38 research outputs found
Inferring the dynamics of underdamped stochastic systems
Many complex systems, ranging from migrating cells to animal groups, exhibit
stochastic dynamics described by the underdamped Langevin equation. Inferring
such an equation of motion from experimental data can provide profound insight
into the physical laws governing the system. Here, we derive a principled
framework to infer the dynamics of underdamped stochastic systems from
realistic experimental trajectories, sampled at discrete times and subject to
measurement errors. This framework yields an operational method, Underdamped
Langevin Inference (ULI), which performs well on experimental trajectories of
single migrating cells and in complex high-dimensional systems, including
flocks with Viscek-like alignment interactions. Our method is robust to
experimental measurement errors, and includes a self-consistent estimate of the
inference error
Cell contraction induces long-ranged stress stiffening in the extracellular matrix
Animal cells in tissues are supported by biopolymer matrices, which typically
exhibit highly nonlinear mechanical properties. While the linear elasticity of
the matrix can significantly impact cell mechanics and functionality, it
remains largely unknown how cells, in turn, affect the nonlinear mechanics of
their surrounding matrix. Here we show that living contractile cells are able
to generate a massive stiffness gradient in three distinct 3D extracellular
matrix model systems: collagen, fibrin, and Matrigel. We decipher this
remarkable behavior by introducing Nonlinear Stress Inference Microscopy
(NSIM), a novel technique to infer stress fields in a 3D matrix from nonlinear
microrheology measurement with optical tweezers. Using NSIM and simulations, we
reveal a long-ranged propagation of cell-generated stresses resulting from
local filament buckling. This slow decay of stress gives rise to the large
spatial extent of the observed cell-induced matrix stiffness gradient, which
could form a mechanism for mechanical communication between cells
Learning the dynamics of cell-cell interactions in confined cell migration
The migratory dynamics of cells in physiological processes, ranging from
wound healing to cancer metastasis, rely on contact-mediated cell-cell
interactions. These interactions play a key role in shaping the stochastic
trajectories of migrating cells. While data-driven physical formalisms for the
stochastic migration dynamics of single cells have been developed, such a
framework for the behavioral dynamics of interacting cells still remains
elusive. Here, we monitor stochastic cell trajectories in a minimal
experimental cell collider: a dumbbell-shaped micropattern on which pairs of
cells perform repeated cellular collisions. We observe different characteristic
behaviors, including cells reversing, following and sliding past each other
upon collision. Capitalizing on this large experimental data set of coupled
cell trajectories, we infer an interacting stochastic equation of motion that
accurately predicts the observed interaction behaviors. Our approach reveals
that interacting non-cancerous MCF10A cells can be described by repulsion and
friction interactions. In contrast, cancerous MDA-MB-231 cells exhibit
attraction and anti-friction interactions, promoting the predominant relative
sliding behavior observed for these cells. Based on these experimentally
inferred interactions, we show how this framework may generalize to provide a
unifying theoretical description of the diverse cellular interaction behaviors
of distinct cell types
Actively crosslinked microtubule networks: mechanics, dynamics and filament sliding
Cytoskeletal networks are foundational examples of active matter and central
to self-organized structures in the cell. In vivo, these networks are active
and heavily crosslinked. Relating their large-scale dynamics to properties of
their constituents remains an unsolved problem. Here we study an in vitro
system made from microtubules and XCTK2 kinesin motors, which forms an aligned
and active gel. Using photobleaching we demonstrate that the gel's aligned
microtubules, driven by motors, continually slide past each other at a speed
independent of the local polarity. This phenomenon is also observed, and
remains unexplained, in spindles. We derive a general framework for coarse
graining microtubule gels crosslinked by molecular motors from microscopic
considerations. Using the microtubule-microtubule coupling, and force-velocity
relationship for kinesin, this theory naturally explains the experimental
results: motors generate an active strain-rate in regions of changing polarity,
which allows microtubules of opposite polarities to slide past each other
without stressing the material
Soft matter roadmap
Soft materials are usually defined as materials made of mesoscopic entities, often self-organised, sensitive to thermal fluctuations and to weak perturbations. Archetypal examples are colloids, polymers, amphiphiles, liquid crystals, foams. The importance of soft materials in everyday commodity products, as well as in technological applications, is enormous, and controlling or improving their properties is the focus of many efforts. From a fundamental perspective, the possibility of manipulating soft material properties, by tuning interactions between constituents and by applying external perturbations, gives rise to an almost unlimited variety in physical properties. Together with the relative ease to observe and characterise them, this renders soft matter systems powerful model systems to investigate statistical physics phenomena, many of them relevant as well to hard condensed matter systems. Understanding the emerging properties from mesoscale constituents still poses enormous challenges, which have stimulated a wealth of new experimental approaches, including the synthesis of new systems with, e.g. tailored self-assembling properties, or novel experimental techniques in imaging, scattering or rheology. Theoretical and numerical methods, and coarse-grained models, have become central to predict physical properties of soft materials, while computational approaches that also use machine learning tools are playing a progressively major role in many investigations. This Roadmap intends to give a broad overview of recent and possible future activities in the field of soft materials, with experts covering various developments and challenges in material synthesis and characterisation, instrumental, simulation and theoretical methods as well as general concepts
The variety of ordering transitions in liquids characterized by a locally favoured structure
We present a new lattice model of liquids in which the energy of a configuration is determined by the local coordination environments rather than pairwise interactions. This model is used to explore how the accumulation of order on cooling depends on the geometry of the locally favoured structure. We find that, while high-symmetry local structures result in ordering that occurs predominantly via a thermodynamic freezing transition, liquids characterised by a low-symmetry local structure exhibit a significant increase in local order on cooling before crystallizing
Learning the non-equilibrium dynamics of Brownian movies
Time-lapse microscopy imaging provides direct access to the dynamics of soft and living systems. At mesoscopic scales, such microscopy experiments reveal intrinsic thermal and non-equilibrium fluctuations. These fluctuations, together with measurement noise, pose a challenge for the dynamical analysis of these Brownian movies. Traditionally, methods to analyze such experimental data rely on tracking embedded or endogenous probes. However, it is in general unclear, especially in complex many-body systems, which degrees of freedom are the most informative about their non-equilibrium nature. Here, we introduce an alternative, tracking-free approach that overcomes these difficulties via an unsupervised analysis of the Brownian movie. We develop a dimensional reduction scheme selecting a basis of modes based on dissipation. Subsequently, we learn the non-equilibrium dynamics, thereby estimating the entropy production rate and time-resolved force maps. After benchmarking our method against a minimal model, we illustrate its broader applicability with an example inspired by active biopolymer gels
Morbidity of thyroid surgery.
BACKGROUND: Morbidity is today\u27s concern in thyroid surgery. The purpose of this paper was to quantify risk factors\u27 contribution to morbidity rates.
METHODS: During 50 months, 1,163 patients undergoing 1,192 thyroidectomies at one hospital were reviewed at follow-up of 8 to 58 months.
RESULTS: There was 1 death (0.08%). Wound morbidity included 19 hematomas (1.6%), 3 chyle leaks (0.2%), and 6 abscesses (0.5%). Mean hospital stay was 4.3 days after surgery without drain and 5.3 days with drain (P \u3c 0.01). Temporary and permanent hypoparathyroidism (TH; PH) rates were 20% and 4%. Parathyroid autografting and excision rates were 19% and 9%. TH rates were higher after parathyroid autografting or accidental excision (P \u3c 0.01). There was no correlation between the severity of TH and the number of lymph nodes at neck dissection nor between postoperative serum calcium levels and the number of parathyroids identified at bilateral surgery. Temporary and permanent recurrent laryngeal nerve (RLN) palsy (TRLNP; PRLNP) rates were 2.9% and 0.5% (0.3% of 2,010 RLNs at risk). PH and TRLNP (not PRLNP) rates were higher after completion or total thyroidectomy with node dissection (P \u3c 0.01). TRLNP and PRLNP rates after RLN exposure and after nonexposure were not statistically different. Surgical volume had no bearing on hematoma, abscess, TH, PH, TRLNP, and PRLNP rates.
CONCLUSIONS: High surgical volume, identifying parathyroids and RLNs, failed to reduce morbidity. Completion and total thyroidectomy with node dissection increased PH and TRLNP (not PRLNP) rates