Vesicle shape transformations driven by confined active filaments

In active matter systems, deformable boundaries provide a mechanism to organize internal active stresses. To study a minimal model of such a system, we perform particle-based simulations of an elastic vesicle containing a collection of polar active filaments. The interplay between the active stress organization due to interparticle interactions and that due to the deformability of the confinement leads to a variety of filament spatiotemporal organizations that have not been observed in bulk systems or under rigid confinement, including highly-aligned rings and caps. In turn, these filament assemblies drive dramatic and tunable transformations of the vesicle shape and its dynamics. We present simple scaling models that reveal the mechanisms underlying these emergent behaviors and yield design principles for engineering active materials with targeted shape dynamics.

Activity-induced instabilities of brain organoids

We present an analytical and numerical investigation of the activity-induced hydrodynamic instabilities in model brain organoids. While several mechanisms have been introduced to explain the experimental observation of surface instabilities in brain organoids, the role of activity has been largely overlooked. Our results show that the active stress generated by the cells can be a, previously overlooked, contributor to the emergence of surface deformations in brain organoids.

Dynamics and instabilities of the free boundary of a two-dimensional dry active nematic aggregate

The dynamics of of the free boundary of a two-dimensional aggregate of active rod-shaped particles in the nematic phase is considered theoretically. The aggregate is in contact with a hard boundary at $y=0$, a free boundary at $y=H(x,t)$, and in the $x$-direction the aggregate is of infinite size. The analysis shows that the behavior for an aggregate with steady-state particle density $\rho _s$, strength of active stress $\chi$, bulk modulus $\rho_s \beta$, and particles aligned perpendicular to the boundaries can be mapped to one with active stress strength $- \chi$, bulk modulus $\rho_s(\beta - \chi)$, and particles aligned parallel to the boundaries. For a contractile aggregate, when the particles are aligned parallel to the boundaries, the system is unstable in long wavelengths at any strength of contractility for any $H$, and the critical wavelength increases as $H$ increases; when the particles are aligned perpendicular to the boundaries, the system acquires a finite-wavelength instability at a critical active stress whose strength decreases as $H$ increases. The stability of an extensile aggregate can be obtained from the analysis for contractile aggregates and the aforementioned mapping, even though the corresponding physical mechanisms for the instabilities are different. Finally, in the limit $H \rightarrow \infty$, the free boundary is unstable for any contractile or extensile systems in the long wavelength limit.

Interplay of Active Stress and Driven Flow in Self-Assembled, Tumbling Active Nematics

Lyotropic chromonic liquid crystals (LCLCs) are a special type of hierarchical material in which self-assembled molecular aggregates are responsible for the formation of liquid crystal phases. Thanks to its unusual material properties and bio compatibility, it has found wide applications including the formation of active nematic liquid crystals. Recent experiments have uncovered tumbling character of certain LCLCs. However, how tumbling behavior modifies structure and flow in driven and active nematics is poorly understood. Here, we rely on continuum simulation to study the interplay of extensile active stress and externally driven flow in a flow-tumbling nematic with a low twist modulus to mimic nematic LCLCs. We find that a spontaneous transverse flow can be developed in a flow-tumbling active nematic confined to a hybrid alignment cell when it is in log-rolling mode at sufficiently high activities. The orientation of the total spontaneous flow is tunable by tuning the active stress. We further show that activity can suppress pressure-driven flow of a flow-tumbling nematic in a planar-anchoring cell but can also promote a transition of the director field under a pressure gradient in a homeotropic-anchoring cell. Remarkably, we demonstrate that the frequency of unsteady director dynamics in a tumbling nematic under Couette flow is invariant against active stress when below a threshold activity but exhibits a discontinuous increase when above the threshold at which a complex, periodic spatiotemporal director pattern emerges. Taken together, our simulations reveal qualitative differences between flow-tumbling and flow-aligning active nematics and suggest potential applications of tumbling nematics in microfluidics.

A Physiology-Guided Classification of Active-Stress and Active-Strain Approaches for Continuum-Mechanical Modeling of Skeletal Muscle Tissue

The well-established sliding filament and cross-bridge theory explain the major biophysical mechanism responsible for a skeletal muscle's active behavior on a cellular level. However, the biomechanical function of skeletal muscles on the tissue scale, which is caused by the complex interplay of muscle fibers and extracellular connective tissue, is much less understood. Mathematical models provide one possibility to investigate physiological hypotheses. Continuum-mechanical models have hereby proven themselves to be very suitable to study the biomechanical behavior of whole muscles or entire limbs. Existing continuum-mechanical skeletal muscle models use either an active-stress or an active-strain approach to phenomenologically describe the mechanical behavior of active contractions. While any macroscopic constitutive model can be judged by it's ability to accurately replicate experimental data, the evaluation of muscle-specific material descriptions is difficult as suitable data is, unfortunately, currently not available. Thus, the discussions become more philosophical rather than following rigid methodological criteria. Within this work, we provide a extensive discussion on the underlying modeling assumptions of both the active-stress and the active-strain approach in the context of existing hypotheses of skeletal muscle physiology. We conclude that the active-stress approach resolves an idealized tissue transmitting active stresses through an independent pathway. In contrast, the active-strain approach reflects an idealized tissue employing an indirect, coupled pathway for active stress transmission. Finally the physiological hypothesis that skeletal muscles exhibit redundant pathways of intramuscular stress transmission represents the basis for considering a mixed-active-stress-active-strain constitutive framework.

Physical Activity, Stress, and Physically Active Stress Management Behaviors Among University Students With Overweight/Obesity

Student physical activity is associated with lower stress. Research gaps remain regarding the types of stress management behaviors students use and how these behaviors are associated with students’ activity levels. This study examined associations between physical activity and stress management behaviors among students (18-35 years). Students with overweight/obesity (n = 405) attending universities in 2 urban locations enrolled in a randomized controlled trial to promote healthy weight and completed the following baseline measurements: perceived stress, stress management behaviors, accelerometer-measured physical activity, and demographic characteristics. Perceived stress did not differ by physical activity status or race. A greater proportion of students meeting moderate-to-vigorous physical activity guidelines used physically active stress management behaviors compared to those not meeting guidelines (74% vs 56%; P = .006), and students using physically active stress management had lower stress scores (13.1 vs 15.5; P = .003). Among Black and White students only (n = 306), a greater proportion of White students used physically active stress management behaviors compared to Black students (77% vs 62%, P = .013). Results indicate differences in stress management behaviors by student activity level and race. During times of high stress, colleges/universities might support students by promoting stress management and physical activity in tandem, and tailoring messages to student activity levels and demographic characteristics.