Enhanced endothelialization for developing artificial vascular networks with a natural vessel mimicking the luminal surface in scaffolds

Soft bioreactor systems: a necessary step toward engineered MSK soft tissue?

A key objective of tissue engineering (TE) is to produce in vitro funcional grafts that can replace damaged tissues or organs in patients. TE uses bioreactors, which are controlled environments, allowing the application of physical and biochemical cues to relevant cells growing in biomaterials. For soft musculoskeletal (MSK) tissues such as tendons, ligaments and cartilage, it is now well established that applied mechanical stresses can be incorporated into those bioreactor systems to support tissue growth and maturation via activation of mechanotransduction pathways. However, mechanical stresses applied in the laboratory are often oversimplified compared to those found physiologically and may be a factor in the slow progression of engineered MSK grafts towards the clinic. In recent years, an increasing number of studies have focused on the application of complex loading conditions, applying stresses of different types and direction on tissue constructs, in order to better mimic the cellular environment experienced in vivo. Such studies have highlighted the need to improve upon traditional rigid bioreactors, which are often limited to uniaxial loading, to apply physiologically relevant multiaxial stresses and elucidate their influence on tissue maturation. To address this need, soft bioreactors have emerged. They employ one or more soft components, such as flexible soft chambers that can twist and bend with actuation, soft compliant actuators that can bend with the construct, and soft sensors which record measurements in situ. This review examines types of traditional rigid bioreactors and their shortcomings, and highlights recent advances of soft bioreactors in MSK TE. Challenges and future applications of such systems are discussed, drawing attention to the exciting prospect of these platforms and their ability to aid development of functional soft tissue engineered grafts.

Branched Channels in Porous β-Tricalcium Phosphate Scaffold Promote Vascularization

Rapid and efficient vascularization is still considerably challenging for a porous β-tricalcium phosphate (β-TCP) scaffold to achieve. To overcome this challenge, branched channels were created in the porous β-TCP scaffold by using 3D printing and a template-casting method to facilitate the instant flow of blood supply. Human bone mesenchymal stem cells (hBMSCs) and human umbilical vein endothelial cells (HUVECs) were seeded in the channeled porous scaffolds and characterized through a double-stranded DNA (dsDNA) assay, alkaline phosphatase (ALP) assay, and cell migration. Channeled porous β-TCP scaffolds were then implanted in the subcutaneous pockets of mice. Histological staining and immunohistochemical staining on vascularization and bone-related markers were carried out on the embedded paraffin sections. Results from in vitro experiments showed that branched channels significantly promoted HUVECs' infiltration, migration, proliferation, and angiogenesis, and also promoted the proliferation and osteogenesis differentiation of hBMSCs. In vivo implantation results showed that, in the early stage after implantation, cells significantly migrated into branched channeled scaffolds. More matured blood vessels formed in the branched channeled scaffolds compared to that in nonchanneled and straight channeled scaffolds. Beside promoting vascularization, the branched channels also stimulated the infiltration of bone-related cells into the scaffolds. These results suggested that the geometric design of branched channels in the porous β-TCP scaffold promoted rapid vascularization and potentially stimulated bone cells recruitment.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

Step-wise CAG@PLys@PDA-Cu2+ modification on micropatterned nanofibers for programmed endothelial healing

Native-like endothelium regeneration is a prerequisite for material-guided small-diameter vascular regeneration. In this study, a novel strategy is proposed to achieve phase-adjusted endothelial healing by step-wise modification of parallel-microgroove-patterned (i.e., micropatterned) nanofibers with polydopamine-copper ion (PDA-Cu²⁺) complexes, polylysine (PLys) molecules, and Cys-Ala-Gly (CAG) peptides (CAG@PLys@PDA-Cu²⁺). Using electrospun poly(l-lactide-co-caprolactone) random nanofibers as the demonstrating biomaterial, step-wise modification of CAG@PLys@PDA-Cu²⁺ significantly enhanced substrate wettability and protein adsorption, exhibited an excellent antithrombotic surface and outstanding phase-adjusted capacity of endothelium regeneration involving cell adhesion, endothelial monolayer formation, and the regenerated endothelium maturation. Upon in vivo implantation for segmental replacement of rabbit carotid arteries, CAG@PLys@PDA-Cu²⁺ modified grafts (2 mm inner diameter) with micropatterns on inner surface effectively accelerated native-like endothelium regeneration within 1 week, with less platelet aggregates and inflammatory response compared to those on non-modified grafts. Prolonged observations at 6- and 12-weeks post-implantation demonstrated a positive vascular remodeling with almost fully covered endothelium and mature smooth muscle layer in the modified vascular grafts, accompanied with well-organized extracellular matrix. By contrast, non-modified vascular grafts induced a disorganized tissue formation with a high risk of thrombogenesis. In summary, step-wise modification of CAG@PLys@PDA-Cu²⁺ on micropatterned nanofibers can significantly promote endothelial healing without inflicting thrombosis, thus confirming a novel strategy for developing functional vascular grafts or other blood-contacting materials/devices.

Electrospun Nanofibers for Manipulating the Soft Tissue Regeneration

Soft tissue damage is a common clinical problem that affects the lives of a large number of patients all over the world. It is of great importance to develop functional...

A pre-conditioning protocol of peripheral blood derived endothelial colony forming cells for endothelialization of tissue engineered constructs

In regenerative medicine, autologous endothelial colony forming cells (ECFCs) bear the greatest potential to be used for surface endothelialization of tissue engineered constructs, as they are easily attainable and possess a high proliferation rate. The aim of this study was to develop a standardized pre-conditioning protocol under dynamic conditions simulating the physiology of human circulation to improve the formation of a flow resistant monolayer of ECFCs and to enhance the antithrombogenicity of the endothelial cells. The main focus of the study was to consequently compare the cellular behavior under a steady laminar flow against a pulsatile flow. Mononuclear cells were isolated out of peripheral blood (PB) buffy coats and plated on uncoated tissue culture flasks in anticipation of guidelines for Advanced Therapy Medicinal Products. ECFCs were identified by typical surface markers such as CD31, CD146 and VE-Cadherin. To explore the effects of dynamic cultivation, ECFCs and human umbilical vein endothelial cells were comparatively cultured under either laminar or pulsatile (1 Hz) flow conditions with different grades of shear stress (5 dyn/cm²versus 20 dyn/cm²). High shear stress of 20 dyn/cm² led to a significant upregulation of the antithrombotic gene marker thrombomodulin in both cell types, but only ECFCs orientated and elongated significantly after shear stress application forming a confluent endothelial cell layer. The work therefore documents a suitable protocol to pre-condition PB-derived ECFCs for sustainable endothelialization of blood contacting surfaces and provides essential knowledge for future cultivations in bioreactor systems.

Research on alginate-polyacrylamide enhanced amnion hydrogel, a potential vascular substitute material

Although traditional synthetic vascular grafts have good mechanical stability, stenosis and even thrombus can be easily caused at the beginning of transplantation due to the material's procoagulant and low cell adhesion rate. In order to address these problems, by combining acellular amnion gel and polyacrylamide-alginate gel, we gained a composite hydrogel with high elasticity, mechanical stability, high bioactivity and low swelling ratio. The results showed that the composite gel had excellent mechanical strength, resistance to enzymatic degradation and anti-calcification ability. Also, it could significantly inhibit the adhesion, aggregation and activation of platelet and hemolysis. What is more, this composite hydrogel could significantly promote the adhesion and proliferation of ECs, as well as inducing the migration of ECs to the surface of the hydrogel. It could also stimulate the secretion of NO and PGI₂ from seeded HUVECs, which were important factors involved in vascular remodelling and repair. All the results indicated that prepared AlgSr/PAM-AM hydrogel was an excellent biomaterial with properties for potential use in vascular repair.

Vascularisation for cardiac tissue engineering: the extracellular matrix

Cardiovascular diseases present a major socio-economic burden. One major problem underlying most cardiovascular and congenital heart diseases is the irreversible loss of contractile heart muscle cells, the cardiomyocytes. To reverse damage incurred by myocardial infarction or by surgical correction of cardiac malformations, the loss of cardiac tissue with a thickness of a few millimetres needs to be compensated. A promising approach to this issue is cardiac tissue engineering. In this review we focus on the problem of in vitro vascularisation as implantation of cardiac patches consisting of more than three layers of cardiomyocytes (> 100 μm thick) already results in necrosis. We explain the need for vascularisation and elaborate on the importance to include non-myocytes in order to generate functional vascularised cardiac tissue. We discuss the potential of extracellular matrix molecules in promoting vascularisation and introduce nephronectin as an example of a new promising candidate. Finally, we discuss current biomaterial- based approaches including micropatterning, electrospinning, 3D micro-manufacturing technology and porogens. Collectively, the current literature supports the notion that cardiac tissue engineering is a realistic option for future treatment of paediatric and adult patients with cardiac disease.

The Rapidly Evolving Concept of Whole Heart Engineering

Whole heart engineering represents an incredible journey with as final destination the challenging aim to solve end-stage cardiac failure with a biocompatible and living organ equivalent. Its evolution started in 2008 with rodent organs and is nowadays moving closer to clinical application thanks to scaling-up strategies to human hearts. This review will offer a comprehensive examination on the important stages to be reached for the bioengineering of the whole heart, by describing the approaches of organ decellularization, repopulation, and maturation so far applied and the novel technologies of potential interest. In addition, it will carefully address important demands that still need to be satisfied in order to move to a real clinical translation of the whole bioengineering heart concept.

Construction of Large-Volume Tissue Mimics with 3D Functional Vascular Networks

We used indirect stereolithography (SL) to form inner-layered fluidic networks in a porous scaffold by introducing a hydrogel barrier on the luminal surface, then seeded the networks separately with human umbilical vein endothelial cells and human lung fibroblasts to form a tissue mimic containing vascular networks. The artificial vascular networks provided channels for oxygen transport, thus reducing the hypoxic volume and preventing cell death. The endothelium of the vascular networks significantly retarded the occlusion of channels during whole-blood circulation. The tissue mimics have the potential to be used as an in vitro platform to examine the physiologic and pathologic phenomena through vascular architecture.

Computer-aided multiple-head 3D printing system for printing of heterogeneous organ/tissue constructs

Recently, much attention has focused on replacement or/and enhancement of biological tissues via the use of cell-laden hydrogel scaffolds with an architecture that mimics the tissue matrix, and with the desired three-dimensional (3D) external geometry. However, mimicking the heterogeneous tissues that most organs and tissues are formed of is challenging. Although multiple-head 3D printing systems have been proposed for fabricating heterogeneous cell-laden hydrogel scaffolds, to date only the simple exterior form has been realized. Here we describe a computer-aided design and manufacturing (CAD/CAM) system for this application. We aim to develop an algorithm to enable easy, intuitive design and fabrication of a heterogeneous cell-laden hydrogel scaffolds with a free-form 3D geometry. The printing paths of the scaffold are automatically generated from the 3D CAD model, and the scaffold is then printed by dispensing four materials; i.e., a frame, two kinds of cell-laden hydrogel and a support. We demonstrated printing of heterogeneous tissue models formed of hydrogel scaffolds using this approach, including the outer ear, kidney and tooth tissue. These results indicate that this approach is particularly promising for tissue engineering and 3D printing applications to regenerate heterogeneous organs and tissues with tailored geometries to treat specific defects or injuries.

Effect of immobilized collagen type IV on biological properties of endothelial cells for the enhanced endothelialization of synthetic vascular graft materials

Regeneration of healthy endothelium onto vascular graft materials is imperative for prevention of intimal hyperplasia and thrombogenesis. In this study, we investigated the effect of collagen type IV (COL-IV) immobilized onto electrospun nanofibers on modulation of endothelial cell (EC) function, as a potential signal to rapid endothelialization of vascular grafts. COL-IV is assembled in basement membrane underneath intimal layer and regulates morphogenesis of blood vessels. For immobilization of COL-IV, poly(l-lactic acid) (PLLA) nanofibers (PL) were prepared as a model vascular graft substrate, onto which acrylic acid (AAc) was then grafted by using gamma-ray irradiation. AAc graft was dependent on irradiation doses and AAc concentrations, which allowed us to select the condition of 5% (v/v) AAc and 10 kGy for further conjugation of COL-IV. COL-IV immobilization was proportionally controlled as a function of its concentration. Atomic force microscope (AFM) analysis qualitatively supported immobilization of COL-IV, demonstrating increase in root mean square roughness of the PL from 665.37 ± 13.20 nm to 1440.74 ± 33.24. However, the Young's modulus of nanofibers was retained as approximately 1 MPa, regardless of surface modification. The number of ECs attached on the nanofibers with immobilized COL-IV was significantly increased by 5 times (1052 ± 138 cells/mm(2)) from pristine PL (234 ± 41 cells/mm(2)). In addition, the effect of immobilized COL-IV was profound for enhancing proliferation and up-regulation of markers implicated in rapid endothelialization. Collectively, our results suggest that COL-IV immobilized onto electrospun PLLA nanofibers may serve as a promising instructive cue used in vascular graft materials.

Accelerating in situ endothelialisation of cardiovascular bypass grafts

The patency of synthetic cardiovascular grafts in the long run is synonymous with their ability to inhibit the processes of intimal hyperplasia, thrombosis and calcification. In the human body, the endothelium of blood vessels exhibits characteristics that inhibit such processes. As such it is not surprising that research in tissue engineering is directed towards replicating the functionality of the natural endothelium in cardiovascular grafts. This can be done either by seeding the endothelium within the lumen of the grafts prior to implantation or by designing the graft such that in situ endothelialisation takes place after implantation. Due to certain difficulties identified with in vitro endothelialisation, in situ endothelialisation, which will be the focus of this article, has garnered interest in the last years. To promote in situ endothelialisation, the following aspects can be taken into account: (1) Endothelial progenital cell mobilization, adhesion and proliferation; (2) Regulating differentiation of progenitor cells to mature endothelium; (3) Preventing thrombogenesis and inflammation during endothelialisation. This article aims to review and compile recent developments to promote the in situ endothelialisation of cardiovascular grafts and subsequently improve their patency, which can also have widespread implications in the field of tissue engineering.

Fabrication of poly(ethylene glycol): gelatin methacrylate composite nanostructures with tunable stiffness and degradation for vascular tissue engineering

Although synthetic polymers are desirable in tissue engineering applications for the reproducibility and tunability of their properties, synthetic small diameter vascular grafts lack the capability to endothelialize in vivo. Thus, synthetically fabricated biodegradable tissue scaffolds that reproduce important aspects of the extracellular environment are required to meet the urgent need for improved vascular grafting materials. In this study, we have successfully fabricated well-defined nanopatterned cell culture substrates made of a biodegradable composite hydrogel consisting of poly(ethylene glycol) dimethacrylate (PEGDMA) and gelatin methacrylate (GelMA) by using UV-assisted capillary force lithography. The elasticity and degradation rate of the composite PEG-GelMA nanostructures were tuned by varying the ratios of PEGDMA and GelMA. Human umbilical vein endothelial cells (HUVECs) cultured on nanopatterned PEG-GelMA substrates exhibited enhanced cell attachment compared with those cultured on unpatterned PEG-GelMA substrates. Additionally, HUVECs cultured on nanopatterned PEG-GelM substrates displayed well-aligned, elongated morphology similar to that of native vascular endothelial cells and demonstrated rapid and directionally persistent migration. The ability to alter both substrate stiffness and degradation rate and culture endothelial cells with increased elongation and alignment is a promising next step in recapitulating the properties of native human vascular tissue for tissue engineering applications.

Strategies affording prevascularized cell-based constructs for myocardial tissue engineering

The production of a functional cardiac tissue to be transplanted in the injured area of the infarcted myocardium represents a challenge for regenerative medicine. Most cell-based grafts are unviable because of inadequate perfusion; therefore, prevascularization might be a suitable approach for myocardial tissue engineering. To this aim, cells with a differentiation potential towards vascular and cardiac muscle phenotypes have been cocultured in 2D or 3D appropriate scaffolds. In addition to these basic approaches, more sophisticated strategies have been followed employing mixed-cell sheets, microvascular modules, and inosculation from vascular explants. Technologies exerting spatial control of vascular cells, such as topographical surface roughening and ordered patterning, represent other ways to drive scaffold vascularization. Finally, microfluidic devices and bioreactors exerting mechanical stress have also been employed for high-throughput scaling-up production in order to accelerate muscle differentiation and speeding the endothelialization process. Future research should address issues such as how to optimize cells, biomaterials, and biochemical components to improve the vascular integration of the construct within the cardiac wall, satisfying the metabolic and functional needs of the myocardial tissue.

In-vivo efficacy of compliant 3D nano-composite in critical-size bone defect repair: a six month preclinical study in rabbit

Bone defects above critical size do not heal completely by itself and thus represent major clinical challenge to reconstructive surgery. Numerous bone substitutes have already been used to promote bone regeneration, however their use, particularly for critical-sized bone defects along with their long term in vivo safety and efficacy remains a concern. The present study was designed to obtain a complete healing of critical-size defect made in the proximal tibia of New Zealand White rabbit, using nano-hydroxyapatite/gelatin and chemically carboxymethylated chitin (n-HA/gel/CMC) scaffold construct. The bone-implant interfaces and defect site healing was evaluated for a period up to 25 weeks using radiography, micro-computed tomography, fluorescence labeling, and histology and compared with respective SHAM (empty contra lateral control). The viscoelastic porous scaffold construct allows easy surgical insertion and post-operatively facilitate oxygenation and angiogenesis. Radiography of defect treated with scaffold construct suggested expedited healing at defect edges and within the defect site, unlike confined healing at edges of the SHAM sites. The architecture indices analyzed by micro-computed tomography showed a significant increase in percentage of bone volume fraction, resulted in reconciled cortico-trabecular bone formation at n-HA/gel/CMC constructs treated site (15.2% to 52.7%) when compared with respective SHAM (10.2% to 31.8%). Histological examination and fluorescence labeling revealed that the uniformly interconnected porous surface of scaffold construct enhanced osteoblasts' activity and mineralization. These preclinical data suggest that, n-HA/gel/CMC construct exhibit stimulation of bone's innate regenerative capacity, thus underscoring their use in guided bone regeneration.