Tropoelastin coated PLLA-PLGA scaffolds promote vascular network formation

Directed Assembly of Elastic Fibers via Coacervate Droplet Deposition on Electrospun Templates

Elastic fibers provide critical elasticity to the arteries, lungs, and other organs. Elastic fiber assembly is a process where soluble tropoelastin is coacervated into liquid droplets, cross-linked, and deposited onto and into microfibrils. While much progress has been made in understanding the biology of this process, questions remain regarding the timing of interactions during assembly. Furthermore, it is unclear to what extent fibrous templates are needed to guide coacervate droplets into the correct architecture. The organization and shaping of coacervate droplets onto a fiber template have never been previously modeled or employed as a strategy for shaping elastin fiber materials. Using an in vitro system consisting of elastin-like polypeptides (ELPs), genipin cross-linker, electrospun polylactic-co-glycolic acid (PLGA) fibers, and tannic acid surface coatings for fibers, we explored ELP coacervation, cross-linking, and deposition onto fiber templates. We demonstrate that integration of coacervate droplets into a fibrous template is primarily influenced by two factors: (1) the balance of coacervation and cross-linking and (2) the surface energy of the fiber templates. The success of this integration affects the mechanical properties of the final fiber network. Our resulting membrane materials exhibit highly tunable morphologies and a range of elastic moduli (0.8-1.6 MPa) comparable to native elastic fibers.

Chitin whisker/chitosan liquid crystal hydrogel assisted scaffolds with bone-like ECM microenvironment for bone regeneration

Natural bone exhibits a complex anisotropic and micro-nano hierarchical structure, more importantly, bone extracellular matrix (ECM) presents liquid crystal (LC) phase and viscoelastic characteristics, providing a unique microenvironment for guiding cell behavior and regulating osteogenesis. However, in bone tissue engineering scaffolds, the construction of bone-like ECM microenvironment with exquisite microstructure is still a great challenge. Here, we developed a novel polysaccharide LC hydrogel supported 3D printed poly(l-lactide) (PLLA) scaffold with bone-like ECM microenvironment and micro-nano aligned structure. First, we prepared a chitin whisker/chitosan polysaccharide LC precursor, and then infuse it into the pores of 3D printed PLLA scaffold, which was previously surface modified with a polydopamine layer. Next, the LC precursor was chemical cross-linked by genipin to form a hydrogel network with bone-like ECM viscoelasticity and LC phase in the scaffold. Subsequently, we performed directional freeze-casting on the composite scaffold to create oriented channels in the LC hydrogel. Finally, we soaked the composite scaffold in phytic acid to further physical cross-link the LC hydrogel through electrostatic interactions and impart antibacterial effects to the scaffold. The resultant biomimetic scaffold displays osteogenic activity, vascularization ability and antibacterial effect, and is expected to be a promising candidate for bone repair.

Surface Modification Progress for PLGA-Based Cell Scaffolds

Poly(lactic-glycolic acid) (PLGA) is a biocompatible bio-scaffold material, but its own hydrophobic and electrically neutral surface limits its application as a cell scaffold. Polymer materials, mimics ECM materials, and organic material have often been used as coating materials for PLGA cell scaffolds to improve the poor cell adhesion of PLGA and enhance tissue adaptation. These coating materials can be modified on the PLGA surface via simple physical or chemical methods, and coating multiple materials can simultaneously confer different functions to the PLGA scaffold; not only does this ensure stronger cell adhesion but it also modulates cell behavior and function. This approach to coating could facilitate the production of more PLGA-based cell scaffolds. This review focuses on the PLGA surface-modified materials, methods, and applications, and will provide guidance for PLGA surface modification.

Kartogenin delivery systems for biomedical therapeutics and regenerative medicine

Kartogenin, a small and heterocyclic molecule, has emerged as a promising therapeutic agent for incorporation into biomaterials, owing to its unique physicochemical and biological properties. It holds potential for the regeneration of cartilage-related tissues in various common conditions and injuries. Achieving sustained release of kartogenin through appropriate formulation and efficient delivery systems is crucial for modulating cell behavior and tissue function. This review provides an overview of cutting-edge kartogenin-functionalized biomaterials, with a primarily focus on their design, structure, functions, and applications in regenerative medicine. Initially, we discuss the physicochemical properties and biological functions of kartogenin, summarizing the underlying molecular mechanisms. Subsequently, we delve into recent advancements in nanoscale and macroscopic materials for the carriage and delivery of kartogenin. Lastly, we address the opportunities and challenges presented by current biomaterial developments and explore the prospects for their application in tissue regeneration. We aim to enhance the generation of insightful ideas for the development of kartogenin delivery materials in the field of biomedical therapeutics and regenerative medicine by providing a comprehensive understanding of common preparation methods.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal muscle tissue engineering.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Development of tropoelastin-functionalized anisotropic PCL scaffolds for musculoskeletal tissue engineering

The highly organized extracellular matrix (ECM) of musculoskeletal tissues, encompassing tendons, ligaments and muscles, is structurally anisotropic, hierarchical and multi-compartmental. These features collectively contribute to their unique function. Previous studies have investigated the effect of tissue-engineered scaffold anisotropy on cell morphology and organization for musculoskeletal tissue repair and regeneration, but the hierarchical arrangement of ECM and compartmentalization are not typically replicated. Here, we present a method for multi-compartmental scaffold design that allows for physical mimicry of the spatial architecture of musculoskeletal tissue in regenerative medicine. This design is based on an ECM-inspired macromolecule scaffold. Polycaprolactone (PCL) scaffolds were fabricated with aligned fibers by electrospinning and mechanical stretching, and then surface-functionalized with the cell-supporting ECM protein molecule, tropoelastin (TE). TE was attached using two alternative methods that allowed for either physisorption or covalent attachment, where the latter was achieved by plasma ion immersion implantation (PIII). Aligned fibers stimulated cell elongation and improved cell alignment, in contrast to randomly oriented fibers. TE coatings bound by physisorption or covalently following 200 s PIII treatment promoted fibroblast proliferation. This represents the first cytocompatibility assessment of novel PIII-treated TE-coated PCL scaffolds. To demonstrate their versatility, these 2D anisotropic PCL scaffolds were assembled into 3D hierarchical constructs with an internally compartmentalized structure to mimic the structure of musculoskeletal tissue.

Engineering Biomimetic Extracellular Matrix with Silica Nanofibers: From 1D Material to 3D Network

Biomaterials at nanoscale is a fast-expanding research field with which extensive studies have been conducted on understanding the interactions between cells and their surrounding microenvironments as well as intracellular communications. Among many kinds of nanoscale biomaterials, mesoporous fibrous structures are especially attractive as a promising approach to mimic the natural extracellular matrix (ECM) for cell and tissue research. Silica is a well-studied biocompatible, natural inorganic material that can be synthesized as morpho-genetically active scaffolds by various methods. This review compares silica nanofibers (SNFs) to other ECM materials such as hydrogel, polymers, and decellularized natural ECM, summarizes fabrication techniques for SNFs, and discusses different strategies of constructing ECM using SNFs. In addition, the latest progress on SNFs synthesis and biomimetic ECM substrates fabrication is summarized and highlighted. Lastly, we look at the wide use of SNF-based ECM scaffolds in biological applications, including stem cell regulation, tissue engineering, drug release, and environmental applications.

Microcomputed Tomography-Based Analysis of Neovascularization within Bioengineered Vascularized Tissues

In the field of tissue engineering, evaluating newly formed vascular networks is considered a fundamental step in deciphering the processes underlying tissue development. Several common modalities exist to study vessel network formation and function. However, a proper methodology that allows through three-dimensional visualization of neovessels in a reproducible manner is required. Here, we describe in-depth exploration, visualization, and analysis of vessels within newly formed tissues by utilizing a contrast agent perfusion protocol and high-resolution microcomputed tomography. Bioengineered constructs consisting of porous, biocompatible, and biodegradable scaffolds are loaded with cocultures of adipose-derived microvascular endothelial cells (HAMECs) and dental pulp stem cells (DPSCs) and implanted in a rat femoral bundle model. After 14 days of in vivo maturation, we performed the optimized perfusion protocol to allow host penetrating vascular visualization and assessment within neotissues. Following high-resolution microCT scanning of DPSC:HAMEC explants, we performed the volumetric and spatial analysis of neovasculature. Eventually, the process was repeated with a previously published coculture system for prevascularization based on adipose-derived mesenchymal stromal cells (MSCs) and HAMECs. Overall, our approach allows a comprehensive understanding of vessel organization during engraftment and development of neotissues.

Emerging concepts in bone repair and the premise of soft materials

Human bone has a strong regenerative capacity that allows for restoration of its function and structure after damage. For degenerative bone diseases or large defects, bone regeneration requirements exceed the natural potential for self-healing, so bone grafts or bone substitute materials are required to support the regeneration of bone tissue. Compared to the plethora of endogenous bioactive molecules and cells in native bone grafts, the regenerative capacity of tissue-engineered materials is limited. The modest clinical impact of tissue-engineered strategies in this domain can be attributed to a failure to fully recognize key physical and biological events during bone healing, and to recapitulate the structure and composition of the target tissue to generate truly biomimetic grafts. This limitation has motivated the emergence of new strategies such as immunomodulation, endochondral ossification routes, engineered microtissues and hematoma regulation, and the development of advanced biomaterials including gene-activated matrices, soft microgels and hierarchically designed materials.

3D Bioprinting of Engineered Tissue Flaps with Hierarchical Vessel Networks (VesselNet) for Direct Host-To-Implant Perfusion

Engineering hierarchical vasculatures is critical for creating implantable functional thick tissues. Current approaches focus on fabricating mesoscale vessels for implantation or hierarchical microvascular in vitro models, but a combined approach is yet to be achieved to create engineered tissue flaps. Here, millimetric vessel-like scaffolds and 3D bioprinted vascularized tissues interconnect, creating fully engineered hierarchical vascular constructs for implantation. Endothelial and support cells spontaneously form microvascular networks in bioprinted tissues using a human collagen bioink. Sacrificial molds are used to create polymeric vessel-like scaffolds and endothelial cells seeded in their lumen form native-like endothelia. Assembling endothelialized scaffolds within vascularizing hydrogels incites the bioprinted vasculature and endothelium to cooperatively create vessels, enabling tissue perfusion through the scaffold lumen. Using a cuffing microsurgery approach, the engineered tissue is directly anastomosed with a rat femoral artery, promoting a rich host vasculature within the implanted tissue. After two weeks in vivo, contrast microcomputer tomography imaging and lectin perfusion of explanted engineered tissues verify the host ingrowth vasculature's functionality. Furthermore, the hierarchical vessel network (VesselNet) supports in vitro functionality of cardiomyocytes. Finally, the proposed approach is expanded to mimic complex structures with native-like millimetric vessels. This work presents a novel strategy aiming to create fully-engineered patient-specific thick tissue flaps.

Tropoelastin Promotes the Formation of Dense, Interconnected Endothelial Networks

Tropoelastin, the soluble precursor of elastin, has been used for regenerative and wound healing purposes and noted for its ability to accelerate wound repair by enhancing vascularization at the site of implantation. However, it is not clear whether these effects are directly due to the interaction of tropoelastin with endothelial cells or communicated to endothelial cells following interactions between tropoelastin and neighboring cells, such as mesenchymal stem cells (MSCs). We adapted an endothelial tube formation assay to model in vivo vascularization with the goal of exploring the stimulatory mechanism of tropoelastin. In the presence of tropoelastin, endothelial cells formed less tubes, with reduced spreading into capillary-like networks. In contrast, conditioned media from MSCs that had been cultured on tropoelastin enhanced the formation of more dense, complex, and interconnected endothelial tube networks. This pro-angiogenic effect of tropoelastin is mediated indirectly through the action of tropoelastin on co-cultured cells. We conclude that tropoelastin inhibits endothelial tube formation, and that this effect is reversed by pro-angiogenic crosstalk from tropoelastin-treated MSCs. Furthermore, we find that the known in vivo pro-angiogenic effects of tropoelastin can be modeled in vitro, highlighting the value of tropoelastin as an indirect mediator of angiogenesis.

Applications of Engineering Techniques in Microvasculature Design

Achieving successful microcirculation in tissue engineered constructs in vitro and in vivo remains a challenge. Engineered tissue must be vascularized in vitro for successful inosculation post-implantation to allow instantaneous perfusion. To achieve this, most engineering techniques rely on engineering channels or pores for guiding angiogenesis and capillary tube formation. However, the chosen materials should also exhibit properties resembling the native extracellular matrix (ECM) in providing mechanical and molecular cues for endothelial cells. This review addresses techniques that can be used in conjunction with matrix-mimicking materials to further advance microvasculature design. These include electrospinning, micropatterning and bioprinting. Other techniques implemented for vascularizing organoids are also considered for their potential to expand on these approaches.

Prevascularized Scaffolds Bearing Human Dental Pulp Stem Cells for Treating Complete Spinal Cord Injury

The regeneration of injured spinal cord is hampered by the lack of vascular supply and neurotrophic support. Transplanting tissue-engineered constructs with developed vascular networks and neurotrophic factors, and further understanding the pattern of vessel growth in the remodeled spinal cord tissue are greatly desired. To this end, highly vascularized scaffolds embedded with human dental pulp stem cells (DPSCs) are fabricated, which possess paracrine-mediated angiogenic and neuroregenerative potentials. The potent pro-angiogenic effect of the prevascularized scaffolds is first demonstrated in a rat femoral bundle model, showing robust vessel growth and blood perfusion induced within these scaffolds postimplantation, as evidenced by laser speckle contrast imaging and 3D microCT dual imaging modalities. More importantly, in a rat complete spinal cord transection model, the implantation of these scaffolds to the injured spinal cords can also promote revascularization, as well as axon regeneration, myelin deposition, and sensory recovery. Furthermore, 3D microCT imaging and novel morphometric analysis on the remodeled spinal cord tissue demonstrate substantial regenerated vessels, more significantly in the sensory tract regions, which correlates with behavioral recovery following prevascularization treatment. Taken together, prevascularized DPSC-embedded constructs bear angiogenic and neurotrophic potentials, capable of augmenting and modulating SCI repair.

Tubular Fibrous Scaffolds Functionalized with Tropoelastin as a Small-Diameter Vascular Graft

Cardiovascular disorders are a healthcare problem in today's society. The clinically available synthetic vascular grafts are thrombogenic and could induce intimal hyperplasia. Rapid endothelialization and matched mechanical properties are two major requirements to be considered when designing functional vascular grafts. Herein, an electrospun tubular fibrous (eTF) scaffold was biofunctionalized with tropoelastin at the luminal surface. The luminal surface functionalization was confirmed by an increase of the zeta potential and by the insertion of NH₂ groups. Tropoelastin was immobilized via its -NH₂ or -COOH groups at the activated or aminolysed eTF scaffolds, respectively, to study the effect of exposed functional groups on human endothelial cells (ECs) behavior. Tensile properties demonstrated that functionalized eTF scaffolds presented strength and stiffness within the range of those of native blood vessels. Tropoelastin immobilized on activated eTF scaffolds promoted higher metabolic activity and proliferation of ECs, whereas when immobilized on aminolysed eTF scaffolds, significantly higher protein synthesis was observed. These biofunctional eTF scaffolds are a promising small-diameter vascular graft that promote rapid endothelialization and have compatible mechanical properties.

Engineering functional microvessels in synthetic polyurethane random-pore scaffolds by harnessing perfusion flow

Recently reported biomaterial-based approaches toward prevascularizing tissue constructs rely on biologically or structurally complex scaffolds that are complicated to manufacture and sterilize, and challenging to customize for clinical applications. In the current work, a prevascularization method for soft tissue engineering that uses a non-patterned and non-biological scaffold is proposed. Human fibroblasts and HUVECs were seeded on an ionomeric polyurethane-based hydrogel and cultured for 14 days under medium perfusion. A flow rate of 0.05 mL/min resulted in a greater lumen density in the constructs relative to 0.005 and 0.5 mL/min, indicating the critical importance of flow magnitude in establishing microvessels. Constructs generated at 0.05 mL/min perfusion flow were implanted in a mouse subcutaneous model and intravital imaging was used to characterize host blood perfusion through the construct after 2 weeks. Engineered microvessels were functional (i.e. perfused with host blood and non-leaky) and neovascularization of the construct by host vessels was enhanced relative to non-prevascularized constructs. We report on the first strategy toward engineering functional microvessels in a tissue construct using non-bioactive, non-patterned synthetic polyurethane materials.

An Integrative Dual-Layer Poly-L-Lactic Acid Fibrous Membrane Prevents Peritendinous Adhesions

Anti-adhesion membranes are prospective scaffolds for preventing peritendinous adhesion after injury. However, currently available scaffolds have some limitations, such as low efficacy for anti-adhesion, low quality of tendon healing, and unknown drug interactions. Thus, in this study, we designed an innovative structure involving an integrated dual-layer poly(L-lactic acid) (PLLA) electrospun membrane for preventing peritendonous adhesion by promoting tendon gliding. We investigated the surface morphology and wettability of the fiber scaffold. The adhesion and proliferation of fibroblasts were low on the PLLA fibrous membrane. Compared with single-layer membranes, the dual-layer PLLA fiber scaffold reduced adhesion to the tissues. The gliding space persisted until recovery in chicken extensor flexor tendons in vivo. Thus, this innovative PLLA membrane scaffold could prevent adhesion and promote gliding to facilitate tendon healing.

Rapid prototyping fabrication of soft and oriented polyester scaffolds for axonal guidance

Biodegradable polyesters have been extensively used for preparation of nerve guidance scaffolds, due to their high biocompatibility and defined degradation periods. However, conventional methods for fabrication of porous polyester scaffolds provide limited control over shape and micro-architecture. Here, a fabrication procedure based on 3D printing was developed to generate highly ordered and anatomically personalized, polyester scaffolds for soft tissue regeneration. Scaffolds composed of Poly-lactic-glycolic acid (PLGA) and poly-L-lactic acid (PLLA) were specifically customized for nerve injuries. This was obtained by using an oriented multi-layer printing pattern which established a linear structure in the fabricated scaffolds to match the aligned topography of nerve tissues. The oriented scaffold was shown to guide regenerating axons to linear conformations and support growth of induced pluripotent stem cell-derived neurons in vitro and in vivo in a model of spinal cord injury. The described scaffolds may advance the field of nerve regeneration. Furthermore, modifications could be integrated to generate soft implants for various types of tissues.

Albumin-Enriched Fibrin Hydrogel Embedded in Active Ferromagnetic Networks Improves Osteoblast Differentiation and Vascular Self-Organisation

Porous coatings on prosthetic implants encourage implant fixation. Enhanced fixation may be achieved using a magneto-active porous coating that can deform elastically in vivo on the application of an external magnetic field, straining in-growing bone. Such a coating, made of 444 ferritic stainless steel fibres, was previously characterised in terms of its mechanical and cellular responses. In this work, co-cultures of human osteoblasts and endothelial cells were seeded into a novel fibrin-based hydrogel embedded in a 444 ferritic stainless steel fibre network. Albumin was successfully incorporated into fibrin hydrogels improving the specific permeability and the diffusion of fluorescently tagged dextrans without affecting their Young’s modulus. The beneficial effect of albumin was demonstrated by the upregulation of osteogenic and angiogenic gene expression. Furthermore, mineralisation, extracellular matrix production, and formation of vessel-like structures were enhanced in albumin-enriched fibrin hydrogels compared to fibrin hydrogels. Collectively, the results indicate that the albumin-enriched fibrin hydrogel is a promising bio-matrix for bone tissue engineering and orthopaedic applications.

Interactions at scaffold interfaces: Effect of surface chemistry, structural attributes and bioaffinity

Effective regenerative medicine relies on understanding the interplay between biomaterial implants and the adjoining cells. Scaffolds contribute by presenting sites for cellular adhesion, growth, proliferation, migration, and differentiation which lead to regeneration of tissues over desired periods of time. The fabrication and recruitment of scaffolds often fail to consider the interactions that occur at the interfaces, thereby risking rejection. This lack of knowledge on interfacial microenvironments and related exchanges often causes reduced cellular interactions, poor cell survival and intervention failure. Successful regenerative therapy requires scaffolds with bespoke biocompatibility, optimum pore structure, and cues for cell attachments. These factors determine the development of cellular affinity in scaffolds. For biomedical applications, a detailed understanding of scaffolds and their interfaces is required for better tuning of biomaterials to suit the microenvironments. In this review, we discuss the role of biointerfaces with a focus on surface chemistry, pore structure, scaffold hydro-affinity and their biointeractions. An understanding of the effect of scaffold interfacial properties is crucial for enhancing the progress of tissue engineering towards clinical applications.

Tropoelastin-Coated Tendon Biomimetic Scaffolds Promote Stem Cell Tenogenic Commitment and Deposition of Elastin-Rich Matrix

Tendon tissue engineering strategies that recreate the biophysical and biochemical native microenvironment have a greater potential to achieve regeneration. Here, we developed tendon biomimetic scaffolds using mechanically competent yarns of poly-ε-caprolactone, chitosan, and cellulose nanocrystals to recreate the inherent tendon hierarchy from a nano-to-macro scale. These were then coated with tropoelastin (TROPO) through polydopamine (PDA) linking, to mimic the native extracellular matrix (ECM) composition and elasticity. Both PDA and TROPO coatings decreased surface stiffness without masking the underlying substrate. We found that human adipose-derived stem cells (hASCs) seeded onto these TROPO biomimetic scaffolds more rapidly acquired their spindle-shape morphology and high aspect ratio characteristic of tenocytes. Immunocytochemistry shows that the PDA and TROPO-coated surfaces boosted differentiation of hASCs toward the tenogenic lineage, with sustained expression of the tendon-related markers scleraxis and tenomodulin up to 21 days of culture. Furthermore, these surfaces enabled the deposition of a tendon-like ECM, supported by the expression of collagens type I and III, tenascin, and decorin. Gene expression analysis revealed a downregulation of osteogenic and fibrosis markers in the presence of TROPO when compared with the control groups, suggesting proper ECM deposition. Remarkably, differentiated cells exposed to TROPO acquired an elastogenic profile due to the evident elastin synthesis and deposition, contributing to the formation of a more mimetic matrix in comparison with the PDA-coated and uncoated conditions. In summary, our biomimetic substrates combining biophysical and biological cues modulate stem cell behavior potentiating their long-term tenogenic commitment and the production of an elastin-rich ECM.

Mature vessel networks in engineered tissue promote graft-host anastomosis and prevent graft thrombosis

Graft vascularization remains one of the most critical challenges facing tissue-engineering experts in their attempt to create thick transplantable tissues and organs. In vitro prevascularization of engineered tissues has been suggested to promote rapid anastomosis between the graft and host vasculatures; however, thrombotic events have been reported upon graft implantation. Here, we aimed to determine whether in vitro vessel maturation in transplantable grafts can accelerate vascular integration and graft perfusion and prevent thrombotic events in the grafts. To this end, endothelial cells and fibroblasts were cocultured on 3D scaffolds for 1, 7, or 14 d to form vasculature with different maturation degrees. Monitoring graft-host interactions postimplantation demonstrated that the 14-d in vitro-cultured grafts, bearing more mature and complex vessel networks as indicated by elongated and branched vessel structures, had increased graft-host vessel anastomosis; host vessel penetration into the graft increased approximately eightfold, and graft perfusion increased sixfold. The presence of developed vessel networks prevented clot accumulation in the grafts. Conversely, short-term cultured constructs demonstrated poor vascularization and increased thrombus formation. Elevated expression levels of coagulation factors, von Willebrand factor (vWF), and tissue factor (TF), were demonstrated in constructs bearing less mature vasculature. To conclude, these findings demonstrate the importance of establishing mature and complex vessel networks in engineered tissues before implantation to promote anastomosis with the host and accelerate graft perfusion.

Oscillatory Strain Promotes Vessel Stabilization and Alignment through Fibroblast YAP-Mediated Mechanosensitivity

Endothelial cells form the interior layer of blood vessels and, as such, are constantly exposed to shear stress and mechanical strain. While the impact of shear stress on angiogenesis is widely studied, the role of mechanical strain is less understood. To this end, endothelial cells and fibroblasts are cocultured under oscillatory strain to create a vessel network. The two cell types show distinctly different sensitivities to the mechanical stimulation. The fibroblasts, sense the stress directly, and respond by increased alignment, proliferation, differentiation, and migration, facilitated by YAP translocation into the nucleus. In contrast, the endothelial cells form aligned vessels by tracking fibroblast alignment. YAP inhibition in constructs under mechanical strain results in vessel destruction whereas less damage is observed in the YAP-inhibited static control. Moreover, the mechanical stimulation enhances vessel development and stabilization. Additionally, vessel orientation is preserved upon implantation into a mouse dorsal window chamber and promotes the invading host vessels to orient in the same manner. This study sheds light on the mechanisms by which mechanical strain affects the development of blood vessels within engineered tissues. This can be further utilized to engineer a more organized and stable vasculature suitable for transplantation of engineered grafts.

Promoting Endothelial Cell Affinity and Antithrombogenicity of Polytetrafluoroethylene (PTFE) by Mussel-Inspired Modification and RGD/Heparin Grafting

When used as small-diameter vascular grafts (SDVGs), synthetic biomedical materials like polytetrafluoroethylene (PTFE) may induce thrombosis and intimal hyperplasia due to the lack of an endothelial cell layer. Modification of the PTFE in an aqueous solution is difficult because of its hydrophobicity. Herein, aiming to simultaneously promote endothelial cell affinity and antithrombogenicity, a mussel-inspired modification approach was employed to enable the grafting of various bioactive molecules like RGD and heparin. This approach involves a series of pragmatic steps including oxygen plasma treatment, dopamine (DA) coating, polyethylenimine (PEI) grafting, and RGD or RGD/heparin immobilization. Successful modification in each step was verified via Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Plasma treatment increased the hydrophilicity of PTFE, thereby allowing it to be efficiently coated with dopamine. Grafting of dopamine, RGD, and heparin led to an increase in surface roughness and a decrease in water contact angle due to increased surface energy. Platelet adhesion increased after dopamine and RGD modification, but it dramatically decreased when heparin was introduced. All of these modifications, especially the incorporation of RGD, showed favorable effects on endothelial cell attachment, viability, and proliferation. Due to strong cell-substrate interactions between endothelial cells and RGD, the RGD/heparin-grafted PTFE demonstrated high endothelial cell affinity. This facile modification method is highly suitable for all hydrophobic surfaces and provides a promising technique for SDVG modification to stimulate fast endothelialization and effective antithrombosis.

Synergistic Effects of Electrical Stimulation and Aligned Nanofibrous Microenvironment on Growth Behavior of Mesenchymal Stem Cells

Incontrollable cellular growth behavior is a significant issue, which severely affects the functional tissue formation and cellular protein expression. Development of natural extracellular matrix (ECM) like biomaterials to present microenvironment cues for regulation of cell responses can effectively overcome this problem. The external simulation and topological characteristics as typical guiding cues are capable of providing diverse influences on cellular growth. Herein, we fabricated two-dimensional aligned conductive nanofibers (2D-ACNFs) by an electrospinning process and surface polymerization, and the obtained 2D-ACNFs provided the effects of both alignment and electrical stimulation (ES) on cellular response of human mesenchymal cells (hMSCs). The results of cellular responses implied that the obtained 2D-ACNFs could offer a synergistic effect of both ES and aligned nanopattern on hMSC growth behavior. The effects could not only promote hMSCs to contact each other and maintain cellular activity but also provide positive promotion to regulate cellular proliferation. Thus, we believe that the obtained 2D-ACNFs will have a broad application in the biomedical field, such as cell culture with ES, directional induction for cell growth, and damaged tissue repair, etc.

Tropoelastin Implants That Accelerate Wound Repair

A novel, pure, synthetic material is presented that promotes the repair of full-thickness skin wounds. The active component is tropoelastin and leverages its ability to promote new blood vessel formation and its cell recruiting properties to accelerate wound repair. Key to the technology is the use of a novel heat-based, stabilized form of human tropoelastin which allows for tunable resorption. This implantable material contributes a tailored insert that can be shaped to the wound bed, where it hydrates to form a conformable protein hydrogel. Significant benefits in the extent of wound healing, dermal repair, and regeneration of mature epithelium in healthy pigs are demonstrated. The implant is compatible with initial co-treatment with full- and split-thickness skin grafts. The implant's superiority to sterile bandaging, commercial hydrogel and dermal regeneration template products is shown. On this basis, a new concept for a prefabricated tissue repair material for point-of-care treatment of open wounds is provided.

Fabrication of engineered tubular tissue for small blood vessels via three-dimensional cellular assembly and organization ex vivo

Although there is a great need for suitable vascular replacements in clinical practice, much progress needs to be made toward the development of a fully functional tissue-engineered construct. We propose a fabrication method of engineered tubular tissue for small blood vessels via a layer-by-layer cellular assembly technique using mouse smooth muscle cells, the construction of a poly-(l-lactide-co-ε-caprolactone) (PLCL) scaffold, and integration in a microfluidic perfusion culture system. The cylindrical PLCL scaffold is incised, expanded, and its surface is laminated with the cell layers. The construct confirms into tubular structures due to residual stress imposed by the cylindrical PLCL scaffold. The perfusion culture system allows simulation of static, perfusion (laminar flow), and perfusion with pulsatile pressure (Pulsatile flow) conditions in which mimicking the in vivo environments. The aim of this evaluation was to determine whether fabricated tubular tissue models developed their mechanical properties. The cellular response to hemodynamic stimulus imposed by the dynamic culture system is monitored through expression analysis of fibrillin-1 and fibrillin-2, elastin and smooth muscle myosin heavy chains isoforms transcription factors, which play an important role in tissue elastogenesis. Among the available materials for small blood vessel construction, these cellular hybrid vascular scaffolds hold much potential due to controllability of the mechanical properties of synthetic polymers and biocompatibility of integrated cellular components.

Mesenchymal stem cell seeded, biomimetic 3D printed scaffolds induce complete bridging of femoral critical sized defects

No current clinical treatments provide an ideal long-term solution for repair of long bone segment defects. Incomplete healing prevents patients from returning to preinjury activity and ultimately requires additional surgery to induce healing. Obtaining autologous graft material is costly, incurs morbidity, requires surgical time, and quality material is finite. In this pilot study, 3D printed biomimetic scaffolds were used to facilitate rapid bone bridging in critical sized defects in a sheep model. An inverse trabecular pattern based on micro-CT scans of sheep trabecular bone was printed in polybutylene terephthalate. Scaffolds were coated with micron-sized tricalcium phosphate particles to induce osteoconductivity. Mesenchymal stem cells (MSCs) were isolated from sheep inguinal and tail fat, in one group of sheep and scaffolds were infiltrated with MSCs in a bioreactor. Controls did not undergo surgery for cell extraction. Scaffolds were implanted into two experimental and two control adult sheep, and followed for either 3 or 6 months. Monthly radiographs and post explant micro-CT scanning demonstrated bone formation on the lateral, anterior, medial, and posterior-medial aspects along the entire length of the defect. Bone formation was absent on the posterior-lateral aspect where a muscle is generally attached to the bone. The 3-month time point showed 15.5% more cortical bone deposition around the scaffold circumference while the 6-month time point showed 40.9% more bone deposition within scaffold pores. Control sheep failed to unite. Serum collagen type-1C-terminus telopeptides (CTX-1) showed time-dependent levels of bone resorption, and calcein labeling demonstrated an increase in bone formation rate in treated animals compared with controls. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 242-252, 2019.

Localization of Engineered Vasculature within 3D Tissue Constructs

Today, in vitro vessel network systems frequently serve as models for investigating cellular and functional mechanisms underlying angiogenesis and vasculogenesis. Understanding the cues triggering the observed cell migration, organization, and differentiation, as well as the time frame of these processes, can improve the design of engineered microvasculature. Here, we present first evidence of the migration of endothelial cells into the depths of the scaffold, where they formed blood vessels surrounded by extracellular matrix and supporting cells. The supporting cells presented localization-dependent phenotypes, where cells adjacent to blood vessels displayed a more mature phenotype, with smooth muscle cell characteristics, whereas cells on the scaffold surface showed a pericyte-like phenotype. Yes-associated protein (YAP), a transcription activator of genes involved in cell proliferation and tissue growth, displayed spatially dependent expression, with cells on the surface showing more nuclear YAP than cells situated deeper within the scaffold.

Emerging bone tissue engineering via Polyhydroxyalkanoate (PHA)-based scaffolds

Polyhydroxyalkanoates (PHAs) are a class of biodegradable polymers derived from microorganisms. On top of their biodegradability and biocompatibility, different PHA types can contribute to varying mechanical and chemical properties. This has led to increasing attention to the use of PHAs in numerous biomedical applications over the past few decades. Bone tissue engineering refers to the regeneration of new bone through providing mechanical support while inducing cell growth on the PHA scaffolds having a porous structure for tissue regeneration. This review first introduces the various properties PHA scaffold that make them suitable for bone tissue engineering such as biocompatibility, biodegradability, mechanical properties as well as vascularization. The typical fabrication techniques of PHA scaffolds including electrospinning, salt-leaching and solution casting are further discussed, followed by the relatively new technology of using 3D printing in PHA scaffold fabrication. Finally, the recent progress of using different types of PHAs scaffold in bone tissue engineering applications are summarized in intrinsic PHA/blends forms or as composites with other polymeric or inorganic hybrid materials.