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1. Gas‐Induced Electrical and Magnetic Modulation of Two‐Dimensional Conductive Metal–Organic Framework

   https://doi.org/10.1002/anie.202404290  

   Meng, Zheng; Stolz, Robert M; De_Moraes, Lygia Silva; Jones, Christopher G; Eagleton, Aileen M; Nelson, Hosea M; Mirica, Katherine A (June 2024, Angewandte Chemie International Edition)

   Abstract Controlled modulation of electronic and magnetic properties in stimuli‐responsive materials provides valuable insights for the design of magnetoelectric or multiferroic devices. This paper demonstrates the modulation of electrical and magnetic properties of a semiconductive, paramagnetic metal−organic framework (MOF) Cu₃(C₆O₆)₂with small gaseous molecules, NH₃, H₂S, and NO. This study merges chemiresistive and magnetic tests to reveal that the MOF undergoes simultaneous changes in electrical conductance and magnetization that are uniquely modulated by each gas. The features of response, including direction, magnitude, and kinetics, are modulated by the physicochemical properties of the gaseous molecules. This study advances the design of multifunctional materials capable of undergoing simultaneous changes in electrical and magnetic properties in response to chemical stimuli. 

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2. Bioinspired Smart Materials With Externally-Stimulated Switchable Adhesion

   https://doi.org/10.3389/fnano.2021.667287  

   Wang, Jie; Wan, Yiyang; Wang, Xiaowei; Xia, Zhenhai (September 2021, Frontiers in Nanotechnology)

   Living organisms have evolved, over billions of years, to develop specialized biostructures with switchable adhesion for various purposes including climbing, perching, preying, sensing, and protecting. According to adhesion mechanisms, switchable adhesives can be divided into four categories: mechanically-based adhesion, liquid-mediated adhesion, physically-actuated adhesion and chemically-enhanced adhesion. Mimicking these biostructures could create smart materials with switchable adhesion, appealing for many engineering applications in robotics, sensors, advanced drug-delivery, protein separation, etc. Progress has been made in developing bioinspired materials with switchable adhesion modulated by external stimuli such as electrical signal, magnetic field, light, temperature, pH value, etc. This review will be focused on new advance in biomimetic design and synthesis of the materials and devices with switchable adhesion. The underlying mechanisms, design principles, and future directions are discussed for the development of high-performance smart surfaces with switchable adhesion. 

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3. Sharkskin-Inspired Magnetoactive Reconfigurable Acoustic Metamaterials

   https://doi.org/10.34133/2020/4825185  

   Lee, Kyung Hoon; Yu, Kunhao; Al Ba’ba’a, Hasan; Xin, An; Feng, Zhangzhengrong; Wang, Qiming (February 2020, Research)

   Most of the existing acoustic metamaterials rely on architected structures with fixed configurations, and thus, their properties cannot be modulated once the structures are fabricated. Emerging active acoustic metamaterials highlight a promising opportunity to on-demand switch property states; however, they typically require tethered loads, such as mechanical compression or pneumatic actuation. Using untethered physical stimuli to actively switch property states of acoustic metamaterials remains largely unexplored. Here, inspired by the sharkskin denticles, we present a class of active acoustic metamaterials whose configurations can be on-demand switched via untethered magnetic fields, thus enabling active switching of acoustic transmission, wave guiding, logic operation, and reciprocity. The key mechanism relies on magnetically deformable Mie resonator pillar (MRP) arrays that can be tuned between vertical and bent states corresponding to the acoustic forbidding and conducting, respectively. The MRPs are made of a magnetoactive elastomer and feature wavy air channels to enable an artificial Mie resonance within a designed frequency regime. The Mie resonance induces an acoustic bandgap, which is closed when pillars are selectively bent by a sufficiently large magnetic field. These magnetoactive MRPs are further harnessed to design stimuli-controlled reconfigurable acoustic switches, logic gates, and diodes. Capable of creating the first generation of untethered-stimuli-induced active acoustic metadevices, the present paradigm may find broad engineering applications, ranging from noise control and audio modulation to sonic camouflage. 

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4. Encoding reprogrammable properties into magneto-mechanical materials via topology optimization

   https://doi.org/10.1038/s41524-023-00980-2  

   Zhao, Zhi; Zhang, Xiaojia Shelly (December 2023, npj Computational Materials)

   Abstract The properties of materials and structures typically remain fixed after being designed and manufactured. There is a growing interest in systems with the capability of altering their behaviors without changing geometries or material constitutions, because such reprogrammable behaviors could unlock multiple functionalities within a single design. We introduce an optimization-driven approach, based on multi-objective magneto-mechanical topology optimization, to design magneto-active metamaterials and structures whose properties can be seamlessly reprogrammed by switching on and off the external stimuli fields. This optimized material system exhibits one response under pure mechanical loading, and switches to a distinct response under joint mechanical and magnetic stimuli. We discover and experimentally demonstrate magneto-mechanical metamaterials and metastructures that realize a wide range of reprogrammable responses, including multi-functional actuation responses, adaptable snap-buckling behaviors, switchable deformation modes, and tunable bistability. The proposed approach paves the way for promising applications such as magnetic actuators, soft robots, and energy harvesters. 

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5. Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels

   https://doi.org/10.1021/acs.biomac.4c00450  

   Ollier, Rachel C; Webber, Matthew J (July 2024, Biomacromolecules)

   Mechanical stimuli such as strain, force, and pressure are pervasive within and beyond the human body. Mechanoresponsive hydrogels have been engineered to undergo changes in their physicochemical or mechanical properties in response to such stimuli. Relevant responses can include strain-stiffening, self-healing, strain-dependent stress relaxation, and shear rate-dependent viscosity. These features are a direct result of dynamic bonds or non- covalent/physical interactions within such hydrogels. The contributions of various types of bonds and intermolecular interactions to these behaviors are important to more fully understand the resulting materials and engineer their mechanoresponsive features. Here, strain-stiffening in carboxymethylcellulose hydrogels crosslinked with pendant dynamic-covalent boronate esters using tannic acid is studied and modulated as a function of polymer concentration, temperature, and effective crosslink density. Furthermore, these materials are found to exhibit self-healing and strain- memory, as well as strain-dependent stress relaxation and shear rate-dependent changes in gel viscosity. These features are attributed to the dynamic nature of the boronate ester crosslinks, inter-chain hydrogen bonding and bundling, or a combination of these two intermolecular interactions. This work provides insight into the interplay of such interactions in the context of mechanoresponsive behaviors, particularly informing the design of hydrogels with tunable strain- stiffening. The multi-responsive and tunable nature of this hydrogel system therefore presents a promising platform for a variety of applications. 

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