Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P11: Mechanics of Cells and Tissues IVFocus Session Live
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Sponsoring Units: DBIO DSOFT GSNP DPOLY Chair: Itai Cohen; Moumita Das, Rochester Institute of Technology |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P11.00001: Correlated Dilution of Filamentous Networks Leads to Reentrant Rigidity Percolation Invited Speaker: Jonathan Michel Networks of stiff, cross-linked polymers are widespread in biological tissues. A remarkable phenomenon exhibited by such tissues is an abrupt increase in elastic moduli as a result of a comparatively small increase in polymer density. Biopolymer network models commonly consist of an isotropic and homogeneous arrangement of bonds with harmonic stretching and bending stiffness, with a certain fraction of bonds removed at random. Whereas dilution of bonds has customarily been spatially uniform, many real tissues, such as articular cartilage, exhibit alternating dense and sparse regions. To capture spatially varying density, we modify the dilution of bonds to introduce structural correlation, so that a bond is more likely to be retained if neighboring bonds have already been retained, following recent work on colloidal gels. With greater structural correlation, we initially find a network reaches a non-zero shear modulus with fewer bonds, but that for high correlation, rigidity demands even more material be retained than in the case of spatially uniform dilution. We again find a peak in the measure of non-affinity of network displacement to coincide with rigidity percolation, but observe that this peak becomes higher and broader with increasing correlation. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P11.00002: Detecting force transmission pathways in 3D fibrin networks Qingda Hu, Elliot Botvinick Cells receive mechanical cues from their surroundings. Of particular interest are 3D fibrous matrices, offering a more physiological microenvironment. The path of force transmission through fibrous extracellular matrices is difficult to predict but of great importance to cell biology. Force transmission in bulk fibrous matrices is well studied, but the local fiber network response to forces is yet to be fully understood. Modelling has provided insight into how these 3D fibrous networks transmit forces, however direct measurement of force transmission through these networks remains a challenge. Here we detect which fibers respond to an applied force using fluorescence confocal microscopy and optical tweezers in a 3D fibrin hydrogel. Embedded microbeads are oscillated by optical tweezers at low frequency to explore force transmission while avoiding the nonlinearity of the network observed at higher frequencies. Pixel intensity fluctuations in time are used to detect the subset of fibers within a local network that oscillate in phase with the microbead at the drive frequency, and thus carry tension. With some extension of this approach, the results can be used to further inform our understanding of fibrous biological materials at the mesoscale. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P11.00003: Rigidity and fracture of biopolymer double networks Pancy Lwin, Andrew B Sindermann, Leo Sutter, Thomas Wyse Jackson, Itai Cohen, Lawrence Bonassar, Moumita Das Composite biopolymer networks in soft tissues such as cartilage have remarkable tunable mechanics and resistance to failure. To achieve these properties in engineered materials, we need a mechanistic understanding of the structure-function properties that determine the workable range of strains and stresses over which the system maintains its integrity and mechanisms that facilitate protection against fracture. We study a model that combines two structure-function frameworks - a double network (DN) made of a stiff network and a flexible network, and rigidity percolation theory. We find that the rigidity percolation threshold for the stiff network can be varied, even significantly lowered by changing the flexible network's concentration. Second, the flexible network can modulate the mechanics of the DN (strength, extensibility, and toughness) far more efficiently when the stiff network is just above its rigidity percolation threshold. Third, the DN can further be tuned to either be more extensible for low concentrations of the flexible network, breaking gradually, or be stronger, breaking in a more brittle fashion for high concentrations of the flexible network. Our results show how structure and composition can be tuned to resist cracks. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P11.00004: Optimal elasticity of biological networks Henrik Ronellenfitsch Reinforced elastic sheets surround us in daily life, from concrete shell buildings |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P11.00005: Mechanobiology of hyaluronan-rich glycocalyx: How do giant polymers modulate cell adhesion. Yu Jing, Shlomi Cohen, Jessica Faubel, Wenbin Wei, Jennifer E. Curtis The overexpression of macromolecules in the cellular glycocalyx is strongly associated with cells in a pathological state, especially in cancer cell metastasis. More generally, hyaluronan-rich glycocalyx is upregulated during physiological processes that involve changes in adhesion or cell migration. In these scenarios, hyaluronan takes the form of giant megaDalton polymers that play a central physiochemical role in glycocalyx function. To examine the physical role of HA glycocalyx, we developed a biomimetic glycocalyx consisting of a microns-thick hyaluronan polymer brush. Our studies provide strong evidence that cells easily compress hyaluronan glycocalyx in order to establish molecular adhesions to the substrate. Further, we demonstrate that the compressed hyaluronan matrix modulates biophysical quantities such as the kinetics of cell adhesion, the topography of the cell membrane, and therefore, the tension on molecular adhesions. This work is instrumental in establishing a mechanistic view of how glycocalyx mediates cellular behavior. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P11.00006: Role of cellular rearrangement time delays on the rheology of vertex models for confluent tissues Gonca Erdemci-Tandogan, M. Lisa Manning Morphogenesis involve global-scale changes to the tissue, and in confluent tissues, large-scale deformation requires cell rearrangements. In its simplest form, a cell rearrangement involves neighbor exchanges among four cells, called a T1 transition. In order to complete the T1, a sequence of molecular processes, such as endocytosis of adhesion molecules, must occur over a finite time. In this work, we incorporate this idea by augmenting vertex models to require a fixed, finite time for T1 transitions, which we call the “T1 delay time”. We study how variations in T1 delay affect tissue mechanics, by quantifying the relaxation time of tissues in the presence of T1 delays and comparing to the cell-shape based timescale that characterizes fluidity in the absence of any T1 delays. We show the molecular-scale T1 delay timescale dominates over the cell shape-scale collective response timescale when the T1 delay is the larger of the two. We extend this analysis to anisotropic tissues under convergent extension, finding similar results. We also find correlations between the rate of extension and rosette formation. The T1 delay time could be a mechanism to regulate tissue mechanics and rosette formation during morphogenesis. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P11.00007: A Rigidity Percolation Framework to Understand How Biologically Induced Changes in Constituent Composition Alter Cartilage Shear Mechanics Thomas Wyse Jackson, Jonathan Michel, Pancy Lwin, Lena Bartell, Lisa Fortier, Moumita Das, Lawrence Bonassar, Itai Cohen Cartilage can sustain millions of loading cycles over decades of use and outperforms any synthetic substitute. The bulk properties of this tissue primarily reflect the mechanics of an extracellular matrix, comprised of only two components: a collagen network and a reinforcing proteoglycan network. Diseases of cartilage involve loss of extracellular matrix constituents due to mechanical overloading or biochemical processes. A mystery associated with disease is why the same amount of degradation in some cases leads to minor changes in modulus, while in other cases leads to tissue collapse. We present experiments and theory in support of a rigidity percolation framework that explains how the shear properties of cartilage depend on the concentrations of both constituents. This framework predicts a sensitivity to degradation that depends on the collagen concentration. When the collagen network is sparse, changes in aggrecan concentration create dramatic changes in modulus yet deeper into the tissue, similar changes in aggrecan leave properties nearly unchanged. This framework provides a tool for understanding the effect of degradation of cartilage on its shear properties, and its function in vivo. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P11.00008: Variations in the extensibility of fibrin fibers Christine Helms Fibrin fibers, a major structural component of blood clots, are one of the most extensible natural fibers. However, the mechanisms responsible for their extensibility and their internal structure are not fully known. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P11.00009: Membrane deflection due to ultrasound results in cellular depolarization Aditya Vasan, Jeremy Orosco, Uri Magaram, Mark Duque Ramirez, Sreekanth Chalasani, James R Friend Stimulating neural activity currently uses electrical, optical or chemical techniques. They are either invasive or have poor spatiotemporal resolution. Megahertz-order ultrasound noninvasively improves the resolution of neural stimulation and sonogenetic techniques improve sensitivity. However, the mechanism tying ultrasound to neural activity is poorly understood. The few models of the phenomenon to date have intrinsic errors. Experiments so far lack sufficient imaging speed to resolve the phenomena. We have developed a model that predicts membrane deflection due to an ultrasound stimulus, and verified the results of this model with novel measurements of the membrane's motion using high-speed digital holographic microscopy. Our experiments reveal that neuronal membranes can deflect by as much as 150 nm. We have used single neuron current clamp electrophysiology to verify transmembrane voltage changes predicted by the model. Our results form the foundation of our work in developing ultrasound-based neuromodulation devices for freely-moving mice and further exploration into sonogenetic tools for communication, diagnostics, and disease treatment. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P11.00010: Real-time imaging of fibrin-bead networks under compression Bobby Carroll, Alison Patteson Mechanical forces are an essential aspect of development and healthy tissue maintenance. While the response of tissues and biopolymer networks under shear has received much attention, we are not yet able to deduce the mechanical properties of tissue from the microstructure alone. An emerging in vitro model system for tissues are fibrin networks containing inert particle inclusions. These networks mimic the cell-loaded structure of the extracellular matrix (ECM) and recapitulate distinct mechanical behavior of tissues, such as an apparent bulk stiffening of the sample under uniaxial compression. Here, we develop a custom compression device that allows for real-time imaging of the local micro-structure in the fibrin-bead network. We map the emergence of a densified network compaction near the driving plate and observe poro-elastic network relaxations for sufficiently high compression rates. Our results indicate a predominate role of non-uniform network compaction in modeling the compression-stiffening behavior of fibrin-bead networks and tissues. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P11.00011: Viscoelastic properties of tissues in the vertex model Sijie Tong, Navreeta Singh, Rastko Sknepnek, Andrej Kosmrlj Epithelial cell sheets have been studied extensively in various contexts ranging from embryonic development to cancer metastasis, where collective groups of cells can drastically reorganize and move over substantial distances like a fluid. The collective behavior of epithelial cells is commonly simulated with the vertex model. It was previously demonstrated that the vertex model can describe both the solid- and fluid-like behavior by tuning the target cell-shape parameter p0=P0/sqrt[A0], where P0 and A0 are the preferred perimeter and area of cells, respectively. The shear modulus is finite for low values of p0 and it vanishes beyond the critical value of the cell-shape parameter (p0>pc). However, the viscoelastic properties of tissues in the vertex model have not been yet studied systematically. Here, we performed rheological tests by applying an oscillatory shear strain and measuring the shear stress. We found that tissues can be described with the standard linear solid model in the solid phase (p0<pc) and with Burgers material model in the fluid phase (p0>pc). In both regimes, the values of elastic spring constants decrease towards 0 and the relaxation timescales diverge as p0 approaches the critical value pc. |
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