Biomaterials to prevascularize engineered tissues

Engineering blood and lymphatic microvascular networks

The human vasculature plays a crucial role in the blood supply of nearly all organs as well as the drainage of the interstitial fluid. Consequently, if these physiological systems go awry, pathological changes might occur. Hence, the regeneration of existing vessels, as well as approaches to engineer artificial blood and lymphatic structures represent current challenges within the field of vascular research. In this review, we provide an overview of both the vascular blood circulation and the long-time neglected but equally important lymphatic system, with regard to their organotypic vasculature. We summarize the current knowledge within the field of vascular tissue engineering focusing on the design of co-culture systems, thereby mainly discussing suitable cell types, scaffold design and disease models. This review will mainly focus on addressing those subjects concerning atherosclerosis. Moreover, current technological approaches such as vascular organ-on-a-chip models and microfluidic devices will be discussed.

Human Induced Pluripotent Spheroids' Growth Is Driven by Viscoelastic Properties and Macrostructure of 3D Hydrogel Environment

Stem cells, particularly human iPSCs, constitute a powerful tool for tissue engineering, notably through spheroid and organoid models. While the sensitivity of stem cells to the viscoelastic properties of their direct microenvironment is well-described, stem cell differentiation still relies on biochemical factors. Our aim is to investigate the role of the viscoelastic properties of hiPSC spheroids' direct environment on their fate. To ensure that cell growth is driven only by mechanical interaction, bioprintable alginate-gelatin hydrogels with significantly different viscoelastic properties were utilized in differentiation factor-free culture medium. Alginate-gelatin hydrogels of varying concentrations were developed to provide 3D environments of significantly different mechanical properties, ranging from 1 to 100 kPa, while allowing printability. hiPSC spheroids from two different cell lines were prepared by aggregation (⌀ = 100 µm, n > 1 × 104), included and cultured in the different hydrogels for 14 days. While spheroids within dense hydrogels exhibited limited growth, irrespective of formulation, porous hydrogels prepared with a liquid-liquid emulsion method displayed significant variations of spheroid morphology and growth as a function of hydrogel mechanical properties. Transversal culture (adjacent spheroids-laden alginate-gelatin hydrogels) clearly confirmed the separate effect of each hydrogel environment on hiPSC spheroid behavior. This study is the first to demonstrate that a mechanically modulated microenvironment induces diverse hiPSC spheroid behavior without the influence of other factors. It allows one to envision the combination of multiple formulations to create a complex object, where the fate of hiPSCs will be independently controlled by their direct microenvironment.

Application of mesenchymal stem cell sheet for regeneration of craniomaxillofacial bone defects

Bone defects are among the most common damages in human medicine. Due to limitations and challenges in the area of bone healing, the research field has turned into a hot topic discipline with direct clinical outcomes. Among several available modalities, scaffold-free cell sheet technology has opened novel avenues to yield efficient osteogenesis. It is suggested that the intact matrix secreted from cells can provide a unique microenvironment for the acceleration of osteoangiogenesis. To the best of our knowledge, cell sheet technology (CST) has been investigated in terms of several skeletal defects with promising outcomes. Here, we highlighted some recent advances associated with the application of CST for the recovery of craniomaxillofacial (CMF) in various preclinical settings. The regenerative properties of both single-layer and multilayer CST were assessed regarding fabrication methods and applications. It has been indicated that different forms of cell sheets are available for CMF engineering like those used for other hard tissues. By tackling current challenges, CST is touted as an effective and alternative therapeutic option for CMF bone regeneration.

Synthesis of a zwitterionic N-Ser-Ser-C dimethacrylate cross-linker and evaluation in polyampholyte hydrogels

Polyampholyte hydrogels are attractive materials for tissue engineering scaffolds as they offer a wide variety of features including nonfouling, selective protein delivery, and tunable physical characteristics. However, to improve the potential performance of these materials for in vivo applications, there is a need for a higher diversity of zwitterionic cross-linker species to replace commonly used ethylene glycol (EG) based chemistries. Towards this end, the synthesis of a dipeptide based zwitterionic cross-linker, N-Ser-Ser-C dimethacrylate (S-S) from N-Boc-l-serine is presented. The strategy utilized a convergent coupling of methacrylated serine partners followed by careful global deprotection to yield the zwitterionic cross-linker with good overall yields. This novel cross-linker was incorporated into a polyampholyte hydrogel and its physical properties and biocompatibility were compared against a polyampholyte hydrogel synthesized with an EG-based cross-linker. The S-S cross-linked hydrogel demonstrated excellent nonfouling performance, while promoting enhanced cellular adhesion to fibrinogen delivered from the hydrogel. Therefore, the results suggest that the S-S cross-linker will demonstrate superior future performance for in vivo applications.

3D Bioprinting of Reinforced Vessels by Dual-Cross-linked Biocompatible Hydrogels

3D bioprinting offers a powerful tool to fabricate vessel channels in tissue engineering applications, but inadequate strength of the vascular walls limited the development of this strategy and reinforced channels were highly desired for vascular constructions. Herein, we demonstrated a dual cross-linking system for 3D bioprinting of tubular structures, achieved by a combination of photo-cross-linking and enzymatic cross-linking. Photo-cross-linking of gelatin methacryloyl (GelMA) was achieved with a photoactive conjugated polymer PBF under 550 nm irradiation. Enzymatic cross-linking utilized cascade reactions catalyzed by glucose peroxidase and horseradish peroxidase that can cross-link both methacrylate and tyrosine moieties of GelMA. After removing the 3D-printed sacrificial layer (Pluronic F-127), the obtained perfusable channels showed great biocompatibility that allowed endothelial cells to adhere and proliferate. Our dual cross-linking strategy has great potential in 3D bioprinting of tubular structure for biomedical applications, especially for artificial blood vessels.

Beyond Growth Factors: Macrophage-Centric Strategies for Angiogenesis

Functional angiogenesis is a critical therapeutic goal in many pathological conditions. Logically, the use of pro-angiogenic growth factors has been the mainstay approach despite obvious limitations and modest success. Recently, macrophages have been identified as key regulators of the host response to implanted materials. Particularly, our understanding of dynamically plastic macrophage phenotypes, their interactions with biomaterials, and varied roles in different stages of angiogenic processes is evolving rapidly. In this review, we discuss changing perspectives on therapeutic angiogenesis, in relation to implantable materials and macrophage-centric strategies therein. Harnessing the different mechanisms through which the macrophage-driven host response is involved in angiogenesis has great potential for improving clinical outcome.

Development of a Dual Hydrogel Model System for Vascularization

Numerous hydrogel-based culture systems are used to create in vitro model for prevascularization. Hydrogels used to induce a microenvironment conducive to microvessel formation are typically soft and fast degradable, but often suffer from maintaining a lasting perfusable channel in vitro. Here, a dual hydrogel system that consists of photo-crosslinkable gelatin methacrylate (GelMA) and polyethylene glycol dimethacrylate (PEGDMA) is reported. GelMA hydrogels present soft and rapidly degradable properties and show microporous structures while PEGDMA is relatively stiff, almost nondegradable in vitro, and less porous. The dual hydrogel system is sequentially photo-crosslinked to construct an endothelial cell (EC)-lined perfusable PEGDMA channel and surrounding GelMA for endothelial vascular networks. Such dual hydrogel system exhibits seamless integration of the stiff PEGDMA channel and the surrounding soft GelMA, and facilitates rapid EC sprouting and extensive microvessel formation from a stable endothelium on the PEGDMA channel into the GelMA. Furthermore, diffusivity of biomolecules in the perfusable dual hydrogel system is affected by both the structural and physicochemical properties of the hydrogel system and the microvascular networks formed in the system. The establishment of the dual hydrogel system for vascularization holds great promise as an in vitro angiogenesis model and prevascularization strategy of large tissue constructs.

Osteogenic and angiogenic lineage differentiated adipose-derived stem cells for bone regeneration of calvarial defects in rabbits

Cell sheet techniques are widely used in bone engineering. However, vascularization remains a challenge in fabricating vascularized engineered bone. The goal of this study was to induce adipose-derived stem cell (ADSC) osteogenic and angiogenic lineage differentiation and investigate the use of bidiretionally differentiated ADSCs for bone regeneration. ADSCs were cultured to form an osteogenic cell sheet. Other ADSCs were induced to differentiate into endothelial progenitor cells (EPCs), which were identified and characterized by morphological observation and CD31 immunofluorescent staining. Then, the ADSC sheet-EPC complexes were implanted subcutaneously into nude mice, while ADSC sheets alone were implanted as a control. After 8 weeks of transplantation, microcomputed tomography (micro-CT) and histological observation were used to assess bone formation. We then implanted the complexes in calvarial defects in rabbits and assessed bone repair by micro-CT and histological analysis. The ADSC sheets consisted of multiple layers of cells and extracellular matrix. The obtained EPCs formed capillary-like structures and expressed the specific antigen marker CD31. The osteogenic ADSC sheet-EPC complexes formed dense and well-vascularized new bone tissue at 8 weeks after implantation. Bone density was significantly lower in the control group than in the complex group (p < .05). In addition, the reconstruction of calvarial defects in rabbits in complex group was obviously greater than that in the control group (p < .05). These results suggested that the approach of engineering bone tissue with bidiretionally differentiated ADSCs enabled bone regeneration, thus offering a promising strategy for repairing bone defects.

Silk fibroin-poly(lactic acid) biocomposites: Effect of protein-synthetic polymer interactions and miscibility on material properties and biological responses

A protein-polymer blend system based on silkworm silk fibroin (SF) and polylactic acid (PLA) was systematically investigated to understand the interaction and miscibility of proteins and synthetic biocompatible polymers in the macro- and micro-meter scales, which can dramatically control the cell responses and enzyme biodegradation on the biomaterial interface. Silk fibroin, a semicrystalline protein with beta-sheet crystals, provides controllable crystal content and biodegradability; while noncrystallizable PDLLA provides hydrophobicity and thermal stability in the system. Differential scanning calorimetry (DSC) combined with scanning electron microscope (SEM) showed that the morphology of the blend films was uniform on a macroscopic scale, yet with tunable micro-phase patterns at different mixing ratios. Fourier transform infrared analysis (FTIR) revealed that structures of the blend system, such as beta-sheet crystal content, gradually changed with the mixing ratios. All blended samples have better stability than pure SF and PLA samples as evidenced by thermogravimetric analysis. Protease XIV enzymatic study showed that the biodegradability of the blend samples varied with their blending ratios and microscale morphologies. Significantly, the topology of the micro-phase patterns on the blends can promote cell attachment and manipulate the cell growth and proliferation. This study provided a useful platform for understanding the fabrication strategies of protein-synthetic polymer composites that have direct biomedical and green chemistry applications.

Coaxial bioprinting of cell-laden vascular constructs using a gelatin–tyramine bioink

The study revealed that linear distribution of multiple vascular cells could be achieved using synthetic bioink with short gelling time and a coaxial printing system.

Comparison of covalently and physically cross-linked collagen hydrogels on mediating vascular network formation for engineering adipose tissue

Timely tissue vascularization and integration of engineered tissues into a patient plays an important role in the successful translation of engineered tissues into clinically relevant therapies. To decrease the time needed to vascularize an engineered adipose tissue, suitable local microenvironments provided by hydrogels to support cell-based functional vascular network formation have been investigated. Using the same biomolecule in solution, two types of hydrogels can be obtained: a "physical hydrogel" which is thermal-induced self-assemble fibril initiation and growth, due to amino and carboxyl telopeptides on collagen chains, and a "chemical hydrogel" which results from the covalently cross-linking of the side chains induced by one step enzyme mediation in aqueous solution. In this paper, we compare the capability of engineering vascular network and large-sized vascularized adipose tissue in vivo in different types of collagen hydrogels, physical and chemical crosslinking. The relationships between vascular network formation and hydrogel properties for the two types of hydrogels are discussed. Finally, we successfully engineered a vascularized adipose tissue construct (∼877.6 adipocytes/mm²; 94% area of a construct) in the absence of exogenous cytokines in chemical covalently crosslinking cell-laden hydrogel. These results show manipulating the polymerized methods of a hydrogel could not only modulate vascular network formation, but also regenerate adipose tissue in vivo.

Evaluation of a Miniaturized Biologically Vascularized Scaffold in vitro and in vivo

In tissue engineering, the generation and functional maintenance of dense voluminous tissues is mainly restricted due to insufficient nutrient supply. Larger three-dimensional constructs, which exceed the nutrient diffusion limit become necrotic and/or apoptotic in long-term culture if not provided with an appropriate vascularization. Here, we established protocols for the generation of a pre-vascularized biological scaffold with intact arterio-venous capillary loops from rat intestine, which is decellularized under preservation of the feeding and draining vascular tree. Vessel integrity was proven by marker expression, media/blood reflow and endothelial LDL uptake. In vitro maintenance persisted up to 7 weeks in a bioreactor system allowing a stepwise reconstruction of fully vascularized human tissues and successful in vivo implantation for up to 4 weeks, although with time-dependent decrease of cell viability. The vascularization of the construct lead to a 1.5× increase in cellular drug release compared to a conventional static culture in vitro. For the first time, we performed proof-of-concept studies demonstrating that 3D tissues can be maintained within a miniaturized vascularized scaffold in vitro and successfully implanted after re-anastomosis to the intrinsic blood circulation in vivo. We hypothesize that this technology could serve as a powerful platform technology in tissue engineering and regenerative medicine.

Systematic Analysis of mRNA and miRNA Expression of 3D-Cultured Neural Stem Cells (NSCs) in Spaceflight

Recently, with the development of the space program there are growing concerns about the influence of spaceflight on tissue engineering. The purpose of this study was thus to determine the variations of neural stem cells (NSCs) during spaceflight. RNA-Sequencing (RNA-Seq) based transcriptomic profiling of NSCs identified many differentially expressed mRNAs and miRNAs between space and earth groups. Subsequently, those genes with differential expression were subjected to bioinformatic evaluation using gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) and miRNA-mRNA network analyses. The results showed that NSCs maintain greater stemness ability during spaceflight although the growth rate of NSCs was slowed down. Furthermore, the results indicated that NSCs tended to differentiate into neuron in outer space conditions. Detailed genomic analyses of NSCs during spaceflight will help us to elucidate the molecular mechanisms behind their differentiation and proliferation when they are in outer space.

Proangiogenic Activity of Endometrial Epithelial and Stromal Cells in Response to Estradiol in Gelatin Hydrogels

Biomaterial vascularization remains a major focus in the field of tissue engineering. Biomaterial culture of endometrial cells is described as a platform to inform the design of proangiogenic biomaterials. The endometrium undergoes rapid growth and shedding of dense vascular networks during each menstrual cycle mediated via estradiol and progesterone in vivo. Cocultures of endometrial epithelial and stromal cells encapsulated within a methacrylamide-functionalized gelatin hydrogel are employed. It is reported that proangiogenic gene expression profiles and vascular endothelial growth factor production are hormone dependent in endometrial epithelial cells, but that hormone signals have no effect on human telomerase reverse transcriptase (hTERT)-immortalized endometrial stromal cells. This study subsequently examines whether the magnitude of epithelial cell response is sufficient to induce changes in human umbilical vein endothelial cell network formation. Incorporation of endometrial stromal cells improves vessel formation, but co-culture with endometrial epithelial cells leads to a decrease in vascular formation, suggesting the need for stratified cocultures of endometrial epithelial and stromal cells with endothelial cells. Given the transience of hormonal signals within 3D biomaterials, the inclusion of sex hormone binding globulin (SHBG) to alter the bioavailability of estradiol within the hydrogel is reported, demonstrating a strategy to reduce diffusive losses via SHBG-mediated estradiol sequestration.

"Data characterizing microfabricated human blood vessels created via hydrodynamic focusing"

This data article provides further detailed information related to our research article titled “Microfabricated Blood Vessels Undergo Neovascularization” (DiVito et al., 2017) [1], in which we report fabrication of human blood vessels using hydrodynamic focusing (HDF). Hydrodynamic focusing with advection inducing chevrons were used in concert to encase one fluid stream within another, shaping the inner core fluid into ‘bullseye-like” cross-sections that were preserved through click photochemistry producing streams of cellularized hollow 3-dimensional assemblies, such as human blood vessels (Daniele et al., 2015a, 2015b, 2014, 2016; Roberts et al., 2016) [2], [3], [4], [5], [6]. Applications for fabricated blood vessels span general tissue engineering to organ-on-chip technologies, with specific utility in in vitro drug delivery and pharmacodynamics studies. Here, we report data regarding the construction of blood vessels including cellular composition and cell positioning within the engineered vascular construct as well as functional aspects of the tissues.

Novel Compound-Forming Technology Using Bioprinting and Electrospinning for Patterning a 3D Scaffold Construct with Multiscale Channels

One of the biggest challenges for tissue engineering is to efficiently provide oxygen and nutrients to cells on a three-dimensional (3D) engineered scaffold structure. Thus, achieving sufficient vascularization of the structure is a critical problem in tissue engineering. This facilitates the need to develop novel methods to enhance vascularization. Use of patterned hydrogel structures with multiscale channels can be used to achieve the required vascularization. Patterned structures need to be biocompatible and biodegradable. In this study, gelatin was used as the main part of a hydrogel to prepare a biological structure with 3D multiscale channels using bioprinting combined with selection of suitable materials and electrostatic spinning. Human umbilical vein endothelial cells (HUVECs) were then used to confirm efficacy of the structure, inferred from cell viability on different engineered construct designs. HUVECs were seeded on the surface of channels and cultured in vitro. HUVECs showed high viability and diffusion within the construct. This method can be used as a practical platform for the fabrication of engineered construct for vascularization.

Computational Model-Based Analysis of Strategies to Enhance Scaffold Vascularization

Stable and extensive blood vessel networks are required for cell function and survival in engineered tissues. A number of different strategies are currently being investigated to enhance biomaterial vascularization with screening primarily through extensive in vitro and in vivo experiments. In this article, we describe an agent-based model (ABM) developed to evaluate various strategies in silico, including design of optimal biomaterial structure, delivery of angiogenic factors, and application of prevascularized biomaterials. The model predictions are evaluated using experimental data. The ABM developed provides insight into different strategies currently applied for scaffold vascularization and will enable researchers to rapidly screen new hypotheses and explore alternative strategies for enhancing vascularization.

Synergizing Engineering and Biology to Treat and Model Skeletal Muscle Injury and Disease

Although skeletal muscle is one of the most regenerative organs in our body, various genetic defects, alterations in extrinsic signaling, or substantial tissue damage can impair muscle function and the capacity for self-repair. The diversity and complexity of muscle disorders have attracted much interest from both cell biologists and, more recently, bioengineers, leading to concentrated efforts to better understand muscle pathology and develop more efficient therapies. This review describes the biological underpinnings of muscle development, repair, and disease, and discusses recent bioengineering efforts to design and control myomimetic environments, both to study muscle biology and function and to aid in the development of new drug, cell, and gene therapies for muscle disorders. The synergy between engineering-aided biological discovery and biology-inspired engineering solutions will be the path forward for translating laboratory results into clinical practice.

Synthetic Capillaries to Control Microscopic Blood Flow

Capillaries pervade human physiology. The mean intercapillary distance is only about 100 μm in human tissue, which indicates the extent of nutrient diffusion. In engineered tissue the lack of capillaries, along with the associated perfusion, is problematic because it leads to hypoxic stress and necrosis. However, a capillary is not easy to engineer due to its complex cytoarchitecture. Here, it is shown that it is possible to create in vitro, in about 30 min, a tubular microenvironment with an elastic modulus and porosity consistent with human tissue that functionally mimicks a bona fide capillary using "live cell lithography"(LCL) to control the type and position of cells on a composite hydrogel scaffold. Furthermore, it is established that these constructs support the forces associated with blood flow, and produce nutrient gradients similar to those measured in vivo. With LCL, capillaries can be constructed with single cell precision-no other method for tissue engineering offers such precision. Since the time required for assembly scales with the number of cells, this method is likely to be adapted first to create minimal functional units of human tissue that constitute organs, consisting of a heterogeneous population of 100-1000 cells, organized hierarchically to express a predictable function.

Three Dimensional Collagen Scaffold Promotes Intrinsic Vascularisation for Tissue Engineering Applications

Here, we describe a porous 3-dimensional collagen scaffold material that supports capillary formation in vitro, and promotes vascularization when implanted in vivo. Collagen scaffolds were synthesized from type I bovine collagen and have a uniform pore size of 80 μm. In vitro, scaffolds seeded with primary human microvascular endothelial cells suspended in human fibrin gel formed CD31 positive capillary-like structures with clear lumens. In vivo, after subcutaneous implantation in mice, cell-free collagen scaffolds were vascularized by host neovessels, whilst a gradual degradation of the scaffold material occurred over 8 weeks. Collagen scaffolds, impregnated with human fibrinogen gel, were implanted subcutaneously inside a chamber enclosing the femoral vessels in rats. Angiogenic sprouts from the femoral vessels invaded throughout the scaffolds and these degraded completely after 4 weeks. Vascular volume of the resulting constructs was greater than the vascular volume of constructs from chambers implanted with fibrinogen gel alone (42.7±5.0 μL in collagen scaffold vs 22.5±2.3 μL in fibrinogen gel alone; p<0.05, n = 7). In the same model, collagen scaffolds seeded with human adipose-derived stem cells (ASCs) produced greater increases in vascular volume than did cell-free collagen scaffolds (42.9±4.0 μL in collagen scaffold with human ASCs vs 25.7±1.9 μL in collagen scaffold alone; p<0.05, n = 4). In summary, these collagen scaffolds are biocompatible and could be used to grow more robust vascularized tissue engineering grafts with improved the survival of implanted cells. Such scaffolds could also be used as an assay model for studies on angiogenesis, 3-dimensional cell culture, and delivery of growth factors and cells in vivo.

Hypoxia Created Human Mesenchymal Stem Cell Sheet for Prevascularized 3D Tissue Construction

3D tissue based on human mesenchymal stem cell (hMSC) sheets offers many interesting opportunities for regenerating multiple types of connective tissues. Prevascularizing hMSC sheets with endothelial cells (ECs) will improve 3D tissue performance by supporting cell survival and accelerating integration with host tissue. It is hypothesized that hypoxia cultured hMSC sheets can promote microvessel network formation and preserve stemness of hMSCs. This study investigates the vascularization of hMSC sheets under different oxygen tensions. It is found that the HN condition, in which hMSC sheets formed under physiological hypoxia (2% O2 ) and then cocultured with ECs under normoxia (20% O2 ), enables longer and more branched microvessel network formation. The observation is corroborated by higher levels of angiogenic factors in coculture medium. Additionally, the hypoxic hMSC sheet is more uniform and less defective, which facilitates fabrication of 3D prevascularized tissue construct by layering the prevascularized hMSC sheets and maturing in rotating wall vessel bioreactor. The hMSCs in the 3D construct still maintain multilineage differentiation ability, which indicates the possible application of the 3D construct for various connective tissues regeneration. These results demonstrate that hypoxia created hMSC sheets benefit the microvessel growth and it is feasible to construct 3D prevascularized tissue construct using the prevascularized hMSC sheets.

Endothelial sprouting and network formation in collagen- and fibrin-based modular microbeads

A modular tissue engineering approach may have advantages over current therapies in providing rapid and sustained revascularization of ischemic tissue. In this study, modular protein microbeads were prepared from pure fibrin (FIB) and collagen-fibrin composites (COL-FIB) using a simple water-in-oil emulsification technique. Human endothelial cells and fibroblasts were embedded directly in the microbead matrix. The resulting microbeads were generally spheroidal with a diameter of 100-200μm. Cell viability was high (75-80% viable) in microbeads, but was marginally lower than in bulk hydrogels of corresponding composition (85-90% viable). Cell proliferation was significantly greater in COL-FIB microbeads after two weeks in culture, compared to pure FIB microbeads. Upon embedding of microbeads in a surrounding fibrin hydrogel, endothelial cell networks formed inside the microbead matrix and extended into the surrounding matrix. The number of vessel segments, average segment length, and number of branch points was higher in FIB samples, compared to COL-FIB samples, resulting in significantly longer total vessel networks. Anastomosis of vessel networks from adjacent microbeads was also observed. These studies demonstrate that primitive vessel networks can be formed by modular protein microbeads containing embedded endothelial cells and fibroblasts. Such microbeads may find utility as prevascularized tissue modules that can be delivered minimally invasively as a therapy to restore blood flow to ischemic tissues.

STATEMENT OF SIGNIFICANCE

Vascularization is critically important for tissue engineering and regenerative medicine, and materials that support and/or promote neovascularization are of value both for translational applications and for mechanistic studies and discovery-based research. Therefore, we fabricated small modular microbeads formulated from pure fibrin (FIB) and collagen-fibrin (COL-FIB) containing endothelial cells and supportive fibroblasts. We explored how cells encapsulated within these materials form microvessel-like networks both within and outside of the microbeads when embedded in larger 3D matrices. FIB microbeads were found to initiate more extensive sprouting into the surrounding ECM in vitro. These results represent an important step towards our goal of developing injectable biomaterial modules containing preformed vascular units that can rapidly restore vascularization to an ischemic tissue in vivo.

In vivo angiogenesis in tissues penetrating into porous β-tricalcium phosphate scaffolds

In vivo angiogenesis in a three-dimensional bone graft after the implantation of spherical porous β-tricalcium phosphate scaffolding materials into lumbodorsal fascia of New Zealand rabbits.

Bioengineering vascularized tissue constructs using an injectable cell-laden enzymatically crosslinked collagen hydrogel derived from dermal extracellular matrix

Tissue engineering promises to restore or replace diseased or damaged tissue by creating functional and transplantable artificial tissues. The development of artificial tissues with large dimensions that exceed the diffusion limitation will require nutrients and oxygen to be delivered via perfusion instead of diffusion alone over a short time period. One approach to perfusion is to vascularize engineered tissues, creating a de novo three-dimensional (3D) microvascular network within the tissue construct. This significantly shortens the time of in vivo anastomosis, perfusion and graft integration with the host. In this study, we aimed to develop injectable allogeneic collagen-phenolic hydroxyl (collagen-Ph) hydrogels that are capable of controlling a wide range of physicochemical properties, including stiffness, water absorption and degradability. We tested whether collagen-Ph hydrogels could support the formation of vascularized engineered tissue graft by human blood-derived endothelial colony-forming cells (ECFCs) and bone marrow-derived mesenchymal stem cells (MSC) in vivo. First, we studied the growth of adherent ECFCs and MSCs on or in the hydrogels. To examine the potential formation of functional vascular networks in vivo, a liquid pre-polymer solution of collagen-Ph containing human ECFCs and MSCs, horseradish peroxidase and hydrogen peroxide was injected into the subcutaneous space or abdominal muscle defect of an immunodeficient mouse before gelation, to form a 3D cell-laden polymerized construct. These results showed that extensive human ECFC-lined vascular networks can be generated within 7 days, the engineered vascular density inside collagen-Ph hydrogel constructs can be manipulated through refinable mechanical properties and proteolytic degradability, and these networks can form functional anastomoses with the existing vasculature to further support the survival of host muscle tissues. Finally, optimized conditions of the cell-laden collagen-Ph hydrogel resulted in not only improving the long-term differentiation of transplanted MSCs into mineralized osteoblasts, but the collagen-Ph hydrogel also improved an increased of adipocytes within the vascularized bioengineered tissue in a mouse after 1 month of implantation.

STATEMENT OF SIGNIFICANCE

We reported a method for preparing autologous extracellular matrix scaffolds, murine collagen-Ph hydrogels, and demonstrated its suitability for use in supporting human progenitor cell-based formation of 3D vascular networks in vitro and in vivo. Results showed extensive human vascular networks can be generated within 7 days, engineered vascular density inside collagen-Ph constructs can be manipulated through refinable mechanical properties and proteolytic degradability, and these networks can form functional anastomoses with existing vasculature to further support the survival of host muscle tissues. Moreover, optimized conditions of cell-laden collagen-Ph hydrogel resulted in not only improving the long-term differentiation of transplanted MSCs into mineralized osteoblasts, but the collagen-Ph hydrogel also improved an increased of adipocytes within the vascularized bioengineered tissue in a mouse.

Emulating human microcapillaries in a multi-organ-chip platform

Current microfluidic chip-based tissue culture systems lack a capillary endothelial vessel system, which would enable perfusion with blood. We utilise spatial cell cultures to populate a perfused multi-organ-chip platform-a microfluidic device recently introduced for substance testing. Complete biological vascularization of such culture systems is vital to properly emulate physiological tissue behaviour. In this study, we incorporated a fibrin scaffold into the two-organ-chip design. Herein, adipose-derived stromal cells (ASCs) directed human umbilical vein endothelial cells (HUVECs) to organise into tube-like structures. The ASCs induced tube formation of HUVECs in static and dynamic conditions. The replacement of full medium enriched with growth factors and foetal calf serum with basal medium resulted in viable cells with similar gene expression profiles. We regard this as a prerequisite for studies with organ constructs that have a need for a different medium formulation. Furthermore, we here address stability issues of the fibrin gel and fibrin composition for optimal microvessel formation.

The induction of pro-angiogenic processes within a collagen scaffold via exogenous estradiol and endometrial epithelial cells

Nutrient transport remains a major limitation in the design of biomaterials. One approach to overcome this constraint is to incorporate features to induce angiogenesis-mediated microvasculature formation. Angiogenesis requires a temporal presentation of both pro- and anti-angiogenic factors to achieve stable vasculature, leading to increasingly complex biomaterial design scheme. The endometrium, the lining of the uterus and site of embryo implantation, exemplifies a non-pathological model of rapid growth, shedding, and re-growth of dense vascular networks regulated by the dynamic actions of estradiol and progesterone. In this study, we examined the individual and combined response of endometrial epithelial cells and human umbilical vein endothelial cells to exogenous estradiol within a three-dimensional collagen scaffold. While endothelial cells did not respond to exogenous estradiol, estradiol directly stimulated endometrial epithelial cell transduction pathways and resulted in dose-dependent increases in endogenous VEGF production. Co-culture experiments using conditioned media demonstrated estradiol stimulation of endometrial epithelial cells can induce functional changes in endothelial cells within the collagen biomaterial. We also report the effect of direct endometrial epithelial and endothelial co-culture as well as covalent immobilization of estradiol within the collagen biomaterial. These efforts establish the suitability of an endometrial-inspired model for promoting pro-angiogenic events within regenerative medicine applications. These results also suggest the potential for developing biomaterial-based models of the endometrium.

Manipulation of a quasi-natural cell block for high-efficiency transplantation of adherent somatic cells

Recent advances have raised hope that transplantation of adherent somatic cells could provide dramatic new therapies for various diseases. However, current methods for transplanting adherent somatic cells are not efficient enough for therapeutic applications. Here, we report the development of a novel method to generate quasi-natural cell blocks for high-efficiency transplantation of adherent somatic cells. The blocks were created by providing a unique environment in which cultured cells generated their own extracellular matrix. Initially, stromal cells isolated from mice were expanded in vitro in liquid cell culture medium followed by transferring the cells into a hydrogel shell. After incubation for 1 day with mechanical agitation, the encapsulated cell mass was perforated with a thin needle and then incubated for an additional 6 days to form a quasi-natural cell block. Allograft transplantation of the cell block into C57BL/6 mice resulted in perfect adaptation of the allograft and complete integration into the tissue of the recipient. This method could be widely applied for repairing damaged cells or tissues, stem cell transplantation, ex vivo gene therapy, or plastic surgery.

Characterization and angiogenic potential of human neonatal and infant thymus mesenchymal stromal cells

Resident mesenchymal stromal cells (MSCs) are involved in angiogenesis during thymus regeneration. We have previously shown that MSCs can be isolated from enzymatically digested human neonatal and infant thymus tissue that is normally discarded during pediatric cardiac surgical procedures. In this paper, we demonstrate that thymus MSCs can also be isolated by explant culture of discarded thymus tissue and that these cells share many of the characteristics of bone marrow MSCs. Human neonatal thymus MSCs are clonogenic, demonstrate exponential growth in nearly 30 population doublings, have a characteristic surface marker profile, and express pluripotency genes. Furthermore, thymus MSCs have potent proangiogenic behavior in vitro with sprout formation and angiogenic growth factor production. Thymus MSCs promote neoangiogenesis and cooperate with endothelial cells to form functional human blood vessels in vivo. These characteristics make thymus MSCs a potential candidate for use as an angiogenic cell therapeutic agent and for vascularizing engineered tissues in vitro.

Porcine Dermis-Derived Collagen Membranes Induce Implantation Bed Vascularization Via Multinucleated Giant Cells: A Physiological Reaction?

In this study, the tissue reactions to 2 new porcine dermis-derived collagen membranes of different thickness were analyzed. The thicker material (Mucoderm) contained sporadically preexisting vessel skeletons and fatty islands. The thinner membrane (Collprotect) had a bilayered structure (porous and occlusive side) without any preexisting structures. These materials were implanted subcutaneously in mice to analyze the tissue reactions and potential transmembranous vascularization. Histological and histomorphometrical methodologies were performed at 4 time points (3, 10, 15, and 30 days). Both materials permitted stepwise connective tissue ingrowth into their central regions. In the Mucoderm matrix, newly built microvessels were found within the preexisting vessel and fatty island skeletons after 30 days. This vascularization was independent of the inflammation-related vascularization on both material surfaces. The Collprotect membrane underwent material disintegration by connective tissue strands in combination with vessels and multinucleated giant cells. The histomorphometric analyses revealed that the thickness of Mucoderm did not decrease significantly, while an initial significant decrease of membrane thickness in the case of Collprotect was found at day 15. The present results demonstrate that the 2 analyzed collagen membranes underwent a multinucleated giant cell-associated vascularization. Neither of the materials underwent transmembraneous vascularization. The microvessels were found within the preexisting vessel and fatty island skeletons. Additional long-term studies and clinical studies are necessary to determine how the observed foreign body giant cells affect tissue regeneration.

Evaluation of a thermoresponsive polycaprolactone scaffold for in vitro three-dimensional stem cell differentiation

Tissue engineering (TE) strategies aim at imitating the natural process of regeneration by using bioresorbable scaffolds that support cellular attachment, migration, proliferation, and differentiation. Based on the idea of combining a fully degradable polymer [poly(ɛ-caprolactone)] with a thermoresponsive polymer (polyethylene glycol methacrylate), a scaffold was developed, which liquefies below 20°C and solidifies at 37°C. In this study, this scaffold was evaluated for its ability to support C2C12 cells and human adipose-derived stem cells (ASCs) to generate an expandable three-dimensional (3D) construct for soft or bone TE. As a first step, biomaterial seeding was optimized and cellular attachment, survival, distribution, and persistence within the 3D material were characterized. C2C12 cells were differentiated toward the osteogenic as well as myogenic lineage, while ASCs were cultured in control, adipogenic, or osteogenic differentiation media. Differentiation was examined using quantitative real-time PCR for the expression of osteogenic, myogenic, and adipogenic markers and by enzyme activity and immunoassays. Both cell types attached and were found evenly distributed within the material. C2C12 cells and ASCs demonstrated the potential to differentiate in all tested lineages under 2D conditions. Under 3D osteogenic conditions for C2C12 cells, only osteocalcin expression (fold induction: 16.3±0.2) and alkaline phosphatase (ALP) activity (p<0.001) were increased compared with the control C2C12 cells. Three-dimensional osteogenic differentiation of ASC was limited and donor dependent. Only one donor showed an increase in the osteogenic markers osteocalcin (p=0.027) and osteopontin (p=0.038). In contrast, differentiation toward the myogenic or adipogenic lineage showed expression of specific markers in 3D, at least at the level of the 2D culture. In 3D culture, strong induction of myogenin (p<0.001) as well as myoD (p<0.001) was found in C2C12 cells. The adipogenic differentiation of one donor showed greater expression of peroxisome proliferative-activated receptor gamma (PPARγ) (p=0.004), fatty acid binding protein 4 (FABP4) (p=0.008), and adiponectin (p=0.045) in 3D compared with 2D culture. Leptin levels in the supernatant of the ASC cultures were elevated in the 3D cultures in both donors at day 14 and 21. In conclusion, the thermoresponsive scaffold was found suitable for 3D in vitro differentiation toward soft tissue.

Human immunity in vitro - solving immunogenicity and more

It has been widely recognised that the phylogenetic distance between laboratory animals and humans limits the former's predictive value for immunogenicity testing of biopharmaceuticals and nanostructure-based drug delivery and adjuvant systems. 2D in vitro assays have been established in conventional culture plates with little success so far. Here, we detail the status of various 3D approaches to emulate innate immunity in non-lymphoid organs and adaptive immune response in human professional lymphoid immune organs in vitro. We stress the tight relationship between the necessarily changing architecture of professional lymphoid organs at rest and when activated by pathogens, and match it with the immunity identified in vitro. Recommendations for further improvements of lymphoid tissue architecture relevant to the development of a sustainable adaptive immune response in vitro are summarized. In the end, we sketch a forecast of translational innovations in the field to model systemic innate and adaptive immunity in vitro.

Vascular Tissue Engineering: Building Perfusable Vasculature for Implantation

Tissue and organ replacement is required when there are no alternative therapies available. Although vascular tissue engineering was originally developed to meet the clinical demands of small-diameter vascular conduits as bypass grafts, it has evolved into a highly advanced field where perfusable vasculatures are generated for implantation. Herein, we review several cutting-edge techniques that have led to implantable human blood vessels in clinical trials, the novel approaches that build complex perfusable microvascular networks in functional tissues, the use of stem cells to generate endothelial cells for vascularization, as well as the challenges in bringing vascular tissue engineering technologies into the clinics.

Boosting angiogenesis and functional vascularization in injectable dextran-hyaluronic acid hydrogels by endothelial-like mesenchymal stromal cells

Angiogenesis and neovascularization are fundamental for the success of clinically relevant-sized tissue-engineered (TE) constructs. The next generation of TE constructs relies on providing instructive materials combined with the delivery of angiogenic growth factors and cells to avoid tissue ischemia. However, the majority of materials and cell types screened so far show limited clinical relevance, either due to insufficient number of cells or due to the use of animal-derived matrixes. Here, we investigated whether endothelial-like cells derived from mesenchymal stromal cells (EL-MSCs) can be used for vascular TE in combination with injectable dextran-hyaluronic acid (Dex-g-HA) hydrogels. These hydrogels can be easily modified, as demonstrated by the incorporation of vascular endothelial growth factor (VEGF). We examined in vitro the reciprocal influences between cells and matrix. Dex-g-HA enabled higher EL-MSC metabolic rates associated with optimal cell sprouting in vitro compared to human umbilical vein endothelial cells. In vivo evaluation demonstrated the absence of an acute inflammatory response, and EL-MSCs incorporated within Dex-g-HA formed a functional vascular network integrated with the host vascular system. This work demonstrates that Dex-g-HA is an efficient delivery method of VEGF to induce angiogenesis. Additionally, functional neovascularization can be achieved in vitro and in vivo by the combination of Dex-g-HA with EL-MSC.

Integrating biological vasculature into a multi-organ-chip microsystem

A chip-based system mimicking the transport function of the human cardiovascular system has been established at minute but standardized microsystem scale. A peristaltic on-chip micropump generates pulsatile shear stress in a widely adjustable physiological range within a microchannel circuit entirely covered on all fluid contact surfaces with human dermal microvascular endothelial cells. This microvascular transport system can be reproducibly established within four days, independently of the individual endothelial cell donor background. It interconnects two standard tissue culture compartments, each of 5 mm diameter, through microfluidic channels of 500 μm width. Further vessel branching and vessel diameter reduction down to a microvessel scale of approximately 40 μm width was realised by a two-photon laser ablation technique applied to inserts, designed for the convenient establishment of individual organ equivalents in the tissue culture compartments at a later time. The chip layout ensures physiological fluid-to-tissue ratios. Moreover, an in-depth microscopic analysis revealed the fine-tuned adjustment of endothelial cell behaviour to local shear stresses along the microvasculature of the system. Time-lapse and 3D imaging two-photon microscopy were used to visualise details of spatiotemporal adherence of the endothelial cells to the channel system and to each other. The first indicative long-term experiments revealed stable performance over two and four weeks. The potential application of this system for the future establishment of human-on-a-chip systems and basic human endothelial cell research is discussed.

Nanospiderwebs: artificial 3D extracellular matrix from nanofibers by novel clinical grade electrospinning for stem cell delivery

Novel clinical grade electrospinning methods could provide three-dimensional (3D) nanostructured biomaterials comprising of synthetic or natural biopolymer nanofibers. Such advanced materials could potentially mimic the natural extracellular matrix (ECM) accurately and may provide superior niche-like spaces on the subcellular scale for optimal stem-cell attachment and individual cell homing in regenerative therapies. The goal of this study was to design several novel "nanofibrous extracellular matrices" (NF-ECMs) with a natural mesh-like 3D architecture through a unique needle-free multi-jet electrospinning method in highly controlled manner to comply with good manufacturing practices (GMP) for the production of advanced healthcare materials for regenerative medicine, and to test cellular behavior of human mesenchymal stem cells (HMSCs) on these. Biopolymers manufactured as 3D NF-ECM meshes under clinical grade GMP-like conditions show higher intrinsic cytobiocompatibility with superior cell integration and proliferation if compared to their 2D counterparts or a clinically-approved collagen membrane.

Effects of silk fibroin fiber incorporation on mechanical properties, endothelial cell colonization and vascularization of PDLLA scaffolds

Attainment of functional vascularization of engineered constructs is one of the fundamental challenges of tissue engineering. However, the development of an extracellular matrix in most tissues, including bone, is dependent upon the establishment of a well developed vascular supply. In this study a poly(d,l-lactic acid) (PDLLA) salt-leached sponge was modified by incorporation of silk fibroin fibers to create a multicomponent scaffold, in an effort to better support endothelial cell colonization and to promote in vivo vascularization. Scaffolds with and without silk fibroin fibers were compared for microstructure, mechanical properties, ability to maintain cell populations in vitro as well as to permit vascular ingrowth into acellular constructs in vivo. We demonstrated that adding silk fibroin fibers to a PDLLA salt-leached sponge enhanced scaffold properties and heightened its capacity to support endothelial cells in vitro and to promote vascularization in vivo. Therefore refinement of scaffold properties by inclusion of materials with beneficial attributes may promote and shape cellular responses.

Upcyte® microvascular endothelial cells repopulate decellularized scaffold

A general problem in tissue engineering is the poor and insufficient blood supply to guarantee tissue cell survival as well as physiological tissue function. To address this limitation, we have developed an in vitro vascularization model in which a decellularized porcine small bowl segment, representing a capillary network within a collagen matrix (biological vascularized scaffold [BioVaSc]), is reseeded with microvascular endothelial cells (mvECs). However, since the supply of mvECs is limited, in general, and as these cells rapidly dedifferentiate, we have applied a novel technology, which allows the generation of large batches of quasi-primary cells with the ability to proliferate, whilst maintaining their differentiated functionality. These so called upcyte mvECs grew for an additional 15 population doublings (PDs) compared to primary cells. Upcyte mvECs retained endothelial characteristics, such as von Willebrandt Factor (vWF), CD31 and endothelial nitric oxide synthase (eNOS) expression, as well as positive Ulex europaeus agglutinin I staining. Upcyte mvECs also retained biological functionality such as tube formation, cell migration, and low density lipoprotein (LDL) uptake, which were still evident after PD27. Initial experiments using MTT and Live/Dead staining indicate that upcyte mvECs repopulate the BioVaSc Scaffold. As with conventional cultures, these cells also express key endothelial molecules (vWF, CD31, and eNOS) in a custom-made bioreactor system even after a prolonged period of 14 days. The combination of upcyte mvECs and the BioVaSc represents a novel and promising approach toward vascularizing bioreactor models which can better reflect organs, such as the liver.

Human adipose-derived cells can serve as a single-cell source for the in vitro cultivation of vascularized bone grafts

Orthopaedic surgery often requires bone grafts to correct large defects resulting from congenital defects, surgery or trauma. Great improvements have been made in the tissue engineering of bone grafts. However, these grafts lack the vascularized component that is critical for their survival and function. From a clinical perspective, it would be ideal to engineer vascularized bone grafts starting from one single-cell harvest obtained from the patient. To this end, we explored the potential of human adipose-derived mesenchymal stem cells (hASCs) as a single-cell source for osteogenic and endothelial differentiation and the assembly of bone and vascular compartments within the same scaffold. hASCs were encapsulated in fibrin hydrogel as an angioinductive material for vascular formation, combined with a porous silk fibroin sponge to support osteogenesis, and subjected to sequential application of growth factors. Three strategies were evaluated by changing spatiotemporal cues: (a) induction of osteogenesis prior to vasculogenesis; (b) induction of vasculogenesis prior to osteogenesis; or (c) simultaneous induction of osteogenesis and vasculogenesis. By 5 weeks of culture, bone-like tissue development was evidenced by the deposition of bone matrix proteins, alkaline phosphatase activity and calcium deposition, along with the formation of vascular networks, evidenced by endothelial cell surface markers, such as CD31 and von Willebrand factor, and morphometric analysis. Most robust development of the two tissue compartments was achieved by sequential induction of osteogenesis followed by the induction of vasculogenesis. Taken together, the collected data strongly support the utility of hASCs as a single-cell source for the formation of vascularized bone tissue.

Orchestrating cell/material interactions for tissue engineering of surgical implants

Research groups are currently recognising a critical clinical need for innovative approaches to organ failure and agenesis. Allografting, autologous reconstruction and prosthetics are hampered with severe limitations. Pertinently, readily available 'laboratory-grown' organs and implants are becoming a reality. Tissue engineering constructs vary in their design complexity depending on the specific structural and functional demands. Expeditious methods on integrating autologous stem cells onto nanoarchitectured 3D nanocomposites, are being transferred from lab to patients with a number of successful first-in-man experiences. Despite the need for a complete understanding of cell/material interactions tissue engineering is offering a plethora of exciting possibilities in regenerative medicine.