April 21-25, 2014 | San Francisco
Meeting Chairs: Jose A. Garrido, Sergei V. Kalinin, Edson R. Leite, David Parrillo, Molly Stevens
Tessellated patterns, realistic animals, and curved polygonal shapes are all examples of the beautiful and amazing sculptures that can now be made using Origami, the art of paper folding. This art form has experienced tremendous growth with the advent of mathematical techniques that allow the basic structure of any new sculpture to be plotted out before any folding occurs, and laser cutter technologies that have made it easier to create folds in a variety of materials. In addition to their static properties, Origami sculptures can be designed to have a wide variety of mechanical properties making them responsive and tunable. Here, we will present a work-flow pipeline for materials design that uses Origami as a means of devising basic modular building blocks that can be assembled into larger-scale mechanical meta-materials. We start by working with origami artists to identify and generate candidate folding patterns for study. Next, we develop full-scale models using laser cut Mylar and paper sheets for rapid design, testing, and redesign. Mechanical measurements of these prototypes are combined with numerical simulations to identify the key relations between mechanical properties and geometric structure that give rise to the measured properties. Once a desirable pattern is identified, it is scaled down to a sub-mm tri-layer temperature-responsive polymer sheet using photolithographic techniques. The polymer sheet is capable of folding and unfolding as a function of temperature, and moreover, exhibits similar geometry-driven mechanical properties as the bench top prototypes. Stepping-back, we see this work-flow from design to synthesis as a conceptual tool that will help expedite origami-inspired materials from the minds of artists into the realm of engineering technology.
Self-folding is a self-assembly process that causes a predefined 2D template to fold into a desired 3D structure with high fidelity. Self-folding can be applied in the fields of actuation, sensors, and packaging. We have developed a simple method of self-folding that uses predefined ink “hinges” printed onto pre-strained polymer sheets via a desk top printer. The ink absorbs external light, causing the area underneath the hinge to heat up and relax the strain in the hinge regions gradually across the sheet thickness. This process results in folding the sheet at the hinge region. We will demonstrate that sequential folding of multiple hinges on the same sample can be programmed by changing the light source and ink color of the hinge. We have successfully employed this strategy to produce complex origami shapes.
Traditional shape memory materials are initialized with a several-state “program” via mechanical constraints applied at various temperatures. The program is then initiated by the uniform stimulus of temperature change. Photo-origami, demonstrated recently by one of us (Qi), instead uses a complex stimulus - one or more patterns of structured illumination - to locally relax an applied stress. The shape change is accomplished by polymer network relaxation via photo-initiation of a reversible addition-fragmentation chain transfer (RAFT) process. This method has the advantage of potentially complex shape response but the limitation that the stimulus, in the form of the applied optical pattern, must be similarly complex. Here we explore photo-origami using the stress-relaxing RAFT mechanism with the goal of complex programmed shape in response to simple stimulus, e.g. uniform optical exposure.This goal may be accomplished by locally modifying the optical or mechanical properties of the host polymer prior to initiation of the RAFT mechanism. We demonstrate that a low molecular-weight monomer freely diffusing through the solid polymer network can be locally attached to this network via radical photo-polymerization. Subsequent diffusion of the small monomer results in compositional gradients approximately proportional to the local optical dose. Finally, illumination can then initiate the RAFT mechanism of this inhomogeneous polymer volume which then exhibits bending based on the pre-programed internal structure. We report on a predictive model for the material structure in response to the first illumination and show that the the RAFT mechanism is maintained through this programming step. This internal mechanical (e.g. modulus) and/or optical (e.g. refractive index) structure can be arbitrarily patterned within the limits imposed by diffraction of the writing optical field and the reaction/diffusion kinetics of the photo-polymerization process. We show that this large design space maps well to the numerical design method known as Topology Optimization, which efficiently finds optimal distributions of 3D material properties to meet a design goal such as final shape.
Self-folding materials can enable new forms of manufacturing by harnessing the high speed and low cost of planar fabrication techniques to build three-dimensional structures. Construction by folding provides excellent strength-to-weight ratios and several folded components and machines have already been demonstrated. Furthermore, there is substantial research into the mathematics of folded structures, providing the basis for computational design tools to aid in fold planning. In order to capitalize on this, a self-folding composite must be capable of complex and precise geometries through sequential folding. We have developed a new self-folding composite that integrates prestretched shape memory polymers (SMPs) and paper . Self-folding hinges are programmed into the composite by cutting a gap in the paper along a line. When the SMP is heated above its glass transition temperature, it contracts bidirectionally, causing the composite to fold at that line. The final fold angle can be mechanically programmed into the composite by varying the gap width in the substrate, or by including mechanical stops in the form of additional folding tabs. In order to apply heat and activate folding in a localized and controlled manner, resistive heating circuits are embedded in the composite on a flexible printed circuit board (PCB). Electrical activation enables sequential activation and controlled folding, making autonomous assembly possible. These self-folding composites are easy and inexpensive to make, even by hand. The PCB is created by masking a sheet of copper-polyimide with a solid ink printer and etching with ferric chloride. The PCB, as well as the SMP and paper layers are then laser machined individually to program the hinge pattern into the composite. The layers are bonded with silicone tape, and the final composite is laser-machined again. Using this technique, we have produced three structures to demonstrate its versatility. We have built a self-folding pyramid to demonstrate an enclosed polyhedron acheived through simultanuous folding. We have also constructed a self-folding lock-and-tab mechanism to demonstrate folds that are precise enough to join together and sequential folding to create interlocking structures. Finally, we built an origami-inspired crane to demonstrate the complexity that this technique is capable of. These structures incorporated hinge lengths from one to nine centimeters, and each structure took between one and four minutes to fold. We believe that with the development of appropriate materials, this technique can be tailored to a wide range of sizes and functions.  S. Felton et al., Soft Matter 9, 7688 (2013).
The goal of this work is to derive inspiration from origami principles to create 3-dimensional multi-field responsive engineering structures. The key to achieving this unique goal is the synergistic use of active materials, compliant mechanisms and multiple actuating stimuli. Our current research is focused on electroactive polymers (EAPs) and magneto active elastomers to realize these multi-field responsive structures that fold and unfold in response to electric and/or magnetic field. In this presentation, we focus on a new class of relaxor ferroelectric terpolymers (PVDF-TrFE-CTFE), and conduct both fundamental studies and applied experiments to illustrate the promise of these types of EAPs in realizing origami-inspired 3D structures. An exhaustive study on the fundamentals of EAP actuation is completed by exploring static and dynamic thickness actuation mechanisms and quantifying changes in dielectric permittivity under strain. Input and output energy densities are also quantified for various multilayered thickness-extension and bender configurations. These fundamental studies are necessary to determine ability and limitation of polymer-based materials in a folding configuration. Finally, to demonstrate active folding/unfolding, bending using EAP actuators is transformed into folding by exploring a variety of geometric approaches. On demand folding and unfolding using EAPs is a step towards realizing active 3D origami-inspired structures. In our future work we will focus on improving the electromechanical properties of PVDF terpolymers, enabled through the graded distribution of nano particles; the gradient morphology will give rise to an intrinsic bimorph response where parts of the materials contract while others expand, resulting in high bending angles. Presence of nano particles will also reduce the required actuation voltage while potentially increasing actuation force.
In the aerospace field, systems in operation must meet a number of conflicting design requirements, and as such they typically compromise performance across the operating envelope in order to achieve the full set of mission requirements. Reconfiguration, or morphing, represents a paradigm by changing shape to match mission needs at different points in the flight envelope. Motivation for morphing is clearly bio-inspired, as birds, bats, and other flying organisms all vary wing shape to perform different maneuvers or flight activities. Shape adaptive structures, compliant structures, reconfigurable systems, and morphing are all terms used to describe systems for which shape is not a fixed attribute, but is changed in order to perform tasks for which they were designed.Improving performance across the flight envelope is not the only reason why reconfiguration might be used in a vehicle system. Packaging is a way of reducing the cost of inserting the system into its operational environment. Space structures, especially solar panels and antenna, are large, lightly-loaded structures in their operational environment. However, launch loads and volume restrictions require many space structures to be packaged and deployed after launch. Some air vehicles and systems have similar requirements. Missiles, for instance, many times have winglets, fins, or other control surfaces that must be deployed after launch.An interesting recent development related to adaptive structures is the use of origami in engineering design. Origami represents the process of construction by folding of paper. Traditional origami requires an artist or practitioner to create the folds and construct the desired object. Principles of origami can be adapted for use in shape adaptive structures, both in terms of design/manufacturing of complex three-dimensional components from two-dimensional materials, and in terms of active systems, where one or more shapes can be achieved by a system containing intrinsic actuation along fold lines.This talk will address motivation and applications to adaptive origami structures in aerospace structures, and highlight efforts at AFRL in development of origami through material, geometry, and system-level design optimization.
Soft actuators are of interest in areas ranging from advanced robotics to surgical tools. This talk describes some schemes for using embedded electronic actuators to control the shapes of three dimensional structures in hydrogels and shape memory polymers. Attention to the mechanics of these integrated systems allows sophisticated levels of control, without significant constraint on the motions of the soft materials by the hard electronic components.
Nafion, owing to its exceptional combination of chemical durability and proton conductivity, has been the state of art material of choice for use as fuel cell proton exchange membranes. Despite its thermoplastic nature and thermal transition between c.a. 55 °C and 135 °C, the material is thermally intractable due to its ionic interactions. Whereas this thermal intractability may be viewed unfavorably from the standpoint of membrane processing, its combination with the thermal transition makes it a potential and unexpected candidate for use as a shape memory polymer. Indeed, we report here that its broad thermal transition can be utilized as a highly versatile platform for physically programmable multishape material as well as temperature memory material. Both the multi-shape and temperature memory behaviors are one way shape memory in nature. In contract, the perhaps even more surprising discovery is that Nafion exhibits the two way shape memory behavior previously known for liquid crystalline elastomers and crosslinked semi-crystalline network, thus shedding new light on the structural ordering in Nafion.
Currently, most fabricated elastomeric materials are isotropic. However, there is a need and demand for anisotropic elastomeric materials that mimic anisotropic materials found in nature, such as tendons, wood, and bat wings. Our group recently developed an anisotropic elastomeric composite with shape memory (SM) properties. Composed of highly aligned poly(vinyl acetate) (PVAc) fibrous webs embedded in an elastomeric matrix, the anisotropic shape memory elastomeric composite (A-SMEC) was shown to have mechanical and SM properties that depend on fiber orientation. Specifically, the composite&’s ability to fix a temporary elongated shape when stretched at room temperature (RT) was found to vary with the orientation of the PVAc fibers. In our investigations, due to differences in shape fixing, two A-SMEC lamellae laminated together curl after stretching if a mismatch in fiber orientation exists between the layers. Here, we present our findings on the preparation and characterization of this mechanically activated shape change system. We correlate the curvature of the bilayer system to its fiber directions and the degree of straining. Characterization of curling is based on measurements of pitch and radius of curvature. Results of the curvature characterization, along with thermal and mechanical properties of the bilayer A-SMEC system will be reported. Additionally, we will show that the laminated composite maintains its shape memory capability and is triggered to return to its permanent flat shape from its temporary curled geometry by a thermal stimulus.
Shape memory polymers (SMPs) have been extensively exploited for a wide spectrum of technological applications ranging from biomedical devices (e.g., smart surgical stents and sutures) to aerospace morphing structures. Temperature-responsive SMPs are mostly studied and employed in practical applications. Although pressure is an easily adjustable parameter like temperature in many processes, pressure-responsive SMPs are largely unexplored. Here we demonstrate a new class of pressure-sensitive SMPs that enable the immediate recovery of the memorized shapes by applying a very small pressure (less than 60 kPa). By combining these novel SMPs with self-assembled colloidal crystals, we have developed tunable macroporous photonic crystals showing large optical stop band shifts triggered by a small pressure change. Rewritable photonic crystal microstructures, such as optical waveguides, can be easily printed on the pressure-responsive SMP films. We have also demonstrated that nanoporous smart coatings exhibiting tunable anti-glare properties can be fabricated by using the new SMPs. These pressure-responsive SMPs could find important applications in counterfeiting and biometric recognition markets.
The possibility of using photo-stimulus to control actuation is very appealing as light can provide contactless stimulation, is biocompatible and can be applied in a non-invasive and highly precise manner. Photo-responsive hydrogels are highly favoured for making light-stimulated polymer actuators as they have the ability to undergo significant volumetric changes in response to an external light stimulus. Hydrogels possess biocompatibility and a degree of flexibility, very similar to natural tissue, due to their significant water content. Photo-responsive engineered hydrogels can be made to collapse and thereby release a percentage of their water content upon light irradiation.The most widely used photo-sensitive molecules for photo-actuation of hydrogels are spiropyrans as they allow for reversible conformational, hydrophobicity/hydrophilicity and polarity changes upon irradiation with light of particular wavelengths. Taking advantage of this photo-induced change of hydrophilic/hydrophobic character, photoresponsive gels based on N-isopropylacrylamide and spiropyran have been realised.[1,3]In this work, a new generation of photoresponsive hydrogels are proposed, based on N-isopropylacrylamide copolymerised with pendant spiropyran groups and acrylic acid. These hydrogels actuate on the following principle: When the hydrogel is immersed in water, the copolymerised acrylic acid dissociates and the proton is taken by the spiropyran unit present in the copolymer causing it to change from spiropyran (SP) to the protonated merocyanine (MCH+) form. When the copolymer is irradiated with light matching the absorbance of MCH+, the MCH+ switches back to the closed hydrophobic SP form, releasing a proton in the process; As result of the formation of the more hydrophobic SP isomer, dehydration of the main polymer chain occurs and the hydrogel shrinks. When the light source is removed, the SP is spontaneously reprotonated and the hydrogel reswells.Due to the relative pKa values of acrylic acid, and of the spiropyran and merocyanine isomers, the protonation and deprotonation processes occur internally within the gel and there is no need for an external source of protons. In contrast to previous formulations, these gels do not show degradation of their photo-induced shrinking ability after multiple washings in deionised water and repeated switching over a two month period. Moreover, improved reswelling performance of these hydrogels has been realised by inducing porosity through the use of poly(ethylene glycol) as a pore forming agent. In this way, significantly faster kinetics for the hydrogel shrinking and re-swelling processes have been obtained compared to conventional hydrogels.1. Sugiura, S. et al., Sens. Act. A 2007, 140, 176.2. Florea, L.; Diamond, D.; Benito-Lopez, F. Macromol. Mater. Eng. 2012, 297, 1148.3. Ziolkowski, B.; Florea, L.; Theobald, J.; Benito-Lopez, F.; Diamond, D. Soft Matter 2013, 9, 8754.
Compliant mechanisms achieve their motion from the deflection of flexible components rather than from traditional motion elements such as hinges and bearings. Origami models can be thought of as compliant mechanisms during folding because they achieve their motion from bending at folds and flexing panels. Advantages of compliant mechanisms include characteristics associated with efficiency such as reduced part count and ease of manufacture, as well as compactness (also shared with origami), low weight, low wear, reduced maintenance, improved recyclability, and high precision motion. Uniting origami and compliant mechanisms principles could enable innovative and cost-effective devices that are capable of accomplishing sophisticated mechanical tasks. Origami-inspired compliant mechanisms have the potential advantages of planar fabrication (they can be fabricated from planar sheets of material and allow the use of planar fabrication methods); a flat initial state (which allows compactness for volume critical applications); and monolithic composition (which provides the advantages associated with compliant mechanisms noted above). Many recent origami-inspired compliant mechanisms have been manually or passively actuated, but some applications would be improved, and many others made feasible, if they were actuated by integrated actuators. Origami-inspired compliant mechanisms have characteristics that make them an ideal test bed for shape programmable materials, including the following: 1) Because of their nature, it may be possible to make entire compliant mechanisms, or at least many of their elements, of programmable materials; 2) They provide a cost effective way to evaluate, validate, and refine programmable materials; 3) Their basic designs (e.g. crease patterns) are transferrable, enabling sharing between labs across the world; 4) Their applications offer specifications and performance goals to guide material development; 5) Successful integration with applications can lead to mechanisms that can make a positive societal, scientific, or economic impact. Specific compliant mechanisms proposed as potential test beds include the following: 1) Bistable waterbomb base (a straightforward origami base that has interesting bistable behavior); 2) Lamina emergent mechanisms (compliant mechanisms that are fabricated in a plane but have motion that emerges out of the plane of fabrication); 3) Two-degree-of-freedom positioner (a monolithic compliant mechanism originally developed for space applications); 4) Deployable solar panel array (compact when stored and a large surface area when deployed); 5) Minimally invasive surgery devices (compact during transport, then deployed when at the surgery site).
Origami engineering--the practice of creating useful 3D structures through folding and fold-like operations on 2D building-blocks--has the potential to improve the design of engineered systems in many ways. Potential advantages include reduced manufacturing complexity (reduced part count, improved assembly via collapsible/deployable parts), the capability to create structures at small scales (microscale and nanoscale folding and self-assembly), the capability to create deployable structures that fold compactly for storage (solar arrays and other space structures), and the potential to create structures that are highly resilient through their capability to perform in-situ reconfiguration. This talk will describe a concept for a self-folding reprogrammable sheet for use in origami engineering applications. The sheet is a laminate structure consisting of a compliant medium sandwiched between two shape memory alloy (SMA) mesh layers. The SMA layers are thermally actuated, with the final 3D structure determined by the locations, durations, and sequencing of applied heat. The direction of actuation is determined by whether heat is applied to the top or bottom SMA layer. Folds are approximated by localized deformations in the sheet. These are fully reversible, allowing the sheets to be fully reprogrammable. The talk will cover the design and analysis of the sheets and present results from fabrication, testing, and applications.
Folded structures abound in nature (e.g. DNA, folded membranes comprising rods and cones in the retina, plant leaves and insect wings, lungs), span many length scales, and maximize performance while minimizing energy and material waste. Inspired by these structures and by the discipline of origami, we study the physics of folding and unfolding across multiple length scales, from human-sized to the molecular, and work to develop origami-inspired mechanisms, materials, and methodologies for energy conversion and communications. As part of this talk, we will discuss cut structures (kirigami) that can be actuated to enable optics enabling large tracking angles and high speed, with potential for performance improvements and cost reduction in solar energy harvesting, beam steering, and other applications.
Polymer gels swollen with macromers represent materials that enable precise, non-invasive adjustment of their shape, modulus or refractive index. This ability is used clinically to achieve optimal refractive outcomes despite the unpredictability of wound healing after cataract surgery—the most common surgical procedure on adults age 65 and older. A crosslinked silicone matrix dictates the initial shape of the intra-ocular lens (IOL), while macromers—short silicone chains with polymerizable end groups—and photoinitiator enable shape adjustment using light. Spatially resolved photopolymerization of macromer, followed by diffusion of free macromer, causes local swelling in irradiated regions. The resulting shape change occurs without external forces and no material enters or leaves the gel. Here, a predictive model of the transient shape change with no adjustable parameters is presented, which predicts shape change directly from the photopolymerization profile by accounting for the coupling between diffusion and deformation. Using connections between thermodynamics, constitutive equations and equations of motion eliminates adjustable parameters. Instead, parameter values are drawn from our prior experimental studies of the mechanical properties, equilibrium swelling, penetrant diffusivities and interaction parameters of systematically varied polydimethylsiloxane (PDMS) networks and acrylate-ended PDMS macromers. Preliminary computational studies explain the surprising transients that had been observed experimentally and show semiquantitative accord with established dose-response relationships for light-adjustable lenses.
Stimuli responsive hydrogel networks are ideal biocompatible shape programmable materials due to their ability to transduce chemical energy into mechanical motion without the use of external mechanical input. Hydrogels can also reversibly acquire complex 3D shapes by directional interactions and stresses within the polymer network in an anisotropic response to external stimuli. The ability to pattern, structure, re-shape and actuate hydrogels is important for biomedical applications, soft robotics, sensing elements and biomaterials. Our work focuses on manipulating the local ionic interactions within hydrogels by external electric fields. Electrical fields can induce osmotic pressure and electrostatic interactions between the fixed charges within gel networks for controlled motion and response. First, we introduce an ‘ionoprinting&’ technique with the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions via their directed injection and complexation. The mechanically patterned hydrogels exhibit programmable temporal and spatial shape transitions and serve as a basis for a new class of soft actuators that can gently manipulate objects both in air and in liquid. In addition, we present a novel class of "walking" gel actuators comprised of cationic and anionic gel legs which bend in response to the redistribution of mobile ions in the external field. The sign of the fixed charges on the polyelectrolyte network determines the direction of bending, which we harness to control the motion of the gel legs in opposing directions. These simple soft matter actuator devices and robotic components utilizing conventional polymers and external stimuli may serve as active components where conventional, stiff materials are inadequate.
The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. One of the most promising candidates for this strong photo-switching effect, for reversibility, speed, and simplicity of incorporation, is azobenzene. This paper outlines recent design and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene, such as single crystals, amorphous polymers, and liquid crystal elastomers, all of which experience in a significant macroscopic mechanical deformation of the host material. Measurements will be presented of associated parameters of this opto-mechanical effect, such as the pressure and stress measured by high pressure spectroscopy, AFM cantilever bending, visco-elastic changes by nano-indentation, and density studies by neutron reflectometry. Single crystal bending is perhaps the most illustrative as it can be used to elucidate the relationship between incident light parameters and polarizations, and space groups and geometries of crystal-to-crystal opto-mechanical transitions.
A soft elastic material placed under compressive stress can undergo a surface creasing instability in which the free interface becomes directly unstable to the formation of sharp self-contacting features. The resulting changes in contact between neighboring surface regions, and the non-affine in-plane displacement of material elements, provide unique opportunities for designing shape-programmable materials. This presentation will highlight several recent advances related to the formation of creases on the surfaces of multilayer polymer films. In one example, we study the influence of pre-compression of the substrate in a soft elastic bilayer as a means to tune hysteresis between crease onset and disappearance, yielding well-defined bistability. In another example, we consider the behavior of patterned stiff films on soft substrates, wherein the interplay between wrinkling and creasing modes offers promise for the design of new types of electronic switches with potential applications in flexible electronics.
Recently polymer chains of photoisomerizing complexes linked by soft superamolecular connectors have been synthesized, and condensed into fiber bundles that can template and amplify mechanical actuation by the photoisomerizing molecules. We describe the use of a heuristic Burridge-knopoff--type model to examine the collective behavior of this system. We examine the extent to which the random isomerization events become correlated because of the strain that a transformed isomer exerts on their neighbouring chromophores, and the change that this strain imparts on their optical gap. The purpose of this model is two-fold: to understand the limiting constraint that prevents isomerization, and to examine if the collective response to constraint in the system can increase the efficiency of solar energy conversion to mechanical work.
CP2 is a high-Tg polyimide that was developed in 1980&’s from the “Colorless Polyimides” R&D program at the NASA-Langley Research Center, with the objective toward such space applications as thin film membranes or coatings and where high optical transparency in 300-600 nm region was an important requirement. In addition to its excellent thermal, mechanical and optical properties, we have recently shown that CP2 is also promising as a high-temperature shape memory (SM) polymer, and when incorporated with an azobenzene-containing amine crosslinker, a CP2 cantilever, and upon exposure to a polarized blue light at room temperature, it can be rendered photomechanically active as well. CP2 is synthesized by the linear polymerization of 1,1,1,3,3,3-hexafluoro-2,2-bis(4-phthalic anhydride)-propane (6FDA) and 1,3-bis(3-aminophenoxy)benzene (APB) in DMAc to generate the poly(amic acid) precursor, which is then cast into film and thermally imidized. However, it was puzzling that when a CP2 film was placed in a heat-bath at temperatures above its Tg (~199 oC), it could be easily stretched, bend or twisted, and the temporary shape could be fixed by simply removing the film from the hot zone under a constant stress. When re-immersing in the heat-bath (~210-215oC) without being under stress, the CP2 film would fully recover its original shape. This SM effect could be repeated (>10 times) and was observed also in CP2-polymers crosslinked with a triarylamine compound. Interestingly, such SM effect was not observed in other similarly prepared (without a crosslinker added) polyimide films (e.g. CP1 and Ultem). Thus, in this work, we have attempted to delineate the source and mechanism of crosslinking, and correlate the crosslinking chemistry and the SM effect apparently unique to CP2. We came to a tentative conclusion that the high temperature shape-memory effect CP2 polyimide is originated from the small amount of “unexpected/deliberate” crosslinks stemming from the slight excess of APB (diamine). The results for supporting such conclusion and proposed crosslinking mechanism will be presented. H. Koerner, R.J. Strong, M.L. Smith, D.H. Wang, L.-S. Tan, K.M. Lee, T. J. White, and R. A. Vaia, Polymer 2013, 54(1), 391-402. D.H. Wang, K.M. Lee, Z. Yu, H. Koerner, R.A. Vaia, T.J. White, and L.-S. Tan, Macromolecules, 2011, 44(10), 3840-846. CP1 was prepared from 6FDA and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane. Ultem was prepared from 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine.
In micrometer scale, liquids can be handled as discrete elements bounded by interfacial surfaces, the elements typically being hemispherical or disk-shaped droplets. Unlike the solid walls confining the liquids in macro-scale, in micro-scale the virtual walls between the liquid and surrounding fluid open the possibility of actively reshaping individual droplets or re-configuring a system consisting of the droplets. Compared with other actuation methods, an electrostatic manipulation of droplets is more attractive because of its simplicity to implement for devices and systems. After examining how electrostatic attractions affect a liquid droplet on substrate and discussing the popular electrowetting-on-dielectric (EWOD) actuation, a series of application examples will be reviewed. By deforming individual droplets, we have developed a miniature rheometer for scarce samples and a micromechanical switch for radio-frequency (RF) electronics. By sliding droplets on a substrate, we have developed a micromechanical toggle switch for reconfigurable integrated circuits (IC) and a lab-on-a-chip platform for a wide range of biochemical analyses and syntheses. In addition to the simplicity, the low power consumption of the electrostatic manipulation makes the approach especially attractive for portable applications.
Recent advances in additive manufacturing allow the precise placement of multiple materials at micrometer resolution with essentially no restrictions on the geometric complexity of the spatial arrangement. Complex 3D solids thus can be created with highly non-regular material distributions in an optimal fashion, enabling the fabrication of devices with unprecedented multifunctional performance. In this work, we exploit these advances to and introduce a paradigm of active composites by 4D printing in the spirit of the recent developments of Tibbits, although our work differs significantly in terms of the physical phenomena at play and the emphasis on understanding them. We directly print a composite in its initial 3D configuration from a CAD file that specifies the shape memory fiber (SMF) architecture at the lamina and laminate level. Later, the programmed action of the SMFs creates time dependence of the composite configuration - the 4D aspect. This process has considerable design freedom to enable creation of composites with complex and controllable anisotropic thermomechanical behavior via the prescribed fiber architecture, shape, size, orientation and even spatial variation of these parameters. . We design and print laminates in thin plate form that can be thermomechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted strips, folded shapes, and complex contoured shapes with nonuniform, spatially-varying curvature. The original flat plate shape can be recovered by heating the material again. We also show how the printed active composites can be directly integrated with other printed functionalities to create devices; here we demonstrate this by creating a structure that can assemble itself, such as printed origami.
Finding materials with combinations of several extreme properties is one of the key requirements for the successful engineering of adaptive systems. Many of such challenges are represented by the blank areas of Ashby plots, but others are less known. Successful realization of such materials requires new choices for materials components and new approaches for materials engineering including their microfabrication in biomimetic origami-like structures. Layer-by-layer assembly (LBL) is materials manufacturing technique from nanomaterials that affords engineering of nanocomposite materials based on sequential adsorption of nanometer scale layers of polymers and inorganic particle, nanowires, nanotubes, sheets, etc. In this presentation it will be demonstrated that LBL and related techniques can lead to the materials with seemingly “impossible” combinations of physical properties encompassing mechanical, electrical, optical, and biological properties. We will make particular emphasis in this presentation on subsequent alteration of these properties using stretchable and shape-changing materials with micro-scale reconfigurability. Finding composites with high stiffness + high damping, high stretchability+high conductance and as well as high stiffness + transparency will be demonstrated.
The ability to control the shape liquid metals on the sub-mm scale is important for many applications, including flexible electronics, optics, metamaterials, and microfluidics. While mercury has drawn much of the focus of liquid metal research, its toxicity and incompatibility with microfluidic systems limit its potential for applications. Gallium and gallium-based alloys, however, are perfectly suitable for use with biocompatible flexible electronic devices, but have traditionally been avoided due to a passivating surface oxide layer that limits their use in hydrodynamics and electrochemistry. Here, we present a novel way to utilize this surface oxide to tune the surface tension and create unprecedented electrohydrodynamic control of the liquid metal using only modest voltages.
Adaptive nanomaterials and bio-mimicking nanodevices all require the underlying structure be able to reconfigure in response to environmental cues such as light, pH or other external "field". While it is evident that the shape of constituent building blocks plays a crucial role in determining the morphology of assembled structures , only recently has this parameter been considered as a "knob" to optimizing assembly propensity of a target structure, or for reconfiguring among multiple crystal polymorphs. In this talk, we show how shape-shifting at the building block level can promote assembly and shape-shifting at mesoscopic and macroscopic scales in computer simulations [2,3]. Starting from ordered structures made of spherical and rod-like particles, we show how morphing the original particle shapes into square, triangle or zigzag geometries (in 2D)  or tetrahedron, tetrapod and octopod shapes (in 3D) shift the morphology of the structure as a function of the initial and target shapes and shifting rate. We demonstrate that in many cases particle shape shifting yields more efficient assembly pathways, higher packing fractions, and ordered structures with fewer defects than self-assembly without shape shifting. We show that otherwise unattainable structures can be achieved via shape shifting. We explore the idea of colloidal shape shifting for information applications . Finally, we introduce the idea, and explore the thermodynamics of shape neighborhoods  for packing and assembly via shape shifting.References:1. P. F. Damasceno, M. Engel and S. C. Glotzer, Science, 337, 6093, 2012.2. T. D. Nguyen and S. C. Glotzer, ACS Nano, 4, 2584-2594, 2010.3. T. D. Nguyen, E. Jankowski and S. C. Glotzer, ACS Nano, 5, 8892-8903, 2011.4. C.L. Phillips, E. Jankowski, Bhaskar, Kazem, Sacanna, Grier, Pine, Glotzer, preprint.5. E. Chen, D. Klotsa, M. Engel, P. F. Damasceno and S. C. Glotzer, arXiv:1309.2662, 2013.
Actuators based on soft, water-containing polymeric network, such as hydrogels, are currently being explored as soft robotic components, biosensors, and controlled drug delivery applications. Here, we report the development of a new pH-responsive hydrogel actuator based on chemistry found in mussel adhesive proteins. Mussels secrete a protienaceous fluid that hardens to form an adhesive plaque and byssal thread complex that enables these organisms to anchor themselves to surfaces in wet, saline and turbulent environments. One unique structural component found in these proteins is 3,4-dihydroxyphenylalanine (DOPA), a catecholic amino acid that is responsible for both the moisture-resistant adhesion and the energy dissipative properties of the byssal thread. Breaking and reformation of strong, reversible catechol-metal ion interactions contributes to the wear resistance properties of the protective coatings on mussel byssus threads, which experience large cyclic strains in the turbulent intertidal zone. The catechol moiety is capable of forming strong complexes with metal ions (i.e., Fe3+) with log stability constants greater than 40. Hydrogels were prepared with network-bound catechol by copolymerizing dopamine methacrylamide (DMA) with a neutral monomer (i.e., hydroxyethyl acrylamide) using photopolymerization and Fe3+ was introduced by soaking the hydrogel in a FeCl3 solution. Dopamine contains a catechol side chain mimicking that of DOPA. Catechol and Fe3+ transitioned from a mono-complex to a bis- and then a tris-complex with increasing pH, which drastically increased the crosslinking density of the hydrogel. Hydrogels containing both DMA and Fe3+ demonstrated reduced water content and increased storage modulus under dynamic compression testing. Additionally, transitional metal ions (Fe3+ or V3+) can be incorporated locally by applying electric potential (3-20 V) using an iron or vanadium wire (anode), respectively, while using aluminum foil as the counter electrode (cathode). The oxidative bias generated from the applied electric potential resulted in oxidation of the anode and the released metal ions were locally captured by the dopamine side chain. When the hydrogels were submerged in a basic medium, formation of the tris-complex increased local crosslinking density at the site of ionoprinting and resulted in the hydrogel shape transition. Localized stress was large enough to cause the hydrogel to fold like a hinge. When the hydrogel was submerged in an acidic medium, the catechol forms a mono-complex with reduced crosslinking density and the hydrogel retuned to its original shape. To our knowledge, this is the first demonstration of a hydrogel actuator inspired by mussel adhesive chemistry.
In nature we find examples of a relatively small set of building blocks arranged in ways that give rise to properties that range orders of magnitudes. The large variation in mechanical properties, such as strength and extensibility is enabled by secondary structures that undergo different degrees of folding reversibly. Using the same idea, we demonstrate the design and fabrication of tunable mechanical properties over orders of magnitude by both macroscopic and microscopic patterning of “secondary structures”. Here we report using three types of “paper”, including nanocomposite, to experimentally demonstrate using hierarchical structures to induce elastic instability. FEM results are compared to elucidate the effect of different geometries and several relevant design parameters. The dynamic folding and the associated conformational change can revolutionize the manufacturing of origami materials and deployable structures that undergo rapid and reversible transformation with high fidelity.
While origami has been practiced as an art form since the 17th century, recent advances have suggested a rich set of possibilities for tuning the mechanics of thin sheets through fold geometry. Though well studied on developable surfaces, origami on non-Euclidean surfaces has not been widely explored owing to difficulties in preparing curved shells suitable for folding. Using a recently developed approach to fabricate polymer shells, we present a combined experimental and theoretical study of folding on surfaces with Gaussian curvature. The most important finding involves the possibility for ‘snap-through&’ transitions between nearly isometric states whenever crease lines lie along directions of non-zero normal curvature. We study the geometrical dependencies of this snap-through transition on a variety of non-Euclidean surfaces, as well as on the material properties of the shell. Under appropriate conditions, we find robust bi-stability between folded and unfolded states, with rapid switching between states. We anticipate this approach will lead to new avenues in the design of MEMS devices, self-folding structures, and mechanical meta-materials.
The key metric to benchmark any solar energy technology is the cost in terms of dollars per watt ($/Wp) or levelized cost of electricity (LCOE). Reducing the $/Wp requires lowering the materials cost, e.g. by using the concentrated photovoltaics (CPV) technology. However, the CPV technology requires a costly tracking system. A parabolic trough with one axis tracking can cost more than $200/m2. Furthermore, one cannot directly apply the CPV tracking system to a flat panel system to reduce the materials cost. To this end, we have designed a self-actuated and self- tracking origami structure that can be integrated with a flat panel system to reduce the overall materials cost.In our design, we consider a semiconductor thin-film absorber being transferred onto a paper-like flexible substrate, for example a micron-thin GaAs PV structure transferred onto a Kapton film. This PV “paper” will be cut by laser cutting technology and then folded into an array of concentrating collectors. Each origami collector will be actuated by another origami structure also formed from a sheet of paper-like film. The temperature difference between the two sides of actuation structure induced by the Sunlight will be converted into a differential force to guide the collector toward the Sun at all times. According to our optical simulation, the collector with six surfaces coated with aluminum can collect 88% of light intercepting the collector aperture. Assuming the added cost to make the origami structure is 15% of the original PV materials cost, the total materials cost can be reduced by 68%. In the presentation, we will show our experimental results.
We demonstrate the design and fabrication of tilted pillar arrays on wrinkled elastomeric polydimethylsiloxane (PDMS) as a reversibly switchable optical window. Previously reported methods for tilting polymeric pillars, such as oblique metal deposition, ion beam irradiation and shearing of shape memory polymers, have only achieved non-cyclical one-dimensional tilting. In this study, we exploited the surface wrinkling effect to reversibly tilt PDMS micro-pillars. Since the morphology of these hierarchically-patterned substrates can be controlled reversibly using mechanical force, these substrates are thus versatile materials for controlled, switchable transparency, adhesion, and wetting. . Using square arrays of microposts (diameter 1mu;m, pitch 2mu;m, and height 4mu;m), the wrinkle formation can be confined to the micropattern. We show that the pattern and wrinkle morphology, orientation and dimensions were controlled by varying the treatment duration of the oxygen plasma and the angle of uniaxial stretching with respect to the pillar array axis. While the original PDMS film exhibited angle-dependent colorful reflection due to Bragg diffraction of light from the periodic pillar array, the tilted pillar film appeared opaque. Upon re-stretching the film to the original pre-strain, the grating color is restored and ~50% transmittance is recovered.
Layered, polymer-based materials can be constructed by depositing a film of polydiallyldimethylammonium chloride (pDADMAC) on a single layer of poly (N-isopropylacrylamide) (pNIPAm) - based microgels attached to a Au-coated semi-rigid polymer substrate. Interestingly, when the pDADMAC layer is completely dried, the complete assembly bends and deforms; the bending is completely reversible upon increasing the humidity of the environment. We found that this material was able to do work and lift many times its own mass simply by changing the humidity. We further investigated how the size and aspect ratio of the polymer substrate affected the self-folding behavior of the materials. From experimental observations, a set of empirical rules were developed that can be applied to predict the self-folding behavior. From these rules, we were able to direct the folding of these materials into discrete three-dimensional objects, which are fully capable of unfolding and folding in response to humidity.
Stimuli-responsive polymer hydrogels (SPHs) can largely and reversibly change their volume or shapes under slight external stimuli, e.g. temperature, pH, special chemicals, and so on. Due to this property, SPHs have been widely used in diverse industrial and biomedical applications. Most of the SPHs provide only isotropic swelling/shrinking in response to uniform stimuli, since they are usually of homogeneous structures. Recently, we used a macroscopic self-assembly (MSA) technique, which is based on host/guest supramolecular interaction between β-cyclodextrin (β-CD) and adamantly moieties respectively provided by the interfaces of the two hydrogel strips, to combine a thermo-sensitive hydrogel strip with a non-thermo-sensitive one. Upon heating, the combined bi-strip hydrogel was able to provide a bending activity because of the different volume change abilities of the two strips. The MSA technique showed advantages such as simple sample preparation, arbitrary combination of ‘building blocks&’. In this case, more complex movements are possible to be conveniently provided. However, the association force between β-cyclodextrin and adamantly moieties are too stronge that the bi-strip hydrogels could hardly be separated and recovered into the two original strips intactly In this study, we prepared a bi-strip hydrogel which provided bending movements in response to pH values and ionic strength. One strip was functionalized with β-CD host groups and carboxyl groups, which provided pH/ionic strength-sensitive isotropic volume change behavior. The other strip was a non-sensitive hdyrogel functionalized with ferrocene (Fc) guest groups. Based on the macroscopic host-guest inclusions, the two hydrogel blocks were integrated into a bi-strip hydrogel. Upon variation of pH values or ionic strength, bending actions could be reversibly achieved. The bi-strip can adhere/detach reversibly in response to an appropriate redox state because the β-CD-ferrocene inclusion would be dynamically dissociated at an oxidized state. Swelling ratio and bending kinetics in various pH values and ionic strength were investigated. In addition, an electrical device based on the bi-strip hydrogel for monitoring the pH/ionic strength variation of the environment was demonstrated.
Form-finding describes the process of finding a stable equilibrium shape for a structure under a specific set of loading for a set of boundary conditions. Physical and numerical form-finding methods have been employed by structural engineers and architects for the design of shape-resistant structures: structures whose behavior depends mostly on their global spatial configuration and less on the properties of their individual components. The shape of dielectric elastomer minimum energy structures (DEMES) depends on the equilibrium between the pre-stressed elastomeric membrane and its inextensible frame. Therefore, DEMES can be modeled and analyzed using structural form-finding techniques . We applied dynamic relaxation (DR), a well-established explicit and efficient numerical form-finding and analysis method, to simulate DEMES equilibrium shapes and predict the elastic energy of DEMES . The DR-DEMES model shows generally good agreement with its physical implementation counterpart, as it captures the equilibrium shape and also the elastic energy in function of shape. However, we find that the numerical and the physical models differ in the pre-stress that is required to obtain a specific equilibrium shape. Therefore we introduced materials laws, including hyper-visco-elasticity for the membrane and hyper-elasticity for the frame. With these refinements in physical parameters the DR-DEMES model approaches the pre-stress state of the physical DEMES implementation more closely, while it maintains the computational efficiency of the form-finding approach. We conclude that dynamic relaxation, with its low computational cost, is a powerful tool for the design of novel DEMES applications.References: G. Kofod et al., Appl. Phys. A, 85, 141 (2006). S. Siu, et al., Appl. Phys. Lett. 103, 171906 (2013).
A form of self-assembly, “self-folding” presents an alternative approach to the creation of reconfigurable, responsive materials with applications ranging from robotics to drug design. However, the complexity of interactions present in biological and engineered systems that undergo folding makes it challenging to isolate the main factors controlling their assembly and dis-assembly. Here we use computer simulations of simple, minimalistic self-foldable structures and investigate their stochastic folding process. By dynamically accessing all the states that lead to, or inhibit, successful folding, we show that the mechanisms by which general stochastic systems can achieve their “native” structures can be identified and used to design rules for optimized folding propensity.
Despite the small set of building blocks used for their assembly, naturally occurring materials such as proteins show remarkable diversity in their mechanical properties ranging from something resembling rubber - low stiffness, high resilience and extensibility - to silk - high stiffness and strength. Moreover, their self-folding properties inspire the design of structures capable of tunable reconfiguration. Motivated by such versatility, we report on simulations and experiments for the design of nanocomposites sheets whose mechanical properties can be made tunable via “secondary structures” patterning. Our simulations reveal the main cutting features needed to obtain desired material extensibility. Additionally, we study how similar sheets could self-fold into their desired “native” structure via stochastic forces. Our results open the possibilities for manufacture of flexible and reconfigurable materials with targeted strength and extensibility.
We present a multilayer microfluidic system having a laser-micromachined thermo-responsive poly-(N-isopropylacrylamide) (PNIPAAm)-based hydrogel layer integrated as freestanding component which operates as a temperature-triggered cell sorting actuator for single cell assays applications. PNIPAAm based thin layers (50 divide; 300 mu;m thick) are synthesized and manufactured by an injection/compression molding technique in order to obtain flat hydrogels attached to rigid polyvinyl chloride (PVC) substrates to facilitate laser focusing. Laser machining parameters were empirically optimized to fabricate through-holes with entrance diameter of 150 mu;m and different exit diameter (from 10 to 20 mu;m) on the PNIPAAm using a stencil aluminum mask. After laser processing, the micro-structured layers are detached from the PVC using a chemical treatment and then swollen in pure water.By employing mechanical fastening as the packaging strategy, the hydrated hydrogel is sealed between two micro-milled poly-methyl methacrylate (PMMA) components, which are providing the fluid accesses and ducts to the cell suspension to be flown over the thermo-responsive actuator (top layer) and the well to collect the sorted sample (bottom layer). The device is also endowed with a thin transparent heater to control the microfluidic chip temperature. The size of the laser-machined microstructures can be reversibly modulated as a consequence of the PNIPAAm volumetric phase transition induced by heating the device above the critical temperature value of 32 °C; due to the polymer water loss, the shrinkage of the layer caused the hole to homogeneously shrink along with it, thus reducing its original size of about the 40% in the polymer collapsed state. This actuation mechanism was exploited to firstly trap a cellular sample in the shrunken exit hole on the top of the hydrogel layer by applying a negative pressure across the film via the bottom PMMA component while keeping the system at 37 °C. Subsequently, the sorting of the trapped cells took place through the micro-capillary when the polymer natural relaxation toward its initial state occurred at room temperature. The functionality of the device was proved using MG63 cells as a model cell line, by monitoring the sorting through the size-modulating structure using optical microscopy imaging.
Origami is in part a realization of the study of flat-foldable crease patterns. Interestingly, techniques to quantify what makes a “good” foldable material, let alone to optimize substrates of engineered or adaptive origami structures, are lacking. With respect to the external deformation, a “fold” has two states: uniform where the deformation is distributed around the bend radius; and localized where the deformation is concentrated to form a crease, akin to necking in tensile deformation. Depending on the material and film geometry, the fold response can range from elastic, to elasto-plastic, to dissipative and failure. To begin to establish correlations between fold response and thermomechanical properties of polymers, fold performance was categorized into three areas: 1) formation of the crease, 2) stability of the crease, and 3) the failure mechanism at the crease. Paper, the classic material for origami, served as a baseline. Folding of seven common thermoplastics and nanocomposites was simulated by compressing thin rectangular specimens between parallel plates. This technique mimics flexural loading with no transverse loading applied along the crease. Formation of creases corresponded to bending the specimens beyond their yield or failure point. Paper exhibited a lower bending yield stress at lower strains than polymers, and hence easier crease formation. Crease stability was reflected in the equilibrium fold radius and relative reduction in deformation energy of sequential fold cycles. Equilibrium fold radius was generally lower in paper than polymers. This was attributed mainly to the difference in failure mechanisms. In contrast to buckling and delamination observed in paper, polymers plastically flow within the crease resulting in wrinkle-like features on both interior and exterior fold surfaces. The wrinkle frequency increased with decreased fold radius. These metrics provide a valuable tool to establish guidelines for material selection in the design of origami structures.
We live in a world where computers and machines are increasingly reducing the need for manual labor. With a ‘hands off&’ approach in mind, and with origami as a source of inspiration, we have developed a water-triggered origami system that enables assembly of structures with minimal handling. Utilizing the decrease in the glass transition temperature (Tg) upon hydration of poly(vinyl acetate) (PVAc), we have found that electrospun PVAc fibrous webs fold when stripes of water are drawn on the webs. As water diffuses through the web and is absorbed by the PVAc chains, a gradient of shrinkage forms through the thickness of the web, causing folding. The folding kinetics of a basic rectangular shape are studied and related to the average fiber diameter and thickness of the sample. To support the kinetic results, we have measured the contractile force generated when strips of PVAc fibrous webs are submerged in water. Modeling the kinetics in order to enable the estimation of assembly times, along with investigation of folding reversal without manual intervention are being investigated. We will present quantitative results of these studies and present new, complex geometries that mimic conventional, paper-based origami. The combination of strategic sample cutting and water line placement allow for the self-assembly of more intricate structures.
A hybrid material system combining a Nickel-Titanium shape memory alloy (NiTi SMA) with a soft polymer layer of polydimethylsiloxane (PDMS) was generated to elicit temperature induced switchable properties of wettability or adhesion. In this system the indentation induced two way shape memory effect was used to reversibly change the surface topography of the NiTi SMA. The switchable NiTi surface was covered with a PDMS layer, which acts as an adhesion mediating contact element. By the use of different bioinspired adhesive or hydrophobic structures and an accurate thermal stimulus, the adhesive properties of the “gecko surface” or the hydrophobicity of the “lotus-effect surface” could be switched. It was experimentally shown that with increasing temperature and bulging surface structures the adhesion of this system can be reversibly changed by an order of magnitude. The adhesion was reduced by more than 80% if the temperature was increased from room temperature to 80°C. In case of using hydrophobic surface structures the contact angle and water distribution could be guided by the surface protrusions induced by heating. Cooling down the system nearly completely restored the initial adhesive or wetting properties. Additionally, the influence of different adhesive or hydrophobic PDMS structures and the spatial arrangement of the underlying NiTi shape memory bumps were investigated. The results show a significant interaction of the switchable surface properties and the bulging NiTi surface. While the NiTi SMA surface with high aspect ratio structures lead to stronger changes in the surface functionalities, shape memory surfaces with low aspect ratio structures show stronger switch ability. When covered with bioinspired adhesive or hydrophobic PDMS structures, already small aspect ratio NiTi structures lead into a significant decrease of overall adhesion and contact angles during heating. During cooling the system recovers almost entirely. This hybrid material shows promising performance to receive a switchable adhesive “gecko surface” or a surface with switchable wetting behavior using “lotus-effect” structures.
Devices based on smart materials that enable stimulus-triggered mechanical adaptability have been proposed for a diverse set of devices in areas from robotics to biomedical devices. A limitation to the employment of phototresponsive amorphous polymers and liquid crystal networks in applications as shape programmable materials is the magnitude of the photogenerated strain. Towards this end, we report on the preparation and characterization of a number of distinct materials chemistries - primarily employing acrylate and thiol-ene chemistry. The underlying thermomechanical properties and resulting photomechanical responses of these materials were characterized. For networks exhibiting liquid crystallinity, the order parameter of these networks is evaluated. Dynamic mechanical analysis is used to evaluate network uniformity and crosslink density, and the effect of these factors is discussed in the context of maximizing the efficacy of photoinduced stimulus-response. The potential of this photomechanical response is evaluated in the context of achieving polymer films that can controllably deform into complex three dimensional shapes, mimicking the structures achieved through origami.
An embryonic pluripotent stem cell can differentiate into any type of cell in the body. Arguably, the stem cell is one of the most complex pluripotential structures. We demonstrate that relatively simple materials, such as rubber sheets, can also be made to differentiate into a wide range of forms with a wide range of properties via a very simple process - origami (folding) and/or kirigami (cutting). The design of the origami and kirigami pattern together with the intrinsic properties of the material allows for precise control of the differentiated material properties. As a proof-of-concept, we performed a series of numerical calculations with a focus on prescribed expandability and shape control that effectively broadens the design space of engineered materials and systems. While the present results focused on 2-dimensional (2D) sheets as a starting point, the same approaches can be applied in 3-dimensional (3D) materials where the initial space-filling unit shape, the pattern geometry and the types of hinges are included. With a responsive material, self-transformable 2D/3D structures could also be realized.
Fiber Bragg gratings (FBGs) are longitudinal periodic variations of the refractive index along an optical fiber. Broadband light propagating in the fiber waveguide is transmitted except for a narrow stopband, which is reflected backwards. The reflected wavelength (termed the Bragg wavelength) is determined by the grating period and refractive index. Thus a Bragg grating can be considered an optical notch filter or wavelength-selective mirror.Bragg gratings in glass optical fibers have found widespread use in telecommunications and optoelectronics, with applications as sensors, logic elements, optical wavelength multiplexers, and fiber lasers. Recently, FBGs have been incorporated into polymer fibers. Polymer fibers are more attractive than their glass counterparts for many FBG applications due to their lower Young&’s modulus (allowing for easier stretching and wavelength tunability), larger nonlinear optical susceptibilities, and larger photomechanical effects.We report on the fabrication of FBGs in poly(methyl methacrylate) (PMMA) polymer fibers doped with azo dyes. The fabrication process that we are developing is amenable to making polymer FBGs that incorporate liquid-crystal elastomers and photo-responsive gels to leverage the magnitude of the photomechanical effect that can lead to photo-induced bending. Fibers with multiple cores can be made to bend to the desired angle and direction when excited by the correct total intensity and intensity ratios between the cores. Our target is the development of Photomechanical Optical Devices (PODs), comprised of two FBGs in series, separated by a Fabry-Perot cavity of photomechanical material. PODs exhibit photomechanical multi-stability, with the capacity to access multiple length states for the same optical input when a mechanical shock is applied. Consequently, networks of PODs may provide a framework for very fast optical computation and smart shape-shifting materials where the decision process is distributed over the entire network.Using coupled-mode theory and finite-difference time-domain (FDTD) numerical methods, we have modeled the optical reflectance and transmittance of uniform sinusoidal Bragg gratings in polymer optical fibers. Our model correctly predicts the essential optical features of FBGs, including narrow reflectance at the Bragg wavelength. Further additions to our model incorporate the effects of temperature and mechanical strain. Because these parameters influence the length of the polymer fiber, dynamic modification of these parameters can be used to tune the Bragg wavelength. Finally, we expand the model both to include coupled networks of several serial Bragg gratings in a single fiber, and to address multi-stability and interactivity between PODs. Such models will inform and accelerate targeted development of novel polymer FBGs for a variety of applications.
Precise positioning of particles has wide implications for a variety of applications such as in device fabrication, controlled crystallization, and tuning of materials properties. We have developed a versatile particle assembly technique with precision down to the single particle level, allowing fabrication of limitless particle configurations. This method was demonstrated with micron sized polystyrene beads by utilizing a templated electrode to electrophoretically deposit particles in a pre-designed pattern. The same technique can be applied to nano sized particles, such as metal nanoparticles, where their surface plasmon resonance (SPR) is highly sensitive to the metal shape and geometry. By electric field manipulation, multi-material deposition on the same 2-dimensional platform was performed where precise position and assembly of the different materials with respect to each other was shown with a high degree of control. Programmable particle assembly push the limits of precise control of materials properties and has direct impact for a wide range of applications.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
We introduce a patterning method to produce PDMS-based microstructures using a single step UV exposure. The process requires photosensitive PDMS (P-PDMS) spun on a carrier wafer, a chromium photomask, a UV lamp and a curing oven. Microchannels with sub-millimeter cross-section can be patterned within the elastomer membrane after UV exposure and curing under mild temperature (<200°C). No development step is required.P-PDMS is prepared by adding to the silicone base a photoinhibitor compound (benzophenone). To date, P-PDMS has been used to pattern PDMS using UV photolithography, i.e. exposing the polymer to UV through a mask then developing it in a solvent, or to modulate locally the elasticity of the elastomer. Here we demonstrate that patterning of structures and elastic modulus of the elastomer may be achieved simultaneously and without a development step. The positive P-PDMS mixture is prepared by mixing un-crosslinked PDMS with benzophenone flakes (3 wt. % of the elastomer mixture) dissolved in xylene (4.3 wt. % of the elastomer mixture). The P-PDMS mixture is spin-coated on a wafer to obtain a layer of 70µm thickness, subsequently exposed to UV light (lambda;=365nm) through a chromium mask and cured on a hotplate for 24h. We report on the patterning resolution and mechanical properties of the P-PDMS structures as a function of the Cr mask feature size (tested from 200µm to >1 mm), UV exposure energy (from 0 to 18J/cm2), curing temperature (80 and 150°C), and pre-polymer:curing agent ratio (from 10:1 to 5:1 ). This technique allows for the creation of motifs of controlled shapes and elastic moduli within a single elastomer membrane thus promises applications in stretchable electronics and soft interfaces.
Polymer networks with unique capabilities including the capacity for rapid and dramatic shape changes will be discussed through light illumination and through mechanical folding. These networks represent a paradigm in polymer network fabrication aimed at the rational design of structural materials possessing dynamic characteristics for specialty applications and functions. Here, we will explore two distinct classes of shape changing materials -- one based on chemical reactions and a second based on foldable materials. Photoinduced plasticization, lithography, and stress relaxation will be discussed and implemented. Secondly, networks that are capable of shape memory programming while remaining in their glassy state will be discussed. Ultimately, the potential for these materials to impact smart materials applications will be discussed, considering the range of possible material properties and responses.
A “photomorphon” is an optical-fiber-based structure that forms the basic building block of a smart material that intelligently changes shape in response to light. Photomorphons are made of patterned dye-doped elastomers and serve several functions simultaneously, such as controlling the flow of light in response to external stimuli -- e.g. stress and temperature; performing optical logic operations on light pulses; changing the material's shape based on light intensity and external stimuli; and exhibiting optical multi-stability so that for one set of inputs the fiber can be in many shape configurations and internal light intensity levels.A series of photomorphon devices can be fabricated into a single strand of polymer optical fiber that communicate with each other using light and time-division multiplexing or wavelength multiplexing to access specific units in the network. Like a transistor, which endows integrated circuits with complex functionality, photomorphons interconnected with light will make photonic circuits with exceptional computational capabilities. Each element within such a photomorphon network will also be both a sensor and an actuator, making a morphing material that intelligently changes shape in response to external stimuli. We are developing new materials with enhanced photomechanical response and integrating them together to explore applications that can take advantage of morphing materials. Applications will include tactile haptics and sensors, ultra-smart active textiles, and adaptive stretchable electronics, antennas and mirrors.The simplest photomorphon is a Bragg grating fabricated in a photomechanical fiber. The grating reflects a single wavelength at twice the grating period; but, the photomechanical effect changes the grating period and alters the wavelength of light that is reflected. The interplay between light and grating induces an intensity dependent change in the grating's period, leading to complex behavior. A Photomechanical Optical Device (POD) is a multi-grating device made of two or more parallel reflectors surrounding a photomechanical material. A POD is essentially a nonlinear Fabry-Perot cavity that intensifies the light within to leverage the photomechanical effect, and acts as a sensor and actuator. In addition, it exhibits multistability in its light output and mechanical state.This talk will describe the experimental implementation of a POD as well as the next generation of PODs made with Bragg grating reflectors. The theory of multistability, sensing, and actuation in a POD will be discussed and experimental results presented. Also reported will be our progress on making a photomorphon-based deep brain stimulater with electrodes to apply an electric field and a photomechanical actuator section for steering. Preliminary animal studies on early prototypes will also be reported.
The ability of thin sheets of glassy nematic solid to predictably and reversibly deform into potentially complex shapes holds the potential to open new avenues to device design. This shape change is controlled by writing and blueprinting disclination defects in a prescribed way into the director fields of these sheets in advance; through careful choices in the written pattern and arrangement of defects, a chosen target shape can be then be switched on and off again by the imposition of external stimuli, such as heat, light, or chemical solvent. We review this method of shape design and, as an example, show how certain simple patterns and sheet geometries can result in a device that, when activated, folds itself from flat into a polyhedral container and back again.
Stimuli-responsive, mechanically active polymeric materials are of interest in a variety of end uses. One of the salient features of stimuli-responsive liquid crystalline polymer networks (LCNs) is the ability to fabricate monolithic (e.g. jointless, creaseless) structures tailored and engineered for a desired response. Here we present upon recent work in our group that seeks to realize this promise by preparing LCN materials with director and defect profiles capable of distinctive functional responses. Towards this end, employing photoalignment materials and a variety of optical patterning techniques that will be discussed at length, we have prepared and characterized films with subsumed defects ranging from ±½-10 and demonstrated simple methods of printing arrays of interconnected defects. Recently, we have extended upon this technique by developing a optical patterning capability that allows for precise control of the linear polarization of the aligning laser that ultimately is purposed to generate films with intricate and complex director and defect profiles. The patterning capability has been employed to demonstrate a number of stimuli-induced programed shapes.
When the underlying microstructure of an actuatable material varies in space, simple sheets can transform into complex shapes. Using nonlinear finite element elastodynamic simulations, we explore the design space of two such materials: liquid crystal elastomers and swelling polymer gels. Liquid crystal elastomers (LCE) undergo shape transformations induced by stimuli such as heating/cooling or illumination; complex deformations may be programmed by "blueprinting" a non-uniform director field in the sample when the polymer is cross-linked, e.g. by forming it between patterned substrates which impose anchoring forces  . Similarly, swellable gels can undergo shape change when they are swollen anisotropically as programmed by recently developed halftone gel lithography techniques . For each of these materials we design and test programmable motifs which give rise to complex deformation trajectories including folded structures, soft swimmers, apertures that open and close, bas relief patterns, and other shape transformations inspired by art and nature. Comparison with relevant experiments shows our model&’s reliability. In order to accommodate the large computational needs required to model these materials, our 3-d nonlinear finite element elastodynamics simulation algorithm is implemented in CUDA, running on a single GPU-enabled workstation. C.D. Modes and M. Warner, Phys. Rev. E 84021711 (2011).J. Kim, J. A. Hanna, M. Byun, C. D. Santangelo, and R. C. Hayward, Science 9 March 2012: 335 (6073), 1201-1205.
Throughout the course of evolution, living creatures have developed a wide range of motions and survival strategies based on the unique topologic patterns on their surfaces. Gecko lizards climb on tilted, vertical and inverted surfaces yet can still easily detach by adapting the microstructures on their feet. Thanks to the complex and fine architectures on their surface, lotus flower leaves are highly water repellent and self-cleaning by rolling water droplets. And even our human respiratory system makes use of little moving protrusions and cilia to protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter nose with breathed air.Inspired by nature, we developed responsive surface topographies that can be switched on and off in a pre-designed manner. Here we provide a general method to generate surface topographies. It is based on the change of molecule organization in ordered liquid crystal polymer networks. A local change in the degree of molecular orderresults in a local expansion of the film leading to the desired protrusions.Previously, we published the formation of dynamic protrusions in liquid crystal networks modified with azobenzene moieties as crosslinks. The protrusions can be actuated by either uniform light exposure or by localized exposure. Here, we investigated the changes in density upon reduction of LC order in the polymer networks. In general, this topic remained underexposed in photo-triggered LC networks until now. But here it proves one of the leading mechanisms for protrusion formation leading to expansions of the order of 10 % of the initial thickness of the film. This is supported further by density measurements. The combination density effects with director-driven expansion even could increase protrusion formation up to 20 % of the initial thickness.In the presentation new strategies will be elucidated that lead to lateral dimensions of the dynamic protrusions down to micrometer level while further improving the height of the structures to values exceeding 20% of the initial film thickness. We will also demonstrate some properties of the films leading to applications in robotics and self-cleaning.
In-situ photopolymerization of liquid crystalline (LC) monomers in their aligned state has proven to be a valuable technique for the formation of well-ordered polymer networks in a single-step process. Their anisotropic properties led to a variety of applications in optics, electronics and mechanics. The use of light to initiate polymerization enables lithographic approaches for patterning. The LC behavior enables formation of complex morphologies on molecular level. Controlling the director profile of an LC network film in transversal direction gives geometrical morphing upon minor changes in order parameter. Examples of suited profiles of molecular orientation are twisted or splayed director configurations tied up in the network configuration. Reversible order parameter changes can be induced by a variety of means. It can be simply induced by temperature changes resulting in gradients in thermal expansion over the cross-section of the film. But more sophisticated and of interest for applications is a light induced change as a result of the E-Z isomerization of a built-in azo group. In conjunction with the change in order parameter the density of the LC network changes. When this density change is confined to a small volume and the film is attached to a solid, high modulus, substrate this results in localized formation of protrusions. In other words the films have switchable surface topographies which size and shape can either be controlled by mask exposure or by a pre-set director pattern.Rather than composing the monomers of covalent bonds alone, one can chose to replace some bonds by secondary interactions such as hydrogen bridges, thus providing responsive molecularly organized hydrogels. We applied the H-bridge based dimerization of benzoic acid to form nematic LC acrylate monomers. By a controlled and reversible rupture of the H-bridges mechanical responses can be initiated. Photopolymerization of smectic LC monomers lock in structures of different length scales. The first length scale is the resolution of lithography, and goes down to a few micrometers. The second length scale is set by the spacing of smectic layers and is typically a few nanometers. The third length scale is the intermolecular distance in the layers, usually around 1 nm or below. By modifying smectic molecules with H-bridges the smectic periodicity can break-up into separated layers with a well-defined spacing, also in the nanometer range. The nanopores form by breaking the H-bridges at elevated temperatures or by contact with an alkaline solution. The integrity of the film is maintained by copolymerizing with fully covalent smectic crosslinkers.The size of the nanopores can be controlled by changing the pH or by photochemistry at the crosslinking bridges.
Low-cost and scalable therapies are needed to treat heart failure, which affects around 5.7 million people in the US. Current therapies rely on ventricular assist devices which restrict patient mobility, but biocompatible stimuli-responsive biomaterials provide a potential alternative. Here, we report the development of liquid crystal elastomer (LCE) composite materials as electrically responsive materials for implantable mechanical assist devices. Conductive monodomain LCEs are produced using a two-step crosslinking method along with the addition of carbon black particles both before and after crosslinking, resulting in conductive LCEs (resistivity ~ 4 - 5 Omega; m) with as little as 2 wt % carbon black nanoparticles. The resulting LCE exhibits rapid (~ 0.1 - 1 Hz) and reversible shape and topography changes in response to modest voltages (10 - 40 V). The response of the substrate can be controlled by variation of the pulse amplitude, duration, and frequency to achieve reversible strains from 1 - 20 % with a response time as fast as 0.5s. Electro-mechancial epxansion of conductive LCEs was completely reversible with no hysteresis, even after 12 hours of electrical stimulation. Electrospun LCE meshes provide porous, stimuli-responsive scaffolds. These materials potentially provide a straightforward and scalable route to stimuli-responsive biomaterials for mechanical assist devices, dynamic substrates, or tissue scaffolds.
Liquid crystal elastomers and networks (LCEs) and can exhibit strikingly unusual motion and locomotion when activated by light, heat or the presence of chemicals. The response originates in volume conserving changes of sample shape, and locomotion is associated with a localized source of energy/excitation. I will discuss some simple examples of translation and rotation originating in shape changes, and indicate unusual aspects of the dynamics. Examples from experiments will be presented, together with models and model predictions. The potential and actual energy conversion efficiency of LCE motors will be discussed.
Liquid crystal polymer networks (LCNs) are distinctive shape adaptive materials in that the local director profile can be spatially patterned to produce monolithic materials that yield an engineered output. Recent work from our group has focused on thermally and optically induced torsional responses in LCNs in the twisted nematic orientation. In this contribution, we will overview our prior work in the topic and discuss recent efforts in which the torsional deflection of these materials is used to induce locomotion of polymer films across surfaces.
“Shape-shifting” and self-propelled motion of spirobenzopyran-functionalized self-oscillating gelsA remarkable feature of certain biological species is their ability to dramatically alter their shape in response to environmental cues. Herein, we show that this shape-shifting ability can be replicated in synthetic polymer gels that combine two distinct functionalities. First, these gels contain spirobenzopyran (SP) chromophores grafted onto the polymer matrix; the SP moieties are hydrophilic in the dark in acidic aqueous solution, but become hydrophobic under illumination with blue light. Hence, with the incorporation of these chromophores into the gel, light can be harnessed to control the gel's swelling or shrinking and, thereby, dynamically alter the gel's shape. Second, these gels also contain Ru catalyst grafted onto the polymer matrix; when placed into a solution with all the reagents needed for the Belousov-Zhabotinsky (BZ) reaction, these BZ gels are know to oscillate; the degree of swelling increases when the catalyst is in oxidized state and decreases when it is in the reduced state. Notably, the dynamics of BZ gels can also be controlled with low intensity blue light; shining uniform light with an intensity higher than a critical value suppresses the oscillations. Herein, we develop the first model for gels that combine both (SPBZ) functionalities and show that these systems can undergo large scale shape changes, with an initially flat sheet morphing into a variety of complicated 3D forms. Owing to functionalization with the Ru catalyst, SPBZ gels undergo rhythmic swelling-deswelling in response to oxidation and reduction of the catalyst and hence, are capable of self-propelled motion. Owing to functionalization with SP chromophores, SPBZ gels undergo dramatic deswelling if placed in non-uniform illumination. Our results show that combination of both functionalities results in active, remarkably large scale shape-shifting in response to non-uniform illumination. Moreover, the SPBZ gels undergo unprecedented self-propelled motion with the mode of motion being controlled by external light. Our results point to a novel class of active self-oscillating materials and to a robust method for controllably reconfiguring their 3D shapes and their self-propelled motion.
Three dimensional (3D) assembly and reversible untethered actuation at small size scales represent important challenges in microengineering, robotics, drug delivery and surgery. I will discuss self-folding approaches wherein differential swelling of bilayer or cross-link gradient polymer thin films cause them to spontaneously curve or fold in response to a variety of stimuli such as temperature, solvents or pH. Further, by utilizing photopatterning with precisely defined CAD masks, we are able to shape these films with high resolution in 2D so that complex 3D assemblies (such as capsules, cylinders, and bidirectionally folded sheets) as well as reversibly actuating multi-fingered gripping devices can be formed. I will highlight a number of self-folded polymeric devices including bio-origami hydrogels and curved / folded microfluidic devices for tissue engineering applications and therapeutic grippers as well as smart bioinspired actuators for drug delivery and surgery.
Engineering structures with adaptive geometry require an optimized integration of actuation, sensing, and packaging. Origami structures, by definition, can “shape-shift” between multiple geometric configurations that are predefined by a pattern of folds. However, most origami models assume ‘rigid origami&’, meaning that the facets between fold lines cannot deform. Although a useful, simplifying assumption, in reality no origami pattern is completely rigid. Furthermore, facet compliance could broaden the design space of origami devices. Together, these motivate our investigation of the transition from rigid to compliant origami for a given fold pattern. We approximate the fold pattern as a truss and incorporate relative stiffness magnitudes associated with bar elongation (10^8), folding (10^2), and facet bending (10^1 -10^12). Following Schenk et al&’s formalism (PNAS 2012) to relate nodal forces to the nodal displacement, we determine the eigenvalues and eigenmodes of the stiffness matrix, which correspond to the natural modes of the structure. Due to the significant difference in stiffness values, we are able to separate the eigenmodes associated only with folding from those with bar elongation and/or facet bending. As expected, rigid origami modes appear as “fold-only” modes when facet stiffness is much greater than fold stiffness. However, as facet stiffness approaches fold stiffness, these mode shapes adopt a combination of fold and facet deformations. By identifying these fold/facet bending modes we can determine how to redistribute bar, fold or facet stiffness in the surrounding structure in order to mitigate or amplify their effect. These results have important implications for not only origami topology design, but also for optimal placement of local material properties.
Origami is a powerful technique for creating three-dimensional structures from two-dimensional sheets, and correspondingly, self-folding materials have attracted considerable interest for the design of actuators and remotely deployable devices. Especially for folding on the micro-scale, achieving reversible stimuli-responsive folds with good control over dihedral angle and mountain/valley assignments has remained a challenge. We describe a simple approach to fabricate self-folding origami based on tri-layer films of photo-crosslinkable copolymers. Stresses developed during swelling of a thermally-responsive hydrogel layer bonded to thin rigid polymer layers allows for the definition of micrometer-scale hinges that reversibly fold to well-defined angles, with control of mountain and valley assignments. This method allows for the preparation of arbitrarily complex crease patterns with high fold densities, offering promise for applications in biomimetic systems, soft robotics, and mechanical meta-materials.
PopupCAD: a New Design Tool for Developing Self-folding DevicesDaniel McConnell AukesWhile the promise of printable, popup-capable, and self-folding robots is being demonstrated through innovative designs and new manufacturing methods such as PC-MEMS, the practical aspects of design are limiting the development process of such devices. Traditional three-dimensional CAD workflows are cumbersome for generating two-dimensional geometries of layered composites, requiring an intimate knowledge of the PC-MEMS manufacturing process to ensure a successful, manufacturable design. Because the manufacturing rules of this new paradigm are still mostly internalized by the designer during the design process, errors are frequent, requiring more design iterations and restricting the pool of designers to those with a high level of manufacturing expertise.To facilitate faster prototyping and development of printable, popup, and self-folding devices, we have developed a design tool called popupCAD. By building this design tool around the specifics of the layered manufacturing PC-MEMS paradigm, we hope to speed workflow by encouraging design methods which are inherently manufacturable. The object-oriented structure of designs produced with popupCAD has the potential to reduce design flaws by encouraging modularity, reusing successful components rather than requiring designers to redraw them. By encapsulating knowledge of each layers&’ material properties and functional characteristics, popupCAD can also help build intuition in kinematics, stiffness, and manufacturability of the robots designed. This is accomplished via a suite of analytical tools built into the software. We hope that this analysis suite will give designers more immediate comparisons across differing design strategies, reducing the number of physical build iterations required.PopupCAD&’s manufacturing analysis tools are presented in greater detail, outlining the steps required to understand how to manufacture any device. With the goal of outputting a full manufacturing roadmap for designs, consisting of a human-readable set of process instructions alongside the output files required to cut and laminate the design, we introduce the algorithms required to understand whether such a design can be cut, laminated, assembled, and removed efficiently. Because printable, self-folding robots are not necessarily stiff enough to be analyzed using ideal methods, we also introduce a framework for analyzing such folded, hinged composite structures in a uniform way. Such a framework could incorporate arbitrary forces at the joints, allowing this tool to help designers understand the order of assembly for self-folding. Exemplar designs produced with popupCAD are demonstrated, highlighting the workflow, analysis tools used, and manufacturing process required to produce these designs.
Control over the composition, shape, spatial location and/or geometrical configuration of semiconductor nanostructures is important for nearly all applications of these materials. Although various techniques have been developed for defining the composition, diameter, length and position of nanowires and nanoribbons, there are relatively few approaches for controlling their 3D configurations. We have designed a strategy that creates certain classes of 3D shapes in nanoribbons that would be difficult to generate in other ways. Lithographically defined surface chemistry was used to provide spatial control over adhesion sites, and with compression from a prestrained supporting substrate, buckling configurations with deterministic control over their geometries could be achieved. Precisely engineered buckling geometries can be created in Si nanoribbons in this manner and these configurations can be described quantitatively with analytical models of the mechanics. Periodic or aperiodic designs are possible, for any selected set of individual nanoribbons, in large-scale organized arrays of such structures. Specialized geometries designed for stretchable electronics enable strain ranges of up to nearly 150%, even in brittle materials such as Si, which is consistent with analytical modeling of the mechanics, and as much as ten times larger than previously reported results. These hierarchical structures can potentially serve as effective scaffolds in three-dimensional sensor networks, extra cellular matrix, and electromagnetic devices.
Plants exhibit remarkable properties and adaptability of biological systems,which that are typically limited in space and life span. A unique ability of plant organs tochange their shape and move in response to the changes in ambient conditionsoffers a new paradigm for creating adaptable materials by-design. Beinginspired by the remarkable structure-property relationships in fibrous tissuesof plants, we developed a nature-inspired strategy for the generation ofcomplex 3D structures by programming stimuli-responsive deformations ofcomposite hydrogel sheets. We used a combination of different materials andstimuli to predict and program well-defined shape transformations of theplanar hydrogel sheets. This work constitutes a major step toward the designof adaptable soft materials with applications in sensing, actuation andlocomotion.
An elusive goal in materials science is designing systems that mimic the remarkable ability of amphibians to re-grow limbs. While self-healing materials can mend local defects, there are virtually no examples of materials that can regenerate themselves. The advent of such regenerative materials could dramatically extend the useful lifetime of manufactured products. Through new computational models, we design a nanorod-filled gel that effectively regenerates the gel matrix when a layer of the material is sliced-off. With this layer removed, the nanorods diffuse to the newly formed interface and extend into the outer solution, which contains monomers and a small fraction of cross-linkers. Polymerization initiated from the rods&’ surfaces leads to chains that become cross-linked to form a new gel that resembles the severed layer. After the initial cut, the regeneration requires no external intervention; synergistic interactions among all components in this system enable the vital processes leading to re-growth, which could be repeated with subsequent cuts.
Composite Belousov-Zhabotinsky (BZ) hydrogels show great promise as autonomous mechano-chemical actuators and sensors, with applications ranging from encryption and microscale mass transport to artificial skin. Composite BZ gels contain localized regions of active Ru-immobilized gel encased within a non-active gel matrix. The structure and arrangement of the active nodes regulates the autonomous chemical (Ru oxidation) and mechanical (swelling) oscillations of the composite. However, the relationship between structure, arrangement and autonomic response of the nodes is not well understood, and thus no design criteria exist for patterning composite BZ gels with predictable responses. To investigate these questions, we fabricated composite gel sheets (thickness ~0.5 mm) with a range of configurations of fully and partially embedded square active nodes. Node pairs, both embedded and non-embedded, were more likely to synch at short separation distances, emphasizing that chemical communication is intrinsically tied to the diffusion length. However, embedment increased the synch probability over non-embedment at similar separation distances, suggesting that mechanical communication between nodes can enhance their synchrony. Additionally, the proximity of the active node to the edge of the composite substantially effects its initial transient oscillations as well as the final steady-state behavior. For example when active nodes are asymmetrically embedded (3 sides embedded or 2 adjacent sides embedded), steady state oxidation waves originate at the embedded corner or edge and travel toward the free edge. The wave direction though depends on the aspect ratio of the active node. For rectangular active nodes (aspect ratio >3) the steady-state waves propagates along the long axis of the node, irrespective of embedding conditions. This reflects the balance between reactant diffusion gradient and the boundary conditions of the active node. Together, these results provide tools to design composite BZ gels with predictable initiation point and direction of oxidation waves. The design criteria from this study contribute toward making self-oscillating, mechano-chemical hydrogels a future engineering material.
Light driven soft actuators are ideal for artificial muscles and remote optical switching applications in MEMS and microfluidics. They are lightweight, easy to miniaturize and can be triggered remotely. However, their use is limited by sluggish actuation times and small power densities. We overcome these difficulties by using mechanical instabilities to trigger motion, which causes fast and large displacement actuations. In particular, we investigate the snap-through buckling instability of bistable actuators designed from azo-benzene functionalized polymers. Such actuators provide orders of magnitude increase in actuation times and power densities. The performance of these actuators is determined by actuator geometry, mechanical properties and irradiation intensities. We will describe the different design parameters affecting actuation timescales, power densities and energy conversion efficiency in these actuators.
Snap-through buckling in mechanical structures is commonly associatedwith catastrophic failure, but if purposely programmed into fabricatedmaterials these transitions provide a fast and tunable mechanism forswitching between states characterized by different properties.Drawing inspiration from the Japanese art of origami, we discuss howthe design of fold patterns on 2-D sheets and non-Euclidean shells can be used to generatestructures with many stable local minima and multiple controllableregimes of mechanical response. By exploring theoretical andexperimental methods by which these regimes of mechanical response andstability can be accessed through fold patterning on thin sheets andcurved shells, we delineate future design strategies for programmable structures with tuned material characteristics.
Shape programmable structures and devices are typically fabricated using shape memory materials and are attracting increasing interest since they can be designed to have multiple properties and functions. However, shape memory materials, such as NiTi-based metal alloys and block copolymers, suffer from severe structural fatigue due to the microstructural changes occurring during each thermal cycle. Moreover, there is only a limited set of shape memory material systems, limiting the possible number of applications for reconfigurable structures and devices. To overcome all these issues and expand the range of applications of shape programmable structures, we designed a library of 2D elastic metamaterials that can be programmed to retain multiple shapes simply by applying a force. The building block of our metamaterial consists of a unit comprising interconnected elastic arches and we demonstrate both numerically and experimentally that geometric non-linearity and snap-through instabilities can be effectively exploited to reconfigure the system into multiple shapes. Since our system exploits mechanical instability, our findings can be extended to different materials and length scales, outlining a general strategy to effectively design a new generation of shape programmable metamaterials.