Mechanical properties of brain tissue based on microstructure

Heterogeneity of brain extracellular matrix and astrocyte activation

From the blood brain barrier to the synaptic space, astrocytes provide structural, metabolic, ionic, and extracellular matrix (ECM) support across the brain. Astrocytes include a vast array of subtypes, their phenotypes and functions varying both regionally and temporally. Astrocytes' metabolic and regulatory functions poise them to be quick and sensitive responders to injury and disease in the brain as revealed by single cell sequencing. Far less is known about the influence of the local healthy and aging microenvironments on these astrocyte activation states. In this forward-looking review, we describe the known relationship between astrocytes and their local microenvironment, the remodeling of the microenvironment during disease and injury, and postulate how they may drive astrocyte activation. We suggest technology development to better understand the dynamic diversity of astrocyte activation states, and how basal and activation states depend on the ECM microenvironment. A deeper understanding of astrocyte response to stimuli in ECM-specific contexts (brain region, age, and sex of individual), paves the way to revolutionize how the field considers astrocyte-ECM interactions in brain injury and disease and opens routes to return astrocytes to a healthy quiescent state.

Evaluating the Effects of Pulsed Electrical Stimulation on the Mechanical Behavior and Microstructure of Medulla Oblongata Tissues

The development of remote surgery hinges on comprehending the mechanical properties of the tissue at the surgical site. Understanding the mechanical behavior of the medulla oblongata tissue is instrumental for precisely determining the remote surgery implementation site. Additionally, exploring this tissue's response under electric fields can inform the creation of electrical stimulation therapy regimens. This could potentially reduce the extent of medulla oblongata tissue damage from mechanical compression. Various types of pulsed electric fields were integrated into a custom-built indentation device for this study. Experimental findings suggested that applying pulsed electric fields amplified the shear modulus of the medulla oblongata tissue. In the electric field, the elasticity and viscosity of the tissue increased. The most significant influence was noted from the low-frequency pulsed electric field, while the burst pulsed electric field had a minimal impact. At the microstructural scale, the application of an electric field led to the concentration of myelin in areas distant from the surface layer in the medulla oblongata, and the orderly structure of proteoglycans became disordered. The alterations observed in the myelin and proteoglycans under an electric field were considered to be the fundamental causes of the changes in the mechanical behavior of the medulla oblongata tissue. Moreover, cell polarization and extracellular matrix cavitation were observed, with transmission electron microscopy results pointing to laminar separation within the myelin at the ultrastructure scale. This study thoroughly explored the impact of electric field application on the mechanical behavior and microstructure of the medulla oblongata tissue, delving into the underlying mechanisms. This investigation delved into the changes and mechanisms in the mechanical behavior and microstructure of medulla oblongata tissue under the influence of electric fields. Furthermore, this study could serve as a reference for the development of electrical stimulation regimens in the central nervous system. The acquired mechanical behavior data could provide valuable baseline information to aid in the evolution of remote surgery techniques involving the medulla oblongata tissue.

Mechanical behavior and microstructure of porcine brain tissues under pulsed electric fields

Pulsed electric fields are extensively utilized in clinical treatments, such as subthalamic deep brain stimulation, where electric field loading is in direct contact with brain tissue. However, the alterations in brain tissue's mechanical properties and microstructure due to changes in electric field parameters have not received adequate attention. In this study, the mechanical properties and microstructure of the brain tissue under pulsed electric fields were focused on. Herein, a custom indentation device was equipped with a module for electric field loading. Parameters such as pulse amplitude and frequency were adjusted. The results demonstrated that following an indentation process lasting 5 s and reaching a depth of 1000 μm, and a relaxation process of 175 s, the average shear modulus of brain tissue was reduced, and viscosity decreased. At the same amplitude, high-frequency pulsed electric fields had a smaller effect on brain tissue than low-frequency ones. Furthermore, pulsed electric fields induced cell polarization and reduced the proteoglycan concentration in brain tissue. As pulse frequency increased, cell polarization diminished, and proteoglycan concentration decreased significantly. High-frequency pulsed electric fields applied to brain tissue were found to reduce impedance fluctuation amplitude. This study revealed the effect of pulsed electric fields on the mechanical properties and microstructure of ex vivo brain tissue, providing essential information to promote the advancement of brain tissue electrotherapy in clinical settings.

Changes in myelinated nerve fibers induced by pulsed electrical stimulation: A microstructural perspective on the causes of electrical stimulation side effects

Electrical brain stimulation technology is widely used in the clinic to treat brain neurological disorders. However, during treatment, patients may experience side effects such as pain, poor limb coordination, and skin rash. Previous reports have focused on the brilliant chapter on electrical brain stimulation technology and have not paid attention to patients' suffering caused by side effects during treatment. In this study, electrodes were arranged on the medulla oblongata. Pulsed electric fields of different frequencies were used to perform electrical stimulation to study the impact of electric fields on myelinated nerve fibers and reveal the possible microstructural origin of side effects. Transmission electron microscopy was used to analyze and quantify the changes in microstructure. The results illustrated that myelinated nerve fibers underwent atrophy under pulsed electric fields, with the mildest degree of atrophy under high-frequency (400 Hz) electric fields. Myelin sheaths experienced plate separation under pulsed electric fields, and a distinct laminar structure appeared. The microstructure changes may be related to the side effects of clinical electrical stimulation. This study can provide pathological possibilities for exploring the causes of the side effects of electrical stimulation and supply guidance for selecting electrical parameters for clinical electrical stimulation therapy from a distinctive perspective.

Biofabrication methods for reconstructing extracellular matrix mimetics

In the human body, almost all cells interact with extracellular matrices (ECMs), which have tissue and organ-specific compositions and architectures. These ECMs not only function as cellular scaffolds, providing structural support, but also play a crucial role in dynamically regulating various cellular functions. This comprehensive review delves into the examination of biofabrication strategies used to develop bioactive materials that accurately mimic one or more biophysical and biochemical properties of ECMs. We discuss the potential integration of these ECM-mimics into a range of physiological and pathological in vitro models, enhancing our understanding of cellular behavior and tissue organization. Lastly, we propose future research directions for ECM-mimics in the context of tissue engineering and organ-on-a-chip applications, offering potential advancements in therapeutic approaches and improved patient outcomes.

Multi-scale measurement of stiffness in the developing ferret brain

Cortical folding is an important process during brain development, and aberrant folding is linked to disorders such as autism and schizophrenia. Changes in cell numbers, size, and morphology have been proposed to exert forces that control the folding process, but these changes may also influence the mechanical properties of developing brain tissue. Currently, the changes in tissue stiffness during brain folding are unknown. Here, we report stiffness in the developing ferret brain across multiple length scales, emphasizing changes in folding cortical tissue. Using rheometry to measure the bulk properties of brain tissue, we found that overall brain stiffness increases with age over the period of cortical folding. Using atomic force microscopy to target the cortical plate, we found that the occipital cortex increases in stiffness as well as stiffness heterogeneity over the course of development and folding. These findings can help to elucidate the mechanics of the cortical folding process by clarifying the concurrent evolution of tissue properties.

Regulation of local alternating electric fields on synaptic plasticity in brain tissue

Purpose

External electric fields can regulate the neural network and change the excitability of the in-vivo cerebral cortex. Here, to prove the effect of alternating electric fields on the synaptic plasticity of ex-vivo tissues, the regular changes in the synaptic structure under alternating electric fields were studied.

Methods

This study applied alternating electric fields with a peak voltage of 20 V and frequencies of 5, 20, 50, and 80 Hz to the porcine cerebral cortex. Relying on transmission electron microscopy (TEM), the ultrastructure of synapses was observed, and the curvature radius of post-synaptic density (PSD) and the synaptic gap distance was quantified.

Results

The results indicated that under alternating electric fields, the average synaptic curvature of the PSD decreased by 30-59% with increasing frequency, and the average synaptic gap distance became narrower.

Conclusion

In ex-vivo brain tissue, synaptic plasticity can be regulated by alternating electric fields of different frequencies. This study can provide reference data for the storage and regulation of ex-vivo organs, as well as comparable data for in-vivo studies.

Biomechanics Characterization of Autonomic and Somatic Nerves by High Dynamic Closed-Loop MEMS force sensing

The biomechanics of peripheral nerves are determined by the blood-nerve barrier (BNB), together with the epineural barrier, extracellular matrix, and axonal composition, which maintain structural and functional stability. These elements are often ignored in the fabrication of penetrating devices, and the implant process is traumatic due to the mechanical distress, compromising the function of neuroprosthesis for sensory-motor restoration in amputees. Miniaturization of penetrating interfaces offers the unique opportunity of decoding individual nerve fibers associated to specific functions, however, a main issue for their implant is the lack of high-precision standardization of insertion forces. Current automatized electromechanical force sensors are available; however, their sensitivity and range amplitude are limited (i.e. mN), and have been tested only in-vitro. We previously developed a high-precision bi-directional micro-electromechanical force sensor, with a closed-loop mechanism (MEMS-CLFS), that while measuring with high-precision (-211.7μN to 211.5μN with a resolution of 4.74nN), can be used in alive animal. Our technology has an on-chip electrothermal displacement sensor with a shuttle beam displacement amplification mechanism, for large range and high-frequency resolution (dynamic range of 92.9 dB), which eliminates the adverse effect of flexural nonlinearity measurements, observed with other systems, and reduces the mechanical impact on delicate biological tissue. In this work, we use the MEMS-CLFS for in-vivo bidirectional measurement of biomechanics in somatic and autonomic nerves. Furthermore we define the mechanical implications of irrigation and collagen VI in the BNB, which is different for both autonomic and somatic nerves (~ 8.5-8.6 fold density of collagen VI and vasculature CD31+ in the VN vs ScN). This study allowed us to create a mathematical approach to predict insertion forces. Our data highlights the necessity of nerve-customization forces to prevent injury when implanting interfaces, and describes a high precision MEMS technology and mathematical model for their measurements.

Effects of Different Types of Electric Fields on Mechanical Properties and Microstructure of Ex Vivo Porcine Brain Tissues

Electrotherapy plays a crucial role in regulating neuronal activity. Nevertheless, the relevant therapeutic mechanisms are still unclear; thus, the effects of electric fields on brain tissue's mechanical properties and microstructure need to be explored. In this study, focusing on the changes in mechanical properties and microstructure of ex vivo porcine brain tissues under different types of electric fields, directional and alternating electric fields (frequencies of 5, 20, 50, and 80 Hz, respectively) integrate with a custom-designed indentation device. The experimental results showed that for the ex vivo brain tissue, the directional electric field (DEF) can reduce the elastic properties of brain tissue. Simultaneously, the DEF can increase the cell spacing and reduce the proteoglycan content. The transmission electron microscope (TEM) analysis observed that the DEF can reduce the integrity of the plasma membrane, the endoplasmic reticulum's stress response, and the myelin lamella's separation. The alternating electric field (AEF) can accelerate the stress relaxation process of brain tissue and change the time-dependent mechanical properties of brain tissue. Meanwhile, with the increase in frequency, the cell spacing decreased, and the proteoglycan content gradually approached the control group without electric fields. TEM analysis observed that with the increase in frequency, the integrity of the plasma membrane increases, and the separation of the myelin lamella gradually disappears. Understanding the changes in the mechanical properties and microstructure of brain tissue under AEF and DEF enables a preliminary exploration of the therapeutic mechanism of electrotherapy. Simultaneously, the essential data was provided to support the development of embedded electrodes. In addition, the ex vivo experiments build a solid foundation for future in vivo experiments.

Biochemical Pathways of Cellular Mechanosensing/Mechanotransduction and Their Role in Neurodegenerative Diseases Pathogenesis

In this review, we shed light on recent advances regarding the characterization of biochemical pathways of cellular mechanosensing and mechanotransduction with particular attention to their role in neurodegenerative disease pathogenesis. While the mechanistic components of these pathways are mostly uncovered today, the crosstalk between mechanical forces and soluble intracellular signaling is still not fully elucidated. Here, we recapitulate the general concepts of mechanobiology and the mechanisms that govern the mechanosensing and mechanotransduction processes, and we examine the crosstalk between mechanical stimuli and intracellular biochemical response, highlighting their effect on cellular organelles' homeostasis and dysfunction. In particular, we discuss the current knowledge about the translation of mechanosignaling into biochemical signaling, focusing on those diseases that encompass metabolic accumulation of mutant proteins and have as primary characteristics the formation of pathological intracellular aggregates, such as Alzheimer's Disease, Huntington's Disease, Amyotrophic Lateral Sclerosis and Parkinson's Disease. Overall, recent findings elucidate how mechanosensing and mechanotransduction pathways may be crucial to understand the pathogenic mechanisms underlying neurodegenerative diseases and emphasize the importance of these pathways for identifying potential therapeutic targets.