Mechanical heterogeneity of the rat hippocampus measured by atomic force microscope indentation

A method for analyzing AFM force mapping data obtained from soft tissue cryosections

Atomic force microscopy (AFM) is a valuable tool for assessing mechanical properties of biological samples, but interpretations of measurements on whole tissues can be difficult due to the tissue's highly heterogeneous nature. To overcome such difficulties and obtain more robust estimates of tissue mechanical properties, we describe an AFM force mapping and data analysis pipeline to characterize the mechanical properties of cryosectioned soft tissues. We assessed this approach on mouse optic nerve head and rat trabecular meshwork, cornea, and sclera. Our data show that the use of repeated measurements, outlier exclusion, and log-normal data transformation increases confidence in AFM mechanical measurements, and we propose that this methodology can be broadly applied to measuring soft tissue properties from cryosections.

Biomechanical Effects of Seizures on Cerebral Dynamics and Brain Stress

Epilepsy is one of the most common neurological disorders globally, affecting about 50 million people, with nearly 80% of those affected residing in low- and middle-income countries. It is characterized by recurrent seizures that result from abnormal electrical brain activity, with seizures varying widely in manifestation. The exploration of the biomechanical effects that seizures have on brain dynamics and stress levels is relevant for the development of more effective treatments and protective strategies. This study uses a blend of experimental data and computational simulations to assess the brain's physical response during seizures, particularly focusing on the behavior of cerebrospinal fluid and the resulting mechanical stresses on different brain regions. Notable findings show increases in stress, predominantly in the posterior gyri and brainstem, during seizures and an evidence of brain displacement relative to the skull. These observations suggest a dynamic and complex interaction between the brain and skull, with maximum shear stress regions demonstrating the limited yet essential protective role of the CSF. By providing a deeper understanding of the mechanical changes occurring during seizures, this research supports the goal of advancing diagnostic tools, informing more targeted treatment interventions, and guiding the creation of customized therapeutic strategies to enhance neurological care and protect against the adverse effects of seizures.

Mechanics in the nervous system: From development to disease

Physical forces are ubiquitous in biological processes across scales and diverse contexts. This review highlights the significance of mechanical forces in nervous system development, homeostasis, and disease. We provide an overview of mechanical signals present in the nervous system and delve into mechanotransduction mechanisms translating these mechanical cues into biochemical signals. During development, mechanical cues regulate a plethora of processes, including cell proliferation, differentiation, migration, network formation, and cortex folding. Forces then continue exerting their influence on physiological processes, such as neuronal activity, glial cell function, and the interplay between these different cell types. Notably, changes in tissue mechanics manifest in neurodegenerative diseases and brain tumors, potentially offering new diagnostic and therapeutic target opportunities. Understanding the role of cellular forces and tissue mechanics in nervous system physiology and pathology adds a new facet to neurobiology, shedding new light on many processes that remain incompletely understood.

Biomaterials in Traumatic Brain Injury: Perspectives and Challenges

Traumatic brain injury (TBI) is a leading cause of mortality and long-term impairment globally. TBI has a dynamic pathology, encompassing a variety of metabolic and molecular events that occur in two phases: primary and secondary. A forceful external blow to the brain initiates the primary phase, followed by a secondary phase that involves the release of calcium ions (Ca2+) and the initiation of a cascade of inflammatory processes, including mitochondrial dysfunction, a rise in oxidative stress, activation of glial cells, and damage to the blood-brain barrier (BBB), resulting in paracellular leakage. Currently, there are no FDA-approved drugs for TBI, but existing approaches rely on delivering micro- and macromolecular treatments, which are constrained by the BBB, poor retention, off-target toxicity, and the complex pathology of TBI. Therefore, there is a demand for innovative and alternative therapeutics with effective delivery tactics for the diagnosis and treatment of TBI. Tissue engineering, which includes the use of biomaterials, is one such alternative approach. Biomaterials, such as hydrogels, including self-assembling peptides and electrospun nanofibers, can be used alone or in combination with neuronal stem cells to induce neurite outgrowth, the differentiation of human neural stem cells, and nerve gap bridging in TBI. This review examines the inclusion of biomaterials as potential treatments for TBI, including their types, synthesis, and mechanisms of action. This review also discusses the challenges faced by the use of biomaterials in TBI, including the development of biodegradable, biocompatible, and mechanically flexible biomaterials and, if combined with stem cells, the survival rate of the transplanted stem cells. A better understanding of the mechanisms and drawbacks of these novel therapeutic approaches will help to guide the design of future TBI therapies.

Glioma Cells Secrete Collagen VI to Facilitate Invasion

While glioblastoma (GBM) progression is associated with extensive extracellular matrix (ECM) secretion, the causal contributions of ECM secretion to invasion remain unclear. Here we investigate these contributions by combining engineered materials, proteomics, analysis of patient data, and a model of bevacizumab-resistant GBM. We find that GBM cells cultured in engineered 3D hyaluronic acid hydrogels secrete ECM prior to invasion, particularly in the absence of exogenous ECM ligands. Proteomic measurements reveal extensive secretion of collagen VI, and collagen VI-associated transcripts are correspondingly enriched in microvascular proliferation regions of human GBMs. We further show that bevacizumab-resistant GBM cells deposit more collagen VI than their responsive counterparts, which is associated with marked cell-ECM stiffening. COL6A3 deletion in GBM cells reduces invasion, β-catenin signaling, and expression of mesenchymal markers, and these effects are amplified in hypoxia. Our studies strongly implicate GBM cell-derived collagen VI in microenvironmental remodeling to facilitate invasion.

Regenerative capacity of neural tissue scales with changes in tissue mechanics post injury

Spinal cord injuries have devastating consequences for humans, as mammalian neurons of the central nervous system (CNS) cannot regenerate. In the peripheral nervous system (PNS), however, neurons may regenerate to restore lost function following injury. While mammalian CNS tissue softens after injury, how PNS tissue mechanics changes in response to mechanical trauma is currently poorly understood. Here we characterised mechanical rat nerve tissue properties before and after in vivo crush and transection injuries using atomic force microscopy-based indentation measurements. Unlike CNS tissue, PNS tissue significantly stiffened after both types of tissue damage. This nerve tissue stiffening strongly correlated with an increase in collagen I levels. Schwann cells, which crucially support PNS regeneration, became more motile and proliferative on stiffer substrates in vitro, suggesting that changes in tissue stiffness may play a key role in facilitating or impeding nervous system regeneration.

A Method for Analyzing AFM Force Mapping Data Obtained from Soft Tissue Cryosections

Atomic force microscopy (AFM) is a valuable tool for assessing mechanical properties of biological samples, but interpretations of measurements on whole tissues can be difficult due to the tissue's highly heterogeneous nature. To overcome such difficulties and obtain more robust estimates of tissue mechanical properties, we describe an AFM force mapping and data analysis pipeline to characterize the mechanical properties of cryosectioned soft tissues. We assessed this approach on mouse optic nerve head and rat trabecular meshwork, cornea, and sclera. Our data show that the use of repeated measurements, outlier exclusion, and log-normal data transformation increases confidence in AFM mechanical measurements, and we propose that this methodology can be broadly applied to measuring soft tissue properties from cryosections.

Tunable hydrogel viscoelasticity modulates human neural maturation

Human-induced pluripotent stem cells (hiPSCs) have emerged as a promising in vitro model system for studying neurodevelopment. However, current models remain limited in their ability to incorporate tunable biomechanical signaling cues imparted by the extracellular matrix (ECM). The native brain ECM is viscoelastic and stress-relaxing, exhibiting a time-dependent response to an applied force. To recapitulate the remodelability of the neural ECM, we developed a family of protein-engineered hydrogels that exhibit tunable stress relaxation rates. hiPSC-derived neural progenitor cells (NPCs) encapsulated within these gels underwent relaxation rate-dependent maturation. Specifically, NPCs within hydrogels with faster stress relaxation rates extended longer, more complex neuritic projections, exhibited decreased metabolic activity, and expressed higher levels of genes associated with neural maturation. By inhibiting actin polymerization, we observed decreased neuritic projections and a concomitant decrease in neural maturation gene expression. Together, these results suggest that microenvironmental viscoelasticity is sufficient to bias human NPC maturation.

Surface Tension and Neuronal Sorting in Magnetically Engineered Brain-Like Tissue

Engineered 3D brain-like models have advanced the understanding of neurological mechanisms and disease, yet their mechanical signature, while fundamental for brain function, remains understudied. The surface tension for instance controls brain development and is a marker of cell-cell interactions. Here, 3D magnetic brain-like tissue spheroids composed of intermixed primary glial and neuronal cells at different ratios are engineered. Remarkably, the two cell types self-assemble into a functional tissue, with the sorting of the neuronal cells toward the periphery of the spheroids, whereas the glial cells constitute the core. The magnetic fingerprint of the spheroids then allows their deformation when placed under a magnetic field gradient, at a force equivalent to a 70 g increased gravity at the spheroid level. The tissue surface tension and elasticity can be directly inferred from the resulting deformation, revealing a transitional dependence on the glia/neuron ratio, with the surface tension of neuronal tissue being much lower. The results suggest an underlying mechanical contribution to the exclusion of the neurons toward the outer spheroid region, and depict the glia/neuron organization as a sophisticated mechanism that should in turn influence tissue development and homeostasis relevant in the neuroengineering field.

Adipose-Derived Stem Cells Spontaneously Express Neural Markers When Grown in a PEG-Based 3D Matrix

Neurological diseases are among the leading causes of disability and death worldwide and remain difficult to treat. Tissue engineering offers avenues to test potential treatments; however, the development of biologically accurate models of brain tissues remains challenging. Given their neurogenic potential and availability, adipose-derived stem cells (ADSCs) are of interest for creating neural models. While progress has been made in differentiating ADSCs into neural cells, their differentiation in 3D environments, which are more representative of the in vivo physiological conditions of the nervous system, is crucial. This can be achieved by modulating the 3D matrix composition and stiffness. Human ADSCs were cultured for 14 days in a 1.1 kPa polyethylene glycol-based 3D hydrogel matrix to assess effects on cell morphology, cell viability, proteome changes and spontaneous neural differentiation. Results showed that cells continued to proliferate over the 14-day period and presented a different morphology to 2D cultures, with the cells elongating and aligning with one another. The proteome analysis revealed 439 proteins changed in abundance by >1.5 fold. Cyclic nucleotide 3'-phosphodiesterase (CNPase) markers were identified using immunocytochemistry and confirmed with proteomics. Findings indicate that ADSCs spontaneously increase neural marker expression when grown in an environment with similar mechanical properties to the central nervous system.

Spatially controlled construction of assembloids using bioprinting

The biofabrication of three-dimensional (3D) tissues that recapitulate organ-specific architecture and function would benefit from temporal and spatial control of cell-cell interactions. Bioprinting, while potentially capable of achieving such control, is poorly suited to organoids with conserved cytoarchitectures that are susceptible to plastic deformation. Here, we develop a platform, termed Spatially Patterned Organoid Transfer (SPOT), consisting of an iron-oxide nanoparticle laden hydrogel and magnetized 3D printer to enable the controlled lifting, transport, and deposition of organoids. We identify cellulose nanofibers as both an ideal biomaterial for encasing organoids with magnetic nanoparticles and a shear-thinning, self-healing support hydrogel for maintaining the spatial positioning of organoids to facilitate the generation of assembloids. We leverage SPOT to create precisely arranged assembloids composed of human pluripotent stem cell-derived neural organoids and patient-derived glioma organoids. In doing so, we demonstrate the potential for the SPOT platform to construct assembloids which recapitulate key developmental processes and disease etiologies.

Biomaterial-based in vitro 3D modeling of glioblastoma multiforme

Adult-onset brain cancers, such as glioblastomas, are particularly lethal. People with glioblastoma multiforme (GBM) do not anticipate living for more than 15 months if there is no cure. The results of conventional treatments over the past 20 years have been underwhelming. Tumor aggressiveness, location, and lack of systemic therapies that can penetrate the blood-brain barrier are all contributing factors. For GBM treatments that appear promising in preclinical studies, there is a considerable rate of failure in phase I and II clinical trials. Unfortunately, access becomes impossible due to the intricate architecture of tumors. In vitro, bioengineered cancer models are currently being used by researchers to study disease development, test novel therapies, and advance specialized medications. Many different techniques for creating in vitro systems have arisen over the past few decades due to developments in cellular and tissue engineering. Later-stage research may yield better results if in vitro models that resemble brain tissue and the blood-brain barrier are used. With the use of 3D preclinical models made available by biomaterials, researchers have discovered that it is possible to overcome these limitations. Innovative in vitro models for the treatment of GBM are possible using biomaterials and novel drug carriers. This review discusses the benefits and drawbacks of 3D in vitro glioblastoma modeling systems.

Bridging the Gap in Ashby's Map for Soft Material Properties for Tissue Engineering

Ashby's map's role in rationally selecting materials for optimal performance is well-established in traditional engineering applications. However, there is a major gap in Ashby's maps in selecting materials for tissue engineering, which are very soft with an elastic modulus of less than 100 kPa. To fill the gap, we create an elastic modulus database to effectively connect soft engineering materials with biological tissues such as the cardiac, kidney, liver, intestine, cartilage, and brain. This soft engineering material mechanical property database is created for widely applied agarose hydrogels based on big-data screening and experiments conducted using ultra-low-concentration (0.01-0.5 wt %) hydrogels. Based on that, an experimental and analysis protocol is established for evaluating the elastic modulus of ultra-soft engineering materials. Overall, we built a mechanical bridge connecting soft matter and tissue engineering by fine-tuning the agarose hydrogel concentration. Meanwhile, a soft matter scale (degree of softness) is established to enable the manufacturing of implantable bio-scaffolds for tissue engineering.

Matching mechanical heterogeneity of the native spinal cord augments axon infiltration in 3D-printed scaffolds

Scaffolds delivered to injured spinal cords to stimulate axon connectivity often match the anisotropy of native tissue using guidance cues along the rostral-caudal axis, but current approaches do not mimic the heterogeneity of host tissue mechanics. Although white and gray matter have different mechanical properties, it remains unclear whether tissue mechanics also vary along the length of the cord. Mechanical testing performed in this study indicates that bulk spinal cord mechanics do differ along anatomical level and that these differences are caused by variations in the ratio of white and gray matter. These results suggest that scaffolds recreating the heterogeneity of spinal cord tissue mechanics must account for the disparity between gray and white matter. Digital light processing (DLP) provides a means to mimic spinal cord topology, but has previously been limited to printing homogeneous mechanical properties. We describe a means to modify DLP to print scaffolds that mimic spinal cord mechanical heterogeneity caused by variation in the ratio of white and gray matter, which improves axon infiltration compared to controls exhibiting homogeneous mechanical properties. These results demonstrate that scaffolds matching the mechanical heterogeneity of white and gray matter improve the effectiveness of biomaterials transplanted within the injured spinal cord.

The mechanosensitive ion channel Piezo1 modulates the migration and immune response of microglia

Microglia are the brain's resident immune cells, performing surveillance to promote homeostasis and healthy functioning. While microglial chemical signaling is well-studied, mechanical cues regulating their function are less well-understood. Here, we investigate the role of the mechanosensitive ion channel Piezo1 in microglia migration, pro-inflammatory cytokine production, and stiffness sensing. In Piezo1 knockout transgenic mice, we demonstrated the functional expression of Piezo1 in microglia and identified genes whose expression was consequently affected. Functional assays revealed that Piezo1 deficiency in microglia enhanced migration toward amyloid β-protein, and decreased levels of pro-inflammatory cytokines produced upon stimulation by lipopolysaccharide, both in vitro and in vivo. The phenomenon could be mimicked or reversed chemically using a Piezo1-specific agonist or antagonist. Finally, we also showed that Piezo1 mediated the effect of substrate stiffness-induced migration and cytokine expression. Altogether, we show that Piezo1 is an important molecular mediator for microglia, its activation modulating microglial migration and immune responses.

Modulation of self-organizing circuits at deforming membranes by intracellular and extracellular factors

Mechanical forces exerted to the plasma membrane induce cell shape changes. These transient shape changes trigger, among others, enrichment of curvature-sensitive molecules at deforming membrane sites. Strikingly, some curvature-sensing molecules not only detect membrane deformation but can also alter the amplitude of forces that caused to shape changes in the first place. This dual ability of sensing and inducing membrane deformation leads to the formation of curvature-dependent self-organizing signaling circuits. How these cell-autonomous circuits are affected by auxiliary parameters from inside and outside of the cell has remained largely elusive. Here, we explore how such factors modulate self-organization at the micro-scale and its emerging properties at the macroscale.

Modulation of DRG neurons response to semaphorin 3A via substrate stiffness

Semaphorin 3A (Sema3a) is a chemotropic protein that acts as a neuronal guidance cue and plays a major role in dorsal root ganglion (DRG) sensory neurons projection during embryo development. The present study evaluated the impact of stiffness in the repulsive response of DRG neurons to Sema3a when cultured over substrates of variable stiffness. Stiffness modified DRG neurons morphology and regulated their response to Sema3a, reducing the collapse of growth cones when they were cultured on softer substrates. Sema3a receptors expression was also regulated by stiffness, neuropilin-1 was overexpressed and plexin A4 mRNA was downregulated in stiffer substrates. Cytoskeleton distribution was also modified by stiffness. In softer substrates, βIII-tubulin and actin co-localized up to the leading edge of the growth cones, and as the substrate became stiffer, βIII-tubulin was confined to the transition and peripheral domains of the growth cone. Moreover, a decrease in the α-actinin adaptor protein was also observed in softer substrates. Our results show that substrate stiffness plays an important role in regulating the collapse response to Sema3a and that the modulation of cytoskeleton distribution and Sema3a receptors expression are related to the differential collapse responses of the growth cones.

Mechanical Properties of the Extracellular Environment of Human Brain Cells Drive the Effectiveness of Drugs in Fighting Central Nervous System Cancers

The evaluation of nanomechanical properties of tissues in health and disease is of increasing interest to scientists. It has been confirmed that these properties, determined in part by the composition of the extracellular matrix, significantly affect tissue physiology and the biological behavior of cells, mainly in terms of their adhesion, mobility, or ability to mutate. Importantly, pathophysiological changes that determine disease development within the tissue usually result in significant changes in tissue mechanics that might potentially affect the drug efficacy, which is important from the perspective of development of new therapeutics, since most of the currently used in vitro experimental models for drug testing do not account for these properties. Here, we provide a summary of the current understanding of how the mechanical properties of brain tissue change in pathological conditions, and how the activity of the therapeutic agents is linked to this mechanical state.

Protocol to assess human glioma propagating cell migration on linear micropatterns mimicking brain invasion tracks

Glioblastoma (GBM) cells invade the brain by following linear structures like blood vessel walls and white matter tracts by using specific motility modes. In this protocol, we describe two micropatterning techniques allowing recapitulation of these linear tracks in vitro: micro-contact printing and deep UV photolithography. We also detail how to maintain, transfect, and prepare human glioma propagating cells (hGPCs) for migration assays on linear tracks, followed by image acquisition and analysis, to measure key parameters of their motility.

For complete details on the use and execution of this protocol, please refer to Monzo et al. (2016) and Monzo et al. (2021a).

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Micropatterning of linear tracks on imaging dishes

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Maintenance and preparation of human glioma propagating cells (hGPC) for transfection

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Transfection of hGPC by electroporation and preparation for imaging

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Imaging of hGPC migration on linear tracks, cell tracking, and analysis

Current State of Potential Mechanisms Supporting Low Intensity Focused Ultrasound for Neuromodulation

Low intensity focused ultrasound (LIFU) has been gaining traction as a non-invasive neuromodulation technology due to its superior spatial specificity relative to transcranial electrical/magnetic stimulation. Despite a growing literature of LIFU-induced behavioral modifications, the mechanisms of action supporting LIFU's parameter-dependent excitatory and suppressive effects are not fully understood. This review provides a comprehensive introduction to the underlying mechanics of both acoustic energy and neuronal membranes, defining the primary variables for a subsequent review of the field's proposed mechanisms supporting LIFU's neuromodulatory effects. An exhaustive review of the empirical literature was also conducted and studies were grouped based on the sonication parameters used and behavioral effects observed, with the goal of linking empirical findings to the proposed theoretical mechanisms and evaluating which model best fits the existing data. A neuronal intramembrane cavitation excitation model, which accounts for differential effects as a function of cell-type, emerged as a possible explanation for the range of excitatory effects found in the literature. The suppressive and other findings need additional theoretical mechanisms and these theoretical mechanisms need to have established relationships to sonication parameters.

Egr1 is a 3D matrix-specific mediator of mechanosensitive stem cell lineage commitment

While extracellular matrix (ECM) mechanics strongly regulate stem cell commitment, the field's mechanistic understanding of this phenomenon largely derives from simplified two-dimensional (2D) culture substrates. Here, we found a 3D matrix-specific mechanoresponsive mechanism for neural stem cell (NSC) differentiation. NSC lineage commitment in 3D is maximally stiffness sensitive in the range of 0.1 to 1.2 kPa, a narrower and more brain-mimetic range than we had previously identified in 2D (0.75 to 75 kPa). Transcriptomics revealed stiffness-dependent up-regulation of early growth response 1 (Egr1) in 3D but not in 2D. Egr1 knockdown enhanced neurogenesis in stiff ECMs by driving β-catenin nuclear localization and activity in 3D, but not in 2D. Mechanical modeling and experimental studies under osmotic pressure indicate that stiff 3D ECMs are likely to stimulate Egr1 via increases in confining stress during cell volumetric growth. To our knowledge, Egr1 represents the first 3D-specific stem cell mechanoregulatory factor.

Tissue-Scale Biomechanical Testing of Brain Tissue for the Calibration of Nonlinear Material Models

Brain tissue is one of the most complex and softest tissues in the human body. Due to its ultrasoft and biphasic nature, it is difficult to control the deformation state during biomechanical testing and to quantify the highly nonlinear, time-dependent tissue response. In numerous experimental studies that have investigated the mechanical properties of brain tissue over the last decades, stiffness values have varied significantly. One reason for the observed discrepancies is the lack of standardized testing protocols and corresponding data analyses. The tissue properties have been tested on different length and time scales depending on the testing technique, and the corresponding data have been analyzed based on simplifying assumptions. In this review, we highlight the advantage of using nonlinear continuum mechanics based modeling and finite element simulations to carefully design experimental setups and protocols as well as to comprehensively analyze the corresponding experimental data. We review testing techniques and protocols that have been used to calibrate material model parameters and discuss artifacts that might falsify the measured properties. The aim of this work is to provide standardized procedures to reliably quantify the mechanical properties of brain tissue and to more accurately calibrate appropriate constitutive models for computational simulations of brain development, injury and disease. Computational models can not only be used to predictively understand brain tissue behavior, but can also serve as valuable tools to assist diagnosis and treatment of diseases or to plan neurosurgical procedures. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

Engineering Tissues of the Central Nervous System: Interfacing Conductive Biomaterials with Neural Stem/Progenitor Cells

Conductive biomaterials provide an important control for engineering neural tissues, where electrical stimulation can potentially direct neural stem/progenitor cell (NS/PC) maturation into functional neuronal networks. It is anticipated that stem cell-based therapies to repair damaged central nervous system (CNS) tissues and ex vivo, "tissue chip" models of the CNS and its pathologies will each benefit from the development of biocompatible, biodegradable, and conductive biomaterials. Here, technological advances in conductive biomaterials are reviewed over the past two decades that may facilitate the development of engineered tissues with integrated physiological and electrical functionalities. First, one briefly introduces NS/PCs of the CNS. Then, the significance of incorporating microenvironmental cues, to which NS/PCs are naturally programmed to respond, into biomaterial scaffolds is discussed with a focus on electrical cues. Next, practical design considerations for conductive biomaterials are discussed followed by a review of studies evaluating how conductive biomaterials can be engineered to control NS/PC behavior by mimicking specific functionalities in the CNS microenvironment. Finally, steps researchers can take to move NS/PC-interfacing, conductive materials closer to clinical translation are discussed.

Effect of Velocity and Contact Stress Area on the Dynamic Behavior of the Spinal Cord Under Different Testing Conditions

Knowledge of the dynamic behavior of the spinal cord under different testing conditions is critical for our understanding of biomechanical mechanisms of spinal cord injury. Although velocity and contact stress area are known to affect external mechanical stress or energy upon sudden traumatic injury, quantitative investigation of the two clinically relevant biomechanical variables is limited. Here, freshly excised rat spinal-cord–pia-arachnoid constructs were tested through indentation using indenters of different sizes (radii: 0.25, 0.50, and 1.00 mm) at various loading rates ranging from 0.04 to 0.20 mm/s. This analysis found that the ex vivo specimen displayed significant nonlinear viscoelasticity at <10% of specimen thickness depth magnitudes. At higher velocity and larger contact stress area, the cord withstood a higher peak load and exhibited more sensitive mechanical relaxation responses (i.e., increasing amplitude and speed of the drop in peak load). Additionally, the cord became stiffer (i.e., increasing elastic modulus) and softer (i.e., decreasing elastic modulus) at a higher velocity and larger contact stress area, respectively. These findings will improve our understanding of the real-time complex biomechanics involved in traumatic spinal cord injury.

Live 3D imaging and mapping of shear stresses within tissues using incompressible elastic beads

To investigate the role of mechanical constraints in morphogenesis and development, we have developed a pipeline of techniques based on incompressible elastic sensors. These techniques combine the advantages of incompressible liquid droplets, which have been used as precise in situ shear stress sensors, and of elastic compressible beads, which are easier to tune and to use. Droplets of a polydimethylsiloxane mix, made fluorescent through specific covalent binding to a rhodamin dye, are produced by a microfluidics device. The elastomer rigidity after polymerization is adjusted to the tissue rigidity. Its mechanical properties are carefully calibrated in situ, for a sensor embedded in a cell aggregate submitted to uniaxial compression. The local shear stress tensor is retrieved from the sensor shape, accurately reconstructed through an active contour method. In vitro, within cell aggregates, and in vivo, in the prechordal plate of the zebrafish embryo during gastrulation, our pipeline of techniques demonstrates its efficiency to directly measure the three dimensional shear stress repartition within a tissue.

Adaptive mechanoproperties mediated by the formin FMN1 characterize glioblastoma fitness for invasion

Glioblastoma are heterogeneous tumors composed of highly invasive and highly proliferative clones. Heterogeneity in invasiveness could emerge from discrete biophysical properties linked to specific molecular expression. We identified clones of patient-derived glioma propagating cells that were either highly proliferative or highly invasive and compared their cellular architecture, migratory, and biophysical properties. We discovered that invasiveness was linked to cellular fitness. The most invasive cells were stiffer, developed higher mechanical forces on the substrate, and moved stochastically. The mechano-chemical-induced expression of the formin FMN1 conferred invasive strength that was confirmed in patient samples. Moreover, FMN1 expression was also linked to motility in other cancer and normal cell lines, and its ectopic expression increased fitness parameters. Mechanistically, FMN1 acts from the microtubule lattice and promotes a robust mechanical cohesion, leading to highly invasive motility.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Micromechanical mapping of the intact ovary interior reveals contrasting mechanical roles for follicles and stroma

Follicle development in the ovary must be tightly regulated to ensure cyclical release of oocytes (ovulation). Disruption of this process is a common cause of infertility, for example via polycystic ovary syndrome (PCOS) and premature ovarian insufficiency (POI). Recent ex vivo studies suggest that follicle growth is mechanically regulated, however, crucially, the actual mechanical properties of the follicle microenvironment have remained unknown. Here we use atomic force microscopy (AFM) spherical probe indentation to map and quantify the mechanical microenvironment in the mouse ovary, at high resolution and across the entire width of the intact (bisected) ovarian interior. Averaging over the entire organ, we find the ovary to be a fairly soft tissue comparable to fat or kidney (mean Young's Modulus 3.3±2.5 kPa). This average, however, conceals substantial spatial variations, with the overall range of tissue stiffnesses from c. 0.5-10 kPa, challenging the concept that a single Young's Modulus can effectively summarize this complex organ. Considering the internal architecture of the ovary, we find that stiffness is low at the edge and centre which are dominated by stromal tissue, and highest in an intermediate zone that is dominated by large developmentally-advanced follicles, confirmed by comparison with immunohistology images. These results suggest that large follicles are mechanically dominant structures in the ovary, contrasting with previous expectations that collagen-rich stroma would dominate. Extending our study to the highest resolutions (c. 5 μm) showed substantial mechanical variations within the larger zones, even over very short (sub-100 μm) lengths, and especially within the stiffer regions of the ovary. Taken together, our results provide a new, physiologically accurate, framework for ovarian biomechanics and follicle tissue engineering.

Signal Generation, Acquisition, and Processing in Brain Machine Interfaces: A Unified Review

Brain machine interfaces (BMIs), or brain computer interfaces (BCIs), are devices that act as a medium for communications between the brain and the computer. It is an emerging field with numerous applications in domains of prosthetic devices, robotics, communication technology, gaming, education, and security. It is noted in such a multidisciplinary field, many reviews have surveyed on various focused subfields of interest, such as neural signaling, microelectrode fabrication, and signal classification algorithms. A unified review is lacking to cover and link all the relevant areas in this field. Herein, this review intends to connect on the relevant areas that circumscribe BMIs to present a unified script that may help enhance our understanding of BMIs. Specifically, this article discusses signal generation within the cortex, signal acquisition using invasive, non-invasive, or hybrid techniques, and the signal processing domain. The latest development is surveyed in this field, particularly in the last decade, with discussions regarding the challenges and possible solutions to allow swift disruption of BMI products in the commercial market.

Atomic Force Microscope Nanoindentation Analysis of Diffuse Astrocytic Tumor Elasticity: Relation with Tumor Histopathology

This study aims to investigate the influence of isocitrate dehydrogenase gene family (IDH) mutations, World Health Organization (WHO) grade, and mechanical preconditioning on glioma and adjacent brain elasticity through standard monotonic and repetitive atomic force microscope (AFM) nanoindentation. The elastic modulus was measured ex vivo on fresh tissue specimens acquired during craniotomy from the tumor and the peritumoral white matter of 16 diffuse glioma patients. Linear mixed-effects models examined the impact of tumor traits and preconditioning on tissue elasticity. Tissues from IDH-mutant cases were stiffer than those from IDH-wildtype ones among anaplastic astrocytoma patients (p = 0.0496) but of similar elasticity to IDH-wildtype cases for diffuse astrocytoma patients (p = 0.480). The tumor was found to be non-significantly softer than white matter in anaplastic astrocytomas (p = 0.070), but of similar elasticity to adjacent brain in diffuse astrocytomas (p = 0.492) and glioblastomas (p = 0.593). During repetitive indentation, both tumor (p = 0.002) and white matter (p = 0.003) showed initial stiffening followed by softening. Stiffening was fully reversed in white matter (p = 0.942) and partially reversed in tumor (p = 0.015). Tissue elasticity comprises a phenotypic characteristic closely related to glioma histopathology. Heterogeneity between patients should be further explored.

Mechanosensitive axon outgrowth mediated by L1-laminin clutch interface

Mechanical properties of the extracellular environment modulate axon outgrowth. Growth cones at the tip of extending axons generate traction force for axon outgrowth by transmitting the force of actin filament retrograde flow, produced by actomyosin contraction and F-actin polymerization, to adhesive substrates through clutch and cell adhesion molecules. A molecular clutch between the actin filament flow and substrate is proposed to contribute to cellular mechanosensing. However, the molecular identity of the clutch interface responsible for mechanosensitive growth cone advance is unknown. We previously reported that mechanical coupling between actin filament retrograde flow and adhesive substrates through the clutch molecule shootin1a and the cell adhesion molecule L1 generates traction force for axon outgrowth and guidance. Here, we show that cultured mouse hippocampal neurons extend longer axons on stiffer substrates under elastic conditions that correspond to the soft brain environments. We demonstrate that this stiffness-dependent axon outgrowth requires actin-adhesion coupling mediated by shootin1a, L1, and laminin on the substrate. Speckle imaging analyses showed that L1 at the growth cone membrane switches between two adhesive states: L1 that is immobilized and that undergoes retrograde movement on the substrate. The duration of the immobilized phase was longer on stiffer substrates; this was accompanied by increases in actin-adhesion coupling and in the traction force exerted on the substrate. These data suggest that the interaction between L1 and laminin is enhanced on stiffer substrates, thereby promoting force generation for axon outgrowth.

A Shift in Tissue Stiffness During Hippocampal Maturation Correlates to the Pattern of Neurogenesis and Composition of the Extracellular Matrix

Aging changes the mechanical properties of brain tissue, such as stiffness. It has been proposed that the maintenance and differentiation of neural stem cells (NSCs) are regulated in accordance with extracellular stiffness. Neurogenesis is observed in restricted niches, including the dentate gyrus (DG) of the hippocampus, throughout mammalian lifetimes. However, profiles of tissue stiffness in the DG in comparison with the activity of NSCs from the neonatal to the matured brain have rarely been addressed so far. Here, we first applied ultrasound-based shear-wave elasticity imaging (SWEI) in living animals to assess shear modulus as in vivo brain stiffness. To complement the assay, atomic force microscopy (AFM) was utilized to determine the Young’s modulus in the hippocampus as region-specific stiffness in the brain slice. The results revealed that stiffness in the granule cell layer (GCL) and the hilus, including the subgranular zone (SGZ), increased during hippocampal maturation. We then quantified NSCs and immature neural cells in the DG with differentiation markers, and verified an overall decrease of NSCs and proliferative/immature neural cells along stages, showing that a specific profile is dependent on the subregion. Subsequently, we evaluated the amount of chondroitin sulfate proteoglycans (CSPGs), the major extracellular matrix (ECM) components in the premature brain by CS-56 immunoreactivity. We observed differential signal levels of CSPGs by hippocampal subregions, which became weaker during maturation. To address the contribution of the ECM in determining tissue stiffness, we manipulated the function of CSPGs by enzymatic digestion or supplementation with chondroitin sulfate, which resulted in an increase or decrease of stiffness in the DG, respectively. Our results illustrate that stiffness in the hippocampus shifts due to the composition of ECM, which may affect postnatal neurogenesis by altering the mechanical environment of the NSC niche.

Gradient hydrogels for screening stiffness effects on patient-derived glioblastoma xenograft cellfates in 3D

Brain cancer is a devastating disease given its extreme invasiveness and intricate location. Glioblastoma multiforme (GBM) is one of the most common forms of brain cancer, and cancer progression is often correlated with significantly altered tissue stiffness. To elucidate the effect of matrix stiffness on GBM cell fates, previous research is largely limited to 2D studies using immortalized cell lines, which has limited physiological relevance. The objective of the study is to develop gradient hydrogels with brain-mimicking stiffness range as a 3Din vitro GBM model for screening of the effects of matrix stiffness on GBM. To increase the physiological relevance, patient-derived tumor xenograft (PDTX) GBM cells were used. Our gradient platform allows formation of cell-containing hydrogels with stiffness ranging from 40 Pa to 1,300 Pa within a few minutes. By focusing on a brain-mimicking stiffness range, this gradient hydrogel platform is designed for investigating brain cancer. Increasing stiffness led to decreased GBM proliferation and less spreading, which is accompanied by downregulation of matrix-metalloproteinases (MMPs). Using temozolomide (TMZ) as a model drug, we demonstrate that increasing stiffness led to higher drug resistance by PDTX GBM cells in 3D, suggesting matrix stiffness can directly modulate how GBM cells respond to drug treatment. While the current study focuses on stiffness gradient, the setup may also be adapted for screening other cancer niche cues such as how biochemical ligand gradient modulates brain cancer progression and drug responses using reduced materials and time.

The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics

Neurological diseases are a prevalent cause of global mortality and are of growing concern when considering an ageing global population. Traditional treatments are accompanied by serious side effects including repeated treatment sessions, invasive surgeries, or infections. For example, in the case of deep brain stimulation, large, stiff, and battery powered neural probes recruit thousands of neurons with each pulse, and can invoke a vigorous immune response. This paper presents challenges in engineering and neuroscience in developing miniaturized and biointegrated alternatives, in the form of microelectrode probes. Progress in design and topology of neural implants has shifted the goal post toward highly specific recording and stimulation, targeting small groups of neurons and reducing the foreign body response with biomimetic design principles. Implantable device design recommendations, fabrication techniques, and clinical evaluation of the impact flexible, integrated probes will have on the treatment of neurological disorders are provided in this report. The choice of biocompatible material dictates fabrication techniques as novel methods reduce the complexity of manufacture. Wireless power, the final hurdle to truly implantable neural interfaces, is discussed. These aspects are the driving force behind continued research: significant breakthroughs in any one of these areas will revolutionize the treatment of neurological disorders.

A comparison of insertion methods for surgical placement of penetrating neural interfaces

Many implantable electrode arrays exist for the purpose of stimulating or recording electrical activity in brain, spinal, or peripheral nerve tissue, however most of these devices are constructed from materials that are mechanically rigid. A growing body of evidence suggests that the chronic presence of these rigid probes in the neural tissue causes a significant immune response and glial encapsulation of the probes, which in turn leads to gradual increase in distance between the electrodes and surrounding neurons. In recording electrodes, the consequence is the loss of signal quality and, therefore, the inability to collect electrophysiological recordings long term. In stimulation electrodes, higher current injection is required to achieve a comparable response which can lead to tissue and electrode damage. To minimize the impact of the immune response, flexible neural probes constructed with softer materials have been developed. These flexible probes, however, are often not strong enough to be inserted on their own into the tissue, and instead fail via mechanical buckling of the shank under the force of insertion. Several strategies have been developed to allow the insertion of flexible probes while minimizing tissue damage. It is critical to keep these strategies in mind during probe design in order to ensure successful surgical placement. In this review, existing insertion strategies will be presented and evaluated with respect to surgical difficulty, immune response, ability to reach the target tissue, and overall limitations of the technique. Overall, the majority of these insertion techniques have only been evaluated for the insertion of a single probe and do not quantify the accuracy of probe placement. More work needs to be performed to evaluate and optimize insertion methods for accurate placement of devices and for devices with multiple probes.

Theory for Durotactic Axon Guidance

During the development of the nervous system, neurons extend bundles of axons that grow and meet other neurons to form the neuronal network. Robust guidance mechanisms are needed for these bundles to migrate and reach their functional target. Directional information depends on external cues such as chemical or mechanical gradients. Unlike chemotaxis that has been extensively studied, the role and mechanism of durotaxis, the directed response to variations in substrate rigidity, remain unclear. We model bundle migration and guidance by rigidity gradients by using the theory of morphoelastic rods. We show that, at a rigidity interface, the motion of axon bundles follows a simple behavior analogous to optic ray theory and obeys Snell's law for refraction and reflection. We use this powerful analogy to demonstrate that axons can be guided by the equivalent of optical lenses and fibers created by regions of different stiffnesses.

Longitudinal study of sub-regional cerebral viscoelastic properties of 5XFAD Alzheimer's disease mice using multifrequency MR elastography

PURPOSE

To study sub-regional, longitudinal changes occurring inside brains of 5XFAD mice, an Alzheimer's disease (AD) model, based on viscoelastic parameters derived using MR elastography and their spatial variation.

METHODS

Female 5XFAD and non-transgenic B6SJLF1/J mice as controls (n = 9 for both groups) were used for the study. Scans were performed inside a 9.4T preclinical MRI scanner using SampLe Interval Modulation-magnetic resonance elastography (SLIM-MRE). Experiments were performed at ages 2, 4, and 6 mo, and by using three actuation frequencies: 900, 1000, and 1100 Hz. Multifrequency dual elasto-visco (MDEV) reconstruction was used to combine 3D multifrequency MRE data and calculate magnitude G ∗ , and phase angle φ, of the complex shear modulus G ∗ . Mean values were measured for the overall brain and sub-regions associated with the early onset of AD, to check for the effect of aging and mouse model. Spatial coefficient of variation (CV) of both parameters across different age-groups were analyzed.

RESULTS

G ∗ and φ values reduced with age for overall brain in 5XFAD mice with significant difference in mean G ∗ between 5XFAD and control mice at 6 mo (P = .029). Analyzing values from the hippocampal region highlighted drop in mean G ∗ and φ values. The CV of G ∗ inside hippocampus enabled differentiation at 4 mo with it being significantly lower in 5XFAD mice (P = .0007).

CONCLUSION

Multifrequency 3D MRE revealed longitudinal viscoelastic changes in 5XFAD mice and the CV of G ∗ in brain sub-regions may qualify as biomarker for early diagnosis of AD.

Rapid and Reversible Development of Axonal Varicosities: A New Form of Neural Plasticity

Axonal varicosities are enlarged, heterogeneous structures along axonal shafts, profoundly affecting axonal conduction and synaptic transmission. They represent a key pathological feature believed to develop via slow accumulation of axonal damage that occurs during irreversible degeneration, for example in mild traumatic brain injury (mTBI), Alzheimer's and Parkinson's diseases, and multiple sclerosis. Here this review first discusses recent in vitro results showing that axonal varicosities can be rapidly and reversibly induced by mechanical stress in cultured primary neurons from the central nervous system (CNS). This notion is further supported by in vivo studies revealing the induction of axonal varicosities across various brain regions in different mTBI mouse models, as a prominent feature of axonal pathology. Limited progress in understanding intrinsic and extrinsic regulatory mechanisms of axonal varicosity induction and development is further highlighted. Rapid and reversible formation of axonal varicosities likely plays a key role in CNS neuron mechanosensation and is a new form of neural plasticity. Future investigation in this emerging research field may reveal how to reverse axonal injury, contributing to the development of new strategies for treating brain injuries and related neurodegenerative diseases.

Viscoelastic mapping of mouse brain tissue: Relation to structure and age

There is growing evidence that mechanical factors affect brain functioning. However, brain components responsible for regulating the physiological mechanical environment are not completely understood. To determine the relationship between structure and stiffness of brain tissue, we performed high-resolution viscoelastic mapping by dynamic indentation of the hippocampus and the cerebellum of juvenile mice brains, and quantified relative area covered by neurons (NeuN-staining), axons (neurofilament NN18-staining), astrocytes (GFAP-staining), myelin (MBP-staining) and nuclei (Hoechst-staining) of juvenile and adult mouse brain slices. Results show that brain subregions have distinct viscoelastic parameters. In gray matter (GM) regions, the storage modulus correlates negatively with the relative area of nuclei and neurons, and positively with astrocytes. The storage modulus also correlates negatively with the relative area of myelin and axons (high cell density regions are excluded). Furthermore, adult brain regions are ∼ 20%-150% stiffer than the comparable juvenile regions which coincide with increase in astrocyte GFAP-staining. Several linear regression models are examined to predict the mechanical properties of the brain tissue based on (immuno)histochemical stainings.

Nanomechanics and Histopathology as Diagnostic Tools to Characterize Freshly Removed Human Brain Tumors

Background

The tissue-mechanics environment plays a crucial role in human brain physiological development and the pathogenesis of different diseases, especially cancer. Assessment of alterations in brain mechanical  properties during cancer progression might provide important information about possible tissue abnormalities with clinical relevance.

Methods

With atomic force microscopy (AFM), the stiffness of freshly removed human brain tumor tissue was determined on various regions of the sample and compared to the stiffness of healthy human brain tissue that was removed during neurosurgery to gain access to tumor mass. An advantage of indentation measurement using AFM is the small volume of tissue required and high resolution at the single-cell level.

Results

Our results showed great heterogeneity of stiffness within metastatic cancer or primary high-grade gliomas compared to healthy tissue. That effect was not clearly visible in lower-grade tumors like meningioma.

Conclusion

Collected data indicate that AFM might serve as a diagnostic tool in the assessment of human brain tissue stiffness in the process of recognizing tumors.

Enhancement of the Mechanical Properties of Hydrogels with Continuous Fibrous Reinforcement

Reinforcing mechanically weak hydrogels with fibers is a promising route to obtain strong and tough materials for biomedical applications while retaining a favorable cell environment. The resulting hierarchical structure recreates structural elements of natural tissues such as articular cartilage, with fiber diameters ranging from the nano- to microscale. Through control of properties such as the fiber diameter, orientation, and porosity, it is possible to design materials which display the nonlinear, synergistic mechanical behavior observed in natural tissues. In order to fully exploit these advantages, it is necessary to understand the structure-property relationships in fiber-reinforced hydrogels. However, there are currently limited models which capture their complex mechanical properties. The majority of reported fiber-reinforced hydrogels contain fibers obtained by electrospinning, which allows for limited spatial control over the fiber scaffold and limits the scope for systematic mechanical testing studies. Nevertheless, new manufacturing techniques such as melt electrowriting and bioprinting have emerged, which allow for increased control over fiber deposition and the potential for future investigations on the effect of specific structural features on mechanical properties. In this review, we therefore explore the mechanics of fiber-reinforced hydrogels, and the evolution of their design and manufacture from replicating specific features of biological tissues to more complex structures, by taking advantage of design principles from both tough hydrogels and fiber-reinforced composites. By highlighting the overlap between these fields, it is possible to identify the remaining challenges and opportunities for the development of effective biomedical devices.

Comparative Analysis of Brain Stiffness Among Amniotes Using Glyoxal Fixation and Atomic Force Microscopy

Brain structures are diverse among species despite the essential molecular machinery of neurogenesis being common. Recent studies have indicated that differences in the mechanical properties of tissue may result in the dynamic deformation of brain structure, such as folding. However, little is known about the correlation between mechanical properties and species-specific brain structures. To address this point, a comparative analysis of mechanical properties using several animals is required. For a systematic measurement of the brain stiffness of remotely maintained animals, we developed a novel strategy of tissue-stiffness measurement using glyoxal as a fixative combined with atomic force microscopy. A comparison of embryonic and juvenile mouse and songbird brain tissue revealed that glyoxal fixation can maintain brain structure as well as paraformaldehyde (PFA) fixation. Notably, brain tissue fixed by glyoxal remained much softer than PFA-fixed brains, and it can maintain the relative stiffness profiles of various brain regions. Based on this method, we found that the homologous brain regions between mice and songbirds exhibited different stiffness patterns. We also measured brain stiffness in other amniotes (chick, turtle, and ferret) following glyoxal fixation. We found stage-dependent and species-specific stiffness in pallia among amniotes. The embryonic chick and matured turtle pallia showed gradually increasing stiffness along the apico-basal tissue axis, the lowest region at the most apical region, while the ferret pallium exhibited a catenary pattern, that is, higher in the ventricular zone, the inner subventricular zone, and the cortical plate and the lowest in the outer subventricular zone. These results indicate that species-specific microenvironments with distinct mechanical properties emerging during development might contribute to the formation of brain structures with unique morphology.

A microfiber scaffold-based 3D in vitro human neuronal culture model of Alzheimer's disease

Increasing evidence indicates superiority of three-dimensional (3D) in vitro cell culture systems over conventional two-dimensional (2D) monolayer cultures in mimicking native in vivo microenvironments. Tissue-engineered 3D culture models combined with stem cell technologies have advanced Alzheimer's disease (AD) pathogenesis studies. However, existing 3D neuronal models of AD overexpress mutant genes or have heterogeneities in composition, biological properties and cell differentiation stages. Here, we encapsulate patient induced pluripotent stem cell (iPSC) derived neural progenitor cells (NPC) in poly(lactic-co-glycolic acid) (PLGA) microtopographic scaffolds fabricated via wet electrospinning to develop a novel 3D culture model of AD. First, we enhanced cellular infiltration and distribution inside the scaffold by optimizing various process parameters such as fiber diameter, pore size, porosity and hydrophilicity. Next, we compared key neural stem cell features including viability, proliferation and differentiation in 3D culture with 2D monolayer controls. The 3D microfibrous substrate reduces cell proliferation and significantly accelerates neuronal differentiation within seven days of culture. Furthermore, 3D culture spontaneously enhanced pathogenic amyloid-beta 42 (Aβ42) and phospho-tau levels in differentiated neurons carrying familial AD (FAD) mutations, compared with age-matched healthy controls. Overall, our tunable scaffold-based 3D neuronal culture platform serves as a suitable in vitro model that robustly recapitulates and accelerates the pathogenic characteristics of FAD-iPSC derived neurons.