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Abstract Among the wide range of additive manufacturing — or “three-dimensional (3D) printing” — technologies, “material jetting” approaches are distinctively suited for multi-material fabrication. Because material jetting strategies, such as “PolyJet 3D printing”, harness inkjets that allow for multiple photopolymer droplets (and sacrificial support materials) to be dispensed in parallel to build 3D objects, distinct materials with unique properties can be readily unified in a single print akin to combining multiple-colored inks using a conventional 2D color printer. Although researchers have leveraged this multi-material capability to achieve, for example, 3D functionally graded and bi-material composite systems, there are cases in which the interface between distinct materials can become a key region of mechanical failure if not designed properly. To elucidate potential design factors that contribute to such failure modes, here we investigate the relationship between the interface design and tensile mechanical failure dynamics for PolyJet-printed bi-material coupons. Experimental results for a select set of bi-material sample designs that were 3D printed using a Stratasys Objet500 Connex3 PolyJet 3D printer and subjected to uniaxial tensile testing using a Tinius Olsen H25K-T benchtop universal testing machine under uniaxial strain revealed that increasing the surface contact area between two distinct materials via changes in geometric design does not necessarily increase the interface strength based on the length scales and loading conditions investigated in the current study and that further studies of the role of multi-material geometric designs in interface integrity are warranted to understand potential mechanisms underlying these results. Given the increasing interest in material jetting — and PolyJet 3D printing in particular — as a pathway to multi-material manufacturing in fields including robotics and fluidic circuitry, this study suggests that multi-material interface geometry should be considered appropriately for future applications. 

Abstract Microinjection protocols are ubiquitous throughout biomedical fields, with hollow microneedle arrays (MNAs) offering distinctive benefits in both research and clinical settings. Unfortunately, manufacturing‐associated barriers remain a critical impediment to emerging applications that demand high‐density arrays of hollow, high‐aspect‐ratio microneedles. To address such challenges, here, a hybrid additive manufacturing approach that combines digital light processing (DLP) 3D printing with “ex situ direct laser writing (esDLW)” is presented to enable new classes of MNAs for fluidic microinjections. Experimental results foresDLW‐based 3D printing of arrays of high‐aspect‐ratio microneedles—with 30 µm inner diameters, 50 µm outer diameters, and 550 µm heights, and arrayed with 100 µm needle‐to‐needle spacing—directly onto DLP‐printed capillaries reveal uncompromised fluidic integrity at the MNA‐capillary interface during microfluidic cyclic burst‐pressure testing for input pressures in excess of 250 kPa (n = 100 cycles). Ex vivo experiments perform using excised mouse brains reveal that the MNAs not only physically withstand penetration into and retraction from brain tissue but also yield effective and distributed microinjection of surrogate fluids and nanoparticle suspensions directly into the brains. In combination, the results suggest that the presented strategy for fabricating high‐aspect‐ratio, high‐density, hollow MNAs could hold unique promise for biomedical microinjection applications. 

Fluidically actuated soft robotic devices have attracted increasing interest due to their ability to provide benefits over traditional rigid systems in biomedical applications such as minimally invasive surgery, rehabilitative devices, and prosthetics. Unfortunately, challenges remain for controlling the fluidic operations of such systems, driving a critical need for new classes of fluidic circuit elements. Here we explore the use of “Liquid Crystal Display (LCD)” 3D printing—a low-cost vat photopolymerization (VPP) approach—to additively manufacture “normally closed” fluidic transistors with operations analogous to their electronic counterparts. Specifically, we leverage an “additive assembly” strategy wherein part components are printed separately and assembled post hoc. Experimental results for higher source pressure magnitudes revealed that in the absence of an applied gate pressure, the element obstructed source-to-drain fluid flow—i.e., normally closed behavior—however, by applying a gate pressure of ≥25 kPa, the element permitted source-to-drain fluid flow. Thus, this work establishes the efficacy for VPP-based additive assembly of fluidic circuit elements, which could help to advance and democratize fluidic circuit-based soft robotic technologies. 

A novel two-photon direct laser writing-based hybrid strategy for 3D nanoprinting microfluidic vessels with sophisticated 3D architectures and custom-designed micropores. 

Microinjection protocols that involve using a hollow, high-aspect-ratio microneedle to deliver foreign material (e.g., cells, DNA, viruses, and micro/nanoparticles) into biological targets (e.g., embryos, tissues, and organisms) are essential to diverse biomedical applications in both research and clinical settings. A key deficit of such protocols, however, is that standard microneedle architectures are inherently susceptible to clogging-induced failure modes, which can diminish experimental rigor and lead to failed microinjections. Additive manufacturing (or “three-dimensional (3D) printing”) strategies based on “Two-Photon Direct Laser Writing (DLW)” offer a promising route to address clogging failure phenomena by rearchitecting the needle tip, yet achieving 3D-printed microneedles with the mechanical strength necessary to penetrate into biological targets (e.g., embryos) has remained a critical barrier to efficacy. To overcome this barrier, here we harness a recently reported polyhedral oligomeric silsequioxane (POSS) photomaterial to DLW-print fused silica glass high-aspect-ratio microinjection needles with enhanced mechanical strength. Experimental results for POSS-based 3D-nanoprinted microneedles with inner and outer diameters of 10 μm and 15 μm, respectively, and heights ranging from 500–750 μm revealed that the needles not only enabled successful puncture and penetration into early-stage zebrafish embryos, but also significantly reduced the magnitude of undesired deformations to the embryos during needle puncture and penetration from 61.0±12.1 μm for standard glass-pulled control microneedles to 42.4±11.5 μm for the POSS-enabled 3D microneedles (p < 0.01). In combination, these results suggest that wide-ranging biomedical fields could benefit from the presented 3D microinjection needles. 

This Tutorial Review highlights strategies for leveraging the micron-to-submicron-scale additive manufacturing technique, “direct laser writing”, to enable 3D microfluidic technologies.