3D Actuator Research Articles

Polyelectrolyte‐Mediated Modulation of Spatial Internal Stresses of Hydrogels for Complex 3D Actuators

Hydrogel actuators with complex 3D initial shapes show numerous important applications, but it remains challenging to fabricate such actuators. This article describes a polyelectrolyte‐based strategy for modulating small‐scale internal stresses within hydrogels to construct complex actuators with tailored 3D initial shapes. Introducing polyelectrolytes into precursor solutions significantly enhances the volume shrinkage of hydrogel networks during polymerization, allowing us to modulate internal stresses. Photopolymerization of these polyelectrolyte‐containing solutions through a mask produces mechanically strong hydrogel sheets with large patterned internal stresses. Consequently, these hydrogel sheets attain complex 3D initial shapes at equilibrium, in contrast to the planar initial configuration of 2D actuators. We demonstrate that these 3D actuators can reversibly transform into other 3D shapes (i.e., 3D‐to‐3D shape transformations) in response to external stimuli. Additionally, we develop a predictive model based on the Flory‐Rehner theory to analyze the polyelectrolyte‐mediated shrinking behaviors of hydrogel networks during polymerization, allowing precise modulation of shrinkage and internal stress. This polyelectrolyte‐boosted shrinking mechanism paves a route to the fabrication of high‐performance 3D hydrogel actuators.

Polyelectrolyte-Mediated Modulation of Spatial Internal Stresses of Hydrogels for Complex 3D Actuators.

Hydrogel actuators with complex 3D initial shapes show numerous important applications, but it remains challenging to fabricate such actuators. This article describes a polyelectrolyte-based strategy for modulating small-scale internal stresses within hydrogels to construct complex actuators with tailored 3D initial shapes. Introducing polyelectrolytes into precursor solutions significantly enhances the volume shrinkage of hydrogel networks during polymerization, allowing us to modulate internal stresses. Photopolymerization of these polyelectrolyte-containing solutions through a mask produces mechanically strong hydrogel sheets with large patterned internal stresses. Consequently, these hydrogel sheets attain complex 3D initial shapes at equilibrium, in contrast to the planar initial configuration of 2D actuators. We demonstrate that these 3D actuators can reversibly transform into other 3D shapes (i.e., 3D-to-3D shape transformations) in response to external stimuli. Additionally, we develop a predictive model based on the Flory-Rehner theory to analyze the polyelectrolyte-mediated shrinking behaviors of hydrogel networks during polymerization, allowing precise modulation of shrinkage and internal stress. This polyelectrolyte-boosted shrinking mechanism paves a route to the fabrication of high-performance 3D hydrogel actuators.

Robust Hydrogel Actuators Functioning in Multi-Environments Enabled by Thermo-Responsive Polymer Nanoparticle Coatings on Hydrogel Surfaces.

Hydrogel actuators with anisotropic structures exhibit reversible responsiveness upon the trigger of various external stimuli, rendering them promising for applications in many fields including artificial muscles and soft robotics. However, their effective operation across multiple environments remains a persistent challenge, even for widely studied thermo-responsive polymers like poly(N-isopropyl acrylamide) (PNIPAm). Current attempts to address this issue are hindered by complex synthetic procedures or specific substrates. This study introduces a straightforward methodology to grow a thin, dense PNIPAm nanoparticle layer on diverse hydrogel surfaces, creating a highly temperature-sensitive hydrogel actuator. This actuator demonstrates adaptability across various environments, including water, oil, and open air, owing to its distinct structure facilitating self-water circulation during actuation. The thin PNIPAm layer consists of interconnected PNIPAm nanoparticles synthesized via in situ interfacial precipitation polymerization, seamlessly bonded to the hydrogel substrate through an interfacial layer containing hybrid hydrogel/PNIPAm nanoparticles. This unique anisotropic structure ensures exceptional structural stability without interfacial delamination, even enduring harsh treatments such as freezing, ultrasonic irradiation, and prolonged water immersion. Remarkably, PNIPAm films on hydrogel surfaces which enable programmable 3D actuation can also be precisely patterned. This synthetic approach opens a novel pathway for fabricating advanced hydrogel actuators with broad-ranging applications.

Photothermal porous monodomain liquid crystal elastomer actuator based on confined expansion method

Light-driven monodomain liquid crystal elastomers (mLCEs) have the advantages of remote operation and local control, but the response time will increase rapidly with the increase of thickness due to the limitation of the incident light depth. In order to achieve fast response, porous AuNPs-mLCEs actuators are proposed and fabricated by a confined expansion method. The average pore size of the prepared rod-shaped porous mLCEs is 200 μm, the porosity is 46.5%, and the order parameter of mesogens is 0.41. When the diameter is 3 mm, the response time is 1.2 s at a wavelength of 532 nm and a light intensity of 2.6 W•cm−2. When the diameter increases to 10 mm, the response time only increases to 2.2 s. The contraction of porous mLCEs actuator caused by the photothermal effect enables it can not only to lift an object 321.5 times heavier than itself by contraction, but also raise-up an object 5.2 times heavier than itself by bending. This method can be used to design and manufacture various 3D porous photothermal actuators with fast response.

Melt electrowriting enabled 3D liquid crystal elastomer structures for cross-scale actuators and temperature field sensors.

Liquid crystal elastomers (LCEs) have garnered attention for their remarkable reversible strains under various stimuli. Early studies on LCEs mainly focused on basic dimensional changes in macrostructures or quasi-three-dimensional (3D) microstructures. However, fabricating complex 3D microstructures and cross-scale LCE-based structures has remained challenging. In this study, we report a compatible method named melt electrowriting (MEW) to fabricate LCE-based microfiber actuators and various 3D actuators on the micrometer to centimeter scales. By controlling printing parameters, these actuators were fabricated with high resolutions (4.5 to 60 μm), actuation strains (10 to 55%), and a maximum work density of 160 J/kg. In addition, through the integration of a deep learning-based model, we demonstrated the application of LCE materials in temperature field sensing. Large-scale, real-time, LCE grid-based spatial temperature field sensors have been designed, exhibiting a low response time of less than 42 ms and a high precision of 94.79%.

Topology Optimization of Shape Memory Alloy Actuators for Prescribed Two-Way Transforming Shapes

This paper proposes a new topology optimization formulation for obtaining shape memory alloy actuators which are designed with prescribed two-way transforming shapes. The actuation behaviors of shape memory alloy structures are governed by austenite-martensite phase transformations effected by thermal-mechanical loading processes; therefore, to realize the precise geometric shape variations of shape memory alloy actuators, traditional methods involve iteration processes including heuristic structural design, numerical predictions and experimental validation. Although advanced structural optimization methods such as topology optimization have been used to design three-dimensional (3D) shape memory alloy actuators, the maximization/minimization of quantities such as structural compliance or inaccurate stroke distances has usually been selected as the optimization objective to obtain feasible solutions. To bridge the gap between precise shape-morphing requirements and efficient shape memory alloy actuator designs, this paper formulates optimization criteria with quantitatively desired geometric shapes, and investigates the automatic designs of two-way prescribed shape morphing shape memory alloy structures based on the proposed topology optimization method. The super element method and adjoint method are used to derive the analytical sensitivities of the objective functions with respect to the design variables. Numerical examples demonstrate that the proposed method can obtain 3D actuator designs that have the desired two-way transforming shapes.

SPADA: A Toolbox of Designing Soft Pneumatic Actuators for Shape Matching Based on Surrogate Modeling.

Soft pneumatic actuators (SPAs) produce motions for soft robots with simple pressure input, however, they require to be appropriately designed to fit the target application. Available design methods employ kinematic models and optimization to estimate the actuator response and the optimal design parameters to achieve a target actuator's shape. Within SPAs, bellow SPAs excel in rapid prototyping and large deformation, yet their kinematic models often lack accuracy due to the geometry complexity and the material nonlinearity. Furthermore, existing shape-matching algorithms are not providing an end-to-end solution from the desired shape to the actuator. In addition, despite the availability of computational design pipelines, an accessible and user-friendly toolbox for direct application remains elusive. This article addresses these challenges, offering an end-to-end shape-matching design framework for bellow SPAs to streamline the design process, and the open-source toolbox SPADA (Soft Pneumatic Actuator Design frAmework) implementing the framework with a graphic user interface for easy access. It provides a kinematic model grounded on a modular design to improve accuracy, finite element method (FEM) simulations, and piecewise constant curvature (PCC) approximation. An artificial neural network-trained surrogate model, based on FEM simulation data, is trained for fast computation in optimization. A shape-matching algorithm, merging three-dimensional (3D) PCC segmentation and a surrogate model-based genetic algorithm, identifies optimal actuator design parameters for desired shapes. The toolbox, implementing the proposed design framework, has proven its end-to-end capability in designing actuators to precisely match two-dimensional shapes with root-mean-squared-errors of 4.16, 2.70, and 2.51 mm, and demonstrating its potential by designing a 3D deformable actuator.

A new strategy for reconfigurable actuators based on oxidation of fast responsive shape memory polyarylene sulfide sulfone

Actuators have shown great potential application in many fields. Till now, it remains a challenge to design and exploit diverse modes actuators which can endure in hard environment by reconfiguration. Here, a series of shape memory polyarylene sulfide sulfone (SMP-PASS) shows excellent thermal properties, good mechanical properties and good shape memory behaviors were synthesized. The shape recovery temperature of can further increased by oxidizing thioether bonds to sulfone groups by facile oxidation treatment. According to the different temperature stimulus response of the samples before and after oxidation, herein, we propose a simple strategy to fabricate reconfigurable actuators that can be used in extreme environments by patterns in both plane and depth directions of SMP-PASS films by post-oxidation modification. SMP-PASS films after programming anisotropic can transform planar films into various 3D structure actuators under heating stimulation, unambiguous confirming that the facile oxidation method supplies an efficient route to yield multiple and thermal-resistant actuators. That not only supplies a novel, re-processable and fast-responsive shape memory polymer candidate, but also provides a facile and effective strategy to fabricate smart 3D actuators.

An AutomationML extension towards interoperability of 3D virtual commissioning software applications

ABSTRACT To achieve interoperability between different 3D virtual commissioning software, a generic virtual commissioning data model is required. AutomationML is a standard neutral format for interoperability in the engineering phase. However, the current AutomationML standard is not sufficient for full-scope 3D-based virtual commissioning data exchange, as attributes and modeling method of 3D virtual commissioning-related sensors, actuators and signal connections are not standardized in AutomationML. To fill this gap, the authors suggest extending AutomationML for interoperability in 3D virtual commissioning. In this paper, a case-driven iterative approach is introduced to evolve towards an AutomationML extension. This extension is gradually developed by taking the union of all virtual commissioning-related functions and attributes of 3D virtual commissioning software. During the iteration, naming rules are applied when a new attribute is added to the extension. With this approach, an initial AutomationML extension is created by implementing a first iteration. The interoperability performance of this extension is subsequently evaluated by conducting data exchange of a representative set of 3D emulation models between two 3D virtual commissioning software, namely Siemens NX and Visual Components, via self-developed ‘Import’ and ‘Export’ plug-ins. It shows that AutomationML extension-based data exchange converts 70% more attributes than that only based on AutomationML.

Development of inverse static model of continuum robots based on absolute nodal coordinates formulation for large deformation applications

AbstractContinuum robotics has emerged as a prominent trend within the robotics field for about two decades. Nevertheless, motion analysis of continuum robots based on dynamic modelling remains severely limited. In the realm of continuum robots, it is obvious that simplified theoretical models or first-order approximations utilizing springs and dampers are insufficient for capturing the dynamics of soft-bodied structures. This inadequacy is especially apparent in applications involving significant deformations in 3D space, continuous actuation, and interface effects. In this regard, the development of precise theoretical models is crucial. Within this particular framework, the paper presents the absolute nodal coordinates formulation (ANCF) to construct the kinematic models of continuum robots. The transformation between the structural, body, and element coordinate systems, as well as the derivation of the generalized external forces and moments using the principle of virtual work and the velocity gradient tensor, are demonstrated. Additionally, it proceeds to develop the strain measures required for the computation of the elastic forces of elements with circular cross sections, ultimately enabling the construction of the Forward Static Model (FSM) of continuum robots. A procedure based on B-spline surface geometry is proposed to figure out the robot shapes in space and generate the necessary nodal coordinates and gradients of ANCF elements. Finally, the inverse static model (ISM) of the ANCF-B-spline generated structure is presented using an iterative solution approach. The effectiveness of the proposed model is evaluated by the analysis of numerical examples. According to the numerical results, it can be inferred that the ANCF method well captures the modelling aspects of continuum robots. The results obtained from the FSM demonstrate a high level of accuracy, moreover, the driving forces based on the ISM provide, from a practical standpoint, satisfactory results.

Biomimetic 3D Color-Changing Hydrogel Actuators Constructed Based on Soft Permeable Photonic Crystals.

The integration of photonic crystals and self-shaping actuators is a promising method for constructing powerful biomimetic color-changing actuators. The major barrier is that common photonic crystals generally block the transfer/orientation of monomers/fillers and hence hinder the formation of heterogeneous structures for programmed 3D deformations as well as degrade the deformation capacity and mechanical properties of actuators. Herein, we present the construction of complex and strong 3D color-changing hydrogel actuators by asymmetric photolithography based on soft, permeable photonic crystals. The soft permeable photonic crystals are assembled by hydrogel microspheres with an ultralow volume fraction. During the asymmetric photolithography, the monomers in precursor solutions can thus transfer freely to generate heterogeneous microstructures, spatially patterned internal stresses, and interpenetrating networks for programming the deformation trajectories and initial 3D configurations and enhancing mechanical properties of actuators. Various 3D color-changing hydrogel actuators (e.g., flower and scroll painting) are constructed for applications such as information encryption and display.

Polarization-driven reversible actuation in a photo-responsive polymer composite

Light-responsive polymers and especially amorphous azopolymers with intrinsic anisotropic and polarization-dependent deformation photo-response hold great promises for remotely controlled, tunable devices. However, dynamic control requires reversibility characteristics far beyond what is currently obtainable via plastic deformation of such polymers. Here, we embed azopolymer microparticles in a rubbery elastic matrix at high density. In the resulting composite, cumulative deformations are replaced by reversible shape switching – with two reversible degrees of freedom defined uniquely by the writing beam polarization. We quantify the locally induced strains, including small creeping losses, directly by means of a deformation tracking algorithm acting on microscope images of planar substrates. Further, we introduce free-standing 3D actuators able to smoothly undergo multiple configurational changes, including twisting, roll-in, grabbing-like actuation, and even continuous, pivot-less shape rotation, all dictated by a single wavelength laser beam with controlled polarization.

Opto‐Hydrodynamic Driven 3D Dynamic Microswarm Petals

AbstractArtificial microswarms with a collective intelligence that can execute cooperative tasks will serve as intelligent micro/nanorobot systems for many biomedical and microengineering applications. However, it remains challenging to construct microswarms with 3D dynamic and reconfigurable structures that can execute complex spatiotemporal‐dependent tasks. Here, simply using a tapered optical fiber (TOF) with 1.55 µm wavelength light irradiation, a convenient opto‐hydrodynamic strategy for 3D dynamic microswarm actuation based on photothermal gradient‐induced Marangoni effect is reported. With light irradiation at the water‐air interface, randomly distributed microparticles are reorganized into firework‐like 3D swarms with four petals. Such petals in the microswarm are controllably reconfigurable by adjusting the angle between TOF and water‐air interface. These microswarms are also deformable and capable of performing stable migration by simply moving the TOF. Importantly, this opto‐hydrodynamic strategy is applicable for the formation of artificial 3D‐dynamic bio‐microswarms using different biological cells, which further facilitate the regulation of biological processes such as bacteria growth/division. This opto‐hydrodynamic strategy provides a new solution for 3D dynamic microswarm formation, with many potentials for biomedical and microengineering applications that need spatiotemporal‐dependent individual cooperation.

A custom-built planar biaxial system for soft tissue material testing

Accurate material characterization of soft tissues is crucial for understanding the physiopathology of cardiovascular diseases. However, commercial biaxial testing systems are expensive, prompting the need for affordable custom solutions. This study aimed to develop a low-cost custom biaxial system capable of accurately characterizing the mechanical behavior of soft tissues. The biaxial system was constructed using 3D printing technology and non-captive linear actuators for precise displacement control. A real-time marker tracking system was implemented to estimate dis-placements without the need for costly hardware. The system's performance was evaluated through tests on a calibration spring and frozen porcine aorta samples. The linear actuators demonstrated excellent response to user position input after motor tuning, showing no discrepancies between commands and actual positions. The experimental testing of the calibration spring showed good agreement with the analytical solution, validating the system's ability to accurately test materials. Testing on porcine aorta samples revealed stress–strain responses consistent with existing literature, accounting for potential variations due to tissue preservation and regional material property heterogeneity. Overall, this custom biaxial system demonstrates promising performance in accurately assessing the mechanical behavior of soft tissues, providing researchers with a valuable tool for cardiovascular disease research and tissue engineering applications.

3D printing of magnetostrictive property in 17/4 ph stainless steel

Additive manufacturing (AM) of metallic alloys significantly advances diverse areas of science/engineering with a prosperous future trend. Although this technology is also very attractive for smart materials development, the creation of a pre-programmed “4th dimension” property to appear in response to external stimuli represents an important challenge. Here we report on 3D printing of stainless steel based magnetic alloy and detail the ways to optimize the magnetisation and magnetostriction to be used for actuation and sensing. By controlling the printing parameters and sintering process, we are able to tune the magnetic and magnetoelastic properties of the 17/4 ph stainless steel, demonstrating material fabrication multi-function with a cost effective advantage. The stainless steel 17/4 ph samples in the as-printed (AP) and sintered steel (SS) stages are compared. In the developed SS sample, a remarkable increase of 18% in saturation magnetisation yet with 12.6 % lower coercivity was achieved. Moreover, the 54% higher magnetostriction was developed for the SS sample compared to the AP sample. Plus, the difference in anisotropy energy K1 was also lower for the SS sample. The structural and magnetic properties control are reported at each stage of the printing process, demonstrating development and optimization prospects of 3D printable metallic sensors and actuators.

3D‐Printed Electrostatic Microactuators for Flexible Microsystems

AbstractDeveloping small‐scale, lightweight, and flexible devices with integrated microactuators is one of the critical challenges in wearable haptic devices, soft robotics, and microrobotics. In this study, a novel fabrication process that leverages the benefits of 3D printing with two‐photon polymerization and flexible printed circuit boards (FPCBs) is presented. This method enables flexible microsystems with 3D‐printed electrostatic microactuators, which are demonstrated in a flexible integrated micromirror array and a legged microrobot with a mass of 4 mg. 3D electrostatic actuators on FPCBs are robust enough to actuate the micromirrors while the device is deformed, and they are easily integrated with off‐the‐shelf electronics. The crawling robot is one of the lightest legged microrobots actuated without external fields, and the legs actuated with 3D electrostatic actuators enable a locomotion speed of 0.27 body length per second. The proposed fabrication framework opens up a pathway toward a variety of highly integrated flexible microsystems.

Attitude control of 3D soft pneumatic actuators based on BP neural network

Soft pneumatic robotics have attracted considerable attention in recent years due to their deformation capabilities, which far exceed those of conventional robotics. However, precise control of soft pneumatic actuators remains a challenge due to the lack of model-based control techniques. This work aims to employ a high-precision and low-cost backpropagation (BP) neural network-based model method to control a 3D soft pneumatic actuator. Experiments show that this BP neural network-based model control method performs well in terms of precision, in which the errors of bending angle and deflection angle are within 0.8° and 1.2°, respectively, and the end point position error of the soft actuator is less than 2.5 mm, which is significantly better than traditional modeling methods, demonstrating the application potential of soft robots for high-precision operations.

3D Knitting for Pneumatic Soft Robotics

AbstractSoft robots adapt passively to complex environments due to their inherent compliance, allowing them to interact safely with fragile or irregular objects and traverse uneven terrain. The vast tunability and ubiquity of textiles has enabled new soft robotic capabilities, especially in the field of wearable robots, but existing textile processing techniques (e.g., cut‐and‐sew, thermal bonding) are limited in terms of rapid, additive, accessible, and waste‐free manufacturing. While 3D knitting has the potential to address these limitations, an incomplete understanding of the impact of structure and material on knit‐scale mechanical properties and macro‐scale device performance has precluded the widespread adoption of knitted robots. In this work, the roles of knit structure and yarn material properties on textile mechanics spanning three regimes–unfolding, geometric rearrangement, and yarn stretching–are elucidated and shown to be tailorable across unique knit architectures and yarn materials. Based on this understanding, 3D knit soft actuators for extension, contraction, and bending are constructed. Combining these actuation primitives enables the monolithic fabrication of entire soft grippers and robots in a single‐step additive manufacturing procedure suitable for a variety of applications. This approach represents a first step in seamlessly “printing” conformal, low‐cost, customizable textile‐based soft robots on‐demand.

Effect of water content on the constitutive response of a cellulose foam

Cellulose foams can undergo a large one-way actuation strain when taken from a dry, pre-compressed state to a damp state by the addition of water. In order to develop a predictive capability for the 3D actuation of a foam-based structure with a non-uniform water concentration, it is first necessary to understand the mechanical properties of the foam (including actuation strain) as a function of water content. Here, we report the anisotropic constitutive response of a cellulose foam as a function of water content. Over the range of strain rate and water content considered, the cellulose foam is adequately approximated by a visco-plastic, transversely isotropic, compressible constitutive model. Unloading from the plastic state by a stress drop equal to the initial yield stress leads to reversed plastic flow: this is an extreme form of kinematic hardening.

3D actuation of foam-core liquid metal droplets.

Precise manipulation of liquid metal (LM) droplets possesses the potential to enable a wide range of applications in reconfigurable electronics, robotics, and microelectromechanical systems. Although a variety of methods have been explored to actuate LM droplets on a 2D plane, versatile 3D manipulation remains a challenge due to the difficulty in overcoming their heavy weight. Here, foam-core liquid metal (FCLM) droplets that can maintain the surface properties of LM while significantly reducing the density are developed, enabling 3D manipulation in an electrolyte. The FCLM droplet is fabricated by coating LM on the surface of a copper-grafted foam sphere. The actuation of the FCLM droplet is realized by electrically inducing Marangoni flow on the LM surface. Two motion modes of the FCLM droplet are observed and studied and the actuation performance is characterized. Multiple FCLM droplets can be readily controlled to form 3D structures, demonstrating their potential to be further developed to form collaborative robots for enabling wider applications.