1,721,002 research outputs found
Optimal design of planar shapes with active materials
No full textActive materials have emerged as valuable candidates for shape morphing applications, where a body reconfiguration is achieved upon triggering its active response. Given a desired shape change, a natural question is to compare different morphing mechanisms to select the most effective one with respect to an optimality criterion. We introduce an optimal control problem to determine the active strains suitable to attain a target equilibrium shape while minimizing the complexity of the activation. Specifically, we discuss the planar morphing of active, hyperelastic bodies in the absence of external forces and exploit the notion of target metric to encompass a broad set of active materials in a unifying approach. For the case of affine shape changes, we derive explicit conditions on the body reference configuration for the optimality of homogeneous target metrics. More complex shape changes are analysed via numerical simulations to explore the impact on optimal solutions of different objective functionals inspired by features of existing materials. We show how stresses arising from incompatibilities contribute to reduce the complexity of the controls. We believe that our approach may be exploited for the optimal design of active systems and may contribute to gather insight into the morphing strategies of biological systems
Coupled swelling and nematic reordering in liquid crystal gels
Full text linkWe derive a multiphysics model that accounts for network elasticity with spontaneous strains, swelling and nematic interactions in liquid crystal gels (LCGs). We discuss the coupling among the various physical mechanisms, with particular reference to the effects of nematic interactions on chemical equilibrium and that of swelling on the nematic-isotropic transition. Building upon this discussion and using numerical simulations, we explore the transient phenomena involving concurrent swelling and phase transition in LCGs subject to a temperature change. Specifically, we demonstrate separation in time scales between solvent uptake and phase change, in agreement with experiments, which determines a kinetic decoupling between shape and volume changes. Finally, we discuss possible applications in the context of microswimmers, where such a kinetic decoupling is exploited to achieve non-reciprocal actuation and net motion in Stokes flow
Concurrent factors determine toughening in the hydraulic fracture of poroelastic composites
Full text linkBrittle materials fail catastrophically. In consequence of their limited flaw-tolerance, failure occurs by localized fracture and is typically a dynamic process. Recently, experiments on epithelial cell monolayers have revealed that this scenario can be significantly modified when the material susceptible to cracking is adhered to a hydrogel substrate. Thanks to the hydraulic coupling between the brittle layer and the poroelastic substrate, such a composite can develop a toughening mechanism that relies on the simultaneous growth of multiple cracks. Here, we study this remarkable behaviour by means of a detailed model, and explore how the material and loading parameters concur in determining the macroscopic toughness of the system. By extending a previous study, our results show that rapid loading conveys material toughness by promoting distributed cracking. Moreover, our theoretical findings may suggest innovative architectures of flaw-insensitive materials with higher toughness. ArXI
Reduced models of swelling-induced bending of gel bars
No full textThe purpose of the paper is to present a class of reduced models borrowed from structural mechanics aimed at specifically describing swelling-induced bending deformations in a gel bar. A distinct bending pattern to be confirmed by an appropriate experimental setup is evidenced through the analysis of a plane stress-diffusion model. Moreover, a further reduced 1D stress-diffusion model driven by an integro-differential equation is derived. In particular, it is shown that there exists a range of the material parameters where the standard 10 diffusion equation holds. (C) 2012 Elsevier Ltd. All rights reserved
To What Extent Implanting Single vs Pairs of Magnets Per Muscle Affect the Localization Accuracy of the Myokinetic Control Interface? Evidence From a Simulated Environment
Full text linkObjective: We recently proposed a new concept of human-machine interface to control hand prostheses which we dubbed the myokinetic control interface. Such interface detects muscle displacement during contraction by localizing permanent magnets implanted in the residual muscles. So far, we evaluated the feasibility of implanting one magnet per muscle and monitoring its displacement relative to its initial position. However, multiple magnets could actually be implanted in each muscle, as using their relative distance as a measure of muscle contraction could improve the system robustness against environmental disturbances. Methods: Here, we simulated the implant of pairs of magnets in each muscle and we compared the localization accuracy of such system with the one magnet per muscle approach, considering first a planar and then an anatomically appropriate configuration. Such comparison was also performed when simulating different grades of mechanical disturbances applied to the system (i.e., shift of the sensor grid). Results: We found that implanting one magnet per muscle always led to lower localization errors under ideal conditions (i.e., no external disturbances). Differently, when mechanical disturbances were applied, magnet pairs outperformed the single magnet approach, confirming that differential measurements are able to reject common mode disturbances. Conclusion: We identified important factors affecting the choice of the number of magnets to implant in a muscle. Significance: Our results provide important guidelines for the design of disturbance rejection strategies and for the development of the myokinetic control interface, as well as for a whole range of biomedical applications involving magnetic tracking
Swelling-induced and controlled curving in layered gel beams
No full textWe describe swelling-driven curving in originally straight and non-homogeneous beams. We present and verify a structural model of swollen beams, based on a new point of view adopted to describe swelling-induced deformation processes in bilayered gel beams, that is based on the split of the swelling-induced deformation of the beam at equilibrium into two components, both depending on the elastic properties of the gel. The method allows us to: (i) determine beam stretching and curving, once assigned the characteristics of the solvent bath and of the non-homogeneous beam, and (ii) estimate the characteristics of non-homogeneous flat gel beams in such a way as to obtain, under free-swelling conditions, three-dimensional shapes. The study was pursued by means of analytical, semi-analytical and numerical tools; excellent agreement of the outcomes of the different techniques was found, thus confirming the strength of the method
Multiphysics Modeling of Swelling Gels
No full textPolymer gels belong to the realm of
soft active materials as they are capable of
responding to a non-mechanical stimulus – the
permeation of a solvent – with a mechanical
action – a volume change, thanks to the coupling
between different physics. This mechanism of
coupling can be exploited in a wide range of
applications, including biomedical devices,
making crucial the understanding of the
dynamics of these systems. To this aim, we
develop a nonlinear multiphysics theory and
solve numerically the resulting model using the
finite element method
Harnessing Mechanical Instabilities in the Development of an Efficient Soft Pump for an Artificial Heart Ventricle Simulator
Full text linkWhile mechanical instabilities were traditionally considered as failure events, triggering them in a controlled fashion recently paved the way to novel functionalities and improved performance, especially in systems made of soft materials. In this article, we present a novel cable-driven compliant mechanism whose pumping function is based on mechanical instabilities. Specifically, the cables are arranged in helices wrapped around a soft shell chamber that hosts the fluid, and upon pulling, they cause its dramatic volumetric reduction by inducing a torsional instability that maximizes the pumping action. We introduce a geometrical model to describe the deformation kinematics of the soft pump and a finite element model to investigate the detailed postbuckling behavior of the shell. Both models show very good agreement with the experiments. The computational model allowed us to perform a parametric study of the behavior of the soft pump as a function of the number of turns of the cables and their displacement upon pulling. Finally, we demonstrate experimentally the applicability of our soft pump as an artificial ventricle simulator, since the pumped volumes at physiologically relevant afterload pressures approach those found in left and right human ventricles
The multiplicative decomposition of the deformation gradient in the multiphysics modeling of ionic polymers
No full textA thermodynamically consistent modeling of the non-linear multiphysics behavior of ionic polymers is presented and discussed. A key ingredient of the model is the treatment of the electrically induced deformations, resulting in a large bending motion when ionic polymers are employed in IPMCs (Ionic Polymer-Metal Composites). With specific reference to the mechanics, the position of the linear models for ionic polymers is discussed and set within a formalized linearization procedure. A set of numerical experiments is developed, showing the capability of the model to capture the fundamental features of the actuation mechanism
Transient analysis of swelling–induced large deformations in polymer gels.
No full textThe purpose of the research is to describe the swelling-induced large deformations in polymer gels: a theoretical model is developed, and then implemented and solved using the finite element method. The model is firstly assessed with two well-known bench- mark problems; moreover, the proposed approach is benchmarked against a recent experiment involving localized exposure of the gel boundary to a solvent, where large bending deformations appear during solvent absorption. In both cases, our results are quite satisfying
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