Soft Functionally Gradient Materials and Structures – Natural and Manmade: A Review

Abstract

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

Polydimethylsiloxane Polymerized Emulsions for Acoustic Materials Prepared Using Reactive Triblock Copolymer Surfactants

“…Acoustic metasurfaces consist of a single material with a subwavelength thickness that has a gradient of distinctly different acoustic properties, e.g., acoustic index, to act as a lens to focus or shape acoustic waves, and this can be achieved by producing gradients of stiffness or porosity using polymer-based acoustic metasurfaces. 26 The design and preparation of such subwavelength materials are important in obtaining novel devices for applications in metamaterials, acoustic cloaks, and underwater impedance matching. One method to prepare porous PDMSs with controlled porosity values is the emulsion templating technique known as polymerized high internal phase emulsions (polyHIPEs).…”

“…Porous polysiloxanes have now been widely reported as acoustic materials, − but there remains a need for the development of porous siloxanes that can obtain a wide range of porosities, especially at higher porosities, and mechanical properties to access materials with new properties for acoustic lenses and metasurfaces. Acoustic metasurfaces consist of a single material with a subwavelength thickness that has a gradient of distinctly different acoustic properties, e.g., acoustic index, to act as a lens to focus or shape acoustic waves, and this can be achieved by producing gradients of stiffness or porosity using polymer-based acoustic metasurfaces . The design and preparation of such subwavelength materials are important in obtaining novel devices for applications in metamaterials, acoustic cloaks, and underwater impedance matching …”

A Bioinspired Gradient Design Strategy for Cellulose-Based Electromagnetic Wave Absorbing Structural Materials

“…During the billions of years of evolution, many gradient functional materials have been developed and play vital roles in various biological processes. 18 For example, with a chemical gradient of chitin, water, and His-rich proteins, the hydrated beak of the Humboldt squid Dosidicus gigas exhibits a huge stiffness gradient that spans 2 orders of magnitude from the base to the tip. This characteristic enables a mild transmission of impact forces between the beak and the soft envelope, making it possible to easily cut prey with a sharp beak and avoid hurting itself.…”

“…20 Therefore, gradient design plays a vital role in both mechanical and functional properties, inspiring us to achieve the synergy of functional and mechanical properties. 18 Here, as an example of structural materials with integrated structure and function, a bioinspired CNF-based gradient structural material with excellent mechanical performance and functional properties is developed through a hydrogel layer-bylayer assembly method. The electromagnetic wave absorption To develop a general preparation strategy for CNF-based structure−function integrated materials, we select functional building units whose nanoscale performance is difficult to scale up to the macroscale.…”

“…In fact, nature has already answered us. During the billions of years of evolution, many gradient functional materials have been developed and play vital roles in various biological processes . For example, with a chemical gradient of chitin, water, and His-rich proteins, the hydrated beak of the Humboldt squid Dosidicus gigas exhibits a huge stiffness gradient that spans 2 orders of magnitude from the base to the tip.…”