State of the art, challenges, and future prospects for the multi-material 3D printing of plant-based meat.

The emergence of innovative plant-based meat analogs, replicating the flavor, texture, and appearance of animal meat cuts, is deemed crucial for sustainably feeding a growing population while mitigating the environmental impact associated with livestock farming. Multi-material 3D food printing (MM3DFP) has been proposed as a potentially disruptive technology for manufacturing the next generation of plant-based meat analogs. This article provides a comprehensive review of the state of the art, addressing various aspects of 3D printing in the realm of plant-based meat. The disruptive potential of printed meat analogs is discussed with particular emphasis on protein-rich, lipid-rich, and blood-mimicking food inks. The printing parameters, printing requirements, and rheological properties at the different printing stages are addressed in detail. As food rheology plays a key role in the printing process, an appraisal of this subject is performed. Post-printing treatments are assessed based on the extent of improvement in the quality of 3D-printed plant-based meat analogs. The meat-mimicking potential is revealed through sensory attributes, such as texture and flavor. Furthermore, there has been limited research into food safety and nutrition. Economically, the 3D printing of plant-based meat analogs demonstrates significant market potential, contingent upon innovative decision-making strategies and supportive policies to enhance consumer acceptance. This review examines the current limitations of this technology and highlights opportunities for future developments.

Fabrication of Multi-Material Pneumatic Actuators and Microactuators Using Stereolithography.

Pneumatic actuators are of great interest for device miniaturization, microactuators, soft robots, biomedical engineering, and complex control systems. Recently, multi-material actuators have become of high interest to researchers due to their comprehensive range of suitable applications. Three-dimensional (3D) printing of multi-material pneumatic actuators would be the ideal way to fabricate customized actuators, but so far, this is mostly limited to deposition-based methodologies, such as fused deposition modeling (FDM) or Polyjetting. Vat-based stereolithography is one of the most relevant high-resolution 3D printing methods but is only rarely utilized in the multi-material 3D printing of materials. This study demonstrated multi-material stereolithography using combinations of materials with different Young's moduli, i.e., 0.5 MPa and 1.1 GPa, for manufacturing pneumatic actuators and microactuators with a resolution as small as 200 µm. These multi-material actuators have advantages over single-material actuators in terms of their deformation controllability and ease of assembly.

Dual-Material 3D-Printed Intestinal Model Devices with Integrated Villi-like Scaffolds.

In vitro small intestinal models aim to mimic the in vivo intestinal function and structure, including the villi architecture of the native tissue. Accurate models in a scalable format are in great demand to advance, for example, the development of orally administered pharmaceutical products. Widely used planar intestinal cell monolayers for compound screening applications fail to recapitulate the three-dimensional (3D) microstructural characteristics of the intestinal villi arrays. This study employs stereolithographic 3D printing to manufacture biocompatible hydrogel-based scaffolds with villi-like micropillar arrays of tunable dimensions in poly(ethylene glycol) diacrylates (PEGDAs). The resulting 3D-printed microstructures are demonstrated to support a month-long culture and induce apicobasal polarization of Caco-2 epithelial cell layers along the villus axis, similar to the native intestinal microenvironment. Transport analysis requires confinement of compound transport to the epithelial cell layer within a compound diffusion-closed reservoir compartment. We meet this challenge by sequential printing of PEGDAs of different molecular weights into a monolithic device, where a diffusion-open villus-structured hydrogel bottom supports the cell culture and mass transport within the confines of a diffusion-closed solid wall. As a functional demonstrator of this scalable dual-material 3D micromanufacturing technology, we show that Caco-2 cells seeded in villi-wells form a tight epithelial barrier covering the villi-like micropillars and that compound-induced challenges to the barrier integrity can be monitored by standard high-throughput analysis tools (fluorescent tracer diffusion and transepithelial electrical resistance).

Development of a Multi-Material 3D Printer for Functional Anatomic Models.

Anatomic models are important in medical education and pre-operative planning as they help students or doctors prepare for real scenarios in a risk-free way. Several experimental anatomic models were made with additive manufacturing techniques to improve geometric, radiological, or mechanical realism. However, reproducing the mechanical behavior of soft tissues remains a challenge. To solve this problem, multi-material structuring of soft and hard materials was proposed in this study, and a three-dimensional (3D) printer was built to make such structuring possible. The printer relies on extrusion to deposit certain thermoplastic and silicone rubber materials. Various objects were successfully printed for testing the feasibility of geometric features such as thin walls, infill structuring, overhangs, and multi-material interfaces. Finally, a small medical image-based ribcage model was printed as a proof of concept for anatomic model printing. The features enabled by this printer offer a promising outlook on mimicking the mechanical properties of various soft tissues.

Emerging 3D printing technologies for drug delivery devices: Current status and future perspective.

The 'one-size-fits-all' approach followed by conventional drug delivery platforms often restricts its application in pharmaceutical industry, due to the incapability of adapting to individual pharmacokinetic traits. Driven by the development of additive manufacturing (AM) technology, three-dimensional (3D) printed drug delivery medical devices have gained increasing popularity, which offers key advantages over traditional drug delivery systems. The major benefits include the ability to fabricate 3D structures with customizable design and intricate architecture, and most importantly, ease of personalized medication. Furthermore, the emergence of multi-material printing and four-dimensional (4D) printing integrates the benefits of multiple functional materials, and thus provide widespread opportunities for the advancement of personalized drug delivery devices. Despite the remarkable progress made by AM techniques, concerns related to regulatory issues, scalability and cost-effectiveness remain major hurdles. Herein, we provide an overview on the latest accomplishments in 3D printed drug delivery devices as well as major challenges and future perspectives for AM enabled dosage forms and drug delivery systems.

3D Printing in Digital Prosthetic Dentistry: An Overview of Recent Developments in Additive Manufacturing.

Popular media now often present 3D printing as a widely employed technology for the production of dental prostheses. This article aims to show, based on factual information, to what extent 3D printing can be used in dental laboratories and dental practices at present. It attempts to present a rational evaluation of todays´ applications of 3D printing technology in the context of dental restorations. In addition, the article discusses future perspectives and examines the ongoing viability of traditional dental laboratory services and manufacturing processes. It also shows which expertise is needed for the digital additive manufacturing of dental restorations.

Design of an Inkjet-Printed Rotary Bellows Actuator and Simulation of its Time-Dependent Deformation Behavior.

State-of-the-art Additive Manufacturing processes such as three-dimensional (3D) inkjet printing are capable of producing geometrically complex multi-material components with integrated elastomeric features. Researchers and engineers seeking to exploit these capabilities must handle the complex mechanical behavior of inkjet-printed elastomers and expect a lack of suitable design examples. We address these obstacles using a pneumatic actuator as an application case. First, an inkjet-printable actuator design with elastomeric bellows structures is presented. While soft robotics research has brought forward several examples of inkjet-printed linear and bending bellows actuators, the rotary actuator described here advances into the still unexplored field of additively manufactured pneumatic lightweight robots with articulated joints. Second, we demonstrate that the complex structural behavior of the actuator's elastomeric bellows structure can be predicted by Finite Element (FE) simulation. To this end, a suitable hyperviscoelastic material model was calibrated and compared to recently published models in a multiaxial-state-of-stress relaxation experiment. To verify the material model, Finite Element simulations of the actuator's deformation behavior were conducted, and the results compared to those of corresponding experiments. The simulations presented here advance the materials science of inkjet-printed elastomers by demonstrating use of a hyperviscoelastic material model for estimating the deformation behavior of a prototypic robotic component. The results obtained contribute to the long-term goal of additively manufactured and pneumatically actuated lightweight robots.

Corrigendum: Design of an Inkjet-Printed Rotary Bellows Actuator and Simulation of its Time-Dependent Deformation Behavior.

[This corrects the article DOI: 10.3389/frobt.2021.663158.].

Development of a multi-material stereolithography 3D printing device.

Additive manufacturing, or nowadays more popularly entitled as 3D printing, enables a fast realization of polymer, metal, ceramic or composite devices, which often cannot be fabricated with conventional methods. One critical issue for a continuation of this success story is the generation of multi-material devices. Whilst in fused filament fabrication or 3D InkJet printing, commercial solutions have been realized, in stereolithography only very few attempts have been seen. In this work, a comprehensive approach, covering the construction, material development, software control and multi-material printing is presented for the fabrication of structural details in the micrometer range. The work concludes with a critical evaluation and possible improvements.

Tensile Mechanical Behaviour of Multi-Polymer Sandwich Structures via Fused Deposition Modelling.

The application of single homogeneous materials produced through the fused deposition modelling (FDM) technology restricts the production of high-level multi-material components. The fabrication of a sandwich-structured specimen with different material combinations using conventional thermoplastics such as poly (lactic acid) (PLA), acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) through the filament-based extrusion process can demonstrate an improvement on its properties. This paper aims to assess among these materials, the best material sandwich-structured arrangement design, to enhance the mechanical properties of a part and to compare the results with the homogeneous materials selected. The samples were subjected to tensile testing to identify the tensile strength, elongation at break and Young's modulus of each material combination. The experimental results demonstrate that applying the PLA-ABS-PLA sandwich arrangement leads to the best mechanical properties between these materials. This study enables users to consider sandwich structure designs as an alternative to manufacturing multi-material components using conventional and low-cost materials. Future work will consider the flexural tests to identify the maximum stresses and bending forces under pressure.

Multi-material 3D bioprinting of porous constructs for cartilage regeneration.

The current gold standard for nasal reconstruction after rhinectomy or severe trauma includes transposition of autologous cartilage grafts in conjunction with coverage using an autologous skin flap. Harvesting autologous cartilage requires a major additional procedure that may create donor site morbidity. Major nasal reconstruction also requires sculpting autologous cartilages to form a cartilage framework, which is complex, highly skill-demanding and very time consuming. These limitations have prompted facial reconstructive surgeons to explore different techniques such as tissue engineered cartilage. This work explores the use of multi-material 3D bioprinting with chondrocyte-laden gelatin methacrylate (GelMA) and polycaprolactone (PCL) to fabricate constructs that can potentially be used for nasal reconstruction. In this study, we have investigated the effect of 3D manufacturing parameters including temperature, needle gauge, UV exposure time, and cell carrier formulation (GelMA) on the viability and functionality of chondrocytes in bioprinted constructs. Furthermore, we printed chondrocyte-laden GelMA and PCL into composite constructs to combine biological and mechanical properties. It was found that 20% w/v GelMA was the best concentration for the 3D bioprinting of the chondrocytes without comprising the scaffold's porous structure and cell functionality. In addition, the 3D bioprinted constructs showed neocartilage formation and similar mechanical properties to nasal alar cartilage after a 50-day culture period. Neocartilage formation was also observed in the composite constructs evidenced by the presence of glycosaminoglycans and collagen type II. This study shows the feasibility of manufacturing neocartilage using chondrocytes/GelMA/PCL 3D bioprinted porous constructs which could be applied as a method for fabricating implants for nose reconstruction.

Fracture Behavior of Bio-Inspired Functionally Graded Soft-Hard Composites Made by Multi-Material 3D Printing: The Case of Colinear Cracks.

The functional gradient is a concept often occurring in nature. This concept can be implemented in the design and fabrication of advanced materials with specific functionalities and properties. Functionally graded materials (FGMs) can effectively eliminate the interface problems in extremely hard-soft connections, and, thus, have numerous and diverse applications in high-tech industries, such as those in biomedical and aerospace fields. Here, using voxel-based multi-material additive manufacturing (AM, = 3D printing) techniques, which works on the basis of material jetting, we studied the fracture behavior of functionally graded soft-hard composites with a pre-existing crack colinear with the gradient direction. We designed, additively manufactured, and mechanically tested the two main types of functionally graded composites, namely, composites with step-wise and continuous gradients. In addition, we changed the length of the transition zone between the hard and soft materials such that it covered 5%, 25%, 50%, or 100% of the width (W) of the specimens. The results showed that except for the fracture strain, the fracture properties of the graded specimens decreased as the length of the transition zone increased. Additionally, it was found that specimens with abrupt hard-soft transitions have significantly better fracture properties than those with continuous gradients. Among the composites with gradients, those with step-wise gradients showed a slightly better fracture resistance compared to those with continuous gradients. In contrast, FGMs with continuous gradients showed higher values of elastic stiffness and fracture energy, which makes each gradient function suitable for different loading scenarios. Moreover, regardless of the gradient function used in the design of the specimens, decreasing the length of the transition zone from 100%W to 5%W increased the fracture resistance of FGMs. We discuss the important underlying fracture mechanisms using data collected from digital image correlation (DIC), digital image microscopy, and scanning electron microscopy (SEM), which were used to analyze the fracture surface.

A Fully Multi-Material Three-Dimensional Printed Soft Gripper with Variable Stiffness for Robust Grasping.

Multi-material three-dimensional (3D) printing has provided the possibility of direct 3D printing of soft actuators with high complexity and functionality in a fast and easy fabrication process. In this article, we present the design of a multi-material 3D printed variable stiffness soft robotic gripper to ensure grasping robustness during high acceleration. The proposed gripper contains two identical soft fingers, with each finger including a pneumatic actuator and an integrated layer jamming unit. Prototypes of the soft finger, with material hardness transfer from the soft-bodied actuator to the hard pneumatic tubing and layer jamming unit, are fully fabricated by one-step 3D printing. A multi-material 3D printer, Objet350Connex, is used to directly print out the whole finger without the need for an additional casting process. The printed soft finger has a complex inner geometry, which integrates a small, light, and flexible layer jamming unit. The proposed finger can freely deform at low stiffness and maintain its grasping robustness at high stiffness during high acceleration. To demonstrate the effectiveness of the proposed design, the gripper is mounted on a robotic arm to evaluate its grasping robustness. With the aid of the integrated layer jamming unit, grasping robustness can be guaranteed when the robotic arm is moving at acceleration up to 8 m/s2. The results show that the proposed soft gripper is an effective design, which can guarantee grasping robustness during high acceleration.

PolyJet-Printed Bellows Actuators: Design, Structural Optimization, and Experimental Investigation.

Pneumatic bellows actuators are exceptionally suitable for Additive Manufacturing (AM) as the required geometrical complexity can easily be obtained and their functionality is not affected by rough surfaces and small dimensional accuracy. This paper is an extended version of a previously published contribution to the RoboSoft2018 conference in Livorno, Italy. The original paper (Dämmer et al., 2018) contains a simulation-driven design approach as well as experimental investigations of the structural and fatigue behavior of pneumatic multi-material PolyJet™ bellows actuators. This extended version is enhanced with investigations on the relaxation behavior of PolyJet bellows actuators. The presented results are useful for researchers and engineers considering the application of PolyJet bellows actuators for pneumatic robots.

A Biologically Inspired, Functionally Graded End Effector for Soft Robotics Applications.

Soft robotic actuators offer many advantages over their rigid counterparts, but they often are unable to apply highly localized point loads. In contrast, many invertebrates have not only evolved extremely strong "hybrid appendages" that are composed of rigid ends that can grasp, puncture, and anchor into solid substrates, but they also are compliant and resilient, owing to the functionally graded architecture that integrates rigid termini with their flexible and highly extensible soft musculatures. Inspired by the design principles of these natural hybrid appendages, we demonstrate a synthetic hybrid end effector for soft-bodied robots that exhibits excellent piercing abilities. Through the incorporation of functionally graded interfaces, this design strategy minimizes stress concentrations at the junctions adjoining the fully rigid and soft components and optimizes the bending stiffness to effectively penetrate objects without interfacial failure under shear and compressive loading regimes. In this composite architecture, the radially aligned tooth-like elements apply balanced loads to maximize puncturing ability, resulting in the coordinated fracture of an object of interest.

A methodology for developing anisotropic AAA phantoms via additive manufacturing.

An Abdominal Aortic Aneurysm (AAA) is a permanent focal dilatation of the abdominal aorta at least 1.5 times its normal diameter. The criterion of maximum diameter is still used in clinical practice, although numerical studies have demonstrated the importance of biomechanical factors for rupture risk assessment. AAA phantoms could be used for experimental validation of the numerical studies and for pre-intervention testing of endovascular grafts. We have applied multi-material 3D printing technology to manufacture idealized AAA phantoms with anisotropic mechanical behavior. Different composites were fabricated and the phantom specimens were characterized by biaxial tensile tests while using a constitutive model to fit the experimental data. One composite was chosen to manufacture the phantom based on having the same mechanical properties as those reported in the literature for human AAA tissue; the strain energy and anisotropic index were compared to make this choice. The materials for the matrix and fibers of the selected composite are, respectively, the digital materials FLX9940 and FLX9960 developed by Stratasys. The fiber proportion for the composite is equal to 0.15. The differences between the composite behavior and the AAA tissue are small, with a small difference in the strain energy (0.4%) and a maximum difference of 12.4% in the peak Green strain ratio. This work represents a step forward in the application of 3D printing technology for the manufacturing of AAA phantoms with anisotropic mechanical behavior.