Emergence of biphasic versus monotonic response of actin retrograde flow and cell traction force with varying substrate rigidity.

The transmission of cytoskeletal forces to the extracellular matrix through focal adhesion complexes is essential for a multitude of biological processes, such as cell migration, cell differentiation, tissue development, and cancer progression, among others. During migration, focal adhesions arrest the actin retrograde flow towards the cell interior, allowing the cell front to move forward. Here, we address a puzzling observation of the existence of two distinct phenomena: a biphasic vs a monotonic relationship of the retrograde flow and cell traction force with substrate rigidity.

Inertial effect on evasion and pursuit dynamics of prey swarms: the emergence of a favourable mass ratio for the predator-prey arms race.

We show, based on a theoretical model, how inertia plays a pivotal role in the survival dynamics of a prey swarm while chased by a predator. With the varying mass of the prey and predator, diverse escape patterns emerge, such as circling, chasing, maneuvering, dividing into subgroups, and merging into a unitary group, similar to the escape trajectories observed in nature. Moreover, we find a transition from non-survival to survival of the prey swarm with increasing predator mass.

Collective motion: Influence of local behavioural interactions among individuals.

Fascinating patterns are displayed in nature due to the collective coherent motion of many living organisms. The origin of collective behaviours is diverse as the group members benefit in various ways: large resources of food, mating choices, nesting, and protection from predators, to name a few. It is still not well understood how complex behaviours emerge from a collective group that are otherwise not displayed at the level of solitary individuals.

Cholate-conjugated cationic polymers for regulation of actin dynamics.

Cytoskeletal movement is a compulsory necessity for proper cell functioning and is largely controlled by actin filament dynamics. The actin dynamics can be fine-tuned by various natural and artificial materials including cationic proteins, polymers, liposomes, and lipids, although most of the synthetic substrates have toxicity issues. Herein, we show actin nucleation and stabilization with a synthetic family of cholic acid (CA)-conjugated cationic macromolecules.

Fatty acid-based polymeric micelles to ameliorate amyloidogenic disorders.

To develop anti-amyloidogenic inhibitors for ameliorating the treatment of diabetes, herein, we have synthesised amphiphilic block copolymers with side-chain fatty acid (FA) moieties reversible addition fragmentation chain-transfer (RAFT) polymerization. We addressed the unexplored role of FA pendants in the FA-tethered block copolymers (FABC) towards modulating the insulin fibrillation process with the aid of different biophysical techniques. Experimental findings established that FABC micelles can elongate the lag phase time to a greater extent and exhibit significant inhibitory potencies, with the more pronounced effect observed in stearic acid-based polymeric micelles (SABC475).

Growth kinetics and power laws indicate distinct mechanisms of cell-cell interactions in the aggregation process.

Cellular aggregation is a complex process orchestrated by various kinds of interactions depending on the environment. Different interactions give rise to different pathways of cellular rearrangement and the development of specialized tissues. To distinguish the underlying mechanisms, in this theoretical work, we investigate the spontaneous emergence of tissue patterns from an ensemble of single cells on a substrate following three leading pathways of cell-cell interactions, namely, direct cell adhesion contacts, matrix-mediated mechanical interaction, and chemical signaling.

Efficient flocking: metric versus topological interactions.

Flocking is a fascinating phenomenon observed across a wide range of living organisms. We investigate, based on a simple self-propelled particle model, how the emergence of ordered motion in a collectively moving group is influenced by the local rules of interactions among the individuals, namely, metric versus topological interactions as debated in the current literature. In the case of the metric ruling, the individuals interact with the neighbours within a certain metric distance; by contrast, in the topological ruling, interaction is confined within a number of fixed nearest neighbours.

Survival probability of a lazy prey on lattices and complex networks.

We develop a framework to analyse the survival probability of a prey following a minimal effort evasion strategy, that is being chased by N predators on finite lattices or complex networks. The predators independently perform random walks if the prey is not within their sighting radius, whereas, the prey only moves when a predator moves onto a node within its sighting radius. We verify the proposed framework on three different finite lattices with periodic boundaries through numerical simulations and find that the survival probability (P) decays exponentially with a decay rate proportional to P(N, k) (number of permutations), where k is the minimum number of predators required to capture a prey.

Survival chances of a prey swarm: how the cooperative interaction range affects the outcome.

A swarm of prey, when attacked by a predator, is known to rely on their cooperative interactions to escape. Understanding such interactions of collectively moving prey and the emerging patterns of their escape trajectories still remain elusive. In this paper, we investigate how the range of cooperative interactions within a prey group affects the survival chances of the group while chased by a predator.

Stick-slip dynamics of migrating cells on viscoelastic substrates.

Stick-slip motion, a common phenomenon observed during crawling of cells, is found to be strongly sensitive to the substrate stiffness. Stick-slip behaviors have previously been investigated typically using purely elastic substrates. For a more realistic understanding of this phenomenon, we propose a theoretical model to study the dynamics on a viscoelastic substrate.

Aggregation dynamics of active cells on non-adhesive substrate.

Cellular self-assembly and organization are fundamental steps for the development of biological tissues. In this paper, within the framework of a cellular automata model, we address how an ordered tissue pattern spontaneously emerges from a randomly migrating single cell population without the influence of any external cues. This model is based on the active motility of cells and their ability to reorganize due to cell-cell cohesivity as observed in experiments.

A general model of focal adhesion orientation dynamics in response to static and cyclic stretch.

Understanding cellular response to mechanical forces is immensely important for a plethora of biological processes. Focal adhesions are multimolecular protein assemblies that connect the cell to the extracellular matrix and play a pivotal role in cell mechanosensing. Under time-varying stretches, focal adhesions dynamically reorganize and reorient and as a result, regulate the response of cells in tissues.

Theoretical concepts and models of cellular mechanosensing.

Recent discoveries have established that mechanical properties of the cellular environment such as its rigidity, geometry, and external stresses play an important role in determining the cellular function and fate. Mechanical properties have been shown to influence cell shape and orientation, regulate cell proliferation and differentiation, and even govern the development and organization of tissues. In recent years, many theoretical and experimental investigations have been carried out to elucidate the mechanisms and consequences of the mechanosensitivity of cells.

Nonlinear dynamics of cell orientation.

The nonlinear dependence of cellular orientation on an external, time-varying stress field determines the distribution of orientations in the presence of noise and the characteristic time, tauc, for the cell to reach its steady-state orientation. The short, local cytoskeletal relaxation time distinguishes between high-frequency (nearly perpendicular) and low-frequency (random or parallel) orientations. However, tauc is determined by the much longer, orientational relaxation time.

Intermittent peel front dynamics and the crackling noise in an adhesive tape.

We report a comprehensive investigation of a model for peeling of an adhesive tape along with a nonlinear time series analysis of experimental acoustic emission signals in an effort to understand the origin of intermittent peeling of an adhesive tape and its connection to acoustic emission. The model represents the acoustic energy dissipated in terms of Rayleigh dissipation functional that depends on the local strain rate. We show that the nature of the peel front exhibits rich spatiotemporal patterns ranging from smooth, rugged, and stuck-peeled configurations that depend on three parameters, namely the ratio of inertial time scale of the tape mass to that of the roller, the dissipation coefficient, and the pull velocity.

Dynamical theory of active cellular response to external stress.

We present a comprehensive, theoretical treatment of the orientational response to external stress of active, contractile cells embedded in a gel-like elastic medium. The theory includes both the forces that arise from the deformation of the matrix as well as forces due to the internal regulation of the stress fibers and focal adhesions of the cell. We calculate the time-dependent response of both the magnitude and the direction of the elastic dipole that characterizes the active forces exerted by the cell, for various situations.

Do cells sense stress or strain? Measurement of cellular orientation can provide a clue.

We predict theoretically the steady-state orientation of cells subject to dynamical stresses that vary more quickly than the cell relaxation time. We show that the orientation is a strong function of the Poisson's ratio, nu, of the matrix when cell activity is governed by the matrix strain; if cell activity is governed by the matrix stress, the orientation depends only weakly on nu. These results can be used to differentiate systems in which the strain or the stress determine the setpoint for the mechanosensitivity of cells.

Dynamics of the peel front and the nature of acoustic emission during peeling of an adhesive tape.

We investigate the peel front dynamics and acoustic emission (AE) of an adhesive tape within the context of a recent model by including an additional dissipative energy that mimics bursts of acoustic signals. We find that the nature of the peeling front can vary from a smooth to a stuck-peeled configuration depending on the values of dissipation coefficient, inertia of the roller, and mass of the tape. Interestingly, we find that the distribution of AE bursts shows power law statistics with two scaling regimes with increasing pull velocity as observed in experiments.

Missing physics in stick-slip dynamics of a model for peeling of an adhesive tape.

It is now known that the equations of motion for the contact point during peeling of an adhesive tape mounted on a roll introduced earlier are singular and do not support dynamical jumps across the two stable branches of the peel force function. By including the kinetic energy of the tape in the Lagrangian, we derive equations of motion that support stick-slip jumps as a natural consequence of the inherent dynamics. In the low mass limit, these equations reproduce solutions obtained using a differential-algebraic algorithm introduced for the earlier equations.

Dynamics of stick-slip in peeling of an adhesive tape.

We investigate the dynamics of peeling of an adhesive tape subjected to a constant pull speed. We derive the equations of motion for the angular speed of the roller tape, the peel angle and the pull force used in earlier investigations using a Lagrangian. Due to the constraint between the pull force, peel angle and the peel force, it falls into the category of differential-algebraic equations requiring an appropriate algorithm for its numerical solution.