Facile Fabrication of High-Definition Hierarchical Wrinkle Structures for Investigating the Geometry-Sensitive Fate Commitment of Human Neural Stem Cells

Dynamic light-responsive RhoA activity regulates mechanosensitive stem cell fate decision in 3D matrices

The behavior of stem cells is regulated by mechanical cues in their niche that continuously vary due to extracellular matrix (ECM) remodeling, pulsated mechanical stress exerted by blood flow, and/or cell migration. However, it is still unclear how dynamics of mechanical cues influence stem cell lineage commitment, especially in a 3D microenvironment where mechanosensing differs from that in a 2D microenvironment. In the present study, we investigated how temporally varying mechanical signaling regulates expression of the early growth response 1 gene (Egr1), which we recently discovered to be a 3D matrix-specific mediator of mechanosensitive neural stem cell (NSC) lineage commitment. Specifically, we temporally controlled the activity of Ras homolog family member A (RhoA), which is known to have a central role in mechanotransduction, using our previously developed Arabidopsis thaliana cryptochrome-2-based optoactivation system. Interestingly, pulsed RhoA activation induced Egr1 upregulation in stiff 3D gels only, whereas static light stimulation induced an increase in Egr1 expression across a wide range of 3D gel stiffnesses. Actin assembly inhibition limited Egr1 upregulation upon RhoA activation, implying that RhoA signaling requires an actin-involved process to upregulate Egr1. Consistently, static-light RhoA activation rather than pulsed-light activation restricted neurogenesis in soft gels. Our findings indicate that the dynamics of RhoA activation influence Egr1-mediated stem cell fate within 3D matrices in a matrix stiffness-dependent manner.

Nanoscale surface coatings and topographies for neural interfaces

With the lack of minimally invasive tools for probing neuronal systems across spatiotemporal scales, understanding the working mechanism of the nervous system and limited assessments available are imperative to prevent or treat neurological disorders. In particular, nanoengineered neural interfaces can provide a solution to this technological barrier. This review covers recent surface engineering approaches, including nanoscale surface coatings, and a range of topographies from the microscale to the nanoscale, primarily focusing on neural-interfaced biosystems. Specifically, the immobilization of bioactive molecules to fertilize the neural cell lineage, topographical engineering to induce mechanotransduction in neural cells, and enhanced cell-chip coupling using three-dimensional structured surfaces are highlighted. Advances in neural interface design will help us understand the nervous system, thereby achieving the effective treatments for neurological disorders. STATEMENT OF SIGNIFICANCE: • This review focuses on designing bioactive neural interface with a nanoscale chemical modification and topographical engineering at multiscale perspective. • Versatile nanoscale surface coatings and topographies for neural interface are summarized. • Recent advances in bioactive materials applicable for neural cell culture, electrophysiological sensing, and neural implants are reviewed.

Micro-/Nanohierarchical Structures Physically Engineered on Surfaces: Analysis and Perspective

The high demand for micro-/nanohierarchical structures as components of functional substrates, bioinspired devices, energy-related electronics, and chemical/physical transducers has inspired their in-depth studies and active development of the related fabrication techniques. In particular, significant progress has been achieved in hierarchical structures physically engineered on surfaces, which offer the advantages of wide-range material compatibility, design diversity, and mechanical stability, and numerous unique structures with important niche applications have been developed. This review categorizes the basic components of hierarchical structures physically engineered on surfaces according to function/shape and comprehensively summarizes the related advances, focusing on the fabrication strategies, ways of combining basic components, potential applications, and future research directions. Moreover, the physicochemical properties of hierarchical structures physically engineered on surfaces are compared based on the function of their basic components, which may help to avoid the bottlenecks of conventional single-scale functional substrates. Thus, the present work is expected to provide a useful reference for scientists working on multicomponent functional substrates and inspire further research in this field.

Hierarchical Topography with Tunable Micro- and Nanoarchitectonics for Highly Enhanced Cardiomyocyte Maturation via Multi-Scale Mechanotransduction

Enhancing cardiomyocyte (CM) maturation by topographical cues is a critical issue in cardiac tissue engineering. Thus far, single-scale topographies with a broad range of feature shapes and dimensions have been utilized including grooves, pillars, and fibers. This study reports for the first time a hierarchical structure composed of nano-pillars (nPs) on micro-wrinkles (µWs) for effective maturation of CMs. Through capillary force lithography followed by a wrinkling process, vast size ranges of topographies are fabricated, and the responses of CMs are systematically investigated. Maturation of CMs on the hierarchical structures is highly enhanced compared to a single-scale topography: cardiac differentiation of H9C2s (rat cardiomyocytes) on the hierarchical topography is ≈ 2.8 and ≈ 1.9 times higher than those consisting of single-scale µWs and nPs. Both nPs and µWs have important roles in cardiac maturation, and the aspect ratio (height/diameter) of the nPs and the wavelength of the µWs are important in CM maturation. This enhancement is caused by strong focal adhesion and nucleus mediated mechanotransduction of CMs from the confinement effects of the different wavelengths of µWs and the cellular membrane protrusion on the nPs. This study demonstrates how a large family of hierarchical structures is used for cardiac maturation.

N-Cadherin adhesive ligation regulates mechanosensitive neural stem cell lineage commitment in 3D matrices

During differentiation, neural stem cells (NSCs) encounter diverse cues from their niche, including not only biophysical cues from the extracellular matrix (ECM) but also cell-cell communication. However, it is still poorly understood how these cues cumulatively regulate mechanosensitive NSC fate commitment, especially in 3D matrices that better mimic in vivo systems. Here, we develop a click chemistry-based 3D hydrogel material system to fully decouple cell-cell and cell-ECM interactions by functionalizing small peptides: the HAVDI motif from N-cadherin and RGD motif from fibronectin. The hydrogel is engineered to range in stiffness from 75 Pa to 600 Pa. Interestingly, HAVDI-mediated interaction shows increased neurogenesis, except for the softest gel (75 Pa). Moreover, the HAVDI ligation attenuates the mechanosensing state of NSCs, exhibiting restricted cytoskeletal formation and RhoA signaling. Given that mechanosensitive neurogenesis has been reported to be regulated by cytoskeletal formation, our finding suggests that the enhanced neurogenesis in the HAVDI-modified gel may be highly associated with the HAVDI interaction-mediated attenuation of mechanosensing. Furthermore, NSCs in the HAVDI gel shows higher β-catenin activity, which has been known to promote neurogenesis. Our findings provide critical insights into how mechanosensitive NSC fate commitment is regulated as a consequence of diverse interactions in 3D microenvironments.

Vapor-phase synthesis of a reagent-free self-healing polymer film with rapid recovery of toughness at room temperature and under ambient conditions

A rapidly self-healable polymer is highly desirable but challenging to achieve. Herein, we developed an elastomeric film with instant self-healing ability within 10 s at room temperature. For this purpose, a series of copolymers of poly(glycidyl methacrylate-co-2-hydroxyethyl acrylate) (poly(GMA-co-HEA), or pGH) were synthesized in the vapor phase via an initiated chemical vapor deposition (iCVD) process. The elastomer includes a large amount of hydroxyl groups in the 2-hydroxyethyl acrylate (HEA) moiety capable of forming rapid, reversible hydrogen bonding at room temperature, while glycidyl methacrylate (GMA) with a rigid methacrylic backbone chain in the copolymer provides mechanical robustness to the elastic copolymer. With the optimized copolymer composition, pGH indeed showed instant recovery of the toughness within a minute; a completely divided specimen could be welded within a minute at room temperature and under ambient conditions simply by placing the pieces in close contact, which showed the outstanding recovery performance of elastic modulus (93.2%) and toughness (15.6 MJ m^-3). The rapid toughness recovery without supplementing any external energy or reagents (e.g. light, temperature, or catalyst) at room temperature and under ambient conditions will be useful in future wearable electronics and soft robotics applications.

A Versatile Surface Modification Method via Vapor-phase Deposited Functional Polymer Films for Biomedical Device Applications

For last two decades, the demand for precisely engineered three-dimensional structures has increased continuously for the developments of biomaterials. With the recent advances in micro- and nano-fabrication techniques, various devices with complex surface geometries have been devised and produced in the pharmaceutical and medical fields for various biomedical applications including drug delivery and biosensors. These advanced biomaterials have been designed to mimic the natural environments of tissues more closely and to enhance the performance for their corresponding biomedical applications. One of the important aspects in the rational design of biomaterials is how to configure the surface of the biomedical devices for better control of the chemical and physical properties of the bioactive surfaces without compromising their bulk characteristics. In this viewpoint, it of critical importance to secure a versatile method to modify the surface of various biomedical devices. Recently, a vapor phase method, termed initiated chemical vapor deposition (iCVD) has emerged as damage-free method highly beneficial for the conformal deposition of various functional polymer films onto many kinds of micro- and nano-structured surfaces without restrictions on the substrate material or geometry, which is not trivial to achieve by conventional solution-based surface functionalization methods. With proper structural design, the functional polymer thin film via iCVD can impart required functionality to the biomaterial surfaces while maintaining the fine structure thereon. We believe the iCVD technique can be not only a valuable approach towards fundamental cell-material studies, but also of great importance as a platform technology to extend to other prospective biomaterial designs and material interface modifications for biomedical applications.

Synergistic Effect of Cell-Derived Extracellular Matrices and Topography on Osteogenesis of Mesenchymal Stem Cells

Cell-derived matrices (CDMs) are an interesting alternative to conventional sources of extracellular matrices (ECMs) as CDMs mimic the natural ECM composition better and are therefore attractive as a scaffolding material for regulating the functions of stem cells. Previous research on stem cell differentiation has demonstrated that both surface topography and CDMs have a significant influence. However, not much focus has been devoted to elucidating possible synergistic effects of CDMs and topography on osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBM-MSCs). In this study, polydimethylsiloxane (PDMS)-based anisotropic topographies (wrinkles) with various topography dimensions were prepared and subsequently combined with native ECMs produced by human fibroblasts that remained on the surface topography after decellularization. The synergistic effect of CDMs combined with topography on osteogenic differentiation of hBM-MSCs was investigated. The results showed that substrates with specific topography dimensions, coated with aligned CDMs, dramatically enhanced the capacity of osteogenesis as investigated using immunofluorescence staining for identifying osteopontin (OPN) and mineralization. Furthermore, the hBM-MSCs on the substrates decorated with CDMs exhibited a higher percentage of (Yes-associated protein) YAP inside the nucleus, stronger cell contractility, and greater formation of focal adhesions, illustrating that enhanced osteogenesis is partly mediated by cellular tension and mechanotransduction following the YAP pathway. Taken together, our findings highlight the importance of ECMs mediating the osteogenic differentiation of stem cells, and the combination of CDMs and topography will be a powerful approach for material-driven osteogenesis.

High-Throughput Screening and Hierarchical Topography-Mediated Neural Differentiation of Mesenchymal Stem Cells

Biophysical factors such as anisotropic topography composed of micro/nanosized structures are important for directing the fate of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) and have been applied to neuronal differentiation. Via high-throughput screening (HTS) methods based on topography gradients, the optimum topography is determined and translated toward a hierarchical architecture designed to mimic the nerve nano/microstructure. The polydimethylsiloxane (PDMS)-based topography gradient with amplitudes (A) from 541 to 3073 nm and wavelengths (W) between 4 and 30 µm is developed and the fate commitment of MSC toward neuron lineage is investigated. The hierarchical structures, combining nano- and microtopography (W0.3/W26 parallel/perpendicular) are fabricated to explore the combined topography effects on neuron differentiation. From the immunofluorescent staining results (Tuj1 and MAP2), the substrate characterized by W: 26 µm; A: 2.9 µm shows highest potential for promoting neurogenesis. Furthermore, the hierarchical features (W0.3/W26 parallel) significantly enhance neural differentiation. The hBM-MSCs on the hierarchical substrates exhibit a significantly lower percentage of nuclear Yes-associated protein (YAP)/TAZ and weaker cell contractility indicating that the promoted neurogenesis is mediated by the cell tension and YAP/TAZ pathway. This research provides new insight into designing biomaterials for applications in neural tissue engineering and contributes to the understanding of topography-mediated neuronal differentiation.

Biointerface topography mediates the interplay between endothelial cells and monocytes

Endothelial cell (EC) monolayers located in the inner lining of blood vessels serve as a semipermeable barrier between circulating blood and surrounding tissues. The structure and function of the EC monolayer affect the recruitment and adhesion of monocytes, which plays a pivotal role in the development of inflammation and atherosclerosis. Here we investigate the effect of material wrinkled topographies on the responses of human umbilical vein endothelial cells (HUVECs) and adhesion of monocytes to HUVECs. It is found that HUVEC responses are non-linearly mediated by surface topographies with different dimensions. Specifically, more cell elongation and better cell orientation on the wrinkled surface with a 3.5 μm amplitude and 10 μm wavelength (W10) are observed compared to other surfaces. The proliferation rate of HUVECs on the W10 surface is higher than that on other surfaces due to more 5-ethynyl-2′-deoxyuridine (EdU) detected on the W10 surface. Also, greater expression of inflammatory cytokines from HUVECs and adhesion of monocytes to HUVECs on the W10 surface is shown than other surfaces due to greater expression of p-AKT and ICAM, respectively. This study offers a new in vitro system to understand the interplay between HUVEC monolayers and monocytes mediated by aligned topographies, which may be useful for vascular repair and disease modeling for drug testing.

This study offers a new in vitro system to understand the interplay between HUVEC monolayer and monocytes mediated by aligned topographies, which may be useful for vascular repair and disease modeling for drug testing.

Topography induced stiffness alteration of stem cells influences osteogenic differentiation

Topography-driven alterations to single cell stiffness rather than alterations in cell morphology, is the underlying driver for influencing cell biological processes, particularly stem cell differentiation.

Mechanotransduction in neuronal cell development and functioning

Although many details remain still elusive, it became increasingly evident in recent years that mechanosensing of microenvironmental biophysical cues and subsequent mechanotransduction are strongly involved in the regulation of neuronal cell development and functioning. This review gives an overview about the current understanding of brain and neuronal cell mechanobiology and how it impacts on neurogenesis, neuronal migration, differentiation, and maturation. We will focus particularly on the events in the cell/microenvironment interface and the decisive extracellular matrix (ECM) parameters (i.e. rigidity and nanometric spatial organisation of adhesion sites) that modulate integrin adhesion complex-based mechanosensing and mechanotransductive signalling. It will also be outlined how biomaterial approaches mimicking essential ECM features help to understand these processes and how they can be used to control and guide neuronal cell behaviour by providing appropriate biophysical cues. In addition, principal biophysical methods will be highlighted that have been crucial for the study of neuronal mechanobiology.