Additive manufacturing for structuring stimuli-responsive materials

Despite great progress in four-dimensional (4D) printing, i.e. three-dimensional (3D) printing of active (stimuli-responsive) materials, the relatively low actuation force of the 4D printed structures often impedes their engineering applications. In this study, we use multimaterial inkjet 3D printing technology to fabricate shape memory structures, including a morphing wing flap and a deployable structure, which consist of active and flexible hinges joining rigid (non-active) parts. The active hinges, printed from a shape memory polymer (SMP), lock the structure into a second temporary shape during a thermomechanical programming process, while the flexible hinges, printed from an elastomer, effectively increase the actuation force and the load-bearing capacity of the printed structure as reflected in the recovery ratio. A broad range of mechanical properties such as modulus and failure strain can be achieved for both active and flexible hinges by varying the composition of the two base materials, i.e. the SMP and the elastomer, to accommodate large deformation induced during programming step, and enhance the recovery in the actuating step. To find the important design parameters, including local deformation, shape fixity and recovery ratio, we conduct high fidelity finite element simulations, which are able to accurately predict the nonlinear deformation of the printed structures. In addition, a coupled thermal-electrical finite element analysis was performed to model the heat transfer within the active hinges during the localized Joule heating process. The model predictions showed good agreement with the measured temperature data and were used to find the major parameters affecting temperature distribution including the applied voltage and the convection rate.

From Responsive Molecules to Interactive Materials.

In the Fused Filament Fabrication (FFF/FDM) technology, the multi-material manufacturing additive method is achieved by a single nozzle or multiple nozzles working simultaneously with different materials. However, the adhesion between different materials at the boundary interface in FDM multi-material printing is a limiting factor. These studies are concerned with improving and study the adhesion between two polymers.Due to the numerous applications and possibilities of 3D printed objects, combining different materials has become a subject of interest. PLA is an alternative to the use of petrochemical-based polymers. Thermoplastic Polyurethane is a flexible material that can achieve different characteristics when combined with a rigid filament, such as PLA. To improve the adhesion between PLA and TPU in multi-material FFF/FDM, we propose the comparison of different processes: post-processing with acetone immersion, surface activation during printing with Acetone, surface activation during printing with tetrahydrofuran, post-processing annealing, and connection of printed parts with tetrahydrofuran.Modifying the 3D printing process improved the quality of the adhesive bond between the two different polymers. Activation of the surface with THF is the treatment method recommended by the authors due to the low impact on the deformation/degradation of the object.In the study, adhesion was considered in relation to the circular pattern of surface development. Further analysis should include other surface development patterns and changes in printing parameters, e.g. process temperatures and layer application speed.3D printing with multi-materials, such as PLA biopolymer and thermoplastic polyurethane, allows for the creation of flexible connections. The strengthening of the biopolymer broadens the possibilities of using polylactide. Examples of applications include: automotive (elements, where flexible TPU absorbs vibrations and protects PLA from cracking), medicine (prostheses with flexible elements ensuring mobility in the joints).Multi-material printing is a new trend in 3D printing research, and this research is aimed at promoting the use and expanding the possibilities of using PLA biopolymer.

Multiscale and multimaterial three-dimensional (3D) printing is new frontier in additive manufacturing (AM). It has shown great potential to implement the simultaneous and full control for fabricated object including external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano-, micro-, to macro-scale, embedded components and electrocircuit, etc. Furthermore, it has the ability to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. That paves the way and may result in great breakthrough in various applications, e.g., functional tissue and organ, functionally graded (FG) material/structure, wearable devices, soft robot, functionally embedded electronics, metamaterial, multifunctionality product, etc. However, very few of the established AM processes have now the capability to implement the multimaterial and multiscale 3D printing. This paper presented a single nozzle-based multiscale and multimaterial 3D printing process by integrating the electrohydrodynamic jet printing and the active mixing multimaterial nozzle. The proposed AM technology has the capability to create multifunctional heterogeneously structured objects with control of the macroscale external geometry and microscale internal structures as well as functional surface features, particularly, the potential to dynamically mix, grade, and vary the ratios of different materials. An active mixing nozzle, as a core functional component of the 3D printer, is systematically investigated by combining with the theoretical analysis, numerical simulation, and experimental verification. The study aims at exploring a feasible solution to implement the multiscale and multimaterial 3D printing at low cost.

We present development of multi-material Fused deposition modeling (FDM) 3D printer. Most of the FDM type of 3d printers are able to print plastics such as ABS, PLA and Nylon in a single run. In order to overcome this problem, we have developed an affordable multi-material 3D printer that can print an object with a different kind of soft and hard material in a single run. The combination of printing soft and hard materials in a single run will open the doors to various applications such as soft robotics, actuators and sensors. One of the main printing materials for this printer is the conductive based soft filament made by our laboratory "Soft & Wet Matter Engineering Laboratory", along with other different soft filament and hard plastic filament to create a fully functional, multi-material printer to print objects in a single printing run with greater diversity and lower cost than any other single material printer. We have developed a single screw-based extruder 3D printer extruder type 3D printer that can print both soft and hard materials. Instead of using a filament to 3D print objects, we have used pellets of different materials, including carbon-based conductive composites. Such type of single screw base extruder 3D printer will be a new addition to the additive manufacturing process.

The advancement of four-dimensional (4D) printing has been fueled by the rise in demand for additive manufacturing and the expansion in shape-memory materials. The printing of smart substances that respond to external stimuli is known as 4D printing. 4D printing allows highly controlled shapes to simulate the physiological milieu by adding time dimensions. The 4D printing is suitable with current progress in smart compounds, printers, and its mechanism of action. The 4D printing paradigm, a revolutionary enhancement of 3D printing, was anticipated by various engineering disciplines. Tissue engineering, medicinal, consumer items, aerospace, and organ engineering use 4D printing technology. The current review mainly focuses on the basics of 4D printing and the methods used therein. It also discusses the time-dependent behavior of stimulus-sensitive compounds, which are widely used in 4D printing. In addition, this review highlights material aspects, specifically related to shape-memory polymers, stimuli-responsive materials (classified as physical, chemical, and biological), and modified materials, the backbone of 4D printing technology. Finally, potential applications of 4D printing in the biomedical sector are also discussed with challenges and future perspectives.

Printed silicone pneumatic actuators for soft robotics

In these times of Industrial 4.0 and Health 4.0, people currently want to enhance the ability of science and technology, to focus on patient aspects. However, with intelligent, green energy and biomedicine these days, traditional three-dimensional (3D) printing technology has been unable to meet our needs, so 4D printing has now arisen. In this research, a shape-memory composite material with 3D printing technology was used for 4D printing technology. The authors used fused deposition modeling (FDM) to print a polylactic acid (PLA) strip onto the surface of paper to create a shape-memory composite material, and a stimulus (heat) was used to deform and recover the shape of this material. The deformation angle and recovery angle of the material were studied with various processing parameters (heating temperature, heating time, pitch, and printing speed). This research discusses optimal processing related to shape-memory situations of stimulus-responsive composite materials. The optimal deformation angle (maximum) of the stimulus-responsive composite material was found with a thermal stimulus for an optimal heating temperature of 190 °C, a heating time of 20 s, a pitch of 1.5 mm, and a printing speed of 80 mm/s. The optimal recovery angle (minimum) of this material was found with a thermal stimulus for an optimal heating temperature of 170 °C, a heating time of 90 s, a pitch of 2.0 mm, and a printing speed of 80 mm/s. The most important factor affecting both the deformation and recovery angle of the stimulus-responsive composite material was the heating temperature.

Multi-material vat photopolymerization (MMVPP) is an emerging additive manufacturing technology with great potential in biomedical engineering, soft robotics, electronics, and customized manufacturing, particularly where functional gradients and spatially varying material properties are essential. The capability to precisely control composition at the voxel level enables the fabrication of bioinspired structures and multifunctional components with tailored mechanical and functional performance. This study presents the conceptual design and development of a compact and commercially viable MMVPP system that addresses key challenges in current state-of-the-art technologies. A two-vat prototype printer was fabricated to demonstrate the feasibility of precise multi-material printing. Critical challenges, including resin compatibility, interfacial adhesion, and mechanical property optimization, were systematically investigated. Novel strategies such as variable layer height exposure control, overlapped layer printing, and optimized curing parameters were introduced to improve interfacial bonding and overall structural integrity. The proposed methods were validated through mechanical testing, confirming enhanced interface strength and material cohesion. The study also details system-level innovations, including efficient vat-switching mechanisms and process synchronization for rapid material transition. These advances establish foundational methodologies for reliable multi-material photopolymerization and expand the design space of photopolymer-based additive manufacturing. The results demonstrate MMVPP’s transformative potential to enable next-generation manufacturing of functionally graded and multi-material components with superior performance and design freedom.  

Stimuli-responsive materials are smart materials capable to adjust their properties (e.g. shape and size) in response to an external or internal stimulus including temperature, light, magnetic field, mechanical stress, and pH. Shape memory polymers (SMPs) are attractive class of stimuli-responsive, programmable smart materials with shape-memory behavior: memorize their original, permanent shape; deform and fix into a secondary, temporary shape; and recover the original, permanent shape upon applying an external stimulus. In recent years, advancing intelligent supercapacitors and batteries have attracted significant attention whereas stimuli-responsive materials were integrated to induce stimuli-responsive properties to introduce new properties or mitigate some of the existing issues such as thermal management, mitigating volume change-induced performance degradation, recovery from a mechanical deformation, self-healing, electrochromism. Indeed, integrating energy storage technologies with a variety of stimuli are a fascinating method to realize smart functions [1-3].Herein, we aim to develop a heat-responsive battery capable of mitigating the impact of incidental mechanical abuse or deformation. To achieve this, a shape-memory solid polymer electrolyte (SMSPE) is engineered and incorporated into lithium metal batteries (LMBs). In response to an external heat, the engineered SMSPEs recover their original shape and size from mechanical deformations such as bending and elongation. This study opens-up a promising approach for engineering intelligent batteries responsive to unfavorable internal or external stimuluses, with potential to have a broad impact on other energy storage technologies in different sizes and shapes. The Li|SPSPEs|LFP full cell batteries display a long-term cycle life with a discharge capacity of ~120 mAh g−1 at C/10 in ambient temperature with around 90% capacity retention after 100 cycles. Moreover, the Li|SPSPEs|Li symmetrical cells exhibit a long-term stability with a low voltage polarization at a current density of 0.05 mAh cm−2 for up to 1000 cycles.Overall, the proposed approach shows a promising design principle which can open up new possibilities for the next-generation batteries and their safe operation. With vast range of available SMPs with tunable chemistries, transition temperatures, and high shape-memory performance, and opens up new frameworks to address a variety of detrimental, unresolved issues such as thermal runaway, with potential expansion to the other energy storage technologies.

The growing interest in soft robots comes from the new possibilities offered by these systems to cope with problems that cannot be addressed by robots built from rigid bodies. Many innovative solutions have been developed in recent years to design soft components and systems. They all demonstrate how soft robotics development is closely dependent on advanced manufacturing processes. This review aims at giving an insight on the current state of the art in soft robotics manufacturing. It first puts in light the elementary components that can be used to develop soft actuators, whether they use fluids, shape memory alloys, electro-active polymers or stimuli-responsive materials. Other types of elementary components, such as soft smart structures or soft-rigid hybrid systems, are then presented. The second part of this review deals with the manufacturing methods used to build complete soft structures. It includes molding, with possibly reinforcements and inclusions, additive manufacturing, thin-film manufacturing, shape deposition manufacturing, and bonding. The paper conclusions sums up the pros and cons of the presented techniques, and open to developing topics such as design methods for soft robotics and sensing technologies.

Concentrated on additive manufacturing, this chapter discusses materials research in the Manufacturing 4.0 context. In addition to providing a brief review on stimuli-responsive materials and their applications under Manufacturing 4.0, this chapter also looks ahead and discusses in detail the recent developments in additive manufacturing of machines, particularly machines based on the special characteristics of certain stimuli-responsive materials, such as thermally activated shape memory alloys, magnetic shape memory alloys and magnetocaloric materials. Furthermore, this chapter emphasizes possible future directions for the additive manufacturing of metallic materials in general.

Additive manufacturing (AM), commonly known as 3D printing, has emerged as a transformative technology with profound implications for multiple industries. The convergence of AM with Industry 4.0 principles and advanced technologies has given rise to Industry 5.0, a new era of manufacturing characterized by enhanced integration and digitalization. This chapter explores the dynamic landscape of AM within the context of Industry 5.0, highlighting research trends, innovations, and challenges. Key developments include materials advancements, multi-material printing, digital twins, bioprinting, AI-driven design, and sustainability initiatives. Industry 5.0's impact is felt globally, with applications spanning aerospace, healthcare, fashion, and beyond. Collaboration between academia and industry, regulatory frameworks, and the pursuit of sustainable practices are driving forces shaping the future of AM in Industry 5.0.

Millions of years’ evolution has imparted life on earth with excellent environment adaptability. Of particular interest to scientists are some plants capable of macroscopically and reversibly altering their morphological and mechanical properties in response to external stimuli from the surrounding environment. These intriguing natural phenomena and underlying actuation mechanisms have provided important design guidance and principles for man-made soft robotic systems. Constructing bio-inspired soft robotic systems with effective actuation requires the efficient supply of mechanical energy generated from external inputs, such as temperature, light, and electricity. By combining bio-inspired designs with stimuli-responsive materials, various intelligent soft robotic systems that demonstrate promising and exciting results have been developed. As one of the building materials for soft robotics, hydrogels are gaining increasing attention owing to their advantageous properties, such as ultra-tunable modulus, high compliance, varying stimuli-responsiveness, good biocompatibility, and high transparency. In this review article, we summarize the recent progress on plant-inspired soft robotics assembled by stimuli-responsive hydrogels with a particular focus on their actuation mechanisms, fabrication, and application. Meanwhile, some critical challenges and problems associated with current hydrogel-based soft robotics are briefly introduced, and possible solutions are proposed. We expect that this review would provide elementary tutorial guidelines to audiences who are interested in the study on nature-inspired soft robotics, especially hydrogel-based intelligent soft robotic systems.

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.