The effect of Young's modulus on the neuronal differentiation of mouse embryonic stem cells

Biomimetic Dual-Network Collagen Fibers with Porous and Mechanical Cues Reconstruct Neural Stem Cell Niche via AKT/YAP Mechanotransduction after Spinal Cord Injury

Tissue engineering scaffolds can mediate the maneuverability of neural stem cell (NSC) niche to influence NSC behavior, such as cell self-renewal, proliferation, and differentiation direction, showing the promising application in spinal cord injury (SCI) repair. Here, dual-network porous collagen fibers (PCFS) are developed as neurogenesis scaffolds by employing biomimetic plasma ammonia oxidase catalysis and conventional amidation cross-linking. Following optimizing the mechanical parameters of PCFS, the well-matched Young's modulus and physiological dynamic adaptability of PCFS (4.0 wt%) have been identified as a neurogenetic exciter after SCI. Remarkably, porous topographies and curving wall-like protrusions are generated on the surface of PCFS by simple and non-toxic CO2 bubble-water replacement. As expected, PCFS with porous and matched mechanical properties can considerably activate the cadherin receptor of NSCs and induce a series of serine-threonine kinase/yes-associated protein mechanotransduction signal pathways, encouraging cellular orientation, neuron differentiation, and adhesion. In SCI rats, implanted PCFS with matched mechanical properties further integrated into the injured spinal cords, inhibited the inflammatory progression and decreased glial and fibrous scar formation. Wall-like protrusions of PCFS drive multiple neuron subtypes formation and even functional neural circuits, suggesting a viable therapeutic strategy for nerve regeneration and functional recovery after SCI.

Neurogenic and angiogenic poly(N-acryloylglycine)-co-(acrylamide)-co-(N-acryloyl-glutamate) hydrogel: preconditioning effect under oxidative stress and use in neuroregeneration

Traumatic injuries, neurodegenerative diseases and oxidative stress serve as the early biomarkers for neuronal damage and impede angiogenesis and subsequently neuronal growth. Considering this, the present work aimed to develop a poly(N-acryloylglycine)-co-(acrylamide)-co-(N-acryloylglutamate) hydrogel [p(NAG-Ac-NAE)] with angiogenesis/neurogenesis properties. As constituents of this polymer modulate their vital role in biological functions, inhibitory neurotransmitter glycine regulates neuronal homeostasis, and glutamatergic signalling regulates angiogenesis. The p(NAG-Ac-NAE) hydrogel is a highly branched, biodegradable and pH-responsive polymer with a very high swelling behavior of 6188%. The mechanical stability (G', 2.3-2.7 kPa) of this polymeric hydrogel is commendable in the differentiation of mature neurons. This hydrogel is biocompatible (as tested in HUVEC cells) and helps to proliferate PC12 cells (152.7 ± 13.7%), whereas it is cytotoxic towards aggressive cancers such as glioblastoma (LN229 cells) and triple negative breast cancer (TNBC; MDA-MB-231 cells) and helps to maintain the healthy cytoskeleton framework structure of primary cortical neurons by facilitating the elongation of the axonal pathway. Furthermore, FACS results revealed that the synthesized hydrogel potentiates neurogenesis by inducing the cell cycle (G0/G1) and arresting the sub-G1 phase by limiting apoptosis. Additionally, RT-PCR results revealed that this hydrogel induced an increased level of HIF-1α expression, providing preconditioning effects towards neuronal cells under oxidative stress by scavenging ROS and initiating neurogenic and angiogenic signalling. This hydrogel further exhibits more pro-angiogenic activities by increasing the expression of VEGF isoforms compared to previously reported hydrogels. In conclusion, the newly synthesized p(NAG-Ac-NAE) hydrogel can be one of the potential neuroregenerative materials for vasculogenesis-assisted neurogenic applications and paramount for the management of neurodegenerative diseases.

Atomic force microscopy correlates mechanical and electrical properties of HepG2 cells with curcumin concentration

Hepatocellular carcinoma (HCC) is a highly prevalent cancer with a significant impact on human health. Curcumin, a natural compound, induces cytoskeletal changes in liver cancer cells and modifies the distribution of lipids, proteins, and polysaccharides on plasma membranes, affecting their mechanical and electrical properties. In this study, we used nanomechanical indentation techniques and Kelvin probe force microscopy (KPFM) based on atomic force microscopy (AFM) to investigate the changes in surface nanomechanical and electrical properties of nuclear and cytoplasmic regions of HepG2 cells in response to increasing curcumin concentrations. CCK-8 assays and flow cytometry results demonstrated time- and concentration-dependent inhibition of HepG2 cell proliferation by curcumin. Increasing curcumin concentration led to an initial increase and then decrease in the mechanical properties of nuclear and cytoplasmic regions of HepG2 cells, represented by the Young's modulus (E), as observed through nanoindentation. KPFM measurements indicated decreasing trends in both cell surface potential and height. Fluorescence microscopy results indicated a positive correlation between curcumin concentration and phosphatidylserine translocation from the inner to the outer membrane, which influenced the electrical properties of HepG2 cells. This study provides valuable insights into curcumin's mechanisms against cancer cells and aids nanoscale evaluation of therapeutic efficacy and drug screening.

Poly(N-acryloylglycine-acrylamide) Hydrogel Mimics the Cellular Microenvironment and Promotes Neurite Growth with Protection from Oxidative Stress

In this work, the glycine-based acryloyl monomer is polymerized to obtain a neurogenic polymeric hydrogel for regenerative applications. The synthesized poly(N-acryloylglycine-acrylamide) [poly(NAG-b-A)] nanohydrogel exhibits high swelling (∼1500%) and is mechanically very stable, biocompatible, and proliferative in nature. The poly(NAG-b-A) nanohydrogel provides a stable 3D extracellular mimetic environment and promotes healthy neurite growth for primary cortical neurons by facilitating cellular adhesion, proliferation, actin filament stabilization, and neuronal differentiation. Furthermore, the protective role of the poly(NAG-b-A) hydrogel for the neurons in oxidative stress conditions is revealed and it is found that it is a clinically relevant material for neuronal regenerative applications, such as for promoting nerve regeneration via GSK3β inhibition. This hydrogel additionally plays an important role in modulating the biological microenvironment, either as an agonist and antagonist or as an antioxidant. Furthermore, it favors the physiological responses and eases the neurite growth efficiency. Additionally, we found out that the conversion of glycine-based acryloyl monomers into their corresponding polymer modulates the mechanical performance, mimics the cellular microenvironment, and accelerates the self-healing capability due to the responsive behavior towards reactive oxygen species (ROS). Thus, the p(NAG-b-A) hydrogel could be a potential candidate to induce neuronal regeneration since it provides a physical cue and significantly boosts neurite outgrowth and also maintains the microtubule integrity in neuronal cells.

Micro 3D Printing Elastomeric IP-PDMS Using Two-Photon Polymerisation: A Comparative Analysis of Mechanical and Feature Resolution Properties

The mechanical properties of two-photon-polymerised (2PP) polymers are highly dependent on the employed printing parameters. In particular, the mechanical features of elastomeric polymers, such as IP-PDMS, are important for cell culture studies as they can influence cell mechanobiological responses. Herein, we employed optical-interferometer-based nanoindentation to characterise two-photon-polymerised structures manufactured with varying laser powers, scan speeds, slicing distances, and hatching distances. The minimum reported effective Young's modulus (YM) was 350 kPa, while the maximum one was 17.8 MPa. In addition, we showed that, on average, immersion in water lowered the YM by 5.4%, a very important point as in the context of cell biology applications, the material must be employed within an aqueous environment. We also developed a printing strategy and performed a scanning electron microscopy morphological characterisation to find the smallest achievable feature size and the maximum length of a double-clamped freestanding beam. The maximum reported length of a printed beam was 70 µm with a minimum width of 1.46 ± 0.11 µm and a thickness of 4.49 ± 0.05 µm. The minimum beam width of 1.03 ± 0.02 µm was achieved for a beam length of 50 µm with a height of 3.00 ± 0.06 µm. In conclusion, the reported investigation of micron-scale two-photon-polymerized 3D IP-PDMS structures featuring tuneable mechanical properties paves the way for the use of this material in several cell biology applications, ranging from fundamental mechanobiology to in vitro disease modelling to tissue engineering.

The Combined Effects of Topography and Stiffness on Neuronal Differentiation and Maturation Using a Hydrogel Platform

Biophysical parameters such as substrate topography and stiffness have been shown independently to elicit profound effects on neuronal differentiation and maturation from neural progenitor cells (NPCs) yet have not been investigated in combination. Here, the effects of various micrograting and stiffness combinations on neuronal differentiation and maturation were investigated using a polyacrylamide and N-acryloyl-6-aminocaproic acid copolymer (PAA-ACA) hydrogel with tunable stiffness. Whole laminin was conjugated onto the PAA-ACA surface indirectly or directly to facilitate long-term mouse and human NPC-derived neuron attachment. Three micrograting dimensions (2-10 µm) were patterned onto gels with varying stiffness (6.1-110.5 kPa) to evaluate the effects of topography, stiffness, and their interaction. The results demonstrate that the extracellular matrix (ECM)-modified PAA-ACA gels support mouse and human neuronal cell attachment throughout the differentiation and maturation stages (14 and 28 days, respectively). The interaction between topography and stiffness is shown to significantly increase the proportion of β-tubulin III (TUJ1) positive neurons and microtubule associated protein-2 (MAP2) positive neurite branching and length. Thus, the effects of topography and stiffness cannot be imparted. These results provide a novel platform for neural mechanobiology studies and emphasize the utility of optimizing numerous biophysical cues for improved neuronal yield in vitro.

Substrate stiffness in nerve cells

Recently, substrate stiffness has been involved in the physiology and pathology of the nervous system. However, the role and function of substrate stiffness remain unclear. Here, we review known effects of substrate stiffness on nerve cell morphology and function in the central and peripheral nervous systems and their involvement in pathology. We hope this review will clarify the research status of substrate stiffness in nerve cells and neurological disorder.

Application of artificial neural networks to predict Young's moduli of cartilage scaffolds: An in-vitro and micromechanical study

In this study, four-phase Gelatin-Polypyrrole-Akermanite-Magnetite scaffolds were fabricated and analyzed using in-vitro tests and numerical simulations. Such scaffolds contained various amounts of Magnetite bioceramics as much as 0, 5, 10, and 15 wt% of Gelatin-Polypyrrole-Akermanite biocomposite. X-ray diffraction analysis and Fourier transform infrared spectroscopy were conducted. Swelling and degradation of the scaffolds were studied by immersing them in phosphate-buffered saline, PBS, solution. Magnetite bioceramics decreased the swelling percent and degradation duration. By immersing scaffolds in simulated body fluid, the highest formation rate of Apatite was observed in the 15 wt% Magnetite samples. The mean pore size was in an acceptable range to provide suitable conditions for cell proliferation. MG-63 cells were cultured on extracts of the scaffolds for 24, 48, and 72 h and their surfaces for 24 h. Cell viabilities and cell morphologies were assessed. Afterward, micromechanical models with spherical and polyhedral voids and artificial neural networks were employed to predict Young's moduli of the scaffolds. Based on the results of finite element analyses, spherical-shaped void models made the best predictions of elastic behavior in the 0, 5 wt% Magnetite scaffolds compared to the experimental data. Results of the simulations and experimental tests for the ten wt% Magnetite samples were well matched in both micromechanical models. In the 15 wt% Magnetite sample, models with polyhedral voids could precisely predict Young's modulus of such scaffolds.

Graphene Microelectrodes for Real-Time Impedance Spectroscopy of Neural Cells

Understanding the changes in the electrochemical properties of neural cells upon exposure to stress factors imparts vital information about the conditions prior to their death. This study presents a graphene-based biosensor for real-time monitoring of N27 rat dopaminergic neural cells which characterizes cell adhesion and cytotoxicity factors through impedance spectroscopy. The aim was to monitor the growth of the entire cell network via a nonmetallic flexible electrode array. Therefore, a water-based graphene solution was formulized as a conductive ink, 3D-printed into a flexible substrate through an electrohydrodynamic approach, resulting in electrodes with a conductivity of 6750 s/m. The presented high-throughput method enabled microscale monitoring of the entire cell network via the design of PDMS-based growth channels. The electrical resistance of the cell network was measured continuously along with their network density, constituting a mean density of 1890 cell/mm² at full cell confluency. The results demonstrate the applicability of the impedance-based sensing of the cell network for rapid screening of the cytotoxic elements, and the real-time effect of UV exposure on dopaminergic neural cells was reported as an immediate application of the device.

Intrinsic Mechanical Cues and Their Impact on Stem Cells and Embryogenesis

Although understanding how soluble cues direct cellular processes revolutionised the study of cell biology in the second half of the 20th century, over the last two decades, new insights into how mechanical cues similarly impact cell fate decisions has gained momentum. During development, extrinsic cues such as fluid flow, shear stress and compressive forces are essential for normal embryogenesis to proceed. Indeed, both adult and embryonic stem cells can respond to applied forces, but they can also detect intrinsic mechanical cues from their surrounding environment, such as the stiffness of the extracellular matrix, which impacts differentiation and morphogenesis. Cells can detect changes in their mechanical environment using cell surface receptors such as integrins and focal adhesions. Moreover, dynamic rearrangements of the cytoskeleton have been identified as a key means by which forces are transmitted from the extracellular matrix to the cell and vice versa. Although we have some understanding of the downstream mechanisms whereby mechanical cues are translated into changes in cell behaviour, many of the signalling pathways remain to be defined. This review discusses the importance of intrinsic mechanical cues on adult cell fate decisions, the emerging roles of cell surface mechano-sensors and the cytoskeleton in enabling cells to sense its microenvironment, and the role of intracellular signalling in translating mechanical cues into transcriptional outputs. In addition, the contribution of mechanical cues to fundamental processes during embryogenesis such as apical constriction and convergent extension is discussed. The continued development of tools to measure the biomechanical properties of soft tissues in vivo is likely to uncover currently underestimated contributions of these cues to adult stem cell fate decisions and embryogenesis, and may inform on regenerative strategies for tissue repair.

Synthesis of an Enzyme-Mediated Reversible Cross-linked Hydrogel for Cell Culture

Detachment of fragile cell types cultured on two-dimensional (2D) surfaces has been shown to be detrimental to their viability. For example, detachment of induced pluripotent stem cell (iPSC)-derived neurons grown in vitro in 2D typically results in loss of neuronal connections and/or cell death. Avoiding cell detachment altogether by changing the properties of the substrate on which the cells are grown is a compelling strategy to maintain cell viability. Here, we present the synthesis of a reversible cross-linked hydrogel that is sufficiently stable for cell culture and differentiation and is cleaved by an external stimulus, facilitating injection. Specifically, hyaluronan (HA) and methylcellulose (MC) were modified with ketone and aldehyde groups, respectively, and a TEV protease-degradable peptide was synthesized via solid-state synthesis and modified at both termini with oxyamine groups to cross-link HA-ketone and MC-aldehyde to produce oxime-cross-linked HA × MC. The HA × MC hydrogel demonstrated good stability, enzyme-sensitive degradation, and cytocompatibility with iPSC-derived neural progenitor cells, laying the framework for broad applicability.

StemBond hydrogels control the mechanical microenvironment for pluripotent stem cells

Studies of mechanical signalling are typically performed by comparing cells cultured on soft and stiff hydrogel-based substrates. However, it is challenging to independently and robustly control both substrate stiffness and extracellular matrix tethering to substrates, making matrix tethering a potentially confounding variable in mechanical signalling investigations. Moreover, unstable matrix tethering can lead to poor cell attachment and weak engagement of cell adhesions. To address this, we developed StemBond hydrogels, a hydrogel in which matrix tethering is robust and can be varied independently of stiffness. We validate StemBond hydrogels by showing that they provide an optimal system for culturing mouse and human pluripotent stem cells. We further show how soft StemBond hydrogels modulate stem cell function, partly through stiffness-sensitive ERK signalling. Our findings underline how substrate mechanics impact mechanosensitive signalling pathways regulating self-renewal and differentiation, indicating that optimising the complete mechanical microenvironment will offer greater control over stem cell fate specification.

Influence of dityrosine nanotubes on the expression of dopamine and differentiation in neural cells

In this study, we report the synthesis of self-assembled dityrosine nanotubes as a biologically functional scaffold and their interactions with neural cells. Quantum chemical methods were used to determine the forces involved in the self-assembly process. The physicochemical properties of the nanostructures relevant to their potential as bioactive scaffolds were characterized. The morphology, secondary structure, crystallinity, mechanical properties, and thermal characteristics of YY nanotubes were analyzed. The influence of these nanotubes as scaffolds for neural cells was studied in vitro to understand their effects on cell proliferation, morphology, and gene expression. The scanning electron microscopy and fluorescence confocal microscopy demonstrated the feasibility of nanotube scaffolds for enhanced adhesion to rat and human neural cells (PC12 and SH-SY5Y). Preliminary ELISA and qPCR analyses demonstrate the upregulation of dopamine synthesis and genes involved in dopamine expression and differentiation. The expression levels of DβH, AADC, VMAT2 and MAOA in SH-SY5Y cells cultured on the nanotube scaffolds for 7 days were elevated in comparison to the control cells.

Compounded topographical and physicochemical cueing by micro-engineered chitosan substrates on rat dorsal root ganglion neurons and human mesenchymal stem cells

Given the intertwined physicochemical effects exerted in vivo by both natural and synthetic (e.g., biomaterial) interfaces on adhering cells, the evaluation of structure-function relationships governing cellular response to micro-engineered surfaces for applications in neuronal tissue engineering requires the use of in vitro testing platforms which consist of a clinically translatable material with tunable physiochemical properties. In this work, we micro-engineered chitosan substrates with arrays of parallel channels with variable width (20 and 60 μm). A citric acid (CA)-based crosslinking approach was used to provide an additional level of synergistic cueing on adhering cells by regulating the chitosan substrate's stiffness. Morphological and physicochemical characterization was conducted to unveil the structure-function relationships which govern the activity of rat dorsal root ganglion neurons (DRGs) and human mesenchymal stem cells (hMSCs), ultimately singling out the key role of microtopography, roughness and substrate's stiffness. While substrate's stiffness predominantly affected hMSC spreading, the modulation of the channels' design affected the neuronal architecture's complexity and guided the morphological transition of hMSCs. Finally, the combined analysis of tubulin expression and cell morphology allowed us to cast new light on the predominant role of the microtopography over substrate's stiffness in the process of hMSCs neurogenic differentiation.

Polyaniline-polycaprolactone fibers for neural applications: Electroconductivity enhanced by pseudo-doping

Replenishing neurons in patients with neurodegenerative diseases is one of the ultimate therapies for these progressive, debilitating and fatal diseases. Electrical stimulation can improve neuron stem cell differentiation but requires a reliable nanopatterned electroconductive substrate. Potential candidate substrates are polycaprolactone (PCL) - polyaniline:camphorsulfonic acid (PANI:CSA) nanofibers, but their nanobiophysical properties need to be finetuned. The present study investigates the use of the pseudo-doping effect on the optimization of the electroconductivity of these polyaniline-based electrospun nanofibers. This was performed by developing a new solvent system that comprises a mixture of hexafluoropropanol (HFP) and trifluoroethanol (TFE). For the first time, an electroconductivity so high as 0.2 S cm^-1 was obtained for, obtained from a TFE:HFP 50/50 vol% solution, while maintaining fiber biocompatibility. The physicochemical mechanisms behind these changes were studied. The results suggest HFP promotes changes on PANI chains conformations through pseudo-doping, leading to the observed enhancement in electroconductivity. The consequences of such change in the nanofabrication of PCL-PANI fibers include an increase in fiber diameter (373 ± 172 nm), a decrease in contact angle (42 ± 3°) and a decrease in Young modulus (1.6 ± 0.5 MPa), making these fibers interesting candidates for neural tissue engineering. Electrical stimulation of differentiating neural stem cells was performed using AC electrical current. Positive effects on cell alignment and gene expression (DCX, MAP2) are observed. The novel optimized platform shows promising applications for (1) building in vitro platforms for drug screening, (2) interfaces for deep-brain electrodes; and (3) fully grown and functional neurons transplantation.

Fabrication of a biomimetic spinal cord tissue construct with heterogenous mechanical properties using intrascaffold cell assembly

In tissue engineering studies, scaffolds play a very important role in offering both physical and chemical cues for cell growth and tissue regeneration. However, in some cases, tissue regeneration requires scaffolds with high mechanical properties (e.g., bone and cartilage), while cells need a soft mechanical microenvironment. In this study, to mimic the heterogenous mechanical properties of a spinal cord tissue, a biomimetic rat tissue construct is fabricated. A collagen-coated poly(lactic-co-glycolic acid) scaffold is manufactured using thermally induced phase separation casting. Primary rat neural cells (P01 Wistar rat cortex) with soft hydrogels are later printed within the scaffold using an image-guided intrascaffold cell assembly technique. The scaffolds have unidirectional microporous structure with parallel axial macrochannels (260 ± 4 µm in diameter). Scaffolds showed mechanical properties similar to rat spine (ultimate tensile strength: 0.085 MPa, Young's modulus [stretch]: 0.31 MPa). The bioink composed of gelatin/alginate/fibrinogen is precisely printed into the macrochannels and showed mechanical properties suitable for neural cells (Young's modulus [compressive]: 3.814 kPa). Scaffold interface, cell viability, and immunostaining analyses show uniform distribution of stable, healthy, and elongated neural cells and neurites over 14 culture days in vitro. The results demonstrated that this method can serve as a valuable tool to aid manufacturing of tissue constructs requiring heterogenous mechanical properties for complex cell and/or biomolecule assembly.

Gelator length precisely tunes supramolecular hydrogel stiffness and neuronal phenotype in 3D culture

The brain is one of the softest tissues in the body with storage moduli (G') that range from hundreds to thousands of pascals (Pa) depending upon the anatomic region. Furthermore, pathological processes such as injury, aging and disease can cause subtle changes in the mechanical properties throughout the central nervous system. However, these changes in mechanical properties lie within an extremely narrow range of moduli and there is great interest in understanding their effect on neuron biology. We report here the design of supramolecular hydrogels based on anionic peptide amphiphile nanofibers using oligo-L-lysines of different molecular lengths to precisely tune gel stiffness over the range of interest and found that G' increases by 10.5 Pa for each additional lysine monomer in the oligo-L-lysine chain. We found that small changes in storage modulus on the order of 70 Pa significantly affect survival, neurite growth and tyrosine hydroxylase-positive population in dopaminergic neurons derived from induced pluripotent stem cells. The work reported here offers a strategy to tune mechanical stiffness of hydrogels for use in 3D neuronal cell cultures and transplantation matrices for neural regeneration.

Three-dimensional differentiation of human pluripotent stem cell-derived neural precursor cells using tailored porous polymer scaffolds

This study investigates the utility of a tailored poly(ethylene glycol) diacrylate-crosslinked porous polymeric tissue engineering scaffold, with mechanical properties specifically optimised to be comparable to that of mammalian brain tissue for 3D human neural cell culture. Results obtained here demonstrate the attachment, proliferation and terminal differentiation of both human induced pluripotent stem cell- and embryonic stem cell-derived neural precursor cells (hPSC-NPCs) throughout the interconnected porous network within laminin-coated scaffolds. Phenotypic data and functional analyses are presented demonstrating that this material supports terminal in vitro neural differentiation of hPSC-NPCs to a mixed population of viable neuronal and glial cells for periods of up to 49 days. This is evidenced by the upregulation of TUBB3, MAP2, SYP and GFAP gene expression, as well as the presence of the proteins βIII-TUBULIN, NEUN, MAP2 and GFAP. Functional maturity of neural cells following 49 days 3D differentiation culture was tested via measurement of intracellular calcium. These analyses revealed spontaneously active, synchronous and rhythmic calcium flux, as well as response to the neurotransmitter glutamate. This tailored construct has potential application as an improved in vitro human neurogenesis model with utility in platform drug discovery programs. STATEMENT OF SIGNIFICANCE: The interconnected porosity of polyHIPE scaffolds exhibits the ability to support three-dimensional neural cell network formation due to limited resistance to cellular migration and re-organisation. The previously developed scaffold material displays mechanical properties similar to that of the mammalian brain. This research also employs the utility of pluripotent stem cell-derived neural cells which are of greater clinical relevance than primary neural cell lines. This scaffold material has future potential in better mimicking three-dimensional neural networks found in the human brain and may result in improved in vitro models for disease modelling and drug screening applications.

Matrix stiffness-regulated cellular functions under different dimensionalities

The microenvironments that cells encounter with in vitro.

Substrate stiffness affects the morphology and gene expression of epidermal neural crest stem cells in a short term culture

According to the intrinsic plasticity of stem cells, controlling their fate is a critical issue in cell-based therapies. Recently, a growing body of evidence has suggested that substrate stiffness can affect the fate decisions of various stem cells. Epidermal neural crest stem cells as one of the main neural crest cell derivatives hold great promise for cell therapies due to presenting a high level of plasticity. This study was conducted to define the influence of substrate stiffness on the lineage commitment of these cells. Here, four different polyacrylamide hydrogels with elastic modulus in the range of 0.7-30 kPa were synthesized and coated with collagen and stem cells were seeded on them for 24 hr. The obtained data showed that cells can attach faster to hydrogels compared with culture plate and cells on <1 kPa stiffness show more neuronal-like morphology as they presented several branches and extended longer neurites over time. Moreover, the transcription of actin downregulated on all hydrogels, while the expression of Nestin, Tubulin, and PDGFR-α increased on all of them and SOX-10 and doublecortin gene expression were higher only on <1 kPa. Also, it was revealed that soft hydrogels can enhance the expression of glial cell line-derived neurotrophic factor, neurotrophin-3, and vascular endothelial growth factor in these stem cells. On the basis of the results, these cells can respond to the substrate stiffness in the short term culture and soft hydrogels can alter their morphology and gene expression. These findings suggested that employing proper substrate stiffness might result in cells with more natural profiles similar to the nervous system and superior usefulness in therapeutic applications.

Promoting neural cell proliferation and differentiation by incorporating lignin into electrospun poly(vinyl alcohol) and poly(glycerol sebacate) fibers

Electrospinning of natural and synthetic polymers open a new practical approach to tissue engineering by producing fibers. In this study, aligned electrospun poly(vinyl alcohol) (PVA)-poly(glycerol sebacate) (PGS) fibers with various percentages of lignin (0, 1, 3, and 5%wt) fabricated for nerve tissue engineering. The effect of the different amount of lignin on the morphology and diameter of the fibers was investigated by scanning electron microscopy (SEM). The physicochemical properties of fibers were studied using FTIR, tensile strain, contact angle, water uptake, and degradation test. MTT assay and SEM were employed to evaluate PC12 cell proliferation and adhesion, respectively. Immunocytochemistry and gene expression were utilized to study how the lignin affected on cell differentiation. The results revealed the smooth with a uniform diameter of the fabricated fibers, and the increased amount of lignin reduced the fiber diameter from 530 to 370 nm. The modulus of elasticity increased from 0.1 to 0.4 MPa by increasing the lignin percentage. The PC12 cell culture indicated that the lignin enhanced cell proliferation. The mRNA expression level for Gfap, β-Tub III, and Map2 and immunocytochemistry (Map2) revealed the positive effect of lignin on neural cell differentiation. Finally, the results suggest PVA-PGS/5% lignin as a promising material for nerve tissue engineering.

Differential neural cell adhesion and neurite outgrowth on carbon nanotube and graphene reinforced polymeric scaffolds

Carbon nanomaterials, such as graphene nanoplatelets (GNPs) and multiwalled carbon nanotubes (MWCNTs) are potential candidates in a large number of biomedical applications. The present study investigates the effect of the difference in morphology of these materials on neural cell regeneration on a biodegradable scaffold. Electrical conductivities of all the hybrid scaffolds are found to be in between that of MWCNT/chitosan scaffold (highest-conductivity) and GNP/chitosan scaffold (lowest-conductivity). While, hybrid scaffolds show improvement in elastic modulus and ultimate tensile strength over MWCNT/chitosan and GNP/chitosan scaffolds. The protein adsorption isotherms of bovine serum albumin (BSA) show greater equilibrium constant (Keq) on GNP/chitosan composites as compared to MWCNT/chitosan composites, proving more potential for cell adhesion in the former. Interactions of HT-22 hippocampal neurons with MWCNT/chitosan, GNP/chitosan and various MWCNT/GNP hybrid chitosan matrices prove cytocompatibility. The neurons acquire elongated geometry on the MWCNT/chitosan scaffold, while GNP reinforcement drives the neurons to spread cellular processes radially.

Functional and Sustainable 3D Human Neural Network Models from Pluripotent Stem Cells

Three-dimensional in vitro cell culture models, particularly for the central nervous system, allow for the exploration of mechanisms of organ development, cellular interactions, and disease progression within defined environments. Here we describe the development and characterization of three-dimensional tissue models that promote the differentiation and long-term survival of functional neural networks. These tissue cultures show diverse cell populations including neurons and glial cells (astrocytes) interacting in 3D with spontaneous neural activity confirmed through electrophysiological recordings and calcium imaging over at least 8 months. This approach allows for the direct integration of pluripotent stem cells into the 3D construct bypassing early neural differentiation steps (embryoid bodies and neural rosettes), which streamlines the process while also providing a system that can be manipulated to support a variety of experimental applications. This tissue model has been tested in stem cells derived from healthy individuals as well as Alzheimer's and Parkinson's disease patients, with similar growth and gene expression responses indicating potential use in the modeling of disease states related to neurodegenerative diseases.

Initial Priming on Soft Substrates Enhances Subsequent Topography-Induced Neuronal Differentiation in ESCs but Not in MSCs

Differentiation of stem cells into neurogenic lineage is of great interest for treatment of neurodegenerative diseases. While the role of chemical cues in regulating stem cell fate is well appreciated, the identification of physical cues has revolutionized the field of tissue engineering leading to development of scaffolds encoding one or more physical cues for inducing stem cell differentiation. In this study, using human mesenchymal stem cells (hMSCs) and mouse embryonic stem cells (mESCs), we have tested if stiffness and topography can be collectively tuned for inducing neuronal differentiation by culturing these cells on polyacrylamide hydrogels of varying stiffness (5, 10, and 20 kPa) containing rectangular grooves (10, 15, and 25 μm in width). While hMSCs maximally elongate and express neuronal markers on soft 5 kPa gels containing 10/15 μm grooves, single mESCs are unable to sense topographical features when cultured directly on grooved gels. However, this inability to sense topography is rescued by priming mESCs initially on soft 1 kPa flat gels and then replating these cells onto the grooved gels. Compared to direct culture on the grooved gels, this sequential adaptation increases both viability as well as neuronal differentiation. However, this two-step process does not enhance neuronal marker expression in hMSCs. In addition to highlighting important differences between hMSCs and mESCs in their responsiveness to physical cues, our study suggests that conditioning on soft substrates is essential for inducing topography-mediated neuronal differentiation in mESCs.

Bundle Gel Fibers with a Tunable Microenvironment for in Vitro Neuron Cell Guiding

As scaffolds for neuron cell guiding in vitro, gel fibers with a bundle structure, comprising multiple microfibrils, were fabricated using a microfluidic device system by casting a phase-separating polymer blend solution comprising hydroxypropyl cellulose (HPC) and sodium alginate (Na-Alg). The topology and stiffness of the obtained bundle gel fibers depended on their microstructure derived by the polymer blend ratio of HPC and Na-Alg. High concentrations of Na-Alg led to the formation of small microfibrils in a one-bundle gel fiber and stiff characteristics. These bundle gel fibers permitted for the elongation of the neuron cells along their axon orientation with the long axis of fibers. In addition, human-induced pluripotent-stem-cell-derived dopaminergic neuron progenitor cells were differentiated into neuronal cells on the bundle gels. The bundle gel fibers demonstrated an enormous potential as cell culture scaffold materials with an optimal microenvironment for guiding neuron cells.

Highly Stretchable, Compliant, Polymeric Microelectrode Arrays for In Vivo Electrophysiological Interfacing

Polymeric microelectrode arrays (MEAs) are emerging as a new generation of biointegrated microelectrodes to transduce original electrochemical signals in living tissues to external electrical circuits, and vice versa. So far, the challenge of stretchable polymeric MEAs lies in the competition between high stretchability and good electrode-substrate adhesion. The larger the stretchability, the easier the delamination of electrodes from the substrate due to the mismatch in their Young's modulus. In this work, polypyrrole (PPy) electrode materials are designed, with PPy nanowires integrated on the high conductive PPy electrode arrays. By utilizing this electrode material, for the first time, stretchable polymeric MEAs are fabricated with both high stretchability (≈100%) and good electrode-substrate adhesion (1.9 MPa). In addition, low Young's modulus (450 kPa), excellent recycling stability (10 000 cycles of stretch), and high conductivity of the MEAs are also achieved. As a proof of concept, the as-prepared polymeric MEAs are successfully used for conformally recording the electrocorticograph signals from rats in normal and epileptic states, respectively. Further, these polymeric MEAs are also successful in stimulating the ischiadic nerve of the rat. This strategy provides a new perspective to the highly stretchable and mechanically stable polymeric MEAs, which are vital for compliant neural electrodes.

Scaffolds for 3D in vitro culture of neural lineage cells

Understanding how neurodegenerative disorders develop is not only a key challenge for researchers but also for the wider society, given the rapidly aging populations in developed countries. Advances in this field require new tools with which to recreate neural tissue in vitro and produce realistic disease models. This in turn requires robust and reliable systems for performing 3D in vitro culture of neural lineage cells. This review provides a state of the art update on three-dimensional culture systems for in vitro development of neural tissue, employing a wide range of scaffold types including hydrogels, solid porous polymers, fibrous materials and decellularised tissues as well as microfluidic devices and lab-on-a-chip systems. To provide some context with in vivo development of the central nervous system (CNS), we also provide a brief overview of the neural stem cell niche, neural development and neural differentiation in vitro. We conclude with a discussion of future directions for this exciting and important field of biomaterials research.

STATEMENT OF SIGNIFICANCE

Neurodegenerative diseases, including dementia, Parkinson's and Alzheimer's diseases and motor neuron diseases, are a major societal challenge for aging populations. Understanding these conditions and developing therapies against them will require the development of new physical models of healthy and diseased neural tissue. Cellular models resembling neural tissue can be cultured in the laboratory with the help of 3D scaffolds - materials that allow the organization of neural cells into tissue-like structures. This review presents recent work on the development of different types of scaffolds for the 3D culture of neural lineage cells and the generation of functioning neural-like tissue. These in vitro culture systems are enabling the development of new approaches for modelling and tackling diseases of the brain and CNS.

Characterization of SH-SY5Y human neuroblastoma cell growth over glass and SU-8 substrates

The physical properties of substrates can have profound effects on the structure and function of cultured cells. In this study, we aimed to examine the viability, adherence, and morphological and functional variations between SH-SY5Y human neuroblastoma cells cultured on SU-8 surfaces compared with control surfaces composed of borosilicate glass, which are routinely used for cell culture. The SU-8 polymer has been extensively studied for its biocompatibility, but there has been little investigation into the characteristic differences between cells cultured on SU-8 when compared with glass. SH-SY5Y cells were cultured within polydimethylsiloxane wells on both SU-8 and glass substrates for up to 72 h after which flow cytometry and enzyme-linked immunosorbent assay analysis was performed to examine cell viability and neurotoxicity. Immunocytochemistry was also performed to analyze the morphological and functional characteristics of the cells. Atomic force microscopy was performed to measure surface roughness and to map cell-substrate interactions. Nanoindentation testing was used to characterize the mechanical properties of polymer surface. Results showed that SH-SY5Y cells grown on SU-8 have significantly improved viability and increased morphological and functional characteristics of neurodevelopment. The results from this study suggest that the mechanical properties of the polymer are optimal for the study of cultured cell lines, which could account for the increased viability, adherence, and morphological and functional characteristics of neurodevelopment. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2129-2138, 2017.

The rotation of mouse myoblast nuclei is dependent on substrate elasticity

The complex interplay of biochemical signaling and mechanical traction forces regulate the position of cellular nuclei. Although the phenomenon of nuclear rotation has been observed for many years, the influence of substrate elasticity was unknown. We discovered another layer of complexity to this phenomenon: nuclear rotation is dependent on substrate elasticity. Nuclear rotation is drastically reduced on physiologically relevant stiffnesses. Here, we studied nuclear rotation in mouse C2C12 myoblasts cultured on soft substrates designed to mimic resting tissue (∼26 kPa) and on hard glass substrates. We examined the roles of the actin and microtubule cytoskeleton on the presence and dynamics of nuclear rotation in these two different microenvironments. We demonstrated the clear dependence of nuclear rotation dynamics on matrix stiffness. These results will have important implications for the design of future studies of nuclear rotation and our understanding of the phenomenon as a whole. Unnaturally, hard substrates do not only fail to mimic the in vivo microenvironment, but can also induce cellular processes that would not normally occur in the natural cellular environment.

Tailored emulsion-templated porous polymer scaffolds for iPSC-derived human neural precursor cell culture

The work here describes the synthesis of tailor-made, porous, polymeric materials with elastic moduli in the range associated with mammalian brain tissue (0.1–24 kPa).

Effects of methotrexate on the viscoelastic properties of single cells probed by atomic force microscopy

Methotrexate is a commonly used anti-cancer chemotherapy drug. Cellular mechanical properties are fundamental parameters that reflect the physiological state of a cell. However, so far the role of cellular mechanical properties in the actions of methotrexate is still unclear. In recent years, probing the behaviors of single cells with the use of atomic force microscopy (AFM) has contributed much to the field of cell biomechanics. In this work, with the use of AFM, the effects of methotrexate on the viscoelastic properties of four types of cells were quantitatively investigated. The inhibitory and cytotoxic effects of methotrexate on the proliferation of cells were observed by optical and fluorescence microscopy. AFM indenting was used to measure the changes of cellular viscoelastic properties (Young's modulus and relaxation time) by using both conical tip and spherical tip, quantitatively showing that the stimulation of methotrexate resulted in a significant decrease of both cellular Young's modulus and relaxation times. The morphological changes of cells induced by methotrexate were visualized by AFM imaging. The study improves our understanding of methotrexate action and offers a novel way to quantify drug actions at the single-cell level by measuring cellular viscoelastic properties, which may have potential impacts on developing label-free methods for drug evaluation.

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.

Engineering Embryonic Stem Cell Microenvironments for Tailored Cellular Differentiation

The rapid progress of embryonic stem cell (ESCs) research offers great promise for drug discovery, tissue engineering, and regenerative medicine. However, a major limitation in translation of ESCs technology to pharmaceutical and clinical applications is how to induce their differentiation into tailored lineage commitment with satisfactory efficiency. Many studies indicate that this lineage commitment is precisely controlled by the ESC microenvironment in vivo. Engineering and biomaterial-based approaches to recreate a biomimetic cellular microenvironment provide valuable strategies for directing ESCs differentiation to specific lineages in vitro. In this review, we summarize and examine the recent advances in application of engineering and biomaterial-based approaches to control ESC differentiation. We focus on physical strategies (e.g., geometrical constraint, mechanical stimulation, extracellular matrix (ECM) stiffness, and topography) and biochemical approaches (e.g., genetic engineering, soluble bioactive factors, coculture, and synthetic small molecules), and highlight the three-dimensional (3D) hydrogel-based microenvironment for directed ESC differentiation. Finally, future perspectives in ESCs engineering are provided for the subsequent advancement of this promising research direction.

Response to di-functionalized hyaluronic acid with orthogonal chemistry grafting at independent modification sites in rodent models of neural differentiation and spinal cord injury

Hyaluronic acid functionalized with two orthogonal chemistries at different targets expedites neural maturation in vitro, while reducing inflammation in vivo.