3D Printing Magnetic Actuators for Biomimetic Applications

Silk Fibroin Film Decorated with Ultralow FeCo Content by Sputtering Deposition Results in a Flexible and Robust Biomaterial for Magnetic Actuation

Magnetically responsive soft biomaterials are at the forefront of bioengineering and biorobotics. We have created a magnetic hybrid material by coupling silk fibroin─i.e., a natural biopolymer with an optimal combination of biocompatibility and mechanical robustness─with the FeCo alloy, the ferromagnetic material with the highest saturation magnetization. The material is in the form of a 6 μm-thick silk fibroin film, coated with a FeCo layer (nominal thickness: 10 nm) grown by magnetron sputtering deposition. The sputtering deposition technique is versatile and eco-friendly and proves effective for growing the magnetic layer on the biopolymer substrate, also allowing one to select the area to be decorated. The hybrid material is biocompatible, lightweight, flexible, robust, and water-resistant. Electrical, structural, mechanical, and magnetic characterization of the material, both as-prepared and after being soaked in water, have provided information on the adhesion between the silk fibroin substrate and the FeCo layer and on the state of internal mechanical stresses. The hybrid film exhibits a high magnetic bending response under a magnetic field gradient, thanks to an ultralow fraction of the FeCo component (less than 0.1 vol %, i.e., well below 1 wt %). This reduces the risk of adverse health effects and makes the material suitable for bioactuation applications.

Recent Advances in 4D Printing of Advanced Materials and Structures for Functional Applications

4D printing has attracted tremendous worldwide attention during the past decade. This technology enables the shape, property, or functionality of printed structures to change with time in response to diverse external stimuli, making the original static structures alive. The revolutionary 4D-printing technology offers remarkable benefits in controlling geometric and functional reconfiguration, thereby showcasing immense potential across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Here, a comprehensive review of the latest achievements in 4D printing using various types of materials and different additive manufacturing techniques is presented. The state-of-the-art strategies implemented in harnessing various 4D-printed structures are highlighted, which involve materials design, stimuli, functionalities, and applications. The machine learning approach explored for 4D printing is also discussed. Finally, the perspectives on the current challenges and future trends toward further development in 4D printing are summarized.

Reprogrammable Magnetic Soft Actuators with Microfluidic Functional Modules via Pixel-Assembly

Magnetic soft actuators and robots have attracted considerable attention in biomedical applications due to their speedy response, programmability, and biocompatibility. Despite recent advancements, the fabrication process of magnetic actuators and the reprogramming approach of their magnetization profiles continue to pose challenges. Here, a facile fabrication strategy is reported based on arrangements and distributions of reusable magnetic pixels on silicone substrates, allowing for various magnetic actuators with customizable architectures, arbitrary magnetization profiles, and integration of microfluidic technology. This approach enables intricate configurations with decent deformability and programmability, as well as biomimetic movements involving grasping, swimming, and wriggling in response to magnetic actuation. Moreover, microfluidic functional modules are integrated for various purposes, such as on/off valve control, curvature adjustment, fluid mixing, dynamic microfluidic architecture, and liquid delivery robot. The proposed method fulfills the requirements of low-cost, rapid, and simplified preparation of magnetic actuators, since it eliminates the need to sustain pre-defined deformations during the magnetization process or to employ laser heating or other stimulation for reprogramming the magnetization profile. Consequently, it is envisioned that magnetic actuators fabricated via pixel-assembly will have broad prospects in microfluidics and biomedical applications.

Enhanced Magnetic Soft Robotics: Integrating Fiber Optics and 3D Printing for Rapid Actuation and Precision Sensing

Timely, accurate, and rapid grasping of dynamic change information in magnetic actuation soft robots is essential for advancing their evolution toward intelligent, integrated, and multifunctional systems. However, existing magnetic-actuation soft robots lack effective functions for integrating sensing and actuation. Herein, we demonstrate the integration of distributed fiber optics technology with advanced-programming 3D printing techniques. This integration provides our soft robots unique capabilities such as integrated sensing, precise shape reconstruction, controlled deformation, and sophisticated magnetic navigation. By utilizing an improved magneto-mechanical coupling model and an advanced inversion algorithm, we successfully achieved real-time reconstruction of complex structures, such as 'V', 'N', and 'M' shapes and gripper designs, with a notable response time of 34 ms. Additionally, our robots demonstrate proficiency in magnetic navigation and closed-loop deformation control, making them ideal for operation in confined or obscured environments. This work thus provides a transformative strategy to meet unmet demands in the rapidly growing field of soft robotics, especially in establishing the theoretical and technological foundation for constructing digitized robots.

Untethered soft actuators for soft standalone robotics

Soft actuators produce the mechanical force needed for the functional movements of soft robots, but they suffer from critical drawbacks since previously reported soft actuators often rely on electrical wires or pneumatic tubes for the power supply, which would limit the potential usage of soft robots in various practical applications. In this article, we review the new types of untethered soft actuators that represent breakthroughs and discuss the future perspective of soft actuators. We discuss the functional materials and innovative strategies that gave rise to untethered soft actuators and deliver our perspective on challenges and opportunities for future-generation soft actuators.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

A Printable Magnetic-Responsive Iron Oxide Nanoparticle (ION)-Gelatin Methacryloyl (GelMA) Ink for Soft Bioactuator/Robot Applications

The features or actuation behaviors of nature's creatures provide concepts for the development of biomimetic soft bioactuators/robots with stimuli-responsive capabilities, design convenience, and environmental adaptivity in various fields. Mimosa pudica is a mechanically responsive plant that can convert pressure to the motion of leaves. When the leaves receive pressure, the occurrence of asymmetric turgor in the extensor and flexor sides of the pulvinus from redistributing the water in the pulvinus causes the bending of the pulvinus. Inspired by the actuation of Mimosa pudica, designing soft bioactuators can convert external stimulations to driving forces for the actuation of constructs which has been receiving increased attention and has potential applications in many fields. 4D printing technology has emerged as a new strategy for creating versatile soft bioactuators/robots by integrating printing technologies with stimuli-responsive materials. In this study, we developed a hybrid ink by combining gelatin methacryloyl (GelMA) polymers with iron oxide nanoparticles (IONs). This hybrid ION-GelMA ink exhibits tunable rheology, controllable mechanical properties, magnetic-responsive behaviors, and printability by integrating the internal metal ion-polymeric chain interactions and photo-crosslinking chemistries. This design offers the inks a dual crosslink mechanism combining the advantages of photocrosslinking and ionic crosslinking to rapidly form the construct within 60 s of UV exposure time. In addition, the magnetic-responsive actuation of ION-GelMA constructs can be regulated by different ION concentrations (0-10%). Furthermore, we used the ION-GelMA inks to fabricate a Mimosa pudica-like soft bioactuator through a mold casting method and a direct-ink-writing (DIW) printing technology. Obviously, the pinnule leaf structure of printed constructs presents a continuous reversible shape transformation in an air phase without any liquid as a medium, which can mimic the motion characteristics of natural creatures. At the same time, compared to the model casting process, the DIW printed bioactuators show a more refined and biomimetic transformation shape that closely resembles the movement of the pinnule leaf of Mimosa pudica in response to stimulation. Overall, this study indicates the proof of concept and the potential prospect of magnetic-responsive ION-GelMA inks for the rapid prototyping of biomimetic soft bioactuators/robots with untethered non-contact magneto-actuations.

One-Shot 3D Printed Soft Device Actuated Using Metal-Filled Channels and Sensed with Embedded Strain Gauge

In this article, the multimaterial extrusion (M-MEX) technology is used to fabricate, in a single step, a three-dimensional printed soft electromagnetic (EM) actuator, based on internal channels, filled with soft liquid metal (Galinstan) and equipped with an embedded strain gauge, for the first time. At the state of the art, M-MEX techniques result underexploited for the manufacture of soft EM actuators: only traditional manufacturing approaches are used, resulting in many assembly steps. The main features of this work are as follows: (1) one shot fabrication, (2) smart structure equipped with sensor unit, and (3) scalability. The actuator was tested in conjunction with a commercial magnet, showing a bending angle of 22.4° (when activated at 4A), a relative error of 0.7%, and a very high sensor sensitivity of 49.7 Two more examples, showing all the potentialities of the proposed approach, are presented: a jumping frog-inspired soft robot and a dual independent two-finger actuator. This article aims to push the role of extrusion-based additive manufacturing for the fabrication of EM soft robots: several advantages such as portability, no cooling systems, fast responses, and noise reduction can be achieved by exploiting the proposed actuation system compared to the traditional and widespread actuation mechanisms (shape memory polymers, shape memory alloys, pneumatic actuation, and cable-driven actuation).

Fabrication and Applications of Magnetic Polymer Composites for Soft Robotics

The emergence of magnetic polymer composites has had a transformative impact on the field of soft robotics. This overview will examine the various methods by which innovative materials can be synthesized and utilized. The advancement of soft robotic systems has been significantly enhanced by the utilization of magnetic polymer composites, which amalgamate the pliability of polymers with the reactivity of magnetic materials. This study extensively examines the production methodologies involved in dispersing magnetic particles within polymer matrices and controlling their spatial distribution. The objective is to gain insights into the strategies required to attain the desired mechanical and magnetic properties. Additionally, this study delves into the potential applications of these composites in the field of soft robotics, encompassing various devices such as soft actuators, grippers, and wearable gadgets. The study emphasizes the transformative capabilities of magnetic polymer composites, which offer a novel framework for the advancement of biocompatible, versatile soft robotic systems that utilize magnetic actuation.

3D Printing Technologies for Fabrication of Magnetic Materials Based on Metal-Polymer Composites: A Review

Additive manufacturing is a very rapidly developing industrial field. It opens many possibilities for the fast fabrication of complex-shaped products and devices, including functional materials and smart structures. This paper presents an overview of polymer 3D printing technologies currently used to produce magnetic materials and devices based on them. Technologies such as filament-fused modeling (FDM), direct ink writing (DIW), stereolithography (SLA), and binder jetting (BJ) are discussed. Their technological features, such as the optimal concentration of the filler, the shape and size of the filler particles, printing modes, etc., are considered to obtain bulk products with a high degree of detail and with a high level of magnetic properties. The polymer 3D technologies are compared with conventional technologies for manufacturing polymer-bonded magnets and with metal 3D technologies. This paper shows prospective areas of application of 3D polymer technologies for fabricating the magnetic elements of complex shapes, such as shim elements with an optimized shape and topology; advanced transformer cores; sensors; and, in particular, the fabrication of soft robots with a fast response to magnetic stimuli and composites based on smart fillers.

Research Progress on Low Damage Grasping of Fruit, Vegetable and Meat Raw Materials

The sorting and processing of food raw materials is an important step in the food production process, and the quality of the sorting operation can directly or indirectly affect the quality of the product. In order to improve production efficiency and reduce damage to food raw materials, some food production enterprises currently use robots for sorting operations of food raw materials. In the process of robot grasping, some food raw materials such as fruits, vegetables and meat have a soft appearance, complex and changeable shape, and are easily damaged by the robot gripper. Therefore, higher requirements have been put forward for robot grippers, and the research and development of robot grippers that can reduce damage to food raw materials and ensure stable grasping has been a major focus. In addition, in order to grasp food raw materials with various shapes and sizes with low damage, a variety of sensors and control strategies are required. Based on this, this paper summarizes the low damage grasp principle and characteristics of electric grippers, pneumatic grippers, vacuum grippers and magnetic grippers used in automated sorting production lines of fruit, vegetable and meat products, as well as gripper design methods to reduce grasp damage. Then, a grasping control strategy based on visual sensors and tactile sensors was introduced. Finally, the challenges and potential future trends faced by food robot grippers were summarized.

Advanced Composite Materials Utilized in FDM/FFF 3D Printing Manufacturing Processes: The Case of Filled Filaments

The emergence of additive manufacturing technologies has brought about a significant transformation in several industries. Among these technologies, Fused Deposition Modeling/Fused Filament Fabrication (FDM/FFF) 3D printing has gained prominence as a rapid prototyping and small-scale production technique. The potential of FDM/FFF for applications that require improved mechanical, thermal, and electrical properties has been restricted due to the limited range of materials that are suitable for this process. This study explores the integration of various reinforcements, including carbon fibers, glass fibers, and nanoparticles, into the polymer matrix of FDM/FFF filaments. The utilization of advanced materials for reinforcing the filaments has led to the enhancement in mechanical strength, stiffness, and toughness of the 3D-printed parts in comparison to their pure polymer counterparts. Furthermore, the incorporation of fillers facilitates improved thermal conductivity, electrical conductivity, and flame retardancy, thereby broadening the scope of potential applications for FDM/FFF 3D-printed components. Additionally, the article underscores the difficulties linked with the utilization of filled filaments in FDM/FFF 3D printing, including but not limited to filament extrusion stability, nozzle clogging, and interfacial adhesion between the reinforcement and matrix. Ultimately, a variety of pragmatic implementations are showcased, wherein filled filaments have exhibited noteworthy benefits in comparison to standard FDM/FFF raw materials. The aforementioned applications encompass a wide range of industries, such as aerospace, automotive, medical, electronics, and tooling. The article explores the possibility of future progress and the incorporation of innovative reinforcement materials. It presents a plan for the ongoing growth and application of advanced composite materials in FDM/FFF 3D printing.

Coaxially printed magnetic mechanical electrical hybrid structures with actuation and sensing functionalities

Soft electromagnetic devices have great potential in soft robotics and biomedical applications. However, existing soft-magneto-electrical devices would have limited hybrid functions and suffer from damaging stress concentrations, delamination or material leakage. Here, we report a hybrid magnetic-mechanical-electrical (MME) core-sheath fiber to overcome these challenges. Assisted by the coaxial printing method, the MME fiber can be printed into complex 2D/3D MME structures with integrated magnetoactive and conductive properties, further enabling hybrid functions including programmable magnetization, somatosensory, and magnetic actuation along with simultaneous wireless energy transfer. To demonstrate the great potential of MME devices, precise and minimally invasive electro-ablation was performed with a flexible MME catheter with magnetic control, hybrid actuation-sensing was performed by a durable somatosensory MME gripper, and hybrid wireless energy transmission and magnetic actuation were demonstrated by an untethered soft MME robot. Our work thus provides a material design strategy for soft electromagnetic devices with unexplored hybrid functions.

Recent Advances in Magnetic Polymer Composites for BioMEMS: A Review

The objective of this review is to investigate the potential of functionalized magnetic polymer composites for use in electromagnetic micro-electro-mechanical systems (MEMS) for biomedical applications. The properties that make magnetic polymer composites particularly interesting for application in the biomedical field are their biocompatibility, their adjustable mechanical, chemical, and magnetic properties, as well as their manufacturing versatility, e.g., by 3D printing or by integration in cleanroom microfabrication processes, which makes them accessible for large-scale production to reach the general public. The review first examines recent advancements in magnetic polymer composites that possess unique features such as self-healing capabilities, shape-memory, and biodegradability. This analysis includes an exploration of the materials and fabrication processes involved in the production of these composites, as well as their potential applications. Subsequently, the review focuses on electromagnetic MEMS for biomedical applications (bioMEMS), including microactuators, micropumps, miniaturized drug delivery systems, microvalves, micromixers, and sensors. The analysis encompasses an examination of the materials and manufacturing processes involved and the specific fields of application for each of these biomedical MEMS devices. Finally, the review discusses missed opportunities and possible synergies in the development of next-generation composite materials and bioMEMS sensors and actuators based on magnetic polymer composites.

Silicon Distribution-Induced Actuation Film with Bidirectional Bending Deformation and Versatile Bionic Applications

As an important branch of intelligent materials, the research and development of stimulus-responsive flexible intelligent actuation materials is of great significance to promote the industrialization of intelligent materials. In this study, the asymmetric PVA-co-PE/silicon nanoparticle (PPSN) composite films and PVA-co-PE/silicon sol (PPSS) composite film with different silicon distributions were prepared by a simple spraying method. The silicon nanoparticle layer in the PPSN composite film was similar to the sand-like water-absorbing layer, which can quickly absorb water and permeate it into the interior region, leading to the hygroscopic expansion behavior on one side of the nanofiber film. Then, the PPSN composite film would quickly bend and deform to the silicon nanoparticle side. However, in the PPSS composite film, due to the excellent hygroscopicity and swelling characteristics of the silica sol layer, the composite film can be rapidly deformed to the PVA-co-PE nanofiber film side under moisture stimulation. The above results subvert the traditional asymmetric actuation film, which mainly depends on the hydrophilicity difference to determine the hygroscopic responsiveness and deformation direction, and ignore that the swelling degree is the main factor determining the bending direction of actuator. In addition, both the composite films can quickly respond to moisture stimulation (<1 s) and produce large-scale bending deformation (180°). Furthermore, due to the excellent interface adhesion formed by the continuity structure in the PPSS composite film, it has better actuation stability than the PPSN composite film. The excellent actuation characteristics and different bending directions of the PPSN and PPSS composite films make it a great application prospect in the field of bionics in the future.

Robust, Healable, Self-Locomotive Integrated Robots Enabled by Noncovalent Assembled Gradient Nanostructure

Integration, being lightweight, and intelligence are important orientations for the future advancement of soft robots. However, existing soft robots are generally hydrogels or silicone rubber, which are inherently mechanically inferior and easily damaged and difficult to integrate functions. Here, inspired by nacre, an elastomer actuator with sulfonated graphene-based gradient nanostructures is constructed via supramolecular multiscale assembly. The resulting nanocomposite possesses an ultrahigh toughness of 141.19 MJ/m³ and high room-temperature self-healing efficiency (89%). The proof-of-concept robot is demonstrated to emphasize its maximum swimming speed of 2.67 body length per second, whose speed is comparable to that of plankton, representing the outperformance of most artificial soft robots. Furthermore, the robot can stably absorb pollutants and recover its robustness and functionality even when damaged. This study breaks the mutual exclusivity of functional execution and fast locomotions, and we anticipate that our nanostructural design will offer an effective extended path to other integrated robots that required multifunction integration.

3D Printing Ultraflexible Magnetic Actuators via Screw Extrusion Method

Soft magnetic actuators with programmable structure design and controllable deformation ability based on 3D printing technology have attracted extensive attention. In this paper, a novel 3D printing strategy is developed to manufacture the ultraflexible magnetic actuator, in which the printed material is composed of magnetic particles and thermoplastic rubber materials. Different from the traditional fused deposition printing, this printing strategy introduces screw extrusion technology to the heating components of the printer to overcome the problem of filament buckling in the flexible material. Thus, the tensile modulus of the printed products can be reduced to as low as ≈2 MPa. Based on the above method, biomimetic magnetic actuators of the sucker and the pump are constructed for adhering and releasing object and pumping liquid. The contraction performance of the magnetic actuator is studied via a series of experiments and the magnetic field-induced deformation is analyzed by the multiphysics-based finite element model. This work proves that ultraflexible magnetic actuators fabricated by this 3D printing strategy show broad prospects in the fields of soft robotics and bionics.

Towards 4D Printing of Very Soft Heterogeneous Magnetoactive Layers for Morphing Surface Applications via Liquid Additive Manufacturing

This work explores the use of liquid additive manufacturing (LAM) to print heterogeneous magnetoactive layers. A general method is proposed where, by studying the printing of pure silicone lines, the successful printing of closed shapes, open shapes, and a combination thereof, can be achieved while accounting for the continuous deposition that is specific to LAM. The results of this characterization are subsequently exploited for the printing of a heterogeneous layer composed of four magnetoactive discs embedded in a pure silicone square. Such a layer, when affixed to a softer silicone substrate, yields a system that produces truly three-dimensional surface patterns upon application of a magnetic field. Hence, this work demonstrates that LAM is a promising approach for the rapid 4D printing of morphing surfaces exhibiting 3D surface patterns that can be actuated remotely and reversibly via a magnetic field. Such heterogenous layers have a wide range of applications, ranging from haptics to camouflage to differential cell growth.

Shape-programmable magneto-active elastomer composites for curve and biomimetic behavior imitation

A programming methodology, which can be applied to soft-magnetic-material-based magneto-active elastomers (MAEs), to catch the predefined specific objective curves is proposed in this study. The objective curves have been equally separated into a couple of segments, which will be filled by the designed MAE elements. Furthermore, the designed MAE segments with different chain angles, in which the deformation orientation of each element under applied homogeneous magnetic fields has been investigated based on the designed experimental setup, are arrayed based on the proposed programming methodology to constitute the MAE composite to catch the orientation of the objective curve. The experimental results show that based on the proposed programming methodology, the MAE composites can describe different curves, which include harmonic, tangential and arc tangential functions under applied homogeneous magnetic fields with good agreement. Furthermore, on the basis of the proposed programming methodology, the MAE composites are utilized to mimic the typical biomimetic behavior (the peeking-up behavior of snakes and the flapping behavior of birds) with smooth curvature properties, in which the dynamic procedures present continuous curves.

3D/4D Printing of Polymers: Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA)

Additive manufacturing (AM) or 3D printing is a digital manufacturing process and offers virtually limitless opportunities to develop structures/objects by tailoring material composition, processing conditions, and geometry technically at every point in an object. In this review, we present three different early adopted, however, widely used, polymer-based 3D printing processes; fused deposition modelling (FDM), selective laser sintering (SLS), and stereolithography (SLA) to create polymeric parts. The main aim of this review is to offer a comparative overview by correlating polymer material-process-properties for three different 3D printing techniques. Moreover, the advanced material-process requirements towards 4D printing via these print methods taking an example of magneto-active polymers is covered. Overall, this review highlights different aspects of these printing methods and serves as a guide to select a suitable print material and 3D print technique for the targeted polymeric material-based applications and also discusses the implementation practices towards 4D printing of polymer-based systems with a current state-of-the-art approach.