In Situ Tissue Engineering of Functional Small-Diameter Blood Vessels by Host Circulating Cells Only

Fabrication of small-diameter in situ tissue engineered vascular grafts with core/shell fibrous structure and a one-year evaluation via rat abdominal vessel replacement model

A high vascular patency was realized in the bulk or surface heparinized small-diameter in situ tissue-engineered vascular grafts (TEVGs) via a rabbit carotid artery replacement model in our previous studies. Those surface heparinized TEVGs could reduce the occurrence of aneurysms, but with a low level of the remodeled elastin, whereas those bulk heparinized TEVGs displayed a faster degradation and an increasing occurrence of aneurysms, but with a high level of the regenerated elastin. To combine the advantages of the bulk and surface graft heparinization to boost the remodeling of elastin and defer the occurrence of aneurysms, a coaxial electro-spinning technique was used to fabricate a kind of small-diameter core/shell fibrous structural in situ TEVGs with a faster degradable poly(lactic-co-glycolic acid) (PLGA) as a core layer and a relatively lower degradable poly(ε-caprolactone) (PCL) as a shell layer followed by the surface heparinization. The in vitro mechanical performance and enzymatic degradation tests revealed the resulting PLGA@PCL-Hep in situ TEVGs possessing not only a faster degradation rate, but also the mechanical properties comparable to those of human saphenous veins. After implanted in the rat abdominal aorta for 12 months, the good endothelialization, low inflammation, and no calcification were evidenced. Furthermore, the neointima layer of regenerated new blood vessels was basically constructed with a well-organized arrangement of elastin and collagen proteins. The results showed the great potential of these in situ TEVGs to be used as a novel type of long-term small-diameter vascular grafts.

Transmural macrophage migration into an arterial bioresorbable vascular graft promotes inflammatory-mediated response and collagen deposition for vascular remodeling

Macrophages are the primary cell type orchestrating bioresorbable vascular graft (BVG) remodeling and infiltrate from three sources: the adjacent native vessel, circulating blood, and transmural migration from outer surface of the graft. To elucidate the kinetics of macrophage infiltration into the BVG, we fabricated two different bilayer arterial BVGs consisting of a macroporous sponge layer and a microporous electrospun (ES) layer. The Outer ES graft was designed to reduce transmural cell infiltration from the outer surface and the Inner ES graft was designed to reduce cell infiltration from the circulation. These BVGs were implanted in mice as infrarenal abdominal aorta grafts and extracted at 1, 4, and 8 weeks (n = 5, 10, and 10 per group, respectively) for evaluation. Cell migration into BVGs was higher in the Inner ES graft than in the Outer ES graft. For Inner ES grafts, the majority of macrophage largely expressed a pro-inflammatory M1 phenotype but gradually changed to tissue-remodeling M2 macrophages. In contrast, in Outer ES grafts macrophages primarily maintained an M1 phenotype. The luminal surface endothelialized faster in the Inner ES graft; however, the smooth muscle cell layer was thicker in the Outer ES graft. Collagen fibers were more abundant and matured faster in the Inner ES graft than that in the Outer ES graft. In conclusion, compared to macrophages infiltrating from the circulating blood, transmural macrophages from outside promote the acute inflammatory-mediated response for vascular remodeling and subsequent collagen deposition within BVGs. STATEMENT OF SIGNIFICANCE: To elucidate the kinetics of macrophage infiltration into the bioresorbable vascular graft (BVG), two different bilayer arterial BVGs were implanted in mice as infrarenal abdominal aorta grafts. Cell migration into BVGs was higher in the inner electrospun graft which cells mainly infiltrate from outer surface than in the outer electrospun graft which cells mainly infiltrate from the circulating blood. In the inner electrospun grafts, the majority of macrophages changed from the M1 phenotype to the M2 phenotype, however, outer electrospun grafts maintained the M1 phenotype. Collagen fibers matured faster in the Inner electrospun graft. Compared to macrophages infiltrating from the circulating blood, transmural macrophages from outside promote the acute inflammatory-mediated response for vascular remodeling and subsequent collagen deposition within BVGs.

Off-the-Shelf Synthetic Biodegradable Grafts Transform In Situ into a Living Arteriovenous Fistula in a Large Animal Model

Current vascular access options require frequent interventions. In situ tissue engineering (TE) may overcome these limitations by combining the initial success of synthetic grafts with long-term advantages of autologous vessels by using biodegradable grafts that transform into autologous vascular tissue at the site of implantation. Scaffolds (6 mm-Ø) made of supramolecular polycarbonate-bisurea (PC-BU), with a polycaprolactone (PCL) anti-kinking-coil, are implanted between the carotid artery and jugular vein in goats. A subset is bio-functionalized using bisurea-modified-Stromal cell-derived factor-1α (SDF1α) derived peptides and ePTFE grafts as controls. Grafts are explanted after 1 and 3 months, and evaluated for material degradation, tissue formation, compliance, and patency. At 3 months, the scaffold is resorbed and replaced by vascular neo-tissue, including elastin, contractile markers, and endothelial lining. No dilations, ruptures, or aneurysms are observed and grafts are successfully cannulated at termination. SDF-1α-peptide-biofunctionalization does not influence outcomes. Patency is lower in TE grafts (50%) compared to controls (100% patency), predominantly caused by intimal hyperplasia. Rapid remodeling of a synthetic, biodegradable vascular scaffold into a living, compliant arteriovenous fistula is demonstrated in a large animal model. Despite lower patency compared to ePTFE, transformation into autologous and compliant living tissue with self-healing capacity may have long-term advantages.

Tissue formation and host remodeling of an elastomeric biodegradable scaffold in an ovine pulmonary leaflet replacement model

In pursuit of a suitable scaffold material for cardiac valve tissue engineering applications, an acellular, electrospun, biodegradable polyester carbonate urethane urea (PECUU) scaffold was evaluated as a pulmonary valve leaflet replacement in vivo. In sheep (n = 8), a single pulmonary valve leaflet was replaced with a PECUU leaflet and followed for 1, 6, and 12 weeks. Implanted leaflet function was assessed in vivo by echocardiography. Explanted samples were studied for gross pathology, microscopic changes in the extracellular matrix, host cellular re-population, and immune responses, and for biomechanical properties. PECUU leaflets showed normal leaflet motion at implant, but decreased leaflet motion and dimensions at 6 weeks. The leaflets accumulated α-SMA and CD45 positive cells, with surfaces covered with endothelial cells (CD31+). New collagen formation occurred (Picrosirius Red). Accumulated tissue thickness correlated with the decrease in leaflet motion. The PECUU scaffolds had histologic evidence of scaffold degradation and an accumulation of pro-inflammatory/M1 and anti-inflammatory/M2 macrophages over time in vivo. The extent of inflammatory cell accumulation correlated with tissue formation and polymer degradation but was also associated with leaflet thickening and decreased leaflet motion. Future studies should explore pre-implant seeding of polymer scaffolds, more advanced polymer fabrication methods able to more closely approximate native tissue structure and function, and other techniques to control and balance the degradation of biomaterials and new tissue formation by modulation of the host immune response.

Biofunctionalized Electrospun Vascular Scaffolds for Enhanced Antithrombotic Properties and In Situ Endothelialization

The development of innovative vascular substitutes has become increasingly significant due to the prevalence of vascular diseases. In this study, we designed a biofunctionalized electrospun vascular scaffold by chemically conjugating heparin molecules as an antithrombotic agent with an endothelial cell (EC)-specific antibody to promote in situ endothelialization. To optimize this biofunctionalized electrospun vascular scaffolding system, we examined various parameters, including material compositions, cross-linker concentrations, and cross-linking and conjugation processes. The findings revealed that a higher degree of heparin conjugation onto the vascular scaffold resulted in improved antithrombotic properties, as confirmed by the platelet adhesion test. Additionally, the flow chamber study demonstrated that the EC-specific antibody immobilization enhanced the scaffold's EC-capturing capability compared to a nonconjugated vascular scaffold. The optimized biofunctionalized vascular scaffolds also displayed exceptional mechanical properties, such as suture retention strength and tensile properties. Our research demonstrated that the biofunctionalized vascular scaffolds and the directed immobilization of bioactive molecules could provide the necessary elements for successful acellular vascular tissue engineering applications.

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

Bio-Inspired Fiber Reinforcement for Aortic Valves: Scaffold Production Process and Characterization

The application of tissue-engineered heart valves in the high-pressure circulatory system is still challenging. One possible solution is the development of biohybrid scaffolds with textile reinforcement to achieve improved mechanical properties. In this article, we present a manufacturing process of bio-inspired fiber reinforcement for an aortic valve scaffold. The reinforcement structure consists of polyvinylidene difluoride monofilament fibers that are biomimetically arranged by a novel winding process. The fibers were embedded and fixated into electrospun polycarbonate urethane on a cylindrical collector. The scaffold was characterized by biaxial tensile strength, bending stiffness, burst pressure and hemodynamically in a mock circulation system. The produced fiber-reinforced scaffold showed adequate acute mechanical and hemodynamic properties. The transvalvular pressure gradient was 3.02 ± 0.26 mmHg with an effective orifice area of 2.12 ± 0.22 cm2. The valves sustained aortic conditions, fulfilling the ISO-5840 standards. The fiber-reinforced scaffold failed in a circumferential direction at a stress of 461.64 ± 58.87 N/m and a strain of 49.43 ± 7.53%. These values were above the levels of tested native heart valve tissue. Overall, we demonstrated a novel manufacturing approach to develop a fiber-reinforced biomimetic scaffold for aortic heart valve tissue engineering. The characterization showed that this approach is promising for an in situ valve replacement.

The effect of chronic kidney disease on tissue formation of in situ tissue-engineered vascular grafts

Vascular in situ tissue engineering encompasses a single-step approach with a wide adaptive potential and true off-the-shelf availability for vascular grafts. However, a synchronized balance between breakdown of the scaffold material and neo-tissue formation is essential. Chronic kidney disease (CKD) may influence this balance, lowering the usability of these grafts for vascular access in end-stage CKD patients on dialysis. We aimed to investigate the effects of CKD on in vivo scaffold breakdown and tissue formation in grafts made of electrospun, modular, supramolecular polycarbonate with ureido-pyrimidinone moieties (PC-UPy). We implanted PC-UPy aortic interposition grafts (n = 40) in a rat 5/6th nephrectomy model that mimics systemic conditions in human CKD patients. We studied patency, mechanical stability, extracellular matrix (ECM) components, total cellularity, vascular tissue formation, and vascular calcification in CKD and healthy rats at 2, 4, 8, and 12 weeks post-implantation. Our study shows successful in vivo application of a slow-degrading small-diameter vascular graft that supports adequate in situ vascular tissue formation. Despite systemic inflammation associated with CKD, no influence of CKD on patency (Sham: 95% vs CKD: 100%), mechanical stability, ECM formation (Sirius red⁺, Sham 16.5% vs CKD 25.0%-p:0.83), tissue composition, and immune cell infiltration was found. We did find a limited increase in vascular calcification at 12 weeks (Sham 0.08% vs CKD 0.80%-p:0.02) in grafts implanted in CKD animals. However, this was not associated with increased stiffness in the explants. Our findings suggest that disease-specific graft design may not be necessary for use in CKD patients on dialysis.

Designing Biocompatible Tissue Engineered Heart Valves In Situ: JACC Review Topic of the Week

Valvular heart disease is a globally prevalent cause of morbidity and mortality, with both congenital and acquired clinical presentations. Tissue engineered heart valves (TEHVs) have the potential to radically shift the treatment landscape for valvular disease by functioning as life-long valve replacements that overcome the current limitations of bioprosthetic and mechanical valves. TEHVs are envisioned to meet these goals by functioning as bioinstructive scaffolds that guide the in situ generation of autologous valves capable of growth, repair, and remodeling within the patient. Despite their promise, clinical translation of in situ TEHVs has proven challenging largely because of the unpredictable and patient-specific nature of the TEHV and host interaction following implantation. In light of this challenge, we propose a framework for the development and clinical translation of biocompatible TEHVs, wherein the native valvular environment actively informs the valve's design parameters and sets the benchmarks by which it is functionally evaluated.

Strategies to counteract adverse remodeling of vascular graft: A 3D view of current graft innovations

Vascular grafts are widely used for vascular surgeries, to bypass a diseased artery or function as a vascular access for hemodialysis. Bioengineered or tissue-engineered vascular grafts have long been envisioned to take the place of bioinert synthetic grafts and even vein grafts under certain clinical circumstances. However, host responses to a graft device induce adverse remodeling, to varied degrees depending on the graft property and host's developmental and health conditions. This in turn leads to invention or failure. Herein, we have mapped out the relationship between the design constraints and outcomes for vascular grafts, by analyzing impairment factors involved in the adverse graft remodeling. Strategies to tackle these impairment factors and counteract adverse healing are then summarized by outlining the research landscape of graft innovations in three dimensions-cell technology, scaffold technology and graft translation. Such a comprehensive view of cell and scaffold technological innovations in the translational context may benefit the future advancements in vascular grafts. From this perspective, we conclude the review with recommendations for future design endeavors.

3D Human iPSC Blood Vessel Organoids as a Source of Flow-Adaptive Vascular Cells for Creating a Human-Relevant 3D-Scaffold Based Macrovessel Model

3D-scaffold based in vitro human tissue models accelerate disease studies and screening of pharmaceutics while improving the clinical translation of findings. Here is reported the use of human induced pluripotent stem cell (hiPSC)-derived vascular organoid cells as a new cell source for the creation of an electrospun polycaprolactone-bisurea (PCL-BU) 3D-scaffold-based, perfused human macrovessel model. A separation protocol is developed to obtain monocultures of organoid-derived endothelial cells (ODECs) and mural cells (ODMCs) from hiPSC vascular organoids. Shear stress responses of ODECs versus HUVECs and barrier function (by trans endothelial electrical resistance) are measured. PCL-BU scaffolds are seeded with ODECs and ODMCs, and tissue organization and flow adaptation are evaluated in a perfused bioreactor system. ODECs and ODMCs harvested from vascular organoids can be cryopreserved and expanded without loss of cell purity and proliferative capacity. ODECs are shear stress responsive and establish a functional barrier that self-restores after the thrombin challenge. Static bioreactor culture of ODECs/ODMCs seeded scaffolds results in a biomimetic vascular bi-layer hierarchy, which is preserved under laminar flow similar to scaffolds seeded with primary vascular cells. HiPSC-derived vascular organoids can be used as a source of functional, flow-adaptive vascular cells for the creation of 3D-scaffold based human macrovascular models.

Monocyte Recruitment for Vascular Tissue Regeneration

A strategy to recruit monocytes (MCs) from blood to regenerate vascular tissue from unseeded (cell-free) tissue engineered vascular grafts is presented. When immobilized on the surface of vascular grafts, the fusion protein, H2R5 can capture blood-derived MC under static or flow conditions in a shear stress dependent manner. The bound MC turns into macrophages (Mϕ) expressing both M1 and M2 phenotype specific genes. When H2R5 functionalized acellular-tissue engineered vessels (A-TEVs) are implanted into the mouse aorta, they remain patent and form a continuous endothelium expressing both endothelial cell (EC) and MC specific proteins. Underneath the EC layer, multiple cells layers are formed coexpressing both smooth muscle cell (SMC) and MC specific markers. Lineage tracing analysis using a novel CX3CR1-confetti mouse model demonstrates that fluorescently labeled MC populates the graft lumen by two and four weeks postimplantation, providing direct evidence in support of MC/Mϕ recruitment to the graft lumen. Given their abundance in the blood, circulating MCs may be a great source of cells that contribute directly to the endothelialization and vascular wall formation of acellular vascular grafts under the right chemical and biomechanical cues.

In Vivo Evaluation of PCL Vascular Grafts Implanted in Rat Abdominal Aorta

Electrospun tissue-engineered grafts made of biodegradable materials have become a perspective search field in terms of vascular replacement, and more research is required to describe their in vivo transformation. This study aimed to give a detailed observation of hemodynamic and structural properties of electrospun, monolayered poly-ε-caprolactone (PCL) grafts in an in vivo experiment using a rat aorta replacement model at 10, 30, 60 and 90 implantation days. It was shown using ultrasound diagnostic and X-ray tomography that PCL grafts maintain patency throughout the entire follow-up period, without stenosis or thrombosis. Vascular compliance, assessed by the resistance index (RI), remains at the stable level from the 10th to the 90th day. A histological study using hematoxylin-eosin (H&E), von Kossa and Russell–Movat pentachrome staining demonstrated the dynamics of tissue response to the implant. By the 10th day, an endothelial monolayer was forming on the graft luminal surface, followed by the gradual growth and compaction of the neointima up to the 90th day. The intense inflammatory cellular reaction observed on the 10th day in the thickness of the scaffold was changed by the fibroblast and myofibroblast penetration by the 30th day. The cellularity maximum was reached on the 60th day, but by the 90th day the cellularity significantly (p = 0.02) decreased. From the 60th day, in some samples, the calcium phosphate depositions were revealed at the scaffold-neointima interface. Scanning electron microscopy showed that the scaffolds retained their fibrillar structure up to the 90th day. Thus, we have shown that the advantages of PCL scaffolds are excellent endothelialization and good surgical outcome. The disadvantages include their slow biodegradation, ineffective cellularization, and risks for mineralization and intimal hyperplasia.

Recombinant DTβ4-inspired porous 3D vascular graft enhanced antithrombogenicity and recruited circulating CD93+/CD34+ cells for endothelialization

Matching material degradation with host remodeling, including endothelialization and muscular remodeling, is important to vascular regeneration. We fabricated 3D PGS-PCL vascular grafts, which presented tunable polymer components, porosity, mechanical strength, and degrading rate. Furthermore, highly porous structures enabled 3D patterning of conjugated heparin-binding peptide, dimeric thymosin β4 (DTβ4), which played key roles in antiplatelets, fibrinogenesis inhibition, and recruiting circulating progenitor cells, thereafter contributed to high patency rate, and unprecedentedly acquired carotid arterial regeneration in rabbit model. Through single-cell RNA sequencing analysis and cell tracing studies, a subset of endothelial progenitor cells, myeloid-derived CD93⁺/CD34⁺ cells, was identified as the main contributor to final endothelium regeneration. To conclude, DTβ4-inspired porous 3DVGs present adjustable physical properties, superior anticoagulating, and re-endothelializing potentials, which leads to the regeneration of small-caliber artery, thus offering a promising tool for vessel replacement in clinical applications.

Donor Heterogeneity in the Human Macrophage Response to a Biomaterial under Hyperglycemia in vitro

Macrophages have a commanding role in scaffold-driven in situ tissue regeneration. Depending on their polarization state, macrophages mediate the formation and remodeling of new tissue by secreting growth factors and cytokines. Therefore, successful outcomes of material-driven in situ tissue vascular tissue engineering depends largely on the immuno-regenerative potential of the recipient. A large cohort of patients requiring vascular replacements suffers from systemic multifactorial diseases, like diabetes, which gives rise to a hyperglycemic and aggressive oxidative inflammatory environment that is hypothesized to hamper a well-balanced regenerative process. Here, we aimed to fundamentally explore the effects of hyperglycemia, as one of the hallmarks of diabetes, on the macrophage response to 3D electrospun synthetic biomaterials for in situ tissue engineering, in terms of inflammatory profile and tissue regenerative capacity. To simulate the early phases of the in situ regenerative cascade, we used a bottom-up in vitro approach. Primary human macrophages (n=8 donors) and (myo)fibroblasts in mono- or co-culture were seeded in 2D, as well as in a 3D electrospun resorbable polycaprolactone bisurea (PCL-BU) scaffold and exposed to normoglycemic (5.5 mM glucose), hyperglycemic (25 mM glucose) and osmotic control conditions (5.5 mM glucose, 19.5 mM mannitol). The results showed that macrophage polarization by biochemical stimuli was effective under all glycemic conditions and that the polarization states dictated expression of the receptors SCL2A1 (glucose transporter 1) and CD36 (fatty acid transporter). In 3D, the macrophage response to hyperglycemic conditions was strongly donor-dependent in terms of phenotype, cytokine secretion profile, and metabolic receptor expression. When co-cultured with (myo)fibroblasts, hyperglycemic conditions led to an increased expression of fibrogenic markers (ACTA2, COL1, COL3, IL-1β). Together, these findings show that the hyperglycemic and hyperosmotic conditions may indeed influence the process of macrophage-driven in situ tissue engineering, and that the extent of this is likely to be patient-specific.

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.

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

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

Tissue-engineered vascular grafts and regeneration mechanisms

Cardiovascular diseases (CVDs) are life-threatening diseases with high morbidity and mortality worldwide. Vascular bypass surgery is still the ultimate strategy for CVD treatment. Autografts are the gold standard for graft transplantation, but insufficient sources limit their widespread application. Therefore, alternative tissue engineered vascular grafts (TEVGs) are urgently needed. In this review, we summarize the major strategies for the preparation of vascular grafts, as well as the factors affecting their patency and tissue regeneration. Finally, the underlying mechanisms of vascular regeneration that are mediated by host cells are discussed.

Distinct Effects of Heparin and Interleukin-4 Functionalization on Macrophage Polarization and In Situ Arterial Tissue Regeneration Using Resorbable Supramolecular Vascular Grafts in Rats

Two of the greatest challenges for successful application of small-diameter in situ tissue-engineered vascular grafts are 1) preventing thrombus formation and 2) harnessing the inflammatory response to the graft to guide functional tissue regeneration. This study evaluates the in vivo performance of electrospun resorbable elastomeric vascular grafts, dual-functionalized with anti-thrombogenic heparin (hep) and anti-inflammatory interleukin 4 (IL-4) using a supramolecular approach. The regenerative capacity of IL-4/hep, hep-only, and bare grafts is investigated as interposition graft in the rat abdominal aorta, with follow-up at key timepoints in the healing cascade (1, 3, 7 days, and 3 months). Routine analyses are augmented with Raman microspectroscopy, in order to acquire the local molecular fingerprints of the resorbing scaffold and developing tissue. Thrombosis is found not to be a confounding factor in any of the groups. Hep-only-functionalized grafts resulted in adverse tissue remodeling, with cases of local intimal hyperplasia. This is negated with the addition of IL-4, which promoted M2 macrophage polarization and more mature neotissue formation. This study shows that with bioactive functionalization, the early inflammatory response can be modulated and affect the composition of neotissue. Nevertheless, variability between graft outcomes is observed within each group, warranting further evaluation in light of clinical translation.

Inflammatory and regenerative processes in bioresorbable synthetic pulmonary valves up to two years in sheep-Spatiotemporal insights augmented by Raman microspectroscopy

In situ heart valve tissue engineering is an emerging approach in which resorbable, off-the-shelf available scaffolds are used to induce endogenous heart valve restoration. Such scaffolds are designed to recruit endogenous cells in vivo, which subsequently resorb polymer and produce and remodel new valvular tissue in situ. Recently, preclinical studies using electrospun supramolecular elastomeric valvular grafts have shown that this approach enables in situ regeneration of pulmonary valves with long-term functionality in vivo. However, the evolution and mechanisms of inflammation, polymer absorption and tissue regeneration are largely unknown, and adverse valve remodeling and intra- and inter-valvular variability have been reported. Therefore, the goal of the present study was to gain a mechanistic understanding of the in vivo regenerative processes by combining routine histology and immunohistochemistry, using a comprehensive sheep-specific antibody panel, with Raman microspectroscopy for the spatiotemporal analysis of in situ tissue-engineered pulmonary valves with follow-up to 24 months from a previous preclinical study in sheep. The analyses revealed a strong spatial heterogeneity in the influx of inflammatory cells, graft resorption, and foreign body giant cells. Collagen maturation occurred predominantly between 6 and 12 months after implantation, which was accompanied by a progressive switch to a more quiescent phenotype of infiltrating cells with properties of valvular interstitial cells. Variability among specimens in the extent of tissue remodeling was observed for follow-up times after 6 months. Taken together, these findings advance the understanding of key events and mechanisms in material-driven in situ heart valve tissue engineering. STATEMENT OF SIGNIFICANCE: This study describes for the first time the long-term in vivo inflammatory and regenerative processes that underly in situ heart valve tissue engineering using resorbable synthetic scaffolds. Using a unique combinatorial analysis of immunohistochemistry and Raman microspectroscopy, important spatiotemporal variability in graft resorption and tissue formation was pinpointed in in situ tissue-engineered heart valves, with a follow-up time of up to 24 months in sheep. This variability was correlated to heterogenous regional cellular repopulation, most likely instigated by region-specific differences in surrounding tissue and hemodynamics. The findings of this research contribute to the mechanistic understanding of in situ tissue engineering using resorbable synthetics, which is necessary to enable rational design of improved grafts, and ensure safe and robust clinical translation.

Immuno-regenerative biomaterials for in situ cardiovascular tissue engineering - Do patient characteristics warrant precision engineering?

In situ tissue engineering using bioresorbable material implants - or scaffolds - that harness the patient's immune response while guiding neotissue formation at the site of implantation is emerging as a novel therapy to regenerate human tissues. For the cardiovascular system, the use of such implants, like blood vessels and heart valves, is gradually entering the stage of clinical translation. This opens up the question if and to what extent patient characteristics influence tissue outcomes, necessitating the precision engineering of scaffolds to guide patient-specific neo-tissue formation. Because of the current scarcity of human in vivo data, herein we review and evaluate in vitro and preclinical investigations to predict the potential role of patient-specific parameters like sex, age, ethnicity, hemodynamics, and a multifactorial disease profile, with special emphasis on their contribution to the inflammation-driven processes of in situ tissue engineering. We conclude that patient-specific conditions have a strong impact on key aspects of in situ cardiovascular tissue engineering, including inflammation, hemodynamic conditions, scaffold resorption, and tissue remodeling capacity, suggesting that a tailored approach may be required to engineer immuno-regenerative biomaterials for safe and predictive clinical applicability.

Chemokine-Induced PBMC and Subsequent MSC Migration Toward Decellularized Heart Valve Tissue

PURPOSE

Enhancing the recellularization of a decellularized heart valve in situ may lead to an improved or ideal heart valve replacement. A promising approach is leveraging the immune response for inflammation-mediated recellularization. However, this mechanism has not been previously demonstrated in vitro.

METHODS

This study investigated loading the chemokine MCP-1 into decellularized porcine heart valve tissue and measured the migration of human peripheral blood mononuclear cells (PBMCs) and mesenchymal stem cells (MSCs) toward the chemokine loaded valve tissue.

RESULTS

The results of this study demonstrate that MCP-1-loaded tissues increase PBMC migration compared to non-loaded tissues. Additionally, we demonstrate MCP-1-loaded tissues that have recruited PBMCs lead to increased migration of MSCs compared to decellularized tissue alone.

CONCLUSION

The results of this study provide evidence for the inflammation-mediated recellularization mechanism. Furthermore, the results support the use of such an approach for enhancing the recellularization of a decellularized heart valve.

Sonication-Assisted Method for Decellularization of Human Umbilical Artery for Small-Caliber Vascular Tissue Engineering

Decellularized vascular grafts are useful for the construction of biological small-diameter tissue-engineered vascular grafts (≤6 mm). Traditional chemical decellularization requires a long treatment time, which may damage the structure and alter the mechanical properties. Decellularization using sonication is expected to solve this problem. The aim of this study was to develop an effective decellularization method using ultrasound followed by washing. Different power values of sonication at 40 kHz were tested for 2, 4, and 8 h followed by a washing procedure. The efficacy of sonication of decellularized human umbilical artery (sDHUA) was evaluated via DNA content, histological staining, mechanical properties, and biocompatibility. The sDHUAs were further implanted into rats for up to 90 days and magnetic resonance angiography (MRA) was performed for the implanted grafts. The results demonstrated that treatment of human umbilical artery (HUA) by sonication at ultrasonic power of 204 W for 4 h followed by washing for 24 h in 2% SDS buffer could eliminate more than 90% of cells and retain similar mechanical properties of the HUA. Recellularization was assessed by scanning electron microscopy (SEM), which indicated that sDHUA provided niches for human umbilical vein endothelial cells (HUVECs) to reside, indicating in vitro cytocompatibility. Further implantation tests also indicated the fitness of the sonication-treated HUA as a scaffold for small-caliber tissue engineering vascular grafts.

bFGF and SDF-1α Improve In Vivo Performance of VEGF-Incorporating Small-Diameter Vascular Grafts

Tissue-engineered vascular grafts are widely tested as a promising substitute for both arterial bypass and replacement surgery. We previously demonstrated that incorporation of VEGF into electrospun tubular scaffolds from poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(ε-caprolactone) enhances formation of an endothelial cell monolayer. However, an overdose of VEGF can induce tumor-like vasculature; thereby, other bioactive factors are needed to support VEGF-driven endothelialization and successful recruitment of smooth muscle cells. Utilizing emulsion electrospinning, we fabricated one-layer vascular grafts with either VEGF, bFGF, or SDF-1α, and two-layer vascular grafts with VEGF incorporated into the inner layer and bFGF and SDF-1α incorporated into the outer layer with the following structural evaluation, tensile testing, and in vivo testing using a rat abdominal aorta replacement model. The latter graft prototype showed higher primary patency rate. We found that the two-layer structure improved surface topography and mechanical properties of the grafts. Further, the combination of bFGF, SDF-1α, and VEGF improved endothelialization compared with VEGF alone, while bFGF induced a rapid formation of a smooth muscle cell layer. Taken together, these findings show that the two-layer structure and incorporation of bFGF and SDF-1α into the vascular grafts in combination with VEGF provide a higher primary patency and therefore improved in vivo performance.

Review: Tissue Engineering of Small-Diameter Vascular Grafts and Their In Vivo Evaluation in Large Animals and Humans

To date, a wide range of materials, from synthetic to natural or a mixture of these, has been explored, modified, and examined as small-diameter tissue-engineered vascular grafts (SD-TEVGs) for tissue regeneration either in vitro or in vivo. However, very limited success has been achieved due to mechanical failure, thrombogenicity or intimal hyperplasia, and improvements of the SD-TEVG design are thus required. Here, in vivo studies investigating novel and relative long (10 times of the inner diameter) SD-TEVGs in large animal models and humans are identified and discussed, with emphasis on graft outcome based on model- and graft-related conditions. Only a few types of synthetic polymer-based SD-TEVGs have been evaluated in large-animal models and reflect limited success. However, some polymers, such as polycaprolactone (PCL), show favorable biocompatibility and potential to be further modified and improved in the form of hybrid grafts. Natural polymer- and cell-secreted extracellular matrix (ECM)-based SD-TEVGs tested in large animals still fail due to a weak strength or thrombogenicity. Similarly, native ECM-based SD-TEVGs and in-vitro-developed hybrid SD-TEVGs that contain xenogeneic molecules or matrix seem related to a harmful graft outcome. In contrast, allogeneic native ECM-based SD-TEVGs, in-vitro-developed hybrid SD-TEVGs with allogeneic banked human cells or isolated autologous stem cells, and in-body tissue architecture (IBTA)-based SD-TEVGs seem to be promising for the future, since they are suitable in dimension, mechanical strength, biocompatibility, and availability.

Basement membrane proteins modulate cell migration on bovine pericardium extracellular matrix scaffold

Native bovine pericardium (BP) exhibits anisotropy of its surface ECM niches, with the serous surface (i.e., parietal pericardium) containing basement membrane components (e.g., Laminin, Col IV) and the fibrous surface (i.e., mediastinal side) being composed primarily of type I collagen (Col I). Native BP surface ECM niche anisotropy is preserved in antigen removed BP (AR-BP) extracellular matrix (ECM) scaffolds. By exploiting sideness (serous or fibrous surface) of AR-BP scaffolds, this study aims to determine the mechanism by which ECM niche influences human mesenchymal stem cells (hMSCs) migration. Human mesenchymal stem cells (hMSC) seeding on serous surface promoted more rapid cell migration than fibrous surface seeding. Gene analysis revealed that expression of integrin α3 and α11 were increased in cells cultured on serous surface compared to those on the fibrous side. Monoclonal antibody blockade of α3β1 (i.e., laminin binding) inhibited early (i.e. ≤ 6 h) hMSC migration following serous seeding, while having no effect on migration of cells on the fibrous side. Blockade of α3β1 resulted in decreased expression of integrin α3 by cells on serous surface. Monoclonal antibody blockade of α11β1 (i.e., Col IV binding) inhibited serous side migration at later time points (i.e., 6-24 h). These results confirmed the role of integrin α3β1 binding to laminin in mediating early rapid hMSCs migration and α11β1 binding to Col IV in mediating later hMSCs migration on the serous side of AR-BP, which has critical implications for rate of cellular monolayer formation and use of AR-BP as blood contacting material for clinical applications.

Transcatheter-Delivered Expandable Bioresorbable Polymeric Graft With Stenting Capacity Induces Vascular Regeneration

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We designed a transcatheter balloon-expandable resorbable vascular graft with support capacity.

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After 2 months in vivo, grafts show native-like tissue reconstruction with endoluminal elastin.

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The concept convenes regenerative grafting, minimally invasive delivery, and clinical stenting.

As the next step in the translation of vascular tissue engineering, this study uniquely combines transcatheter delivery and in situ tissue regeneration using a novel bioresorbable electrospun polymer graft that can be implanted minimally invasively. Once delivered inside a small-diameter vessel, the electrospun microstructure supports the vessel wall, facilitates cellular infiltration, and guides organized tissue formation.

A Novel Hybrid Silk Fibroin/Polyurethane Arteriovenous Graft for Hemodialysis: Proof-of-Concept Animal Study in an Ovine Model

To solve the problem of vascular access failure, a novel semi-degradable hybrid vascular graft, manufactured by electrospinning using silk fibroin and polyurethane (Silkothane), has been previously developed and characterized in vitro. This proof-of-concept animal study aims at evaluating the performances of Silkothane grafts in a sheep model of arteriovenous shunt, in terms of patency and short-term remodeling. Nine Silkothane grafts are implanted between the common carotid artery and the external jugular vein of nine sheep, examined by palpation three times per week, by echo-color Doppler every two weeks, and euthanized at 30, 60, and 90 days (N = 3 per group). At sacrifice, grafts are harvested and submitted for histopathology and/or scanning electron microcopy (SEM). No cases of graft-related complications are recorded. Eight of nine sheep (89%) show 100% primary unassisted patency at the respective time of sacrifice (flow rate 1.76 ± 0.61 L min^-1 , one case of surgery-related thrombosis excluded). Histopathology and SEM analysis evidence signs of inflammation and pseudointima inside the graft lumen, especially at the venous anastomosis; however, endoluminal stenosis never impairs the functionality of the shunt and coverage by endothelial cells is observed. In this model, Silkothane grafts grant safety and 100% patency up to 90 days.

Slow degrading poly(glycerol sebacate) derivatives improve vascular graft remodeling in a rat carotid artery interposition model

Porous synthetic grafts made of poly (glycerol sebacate) (PGS) can transform into autologous vascular conduits in vivo upon degradation of PGS. A long-held doctrine in tissue engineering is the necessity to match degradation of the scaffolds to tissue regeneration. Here, we tested the impact of degradation of PGS and its derivative in an interposition model of rat common carotid artery (CCA). Previous work indicates a complete degradation of PGS within approximately 2 weeks, likely at the fast end of the spectrum. Thus, the derivation of PGS focuses on delay degradation by conjugating the free hydroxy groups in PGS with a long chain carboxylic acid: palmitic acid, one of the most common lipid components. We evaluated two of the resultant palmitate-PGS (PPGS) in this study: one containing 9% palmitate (9-PPGS) and the other16% palmitate (16-PPGS). 16-PPGS grafts had the highest patency. Ultrasound imaging showed that the lumens of 16-PPGS grafts were similar to CCA and smaller than 9-PPGS and PGS grafts 12 weeks post-operation. Immunohistological and histological examination showed an endothelialized lumens in all three types of grafts within 4 weeks. Inflammatory responses to 16-PPGS grafts were limited to the adventitial space in contrast to a more diffusive infiltration in 9-PPGS and PGS grafts in week 4. Examination of calponin+ and αSMA+ cells revealed that 16-PPGS grafts remodeled into a distinctive bi-layered wall, while the walls of 9-PPGS grafts and PGS grafts only had one thick layer of smooth muscle-like cells. Correspondingly, the expression of collagen III and elastin displayed an identical layered structure in the remodeled 16-PPGS grafts, in contrast to a more spread distribution in 9-PPGS and PGS grafts. All the three types of grafts exhibited the same collagen content and burst pressure after 12 weeks of host remodeling. However, the compliance and elastin content of 16-PPGS grafts in week 12 were closest to those of CCA. Overall, placing the degradation of PGS derived elastomer to a window of 4-12 weeks results in vascular conduits closer to arteries in a rat carotid artery interposition model over a 12-week observation period.

Imaging the In Vivo Degradation of Tissue Engineering Implants by Use of Supramolecular Radiopaque Biomaterials

For in situ tissue engineering (TE) applications it is important that implant degradation proceeds in concord with neo-tissue formation to avoid graft failure. It will therefore be valuable to have an imaging contrast agent (CA) available that can report on the degrading implant. For this purpose, a biodegradable radiopaque biomaterial is presented, modularly composed of a bisurea chain-extended polycaprolactone (PCL2000-U4U) elastomer and a novel iodinated bisurea-modified CA additive (I-U4U). Supramolecular hydrogen bonding interactions between the components ensure their intimate mixing. Porous implant TE-grafts are prepared by simply electrospinning a solution containing PCL2000-U4U and I-U4U. Rats receive an aortic interposition graft, either composed of only PCL2000-U4U (control) or of PCL2000-U4U and I-U4U (test). The grafts are explanted for analysis at three time points over a 1-month period. Computed tomography imaging of the test group implants prior to explantation shows a decrease in iodide volume and density over time. Explant analysis also indicates scaffold degradation. (Immuno)histochemistry shows comparable cellular contents and a similar neo-tissue formation process for test and control group, demonstrating that the CA does not have apparent adverse effects. A supramolecular approach to create solid radiopaque biomaterials can therefore be used to noninvasively monitor the biodegradation of synthetic implants.

In situ Enabling Approaches for Tissue Regeneration: Current Challenges and New Developments

In situ tissue regeneration can be defined as the implantation of tissue-specific biomaterials (by itself or in combination with cells and/or biomolecules) at the tissue defect, taking advantage of the surrounding microenvironment as a natural bioreactor. Up to now, the structures used were based on particles or gels. However, with the technological progress, the materials' manipulation and processing has become possible, mimicking the damaged tissue directly at the defect site. This paper presents a comprehensive review of current and advanced in situ strategies for tissue regeneration. Recent advances to put in practice the in situ regeneration concept have been mainly focused on bioinks and bioprinting techniques rather than the combination of different technologies to make the real in situ regeneration. The limitation of conventional approaches (e.g., stem cell recruitment) and their poor ability to mimic native tissue are discussed. Moreover, the way of advanced strategies such as 3D/4D bioprinting and hybrid approaches may contribute to overcome the limitations of conventional strategies are highlighted. Finally, the future trends and main research challenges of in situ enabling approaches are discussed considering in vitro and in vivo evidence.

In situ Enabling Approaches for Tissue Regeneration: Current Challenges and New Developments

In situ tissue regeneration can be defined as the implantation of tissue-specific biomaterials (by itself or in combination with cells and/or biomolecules) at the tissue defect, taking advantage of the surrounding microenvironment as a natural bioreactor. Up to now, the structures used were based on particles or gels. However, with the technological progress, the materials' manipulation and processing has become possible, mimicking the damaged tissue directly at the defect site. This paper presents a comprehensive review of current and advanced in situ strategies for tissue regeneration. Recent advances to put in practice the in situ regeneration concept have been mainly focused on bioinks and bioprinting techniques rather than the combination of different technologies to make the real in situ regeneration. The limitation of conventional approaches (e.g., stem cell recruitment) and their poor ability to mimic native tissue are discussed. Moreover, the way of advanced strategies such as 3D/4D bioprinting and hybrid approaches may contribute to overcome the limitations of conventional strategies are highlighted. Finally, the future trends and main research challenges of in situ enabling approaches are discussed considering in vitro and in vivo evidence.

Imatinib attenuates neotissue formation during vascular remodeling in an arterial bioresorbable vascular graft

Background

Bioresorbable vascular grafts (BVGs) can transform biologically into active blood vessels and represent an alternative to traditional synthetic conduits, which are prone to complications such as infection and thrombosis. Although platelet-derived growth factors and c-Kit positive cells play an important role in smooth muscle cell (SMC) migration and proliferation in vascular injury, atherosclerosis, or allograft, their roles in the vascular remodeling process of an arterial BVG remains unknown. Thus, we assessed the neottisue formation on arterial BVG remodeling by administrating imatinib, which is both a platelet-derived growth factor receptor kinase inhibitor and c-Kit receptor kinase inhibitor, in a murine model.

Methods

BVGs were composed of an inner poly(_L-lactic-co-ε-caprolactone) copolymer sponge layer and an outer electrospun poly(_L-lactic acid) nanofiber layer, which were implanted into the infrarenal abdominal aortas of C57BL/6 mice. After graft implantation, saline or 100 mg/kg of imatinib was administrated intraperitoneally daily for 2 weeks (n = 20 per group). Five mice in each group were scheduled to be humanely killed at 3 weeks and 15 at 8 weeks, and BVGs were explanted for histologic assessments.

Results

Graft patency during the 8-week observational period was not significantly different between groups (control, 86.7% vs imatinib, 80.0%; P > .999). Neotissue formation consisting of endothelialization, smooth muscle proliferation, and deposition of collagen and elastin was not observed in either group at 3 weeks. Similar endothelialization was achieved in both groups at 8 weeks, but thickness and percent area of neotissue formation were significantly higher in the control group than in the imatinib group, (thickness, 30.1 ± 7.2 μm vs 19.6 ± 4.5 μm [P = .001]; percent area, 9.8 ± 2.7% vs 6.8 ± 1.8% [P = .005]). Furthermore, SMC layer and deposition of collagen and elastin were better organized at 8 weeks in the control group compared with the imatinib group. The thickness of SMC layer and collagen fiber area were significantly greater at 8 weeks in the control group than in the imatinib group (P < .001 and P = .026, respectively). Because there was no difference in the inner diameter of explanted BVGs (831.7 ± 63.4 μm vs 841.8 ± 41.9 μm; P = .689), neotissue formation was thought to advance toward the outer portion of the BVG with degradation of the polymer scaffold.

Conclusions

Imatinib attenuates neotissue formation during vascular remodeling in arterial bioresorbable vascular grafts (BVGs) by inhibiting SMC layer formation and extracellular matrix deposition.

This study demonstrated that imatinib attenuated neotissue formation during vascular remodeling in arterial Bioresorbable vascular graft (BVG) by inhibiting smooth muscle cell formation and extracellular matrix deposition. In addition, as imatinib did not modify the inner diameter of BVG, neotissue advanced circumferentially toward the outer portion of the neovessel. Currently, BVGs have not yet been clinically applied to the arterial circulation. The results of this study are helpful for the design of BVG that can achieve an optimal balance between polymer degradation and neotissue formation.

Differential Leaflet Remodeling of Bone Marrow Cell Pre-Seeded Versus Nonseeded Bioresorbable Transcatheter Pulmonary Valve Replacements

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Bone marrow mononuclear cell pre-seeding of polycarbonate bisurea–based tissue-engineered heart valves has detrimental effects on long-term performance and in situ remodeling and, therefore, should be avoided.

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Leaflet-specific analysis revealed pronounced remodeling differences with regard to cell infiltration, scaffold resorption, and extracellular matrix deposition within the same valve explant.

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The heterogeneity in remodeling of polycarbonate bisurea–based tissue-engineered heart valves may have important safety implications in terms of clinical translation.

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An in-depth understanding of the mechanobiological mechanisms involved in the in situ remodeling is required to limit the risk of unpredictable (maladaptive) remodeling.

This study showed that bone marrow mononuclear cell pre-seeding had detrimental effects on functionality and in situ remodeling of bioresorbable bisurea-modified polycarbonate (PC-BU)-based tissue-engineered heart valves (TEHVs) used as transcatheter pulmonary valve replacement in sheep. We also showed heterogeneous valve and leaflet remodeling, which affects PC-BU TEHV safety, challenging their potential for clinical translation. We suggest that bone marrow mononuclear cell pre-seeding should not be used in combination with PC-BU TEHVs. A better understanding of cell–scaffold interaction and in situ remodeling processes is needed to improve transcatheter valve design and polymer absorption rates for a safe and clinically relevant translation of this approach.

Biomaterials for In Situ Tissue Regeneration: A Review

This review focuses on a somewhat unexplored strand of regenerative medicine, that is in situ tissue engineering. In this approach manufactured scaffolds are implanted in the injured region for regeneration within the patient. The scaffold is designed to attract cells to the required volume of regeneration to subsequently proliferate, differentiate, and as a consequence develop tissue within the scaffold which in time will degrade leaving just the regenerated tissue. This review highlights the wealth of information available from studies of ex-situ tissue engineering about the selection of materials for scaffolds. It is clear that there are great opportunities for the use of additive manufacturing to prepare complex personalized scaffolds and we speculate that by building on this knowledge and technology, the development of in situ tissue engineering could rapidly increase. Ex-situ tissue engineering is handicapped by the need to develop the tissue in a bioreactor where the conditions, however optimized, may not be optimum for accelerated growth and maintenance of the cell function. We identify that in both methodologies the prospect of tissue regeneration has created much promise but delivered little outside the scope of laboratory-based experiments. We propose that the design of the scaffolds and the materials selected remain at the heart of developments in this field and there is a clear need for predictive modelling which can be used in the design and optimization of materials and scaffolds.

Biomaterials for In Situ Tissue Regeneration: A Review

This review focuses on a somewhat unexplored strand of regenerative medicine, that is in situ tissue engineering. In this approach manufactured scaffolds are implanted in the injured region for regeneration within the patient. The scaffold is designed to attract cells to the required volume of regeneration to subsequently proliferate, differentiate, and as a consequence develop tissue within the scaffold which in time will degrade leaving just the regenerated tissue. This review highlights the wealth of information available from studies of ex-situ tissue engineering about the selection of materials for scaffolds. It is clear that there are great opportunities for the use of additive manufacturing to prepare complex personalized scaffolds and we speculate that by building on this knowledge and technology, the development of in situ tissue engineering could rapidly increase. Ex-situ tissue engineering is handicapped by the need to develop the tissue in a bioreactor where the conditions, however optimized, may not be optimum for accelerated growth and maintenance of the cell function. We identify that in both methodologies the prospect of tissue regeneration has created much promise but delivered little outside the scope of laboratory-based experiments. We propose that the design of the scaffolds and the materials selected remain at the heart of developments in this field and there is a clear need for predictive modelling which can be used in the design and optimization of materials and scaffolds.

Small Caliber Compliant Vascular Grafts Based on Elastin-Like Recombinamers for in situ Tissue Engineering

Vascular disease is a leading cause of death worldwide, but surgical options are restricted by the limited availability of autologous vessels, and the suboptimal performance of prosthetic vascular grafts. This is especially evident for coronary artery by-pass grafts, whose small caliber is associated with a high occlusion propensity. Despite the potential of tissue-engineered grafts, compliance mismatch, dilatation, thrombus formation, and the lack of functional elastin are still major limitations leading to graft failure. This calls for advanced materials and fabrication schemes to achieve improved control on the grafts' properties and performance. Here, bioinspired materials and technical textile components are combined to create biohybrid cell-free implants for endogenous tissue regeneration. Clickable elastin-like recombinamers are processed to form an open macroporous 3D architecture to favor cell ingrowth, while being endowed with the non-thrombogenicity and the elastic behavior of the native elastin. The textile components (i.e., warp-knitted and electrospun meshes) are designed to confer suture retention, long-term structural stability, burst strength, and compliance. Notably, by controlling the electrospun layer's thickness, the compliance can be modulated over a wide range of values encompassing those of native vessels. The grafts support cell ingrowth, extracellular matrix deposition and endothelium development in vitro. Overall, the fabrication strategy results in promising off-the-shelf hemocompatible vascular implants for in situ tissue engineering by addressing the known limitations of bioartificial vessel substitutes.

Bovine pericardial extracellular matrix niche modulates human aortic endothelial cell phenotype and function

Xenogeneic biomaterials contain biologically relevant extracellular matrix (ECM) composition and organization, making them potentially ideal surgical grafts and tissue engineering scaffolds. Defining the effect of ECM niche (e.g., basement membrane vs. non-basement membrane) on repopulating cell phenotype and function has important implications for use of xenogeneic biomaterials, particularly in vascular applications. We aim to understand how serous (i.e., basement membrane) versus fibrous (i.e., non-basement membrane) ECM niche of antigen-removed bovine pericardium (AR-BP) scaffolds influence human aortic endothelial cell (hAEC) adhesion, growth, phenotype, inflammatory response and laminin production. At low and moderate seeding densities hAEC proliferation was significantly increased on the serous side. Similarly, ECM niche modulated cellular morphology, with serous side seeding resulting in a more rounded aspect ratio and intact endothelial layer formation. At moderate seeding densities, hAEC production of human laminin was enhanced following serous seeding. Finally, inflammatory marker and pro-inflammatory cytokine expression decreased following long-term cell growth regardless of seeding side. This work demonstrates that at low and moderate seeding densities AR-BP sidedness significantly impacts endothelial cell growth, morphology, human laminin production, and inflammatory state. These findings suggest that ECM niche has a role in modulating response of repopulating recipient cells toward AR-BP scaffolds for vascular applications.

Initial scaffold thickness affects the emergence of a geometrical and mechanical equilibrium in engineered cardiovascular tissues

In situ cardiovascular tissue-engineering can potentially address the shortcomings of the current replacement therapies, in particular, their inability to grow and remodel. In native tissues, it is widely accepted that physiological growth and remodelling occur to maintain a homeostatic mechanical state to conserve its function, regardless of changes in the mechanical environment. A similar homeostatic state should be reached for tissue-engineered (TE) prostheses to ensure proper functioning. For in situ tissue-engineering approaches obtaining such a state greatly relies on the initial scaffold design parameters. In this study, it is investigated if the simple scaffold design parameter initial thickness, influences the emergence of a mechanical and geometrical equilibrium state in in vitro TE constructs, which resemble thin cardiovascular tissues such as heart valves and arteries. Towards this end, two sample groups with different initial thicknesses of myofibroblast-seeded polycaprolactone-bisurea constructs were cultured for three weeks under dynamic loading conditions, while tracking geometrical and mechanical changes temporally using non-destructive ultrasound imaging. A mechanical equilibrium was reached in both groups, although at different magnitudes of the investigated mechanical quantities. Interestingly, a geometrically stable state was only established in the thicker constructs, while the thinner constructs' length continuously increased. This demonstrates that reaching geometrical and mechanical stability in TE constructs is highly dependent on functional scaffold design.

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.

Electrospun fibroin/polyurethane hybrid meshes: Manufacturing, characterization, and potentialities as substrates for haemodialysis arteriovenous grafts

Several attempts made so far to combine silk fibroin and polyurethane, in order to prepare scaffolds encompassing the bioactivity of the former with the elasticity of the latter, suffer from critical drawbacks concerning industrial and clinical applicability (e.g., separation of phases upon processing, use of solvents unaddressed by the European Pharmacopoeia, and use of degradable polyurethanes). Overcoming these limitations, in this study, we report the successful blending of regenerated silk fibroin with a medical-grade, non-degradable polyurethane using formic acid and dichloromethane, and the manufacturing of hybrid, semi-degradable electrospun tubular meshes with different ratios of the two materials. Physicochemical analyses demonstrated the maintenance of the characteristic features of fibroin and polyurethane upon solubilization, blending, electrospinning, and postprocessing with ethanol or methanol. Envisioning their possible application as semidegradable substrates for haemodialysis arteriovenous grafts, tubular meshes were further characterized, showing submicrometric fibrous morphologies, tunable mechanical properties, permeability before and after puncture in the same order of magnitude as commercial grafts currently used in the clinics. Results demonstrate the potential of this material for the development of hybrid, new-generation vascular grafts with disruptive potential in the field of in situ tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 807-817, 2019.

Host Response and Neo-Tissue Development during Resorption of a Fast Degrading Supramolecular Electrospun Arterial Scaffold

In situ vascular tissue engineering aims to regenerate vessels "at the target site" using synthetic scaffolds that are capable of inducing endogenous regeneration. Critical to the success of this approach is a fine balance between functional neo-tissue formation and scaffold degradation. Circulating immune cells are important regulators of this process as they drive the host response to the scaffold and they play a central role in scaffold resorption. Despite the progress made with synthetic scaffolds, little is known about the host response and neo-tissue development during and after scaffold resorption. In this study, we designed a fast-degrading biodegradable supramolecular scaffold for arterial applications and evaluated this development in vivo. Bisurea-modified polycaprolactone (PCL2000-U4U) was electrospun in tubular scaffolds and shielded by non-degradable expanded polytetrafluoroethylene in order to restrict transmural and transanastomotic cell ingrowth. In addition, this shield prevented graft failure, permitting the study of neo-tissue and host response development after degradation. Scaffolds were implanted in 60 healthy male Lewis rats as an interposition graft into the abdominal aorta and explanted at different time points up to 56 days after implantation to monitor sequential cell infiltration, differentiation, and tissue formation in the scaffold. Endogenous tissue formation started with an acute immune response, followed by a dominant presence of pro-inflammatory macrophages during the first 28 days. Next, a shift towards tissue-producing cells was observed, with a striking increase in α-Smooth Muscle Actin-positive cells and extracellular matrix by day 56. At that time, the scaffold was resorbed and immune markers were low. These results suggest that neo-tissue formation was still in progress, while the host response became quiescent, favoring a regenerative tissue outcome. Future studies should confirm long-term tissue homeostasis, but require the strengthening of the supramolecular scaffold if a non-shielded model will be used.

Quickening: Translational design of resorbable synthetic vascular grafts

Traditional tissue-engineered vascular grafts have yet to gain wide clinical use. The difficulty of scaling production of these cell- or biologic-based products has hindered commercialization. In situ tissue engineering bypasses such logistical challenges by using acellular resorbable scaffolds. Upon implant, the scaffolds become remodeled by host cells. This review describes the scientific and translational advantages of acellular, synthetic vascular grafts. It surveys in vivo results obtained with acellular synthetics over their fifty years of technological development. Finally, it discusses emerging principles, highlights strategic considerations for designers, and identifies questions needing additional research.

Histone Deacetylase 7-Derived Peptides Play a Vital Role in Vascular Repair and Regeneration

Cardiovascular disease is a leading cause of morbidity and mortality globally. Accumulating evidence indicates that local resident stem/progenitor cells play an important role in vascular regeneration. Recently, it is demonstrated that a histone deacetylase 7-derived 7-amino acid peptide (7A, MHSPGAD) is critical in modulating the mobilization and orientated differentiation of these stem/progenitor cells. Here, its therapeutic efficacy in vascular repair and regeneration is evaluated. In vitro functional analyses reveal that the 7A peptide, in particular phosphorylated 7A (7Ap, MH[pSer]PGAD), could increase stem cell antigen-1 positive (Sca1+) vascular progenitor cell (VPC) migration and differentiation toward an endothelial cell lineage. Furthermore, local delivery of 7A as well as 7Ap could enhance angiogenesis and ameliorate vascular injury in ischaemic tissues; these findings are confirmed in a femoral artery injury model and a hindlimb ischaemia model, respectively. Importantly, sustained delivery of 7A, especially 7Ap, from tissue-engineered vascular grafts could attract Sca1+-VPC cells into the grafts, contributing to endothelialization and intima/media formation in the vascular graft. These results suggest that this novel type of peptides has great translational potential in vascular regenerative medicine.