Progress in Bioinspired Dry and Wet Gradient Materials from Design Principles to Engineering Applications

Abstract

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.

Gradient Micropillar Array Inspired by Tree Frog for Robust Adhesion on Dry and Wet Surfaces

“…To maximize the survival in the complex, dynamic natural environments, functional gradient structures have been developed in many creatures [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. For instance, the microscale setae of the ladybird beetle Coccinella septempunctata have a gradient modulus from 7.2 GPa at the root to 1.2 MPa at the tip, endowing a high flexibility at the seta tip to enhance contact formation and a stiff stalk to maintain mechanical stability [ 6 ].…”

Hierarchical Lattice Modeling Method with Gradient Functions

“…In fact, creatures in nature have evolved over tens of thousands of years and gradually developed unique structural and mechanical properties. ese properties allow them to function beyond the man-made systems through using their complex multiscale and multiphase structures [1][2][3][4][5].…”

A Bioinspired Fibrous Helix with Periodic Gradient for Directional Fluidic Gates

“…The transportation of liquid is ubiquitous in nature, [1,2] such as the concave-convex hydrophilic and hydrophobic structure on the desert beetle, [3] the opening of Nepenthes, [4] and the cactus needle structure [5,6] for the collection and directional transportation of water as well as vascular plants for anti-gravity water transpiration. [7,8] The unique multiscale structure gives them excellent liquid transport effects, making them better adapt to the natural environment.…”