Molecular Dynamics Study on the Photothermal Actuation of a Glassy Photoresponsive Polymer Reinforced with Gold Nanoparticles with Size Effect

Azobenzene-Functionalized Semicrystalline Liquid Crystal Elastomer Springs for Underwater Soft Robotic Actuators

As actuated devices become smaller and more complex, there is a need for smart materials and structures that directly function as complete mechanical units without an external power supply. The strategy uses light-powered, twisted, and coiled azobenzene-functionalized semicrystalline liquid crystal elastomer (AC-LCE) springs. This twisting and coiling, which has previously been used for only thermally, electrochemically, or absorption-powered muscles, maximizes uniaxial and radial actuation. The specially designed photochemical muscles can undergo about 60% tensile stroke and provide 15 kJ m-3 of work capacity in response to light, thus providing about three times and two times higher performance, respectively, than previous azobenzene actuators. Since this actuation is photochemical, driven by ultraviolet (UV) light and reversed by visible light, isothermal actuation can occur in a range of environmental conditions, including underwater. In addition, photoisomerization of the AC-LCEs enables unique latch-like actuation, eliminating the need for continuous energy application to maintain the stroke. Also, as the light-powered muscles processed to be either homochiral or heterochiral, the direction of actuation can be reversed. The presented approach highlights the novel capabilities of photochemical actuator materials that can be manipulated in untethered, isothermal, and wet environmental conditions, thus suggesting various potential applications, including underwater soft robotics.

Photo-activated dynamic isomerization induced large density changes in liquid crystal polymers: A molecular dynamics study

We use molecular dynamics simulations to unravel the physics underpinning the light-induced density changes caused by the dynamic trans-cis-trans isomerization cycles of azo-mesogens embedded in a liquid crystal polymer network, an intriguing experimental observation reported in the literature. We employ two approaches, cyclic and probabilistic switching of isomers, to simulate dynamic isomerization. The cyclic switching of isomers confirms that dynamic isomerization can lead to density changes at specific switch-time intervals. The probabilistic switching approach further deciphers the physics behind the non-monotonous relation between density reduction and light intensities observed in experiments. Light intensity variations in experiments are accounted for in simulations by varying the trans-cis and cis-trans isomerization probabilities. The simulations show that an optimal combination of these two probabilities results in a maximum density reduction, corroborating the experimental observations. At such an optimal combination of probabilities, the dynamic trans-cis-trans isomerization cycles occur at a specific frequency, causing significant distortion in the polymer network, resulting in a maximum density reduction.

Recent Trends in Continuum Modeling of Liquid Crystal Networks: A Mini-Review

This work aims to provide a comprehensive review of the continuum models of the phase behaviors of liquid crystal networks (LCNs), novel materials with various engineering applications thanks to their unique composition of polymer and liquid crystal. Two distinct behaviors are primarily considered: soft elasticity and spontaneous deformation found in the material. First, we revisit these characteristic phase behaviors, followed by an introduction of various constitutive models with diverse techniques and fidelities in describing the phase behaviors. We also present finite element models that predict these behaviors, emphasizing the importance of such models in predicting the material's behavior. By disseminating various models essential to understanding the underlying physics of the behavior, we hope to help researchers and engineers harness the material's full potential. Finally, we discuss future research directions necessary to advance our understanding of LCNs further and enable more sophisticated and precise control of their properties. Overall, this review provides a comprehensive understanding of the state-of-the-art techniques and models used to analyze the behavior of LCNs and their potential for various engineering applications.

Thermal/Near-Infrared Light Dual-Responsive Reconfigurable and Recyclable Polythiourethane/CNT Composite with Simultaneously Enhanced Strength and Toughness

Thermoset polymers cross-linked by dynamic covalent bonds are recyclable and reconfigurable based on solid-state plasticity, resulting in less waste and environmental pollution. However, most thermoset polymers previously reported show thermal-responsive solid-state plasticity, depending much on external conditions and not allowing for local shape modulation. Here, the isocyanate modified carbon nanotubes (CNTs-NCO) are introduced into the polythiourethane (PCTU) network with multiple dynamic covalent bonds by in situ polymerization to prepare the composite with thermal/light dual-responsive solid-state plasticity, reconfigurability, and recyclability. The introduction of CNTs-NCO simultaneously strengthens and toughens the PCTU composite. Moreover, based on the photothermal properties and light-responsive solid-state plasticity, the PCTU/CNTs composite or bilayer sample could achieve complex permanent shape by locally precise shape regulation without affecting other parts. This work provides a simple and reliable method for preparing high-performance polymer composite with light-responsive solid-state plasticity, which may be applied in the fields of sensing and flexible electronics.

The interaction mechanism between gold nanoparticles and proteins: Lysozyme, trypsin, pepsin, γ-globulin, and hemoglobin

In this study, the interaction between gold nanoparticles (AuNPs) and proteins (including lysozyme, trypsin, pepsin, γ-globulin and hemoglobin) was investigated by UV-visible absorption spectroscopy, fluorescence spectroscopy, circular dichroism (CD) spectroscopy and protein activity assay. AuNPs was synthesized using reduction of HAuCl₄ with sodium citrate. The formation of AuNPs was confirmed from the characteristic surface plasmon resonance band at 521 nm and transmission electron microscopy revealed the average particle size was about 10 nm. The results reveal that AuNPs can interact with proteins to form a "protein corona (PC)", but the protein concentration required to form a relatively stable PC is not the same. The quenching mechanism of proteins by AuNPs is arisen from static quenching. The binding constants of AuNPs with proteins are in the range from 10⁶ to 10¹⁰ L mol^-1, and the order is pepsin > γ-globulin > hemoglobin > trypsin > lysozyme at 298 K. Van der Waals forces and hydrogen bonds are the main forces for the lysozyme-AuNPs system. The interaction between trypsin/pepsin/γ-globulin/hemoglobin and AuNPs is mainly by hydrophobic interaction. The addition of AuNPs has an effect on the secondary structure of proteins as confirmed from CD spectra. The change in secondary structure of different proteins is different and seems to have little relation with the binding constant. The activity of lysozyme/trypsin/pepsin decreases with the addition of AuNPs.

Investigation of molecular mechanisms of polyvinylidene fluoride under the effects of temperature, electric poling, and mechanical stretching using molecular dynamics simulations

This study uses molecular dynamics (MD) simulations to investigate the molecular mechanisms of polyvinylidene fluoride (PVDF) influenced by temperature, electric poling, and mechanical stretching. The β-phase, with all-trans ⟨T⟩ planar zigzag conformation, is known to have the best potential of energy harvesting, while α-phase, with alternating trans ⟨T⟩ and gauche ⟨G⟩ linkages, is more stable in terms of potential energy. By applying an electric field and uniaxial deformation to an amorphous PVDF system, we study the transformation from α- to β-phase and corresponding molecular mechanisms by tracking the molecular chain conformation using the trans percentages (P_T). After complete relaxation of molecular chains, the chain conformations and P_T values indicate a typical distribution pattern of α-phase. Next, we observe that the dipole moment of the system increases significantly with the presence of a strong electric field without immediately affecting the chain conformations. The increment of dipole moment is due to the aligning of side atoms within the chains and the increment becomes more significant with elevated temperature. In contrast, chain conformations change significantly under mechanical stretching. Specifically, before yielding, the total dipole moments are still governed by local orientations of atoms. Later, the chain segments begin to straighten in the large deformation stage, which leads to the increment of the total dipole moment. Our results also show that there exists an optimal temperature window for maximum ⟨G⟩ to ⟨T⟩ transformation rate. Moreover, we look into the synergistic effect of electric poling and mechanical stretching and explain molecular-level mechanisms for this effect. This study contributes to the fundamental understanding of the underlying molecular mechanisms for the piezoelectric PVDF system under different processing conditions.

Cyclic Photoisomerization of Azobenzene in Atomistic Simulations: Modeling the Effect of Light on Columnar Aggregates of Azo Stars

This computational study investigates the influence of light on supramolecular aggregates of three-arm azobenzene stars. Every star contains three azobenzene (azo) moieties, each able to undergo reversible photoisomerization. In solution, the azo stars build column-shaped supramolecular aggregates. Previous experimental works report severe morphological changes of these aggregates under UV-Vis light. However, the underlying molecular mechanisms are still debated. Here we aim to elucidate how light affects the structure and stability of the columnar stacks on the molecular scale. The system is investigated using fully atomistic molecular dynamics (MD) simulations. To implement the effects of light, we first developed a stochastic model of the cyclic photoisomerization of azobenzene. This model reproduces the collective photoisomerization kinetics of the azo stars in good agreement with theory and previous experiments. We then apply light of various intensities and wavelengths on an equilibrated columnar stack of azo stars in water. The simulations indicate that the aggregate does not break into separate fragments upon light irradiation. Instead, the stack develops defects in the form of molecular shifts and reorientations and, as a result, it eventually loses its columnar shape. The mechanism and driving forces behind this order-disorder structural transition are clarified based on the simulations. In the end, we provide a new interpretation of the experimentally observed morphological changes.

Light-Responsive Inorganic Biomaterials for Biomedical Applications

Light-responsive inorganic biomaterials are an emerging class of materials used for developing noninvasive, noncontact, precise, and controllable medical devices in a wide range of biomedical applications, including photothermal therapy, photodynamic therapy, drug delivery, and regenerative medicine. Herein, a range of biomaterials is discussed, including carbon-based nanomaterials, gold nanoparticles, graphite carbon nitride, transition metal dichalcogenides, and up-conversion nanoparticles that are used in the design of light-responsive medical devices. The importance of these light-responsive biomaterials is explored to design light-guided nanovehicle, modulate cellular behavior, as well as regulate extracellular microenvironments. Additionally, future perspectives on the clinical use of light-responsive biomaterials are highlighted.

Biological Behavior Regulation of Gold Nanoparticles via the Protein Corona

One of the difficulties in the translation of gold nanoparticles (GNPs) into clinical practice is the formation of the protein corona (PC) that causes the discrepancy between the in vitro and in vivo performance of GNPs. The PC formed on the surface of GNPs gives them a biological identity instead of an initial synthetic one. In most instances, this biological identity increases the particle size, leads to more clearance by the reticuloendothelial system, and causes less uptake by target cells. However, the performance of GNPs can still be improved by rewriting their original surface chemistry via the PC. This review specifically focuses on discussing the main influence factors, including the biological environment and physicochemical properties of GNPs, which affect the production and status of the PC. The status of the PC such as the amount, thickness, and composition subsequently influence the biological behavior of GNPs, especially their cellular uptake, cytotoxicity, biodistribution, and tumor targeting. Further understanding and revealing the impacts of the PC on the biological behavior of GNPs can be a promising and important strategy to regulate and improve the performance of GNP-based biosystems in the future.

Do Columns of Azobenzene Stars Disassemble under Light Illumination?

The clustering properties of star-shaped molecules comprising three photochromic azobenzene-containing arms are investigated with specific focus on the influence of light on these structures. Previous experimental works report self-assembly of azobenzene stars in aqueous solution into long columnar clusters that are detectable using optical microscopy. These clusters appear to vanish under UV irradiation, which is known to induce trans-to-cis photoisomerization of the azobenzene groups. We have performed MD simulations, density functional theory, and density functional tight binding calculations to determine conformational properties and binding energies of these clusters. Our simulation data suggest that the binding strength of the clusters is large enough to prevent a breaking along their main axis. We conclude that very likely other mechanisms lead to the apparent disappearance of the clusters.