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In this work, an innovative model is proposed as a design tool to predict both the inner and outer radii in rolled structures based on polydimethylsiloxane bilayers. The model represents an improvement of Timoshenko's formula taking into account the friction arising from contacts between layers arising from rolling by more than one turn, hence broadening its application field towards materials based on elastomeric bilayers capable of large deformations. The fabricated structures were also provided with surface topographical features that would make them potentially usable in different application scenarios, including cell/tissue engineering ones. The bilayer design parameters were varied, such as the initial strain (from 20 to 60%) and the bilayer thickness (from 373 to 93 µm). The model matched experimental data on the inner and outer radii nicely, especially when a high friction condition was implemented in the model, particularly reducing the error below 2% for the outer diameter while varying the strain. The model outperformed the current literature, where self-penetration is not excluded, and a single value of the radius of spontaneous rolling is used to describe multiple rolls. A complex 3D bioinspired hierarchical elastomeric microstructure made of seven spirals arranged like a hexagon inscribed in a circumference, similar to typical biological architectures (e.g., myofibrils within a sarcolemma), was also developed. In this case also, the model effectively predicted the spirals' features (error smaller than 18%), opening interesting application scenarios in the modeling and fabrication of bioinspired materials. [ABSTRACT FROM AUTHOR]

We analyze temperature dynamics in anatomic pathology samples to identify the most efficient refrigeration method and to predict the time available for optimal sectioning before sample heating, thus getting appropriate information for a correct diagnosis by anatomopathologists. A thermal finite element (FE) analysis was carried out with comsolmultiphysics to evaluate temperature variations in paraffin-embedded tissues, i.e., muscle, bone and fat, and the corresponding thermal stresses. Experiments with different tissues and thermocouple-based measurements allowed validating the FE simulations. Simulations allowed to estimate the time needed to bring the sample at the optimal temperature for sectioning (-8 to -4 °C) in different conditions: refrigeration on a cold plate, refrigeration in a cooled environment, and refrigeration in an environment with forced convection. Among the three cooling methods tested, the forced convection at -20 °C and with an air-flow speed of 5 m/s resulted in the shortest cooling time. As compared to the other methods, thermal stresses can be modulated by varying the air-flow speed. For the different conditions, the time needed for the surface of the tissue block to exit from a temperature corresponding to an optimal cutting, when leaving the sample exposed to room temperature after refrigeration, ranged from 12 to 310 s. We quantify the time needed to adequately refrigerate paraffin-embedded tissue samples and the time available before they leave the optimal temperature window for sectioning. We also evaluate the maximum stress attained in the paraffin block during the cooling and the heating transients. This information will help optimize anatomic pathology processes. [ABSTRACT FROM AUTHOR]

Based on their shape-shifting capabilities, soft active materials have enabled new possibilities for the engineering of sensing and actuation devices. While the relation between active strains and emergent equilibrium shapes has been fully characterized, the transient morphing of thin structures is a rather unexplored topic. Here, we focus on polymer gel plates and derive a reduced linear model to study their time-dependent response to changes in the fluid environment. We show that independent control of stretching and bending deformations in stress-free conditions allows to realize spherical shapes with prescribed geometry of the mid-plane. Furthermore, we demonstrate that tensile (compressive) membrane stresses delay (accelerate) swelling-induced shape transitions compared to the stress-free evolution. We believe that these effects should be considered for the accurate design of smart systems and may contribute to explain the complexity of natural shapes. [ABSTRACT FROM AUTHOR]

Nature and technology often adopt structures that can be described as tubular helical assemblies. However, the role and mechanisms of these structures remain elusive. In this paper, we study the mechanical response under compression and extension of a tubular assembly composed of 8 helical Kirchhoff rods, arranged in pairs with opposite chirality and connected by pin joints, both analytically and numerically. We first focus on compression and find that, whereas a single helical rod would buckle, the rods of the assembly deform coherently as stable helical shapes wound around a common axis. Moreover, we investigate the response of the assembly under different boundary conditions, highlighting the emergence of a central region where rods remain circular helices. Secondly, we study the effects of different hypotheses on the elastic properties of rods, i.e., stress-free rods when straight versus when circular helices, Kirchhoff's rod model versus Sadowsky's ribbon model. Summing up, our findings highlight the key role of mutual interactions in generating a stable ensemble response that preserves the helical shape of the individual rods, as well as some interesting features, and they shed some light on the reasons why helical shapes in tubular assemblies are so common and persistent in nature and technology. We study the mechanical response under compression/extension of an assembly composed of 8 helical rods, pin-jointed and arranged in pairs with opposite chirality. In compression we find that, whereas a single rod buckles (a), the rods of the assembly deform as stable helical shapes (b). We investigate the effect of different boundary conditions and elastic properties on the mechanical response, and find that the deformed geometries exhibit a common central region where rods remain circular helices. Our findings highlight the key role of mutual interactions in the ensemble response and shed some light on the reasons why tubular helical assemblies are so common and persistent. [ABSTRACT FROM AUTHOR]

We rigorously derive a Kirchhoff plate theory, via Γ-convergence, from a three-dimensional model that describes the finite elasticity of an elastically heterogeneous, thin sheet. The heterogeneity in the elastic properties of the material results in a spontaneous strain that depends on both the thickness and the plane variables x′. At the same time, the spontaneous strain is h-close to the identity, where h is the small parameter quantifying the thickness. The 2D Kirchhoff limiting model is constrained to the set of isometric immersions of the mid-plane of the plate into ℝ3, with a corresponding energy that penalizes deviations of the curvature tensor associated with a deformation from an x′-dependent target curvature tensor. A discussion on the 2D minimizers is provided in the case where the target curvature tensor is piecewise constant. Finally, we apply the derived plate theory to the modeling of swelling-induced shape changes in heterogeneous thin gel sheets. [ABSTRACT FROM AUTHOR]

We derive a large-strain plate model that allows to describe transient, coupled processes involving elasticity and solvent migration, by performing a dimensional reduction of a three-dimensional poroelastic theory. We apply the model to polymer gel plates, for which a specific kinematic constraint and constitutive relations hold. Finally, we assess the accuracy of the plate model with respect to the parent three-dimensional model through two numerical benchmarks, solved by means of the finite element method. Our results show that the theory offers an efficient computational framework for the study of swelling-induced morphing of composite gel plates. [ABSTRACT FROM AUTHOR]

Motivated by pine cones and plant seeds that open or close in response to changes in humidity thanks to the shrinking or swelling of hygroscopic bilayers, and given the recent interest in artificial systems that mimic the hygroscopic motility of seeds, we consider bilayers consisting of two adhering elongated thin layers made of hygroscopic gels, modeled as active, transversally isotropic elastic materials. The direction of transverse isotropy is set as one of the fibers that are present in both biological and synthetic materials. As a special, biologically relevant case, we consider one of the two layers passive. This work develops a three-dimensional model for the coupled evolution of elastic deformations and moisture content (solvent concentration). Based on this model, a numerical scheme for solving the evolution equations coupling mechanical equilibrium and diffusion has been constructed based on the Finite Element Method. The equations are formulated in weak form and solved using Comsol Multiphysics. We have applied our model to compute transient configurations and their steady-state equilibrium limits in some relevant test cases. These configurations are then analyzed to understand the influence of the various geometric and physical parameters. • We study the hygroscopic morphing of gel bilayers that mimic seed motility induced by humidity changes. • The bilayer consists of the top active, transversally isotropic, the bottom passive, and isotropic. • Using (3D) non-linear elasticity, we create a model coupling mechanics and moisture diffusion. • Model predictions show good qualitative agreement with experimental evidence from the literature. [ABSTRACT FROM AUTHOR]

Light actuation is one of the preferred and advantageous approaches to remotely induce and control deformations in soft materials such as photoactive Liquid Crystal Elastomers (LCEs). Various experimental and numerical works have been carried out in the literature to study the actuation of photoactive LCE sheets under illumination. In this study, we have developed a reduced multi-physics model to predict the equilibrium and dynamic response of photoactive LCE beams under illumination. We test our model against an experiment in which a double-clamped thin nematic LCE beam is subjected to UV light, and the stress is generated in the beam due to induced contraction under illumination. Our numerical results demonstrate reasonable agreement with the experiment regarding stress evolution trend and saturation time. We also investigate the bending response of a photoactive LCE beam subjected to UV light. Based on our parameters, we observe that the nematic beam bends towards the light only due to the photochemical strain gradient along the thickness. Finally, to test our model in a dynamic situation, we perform the simulation for the self-oscillations of an LCE beam under illumination. We show that the alternate activation of the top and bottom surfaces of the LCE beam by uniform steady illumination can pump energy into the system, resulting in the phenomenon of self-oscillations. • Developing a multiphysics model to predict the response of photoactive LCE beam under illumination • Considering both thermal and chemical effects of illumination in the modeling • Generating stress in a double-clamped nematic beam under illumination • Generating self-oscillation in the beam due to alternate illumination of top and bottom surfaces [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Active 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. [ABSTRACT FROM AUTHOR]

• We study numerically the shape programming of gel plates that swell heterogeneously. • Curvature of the plate can be induced by in-plane patterning of the shear modulus. • We set an inverse problem for the shear modulus field corresponding to a target shape. • We solve numerically the inverse problem for cones and saddles. Polymer gel plates may be programmed to morph into three-dimensional configurations upon swelling. An effective strategy to control such shape transformations consists in patterning the in-plane cross-linking density of the polymer network to realize non-homogeneous swelling. In general, one needs to solve an inverse problem to determine the shear modulus field that produces a given target shape. Here, we propose a computational framework for the solution of such an inverse problem, which we validate against two benchmark problems, i.e. making cones and saddles from gel disks. [ABSTRACT FROM AUTHOR]

Highlights • We classify the equilibrium shapes of pre-stretched bilayers for various aspect ratios. • The principal curvatures exhibit a bifurcation as the bilayer becomes thinner/wider. • Curvatures in rectangular samples bifurcate earlier than in square ones. Graphical abstract Abstract An elastic bilayer, consisting of an equibiaxially pre-stretched sheet bonded to a stress-free one, spontaneously morphs into curved shapes in the absence of external loads or constraints. Using experiments and numerical simulations, we explore the role of geometry for square and rectangular samples in determining the equilibrium shape of the system, for a fixed pre-stretch. We classify the observed shapes over a wide range of aspect ratios according to their curvatures and compare measured and computed values, which show good agreement. In particular, as the bilayer becomes thinner, a bifurcation of the principal curvatures occurs, which separates two scaling regimes for the energy of the system. We characterize the transition between these two regimes and show the peculiar features that distinguish square from rectangular samples. The results for our model bilayer system may help explaining morphing in more complex systems made of active materials. [ABSTRACT FROM AUTHOR]

Hydrothermal growth of ZnO nanorods has been widely used for the development of tactile sensors, with the aid of ZnO seed layers, favoring the growth of dense and vertically aligned nanorods. However, seed layers represent an additional fabrication step in the sensor design. In this study, a seedless hydrothermal growth of ZnO nanorods was carried out on Au-coated Si and polyimide substrates. The effects of both the Au morphology and the growth temperature on the characteristics of the nanorods were investigated, finding that smaller Au grains produced tilted rods, while larger grains provided vertical rods. Highly dense and high-aspect-ratio nanorods with hexagonal prismatic shape were obtained at 75 °C and 85 °C, while pyramid-like rods were grown when the temperature was set to 95 °C. Finite-element simulations demonstrated that prismatic rods produce higher voltage responses than the pyramid-shaped ones. A tactile sensor, with an active area of 1 cm2, was fabricated on flexible polyimide substrate and embedding the nanorods forest in a polydimethylsiloxane matrix as a separation layer between the bottom and the top Au electrodes. The prototype showed clear responses upon applied loads of 2–4 N and vibrations over frequencies in the range of 20–800 Hz. [ABSTRACT FROM AUTHOR]