Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L22: Biomaterials III: Tissue-Scale PhysicsFocus
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Sponsoring Units: DBIO DCP DMP DPOLY Chair: Esther Amstad Room: 303 |
Wednesday, March 4, 2020 8:00AM - 8:36AM |
L22.00001: Filament Nucleation Tunes Mechanical Memory in Active Polymer Networks Invited Speaker: Michael Murrell Incorporating growth into contemporary material functionality presents a grand challenge in materials design. The F-actin cytoskeleton is an active polymer network which serves as the mechanical scaffolding for eukaryotic cells, growing and remodeling in order to determine changes in cell shape. Nucleated from the membrane, filaments polymerize and grow into a dense network whose dynamics of assembly and disassembly, or ‘turnover’, coordinates both fluidity and rigidity. Here, we vary the extent of F-actin nucleation from a membrane surface in a biomimetic model of the cytoskeleton constructed from purified protein. We find that nucleation of F-actin mediates the accumulation and dissipation of polymerization-induced F-actin bending energy. At high and low nucleation, bending energies are low and easily relaxed yielding an isotropic material. However, at an intermediate critical nucleation, stresses are not relaxed by turnover and the internal energy accumulates 100-fold. In this case, high filament curvatures template further assembly of F-actin, driving the formation and stabilization of vortex-like topological defects. Thus, nucleation coordinates mechanical and chemical timescales to encode shape memory into active materials. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L22.00002: Bioinspired materials with self-adaptable mechanical behaviors Santiago Orrego, Zhezhi Chen, Urszula Krekora, Decheng Hou, Seung-Yeol Jeon, Matthew Pittman, Carolina Montoya, Yun Chen, Sung Kang Nature produces outstanding biomaterials for structural applications such as bones and woods that can adapt to their surrounding environment. However, it is a challenge for synthetic materials to change and adapt their structures and properties to address the changes in loading conditions. To overcome the issue, we have investigated synthetic materials inspired by bone that trigger mineral syntheses from ionic solutions on scaffolds upon mechanical loadings so that they can self-adapt to mechanical loadings. For example, we observed a 30-180% increase in the modulus of the material upon different magnitudes of periodic loadings. Moreover, the mechanism allows a one-step route for making graded materials by controlling stress distribution along the scaffold. The findings can contribute to addressing the current challenges of synthetic materials for load-bearing applications from self-adaptive capabilities. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L22.00003: The continuum of allosteric behavior in mechanical networks Jason Rocks, Andrea Jo-Wei Liu, Eleni Katifori Allosteric regulation in proteins is often accompanied by conformational transitions, facilitating the transmission of mechanical signals between distant ligand binding sites. Analyses of allosteric proteins have revealed a variety of archetypal motions ranging from hinge-like or shear mechanisms to allosteric strain pathways connecting different binding sites. Here we investigate the range of possible motions that can be achieved in mechanical networks tuned to perform allostery. Using an analysis based on persistent homology, we develop a description of allosteric motion which quantifies and unifies all mechanisms into a single framework. We show that while some networks fall into distinct classes of archetypal designs, most fall along a continuum consisting of combinations of hinges, strain pathways and isostatic architectures. We apply this analysis to a collection of proteins, allowing us to identify potential sets of residues that are important for facilitating allosteric communication between different binding sites. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L22.00004: A microfluidic model of periarterial spaces in the glymphatic system Keelin Quirk, Kerstin N. Nordstrom, Douglas H Kelley In the glymphatic system, cerebrospinal fluid enters through periarterial spaces and removes metabolic waste from the brain’s interstitial spaces. Previous experiments in live mice have found that a wave along artery walls produced by the heartbeat induces flow in the surrounding perivascular space. However, the mechanisms driving the flow are still not well understood, as many mechanisms may be acting simultaneously. We have designed microfluidic devices to serve as two-dimensional models of periarterial spaces. Using particle tracking velocimetry, we analyze induced flow driven by a peristaltic wave at the frequency range representative of human heartbeats. We find the overall bulk flow induced by the membrane wave travels in the same direction as the wave and increases with frequency. However, during an individual pump cycle, we observe both forward and backwards flow. We also measure the phase shift of the induced flow. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L22.00005: Ultrafast Finger Snap is Mediated by a Frictional Skin Latch Raghav Acharya, Elio Challita, Saad Bhamla The snap of a finger is a ubiquitous motion that has been seen across cultures and times. Using high-speed imaging, we analyze finger snap dynamics for the first time. We find that the mechanics of the snap are strongly mediated by human skin friction, which acts as a latch to generate rapid motion. The skin frictional latch is optimally tuned to enable maximum kinematic performance as the angular accelerations observed during a snap are one of the fastest human motions known. A simple scaling relationship is found that links the latch geometry to the performance of snapping motion across multiple organisms from termites to humans. Ultimately, our work reveals how friction between surfaces can be harnessed as tunable and scalable latching mechanism, with applications ranging from increasing grip in biomedical prosthetic surfaces to generating high force and accelerations in tiny robots. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L22.00006: Unveiling Interfacial Properties of Surfactant Assemblies Mimicking Healthy and Diseased States in Lung Membranes Marilyn Porras-Gomez, Cecilia Leal Lipid-protein complexes conform the basis of pulmonary surfactants covering the respiratory surface and mediating gas exchange in lungs, yet how they contribute to alveoli membrane functions in healthy and diseased conditions is not sufficiently understood. Alveolar stability appears to be controlled by the passive elastic properties of the pulmonary tissue as well as the mechanical performance of the surfactant membranes and an unbalance of these is associated with different respiratory dysfunctions and pathologies. Cardiolipin is a mitochondrial lipid overexpressed in mammalian lungs infected by bacterial pneumonia, likely to play a role in alveolar stability. We performed structural and mechanical characterization by GISAXS, AFM and Fast Force Mapping on lipid-based mimicking pulmonary membranes in healthy and diseased states. Our preliminary results unveiled that pulmonary membranes suffer structural transformations induced by cardiolipin and calcium ions. Membrane contacts, or stalks, might induce a significant increase in oxygen gas permeation that can lead to imbalance in alveoli gas exchange. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L22.00007: Hybrid active matter: particles and cellular aggregates Francoise Brochard-Wyart We investigate the collective migration of cell on adhesive substrates, using 3D cellular aggregates as a model system. Aggregates spread by expanding outwards a cell monolayer, which may dewet, causing the aggregates to move as “Giant Keratocytes”. We interpret this motion by a symmetry-breaking of cell polarity in analogy to active droplets. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L22.00008: The role of heterogeneous environments and docetaxel gradients in the emergence of polyploid, mesenchymal and resistant prostate cancer cells. Robert Austin The ability of a population of PC3 prostate epithelial cancer cells to become resistant to docetaxel therapy and progress to a mesenchymal state remains a fundamental problem. The progression towards resistance is difficult to directly study in hetero- geneous ecological environments such as tumors. In this work, we use a micro-fabricated “evolution accelerator” environment to create a complex heterogeneous yet controllable in-vitro environment with a spatially-varying drug concentration. With such a structure we observe the rapid emergence of a surprisingly large number of polyploid giant cancer cells (PGCCs) in regions of very high drug concentration, which does not occur in conventional cell culture of uniform concentration. This emergence of PGCCs in a high drug environment is due to migration of diploid epithelial cells from regions of low drug concentration, where they proliferate, to regions of high drug concentration, where they rapidly convert to PGCCs. Such a mechanism can only occur in spatially-varying rather than homogeneous environments. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L22.00009: Predicting Pericellular Matrix Structure from Simple Models of Hyaluronan Secretion Jan Scrimgeour The synthesis and release of hyaluronan (HA) from the surface of living cells is essential to the maintenance of living tissues and fluids. HA synthesis is controlled by the hyaluronan synthase enzyme, which assembles the polymer chain, and extrudes it through the cell membrane before it is released into the extracellular space. This enzymatic process forms a critical link between the state of the cellular system (protein expression, metabolism, etc) and the properties of cellular interfaces, the extracellular matrix and many biofluids. I present a simple kinetic model that allows an examination of the secretion process. This model enables simulation of the synthesis and release process, enabling prediction of the molecular weight distributions of HA that are tethered to, and released from the cell surface. In so doing this model offers direct insight into the structure of the pericellular matrix, a cell tethered, hyaluronan-rich interface that mediates many cell processes such as adhesion and cell surface access. In addition, the simulations suggest that time-resolved analysis of the hyaluronan molecular weight distributions produced by living cells after PCM digestion can reveal critical details of hyaluronan synthase function. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L22.00010: Electrical detection of Hachimoji nucleobases via a nanopore device incorporated in a graphene/h-BN heterostructure Ganesh Sivaraman, Fabio Arthur Leao de Souza, Maria Fyta, Ralph Hendrik Scheicher, Wanderla L. Scopel, Rodrigo G. Amorim Sold-state nanopores based on 2D materials have been proposed as a candidate for next generation sequencing (NGS) to achieve high throughput, label-free DNA and protein sequencing at low costs. A recent endeavor in synthetic biology resulted in the creation of stable DNA/RNA systems based on 8 (`hachi`) letter (`moji`) building blocks (i.e. 4 synthetic and 4 natural nucleobases) [1]. Although mutations and methylations of the natural nucleobases have been extensively investigated, synthetic nucleobases reported in the hachimoji system remain unexplored with such NGS methods. Hence in this talk, we propose a computational study based on density functional theory and non-equilibrium Green’s function formalism, to unravel the electrical read-out of synthetic and natural nucleobases. To this end, we propose a hybrid 2D nanopore formed in a graphene nanorod embedded within hexagonal boron nitride to perform the electrical read. We will go on to show that the proposed hybrid 2D nanopore can qualitatively discriminate natural and synthetic nucleobases. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L22.00011: Modeling the rheology of dense biological tissues Junxiang Huang, Dapeng Bi Shear forces in tissues are prevalent in many important biological processes including embryonic development, organogenesis and tumor invasion. However, the intercellular transmission of shear forces and the rheological response of a tissue remains poorly understood. In this work, we use a minimal vertex-based model to investigate the rheology of confluent epithelial tissues. We systematically probe the effects single-cell stiffness, polarized cell motility and the strain rate on the monolayer stress. We also elucidate how the interplay of these parameters affect the cellular rearrangements such as T1 transitions and the statistics of cell shapes in the tissue. |
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