A functional microvascular system is imperative to build and maintain healthy tissue. Impaired microvasculature results in ischemia, thereby limiting the tissue’s intrinsic regeneration capacity. Therefore, the ability to regenerate microvascular networks is key to the development of effective cardiovascular therapies. To stimulate the formation of new microvasculature, researchers have focused on fabricating materials that mimic the angiogenic properties of the native extracellular matrix (ECM). Here, we will review biomaterials that seek to imitate the physical cues that are natively provided by the ECM to encourage the formation of microvasculature in engineered constructs and ischemic tissue in the body.

Induced pluripotent stem cell-derived endothelial progenitors (iPSC-EPs) have emerged as a promising candidate cell source for patient-specific ischemic therapies. Before these cells can be appropriately deployed in a clinical setting, it is imperative to study their assembly into functional vascular networks in extracellular matrix (ECM)-mimicking, three-dimensional (3D) microenvironments. To elucidate the interactions of iPSC-EPs with the ECM, we examined how in vitro modulation of structural protein density, the presence of angiogenic growth factors, and relative proteolytic activity affected the vasculogenic potential of these progenitors, that is, their ability to self-assemble into vessel-like networks. We found that the addition of a ROCK pathway inhibitor and exogenous vascular endothelial growth factor (VEGF) are imperative for inducing robust iPSC-EP vasculogenesis in collagen hydrogels. Under these conditions, 3D vascular-like networks containing VE-cadherin-expressing lumens formed within a week of culture. To quantify this 3D vessel-like network, we developed a computational pipeline to analyze network length, connectivity, and average lumen diameter. Increasing the concentration of collagen in the hydrogels abrogated network formation and encouraged the formation of disconnected, largediameter lumens. This phenomenon was in part related to the cells' proteolytic capacity and the hydrogels' properties, specifically hydrogel deformability and pore size. In conclusion, we demonstrate that the vasculogenic potential of iPSC-EPs is regulated by cell-matrix interactions and the matrix properties of collagen hydrogels.

Cell alignment in muscle, nervous tissue, and cartilage is requisite for proper tissue function; however, cell sheeting techniques using the thermosensitive polymer poly(N-isopropyl acrylamide) (PNIPAAm) can only produce anisotropic cell sheets with delicate and resource-intensive modifications. We hypothesized that electrospinning, a relatively simple and inexpensive technique to generate aligned polymer fibers, could be used to fabricate anisotropic PNIPAAm and poly(caprolactone) (PCL) blended surfaces that both support cell viability and permit cell sheet detachment via PNIPAAm dissolution. Aligned electrospun PNIPAAm/PCL fibers (0%, 25%, 50%, 75%, 90%, and 100% PNIPAAm) were electrospun and characterized. Fibers ranged in diameter from 1–3 μm, and all fibers had an orientation index greater than 0.65. Fourier transform infrared spectroscopy was used to confirm the relative content of PNIPAAm and PCL. For advancing water contact angle and mass loss studies, only high PNIPAAm-content fibers (75% and greater) exhibited, temperature-dependent properties like 100% PNIPAAm fibers, whereas 25% and 50% PNIPAAm fibers behaved similarly to PCL-only fibers. 3T3 fibroblasts seeded on all PNIPAAm/PCL fibers had high cell viability and spreading except for the 100% PNIPAAm fibers. Cell sheet detachment by incubation with cold medium was successful only for 90% PNIPAAm fibers, which had a sufficient amount of PCL to allow cell attachment and spreading but not enough to prevent detachment upon PNIPAAm dissolution. This study demonstrates the feasibility of using anisotropic electrospun PNIPAAm/PCL fibers to generate aligned cell sheets that can potentially better recapitulate anisotropic architecture to achieve proper tissue function.

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Induced pluripotent stem cells are a patient-specific, proliferative cell source that can differentiate into any somatic cell type. Bipotent endothelial progenitors, which can differentiate into the cell types necessary to assemble mature, functional vasculature, have been derived from both embryonic and induced pluripotent stem cells. However, these cells have not been rigorously evaluated in three-dimensional environments, and a quantitative measure of their vasculogenic potential remains elusive. Here, the generation and isolation of iPSC-EPs via fluorescent-activated cell sorting are first outlined, followed by a description of the encapsulation and culture of iPSC-EPs in collagen hydrogels. This extracellular matrix (ECM)-mimicking microenvironment encourages a robust vasculogenic response; vascular networks form after a week of culture. The creation of a computational pipeline that utilizes open-source software to quantify this vasculogenic response is delineated. This pipeline is specifically designed to preserve the 3D architecture of the capillary plexus to robustly identify the number of branches, branching points, and the total network length with minimal user input.

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Recent advances in developmental biology and biomedical engineering have significantly improved the efficiency and purity of cardiomyocytes (CMs) generated from pluripotent stem cells (PSCs). Regardless of the protocol used to derive CMs, these cells exhibit hallmarks of functional immaturity. In this Opinion, we focus on reactive oxygen species (ROS), signaling molecules that can potentially modulate cardiac maturation. We outline how ROS impacts nearly every aspect associated with cardiac maturation, including contractility, calcium handling, metabolism, and hypertrophy. Though the precise role of ROS in cardiac maturation has yet to be elucidated, ROS may provide a valuable perspective for understanding the molecular mechanisms for cardiac maturation under various conditions.