Soft Actuator with Integrated and Localized Sensing Properties through Parameter-Encoded 4D Printing

6 - 4D printing of autonomously controlled soft actuators for tremor vibration suppression

Four-dimensional printing has set the stage for a new generation of soft robotics. The applications of rigid planar parallel robotic manipulators are also significant because of their various desirable characteristics, such as lower inertia, higher payload, and high accuracy. However, rigid planar parallel robots are heavy and require different actuators and components. This study introduces a novel technique to produce a light three degrees of freedom soft parallel manipulator at a low cost, which can be stimulated easily. This technique allows researchers to customize the actuator’s design based on the requirement. The robot is made by 3D printing based on fused deposition modelling and a direct ink writing process. The design, development, and additive manufacturing of a soft parallel robot electrothermally driven by a linear silicon-based actuator and polylactic acid parts are presented. Silicon-based soft actuators replace the rigid conventional linear actuators in this study to drive the planar parallel manipulator. The actuation of actuators is conducted using simple heating compared to the conventional rigid actuator. Various heating approaches and configurations are compared and analysed to find the most suitable one for the effective linear stroke of the soft actuator. The finite element model is used to analyse the performance of the electrothermally silicon-ethanol soft actuators in ABAQUS. The kinematics of the planar parallel robotic manipulator are simulated in MATLAB to achieve its workspace. The final soft parallel robot mechanism and the active and passive links are fabricated and tested experimentally.

Controls over stimuli-responsive functional materials and programmable shape deformations are key features in the four-dimensional (4D) printing of soft actuators. Instead of using random copolymers, homopolymers, or natural polymers, this paper reports the first use of amphiphilic, photocurable, and pH-responsive block copolymer (BCP) nanoassemblies in digital light processing (DLP) 4D printing to fabricate smart and programmable soft actuators. Programmable actuation was studied via a bottom-up approach: (1) designed synthesis of pH-responsive BCPs, (2) nanoassembly structures of BCPs, and (3) networking of nanoassemblies via the photocuring process in DLP. As a proof-of-concept, bilayered grippers, ring-shaped actuators, and octopus-like actuators were programmed to produce a range of bending angles and actuation patterns. pH-responsive BCP nanoassemblies were also combined with commercially available three-dimensional printing liquid resin (PlasClear) to produce stimulus-responsive printing ink that was successfully used for 4D printing applications. Thus, smart and programmable materials were developed for 4D printing applications.

4D printing soft robots guided by machine learning and finite element models

Four-dimensional (4D) printing technology, as a next-generation additive manufacturing method, enables printed objects to further change their shapes, functionalities, or properties upon exposure to external stimuli. The 4D printing of programmable and deformable materials such as thermo-responsive shape-memory polymers (trSMPs), which possess the ability to change shape by exposure to heat, has attracted particular interest in recent years. Three-dimensional objects based on SMPs have been proposed for various potential applications in different fields, including soft robotics, smart actuators, biomedical and electronics. To enable the manufacturing of complex multifunctional 3D objects, SMPs are often coupled with other functional polymers or fillers during or before the 3D printing process. This review highlights the 4D printing of state-of-the-art multi-component SMP formulations. Commonly used 4D printing technologies such as material extrusion techniques including fused filament fabrication (FFF) and direct ink writing (DIW), as well as vat photopolymerization techniques such as stereolithography (SLA), digital light processing (DLP), and multi-photon polymerization (MPP), are discussed. Different multicomponent SMP systems, their actuation methods, and potential applications of the 3D printed objects are reviewed. Finally, current challenges and prospects for 4D printing technology are summarized.

During the last years, great progress was made in material science in terms of concept, design and fabrication of new composite materials with conferred properties and desired functionalities. The scientific community paid particular interest to active soft materials, such as soft actuators, for their potential as transducers responding to various stimuli aiming to produce mechanical work. Inspired by this, materials engineers today are developing multidisciplinary approaches to produce new active matters, focusing on the kinematics allowed by the material itself more than on the possibilities offered by its design. Traditionally, more complex motions beyond pure elongation and bending are addressed by the robotics community. The present review targets encompassing and rationalizing a framework which will help a wider scientific audience to understand, sort and design future soft actuators and methods enabling complex motions. Special attention is devoted to recent progress in developing innovative stimulus-responsive materials and approaches for complex motion programming for soft robotics. In this context, a challenging overview of the new materials as well as their classification and comparison (performances and characteristics) are proposed. In addition, the great potential of soft transducers are outlined in terms of kinematic capabilities, illustrated by the related application. Guidelines are provided to design actuators and to integrate asymmetry enabling motions along any of the six basic degrees of freedom (translations and rotations), and strategies towards the programming of more complex motions are discussed. As a final note, a series of manufacturing methods are described and compared, from molding to 3D and 4D printing. The review ends with a Perspectives section, from material science and microrobotic points of view, on the soft materials’ future and close future challenges to be overcome.

The goal of the robotics industry has been to build devices that mirror the remarkable powers of the human body ever since it was founded. In an attempt to emulate the compliance and deformability of genuine biological tissue, efforts have been made to construct actuators and electronics out of elastomers, textiles, and other soft materials as early as the 1940s. Since then, the field of soft robotics has advanced extraordinarily, with recent work concentrating on actuation, sensing, and application. In this review, recent advancement in the soft robotically field from the perspective of soft actuators, soft material, and biomedical application is highlighted. Soft actuators had gone decent development along with different advantages and setbacks. The need for chemistry and material science has been increasingly large in recent years and the material for 4D printing manufacturing is a major focus. Biomedical applications can be advanced by introducing soft robotics combined with existing technology. The biocompatibility issue is a major challenge to be faced, and perspectives on such regard will be discussed.

In the contemporary era, novel manufacturing technologies like additive manufacturing (AM) have revolutionized the different engineering sectors including biomedical, aerospace, electronics, etc. Four-dimensional (4D) printing aka AM of smart materials is gaining popularity among the scientific community, which has the excellent ability to make soft structures such as soft robots, actuators, and grippers. These soft structures are developed by applying various stimuli such as pH, temperature, magnetic field, and many combinations onto soft materials. Stimuli in 3D printing permit various shape-morphing behaviors such as bending, twisting, folding, swelling, rolling, shrinking, origami, or locomotion. A wide variety of soft magnetic structures can be fabricated through the incorporation of soft or hard magnetic particles into soft materials resulting in magneto-active soft materials (MASMs). With this integration, magneto-thermal coupling actuation allows diverse magneto-deformations, facilitating the development of personalized devices that are capable of enhanced deformation. In this review, guidelines are provided on the 3D printing for MASMs such as magneto-active polymers (MAPs), magneto-active composites, and magneto-active hydrogels (MAHs) on the booming development of various smart and flexible devices such as soft robots, wearable electronics, and biomimetic devices. Moreover, 3D-printed soft robotics have an outstanding capacity to adapt to complicated situations for many advanced actuating applications. Finally, some current challenges and emerging areas in this exciting technology have been proposed. Lastly, it is anticipated that technological advancements in developing smart and intelligent magneto-active structures will have a significant impact on the design of real-world applications.

One of the avenues for the development of functional gradient additive manufacturing is the creation of four-dimensional (4D) printed structures for soft robotic gripping, achieved by combining fused deposition modeling 3D printing with soft hydrogel actuators. This work proposes a conceptual approach to creating an energy-independent soft robotic gripper, consisting of a modified 3D printed holder substrate made from thermoplastic polyurethane (TPU) and an actuator based on a gelatin hydrogel, allowing programmed hygroscopic deformation without using complex mechanical constructions. The use of a 20% gelatin-based hydrogel imparts soft robotic biomimetic functionality to the structure and is responsible for the intelligent stimulus-responsive mechanical functionality of the printed object by responding to swelling processes in liquid environments. The targeted surface functionalization of thermoplastic polyurethane in an argon–oxygen environment for 90 s, at a power of 100 W and a pressure of 26.7 Pa, facilitates changes in its microrelief, thus improving the adhesion and stability of the swollen gelatin on its surface. In this work, the morphological (scanning electron microscopy and atomic force microscopy), colloidal, chemical (FTIR), physicochemical (swelling), biological (cell biocompatibility), and performance evaluations of the TPU/gelatin gripper were studied. The realized concept of creating 4D printed biocompatible comb structures for macroscopic underwater soft robotic gripping can provide noninvasive local gripping, transport small objects, and release bioactive substances upon swelling in water. The resulting product can therefore be used as a self-powered biomimetic actuator, an encapsulation system, or soft robotics.

Over the last few years, 4D printing of liquid crystal elastomers (LCEs) has been explored to develop actuators with significant, reversible, and anisotropic shape changes upon external stimuli such as heat or light. Most reported ink formulations rely on photo-curable liquid crystal materials derived from high molecular weight (MW) main-chain macromers. Their rheological properties allow to program molecular alignment during direct ink writing. In contrast, formulations composed mainly of monoacrylate or diacrylate low MW mesogens are unsuitable for extrusion-based 4D printing as they fail to promote mesogen alignment during printing. This shortcoming restricts the range of chemical and physical compositions accessible to 4D printing. Here, we introduce a practical route for formulating 4D printable inks that integrate low MW mesogens. By adding an acrylate-ended macromer to the low MW constituents, these mixtures become suitable for direct ink writing. This enables the preparation of multi-functional actuators with digitally controlled director morphology and tailored properties, for example, stiffness and actuation strain, as well as light-responsive behavior when photoactive molecules are used. These results highlight the versatility of the method and suggest its potential compatibility with other functional low MW units, such as those responsive to electrical or pH stimuli.

Fiber-reinforced soft composites (FrSCs) are seeing increasing use in applications involving soft actuators, four-dimensional printing, biomimetic composites, and embedded sensing. The three-dimensional (3D) printing of FrSCs is a layer-by-layer material deposition process that alternates between inkjet deposition of an ultraviolet (UV) curable polymer layer and the stamping of electrospun fibers onto the layer, to build the final part. While this process has been proven for complex 3D geometries, it suffers from poor fiber transfer efficiencies (FTEs) that affect the eventual fiber content in the printed part. In order to address this issue, it is critical to first understand the mechanics of the fiber transfer process. To this end, the objective of this paper is to develop a cohesive zone-based finite element model that captures the competition between the “fiber–carrier substrate” adhesion and the “fiber–polymer matrix” adhesion, encountered during the stamping process used for 3D printing FrSCs. The cohesive zone model (CZM) parameters are first calibrated using independent microscale fiber peeling experiments involving both the thin-film aluminum carrier substrate and the UV curable polymer matrix. The predictions of the calibrated model are then validated using fiber transfer experiments. The model parametric studies suggest the use of a roller-based stamping unit design to improve the FTE of the FrSC 3D printing process. Preliminary experiments confirm that for a 0.5 in diameter roller, this new design can increase the FTE to ∼97%, which is a substantial increase from the 55% efficiency value seen for the original flat-plate stamping platen design. The model has broader applications for the transfer-printing of soft material constructs at the submicron scale.

Bi-Directional Deformation, Stiffness-Tunable, and Electrically Controlled Soft Actuators Based on LCEs 4D Printing

Untethered soft actuators that operate without physical connections to external power sources or control system cables are an active area of research for their potential applications in human-robot interaction, haptic devices, and 4D printing due to their wide range of motion and high degrees of freedom. Among these actuators, liquid-to-gas phase change actuators have a simple structure and can generate higher output force using photothermal conversion. In this study, we propose a novel control method for untethered soft actuators by dyeing the low-boiling-point liquids used in liquid-to-gas phase change actuators. The dyed low-boiling-point liquids exhibit different light absorption spectra matching their color, enabling wavelength-selective actuation using light sources with corresponding wavelengths. We evaluated and discussed this control method in terms of the relationship between irradiance and output, actuation speed, and continuous operation. Additionally, we prototyped a robotic arm to demonstrate selective motion according to the light color.