization of cross-linked polyester networks based on Diels-Alder coupling reactions. Hyperbranched furan-modified polyester precursors based on poly(glycerol-co-sebacate) are coupled with bifunctional maleimide cross-linking agents. The chemical and thermomechanical properties of the elastomers are characterized at various stages of network formation. Experimental observations of gel formation are compared to theoretical predictions derived from Flory-Stockmayer relationships. This cross-linking strategy confers unique advantages in processing and properties including the ability to fabricate biodegradable reconfigurable covalent networks without additional catalysts or reaction byproducts. Reconfigurable biodegradable networks using Diels-Alder cycloaddition reactions permit the fabrication of shape-memory polymers with complex permanent geometries. Biodegradable elastomers based on polyester networks with molecular reconfigurability achieve vastly expanded properties and processing capabilities for potential applications in medicine and beyond.

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).

Strategies and techniques to enhance the in situ endothelialization of small-diameter biodegradable polymeric vascular grafts

Due to the lack of success in small-diameter (<6 mm) prosthetic vascular grafts, a variety of strategies have evolved utilizing a tissue-engineering approach. Much of this work has focused on enhancing the endothelialization of these grafts. A healthy, confluent endothelial layer provides dynamic control over homeo-stasis, influencing and preventing thrombosis and smooth muscle cell proliferation that can lead to intimal hyperplasia. Strategies to improve endothelialization of biodegradable polymeric grafts have encompassed both chemical and physical modifications to graft surfaces, many focusing on the recruitment of endothelial and endothelial progenitor cells. This review aims to provide a compilation of current and developing strategies that utilize in situ endothelialization to improve vascular graft outcomes, providing a context for the future directions of vascular tissue-engineering strategies that do not require preprocedural cell seeding.

Muscle-derived stem cells promote angiogenesis and attenuate intimal hyperplasia in different murine vascular disease models

Muscle-derived stem cells (MDSCs) are known to promote angiogenesis, but have never been studied in vascular diseases. We differentiated MDSCs into endothelial lineage cells in vitro by stimulation with shear stress and vascular endothelial growth factor. Such differentiated MDSCs (diff-MDSC) showed strong angiogenic potential in vitro. When tested in ischemic hindlimbs of mice, diff-MDSCs increased perfusion and decreased necrosis of the ischemic limbs, by promoting new vessel formation and by upregulating genes involved in endothelial expression. Such effects were not observed with native MDSCs (without endothelial stimulation in vitro). Diff-MDSCs were also injected into carotid arteries of rats after balloon denudation of the intima layer to induce intimal hyperplasia. The cell-treated group had significantly reduced intima-to-media thickness ratio compared to control, thus attenuating intimal hyperplasia by early re-endothelialization of the intima layer. Our findings suggest that MDSCs are a potential source of stem cell therapy for treatment of various vascular diseases, by inducing angiogenesis to improve perfusion in sites of ischemia, and by preventing intimal hyperplasia in sites of vessel injury.

Tissue engineered vascular grafts--preclinical aspects

Tissue engineering enables the development of fully biological vascular substitutes that restore, maintain and improve tissue function in a manner identical to natural host tissue. However the development of the appropriate preclinical evaluation techniques for the generation of fully functional tissue-engineered vascular graft (TEVG) is required to establish their safety for use in clinical trials and to test clinical effectiveness. This review gives an insight on the various preclinical studies performed in the area of tissue engineered vascular grafts highlighting the different strategies used with respect to cells and scaffolds, typical animal models used and the major in vivo evaluation studies that have been carried out. The review emphasizes the combined effort of engineers, biologists and clinicians which can take this clinical research to new heights of regenerative therapy.

Stem cell sources for vascular tissue engineering and regeneration

This review focuses on the stem cell sources with the potential to be used in vascular tissue engineering and to promote vascular regeneration. The first clinical studies using tissue-engineered vascular grafts are already under way, supporting the potential of this technology in the treatment of cardiovascular and other diseases. Despite progress in engineering biomaterials with the appropriate mechanical properties and biological cues as well as bioreactors for generating the correct tissue microenvironment, the source of cells that make up the vascular tissues remains a major challenge for tissue engineers and physicians. Mature cells from the tissue of origin may be difficult to obtain and suffer from limited proliferative capacity, which may further decline as a function of donor age. On the other hand, multipotent and pluripotent stem cells have great potential to provide large numbers of autologous cells with a great differentiation capacity. Here, we discuss the adult multipotent as well as embryonic and induced pluripotent stem cells, their differentiation potential toward vascular lineages, and their use in engineering functional and implantable vascular tissues. We also discuss the associated challenges that need to be addressed in order to facilitate the transition of this technology from the bench to the bedside.

Immediate production of a tubular dense collagen construct with bioinspired mechanical properties

The intrinsic complexity of tissues and organs demands tissue engineering approaches that extend beyond planar constructs currently in clinical use. However, the engineering of cylindrical or tubular tissue constructs with a hollow lumen presents significant challenges arising from geometrical and architectural considerations required to tailor biomaterials for tissue and organ repair. Type I collagen is an ideal scaffolding material due to its outstanding biocompatibility and high processability. However, the highly hydrated nature of collagen hydrogels results in their lack of mechanical properties and instability, as well as extensive cell-mediated contraction, which must be overcome to achieve process control. Herein, tubular dense collagen constructs (TDCCs) were produced simply and rapidly (in less than 1h) by circumferentially wrapping plastically compressed dense collagen gel sheets around a cylindrical support. The effects of collagen source, i.e. rat-tail tendon and bovine dermis-derived acid solubilized collagen, and concentration on TDCC properties were investigated through morphological, mechanical and chemical characterizations. Both tensile strength and apparent modulus correlated strongly with physiologically relevant collagen gel fibrillar densities. The clinical potential of TDCC as a tubular tissue substitute was demonstrated mechanically, through circumferential tensile properties, theoretical burst pressure, which ranged from 1225 to 1574 mm Hg, compliance values of between 8.3% to 14.2% per 100mm Hg and suture retention strength in the range of 116-151 grams-force, which were compatible with surgical procedures. Moreover, NIH/3T3 fibroblast viability and uniform distribution within the construct wall were confirmed up to day 7 in culture. TDCCs with fibrillar densities equivalent to native tissues can be readily engineered in various dimensions with tunable morphological and mechanical properties, which can be easily handled for use as tissue models and adapted to clinical needs.

In vivo biofunctionality comparison of different topographic PLLA scaffolds

In this work, the in vivo biodegradation of, biocompatibility of, and host response to various topographic scaffolds were investigated. Randomly oriented fibrous poly(L-lactide) (PLLA) nanofibers were fabricated using the electrospinning technique. A PLLA scaffold was obtained by salt leaching. Both the electrospun PLLA nanofibers and the salt-leaching PLLA scaffolds formed three-dimensional pore structures. Cytotoxicity studies, in which rat muscle-derived stem cells (rMDSCs) were grown on electrospun PLLA nanofibers or the salt-leaching PLLA scaffolds, revealed that the rMDSCs cell count on the PLLA nanofibers was slightly higher than that on the salt-leaching PLLA scaffolds. An in vivo study was carried out by implanting the scaffolds subcutaneously into rats to test the biodegradation, biocompatibility, and host response at regular intervals over 0-4 weeks. The degradation of the PLLA nanofibers 1, 2, and 4 weeks after initial implantation was more extensive than that observed for the salt-leaching PLLA scaffolds. PLLA nanofibers seeded the growth of larger fibrous tissue masses due to in vivo cellular infiltration into the randomly oriented fibrillar structures of the PLLA nanofibers. In addition, the inflammatory cell accumulation in PLLA nanofibers was lower than that in the salt-leaching PLLA scaffolds. These results indicate that the electrospun PLLA nanofibers may serve as a good scaffold to elicit fibrous cellular infiltration, to minimize host response, and to enhance tissue-scaffold integration.

Evaluation of the use of an induced puripotent stem cell sheet for the construction of tissue-engineered vascular grafts

OBJECTIVE

The development of a living, tissue-engineered vascular graft (TEVG) holds great promise for advancing the field of cardiovascular surgery. However, the ultimate source and time needed to procure these cells remain problematic. Induced puripotent stem (iPS) cells have recently been developed and have the potential for creating a pluripotent cell line from a patient's own somatic cells. In the present study, we evaluated the use of a sheet created from iPS cell-derived vascular cells as a potential source for the construction of TEVG.

METHODS

Male mouse iPS cells were differentiated into embryoid bodies using the hanging-drop method. Cell differentiation was confirmed by a decrease in the proportion of SSEA-1-positive cells over time using fluorescence-activated cell sorting. The expression of endothelial cell and smooth muscle cell markers was detected using real-time polymerase chain reaction (PCR). The differentiated iPS cell sheet was made using temperature-responsive dishes and then seeded onto a biodegradable scaffold composed of polyglycolic acid-poly-l-lactide and poly(l-lactide-co-ε-caprolactone) with a diameter of 0.8 mm. These scaffolds were implanted as interposition grafts in the inferior vena cava of female severe combined immunodeficiency/beige mice (n = 15). Graft function was serially monitored using ultrasonography. The grafts were analyzed at 1, 4, and 10 weeks with histologic examination and immunohistochemistry. The behavior of seeded differentiated iPS cells was tracked using Y-chromosome fluorescent in situ hybridization and SRY real-time PCR.

RESULTS

All mice survived without thrombosis, aneurysm formation, graft rupture, or calcification. PCR evaluation of iPS cell sheets in vitro demonstrated increased expression of endothelial cell markers. Histologic evaluation of the grafts demonstrated endothelialization with von Willebrand factor and an inner layer with smooth muscle actin- and calponin-positive cells at 10 weeks. The number of seeded differentiated iPS cells was found to decrease over time using real-time PCR (42.2% at 1 week, 10.4% at 4 weeks, 9.8% at 10 weeks). A fraction of the iPS cells were found to be Y-chromosome fluorescent positive at 1 week. No iPS cells were found to co-localize with von Willebrand factor or smooth muscle actin-positive cells at 10 weeks.

CONCLUSIONS

Differentiated iPS cells offer an alternative cell source for constructing TEVG. Seeded iPS cells exerted a paracrine effect to induce neotissue formation in the acute phase and were reduced in number by apoptosis at later time points. Sheet seeding of our TEVG represents a viable mode of iPS cell delivery over time.