Actuation Cycles Research Articles

Periodic concentration-polarization-based formation of a biomolecule preconcentrate for enhanced biosensing.

Ionic concentration-polarization (CP)-based biomolecule preconcentration is an established method for enhancing the detection sensitivity of target biomolecules. However, the formed preconcentrated biomolecule plug rapidly sweeps over the surface-immobilized antibodies, resulting in a short-term overlap between the capture agent and the analyte, and subsequently suboptimal binding. To overcome this, we designed a setup allowing for the periodic formation of a preconcentrated biomolecule plug by activating the CP for predetermined on/off intervals. This work demonstrated the feasibility of cyclic CP actuation and optimized the sweeping conditions required to obtain the maximum retention time of a preconcentrated plug over a desired sensing region and enhanced detection sensitivity. The ability of this method to efficiently preconcentrate different analytes and to successfully increase immunoassay sensitivity underscore its potential in immunoassays serving the clinical and food testing industries.

Simulation of Operating a Cyclic Actuator Based on a Circular Force Element Made of Shape Memory Material

We consider a linear cyclic unilateral actuator based on a force element made of a material with the shape memory (SM) effect and a return spring. We have developed a model of such an actuator, which makes it possible to connect its characteristics with functional properties of the SM material. The model makes it possible to perform engineering design of the main characteristics of the cyclic actuator.

A Focus on Soft Actuation

The present editorial paper analyzes the hundred recent research works on soft actuation to understand the current main research focus in the light of the grand challenges in the field. Two characteristic paper types were obtained: one focuses on soft actuator design, manufacturing and demonstration, while another includes in addition the development of functional materials. Although vast majority of the works showcased soft actuation, evaluation of its robustness by multi-cyclic actuation was reported in less than 50% of the works, while only 10% described successful actuation for more than 1000 cycles. It is suggested that broadening the research focus to include investigation of mechanisms underlying the degradation of soft functional material performance in real cyclic actuation conditions, along with application of artificial intelligence methods for prediction of muscle behavior, may allow overcoming the reliability issues and developing robust soft-material actuators. The outcomes of the present work might be applicable to the entire soft robotics domain.

Reprocessable Thermoset Soft Actuators

AbstractWidely used traditional thermosets are good candidates for construction of 3D soft actuators because of their excellent stability; however, it is generally acknowledged that they cannot be reprocessed. The time–temperature equivalence principle enables reprocessing of traditional liquid crystalline epoxy thermosets (LCETs) into 3D soft actuators. Even though the transesterification reaction of LCETs is extremely slow, it is fast enough to induce a topology rearrangement and subsequent reprocessing when prolonging the transesterification time according to aforementioned principle. Therefore, LCETs can be aligned by a simple procedure. The alignment is quite stable at high temperature and remains after more than 1000 heating–cooling actuation cycles. The resulting 3D soft actuators are remouldable, reprogrammable, reconfigurable, weldable, self‐healable, recyclable, and stable, which is impossible for any traditional thermosets and is therefore a compelling advance in terms of the applications open to 3D soft actuators.

Reprocessable Thermoset Soft Actuators.

Widely used traditional thermosets are good candidates for construction of 3D soft actuators because of their excellent stability; however, it is generally acknowledged that they cannot be reprocessed. The time-temperature equivalence principle enables reprocessing of traditional liquid crystalline epoxy thermosets (LCETs) into 3D soft actuators. Even though the transesterification reaction of LCETs is extremely slow, it is fast enough to induce a topology rearrangement and subsequent reprocessing when prolonging the transesterification time according to aforementioned principle. Therefore, LCETs can be aligned by a simple procedure. The alignment is quite stable at high temperature and remains after more than 1000 heating-cooling actuation cycles. The resulting 3D soft actuators are remouldable, reprogrammable, reconfigurable, weldable, self-healable, recyclable, and stable, which is impossible for any traditional thermosets and is therefore a compelling advance in terms of the applications open to 3D soft actuators.

Enhanced dynamic performance of twisted and coiled soft actuators using graphene coating

Twisted and coiled actuators (TCAs) composed of fishing lines or sewing threads are one of the most promising artificial muscles because of their high specific power, large stroke, and low fabrication cost. Since the TCA can be thermally actuated, the thermal properties of the actuator determine its actuation performance. Here, graphene materials with high thermal conductivity and low thermal mass were used as coating materials on nylon TCA surfaces to enhance the dynamic responses of the TCA. Large-area monolayer graphene and porous graphene foam were wrapped on the TCA surface and provided a reduction of the actuation cycle times by 10.1 and 17.8%, respectively. Furthermore, high-quality multi-layer graphene flakes were spray-coated on the surface of the TCA, showing a dramatic decrease in the cycle time by 30.9%. The enhanced heat transfer due to the graphene flake coating also increased the maximum strain of the TCA over 3 times under a duty cycle of a pulsed current input. This demonstrates an alternative way to improve the dynamic performance of the TCA using a simple coating method.

Investigation into post constrained recovery properties of nickel titanium shape memory alloys

Experimental investigations have found that nickel titanium (NiTi) shape memory alloys (SMAs) exhibit a phenomenon wherein the material can be recoverably deformed in its low temperature martensitic state, thermally actuated to its austenite state while constrained from returning to its parent geometry, and upon ceasing actuation and return to the low temperature martensite state it will continue to generate post constrained recovery residual stresses (PCRRS). The ability to generate a PCRRS creates a new way for SMAs to be used but the stability of the PCRRS, response to subsequent loading, and underlying microstructural mechanisms that generate the PCRRS are poorly understood. This paper presents an experimental investigation into the thermomechanical response of NiTi beginning from a PCRRS state. Multiple formulations of NiTi were exposed to training and sequences of cyclic deformation and thermal actuation in this study. It was found that repeated application of 0.5% strain reduced the residual stress with each application and thermal actuation would restore the original PCRRS. Training and mechanically working the material stabilized the thermomechanical response and did not significantly degrade the magnitude of the PCRRS. Understanding of PCRRS supports a new way for materials to be used as actuators, storing potential energy in material structures that may be beneficial to self-healing material, fatigue crack prevention and other applications.

Peristaltic locomotion without digital controllers: Exploiting multi-stability in origami to coordinate robotic motion

This study examines a novel approach to generate peristaltic-like locomotion in a segmented origami robot. Specifically, we demonstrate the use of multi-stability embedded in the origami skeleton to eliminate the need for multiple actuators or digital controllers to coordinate the complex robotic movements in peristaltic crawling. The crawling robot in this study consists of two serially connected bistable origami segments, each featuring a generalized Kresling design and a foldable anchoring mechanism. Mechanics analysis and experimental testing reveal that the nonlinear elastic behaviors of this dual-segment module, especially its rapid deformation due to the non-monotonic energy landscape and force–displacement relationship, can create a deterministic deformation sequence or actuation cycle. This cycle can then be used to generate the different phases in a peristaltic-like locomotion gait. Instead of individually controlling the segment deformation like in earthworm and other crawling robots, we only control the total length of this robot. Therefore, this approach can significantly reduce the total number of actuators needed for locomotion and simplify the control requirements. Moreover, the richness in Kresling origami design offers us substantial freedom to tailor the locomotion performance. The results of this study will contribute to a paradigm shift in how we can use the mechanics of multi-stability for robotic actuation and control.

Reliability of platinum contacts in a cold operated MEMS switch

Lifetime testing results of an electrostatically actuated microelectromechanical systems (MEMS) switch with Pt-Pt contact are presented. Moveable electrode of the switch is an aluminium beam with platinum contact bumps, which comes in contact with platinum thin-film electrodes. The switch operates in a cold DC mode. Testing is performed at several levels of the input current. Dependence of the switch resistance in the “on” state on the number of actuation cycles is measured. Lifetime of the device is limited by the sharp increase of the on-resistance up to 100 MΩ and depends on the switch design and the input current. Morphology and chemical composition of the contacting surfaces are investigated and mechanisms of the contact degradation are determined.

Energetic electron assisted synthesis of highly tunable temperature-responsive collagen/elastin gels for cyclic actuation: macroscopic switching and molecular origins

Thermoresponsive bio-only gels that yield sufficiently large strokes reversibly and without large hysteresis at a well-defined temperature in the physiological range, promise to be of value in biomedical application. Within the present work we demonstrate that electron beam modification of a blend of natural collagen and elastin gels is a route to achieve this goal, viz. to synthesize a bioresorbable gel with largely reversible volume contractions as large as 90% upon traversing a transition temperature that can be preadjusted between 36 °C and 43 °C by the applied electron dose. Employing circular dichroism and temperature depending confocal laser scanning microscopy measurements, we furthermore unravel the mechanisms underlying this macroscopic behavior on a molecular and network level, respectively and suggest a stringent picture to account for the experimental observations.

Mesoscale Shape Memory Alloy Actuator for Visual Clarity of Surgical Cameras in Minimally Invasive Robotic Surgery

This paper proposes a mesoscale shape memory alloy (SMA) actuator for cleaning the contaminated lenses of surgical cameras during minimally invasive robotic surgery. The proposed design features minimal intraopertive interruption to maintain visual clarity of fully insertable surgical cameras by quickly removing vapor condensation, particle debris, body fluids, etc. that adhere to the cover lenses. Inspired by the mechanical properties of 1-D SMA materials, a set of SMA wires are employed to drive an eyelid wiper that is concealed in the module’s edge. By analyzing the actuation mechanism, we provide a design guideline for developing a functional modular actuator. A numerical computation model is also proposed to predict the actuation cycles according to various combinations of design parameters. The performance of an initial prototype is experimentally evaluated by introducing three different contamination sources, i.e., fogging, bone dusts, and blood. The predicted actuation cycle is experimentally validated with 95.2% prediction accuracy. And the actuation cycle of the eyelid wiper is about 2 s.

A multi-segmented shape memory alloy-based actuator system for endoscopic applications

This paper describes the design, fabrication and testing of a shape memory alloy based actuator system to provide three degrees of freedom bending with high flexibility and repeated use capability without any performance loss for minimally invasive surgery. The flexibility, long-term endurance and thermal characteristics of this system were studied. The system consisted of 3 segments, front, middle and back. Thermal simulations showed an increase in temperature of 1–2 °C at the segment nearest to the activated segment and a temperature increase of 1 °C at the segment furthest from the activated segment. Transformation strain simulations showed that the 8 mm width design has the lowest strain (2%). The average maximum bending angle was above 30° across most actuator segments but a reformation angle of only 50% of the maximum angle. The endurance test showed that all actuators can operate for 7000 actuation cycles with a standard deviation of 0.6154, 0.8293 and 0.0364 mm for sections A, B and C, respectively. The system was capable of high angle of bending across all segments and capable of continuous long-term use with little performance deviation. The system will allow surgeons to have more flexibility during surgery and thus enable reaching difficult regions in minimally invasive surgery.

Development of Direct-Laser-Printable Light-Powered Nanocomposites.

Four-dimensional (4D) printable light-powered materials have emerged as a new generation of materials for the development of functional devices. The design of these types of materials is mostly based on the trans-cis transformation of azobenzene moieties in a liquid crystalline elastomer (LCE) matrix, in which the motion is triggered by ultraviolet (UV) irradiation. In this paper, we first report on a direct laser printable photoresist for producing light-powered 4D structures with enhanced mechanical properties and near-infrared (NIR) responsive mechanical deformation. The reported nanocomposite design is based on the photothermal effects of gold nanorods (AuNRs), which can induce the nematic-to-isotropic transition of LCE upon exposure to NIR irradiation. The miscibility between AuNRs and LCE is enhanced by thiol functionalization. Appropriate printing parameters are determined, and nanocomposites containing 0-3 wt % of AuNR loading are fabricated via femtosecond two-photon direct laser writing. The effects of the AuNR loading fraction and laser power on the light-powered actuating performance are evaluated. It is found that the nanocomposite with AuNR loading of 3 wt % demonstrates the maximum percentage (20%) of elongation under an NIR laser power of 2 W. An increase in laser power can lead to faster deformation but slower restoration. The nanocomposites demonstrate relatively good stability. Even after 300 actuation cycles, 80% of the elongation magnitude can be retained. In addition, an improvement of 80% in the complex modulus of the nanocomposites, due to the inclusion of AuNRs, is observed.

Experimental Study on Fast and Energy-Efficient Direct Driven Hydraulic Actuator Unit

In this experimental study, a Direct Driven Hydraulics (DDH) system of the closed circuit type was utilized for cyclic vertical actuation in heavy load material handling. The actuator was controlled by a speed-controlled fixed displacement pump. The high energy saving potential of this system has been demonstrated in previous studies by the authors, but the dynamic characteristics of the ramped and P-controlled base system were considered unsatisfactory. Therefore, the system was implemented with an open-loop S-curve control that utilized a pre-calculated RPM (revolutions per minute) profile for the electric motor in order to realize a smooth actuator and load transition as a function of time. The results indicate that S-curve control is exceptionally well suited for producing a controlled lifting–lowering rapid motion with a heavy load, while still keeping the actuator chamber pressures within acceptable limits. In comparison, the motion produced by P-control was characterized by large unwanted pressure peaks together with velocity fluctuations and vibrations at the end of the stroke. Using a combination of S-curve control and hydraulic load compensation, a mass of 1325 kg could be moved 0.26 m in less than 0.5 s. The load compensation reduced the energy consumption by 64%, which would allow downsizing the electric motor and enable cost-efficient DDH implementation.

Bio-Inspired Terrestrial Motion of Magnetic Soft Millirobots

Magnetic soft robots have the combined advantages of contactless actuation, requiring no on-board power source, and having flexible bodies that can adapt to unstructured environments. In this study, four milli-scale soft robots are designed (Inchworm, Turtle, Quadruped, and Millipede) and their actuation under external magnetic fields is investigated with the objective of reproducing multi-limbed motion patterns observed in nature. Magnetic properties are incorporated into a silicone polymer by mixing in ferromagnetic microparticles before curing. The magnet-polymer composite is used to fabricate soft magnetic parts, with pre-determined magnetization profiles achieved using a 1 T field. The resulting soft robots are actuated under external magnetic fields of 10–35 mT which are controlled using an array of six electromagnetic coils. The achieved motion patterns are analyzed over five iterations and the motions are quantified in terms of body lengths traversed per actuation cycle and speed of displacement. The speed of the specimens is calculated to be in the range of 0.15–0.37 mm/s for the actuation field used here. The ability of the soft robots to traverse uneven terrain is also tested, with the Turtle and the Millipede demonstrating successful motion.

Hydrolytic Degradation of Actuators Based on Copolymer Networks From Oligo(ε-caprolactone) Dimethacrylate and n-Butyl Acrylate

ABSTRACTShape-memory polymer actuators often contain crystallizable polyester segments. Here, the influence of accelerated hydrolytic degradation on the actuation performance in copolymer networks based on oligo(ε-caprolactone) dimethacrylate (OCL) and n-butyl acrylate is studied. The semi-crystalline OCL was utilized as crosslinker with molecular weights of 2.3 and 15.2 kg∙mol−1 (ratio: 1:1 wt%) and n-butyl acrylate (25 wt% relative to OCL content) acted as softening agent creating the polymer main chain segments within the network architecture. The copolymer networks were programmed by 50% elongation and were degraded by means of alkaline hydrolysis utilizing sodium hydroxide solution (pH = 13). Experiments were performed in the range of the broad melting range of the actuators at 40 °C. The degradation of test specimen was monitored by the sample mass, which was reduced by 25 wt% within 105 d. As degradation products, fragments of OCL with molecular masses ranging from 400 to 50.000 g·mol-1 could be detected by NMR spectroscopy and GPC measurements. The cleavage of ester groups included in OCL segments resulted in a decrease of the melting temperature (Tm) related to the actuator domains (amorphous at the temperature of degradation) and simultaneously, the Tm associated to the skeleton domain was increased (semi-crystalline at the temperature of degradation).The alkaline hydrolysis decreased the polymer chain orientation of OCL domains until a random alignment of crystalline domains was obtained. This result was confirmed by cyclic thermomechanical actuation tests. The performance of directed movements decreased almost linearly as function of degradation time resulting in the loss of functionality when the orientation of polymer chains disappeared. Here, actuators were able to provide reversible movements until 91 d when the accelerated bulk degradation procedure using alkaline hydrolysis (pH = 13) was applied. Accordingly, a lifetime of more than one year can be guaranteed under physiological conditions (pH = 7.4) when, e.g., artificial muscles for biomimetic robots as potential application for these kind of shape-memory polymer actuators will be addressed.

Clearance angle and evolution of depth of cut in actuated disc cutting

This paper investigates the effect of cutter clearance angle on variation of depth of cut and cutting process with an actuated disc cutting (ADC). ADC is a cyclic cutting method with two main characteristics: (i) a disk-shape cutter is used to attack the rock in an undercutting mechanism; and (ii) the cutter is dynamically actuated as it is moved across the rock. Hence, the cutting process of such system is periodic, each recurrence known as actuation cycle. The first ADC model, developed in 2016, represented an idealization of the technology with a flat disc cutter, where no clearance angle was considered. The evolution of the contact between the disc and the rock was, therefore, computed only on horizontal x-y plane, ignoring the effect of normal component of the force acting on cartridge. This article reports on a study that incorporates the cutter inclination angle in derivation of cutter/rock interface laws. It extends the proposed kinematic and geometry based model to take into account the variable depth of cut in estimating the forces associated with cutting in one actuation cycle. Experiments were conducted using Wobble to test the predictions of the improved model at various operating conditions. The model predictions are matched with the experimental results and effects of various factors are analysed.

Dynamic Modeling and Feedforward Control of Jaw Movements Driven by Viscoelastic Artificial Muscles

Artificial muscles based on dielectric elastomer actuators (DEAs) have been used to mimic the motion of the human jaw. However, DEAs show strong nonlinear behavior coupled with rate dependent viscoelastic phenomena. Under cyclic actuation, the viscoelastic creep coupled with hysteresis further makes control of these viscoelastic membranes difficult. In this paper, we develop a nonlinear dynamic model based on the principles of nonequilibrium thermodynamics to account for viscoelasticity. Furthermore, the damping and inertial effects of the jaw and membrane are taken into account, relating the deformation of the membrane to the voltage applied. Experimental results are found to be consistent with theoretical predictions. The feedforward controller is then developed based on a viscoelastic nonlinear dynamic model. The controller can be used to track the sinusoidal, triangular, and staircase trajectories accurately. Finally, a video demonstrating the jaw movement in the speech, “I Have a Dream,” by Dr. Martin Luther King Jr. has been shown to illustrate the effectiveness of the controller. This is one of the first efforts to control of soft robots driven by viscoelastic DEAs.

Application of SMA spring tendons for improved grasping performance

Shape memory alloy (SMA)-based soft actuators and grippers have generally used SMA wires due to design restrictions, limited actuation force and poor cooling performance of SMA springs. This work demonstrates that SMA springs can be used for high performance soft actuation by positioning them externally as a tendon with the force transmitted to the polymeric matrix using a tendon. Through improvements by using active cooling through fans, a spring longer than the matrix and a matrix with a trapezoidal cross-section, the actuator was capable of cyclic actuation at 0.25 Hz, a bending angle of 270° and a tip force of 0.21 N. The same actuator design was used to build a soft robotic gripper capable of grasping objects with a wide range of shapes and weights, and its maximum pull-off force was measured to be 30 N. The gripper was also capable of holding a weight of 2 kg. The increased design freedom of the proposed design would also enable further improvements to the bending and grasping performance of the actuator, and the use of SMA-based soft actuation to more complex designs and a wider range of robotic applications.

A freely yawing axisymmetric bluff body controlled by near-wake flow coupling

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.