Targeting Heparin to Collagen within Extracellular Matrix Significantly Reduces Thrombogenicity and Improves Endothelialization of Decellularized Tissues

Cell-Derived Basal Membrane-Like Extracellular Matrix Promotes Endothelial Cell Expansion and Functionalization

Engineering cellular microenvironments with biomaterials is an effective strategy for endothelial cell expansion and functionality in vascular tissue engineering. The basement membrane (BM) is a natural vascular endothelium microenvironment that plays an important role in promoting rapid expansion and function of endothelial cells. However, mimicking the crucial function of BM with an ideal biomaterial remains challenging. In this study, we developed a cell-derived decellularized extracellular matrix (c-dECM) paper to mimic the role of BM in endothelial cell expansion and function. The results showed that c-dECM paper was a stable, biocompatible, and biodegradable scaffold that significantly promoted endothelial cell expansion by modulating cell migration, adhesion, and proliferation both in vivo and in vitro. Moreover, the biomimetic c-dECM paper can profoundly promote endothelial cell function by increasing the synthesis and release of nitric oxide (NO) and prostaglandin I2 (PGI2) and upregulating the expression of anticoagulant and vascularized genes, including thrombomodulin (THBD), tissue factor pathway inhibitor (TFPI), endothelial growth factor (VEGF) and endoglin (CD105). These data indicate that the c-dECM is a potential biomaterial for constructing vascular tissue engineering scaffolds or developing in vitro models to study the functional mechanisms of endothelial cells.

Vascular reconstruction of the decellularized biomatrix for whole-organ engineering-a critical perspective and future strategies

Whole-organ re-engineering is the most challenging goal yet to be achieved in tissue engineering and regenerative medicine. One essential factor in any transplantable and functional tissue engineering is fabricating a perfusable vascular network with macro- and micro-sized blood vessels. Whole-organ development has become more practical with the use of the decellularized organ biomatrix (DOB) as it provides a native biochemical and structural framework for a particular organ. However, reconstructing vasculature and re-endothelialization in the DOB is a highly challenging task and has not been achieved for constructing a clinically transplantable vascularized organ with an efficient perfusable capability. Here, we critically and articulately emphasized factors that have been studied for the vascular reconstruction in the DOB. Furthermore, we highlighted the factors used for vasculature development studies in general and their application in whole-organ vascular reconstruction. We also analyzed in detail the strategies explored so far for vascular reconstruction and angiogenesis in the DOB for functional and perfusable vasculature development. Finally, we discussed some of the crucial factors that have been largely ignored in the vascular reconstruction of the DOB and the future directions that should be addressed systematically.

The application of composite scaffold materials based on decellularized vascular matrix in tissue engineering: a review

Decellularized vascular matrix is a natural polymeric biomaterial that comes from arteries or veins which are removed the cellular contents by physical, chemical and enzymatic means, leaving only the cytoskeletal structure and extracellular matrix to achieve cell adhesion, proliferation and differentiation and creating a suitable microenvironment for their growth. In recent years, the decellularized vascular matrix has attracted much attention in the field of tissue repair and regenerative medicine due to its remarkable cytocompatibility, biodegradability and ability to induce tissue regeneration. Firstly, this review introduces its basic properties and preparation methods; then, it focuses on the application and research of composite scaffold materials based on decellularized vascular matrix in vascular tissue engineering in terms of current in vitro and in vivo studies, and briefly outlines its applications in other tissue engineering fields; finally, it looks into the advantages and drawbacks to be overcome in the application of decellularized vascular matrix materials. In conclusion, as a new bioactive material for building engineered tissue and repairing tissue defects, decellularized vascular matrix will be widely applied in prospect.

Free-aldehyde neutralized and oligohyaluronan loaded bovine pericardium with improved anti-calcification and endothelialization for bioprosthetic heart valves

The number of patients with valvular heart disease is increasing yearly, and valve replacement is the most effective treatment, during which bioprosthetic heart valves (BHVs) are the most widely used. Commercial BHVs are mainly prepared with glutaraldehyde (Glut) cross-linked bovine pericardial or porcine aortic valves, but the residual free aldehyde groups in these tissues can cause calcification and cytotoxicity. Moreover, insufficient glycosaminoglycans (GAGs) in tissues can further reduce biocompatibility and durability. However, the anti-calcification performance and biocompatibility might be improved by blocking the free aldehyde groups and increasing the GAGs content in Glut-crosslinked tissues. In our study, adipic dihydrazide (ADH) was used to neutralize the residual free aldehyde groups in tissues and provide sites to blind with oligohyaluronan (OHA) to increase the content of GAGs in tissues. The modified bovine pericardium was evaluated for its content of residual aldehyde groups, the amount of OHA loaded, physical/chemical characteristics, biomechanical properties, biocompatibility, and in vivo anticalcification assay and endothelialization effects in juvenile Sprague-Dawley rats. The results showed that ADH could completely neutralize the free aldehyde groups in the Glut-crosslinked bovine pericardium, the amount of OHA loaded increased and the cytotoxicity was reduced. Moreover, the in vivo results also showed that the level of calcification and inflammatory response in the modified pericardial tissue was significantly reduced in a rat subcutaneous implantation model, and the results from the rat abdominal aorta vascular patch repair model further demonstrated the improved capability of the modified pericardial tissues for endothelialization. Furthermore, more α-SMA+ smooth muscle cells and fewer CD68+ macrophages infiltrated in the neointima of the modified pericardial patch. In summary, blocking free-aldehydes and loading OHA improved the anti-calcification, anti-inflammation and endothelialization properties of Glut-crosslinked BHVs and in particularly, this modified strategy may be a promising candidate for the next-generation of BHVs.

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Development of Antithrombogenic ECM-Based Nanocomposite Heart Valve Leaflets

Thrombogenicity, which is commonly encountered in artificial heart valves after replacement surgeries, causes valvular failure. Even life-long anticoagulant drug use may not be sufficient to prevent thrombogenicity. In this study, it was aimed to develop a heart valve construct with antithrombogenic properties and suitable mechanical strength by combining multiwalled carbon nanotubes within a decellularized bovine pericardium. In this context, the decellularization process was performed by using the combination of freeze–thawing and sodium dodecyl sulfate (SDS). Evaluation of decellularization efficiency was determined by histology (Hematoxylin and Eosin, DAPI and Masson’s Trichrome) and biochemical (DNA, sGAG and collagen) analyses. After the decellularization process of the bovine pericardium, composite pericardial tissues were prepared by incorporating −COOH-modified multiwalled carbon nanotubes (MWCNTs). Characterization of MWCNT incorporation was performed by ATR-FTIR, TGA, and mechanical analysis, while SEM and AFM were used for morphological evaluations. Thrombogenicity assessments were studied by platelet adhesion test, Calcein-AM staining, kinetic blood clotting, hemolysis, and cytotoxicity analyses. As a result of this study, the composite pericardial material revealed improved mechanical and thermal stability and hemocompatibility in comparison to decellularized pericardium, without toxicity. Approximately 100% success is achieved in preventing platelet adhesion. In addition, kinetic blood-coagulation analysis demonstrated a low rate and slow coagulation kinetics, while the hemolysis index was below the permissible limit for biomaterials.

An overview of post transplantation events of decellularized scaffolds

Regenerative medicine and tissue engineering are reasonable techniques for repairing failed tissues and could be a suitable alternative to organ transplantation. One of the most widely used methods for preparing bioscaffolds is the decellularization procedure. Although cell debris and DNA are removed from the decellularized tissues, important compositions of the extracellular matrix including proteins, proteoglycans, and glycoproteins are nearly preserved. Moreover, the obtained scaffolds have a 3-dimensional (3D) structure, appropriate naïve mechanical properties, and good biocompatibility. After transplantation, different types of host cells migrate to the decellularized tissues. Histological and immunohistochemical assessment of the different bioscaffolds after implantation reveals the migration of parenchymal cells, angiogenesis, as well as the invasion of inflammatory and giant foreign cells. In this review, the events after transplantation including angiogenesis, scaffold degradation, and the presence of immune and tissue-specific progenitor cells in the decellularized scaffolds in various hosts, are discussed.

Chorion-derived extracellular matrix hydrogel and fibronectin surface coatings show similar beneficial effects on endothelialization of expanded polytetrafluorethylene vascular grafts

The endothelium plays an important regulatory role for cardiovascular homeostasis. Rapid endothelialization of small diameter vascular grafts (SDVGs) is crucial to ensure long-term patency. Here, we assessed a human placental chorionic extracellular matrix hydrogel (hpcECM-gel) as coating material and compared it to human fibronectin in-vitro. hpcECM-gels were produced from placental chorion by decellularization and enzymatic digestion. Human umbilical vein endothelial cells (HUVECs) were seeded to non-, fibronectin- or hpcECM-gel-coated expanded polytetrafluorethylene (ePTFE) SDVGs. Coating efficiency as well as endothelial cell proliferation, migration and adhesion studies on grafts were performed. hpcECM-gel depicted high collagen and glycosaminoglycan content and neglectable DNA amounts. Laminin and fibronectin were both retained in the hpcECM-gel after the decellularization process. HUVEC as well as endothelial progenitor cell attachment were both significantly enhanced on hpcECM-gel coated grafts. HUVECs seeded to hpcECM-gel depicted significantly higher platelet endothelial cell adhesion molecule-1 (PECAM-1) expression in the perinuclear region. Cell retention to flow was enhanced on fibronectin and hpcECM-gel coated grafts. Since hpcECM-gel induced a significantly higher endothelial cell adhesion to ePTFE than fibronectin, it represents a possible alternative for SDVG modification to improve endothelialization.

Recent Advances in Liver Engineering With Decellularized Scaffold

Liver transplantation is currently the only effective treatment for patients with end-stage liver disease; however, donor liver scarcity is a notable concern. As a result, extensive endeavors have been made to diversify the source of donor livers. For example, the use of a decellularized scaffold in liver engineering has gained considerable attention in recent years. The decellularized scaffold preserves the original orchestral structure and bioactive chemicals of the liver, and has the potential to create a de novo liver that is fit for transplantation after recellularization. The structure of the liver and hepatic extracellular matrix, decellularization, recellularization, and recent developments are discussed in this review. Additionally, the criteria for assessment and major obstacles in using a decellularized scaffold are covered in detail.

Heparin modification improves the re-endothelialization and angiogenesis of decellularized kidney scaffolds through antithrombosis and anti-inflammation in vivo

Background

Constructing tissue-engineered kidneys using decellularized kidney scaffolds (DKS) has attracted widespread attention as it is expected to be the key to solving the shortage of donor kidneys. However, thrombosis and the host inflammatory response are unfavorable factors that hider the re-endothelialization and vascularization of the decellularized scaffolds.

Methods

Heparin was immobilized into the DKS using the method of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide/N-hydroxysuccinimide (EDC/NHS) activation. Fourier-transform infrared (FTIR) spectra were used to verify the heparinization of DKS. Human umbilical vein endothelial cells (HUVECs) were seeded and cultured in the DKS, then the sliced scaffolds were transplanted subcutaneously into nude mouse. Scanning electron microscopy and a series of histochemical stains including hematoxylin and eosin (H&E), elastic Verhöeff-Van Gieson (EVG), Sirius red, Masson’s trichrome, and toluidine blue (TB) staining were used for morphological characterization. The qRT-PCR analysis, immunohistochemistry (IHC), and immunofluorescence (IF) staining were used to determine the expression of related molecular markers.

Results

The rat DKS completely retained the extracellular matrix and heparinized modification. The H&E staining results showed there were more HUVECs covering the internal surfaces of tubular structures in the HEP-DKS group compared with the DKS group. The IF analysis results revealed that CD31, Ki67, and CD206 had higher positive rates in HUVECs in the HEP-DKS group compared to the DKS group. Both groups of scaffolds showed blood vessel formation via H&E staining, and there were more blood vessels in the HEP-DKS group compared with the native DKS group (P<0.05). The qRT-PCR results showed that the levels of IL-1β, IL-6, and TNF-α in the HEP-DKS group were significantly lower than those of the native DKS group, while the expression level of IL-10 was significantly higher than that in the native DKS group (P<0.05).

Conclusions

Heparin modification improves the re-endothelialization and vascular regeneration of the DKS through anticoagulation in vitro and in vivo. The anti-inflammatory effect of heparin on the transplanted host was initially confirmed, and it is considered that this effect may play a non-negligible role in promoting DKS re-endothelialization and angiogenesis. Heparinized DKS is therefore a promising candidate for kidney tissue engineering.

Strategies for improving endothelial cell adhesion to blood-contacting medical devices

The endothelium is a critical mediator of homeostasis on blood-contacting surfaces in the body, serving as a selective barrier to regulate processes such as clotting, immune cell adhesion, and cellular response to fluid shear stress. Implantable cardiovascular devices including stents, vascular grafts, heart valves, and left ventricular assist devices are in direct contact with circulating blood and carry a high risk for platelet activation and thrombosis without a stable endothelial cell (EC) monolayer. Development of a healthy endothelium on the blood-contacting surface of these devices would help ameliorate risks associated with thrombus formation and eliminate the need for long-term anti-platelet or anti-coagulation therapy. Although ECs have been seeded onto or recruited to these blood-contacting surfaces, most ECs are lost upon exposure to shear stress due to circulating blood. Many investigators have attempted to generate a stable EC monolayer by improving EC adhesion using surface modifications, material coatings, nanofiber topology, and modifications to the cells. Despite some success with enhanced EC retention in vitro and in animal models, no studies to date have proven efficacious for routinely creating a stable endothelium in the clinical setting. This review summarizes past and present techniques directed at improving the adhesion of ECs to blood-contacting devices.

Tissue engineered bovine saphenous vein extracellular matrix scaffolds produced via antigen removal achieve high in vivo patency rates

Diseases of small diameter blood vessels encompass the largest portion of cardiovascular diseases, with over 4.2 million people undergoing autologous vascular grafting every year. However, approximately one third of patients are ineligible for autologous vascular grafting due to lack of suitable donor vasculature. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissue have potential to serve as ideal biomaterials for production of off-the-shelf vascular grafts capable of eliminating the need for autologous vessel harvest. A modified antigen removal (AR) tissue process, employing aminosulfabetaine-16 (ASB-16) was used to create off-the-shelf small diameter (< 3 mm) vascular graft from bovine saphenous vein ECM scaffolds with significantly reduced antigenic content, while retaining native vascular ECM protein structure and function. Elimination of native tissue antigen content conferred graft-specific adaptive immune avoidance, while retention of native ECM protein macromolecular structure resulted in pro-regenerative cellular infiltration, ECM turnover and innate immune self-recognition in a rabbit subpannicular model. Finally, retention of the delicate vascular basement membrane protein integrity conferred endothelial cell repopulation and 100% patency rate in a rabbit jugular interposition model, comparable only to Autograft implants. Alternatively, the lack of these important basement membrane proteins in otherwise identical scaffolds yielded a patency rate of only 20%. We conclude that acellular antigen removed bovine saphenous vein ECM scaffolds have potential to serve as ideal off-the-shelf small diameter vascular scaffolds with high in vivo patency rates due to their low antigen content, retained native tissue basement membrane integrity and preserved native ECM structure, composition and functional properties. STATEMENT OF SIGNIFICANCE: The use of autologous vessels for the treatment of small diameter vascular diseases is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use for the treatment of small diameter vessel diseases can potentially increase the success rate of autologous vascular grafting by eliminating complications related to the use of autologous vessel and increased availability. This manuscript demonstrates the potential of non-antigenic extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissue as off-the-shelf vascular grafts for the treatment of small diameter vascular diseases.

Effect of heparinization on promoting angiogenesis of decellularized kidney scaffolds

Native decellularized extracellular matrix provides an adequate platform for tissues and organs and promotes the development of organogenesis and tissue remodeling. However, thrombosis poses a great challenge that hinders the transplantation for a substantial organ in vivo. Therefore, anticoagulation and re-reendothelialization of organ biological scaffolds are the primary concerns to be addressed before orthotopic transplantation. Herein, a heparinized decellularized kidney scaffold (HEP-DKSs) was prepared using end-point attachment technology, followed by binding the vascular endothelial growth factor (VEGF) to greatly improve the hemocompatibility and angiogenesis of DKSs. Based on the anticoagulant, co-culture of human umbilical vein endothelial cells, and subcapsular transplantation of kidney experiments, HEP-VEGF-DKSs are shown to reduce platelet adhesion, which is crucial for subsequent vascularization and slow release of heparin and VEGF, suggesting its ability of improve neovascularization. Taken together, these data indicated an optimal anticoagulation function of HEP-VEGF-DKSs and the potential of vascularization for regeneration of whole decellularized kidney.

Enlargement, Reduction, and Even Reversal of Relative Migration Speeds of Endothelial and Smooth Muscle Cells on Biomaterials Simply by Adjusting RGD Nanospacing

Although many tissue regeneration processes after biomaterial implantation are related to migrations of multiple cell types on material surfaces, available tools to adjust relative migration speeds are very limited. Herein, we put forward a nanomaterial strategy to employ surface modification with arginine-glycine-aspartate (RGD) nanoarrays to tune in vitro cell migration using endothelial cells (ECs) and smooth muscle cells (SMCs) as demonstrated cell types. We found that migrations of both cell types exhibited a nonmonotonic trend with the increase of RGD nanospacing, yet with different peaks-74 nm for SMCs but 95 nm for ECs. The varied sensitivities afford a facile way to regulate the relative migration speeds. Although ECs migrated at a speed similar to SMCs on a non-nano surface, the migration of ECs could be controlled to be significantly faster or slower than SMCs simply by adjusting the RGD nanospacing. This study suggests a potential application of surface modification of biomaterials on a nanoscale level.

Zwitterionic hydrogel-coated heart valves with improved endothelialization and anti-calcification properties

Valve replacement surgery is the golden standard for end-stage valvular disease due to the lack of self-repair ability. Currently, bioprosthetic heart valves (BHVs) crosslinked by glutaraldehyde (GA) have been the most popular choice in clinic, especially after the emerge of transcatheter aortic valve replacement (TAVR). Nevertheless, the lifespan of BHVs is limited due to severe calcification and deterioration. In this study, to improve the anti-calcification property of BHVs, decellularized heart valves were modified by methacrylic anhydride to introduce double bonds (MADHVs), and a hybrid hydrogel made of sulfobetaine methacrylate (SBMA) and methacrylated hyaluronic acid (MAHA) was then coated onto the surface of MADHVs. Followed by grafting of Arg-Glu-Asp-Val (REDV), an endothelial cell-affinity peptide, the BHVs with improved affinity to endothelial cell (SMHVs-REDV) was obtained. SMHVs-REDV exhibited excellent collagen stability, reliable mechanical property and superior hemocompatibility. Moreover, enhanced biocompatibility and endothelialization potential compared with GA-crosslinked BHVs were achieved. After subcutaneous implantation for 30 days, SMHVs-REDV showed significantly reduced immune response and calcification compared with GA-crosslinked BHVs. Overall, simultaneous endothelialization and anti-calcification can be realized by this strategy, which was supposed to be benefit for improving the main drawbacks for available commercial BHVs products.

Glycosaminoglycans: From Vascular Physiology to Tissue Engineering Applications

Cardiovascular diseases represent the number one cause of death globally, with atherosclerosis a major contributor. Despite the clinical need for functional arterial substitutes, success has been limited to arterial replacements of large-caliber vessels (diameter > 6 mm), leaving the bulk of demand unmet. In this respect, one of the most challenging goals in tissue engineering is to design a "bioactive" resorbable scaffold, analogous to the natural extracellular matrix (ECM), able to guide the process of vascular tissue regeneration. Besides adequate mechanical properties to sustain the hemodynamic flow forces, scaffold's properties should include biocompatibility, controlled biodegradability with non-toxic products, low inflammatory/thrombotic potential, porosity, and a specific combination of molecular signals allowing vascular cells to attach, proliferate and synthesize their own ECM. Different fabrication methods, such as phase separation, self-assembly and electrospinning are currently used to obtain nanofibrous scaffolds with a well-organized architecture and mechanical properties suitable for vascular tissue regeneration. However, several studies have shown that naked scaffolds, although fabricated with biocompatible polymers, represent a poor substrate to be populated by vascular cells. In this respect, surface functionalization with bioactive natural molecules, such as collagen, elastin, fibrinogen, silk fibroin, alginate, chitosan, dextran, glycosaminoglycans (GAGs), and growth factors has proven to be effective. GAGs are complex anionic unbranched heteropolysaccharides that represent major structural and functional ECM components of connective tissues. GAGs are very heterogeneous in terms of type of repeating disaccharide unit, relative molecular mass, charge density, degree and pattern of sulfation, degree of epimerization and physicochemical properties. These molecules participate in a number of vascular events such as the regulation of vascular permeability, lipid metabolism, hemostasis, and thrombosis, but also interact with vascular cells, growth factors, and cytokines to modulate cell adhesion, migration, and proliferation. The primary goal of this review is to perform a critical analysis of the last twenty-years of literature in which GAGs have been used as molecular cues, able to guide the processes leading to correct endothelialization and neo-artery formation, as well as to provide readers with an overall picture of their potential as functional molecules for small-diameter vascular regeneration.

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.

The study of dry biological valve crosslinked with a combination of carbodiimide and polyphenol

The glutaraldehyde crosslinked pericardium has been used in bioprosthetic valves for about 50 years. However, problems such as glutaraldehyde residue and calcification still exist in current commercial products. Non-glutaraldehyde crosslinked dry valve is an important strategy to solve those problems. In this study, a non-glutaraldehyde crosslinked dry biological valve material was obtained by the combined crosslinking of carbodiimide (EDC) and polyphenol. The results showed that the comprehensive properties of EDC and curcumin crosslinked pericardium were superior to glutaraldehyde crosslinked pericardium, including unfolding property, anti-calcification, cytotoxicity, anticoagulant properties, mechanical properties, enzyme degradation resistance and thermal shrinkage temperature. EDC and curcumin crosslinked dry pericardium could flatten after being folded at 40°C for 3 days while glutaraldehyde crosslinked pericardium could not. The calcification of pericardium treated with EDC and curcumin was 1.21 ± 0.36 mg/g in rats after 60 days’ subdermal implantation, much lower than that of glutaraldehyde treated control group (22.06 ± 3.17 mg/g).

Extracellular matrix grafts: From preparation to application (Review)

Recently, the increasing emergency of traffic accidents and the unsatisfactory outcome of surgical intervention are driving research to seek a novel technology to repair traumatic soft tissue injury. From this perspective, decellularized matrix grafts (ECM-G) including natural ECM materials, and their prepared hydrogels and bioscaffolds, have emerged as possible alternatives for tissue engineering and regenerative medicine. Over the past decades, several physical and chemical decellularization methods have been used extensively to deal with different tissues/organs in an attempt to carefully remove cellular antigens while maintaining the non-immunogenic ECM components. It is anticipated that when the decellularized biomaterials are seeded with cells in vitro or incorporated into irregularly shaped defects in vivo, they can provide the appropriate biomechanical and biochemical conditions for directing cell behavior and tissue remodeling. The aim of this review is to first summarize the characteristics of ECM-G and describe their major decellularization methods from different sources, followed by analysis of how the bioactive factors and undesired residual cellular compositions influence the biologic function and host tissue response following implantation. Lastly, we also provide an overview of the in vivoapplication of ECM-G in facilitating tissue repair and remodeling.

Cell migration regulated by RGD nanospacing and enhanced under moderate cell adhesion on biomaterials

While nanoscale modification of a biomaterial surface is known to influence various cell behaviors, it is unclear whether there is an optimal nanospacing of a bioactive ligand with respect to cell migration. Herein, we investigated the effects of nanospacing of arginine-glycine-aspartate (RGD) peptide on cell migration and its relation to cell adhesion. To this end, we prepared RGD nanopatterns with varied nanospacings (31-125 nm) against the nonfouling background of poly(ethylene glycol), and employed human umbilical vein endothelial cells (HUVECs) to examine cell behaviors on the nanopatterned surfaces. While HUVECs adhered well on surfaces of RGD nanospacing less than 70 nm and exhibited a monotonic decrease of adhesion with the increase of RGD nanospacing, cell migration exhibited a nonmonotonic change with the ligand nanospacing: the maximum migration velocity was observed around 90 nm of nanospacing, and slow or very slow migration occurred in the cases of small or large RGD nanospacings. Therefore, moderate cell adhesion is beneficial for fast cell migration. Further molecular biology studies revealed that attenuated cell adhesion and activated dynamic actin rearrangement accounted for the promotion of cell migration, and the genes of small G proteins such as Cdc42 were upregulated correspondingly. The present study sheds new light on cell migration and its relation to cell adhesion, and paves a way for designing biomaterials for applications in regenerative medicine.

Development of Poly(1,8-octanediol-co-citrate-co-ascorbate) Elastomers with Enhanced Ascorbate Performance for Use as a Graft Coating to Prevent Neointimal Hyperplasia

Small-diameter expanded polytetrafluoroethylene (ePTFE) graft surfaces have poor long-term patency due to limited endothelial cell (EC) coverage and anastomotic intimal hyperplasia. Multifunctional elastomers that coat the ePTFE graft surface to promote EC adhesion while simultaneously inhibiting intimal hyperplasia are highly desirable. Poly(diol-co-citrate) (PDC), a thermoset elastomer, is biodegradable, biocompatible, and mimics vascular mechanical properties. Engineering antioxidant components into PDC polymeric structures improves biocompatibility by attenuating oxidative stress yet is limited by bioavailability. Herein, we develop a new ascorbate protection and deprotection strategy (APDS) for loading bioactive ascorbic acid into the structure of PDC elastomers to improve poly(1,8-octanediol-co-citrate-co-ascorbate) (POCA) prepolymer ascorbate activity. Elastomers cured from APDS POCA prepolymers provide twice the active ascorbate sites on the elastomer surface (35.19 ± 1.64 ng mg-1 cm-2) versus unprotected POCA (Un.POCA, 18.31 ± 0.97 ng mg-1 cm-2). APDS POCA elastomers displayed suitable mechanical properties for vascular graft coating [Young's modulus (2.15-2.61 MPa), elongation (189.5-214.6%) and ultimate tensile strength (2.73-3.61 MPa)], and superior surface antioxidant performance through 1,1-diphenyl-2-picrylhydrazyl free radical scavenging and lipid peroxidation inhibition as compared to poly(1,8-octanediol-co-citrate) (POC) and Un.POCA. Hydrolytic degradation of APDS POCA occurred within 12 weeks under physiological conditions with a mass loss of 25.8 ± 3.4% and the degradation product retaining ascorbate activity. APDS POCA elastomer surfaces supported human aortic endothelial cell proliferation while inhibiting human aortic smooth muscle cell proliferation in vitro. APDS POCA elastomer surfaces displayed superior decomposition of S-nitrosothiols compared to POC and Un.POCA. Taken together, these findings indicate the potential of APDS POCA elastomers to serve as bioactive, therapeutic coatings that enhance the long-term patency of small diameter ePTFE grafts.

Polycaprolactone vascular graft with epigallocatechin gallate embedded sandwiched layer-by-layer functionalization for enhanced antithrombogenicity and anti-inflammation

Small-diameter artificial vascular grafts modified with layer-by-layer (LBL) coating show promise in reducing the failure caused by thrombosis and inflammation, but undesirable stability and bioactivity issues of the coating and payload usually limits their long-term efficacy. Herein, inspired by catechol/gallol surface chemistry, a sandwiched layer-by-layer coating constructed by polyethyleneimine (PEI) and heparin with the embedding of epigallocatechin gallate (EGCG)-dexamethasone combination was used to modify the electrospun polycaprolactone (PCL) vascular grafts. Polyphenol embedding endowed the coating with abundant intermolecular interactions between each coating components, mainly contributed by the π-π stacking, weak intermolecular cross-linking and enriched hydrogen bonding, which further enhanced the coating stability and also supported the sustained release of the payloads, like polyelectrolytes and drugs. Compared with the conventional LBL coating, the loading amounts of heparin and dexamethasone in the EGCG embedded LBL coatings doubled and the drug release could be significantly prolonged without serious initial burst. The in vitro and ex vivo assays indicated that the modified PCL vascular grafts would address impressive prolonged anti-platelet adhesion/activation and anti-fibrinogen denaturation ability. Meanwhile, the dexamethasone loading entrusted the sandwiched LBL coating with mild tissue response, in terms of inhibiting the macrophage activation. These results strongly demonstrated that the sandwiched LBL coating with EGCG embedding was an effective method to improve the patency rates of PCL small artificial vascular grafts, which could also be extended to other blood-contacting materials.

Collagenase loaded chitosan nanoparticles for digestion of the collagenous scar in liver fibrosis: The effect of chitosan intrinsic collagen binding on the success of targeting

A variety of hepatic insults result in the accumulation of collagen-rich new extracellular matrix in the liver, ultimately culminating in liver fibrosis and cirrhosis. For such reasons, approaches looking into digestion of the collagen-rich extracellular matrix present an interesting therapeutic approach for cases of chronic liver disease, where the fibrotic scar is well established. Portal collagenase administration has recently led to the successful reversion of cirrhosis in an experimental rabbit model. Notwithstanding, the question of how such a sensitive therapeutic macromolecule could be administered in a less invasive manner, and in a way that preserves its functionality and avoids digestion of other non-hepatic vital collagen presents itself. Chitosan is a biodegradable polymer that has been reported to interact and bind to collagen. Chitosan nanoparticles (CS NPs) have also been reported to encapsulate therapeutic proteins, maintaining their functional form and protecting them from in-vivo degradation. For such reasons, CS NPs were loaded with collagenase and evaluated in-vitro and in-vivo for their ability to target and digest collagen. CS NPs were able to encapsulate collagenase (≈ 60% encapsulation efficiency) and release its content in active form. To determine whether chitosan's collagen interaction would enable NP collagen binding or whether the modification with collagen binding peptides (CBPs) is necessary, CS NPs were modified with the CBP; CCQDSETRTFY. Since the density of targeting ligand and the length of tether play a significant role in the success of active targeting, the surface of NPs was modified with different densities of the CBP either directly or using a polyethylene glycol (PEG) spacer. PEGylated NPs showed higher levels of CBP tagging; high, intermediate and low density of CBPs corresponded to 585.8 ± 33, 252.9 ± 25.3 and 56.5 ± 8.8 µg/mL for PEGylated NPs and 425.56 ± 12.67, 107.91 ± 10.3 and 49.86 ± 3.2 µg/mL for unPEGylated NPs, respectively. In-vitro collagen binding experiments showed that unmodified CS NPs were able to bind collagen and that modification with CBPs either directly or via PEG did not enhance collagen binding. In-vivo experiments demonstrated that unmodified CS NPs were able to reverse fibrosis with a survival rate of 100% at the end of the study, indicating the ability of CS NPs to deliver functional collagenase to the fibrotic liver and making the use of CBPs unnecessary.

Vascularization in tissue engineering: fundamentals and state-of-art

Vascularization is among the top challenges that impede the clinical application of engineered tissues. This challenge has spurred tremendous research endeavor, defined as vascular tissue engineering (VTE) in this article, to establish a pre-existing vascular network inside the tissue engineered graft prior to implantation. Ideally, the engineered vasculature can be integrated into the host vasculature via anastomosis to supply nutrient to all cells instantaneously after surgery. Moreover, sufficient vascularization is of great significance in regenerative medicine from many other perspectives. Due to the critical role of vascularization in successful tissue engineering, we aim to provide an up-to-date overview of the fundamentals and VTE strategies in this article, including angiogenic cells, biomaterial/bio-scaffold design and bio-fabrication approaches, along with the reported utility of vascularized tissue complex in regenerative medicine. We will also share our opinion on the future perspective of this field.

Pre-mounted dry TAVI valve with improved endothelialization potential using REDV-loaded PEGMA hydrogel hybrid pericardium

Schematic diagram for the preparation of hydrogel hybrid dry valve.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Exogenous hyaluronic acid and chondroitin sulfate crosslinking treatment for increasing the amount and stability of glycosaminoglycans in bioprosthetic heart valves

Glutaraldehyde (GLUT) crosslinked bioprosthetic heart valves (BHVs) might fail due to progressive degradation and calcification. GLUT cannot stabilize glycosaminoglycans (GAGs), which are important for BHVs' life time. In this current study we developed a new BHVs preparation strategy using exogenous hyaluronic acid (HA)/chondroitin sulfate (CS) supplement and sodium trimetaphosphate (STP) crosslinking method. Exogenous HA and CS provide additional GAGs for pericardiums. STP could link two GAGs by reacting with hydroxyl groups in GAGs' repeating polysaccharides units. The feeding ratios of HA/CS were optimized. The GAGs content and long-term stability in vitro, biocompatibility, the in vivo GAGs stability and anti-calcification potential of GLUT/HA/CS and STP treated pericardiums were characterized. We demonstrated that GLUT/HA/CS and STP treated pericardiums had sufficiently increased GAGs' amount and stability and decreased calcification. This new exogenous hyaluronic acid/chondroitin sulfate supplement and sodium trimetaphosphate crosslinking strategy would be a promising method to make BHVs with better structural stability and anti-calcification properties.

The tropoelastin and lysyl oxidase treatments increased the content of insoluble elastin in bioprosthetic heart valves

Valvular heart diseases lead to over 300,000 heart valve replacements worldwide each year. Commercially available bioprosthetic heart valves (BHVs) are mostly made from porcine or bovine pericardiums which were crosslinked by glutaraldehyde (GLUT). However, valve failures can occur within 10 years due to progressive degradation and calcification. GLUT could crosslink collagen but it fails to stabilize elastin. In this current study, we developed a new BHVs preparation strategy named as “GLUT/TE/LOXL/EGCG” that utilizes exogenous tropoelastin (TE)/lysyl oxidase (LOXL) and epigallocatechin gallate (EGCG) to increase the elastin content as well as the stabilization of elastin. The feeding ratios of tropoelastin and lysyl oxidase were optimized. The contents of desmosine and insoluble elastin, biomechanics, cytotoxicity, hemocompatibility, in vivo componential stability and anti-calcification potential were characterized. Pericardiums with increased elastin content had improved the mechanical properties. GLUT/TE/LOXL/EGCG-treated pericardiums had similar cytotoxicity and coagulation properties compared to GLUT and GLUT/EGCG control. We demonstrated that GLUT/TE/LOXL/EGCG-treated pericardiums had high amount of insoluble elastin in 90 days’ rat subdermal implantation model, and better resistance for calcification. This new tropoelastin and lysyl oxidase treatments strategy would be a promising method to make BHVs which have better structural stability and anti-calcification properties.

EGCG and enzymatic cross-linking combined treatments for improving elastin stability and reducing calcification in bioprosthetic heart valves

The failures of glutaraldehyde (GLUT) cross-linked bioprosthetic heart valves (BHVs) are mainly due to degeneration and calcification. In this study, we developed a new preparation strategy for BHVs named as "HPA/EDC/EGCG" that utilized 3,4-hydroxyphenylpropionic acid (HPA)-conjugated pericardium, epigallocatechin gallate (EGCG), and horseradish peroxidase (HRP)/hydrogen peroxide (H₂ O₂ ) enzymatic cross-linking. HPA-pericardium conjugation was done by carbodiimide coupling reaction using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). Then HPA-conjugated pericardium was cross-linked by HRP/H₂ O₂ enzyme-catalyzed oxidation. The feeding ratios of HPA and EGCG were optimized. The consumption of amino groups, collagenase and elastase degradation in vitro, biomechanics, extracellular matrix stability, and calcification of HPA-/EDC-/EGCG-treated pericardiums were characterized. We demonstrated that HPA-/EDC-/EGCG-treated pericardiums had better elastin stabilization and less calcification. EGCG and enzymatic cross-linking treated pericardiums showed improved mechanical properties. This new EGCG and enzymatic cross-linking strategy would be a promising method to make BHVs with better elastin stability and anti-calcification property. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1551-1559, 2019.

Immobilization of heparin on decellularized kidney scaffold to construct microenvironment for antithrombosis and inducing reendothelialization

In recent years, rapid development of tissue engineering technology provides possibilities for the construction of artificial tissues or organs. In construction of engineered kidneys, researchers used native decellularized extracellular matrix (ECM) as the scaffolds to recellularization. However, thrombosis has been a great issue that hinders the progress of transplantation in vivo. In this study, heparin was immobilized to the collagen part of decellularized scaffold with collagen-binding peptide (CBP). Through the anticoagulant and endothelial cell reperfusion experiments, it can be demonstrated that the heparinized scaffolds absorbed less platelets and red blood cells which can effectively reduce the formation of thrombosis. Moreover, it is conducive to long-term adhesion of endothelial cells which is important for the formation of subsequent vascularization. Taken together, our results reveal that the whole kidney can be modified by CBP-heparin composite to reduce the thrombosis and provide the better conditions for neovascularization.

Vascular scaffolds with enhanced antioxidant activity inhibit graft calcification

There is a need for off-the-shelf, small-diameter vascular grafts that are safe and exhibit high long-term patency. Decellularized tissues can potentially be used as vascular grafts; however, thrombogenic and unpredictable remodeling properties such as intimal hyperplasia and calcification are concerns that hinder their clinical use. The objective of this study was to investigate the long-term function and remodeling of extracellular matrix (ECM)-based vascular grafts composited with antioxidant poly(1, 8-octamethylene-citrate-co-cysteine) (POCC) with or without immobilized heparin. Rat aortas were decellularized to create the following vascular grafts: 1) ECM hybridized with POCC (Poly-ECM), 2) Poly-ECM subsequently functionalized with heparin (Poly-ECM-Hep), and 3) non-modified vascular ECM. Grafts were evaluated as interposition grafts in the abdominal aorta of adult rats at three months. All grafts displayed antioxidant activity, were patent, and exhibited minimal intramural cell infiltration with varying degrees of calcification. Areas of calcification co-localized with osteochondrogenic differentiation of vascular smooth muscle cells, lipid peroxidation, oxidized DNA damage, and cell apoptosis, suggesting an important role for oxidative stress in the calcification of grafts. The extent of calcification within grafts was inversely proportional to their antioxidant activity: Poly-ECM-Hep > ECM > Poly-ECM. The incorporation of antioxidants into vascular grafts may be a viable strategy to inhibit degenerative changes.

Improving in vivo outcomes of decellularized vascular grafts via incorporation of a novel extracellular matrix

Each year, hundreds of thousands coronary bypass procedures are performed in the US, yet there currently exists no off-the-shelf alternative to autologous vessel transplant. In the present study, we investigated the use of mouse thrombospondin-2 knockout (TSP2 KO) cells, which secrete a non-thrombogenic and pro-migratory extracellular matrix (TSP2 KO ECM), to modify small diameter vascular grafts. To accomplish this, we first optimized the incorporation of TSP2 KO ECM on decellularized rat aortas. Because MMP levels are known to be elevated in TSP2 KO cell culture, it was necessary to probe the effect of the modification process on the graft's mechanical properties. However, no differences were found in suture retention, Young's modulus, or ultimate tensile strength between modified and unmodified grafts. Platelet studies were then performed to determine the time point at which the TSP2 KO ECM sufficiently reduced thrombogenicity. Finally, grafts modified by either TSP2 KO or WT cells or unmodified grafts, were implanted in an abdominal aortic interposition model in rats. After 4 weeks, grafts with incorporated TSP2 KO ECM showed improved endothelial and mural cell recruitment, and a decreased failure rate compared to control grafts. Therefore, our studies show that TSP2 KO ECM could enable the production of off-the-shelf vascular grafts while promoting reconstruction of native vessels.