MULTI-SCALE MECHANICAL BEHAVIOR ANALYSIS ON FLUID–SOLID COUPLING FOR OSTEONS IN VARIOUS GRAVITATIONAL FIELDS

Background: Compact bone mainly consists of cylindrical osteon structures. In microgravity, the change in the mechanical microenvironment of osteocytes might be the root cause of astronauts’ bone loss during space flights. Methods: A multi-scale three-dimensional (3D) fluid–solid coupling finite element model of osteons with a two-stage pore structure was developed using COMSOL software based on the natural structure of osteocytes. Gradients in gravitational fields of [Formula: see text]1, 0, 1, 2.5, and 3.7[Formula: see text]g were used to investigate the changes in the mechanical microenvironment on osteocyte structure. The difference in arteriole pulsating pressure and static compression stress caused by each gravity gradient was investigated. Results: The mechanical response of osteocytes increased with the value of g, compared with the Earth’s gravitational field. For instance, the fluid pressure of osteocytes and the von Mises stress of bone matrix near lacunae decreased by 31.3% and 99.9%, respectively, in microgravity. Under static loading, only about 16.7% of osteocytes in microgravity and 58.3% of osteocytes in the Earth’s gravitational field could reach the fluid shear stress threshold of biological reactions in cell culture experiments. Compared with the Earth’s gravitational field, the pressure gradient inside osteocytes severely decreased in microgravity. Conclusion: The mechanical microenvironment of osteocytes in microgravity might cause significant changes in the mechanical microenvironment of osteocytes, which may lead to disuse osteoporosis in astronauts.

Mechanics-driven nuclear localization of YAP can be reversed by N-cadherin ligation in mesenchymal stem cells

Mesenchymal stem cells adopt differentiation pathways based upon cumulative effects of mechanosensing. A cell’s mechanical microenvironment changes substantially over the course of development, beginning from the early stages in which cells are typically surrounded by other cells and continuing through later stages in which cells are typically surrounded by extracellular matrix. How cells erase the memory of some of these mechanical microenvironments while locking in memory of others is unknown. Here, we develop a material and culture system for modifying and measuring the degree to which cells retain cumulative effects of mechanosensing. Using this system, we discover that effects of the RGD adhesive motif of fibronectin (representative of extracellular matrix), known to impart what is often termed “mechanical memory” in mesenchymal stem cells via nuclear YAP localization, are erased by the HAVDI adhesive motif of the N-cadherin (representative of cell-cell contacts). These effects can be explained by a motor clutch model that relates cellular traction force, nuclear deformation, and resulting nuclear YAP re-localization. Results demonstrate that controlled storage and removal of proteins associated with mechanical memory in mesenchymal stem cells is possible through defined and programmable material systems.

StemBond hydrogels control the mechanical microenvironment for pluripotent stem cells

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

Beating organs-on-chip as technological platforms in drug screening: Advanced in vitro models of human physiology and pathology

Taking advantages of uBeat® technology, 3D beating Organs-on-Chip integrates the native complexity of human mechanical microenvironment into clinically relevant in vitro models of human organs and diseases.

Engineering and Monitoring 3D Cell Constructs with Time-Evolving Viscoelasticity for the Study of Liver Fibrosis In Vitro

Liver fibrosis is generally associated with an over-production and crosslinking of extracellular matrix proteins, causing a progressive increase in both the elastic and viscous properties of the hepatic tissue. We describe a strategy for mimicking and monitoring the mechano-dynamics of the 3D microenvironment associated with liver fibrosis. Cell-laden gelatin hydrogels were crosslinked with microbial transglutaminase using a purpose-designed cytocompatible two-step protocol, which allows for the exposure of cells to a mechanically changing environment during culturing. A bioreactor was re-engineered to monitor the mechanical properties of cell constructs over time. The results showed a shift towards a more elastic (i.e., solid-like) behaviour, which is likely related to an increase in cell stress. The method effectively mimics the time-evolving mechanical microenvironment associated with liver fibrosis and could provide novel insights into pathophysiological processes in which both elastic and viscous properties of tissues change over time.