Rapid Actuation Research Articles

Shape memory effect and rapid reversible actuation of nanocomposite hydrogels with electrochemically controlled local metal ion coordination and crosslinking.

Rapid and reversible actuation and shape memory effects are critical for biomimetic soft actuators based on polymer hydrogels. However, most conventional hydrogel actuators show very slow actuation or deformation rates in water. It remains a challenge to realize rapid actuations, particularly for hydrogels to actuate in air. Here, a novel strategy to create diverse hydrogel devices with shape memory effects and rapid reversible actuations even in air was demonstrated. This strategy relies on a precise definition of local crosslinking by using multivalent metal ion coordination. This is demonstrated by infiltrating Fe3+ ions into stretchable nanocomposite polyacrylamide hydrogels with the amide groups converted into primary amine groups for multivalent coordination and crosslinking. The Fe3+ coordination with amine groups enhanced the crosslink density and modulus, leading to deswelling. By using an iron rod electrode, the Fe3+ coordination and crosslinking were precisely controlled to generate hydrogels with heterogeneous local crosslinking, including Janus hydrogels, S-shaped hydrogels, and cross-shaped hydrogel grippers. These soft devices were reversibly actuated in tens of seconds when cyclically dehydrated in ethanol and rehydrated in water. Most interestingly, very rapid reversible actuations of a hydrogel device in air were demonstrated by using electro-redox reaction of Fe3+ and Fe2+ in the hydrogel, where the reversible local coordination crosslinking and decomposition served as a hinge to actuate the hydrogel. This strategy based on reversible local coordination and crosslinking may open an avenue for rapid fabrication of hydrogel devices with well-defined structures and actuation properties.

Rapid photothermal actuation of light-addressable, arrayed hydrogel columns in a macroporous silicon membrane

This paper presents rapid photothermal actuation of light-addressable, arrayed hydrogel columns in a macroporous silicon membrane. Au nanorods are incorporated into thermo-responsive p-NIPAAm hydrogel to utilize surface plasmon-induced local heating by near-infrared light. By measuring optical transmission through the fabricated membrane structure with Au nanorod embedded hydrogel, we have demonstrated that photothermal actuation of the hydrogel can be done in two-dimensional, pixel-like configuration with high spatial and temporal resolutions. Benefiting from the hydrogel volume confinement within micron-sized pores, we have achieved sub-second response time of hydrogel photothermal actuation and its repeatable photothermal actuation on highly localized illuminated area. Considering that each hydrogel column is confined within each pore and different wavelengths of light can be used to induce photothermal actuation of hydrogel’s deswelling characteristics by modifying the physical dimensions of Au nanorods, it has a potential for optically-addressable, multiplexed drug release systems with the rapid response time.

Reciprocal Control of Hierarchical DNA Origami-Nanoparticle Assemblies.

DNA origami mechanisms offer promising tools for precision nanomanipulation of molecules or nanomaterials. Recent advances have extended the function of individual DNA origami devices to material scales via hierarchical assemblies. However, achieving rapid and precise control of large conformational changes in hierarchical assemblies remains a critical challenge. Here, we demonstrate a method for controlling DNA origami-nanoparticle assemblies through a multiscale approach, in which nanoparticles impart control on the conformation of individual DNA origami mechanisms, whereas DNA origami assemblies control the conformation of nanoparticle arrays. Specifically, we show that the angular distributions of DNA origami hinge mechanisms are tunable as a function of nanoparticle size and distance from the hinge vertex. We selectively adjust the affinity of nanoparticle binding sites, resulting in hinge actuation via DNA melting without releasing the nanoparticle, thereby enabling rapid and reversible temperature-based actuation. Finally, we demonstrate this rapid actuation in DNA origami-nanoparticle arrays of length scales extending over a micron. These results provide guiding principles toward the design of dynamic, DNA-origami hierarchical materials capable of storing and releasing mechanical energy.

Mutually Exclusive p‐Type and n‐Type Hybrid Electrode of MoS2 and Graphene for Artificial Soft Touch Fingers

AbstractFuture smart mobile electronics and wearable robotics that can perform delicate activities controlled by artificial intelligence can require rapid motion actuators working at low voltages with acceptable safety and improved energy efficiency. Accordingly, ionic soft actuators can have great potential over other counterparts because they exhibit gentle movements at low voltages, less than 2 V. However, these actuators currently show deficient performances at sub‐1 V voltages in the high‐frequency range because of the lack of electrode materials with the vital antagonistic properties of high capacitance and good conductivity. Herein, a mutually exclusive nanohybrid electrode (pMoS2‐nSNrGO) is reported consisting of oxide‐doped p‐type molybdenum‐disulfide and sulfur‐nitrogen‐codoped n‐type reduced‐graphene‐oxide. The pMoS2‐nSNrGO electrode derives high capacitance from MoS2 and good charge transfer between the two components from p‐n nano‐junctions, resulting in excellent actuation performances (670% improvement compared with rGO electrode at 0.5 V and 1 Hz, together with fast responses up to 15 Hz). With such excellent performances, these actuators can be successfully applied to realize an artificial soft robotic finger system for delicately touching the fragile surfaces of smartphones and tablets. The mutually exclusive pMoS2‐nSNrGO electrode can open a new way to develop high‐performance soft actuators for soft robotic applications in the future.

Characterization and evaluation of ionic liquids for use in rapidly-actuated hydraulic microvalves

Elastomeric membrane-based microvalves, especially pneumatically-actuated valves, are a core technology responsible for the proliferation of micro total analysis systems (μTAS) due to enabling the precise metering and mixing required by lab-on-a-chip applications. Hydraulic actuation is a promising actuation strategy, analogous to pneumatic actuation, useful in environments where the portability and power requirements of pneumatic actuation systems is non-ideal. Central to the function of hydraulic valves for active applications that require rapid and repeatable actuation, such as microfluidic pumping, are the viscosity, evaporation profile and the hygroscopic nature of the hydraulic fluid. Here the properties of a series of 1-methyl-3-butylimidizaolium (BMIM)-based ionic liquids (ILs) and water are investigated for application in self-contained hydraulically-actuated microfluidic (SCHAM) devices. Evaporation and water uptake of SCHAM devices was monitored daily over a one-month period, with IL-based SCHAMs providing superior performance compared to water-based SCHAMs. Evaporation and water uptake profiles of IL-filled devices were recorded over a six-week time period, subjected to thermocycling of one-week periods between −20 °C and 60 °C, with IL-based SCHAMs exhibiting superior stability in comparison to water-based SCHAMs. As a proof of concept, a SCHAM pump using [BMIM][BF4] was fabricated utilizing a simple and inexpensive rapid prototyping method, and tested over a range of actuation times (150 ms–350 ms, with pumping rates of 5.0 ± 0.1 μL min–1 to 2.6 ± 0.1 μL min–1, respectively). To the best of our knowledge, this is the first demonstration of an IL-filled SCHAM device utilizing active valve elements to attain fluidic transfer.

Thermally stable access to microresonator solitons via slow pump modulation.

Temporal dissipative Kerr solitons (DKSs) in microresonators provide ultra-short optical pulses and low-noise frequency combs with gigahertz to terahertz repetition rates. Owing to their unique properties, they have found application in fields, including optical communications, rapid laser ranging, and optical precision spectroscopy. However, due to the thermal instability encountered when entering the DKS regime, the stable generation of solitons remains challenging for many systems and usually requires rapid actuation of the pump laser detuning, pulsed driving, additional lasers, a particular mode structure and/or active feedback loops to stabilize the system. Here we show that slow pump modulation can remove the thermal instability and enable passively stable soliton states that can be readily accessed via arbitrarily slow laser tuning, thereby greatly reducing the technical complexity of stable DKS generation.

Low-voltage-triggered rapid shape memory actuation with interfacial self-assembled silver nanowires

Shape memory polymeric actuators containing interfacial self-assembled silver nanowires (AgNWs) were fabricated. The morphology and electrothermal effect of the aligned AgNWs were investigated. Voltages as low as less than 2 V enabled rapid shape recovery, in which the speeds as well as nonlinearity of the shape memory-based actuation were comparatively studied. Two distinct actuation stages and the underlying mechanism were disclosed. The findings may greatly benefit the application of self assembly of nano building blocks in flexible electronics and soft robot fields.

Rapid and low power laser actuation of sputter-deposited NiTi shape memory alloy (SMA) MEMS thermal bimorph actuators

NiTi SMA thermal bimorph actuators have potential as high-force, high-displacement MEMS actuators. Historically, even microscale SMA actuation response has been limited to a maximum of ˜100 Hz. As NiTi film and device dimensions are scaled down into the micro and nano scales, heat transfer, and thus device cycling speeds can be significantly improved upon. We have used an in-situ annealed nickel-titanium (NiTi) shape memory alloy (SMA) sputter deposition process to sputter equiatomic NiTi films at 600 °C. We characterized our thin film (270 nm NiTi – 1.6 μm NiTi) material and verified its reversible shape memory effects (SME) using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and wafer bow versus temperature measurements. Upon release, the NiTi material exhibited a reversible phase change around 60 °C with a hysteresis of ˜3 °C. For the substrate confined case (i.e. NiTi on Si or Pt), hysteresis was much larger (i.e. ˜40 °C) with the phase change completed at ˜80 °C. In addition to SMA material characterization, we fabricated NiTi/Pt bimorph actuators at several (100 nm to 1.2 μm) NiTi and Pt thicknesses. Free-standing bimorph actuators were produced via a dry etch release, and temperature dependent curvature of these cantilevers was investigated. To address the low power, and high response time aspects, we performed a dynamic characterization using a 440 mW, 532 nm “green” laser to irradiate devices from 2 to 24 W/cm2 while measuring actuation response times that varied from 3 ms at the highest irradiation fluxes to 240 ms at the lowest. Our results showed decreased actuation powers and faster heating or actuation times compared to past works with NiTi microactuators. The NiTi films with 600 nm thickness on top of 20 nm Pt films exhibited the greatest change in curvature from 200 μm to flat states, and actuated in under 3 ms due to the very small volume of SMA requiring to be heated. These results suggest that NiTi/Pt bimorphs have potential applications as lower-power, faster-response, laser-activated micro shutters or thermal switches without needing a traditional wired power source.

Janus Soft Actuators with On–Off Switchable Behaviors for Controllable Manipulation Driven by Oil

Soft actuators have tremendous applications in diverse fields. Facile preparation, rapid actuation, and versatile actions are always pursued when developing new types of soft actuators. In this paper, we present a facile method integrating laser etching and mechanical cutting to prepare Janus actuators driven by oil. A Janus film with superhydrophobic and hydrophobic sides was fabricated successfully. By cutting the functional layer at the desired positions, a number of quintessential oil-driven soft devices were demonstrated. Furthermore, Janus actuators with surfaces of different wettabilities exhibited different swelling behaviors, and different media manifested different surface extensions; thus, these actuators are promising candidates for soft actuators and also realized on-off switchability between an oil/water mixture and ethanol. This study offers novel insight into the design of soft actuators, and this insight may be helpful for developing an oil-driven soft actuator that can be operated like a human finger to manipulate any object and extending stimuli-responsive applications for soft robotics.

Direct Quantification of Rapid and Efficient Single-Stroke Actuation by a Martensitic Transition in a Thermosalient Crystal.

Molecular dynamic crystals conveniently combine flexibility required for mechanical reconfiguration, strength for effective translation of elastic energy, and long-range order of mechanically coupled molecules for rapid conversion of disordered motion (heat) or photons (light) into ordered motion (work). By direct measurement of the actuation force generated by crystals of a thermosalient solid, here we describe the first direct quantification of the work performedand energy conversion that can be accomplished by using dynamic crystals as supramolecular actuators. Upon reversible α-to-γ phase transition, crystals of (phenylazophenyl)palladium hexafluoroacetylacetonate of submillimeter to millimeter size exert forces in the range of 1-100 mN upon longitudinal and lateral expansion. This work translates to a volumetric power density of about 1-3MW m-3 and efficiency comparable to the existing multicomponent actuators.

Multistable Thermal Actuators Via Multimaterial 4D Printing

Abstract3D printing of smart materials, called “four‐dimensional” (4D) printing, adds active, responsive functions to 3D‐printed structures. Shape memory polymers (SMPs) can be employed as an active material in 4D printing, and this could be useful for a wide range of potential applications. New design for 4D printing is presented here by introducing SMPs into rotational multistable structures. Two different digital SMPs are employed to enable large‐angle, thermal actuation in a controlled manner. It is started with bistable structures and then extending them to quadristable ones. In this design, by adjusting the thickness of SMP beams, a balance is controlled between the energy barrier and shape memory force, and this can enable controlled thermal actuation. Especially, one can control the activation time for thermal actuation. Multistable structures can simplify actuation and motion control without complicated control systems. Therefore, by adopting multistable structures in 4D printing, one can potentially enable precisely controlled, large‐magnitude, rapid actuation beyond the material capability of SMPs. These multistable structures do not require heating in the programming stage and this significantly simplifies the actuation procedure. This work provides new physical insights into 3D‐printable, active structures, and could be useful for various smart actuators responding to the environmental stimuli.

Shape memory alloy–actuated prestressed composites with application to morphing automotive fender skirts

This work presents smart laminated composites that enable morphing vehicle structures. Morphing panels can be effective for drag reduction, for example, adaptive fender skirts. Mechanical prestress provides tailored curvature in composites without the drawbacks of thermally induced residual stress. When driven by smart materials such as shape memory alloys, mechanically-prestressed composites can serve as building blocks for morphing structures. An analytical energy-based model is presented to calculate the curved shape of a composite as a function of force applied by an embedded actuator. Shape transition is modeled by providing the actuation force as an input to a one-dimensional thermomechanical constitutive model of a shape memory alloy wire. A design procedure, based on the analytical model, is presented for morphing fender skirts comprising radially configured smart composite elements. A half-scale fender skirt for a compact passenger car is designed, fabricated, and tested. The demonstrator has a domed unactuated shape and morphs to a flat shape when actuated using shape memory alloys. Rapid actuation is demonstrated by coupling shape memory alloys with integrated quick-release latches; the latches reduce actuation time by 95%. The demonstrator is 62% lighter than an equivalent dome-shaped steel fender skirt.

Snap-Buckling Motivated Controllable Jumping of Thermo-Responsive Hydrogel Bilayers.

Responsive hydrogel actuators have promising applications in diverse fields. Most hydrogel actuators are limited by slow actuation or shape transformations. This work reports on snap-buckling motivated jumping of thermoresponsive hydrogel bilayers. The bilayers are composed of poly(NIPAM- co-DMAPMA)/clay hydrogel with different lower critical solution temperatures in each layer, and thus undergo slow reversible curling/uncurling at temperature changes. The gels are adhesive to numerous materials including aluminum. The adhesion between the gels and an aluminum ratchet is utilized to constrain the thermoresponsive deformation of the bilayers to store elastic energy. When the accumulated elastic energy overwhelms the gel-aluminum adhesion, snap-buckling takes place to abruptly release the accumulated energy, which motivates the bilayer to jump. The jumping direction, start time, height, and distance are controlled by the geometry of the bilayers or the ratchet. This work paves a novel way for the rapid actuation of responsive hydrogels in a controlled manner and may stimulate the development of novel hydrogel devices.

Rapid Recovery Hydrogel Actuators in Air with Bionic Large-Ranged Gradient Structure.

Fast recovery in a nonaqueous environment is a big challenge for hydrogel actuators. In this work, a temperature-responsive hydrogel actuator with outstandingly rapid recovery in air was reported. The hydrogel with bionic large-ranged gradient structure was fabricated by copolymerization of hydrophilic monomer hydroxyethyl acrylate (HEA) and N-isopropylacrylamide in the dispersion of Laponite utilizing a facile electrophoretic method. The deformation degree and time can be regulated by varying the concentration of HEA to change the lower critical solution temperature (LCST) and swelling of the hydrogel. A dynamic equilibrium between the water into and out of the hydrogel was observed, and the hydrogel showed no shrink above LCST. The synthesized hydrogels showed fast response in hot water and rapid recovery in air. Such nonshrink characteristics and excellent reversibility made it possible for these hydrogels to be used as temperature-controlled microfluidic switches. This work provided an approach to design fast recovery hydrogel actuators by the incorporation of hydrophilic monomers and extend the application of the hydrogel actuators into fields such as soft robots, micromanipulation, microfluidics and artificial muscles in various environments.

Photoactivated Polymeric Bilayer Actuators Fabricated via 3D Printing.

4D printing is an emerging additive manufacturing technology that combines the precision of 3D printing with the versatility of smart materials. 4D printed objects can change their shape over time with the application of a stimulus (i.e., heat, light, moisture). Light driven smart materials are attractive because light is wireless, remote, and can induce a rapid shape change. Herein, we present a method for fabricating polymeric bilayer actuators via 3D printing which reversibly change their shape upon exposure to light. The photoactive layer consists of a poly(siloxane) containing pendant azobenzene groups. Two different photoactive polymers were synthesized, and the photomechanical effect displayed by the bilayers was evaluated. These bilayers exhibit rapid actuation with full cycles completed within seconds, and photo generated stresses ranging from 1.03 to 1.70 MPa.

Rapid and flexible actuation of droplets via a low-adhesive and deformable magnetically functionalized membrane

Swift and convenient handling of discrete micro-droplets on digital microfluidics platform has recently attracted plentiful interest in high-throughput bio/chemical analysis. In this work, we developed a novel and facile approach to manipulate the micro-droplet based on a hydrophobic and low-adhesive magnetically functionalized polydimethylsiloxane (MF-PDMS) membrane. The platform is composed of the MF-PDMS membrane as the substrate, integrated with non-magnetic PDMS micro-pillar arrays on-top to ensure the low adhesion between the platform and the resident drops. With applied magnetic field, the localized membrane deformation could induce the unbalanced force imposed on the micro-droplet, leading to free rolling-off for flexible control including the position and velocity, etc. We have experimentally verified that the proposed avenue could be successfully applied for kinds of on-chip micro-droplet manipulations, such as droplet merging, parallel manipulation, flexible trajectory control. With the flexibility and maximum velocity reported herein (~ 79 ± 6 mm/s), we believe that the presented platform could be potentially applied for biological or chemical analysis where precise manipulation of large amounts of individual droplets is critical.

Rapid Prototyping High-Performance MR Safe Pneumatic Stepper Motors

In this paper, we show that magnetic resonance (MR) safe pneumatic stepper motors can be constructed using rapid prototyping techniques such as 3-D printing and laser-cutting. The designs are lightweight, metal-free, and fully customizable. Besides MR safe robotic systems, other potential applications include high-voltage switchgear and nuclear power plant systems, which also restrict electric actuation. In addition, it can be applied to other actuation systems, where pressurized air is available, and lightweight rapid prototypeable actuators are needed. Five pneumatically driven linear and rotational stepper motors have been developed with forces up to $\text{330 N}$ , torques up to 3.7 N $\cdot$ m, the stepping frequency up to 320 Hz, dimensions ranging from 25 to 80 mm, free of backlash, and power up to 26 W. All five motors are constructed from six 3-D printed parts and four seals, held together by nylon screws or clips. The described stepper motors outperform state-of-the-art plastic pneumatic stepper motor designs, both in specifications and in manufacturability.

Thermo‐Responsive Actuation of a DNA Origami Flexor

AbstractNanomachines capable of controlled programmable work at the nanoscale promise to revolutionize a vast range of research and eventually should impact on daily lives. Due to the ease of design and modification, DNA origami is emerging as a natural platform to build such machines. However, one essential challenge is the controlled and rapid actuation of DNA origami using an external biocompatible stimulus. Here, actuation based on temperature‐induced phase transitions of the thermo‐responsive polymer poly(N‐isopropylacrylamide) (PNIPAM) is reported. By incorporating this polymer into DNA origami structures on either side of a flexible region, a “DNA origami flexor” is created that uses the tunable PNIPAM hydrophobicity to reversibly open and close the DNA structures. Such a mechanism has the advantage of being versatile and biocompatible, and possessing strong response to temperature changes of a few degrees Kelvin.

Rapid actuation of hydrogel micro-actuator with porous structures

Rapid actuation of hydrogel micro-actuator with porous structures

Bilayer-type fluorescence hydrogels with intelligent response serve as temperature/pH driven soft actuators

Abstract In this study, we propose a fluorescent bilayer hydrogel soft actuator that responds quickly to multiple environmental stimuli. The two layers of hydrogel contain poly(N-isopropylacrylamide) and poly(2-(dimethylamino) ethyl methacrylate), respectively, which enable their mechanical response to temperature and pH. Additionally, both hydrogel layers contain poly(ethylene oxide) diacrylate, which serves as the crosslinker. Thanks to the anisotropic swelling of two layers, the bilayer hydrogel structure exhibits rapid, reversible, and repeatable bending motion, under varying temperature or pH. In addition, a photo-sensitive coumarin-based unit is introduced into the hydrogel network to add fluorescence. A four-arm gripper made of hydrogel bilayers demonstrates rapid actuation in response to both temperature and pH. Furthermore, we design a circuit switch using the hydrogel bilayer to serve as one “bidirectional alarm” and selectively turn on light emitting diodes in different media. This work demonstrates the applications of the hydrogel bilayer as a soft actuator and a circuit switch in ionic circuitry, enabling potential applications in a wide range of fields, including biomimetic devices, biocompatible/medical devices, and environmental sensors.