Abstract:Geckos have the extraordinary ability to adhere and move across varied surfaces, while keeping their tiny high-aspect-ratio foot-hairs intact for thousands of attachment−detachment cycles. Inspired by the dry adhesive structure of gecko sole, various gecko-inspired artificial mimics have been developed, but many of them suffer from premature failures and short fatigue life. Herein, we discover that individual gecko seta is a functionally graded material. Its Young's modulus gradually decreases from base to tip… Show more
“…Results indicated the fatigue life has been increased, and the shear adhesion force of the fabricated surface in 300 repeated tests for the gradient PDMS-based pillar arrays can still be restored to more than 90% of the original. These have solidly supported the hypothesis that gradient surfaces can dramatically enhance the fatigue life and reversible adhesion performances of the biomimic gecko-inspired surfaces (Dong et al, 2020).…”
Section: Ll Open Accesssupporting
confidence: 55%
“…Results indicated the fatigue life has been increased, and the shear adhesion force of the fabricated surface in 300 repeated tests for the gradient PDMS-based pillar arrays can still be restored to more than 90% of the original. These have solidly supported the hypothesis that gradient surfaces can dramatically enhance the fatigue life and reversible adhesion performances of the biomimic gecko-inspired surfaces ( Dong et al., 2020 ). Figure 5 The Hierarchical Structure of Nature Animals (A) Hierarchical structures of a Tokay gecko, including optical images of a Tokay gecko, its foot, and its toe; SEM images of a setal array, spatula pads, and a magnified view of a spatula pad ( Yu et al., 2011 ).…”
Section: Process Design and Function Classificationsupporting
confidence: 54%
“…discovered that the Young's modulus of each individual seta gradually decreases from the proximal to distal direction along the setal stalk. This effect is caused by the gradient distribution of β-keratin and the gradient distribution of the proteins evident by confocal laser microscope (CLSM) ( Dong et al., 2020 ). Although single seta shows distinctive gradient properties in the out-of-plane direction and are angled, the setae are distributed relatively evenly in-plane.…”
Section: Dry Gradient and Wet Gradientmentioning
confidence: 99%
“…The results show that the protein in the seta presents a gradient distribution. The model was established by theoretically analyzing the geckos' friction and adhesion behavior and the schematic of an XY-plane bionic material preparation process ( Tian et al., 2006 ; Zhou et al., 2015 ; Dong et al., 2020 ). (B) An SEM image and the CLSM maximum intensity projection of a spatula-like adhesive tarsal setae of the second adhesive pad of a foreleg of a female Coccinella septempunctata .…”
Section: Dry Gradient and Wet Gradientmentioning
confidence: 99%
“…Taking a single gecko seta as an example, the β-keratin had a gradient distribution along the length of single gecko seta and plays a key role in enhancing the Young's modulus of the seta. Thus, the single gecko seta showed a gradient Young's modulus distribution along the length of the seta body with a range from 95 MPa at the top to 1725 MPa at the bottom, a difference of 20 times higher ( Dong et al., 2020 ). Similar phenomena was also observed in ladybirds and squid beaks ( Miserez et al., 2008 ; Peisker et al., 2013 ).…”
Section: Simplified Models Of Gradient Phenomenonmentioning
Summary
Nature does nothing in vain. Through millions of years of revolution, living organisms have evolved hierarchical and anisotropic structures to maximize their survival in complex and dynamic environments. Many of these structures are intrinsically heterogeneous and often with functional gradient distributions. Understanding the convergent and divergent gradient designs in the natural material systems may lead to a new paradigm shift in the development of next-generation high-performance bio-/nano-materials and devices that are critically needed in energy, environmental remediation, and biomedical fields. Herein, we review the basic design principles and highlight some of the prominent examples of gradient biological materials/structures discovered over the past few decades. Interestingly, despite the anisotropic features in one direction (i.e., in terms of gradient compositions and properties), these natural structures retain certain levels of symmetry, including point symmetry, axial symmetry, mirror symmetry, and 3D symmetry. We further demonstrate the state-of-the-art fabrication techniques and procedures in making the biomimetic counterparts. Some prototypes showcase optimized properties surpassing those seen in the biological model systems. Finally, we summarize the latest applications of these synthetic functional gradient materials and structures in robotics, biomedical, energy, and environmental fields, along with their future perspectives. This review may stimulate scientists, engineers, and inventors to explore this emerging and disruptive research methodology and endeavors.
“…Results indicated the fatigue life has been increased, and the shear adhesion force of the fabricated surface in 300 repeated tests for the gradient PDMS-based pillar arrays can still be restored to more than 90% of the original. These have solidly supported the hypothesis that gradient surfaces can dramatically enhance the fatigue life and reversible adhesion performances of the biomimic gecko-inspired surfaces (Dong et al, 2020).…”
Section: Ll Open Accesssupporting
confidence: 55%
“…Results indicated the fatigue life has been increased, and the shear adhesion force of the fabricated surface in 300 repeated tests for the gradient PDMS-based pillar arrays can still be restored to more than 90% of the original. These have solidly supported the hypothesis that gradient surfaces can dramatically enhance the fatigue life and reversible adhesion performances of the biomimic gecko-inspired surfaces ( Dong et al., 2020 ). Figure 5 The Hierarchical Structure of Nature Animals (A) Hierarchical structures of a Tokay gecko, including optical images of a Tokay gecko, its foot, and its toe; SEM images of a setal array, spatula pads, and a magnified view of a spatula pad ( Yu et al., 2011 ).…”
Section: Process Design and Function Classificationsupporting
confidence: 54%
“…discovered that the Young's modulus of each individual seta gradually decreases from the proximal to distal direction along the setal stalk. This effect is caused by the gradient distribution of β-keratin and the gradient distribution of the proteins evident by confocal laser microscope (CLSM) ( Dong et al., 2020 ). Although single seta shows distinctive gradient properties in the out-of-plane direction and are angled, the setae are distributed relatively evenly in-plane.…”
Section: Dry Gradient and Wet Gradientmentioning
confidence: 99%
“…The results show that the protein in the seta presents a gradient distribution. The model was established by theoretically analyzing the geckos' friction and adhesion behavior and the schematic of an XY-plane bionic material preparation process ( Tian et al., 2006 ; Zhou et al., 2015 ; Dong et al., 2020 ). (B) An SEM image and the CLSM maximum intensity projection of a spatula-like adhesive tarsal setae of the second adhesive pad of a foreleg of a female Coccinella septempunctata .…”
Section: Dry Gradient and Wet Gradientmentioning
confidence: 99%
“…Taking a single gecko seta as an example, the β-keratin had a gradient distribution along the length of single gecko seta and plays a key role in enhancing the Young's modulus of the seta. Thus, the single gecko seta showed a gradient Young's modulus distribution along the length of the seta body with a range from 95 MPa at the top to 1725 MPa at the bottom, a difference of 20 times higher ( Dong et al., 2020 ). Similar phenomena was also observed in ladybirds and squid beaks ( Miserez et al., 2008 ; Peisker et al., 2013 ).…”
Section: Simplified Models Of Gradient Phenomenonmentioning
Summary
Nature does nothing in vain. Through millions of years of revolution, living organisms have evolved hierarchical and anisotropic structures to maximize their survival in complex and dynamic environments. Many of these structures are intrinsically heterogeneous and often with functional gradient distributions. Understanding the convergent and divergent gradient designs in the natural material systems may lead to a new paradigm shift in the development of next-generation high-performance bio-/nano-materials and devices that are critically needed in energy, environmental remediation, and biomedical fields. Herein, we review the basic design principles and highlight some of the prominent examples of gradient biological materials/structures discovered over the past few decades. Interestingly, despite the anisotropic features in one direction (i.e., in terms of gradient compositions and properties), these natural structures retain certain levels of symmetry, including point symmetry, axial symmetry, mirror symmetry, and 3D symmetry. We further demonstrate the state-of-the-art fabrication techniques and procedures in making the biomimetic counterparts. Some prototypes showcase optimized properties surpassing those seen in the biological model systems. Finally, we summarize the latest applications of these synthetic functional gradient materials and structures in robotics, biomedical, energy, and environmental fields, along with their future perspectives. This review may stimulate scientists, engineers, and inventors to explore this emerging and disruptive research methodology and endeavors.
Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) have been used to design and fabricate FGMs; however, there has been increasing interest in polymer‐based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are not only ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in our everyday lives; they are resilient and can easily deform, absorb energy and adapt to changing environments. Here, we discuss the basic design and functional principles of biological FGMs and their manmade counterparts using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, we highlight the challenges and potential pathways to leverage soft materials' facile processability and unique properties to wards functional FGMs.This article is protected by copyright. All rights reserved
A multiscale modeling approach is used to develop a particle‐based mesoscale gecko spatula model that is able to link atomistic simulations and mesoscale (0.44 µm) simulations. It is used to study the detachment of spatulae from flat as well as nanostructured surfaces. The spatula model is based on microscopical information about spatulae structure and on atomistic molecular simulation results. Target properties for the coarse‐graining result from a united‐atom model of gecko keratin in periodic boundary conditions (PBC), previously developed by the authors. Pull‐off forces necessary to detach gecko keratin under 2D PBC parallel to the surface are previously overestimated when only a small region of a spatula is examined. It is shown here that this is due to the restricted geometry (i.e., missing peel‐off mode) and not model parameters. The spatula model peels off when pulled away from a surface, both in the molecular picture of the pull‐off process and in the force‐extension curve of non‐equilibrium simulations mimicking single‐spatula detachment studied with atomic force microscopy equipment. The force field and spatula model can reproduce experimental pull‐off forces. Inspired by experimental results, the underlying mechanism that causes pull‐off forces to be at a minimum on surfaces of varying roughnesses is also investigated. A clear sigmoidal increase in the pull‐off force of spatulae with surface roughness shows that adhesion is determined by the ratio between spatula pad area and the area between surface peaks. Experiments showed a correlation with root‐mean‐square roughness of the surface, but the results of this work indicate that this is not a causality but depends on the area accessible.
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