Decellularized, xenogeneic small-diameter arteries: transition from a muscular to an elastic phenotype in vivo

Stimuli-responsive hydrogels: smart state of-the-art platforms for cardiac tissue engineering

Biomedicine and tissue regeneration have made significant advancements recently, positively affecting the whole healthcare spectrum. This opened the way for them to develop their applications for revitalizing damaged tissues. Thus, their functionality will be restored. Cardiac tissue engineering (CTE) using curative procedures that combine biomolecules, biomimetic scaffolds, and cells plays a critical part in this path. Stimuli-responsive hydrogels (SRHs) are excellent three-dimensional (3D) biomaterials for tissue engineering (TE) and various biomedical applications. They can mimic the intrinsic tissues' physicochemical, mechanical, and biological characteristics in a variety of ways. They also provide for 3D setup, adequate aqueous conditions, and the mechanical consistency required for cell development. Furthermore, they function as competent delivery platforms for various biomolecules. Many natural and synthetic polymers were used to fabricate these intelligent platforms with innovative enhanced features and specialized capabilities that are appropriate for CTE applications. In the present review, different strategies employed for CTE were outlined. The light was shed on the limitations of the use of conventional hydrogels in CTE. Moreover, diverse types of SRHs, their characteristics, assembly and exploitation for CTE were discussed. To summarize, recent development in the construction of SRHs increases their potential to operate as intelligent, sophisticated systems in the reconstruction of degenerated cardiac tissues.

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.

Acellular Human Placenta Small-Diameter Vessels as a Favorable Source of Super-Microsurgical Vascular Replacements: A Proof of Concept

In this study, we aimed to evaluate the human placenta as a source of blood vessels that can be harvested for vascular graft fabrication in the submillimeter range. Our approach included graft modification to prevent thrombotic events. Submillimeter arterial grafts harvested from the human placenta were decellularized and chemically crosslinked to heparin. Graft performance was evaluated using a microsurgical arteriovenous loop (AVL) model in Lewis rats. Specimens were evaluated through hematoxylin-eosin and CD31 staining of histological sections to analyze host cell immigration and vascular remodeling. Graft patency was determined 3 weeks after implantation using a vascular patency test, histology, and micro-computed tomography. A total of 14 human placenta submillimeter vessel grafts were successfully decellularized and implanted into AVLs in rats. An appropriate inner diameter to graft length ratio of 0.81 ± 0.16 mm to 7.72 ± 3.20 mm was achieved in all animals. Grafts were left in situ for a mean of 24 ± 4 days. Decellularized human placental grafts had an overall patency rate of 71% and elicited no apparent immunological responses. Histological staining revealed host cell immigration into the graft and re-endothelialization of the vessel luminal surface. This study demonstrates that decellularized vascular grafts from the human placenta have the potential to serve as super-microsurgical vascular replacements.

Acellular Vascular Scaffolds Preloaded With Heparin and Hepatocyte Growth Factor for Small-Diameter Vascular Grafts Might Inhibit Intimal Hyperplasia

To develop small-diameter (<6 mm) scaffolds capable of accelerating rapid endothelialization and improving long-term patency rate, we created acellular vascular scaffolds preloaded with heparin and hepatocyte growth factor (HGF). Heparin was conjugated to suppress thrombogenic responses, and HGF was immobilized to induce endothelial cells (ECs) proliferation and migration. The scaffolds immobilized with heparin exhibited highly effective localization and sustained release of HGF for 30 days in vitro. We implanted this modified scaffold into the carotid artery of a rabbit model to investigate the efficacy in vivo. The acellular vascular scaffold with heparin only was used as control. After transplantation, the patency of this modified scaffold was 91.67% at 1, 3, 6, and 12 months, while the patency rate in the group with grafted heparin only was 83.33% at 1, 3, 6, and 12 months. This modified scaffold significantly stimulated ECs proliferation and the endothelium aligned in the direction of flow after 12 months. In addition, intimal hyperplasia was significantly reduced in the grafts coated with HGF compared with the control grafts. The small-diameter vascular grafts with an inner diameter of 2.5 mm preloaded with heparin and HGF may be a substitute for autologous blood vessels in clinic.

Riboflavin-mediated photooxidation to improve the characteristics of decellularized human arterial small diameter vascular grafts

Vascular grafts with a diameter of less than 6 mm are made from a variety of materials and techniques to provide alternatives to autologous vascular grafts. Decellularized materials have been proposed as a possible approach to create extracellular matrix (ECM) vascular prostheses as they are naturally derived and inherently support various cell functions. However, these desirable graft characteristics may be limited by alterations of the ECM during the decellularization process leading to decreased biomechanical properties and hemocompatibility. In this study, arteries from the human placenta chorion were decellularized using two distinct detergents (Triton X-100 or SDS), which differently affect ECM ultrastructure. To overcome biomechanical strength loss and collagen fiber exposure after decellularization, riboflavin-mediated UV (RUV) crosslinking was used to uniformly crosslink the collagenous ECM of the grafts. Graft characteristics and biocompatibility with and without RUV crosslinking were studied in vitro and in vivo. RUV-crosslinked ECM grafts showed significantly improved mechanical strength and smoothening of the luminal graft surfaces. Cell seeding using human endothelial cells revealed no cytotoxic effects of the RUV treatment. Short-term aortic implants in rats showed cell migration and differentiation of host cells. Functional graft remodeling was evident in all grafts. Thus, RUV crosslinking is a preferable tool to improve graft characteristics of decellularized matrix conduits.

In Vivo Performance of Decellularized Vascular Grafts: A Review Article

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

Acellular vascular matrix grafts from human placenta chorion: Impact of ECM preservation on graft characteristics, protein composition and in vivo performance

Small diameter vascular grafts from human placenta, decellularized with either Triton X-100 (Triton) or SDS and crosslinked with heparin were constructed and characterized. Graft biochemical properties, residual DNA, and protein composition were evaluated to compare the effect of the two detergents on graft matrix composition and structural alterations. Biocompatibility was tested in vitro by culturing the grafts with primary human macrophages and in vivo by subcutaneous implantation of graft conduits (n = 7 per group) into the flanks of nude rats. Subsequently, graft performance was evaluated using an aortic implantation model in Sprague Dawley rats (one month, n = 14). In situ graft imaging was performed using MRI angiography. Retrieved specimens were analyzed by electromyography, scanning electron microscopy, histology and immunohistochemistry to evaluate cell migration and the degree of functional tissue remodeling. Both decellularization methods resulted in grafts of excellent biocompatibility in vitro and in vivo, with low immunogenic potential. Proteomic data revealed removal of cytoplasmic proteins with relative enrichment of ECM proteins in decelluarized specimens of both groups. Noteworthy, LC-Mass Spectrometry analysis revealed that 16 proteins were exclusively preserved in Triton decellularized specimens in comparison to SDS-treated specimens. Aortic grafts showed high patency rates, no signs of thrombus formation, aneurysms or rupture. Conduits of both groups revealed tissue-specific cell migration indicative of functional remodeling. This study strongly suggests that decellularized allogenic grafts from the human placenta have the potential to be used as vascular replacement materials. Both detergents produced grafts with low residual immunogenicity and appropriate mechanical properties. Observed differences in graft characteristics due to preservation method had no impact on successful in vivo performance in the rodent model.

Decellularized and Secured Porcine Arteries with NaOH-based Process: Proof of Concept

BACKGROUND

There is a need for small caliber vascular prosthesis. Synthetic grafts are hindered by thrombogenicity and rapid occlusion. Decellularized matrices could be an alternative. We assessed in vitro and in vivo the biocompatibility of porcine artery treated with a chemical/physical process for decellularization and graft securitization with non/conventional pathogens inactivation.

METHODS

Porcine carotid arteries (PCA) were treated. First, biopsies (n = 4/tissue) were performed before/after treatment to assess decellularization (hematoxylin and eosin/-4',6-diamidino-2-phenylindole/DNA/Miller). Second, 5 rats received an abdominal aortic patch of decellularized PCA (DPCA). Four pigs received subcutaneous DPCA implants (n = 2/pig). Half were explanted at day 15 and half at day 30. Finally, 2 pigs received DPCA (n = 2) and polytetrafluoroethylene prosthesis (n = 1), respectively, as carotid interposition. Implants were removed at day 30. Inflammation (CD3 and CD68 immunostaining) calcifications (von Kossa staining), remodeling (hematoxylin and eosin), and vascular characterization (CD31 and alpha-smooth muscle actin immunofluorescent staining) were investigated.

RESULTS

Ninety-five percentage of decellularization was obtained without structural deterioration. No death occurred. Low inflammatory reaction was found in the 2 models for DPCA. Acquisition of vascular identity was confirmed in the rodent and porcine models. Similarity between native PCA and DPCA was observed after 30 days. In contrast, polytetrafluoroethylene graft showed severe calcifications, higher CD3 reaction, and higher intimal hyperplasia (P < 0.05).

CONCLUSIONS

The physical and chemical process ensures decellularization of carotid porcine arteries and their in vivo remodeling with the presence of an endothelium and smooth-muscle-like cells as well as a low level of inflammatory cells.

Mechanocompatible Polymer-Extracellular-Matrix Composites for Vascular Tissue Engineering

Small-diameter vascular grafts developed from vascular extracellular matrix (ECM) can potentially be used for bypass surgeries and other vascular reconstruction and repair procedures. The addition of heparin to the ECM improves graft hemocompatibility but often involves chemical cross-linking, which increases ECM mechanical stiffness compared to native arteries. Herein, the importance of maintaining ECM mechanocompatibility is demonstrated, and a mechanocompatible strategy to immobilize heparin onto the ECM via a biodegradable elastomer is described. Specifically, poly(1,8-octamethylene citrate)-co-cysteine is hybridized to the ECM, forming a polymer-ECM composite that allows for heparin immobilization via maleimide-thiol "click" chemistry. Heparinized composites reduce platelet adhesion by >60% in vitro, without altering the elastic modulus of the ECM. In a rat abdominal aortic interposition model, intimal hyperplasia in heparinized mechanocompatible grafts is 65% lower when compared to ECM-only control grafts at four weeks. In contrast, grafts that are heparinized with carbodiimide chemistry exhibit increased intimal hyperplasia (4.2-fold) and increased macrophage infiltration (3.5-fold) compared to ECM-only control grafts. All grafts show similar, partial endothelial cell coverage and little to no ECM remodeling. Overall, a mechanocompatible strategy to improve ECM thromboresistance is described and the importance of ECM mechanical properties for proper in vivo graft performance is highlighted.

Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements

Biodegradable vascular grafts with sufficient in vivo performance would be more advantageous than permanent non-degradable prostheses. These constructs would be continuously replaced by host tissue, leading to an endogenous functional implant which would adapt to the need of the patient and exhibit only limited risk of microbiological graft contamination. Adequate biomechanical strength and a wall structure which promotes rapid host remodeling are prerequisites for biodegradable approaches. Current approaches often reveal limited tensile strength and therefore require thicker or reinforced graft walls. In this study we investigated the in vitro and in vivo biocompatibility of thin host-vessel-matched grafts (n=34) formed from hard-block biodegradable thermoplastic polyurethane (TPU). Expanded polytetrafluoroethylene (ePTFE) conduits (n=34) served as control grafts. Grafts were analyzed by various techniques after retrieval at different time points (1 week; 1, 6, 12 months). TPU grafts showed significantly increased endothelial cell proliferation in vitro (P<0.001). Population by host cells increased significantly in the TPU conduits within 1 month of implantation (P=0.01). After long-term implantation, TPU implants showed 100% patency (ePTFE: 93%) with no signs of aneurysmal dilatation. Substantial remodeling of the degradable grafts was observed but varied between subjects. Intimal hyperplasia was limited to ePTFE conduits (29%). Thin-walled TPU grafts offer a new and desirable form of biodegradable vascular implant. Degradable grafts showed equivalent long-term performance characteristics compared to the clinically used, non-degradable material with improvements in intimal hyperplasia and ingrowth of host cells.

The in vivo performance of small-caliber nanofibrous polyurethane vascular grafts

BACKGROUND

In a previous in vitro study, we confirmed that small-caliber nanofibrous polyurethane (PU) vascular grafts have favorable mechanical properties and biocompatibility. In the present study, we examined the in vivo biocompatibility and stability of these grafts.

METHODS

Forty-eight adult male beagle dogs were randomly divided into two groups receiving, respectively, polyurethane (PU) or polytetrafluoroethylene (PTFE) grafts (n = 24 animals / group). Each group was studied at 4, 8, 12, and 24 weeks after graft implantation. Blood flow was analyzed by color Doppler ultrasound and computed tomography angiography. Patency rates were judged by animal survival rates. Coverage with endothelial and smooth muscle cells was characterized by hematoxylin-eosin and immunohistological staining, and scanning electron microscopy (SEM).

RESULTS

Patency rates were significantly higher in the PU group (p = 0.02 vs. PTFE group). During the first 8 weeks, endothelial cells gradually formed a continuous layer on the internal surface of PU grafts, whereas coverage of PTFE graft by endothelial cells was inhomogeneous. After 12 weeks, neointimal thickness remained constant in the PU group, while PTFE group showed neointimal hyperplasia. At 24 weeks, some anastomotic sites of PTFE grafts became stenotic (p = 0.013 vs. PU group). Immunohistological staining revealed a continuous coverage by endothelial cells and an orderly arrangement of smooth muscle cells on PU grafts. Further, SEM showed smooth internal surfaces in PU grafts without thrombus or obvious neointimal hyperplasia.

CONCLUSIONS

Small-caliber nanofibrous PU vascular grafts facilitate the endothelialization process, prevent excessive neointimal hyperplasia, and improve patency rates.

Tissue engineering for the oncologic urinary bladder

Urinary diversion after radical cystectomy in patients with bladder cancer normally takes the form of an ileal conduit or neobladder. However, such diversions are associated with a number of complications including increased risk of infection. A plausible alternative is the construction of a neobladder (or bladder tissue) in vitro using autologous cells harvested from the patient. Biomaterials can be used as a scaffold for naturally occurring regenerative stem cells to latch onto to regrow the bladder smooth muscle and epithelium. Such engineered tissues show great promise in urologic tissue regeneration, but are faced with a number of challenges. For example, the differentiation mesenchymal stem cells from various sources can be difficult and the smooth muscle cells formed do not precisely mimic the natural cells.

Integration of multiple cell-matrix interactions into alginate scaffolds for promoting cardiac tissue regeneration

Cardiac tissue engineering aims to repair damaged myocardial tissues by applying heart patches created in vitro. Herein, we explored the possible role of a combination of two matrix-attached peptides, the adhesion peptide G(4)RGDY and heparin-binding peptide G(4)SPPRRARVTY (HBP) in cardiac tissue regeneration. Neonatal rat cardiac cells were seeded into unmodified, single peptide or double peptide-attached alginate scaffolds, all having the same physical features of porosity, hydrogel forming and matrix stiffness. The cardiac tissue developed in the HBP/RGD-attached scaffolds revealed the best features of a functional muscle tissue, as judged by all studied parameters, i.e., immunostaining of cardiac cell markers, histology, western blot of protein expressions and metabolic activity. By day 7, well-developed myocardial fibers were observed in these cell constructs. At 14 days the HBP/RGD-attached constructs presented an isotropic myofiber arrangement, while no such arrangement was seen in the other constructs. The expression levels of α-actinin, N-cadherin and Connexin-43, showing preservation and an increase in Connexin-43 expression (Cx-43) with time, further supported the formation a contractile muscle tissue in the HBP/RGD-attached scaffolds. Collectively, the attachment of combinatorial peptides representing different signaling in ECM-cell interactions proved to play a key role, contributing to the formation of a functional cardiac muscle tissue, in vitro.

Fibroblast and vascular endothelial growth factor coating of decellularized vascular grafts stimulates undesired giant cells and graft encapsulation in a rat model

Replacing an infected prosthesis with a bioimplant provides a hopeful alternative in septic vascular surgery. The objective of this study was to determine the effect of fibroblast endothelial growth factors (FGF) and vascular endothelial growth factors (VEGF) coating on a decellularized vascular graft in a rat model and the possible impact on recellularization processes. Rat aortas were decellularized, crosslinked with genipin, and coated with poly-(D, L) lactide containing either FGF or VEGF. Observation periods were 6 and 12 weeks. Surprisingly, we found moderate accumulation of giant cells around the grafts that contained poly-(D, L) lactide acid. FGF and VEGF grafts showed massive stimulation of giant cells and eosinophils leading to complete graft encapsulation (P < 0.05). Pseudointmal hyperplasia was significantly increased in the FGF group (P < 0.05). Both results can only be interpreted as very negative. We achieved a situation in diametric opposition to that which we had hoped for. These data demonstrate that the use of growth factors may produce harmful side effects.