Preimplantation processing of ex vivo-derived vascular biomaterials: effects on peripheral cell adhesion

Acellular carotid scaffold and evaluation the biological and biomechanical properties for tissue engineering

Introduction

The issues associated with the limitation of appropriate autologous vessels for vascular reconstruction via bypass surgery highlight the need for new alternative strategies based on tissue engineering. The present study aimed to prepare decellularized scaffolds from ovine carotid using chemical decellularization method.

Methods

Ovine carotid were decellularized with Triton X-100 and tri-n-butyl phosphate (TnBP) at 37 °C. Histological analysis, biochemical tests, biomechanical assay and biocompatibility assay were used to investigate the efficacy of decellularization.

Results

Decellularization method could successfully decellularize ovine carotid without leaving any cell remnants. Scaffolds showed minimal destruction of the three-dimensional structure and extracellular matrix, as well as adequate mechanical resistance and biocompatibility for cell growth and proliferation.

Conclusion

Prepared acellular scaffold exhibited the necessary characteristics for clinical applications.

Decellularization for the retention of tissue niches

Decellularization of natural tissues to produce extracellular matrix is a promising method for three-dimensional scaffolding and for understanding microenvironment of the tissue of interest. Due to the lack of a universal standard protocol for tissue decellularization, recent investigations seek to develop novel methods for whole or partial organ decellularization capable of supporting cell differentiation and implantation towards appropriate tissue regeneration. This review provides a comprehensive and updated perspective on the most recent advances in decellularization strategies for a variety of organs and tissues, highlighting techniques of chemical, physical, biological, enzymatic, or combinative-based methods to remove cellular contents from tissues. In addition, the review presents modernized approaches for improving standard decellularization protocols for numerous organ types.

Porcine carotid arteries decellularized with a suitable concentration combination of Triton X-100 and sodium dodecyl sulfate for tissue engineering vascular grafts

Tissue engineering vascular grafts (TEVGs) constructed by decellularized arteries have the potential to replace autologous blood vessels in bypass surgery for patients with cardiovascular disease. There are various methods of decellularization without a standard protocol. Detergents approaches are simple, and easy control of experimental conditions. Non-ionic detergent Triton X-100 and ionic detergent sodium dodecyl sulfate (SDS) are the most commonly used detergents. In this study, we used Triton X-100 and SDS with different concentrations to decellularize porcine carotid arteries. After that, we investigated the acellular effect and mechanical properties of decellularized arteries to find a promising concentration combination for decellularization. Results showed that any detergents' combination would damage the inherent structure of extracellular matrix, and the destruction increased with the increase of detergents' concentration. We concluded that the decellularization approach of 0.5% Triton X-100 for 24 h combined with 0.25% SDS for 72 h could help to obtain decellularized arteries with minimum destruction. This protocol may be able to prepare a clinically suitable vascular scaffold for TEVGs.

Flow with variable pulse frequencies accelerates vascular recellularization and remodeling of a human bioscaffold

Despite significant advances in vascular tissue engineering, the ideal graft has not yet been developed and autologous vessels remain the gold standard substitutes for small diameter bypass procedures. Here, we explore the use of a flow field with variable pulse frequencies over the regeneration of an ex vivo-derived human scaffold as vascular graft. Briefly, human umbilical veins were decellularized and used as scaffold for cellular repopulation with human smooth muscle cells (SMC) and endothelial cells (EC). Over graft development, the variable flow, which mimics the real-time cardiac output of an individual performing daily activities (e.g., resting vs. exercising), was implemented and compared to the commonly used constant pulse frequency. Results show marked differences on SMC and EC function, with changes at the molecular level reflecting on tissue scales. First, variable frequencies significantly increased SMC proliferation rate and glycosaminoglycan production. These results can be tied with the SMC gene expression that indicates a synthetic phenotype, with a significant downregulation of myosin heavy chain. Additionally and quite remarkably, the variable flow frequencies motivated the re-endothelialization of the grafts, with a quiescent-like structure observed after 10 days of conditioning, contrasting with the low surface coverage and unaligned EC observed under constant frequency (CF). Besides, the overall biomechanics of the generated grafts (conditioned with both pulsed and CFs) evidence a significant remodeling after 55 days of culture, depicted by high burst pressure and Young's modulus. These last results demonstrate the positive recellularization and remodeling of a human-derived scaffold toward an arterial vessel.

Sequential adaptation of perfusion and transport conditions significantly improves vascular construct recellularization and biomechanics

Recellularization of ex vivo-derived scaffolds remains a significant hurdle primarily due to the scaffolds subcellular pore size that restricts initial cell seeding to the scaffolds periphery and inhibits migration over time. With the aim to improve cell migration, repopulation, and graft mechanics, the effects of a four-step culture approach were assessed. Using an ex vivo-derived vein as a model scaffold, human smooth muscle cells were first seeded onto its ablumen (Step 1: 3 hr) and an aggressive 0-100% nutrient gradient (lumenal flow under hypotensive pressure) was created to initiate cell migration across the scaffold (Step 2: Day 0 to 19). The effects of a prolonged aggressive nutrient gradient created by this single lumenal flow was then compared with a dual flow (lumenal and ablumenal) in Step 3 (Day 20 to 30). Analyses showed that a single lumenal flow maintained for 30 days resulted in a higher proportion of cells migrating across the scaffold toward the vessel lumen (nutrient source), with improved distribution. In Step 4 (Day 31 to 45), the transition from hypotensive pressure (12/8 mmHg) to normotensive (arterial-like) pressure (120/80 mmHg) was assessed. It demonstrated that recellularized scaffolds exposed to arterial pressures have increased glycosaminoglycan deposition, physiological modulus, and Young's modulus. By using this stepwise conditioning, the challenging recellularization of a vein-based scaffold and its positive remodeling toward arterial biomechanics were obtained.

Tissue-Engineered Grafts from Human Decellularized Extracellular Matrices: A Systematic Review and Future Perspectives

Tissue engineering and regenerative medicine involve many different artificial and biologic materials, frequently integrated in composite scaffolds, which can be repopulated with various cell types. One of the most promising scaffolds is decellularized allogeneic extracellular matrix (ECM) then recellularized by autologous or stem cells, in order to develop fully personalized clinical approaches. Decellularization protocols have to efficiently remove immunogenic cellular materials, maintaining the nonimmunogenic ECM, which is endowed with specific inductive/differentiating actions due to its architecture and bioactive factors. In the present paper, we review the available literature about the development of grafts from decellularized human tissues/organs. Human tissues may be obtained not only from surgery but also from cadavers, suggesting possible development of Human Tissue BioBanks from body donation programs. Many human tissues/organs have been decellularized for tissue engineering purposes, such as cartilage, bone, skeletal muscle, tendons, adipose tissue, heart, vessels, lung, dental pulp, intestine, liver, pancreas, kidney, gonads, uterus, childbirth products, cornea, and peripheral nerves. In vitro recellularizations have been reported with various cell types and procedures (seeding, injection, and perfusion). Conversely, studies about in vivo behaviour are poorly represented. Actually, the future challenge will be the development of human grafts to be implanted fully restored in all their structural/functional aspects.

Approaches to improve integration and regeneration of an ex vivo derived temporomandibular joint disc scaffold with variable matrix composition

Due to their natural biochemical and biomechanical characteristics, using ex vivo tissues as platforms for guided tissue regeneration has become widely accepted, however subsequent attachment and integration of these constructs in vivo is often overlooked. A decellularized porcine temporomandibular joint (TMJ) disc has shown promise as a scaffold to guide disc regeneration and preliminary work has shown the efficacy of surfactant (SDS) treatment within the fibrocartilaginous disc to remove cellular components. The majority of studies focus on the intermediate region of the disc (or disc proper). Using this approach, inherent attachment tissues can be maintained to improve construct stability and integration within the joint. Unlike human disc attachment tissue, the porcine attachment tissues have high lipid content which would require a different processing approach to remove immunogenic components. In order to examine the effect of delipidation on the attachment tissue properties, SDS and two organic solvent mixtures (acetone/ethanol and chloroform/methanol) were compared. Lipid and cellular solubilization, ECM alteration, and seeded human mesenchymal stem cell (MSC) morphology and viability were assessed. Quantitative analysis showed SDS treatments did not effectively delipidate the attachment tissues and cytotoxicity was noted toward MSC in these regions. Acetone/ethanol removed cellular material but not all lipids, while chloroform/methanol removed all visible lipid deposits but residual porcine cells were observed in histological sections. When a combination of approaches was used, no residual lipid or cytotoxicity was noted. Preparing a whole TMJ graft with a combined approach has the potential to improve disc integration within the native joint environment.

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.

Laser micro-ablation of fibrocartilage tissue: Effects of tissue processing on porosity modification and mechanics

The temporomandibular joint disk (TMJd) is an extremely dense and avascular fibrocartilaginous extracellular matrix (ECM) resulting in a limited regenerative capacity. The use of decellularized TMJd as a biocompatible scaffold to guide tissue regeneration is restricted by innate subcellular porosity of the ECM that hinders cellular infiltration and regenerative events. Incorporation of an artificial microporosity through laser micro-ablation (LMA) can alleviate these cell and diffusion based limitations. In this study, LMA was performed either before or after decellularization to assess to effect of surfactant treatment on porosity modification as well as the resultant mechanical and physical scaffold properties. Under convective flow or agitation schemes, pristine and laser ablated disks were decellularized using either low (0.1% w/v) or high (1% w/v) concentrations of sodium dodecyl sulfate (SDS). Results show that lower concentrations of SDS minimized collagen degradation and tissue swelling while retaining its capacity to solubilize cellular content. Regardless of processing scheme, laser ablated channels incorporated after SDS treatment were relatively smaller and more uniform than those incorporated before SDS treatment, indicating an altered laser interaction with surfactant treated tissues. Smaller channels correlated with less disruption of native biomechanical properties indicating surfactant pre-treatment is an important consideration when using LMA to produce artificial porosity in ex vivo derived tissues. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1858-1868, 2018.

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.

Human Umbilical Vessels: Choosing the Optimal Decellularization Method

There is an increasing demand of small-diameter vascular grafts for treatment of circulatory pathologies. Decellularization offers the possibility of using human blood vessels as scaffolds to create vascular grafts. Umbilical vessels have great potential because of their availability and morphological characteristics. Various decellularization techniques have been used in umbilical vessels, but consensus on which is the most appropriate has not yet been reached. The objective of this review is to analyze the morphological and biomechanical characteristics of decellularized human umbilical arteries and veins with different techniques. Evidence indicates that the umbilical vessels are a viable option to develop small-diameter vascular grafts. Detergents are the agents most often used and with most evidence. However, further studies are needed to accurately analyze the components of the extracellular matrix and biomechanical characteristics, as well as the capacity for recellularization and in vivo functionality.

Use of polyvinylpyrrolidone-iodine solution for sterilisation and preservation improves mechanical properties and osteogenesis of allografts

Allografts eliminate the disadvantages associated with autografts and synthetic scaffolds but are associated with a disease-transmission risk. Therefore, allograft sterilisation is crucial. We aimed to determine whether polyvinylpyrrolidone-iodine (PVP-I) can be used for sterilisation and as a new wet-preservation method. PVP-I-sterilised and preserved allografts demonstrated improved mechanical property, osteogenesis, and excellent microbial inhibition. A thigh muscle pouch model of nude mice showed that PVP-I-preserved allografts demonstrated better ectopic formation than Co₆₀-sterilised allografts (control) in vivo (P < 0.05). Furthermore, the PVP-I-preserved group showed no difference between 24 h and 12 weeks of allograft preservation (P > 0.05). PVP-I-preserved allografts showed more hydrophilic surfaces and PVP-I-sterilised tendons showed higher mechanical strength than Co₆₀-sterilised tendons (P < 0.05). The level of residual PVP-I was higher without washing and with prolonged preservation (P < 0.05). In vitro cellular tests showed that appropriate PVP-I concentration was nontoxic to preosteoblast cells, and cellular differentiation measured by alkaline phosphatase activity and osteogenic gene markers was enhanced (P < 0.05). Therefore, the improved biological performance of implanted allografts may be attributable to better surface properties and residual PVP-I, and PVP-I immersion can be a simple, easy method for allograft sterilisation and preservation.

The consequence of biologic graft processing on blood interface biocompatibility and mechanics

Processing ex vivo derived tissues to reduce immunogenicity is an effective approach to create biologically complex materials for vascular reconstruction. Due to the sensitivity of small diameter vascular grafts to occlusive events, the effect of graft processing on critical parameters for graft patency, such as peripheral cell adhesion and wall mechanics, requires detailed analysis. Isolated human umbilical vein sections were used as model allogenic vascular scaffolds that were processed with either: 1. sodium dodecyl sulfate (SDS), 2. ethanol/acetone (EtAc), or 3. glutaraldehyde (Glu). Changes in material mechanics were assessed via uniaxial tensile testing. Peripheral cell adhesion to the opaque grafting material was evaluated using an innovative flow chamber that allows direct observation of the blood-graft interface under physiological shear conditions. All treatments modified the grafts tensile strain and stiffness properties, with physiological modulus values decreasing from Glu 240±12 kPa to SDS 210±6 kPa and EtAc 140±3 kPa, P<.001. Relative to glutaraldehyde treatments, neutrophil adhesion to the decellularized grafts increased, with no statistical difference observed between SDS or EtAc treatments. Early platelet adhesion (% surface coverage) showed no statistical difference between the three treatments; however, quantification of platelet aggregates was significantly higher on SDS scaffolds compared to EtAc or Glu. Tissue processing strategies applied to the umbilical vein scaffold were shown to modify structural mechanics and cell adhesion properties, with the EtAc treatment reducing thrombotic events relative to SDS treated samples. This approach allows time and cost effective prescreening of clinically relevant grafting materials to assess initial cell reactivity.

Effect of Urea and Thiourea on Generation of Xenogeneic Extracellular Matrix Scaffolds for Tissue Engineering

Effective solubilization of proteins by chaotropes in proteomic applications motivates their use in solubilization-based antigen removal/decellularization strategies. A high urea concentration has previously been reported to significantly reduce lipophilic antigen content of bovine pericardium (BP); however, structure and function of the resultant extracellular matrix (ECM) scaffold were compromised. It has been recently demonstrated that in vivo ECM scaffold fate is determined by two primary outcome measures as follows: (1) sufficient reduction in antigen content to avoid graft-specific adaptive immune responses and (2) maintenance of native ECM structural proteins to avoid graft-specific innate responses. In this work, we assessed residual antigenicity, ECM architecture, ECM content, thermal stability, and tensile properties of BP subjected to a gradient of urea concentrations to determine whether an intermediate concentration exists at which both antigenicity and structure-function primary outcome measures for successful in vivo scaffold outcome can simultaneously be achieved. Alteration in tissue structure-function properties at various urea concentrations with decreased effectiveness for antigen removal makes use of urea-mediated antigen removal unlikely to be suitable for functional scaffold generation.

Physiologically Modeled Pulse Dynamics to Improve Function in In Vitro-Endothelialized Small-Diameter Vascular Grafts

The lack of a functional endothelium on small-diameter vascular grafts leads to intimal hyperplasia and thrombotic occlusion. Shear stress conditioning through controlled hydrodynamics within in vitro perfusion bioreactors has shown promise as a mechanism to drive endothelial cell (EC) phenotype from an activated, pro-inflammatory wound state toward a quiescent functional state that inhibits responses that lead to occlusive failure. As part of an overall design strategy to engineer functional vascular grafts, we present a novel two-phase shear conditioning approach to improve graft endothelialization. Axial rotation was first used to seed uniform EC monolayers onto the lumenal surface of decellularized scaffolds derived from the human umbilical vein. Using computer-controlled perfusion circuits, a flow-ramping paradigm was applied to adapt endothelia to arterial levels of fluid shear stress and pressure without graft denudation. The effects of constant pulse frequencies (CF) on EC quiescence were then compared with pulse frequencies modeled from temporal fluctuations in blood flow observed in vivo, termed physiologically modeled pulse dynamics (PMPD). Constructs exposed to PMPD for 72 h expressed a more functional transcriptional profile, lower metabolic activity (39.8% ± 8.4% vs. 62.5% ± 11.5% reduction, p = 0.012), and higher nitric oxide production (80.42 ± 23.93 vs. 48.75 ± 6.93 nmol/10(5) cells, p = 0.028) than those exposed to CF. By manipulating in vitro flow conditions to mimic natural physiology, endothelialized vascular grafts can be stimulated to express a nonactivated phenotype that would better inhibit peripheral cell adhesion and smooth muscle cell hyperplasia, conditions that typically lead to occlusive failure. Development of robust, functional endothelia on vascular grafts by modulation of environmental conditions within perfusion bioreactors may ultimately improve clinical outcomes in vascular bypass grafting.

Blood vessel matrix seeded with cells: a better alternative for abdominal wall reconstruction-a long-term study

PURPOSE

The aim of this study was to present abdominal wall reconstruction using a porcine vascular graft seeded with MSC (mesenchymal stem cells) on rat model.

MATERIAL AND METHODS

Abdominal wall defect was prepared in 21 Wistar rats. Acellular porcine-vascular grafts taken from aorta and prepared with Triton X were used. 14 aortic grafts were implanted in place, of which 7 grafts were seeded with rat MSC cells (Group I), and 7 were acellular grafts (Group II). As a control, 7 standard polypropylene meshes were used for defect augmentation (Group III). The assessment method was performed by HE and CD31 staining after 6 months. The mechanical properties have been investigated by Zwick&Roell Z0.5.

RESULTS

The strongest angiogenesis and lowest inflammatory response were observed in Group I. Average capillaries density was 2.75, 0.75, and 1.53 and inflammatory effect was 0.29, 1.39, and 2.72 for Groups I, II, and III, respectively. The means of mechanical properties were 12.74 ± 1.48, 7.27 ± 1.56, and 14.4 ± 3.7 N/cm in Groups I and II and control, respectively.

CONCLUSIONS

Cell-seeded grafts have better mechanical properties than acellular grafts but worse than polypropylene mesh. Cells improved mechanical and physiological properties of decellularized natural scaffolds.

The effect of terminal sterilization on structural and biophysical properties of a decellularized collagen-based scaffold; implications for stem cell adhesion

Terminal sterilization induces physical and chemical changes in the extracellular matrix (ECM) of ex vivo-derived biomaterials due to their aggressive mechanism of action. Prior studies have focused on how sterilization affects the mechanical integrity of tissue-based biomaterials but have rarely characterized effects on early cellular interaction, which is indicative of the biological response. Using a model fibrocartilage disc scaffold, these investigations compare the effect of three common sterilization methods [peracetic acid (PAA), gamma irradiation (GI), and ethylene oxide (EtO)] on a range of material properties and characterized early cellular interactions. GI and EtO produced unfavorable structural damage that contributed to inferior cell adhesion. Conversely, exposure to PAA resulted in limited structural alterations while inducing chemical modifications that favored cell attachment. Results suggest that the sterilization approach can be selected to modulate biomaterial properties to favor cellular adhesion and has relevance in tissue engineering and regenerative medicine applications. Furthermore, the study of cellular interactions with modified biomaterials in vitro provides information of how materials may react in subsequent clinical applications.

In vitro method for real-time, direct observation of cell-vascular graft interactions under simulated blood flow

In the development of engineered vascular grafts, assessing the material's interactive properties with peripheral blood cells and its capacity to endothelialize are important for predicting in vivo graft behavior. Current in vitro techniques used for characterizing cell adhesion at the surface of engineered scaffolds under flow only facilitate a terminal quantification of cell/surface interactions. Here, we present the design of an innovative flow chamber for real-time analysis of blood-biomaterial interactions under controllable hemodynamic conditions. Decellularized human umbilical veins (dHUV) were used as model vascular allografts to characterize platelet, leukocyte, and endothelial cell (EC) adhesion dynamics. Confluent EC monolayers adhered to the lumenal surface of the grafting material were flow conditioned to resist arterial shear stress levels (up to 24 dynes/cm(2)) over a 48 h period, and shown to maintain viability over the 1 week assessment period. The basement membrane was imaged while whole blood/neutrophil suspensions were perfused across the HUV surface to quantify cell accumulation. This novel method facilitates live visualization of dynamic events, including cell adhesion, migration, and morphological adaptation at the blood-graft interface on opaque materials, and it can be used for preliminary assessment of clinically relevant biomaterials before implantation.

Tuning scaffold mechanics by laminating native extracellular matrix membranes and effects on early cellular remodeling

At approximately 50 µm thin, the human amniotic membrane (hAM) has been shown to be a versatile biomaterial with applications ranging from ocular transplants to skin and nerve regeneration. These investigations describe laminating layers of the hAM into a multilayered, conformation creating a thicker, more robust biomaterial for applications requiring more supportive structures. Amniotic membranes were decellularized using 4 M NaCl and prepared as either flat single-layered sheets or rolled into concentric five-layered configurations. Constructs were seeded with human vascular smooth muscle cells and cultured over 40 days to quantify biological and mechanical changes that occurred during early remodeling events. By day 40 single-layered constructs displayed a decreasing trend in cellular densities and glycosaminoglycan (GAG) concentration, comparative to multilayered constructs with increasing cell densities (from 9.1 to 32 × 10(6) cells/g) and GAG concentrations (from 6.07 to 17.4 mg/g). Oxygen diffusion was calculated and found to be sufficient to maintain cell populations through the constructs full thickness. Although an overall decrease in the modulus of elasticity was noted, the modulus in the failure range of rolled constructs stabilized at values 25 times higher than single-layered constructs. Rolled constructs typically displayed an upregulation of contractile and matrix remodeling markers (α-actin, SM22 and type 1 collagen, MMP-2 respectively) indicating biological adaptation. Considerable design flexibility can be achieved by varying the number of scaffold layers, allowing the possibility of tuning the constructs physical dimensions, shape and tensile properties to suit specific targeted vascular locations.