Porous titanium alloys have been extensively used for diverse engineering applications. However, current additive manufacturing (AM) strategies face significant challenges (e.g., low fabrication efficiency and limited shape diversity) in producing porous titanium alloys. This work aims to develop a distance-controlled direct ink writing (DC-DIW) approach for constructing macroscale 3D architectures from titanium alloy powders. This approach integrates a constant interlayer distance control with traditional DIW, breaking through the angle limit in current porous metal printing from 60° to 30°. Additionally, subsequent heat treatment is applied to control microstructures. To demonstrate the capabilities of this approach, three representative structures, including a bifurcated tube, an orbital implant, and a knee implant, are successfully printed and treated, achieving suitable mechanical properties and high shape fidelity. This work provides a viable and efficient AM strategy for fabricating porous titanium alloys with enhanced shape diversity and controllable porosity suitable for various engineering applications.

Lung cancer is a serious global health issue that requires the development of patient-specific, lung cancer model for surgical planning to train interventionalists and improve the accuracy of biopsies. Although the emergence of three-dimensional (3D) printing provides a promising solution to create customized models with complicated architectures, current 3D printing methods cannot accurately duplicate anatomical-level lung constructs with tumor(s) which are applicable for hands-on training and procedure planning. To address this issue, an embedded printing strategy is proposed to create respiratory bronchioles, blood vessels, and tumors in a photocurable yield-stress matrix bath. After crosslinking, a patient-specific lung cancer analogous model is produced, which has tunable transparency and mechanical properties to mimic lung parenchyma. This engineered model not only enables the practical training of fine-needle aspiration biopsy but also provides the necessary information, such as coordinates of aspiration, wound depth, and interference with surrounding tissues, for procedure optimization.

Embedded ink writing (EIW) is an emerging 3D printing technique that is extensively applied for a variety of engineering applications. This technique is featured by printing a liquid functional ink within a yield-stress fluid bath. Due to the selection of ink and bath materials with different hydrophilicity and hydrophobicity, diverse ink-bath interactions occur that affect the filament formation in EIW. In this work, the interrelationships between ink-bath interactions and three major physical phenomena (including filament diffusion, shrinkage, and breakage) are experimentally and systematically investigated. Several key parameters, such as interfacial tension between ink and bath materials, ink's elasticity, and bath's yield stress, significantly affect the morphology and/or geometry of as-printed filaments. Based on the obtained knowledge, a functional index finger is printed, which demonstrates a good grasping capability in a robotic hand system.

Abstract Embedded ink writing (EIW) is an emerging 3D printing technique that fabricates complex 3D structures from various biomaterial inks but is limited to a printing speed of ∼10 mm s−1 due to suboptimal rheological properties of particulate‐dominated yield‐stress fluids when used as liquid baths. In this work, a particle‐hydrogel interactive system to design advanced baths with enhanced yield stress and extended thixotropic response time for realizing high‐speed EIW is developed. In this system, the interactions between particle additive and three representative polymeric hydrogels enable the resulting nanocomposites to demonstrate different rheological behaviors. Accordingly, the interaction models for the nanocomposites are established, which are subsequently validated by macroscale rheological measurements and advanced microstructure characterization techniques. Filament formation mechanisms in the particle‐hydrogel interactive baths are comprehensively investigated at high printing speeds. To demonstrate the effectiveness of the proposed high‐speed EIW method, an anatomic‐size human kidney construct is successfully printed at 110 mm s−1, which only takes ∼4 h. This work breaks the printing speed barrier in current EIW and propels the maximum printing speed by at least 10 times, providing an efficient and promising solution for organ reconstruction in the future.

Nanocomposites made from alginate and nanoclay are extensively applied for diverse biomedical applications. However, the lack of a clear understanding of the interactions between alginate and nanoclay makes it difficult to rationally design the nanocomposites for different material extrusion-based 3D bioprinting strategies. Here, a combined analytical model is proposed to accurately predict the interaction mechanisms between alginate and nanoclay through small-angle neutron scattering. These mechanisms are summarized into a phase diagram that can guide the design of alginate-nanoclay nanocomposites for different bioprinting applications. The rheological properties of various nanocomposites are measured to validate the proposed interaction mechanisms at the macroscale. Accordingly, three representative extrusion-based bioprinting strategies are linked with the nanocomposite design and applied to freeform fabricate complex structures. A roadmap is summarized to bridge the gap between biomaterial design and bioprinting processes, enabling the rapid and rational selection of biomaterial formula based on available 3D printing methods, and vice versa. A combined analytical model is proposed to accurately predict the interaction mechanisms in alginate-nanoclay nanocomposites through small-angle neutron scattering, which are validated by rheological measurements. Three bioprinting strategies are linked with the nanocomposite design and applied to fabricate complex structures. The findings enable the rapid and rational selection of biomaterial formula based on available 3D bioprinting methods, and vice versa. image

Orbital implants are necessary for reconstructing fractured orbital walls and are traditionally fabricated using titanium or polyethylene, but these materials result in medical complications such as increased risk of implant migration and hemorrhaging. Therefore, orbital implants constructed from biocompatible and biodegradable polymers have been recently researched to mitigate these risks. Material extrusion three-dimensional (3D) printing techniques, especially fused deposition modeling (FDM), can be applied to produce patient-specific orbital implants. However, current structures fabricated by FDM usually possess poor mechanical properties and high surface roughness. In this work, an embedded FDM method is designed and implemented to fabricate polycaprolactone (PCL) orbital implants with increased mechanical properties and surface morphology through the development and utilization of a temperature-stable yield-stress suspension comprised of fumed silica particles and a sunflower oil solvent. The rheological properties of the suspension were measured and tuned to produce a viable support bath material above the melting temperature of PCL. Filaments, single-layer sheets, and tensile test samples were printed to optimize the printing parameters, verify the surface morphology, and validate the mechanical properties, respectively. After that, a numerical simulation was performed to determine the mechanical robustness of the designed orbital implant model. Finally, the orbital implant was printed, measured, and implanted into a mock-up orbital socket to verify the viability of the proposed embedded FDM method.

Micro-injection molding (MiM) is a promising technique for manufacturing biodegradable polymeric vascular stents (BPVSs) at scale, in which a trapezoidal strut cross section is needed to ensure high-quality de-molding. However, there is a lack of research on the influence of the strut cross-sectional shape on its mechanical properties, posing a challenge in determining the key geometries of the strut when using MiM to produce BPVSs. Hence, this work has investigated the relationships between the geometry parameters, including the de-molding angle, and the radial support property of BPVSs using the finite element method. The results reveal that the radial stiffness of BPVSs is significantly affected by the de-molding angle, which can be counteracted by adjusting strut height, bending radius, and strut thickness. Stress distribution analysis underscores the crucial role of the curved portion of the support ring during compression, with the inner side of the curved region experiencing stress concentration. A mathematical model has been established to describe the relationships between the geometry parameters and the radial support property of the BPVSs. Notably, the radius of the neutral layer emerges as a key determinant of radial stiffness. This study is expected to serve as a guideline for the development of BPVSs that can be manufactured using MiM.

Embedded ink writing (EIW), an emerging material extrusion-based three-dimensional (3D) printing strategy, has demonstrated great potential for diverse applications. EIW consists of three key components: support bath, ink, and printing conditions. Filament formation in EIW is significantly affected by the coupling effects of these components, which is still unclear. This work aims to fundamentally unveil the filament formation processes and mechanisms within support bath materials with various rheological properties. A broader range of path speed is utilized to ensure the formation of diverse representative filaments from low- and high-viscosity hydrogel inks. By varying the combination of support bath, ink, and printing conditions, ten types of filaments are observed during EIW and their formation mechanisms are explained in detail. Particularly, when designing a support bath for EIW, thixotropic time serves as the baseline to determine the functionality of the bath. A relatively long thixotropic time can facilitate the formation of well-defined filaments. In addition, the support bath’s yield stress decides the allowed maximum path speed. High yield stress is necessary if path speed is targeted in a higher range. Based on the experimental findings, a position-shape-size evaluation system is established and proposed for comprehensively evaluating printed filaments, which is helpful in filtering suitable filaments for EIW. Finally, complex 3D structures including a human nose and a human ear are printed at a high speed within the support bath with suitable rheological properties to bridge the gap between filament formation mechanisms and practical 3D printing applications.

Embedded ink writing (EIW) and direct ink writing (DIW) constitute the primary strategies for three-dimensional (3D) printing within the realm of material extrusion. These methods enable the rapid fabrication of complex 3D structures, utilizing either yield-stress support baths or self-supporting inks. Both these strategies have been extensively studied across a range of fields, including biomedical, soft robotics, and smart sensors, due to their outstanding print fidelity and compatibility with diverse ink materials. Particle additives capable of forming volume-filling 3D networks are frequently incorporated into polymer solvents. This integration is crucial for engineering the requisite microstructures essential for the formulation of successful support bath and ink materials. The interplay between the particle additives and polymer solvents is critical for achieving rheological tunability in various 3D printing strategies, yet this area has not been systematically reviewed. Therefore, in this critical review, we examined various mechanisms of particle–polymer interactions, the resulting microstructures, and their subsequent impact on mechanical and rheological properties. Overall, this work aims to serve as a foundational guideline for the design of next-generation materials in the field of extrusion additive manufacturing, specifically for EIW and DIW.

Creating tissue and organ equivalents with intricate architectures and multiscale functional feature sizes is the first step toward the reconstruction of transplantable human tissues and organs. Existing embedded ink writing approaches are limited by achievable feature sizes ranging from hundreds of microns to tens of millimeters, which hinders their ability to accurately duplicate structures found in various human tissues and organs. In this study, a multiscale embedded printing (MSEP) strategy is developed, in which a stimuli-responsive yield-stress fluid is applied to facilitate the printing process. A dynamic layer height control method is developed to print the cornea with a smooth surface on the order of microns, which can effectively overcome the layered morphology in conventional extrusion-based three-dimensional bioprinting methods. Since the support bath is sensitive to temperature change, it can be easily removed after printing by tuning the ambient temperature, which facilitates the fabrication of human eyeballs with optic nerves and aortic heart valves with overhanging leaflets on the order of a few millimeters. The thermosensitivity of the support bath also enables the reconstruction of the full-scale human heart on the order of tens of centimeters by on-demand adding support bath materials during printing. The proposed MSEP demonstrates broader printable functional feature sizes ranging from microns to centimeters, providing a viable and reliable technical solution for tissue and organ printing in the future.

Cerebral aneurysm disease has extremely high mortality and morbidity, which can be efficiently treated by implanting a vascular stent. However, due to relatively poor flexibility and biodegradability, current vascular stents still cause some life-threatening complications. Thus, it is of great significance to develop new biodegradable cerebral stents (BCSs) with enhanced flexibility for improving the prognosis of cerebral aneurysm disease. In this work, a flexibility-enhanced BCS architecture has been investigated and designed. Both numerical simulation and experiments have been performed to demonstrate the potential clinical application of the proposed BCSs. First, the effects of the structural parameters on the flexibility have been analyzed by finite element simulation. Then, the mechanical properties of the proposed BCSs have been characterized via both numerical simulation and experiments and compared to those of two representative commercial stents for demonstrating the flexibility and radial stiffness of the proposed design. The knowledge from this work provides a new design methodology.

Surgery is the most frequent treatment for patients with brain tumors. The construction of full-scale human brain models, which is still challenging to realize via current manufacturing techniques, can effectively train surgeons before brain tumor surgeries. This paper aims to develop a set of three-dimensional (3D) printing approaches to fabricate customized full-scale human brain models for surgery training as well as specialized brain patches for wound healing after surgery. First, a brain patch designed to fit a wound's shape and size can be easily printed in and collected from a stimuli-responsive yield-stress support bath. Then, an inverse 3D printing strategy, called "peeling-boiled-eggs," is proposed to fabricate full-scale human brain models. In this strategy, the contour layer of a brain model is printed using a sacrificial ink to envelop the target brain core within a photocurable yield-stress support bath. After crosslinking the contour layer, the as-printed model can be harvested from the bath to photo crosslink the brain core, which can be eventually released by liquefying the contour layer. Both the brain patch and full-scale human brain model are successfully printed to mimic the scenario of wound healing after removing a brain tumor, validating the effectiveness of the proposed 3D printing approaches.

Heart valve disease has become a serious global health problem, which calls for numerous implantable prosthetic valves to fulfill the broader needs of patients. Although current three-dimensional (3D) bioprinting approaches can be used to manufacture customized valve prostheses, they still have some complications, such as limited biocompatibility, constrained structural complexity, and difficulty to make heterogeneous constructs, to name a few. To overcome these challenges, a sacrificial scaffold-assisted direct ink writing approach has been explored and proposed in this work, in which a sacrificial scaffold is printed to temporarily support sinus wall and overhanging leaflets of an aortic valve prosthesis that can be removed easily and mildly without causing any potential damages to the valve prosthesis. The bioinks, composed of alginate, gelatin, and nanoclay, used to print heterogenous valve prostheses have been designed in terms of rheological/mechanical properties and filament formability. The sacrificial ink made from Pluronic F127 has been developed by evaluating rheological behavior and gel temperature. After investigating the effects of operating conditions, complex 3D structures and homogenous/heterogenous aortic valve prostheses have been successfully printed. Lastly, numerical simulation and cycling experiments have been performed to validate the function of the printed valve prostheses as one-way valves.

Three-dimensional (3D) printing technology has been widely developed and utilized to make diverse parts from shape memory polymers (SMPs). However, it is still challenging to fabricate functional devices with both complex geometries and embedded circuits using current 3D printing strategies. The objective of this study is to propose a hybrid direct ink writing (DIW)/embedded 3D printing (e-3DP) method, in which direct ink writing is used to print a self-supporting ink into a complex 3D structure at an uncured state while e-3DP is then applied to deposit a conductive ink within the structure. After crosslinking, the device at a cured state with embedded circuits can be formed in one step. This hybrid 3D printing method can only be realized by using ink materials with required rheological properties. Thus, how to develop the self-supporting ink is mainly discussed. Herein, a shape memory photocurable resin, aliphatic urethane diacrylate (AUD), and its chain builder, tert-butyl acrylate (tBA), are selected as the main components of the polymer precursor and fumed silica is used to tune the rheology for printing purposes. It is found that the tBA/AUD ratio affects the rheological properties, dry-out process, filament geometries, and self-supporting capability significantly. For demonstration, a smart hinge with an embedded circuit as strain sensor is successfully printed, which can program and recover its shape by controlling the ambient temperatures, validating the effectiveness of the proposed method for creating functional devices.

Microfluidic devices made from polydimethylsiloxane (PDMS) have diverse biomedical applications. However, due to the poor printability of PDMS, current 3D printing techniques are rarely used to fabricate microfluidic devices. This study aims to investigate a fumed silica-PDMS suspension that can function as a matrix bath for embedded 3D printing (e-3DP) purposes, making it technically feasible to print microfluidic chips with complex embedded channels via low-cost extrusion 3D printing. The rheological properties, mechanical properties, transparency, and filament fidelity of the fumed silica-PDMS suspension have been systematically studied. It is found that the addition of fumed silica particles can effectively change PDMS from a viscous solution to a yield-stress suspension with suitable rheological properties for e-3DP. Also, the mechanical properties of the crosslinked fumed silica-PDMS are enhanced with an increased concentration of fumed silica. Although the transparency of PDMS has been lessened by mixing it with fumed silica particles, the visibility of the printed microfluidic chips is still acceptable. The filament fidelity has been studied by embedded printing filaments using a sacrificial ink in the fumed silica-PDMS suspension. Finally, two representative microfluidic chips for biomedical applications have been successfully printed to validate the effectiveness of the proposed fumed silica-PDMS suspension-enabled e-3DP method.

Smart hinges fabricated using three-dimensional (3D) printing have been accepted in the aerospace, robotics, and biomedical fields since these devices possess a shape memory characteristic. Shape memory polymers (SMPs) are the preferred materials for creating smart hinges due to their ability to achieve programmable complex geometries. However, fabricating SMPs with embedded components remains a challenge due to the constraints of current 3D printing methods and material limitations. This study investigated the use of a hybrid 3D printing method, direct ink writing (DIW), and embedded 3D printing (e-3DP) to print smart hinges with an embedded circuit to act as a strain sensor. The main components of the SMP included tert-Butyl acrylate (tBA) and aliphatic urethane diacrylate (AUD), but this SMP ink had a low viscosity and could not be used for DIW or e-3DP. Fumed silica (FS) was added to the SMP to tune its rheology, and it was shown that the FS concentration significantly affected the rheological properties, dry-out process, filament geometries, and self-supporting capabilities. This study presents a hybrid 3D printing approach for creating smart hinges with internal strain sensors in one step, demonstrating the versatility of DIW/e-3DP. The findings from this work provide a foundational and reliable technical solution to efficiently fabricate functional, self-monitoring, smart devices from SMPs for diverse applications.

Astronauts suffer skeletal muscle atrophy in microgravity and/or zero-gravity environments. Artificial muscle-actuated exoskeletons can aid astronauts in physically strenuous situations to mitigate risk during spaceflight missions. Current artificial muscle fabrication methods are technically challenging to be performed during spaceflight. The objective of this research is to unveil the effects of critical operating conditions on artificial muscle formation and geometry in a newly developed helical fiber extrusion method. It is found that the fiber outer diameter decreases and pitch increases when the printhead temperature increases, inlet pressure increases, or cooling fan speed decreases. Similarly, fiber thickness increases when the cooling fan speed decreases or printhead temperature increases. Extrusion conditions also affect surface morphology and mechanical properties. Particularly, extrusion conditions leading to an increased polymer temperature during extrusion can result in lower surface roughness and increased tensile strength and elastic modulus. The shape memory properties of an extruded fiber are demonstrated in this study to validate the ability of the fiber from shape memory polymer to act as an artificial muscle. The effects of the operating conditions are summarized into a phase diagram for selecting suitable parameters for fabricating helical artificial muscles with controllable geometries and excellent performance in the future.

Yield-stress support bath-enabled three-dimensional (3D) printing has been widely used in recent years for diverse applications. However, current yield-stress fluids usually possess single microstructures and still face the challenges of on-demand adding and/or removing support bath materials during printing, constraining their application scope. This study aims to propose a concept of stimuli-responsive yield-stress fluids with an interactive dual microstructure as support bath materials. The microstructure from a yield-stress additive allows the fluids to present switchable states at different stresses, facilitating an embedded 3D printing process. The microstructure from stimuli-responsive polymers enables the fluids to have regulable rheological properties upon external stimuli, making it feasible to perfuse additional yield-stress fluids during printing and easily remove residual fluids after printing. A nanoclay-Pluronic F127 nanocomposite is studied as a thermosensitive yield-stress fluid. The key material properties are characterized to unveil the interactions in the formed dual microstructure and microstructure evolutions at different stresses and temperatures. Core scientific issues, including the filament formation principle, surface roughness control, and thermal effects of the newly added nanocomposite, are comprehensively investigated. Finally, three representative 3D structures, the Hall of Prayer, capsule, and tube with changing diameter, are successfully printed to validate the printing capability of stimuli-responsive yield-stress fluids for fabricating arbitrary architectures.

Biodegradable polymer vascular stents (BPVSs) have been widely used in percutaneous coronary interventions for the treatment of coronary artery diseases. The development of BPVSs is an integrated process that combines material design/selection, manufacturing, and performance characterization. Three-dimensional (3D) printing technology is a powerful tool for polymer stent fabrication. Current review studies have focused primarily on the material and structural design of polymer stents but have failed to comprehensively discuss different 3D printing approaches and stent characterization techniques. In this paper, we address these shortcomings by discussing 3D printing methods and their application in BPVSs. First, some commonly used 3D printing methods (including material extrusion, vat polymerization, and powder bed fusion) and potential 3D printing strategies (including material jetting and binder jetting) for fabricating BPVSs are discussed; furthermore, the main post-treatments are summarized. Then, techniques to characterize the morphology, mechanical properties, and biological properties of the printed BPVSs are introduced. Subsequently, representative commercial BPVSs and lab-grade BPVSs are compared. Finally, based on the limitations of stent printing and characterization processes, future perspectives are proposed, which may help develop new techniques to fabricate more customized stents and accurately evaluate their performance.

Rehabilitation and mobility assistance using robotic orthosis or exoskeletons have shown potential in aiding those with musculoskeletal disorders. Artificial muscles are the main component used to drive robotics and bio-assistive devices. However, current fabrication methods to produce artificial muscles are technically challenging and laborious for medical staff at clinics and hospitals. This study aims to investigate a printhead system for material extrusion of helical polymer artificial muscles. In the proposed system, an internal fluted mandrel within the printhead and a temperature control module were used simultaneously to solidify and stereotype polymer filaments prior to extrusion from the printhead with a helical shape. Numerical simulation was applied to determine the optimal printhead design, as well as analyze the coupling effects and sensitivity of the printhead geometries on artificial muscle fabrication. Based on the simulation analysis, the printhead system was designed, fabricated, and operated to extrude helical filaments using polylactic acid. The diameter, thickness, and pitch of the extruded filaments were compared to the corresponding geometries of the mandrel to validate the fabrication accuracy. Finally, a printed filament was programmed and actuated to test its functionality as a helical artificial muscle. The proposed printhead system not only allows for the stationary extrusion of helical artificial muscles but is also compatible with commercial 3D printers to freeform print helical artificial muscle groups in the future.