The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Manufacturing and validation of small-diameter vascular grafts: A mini review

The field of small-diameter vascular grafts remains a challenge for biomaterials scientists. While decades of research have brought us much closer to developing biomimetic materials for regenerating tissues and organs, the physiological challenges involved in manufacturing small conduits that can transport blood while not inducing an immune response or promoting blood clots continue to limit progress in this area. In this short review, we present some of the most recent methods and advancements made by researchers working in the field of small-diameter vascular grafts. We also discuss some of the most critical aspects biomaterials scientists should consider when developing lab-made small-diameter vascular grafts.

Strengthened Decellularized Porcine Valves via Polyvinyl Alcohol as a Template Improving Processability

The heart valve is crucial for the human body, which directly affects the efficiency of blood transport and the normal functioning of all organs. Generally, decellularization is one method of tissue-engineered heart valve (TEHV), which can deteriorate the mechanical properties and eliminate allograft immunogenicity. In this study, removable polyvinyl alcohol (PVA) is used to encapsulate decellularized porcine heart valves (DHVs) as a dynamic template to improve the processability of DHVs, such as suturing. Mechanical tests show that the strength and elastic modulus of DHVs treated with different concentrations of PVA significantly improve. Without the PVA layer, the valve would shift during suture puncture and not achieve the desired suture result. The in vitro results indicate that decellularized valves treated with PVA can sustain the adhesion and growth of human umbilical vein endothelial cells (HUVECs). All results above show that the DHVs treated with water-soluble PVA have good mechanical properties and cytocompatibility to ensure post-treatment. On this basis, the improved processability of DHV treated with PVA enables a new paradigm for the manufacturing of scaffolds, making it easy to apply.

Prevention by the CXCR2 antagonist SCH527123 of the calcification of porcine heart valve cusps implanted subcutaneously in rats

Introduction

Calcification is a main cause of bioprosthetic heart valves failure. It may be promoted by the inflammation developed in the glutaraldehyde (GA)-fixed cusps of the bioprosthesis. We tested the hypothesis that antagonizing the C-X-C chemokines receptor 2 (CXCR2) may prevent the calcification of GA-fixed porcine aortic valves.

Materiel and methods

Four-week-old Sprague Dawley males were transplanted with 2 aortic valve cusps isolated from independent pigs and implanted into the dorsal wall. Four groups of 6 rats were compared: rats transplanted with GA-free or GA-fixed cusps and rats transplanted with GA-fixed cusps and treated with 1 mg/kg/day SCH5217123 (a CXCR2 antagonist) intraperitoneally (IP) or subcutaneously (SC) around the xenograft, for 14 days. Then, rats underwent blood count before xenografts have been explanted for histology and biochemistry analyses.

Results

A strong calcification of the xenografts was induced by GA pre-incubation. However, we observed a significant decrease in this effect in rats treated with SCH527123 IP or SC. Implantation of GA-fixed cusps was associated with a significant increase in the white blood cell count, an effect that was significantly prevented by SCH527123. In addition, the expression of the CD3, CD68 and CXCR2 markers was reduced in the GA-fixed cusps explanted from rats treated with SCH527123 as compared to those explanted from non-treated rats.

Conclusion

The calcification of GA-fixed porcine aortic valve cusps implanted subcutaneously in rats was significantly prevented by antagonizing CXCR2 with SCH527123. This effect may partly result from an inhibition of the GA-induced infiltration of T-cells and macrophages into the xenograft.

Generating robust human valvular interstitial cell cultures: Protocol and considerations

Research in heart valve biology is a growing field that has yet to elucidate the fundamentals of valve disease. Human valvular interstitial cells (hVICs) are the best option for studying the cellular mechanisms behind valvular pathologies. However, there is a wide range of isolation procedures for these cells published in the literature. To what extent various isolation methods, patient pathologies, and seeding densities influence the behaviour of hVICs remains unclear. Here, we present an optimised method of hVIC isolation from diseased human valves donated at the time of surgery. We show that two rounds of 1000 U/mL collagenase digestion for not >2 h results in a phenotypically stable cell culture with a near complete absence of endothelial cell contamination. We also suggest that cells should be seeded at 10,000 cells/cm² for experimentation. We found that patient pathology does not affect the success of the isolation procedure, and that instead, successful cultures are predicted by ensuring >500 mg valve tissue as starting material.

The Sheep as a Large Animal Model for the Investigation and Treatment of Human Disorders

Simple Summary

We review the value of large animal models for improving the translation of biomedical research for human application, focusing primarily on sheep.

Abstract

An essential aim of biomedical research is to translate basic science information obtained from preclinical research using small and large animal models into clinical practice for the benefit of humans. Research on rodent models has enhanced our understanding of complex pathophysiology, thus providing potential translational pathways. However, the success of translating drugs from pre-clinical to clinical therapy has been poor, partly due to the choice of experimental model. The sheep model, in particular, is being increasingly applied to the field of biomedical research and is arguably one of the most influential models of human organ systems. It has provided essential tools and insights into cardiovascular disorder, orthopaedic examination, reproduction, gene therapy, and new insights into neurodegenerative research. Unlike the widely adopted rodent model, the use of the sheep model has an advantage over improving neuroscientific translation, in particular due to its large body size, gyrencephalic brain, long lifespan, more extended gestation period, and similarities in neuroanatomical structures to humans. This review aims to summarise the current status of sheep to model various human diseases and enable researchers to make informed decisions when considering sheep as a human biomedical model.

The loading of C-type natriuretic peptides improved hemocompatibility and vascular regeneration of electrospun poly(ε-caprolactone) grafts

As a result of thrombosis or intimal hyperplasia, synthetic artificial vascular grafts had a low success rate when they were used to replace small-diameter arteries (inner diameter < 6 mm). C-type natriuretic peptides (CNP) have anti-thrombotic effects, and can promote endothelial cell (EC) proliferation and inhibit vascular smooth muscle cell (SMC) over-growth. In this study, poly(ε-caprolactone) (PCL) vascular grafts loaded with CNP (PCL-CNP) were constructed by electrospinning. The PCL-CNP grafts were able to continuously release CNP at least 25 days in vitro. The results of scanning electron microscopy (SEM) and mechanical testing showed that the loading of CNP did not change the microstructure and mechanical properties of the PCL grafts. In vitro blood compatibility analysis displayed that PCL-CNP grafts could inhibit thrombin activity and reduce platelet adhesion and activation. In vitro cell experiments demonstrated that PCL-CNP grafts activated ERK1/2 and Akt signaling in human umbilical vein endothelial cells (HUVECs), as well as increased cyclin D1 expression, enhanced proliferation and migration, and increased vascular endothelial growth factor (VEGF) secretion and nitric oxide (NO) production. The rabbit arteriovenous (AV)-shunt ex vitro indicated that CNP loading significantly improved the antithrombogenicity of PCL grafts. The assessment of vascular grafts in rat abdominal aorta implantation model displayed that PCL-CNP grafts promoted the regeneration of ECs and contractile SMCs, modulated macrophage polarization toward M2 phenotype, and enhanced extracellular matrix remodeling. These findings confirmed for the first time that loading CNP is an effective approach to improve the hemocompatibility and vascular regeneration of synthetic vascular grafts. STATEMENT OF SIGNIFICANCE: : Small-diameter (< 6 mm) vascular grafts (SDVGs) have not been made clinically available due to their prevalence of thrombosis, limited endothelial regeneration and intimal hyperplasia. The incorporation of bioactive molecules into SDVGs serves as an effective solution to improve hemocompatibility and endothelialization. In this study, for the first time, we loaded C-type natriuretic peptides (CNP) into PCL grafts by electrospunning and confirmed the effectiveness of loading CNP on improving the hemocompatibility and vascular regeneration of artificial vascular grafts. Regenerative advantages included enhancement of endothelialization, modulation of macrophage polarization toward M2 phenotypes, and improved contractile smooth muscle cell regeneration. Our investigation brings attention to CNP as a valuable bioactive molecule for modifying cardiovascular biomaterial.

Off-the-Shelf, Heparinized Small Diameter Vascular Graft Limits Acute Thrombogenicity in a Porcine Model

Thrombogenicity poses a challenge to the clinical translation of engineered grafts. Previously, small-diameter vascular grafts (sdVG) composed of fibrin hydrogel microfiber tubes (FMT) with an external poly(ε-caprolactone) (PCL) sheath supported long-term patency in mice. Towards the development of an sdVG with off-the-shelf availability, the FMT's shelf stability, scale-up, and successful conjugation of an antithrombotic drug to the fibrin scaffold are reported here. FMTs maintain mechanical stability and high-water retention after storage for one year in a freezer, in a refrigerator, or at room temperature. Low molecular weight heparin-conjugated fibrin scaffolds enabled local and sustained delivery during two weeks of enzymatic degradation. Upscaled fabrication of sdVGs provides natural biodegradable grafts with size and mechanics suitable for human application. Implantation in a carotid artery interposition porcine model exhibited no rupture with thrombi prevented in all heparinized sdVGs (n=4) over 4-5 weeks. Remodeling of the sdVGs is demonstrated with endothelial cells on the luminal surface and initial formation of the medial layer by 4-5 weeks. However, neointimal hyperplasia at 4-5 weeks led to the stenosis and occlusion of most of the sdVGs, which must be resolved for future long-term in vivo assessments. The off-the-shelf, biodegradable heparinized fibrin sdVG layer limits acute thrombogenicity while mediating extensive neotissue formation as the PCL sheath maintains structural integrity. STATEMENT OF SIGNIFICANCE: : To achieve clinical and commercial utility of small-diameter vascular grafts as arterial conduits, these devices must have off-the-shelf availability for emergency arterial bypass applications and be scaled to a size suitable for human applications. A serious impediment to clinical translation is thrombogenicity. Treatments have focused on long-term systemic drug therapy, which increases the patient's risk of bleeding complications, or coating grafts and stents with anti-coagulants, which minimally improves patient outcomes even when combined with dual anti-platelet therapy. We systematically modified the biomaterial properties to develop anticoagulant embedded, biodegradable grafts that maintain off-the-shelf availability, provide mechanical stability, and prevent clot formation through local drug delivery.

Animal studies for the evaluation of in situ tissue-engineered vascular grafts - a systematic review, evidence map, and meta-analysis

Vascular in situ tissue engineering (TE) is an approach that uses bioresorbable grafts to induce endogenous regeneration of damaged blood vessels. The evaluation of newly developed in situ TE vascular grafts heavily relies on animal experiments. However, no standard for in vivo models or study design has been defined, hampering inter-study comparisons and translational efficiency. To provide input for formulating such standard, the goal of this study was to map all animal experiments for vascular in situ TE using off-the-shelf available, resorbable synthetic vascular grafts. A literature search (PubMed, Embase) yielded 15,896 studies, of which 182 studies met the inclusion criteria (n = 5,101 animals). The reports displayed a wide variety of study designs, animal models, and biomaterials. Meta-analysis on graft patency with subgroup analysis for species, age, sex, implantation site, and follow-up time demonstrated model-specific variations. This study identifies possibilities for improved design and reporting of animal experiments to increase translational value.

Repopulation of decellularised porcine pulmonary valves in the right ventricular outflow tract of sheep: Role of macrophages

The primary objective was to evaluate performance of low concentration SDS decellularised porcine pulmonary roots in the right ventricular outflow tract of juvenile sheep. Secondary objectives were to explore the cellular population of the roots over time. Animals were monitored by echocardiography and roots explanted at 1, 3, 6 (n = 4) and 12 months (n = 8) for gross analysis. Explanted roots were subject to histological, immunohistochemical and quantitative calcium analysis (n = 4 at 1, 3 and 12 months) and determination of material properties (n = 4; 12 months). Cryopreserved ovine pulmonary root allografts (n = 4) implanted for 12 months, and non-implanted cellular ovine roots were analysed for comparative purposes. Decellularised porcine pulmonary roots functioned well and were in very good condition with soft, thin and pliable leaflets. Morphometric analysis showed cellular population by 1 month. However, by 12 months the total number of cells was less than 50% of the total cells in non-implanted native ovine roots. Repopulation of the decellularised porcine tissues with stromal (α-SMA+; vimentin+) and progenitor cells (CD34+; CD271+) appeared to be orchestrated by macrophages (MAC 387+/ CD163low and CD163+/MAC 387-). The calcium content of the decellularised porcine pulmonary root tissues increased over the 12-month period but remained low (except suture points) at 401 ppm (wet weight) or below. The material properties of the decellularised porcine pulmonary root wall were unchanged compared to pre-implantation. There were some changes in the leaflets but importantly, the porcine tissues did not become stiffer. The decellularised porcine pulmonary roots showed good functional performance in vivo and were repopulated with ovine cells of the appropriate phenotype in a process orchestrated by M2 macrophages, highlighting the importance of these cells in the constructive tissue remodelling of cardiac root tissues.

Antiosteoporosis Effect and Possible Mechanisms of the Ingredients of Fructus Psoraleae in Animal Models of Osteoporosis: A Preclinical Systematic Review and Meta-Analysis

Objective

Fructus Psoraleae (FP) and its ingredients (IFP) have a variety of biological activities and are widely used to treat osteoporosis (OP). Herein, we conducted a systematic review to evaluate the efficacy of IFP for an animal model of OP from the current literatures. Potential mechanisms of IFP in the treatment of OP were also summarized.

Materials and Methods

We carried out a search for electronic literature in the PubMed, Chinese National Knowledge Infrastructure, EMBASE, Wanfang, Web of Science, Chinese Biomedical Literature Database, and Cochrane Library, as well as Chinese VIP databases targeting articles published from inception to June 2021. The inclusion criteria were animal studies that assessed the efficacy and safety of IFP for OP, regardless of publication status or language. The exclusion criteria included (1) other types of studies (in vitro studies, case reports, clinical trials, reviews, abstracts, comments, and editorials), (2) combination with other compounds, (3) compared with other traditional Chinese medicine, (4) not osteoporosis or bone loss model, (5) studies with insufficient data, (6) lack of a control group, and (7) duplicate publications. The modified Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Stroke (CAMARADES) 10-item quality checklist was used to evaluate the risk of bias of included studies. We computed the relative risk (RR) and the standard mean difference (SMD) for dichotomous outcomes and continuous outcomes, respectively. When heterogeneity was detected or there was significant statistical heterogeneity (P < 0.05 or I² > 50%), a random-effects model was employed, followed by further subgroup analysis and metaregression estimations to ascertain the origins of heterogeneity. Otherwise, we used a fixed-effects model (P ≥ 0.05 or I² ≤ 50%). The primary outcome measures were bone mineral density (BMD), serum osteocalcin(S-OCN), bone volume over total volume (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), bone maximum load, and elasticity modulus. The secondary outcome measure was the antiosteoporosis mechanisms of IFP. The STATA 12.0 software was used to analyze the data.

Results

Overall, 16 studies focusing on 379 animals were enrolled into the study. The risk of bias score of included studies ranged from 4 to 7 with an average score of 5.25. The present study provided the preliminary preclinical evidence that administration of IFP could significantly increase the S-OCN, BMD, BV/TV, and Tb.N while Tb.Th and Tb.Sp were remarkably decreased by IFP in OP model animals (P < 0.05). Moreover, IFP could significantly improve the bone biomechanical indicator bone maximum load and elasticity modulus (P < 0.05). In terms of the possible mechanisms of treatment of OP, IFP exerts anti-OP effects in animal models probably through osteoprotegerin/receptor activator of the nuclear factor-κB ligand/receptor activator of nuclear factor-κB (OPG/RANKL/RANK), peroxisome proliferator activated receptor γ (PPAR-γ)/Axin2/Wnt, antioxidative stress via forkhead box O3a (FoxO3a)/Axin2/Wnt, phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR), estrogen-like effect, and gamma-aminobutyric acid/gamma-aminobutyric acid receptor (GABA/GABA_BRI) signaling pathway.

Conclusion

Taken together, the findings suggest the possibility of developing IFP as a drug or an ingredient in diet for the clinical treatment of OP. We recommend that rigorous, as well as high-quality, trials involving large sample sizes should be conducted to confirm our findings.

Models of immunogenicity in preclinical assessment of tissue engineered heart valves

Tissue engineered heart valves may one day offer an exciting alternative to traditional valve prostheses. Methods of construction vary, from decellularised animal tissue to synthetic hydrogels, but the goal is the same: the creation of a 'living valve' populated with autologous cells that may persist indefinitely upon implantation. Previous failed attempts in humans have highlighted the difficulty in predicting how a novel heart valve will perform in vivo. A significant hurdle in bringing these prostheses to market is understanding the immune reaction in the short and long term. With respect to innate immunity, the chronic remodelling of a tissue engineered implant by macrophages remains poorly understood. Also unclear are the mechanisms behind unknown antigens and their effect on the adaptive immune system. No silver bullet exists, rather researchers must draw upon a number of in vitro and in vivo models to fully elucidate the effect a host will exert on the graft. This review details the methods by which the immunogenicity of tissue engineered heart valves may be investigated and reveals areas that would benefit from more research. STATEMENT OF SIGNIFICANCE: Both academic and private institutions around the world are committed to the creation of a valve prosthesis that will perform safely upon implantation. To date, however, no truly non-immunogenic valves have emerged. This review highlights the importance of preclinical immunogenicity assessment, and summarizes the available techniques used in vitro and in vivo to elucidate the immune response. To the authors knowledge, this is the first review that details the immune testing regimen specific to a TEHV candidate.

Multispecies Evaluation of a Long-Acting Tenofovir Alafenamide Subdermal Implant for HIV Prophylaxis

New HIV-1 infection rates far outpace the targets set by global health organizations, despite important progress in curbing the progression of the epidemic. Long-acting (LA) formulations delivering antiretroviral (ARV) agents for HIV-1 pre-exposure prophylaxis (PrEP) hold significant promise, potentially facilitating adherence due to reduced dosing frequency compared to oral regimens. We have developed a subdermal implant delivering the potent ARV drug tenofovir alafenamide that could provide protection from HIV-1 infection for 6 months, or longer. Implants from the same lot were investigated in mice and sheep for local safety and pharmacokinetics (PKs). Ours is the first report using these animal models to evaluate subdermal implants for HIV-1 PrEP. The devices appeared safe, and the plasma PKs as well as the drug and metabolite concentrations in dermal tissue adjacent to the implants were studied and contrasted in two models spanning the extremes of the body weight spectrum. Drug and drug metabolite concentrations in dermal tissue are key in assessing local exposure and any toxicity related to the active agent. Based on our analysis, both animal models were shown to hold significant promise in LA product development.

Oral Administration of Quercetin or Its Derivatives Inhibit Bone Loss in Animal Model of Osteoporosis

Objectives. Quercetin (Q) and its derivatives are the major members of the naturally occurring flavonoid family, which possess beneficial effects on disease prevention including osteoporosis. The present study is aimed at further investigating the efficacy of the Q and its derivatives on bone pathology, bone-related parameters under imageology, bone maximum load, and serum bone metabolism indexes in animal model of osteoporosis. Potential mechanisms of Q and its derivatives in the treatment of osteoporosis as well as the existing problems regarding the modeling method and limitations of researches in this area were also summarized. Eight databases were searched from their inception dates to February 2020. Nineteen eligible studies containing 21 comparisons were identified ultimately. The risk of bias and data on outcome measures were analyzed by the CAMARADES 10-item checklist and Rev-Man 5.3 software separately. The results displayed the number of criteria met varied from 3/10 to 7/10 with an average of 5.05. The present study provided the preliminary preclinical evidence that oral administration of Q or its derivatives was capable of improving bone pathology, bone-related parameters under imageology and bone maximum load, increasing serum osteocalcin, alkaline phosphatase, and estradiol, and reducing serum c-terminal cross-linked telopeptide of type I collagen (P < 0.05). No statistical difference was seen in survival rate, index of liver, or kidney function (P > 0.05). Q and its derivatives partially reverse osteopenia probably via antioxidant, anti-inflammatory, promoting osteogenesis, inhibiting osteoclasts, and its estrogen-like effect. The findings reveal the possibility of developing Q or its derivatives as a drug or an ingredient in diet for clinical treatment of osteoporosis.

Preclinical Assessment of Cardiac Valve Substitutes: Current Status and Considerations for Engineered Tissue Heart Valves

Tissue engineered heart valve (TEHV) technology may overcome deficiencies of existing available heart valve substitutes. The pathway by which TEHVs will undergo development and regulatory approval has several challenges. In this communication, we review: (1) the regulatory framework for regulation of medical devices in general and substitute heart valves in particular; (2) the special challenges of preclinical testing using animal models for TEHV, emphasizing the International Standards Organization (ISO) guidelines in document 5840; and (3) considerations that suggest a translational roadmap to move TEHV forward from pre-clinical to clinical studies and clinical implementation.

Experimental Studies for Small Diameter Grafts in an In Vivo Sheep Model-Techniques and Pitfalls

BACKGROUND

 Scientific attempts to create the "ideal" small diameter vascular graft have been compared with the "search of the holy grail." Prosthetic material as expanded polytetrafluoroethylene or Dacron shows acceptable patency rates to large caliber vessels, while small diameter (< 6 mm) prosthetic conduits present unacceptably poor patency rates. Vascular tissue engineering represents a promising option to address this problem.

MATERIAL AND METHODS

 Thirty-two female Texel-sheep aged 6 months to 2 years underwent surgical common carotid artery (CCA) interposition using different tissue-engineered vascular substitutes. Explantation of the grafts was performed 12 (n = 12) and 36 (n = 20) weeks after surgery. Ultrasound was performed on postoperative day 1 and thereafter every 4 weeks to evaluate the graft patency.

RESULTS

 The average length of implanted substitutes was 10.3 ± 2.2 cm. Anesthesia and surgical procedure could be performed without major surgical complications in all cases.The grafts showed a systolic blood flow velocity (BFV) of 28.24 ± 13.5 cm/s, a diastolic BFV of 9.25 ± 4.53 cm/s, and a mean BFV of 17.85 ± 9.25 cm/s. Native vessels did not differ relevantly in hemodynamic measurements (systolic: 29.77 cm/s; diastolic: 7.99 cm/s ±  5.35; mean 15.87 ± 10.75). There was no incidence of neurologic complications or subsequent postoperative occlusion. Perioperative morbidity was low and implantation of conduits was generally well tolerated.

CONCLUSION

 This article aims to give a precise overview of in vivo experiments in sheep for the evaluation of small diameter vascular grafts performing CCA interposition, especially with regard to pitfalls and possible perioperative complications and to discuss advantages and disadvantages of this approach.

Elucidating the role of graft compliance mismatch on intimal hyperplasia using an ex vivo organ culture model

There is a growing clinical need to address high failure rates of small diameter (<6 mm) synthetic vascular grafts. Although there is a strong empirical correlation between low patency rates and low compliance of synthetic grafts, the mechanism by which compliance mismatch leads to intimal hyperplasia is poorly understood. To elucidate this relationship, synthetic vascular grafts were fabricated that varied compliance independent of other graft variables. A computational model was then used to estimate changes in fluid flow and wall shear stress as a function of graft compliance. The effect of compliance on arterial remodeling in an ex vivo organ culture model was then examined to identify early markers of intimal hyperplasia. The computational model prediction of low wall shear stress of low compliance grafts and clinical control correlated well with alterations in arterial smooth muscle cell marker, extracellular matrix, and inflammatory marker staining patterns at the distal anastomoses. Conversely, high compliance grafts displayed minimal changes in fluid flow and arterial remodeling, similar to the sham control. Overall, this work supports the intrinsic link between compliance mismatch and intimal hyperplasia and highlights the utility of this ex vivo organ culture model for rapid screening of small diameter vascular grafts. STATEMENT OF SIGNIFICANCE: We present an ex vivo organ culture model as a means to screen vascular grafts for early markers of intimal hyperplasia, a leading cause of small diameter vascular graft failure. Furthermore, a computational model was used to predict the effect of graft compliance on wall shear stress and then correlate these values to changes in arterial remodeling in the organ culture model. Combined, the ex vivo bioreactor system and computational model provide insight into the mechanistic relationship between graft-arterial compliance mismatch and the onset of intimal hyperplasia.

Three-dimensional microengineered models of human cardiac diseases

In vitro three-dimensional (3D) microengineered tissue models have been the recent focus of pathophysiological studies, particularly in the field of cardiovascular research. These models, as classified by 3D biomimetic tissues within micrometer-scale platforms, enable precise environmental control on the molecular- and cellular-levels to elucidate biological mechanisms of disease progression and enhance efficacy of therapeutic research. Microengineered models also incorporate directed stem cell differentiation and genome modification techniques that warrant derivation of patient-specific and genetically-edited human cardiac cells for precise recapitulation of diseased tissues. Additionally, integration of added functionalities and/or structures into these models serves to enhance the capability to further extract disease-specific phenotypic, genotypic, and electrophysiological information. This review highlights the recent progress in the development of in vitro 3D microengineered models for study of cardiac-related diseases (denoted as CDs). We will primarily provide a brief overview on currently available 2D assays and animal models for studying of CDs. We will further expand our discussion towards currently available 3D microengineered cardiac tissue models and their implementation for study of specific disease conditions.

Endothelialization mechanisms in vascular grafts

Despite the wide variety of tissue-engineered vascular grafts that are currently being developed, autologous vessels, such as the saphenous vein, are still the gold standard grafts for surgical treatment of vascular disease. Recently developed technologies have shown promising results in preclinical studies, but they still do not overcome the issues that native vessels present, and only a few have made the transition into clinical use. The endothelial lining is a key aspect for the success or failure of the grafts, especially on smaller diameter grafts (<5 mm). However, during the design and evaluation of the grafts, the mechanisms for the formation of this layer are not commonly examined. Therefore, a significant amount of established research might not be relevant to the clinical context, due to important differences that exist between the vascular regeneration mechanisms found in animal models and humans. This article reviews current knowledge about endothelialization mechanisms that have been so far identified: in vitro seeding, transanastomotic growth, transmural infiltration, and fallout endothelialization. Emphasis is placed on the models used for study of theses mechanisms and their effects on the development of tissue-engineering vascular conduits.

Sheep-Specific Immunohistochemical Panel for the Evaluation of Regenerative and Inflammatory Processes in Tissue-Engineered Heart Valves

The creation of living heart valve replacements via tissue engineering is actively being pursued by many research groups. Numerous strategies have been described, aimed either at culturing autologous living valves in a bioreactor (in vitro) or inducing endogenous regeneration by the host via resorbable scaffolds (in situ). Whereas a lot of effort is being invested in the optimization of heart valve scaffold parameters and culturing conditions, the pathophysiological in vivo remodeling processes to which tissue-engineered heart valves are subjected upon implantation have been largely under-investigated. This is partly due to the unavailability of suitable immunohistochemical tools specific to sheep, which serves as the gold standard animal model in translational research on heart valve replacements. Therefore, the goal of this study was to comprise and validate a comprehensive sheep-specific panel of antibodies for the immunohistochemical analysis of tissue-engineered heart valve explants. For the selection of our panel we took inspiration from previous histopathological studies describing the morphology, extracellular matrix composition and cellular composition of native human heart valves throughout development and adult stages. Moreover, we included a range of immunological markers, which are particularly relevant to assess the host inflammatory response evoked by the implanted heart valve. The markers specifically identifying extracellular matrix components and cell phenotypes were tested on formalin-fixed paraffin-embedded sections of native sheep aortic valves. Markers for inflammation and apoptosis were tested on ovine spleen and kidney tissues. Taken together, this panel of antibodies could serve as a tool to study the spatiotemporal expression of proteins in remodeling tissue-engineered heart valves after implantation in a sheep model, thereby contributing to our understanding of the in vivo processes which ultimately determine long-term success or failure of tissue-engineered heart valves.

In Vivo Performance of Decellularized Vascular Grafts: A Review Article

Due to poor vessel quality in patients with cardiovascular diseases, there has been an increased demand for small-diameter tissue-engineered blood vessels that can be used as replacement grafts in bypass surgery. Decellularization techniques to minimize cellular inflammation have been applied in tissue engineering research for the development of small-diameter vascular grafts. The biocompatibility of allogenic or xenogenic decellularized matrices has been evaluated in vitro and in vivo. Both short-term and long-term preclinical studies are crucial for evaluation of the in vivo performance of decellularized vascular grafts. This review offers insight into the various preclinical studies that have been performed using decellularized vascular grafts. Different strategies, such as surface-modified, recellularized, or hybrid vascular grafts, used to improve neoendothelialization and vascular wall remodeling, are also highlighted. This review provides information on the current status and the future development of decellularized vascular grafts.

Cardiovascular tissue engineering: From basic science to clinical application

Valvular heart disease is an increasing population health problem and, especially in the elderly, a significant cause of morbidity and mortality. The current treatment options, such as mechanical and bioprosthetic heart valve replacements, have significant restrictions and limitations. Considering the increased life expectancy of our aging population, there is an urgent need for novel heart valve concepts that remain functional throughout life to prevent the need for reoperation. Heart valve tissue engineering aims to overcome these constraints by creating regenerative, self-repairing valve substitutes with life-long durability. In this review, we give an overview of advances in the development of tissue engineered heart valves, and describe the steps required to design and validate a novel valve prosthesis before reaching first-in-men clinical trials. In-silico and in-vitro models are proposed as tools for the assessment of valve design, functionality and compatibility, while in-vivo preclinical models are required to confirm the remodeling and growth potential of the tissue engineered heart valves. An overview of the tissue engineered heart valve studies that have reached clinical translation is also presented. Final remarks highlight the possibilities as well as the obstacles to overcome in translating heart valve prostheses into clinical application.

Ginsenoside Rb1 for Myocardial Ischemia/Reperfusion Injury: Preclinical Evidence and Possible Mechanisms

Ginseng is an important herbal drug that has been used worldwide for many years. Ginsenoside Rb1 (G-Rb1), the major pharmacological extract from ginseng, possesses a variety of biological activities in the cardiovascular systems. Here, we conducted a preclinical systematic review to investigate the efficacy of G-Rb1 for animal models of myocardial ischemia/reperfusion injury and its possible mechanisms. Ten studies involving 211 animals were identified by searching 6 databases from inception to May 2017. The methodological quality was assessed by using the CAMARADES 10-item checklist. All the data were analyzed using RevMan 5.3 software. As a result, the score of study quality ranged from 3 to 7 points. Meta-analyses showed that G-Rb1 can significantly decrease the myocardial infarct size and cardiac enzymes (including lactate dehydrogenase, creatine kinase, and creatine kinase-MB) when compared with control group (P < 0.01). Significant decrease in cardiac troponin T and improvement in the degree of ST-segment depression were reported in one study (P < 0.05). Additionally, the possible mechanisms of G-Rb1 for myocardial infarction are antioxidant, anti-inflammatory, antiapoptosis, promoting angiogenesis and improving the circulation. Thus, G-Rb1 is a potential cardioprotective candidate for further clinical trials of myocardial infarction.

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

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

Compositions Including Synthetic and Natural Blends for Integration and Structural Integrity: Engineered for Different Vascular Graft Applications

Tissue engineering approaches for small-diameter arteries require a scaffold that simultaneously maintains patency by preventing thrombosis and intimal hyperplasia, maintains its structural integrity after grafting, and allows integration. While synthetic and extracellular matrix-derived materials can provide some of these properties individually, developing a scaffold that provides the balanced properties needed for vascular graft survival in the clinic has been particularly challenging. After 30 years of research, there are now several scaffolds currently in clinical trials. However, these products are either being investigated for large-diameter applications or they require pre-seeding of endothelial cells. This progress report identifies important challenges unique to engineering vascular grafts for high pressure arteries less than 4 mm in diameter (e.g., coronary artery), and discusses limitations with the current usage of the term "small-diameter." Next, the composition and processing techniques used for generating tissue engineered vascular grafts (TEVGs) are discussed, with a focus on the benefits of blended materials. Other scaffolds for non-tissue engineering approaches and stents are also briefly mentioned for comparison. Overall, this progress report discusses the importance of defining the most critical challenges for small diameter TEVGs, developing new scaffolds to provide these properties, and determining acceptable benchmarks for scaffold responses in the body.

A Tubular Biomaterial Construct Exhibiting a Negative Poisson's Ratio

Developing functional small-diameter vascular grafts is an important objective in tissue engineering research. In this study, we address the problem of compliance mismatch by designing and developing a 3D tubular construct that has a negative Poisson's ratio νxy (NPR). NPR constructs have the unique ability to expand transversely when pulled axially, thereby resulting in a highly-compliant tubular construct. In this work, we used projection stereolithography to 3D-print a planar NPR sheet composed of photosensitive poly(ethylene) glycol diacrylate biomaterial. We used a step-lithography exposure and a stitch process to scale up the projection printing process, and used the cut-missing rib unit design to develop a centimeter-scale NPR sheet, which was rolled up to form a tubular construct. The constructs had Poisson's ratios of -0.6 ≤ νxy ≤ -0.1. The NPR construct also supports higher cellular adhesion than does the construct that has positive νxy. Our NPR design offers a significant advance in the development of highly-compliant vascular grafts.

Bioabsorbable Bypass Grafts Biofunctionalised with RGD Have Enhanced Biophysical Properties and Endothelialisation Tested In vivo

Small diameter arterial bypass grafts are considered as unmet clinical need since the current grafts have poor patency of 25% within 5 years. We have developed a 3D scaffold manufactured from natural and synthetic biodegradable polymers, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(𝜀-caprolactone) (PCL), respectively. Further to improve the biophysical properties as well as endothelialisation, the grafts were covalently conjugated with arginine-glycine-aspartic acid (RGD) bioactive peptides. The biophysical properties as well as endothelialisation of PHBV/PCL and PCL 2 mm diameter bypass grafts were assessed with and without biofunctionalisation with RGD peptides in vitro and in vivo. Morphology of the grafts was assessed by scanning electron microscopy, whereas physico-mechanical properties were evaluated using a physiological circulating system equipped with a state of art ultrasound vascular wall tracking system. Endothelialisation of the grafts in vitro and in vivo were assessed using a cell viability assay and rat abdominal aorta replacement model, respectively. The biofunctionalisation with RGD bioactive peptides decreased mean fiber diameter and mean pore area in PHBV/PCL grafts; however, this was not the case for PCL grafts. Both PHBV/PCL and PCL grafts with RGD peptides had lower durability compared to those without; these durability values were similar to those of internal mammary artery. Modification of PHBV/PCL and PCL grafts with RGD peptides increased endothelial cell viability in vitro by a factor of eight and enhanced the formation of an endothelial cell monolayer in vivo 1 month postimplantation. In conclusion, PHBV/PCL small-caliber graft can be a suitable 3D scaffold for the development of a tissue engineering arterial bypass graft.

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.

Elastomeric core/shell nanofibrous cardiac patch as a biomimetic support for infarcted porcine myocardium

Heart failure due to Myocardial Infarction (MI) remains the leading cause of death worldwide due to the inability of myocardial tissue to regenerate following infarction. Current therapies could only retard the progression of disease, but fails to bring functional improvement and cardiac regeneration. The present study analyzes the potentials of Poly(glycerol sebacate)/Fibrinogen (PGS/Fib) core/shell fibers as a structural support and initial entrapment of cells in an in vivo porcine model using echocardiography, histology, and immunohistochemistry. The echocardiography results showed the increased ejection fraction (EF) in PGS/Fib/VEGF/Cells compared with MI controls. The percentage increase in the End Diastolic Volume (EDV) dimension from post MI period to 4 weeks follow-up was the least in PGS/Fib/VEGF/Cells groups compared with MI and cell control group proving that the PGS/Fib/VEGF/Cells group restored the left ventricle (LV) function after MI, evident from the improvement in EF and prevention of LV enlargement. Further, immunohistochemistry results demonstrated that most of the transplanted mesenchymal stem cells (MSCs) within the PGS/Fib/VEGF scaffolds expressed cardiac marker proteins troponin and actinin and endothelial cell marker protein CD31 indicating differentiation of human bone marrow MSCs into cardiac cells and endothelial cells. The developed nanofibrous cardiac patch PGS/Fib/VEGF/Cells provides both functional and structural integrity to the infarcted myocardium and also serves as a suitable matrix for the entrapment of MSCs in clinical applications for cardiac tissue engineering.

Suitability of the rat subdermal model for tissue engineering of heart valves

BACKGROUND

Tissue engineering (TE) is a promising approach to overcome problems associated with biological heart valve prosthesis. Currently several animal models are used to advance this method. The rat subdermal model is uncomplicated and widely used, but its suitability for TE has not yet been shown.

MATERIAL AND METHODS

Using the rat subdermal model we implanted two decellularized porcine aortic wall specimens (of which one was endothelialized) and one native porcine aortic wall specimen in 30 Lewis rats, respectively. Endothelial cells (EC) were harvested from the rat jugular veins. After explantation Hematoxylin/Eosin-staining, CD-68-positive cell staining, fibroblast-staining and Von-Willebrand factor staining were performed.

RESULTS

All animals survived without complications. Endothelialization was confirmed to be effective by Giemsa staining. Histological evaluation of specimens in Hematoxylin/Eosin staining showed significant decrease (p<0.05) of inflammatory reaction (confirmed by CD-68-positive cell staining) after decellularization. All specimens showed strongest inflammatory reactions at areas of destroyed extracellular matrix. Fibroblasts could be detected in all specimens, with strongest infiltration in decellularized specimens (p<0.05). Surrounding endothelialized specimens had no monolayer of endothelial cells, but a higher density of blood vessels occurred (p<0.05).

CONCLUSIONS

The subdermal model provides excellent contact of host tissue with implanted specimens leading to rapid cellular infiltration; therefore, we could ascertain reduced inflammatory response to decellularized tissue. Due to the subdermal position, an absence of blood stream and mechanical stress occurs, which influences cellular repopulation; therefore, endothelialization did not lead to an EC monolayer, but rather to increased vascularization. Thus, the model appears ideal for investigating basic biological compatibility, but further questions must be researched using other models.

Predicting in vivo responses to biomaterials via combined in vitro and in silico analysis

The host response to both synthetic and biologically derived biomaterials is a temporally regulated, complex process that involves multiple interacting cell types. This complexity has classically limited the efficacy of in vitro assays for predicting the in vivo outcome, necessitating the use of costly animal models for biomaterial development. The present study addressed these challenges by developing an in vitro assay that characterized the dynamic inflammatory response of human monocyte-derived-macrophages to biomaterials, coupled with quasi-mechanistic analysis in silico analysis: principal component analysis (PCA) and dynamic network analysis (DyNA). Synthetic and extracellular matrix (ECM)-derived materials were evaluated using this method, and were then associated with the in vivo remodeling and macrophage polarization response in a rodent skeletal muscle injury model. PCA and DyNA revealed a distinct in vitro macrophage response to ECM materials that corresponded to constructive remodeling and an increased M2 macrophage presence in vivo. In contrast, PCA and DyNA suggested a response to crosslinked ECM and synthetic materials characteristic of a foreign body reaction and dominant M1 macrophage response. These results suggest that in silico analysis of an in vitro macrophage assay may be useful as a predictor for determining the in vivo host response to implanted biomaterials.

Stem cells accumulate on a decellularized arterial xenograft in vivo

BACKGROUND

Surgical interest in complete arterial revascularization is expanding, but some patients lack suitable conduits for this goal. The field of stem cell biology is rapidly expanding and, together with the concepts of tissue engineering, offers the promise of growing autologous grafts in the laboratory. We aim to develop a model using human arteries as vascular grafts in a murine model and to assess the cellular accumulation on these grafts.

METHODS

Human arterial samples were collected and decellularized using an ionic detergent. These vessel scaffolds were then used as grafts in an in vivo mouse model, and the cellular accumulation on them was examined histologically and by cell culture with assessment of their physiologic properties.

RESULTS

Left internal mammary artery branches were fully decellularized and successfully implanted into a murine model. Grafts were repopulated by cells expressing stem cell markers cluster of differentiation 34 and stage-specific embryonic antigen, and subsequently expressed markers of mature endothelial and smooth muscle cells (cluster of differentiation 31, calponin, and myosin heavy chain). The migratory capacity of the cultured cells was significantly higher than that of mouse smooth muscle cells (p < 0.001).

CONCLUSIONS

We describe the successful use of human arteries in a murine graft model, allowing the study of repopulation. Decellularized grafts are repopulated by cells expressing stem cell markers and subsequently express smooth muscle and endothelial cell markers. This model has the potential to be used for further development of laboratory-grown vascular grafts.

Cross-reactivity of cell-selective CRRETAWAC peptide with human and porcine endothelial cells

We report on the cross-reactivity of the cell adhesive peptide CRRETAWAC between human and porcine endothelial cells (ECs). CRRETAWAC is a phage display derived peptide which has been shown to bind the α5 β1 receptor on human ECs, but does not bind platelets and thus could be incorporated into a coating for cardiovascular biomaterials that resists platelet adhesion and thrombosis, while allowing for endothelialization. To determine the cross-reactivity of the peptide, attachment and growth of human and porcine ECs on CRRETAWAC fluorosurfactant polymer (FSP) coated surfaces was explored. CRRETAWAC FSP was synthesized and characterized by mass spectrometry, NMR, and IR spectroscopy. pEC attachment and growth on CRRETAWAC FSP was similar to the positive controls, human fibronectin and RGD FSP, achieving confluence in 72 h. Initial adhesion on CRRETAWAC FSP was also similar for porcine and human ECs. Blocking with soluble CRRETAWAC peptide reduced adhesion to CRRETAWAC coated surfaces by over 50%, indicating that the pECs specifically bind CRRETAWAC peptide. With this study, we have demonstrated that CRRETAWAC peptide coated surfaces are capable of binding porcine ECs in a specific manner and supporting a confluent layer of pECs. In vitro validation of the porcine model was critical for ensuring the best chance of success for the in vivo testing of CRRETAWAC coated ePTFE vascular grafts.

Heparin loading and pre-endothelialization in enhancing the patency rate of electrospun small-diameter vascular grafts in a canine model

We herein proved that the two commonly used antithrombotic methods, heparin loading and pre-endothelialization could both greatly enhance the patency rate of a small-diameter graft in a canine model. Tubular grafts having an inner diameter of 4 mm were prepared by electrospinning poly(l-lactide-co-ε-caprolactone) (P(LLA-CL)) and heparin through a coaxial electrospinning technique. Seventy-two percent of heparin was found to be released sustainably from the graft within 14 days. To prepare the pre-endothelialized grafts, we seeded endothelial cells isolated from the femoral artery and cultured then dynamically on the lumen until a cell monolayer was formed. Digital subtraction angiography (DSA) and color Doppler flow imaging (CDFI) were used to monitor the patency without sacrificing the animals. Histological analyses revealed that following the direction of blood flow, a cell monolayer was formed at the proximal end of the heparin-loaded grafts, but such a monolayer could be found in the middle or distal region of the grafts. In contrast, the whole luminal surface of the pre-endothelialized graft was covered by a cell monolayer, suggesting the in vivo survival of the preseeded cells. This demonstrated that heparin was a comparatively simple method to achieve good patency, but the pre-endothelialization had better mechanical properties and cellular compatibility.

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.

Calcification resistance for photooxidatively crosslinked acellular bovine jugular vein conduits in right-side heart implantation

This study aimed to investigate the effect of decellularization plus photooxidative crosslinking and ethanol pretreatment on bioprosthetic tissue calcification. Photooxidatively crosslinked acellular (PCA) bovine jugular vein conduits (BJVCs) and their photooxidized controls (n = 5 each) were sterilized in a graded concentration of ethanol solutions for 4 h, and used to reconstruct dog right ventricular outflow tracts. At 1-year implantation, echocardiography showed similar hemodynamic performance, but obvious calcification for the photooxidized BJVC walls. Further histological examination showed intense calcium deposition colocalized with slightly degraded elastic fibers in the photooxidized BJVC walls, with sparsely distributed punctate calcification in the valves and other areas of walls. But PCA BJVCs had apparent degradation of elastic fibers in the walls, with only sparsely distributed punctate calcification in the walls and valves. Content assay demonstrated comparable calcium content for the two groups at preimplantation, whereas less calcium for the PCA group in the walls and similar calcium in the valvular leaflets compared with the photooxidized group at 1-year retrieval. Elastin content assay presented the conduit walls of PCA group had less elastin content at preimplantation, but similar content at 1-year retrieval compared with the photooxidized group. Phospholipid analysis showed phospholipid extraction by ethanol for the PCA group was more efficacious than the photooxidized group. These results indicate that PCA BJVCs resist calcification in right-side heart implantation owing to decellularization, further photooxidative crosslinking, and subsequent phospholipid extraction by ethanol at preimplantation.

Variation in tissue outcome of ovine and human engineered heart valve constructs: relevance for tissue engineering

AIM

Clinical application of tissue engineered heart valves requires precise control of the tissue culture process to predict tissue composition and mechanical properties prior to implantation, and to understand the variation in tissue outcome. To this end we investigated cellular phenotype and tissue properties of ovine (n = 8) and human (n = 7) tissue engineered heart valve constructs to quantify variations in tissue outcome within species, study the differences between species and determine possible indicators of tissue outcome.

MATERIALS & METHODS

Tissue constructs consisted of polyglycolic acid/poly-4-hydroxybutyrate scaffolds, seeded with myofibroblasts obtained from the jugular vein (sheep) or the saphenous vein (from humans undergoing cardiac surgery) and cultured under static conditions. Prior to seeding, protein expression of α-smooth muscle actin, vimentin, nonmuscle myosin heavy chain and heat shock protein 47 were determined to identify differences at an early stage of the tissue engineering process. After 4 weeks of culture, tissue composition and mechanical properties were quantified as indicators of tissue outcome.

RESULTS

After 4 weeks of tissue culture, tissue properties of all ovine constructs were comparable, while there was a larger variation in the properties of the human constructs, especially the elastic modulus and collagen content. In addition, ovine constructs differed in composition from the human constructs. An increased number of α-smooth muscle actin-positive cells before seeding was correlated with the collagen content in the engineered heart valve constructs. Moreover, tissue stiffness increased with increasing collagen content.

CONCLUSION

The results suggest that the culture process of ovine tissues can be controlled, whereas the mechanical properties, and hence functionality, of tissues originating from human material are more difficult to control. On-line evaluation of tissue properties during culture or more early cellular markers to predict the properties of autologous tissues cultured for individual patients are, therefore, of utmost importance for future clinical application of autologous heart valve tissue engineering. As an example, this study shows that α-smooth muscle actin might be an indicator of tissue mechanical properties.

Electrospun PGA/gelatin nanofibrous scaffolds and their potential application in vascular tissue engineering

BACKGROUND AND METHODS

In this study, gelatin was blended with polyglycolic acid (PGA) at different ratios (0, 10, 30, and 50 wt%) and electrospun. The morphology and structure of the scaffolds were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The mechanical properties were also measured by the tensile test. Furthermore, for biocompatibility assessment, human umbilical vein endothelial cells and human umbilical artery smooth muscle cells were cultured on these scaffolds, and cell attachment and viability were evaluated.

RESULTS

PGA with 10 wt% gelatin enhanced the endothelial cells whilst PGA with 30 wt% gelatin increased smooth muscle cell adhesion, penetration, and viability compared with the other scaffold blends. Additionally, with the increase in gelatin content, the mechanical properties of the scaffolds were improved due to interaction between PGA and gelatin, as revealed by Fourier transform infrared spectroscopy and differential scanning calorimetry.

CONCLUSION

Incorporation of gelatin improves the biological and mechanical properties of PGA, making promising scaffolds for vascular tissue engineering.

Use of autologous blood-derived endothelial progenitor cells at point-of-care to protect against implant thrombosis in a large animal model

Titanium (Ti) is commonly utilized in many cardiovascular devices, e.g. as a component of Nitinol stents, intra- and extracorporeal mechanical circulatory assist devices, but is associated with the risk of thromboemboli formation. We propose to solve this problem by lining the Ti blood-contacting surfaces with autologous peripheral blood-derived late outgrowth endothelial progenitor cells (EPCs) after having previously demonstrated that these EPCs adhere to and grow on Ti under physiological shear stresses and functionally adapt to their environment under flow conditions ex vivo. Autologous fluorescently-labeled porcine EPCs were seeded at the point-of-care in the operating room onto Ti tubes for 30 min and implanted into the pro-thrombotic environment of the inferior vena cava of swine (n = 8). After 3 days, Ti tubes were explanted, disassembled, and the blood-contacting surface was imaged. A blinded analysis found all 4 cell-seeded implants to be free of clot, whereas 4 controls without EPCs were either entirely occluded or partially thrombosed. Pre-labeled EPCs had spread and were present on all 4 cell-seeded implants while no endothelial cells were observed on control implants. These results suggest that late outgrowth autologous EPCs represent a promising source of lining Ti implants to reduce thrombosis in vivo.

[Tissue engineering of heart valves]

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

An early study on the mechanisms that allow tissue-engineered vascular grafts to resist intimal hyperplasia

Intimal hyperplasia is one of the prominent failure mechanisms for arteriovenous fistulas and arteriovenous access grafts. Human tissue-engineered vascular grafts (TEVGs) were implanted as arteriovenous grafts in a novel baboon model. Ultrasound was used to monitor flow rates and vascular diameters throughout the study. Intimal hyperplasia in the outflow vein of TEVGs was assessed at the anastomosis and at juxta-anastomotic regions via histological analysis, and was compared to intimal hyperplasia with polytetrafluoroethylene (PTFE) grafts in the baboon model and in literature reports from other animal models. Less venous intimal hyperplasia was observed in histological sections with arteriovenous TEVGs than with arteriovenous PTFE grafts. TEVGs were associated with a mild, noninflammatory intimal hyperplasia. The extent of intimal tissue that formed with TEVG placement correlated with the rate of blood flow through tissue engineered vascular grafts at 2 weeks postimplant. Outflow vein dilatation was observed with increased flow rate. Both mid-graft flow rates and outflow vein diameters reached a plateau by week 4, which suggested that venous remodeling and intimal hyperplasia largely occurred within the first 4 weeks of implant in the baboon model. Given their compliant and noninflammatory nature, TEVGs appear resistant to triggers for venous intimal hyperplasia that are common for PTFE arteriovenous grafts, including (1) abundant proinflammatory macrophage populations that are associated with PTFE grafts and (2) compliance mismatch between PTFE grafts and the outflow vein. Our findings suggest that arteriovenous TEVGs develop only a mild form of venous intimal hyperplasia, which results from the typical hemodynamic changes that are associated with arteriovenous settings.

Readily available tissue-engineered vascular grafts

Autologous or synthetic vascular grafts are used routinely for providing access in hemodialysis or for arterial bypass in patients with cardiovascular disease. However, some patients either lack suitable autologous tissue or cannot receive synthetic grafts. Such patients could benefit from a vascular graft produced by tissue engineering. Here, we engineer vascular grafts using human allogeneic or canine smooth muscle cells grown on a tubular polyglycolic acid scaffold. Cellular material was removed with detergents to render the grafts nonimmunogenic. Mechanical properties of the human vascular grafts were similar to native human blood vessels, and the grafts could withstand long-term storage at 4 °C. Human engineered grafts were tested in a baboon model of arteriovenous access for hemodialysis. Canine grafts were tested in a dog model of peripheral and coronary artery bypass. Grafts demonstrated excellent patency and resisted dilatation, calcification, and intimal hyperplasia. Such tissue-engineered vascular grafts may provide a readily available option for patients without suitable autologous tissue or for those who are not candidates for synthetic grafts.