Session J03: Responsive Polymers, Soft Materials, and Hybrids II

Live

Oil-in-water magnetic nanoemulsions (size ~ 200 nm) containing superparamagnetic nanoparticles (MNPs) in the oil phase are potential candidates for multi-modal hyperthermia therapy due to the possibility of loading with chemotherapeutic and photo-active agents. However, the effect of stabilizing agent and its conformation, especially in the acidic environment near cancerous cells, on the heating efficiency needs to be probed in detail. With this objective, experiments were performed on poly acrylic acid stabilized magnetic nanoemulsions containing Fe₃O₄ MNPs (size ~ 9.6 nm) in the oil phase. Under exposure to an oscillating magnetic field, the MNPs underwent Neel-Brown relaxation and the dissipated thermal energy travelled across the oil-water interface causing a global increase in fluid temperature. On increasing solution pH from 3 to 9, it was observed that heating efficiency decreased by 50%, which was attributed to the pH-dependent conformational changes of the adsorbed PAA molecules that influenced the interfacial heat transfer. At higher pH, the globular conformation of the PAA molecules reduced the overlap of vibrational density of states between oil and PAA molecules that ultimately resulted in a lower interfacial heat transfer leading to a reduced heating efficiency.   

Live

The combined progress in the fields of magneto-elastic theory, additive manufacturing of soft materials and constitutive modeling has recently increased attention on magneto-mechanics. Departing from the study of classical magnetorheological elastomers (MREs) as bulk material, researchers started to investigate composites featuring permanently magnetic particles (h-MREs) and the stability of magneto-elastic structures. Both directions reach for applications far beyond what can be achieved with mere bulk MREs. Our contribution is inspired by experimental observations and highlights a third avenue to potential applications that is based on the physics that emerge from microstructural multistability of very soft MREs: ferroelasticity. The presentation discusses the underlying processes at microscopic scales by means of analytical considerations and representative numerical simulations. Moreover, we provide a characterization of the material properties of the individual phases and of the external loading conditions under which the effective properties of the composite change from elastic to ferroelastic. In that course we also attempt to connect the observed effects to other domains of (multi-)physics.   

Live

Engineering responses of soft materials at hierarchical time and length scales is of great interests to both scientific and engineering communities. By hybridizing soft materials with various functional nanocomponents, the responses of composites can be manipulated via external stimuli such as heat, light, and magnetic/electric fields, etc. We report here on the design of hydrogels containing aligned ferromagnetic nanowires dispersed in a polymer network that change shape in response to light and experience torques in rotating magnetic fields. Such dual responsiveness enables the control of the hydrogel morphologies and its robotic behaviors include walking, steering, climbing, rolling and delivery of cargo. The theoretical description of both light and magnetic response allowed us to program specific trajectories of hydrogel objects that were verified experimentally.   

Live

Tissue adhesion is a fundamental and practically important problem, underpinning board applications ranging from wound management to wearable and implantable devices. Despite the importance, formation of tough adhesion on tissues and other wet surfaces is a long-standing challenge. It is even more difficult to obtain spatial control over the tough adhesion and make it responsive to external stimuli. This talk will present design strategies and material systems to achieve controlled tough responsive tissue adhesion. A family of tough hydrogel adhesives will be shown to achieve an unprecedented combination of cohesive and adhesive properties. Particularly, they can form extremely tough tissue adhesion and respond to external stimuli, contract and heal wounds. Mechanistic understanding will be developed with experimental, theorical and computational approaches. To modulate the tough adhesion, a variety of strategies such as swelling, ultrasound and substrate effects will be presented. This talk will showcase the diverse interplay between the mechanics, chemistry, physics and biology in the development and application of tissue adhesives. The research on tissue adhesion will fuse disciplines and make broad impacts in engineering and medicine.   

Live

Biological cells have long been of interest to researchers due to their ability to actively control their shape. Accordingly, there is significant interest in generating simplified synthetic protocells that can alter their shape in response to stimuli. To better understand the possible mechanisms of local morphological changes in a popular protocell system, the block copolymer vesicle, we developed a reaction-diffusion model that combines Dissipative Particle Dynamics (DPD) and the Split Reactive Brownian Dynamics algorithm (SRBD), which is capable of modeling the dynamics of polymer solution as they undergo chemical reactions. We investigated local morphological change driven by either the microinjection of a stimulus or an enzymatically-produced stimulus. The results suggest that localized inflation can be induced by either a solvent stimulus that swells the vesicle, or by a reactant stimulus that alters the chemistry of the block polymer. The latter technique results in a more persistent local deformation than the former, which we attribute to the slower diffusion of polymer chains relative to the solvent. Additionally, our method expands the capability of simulating the non-equilibrium behavior of polymer solutions on mesoscopic scales to include stochastic chemical kinetics.   

Live

Soft dielectric elastomers with high relative permittivity, very low modulus and high electric breakdown strength have emerged as promising materials for various applications as sensors, actuators and in energy harvesting. A bilayer material(consisting of a stiff thin film on a thick soft substrate), under large compression, exhibit mechanical instabilities like wrinkling and folding. We performed a linear perturbation analysis for the same, to get the first-order approximation curves for the primary and secondary deformed wavelength of the wrinkles due to an applied nominal compression to the system. We developed a fully coupled non-linear FEM code(UEL subroutine in Abaqus/Standard) to model the electromechanical behaviour of the soft substrate. The selection of initial dimensions of the sample for FE simulations is from the first-order approximation curves. The post bifurcation analysis of the system and the effect of applied electric potential, with buckled thin metal film as corrugated electrodes, are studied through FEA, considering Gent hyperelastic model for the substrate. The efficiency of the system(within the failure bounds) and the extent up to which the mechanical instability can be harnessed in this system, under an applied electric field, is studied in this work.   

Live

Mechanophores (MP) are an emerging technology for self-reporting damage sensing applications in polymeric materials in the aeronautical, energy generation, and automotive industries. For some types of MP molecules, a mechanical stimulus leads to isomerization that induces or “activates” a fluorescence response. To date, semi-quantitative studies have shown that the fluorescent intensity increases with applied force, but systematic calibration of the MP response to local stresses remains a current challenge. Here, a methodology is presented to calibrate experimentally determined MP fluorescent activation intensities (I) with local hydrostatic stress (σ) determined through implementing finite element analysis (FEA) using an axisymmetric model composite comprised of a rigid spherical inclusion embedded in a MP-functionalized elastomeric matrix. The experimental approach employs a glass particle in a polydimethylsiloxane (PDMS) matrix with spiropyran (SPN) attached into the PDMS backbone. By modifying the cohesive zone elements and employing a constitutive hyperelastic model, FEA modeled the various stress states for different adhesion levels. The local σ of the PDMS/SPN system calculated through FEA were then compared directly to I to determine the relationship between and I and σ.   

Live

Magnetic fields provide a route for controlling long-range order in diamagnetic materials like homopolymers and block copolymers (BCPs). Prior work has focused on field-driven phase alignment, where high-intensity fields (>5T) or anisotropic liquid crystalline species and/or aromatic groups have been required to ensure sufficient magnetic anisotropy to drive alignment. Our recent experiments with solutions of polyethylene oxide (PEO) and isotropic spherical micelles of PEO-containing BCPs, however, show anomalous responses to low-intensity fields (B>0.05 T) via a mechanism other than alignment. Linear viscoelastic magnetorheology shows a reversible three-to-six order increase in the dynamic moduli under weak fields. Magneto X-ray scattering reveals that a disorder-to-order structural transition causes the change in mechanical properties and that magnetized samples exhibit d-spacings similar to quiescent samples with twice the polymer content. This change in micelle packing parameter appears to be driven by a change in chain conformation induced by the magnetic field, which is stabilized by water reorganization and hydrogen bonding. Assembling materials via this mechanism provides a new approach to develop BCP materials with long-range order using low-intensity magnetic fields.   

Live

The ability to manipulate bending and straining of thin material sheets remains an exciting challenge for reversible nano-scale actuation. While many thin materials are currently in use, most require modification to achieve actuation. In our experiments, we use freely suspended ~10nm thick membranes of self-assembled superparamagnetic nanoparticles. The membranes’ thinness and inherently soft elastic properties allow substantial elastic deformation. Geometric constraints to bending are controlled via the membranes’ boundary conditions along their perimeter or via introducing strain-releasing cuts with a focused ion beam. We find that an applied magnetic field can induce large, micron-scale deflections of the membranes, which we track in 3D with high-resolution confocal microscopy. Beyond a critical field strength we observe evidence of magnetoelastic buckling. We explore particle-scale magnetic effects with Magnetic Force Microscopy (MFM). Combining soft magnetism with soft elasticity, magnetoelastic nanoparticle membranes are a promising platform for reversible actuation of thin materials sheets in the presence of geometric constraints.   

Live

Triggering macroscopic deformation by photochemical molecular configuration changes has been widely studied in the polymer community. A major challenge in this arena is balancing the degree of molecular ordering against the ease of photo-switching and material properties. Typically, materials with higher levels of molecular ordering show more coordinated deformation, thus amplifying microscopic changes to macroscopic level, but usually suffer from difficulties in processing, light-induced fracture, and a lack of photo-switching in the solid state. To help overcome this trade-off, our group developed new crosslinkable semi-crystalline poly(azobenzene)s which provide a high degree of molecular ordering as well as good processability. We have shown the ability to reversibly destroy and restore semi-crystalline order by photo isomerization of azobenzene powered by UV and visible light, respectively. By tuning the chemical structure, and therefore crystalline properties, of the polymers, we show here that photochemical melting/recrystallization can be achieved at room temperature. Aligned fibers of these materials exhibit similar light-induced reversible shape memory properties that were only possible at elevated temperatures with our previously reported polymers.   

Live

Origami is the ancient art of folding paper which has been shown to have great impact in the field of modern science and technology. Self-folding structures can further enhance its influence by allowing for autonomous shape transformation with applications in the fields of robotics, meta-materials, etc. Light can provide a convenient way to remotely manipulate these foldable structures. To this end, we report our theoretical work on the light induced deformations in spiropyran based hydrogel systems and the design of self-folding origami structures using this photo-active material. Using finite element analysis, we can predict the final 3D shape of these structures and the results have been found to be in good agreement with the empirical results.   

Live

Traditionally, synthetic microactuator deformations have remained largely simplistic—stemming from uniform responses to stimuli—and non-reconfigurable—as deformations are prescribed through elaborate architectures relying on cumbersome fabrication procedures. Here, we report a conceptually new approach to reconfigurable microactuators: by employing directional stimuli on a single high-aspect-ratio micropost fabricated from compositionally uniform materials (a photoresponsive liquid crystalline elastomer) with tilted directionality, we elicit reversible, multimodal deformations including light-seeking, light-avoiding, clockwise and counterclockwise twisting. By adjusting irradiation conditions, these deformation modes can be turned into non-linear, non-reciprocal, and self-regulated biomimetic actuations. Conceptually, these deformations are made possible by local, directional disruption of order evolving as a traveling order-to-disorder front across the micropillar, deliberate symmetry-breaking, and spontaneously emerging opto-chemo-mechanical feedback loops. The ease of fabrication, unparalleled level of control, and conceptual simplicity bode well for immediate employment of the presented approach in practical applications.   

Not Participating

The demonstration of programmable actuation performance by the simple remodeling of material properties advances the field of light responsive materials one step closer to the development of “life-like” actuators. We introduce a novel approach for programmable actuation using negatively photochromic molecular photoswitches, termed donor-acceptor Stenhouse adducts (DASAs), capable of generating programmed mechanical energy using photochemically programmed logic. We report a modular Diels-Alder click chemistry approach that enables attachment of different concentrations of DASA conjugates to polymers. We present a visible light-responsive bilayer actuator that can lift weight against gravity, as well as a simple light-powered crawler, that exploits photothermal energy conversion. Slowly converting a highly-absorbing DASA photochrome into a non-absorbing form upon light illumination enables us to demonstrate programmed mechanical energy generation that can be attenuated or terminated when the actuator decolorizes. The absorbance is analogous to a photochemical fuel that can be recharged at elevated temperatures for reprogramming.