Chemical actuation in responsive hydrogels

Thermally Driven Continuous Rolling of a Thick-Walled Cylindrical Rod

Self-sustained motion can take advantage of direct energy extraction from a steady external environment to maintain its own motion, and has potential applications in energy harvesting, robotic motion, and transportation. Recent experiments have found that a thermally responsive rod can perform self-sustained rolling on a flat hot plate with an angular velocity determined by the competition between the thermal driving moment and the friction moment. A rod with a hollow cross section tends to greatly reduce the frictional resistance, while promising improvements in thermal conversion efficiency. In this paper, through deriving the equilibrium equations for steady-state self-sustained rolling of the thick-walled cylindrical rod, estimating the temperature field on the rod cross-section, and solving the analytical solution of the thermally induced driving moment, the dynamic behavior of the thermally driven self-sustained rolling of the thick-walled cylindrical rod is theoretically investigated. In addition, we investigate in detail the effects of radius ratio, heat transfer coefficient, heat flux, contact angle, thermal expansion coefficient, and sliding friction coefficient on the angular velocity of the self-sustained rolling of the thick-walled cylindrical rod to obtain the optimal ratio of internal and external radius. The results are instructive for the application of thick-walled cylindrical rods in the fields of waste heat harvesters and soft robotics.

Chemically Triggered Changes in Mechanical Properties of Responsive Liquid Crystal Polymer Networks with Immobilized Urease

Liquid crystal polymer networks (LCNs) are stimuli-responsive materials that can be programmed to realize spatial variation in mechanical response and undergo shape transformation. Herein, we report a process to introduce chemical specificity to the stimuli response of LCNs by integrating enzymes as molecular triggers. Specifically, the enzyme urease was immobilized in LCN films via acyl fluoride conjugation chemistry. Activity assays and confocal fluorescence imaging confirmed retention of urease activity after immobilization as well as widespread distribution of enzyme on the film. The addition of urea triggered a response in the LCN whereby newly generated ammonia reacted with free acyl fluorides to form benzamide moieties. These moieties were capable of dimerizing through the formation of supramolecular hydrogen bonds, which was reflected in a 4-fold increase in Young's modulus. Through dynamic mechanical analysis and calorimetry, we further confirmed that the degree of hydrogen bonding in the LCNs could be judiciously designed to fine-tune the mechanical properties and glass transition temperature. These findings demonstrate the untapped potential of biochemical mechanisms as molecular triggers in LCNs and open the door to the use of nucleophilic chemistries in modulating the mechanical properties of LCNs.

Wet-Spun Biofiber for Torsional Artificial Muscles

The demands for new types of artificial muscles continue to grow and novel approaches are being enabled by the advent of new materials and novel fabrication strategies. Self-powered actuators have attracted significant attention due to their ability to be driven by elements in the ambient environment such as moisture. In this study, we demonstrate the use of twisted and coiled wet-spun hygroscopic chitosan fibers to achieve a novel torsional artificial muscle. The coiled fibers exhibited significant torsional actuation where the free end of the coiled fiber rotated up to 1155 degrees per mm of coil length when hydrated. This value is 96%, 362%, and 2210% higher than twisted graphene fiber, carbon nanotube torsional actuators, and coiled nylon muscles, respectively. A model based on a single helix was used to evaluate the torsional actuation behavior of these coiled chitosan fibers.

Topography-guided buckling of swollen polymer bilayer films into three-dimensional structures

Thin films that exhibit spatially heterogeneous swelling often buckle into the third dimension to minimize stress. These effects, in turn, offer a promising strategy to fabricate complex three-dimensional structures from two-dimensional sheets. Here we employ surface topography as a new means to guide buckling of swollen polymer bilayer films and thereby control the morphology of resulting three-dimensional objects. Topographic patterns are created on poly(dimethylsiloxane) (PDMS) films selectively coated with a thin layer of non-swelling parylene on different sides of the patterned films. After swelling in an organic solvent, various structures are formed, including half-pipes, helical tubules, and ribbons. We demonstrate these effects and introduce a simple geometric model that qualitatively captures the relationship between surface topography and the resulting swollen film morphologies. The model's limitations are also examined.

PolyWhips: Directional Particle Transport by Gradient-Directed Growth and Stiffening of Supramolecular Assemblies

Growth of rigid rods occurs via supramolecular assembly of a nonconjugated π-donor π-acceptor monomer and is triggered by a NaCl gradient. The mechanical stiffness of this material is controlled by the local salt concentration and is ion specific. The continuous and well-controlled growth process is exploited to power the directional transport of sub-millimeter polymer particles.

Self-organization of hydrophobic-capped triblock copolymers with a polyelectrolyte midblock: a coarse-grained molecular dynamics simulation study

We present the results of a Langevin dynamics simulation study of micellar organization and hydrogel formation in the solutions of coarse-grained ABA copolymer chains. Polymer chains are modeled as bead-spring chains of Lennard-Jones particles by explicit treatment of ionic species in implicit solvent. The studied copolymer is composed of a polyelectrolyte midblock flanked by two hydrophobic endblocks. We explore the self-assembly of copolymer solutions at a fixed polymer concentration and temperature upon systematic variation of the midblock charge fraction, valency of neutralizing counterions, and the stiffness and length of hydrophobic endblocks. Minimization of the surface energy, conformational entropy of the midblock chains, electrostatic repulsion of midblock charges, and the translational entropy of counterions are found to play central roles in controlling the self-organization features of copolymer solutions. Flower-like micelles with A-blocks forming the core of spherical aggregates and B-blocks constituting the micelle corona are established for the neutral midblocks. Increasing the charge content of B chains lowers the fraction of loop conformations and yields a spanning hydrogel network with midblocks bridging the hydrophobic clusters. Counterion valence is shown to exert a strong effect on the micelle size and network structure. The increase in the rigidity of terminal A-blocks increases the fraction of bridging chains and results in the formation of a hydrogel network with bundle-like hydrophobic domains. Longer endblocks are shown to increase the hydrophobic cluster size and enhance the bridged midblock fraction. The qualitative agreement between the experimental and theoretical studies is also discussed. The comprehensive molecular picture provides a framework for the future studies of stimuli-responsive copolymer systems.

Redox-responsive organometallic hydrogels for in situ metal nanoparticle synthesis

A new class of redox active hydrogels composed of poly(ferrocenylsilane) polyanion and poly(ethylene glycol) chains was assembled, using a copper-free azide-alkyne Huisgen cycloaddition reaction. These organometallic hydrogels displayed reversible collapse and reswelling upon chemical oxidation and reduction, respectively, and formed relatively well-defined, unaggregated Pd(0) nanoparticles (8.2 ± 2.2 nm) from K2PdCl4 salts.

Mechanistic studies of an autonomously pulsing hydrogel/enzyme system for rhythmic hormone delivery

Numerous hormones are known to be endogenously secreted in a pulsatile manner. In particular, gonadotropin replacing hormone (GnRH) is released in rhythmic pulses, and disruption of this rhythm is associated with pathologies of reproduction and sexual development. In an effort to develop an implantable, rhythmic delivery system, a scheme has been demonstrated involving a negative feedback instability between a pH-sensitive membrane and enzymes that convert endogenous glucose to hydrogen ion. A bench prototype system based on this scheme was previously shown to produce near rhythmic oscillations in internal pH and in GnRH delivery over a period of one week. In the present work, a systematic study of conditions permitting such oscillations is presented, along with a study of factors causing period of oscillations to increase with time and ultimately cease. Membrane composition, glucose concentration, and surface area of marble (CaCO3), which is incorporated as a reactant, were found to affect the capacity of the system to oscillate, and the pH range over which oscillations occur. Accumulation of gluconate- and Ca2+ in the system over time correlated with lengthening of oscillation period, and possibly with cessation of oscillations. Enzyme degradation may also be a factor. These studies provide the groundwork for future improvements in device design.

Stimulus-responsive polymers and other functional polymer surfaces as components in glass microfluidic channels

The integration of smart stimulus-responsive polymers as functional elements within microfluidic devices has greatly improved the performance capabilities of controlled fluid delivery. For their use as actuators in microfluidic systems, reversible expansion and shrinking are unique mechanisms which can be utilized as both passive and active fluid control elements to establish gate and valve functions (passive) and pumping elements (active). Various constituents in microfluidic glass channels based on stimulus-responsive elements have been reported based on pH-responsive, thermoresponsive and photoresponsive coatings. Fluid control and robust performance have been demonstrated in microfluidic devices in a number of studies. Here we give a brief overview of selected examples from the literature reporting on the use of stimulus response polymers as active or passive elements for fluid control in microfluidic devices, with specific emphasis on glass-based devices. The remaining challenges include improving switching times and achieving local addressability of the responsive constituent. We envisage tackling these challenges by utilizing redox-responsive polymers which offer fast and reversible switching and local addressability in combination with nanofabricated electrodes.

Responsive hydrogels--structurally and dimensionally optimized smart frameworks for applications in catalysis, micro-system technology and material science

Although the technological and scientific importance of functional polymers has been well established over the last few decades, the most recent focus that has attracted much attention has been on stimuli-responsive polymers. This group of materials is of particular interest due to its ability to respond to internal and/or external chemico-physical stimuli, which is often manifested as large macroscopic responses. Aside from scientific challenges of designing stimuli-responsive polymers, the main technological interest lies in their numerous applications ranging from catalysis through microsystem technology and chemomechanical actuators to sensors that have been extensively explored. Since the phase transition phenomenon of hydrogels is theoretically well understood advanced materials based on the predictions can be prepared. Since the volume phase transition of hydrogels is a diffusion-limited process the size of the synthesized hydrogels is an important factor. Consistent downscaling of the gel size will result in fast smart gels with sufficient response times. In order to apply smart gels in microsystems and sensors, new preparation techniques for hydrogels have to be developed. For the up-coming nanotechnology, nano-sized gels as actuating materials would be of great interest.

Progress Toward Robust Polymer Hydrogels

In this review we highlight new developments in tough hydrogel materials in terms of their enhanced mechanical performance and their corresponding toughening mechanisms. These mechanically robust hydrogels have been developed over the past 10 years with many now showing mechanical properties comparable with those of natural tissues. By first reviewing the brittleness of conventional synthetic hydrogels, we introduce each new class of tough hydrogel: homogeneous gels, slip-link gels, double-network gels, nanocomposite gels and gels formed using poly-functional crosslinkers. In each case we provide a description of the fracture process that may be occurring. With the exception of double network gels where the enhanced toughness is quite well understood, these descriptions remain to be confirmed. We also introduce material property charts for conventional and tough synthetic hydrogels to illustrate the wide range of mechanical and swelling properties exhibited by these materials and to highlight links between these properties and the network topology. Finally, we provide some suggestions for further work particularly with regard to some unanswered questions and possible avenues for further enhancement of gel toughness.

Ionogel-based light-actuated valves for controlling liquid flow in micro-fluidic manifolds

We present the fabrication, characterisation and performance of four novel ionic liquid polymer gels (ionogels) as photo-actuated valves incorporated into micro-fluidic manifolds. The ionogels incorporate benzospiropyran units and phosphonium-based ionic liquids. Each ionogel is photo-polymerised in situ in the channels of a poly(methyl methacrylate) micro-fluidic device, generating a manifold incorporating four different micro-valves. The valves are actuated by simply applying localised white light irradiation, meaning that no physical contact between the actuation impulse (light) and the valve structure is required. Through variation of the composition of the ionogels, each of the micro-valves can be tuned to open at different times under similar illumination conditions. Therefore, flows through the manifold can be independently controlled by a single light source. At present, the contraction process to open the channel is relatively rapid (seconds) while the recovery (expansion) process to re-close the channel is relatively slow (minutes), meaning that the valve, in its current form, is better suited for single-actuation events.