Differential effects of cell adhesion, modulus and VEGFR-2 inhibition on capillary network formation in synthetic hydrogel arrays

Harnessing the potential of monocytes/macrophages to regenerate tissue-engineered vascular grafts

Cell-free tissue-engineered vascular grafts provide a promising alternative to treat cardiovascular disease, but timely endothelialization is essential for ensuring patency and proper functioning post-implantation. Recent studies from our lab showed that blood cells like monocytes (MCs) and macrophages (Mϕ) may contribute directly to cellularization and regeneration of bioengineered arteries in small and large animal models. While MCs and Mϕ are leucocytes that are part of the innate immune response, they share common developmental origins with endothelial cells (ECs) and are known to play crucial roles during vessel formation (angiogenesis) and vessel repair after inflammation/injury. They are highly plastic cells that polarize into pro-inflammatory and anti-inflammatory phenotypes upon exposure to cytokines and differentiate into other cell types, including EC-like cells, in the presence of appropriate chemical and mechanical stimuli. This review focuses on the developmental origins of MCs and ECs; the role of MCs and Mϕ in vessel repair/regeneration during inflammation/injury; and the role of chemical signalling and mechanical forces in Mϕ inflammation that mediates vascular graft regeneration. We postulate that comprehensive understanding of these mechanisms will better inform the development of strategies to coax MCs/Mϕ into endothelializing the lumen and regenerate the smooth muscle layers of cell-free bioengineered arteries and veins that are designed to treat cardiovascular diseases and perhaps the native vasculature as well.

Synthesis and Photopatterning of Synthetic Thiol-Norbornene Hydrogels

Hydrogels are a class of soft biomaterials and the material of choice for a myriad of biomedical applications due to their biocompatibility and highly tunable mechanical and biochemical properties. Specifically, light-mediated thiol-norbornene click reactions between norbornene-modified macromers and di-thiolated crosslinkers can be used to form base hydrogels amenable to spatial biochemical modifications via subsequent light reactions between pendant norbornenes in the hydrogel network and thiolated peptides. Macromers derived from natural sources (e.g., hyaluronic acid, gelatin, alginate) can cause off-target cell signaling, and this has motivated the use of synthetic macromers such as poly(ethylene glycol) (PEG). In this study, commercially available 8-arm norbornene-modified PEG (PEG-Nor) macromers were reacted with di-thiolated crosslinkers (dithiothreitol, DTT) to form synthetic hydrogels. By varying the PEG-Nor weight percent or DTT concentration, hydrogels with a stiffness range of 3.3 kPa-31.3 kPa were formed. Pendant norbornene groups in these hydrogels were used for secondary reactions to either increase hydrogel stiffness (by reacting with DTT) or to tether mono-thiolated peptides to the hydrogel network. Peptide functionalization has no effect on bulk hydrogel mechanics, and this confirms that mechanical and biochemical signals can be independently controlled. Using photomasks, thiolated peptides can also be photopatterned onto base hydrogels, and mesenchymal stem cells (MSCs) attach and spread on RGD-functionalized PEG-Nor hydrogels. MSCs encapsulated in PEG-Nor hydrogels are also highly viable, demonstrating the ability of this platform to form biocompatible hydrogels for 2D and 3D cell culture with user-defined mechanical and biochemical properties.

Cyclometalated ruthenium complexes overcome cisplatin resistance through PI3K/mTOR/Nrf2 signaling pathway

Currently, cisplatin resistance remains a primary clinical obstacle in the successful treatment of non-small cell lung cancer. Here, we designed, synthesized, and characterized two novel cyclometalated Ru(II) complexes, [Ru(bpy)2(1-Ph-7-OCH3-IQ)] (PF6) (bpy = 2,2'-bipyridine, IQ = isoquinoline, RuIQ7)and [Ru(bpy)2(1-Ph-6,7-(OCH3)2-IQ)] (PF6) (RuIQ8). As experimental controls, we prepared complex [Ru(bpy)2(1-Ph-IQ)](PF6) (RuIQ6) lacking a methoxy group in the main ligand. Significantly, complexes RuIQ6-8 displayed higher in vitro cytotoxicity when compared to ligands, precursor cis-[Ru(bpy)2Cl2], and clinical cisplatin. Mechanistic investigations revealed that RuIQ6-8 could inhibit cell proliferation by downregulating the phosphorylation levels of Akt and mTOR proteins, consequently affecting the rapid growth of human lung adenocarcinoma cisplatin-resistant cells A549/DDP. Moreover, the results from qRT-PCR demonstrated that these complexes could directly suppress the transcription of the NF-E2-related factor 2 gene, leading to the inhibition of downstream multidrug resistance-associated protein 1 expression and effectively overcoming cisplatin resistance. Furthermore, the relationship between the chemical structures of these three complexes and their anticancer activity, ability to induce cell apoptosis, and their efficacy in overcoming cisplatin resistance has been thoroughly examined and discussed. Notably, the toxicity test conducted on zebrafish embryos indicated that the three Ru-IQ complexes displayed favorable safety profiles. Consequently, the potential of these developed compounds as innovative therapeutic agents for the efficient and low-toxic treatment of NSCLC appears highly promising.

Elastin-like protein hydrogels with controllable stress relaxation rate and stiffness modulate endothelial cell function

Mechanical cues from the extracellular matrix (ECM) regulate vascular endothelial cell (EC) morphology and function. Since naturally derived ECMs are viscoelastic, cells respond to viscoelastic matrices that exhibit stress relaxation, in which a cell-applied force results in matrix remodeling. To decouple the effects of stress relaxation rate from substrate stiffness on EC behavior, we engineered elastin-like protein (ELP) hydrogels in which dynamic covalent chemistry (DCC) was used to crosslink hydrazine-modified ELP (ELP-HYD) and aldehyde/benzaldehyde-modified polyethylene glycol (PEG-ALD/PEG-BZA). The reversible DCC crosslinks in ELP-PEG hydrogels create a matrix with independently tunable stiffness and stress relaxation rate. By formulating fast-relaxing or slow-relaxing hydrogels with a range of stiffness (500-3300 Pa), we examined the effect of these mechanical properties on EC spreading, proliferation, vascular sprouting, and vascularization. The results show that both stress relaxation rate and stiffness modulate endothelial spreading on two-dimensional substrates, on which ECs exhibited greater cell spreading on fast-relaxing hydrogels up through 3 days, compared with slow-relaxing hydrogels at the same stiffness. In three-dimensional hydrogels encapsulating ECs and fibroblasts in coculture, the fast-relaxing, low-stiffness hydrogels produced the widest vascular sprouts, a measure of vessel maturity. This finding was validated in a murine subcutaneous implantation model, in which the fast-relaxing, low-stiffness hydrogel produced significantly more vascularization compared with the slow-relaxing, low-stiffness hydrogel. Together, these results suggest that both stress relaxation rate and stiffness modulate endothelial behavior, and that the fast-relaxing, low-stiffness hydrogels supported the highest capillary density in vivo.

A combination of matrix stiffness and degradability dictate microvascular network assembly and remodeling in cell-laden poly(ethylene glycol) hydrogels

The formation of functional capillary blood vessels that can sustain the metabolic demands of transplanted parenchymal cells remains one of the biggest challenges to the clinical realization of engineered tissues for regenerative medicine. As such, there remains a need to better understand the fundamental influences of the microenvironment on vascularization. Poly(ethylene glycol) (PEG) hydrogels have been widely adopted to interrogate the influence of matrix physicochemical properties on cellular phenotypes and morphogenetic programs, including the formation of microvascular networks, in part due to the ease with which their properties can be controlled. In this study, we co-encapsulated endothelial cells and fibroblasts in PEG-norbornene (PEGNB) hydrogels in which stiffness and degradability were tuned to assess their independent and synergistic effects on vessel network formation and cell-mediated matrix remodeling longitudinally. Specifically, we achieved a range of stiffnesses and differing rates of degradation by varying the crosslinking ratio of norbornenes to thiols and incorporating either one (sVPMS) or two (dVPMS) cleavage sites within the matrix metalloproteinase- (MMP-) sensitive crosslinker, respectively. In less degradable sVPMS gels, decreasing the crosslinking ratio (thereby decreasing the initial stiffness) supported enhanced vascularization. When degradability was increased in dVPMS gels, all crosslinking ratios supported robust vascularization regardless of initial mechanical properties. The vascularization in both conditions was coincident with the deposition of extracellular matrix proteins and cell-mediated stiffening, which was greater in dVPMS conditions after a week of culture. Collectively, these results indicate that enhanced cell-mediated remodeling of a PEG hydrogel, achieved either by reduced crosslinking or increased degradability, leads to more rapid vessel formation and higher degrees of cell-mediated stiffening.

Cell-binding peptides on the material surface guide stem cell fate of adhesion, proliferation and differentiation

Human cells, especially stem cells, need to communicate and interact with extracellular matrix (ECM) proteins, which not only serve as structural components but also guide and support cell fate and properties such as cell adhesion, proliferation, survival and differentiation. The binding of the cells with ECM proteins or ECM-derived peptides via cell adhesion receptors such as integrins activates several signaling pathways that determine the cell fate, morphological change, proliferation and differentiation. The development of synthetic ECM protein-derived peptides that mimic the biological and biochemical functions of natural ECM proteins will benefit academic and clinical application. Peptides derived from or inspired by specific ECM proteins can act as agonists of each ECM protein receptor. Given that most ECM proteins function in cell adhesion via integrin receptors, many peptides have been developed that bind to specific integrin receptors. In this review, we discuss the peptide sequence, immobilization design, reaction method, and functions of several ECM protein-derived peptides. Various peptide sequences derived from mainly ECM proteins, which are used for coating or grafting on dishes, scaffolds, hydrogels, implants or nanofibers, have been developed to improve the adhesion, proliferation or differentiation of stem cells and to culture differentiated cells. This review article will help to inform the optimal choice of ECM protein-derived peptides for the development of scaffolds, implants, hydrogels, nanofibers and 2D cell culture dishes to regulate the proliferation and direct the differentiation of stem cells into specific lineages.

Precisely Tuning the Pore-Wall Surface Composition of Bioceramic Scaffolds Facilitates Angiogenesis and Orbital Bone Defect Repair

Orbital bone damage (OBD) may result in severe post-traumatic enophthalmos, craniomaxillofacial deformities, vision loss, and intracranial infections. However, it is still a challenge to fabricate advanced biomaterials that can match the individual anatomical structure and enhance OBD repair in situ. Herein, we aimed to develop a selective surface modification strategy on bioceramic scaffolds and evaluated the effects of inorganic or organic functional coating on angiogenesis and osteogenesis, ectopically and orthotopically in OBD models. It was shown that the low thermal bioactive glass (BG) modification or layer-by-layer assembly of a biomimetic hydrogel (Biogel) could readily integrate into the pore wall of the bioceramic scaffolds. The BG and Biogel modification showed appreciable enhancement in the initial compressive strength (∼30-75%) or structural stability in vivo, respectively. BG modification could enhance by nearly 2-fold the vessel ingrowth, and the osteogenic capacity was also accelerated, accompanied with a mild scaffold biodegradation after 3 months. Meanwhile, the Biogel-modified scaffolds showed enhanced osteogenic differentiation and mineralization through calcium and phosphorus retention. The potential mechanism of the enhanced bone repair was elucidated via vascular and osteogenic cell responses in vitro, and the cell tests indicated that the Biogel and BG functional layers were both beneficial for in vitro osteoblastic differentiation and mineralization on bioceramics. Totally, these findings demonstrated that the bioactive ions or biomolecules could significantly improve the angiogenic and osteogenic capabilities of conventional bioceramics, and the integration of inorganic or organic functional coating in the pore wall is a highly flexible material toolbox that can be tailored directly to improve orbital bone defect repair.

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

Ovarian Cell Encapsulation in an Enzymatically Crosslinked Silk-Based Hydrogel with Tunable Mechanical Properties

An artificial ovary is a promising approach for preserving fertility in prepubertal girls and women who cannot undergo current cryopreservation strategies. However, this approach is in its infancy, due to the possible challenges of creating a suitable 3D matrix for encapsulating ovarian follicles and stromal cells. To maintain the ovarian stromal cell viability and proliferation, as a first step towards developing an artificial ovary, in this study, a double network hydrogel with a high water swelling capacity (swelling index 15–19) was developed, based on phenol conjugated chitosan (Cs-Ph) and silk fibroin (SF) through an enzymatic crosslinking method using horseradish peroxidase. The addition of SF (1%) to Cs (1%) decreased the storage modulus (G’) from 3500 Pa (Cs1) to 1600 Pa (Cs-SF1), and the hydrogels with a rapid gelation kinetic produced a spatially homogeneous distribution of ovarian cells that demonstrated 167% proliferation after 7 days. This new Cs-SF hydrogel benefits from the toughness and flexibility of SF, and phenolic chemistry could provide the potential microstructure for encapsulating human ovarian stromal cells.

High-Throughput Routes to Biomaterials Discovery

Many existing clinical treatments are limited in their ability to completely restore decreased or lost tissue and organ function, an unenviable situation only further exacerbated by a globally aging population. As a result, the demand for new medical interventions has increased substantially over the past 20 years, with the burgeoning fields of gene therapy, tissue engineering, and regenerative medicine showing promise to offer solutions for full repair or replacement of damaged or aging tissues. Success in these fields, however, inherently relies on biomaterials that are engendered with the ability to provide the necessary biological cues mimicking native extracellular matrixes that support cell fate. Accelerating the development of such "directive" biomaterials requires a shift in current design practices toward those that enable rapid synthesis and characterization of polymeric materials and the coupling of these processes with techniques that enable similarly rapid quantification and optimization of the interactions between these new material systems and target cells and tissues. This manuscript reviews recent advances in combinatorial and high-throughput (HT) technologies applied to polymeric biomaterial synthesis, fabrication, and chemical, physical, and biological screening with targeted end-point applications in the fields of gene therapy, tissue engineering, and regenerative medicine. Limitations of, and future opportunities for, the further application of these research tools and methodologies are also discussed.

Combinatorial screen of dynamic mechanical stimuli for predictive control of MSC mechano-responsiveness

Mechanobiological-based control of mesenchymal stromal cells (MSCs) to facilitate engineering and regeneration of load-bearing tissues requires systematic investigations of specific dynamic mechanical stimulation protocols. Using deformable membrane microdevice arrays paired with combinatorial experimental design and modeling, we probed the individual and integrative effects of mechanical stimulation parameters (strain magnitude, rate at which strain is changed, and duty period) on myofibrogenesis and matrix production of MSCs in three-dimensional hydrogels. These functions were found to be dominantly influenced by a previously unidentified, higher-order interactive effect between strain magnitude and duty period. Empirical models based on our combinatorial cue-response data predicted an optimal loading regime in which strain magnitude and duty period were increased synchronously over time, which was validated to most effectively promote MSC matrix production. These findings inform the design of loading regimes for MSC-based engineered tissues and validate a broadly applicable approach to probe multifactorial regulating effects of mechanobiological cues.

Customized hydrogel substrates for serum-free expansion of functional hMSCs

We describe a screening approach to identify customized substrates for serum-free human mesenchymal stromal cell (hMSC) culture. In particular, we combine a biomaterials screening approach with design of experiments (DOE) and multivariate analysis (MVA) to understand the effects of substrate stiffness, substrate adhesivity, and media composition on hMSC behavior in vitro. This approach enabled identification of poly(ethylene glycol)-based and integrin binding hydrogel substrate compositions that supported functional hMSC expansion in multiple serum-containing and serum-free media, as well as the expansion of MSCs from multiple, distinct sources. The identified substrates were compatible with standard thaw, seed, and harvest protocols. Finally, we used MVA on the screening data to reveal the importance of serum and substrate stiffness on hMSC expansion, highlighting the need for customized cell culture substrates in optimal hMSC biomanufacturing processes.

Evolutionarily conserved sequence motif analysis guides development of chemically defined hydrogels for therapeutic vascularization

Biologically active ligands (e.g., RGDS from fibronectin) play critical roles in the development of chemically defined biomaterials. However, recent decades have shown only limited progress in discovering novel extracellular matrix-protein-derived ligands for translational applications. Through motif analysis of evolutionarily conserved RGD-containing regions in laminin (LM) and peptide-functionalized hydrogel microarray screening, we identified a peptide (a1) that showed superior supports for endothelial cell (EC) functions. Mechanistic studies attributed the results to the capacity of a1 engaging both LM- and Fn-binding integrins. RNA sequencing of ECs in a1-functionalized hydrogels showed ~60% similarities with Matrigel in "vasculature development" gene ontology terms. Vasculogenesis assays revealed the capacity of a1-formulated hydrogels to improve EC network formation. Injectable alginates functionalized with a1 and MMPQK (a vascular endothelial growth factor-mimetic peptide with a matrix metalloproteinase-degradable linker) increased blood perfusion and functional recovery over decellularized extracellular matrix and (RGDS + MMPQK)-functionalized hydrogels in an ischemic hindlimb model, illustrating the power of this approach.

Immobilized RGD concentration and proteolytic degradation synergistically enhance vascular sprouting within hydrogel scaffolds of varying modulus

Insufficient vascularization limits the volume and complexity of engineered tissue. The formation of new blood vessels (neovascularization) is regulated by a complex interplay of cellular interactions with biochemical and biophysical signals provided by the extracellular matrix (ECM) necessitating the development of biomaterial approaches that enable systematic modulation in matrix properties. To address this need poly(ethylene) glycol-based hydrogel scaffolds were engineered with a range of decoupled and combined variations in integrin-binding peptide (RGD) ligand concentration, elastic modulus and proteolytic degradation rate using free-radical polymerization chemistry. The modularity of this system enabled a full factorial experimental design to simultaneously investigate the individual and interaction effects of these matrix cues on vascular sprout formation in 3 D culture. Enhancements in scaffold proteolytic degradation rate promoted significant increases in vascular sprout length and junction number while increases in modulus significantly and negatively impacted vascular sprouting. We also observed that individual variations in immobilized RGD concentration did not significantly impact 3 D vascular sprouting. Our findings revealed a previously unidentified and optimized combination whereby increases in both immobilized RGD concentration and proteolytic degradation rate resulted in significant and synergistic enhancements in 3 D vascular spouting. The above-mentioned findings would have been challenging to uncover using one-factor-at-time experimental analyses.

Surface Modification with Control over Ligand Density for the Study of Multivalent Biological Systems

In the study of multivalent interactions at interfaces, as occur for example at cell membranes, the density of the ligands or receptors displayed at the interface plays a pivotal role, affecting both the overall binding affinities and the valencies involved in the interactions. In order to control the ligand density at the interface, several approaches have been developed, and they concern the functionalization of a wide range of materials. Here, different methods employed in the modification of surfaces with controlled densities of ligands are being reviewed. Examples of such methods encompass the formation of self-assembled monolayers (SAMs), supported lipid bilayers (SLBs) and polymeric layers on surfaces. Particular emphasis is given to the methods employed in the study of different types of multivalent biological interactions occurring at the functionalized surfaces and their working principles.

Linkage Groups within Thiol-Ene Photoclickable PEG Hydrogels Control In Vivo Stability

Thiol-norbornene (thiol-ene) photoclickable poly(ethylene glycol) (PEG) hydrogels are a versatile biomaterial for cell encapsulation, drug delivery, and regenerative medicine. Numerous in vitro studies with these 4-arm ester-linked PEG-norbornene (PEG-4eNB) hydrogels demonstrate robust cytocompatibility and ability to retain long-term integrity with nondegradable crosslinkers. However, when transplanted in vivo into the subcutaneous or intraperitoneal space, these PEG-4eNB hydrogels with nondegradable crosslinkers rapidly degrade within 24 h. This characteristic limits the usefulness of PEG-4eNB hydrogels in biomedical applications. Replacing the ester linkage with an amide linkage (PEG-4aNB) mitigates this rapid in vivo degradation, and the PEG-4aNB hydrogels maintain long-term in vivo stability for months. Furthermore, when compared to PEG-4eNB, the PEG-4aNB hydrogels demonstrate equivalent mechanical properties, crosslinking kinetics, and high cytocompatibility with rat islets and human mesenchymal stem cells. Thus, the PEG-4aNB hydrogels may be a suitable replacement platform without necessitating critical design changes or sacrificing key properties relevant to the well-established PEG-4eNB hydrogels.

Neurovascular Organotypic Culture Models Using Induced Pluripotent Stem Cells to Assess Adverse Chemical Exposure Outcomes

Introduction: Human-induced pluripotent stem cells (iPSCs) represent a promising cell source for the construction of organotypic culture models for chemical toxicity screening and characterization. Materials and Methods: To characterize the effects of chemical exposure on the human neurovasculature, we constructed neurovascular unit (NVU) models consisting of endothelial cells (ECs) and astrocytes (ACs) derived from human-iPSCs, as well as human brain-derived pericytes (PCs). The cells were cocultured on synthetic poly(ethylene glycol) (PEG) hydrogels that guided the self-assembly of capillary-like vascular networks. High-content epifluorescence microscopy evaluated dose-dependent changes to multiple aspects of NVU morphology. Results: Cultured vascular networks underwent quantifiable morphological changes when incubated with vascular disrupting chemicals. The activity of predicted vascular disrupting chemicals from a panel of 38 compounds (U.S. Environmental Protection Agency) was ranked based on morphological features detected in the NVU model. In addition, unique morphological neurovascular disruption signatures were detected per chemical. A comparison of PEG-based NVU and Matrigel^™-based NVU models found greater sensitivity and consistency in chemical detection by the PEG-based NVU models. Discussion: We suspect that specific morphological changes may be used for discerning adverse outcome pathways initiated by chemical exposure and rapid mechanistic characterization of chemical exposure to neurovascular function. Conclusion: The use of human stem cell-derived vascular tissue and PEG hydrogels in the construction of NVU models leads to rapid detection of adverse chemical effects on neurovascular stability. The use of multiple cell types in coculture elucidates potential mechanisms of action by chemicals applied to the model.

Matrix-assisted cell transplantation for tissue vascularization

Cell therapy offers much promise for the treatment of ischemic diseases by augmenting tissue vasculogenesis. Matrix-assisted cell transplantation (MACT) has been proposed as a solution to enhance cell survival and integration with host tissue following transplantation. By designing semi synthetic matrices (sECM) with the correct physical and biochemical signals, encapsulated cells are directed towards a more angiogenic phenotype. In this review, we describe the choice of cells suitable for pro-angiogenic therapies, the properties that should be considered when designing sECM for transplantation and their relative importance. Pre-clinical models where MACT has been successfully applied to promote angiogenesis are reviewed to show the great potential of this strategy to treat ischemic conditions.

Wound healing activity and mechanism of action of antimicrobial and lipopolysaccharide-neutralizing peptides from enzymatic hydrolysates of rice bran proteins

In our previous study, we identified multifunctional cationic peptides from enzymatic hydrolysates of rice bran proteins (RBPs) that have antimicrobial and lipopolysaccharide-neutralizing activities. In this study, we investigated the potential of the peptides RBP-LRR, RBP-EKL, and RBP-SSF to promote proliferation, angiogenesis (tube formation), and migration in human umbilical vein endothelial cells (HUVECs). To determine mechanisms of wound healing actions, angiogenic and migration-promoting activities of these peptides were evaluated following pretreatments of HUVECs with specific inhibitors. In these experiments, the cationic peptides RBP-LRR, RBP-EKL, and RBP-SSF induced cell proliferation at low concentrations of 0.1 μM or 1 μM. Moreover, the three cationic peptides had angiogenic activities at concentrations more than 1 μM in tube formation assays, and their effects were similar to those of LL-37. Subsequent scratch migration assays exhibited that RBP-LRR, RBP-EKL, and RBP-SSF promote wound closure at optimum concentrations of 10, 10, and 0.1 μM, respectively. In further studies, we performed tube formation assays using HUVECs pretreated with SU5416, which inhibits vascular endothelial growth factor (VEGF) receptors, and suggested the possibility that the three cationic peptides induce angiogenesis by activating VEGF receptors. In corresponding scratch migration assays using HUVECs, pretreatment with the proliferation inhibitor mitomycin C did not alter the effects of RBP-LRR and RBP-EKL, and significant contribution to wound closure were mediated by cell migration regardless of proliferation rates. In contrast, RBP-SSF contributed to wound closure exclusively by promoting cell proliferation. The present data indicate that RBP-LRR, RBP-EKL, and RBP-SSF are candidates for use as wound healing agents.

Quantitative Label-Free Imaging of 3D Vascular Networks Self-Assembled in Synthetic Hydrogels

Vascularization is an important strategy to overcome diffusion limits and enable the formation of complex, physiologically relevant engineered tissues and organoids. Self-assembly is a technique to generate in vitro vascular networks, but engineering the necessary network morphology and function remains challenging. Here, autofluorescence multiphoton microscopy (aMPM), a label-free imaging technique, is used to quantitatively evaluate in vitro vascular network morphology. Vascular networks are generated using human embryonic stem cell-derived endothelial cells and primary human pericytes encapsulated in synthetic poly(ethylene glycol)-based hydrogels. Two custom-built bioreactors are used to generate distinct fluid flow patterns during vascular network formation: recirculating flow or continuous flow. aMPM is used to image these 3D vascular networks without the need for fixation, labels, or dyes. Image processing and analysis algorithms are developed to extract quantitative morphological parameters from these label-free images. It is observed with aMPM that both bioreactors promote formation of vascular networks with lower network anisotropy compared to static conditions, and the continuous flow bioreactor induces more branch points compared to static conditions. Importantly, these results agree with trends observed with immunocytochemistry. These studies demonstrate that aMPM allows label-free monitoring of vascular network morphology to streamline optimization of growth conditions and provide quality control of engineered tissues.

Evaluation of PEG-based hydrogel influence on estrogen receptor driven responses in MCF7 breast cancer cells

Extracellular matrix (ECM) mimicking hydrogel scaffolds have greatly improved the physiological relevance of in vitro assays, but introduce another dimension that creates variability in cell related readouts when compared to traditional 2D cells-on-plastic assays. We have developed a synthetic poly(ethylene glycol) (PEG) based ECM mimicking hydrogel and tested it against two gold standard animal-based naturally derived hydrogel scaffolds in MCF7 cell response. We have used the percent coefficient of variation (CV) as a metric to evaluate the reproducibility of said responses. Results indicated that PEG hydrogels performed similarly to naturally derived gold standards, and variance was similar in basic characterization assays, such as viability and cell adherence. PEG based hydrogels had lower CV values in estrogen receptor driven responses to several doses of estrogen in both estrogen receptor transactivation and estrogen induced proliferation.

Aptamer Functionalized DNA Hydrogel for Wise-Stage Controlled Protein Release

With the simple functionalization method and good biocompatibility, an aptamer-integrated DNA hydrogel is used as the protein delivery system with an adjustable release rate and time by using complementary sequences (CSs) as the biomolecular trigger. The aptamer-functionalized DNA hydrogel was prepared via a one-pot self-assembly process from two kinds of DNA building blocks (X-shaped and L-shaped DNA units) and a single-stranded aptamer. The gelling process was achieved under physiological conditions within one minute. In the absence of the triggering CSs, the aptamer grafted in the hydrogel exhibited a stable state for protein-specific capture. While hybridizing with the triggering CSs, the aptamer is turned into a double-stranded structure, resulting in the fast dissociation of protein with a wise-stage controlled release program. Further, the DNA hydrogel with excellent cytocompatibility has been successfully applied to human serum, forming a complex matrix. The whole process of protein capture and release were biocompatible and could not refer to any adverse factor of the protein or cells. Thus, the aptamer-functionalized DNA hydrogel will be a good candidate for controlled protein delivery.

Protease-Sensitive Hydrogel Biomaterials with Tunable Modulus and Adhesion Ligand Gradients for 3D Vascular Sprouting

Biomaterial strategies focused on designing scaffolds with physiologically relevant gradients provide a promising means for elucidating 3D vascular cell responses to spatial and temporal variations in matrix properties. In this study, we present a photopolymerization approach, ascending photofrontal free-radical polymerization, to generate proteolytically degradable hydrogel scaffolds of poly(ethylene) glycol with tunable continuous gradients of (1) elastic modulus (slope of 80 Pa/mm) and uniform immobilized RGD concentration (2.06 ± 0.12 mM) and (2) immobilized concentration of the RGD cell-adhesion peptide ligand (slope of 58.8 μM/mm) and uniform elastic modulus (597 ± 22 Pa). Using a coculture model of vascular sprouting, scaffolds embedded with gradients of elastic modulus induced increases in the number of vascular sprouts in the opposing gradient direction, whereas RGD gradient scaffolds promoted increases in the length of vascular sprouts toward the gradient. Furthermore, increases in vascular sprout length were found to be prominent in regions containing higher immobilized RGD concentration.

Customizable biomaterials as tools for advanced anti-angiogenic drug discovery

The inhibition of angiogenesis is a critical element of cancer therapy, as cancer vasculature contributes to tumor expansion. While numerous drugs have proven to be effective at disrupting cancer vasculature, patient survival has not significantly improved as a result of anti-angiogenic drug treatment. Emerging evidence suggests that this is due to a combination of unintended side effects resulting from the application of anti-angiogenic compounds, including angiogenic rebound after treatment and the activation of metastasis in the tumor. There is currently a need to better understand the far-reaching effects of anti-angiogenic drug treatments in the context of cancer. Numerous innovations and discoveries in biomaterials design and tissue engineering techniques are providing investigators with tools to develop physiologically relevant vascular models and gain insights into the holistic impact of drug treatments on tumors. This review examines recent advances in the design of pro-angiogenic biomaterials, specifically in controlling integrin-mediated cell adhesion, growth factor signaling, mechanical properties and oxygen tension, as well as the implementation of pro-angiogenic materials into sophisticated co-culture models of cancer vasculature.

The antimicrobial and anti-endotoxic peptide AmyI-1-18 from rice α-amylase and its [N3L] analog promote angiogenesis and cell migration

In our previous studies, we showed that AmyI-1-18 and its single amino acid-substituted analogs have antimicrobial, anti-inflammatory, and anti-endotoxic activities and cause little or no hemolysis or cytotoxicity. In this study, we investigated the potential of these peptides to promote proliferation, angiogenesis (tube formation), and migration in human umbilical vein endothelial cells (HUVECs). Among five single amino acid-substituted analogs, [N3L]AmyI-1-18 induced cell proliferation in a concentration-dependent manner with similar efficacy to AmyI-1-18. In tube formation assays, AmyI-1-18 and [N3L]AmyI-1-18 had angiogenic activities at 1 μM and their effects were similar to those of LL-37. Moreover, scratch migration assays showed that AmyI-1-18, [N3L]AmyI-1-18, and LL-37 promote cell migration with optimum concentrations of 10, 1, and 0.1 μM, respectively. Subsequently, we performed tube formation assays using HUVECs pretreated with SU5416, which is an inhibitor of vascular endothelial growth factor (VEGF) receptors, and revealed that AmyI-1-18 and [N3L]AmyI-1-18 induce angiogenesis by activating VEGF receptors. Similarly, after pretreating HUVECs with mitomycin C, which inhibits cell proliferation, [N3L]AmyI-1-18 significantly contributed to wound closure in scratch migration assays. Moreover, enhancements of hydrophobicity following substitution of AmyI-1-18 asparagine with leucine led to greater increases in cell migration. The present data indicate that both peptides, particularly [N3L]AmyI-1-18, are candidates for use as wound healing agents.

Microdevice arrays with strain sensors for 3D mechanical stimulation and monitoring of engineered tissues

Native and engineered tissue development are regulated by the integrative effects of multiple microenvironmental stimuli. Microfabricated bioreactor array platforms can efficiently dissect cue-response networks, and have recently integrated critical 2D and 3D mechanical stimulation for greater physiological relevance. However, a limitation of these approaches is that assessment of tissue functional properties is typically limited to end-point analyses. Here we report a new deformable membrane platform with integrated strain sensors that enables mechanical stretching or compression of 3D cell-hydrogel arrays and simultaneous measurement of hydrogel construct stiffness in situ. We tested the ability of the integrated strain sensors to measure the evolution of the stiffness of cell-hydrogel constructs for two cases. First, we demonstrated in situ stiffness monitoring of degradable poly (ethylene glycol)-norbornene (PEG-NB) hydrogels embedded with mesenchymal stromal cells (MSCs) and cultured with or without cyclic tensile stimulation for up to 15 days. Whereas statically-cultured hydrogels degraded and softened throughout the culture period, mechanically-stimulated gels initially softened and then recovered their stiffness corresponding to extensive cell network and collagen production. Second, we demonstrated in situ measurement of compressive stiffening of MSC-seeded PEG-NB gels cultured statically under osteogenic conditions, corresponding to increased mineralization and cellularization. This measurement technique can be generalized to other relevant bioreactor and organ-on-a-chip platforms to facilitate online, non-invasive, and high-throughput functional analysis, and to provide insights into the dynamics of engineered tissue development that are otherwise not available.

Guiding 3D cell migration in deformed synthetic hydrogel microstructures

The ability of cells to navigate through the extracellular matrix, a network of biopolymers, is controlled by an interplay of cellular activity and mechanical network properties. Synthetic hydrogels with highly tuneable compositions and elastic properties are convenient model systems for the investigation of cell migration in 3D polymer networks. To study the impact of macroscopic deformations on single cell migration, we present a novel method to introduce uniaxial strain in matrices by microstructuring photo-polymerizable hydrogel strips with embedded cells in a channel slide. We find that such confined swelling results in a strained matrix in which cells exhibit an anisotropic migration response parallel to the strain direction. Surprisingly, however, the anisotropy of migration reaches a maximum at intermediate strain levels and decreases strongly at higher strains. We account for this non-monotonic response in the migration anisotropy with a computational model, in which we describe a cell performing durotactic and proteolytic migration in a deformable elastic meshwork. Our simulations reveal that the macroscopically applied strain induces a local geometric anisotropic stiffening of the matrix. This local anisotropic stiffening acts as a guidance cue for directed cell migration, resulting in a non-monotonic dependence on strain, as observed in our experiments. Our findings provide a mechanism for mechanical guidance that connects network properties on the cellular scale to cell migration behaviour.

Aptamer-Patterned Hydrogel Films for Spatiotemporally Programmable Capture and Release of Multiple Proteins

Various hydrogels have been used for protein delivery in the treatment of human diseases. Nevertheless, it is always difficult to control the capture and release of multiple proteins in different regions and periods. This research successfully proves that multiple proteins can be captured and released from the aptamer-patterned hydrogel films with an adjustable release rate at a prospective time and in specific regions utilizing the complementary DNA strand of aptamers via photoclick chemistry and DNA hybridization. The hydrogel film is successfully applied to complex matrixes such as human serum and has excellent cytocompatibility. Thus, the aptamer-patterned hydrogel film will be a good candidate for controlled delivery of multiple proteins.

High-Content Assay Multiplexing for Vascular Toxicity Screening in Induced Pluripotent Stem Cell-Derived Endothelial Cells and Human Umbilical Vein Endothelial Cells

Endothelial cells (ECs) play a major role in blood vessel formation and function. While there is longstanding evidence for the potential of chemical exposures to adversely affect EC function and vascular development, the hazard potential of chemicals with respect to vascular effects is not routinely evaluated in safety assessments. Induced pluripotent stem cell (iPSC)-derived ECs promise to provide a physiologically relevant, organotypic culture model that is amenable for high-throughput (HT) EC toxicant screening and may represent a viable alternative to traditional in vitro models, including human umbilical vein endothelial cells (HUVECs). To evaluate the utility of iPSC-ECs for multidimensional HT toxicity profiling of chemicals, both iPSC-ECs and HUVECs were exposed to selected positive (angiogenesis inhibitors, cytotoxic agents) and negative compounds in concentration response for either 16 or 24 h in a 384-well plate format. Furthermore, chemical effects on vascularization were quantified using EC angiogenesis on biological (Geltrex™) and synthetic (SP-105 angiogenesis hydrogel) extracellular matrices. Cellular toxicity was assessed using high-content live cell imaging and the CellTiter-Glo^® assay. Assay performance indicated good to excellent assay sensitivity and reproducibility for both cell types investigated. Both iPSC-derived ECs and HUVECs formed tube-like structures on Geltrex™ and hydrogel, an effect that was inhibited by angiogenesis inhibitors and cytotoxic agents in a concentration-dependent manner. The quality of HT assays in HUVECs was generally higher than that in iPSC-ECs. Altogether, this study demonstrates the capability of ECs for comprehensive assessment of the biological effects of chemicals on vasculature in a HT compatible format.

Combinatorial screening of 3D biomaterial properties that promote myofibrogenesis for mesenchymal stromal cell-based heart valve tissue engineering

The physical and chemical properties of a biomaterial integrate with soluble cues in the cell microenvironment to direct cell fate and function. Predictable biomaterial-based control of integrated cell responses has been investigated with two-dimensional (2D) screening platforms, but integrated responses in 3D have largely not been explored systematically. To address this need, we developed a screening platform using polyethylene glycol norbornene (PEG-NB) as a model biomaterial with which the polymer wt% (to control elastic modulus) and adhesion peptide types (RGD, DGEA, YIGSR) and densities could be controlled independently and combinatorially in arrays of 3D hydrogels. We applied this platform and regression modeling to identify combinations of biomaterial and soluble biochemical (TGF-β1) factors that best promoted myofibrogenesis of human mesenchymal stromal cells (hMSCs) in order to inform our understanding of regenerative processes for heart valve tissue engineering. In contrast to 2D culture, our screens revealed that soft hydrogels (low PEG-NB wt%) best promoted spread myofibroblastic cells that expressed high levels of α-smooth muscle actin (α-SMA) and collagen type I. High concentrations of RGD enhanced α-SMA expression in the presence of TGF-β1 and cell spreading regardless of whether TGF-β1 was in the culture medium. Strikingly, combinations of peptides that maximized collagen expression depended on the presence or absence of TGF-β1, indicating that biomaterial properties can modulate MSC response to soluble signals. This combination of a 3D biomaterial array screening platform with statistical modeling is broadly applicable to systematically identify combinations of biomaterial and microenvironmental conditions that optimally guide cell responses.

STATEMENT OF SIGNIFICANCE

We present a novel screening platform and methodology to model and identify how combinations of biomaterial and microenvironmental conditions guide cell phenotypes in 3D. Our approach to systematically identify complex relationships between microenvironmental cues and cell responses enables greater predictive power over cell fate in conditions with interacting material design factors. We demonstrate that this approach not only predicts that mesenchymal stromal cell (MSC) myofibrogenesis is promoted by soft, porous 3D biomaterials, but also generated new insights which demonstrate how biomaterial properties can differentially modulate MSC response to soluble signals. An additional benefit of the process includes utilizing both parametric and non parametric analyses which can demonstrate dominant significant trends as well as subtle interactions between biochemical and biomaterial cues.

Versatile synthetic alternatives to Matrigel for vascular toxicity screening and stem cell expansion

The physiological relevance of Matrigel as a cell-culture substrate and in angiogenesis assays is often called into question. Here, we describe an array-based method for the identification of synthetic hydrogels that promote the formation of robust in vitro vascular networks for the detection of putative vascular disruptors, and that support human embryonic stem cell expansion and pluripotency. We identified hydrogel substrates that promoted endothelial-network formation by primary human umbilical vein endothelial cells and by endothelial cells derived from human induced pluripotent stem cells, and used the hydrogels with endothelial networks to identify angiogenesis inhibitors. The synthetic hydrogels show superior sensitivity and reproducibility over Matrigel when evaluating known inhibitors, as well as in a blinded screen of a subset of 38 chemicals, selected according to predicted vascular disruption potential, from the Toxicity ForeCaster library of the US Environmental Protection Agency. The identified synthetic hydrogels should be suitable alternatives to Matrigel for common cell-culture applications.

Crossfire for Two-Photon Photodynamic Therapy with Fluorinated Ruthenium (II) Photosensitizers

Synergistic photodynamic therapy (PDT) that combines photosensitizers (PSs) to attack different key sites in cancer cells is very attractive. However, the use of multiple PSs may increase dark cytotoxicity. Additionally, realizing the multiple vein passage of several PSs through dosing could be a challenge in clinical treatment. To address these issues, a novel strategy that enables a single PS to ablate two key sites (i.e., cytomembranes on the outside and mitochondria on the inside) of cancer cells synergistically was proposed. Five new fluorinated ruthenium (II) complexes (Ru1-Ru5), which possessed excellent two-photon properties and good singlet oxygen quantum yields, were designed and synthesized. When incubated with HeLa cells, the complexes were observed on the cytomembranes at first. With an extension of the treatment time, both the cytomembranes and mitochondria were lit up by the complexes. Under two-photon laser irradiation, the mitochondria and cytomembranes were ablated simultaneously, and the HeLa cells were destroyed effectively by the complexes, whether the cells were in a monolayer or in multicellular spheroids. With the largest phototoxicity index under the two-photon laser, Ru4 was used for two-photon PDT of in vivo xenograft tumors and successfully inhibited the growth of the tumors. Our results emphasized that the strategy of attacking two key sites with a single PS is an efficient method for PDT.

A Genome-wide Analysis of Human Pluripotent Stem Cell-Derived Endothelial Cells in 2D or 3D Culture

A defined protocol for efficiently deriving endothelial cells from human pluripotent stem cells was established and vascular morphogenesis was used as a model system to understand how synthetic hydrogels influence global biological function compared with common 2D and 3D culture platforms. RNA sequencing demonstrated that gene expression profiles were similar for endothelial cells and pericytes cocultured in polyethylene glycol (PEG) hydrogels or Matrigel, while monoculture comparisons identified distinct vascular signatures for each cell type. Endothelial cells cultured on tissue-culture polystyrene adopted a proliferative phenotype compared with cells cultured on or encapsulated in PEG hydrogels. The proliferative phenotype correlated to increased FAK-ERK activity, and knockdown or inhibition of ERK signaling reduced proliferation and expression for cell-cycle genes while increasing expression for “3D-like” vasculature development genes. Our results provide insight into the influence of 2D and 3D culture formats on global biological processes that regulate cell function.

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Defined, high-efficiency differentiation of human PSCs to endothelial cell

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Comprehensive genome-wide comparisons of 2D and 3D cell-culture formats

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Gene expression profiles for endothelial cells and pericytes in 3D cell culture

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Highly proliferative phenotypes on tissue-culture polystyrene surfaces

Biomaterial-mediated strategies targeting vascularization for bone repair

Repair of non-healing bone defects through tissue engineering strategies remains a challenging feat in the clinic due to the aversive microenvironment surrounding the injured tissue. The vascular damage that occurs following a bone injury causes extreme ischemia and a loss of circulating cells that contribute to regeneration. Tissue-engineered constructs aimed at regenerating the injured bone suffer from complications based on the slow progression of endogenous vascular repair and often fail at bridging the bone defect. To that end, various strategies have been explored to increase blood vessel regeneration within defects to facilitate both tissue-engineered and natural repair processes. Developments that induce robust vascularization will need to consolidate various parameters including optimization of embedded therapeutics, scaffold characteristics, and successful integration between the construct and the biological tissue. This review provides an overview of current strategies as well as new developments in engineering biomaterials to induce reparation of a functional vascular supply in the context of bone repair.

Development of peptide-functionalized synthetic hydrogel microarrays for stem cell and tissue engineering applications

Synthetic polymer microarray technology holds remarkable promise to rapidly identify suitable biomaterials for stem cell and tissue engineering applications. However, most of previous microarrayed synthetic polymers do not possess biological ligands (e.g., peptides) to directly engage cell surface receptors. Here, we report the development of peptide-functionalized hydrogel microarrays based on light-assisted copolymerization of poly(ethylene glycol) diacrylates (PEGDA) and methacrylated-peptides. Using solid-phase peptide/organic synthesis, we developed an efficient route to synthesize methacrylated-peptides. In parallel, we identified PEG hydrogels that effectively inhibit non-specific cell adhesion by using PEGDA-700 (M. W.=700) as a monomer. The combined use of these chemistries enables the development of a powerful platform to prepare peptide-functionalized PEG hydrogel microarrays. Additionally, we identified a linker composed of 4 glycines to ensure sufficient exposure of the peptide moieties from hydrogel surfaces. Further, we used this system to directly compare cell adhesion abilities of several related RGD peptides: RGD, RGDS, RGDSG and RGDSP. Finally, we combined the peptide-functionalized hydrogel technology with bioinformatics to construct a library composed of 12 different RGD peptides, including 6 unexplored RGD peptides, to develop culture substrates for hiPSC-derived cardiomyocytes (hiPSC-CMs), a cell type known for poor adhesion to synthetic substrates. 2 out of 6 unexplored RGD peptides showed substantial activities to support hiPSC-CMs. Among them, PMQKMRGDVFSP from laminin β4 subunit was found to support the highest adhesion and sarcomere formation of hiPSC-CMs. With bioinformatics, the peptide-functionalized hydrogel microarrays accelerate the discovery of novel biological ligands to develop biomaterials for stem cell and tissue engineering applications.

STATEMENT OF SIGNIFICANCE

In this manuscript, we described the development of a robust approach to prepare peptide-functionalized synthetic hydrogel microarrays. Combined with bioinformatics, this technology enables us to rapidly identify novel biological ligands for the development of the next generation of functional biomaterials for stem cell and tissue engineering applications.

Tough and Cell-Compatible Chitosan Physical Hydrogels for Mouse Bone Mesenchymal Stem Cells in Vitro

Most hydrogels involve synthetic polymers and organic cross-linkers that cannot simultaneously fulfill the mechanical and cell-compatibility requirements of biomedical applications. We prepared a new type of chitosan physical hydrogel with various degrees of deacetylation (DDs) via the heterogeneous deacetylation of nanoporous chitin hydrogels under mild conditions. The DD of the chitosan physical hydrogels ranged from 56 to 99%, and the hydrogels were transparent and mechanically strong because of the extra intra- and intermolecular hydrogen bonding interactions between the amino and hydroxyl groups on the nearby chitosan nanofibrils. The tensile strength and Young's modulus of the chitosan physical hydrogels were 3.6 and 7.9 MPa, respectively, for a DD of 56% and increased to 12.1 and 92.0 MPa for a DD of 99% in a swelling equilibrium state. In vitro studies demonstrated that mouse bone mesenchymal stem cells (mBMSCs) cultured on chitosan physical hydrogels had better adhesion and proliferation than those cultured on chitin hydrogels. In particular, the chitosan physical hydrogels promoted the differentiation of the mBMSCs into epidermal cells in vitro. These materials are promising candidates for applications such as stem cell research, cell therapy, and tissue engineering.

Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays

Activation of vascular endothelial cells (ECs) by growth factors initiates a cascade of events during angiogenesis in vivo consisting of EC tip cell selection, sprout formation, EC stalk cell proliferation, and ultimately vascular stabilization by support cells. Although EC functional assays can recapitulate one or more aspects of angiogenesis in vitro, they are often limited by undefined substrates and lack of dependence on key angiogenic signaling axes. Here, we designed and characterized a chemically-defined model of endothelial sprouting behavior in vitro using human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs). We rapidly encapsulated iPSC-ECs at high density in poly(ethylene glycol) (PEG) hydrogel spheres using thiol-ene chemistry and subsequently encapsulated cell-dense hydrogel spheres in a cell-free hydrogel layer. The hydrogel sprouting array supported pro-angiogenic phenotype of iPSC-ECs and supported growth factor-dependent proliferation and sprouting behavior. iPSC-ECs in the sprouting model responded appropriately to several reference pharmacological angiogenesis inhibitors of vascular endothelial growth factor, NF-κB, matrix metalloproteinase-2/9, protein kinase activity, and β-tubulin, which confirms their functional role in endothelial sprouting. A blinded screen of 38 putative vascular disrupting compounds from the US Environmental Protection Agency's ToxCast library identified six compounds that inhibited iPSC-EC sprouting and five compounds that were overtly cytotoxic to iPSC-ECs at a single concentration. The chemically-defined iPSC-EC sprouting model (iSM) is thus amenable to enhanced-throughput screening of small molecular libraries for effects on angiogenic sprouting and iPSC-EC toxicity assessment.

STATEMENT OF SIGNIFICANCE

Angiogenesis assays that are commonly used for drug screening and toxicity assessment applications typically utilize natural substrates like Matrigel(TM) that are difficult to spatially pattern, costly, ill-defined, and may exhibit lot-to-lot variability. Herein, we describe a novel angiogenic sprouting assay using chemically-defined, bioinert poly(ethylene glycol) hydrogels functionalized with biomimetic peptides to promote cell attachment and degradation in a reproducible format that may mitigate the need for natural substrates. The quantitative assay of angiogenic sprouting here enables precise control over the initial conditions and can be formulated into arrays for screening. The sprouting assay here was dependent on key angiogenic signaling axes in a screen of angiogenesis inhibitors and a blinded screen of putative vascular disrupting compounds from the US-EPA.

Combinatorial Method/High Throughput Strategies for Hydrogel Optimization in Tissue Engineering Applications

Combinatorial method/high throughput strategies, which have long been used in the pharmaceutical industry, have recently been applied to hydrogel optimization for tissue engineering applications. Although many combinatorial methods have been developed, few are suitable for use in tissue engineering hydrogel optimization. Currently, only three approaches (design of experiment, arrays and continuous gradients) have been utilized. This review highlights recent work with each approach. The benefits and disadvantages of design of experiment, array and continuous gradient approaches depending on study objectives and the general advantages of using combinatorial methods for hydrogel optimization over traditional optimization strategies will be discussed. Fabrication considerations for combinatorial method/high throughput samples will additionally be addressed to provide an assessment of the current state of the field, and potential future contributions to expedited material optimization and design.

Stable engineered vascular networks from human induced pluripotent stem cell-derived endothelial cells cultured in synthetic hydrogels

Here, we describe an in vitro strategy to model vascular morphogenesis where human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) are encapsulated in peptide-functionalized poly(ethylene glycol) (PEG) hydrogels, either on standard well plates or within a passive pumping polydimethylsiloxane (PDMS) tri-channel microfluidic device. PEG hydrogels permissive towards cellular remodeling were fabricated using thiol-ene photopolymerization to incorporate matrix metalloproteinase (MMP)-degradable crosslinks and CRGDS cell adhesion peptide. Time lapse microscopy, immunofluorescence imaging, and RNA sequencing (RNA-Seq) demonstrated that iPSC-ECs formed vascular networks through mechanisms that were consistent with in vivo vasculogenesis and angiogenesis when cultured in PEG hydrogels. Migrating iPSC-ECs condensed into clusters, elongated into tubules, and formed polygonal networks through sprouting. Genes upregulated for iPSC-ECs cultured in PEG hydrogels relative to control cells on tissue culture polystyrene (TCP) surfaces included adhesion, matrix remodeling, and Notch signaling pathway genes relevant to in vivo vascular development. Vascular networks with lumens were stable for at least 14days when iPSC-ECs were encapsulated in PEG hydrogels that were polymerized within the central channel of the microfluidic device. Therefore, iPSC-ECs cultured in peptide-functionalized PEG hydrogels offer a defined platform for investigating vascular morphogenesis in vitro using both standard and microfluidic formats.

STATEMENT OF SIGNIFICANCE

Human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) cultured in synthetic hydrogels self-assemble into capillary networks through mechanisms consistent with in vivo vascular morphogenesis.

Polymer microarray technology for stem cell engineering

Stem cells hold remarkable promise for applications in tissue engineering and disease modeling. During the past decade, significant progress has been made in developing soluble factors (e.g., small molecules and growth factors) to direct stem cells into a desired phenotype. However, the current lack of suitable synthetic materials to regulate stem cell activity has limited the realization of the enormous potential of stem cells. This can be attributed to a large number of materials properties (e.g., chemical structures and physical properties of materials) that can affect stem cell fate. This makes it challenging to design biomaterials to direct stem cell behavior. To address this, polymer microarray technology has been developed to rapidly identify materials for a variety of stem cell applications. In this article, we summarize recent developments in polymer array technology and their applications in stem cell engineering.

STATEMENT OF SIGNIFICANCE

Stem cells hold remarkable promise for applications in tissue engineering and disease modeling. In the last decade, significant progress has been made in developing chemically defined media to direct stem cells into a desired phenotype. However, the current lack of the suitable synthetic materials to regulate stem cell activities has been limiting the realization of the potential of stem cells. This can be attributed to the number of variables in material properties (e.g., chemical structures and physical properties) that can affect stem cells. Polymer microarray technology has shown to be a powerful tool to rapidly identify materials for a variety of stem cell applications. Here we summarize recent developments in polymer array technology and their applications in stem cell engineering.

Hydrogel arrays formed via differential wettability patterning enable combinatorial screening of stem cell behavior

Here, we have developed a novel method for forming hydrogel arrays using surfaces patterned with differential wettability. Our method for benchtop array formation is suitable for enhanced-throughput, combinatorial screening of biochemical and biophysical cues from chemically defined cell culture substrates. We demonstrated the ability to generate these arrays without the need for liquid handling systems and screened the combinatorial effects of substrate stiffness and immobilized cell adhesion peptide concentration on human mesenchymal stem cell (hMSC) behavior during short-term 2-dimensional cell culture. Regardless of substrate stiffness, hMSC initial cell attachment, spreading, and proliferation were linearly correlated with immobilized CRGDS peptide concentration. Increasing substrate stiffness also resulted in increased hMSC initial cell attachment, spreading, and proliferation; however, examination of the combinatorial effects of CRGDS peptide concentration and substrate stiffness revealed potential interplay between these distinct substrate signals. Maximal hMSC proliferation seen on substrates with either high stiffness or high CRGDS peptide concentration suggests that some baseline level of cytoskeletal tension was required for hMSC proliferation on hydrogel substrates and that multiple substrate signals could be engineered to work in synergy to promote mechanosensing and regulate cell behavior.

STATEMENT OF SIGNIFICANCE

Our novel array formation method using surfaces patterned with differential wettability offers the advantages of benchtop array formation for 2-dimensional cell cultures and enhanced-throughput screening without the need for liquid handling systems. Hydrogel arrays formed via our method are suitable for screening the influence of chemical (e.g. cell adhesive ligands) and physical (stiffness, size, shape, and thickness) substrate properties on stem cell behavior. The arrays are also fully compatible with commercially available micro-array add-on systems, which allows for simultaneous control of the insoluble and soluble cell culture environment. This study used hydrogel arrays to demonstrate that synergy between cell adhesion and mechanosensing can be used to regulate hMSC behavior.

A microfabricated platform with hydrogel arrays for 3D mechanical stimulation of cells

Cellular microenvironments present cells with multiple stimuli, including not only soluble biochemical and insoluble matrix cues but also mechanical factors. Biomaterial array platforms have been used to combinatorially and efficiently probe and define two-dimensional (2D) and 3D microenvironmental cues to guide cell functions for tissue engineering applications. However, there are few examples of array platforms that include dynamic mechanical forces, particularly to enable stretching of 3D cell-seeded biomaterials, which is relevant to engineering connective and cardiovascular tissues. Here we present a deformable membrane platform that enables 3D dynamic mechanical stretch of arrayed biomaterial constructs. Cell-seeded polyethylene glycol norbornene (PEG-NB) hydrogels were bound to miniaturized deformable membranes via a thiol-ene reaction with off-stoichiometry thiol-ene based polydimethylsiloxane (OSTE-PDMS) as the membrane material. Bonding to OSTE-PDMS enabled the 3D hydrogel microconstructs to be cyclically deformed and stretched by the membrane. As a first demonstration, human mesenchymal stromal cells (MSCs) embedded in PEG-NB were stretched for several days. They were found to be viable, spread in the 3D hydrogels, and exhibited a contractile myofibroblast phenotype when exposed to dynamic 3D mechanical deformation. This platform, which is readily scalable to larger arrays, enables systematic interrogation of the relationships between combinations of 3D mechanobiological cues and cellular responses, and thus has the potential to identify strategies to predictably control the construction of functional engineered tissues.

STATEMENT OF SIGNIFICANCE

Current high-throughput biomaterial screening approaches fail to consider the effects of dynamic mechanical stimulation, despite its importance in a wide variety of regenerative medicine applications. To meet this need, we developed a deformable membrane platform that enables 3D dynamic stretch of arrayed biomaterial constructs. Our approach combines microtechnologies fabricated with off-stoichiometry thiol-ene based polydimethylsiloxane membranes that can covalently bond cell-seeded polyethylene glycol norbornene 3D hydrogels, a model biomaterial with tunable adhesive, elastic and degradation characteristics. As a first demonstration, we show that human mesenchymal stromal cells embedded in hydrogels and subjected to dynamic mechanical stimulation undergo myofibroblast differentiation. This system is readily scaled up to larger arrays, and will enable systematic and efficient screening of combinations of 3D mechanobiological and biomaterial cues on cell fate and function.

Human vascular tissue models formed from human induced pluripotent stem cell derived endothelial cells

Here we describe a strategy to model blood vessel development using a well-defined induced pluripotent stem cell-derived endothelial cell type (iPSC-EC) cultured within engineered platforms that mimic the 3D microenvironment. The iPSC-ECs used here were first characterized by expression of endothelial markers and functional properties that included VEGF responsiveness, TNF-α-induced upregulation of cell adhesion molecules (MCAM/CD146; ICAM1/CD54), thrombin-dependent barrier function, shear stress-induced alignment, and 2D and 3D capillary-like network formation in Matrigel. The iPSC-ECs also formed 3D vascular networks in a variety of engineering contexts, yielded perfusable, interconnected lumen when co-cultured with primary human fibroblasts, and aligned with flow in microfluidics devices. iPSC-EC function during tubule network formation, barrier formation, and sprouting was consistent with that of primary ECs, and the results suggest a VEGF-independent mechanism for sprouting, which is relevant to therapeutic anti-angiogenesis strategies. Our combined results demonstrate the feasibility of using a well-defined, stable source of iPSC-ECs to model blood vessel formation within a variety of contexts using standard in vitro formats.

Effects of Variations in Ligand Density on Cell Signaling

Multiple simultaneous interactions between receptors and ligands dictate the extracellular and intracellular activities of cells. The concept of programmable ligand display is generally used to study the interaction between ligands, displayed on surfaces at various densities, with receptors present on cell surfaces. Various strategies are discussed here to display ligands on surfaces to study their effect on cell behavior. Only very few strategies have been reported where this display combines precise control over density with lateral spacing of ligands on surfaces. In this review, selected examples of strategies to control ligand density and spacing and their implications for biological functions of cells are discussed.