Bioinspired Temperature-Responsive Multilayer Films and Their Performance under Thermal Fatigue.

biomimetics Article Bioinspired Temperature-Responsive Multilayer Films and Their Performance under Thermal Fatigue Nikolaos Athanasopoulos * ID and Nicolaos J. Siakavellas Department of Mechanical Engineering and Aeronautics, University of Patras, 26500 Patras, Greece; * Correspondence: or ; Tel.: +30-694-663-0065; Fax: +30-261-099-7241 Received: 13 June 2018; Accepted: 29 July 2018; Published: 1 August 2018



 Abstract: The structure of certain nonliving tissues determines their self-shaping and self-folding capabilities in response to a stimulus. Predetermined movements are realized according to changes in the environmental conditions due to the generated stresses of the multilayer anisotropic structure. In this study, we present bioinspired responsive anisotropic multilayer films and their fabrication process which comprises low-cost techniques. The anisotropic multilayer materials are capable of deforming their geometry caused by small temperature changes (<40 ◦ C). The mismatch in the thermo-mechanical properties between three or more anisotropic thin layers creates responsive materials that alter their shape owing to the developed internal stresses. The movements of the material can be controlled by forming anisotropic homogenous metallic strips over an anisotropic thermoplastic layer. As a result, responsive multilayer films made of common materials can be developed to passively react to a temperature stimulus. We demonstrate the ability of the anisotropic materials to transform their geometry and we present a promising fabrication process and the thermal fatigue resistance of the developed materials. The thermal fatigue performance is strongly related to the fabrication method and the thickness of the strips. We studied the thermal fatigue performance of the materials and how the thermal cycling affects their sensitivity, as well as their failure modes and crack formation. Keywords: responsive materials; smart materials; bioinspired materials; nonliving plant tissues; anisotropy; thermal fatigue; microstructure; 4D printing; additive manufacturing 1. Introduction Advances in materials technology have the potential to greatly affect a plethora of applications in different sectors. Urgent needs to be fulfilled are the development of low-weight structures, the integration of different functionalities and sensing abilities, as well as the energy efficiency and financial feasibility in different applications. In nature, extremely complex movements can be realized through the materials’ self-shaping and self-folding capabilities in response to a stimulus [1–9]. The nonliving tissues of various plants are designed to undergo predetermined shape transformations through their anisotropic fibrous structure [1–3,6–9]. The coefficients of hygroscopic expansion are the corresponding parameters characterizing such changes in the physical dimensions of the plants’ nonliving tissues. Pine cones drastically transform their shape using only their anisotropic structure and the mismatch of the coefficients of hygroscopic expansion [1] (Figure 1A). This simple mechanism leads to the bending of the scales, which consequently opens the cone. This system can be regarded as a hygrosensitive bilayer material [1,3]. Biomimetics 2018, 3, 20; doi:10.3390/biomimetics3030020 www.mdpi.com/journal/biomimetics Biomimetics 2018, 3, x FOR PEER REVIEW 2 of 12 The mechanistic behavior/transformation of the aforementioned nonliving tissues inspired Biomimetics 2018, 3, 20 2 of 11 various researchers and can be imitated through the use of multilayered fibrous anisotropic materials, anisotropic nanocomposites, pre-stressed sheets, and nanoreinforced multilayer hydrogels [10–18]. The geometry of these materials can be transformed humidity or temperature stimulus or both, The mechanistic behavior/transformation of under the aforementioned nonliving tissues inspired various researchers and can imitatedcan through the use of multilayered fibrous anisotropic materials, or whereas their initial and finalbeshapes be determined by the geometry, the homogeneous anisotropic nanocomposites, sheets, and multilayernature hydrogels [10–18]. nonhomogeneous nature of the pre-stressed materials’ structure, asnanoreinforced well as the anisotropic of the different The geometry of these materials can be transformed under humidity or temperature stimulus or to layers. Folding structures have been developed using shape-memory alloys (SMAs) in order both, whereas their initial and final shapes can be determined by the geometry, the homogeneous or control their shape [18]. Apart from the well-known SMAs and shape-memory polymers (SMPs), nonhomogeneous nature of the materials’ structure, as well as the anisotropic nature of the different three-dimensional (3D) printed hydrogel architectures have been developed. The shape shift of the layers. Folding structures have been developed using shape-memory alloys (SMAs) in order to biomimetic four-dimensional (4D) printed materials is actuated through the anisotropic swelling control their shape [18]. Apart from the well-known SMAs and shape-memory polymers (SMPs), behavior in water. Various parameters control the shape transformation of the material, such as the three-dimensional (3D) printed hydrogel architectures have been developed. The shape shift of the filament size, orientation, and interfilament [13]. the swelling mechanism has been biomimetic four-dimensional (4D) printedspacing materials is Moreover, actuated through the anisotropic swelling usedbehavior to produce hygroscopic that control the orientation of microplatelets in water. Various multilayer parameters composites control the shape transformation of the material, such as the [14]. Other researchers use layer-by-layer (LBL) spacing techniques the fabrication ofmechanism polymerichas multilayers filament size, orientation, and interfilament [13]. for Moreover, the swelling been used to produce hygroscopic multilayer composites that control the orientation of microplatelets [14]. and that are capable of driving shape transformations in response to environmental humidity Other researchers use In layer-by-layer techniques for the fabrication polymeric temperature variations. this case, a (LBL) hydrophilic multilayer is stackedofwith a lessmultilayers responsivethat carbon are capable of driving shape transformations in response to environmental humidity and temperature nanotube layer. The differential swelling of the two LBL films results in reversible out-of-plane variations.[11]. In this case, a hydrophilic multilayer is stacked with a less responsive carbon nanotube deformations Moreover, complex flower structures have been developed from two-dimensional layer. The differential swelling of the two LBL films results in reversible out-of-plane deformations [11]. (2D) flat anisotropic polymeric sheets, whose shape-shifting behavior is enabled by the coefficient of Moreover, complex flower (...truncated)