Immunomodulation Strategies for the Successful Regeneration of a Tissue-Engineered Vascular Graft

In vitro and in vivo evaluation of biohybrid tissue-engineered vascular grafts with transformative 1H/19F MRI traceable scaffolds

Biohybrid tissue-engineered vascular grafts (TEVGs) promise long-term durability due to their ability to adapt to hosts' needs. However, the latter calls for sensitive non-invasive imaging approaches to longitudinally monitor their functionality, integrity, and positioning. Here, we present an imaging approach comprising the labeling of non-degradable and degradable TEVGs' components for their in vitro and in vivo monitoring by hybrid 1H/19F MRI. TEVGs (inner diameter 1.5 mm) consisted of biodegradable poly(lactic-co-glycolic acid) (PLGA) fibers passively incorporating superparamagnetic iron oxide nanoparticles (SPIONs), non-degradable polyvinylidene fluoride scaffolds labeled with highly fluorinated thermoplastic polyurethane (19F-TPU) fibers, a smooth muscle cells containing fibrin blend, and endothelial cells. 1H/19F MRI of TEVGs in bioreactors, and after subcutaneous and infrarenal implantation in rats, revealed that PLGA degradation could be faithfully monitored by the decreasing SPIONs signal. The 19F signal of 19F-TPU remained constant over weeks. PLGA degradation was compensated by cells' collagen and α-smooth-muscle-actin deposition. Interestingly, only TEVGs implanted on the abdominal aorta contained elastin. XTT and histology proved that our imaging markers did not influence extracellular matrix deposition and host immune reaction. This concept of non-invasive longitudinal assessment of cardiovascular implants using 1H/19F MRI might be applicable to various biohybrid tissue-engineered implants, facilitating their clinical translation.

Mesenchymal Stem Cell-Conditioned Media-Loaded Microparticles Enhance Acute Patency in Silk-Based Vascular Grafts

Coronary artery disease leads to over 360,000 deaths annually in the United States, and off-the-shelf bypass graft options are currently limited and/or have high failure rates. Tissue-engineered vascular grafts (TEVGs) present an attractive option, though the promising mesenchymal stem cell (MSC)-based implants face uncertain regulatory pathways. In this study, "artificial MSCs" (ArtMSCs) were fabricated by encapsulating MSC-conditioned media (CM) in poly(lactic-co-glycolic acid) microparticles. ArtMSCs and control microparticles (Blank-MPs) were incubated over 7 days to assess the release of total protein and the vascular endothelial growth factor (VEGF-A); releasates were also assessed for cytotoxicity and promotion of smooth muscle cell (SMC) proliferation. Each MP type was loaded in previously published "lyogel" silk scaffolds and implanted as interposition grafts in Lewis rats for 1 or 8 weeks. Explanted grafts were assessed for patency and cell content. ArtMSCs had a burst release of protein and VEGF-A. CM increased proliferation in SMCs, but not after encapsulation. TEVG explants after 1 week had significantly higher patency rates with ArtMSCs compared to Blank-MPs, but similar to unseeded lyogel grafts. ArtMSC explants had lower numbers of infiltrating macrophages compared to Blank-MP explants, suggesting a modulation of inflammatory response by the ArtMSCs. TEVG explants after 8 weeks showed no significant difference in patency among the three groups. The ArtMSC explants showed higher numbers of SMCs and endothelial cells within the neotissue layer of the graft compared to Blank-MP explants. In sum, while the ArtMSCs had positive effects acutely, efficacy was lost in the longer term; therefore, further optimization is needed.

ECM-mimetic glucomannan hydrogel promotes pressure ulcer healing by scavenging ROS, promoting angiogenesis and regulating macrophages

Pressure ulcers (PUs) have emerged as a significant burden on both individuals and society. Effective treatment of PUs is a significant clinical challenge due to the compromised tissue microenvironment characterized by extracellular matrix (ECM) depletion, increased levels of reactive oxygen species (ROS), excessive inflammation and impaired angiogenesis. To this end, we have developed a glucomannan hydrogel (GM-Pgel) that mimics the skin's extracellular matrix to accelerate wound healing by regulating chronic inflammation in the PUs. This hydrogel not only faithfully replicates the components and nanofibrous architecture of ECM-like glycoproteins but also exhibits remarkable capabilities in enhancing neovascularization, scavenging ROS, and promoting macrophage polarization toward the M2 phenotype. In summary, this ECM-mimetic multifunctional hydrogel emerges as a promising dressing with diverse functionalities, capable of reshaping the compromised tissue environment without the need for additional drugs, exogenous cytokines, or cells. This presents a compelling and effective strategy for the repair and regeneration of chronic cutaneous wounds.

Peptide functionalized biomimetic gene complexes enhance specificity for vascular endothelial regeneration

Gene delivery presents great potential in endothelium regeneration and prevention of vascular diseases, but its outcome is inevitably limited by high shear stress and instable microenvironment. Highly efficient nanosystems may alleviate the problem with strong dual-specificity for diseased site and targeted cells. Hence, biomimetic coatings incorporating EC-targeting peptides were constructed by platelets and endothelial cells (ECs) for surface modification. A series of biomimetic gene complexes were fabricated by the biomimetic coatings to deliver pcDNA3.1-VEGF165 plasmid (pVEGF) for rapid recovery of endothelium. The gene complexes possessed good biocompatibility with macrophages, stability with serum and showed no evident cytotoxicity for ECs even at very high concentrations. Furthermore, the peptide modified gene complexes achieved selective internalization in ECs and significant accumulation in endothelium-injured site, especially the REDV-modified and EC-derived gene complexes. They substantially enhanced VEGF expression at mRNA and protein levels, thereby enabling a wound to heal completely within 24 h according to wound healing assay. In an artery endothelium-injured mouse model, the REDV-modified and EC-derived gene complexes presented efficient re-endothelialization with the help of enhanced specificity. The biomimetic gene complexes offer an efficient dual-targeting strategy for rapid recovery of endothelium, and hold potential in vascular tissue regeneration.

Modulating the immune system towards a functional chronic wound healing: A biomaterials and Nanomedicine perspective

Chronic non-healing wounds persist as a substantial burden for healthcare systems, influenced by factors such as aging, diabetes, and obesity. In contrast to the traditionally pro-regenerative emphasis of therapies, the recognition of the immune system integral role in wound healing has significantly grown, instigating an approach shift towards immunological processes. Thus, this review explores the wound healing process, highlighting the engagement of the immune system, and delving into the behaviors of innate and adaptive immune cells in chronic wound scenarios. Moreover, the article investigates biomaterial-based strategies for the modulation of the immune system, elucidating how the adjustment of their physicochemical properties or their synergistic combination with other agents such as drugs, proteins or mesenchymal stromal cells can effectively modulate the behaviors of different immune cells. Finally this review explores various strategies based on synthetic and biological nanostructures, including extracellular vesicles, to finely tune the immune system as natural immunomodulators or therapeutic nanocarriers with promising biophysical properties.

Biomimetic Approaches in Scaffold-Based Blood Vessel Tissue Engineering

Cardiovascular diseases remain a leading cause of mortality globally, with atherosclerosis representing a significant pathological means, often leading to myocardial infarction. Coronary artery bypass surgery, a common procedure used to treat coronary artery disease, presents challenges due to the limited autologous tissue availability or the shortcomings of synthetic grafts. Consequently, there is a growing interest in tissue engineering approaches to develop vascular substitutes. This review offers an updated picture of the state of the art in vascular tissue engineering, emphasising the design of scaffolds and dynamic culture conditions following a biomimetic approach. By emulating native vessel properties and, in particular, by mimicking the three-layer structure of the vascular wall, tissue-engineered grafts can improve long-term patency and clinical outcomes. Furthermore, ongoing research focuses on enhancing biomimicry through innovative scaffold materials, surface functionalisation strategies, and the use of bioreactors mimicking the physiological microenvironment. Through a multidisciplinary lens, this review provides insight into the latest advancements and future directions of vascular tissue engineering, with particular reference to employing biomimicry to create systems capable of reproducing the structure-function relationships present in the arterial wall. Despite the existence of a gap between benchtop innovation and clinical translation, it appears that the biomimetic technologies developed to date demonstrate promising results in preventing vascular occlusion due to blood clotting under laboratory conditions and in preclinical studies. Therefore, a multifaceted biomimetic approach could represent a winning strategy to ensure the translation of vascular tissue engineering into clinical practice.

Adaptation process of decellularized vascular grafts as hemodialysis access in vivo

Arteriovenous grafts (AVGs) have emerged as the preferred option for constructing hemodialysis access in numerous patients. Clinical trials have demonstrated that decellularized vascular graft exhibits superior patency and excellent biocompatibility compared to polymer materials; however, it still faces challenges such as intimal hyperplasia and luminal dilation. The absence of suitable animal models hinders our ability to describe and explain the pathological phenomena above and in vivo adaptation process of decellularized vascular graft at the molecular level. In this study, we first collected clinical samples from patients who underwent the construction of dialysis access using allogeneic decellularized vascular graft, and evaluated their histological features and immune cell infiltration status 5 years post-transplantation. Prior to the surgery, we assessed the patency and intimal hyperplasia of the decellularized vascular graft using non-invasive ultrasound. Subsequently, in order to investigate the in vivo adaptation of decellularized vascular grafts in an animal model, we attempted to construct an AVG model using decellularized vascular grafts in a small animal model. We employed a physical-chemical-biological approach to decellularize the rat carotid artery, and histological evaluation demonstrated the successful removal of cellular and antigenic components while preserving extracellular matrix constituents such as elastic fibers and collagen fibers. Based on these results, we designed and constructed the first allogeneic decellularized rat carotid artery AVG model, which exhibited excellent patency and closely resembled clinical characteristics. Using this animal model, we provided a preliminary description of the histological features and partial immune cell infiltration in decellularized vascular grafts at various time points, including Day 7, Day 21, Day 42, and up to one-year post-implantation. These findings establish a foundation for further investigation into the in vivo adaptation process of decellularized vascular grafts in small animal model.

Releasable, Immune-Instructive, Bioinspired Multilayer Coating Resists Implant-Induced Fibrosis while Accelerating Tissue Repair

Implantable biomaterials trigger foreign body reactions (FBRs), which reduces the functional life of medical devices and prevents effective tissue regeneration. Although existing therapeutic approaches can circumvent collagen-rich fibrotic encapsulation secondary to FBRs, they disrupt native tissue repair. Herein, a new surface engineering strategy in which an apoptotic-mimetic, immunomodulatory, phosphatidylserine liposome (PSL) is released from an implant coating to induce the formation of a macrophage phenotype that mitigates FBRs and improves tissue healing is described. PSL-multilayers constructed on implant surfaces via the layer-by-layer method release PSLs over a 1-month period. In rat muscles, poly(etheretherketone) (PEEK), a nondegradable polymer implant model, induces FBRs with dense fibrotic scarring under an aberrant cellular profile that recruits high levels of inflammatory infiltrates, foreign body giant cells (FBGCs), scar-forming myofibroblasts, and inflammatory M1-like macrophages but negligible amounts of anti-inflammatory M2-like phenotypes. However, the PSL-multilayer coating markedly diminishes these detrimental signatures by shifting the macrophage phenotype. Unlike other therapeutics, PSL-multilayered coatings also stimulate muscle regeneration. This study demonstrates that PSL-multilayered coatings are effective in eliminating FBRs and promoting regeneration, hence offering potent and broad applications for implantable biomaterials.

A review of the current state in neointimal hyperplasia development following endovascular intervention and minor emphasis on new horizons in immunotherapy

Endovascular strategies play a vital role in the treatment of peripheral arterial disease (PAD). However, luminal loss or restenosis after endovascular intervention remains a significant challenge. The main underlying mechanisms are negative vascular remodeling and elastic recoil in balloon angioplasty. During stenting, the main reason for this complex is neointimal proliferation. Endothelial cell injury due to endovascular intervention initiates a series of molecular events, such as overexpression of growth factors, cytokine secretion, and adhesion molecules. These induce platelet activation and inflammatory processes, which trigger the proliferation and migration of vascular smooth muscle cells into the intima, resulting in neointimal hyperplasia. During this process, PAD progression is mainly caused by chronic inflammation, in which macrophages play a central role. Of the current strategies, drug release interventions aim to suppress restenosis using antiproliferative drugs, such as sirolimus and paclitaxel, during drug release. These drugs inhibit vascular reendothelialization and reduce late in-stent restenosis. For this reason, immunotherapy can be considered an important alternative. Interventions that polarize macrophages to the M2 subtype are particularly important, as they shape the immune response in an anti-inflammatory direction and contribute to tissue repair. However, there are several challenges to overcome, such as localizing antiproliferative or polarizing agents only to areas of vascular injury. This review discusses, based on the early study observations, immunotherapeutic approaches to prevent restenosis after endovascular intervention for the treatment of PAD.

The construction of elastin-like polypeptides and their applications in drug delivery system and tissue repair

Elastin-like polypeptides (ELPs) are thermally responsive biopolymers derived from natural elastin. These peptides have a low critical solution temperature phase behavior and can be used to prepare stimuli-responsive biomaterials. Through genetic engineering, biomaterials prepared from ELPs can have unique and customizable properties. By adjusting the amino acid sequence and length of ELPs, nanostructures, such as micelles and nanofibers, can be formed. Correspondingly, ELPs have been used for improving the stability and prolonging drug-release time. Furthermore, ELPs have widespread use in tissue repair due to their biocompatibility and biodegradability. Here, this review summarizes the basic property composition of ELPs and the methods for modulating their phase transition properties, discusses the application of drug delivery system and tissue repair and clarifies the current challenges and future directions of ELPs in applications.

Epsin Mimetic UPI Peptide Delivery Strategies to Improve Endothelization of Vascular Grafts

Endothelialization of engineered vascular grafts for replacement of small-diameter coronary arteries remains a critical challenge. The ability for an acellular vascular graft to promote endothelial cell (EC) recruitment in the body would be very beneficial. This study investigated epsins as a target since they are involved in internalization of vascular endothelial growth factor receptor 2. Specifically, epsin-mimetic UPI peptides are delivered locally from vascular grafts to block epsin activity and promote endothelialization. The peptide delivery from fibrin coatings allowed for controlled loading and provided a significant improvement in EC attachment, migration, and growth in vitro. The peptides have even more important impacts after grafting into rat abdominal aortae. The peptides prevented graft thrombosis and failure that is observed with a fibrin coating alone. They also modulated the in vivo remodeling. The grafts are able to remodel without the formation of a thick fibrous capsule on the adventitia with the 100 µg mL-1 peptide-loaded condition, and this condition enabled the formation of a functional EC monolayer in the graft lumen after only 1 week. Overall, this study demonstrated that the local delivery of UPI peptides is a promising strategy to improve the performance of vascular grafts.

MCP-1-Loaded Poly(l-lactide-co-caprolactone) Fibrous Films Modulate Macrophage Polarization toward an Anti-inflammatory Phenotype and Improve Angiogenesis

Tissue engineering approaches such as the electrospinning technique can fabricate nanofibrous scaffolds which are widely used for small-diameter vascular grafting. However, foreign body reaction (FBR) and lack of endothelial coverage are still the main cause of graft failure after the implantation of nanofibrous scaffolds. Macrophage-targeting therapeutic strategies have the potential to address these issues. Here, we fabricate a monocyte chemotactic protein-1 (MCP-1)-loaded coaxial fibrous film with poly(l-lactide-co-ε-caprolactone) (PLCL/MCP-1). The PLCL/MCP-1 fibrous film can polarize macrophages toward anti-inflammatory M2 macrophages through the sustained release of MCP-1. Meanwhile, these specific functional polarization macrophages can mitigate FBR and promote angiogenesis during the remodeling of implanted fibrous films. These studies indicate that MCP-1-loaded PLCL fibers have a higher potential to modulate macrophage polarity, which provides a new strategy for small-diameter vascular graft designing.

Calcification of Synthetic Vascular Grafts: A Systematic Review

Objective

Calcification of vascular grafts, including polyethylene terephthalate (PET) and expanded polytetrafluoroethylene (ePTFE) grafts may contribute to graft failure, but is under reported. The aim of this study was to review the literature to assess whether vascular graft calcification is deleterious to vascular graft outcomes.

Data sources

The Medline and Embase databases were searched.

Review methods

A systematic literature search according to PRISMA Guidelines was performed using a combined search strategy of MeSH terms. The MeSH terms used were "calcification, physiologic", "calcinosis", "vascular grafting", "blood vessel prosthesis", "polyethylene terephthalates", and "polytetrafluoroethylene".

Results

The systematic search identified 17 cases of PET graft calcification and 73 cases of ePTFE graft calcification over a 35 year period. All cases of PET graft calcification were reported in grafts explanted for graft failure. The majority of cases of ePTFE graft calcification were unexpectedly noted in grafts used during cardiovascular procedures and subsequently removed.

Conclusion

Calcification of synthetic vascular grafts is under reported but can compromise the long term performance of the grafts. More data, including specific analysis of radiological findings as well as explant analysis are needed to obtain a more sensitive and specific analysis of the prevalence and incidence of vascular graft calcification and the impact of calcification on synthetic graft outcomes.

Comparison of the Patency and Regenerative Potential of Biodegradable Vascular Prostheses of Different Polymer Compositions in an Ovine Model

The lack of suitable autologous grafts and the impossibility of using synthetic prostheses for small artery reconstruction make it necessary to develop alternative efficient vascular grafts. In this study, we fabricated an electrospun biodegradable poly(ε-caprolactone) (PCL) prosthesis and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(ε-caprolactone) (PHBV/PCL) prosthesis loaded with iloprost (a prostacyclin analog) as an antithrombotic drug and cationic amphiphile with antibacterial activity. The prostheses were characterized in terms of their drug release, mechanical properties, and hemocompatibility. We then compared the long-term patency and remodeling features of PCL and PHBV/PCL prostheses in a sheep carotid artery interposition model. The research findings verified that the drug coating of both types of prostheses improved their hemocompatibility and tensile strength. The 6-month primary patency of the PCL/Ilo/A prostheses was 50%, while all PHBV/PCL/Ilo/A implants were occluded at the same time point. The PCL/Ilo/A prostheses were completely endothelialized, in contrast to the PHBV/PCL/Ilo/A conduits, which had no endothelial cells on the inner layer. The polymeric material of both prostheses degraded and was replaced with neotissue containing smooth-muscle cells; macrophages; proteins of the extracellular matrix such as type I, III, and IV collagens; and vasa vasorum. Thus, the biodegradable PCL/Ilo/A prostheses demonstrate better regenerative potential than PHBV/PCL-based implants and are more suitable for clinical use.

Peritoneal pre-conditioning impacts long-term vascular graft patency and remodeling

There are questions about how well small-animal models for tissue-engineered vascular grafts (TEVGs) translate to clinical patients. Most TEVG studies used grafting times ≤6 months where conduits from generally biocompatible materials like poly(ε-caprolactone) (PCL) perform well. However, longer grafting times can result in significant intimal hyperplasia and calcification. This study tests the hypothesis that differences in pro-inflammatory response from pure PCL conduits will be consequential after long-term grafting. It also tests the long-term benefits of a peritoneal pre-implantation strategy on rodent outcomes. Electrospun conduits with and without peritoneal pre-implantation, and with 0 % and 10 % (w/w) collagen/PCL, were grafted into abdominal aortae of rats for 10 months. This study found that viability of control grafts without pre-implantation was reduced unlike prior studies with shorter grafting times, confirming the relevance of this model. Importantly, pre-implanted grafts had a 100 % patency rate. Further, pre-implantation reduced intimal hyperplasia within the graft. Differences in response between pure PCL and collagen/PCL conduits were observed (e.g., fewer CD80+ and CD3+ cells for collagen/PCL), but only pre-implantation had an effect on the overall graft viability. This study demonstrates how long-term grafting in rodent models can better evaluate viability of different TEVGs, and the benefits of the peritoneal pre-implantation step.

Prosthetic vascular grafts engineered to combat calcification: Progress and future directions

Calcification in prosthetic vascular conduits is a major challenge in cardiac and vascular surgery that compromises the long-term performance of these devices. Significant research efforts have been made to understand the etiology of calcification in the cardiovascular system and to combat calcification in various cardiovascular devices. Novel biomaterial design and tissue engineering strategies have shown promise in preventing or delaying calcification in prosthetic vascular grafts. In this review, we highlight recent advancements in the development of acellular prosthetic vascular grafts with preclinical success in attenuating calcification through advanced biomaterial design. We also discuss the mechanisms of action involved in the designs that will contribute to the further understanding of cardiovascular calcification. Lastly, recent insights into the etiology of vascular calcification will guide the design of future prosthetic vascular grafts with greater potential for translational success.

Cellular energy supply for promoting vascular remodeling of small-diameter vascular grafts: a preliminary study of a new strategy for vascular graft development

Rapid endothelialization is extremely essential for the success of small-diameter tissue-engineered vascular graft (TEVG) (<6 mm) transplantation. However, severe inflammation in situ often causes cellular energy decline of endothelial cells. The cellular energy supply involved in vascular graft therapy remains unclear, and whether promoting energy supply would be helpful in the regeneration of vascular grafts needs to be established. In our work, we generated an AMPK activator (5-aminoimidazole-4-carboxamide ribonucleotide, AICAR) immobilized vascular graft. AICAR-modified vascular grafts were successfully generated by the co-electrospinning technique. In vitro results indicated that AICAR could upregulate energy supply in endothelial cells and reprogram macrophages (MΦ) to assume an anti-inflammatory phenotype. Furthermore, endothelial cells (ECs) co-cultured with AICAR achieved higher survival rates, better migration, and angiogenic capacity than the controls. Concurrently, a rabbit carotid artery transplantation model was used to investigate AICAR-modified vascular grafts at different time points. The results showed that AICAR-modified vascular grafts had higher patency rates (92.9% and 85.7% at 6 and 12 weeks, respectively) than those of the untreated group (11.1% and 0%). In conclusion, AICAR strengthened the cellular energy state and attenuated the adverse effects of inflammation. AICAR-modified vascular grafts achieved better vascular remodeling. This study provides a new perspective on promoting the regeneration of small-diameter vascular grafts.

A textile-reinforced composite vascular graft that modulates macrophage polarization and enhances endothelial cell migration, adhesion and proliferation in vitro

At the present time, there is no successful off-the-shelf small-caliber vascular graft (<6 mm) for the repair or bypass of the coronary or carotid arteries. In this study, we engineer a textile-reinforced hydrogel vascular graft. The textile fibers are circularly knitted into a flexible yet robust conduit to serve as the backbone of the composite vascular graft and provide the primary mechanical support. It is embedded in the hydrogel matrix which seals the open structure of the knitted reinforcement and mediates cellular response toward a faster reendothelialization. The mechanical properties of the composite vascular graft, including bursting strength, suture retention strength and radial compliance, significantly surpass the requirement for the vascular graft application and can be adjusted by altering the structure of the textile reinforcement. The addition of hydrogel matrix, on the other hand, improves the survival, adhesion and proliferation of endothelial cells in vitro. The composite vascular graft also enhances macrophage activation and upregulates M1 and M2 related gene expression, which further improves the endothelial cell migration that might favor the reendothelialization of the vascular graft. Taken together, the textile-reinforced hydrogel shows it potential to be a promising scaffold material to fabricate a tissue engineered vascular graft.

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.

Gelatin-grafted tubular asymmetric scaffolds promote ureteral regeneration via activation of the integrin/Erk signaling pathway

Introduction: The repair of a diseased ureter is an urgent clinical issue that needs to be solved. A tissue-engineered scaffold for ureteral replacement is currently insufficient due to its incompetent bioactivity, especially in long-segment abnormalities. The primary reason is the failure of urothelialization on scaffolds. Methods: In this work, we investigated the ability of gelatin-grafted tubular scaffold in ureteral repairment and its related biological mechanism. We designed various porous asymmetric poly (L-lactic acid) (PLLA)/poly (L-lactide-co-e-caprolactone) (PLCL) tubes with a thermally induced phase separation (TIPS) method via a change in the ratio of solvents (named PP). To regulate the phenotype of urothelial cells and ureteral reconstruction, gelatin was grafted onto the tubular scaffold using ammonolysis and glutaraldehyde crosslinking (named PP-gel). The in vitro and in vivo experiments were performed to test the biological function and the mechanism of the scaffolds. Results and Discussion: The hydrophilicity of the scaffold significantly increased after gelatin grafting, which promoted the adhesion and proliferation of urothelial cells. Through subcutaneous implantation in rats, PP-gel scaffolds demonstrated good biocompatibility. The in vivo replacement showed that PP-gel could improve urothelium regeneration and maintain renal function after the ureter was replaced with an ∼4 cm-long PP-gel tube using New Zealand rabbits as the experimental animals. The related biologic mechanism of ureteral reconstruction was detected in detail. The gelatin-grafted scaffold upgraded the integrin α6/β4 on the urothelial cell membrane, which phosphorylates the focal adhesion kinase (FAK) and enhances urothelialization via the MAPK/Erk signaling pathway. Conclusion: All these results confirmed that the PP46-gel scaffold is a promising candidate for the constitution of an engineered ureter and to repair long-segment ureteral defects.

Vascular endothelial growth factor attenuates neointimal hyperplasia of decellularized small-diameter vascular grafts by modulating the local inflammatory response

Small-diameter vascular grafts (diameter <6 mm) are in high demand in clinical practice. Neointimal hyperplasia, a common complication after implantation of small-diameter vascular grafts, is one of the common causes of graft failure. Modulation of local inflammatory responses is a promising strategy to attenuates neointimal hyperplasia. Vascular endothelial growth factor (VEGF) is an angiogenesis stimulator that also induces macrophage polarization and modulates inflammatory responses. In the present study, we evaluated the effect of VEGF on the neointima hyperplasia and local inflammatory responses of decellularized vascular grafts. In the presence of rhVEGF-165 in RAW264.6 macrophage culture, rhVEGF-165 induces RAW264.6 macrophage polarization to M2 phenotype. Decellularized bovine internal mammary arteries were implanted into the subcutaneous and infrarenal abdominal aorta of New Zealand rabbits, with rhVEGF-165 applied locally to the adventitial of the grafts. The vascular grafts were removed en-bloc and submitted to histological and immunofluorescence analyses on days 7 and 28 following implantation. The thickness of the fibrous capsule and neointima was thinner in the VEGF group than that in the control group. In the immunofluorescence analysis, the number of M2 macrophages and the ratio of M2/M1 macrophages in vascular grafts in the VEGF group were higher than those in the control group, and the proinflammatory factor IL-1 was expressed less than in the control group, but the anti-inflammatory factor IL-10 was expressed more. In conclusion, local VEGF administration attenuates neointimal hyperplasia in decellularized small-diameter vascular grafts by inducing macrophage M2 polarization and modulating the inflammatory response.

Effects of sterilization methods on gelatin methacryloyl hydrogel properties and macrophage gene expressionin vitro

To assure the long-term safety and functional performance after implantation, it is of critical importance to completely sterilize a biomaterial implant. Ineffective sterilization can cause severe inflammation and infection at the implant site, leading to detrimental events of morbidity and even mortality. Macrophages are pivotal players in the inflammatory and foreign body response after implanting a biomaterial in the body. However, the relationship between the sterilization procedure and macrophage response has not been established. In this study, three commonly used sterilization methods, including autoclaving, ethylene oxide gas and ethanol treatment, were used to sterilize a gelatin methacryloyl hydrogel. The impacts of different sterilization methods on the structure and physical properties of the hydrogel were compared. Macrophage responses to the sterilized hydrogel were analyzed based on their morphology, viability andin vitrogene expression. It was found that the sterilization methods only marginally altered the hydrogel morphology, swelling behavior and elastic modulus, but significantly impacted macrophage gene expression within 48 h and over 7 din vitro. Therefore, when selecting sterilization methods for GelMA hydrogel, not only the sterility and hydrogel properties, such as material destruction and degradation caused by temperature and moisture, should be taken into consideration, but also the cellular responses to the sterilized material which could be substantially different.

Small Diameter Cell-Free Tissue-Engineered Vascular Grafts: Biomaterials and Manufacture Techniques to Reach Suitable Mechanical Properties

Vascular grafts (VGs) are medical devices intended to replace the function of a blood vessel. Available VGs in the market present low patency rates for small diameter applications setting the VG failure. This event arises from the inadequate response of the cells interacting with the biomaterial in the context of operative conditions generating chronic inflammation and a lack of regenerative signals where stenosis or aneurysms can occur. Tissue Engineered Vascular grafts (TEVGs) aim to induce the regeneration of the native vessel to overcome these limitations. Besides the biochemical stimuli, the biomaterial and the particular micro and macrostructure of the graft will determine the specific behavior under pulsatile pressure. The TEVG must support blood flow withstanding the exerted pressure, allowing the proper compliance required for the biomechanical stimulation needed for regeneration. Although the international standards outline the specific requirements to evaluate vascular grafts, the challenge remains in choosing the proper biomaterial and manufacturing TEVGs with good quality features to perform satisfactorily. In this review, we aim to recognize the best strategies to reach suitable mechanical properties in cell-free TEVGs according to the reported success of different approaches in clinical trials and pre-clinical trials.