Vascularization strategies for tissue engineering

Magnetic Field Dynamic Strategies for the Improved Control of the Angiogenic Effect of Mesenchymal Stromal Cells

This work shows the ability to remotely control the paracrine performance of mesenchymal stromal cells (MSCs) in producing an angiogenesis key molecule, vascular endothelial growth factor (VEGF-A), by modulation of an external magnetic field. This work compares for the first time the application of static and dynamic magnetic fields in angiogenesis in vitro model, exploring the effect of magnetic field intensity and dynamic regimes on the VEGF-A secretion potential of MSCs. Tissue scaffolds of gelatin doped with iron oxide nanoparticles (MNPs) were used as a platform for MSC proliferation. Dynamic magnetic field regimes were imposed by cyclic variation of the magnetic field intensity in different frequencies. The effect of the magnetic field intensity on cell behavior showed that higher intensity of 0.45 T was associated with increased cell death and a poor angiogenic effect. It was observed that static and dynamic magnetic stimulation with higher frequencies led to improved angiogenic performance on endothelial cells in comparison with a lower frequency regime. This work showed the possibility to control VEGF-A secretion by MSCs through modulation of the magnetic field, offering attractive perspectives of a non-invasive therapeutic option for several diseases by revascularizing damaged tissues or inhibiting metastasis formation during cancer progression.

Electrokinetic Perfusion Through Three-Dimensional Culture Reduces Cell Mortality

Cell proliferation and survival are dependent on mass transfer. In vivo, fluid flow promotes mass transfer through the vasculature and interstitial space, providing a continuous supply of nutrients and removal of cellular waste products. In the absence of sufficient flow, mass transfer is limited by diffusion and poses significant challenges to cell survival during tissue engineering, tissue transplantation, and treatment of degenerative diseases. Artificial perfusion may overcome these challenges. In this work, we compare the efficacy of pressure driven perfusion (PDP) with electrokinetic perfusion (EKP) toward reducing cell mortality in three-dimensional cultures of Matrigel extracellular matrix. We characterize electro-osmotic flow through Matrigel to identify conditions that generate similar interstitial flow rates to those induced by pressure. We also compare changes in cell mortality induced by continuous or pulsed EKP. We report that continuous EKP significantly reduced mortality throughout the perfusion channels more consistently than PDP at similar flow rates, and pulsed EKP decreased mortality just as effectively as continuous EKP. We conclude that EKP has significant advantages over PDP for promoting tissue survival before neovascularization and angiogenesis. Impact statement Interstitial flow helps promote mass transfer and cell survival in tissues and organs. This study generated interstitial flow using pressure driven perfusion (PDP) or electrokinetic perfusion (EKP) to promote cell viability in three-dimensional cultures. EKP through charged extracellular matrices possesses significant advantages over PDP and may promote cell survival during tissue engineering, transplantations, and treatment of degenerative diseases.

Advances in removing mass transport limitations for more physiologically relevant in vitro 3D cell constructs

Spheroids and organoids are promising models for biomedical applications ranging from human disease modeling to drug discovery. A main goal of these 3D cell-based platforms is to recapitulate important physiological parameters of their in vivo organ counterparts. One way to achieve improved biomimetic architectures and functions is to culture cells at higher density and larger total numbers. However, poor nutrient and waste transport lead to low stability, survival, and functionality over extended periods of time, presenting outstanding challenges in this field. Fortunately, important improvements in culture strategies have enhanced the survival and function of cells within engineered microtissues/organs. Here, we first discuss the challenges of growing large spheroids/organoids with a focus on mass transport limitations, then highlight recent tools and methodologies that are available for producing and sustaining functional 3D in vitro models. This information points toward the fact that there is a critical need for the continued development of novel cell culture strategies that address mass transport in a physiologically relevant human setting to generate long-lasting and large-sized spheroids/organoids.

Synthetic and Bone tissue engineering graft substitutes: What is the future?

The management of large segmental bone defects caused by trauma or disease remains clinically challenging within orthopaedics. The major impediment to bone healing with current treatment options is insufficient vascularization and incorporation of graft material. Lack of rapid adequate vascularization leads to cellular necrosis within the inner regions of the implanted material and a failure of bone regeneration. Current treatment options for critical size bone defects include the continued "gold standard" autograft, allograft, synthetic bone graft substitutes, vascularized fibular graft, induced membrane technique, and distraction osteogenesis. Bone tissue engineering (BTE) remains an exciting prospect for the treatment of large segmental bone defects; however, current clinical integration of engineered scaffolds remains low. We believe that the barrier to clinical application of bone tissue engineering constructs lies in the lack of concomitant vascularization of these scaffolds. This mini-review outlines the progress made and the significant limitations remaining in successful clinical incorporation of engineered synthetic bone substitutes for segmental defects.

Biocompatibility and Angiogenic Effect of Chitosan/Graphene Oxide Hydrogel Scaffolds on EPCs

Angiogenesis in the field of tissue engineering has attracted significant attention. Graphene oxide has become a promising nanomaterial in tissue engineering for its unique biochemical properties. Therefore, herein, a series of chitosan (CS)/graphene oxide (GO) hydrogel scaffolds were synthesized by crosslinking CS and GO at different concentrations (0.1, 0.5, and 1.0 wt.%) using genipin. Compared with the CS hydrogel scaffolds, the CS/GO hydrogel scaffolds have a better network structure and mechanical strength. Then, we used endothelial progenitor cells (EPCs) extracted from human umbilical cord blood and cocultured these EPCs with the as-prepared scaffolds. The scaffolds with 0.1 and 0.5 wt.%GO showed no considerable cytotoxicity, could promote the proliferation of EPCs and tube formation, and upregulated the expressions of CD34, VEGF, MMP9, and SDF-1 in EPCs compared to the case of the scaffold with 1.0 wt.%GO. This study shows that the addition of graphene oxide improves the structure of chitosan hydrogel and enhances the proliferation activity and angiogenic capacity of EPCs.

Review of Bioprinting in Regenerative Medicine: Naturally Derived Bioinks and Stem Cells

Regenerative medicine offers the potential to repair or substitute defective tissues by constructing active tissues to address the scarcity and demands for transplantation. The method of forming 3D constructs made up of biomaterials, cells, and biomolecules is called bioprinting. Bioprinting of stem cells provides the ability to reliably recreate tissues, organs, and microenvironments to be used in regenerative medicine. 3D bioprinting is a technique that uses several biomaterials and cells to tailor a structure with clinically relevant geometries and sizes. This technique's promise is demonstrated by 3D bioprinted tissues, including skin, bone, cartilage, and cardiovascular, corneal, hepatic, and adipose tissues. Several bioprinting methods have been combined with stem cells to effectively produce tissue models, including adult stem cells, embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and differentiation techniques. In this review, technological challenges of printed stem cells using prevalent naturally derived bioinks (e.g., carbohydrate polymers and protein-based polymers, peptides, and decellularized extracellular matrix), recent advancements, leading companies, and clinical trials in the field of 3D bioprinting are delineated.

A perfusable vascularized full-thickness skin model for potential topical and systemic applications

Vascularization of reconstructed tissues is one of the remaining hurdles to be considered to improve both the functionality and viability of skin grafts and the relevance ofin vitroapplications. Our study, therefore, sought to develop a perfusable vascularized full-thickness skin equivalent that comprises a more complex blood vasculature compared to existing models. We combined molding, auto-assembly and microfluidics techniques in order to create a vascularized skin equivalent representing (a) a differentiated epidermis with a physiological organization and correctly expressing K14, K10, Involucrin, TGM1 and Filaggrin, (b) three perfusable vascular channels with angiogenic sprouts stained with VE-Caderin and Collagen IV, (c) an adjacent microvascular network created via vasculogenesis and connected to the sprouting macrovessels. Histological analysis and immunostaining of CD31, Collagen IV, Perlecan and Laminin proved the integrity of vascular constructs. In order to validate the vascularized skin potential of topical and systemic applications, caffeine and minoxidil, two compounds with different chemical properties, were topically applied to measure skin permeability and benzo[a]pyrene pollutant was systemically applied to evaluate systemic delivery. Our results demonstrated that perfusion of skin reconstructs and the presence of a complex vascular plexus resulted in a more predictive and reliable model to assess respectively topical and systemic applications. This model is therefore aimed at furthering drug discovery and improving clinical translation in dermatology.

3D Bioprinting of Vascularized Tissues for in vitro and in vivo Applications

With a limited supply of organ donors and available organs for transplantation, the aim of tissue engineering with three-dimensional (3D) bioprinting technology is to construct fully functional and viable tissue and organ replacements for various clinical applications. 3D bioprinting allows for the customization of complex tissue architecture with numerous combinations of materials and printing methods to build different tissue types, and eventually fully functional replacement organs. The main challenge of maintaining 3D printed tissue viability is the inclusion of complex vascular networks for nutrient transport and waste disposal. Rapid development and discoveries in recent years have taken huge strides toward perfecting the incorporation of vascular networks in 3D printed tissue and organs. In this review, we will discuss the latest advancements in fabricating vascularized tissue and organs including novel strategies and materials, and their applications. Our discussion will begin with the exploration of printing vasculature, progress through the current statuses of bioprinting tissue/organoids from bone to muscles to organs, and conclude with relevant applications for in vitro models and drug testing. We will also explore and discuss the current limitations of vascularized tissue engineering and some of the promising future directions this technology may bring.

Angiogenic Properties of Concentrated Growth Factors (CGFs): The Role of Soluble Factors and Cellular Components

Blood-derived concentrated growth factors (CGFs) represent a novel autologous biomaterial with promising applications in regenerative medicine. Angiogenesis is a key factor in tissue regeneration, but the role played by CGFs in vessel formation is not clear. The purpose of this study was to characterize the angiogenic properties of CGFs by evaluating the effects of its soluble factors and cellular components on the neovascularization in an in vitro model of angiogenesis. CGF clots were cultured for 14 days in cell culture medium; after that, CGF-conditioned medium (CGF-CM) was collected, and soluble factors and cellular components were separated and characterized. CGF-soluble factors, such as growth factors (VEGF and TGF-β1) and matrix metalloproteinases (MMP-2 and -9), were assessed by ELISA. Angiogenic properties of CGF-soluble factors were analyzed by stimulating human cultured endothelial cells with increasing concentrations (1%, 5%, 10%, or 20%) of CGF-CM, and their effect on cell migration and tubule-like formation was assessed by wound healing and Matrigel assay, respectively. The expression of endothelial angiogenic mediators was determined using qRT-PCR and ELISA assays. CGF-derived cells were characterized by immunostaining, qRT-PCR and Matrigel assay. We found that CGF-CM, consisting of essential pro-angiogenic factors, such as VEGF, TGF-β1, MMP-9, and MMP-2, promoted endothelial cell migration; tubule structure formation; and endothelial expression of multiple angiogenic mediators, including growth factors, chemokines, and metalloproteinases. Moreover, we discovered that CGF-derived cells exhibited features such as endothelial progenitor cells, since they expressed the CD34 stem cell marker and endothelial markers and participated in the neo-angiogenic process. In conclusion, our results suggest that CGFs are able to promote endothelial angiogenesis through their soluble and cellular components and that CGFs can be used as a biomaterial for therapeutic vasculogenesis in the field of tissue regeneration.

Pressure-driven spreadable deferoxamine-laden hydrogels for vascularized skin flaps

The development of hydrogels that support vascularization to improve the survival of skin flaps, yet establishing homogeneous angiogenic niches without compromising the ease of use in surgical settings remains a challenge. Here, pressure-driven spreadable hydrogels were developed utilizing beta-sheet rich silk nanofiber materials. These silk nanofiber-based hydrogels exhibited excellent spreading under mild pressure to form a thin coating to cover all the regions of the skin flaps. Deferoxamine (DFO) was loaded onto the silk nanofibers to support vascularization and these DFO-laden hydrogels were implanted under skin flaps in rats to fill the interface between the wound bed and the flap using the applied pressure. The thickness of the spread hydrogels was below 200 μm, minimizing the physical barrier effects from the hydrogels. The distribution of the hydrogels provided homogeneous angiogenic stimulation, accelerating rapid blood vessel network formation and significantly improving the survival of the skin flaps. The hydrogels also modulated the immune reactions, further facilitating the regeneration of the skin flaps. Considering the homogeneous distribution at the wound sites, improved vascularization, reduced barrier effects and low inflammation, these hydrogels appear to be promising candidates for use in tissue repair where a high blood supply is in demand. The pressure-driven spreading properties should simplify the use of the hydrogels in surgical settings to facilitate clinical translation.

In Vitro Prevascularization of Self-Assembled Human Bone-Like Tissues and Preclinical Assessment Using a Rat Calvarial Bone Defect Model

In vitro prevascularization has the potential to address the challenge of maintaining cell viability at the core of engineered constructs, such as bone substitutes, and to improve the survival of tissue grafts by allowing quicker anastomosis to the host microvasculature. The self-assembly approach of tissue engineering allows the production of biomimetic bone-like tissue constructs including extracellular matrix and living human adipose-derived stromal/stem cells (hASCs) induced towards osteogenic differentiation. We hypothesized that the addition of endothelial cells could improve osteogenesis and biomineralization during the production of self-assembled human bone-like tissues using hASCs. Additionally, we postulated that these prevascularized constructs would consequently improve graft survival and bone repair of rat calvarial bone defects. This study shows that a dense capillary network spontaneously formed in vitro during tissue biofabrication after two weeks of maturation. Despite reductions in osteocalcin levels and hydroxyapatite formation in vitro in prevascularized bone-like tissues (35 days of culture), in vivo imaging of prevascularized constructs showed an improvement in cell survival without impeding bone healing after 12 weeks of implantation in a calvarial bone defect model (immunocompromised male rats), compared to their stromal counterparts. Globally, these findings establish our ability to engineer prevascularized bone-like tissues with improved functional properties.

Functionalized Strategies and Mechanisms of the Emerging Mesh for Abdominal Wall Repair and Regeneration

Meshes have been the overwhelmingly popular choice for the repair of abdominal wall defects to retrieve the bodily integrity of musculofascial layer. Broadly, they are classified into synthetic, biological and composite mesh based on their mechanical and biocompatible features. With the development of anatomical repair techniques and the increasing requirements of constructive remodeling, however, none of these options satisfactorily manages the conditional repair. In both preclinical and clinical studies, materials/agents equipped with distinct functions have been characterized and applied to improve mesh-aided repair, with the importance of mesh functionalization being highlighted. However, limited information exists on systemic comparisons of the underlying mechanisms with respect to functionalized strategies, which are fundamental throughout repair and regeneration. Herein, we address this topic and summarize the current literature by subdividing common functions of the mesh into biomechanics-matched, macrophage-mediated, integration-enhanced, anti-infective and antiadhesive characteristics for a comprehensive overview. In particular, we elaborate their effects separately with respect to host response and integration and discuss their respective advances, challenges and future directions toward a clinical alternative. From the vastly different approaches, we provide insight into the mechanisms involved and offer suggestions for personalized modifications of these emerging meshes.

Investigation of Vasculogenesis Inducing Biphasic Scaffolds for Bone Tissue Engineering

Vascularization is the main obstacle for the bone tissue engineering strategies since the defect size is generally large. Incorporation of angiogenic factors is one of the strategies employed in order to accelerate vascularization and improve bone healing. In this study, a biphasic scaffold consisting of fibrous poly(lactide-co-glycolide) (PLGA) and poly(lactide-co-glycolide)-block-poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PLGA-PEG-PLGA) hydrogel loaded with vascular endothelial growth factor-A (VEGF) inducer, GS4012, was constructed. Mesenchymal stem cells isolated from rat bone marrow (rBMSCs) were used for differentiation into osteogenic cells, and endothelial cells isolated from rat peripheral blood (rPBECs) were used to test the in vitro endothelial cell recruitment. The biphasic scaffold was tested for cell proliferation, ALP expression, VEGF induction, expression of osteogenic genes by rBMSCs, and recruitment of rPBECs in vitro and for improved bone healing and vascularization in vivo on critical size rat cranial defects. Endothelial migration through porous insert and VEGF induction were obtained in vitro in response to GS4012 as well as the upregulation of ALP, Runx2, Col I, and OC gene expressions. The biphasic scaffold was also shown to be effective in improving endothelial cell recruitment, vascularization, and bone healing in vivo. Thus, the proposed design has a great potential for the healing of critical size bone defect in tissue engineering studies according to both in vitro and in vivo investigations.

The Duo of Osteogenic and Angiogenic Differentiation in ADSC-Derived Spheroids

Bone formation during embryogenesis is driven by interacting osteogenesis and angiogenesis with parallel endothelial differentiation. Thence, all in vitro bioengineering techniques are aimed at pre-vascularization of osteogenic bioequivalents to provide better regeneration outcomes upon transplantation. Due to appearance of cell-cell and cell-matrix interactions, 3D cultures of adipose-derived stromal cells (ADSCs) provide a favorable spatial context for the induction of different morphogenesis processes, including vasculo-, angio-, and osteogenesis and, therefore, allow modeling their communication in vitro. However, simultaneous induction of multidirectional cell differentiation in spheroids from multipotent mesenchymal stromal cells (MMSCs) was not considered earlier. Here we show that arranging ADSCs into spheroids allows rapid and spontaneous acquiring of markers of both osteo- and angiogenesis compared with 2D culture. We further showed that this multidirectional differentiation persists in time, but is not influenced by classical protocols for osteo- or angio-differentiation. At the same time, ADSC-spheroids retain similar morphology and microarchitecture in different culture conditions. These findings can contribute to a better understanding of the fundamental aspects of autonomous regulation of differentiation processes and their cross-talks in artificially created self-organizing multicellular structures. This, in turn, can find a wide range of applications in the field of tissue engineering and regeneration.

Engineering Advanced In Vitro Models of Systemic Sclerosis for Drug Discovery and Development

Systemic sclerosis (SSc) is a complex multisystem disease with the highest case-specific mortality among all autoimmune rheumatic diseases, yet without any available curative therapy. Therefore, the development of novel therapeutic antifibrotic strategies that effectively decrease skin and organ fibrosis is needed. Existing animal models are cost-intensive, laborious and do not recapitulate the full spectrum of the disease and thus commonly fail to predict human efficacy. Advanced in vitro models, which closely mimic critical aspects of the pathology, have emerged as valuable platforms to investigate novel pharmaceutical therapies for the treatment of SSc. This review focuses on recent advancements in the development of SSc in vitro models, sheds light onto biological (e.g., growth factors, cytokines, coculture systems), biochemical (e.g., hypoxia, reactive oxygen species) and biophysical (e.g., stiffness, topography, dimensionality) cues that have been utilized for the in vitro recapitulation of the SSc microenvironment, and highlights future perspectives for effective drug discovery and validation.

Accelerating neovascularization and kidney tissue formation with a 3D vascular scaffold capturing native vascular structure

Establishing an adequate vascularization of three-dimensional (3D) bioengineered tissues remains a critical challenge. We previously fabricated a vascular scaffold using the vascular corrosion casting technique, which provides a similar 3D geometry of native kidney vasculature. In this study, we functionalized the collagen vascular scaffold with a controlled release of vascular endothelial growth factor (VEGF vascular scaffold) to further promote vascularization. The VEGF vascular scaffold showed improved angiogenic capability in 2-dimensional (2D) and 3D in vitro settings. Implantation of the VEGF vascular scaffold seeded with human renal cells into a rat kidney demonstrated enhanced implant vascularization and reduced apoptosis of implanted human renal cells. Hybrid renal tubule-like structures composed of implanted human and migrated host renal cells were formed. This work highlights the critical role of early vascularization of the geometrically mimetic vascular scaffold using the VEGF incorporated vascular scaffold in reducing apoptosis of implanted cells as well as the formation of renal tissue structures.

Fabrication of PCL/keratin composite scaffolds for vascular tissue engineering with catalytic generation of nitric oxide potential

Tissue-engineered vascular grafts (TEVGs) have been proposed as a promising approach to fulfill the need for small-diameter blood vessel substitutes. However, common failure caused by thrombosis and neointimal proliferation after implantation has restricted their use in the clinic. Herein, a NO-generating scaffold for vascular tissue engineering was developed by coelectrospinning poly(ε-caprolactone) (PCL) with keratin. The morphology and surface chemical composition were characterized via SEM, ATR-FTIR spectroscopy and XPS. The biocomposite scaffold selectively enhanced the adhesion and growth of endothelial cells (ECs) while suppressing the proliferation of smooth muscle cells (SMCs) in the presence of GSH and GSNO due to the catalytic generation of NO. In addition, these mats displayed excellent blood compatibility by prolonging the blood-clotting time. In summary, these NO-generating PCL/keratin scaffolds have potential applications in vascular tissue engineering with rapid endothelialization and reduced SMC proliferation.

In vitro vascularization of tissue engineered constructs by non-viral delivery of pro-angiogenic genes

Vascularization is still one of the major challenges in tissue engineering. In the context of tissue regeneration, the formation of capillary-like structures is often triggered by the addition of growth factors which are associated with high cost, bolus release and short half-life. As an alternative to growth factors, we hypothesized that delivering genes-encoding angiogenic growth factors to cells in a scaffold microenvironment would lead to a controlled release of angiogenic proteins promoting vascularization, simultaneously offering structural support for new matrix deposition. Two non-viral vectors, chitosan (Ch) and polyethyleneimine (PEI), were tested to deliver plasmids encoding for vascular endothelial growth factor (pVEGF) and fibroblast growth factor-2 (pFGF2) to human dermal fibroblasts (hDFbs). hDFbs were successfully transfected with both Ch and PEI, without compromising the metabolic activity. Despite low transfection efficiency, superior VEGF and FGF-2 transgene expression was attained when pVEGF was delivered with PEI and when pFGF2 was delivered with Ch, impacting the formation of capillary-like structures by primary human dermal microvascular endothelial cells (hDMECs). Moreover, in a 3D microenvironment, when PEI-pVEGF and Ch-FGF2 were delivered to hDFbs, cells produced functional pro-angiogenic proteins which induced faster formation of capillary-like structures that were retained in vitro for longer time in a Matrigel assay. The dual combination of the plasmids resulted in a downregulation of the production of VEGF and an upregulation of FGF-2. The number of capillary-like segments obtained with this system was inferior to the delivery of plasmids individually but superior to what was observed with the non-transfected cells. This work confirmed that cell-laden scaffolds containing transfected cells offer a novel, selective and alternative approach to impact the vascularization during tissue regeneration. Moreover, this work provides a new platform for pathophysiology studies, models of disease, culture systems and drug screening.

Enhanced Regeneration of Vascularized Adipose Tissue with Dual 3D-Printed Elastic Polymer/dECM Hydrogel Complex

A flexible and bioactive scaffold for adipose tissue engineering was fabricated and evaluated by dual nozzle three-dimensional printing. A highly elastic poly (L-lactide-co-ε-caprolactone) (PLCL) copolymer, which acted as the main scaffolding, and human adipose tissue derived decellularized extracellular matrix (dECM) hydrogels were used as the printing inks to form the scaffolds. To prepare the three-dimensional (3D) scaffolds, the PLCL co-polymer was printed with a hot melting extruder system while retaining its physical character, similar to adipose tissue, which is beneficial for regeneration. Moreover, to promote adipogenic differentiation and angiogenesis, adipose tissue-derived dECM was used. To optimize the printability of the hydrogel inks, a mixture of collagen type I and dECM hydrogels was used. Furthermore, we examined the adipose tissue formation and angiogenesis of the PLCL/dECM complex scaffold. From in vivo experiments, it was observed that the matured adipose-like tissue structures were abundant, and the number of matured capillaries was remarkably higher in the hydrogel–PLCL group than in the PLCL-only group. Moreover, a higher expression of M2 macrophages, which are known to be involved in the remodeling and regeneration of tissues, was detected in the hydrogel–PLCL group by immunofluorescence analysis. Based on these results, we suggest that our PLCL/dECM fabricated by a dual 3D printing system will be useful for the treatment of large volume fat tissue regeneration.

Aligned Silk Sponge Fabrication and Perfusion Culture for Scalable Proximal Tubule Tissue Engineering

Chronic kidney disease and kidney failure are on the rise globally, yet there has not been a corresponding improvement in available therapies. A key challenge in a biological approach to developing kidney tissue is to identify scaffolding materials that support cell growth both in vitro and in vivo to facilitate translational goals. Scaffolds composed of silk fibroin protein possess the biocompatibility, mechanical robustness, and stability required for tissue engineering. Here, we use a silk sponge system to support kidney cells in a perfused bioreactor system. Silk fibroin protein underwent directional freezing to form parallel porous structures that mimic the native kidney structure of aligned tubules and are able to support more cells than nonaligned silk sponges. Adult immortalized renal proximal tubule epithelial cells were seeded into the sponges and cultured under static conditions for 1 week, then grown statically or with perfusion with culture media flowing through the sponge to enhance cell alignment and maturation. The sponges were imaged with confocal and scanning electron microscopies to analyze and quantify cell attachment, alignment, and expression of proteins important to proximal tubule differentiation and function. The perfused tissue constructs showed higher number of cells that are more evenly distributed through the construct and increased gene expression of several key markers of proximal tubule epithelial cell function compared to sponges grown under static conditions. These perfused tissue constructs represent a step toward a scalable approach to engineering proximal tubule structures with the potential to be used as in vitro models or as in vivo implantable tissues to supplement or replace impaired kidney function.

Three-dimensional bioprinted hepatorganoids prolong survival of mice with liver failure

OBJECTIVE

Shortage of organ donors, a critical challenge for treatment of end-stage organ failure, has motivated the development of alternative strategies to generate organs in vitro. Here, we aim to describe the hepatorganoids, which is a liver tissue model generated by three-dimensional (3D) bioprinting of HepaRG cells and investigate its liver functions in vitro and in vivo.

DESIGN

3D bioprinted hepatorganoids (3DP-HOs) were constructed using HepaRG cells and bioink, according to specific 3D printing procedures. Liver functions of 3DP-HOs were detected after 7 days of differentiation in vitro, which were later transplanted into Fah-deficient mice. The in vivo liver functions of 3DP-HOs were evaluated by survival time and liver damage of mice, human liver function markers and human-specific debrisoquine metabolite production.

RESULTS

3DP-HOs broadly acquired liver functions, such as ALBUMIN secretion, drug metabolism and glycogen storage after 7 days of differentiation. After transplantation into abdominal cavity of Fah^-/-Rag2^-/- mouse model of liver injury, 3DP-HOs further matured and displayed increased synthesis of liver-specific proteins. Particularly, the mice acquired human-specific drug metabolism activities. Functional vascular systems were also formed in transplanted 3DP-HOs, further enhancing the material transport and liver functions of 3DP-HOs. Most importantly, transplantation of 3DP-HOs significantly improved the survival of mice.

CONCLUSIONS

Our results demonstrated a comprehensive proof of principle, which indicated that 3DP-HO model of liver tissues possessed in vivo hepatic functions and alleviated liver failure after transplantation, suggesting that 3D bioprinting could be used to generate human liver tissues as the alternative transplantation donors for treatment of liver diseases.

Injectable oxygen-generating nanocomposite hydrogels with prolonged oxygen delivery for enhanced cell proliferation under hypoxic and normoxic conditions

We describe a new organic peroxide-based injectable biomaterial that was prepared by using benzoylperoxide and LAPONITE® incorporated into an alginate hydrogel (BPO-AlgL). BPO-AlgL shows sustained release of O2 over a period of 14 days and reduces the hypoxia-induced cell death. BPO-AlgL also promotes enhanced cell viability by providing sustained O2 within the 3D AlgL scaffold. In addition, BPO-AlgL increases the O2 level in the environment of the cells, which led to a decrease in the proliferation of malignant cells, while it also resulted in an increase in the viability of healthy fibroblast cells. Therefore, our new oxygen generating organic peroxide-based injectable 3D biomaterials have potential to contribute in the development of next generation advanced tissue engineering biomaterials.

Cell spheroids as a versatile research platform: formation mechanisms, high throughput production, characterization and applications

Three-dimensional cell culture has tremendous advantages to closely mimic the in vivo architecture and microenvironment of healthy tissue and organs, as well as of solid tumors. Spheroids are currently the most attractive 3D model to produce uniform reproducible cell structures as well as a potential basis for engineering large tissues and complex organs. In this review we discuss, from an engineering perspective, processes to obtain uniform 3D cell spheroids, comparing dynamic and static cultures and considering aspects such as mass transfer and shear stress. In addition, computational and mathematical modelling of complex cell spheroid systems are discussed. The non-cell-adhesive hydrogel-based method and dynamic cell culture in bioreactors are focused in detail and the myriad of developed spheroid characterization techniques is presented. The main bottlenecks and weaknesses are discussed, especially regarding the analysis of morphological parameters, cell quantification and viability, gene expression profiles, metabolic behavior and high-content analysis. Finally, a vast set of applications of spheroids as tools for in vitro study model systems is examined, including drug screening, tissue formation, pathologies development, tissue engineering and biofabrication, 3D bioprinting and microfluidics, together with their use in high-throughput platforms.

The effects of bone marrow stem and progenitor cell seeding on urinary bladder tissue regeneration

Complications associated with urinary bladder augmentation provide the motivation to delineate alternative bladder tissue regenerative engineering strategies. We describe the results of varying the proportion of bone marrow (BM) mesenchymal stem cells (MSCs) to CD34 + hematopoietic stem/progenitor cells (HSPCs) co-seeded onto synthetic POC [poly(1,8 octamethylene citrate)] or small intestinal submucosa (SIS) scaffolds and their contribution to bladder tissue regeneration. Human BM MSCs and CD34 + HSPCs were co-seeded onto POC or SIS scaffolds at cell ratios of 50 K CD34 + HSPCs/15 K MSCs (CD34-50/MSC15); 50 K CD34 + HSPCs/30 K MSCs (CD34-50/MSC30); 100 K CD34 + HSPCs/15 K MSCs (CD34-100/MSC15); and 100 K CD34 + HSPCs/30 K MSCs (CD34-100/MSC30), in male (M/POC; M/SIS; n = 6/cell seeded scaffold) and female (F/POC; F/SIS; n = 6/cell seeded scaffold) nude rats (n = 96 total animals). Explanted scaffold/composite augmented bladder tissue underwent quantitative morphometrics following histological staining taking into account the presence (S+) or absence (S−) of bladder stones. Urodynamic studies were also performed. Regarding regenerated tissue vascularization, an upward shift was detected for some higher seeded density groups including the CD34-100/MSC30 groups [F/POC S− CD34-100/MSC30 230.5 ± 12.4; F/POC S+ CD34-100/MSC30 245.6 ± 23.4; F/SIS S+ CD34-100/MSC30 278.1; F/SIS S− CD34-100/MSC30 187.4 ± 8.1; (vessels/mm²)]. Similarly, a potential trend toward increased levels of percent muscle (≥ 45% muscle) with higher seeding densities was observed for F/POC S− [CD34-50/MSC30 48.8 ± 2.2; CD34-100/MSC15 53.9 ± 2.8; CD34-100/MSC30 50.7 ± 1.7] and for F/SIS S− [CD34-100/MSC15 47.1 ± 1.6; CD34-100/MSC30 51.2 ± 2.3]. As a potential trend, higher MSC/CD34 + HSPCs cell seeding densities generally tended to increase levels of tissue vascularization and aided with bladder muscle growth. Data suggest that increasing cell seeding density has the potential to enhance bladder tissue regeneration in our model.

Rapid Fabrication of Cell-Laden Microfibers for Construction of Aligned Biomimetic Tissue

Bottom-up engineering of tissue constructs is being rapidly developed and broadly applied in biomanufacturing. As one type of building block, cell-laden microfibers are promising for reconstruction of oriented structures and functions of linear tissues, such as skeletal muscles, myocardia, and spinal cord tissues. Herein, we propose wet-spinning method with agitating collection, wherein alginate-based material is extruded into an agitated CaCl2 bath with a magnetic rotor acting as the microfiber collector. By applying this method, we achieve rapid fabrication and oriented collection of hydrogel microfibers with diameters ranging from 100 to 400 μm. In addition, we encapsulate myoblasts in the hydrogel to form cell-laden microfibers, which show a high viability (more than 94%) during in vitro culture. Moreover, the method allows to fabricate of cell-laden core-sheath microfibers and hollow microfibers. We also fabricate 3D constructs using various methods of microfiber assembly like weaving and braiding. The assembling results suggest that the proposed method is a promising technology for bottom-up engineering of aligned biomimetic tissue constructs.

The impact of delayed surgical intervention following high velocity maxillofacial injuries

Our study compares the number of postoperative complications of Syrian patients admitted to the Galilee Medical Center (GMC) over a 5-year period (May 2013-May 2018) for treatment after initial high-velocity maxillofacial injuries sustained during the Syrian civil war. Specifically, we evaluated complication rates of patients arriving "early," within 24 h, to the GMC versus those who arrived "late," or 14-28 days following high-velocity maxillofacial injuries. Both groups of patients received definitive surgical treatment within 48 h of admission to our hospital with a total of 60 patients included in this study. The mean age was 26 ± 8 years (range: 9-50) and all except one were male. Postoperative complications in the early group were found to be significantly higher compared to the delayed arrival group (p = 0.006). We found that unintentionally delayed treatment may have contributed to a critical revascularization period resulting in improved healing and decreased postoperative morbidity and complications. We discuss potential mechanisms for complication rate variations, including critical vascularization periods. Our study may add to a growing body of work demonstrating the potential benefit of delayed surgical treatment for high-velocity maxillofacial injuries.

Symbiotic Photosynthetic Oxygenation within 3D-Bioprinted Vascularized Tissues

In this study, we present the photosynthetic oxygen (O2) supply to mammalian cells within a volumetric extracellular matrix-like construct, whereby a three-dimensional (3D)-bioprinted fugitive pattern encapsulating unicellular green algae, Chlamydomonas reinhardtii (C. reinhardtii), served as a natural photosynthetic O2-generator. The presence of bioprinted C. reinhardtii enhanced the viability and functionality of mammalian cells while reducing the hypoxic conditions within the tissues. We were able to subsequently endothelialize the hollow perfusable microchannels formed after enzymatic removal of the bioprinted C. reinhardtii-laden patterns from the matrices following the initial oxygenation period, to obtain biologically relevant vascularized mammalian tissue constructs. The feasibility of co-culture of C. reinhardtii with human cells, the printability and the enzymatic degradability of the fugitive bioink, as well as the exploration of C. reinhardtii as a natural, eco-friendly, cost-effective, and sustainable source of O2 would likely promote the development of engineered tissues, tissue models, and food for various applications.

A Multichamber Gas System to Examine the Effect of Multiple Oxygen Conditions on Cell Culture

The classic bone tissue engineering model for bone regeneration combines three elements: scaffolds, biomaterials, and mesenchymal stem cells (MSCs). Incorporation of MSCs and growth factors into a scaffold implanted into the area of bone injury is a proven strategy to achieve successful bone regeneration as demonstrated in the literature. However, a major limitation of using bone grafts or scaffolds is oxygen (O₂) deprivation in the inner sections of the construct, due to lack of adequate vascularization. To address this limitation, we proposed two treatment strategies for MSC-seeded constructs or adipose tissue scaffolds before implantation: (1) O₂ enrichment and (2) acclimation to hypoxia. Based on previous studies, the significance of the different O₂ concentrations on MSC biological characteristics remains controversial. Therefore, the optimal O₂ condition for engineered bone tissues should be determined. Thus, we designed an innovative multichamber gas system aimed to simultaneously assess the effects of different O₂ levels on cell culture. This system was assembled using three isolated chambers integrated into a single incubator. To explore the efficacy of our method, we investigated the effect of hyperoxia, normoxia, and hypoxia, (50-60%, 21%, and 5-7.5% O₂, respectively) on the biological characteristics of human adipose-derived MSCs: immunophenotyping, adhesion, proliferation, and osteogenic, and angiogenic differentiation. Our findings demonstrated that hypoxic adipose-derived mesenchymal stem cells (ASCs) conditions exhibited significantly lower levels of CD34 (p = 0.014), with significantly higher osteogenic and angiogenic differentiation capacities (p = 0.023 and p = 0.0042, respectively) than normoxia. Conversely, hyperoxia-cultured ASCs demonstrated significantly higher levels of CD73 and CD90 expression than both normoxic ASCs (p = 0.006 and p = 0.025, respectively) and hypoxic ASCs (p = 0.003 and p = 0.003, respectively). In addition, hyperoxic ASCs showed significantly reduced proliferation capacity by day 11 (p = 0.032) and significantly enhanced migration rates after 48 h (p = 0.044). The newly developed controllable multichamber gas system was cost-effective and easy to use. Different assays can be performed concurrently while preserving all other conditions identical, and the use of other ranges of O₂ concentrations is feasible and also necessary to determine the ideal O₂ concentration. Furthermore, the multichamber gas system has the potential for wide application, including other cell cultures, grafts, or scaffolds for in vitro and in vivo experimentation. This study was approved by the Galilee Medical Center Helsinki Committee (No. 0009-19-NHR). Impact statement The introduced multichamber gas system provides a custom-made setup for simultaneous control of three oxygen (O₂) levels in a single incubator. The use of our innovative multichamber gas system is essential to determine the ideal O₂ levels for engineered tissues by examining multiple O₂ concentrations on cells in vitro. The determined ideal O₂ concentration will then be used through this system to investigate the engrafted cell survival ex vivo, to ensure successful integration of the engineered tissues and tissue regeneration in vivo. Use of this method may promote a therapeutic tool for a major limitation in tissue engineering due to the problematic O₂ insufficiency in tissue scaffolds.

αv-Class integrin binding to fibronectin is solely mediated by RGD and unaffected by an RGE mutation

Fibronectin (FN) is an essential glycoprotein of the extracellular matrix; binds integrins, syndecans, collagens, and growth factors; and is assembled by cells into complex fibrillar networks. The RGD motif in FN facilitates cell binding- and fibrillogenesis through binding to α5β1 and αv-class integrins. However, whether RGD is the sole binding site for αv-class integrins is unclear. Most notably, substituting aspartate with glutamate (RGE) was shown to eliminate integrin binding in vitro, while mouse genetics revealed that FNRGE preserves αv-class integrin binding and fibrillogenesis. To address this conflict, we employed single-cell force spectroscopy, engineered cells, and RGD motif-deficient mice (Fn1ΔRGD/ΔRGD) to search for additional αv-class integrin-binding sites. Our results demonstrate that α5β1 and αv-class integrins solely recognize the FN-RGD motif and that αv-class, but not α5β1, integrins retain FN-RGE binding. Furthermore, Fn1ΔRGD/ΔRGD tissues and cells assemble abnormal and dysfunctional FNΔRGD fibrils in a syndecan-dependent manner. Our data highlight the central role of FN-RGD and the functionality of FN-RGE for αv-class integrins.

The development of laminin-alginate microspheres encapsulated with Ginsenoside Rg1 and ADSCs for breast reconstruction after lumpectomy

Many technologies have been developed for breast reconstruction after lumpectomy. Although the technologies achieved promising success in clinical, there are still many shortages hanging over and trouble the researchers. Tissue engineering technology was introduced to plastic surgery that gave a light to lumpectomy patients in breast reconstruction. The unexpected absorption rate, resulting from limited vascularization and low cell survival rate, is a major factor that leads to unsatisfactory results for the previous studies in our lab. In the study, the laminin-modified alginate synthesized by a new method of low concertation of sodium periodate would be mixed with ADSCs and Rg1 in the medium; and then sprayed into a calcium chloride (CaCl₂) solution to prepare into microsphere (abbreviated as ADSC–G-LAMS) by bio-electrospray with a power syringe for the mass production and smaller bead size. The developed ADSC–G-LAMS microspheres had the diameter of 232 ± 42 μm. Sustained-release of the Rg1 retained its biological activity. WST-1, live/dead staining, and chromosome aberration assay were evaluated to confirm the safety of the microspheres. In in vivo study, ADSC–G-LAMS microspheres combined with autologous adipocytes were transplanted into the dorsum of rats by subcutaneous injection. The efficacy was investigated by H&E and immunofluorescence staining. The results showed that the bioactive ADSC–G-LAMS microspheres could integrate well into the host adipose tissue with an adequate rate of angiogenesis by constantly releasing Rg1 to enhance the ADSC or adipocyte survival rate to join tissue growth and repair with adipogenesis for breast reconstruction after lumpectomy.

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Laminin-modified alginate was successfully synthesized to mimic early embryonic environment.

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Adipose-derived stem cells (ADSCs) and ginsenoside Rg1 were encapsulated into laminin-alginate microspheres (ADSC–G-LAMS) by bio-electrospray method.

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ADSC–G-LAMS microspheres integrated into the host adipose tissue with an adequate rate of angiogenesis by constantly releasing Rg1.

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The developed bioactive ADSC–G-LAMS microspheres can be potential scaffolds for stem cells and angiogenic factor carriers for tissue engineering.

Signaling molecules orchestrating liver regenerative medicine

The liver is in charge of more than 500 functions in the human body, which any damage and failure to the liver can significantly compromise human life. Numerous studies are being carried out in regenerative medicine, as a potential driving force, toward alleviating the need for liver donors and fabrication of a 3D-engineered transplantable hepatic tissue. Liver tissue engineering brings three main factors of cells, extracellular matrix (ECM), and signaling molecules together, while each of these three factors tries to mimic the physiological state of the tissue to direct tissue regeneration. Signaling molecules play a crucial role in directing tissue fabrication in liver tissue engineering. When mimicking the natural in vivo process of regeneration, it is tightly associated with three main phases of differentiation, proliferation (progression), and tissue maturation through vascularization while directing each of these phases is highly regulated by the specific signaling molecules. The understanding of how these signaling molecules guide the dynamic behavior of regeneration would be a tool for further tailoring of bioengineered systems to help the liver regeneration with many cellular, molecular, and tissue-level functions. Hence, the signaling molecules come to aid all these phases for further improvements toward the clinical use of liver tissue engineering as the goal.

Effect of Nanostructured Scaffold on Human Adipose-Derived Stem Cells: Outcome of In Vitro Experiments

This work is addressed to provide, by in vitro experiments, results on the repercussion that a nanostructured scaffold could have on viability, differentiation and secretion of bioactive factors of human adipose-derived stem cells (hASCs) when used in association to promote angiogenesis, a crucial condition to favour tissue regeneration. To achieve this aim, we evaluated cell viability and morphology by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay and microscopy analysis, respectively. We also investigated the expression of some of those genes involved in angiogenesis and differentiation processes utilizing quantitative polymerase chain reaction (qPCR), whereas the amounts of Vascular Endothelial Growth Factor A, Interleukin 6 and Fatty Acid-Binding Protein 4 secreted in the culture medium, were quantified by enzyme-linked immunosorbent assay (ELISA). Results suggested that, in the presence of the scaffold, cell proliferation and the exocytosis of factors involved in the angiogenesis process are reduced; by contrast, the expression of those genes involved in hASC differentiation appeared enhanced. To guarantee cell survival, the construct dimensions are, generally, smaller than clinically required. Furthermore, being the paracrine event the primary mechanism exerting the beneficial effects on injured tissues, the use of conditioned culture medium instead of cells may be convenient.

Electrospun Microvasculature for Rapid Vascular Network Restoration

BACKGROUND

Sufficient blood supply through neo-vasculature is a major challenge in cell therapy and tissue engineering in order to support the growth, function, and viability of implanted cells. However, depending on the implant size and cell types, the natural process of angiogenesis may not provide enough blood supply for long term survival of the implants, requiring supplementary strategy to prevent local ischemia. Many researchers have reported the methodologies to form pre-vasculatures that mimic in vivo microvessels for implantation to promote angiogenesis. These approaches successfully showed significant enhancement in long-term survival and regenerative functions of implanted cells, yet there remains room for improvement.

METHODS

This paper suggests a proof-of-concept strategy to utilize novel scaffolds of dimpled/hollow electrospun fibers that enable the formation of highly mature pre-vasculatures with adequate dimensions and fast degrading in the tissue.

RESULT

Higher surface roughness improved the maturity of endothelial cells mediated by increased cell-scaffold affinity. The degradation of scaffold material for functional restoration of the neo-vasculatures was also expedited by employing the hollow scaffold design based on co-axial electrospinning techniques.

CONCLUSION

This unique scaffold-based pre-vasculature can hold implanted cells and tissue constructs for a prolonged time while minimizing the cellular loss, manifesting as a gold standard design for transplantable scaffolds.

Electrospun Nanofibers for Improved Angiogenesis: Promises for Tissue Engineering Applications

Angiogenesis (or the development of new blood vessels) is a key event in tissue engineering and regenerative medicine; thus, a number of biomaterials have been developed and combined with stem cells and/or bioactive molecules to produce three-dimensional (3D) pro-angiogenic constructs. Among the various biomaterials, electrospun nanofibrous scaffolds offer great opportunities for pro-angiogenic approaches in tissue repair and regeneration. Nanofibers made of natural and synthetic polymers are often used to incorporate bioactive components (e.g., bioactive glasses (BGs)) and load biomolecules (e.g., vascular endothelial growth factor (VEGF)) that exert pro-angiogenic activity. Furthermore, seeding of specific types of stem cells (e.g., endothelial progenitor cells) onto nanofibrous scaffolds is considered as a valuable alternative for inducing angiogenesis. The effectiveness of these strategies has been extensively examined both in vitro and in vivo and the outcomes have shown promise in the reconstruction of hard and soft tissues (mainly bone and skin, respectively). However, the translational of electrospun scaffolds with pro-angiogenic molecules or cells is only at its beginning, requiring more research to prove their usefulness in the repair and regeneration of other highly-vascularized vital tissues and organs. This review will cover the latest progress in designing and developing pro-angiogenic electrospun nanofibers and evaluate their usefulness in a tissue engineering and regenerative medicine setting.

Plant-Based Scaffolds Modify Cellular Response to Drug and Radiation Exposure Compared to Standard Cell Culture Models

Plant-based scaffolds present many advantages over a variety of biomaterials. Recent studies explored their potential to be repopulated with human cells and thus highlight a growing interest for their use in tissue engineering or for biomedical applications. However, it is still unclear if these in vitro plant-based scaffolds can modify cell phenotype or affect cellular response to external stimuli. Here, we report the characterization of the mechano-regulation of melanoma SK-MEL-28 and prostate PC3 cells seeded on decellularized spinach leaves scaffolds, compared to cells deposited on standard rigid cell culture substrate, as well as their response to drug and radiation treatment. The results showed that YAP/TAZ signaling was downregulated, cellular morphology altered and proliferation rate decreased when cells were cultured on leaf scaffold. Interestingly, cell culture on vegetal scaffold also affected cellular response to external stress. Thus, SK-MEL-28 cells phenotype is modified leading to a decrease in MITF activity and drug resistance, while PC3 cells showed altered gene expression and radiation response. These findings shed lights on the decellularization of vegetal materials to provide substrates that can be repopulated with human cells to better reproduce a soft tissue microenvironment. However, these complex scaffolds mediate changes in cell behavior and in order to exploit the capability of matching physical properties of the various plant scaffolds to diverse physiological functionalities of cells and human tissue constructs, additional studies are required to better characterize physical and biochemical cell-substrate interactions.

ERK and Akt exhibit distinct signaling responses following stimulation by pro-angiogenic factors

Background

Angiogenesis plays an important role in the survival of tissues, as blood vessels provide oxygen and nutrients required by the resident cells. Thus, targeting angiogenesis is a prominent strategy in many different settings, including both tissue engineering and cancer treatment. However, not all of the approaches that modulate angiogenesis lead to successful outcomes. Angiogenesis-based therapies primarily target pro-angiogenic factors such as vascular endothelial growth factor-A (VEGF) or fibroblast growth factor (FGF) in isolation, and there is a limited understanding of how these promoters combine together to stimulate angiogenesis. Targeting one pathway could be insufficient, as alternative pathways may compensate, diminishing the overall effect of the treatment strategy.

Methods

To gain mechanistic insight and identify novel therapeutic strategies, we have developed a detailed mathematical model to quantitatively characterize the crosstalk of FGF and VEGF intracellular signaling. The model focuses on FGF- and VEGF-induced mitogen-activated protein kinase (MAPK) signaling to promote cell proliferation and the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway, which promotes cell survival and migration. We fit the model to published experimental datasets that measure phosphorylated extracellular regulated kinase (pERK) and Akt (pAkt) upon FGF or VEGF stimulation. We validate the model with separate sets of data.

Results

We apply the trained and validated mathematical model to characterize the dynamics of pERK and pAkt in response to the mono- and co-stimulation by FGF and VEGF. The model predicts that for certain ranges of ligand concentrations, the maximum pERK level is more responsive to changes in ligand concentration compared to the maximum pAkt level. Also, the combination of FGF and VEGF indicates a greater effect in increasing the maximum pERK compared to the summation of individual effects, which is not seen for maximum pAkt levels. In addition, our model identifies the influential species and kinetic parameters that specifically modulate the pERK and pAkt responses, which represent potential targets for angiogenesis-based therapies.

Conclusions

Overall, the model predicts the combination effects of FGF and VEGF stimulation on ERK and Akt quantitatively and provides a framework to mechanistically explain experimental results and guide experimental design. Thus, this model can be utilized to study the effects of pro- and anti-angiogenic therapies that particularly target ERK and/or Akt activation upon stimulation with FGF and VEGF.

Fabrication of a self-assembled honeycomb nanofibrous scaffold to guide endothelial morphogenesis

Controlling angiogenesis within tissue engineered constructs remains a critical challenge, especially with regard to the guidance of pre-vascular network formation. Here, we aimed to regulate angiogenesis on a self-assembled honeycomb nanofibrous scaffold. Scaffolds with honeycombs patterns have several desirable properties for tissue engineering, including large surface area, high structural stability and good permeability. Furthermore, the honeycomb pattern resembles early vascular network formation. The self-assembly electrospinning approach to honeycomb scaffolds is a technically simple, rapid, and direct way to realize selective deposition of nanofibers. To evaluate cell compatibility, spreading, proliferation and tube formation, human umbilical vein endothelial cells (HUVECs) were cultured on honeycomb scaffolds, as well as on random scaffolds for comparison. The optimized honeycomb nanofibrous scaffolds were observed to better support cell proliferation and network formation, which can facilitate angiogenesis. Moreover, HUVECs cultured on the honeycomb scaffolds were observed to reorganize their cell bodies into tube-like structures containing a central lumen, while this was not observed on random scaffolds. This work has shown that the angiogenic response can be guided by honeycomb scaffolds, allowing improved early HUVECs organization. The guided organization via honeycomb scaffolds can be utilized for tissue engineering applications that require the formation of microvascular networks.

Cellular Based Strategies for Microvascular Engineering

Vascularization is a major hurdle in complex tissue and organ engineering. Tissues greater than 200 μm in diameter cannot rely on simple diffusion to obtain nutrients and remove waste. Therefore, an integrated vascular network is required for clinical translation of engineered tissues. Microvessels have been described as <150 μm in diameter, but clinically they are defined as <1 mm. With new advances in super microsurgery, vessels less than 1 mm can be anastomosed to the recipient circulation. However, this technical advancement still relies on the creation of a stable engineered microcirculation that is amenable to surgical manipulation and is readily perfusable. Microvascular engineering lays on the crossroads of microfabrication, microfluidics, and tissue engineering strategies that utilize various cellular constituents. Early research focused on vascularization by co-culture and cellular interactions, with the addition of angiogenic growth factors to promote vascular growth. Since then, multiple strategies have been utilized taking advantage of innovations in additive manufacturing, biomaterials, and cell biology. However, the anatomy and dynamics of native blood vessels has not been consistently replicated. Inconsistent results can be partially attributed to cell sourcing which remains an enigma for microvascular engineering. Variations of endothelial cells, endothelial progenitor cells, and stem cells have all been used for microvascular network fabrication along with various mural cells. As each source offers advantages and disadvantages, there continues to be a lack of consensus. Furthermore, discord may be attributed to incomplete understanding about cell isolation and characterization without considering the microvascular architecture of the desired tissue/organ.

Applied Bioengineering in Tissue Reconstruction, Replacement, and Regeneration

IMPACT STATEMENT

The use of autologous tissue in the reconstruction of tissue defects has been the gold standard. However, current standards still face many limitations and complications. Improving patient outcomes and quality of life by addressing these barriers remain imperative. This article provides historical perspective, covers the major limitations of current standards of care, and reviews recent advances and future prospects in applied bioengineering in the context of tissue reconstruction, replacement, and regeneration.

Polyhedral Oligomeric Silsesquioxane-Incorporated Gelatin Hydrogel Promotes Angiogenesis during Vascularized Bone Regeneration

Many approaches have been made toward the development of scaffolds with good biocompatibility and appreciable physicochemical properties to facilitate stem cell adhesion, osteogenic differentiation, and vascularization in tissue engineering. Nowadays, vascularization is a main bottleneck in tissue engineering strategies that is needed to be overcome and developed. Herein, we construct a series of polyhedral oligomeric silsesquioxane (POSS)-modified porous gelatin hydrogels with different POSS concentrations from 0 to 5 wt %, defined as X% POSS hydrogels (X = 0, 1, 2, 3, 4, 5) to support vascularized bone repair. The introduction of POSS into gelatin effectively promoted adhesive protein adsorption and integrin α5β1 expression, subsequently leading to enhanced adhesion of both rat bone marrow mesenchymal stem cells and human umbilical vein endothelial cells (HUVECs). In vitro experiments further demonstrated that POSS-containing hybrid hydrogels more effectively support the angiogenic tube and network formation in HUVECs than the 0% POSS hydrogel. Besides, POSS-containing hybrid hydrogels showed desirable performance as a sustained release system of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2), and they further accelerated vascular network establishment and the formation of a new bone in defect regions. When the hydrogels were implanted into critical-sized rat calvarial defects in vivo, the VEGF/BMP-2-coupled 3% POSS group gained a higher blood vessel volume in the bone defect regions (5.49 ± 0.35 mm³) than the 3% POSS group (3.12 ± 0.20 mm³) and the 0% POSS group (1.57 ± 0.25 mm³), suggesting that the 3% POSS hydrogel with VEGF/BMP-2 would expedite vascularization. Based on these evaluations, our results indicated that the POSS-incorporated gelatin hydrogel would provide a promising bone graft scheme in potential clinical application of large bone defect repair.

Strategies to Improve Nanofibrous Scaffolds for Vascular Tissue Engineering

The biofabrication of biomimetic scaffolds for tissue engineering applications is a field in continuous expansion. Of particular interest, nanofibrous scaffolds can mimic the mechanical and structural properties (e.g., collagen fibers) of the natural extracellular matrix (ECM) and have shown high potential in tissue engineering and regenerative medicine. This review presents a general overview on nanofiber fabrication, with a specific focus on the design and application of electrospun nanofibrous scaffolds for vascular regeneration. The main nanofiber fabrication approaches, including self-assembly, thermally induced phase separation, and electrospinning are described. We also address nanofibrous scaffold design, including nanofiber structuring and surface functionalization, to improve scaffolds' properties. Scaffolds for vascular regeneration with enhanced functional properties, given by providing cells with structural or bioactive cues, are discussed. Finally, current in vivo evaluation strategies of these nanofibrous scaffolds are introduced as the final step, before their potential application in clinical vascular tissue engineering can be further assessed.