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1. Modeling Actuation of Ionomer Cilia in Salt Solution Under an External Electric Field

   https://doi.org/10.1115/1.4046366  

   Boldini, Alain; Rosen, Maxwell; Cha, Youngsu; Porfiri, Maurizio (January 2021, ASME Letters in Dynamic Systems and Control)

   Abstract A recent experiment by Kim’s group from the University of Nevada, Las Vegas, has shown the possibility of actuating ionomer cilia in salt solution. When these actuators are placed between two external electrodes, across which a small voltage is applied, they move toward the cathode. This is in stark contrast with ionic polymer metal composites, where the same ionomers are plated by metal electrodes but bending occurs toward the anode. Here, we seek to unravel the factors underlying the motion of ionomer cilia in salt solution through a physically based model of actuation. In our model, electrochemistry is described through the Poisson–Nernst–Planck system in terms of concentrations of cations and anions and voltage. Through finite element analysis, we establish that Maxwell stress is the main driving force for the motion of the cilia. This study constitutes a first effort toward understanding the motion of ionomer cilia in salt solution, which, in turn, may help elucidate the physical underpinnings of actuation in ionic polymer metal composites. 

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2. On Structural Theories for Ionic Polymer Metal Composites: Balancing Between Accuracy and Simplicity

   https://doi.org/10.1007/s10659-020-09779-4  

   Boldini, Alain; Bardella, Lorenzo; Porfiri, Maurizio (June 2020, Journal of Elasticity)

   Ionic polymer metal composites (IPMCs) are soft electroactive materials that are finding increasing use as actuators in several engineering domains, where there is a need of large compliance and low activation voltage. Similar to traditional sandwich structures, an IPMC comprises a hydrated ionomer core that is sandwiched by two stiffer electrodes. The application of a voltage across the electrodes drives charge migration within the ionomer, which, in turn, contributes to the development of an eigenstress, associated with osmotic pressure and Maxwell stress. Critical to IPMC actuation is the variation of the eigenstress through the thickness of the ionomer, which is responsible for strain localization at the ionomer-electrode interfaces. Despite considerable progress in the development of reliable continuum theories and finite element tools, accurate structural theories that could beget physical insight into the inner workings of IPMC actuation are lacking. Here, we seek to bridge this gap by contributing a principled methodology to structural modeling of IPMC actuation. Our approach begins with the study of the IPMC electrochemistry through the method of matched asymptotic expansions, which yields a semi-analytical expression for the eigenstress as a function of the applied voltage. Hence, we establish a total potential energy that accounts for the strain energy of the ionomer, the strain energy of the electrodes, and the work performed by the eigenstress. By projecting the IPMC kinematics on select beam-like representations and imposing the stationarity of the total potential energy, we formulate rigorous structural theories for IPMC actuation. Not only do we examine classical low-order and higher-order beam theories, but we also propose enriched theories that account for strain localization near the electrodes. The accuracy of these theories is assessed through comparison with finite element simulations on a plane-strain problem of non-uniform bending. Our results indicate that an enriched Euler-Bernoulli beam theory, with three independent field variables, is successful in capturing the main features of IPMC actuation at a limited computational cost. 

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3. Engineered interaction between short elastin-like peptides and perfluorinated sulfonic-acid ionomer

   https://doi.org/10.1039/C8SM00351C  

   Su, Zihang; Pramounmat, Nuttanit; Watson, Skylar T.; Renner, Julie N. (January 2018, Soft Matter)

   Control of ionomer thin films on metal surfaces is important for a range of electrodes used in electrochemical applications. Engineered peptides have emerged as powerful tools in electrode assembly because binding sites and peptide structures can be modulated by changing the amino acid sequence. However, no studies have been conducted showing peptides can be engineered to interact with ionomers and metals simultaneously. In this study, we design a single-repeat elastin-like peptide to bind to gold using a cysteine residue, and bind to a perfluorinated sulfonic-acid ionomer called Nafion® using a lysine guest residue. Quartz crystal microbalance with dissipation monitoring and atomic force microscopy are used to show that an elastin-like peptide monolayer attached to gold facilitates the formation of a thin, phase-separated ionomer layer. Dynamic light scattering confirms that the interaction between the peptide with the lysine residue and the ionomer also happens in solution, and circular dichroism shows that the peptides maintain their secondary structures in the presence of ionomer. These results demonstrate that elastin-like peptides are promising tools for ionomer control in electrode engineering. 

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4. An Epoxy‐Free Sample Preparation Approach to Enable Imaging of Ionomer and Carbon in Polymer Electrolyte Membrane Fuel Cells

   https://doi.org/10.1002/adfm.202209733  

   Soleymani, Amir_Peyman; Reid, Marcia; Jankovic, Jasna (December 2022, Advanced Functional Materials)

   Abstract Performance and durability of electrodes in proton exchange membrane fuel cells (PEMFCs), as one of the most promising zero‐emission power generation technologies, depend on the composition, microstructure, and distribution of its components—metal catalyst, carbon support, and ionomer. Their improvement requires an in‐depth understanding of the electrodes’ structure‐property‐performance relationship, for which transmission electron microscopy (TEM) has been an invaluable tool. However, the conventional TEM sample preparation, namely epoxy‐embedding ultramicrotomy, poses impediments in imaging ionomer and distinguishing it from carbon. Therefore, in this research, an epoxy‐free ultramicrotome technique is implemented on beginning‐of‐life (BOL) and end‐of‐life (EOL) PEMFC samples. For the first time, TEM and electron tomography‐TEM images reveals fascinating details of the ionomer network, carbon particles’ structure, and Pt distribution in BOL, as well as their structural changes after the cell degradation. Finally, the structural descriptors, extracted by a proprietary quantification method, are correlated with visual observations. 

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5. Emergence of traveling waves in linear arrays of electromechanical oscillators

   https://doi.org/10.1038/s42005-018-0086-4  

   Dou, Yong; Pandey, Shashank; Cartier, Charles A.; Miller, Olivia; Bishop, Kyle J. M. (November 2018, Communications Physics)

   Abstract Traveling waves of mechanical actuation provide a versatile strategy for locomotion and transport in both natural and engineered systems across many scales. These rhythmic motor patterns are often orchestrated by systems of coupled oscillators such as beating cilia or firing neurons. Here, we show that similar motions can be realized within linear arrays of conductive particles that oscillate between biased electrodes through cycles of contact charging and electrostatic actuation. The repulsive interactions among the particles along with spatial gradients in their natural frequencies lead to phase-locked states characterized by gradients in the oscillation phase. The frequency and wavelength of these traveling waves can be specified independently by varying the applied voltage and the electrode separation. We demonstrate how traveling wave synchronization can enable the directed transport of material cargo. Our results suggest that simple energy inputs can coordinate complex motions with opportunities for soft robotics and colloidal machines. 

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