Vascular Endothelial Cell Behavior in Complex Mechanical Microenvironments

Mechanical strategies to promote vascularization for tissue engineering and regenerative medicine

Vascularization is a major challenge in the field of tissue engineering and regenerative medicine. Mechanical factors have been demonstrated to play a fundamental role in vasculogenesis and angiogenesis and can affect the architecture of the generated vascular network. Through the regulation of mechanical factors in engineered tissues, various mechanical strategies can be used to optimize the preformed vascular network and promote its rapid integration with host vessels. Optimization of the mechanical properties of scaffolds, including controlling scaffold stiffness, increasing surface roughness and anisotropic structure, and designing interconnected, hierarchical pore structures, is beneficial for the in vitro formation of vascular networks and the ingrowth of host blood vessels. The incorporation of hollow channels into scaffolds promotes the formation of patterned vascular networks. Dynamic stretching and perfusion can facilitate the formation and maturation of preformed vascular networks in vitro. Several indirect mechanical strategies provide sustained mechanical stimulation to engineered tissues in vivo, which further promotes the vascularization of implants within the body. Additionally, stiffness gradients, anisotropic substrates and hollow channels in scaffolds, as well as external cyclic stretch, boundary constraints and dynamic flow culture, can effectively regulate the alignment of vascular networks, thereby promoting better integration of prevascularized engineered tissues with host blood vessels. This review summarizes the influence and contribution of both scaffold-based and external stimulus-based mechanical strategies for vascularization in tissue engineering and elucidates the underlying mechanisms involved.

Massive field-of-view sub-cellular traction force videography enabled by Single-Pixel Optical Tracers (SPOT)

We report a massive field-of-view and high-speed videography platform for measuring the sub-cellular traction forces of more than 10,000 biological cells over 13 mm2 at 83 frames per second. Our Single-Pixel Optical Tracers (SPOT) tool uses 2-dimensional diffraction gratings embedded into a soft substrate to convert cells' mechanical traction force into optical colors detectable by a video camera. The platform measures the sub-cellular traction forces of diverse cell types, including tightly connected tissue sheets and near isolated cells. We used this platform to explore the mechanical wave propagation in a tightly connected sheet of Neonatal Rat Ventricular Myocytes (NRVMs) and discovered that the activation time of some tissue regions are heterogeneous from the overall spiral wave behavior of the cardiac wave.

Biomechanics-mediated endocytosis in atherosclerosis

Biomechanical forces, including vascular shear stress, cyclic stretching, and extracellular matrix stiffness, which influence mechanosensitive channels in the plasma membrane, determine cell function in atherosclerosis. Being highly associated with the formation of atherosclerotic plaques, endocytosis is the key point in molecule and macromolecule trafficking, which plays an important role in lipid transportation. The process of endocytosis relies on the mobility and tension of the plasma membrane, which is sensitive to biomechanical forces. Several studies have advanced the signal transduction between endocytosis and biomechanics to elaborate the developmental role of atherosclerosis. Meanwhile, increased plaque growth also results in changes in the structure, composition and morphology of the coronary artery that contribute to the alteration of arterial biomechanics. These cross-links of biomechanics and endocytosis in atherosclerotic plaques play an important role in cell function, such as cell phenotype switching, foam cell formation, and lipoprotein transportation. We propose that biomechanical force activates the endocytosis of vascular cells and plays an important role in the development of atherosclerosis.

Physiological cyclic stretching potentiates the cell-cell junctions in vascular endothelial layer formed on aligned fiber substrate

In vascular tissue engineering, formation of stable endothelial cell-cell and cell-substrate adhesions is essential for maintaining long-term patency of the tissue-engineered vascular grafts (TEVGs). In this study, sheet-like aligned fibrous substrates of poly(l-lactide-co-caprolactone) (PLCL) were prepared by electrospinning to provide basement membrane-resembling structural support to endothelial cells (ECs). Cyclic stretching at physiological and pathological levels was then applied to human umbilical vein endothelial cells (HUVECs) cultured on chosen fibrous substrate using a force-loading device, from which effects of the cyclic stretching on cell-cell and cell-substrate adhesions were examined. It was found that applying uniaxial 1 Hz cyclic stretch at physiological levels (5 % and 10 % elongation) strengthened the cell-cell junctions, thus leading to improved structural integrity, functional expression and resistance to thrombin-induced damaging impacts in the formed endothelial layer. The cell-cell junctions were disrupted at pathological level (15 % elongation) cyclic stretching, which however facilitated the formation of focal adhesions (FAs) at cell-substrate interface. Mechanistically, the effects of cyclic stretching on endothelial cell-cell and cell-substrate adhesions were identified to be correlated with the RhoA/ROCK signaling pathway. Results from this study highlight the relevance between applying dynamic mechanical stimulation and maintaining the structural integrity of the formed endothelial layer, and implicate a necessity to implement appropriate dynamic mechanical training (i.e., preconditioning) to obtain tissue-engineered blood vessels with long-term patency post-implantation.

Impact of disturbed flow and arterial stiffening on mechanotransduction in endothelial cells

Disturbed flow promotes progression of atherosclerosis at particular regions of arteries where the recent studies show the arterial wall becomes stiffer. Objective of this study is to show how mechanotransduction in subcellular organelles of endothelial cells (ECs) will alter with changes in blood flow profiles applied on ECs surface and mechanical properties of arterial wall where ECs are attached to. We will examine the exposure of ECs to atherogenic flow profiles (disturbed flow) and non-atherogenic flow profiles (purely forward flow), while stiffness and viscoelasticity of arterial wall will change. A multicomponent model of endothelial cell monolayer was applied to quantify the response of subcellular organelles to the changes in their microenvironment. Our results show that arterial stiffening alters mechanotransduction in intra/inter-cellular organelles of ECs by slight increase in the transmitted stresses, particularly over central stress fibers (SFs). We also observed that degradation of glycocalyx and exposure to non-atherogenic flow profiles result in significantly higher stresses in subcellular organelles, while degradation of glycocalyx and exposure to atherogenic flow profiles result in dramatically lower stresses in the organelles. Moreover, we show that increasing the arterial wall viscoelasticity leads to slight increase in the stresses transmitted to subcellular organelles. FAs are particularly influenced with the changes in the arterial wall properties and viscoelasticity. Our study suggests that changes in viscoelasticity of arterial wall and degradation state of glycocalyx have to be considered along with arterial stiffening in designing more efficient treatment strategies for atherosclerosis. Our study provides insight into significant role of mechanotransduction in the localization of atherosclerosis by quantifying the role of ECs mechanosensors and suggests that mechanotransduction may play a key role in design of more efficient and precision therapeutics to slow down or block the progression of atherosclerosis.

Massively Concurrent Sub-Cellular Traction Force Videography enabled by Single-Pixel Optical Tracers (SPOTs)

We report a large field-of-view and high-speed videography platform for measuring the sub-cellular traction forces of more than 10,000 biological cells over 13mm 2 at 83 frames per second. Our Single-Pixel Optical Tracers (SPOT) tool uses 2-dimensional diffraction gratings embedded into a soft substrate to convert cells' mechanical traction stress into optical colors detectable by a video camera. The platform measures the sub-cellular traction forces of diverse cell types, including tightly connected tissue sheets and near isolated cells. We used this platform to explore the mechanical wave propagation in a tightly connected sheet of Neonatal Rat Ventricular Myocytes (NRVMs) and discovered that the activation time of some tissue regions are heterogeneous from the overall spiral wave behavior of the cardiac wave.

One-Sentence Summary

An optical platform for fast, concurrent measurements of cell mechanics at 83 frames per second, over a large area of 13mm 2 .

Development of an Induced Pluripotent Stem Cell-Based Liver-on-a-Chip Assessed with an Alzheimer's Disease Drug

Liver-related drug metabolism is a key aspect of pharmacokinetics and possible toxicity. From this perspective, the availability of advanced in vitro models for drug testing is still an open need, also to the end of reducing the burden of in vivo experiments. In this scenario, organ-on-a-chip is gaining attention as it couples a state-of-the art in vitro approach to the recapitulation of key in vivo physiological features such as fluidodynamics and a tri-dimensional cytoarchitecture. We implemented a novel liver-on-a-chip (LoC) device based on an innovative dynamic device (MINERVA 2.0) where functional hepatocytes (iHep) have been encapsulated into a 3D hydrogel matrix interfaced through a porous membrane with endothelial cells (iEndo)]. Both lines were derived from human-induced pluripotent stem cells (iPSCs), and the LoC was functionally assessed with donepezil, a drug approved for Alzheimer's disease therapy. The presence of iEndo and a 3D microenvironment enhanced the expression of liver-specific physiologic functions as in iHep, after 7 day perfusion, we noticed an increase of albumin, urea production, and cytochrome CYP3A4 expression compared to the iHep static culture. In particular, for donepezil kinetics, a computational fluid dynamic study conducted to assess the amount of donepezil diffused into the LoC indicated that the molecule should be able to pass through the iEndo and reach the target iHep construct. Then, we performed experiments of donepezil kinetics that confirmed the numerical simulations. Overall, our iPSC-based LoC reproduced the in vivo physiological microenvironment of the liver and was suitable for potential hepatotoxic screening studies.

Sex as a Biological Variable in Tissue Engineering and Regenerative Medicine

Although sex differences have been noted in cellular function and behavior, therapy efficacy, and disease incidence and outcomes, the adoption of sex as a biological variable in tissue engineering and regenerative medicine remains limited. Furthering the development of personalized, precision medicine requires considering biological sex at the bench and in the clinic. This review provides the basis for considering biological sex when designing tissue-engineered constructs and regenerative therapies by contextualizing sex as a biological variable within the tissue engineering triad of cells, matrices, and signals. To achieve equity in biological sex within medicine requires a cultural shift in science and engineering research, with active engagement by researchers, clinicians, companies, policymakers, and funding agencies.

Formulation Controls the Potential Neuromuscular Toxicity of Polyethylene Photoproducts in Developing Zebrafish

Sunlight transforms plastic into water-soluble products, the potential toxicity of which remains unresolved, particularly for vertebrate animals. We evaluated acute toxicity and gene expression in developing zebrafish larvae after 5 days of exposure to photoproduced (P) and dark (D) leachates from additive-free polyethylene (PE) film and consumer-grade, additive-containing, conventional, and recycled PE bags. Using a "worst-case" scenario, with plastic concentrations exceeding those found in natural waters, we observed no acute toxicity. However, at the molecular level, RNA sequencing revealed differences in the number of differentially expressed genes (DEGs) for each leachate treatment: thousands of genes (5442 P, 577 D) for the additive-free film, tens of genes for the additive-containing conventional bag (14 P, 7 D), and none for the additive-containing recycled bag. Gene ontology enrichment analyses suggested that the additive-free PE leachates disrupted neuromuscular processes via biophysical signaling; this was most pronounced for the photoproduced leachates. We suggest that the fewer DEGs elicited by the leachates from conventional PE bags (and none from recycled bags) could be due to differences in photoproduced leachate composition caused by titanium dioxide-catalyzed reactions not present in the additive-free PE. This work demonstrates that the potential toxicity of plastic photoproducts can be product formulation-specific.

Restoring endothelial function: shedding light on cardiovascular stent development

Complete endothelialization is highly important for maintaining long-term patency and avoiding subsequent complications in implanting cardiovascular stents. It not only refers to endothelial cells (ECs) fully covering the inserted stents, but also includes the newly formed endothelium, which could exert physiological functions, such as anti-thrombosis and anti-stenosis. Clinical outcomes have indicated that endothelial dysfunction, especially the insufficiency of antithrombotic and barrier functions, is responsible for stent failure. Learning from vascular pathophysiology, endothelial dysfunction on stents is closely linked to the microenvironment of ECs. Evidence points to inflammatory responses, oxidative stress, altered hemodynamic shear stress, and impaired endothelial barrier affecting the normal growth of ECs, which are the four major causes of endothelial dysfunction. The related molecular mechanisms and efforts dedicated to improving the endothelial function are emphasized in this review. From the perspective of endothelial function, the design principles, advantages, and disadvantages behind current stents are introduced to enlighten the development of new-generation stents, aiming to offer new alternatives for restoring endothelial function.

Shear Stress and Metabolic Disorders-Two Sides of the Same Plaque

Significance: Shear stress and metabolic disorder are the two sides of the same atherosclerotic coin. Atherosclerotic lesions are prone to develop at branches and curvatures of arteries, which are exposed to oscillatory and low shear stress exerted by blood flow. Meanwhile, metabolic disorders are pivotal contributors to the formation and advancement of atherosclerotic plaques. Recent Advances: Accumulated evidence has provided insight into the impact and mechanisms of biomechanical forces and metabolic disorder on atherogenesis, in association with mechanotransduction, epigenetic regulation, and so on. Moreover, recent studies have shed light on the cross talk between the two drivers of atherosclerosis. Critical Issues: There are extensive cross talk and interactions between shear stress and metabolic disorder during the pathogenesis of atherosclerosis. The communications may amplify the proatherogenic effects through increasing oxidative stress and inflammation. Nonetheless, the precise mechanisms underlying such interactions remain to be fully elucidated as the cross talk network is considerably complex. Future Directions: A better understanding of the cross talk network may confer benefits for a more comprehensive clinical management of atherosclerosis. Critical mediators of the cross talk may serve as promising therapeutic targets for atherosclerotic vascular diseases, as they can inhibit effects from both sides of the plaque. Hence, further in-depth investigations with advanced omics approaches are required to develop novel and effective therapeutic strategies against atherosclerosis. Antioxid. Redox Signal. 37, 820-841.

An Investigation of the Constructional Design Components Affecting the Mechanical Response and Cellular Activity of Electrospun Vascular Grafts

Cardiovascular disease is anticipated to remain the leading cause of death globally. Due to the current problems connected with using autologous arteries for bypass surgery, researchers are developing tissue-engineered vascular grafts (TEVGs). The major goal of vascular tissue engineering is to construct prostheses that closely resemble native blood vessels in terms of morphological, mechanical, and biological features so that these scaffolds can satisfy the functional requirements of the native tissue. In this setting, morphology and cellular investigation are usually prioritized, while mechanical qualities are generally addressed superficially. However, producing grafts with good mechanical properties similar to native vessels is crucial for enhancing the clinical performance of vascular grafts, exposing physiological forces, and preventing graft failure caused by intimal hyperplasia, thrombosis, aneurysm, blood leakage, and occlusion. The scaffold's design and composition play a significant role in determining its mechanical characteristics, including suturability, compliance, tensile strength, burst pressure, and blood permeability. Electrospun prostheses offer various models that can be customized to resemble the extracellular matrix. This review aims to provide a comprehensive and comparative review of recent studies on the mechanical properties of fibrous vascular grafts, emphasizing the influence of structural parameters on mechanical behavior. Additionally, this review provides an overview of permeability and cell growth in electrospun membranes for vascular grafts. This work intends to shed light on the design parameters required to maintain the mechanical stability of vascular grafts placed in the body to produce a temporary backbone and to be biodegraded when necessary, allowing an autologous vessel to take its place.

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

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

Three-Dimensional Bioprinting of Decellularized Extracellular Matrix-Based Bioinks for Tissue Engineering

Three-dimensional (3D) bioprinting is one of the most promising additive manufacturing technologies for fabricating various biomimetic architectures of tissues and organs. In this context, the bioink, a critical element for biofabrication, is a mixture of biomaterials and living cells used in 3D printing to create cell-laden structures. Recently, decellularized extracellular matrix (dECM)-based bioinks derived from natural tissues have garnered enormous attention from researchers due to their unique and complex biochemical properties. This review initially presents the details of the natural ECM and its role in cell growth and metabolism. Further, we briefly emphasize the commonly used decellularization treatment procedures and subsequent evaluations for the quality control of the dECM. In addition, we summarize some of the common bioink preparation strategies, the 3D bioprinting approaches, and the applicability of 3D-printed dECM bioinks to tissue engineering. Finally, we present some of the challenges in this field and the prospects for future development.

Abnormal Flow Conditions Promote Endocardial Fibroelastosis Via Endothelial-to-Mesenchymal Transition, Which Is Responsive to Losartan Treatment

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EFE is a congenital cardiac pathology contributing to increased morbidity and mortality. The pathologic triggers of EFE remain to be characterized.

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To determine whether abnormal flow promotes EFE development, we used in vivo neonatal rodent surgical models and an in vitro model using human primary endocardial cells

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We established novel surgical model with flow profiles seen in patients that develop EFE. Static and turbulent flow conditions promoted EFE development in neonatal rodent hearts.

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Losartan treatment is shown to significantly ameliorate EFE progression and decreases mRNA and protein expression of EndoMT markers in neonatal rodent hearts.

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RNAseq analysis of human endocardial cells subjected to different flow conditions show that normal flow suppresses gene expression critical for mesenchymal differentiation and Notch signaling.

Endocardial fibroelastosis (EFE) is defined by fibrotic tissue on the endocardium and forms partly through aberrant endothelial-to-mesenchymal transition. However, the pathologic triggers are still unknown. In this study, we showed that abnormal flow induces EFE partly through endothelial-to-mesenchymal transition in a rodent model, and that losartan can abrogate EFE development. Furthermore, we translated our findings to human endocardial endothelial cells, and showed that laminar flow promotes the suppression of genes associated with mesenchymal differentiation. These findings emphasize the role of flow in promoting EFE in endocardial endothelial cells and provide a novel potential therapy to treat this highly morbid condition.

Endothelial Cell Migration Regulated by Surface Topography of Poly(ε-caprolactone) Nanofibers

The study of cell migration on biomaterials is of great significance in tissue engineering and regenerative medicine. In recent years, there has been increasing evidence that the physical properties of the extracellular matrix (ECM), such as surface topography, affect various cellular behaviors such as proliferation, adhesion, and migration. However, the biological mechanism of surface topography influencing cellular behavior is still unclear. In this study, we prepared polycaprolactone (PCL) fibrous materials with different surface microstructures by solvent casting, electrospinning, and self-induced crystallization. The corresponding topographical structure obtained is a two-dimensional (2D) flat surface, 2.5-dimensional (2.5D) fibers, and three-dimensional (3D) fibers with a multilevel microstructure. We then investigated the effects of the complex topographical structure on endothelial cell migration. Our study demonstrates that cells can sense the changes of micro- and nanomorphology on the surface of materials, adapt to the physical environment through biochemical reactions, and regulate actin polymerization and directional migration through Rac1 and Cdc42. The cells on the nanofibers are elongated spindles, and the positive feedback of cell adhesion and actin polymerization along the fiber direction makes the plasma membrane continue to protrude, promoting cell polarization and directional migration. This study might provide new insights into the biomaterial design, especially those used for artificial vascular grafts.

Engineering Vascular Grafts with Multiphase Structures

Blood vessels in the body are multiphasic organs with microenvironmental niches specific to the cells that inhabit each section. Electrospinning is a fabrication technique used to produce nano- to microfibrous architectures capable of mimicking native extracellular matrix structure. Likewise, polycitrate elastomers are favorable luminal materials for vascular applications because of their hemocompatibility and mechanical properties. Here we describe the procedure for fabricating a biphasic polycitrate elastomer, collagen, and elastin electrospun composite to spatially tailor both composition and architecture for recapitulating the intimal and medial layers of the blood vessel in a vascular graft.

The vascular niche in next generation microphysiological systems

In recent years, microphysiological system (MPS, also known as, organ-on-a-chip or tissue chip) platforms have emerged with great promise to improve the predictive capacity of preclinical modeling thereby reducing the high attrition rates when drugs move into trials. While their designs can vary quite significantly, in general MPS are bioengineered in vitro microenvironments that recapitulate key functional units of human organs, and that have broad applications in human physiology, pathophysiology, and clinical pharmacology. A critical next step in the evolution of MPS devices is the widespread incorporation of functional vasculature within tissues. The vasculature itself is a major organ that carries nutrients, immune cells, signaling molecules and therapeutics to all other organs. It also plays critical roles in inducing and maintaining tissue identity through expression of angiocrine factors, and in providing tissue-specific milieus (i.e., the vascular niche) that can support the survival and function of stem cells. Thus, organs are patterned, maintained and supported by the vasculature, which in turn receives signals that drive tissue specific gene expression. In this review, we will discuss published vascularized MPS platforms and present considerations for next-generation devices looking to incorporate this critical constituent. Finally, we will highlight the organ-patterning processes governed by the vasculature, and how the incorporation of a vascular niche within MPS platforms will establish a unique opportunity to study stem cell development.

A Microfluidic Model Artery for Studying the Mechanobiology of Endothelial Cells

Recent vascular mechanobiology studies find that endothelial cells (ECs) convert multiple mechanical forces into functional responses in a nonadditive way, suggesting that signaling pathways such as those regulating cytoskeleton may be shared among the processes of converting individual forces. However, previous in vitro EC-culture platforms are inherent with extraneous mechanical components, which may saturate or insufficiently activate the shared signaling pathways and accordingly, may misguide EC mechanobiological responses being investigated. Here, a more physiologically relevant model artery is reported that accurately reproduces most of the mechanical forces found in vivo, which can be individually varied in any combination to pathological levels to achieve diseased states. Arterial geometries of normal and diseased states are also realized. By mimicking mechanical microenvironments of early-stage atherosclerosis, it is demonstrated that the elevated levels of the different types of stress experienced by ECs strongly correlate with the disruption of barrier integrity, suggesting that boundaries of an initial lesion could be sites for efficient disease progression.

Sex-Specific Response to Combinations of Shear Stress and Substrate Stiffness by Endothelial Cells In Vitro

By using a full factorial design of experiment, the combinatorial effects of biological sex, shear stress, and substrate stiffness on human umbilical vein endothelial cell (HUVEC) spreading and Yes-associated protein 1 (YAP1) activity are able to be efficiently evaluated. Within the range of shear stress (0.5-1.5 Pa) and substrate stiffness (10-100 kPa), male HUVECs are smaller than female HUVECs. Only with sufficient mechanical stimulation do they spread to a similar size. More importantly, YAP1 nuclear localization in female HUVECs is invariant to mechanical stimulation within the range of tested conditions whereas for male HUVECs it increases nonlinearly with increasing shear stress and substrate stiffness. The sex-specific response of HUVECs to combinations of shear stress and substrate stiffness reinforces the need to include sex as a biological variable and multiple mechanical stimuli in experiments, informs the design of precision biomaterials, and offers insight for understanding cardiovascular disease sexual dimorphisms. Moreover, here it is illustrated that different complex mechanical microenvironments can lead to sex-specific phenotypes and sex invariant phenotypes in cultured endothelial cells.

3D Cell Culture: Recent Development in Materials with Tunable Stiffness

It is widely accepted that three-dimensional cell culture systems simulate physiological conditions better than traditional 2D systems. Although extracellular matrix components strongly modulate cell behavior, several studies underlined the importance of mechanosensing in the control of different cell functions such as growth, proliferation, differentiation, and migration. Human tissues are characterized by different degrees of stiffness, and various pathologies (e.g., tumor or fibrosis) cause changes in the mechanical properties through the alteration of the extracellular matrix structure. Additionally, these modifications have an impact on disease progression and on therapy response. Hence, the development of platforms whose stiffness could be modulated may improve our knowledge of cell behavior under different mechanical stress stimuli. In this review, we have analyzed the mechanical diversity of healthy and diseased tissues, and we have summarized recently developed materials with a wide range of stiffness.

Anti-VEGF-R2 Aptamer and RGD Peptide Synergize in a Bifunctional Hydrogel for Enhanced Angiogenic Potential

Hydrogels have gained interest for use in tissue regeneration and wound healing because of their absorbing and swelling properties as well as their ability to mimic the natural extracellular matrix. Their use in wound healing specifically may be in the form of a patch or wound dressing or they may be administered within the wound bed as a filler, gel in situ, to promote healing. Thiolated hyaluronic acid-polyethylene diacrylate (tHA-PEGDA) hydrogels are ideal for this purpose due to their short gelation times at physiological temperature and pH. But these hydrogels alone are not enough and require added components to gain bioactivity. In this work, RGD adhesion peptides and an antivascular endothelial growth factor receptor-2 (VEGF-R2) DNA aptamer are incorporated into a tHA-PEGDA hydrogel to make a bifunctional hyaluronic acid hydrogel. RGD peptides promote attachment and growth of cells while the anti-VEGF-R2 DNA aptamer seems to improve cell viability, induce cell migration, and spur the onset of angiogenesis by tube formation by endothelial cells. This bifunctional hydrogel supports cell culture and has improved biological properties. The data suggest that these hydrogels can be used for advanced tissue regeneration applications such as in wound healing.

Fluid shear stress and endothelial cells synergistically promote osteogenesis of mesenchymal stem cells via integrin β1-FAK-ERK1/2 pathway

Prevascularization and mechanical stimulation have been reported as effective methods for the construction of functional bone tissue. However, their combined effects on osteogenic differentiation and its mechanism remain to be explored. Here, the effects of fluid shear stress (FSS) on osteogenic differentiation of rat bone-marrow-derived mesenchymal stem cells (BMSCs) when cocultured with human umbilical vein endothelial cells (HUVECs) were investigated, and underlying signaling mechanisms were further explored. FSS stimulation for 1-4 h/day increased alkaline phosphatase (ALP) activity and calcium deposition in coculture systems and promoted the proliferation of cocultured cells. FSS stimulation for 2 h/day was selected as the optimized protocol according to osteogenesis in the coculture. In this situation, the mRNA levels of ALP, runt-related transcriptional factor 2 (Runx2) and osteocalcin (OCN), and protein levels of OCN and osteopontin (OPN) in BMSCs were upregulated. Furthermore, FSS and coculture with HUVECs synergistically increased integrin β1 expression in BMSCs and further activated focal adhesion kinases (FAKs) and downstream extracellular signal-related kinase (ERK), leading to the enhancement of Runx2 expression. Blocking the phosphorylation of FAK abrogated FSS-induced ERK phosphorylation and inhibited osteogenesis of cocultured BMSCs. These results revealed that FSS and coculture with HUVECs synergistically promotes the osteogenesis of BMSCs, which was mediated by the integrin β1-FAK-ERK signaling pathway.

Let's Talk About Sex-Biological Sex Is Underreported in Biomaterial Studies

Precision medicine aims to better individualize healthcare. It requires that biomaterials be designed for the physiological characteristics of a specific patient. To make this a reality, biomaterials research and development must address differences of biological sex. More specifically, biomaterials should be designed with properties optimized and appropriate for male and female patients. In analyzing research articles from seven prominent biomaterials journals, sex as a biological variable is missing from an overwhelming majority of in vitro biomaterial studies. From the survey, the reporting of the sex of primary cell cultures happened only 10.3% of the time. Contributing to this trend is that commercial vendors bias cell lines toward one sex or another by not disclosing information of cell line sex at the time of purchase; researchers do not communicate this pertinent information in published studies; and many journal policies have little to no requirements for reporting cell line characteristics. Omitting this valuable information leads to a gap in the understanding of sex-specific cell-biomaterial interactions and it creates a bias in research findings towards one sex or another. To curb this concerning trend and make precision biomaterials a reality will require the biomaterials field to "talk about sex" by reporting cell sex more broadly.

Building Scaffolds for Tubular Tissue Engineering

Hollow organs and tissue systems drive various functions in the body. Many of these hollow or tubular systems, such as vasculature, the intestines, and the trachea, are common targets for tissue engineering, given their relevance to numerous diseases and body functions. As the field of tissue engineering has developed, numerous benchtop models have been produced as platforms for basic science and drug testing. Production of tubular scaffolds for different tissue engineering applications possesses many commonalities, such as the necessity for producing an intact tubular opening and for formation of semi-permeable epithelia or endothelia. As such, the field has converged on a series of manufacturing techniques for producing these structures. In this review, we discuss some of the most common tissue engineered applications within the context of tubular tissues and the methods by which these structures can be produced. We provide an overview of the general structure and anatomy for these tissue systems along with a series of general design criteria for tubular tissue engineering. We categorize methods for manufacturing tubular scaffolds as follows: casting, electrospinning, rolling, 3D printing, and decellularization. We discuss state-of-the-art models within the context of vascular, intestinal, and tracheal tissue engineering. Finally, we conclude with a discussion of the future for these fields.

Self-assembled VEGF-R2 targeting DNA aptamer-collagen fibers stimulate an angiogenic-like endothelial cell phenotype

Vascularization of engineered tissue is one of the hallmark challenges of tissue engineering. Leveraging self-assembled nucleic acid-collagen complexes (NACCs), we mixed a VEGF-R2 targeting aptamer or its receptor agonist divalent assembly with type I collagen to assemble NACC microfibers. Human umbilical vein endothelial cells (HUVECs) quickly remodeled these fibers into tubulogenic-like structures over 48 h. Moreover, NACCs made with the receptor agonist divalent aptamer assembly promoted enhanced expression of von Willebrand factor (vWF), angiopoietin-2 (ANGPT-2), and matrix metalloproteinase-2 (MMP-2) by HUVECs as measured by either immunocytochemistry or ELISA. The findings suggest, endothelial cell phenotype was directed by both biochemical cues afforded by the agonist behavior of the divalent aptamer assembly as well as by the biophysical cues afforded by the fibrous topography. Collectively, these results support the development of an angiogenic endothelial cell phenotype stimulated by the VEGF-R2 agonist NACC fibers. Thus, the combination of engineered DNA aptamer nanotechnology and DNA-collagen complexation phenomena is a promising biofunctional natural scaffold material system for tissue engineering and regenerative medicine applications.

MechanoBioTester: A Decoupled Multistimulus Cell Culture Device for Studying Complex Microenvironments In Vitro

Increasingly being recognized is the role of the complex microenvironment to regulate cell phenotype; however, the cell culture systems used to study these effects in vitro are lagging. The complex microenvironment is host to a combination of biological interactions, chemical factors, and mechanical stimuli. Many devices have been designed to probe the effects of one mechanical stimulus, but few are capable of systematically interrogating all combinations of mechanical stimuli with independent control. To address this gap, we have developed the MechanoBioTester platform, a decoupled, multi-stimulus cell culture model for studying the cellular response to complex microenvironments in vitro. The system uses an engineered elastomeric chamber with a specially defined region for incorporating different target materials to act as the cell culture substrate. We have tested the system with several target materials including: polydimethylsiloxane elastomer, polyacrylamide gel, poly(1,8-octanediol citrate) elastomer, and type I collagen gel for both 2D and 3D co-culture. Additionally, when the chamber is connected to a flow circuit and our stretching device, stimuli in the form of fluid flow, cyclic stretch, and hydrostatic pressure are able to be imparted with independent control. We validated the device using experimental and computational methods to define a range of capabilities relevant to physiological microenvironments. The MechanoBioTester platform promises to function as a model system for mechanobiology, biomaterial design, and drug discovery applications that focus on probing the impact of a complex microenvironment in an in vitro setting. The protocol described within provides the details characterizing the MechanoBioTester system, the steps for fabricating the MechanoBioTester chamber, and the procedure for operating the MechanoBioTester system to stimulate cells.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Oligomer Length Defines the Self-Assembly of Single-Stranded DNA-Collagen Complex Fibers

Collagen and single-stranded DNA (ssDNA) complex to self-assemble into fibers depending on the length of the ssDNA and the relative amounts of collagen and ssDNA in solution. We report for the first time that when monodisperse, random sequences of ssDNA in the range of 15-90 nucleotides and type I collagen were mixed together at room temperature, fibers several tens of micrometers in length and as large as 10 μm in diameter were formed. Fiber formation was rapid and spontaneous, requiring no further treatment after mixing. Most notably, more ssDNA oligomers were incorporated into the fibers formed using shorter ssDNA oligomers. Endothelial cells formed angiogenic-like structures using the fibers with elevated expression of von Willebrand factor for cells in direct contact with the fibers. These fibers open the door to future applications in the administration and functionality of ssDNA and collagen.