The Functional Response of Mesenchymal Stem Cells to Electron-Beam Patterned Elastomeric Surfaces Presenting Micrometer to Nanoscale Heterogeneous Rigidity

Modulated nanowire scaffold for highly efficient differentiation of mesenchymal stem cells

Background

Nanotopographical cues play a critical role as drivers of mesenchymal stem cell differentiation. Nanowire scaffolds, in this regard, provide unique and adaptable nanostructured surfaces with focal points for adhesion and with elastic properties determined by nanowire stiffness.

Results

We show that a scaffold of nanowires, which are remotely actuated by a magnetic field, mechanically stimulates mesenchymal stem cells. Osteopontin, a marker of osteogenesis onset, was expressed after cells were cultured for 1 week on top of the scaffold. Applying a magnetic field significantly boosted differentiation due to mechanical stimulation of the cells by the active deflection of the nanowire tips. The onset of differentiation was reduced to 2 days of culture based on the upregulation of several osteogenesis markers. Moreover, this was observed in the absence of any external differentiation factors.

Conclusions

The magneto-mechanically modulated nanosurface enhanced the osteogenic differentiation capabilities of mesenchymal stem cells, and it provides a customizable tool for stem cell research and tissue engineering.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01488-5.

Cells on Hydrogels with Micron-Scaled Stiffness Patterns Demonstrate Local Stiffness Sensing

Cell rigidity sensing-a basic cellular process allowing cells to adapt to mechanical cues-involves cell capabilities exerting force on the extracellular environment. In vivo, cells are exposed to multi-scaled heterogeneities in the mechanical properties of the surroundings. Here, we investigate whether cells are able to sense micron-scaled stiffness textures by measuring the forces they transmit to the extracellular matrix. To this end, we propose an efficient photochemistry of polyacrylamide hydrogels to design micron-scale stiffness patterns with kPa/µm gradients. Additionally, we propose an original protocol for the surface coating of adhesion proteins, which allows tuning the surface density from fully coupled to fully independent of the stiffness pattern. This evidences that cells pull on their surroundings by adjusting the level of stress to the micron-scaled stiffness. This conclusion was achieved through improvements in the traction force microscopy technique, e.g., adapting to substrates with a non-uniform stiffness and achieving a submicron resolution thanks to the implementation of a pyramidal optical flow algorithm. These developments provide tools for enhancing the current understanding of the contribution of stiffness alterations in many pathologies, including cancer.

Modelling the central nervous system: tissue engineering of the cellular microenvironment

With the increasing prevalence of neurodegenerative diseases, improved models of the central nervous system (CNS) will improve our understanding of neurophysiology and pathogenesis, whilst enabling exploration of novel therapeutics. Studies of brain physiology have largely been carried out using in vivo models, ex vivo brain slices or primary cell culture from rodents. Whilst these models have provided great insight into complex interactions between brain cell types, key differences remain between human and rodent brains, such as degree of cortical complexity. Unfortunately, comparative models of human brain tissue are lacking. The development of induced Pluripotent Stem Cells (iPSCs) has accelerated advancement within the field of in vitro tissue modelling. However, despite generating accurate cellular representations of cortical development and disease, two-dimensional (2D) iPSC-derived cultures lack an entire dimension of environmental information on structure, migration, polarity, neuronal circuitry and spatiotemporal organisation of cells. As such, researchers look to tissue engineering in order to develop advanced biomaterials and culture systems capable of providing necessary cues for guiding cell fates, to construct in vitro model systems with increased biological relevance. This review highlights experimental methods for engineering of in vitro culture systems to recapitulate the complexity of the CNS with consideration given to previously unexploited biophysical cues within the cellular microenvironment.

Electron beam lithography for direct patterning of MoS2 on PDMS substrates

Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography. In this work we show a fabrication technique based on direct electron beam lithography (EBL) on MoS₂ on polydimethylsiloxane (PDMS) substrates. This easy and fast method takes advantage of the interaction of the electron beam with the PDMS, which at high enough doses leads to cross-linking and shrinking of the polymer. At the same time, the adhesion of MoS₂ to PDMS is enhanced in the exposed regions. The EBL acceleration voltages and doses are optimized in order to fabricate well-defined microstructures, which can be subsequently transferred to either a flexible or a rigid substrate, to obtain the negative of the exposed image. The reported procedure greatly simplifies the fabrication process and reduces the number of steps compared to standard lithography and etching. As no additional polymer, such as polymethyl methacrylate (PMMA) or photoresists, are used during the whole process the resulting samples are free of residues.

Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Surface Roughness and Substrate Stiffness Synergize To Drive Cellular Mechanoresponse

Material surface topographic features have been shown to be crucial for tissue regeneration and surface treatment of implanted devices. Many biomaterials were investigated with respect to the response of cells on surface roughness. However, some conclusions even conflicted with each other due to the unclear interplay of surface topographic features and substrate elastic features as well as the lack of mechanistic studies. Herein, wide-scale surface roughness gradient hydrogels, integrating the surface roughness from nanoscale to microscale with controllable stiffness, were developed via soft lithography with precise surface morphology. Based on this promising platform, we systematically studied the mechanosensitive response of human mesenchymal stem cells (MSCs) to a broad range of roughnesses (200 nm to 1.2 μm for R_q) and different substrate stiffnesses. We observed that MSCs responded to surface roughness in a stiffness-dependent manner by reorganizing the surface hierarchical structure. Surprisingly, the cellular mechanoresponse and osteogenesis were obviously enhanced on very soft hydrogels (3.8 kPa) with high surface roughness, which was comparable to or even better than that on smooth stiff substrates. These findings extend our understanding of the interactions between cells and biomaterials, highlighting an effective noninvasive approach to regulate stem cell fate via synergetic physical cues.

The Regulatory Functionality of Exosomes Derived from hUMSCs in 3D Culture for Alzheimer's Disease Therapy

Reducing amyloid-β (Aβ) accumulation could be a potential therapeutic approach for Alzheimer's disease (AD). Particular functional biomolecules in exosomes vested by the microenvironment in which the original cells resided can be transferred to recipient cells to improve pathological conditions. However, there are few reports addressing whether exosomes derived from cells cultured on scaffolds with varying dimension can reduce Aβ deposition or ameliorate cognitive decline for AD therapy. Herein, both 3D graphene scaffold and 2D graphene film are used as the matrix for human umbilical cord mesenchymal stem cell culture, from which the supernatants are obtained to isolate exosomes. The levels of 195 kinds of miRNAs and proteins, including neprilysin, insulin-degrading enzyme and heat shock protein 70, in 3D-cultured exosomes (3D-Exo) are dramatically different from those obtained from 2D culture. Hence, 3D-Exo could up-regulate the expression of α-secretase and down-regulate the β-secretase to reduce Aβ production in both AD pathology cells and transgenic mice, through their special cargo. With rescuing Aβ pathology, 3D-Exo exerts enhanced therapeutic effects on ameliorating the memory and cognitive deficits in AD mice. These findings provide a novel clinical application for scaffold materials and functional exosomes derived from stem cells.

Hierarchical Hydrogel Composite Interfaces with Robust Mechanical Properties for Biomedical Applications

Cells sense and respond to a wide range of external signals, including chemical signals, topography, and interface mechanics, via interactions with the extracellular matrix (ECM), triggering the regulation of behavior and function. The ECM can be considered a hierarchical multiphase porous matrix with various components. Highly porous hydrogel-based biomaterials can mimic the critical ECM properties, to provide mechanical support for tissues and to regulate cellular behaviors, such as adhesion, proliferation, and differentiation. Herein, based on micro/nanoscale-topography-coupled mechanical action, recent advances in the fabrication and application of hydrogel composites with tunable mechanical properties and topography in biomedicine are summarized. In particular, recent findings showing that hydrogels with specifically designed structures not only influence a range of cellular processes and fit the needs of engineered tissues but also have pharmacological effects are emphasized.

Bio-instructive materials for musculoskeletal regeneration

The prevalence and cost of disorders affecting the musculoskeletal system are predicted to rise significantly in the coming years due to the aging global population and the increase of associated risk factors. Despite being the second largest cause of disability, the clinical options for therapeutic intervention remain limited. The clinical translation of cell-based therapies for the treatment of musculoskeletal disorders faces many challenges including maintenance of cell survival in the harsh in vivo environment and the lack of control over regulating cell phenotype upon implantation. In order to address these challenges, the development of bio-instructive materials to modulate cell behavior has taken center stage as a strategy to increase the therapeutic potential of various cell populations. However, the determination of the necessary cues for a specific application and how these signals should be presented from a biomaterial remains elusive. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues. There is a particular emphasis on emerging efforts such as the engineering of immunomodulatory and antibacterial materials, as well as the incorporation of these strategies into biofabrication and organ-on-a-chip approaches. STATEMENT OF SIGNIFICANCE: Disorders affecting the musculoskeletal system affect individuals across the lifespan and have a profound effect on mobility and quality of life. While small defects in many tissues can heal successfully, larger defects are often unable to heal or instead heal with inferior quality fibrous tissue and require clinical intervention. Cell-based therapies are a promising option for clinical translation, yet challenges related to maintaining cell survival and instructing cell phenotype upon implantation have limited the success of this approach. Bio-instructive materials provide an exciting opportunity to modulate cell behavior and enhance the efficacy of cell-based approaches for musculoskeletal repair. However, the identification of critical instructive cues and how to present these stimuli is a focus of intense investigation. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues, while also considering exciting progress in the engineering of immunomodulatory and antibacterial materials.

Boron Nitride Nanotube Addition Enhances the Crystallinity and Cytocompatibility of PVDF-TrFE

Analysis of the cellular response to piezoelectric materials has been driven by the discovery that many tissue components exhibit piezoelectric behavior ex vivo. In particular, polyvinylidene fluoride and the trifluoroethylene co-polymer (PVDF-TrFE) have been identified as promising piezo and ferroelectric materials with applications in energy harvesting and biosensor devices. Critically, the modulation of the structural and crystalline properties of PVDF-TrFE through annealing processes and the addition of particulate or fibrous fillers has been shown to modulate significantly the materials electromechanical properties. In this study, a PVDF-TrFE/boron-nitride nanotube composite was evaluated by modulated differential scanning calorimetry to assess the effects of boron nitride nanotube addition and thermal annealing on the composite structure and crystal behavior. An increased beta crystal formation [f(β) = 0.71] was observed following PVDF-TrFE annealing at the first crystallization temperature of 120°C. In addition, the inclusion of boron nitride nanotubes significantly increased the crystal formation behavior [f(β) = 0.76] and the mechanical properties of the material. Finally, it was observed that BNNT incorporation enhance the adherence and proliferation of human tenocyte cells in vitro.

Controlled Nanoscale Topographies for Osteogenic Differentiation of Mesenchymal Stem Cells

Nanotopography with length scales of the order of extracellular matrix elements offers the possibility of regulating cell behavior. Investigation of the impact of nanotopography on cell response has been limited by the inability to precisely control geometries, especially at high spatial resolutions and across practically large areas. In this paper, we demonstrate well-controlled and periodic nanopillar arrays of silicon and investigate their impact on osteogenic differentiation of human mesenchymal stem cells (hMSCs). Silicon nanopillar arrays with critical dimensions in the range of 40-200 nm, exhibiting standard deviations below 15% across full wafers, were realized using the self-assembly of block copolymer colloids. Immunofluorescence and quantitative polymerase chain reaction measurements reveal clear dependence of osteogenic differentiation of hMSCs on the diameter and periodicity of the arrays. Further, the differentiation of hMSCs was found to be dependent on the age of the donor. While osteoblastic differentiation was found to be promoted by the pillars with larger diameters and heights independent of donor age, they were found to be different for different spacings. Pillar arrays with smaller pitch promoted differentiation from a young donor, while a larger spacing promoted those of an old donor. These findings can contribute for the development of personalized treatments of bone diseases, namely, novel implant nanostructuring depending on patient age.

Anisotropic Nanoscale Presentation of Cell Adhesion Ligand Enhances the Recruitment of Diverse Integrins in Adhesion Structures and Mechanosensing‐Dependent Differentiation of Stem Cells

The nanoscale anisotropic patterns of bioactive ligands in the extracellular matrix regulate cell adhesion behaviors. However, the mechanisms of such regulation remain unclear. Here, RGD‐bearing gold nanorods (AuNRs) are conjugated with different aspect ratios (ARs, from 1 to 7) on cell culture substrates to decouple the effect of nanoscale anisotropic presentation of cell adhesive RGD peptides on cell adhesion. Compared with AuNRs with small ARs, AuNRs with large ARs significantly promote cell spreading, the alignment of the basal cytoskeletal structure, and nanopodia attachment. Furthermore, both ‐β3 and ‐β1 class integrins are recruited to AuNRs with large ARs, thereby promoting the development of focal adhesion toward fibrillar adhesion, whereas the recruitment of diverse integrins and the development of cell adhesion structures are hindered by small ARs AuNRs. The anisotropic presentation of ligands by large AR AuNRs better activates mechanotransduction signaling molecules. These findings are confirmed both in vitro and in vivo. Hence the enhanced mechanotransduction promotes osteogenic differentiation in stem cells. These findings demonstrate the potential use of well‐controlled synthetic nanoplatforms to unravel the fundamental mechanisms of cell adhesion and associated signaling at the molecular level and to provide valuable guidance for the rational design of biomaterials with tailored bioactive functions.

Characterization of Cells Interactions with Patterned Azopolymer-Based Materials using SEM, AFM and Video Microscopy

Introduction:

Artificial Extracellular Matrices (ECMs) are promising tools for the study of cell behaviors.

Methods:

Here, we report a protocol for the use of a reconfigurable biocompatible azopolymer thin film through a photoinduced reconfigurable structuration of its surface to study nerve growth, differentiation and cell guidance.

Results & Discussion:

We show that this protocol combined with a spontaneous self-photoinduced polymer is suitable for time-lapse fluorescence video microscopy and can be easily adapted to electron microscopy techniques (SEM) and near-field imaging techniques (AFM).

An In Situ Reversible Heterodimeric Nanoswitch Controlled by Metal-Ion-Ligand Coordination Regulates the Mechanosensing and Differentiation of Stem Cells

In situ and cytocompatible nanoswitching by external stimuli is highly appealing for reversibly regulating cellular adhesion and functions in vivo. Here, a heterodimeric nanoswitch is designed to facilitate in situ switchable and combinatorial presentation of integrin-binding cell-adhesive moieties, such as Mg²⁺ and Arg-Gly-Asp (RGD) ligand in nanostructures. In situ reversible nanoswitching is controlled by convertible coordination between bioactive Mg²⁺ and bisphosphonate (BP) ligand. A BP-coated gold-nanoparticle monomer (BP-AuNP) on a substrate is prepared to allow in situ assembly of cell-adhesive Mg²⁺ -active Mg-BP nanoparticles (NPs) on a BP-AuNP surface via Mg²⁺ -BP coordination, yielding heterodimeric nanostructures (switching "ON"). Ethylenediaminetetraacetic acid (EDTA)-based Mg²⁺ chelation allows in situ disassembly of Mg²⁺ -BP NP, reverting to Mg²⁺ -free monomer (switching "OFF"). This in situ reversible nanoswitching on and off of cell-adhesive Mg²⁺ presentation allows reversible cell adhesion and release in vivo, respectively, and spatiotemporally controls cyclic cell adhesion. In situ heterodimeric assembly of dual RGD ligand- and Mg²⁺ -active RGD-BP-Mg²⁺ NP (switching "Dual ON") further tunes and promotes focal adhesion, spreading, and differentiation of stem cells. The modular nature of this in situ nanoswitch can accommodate various bioactive nanostructures via metal-ion-ligand coordination to regulate diverse cellular functions in vivo in reversible and compatible manner.