Ergonomic Insect Headgear and Abdominal Buckle with Surface Stimulators Manufactured via Multimaterial 3D Printing: Snap-and-Secure Installation of Noninvasive Sensory Stimulators for Cyborg Insects

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.

Cyborg insects have attracted great attention as the flight performance they have is incomparable by micro aerial vehicles and play a critical role in supporting extensive applications. Approaches to construct cyborg insects consist of two major issues: 1) the stimulating paradigm and 2) the control policy. At present, most cyborg insects are constructed based on invasive methods, requiring the implantation of electrodes into neural or muscle systems, which would harm the insects. As the control policy is basically manual control, the shortcomings of which lie in the requirement of excessive amount of experiments and focused attention. This paper presents the design and implementation of a noninvasive and much safer cyborg insect system based on visual stimulation. The tethered paradigm is adopted here and we look at controlling the flight behavior of bumblebees, especially the abdominal-waving behavior, in the context of a model-free reinforcement learning problem. The problem is formulated as a finite and deterministic Markov decision process, where the agent is designed to change the abdominal-waving behavior from the initial state to the target state. Sarsa with transformed reward function which can speed up the learning process is employed to learn the optimal control policy. Learned policies are compared to the stochastic one by evaluating the results of ten bumblebees, demonstrating that abdominal-waving state can be modulated to approximate the target state quickly with small deviation.

Multi-scale and multi-material 3D printing is new frontier in additive manufacturing. 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 marco-scale, embedded components and electro-circuit, 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 material/structure, wearable devices, soft robot, functionally embedded electronics, metamaterial, multi-functionality product, etc. However, very few of the established additive manufacturing processes have now the capability to implement the multi-material and multi-scale 3D printing. This paper presented a single nozzle-based multi-scale and multi-material 3D printing process by integrating the electrohydrodynamic jet (E-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 macro-scale external geometry and micro-scale 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 multi-scale and multi-material 3D printing at low cost.

Photolithographic patterning of components and integrated circuits based on active polymers for microfluidics is challenging and not always efficient on a laboratory scale using the traditional mask-based fabrication procedures. Here, we present an alternative manufacturing process based on multi-material 3D printing that can be used to print various active polymers in microfluidic structures that act as microvalves on large-area substrates efficiently in terms of processing time and consumption of active materials with a single machine. Based on the examples of two chemofluidic valve types, hydrogel-based closing valves and PEG-based opening valves, the respective printing procedures, essential influencing variables and special features are discussed, and the components are characterized with regard to their properties and tolerances. The functionality of the concept is demonstrated by a specific chemofluidic chip which automates an analysis procedure typical of clinical chemistry and laboratory medicine. Multi-material 3D printing allows active-material devices to be produced on chip substrates with tolerances comparable to photolithography but is faster and very flexible for small quantities of up to about 50 chips.

In recent years, hydrogels have emerged as quintessential 3D printing materials. Coupled with their inherent printability, the unique mechanical properties, network structure, and biocompatibility of hydrogels make them ideal for a wide range of 3D printing applications. Also in recent years, the rise of multimaterial 3D printing, an additive manufacturing technology which involves at least two different materials in the fabrication process, has been witnessed. Advanced multimaterial 3D printing protocols have brought the field closer than ever before to a new industrial revolution. In this review, the use of hydrogels in multimaterial 3D printing is investigated. The review is sectioned by discussing three major multimaterial 3D printing methods: direct‐ink‐writing, vat‐switching, and coaxial extrusion. Subsections detail three common domains of multimaterial hydrogel 3D printing: bioprinting, 4D printing, and particle‐polymer composite printing. In the second part of the review, recent advancements in both multimaterial 3D printing hardware and hydrogel inks which are expected to steer the field in exciting new directions are explored. Finally, a case is made for coaxial extrusion and light‐responsive printers being the best choice for multimaterial hydrogel 3D printing in the long run, due to their gradient and greyscale printing capabilities.

Multimaterial (MM) 3D printing shows great potential for application in metamaterials, flexible electronics, biomedical devices and robots, since it can seamlessly integrate distinctive materials into one printed structure. Among numerous MM 3D printing technologies, digital light processing (DLP) MM 3D printing is compatible with a wide range of materials from hydrogels to ceramics, and can print MM 3D structures with high resolution, high complexity and fast speed. This paper introduces the fundamental mechanisms of DLP 3D printing, and reviews the recent advances of DLP MM 3D printing technologies with emphasis on material switching methods and material contamination issues. It also summarizes a number of typical examples of DLP MM 3D printing systems developed in the past decade, and introduces their system structures, working principles, material switching methods, residual resin removal methods, printing steps, as well as the representative structures and applications. Finally, we provide perspectives on the directions of the further development of DLP MM 3D printing technology.

3D printing a mechanically-tunable acrylate resin on a commercial DLP-SLA printer

This study explores a novel multi-material 3D printing technique for fabricating bioinspired hydrogel-Rochelle salt composites, focusing on optimizing concentration, cooling, and coating parameters to enhance material performance. The hydrogel-Rochelle salt composite is a promising material due to its lightweight, mechanical robustness, and piezoelectric properties, making it suitable for applications in sensors, medical devices, and structural materials. A series of concentration tests was conducted to determine the optimal Rochelle salt concentration for achieving efficient curing depth and exposure time. The results identified 50wt% hydrogel/50wt% Rochelle salt as the optimal concentration, providing a balanced curing profile essential for ensuring reliable layer adhesion and structural consistency. To enable controlled crystallization, a cooling process was introduced, with a cooling time of 15 minutes found to be sufficient for complete crystallization to a depth of 500 microns. Thermal imaging and microscopy confirmed the stability of the crystalline structure within the hydrogel matrix, ensuring the material’s functional integrity. Additionally, applying a coating to the printed structure significantly improved surface uniformity and durability, embedding the crystalline elements more effectively within the hydrogel matrix and enhancing the composite’s overall structural integrity. This coating process allowed the composite to withstand repeated printing cycles, facilitating the construction of layered, multi-material structures with improved mechanical and functional properties. The results highlight the importance of fine-tuning concentration, cooling time, and coating techniques to achieve optimal performance in multi-material 3D printing. By addressing these factors, the study demonstrates a reliable approach to producing hydrogel-Rochelle salt composites with high structural quality and piezoelectric functionality. This method not only enhances the material’s durability and adhesion between layers but also opens new possibilities for creating customized, multifunctional materials. The developed process holds significant promise for applications that require precise control over material properties, such as wearable electronics, medical implants, and lightweight structural components. In conclusion, this research provides valuable insights into the fabrication of hydrogel-Rochelle salt composites through advanced 3D printing techniques. The findings offer a foundation for future exploration in multi-material printing and composite fabrication, paving the way for the development of versatile materials with tailored properties for diverse applications.

Research in animal behavior is increasingly benefiting from the field of robotics, whereby robots are being continuously integrated in a number of hypothesis-driven studies. A variety of robotic fish have been designed after the morphophysiology of live fish to study social behavior. Of the current design factors limiting the mimicry of live fish, size is a critical drawback, with available robotic fish generally exceeding the size of popular fish species for laboratory experiments. Here, we present the design and testing of a novel free-swimming miniature robotic fish for animal-robot studies. The robotic fish capitalizes on recent advances in multi-material three-dimensional printing that afford the integration of a range of material properties in a single print task. This capability has been leveraged in a novel design of a robotic fish, where waterproofing and kinematic functionalities are incorporated in the robotic fish. Particle image velocimetry is leveraged to systematically examine thrust production, and independent experiments are conducted in a water tunnel to evaluate drag. This information is utilized to aid the study of the forward locomotion of the robotic fish, through reduced-order modeling and experiments. Swimming efficiency and turning maneuverability is demonstrated through target experiments. This robotic fish prototype is envisaged as a tool for animal-robot interaction studies, overcoming size limitations of current design.

Cyborg insects are living organisms combined with artificial systems, allowing flexible behavioral control while preserving biological functions. Conventional control methods often electrically stimulate sensory organs like antennae and cerci but these invasive methods can impair vital functions. This study shows a minimally invasive approach using flexible, ultra-thin electrodes on the cockroach’s abdomen, avoiding contact with primary sensory organs. Using liquid evaporation for film adhesion provides a biocompatible process with excellent adhesive strength and electrical durability. Body surface stimulating component structures formed by utilizing an insect’s natural movement showed higher stability than conventional methods. These enable effective control of both turning and straight-line movements. This minimally invasive method maintains the insect’s natural behavior while enhancing cyborg functionality, extending the potential applications.

Solar-powered interfacial evaporation offers a sustainable, low-carbon solution to freshwater scarcity. Aerogels, hydrogels, and foams are common photothermal materials, yet their isotropic 3D structures from conventional fabrication constrain performance optimization, integrated functionality, and user-defined applications. Herein, photothermal matrices are fabricated via multi-material 3D printing, precisely depositing diverse photothermal inks at designated spatial locations. Synergistic engineering of ink formulations, cation-modulated cross-linking, printing fidelity, hierarchical porosity, and matrix integration enables compositional, structural, and functional heterogeneity for high-performance solar desalination and solute separation across a broad salinity range (3.5-25%). Under 1 sun, 3D steam generators (SGs) attain the highest water evaporation rate of 17.9 kg m-2 h-1 in seawater under 2 m s-1 airflow - 10.5% higher than in freshwater and over six times that under calm air. Even in 25% brine, evaporation rates of 6.6 kg m-2 h-1 are retained. Strategic rearrangement of matrix units further produces 3D solar crystallizers (SCs) for localized salt harvesting. The work demonstrates, for the first time, the use of multi-material printing for the flexible fabrication of both SGs and SCs, delivering application-specific photothermal materials that not only enhance evaporation in seawater compared to freshwater, but also operate effectively under extreme salinity with record-level performance.

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.

Multi-material 3D printing: The relevance of materials affinity on the boundary interface performance

Three-dimensional (3D) printing methods, such as vat photopolymerization (VPP) and direct-ink-writing (DIW) processes, are known for their high-resolution and multimaterial capabilities, respectively. Here a novel hybrid 3D printing technique that combines the strengths of VPP and DIW processes to achieve multimaterial and high-resolution printing of functional structures and devices, is presented. The method involves dispensing liquid-like materials via syringes into a photocurable matrix material and subsequently using a Galvano mirror-controlled laser beam to selectively photocure the dispensed material trace or the matrix material surrounding the trace. The laser beam scanning and syringe dispensing are synchronized with a set delay to control liquid diffusion and in situ fixture. The versatility of the method is demonstrated by fabricating intricate 3D ant and wheel prototypes using various materials available for VPP and DIW technologies. The proposed photocuring-while-dispensing strategy offers advantages over conventional multimaterial 3D printing methods, such as integrating materials regardless of photocurability and viscosity, and fabricating heterogeneous structures with complex geometries and high resolution. With its principle demonstrated, this multimaterial 3D printing process will open up a wide range of potential applications with diverse functionalities and materials.

Recent advances in low-cost FDM 3D printing and a range of commercially available materials have enabled integrating different properties into a single object such as flexibility and conductivity, assisting fabrication of a wide variety of interactive devices through multi-material printing. Mechanically different materials such as rigid and flexible filament, however, display issues when adhering to each other making the object vulnerable to coming apart. In this work, we propose Multi-ttach, a low-cost technique to increase the adhesion between different materials utilizing various 3D printing parameters with three specialized geometric structures : (1) bead and (2) lattice structures that interlock layers in vertical material arrangement, and (3) stitching in horizontal material arrangement. We approach this by modifying the geometry of the interface layer at the G-code level and using processing parameters. We validate the result through mechanical testing using off-the-shelf materials and desktop printers and demonstrate the applicability through a range of existing applications that tackle the benefit of multi-material FDM 3D printing.