@article {59, title = {Mechanics of axisymmetric sheets of interlocking and slidable rods}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {141}, year = {2020}, pages = {103969}, abstract = {

In this work, we study the mechanics of metamaterial sheets inspired by the pellicle of Euglenids. They are composed of interlocking elastic rods which can freely slide along their edges. We characterize the kinematics and the mechanics of these structures using the special Cosserat theory of rods and by assuming axisymmetric deformations of the tubular assembly. Through an asymptotic expansion, we investigate both structures that comprise a discrete number of rods and the limit case of a sheet composed by infinitely many rods. We apply our theoretical framework to investigate the stability of these structures in the presence of an axial load. Through a linear analysis, we compute the critical buckling force for both the discrete and the continuous case. For the latter, we also perform a numerical post-buckling analysis, studying the non-linear evolution of the bifurcation through finite elements simulations.\ 

}, keywords = {Biomimetic structures, Elastic structures, Helical rods, Mechanical instabilities, Metamaterials, Post-buckling analysis}, doi = {10.1016/j.jmps.2020.103969}, author = {Davide Riccobelli and Giovanni Noselli and Marino Arroyo and Antonio De Simone} } @article {57, title = {Morphable structures from unicellular organisms with active, shape-shifting envelopes: Variations on a theme by Gauss}, journal = {International Journal of Non{\textendash}Linear Mechanics}, volume = {118}, year = {2020}, pages = {103278}, abstract = {

We discuss some recent results on biological and bio-inspired morphing, and use them to identify promising research directions for the future. In particular, we consider issues related to morphing at microscopic scales inspired by unicellular organisms. We focus on broad conceptual principles and, in particular, on morphing approaches based on the use of Gauss{\textquoteright} theorema egregium (Gaussian morphing). We highlight some connections with biological cell envelopes containing filaments and motors, and discuss ideas for the implementation of Gaussian morphing in surfaces actuated by active shearing or stretching.\ 

}, keywords = {Active shells, cell motility, Deployable structures, Fluid{\textendash}structure interaction, Gaussian morphing, Micro-swimmers, soft robotics, Unicellular swimmers}, doi = {10.1016/j.ijnonlinmec.2019.103278}, author = {Giancarlo Cicconofri and Giovanni Noselli and Marino Arroyo and Antonio De Simone} } @article {40, title = {Hydraulic fracture and toughening of a brittle layer bonded to a hydrogel}, journal = {Physical Review Letters}, volume = {115}, year = {2015}, pages = {188105}, abstract = {

Brittle materials propagate opening cracks under tension. When stress increases beyond a critical magnitude, then quasistatic crack propagation becomes unstable. In the presence of several precracks, a brittle material always propagates only the weakest crack, leading to catastrophic failure. Here, we show that all these features of brittle fracture are fundamentally modified when the material susceptible to cracking is bonded to a hydrogel, a common situation in biological tissues. In the presence of the hydrogel, the brittle material can fracture in compression and can hydraulically resist cracking in tension. Furthermore, the poroelastic coupling regularizes the crack dynamics and enhances material toughness by promoting multiple cracking.

}, keywords = {hydraulic fracture, multiple-cracking, toughening}, doi = {10.1103/PhysRevLett.115.188105}, author = {Alessandro Lucantonio and Giovanni Noselli and Xavier Trepat and Marino Arroyo and Antonio DeSimone} } @article {37, title = {Shape control of active surfaces inspired by the movement of euglenids}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {62}, year = {2014}, pages = {99-112}, abstract = {

We examine a novel mechanism for active surface morphing inspired by the cell body deformations of euglenids. Actuation is accomplished through in-plane simple shear along prescribed slip lines decorating the surface. Under general non-uniform actuation, such local deformation produces Gaussian curvature, and therefore leads to shape changes. Geometrically, a deformation that realizes the prescribed local shear is an isometric embedding. We explore the possibilities and limitations of this bio-inspired shape morphing mechanism, by first characterizing isometric embeddings under axisymmetry, understanding the limits of embeddability, and studying in detail the accessibility of surfaces of zero and constant curvature. Modeling mechanically the active surface as a non-Euclidean plate (NEP), we further examine the mechanism beyond the geometric singularities arising from embeddability, where mechanics and buckling play a decisive role. We also propose a non-axisymmetric actuation strategy to accomplish large amplitude bending and twisting motions of elongated cylindrical surfaces. Besides helping understand how euglenids delicately control their shape, our results may provide the background to engineer soft machines.

}, keywords = {active materials, cell motility, Euglenids, non-Euclidean plates}, doi = {10.1016/j.jmps.2013.09.017}, author = {Marino Arroyo and Antonio DeSimone} } @article {31, title = {Reverse engineering the euglenoid movement}, journal = {Proceedings of the National Academy of Sciences of USA}, volume = {109}, year = {2012}, pages = {17874{\textendash}17879}, abstract = {

Euglenids exhibit an unconventional motility strategy amongst unicellular eukaryotes, consisting of large-amplitude highly concerted deformations of the entire body (euglenoid movement or metaboly). A plastic cell envelope called pellicle mediates these deformations. Unlike ciliary or flagellar motility, the biophysics of this mode is not well understood, including its efficiency and molecular machinery. We quantitatively examine video recordings of four euglenids executing such motions with statistical learning methods. This analysis reveals strokes of high uniformity in shape and pace. We then interpret the observations in the light of a theory for the pellicle kinematics, providing a precise understanding of the link between local actuation by pellicle shear and shape control. We systematically understand common observations, such as the helical conformations of the pellicle, and identify previously unnoticed features of metaboly. While two of our euglenids execute their stroke at constant body volume, the other two exhibit deviations of about 20\% from their average volume, challenging current models of low Reynolds number locomotion. We find that the active pellicle shear deformations causing shape changes can reach 340\%, and estimate the velocity of the molecular motors. Moreover, we find that metaboly accomplishes locomotion at hydrodynamic efficiencies comparable to those of ciliates and flagellates. Our results suggest new quantitative experiments, provide insight into the evolutionary history of euglenids, and suggest that the pellicle may serve as a model for engineered active surfaces with applications in microfluidics.

}, keywords = {active soft matter, microswimmers, self-propulsion, stroke kinematics}, doi = {10.1073/pnas.1213977109}, author = {Marino Arroyo and Luca Heltai and Daniel Mill{\'a}n and Antonio DeSimone} }