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Active gels play an important role in biology and in inspiring biomimetic active materials, due to their ability to change shape, size and create their own morphology; the relevant mechanics behind these changes is driven by self-contraction and liquid flow. Here, we couple contraction and liquid flow within a nonlinear mechanical model of an active gel disc to discuss how contraction dynamics inherits length scales which are typical of the liquid flow processes. The cylindrically symmetric model we present, which recapitulate our previous theoretical modeling in its basic lines, reveals that when also liquid flow is taken into account, the aspect ratio of the disc is not the only geometrical parameter which characterizes the contraction dynamics of the gel. The analyses we present provide important insights into the dependence of contraction dynamics on geometry and allow to make some progress in designing materials which can be adapted for different applications in soft robotics.

We review ideas emerged in the mechanics literature in the last fifty years concerning the problem of controlling shape. The central notion is the multiplicative decomposition of the gradient of the visible deformation into elastic and inelastic parts. We show that, when applied to analyze muscle contraction, this notion is useful to understand the different functions of a muscle as motor or brake, and the key difference in the way skeletal, soft, and cardiac muscles solve the push problem.

Liquid migration in active soft solids is a very common phenomenon in Nature at different scales: from cells to leaves. It can be caused by mechanical as well as chemical actions. The work focuses on the migration of liquid provoked by remodeling processes in an active impermeable gel sphere. Within this context, we present a consistent mathematical theory capable to gain a deep understanding of the phenomenon in both steady and transient conditions.

We characterized motion attributes arising from LV spatio-temporal analysis of motion distributions in myocardial infarction. Time-varying 3D finite element shape models were obtained in 300 Controls and 300 patients with myocardial infarction. Inter-individual left ventricular shape differences were eliminated using parallel transport to the grand mean of all cases. The first three principal component (PC) scores were used to characterize trajectory attributes. Scores were tested with ANOVA/MANOVA using patient disease status (Infarcts vs. Controls) as a factor. Infarcted patients had significantly different magnitude, orientation and shape of left ventricular trajectories in comparison to Controls. Significant differences were found for the angle between PC scores 1 and 2 in the endocardium, and PC scores 1 and 3 in the epicardium. The largest differences were found in the magnitude of endocardial motion. Endocardial PC scores in shape space showed the highest classification power using support vector machine, with higher total accuracy in comparison to previous methods. Shape space performed better than size-and-shape space for both epicardial and endocardial features. In conclusion, LV spatio-temporal motion attributes accurately characterize the presence of infarction. This approach is easily generalizable to different pathologies, enabling more precise study of the pathophysiological consequences of a wide spectrum of cardiac diseases.

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We investigate the swelling dynamics driven by solvent absorption in a hydrogel sphere immersed in a solvent bath, through an accurate computational model and numerical study. We extensively describe the transient process from dry to wet and discuss the onset of surface instabilities through a measure of the lack of smoothness of the outer surface and a morphological pattern of that surface with respect to the two material parameters driving the swelling dynamics., 9 pages, 13 figures

International audience

The enigmatic Early Miocene fossorial mammal Mesoscalops montanensis shows one of the most modified humeri among terrestrial mammals. It has been suggested, on qualitative considerations, that this species has no extant homologues for humerus kinematics and that, functionally, the closest extant group is represented by Chrysochloridae. We combine here three dimensional geometric morphometrics, finite element analysis and phylogenetic comparative methods to explore the shape and mechanical stress states of Mesoscalops montanensis as well as of extant and extinct Talpidae and Chrysochloridae under realistic digging simulations. Evolutionary convergence analyses reveal that the shape of Mesoscalops montanensis represents a unique morphology in the context of fossorial mammals and that its functional performance, albeit superficially similar to that of extant Chrysochloridae, still represents a nonconvergent optimum for adaptation to digging.

International audience; Accurate and reliable modeling of bone elasticity at the organ scale requires to take into account the specific microstructure of bone. X-ray based methods, such as micro-computed tomography (µCT) or synchrotron radiation µCT (SR-µCT) can provide useful and accurate information on bone architecture and composition in vitro. Focusing on cortical bone, the most relevant microstructural information concerns the Haversian Porosity (HP, characteristic length scale of a few to several tens of micrometers) and the mineralization of the solid bone matrix (measured by the Tissue Mineral Density, TMD, characteristic length scale of a few to several micrometers). This information can be used to set up detailed Finite Element (FE) or ho-mogenization models of bone which allow studying the heterogeneous distribution of its elastic properties. Unfortunately, these approaches cannot be applied easily in in vivo. Indeed, spatial resolution of clinical devices (several tens of micrometers) prevents resolving details of bone microstructure accurately and may produce blurry, uncertain data. Thus, uncertainty affecting the experimental input data may question the reliability of the results predicted by the models. Since input data are uncertain, deterministic approaches are limited and new modeling paradigms are required. Probabilistic approaches [1] and fuzzy and interval analysis offer new and powerful ways to face these challenges. In this talk, we will present an overview of the issues related to uncertainty in image-based modeling of bone elasticity and discuss the benefits and drawbacks of some approaches which may be used to tackle them in a multiscale setting.

In this paper we show that trajectory shape analysis should be performed only after obtaining a proper representation before applying ordination methods. In fact, studying the shape of a trajectory means studying how the deformation changes along each path irrespectively of the actual shape to which these deformations apply. The independence of the deformation from the shape to which it is applied is critical: it implies that any shape variation between individuals at the beginning of each trajectories must be completely filtered out. A Parallel Transport, that can be based on various connection types, is necessary to perform such kind of shape data centering. The Levi Civita connection can also be used to transport a deformation. We demonstrate that this procedure does not preserve deformation even in the ane case. We propose a novel procedure called Direct Transport able to perfectly transport deformation in the ane case and to better approximate non ane deformation in comparison to existing tools.

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The aim of the paper is to solve and discuss the representation problem of a special class of integrable distortion fields in fibred cylindrical bodies and to analyze the correspond- ing induced shape changes. We find and discuss the compatibility conditions, i.e. the conditions to be satisfied to get a pair compatible distortion/shape change, when different fields of fibers are assigned on the cylindrical body, through the specification of the fields of fiber angles.

The nematic elastomers with hybrid alignment (HNEs) exhibit large anisotropic and non homogeneous volume changes, which can in- duce noticeable changes in their configuration. Here, we deal with LCEs having hybrid align- ment (HNEs), that is, fabricated with a given non-homogeneous nematic orientation. For such a materials, permanent distortions induced by deswelling can be compensated by those resulting from cooling below TN ; it results the possibility of producing temperature-driven actuators.

Wepresentanelectromechanicalmodelofmyocardiumtissue,coupling finite elasticity, endowed with the capability of describing muscle contractions, with a FitzHugh–Nagumo type system, describing the electrical activity proper to excitable media. Here, we exploit a novel point of view which introduces the notion of active deformation as opposed to that of active stress. The high degree of deformability of the medium makes mandatory to set the diffusion process in a moving domain, thereby producing a direct influence of the deformation on the electrical activity. Various effects of contraction on stationary rotating spiral waves and spiral wave break up are discussed.

We present a continuum model able to describe the mechanism of muscle contractions from a macroscopic point of view.

We present a reduced–order heart model aimed to introduce a novel point of view in the interpretation of the pressure–volume loops. The novelty of the approach is based on the definition of active contraction as opposed to that of active stress. The consequences of such assumption are discussed with reference to a specific pressure-volume loop characteristic of a normal human patient.

We present a model of the dynamics of director rotation in nematic gels under combined electro-mechanical loading. Focusing on a model specimen, we describe the critical voltages that must be exceeded to achieve director reorientation, and the blocking stresses that prevent alignment of the nematic director with the applied electric field. The corresponding phase diagram shows that the dynamic thresholds defined above are different from those predicted on the sole basis of energetics. Multistep loading programs are used to explore the energy landscape of our model specimen, showing the existence of multiple local minima under the same voltage and applied stress. This leads us to conclude that hysteresis should be expected in the electro-mechanical response of nematic gels.

FEMLAB Conference Stockolm, Sweden (2005)