Multimaterial Microfluidic 3D Printing of Textured Composites with Liquid Inclusions

Versatile Patterning of Liquid Metal via Multiphase 3D Printing

This paper presents a scalable and straightforward technique for the immediate patterning of liquid metal/polymer composites via multiphase 3D printing. Capitalizing on the polymer's capacity to confine liquid metal (LM) into diverse patterns. The interplay between distinctive fluidic properties of liquid metal and its self-passivating oxide layer within an oxidative environment ensures a resilient interface with the polymer matrix. This study introduces an inventive approach for achieving versatile patterns in eutectic gallium indium (EGaIn), a gallium alloy. The efficacy of pattern formation hinges on nozzle's design and internal geometry, which govern multiphase interaction. The interplay between EGaIn and polymer within the nozzle channels, regulated by variables such as traverse speed and material flow pressure, leads to periodic patterns. These patterns, when encapsulated within a dielectric polymer polyvinyl alcohol (PVA), exhibit an augmented inherent capacitance in capacitor assemblies. This discovery not only unveils the potential for cost-effective and highly sensitive capacitive pressure sensors but also underscores prospective applications of these novel patterns in precise motion detection, including heart rate monitoring, and comprehensive analysis of gait profiles. The amalgamation of advanced materials and intricate patterning techniques presents a transformative prospect in the domains of wearable sensing and comprehensive human motion analysis.

Soft composites with liquid inclusions: functional properties and theoretical models

Soft materials containing liquid inclusions have emerged as a promising class of materials. Unlike solid inclusions, liquid inclusions possess intrinsic fluidity, which allows them to retain the excellent deformation ability of soft materials. This can prevent compliance mismatches between the inclusions and the matrix, thus leading to improved performance and durability. Various liquids, including metallic, water-based, and ionic liquids, have been selected as inclusions for embedding into soft materials, resulting in unique properties and functionalities that enable a wide range of applications in soft robotics, wearable devices, and other cutting-edge fields. This review provides an overview of recent studies on the functional properties of composites with liquid inclusions and discusses theoretical models used to estimate these properties, aiming to bridge the gap between the microstructure/components and the overall properties of the composite from a theoretical perspective. Furthermore, current challenges and future opportunities for the widespread application of these composites are explored, highlighting their potential in advancing technologies.

Toward Multiscale, Multimaterial 3D Printing

Biological materials and organisms possess the fundamental ability to self-organize, through which different components are assembled from the molecular level up to hierarchical structures with superior mechanical properties and multifunctionalities. These complex composites inspire material scientists to design new engineered materials by integrating multiple ingredients and structures over a wide range. Additive manufacturing, also known as 3D printing, has advantages with respect to fabricating multiscale and multi-material structures. The need for multifunctional materials is driving 3D printing techniques toward arbitrary 3D architectures with the next level of complexity. In this paper, the aim is to highlight key features of those 3D printing techniques that can produce either multiscale or multimaterial structures, including innovations in printing methods, materials processing approaches, and hardware improvements. Several issues and challenges related to current methods are discussed. Ultimately, the authors also provide their perspective on how to realize the combination of multiscale and multimaterial capabilities in 3D printing processes and future directions based on emerging research.

Quantifying local stiffness and forces in soft biological tissues using droplet optical microcavities

Mechanical properties of biological tissues fundamentally underlie various biological processes and noncontact, local, and microscopic methods can provide fundamental insights. Here, we present an approach for quantifying the local mechanical properties of biological materials at the microscale, based on measuring the spectral shifts of the optical resonances in droplet microcavities. Specifically, the developed method allows for measurements of deformations in dye-doped oil droplets embedded in soft materials or biological tissues with an error of only 1 nm, which in turn enables measurements of anisotropic stress inside tissues as small as a few pN/μm2. Furthermore, by applying an external strain, Young's modulus can be measured in the range from 1 Pa to 35 kPa, which covers most human soft tissues. Using multiple droplet microcavities, our approach could enable mapping of stiffness and forces in inhomogeneous soft tissues and could also be applied to in vivo and single-cell experiments. The developed method can potentially lead to insights into the mechanics of biological tissues.

Microfluidic based continuous enzyme immobilization: A comprehensive review

Conventional techniques for enzyme immobilization suffer from suboptimal activity recovery due to insufficient enzyme loading and inadequate stability. Furthermore, these techniques are time-consuming and involve multiple steps which limit the applicability of immobilized enzymes. In contrast, the use of microfluidic devices for enzyme immobilization has garnered significant attention due to its ability to precisely control immobilization parameters, resulting in highly active immobilized enzymes. This approach offers several advantages, including reduced time and energy consumption, enhanced mass-heat transfer, and improved control over the mixing process. It maintains the superior structural configuration in immobilized form which ultimately affects the overall efficiency. The present review article comprehensively explains the design, construction, and various methods employed for enzyme immobilization using microfluidic devices. The immobilized enzymes prepared using these techniques demonstrated excellent catalytic activity, remarkable stability, and outstanding recyclability. Moreover, they have found applications in diverse areas such as biosensors, biotransformation, and bioremediation. The review article also discusses potential future developments and foresees significant challenges associated with enzyme immobilization using microfluidics, along with potential remedies. The development of this advanced technology not only paves the way for novel and innovative approaches to enzyme immobilization but also allows for the straightforward scalability of microfluidic-based techniques from an industrial standpoint.

3D printing of hierarchically micro/nanostructured electrodes for high-performance rechargeable batteries

3D printing, also known as additive manufacturing, is capable of fabricating 3D hierarchical micro/nanostructures by depositing a layer-upon-layer of precursor materials and solvent-based inks under the assistance of computer-aided design (CAD) files. 3D printing has been employed to construct 3D hierarchically micro/nanostructured electrodes for rechargeable batteries, endowing them with high specific surface areas, short ion transport lengths, and high mass loading. This review summarizes the advantages and limitations of various 3D printing methods and presents the recent developments of 3D-printed electrodes in rechargeable batteries, such as lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries. Furthermore, the challenges and perspectives of the 3D printing technique for electrodes and rechargeable batteries are put forward. This review will provide new insight into the 3D printing of hierarchically micro/nanostructured electrodes in rechargeable batteries and promote the development of 3D printed electrodes and batteries in the future.

3D Printing of Ionogels with Complementary Functionalities Enabled by Self-Regulating Ink

Shaping soft and conductive materials into sophisticated architectures through 3D printing is driving innovation in myriad applications, such as robotic counterparts that emulate the synergic functions of biological systems. Although recently developed multi-material 3D printing has enabled on-demand creation of intricate artificial counterparts from a wide range of functional viscoelastic materials. However, directly achieving complementary functionalities in one ink design remains largely unexplored, given the issues of printability and synergy among ink components. In this study, an easily accessible and self-regulating tricomponent ionogel-based ink design to address these challenges is reported. The resultant 3D printed objects, based on the same component but with varying ratios of ink formulations, exhibit distinct yet complementary properties. For example, their Young's modulus can differ by three orders of magnitude, and some structures are rigid while others are ductile and viscous. A theoretical model is also employed for predicting and controlling the printing resolution. By integrating complementary functionalities, one further demonstrates a representative bioinspired prototype of spiderweb, which mimics the sophisticated structure and multiple functions of a natural spiderweb, even working and camouflaging underwater. This ink design strategy greatly extends the material choice and can provide valuable guidance in constructing diverse artificial systems by 3D printing.

Microfluidic Approaches for Microactuators: From Fabrication, Actuation, to Functionalization

Microactuators can autonomously convert external energy into specific mechanical motions. With the feature sizes varying from the micrometer to millimeter scale, microactuators offer many operation and control possibilities for miniaturized devices. In recent years, advanced microfluidic techniques have revolutionized the fabrication, actuation, and functionalization of microactuators. Microfluidics can not only facilitate fabrication with continuously changing materials but also deliver various signals to stimulate the microactuators as desired, and consequently improve microfluidic chips with multiple functions. Herein, this cross-field that systematically correlates microactuator properties and microfluidic functions is comprehensively reviewed. The fabrication strategies are classified into two types according to the flow state of the microfluids: stop-flow and continuous-flow prototyping. The working mechanism of microactuators in microfluidic chips is discussed in detail. Finally, the applications of microactuator-enriched functional chips, which include tunable imaging devices, micromanipulation tools, micromotors, and microsensors, are summarized. The existing challenges and future perspectives are also discussed. It is believed that with the rapid progress of this cutting-edge field, intelligent microsystems may realize high-throughput manipulation, characterization, and analysis of tiny objects and find broad applications in various fields, such as tissue engineering, micro/nanorobotics, and analytical devices.

Advances in the Synthesis of Preceramic Polymers for the Formation of Silicon-Based and Ultrahigh-Temperature Non-Oxide Ceramics

Preceramic polymers (PCPs) are a group of specialty macromolecules that serve as precursors for generating inorganics, including ceramic carbides, nitrides, and borides. PCPs represent interesting synthetic challenges for chemists due to the elements incorporated into their structure. This group of polymers is also of interest to engineers as PCPs enable the processing of polymer-derived ceramic products including high-performance ceramic fibers and composites. These finished ceramic materials are of growing significance for applications that experience extreme operating environments (e.g., aerospace propulsion and high-speed atmospheric flight). This Review provides an overview of advances in the synthesis and postpolymerization modification of macromolecules forming nonoxide ceramics. These PCPs include polycarbosilanes, polysilanes, polysilazanes, and precursors for ultrahigh-temperature ceramics. Following our review of PCP synthetic chemistry, we provide examples of the application and processing of these polymers, including their use in fiber spinning, composite fabrication, and additive manufacturing. The principal objective of this Review is to provide a resource that bridges the disciplines of synthetic chemistry and ceramic engineering while providing both insights and inspiration for future collaborative work that will ultimately drive the PCP field forward.

High Throughput Omnidirectional Printing of Tubular Microstructures from Elastomeric Polymers

Bioelastomers are extensively used in biomedical applications due to their desirable mechanical strength, tunable properties, and chemical versatility; however, three-dimensional (3D) printing bioelastomers into microscale structures has proven elusive. Herein, a high throughput omnidirectional printing approach via coaxial extrusion is described that fabricates perfusable elastomeric microtubes of unprecedently small inner diameter (350-550 µm) and wall thickness (40-60 µm). The versatility of this approach is shown through the printing of two different polymeric elastomers, followed by photocrosslinking and removal of the fugitive inner phase. Designed experiments are used to tune the microtube dimensions and stiffness to match that of native ex vivo rat vasculature. This approach affords the fabrication of multiple biomimetic shapes resembling cochlea and kidney glomerulus and affords facile, high-throughput generation of perfusable structures that can be seeded with endothelial cells for biomedical applications. Post-printing laser micromachining is performed to generate micro-sized holes (520 µm) in the tube wall to tune microstructure permeability. Importantly, for organ-on-a-chip applications, the described approach takes only 3.6 min to print microtubes (without microholes) over an entire 96-well plate device, in contrast to comparable hole-free structures that take between 1.5 and 6.5 days to fabricate using a manual 3D stamping approach.

Multiple configuration transitions of soft actuators under single external stimulus

Soft actuators have a wide range of applications in medical instruments, soft robotics, 3D electronics, and deployable structures, where configuration transitions are crucial for their function realization. However, most soft actuators can only morph from the initial configuration directly to the final configuration under a single external stimulus. Herein, we report a novel soft actuator by 3D printing parallel strips with crescent cross-sections onto a thin PDMS film. Multiple configuration transitions are observed when the soft actuator swells in ethyl acetate. Four factors, i.e., the geometric asymmetry of the strips, the fabrication-induced heterogeneity of the film, the differential swelling ratios of the strips and the film, and the geometric parameters of the actuator, are demonstrated to synergistically regulate the multiple configuration transitions of the actuator. Particularly, the underlying mechanisms for the configuration transitions are systematically investigated through experiments and theoretical analysis, and verified via finite element simulation. Benefitting from the multiple configuration transitions, the grasp-release-re-grab function of the actuator is demonstrated under a single stimulus. This work contributes to fundamental understanding of the morphing behaviors and the novel design of soft actuators.

Physically Entangled Anti-Swelling Hydrogels with High Stiffness

Physically crosslinked hydrogels have great potential for tissue engineering because of their excellent biocompatibility and easy fabrication. However, physical crosslinking points are typically weaker compared to chemical ones and therefore cannot form robust hydrogels with excellent water stability, which greatly hinder their further applications. In this work, we report a novel hydrogel with high stiffness and outstanding anti-swelling performance crosslinked by hydrophobic polymer chains entanglements. The hydrophobic polymer polyimide (PI) was mixed with the hydrophilic polymer poly(vinyl pyrrolidone) (PVP) to form crosslinking points between the chains. At the equilibrium swelling state, tensile moduli of the hydrogel can be up to 22.57 MPa (higher than most existing hydrogels) and the equilibrium water swelling ratio (ESR) can be as low as 125.0%. By decreasing the PI mass ratio, tensile moduli and ESR of the hydrogel can be tuned in a wide range from 22.57 MPa to 0.005 MPa and 125.0% to 765.6%, respectively. Using PVP/PI solutions as inks, we fabricate uniform structures and multi-material structures whose mechanical properties are close to cartilage through a direct ink writing 3D printing platform. The current work demonstrates that entangled PVP/PI hydrogels have excellent tailoring capabilities and are promising candidates for tissue engineering applications. This article is protected by copyright. All rights reserved.

Dynamics of an impacting emulsion droplet

Emulsions are widely used in agriculture where oil-based pesticides are sprayed as an emulsion. However, emulsion droplets can bounce off hydrophobic plant surfaces, leading to major health and environmental issues as pesticides pollute water sources and soils. Here, we report an unexpected transition from bouncing to sticking to bouncing as the droplet impact speed increases. We show that the physics are governed by an in situ, self-generated lubrication of the surface leading to a suction force from the nascent oil layer around the droplet. We demonstrate that this phenomenon can be controlled by a careful balance of three time scales: the contact time of the droplet, the impregnation time scale of the oil, and the oil ridge formation time scale. We lastly build a design map to precisely control the bouncing of droplets and the oil coverage of the target surface. These insights have broad applicability in agriculture, cooling sprays, combustion, and additive manufacturing.

Microfluidics-enabled functional 3D printing

Microfluidic technology has established itself as a powerful tool to enable highly precise spatiotemporal control over fluid streams for mixing, separations, biochemical reactions, and material synthesis. 3D printing technologies such as extrusion-based printing, inkjet, and stereolithography share similar length scales and fundamentals of fluid handling with microfluidics. The advanced fluidic manipulation capabilities afforded by microfluidics can thus be potentially leveraged to enhance the performance of existing 3D printing technologies or even develop new approaches to additive manufacturing. This review discusses recent developments in integrating microfluidic elements with several well-established 3D printing technologies, highlighting the trend of using microfluidic approaches to achieve functional and multimaterial 3D printing as well as to identify potential future research directions in this emergent area.

Robust design of an asymmetrically absorbing Willis acoustic metasurface subject to manufacturing-induced dimensional variations

Advancements in additive manufacturing (AM) technology are promising for the creation of acoustic materials. Acoustic metamaterials and metasurfaces are of particular interest for the application of AM technologies as theoretical predictions suggest the need for precise arrangements of dissimilar materials within specified regions of space to reflect, transmit, guide, or absorb acoustic waves in ways that exceed the capabilities of currently available acoustic materials. This work presents the design of an acoustic metasurface (AMS) with Willis constitutive behavior, which is created from an array of multi-material inclusions embedded in an elastomeric matrix, which displays the asymmetric acoustic absorption. The finite element models of the AMS show that the asymmetric absorption is dependent on asymmetry in the distribution of materials within the inclusion and highly sensitive to small changes in the inclusion geometry. It is shown that the performance variability can be used to place constraints on the manufacturing-induced variability to ensure that an as-built AMS will perform using the as-designed parameters. The evaluation of the AMS performance is computationally expensive, thus, the design is performed with a classifier-based metamodel to support more efficient Monte Carlo simulations and quantify the sensitivity of the candidate design performance to the manufacturing variability. This work explores combinations of material choices and dimensional accuracies to demonstrate how a robust design approach can be used to help select AM fabrication methods or guide process development toward an AM process that is capable of fabricating acoustic material structures.

Spray characteristics of an ultrasonic microdroplet generator with a continuously variable operating frequency

Droplet spraying is utilized in diverse industrial processes and biomedical applications, including nanomaterial synthesis, biomaterial handling, and inhalation drug delivery. Ultrasonic droplet generators transfer energy into bulk liquids using acoustic waves to disrupt the free liquid surface into fine microdroplets. We previously established a method combining ultrasonic actuation, resonant operation, and acoustic wave focusing for efficient spraying of various liquids (e.g., low surface tension fuels, high viscosity inks, and suspensions of biological cells). The microfabricated device comprises a piezoelectric transducer, sample reservoir, and an array of acoustic horn structures terminated by microscale orifices. Orifice size roughly dictates droplet diameter, and a fixed reservoir height prescribes specific device resonant frequencies of operation. Here, we incorporate a continuously variable liquid reservoir height for dynamic adjustment of operating parameters to improve spray efficiency in real-time and potentially tune the droplet size. Computational modeling predicts the system harmonic response for a range of reservoir heights from 0.5 to 3 mm (corresponding to operating frequencies from ∼500 kHz to 2.5 MHz). Nozzle arrays with 10, 20, and 40 μm orifices are evaluated for spray uniformity and stability of the active nozzles, using model predictions to explain the experimental observations.

[Research advances of high-throughput cell-based drug screening systems based on microfluidic technique]

Drug screening is the process of screening new drugs or leading compounds with biological activity from natural products or synthetic compounds, and it plays an essential role in drug discovery. The discovery of innovative drugs requires the screening of a large number of compounds with appropriate drug targets. With the development of genomics, proteomics, metabolomics, combinatorial chemistry, and other disciplines, the library of drug molecules has been largely expanded, and the number of drug targets is continuously increasing. High-throughput screening systems enable the parallel analysis of thousands of reactions through automated operation, thereby enhancing the experimental scale and efficiency of drug screening. Among them, cell-based high-throughput drug screening has become the main screening mode because it can provide a microenvironment similar to human physiological conditions. However, the current high-throughput screening systems are mainly built based on multiwell plates, which have several disadvantages such as simple cell culture conditions, laborious and time-consuming operation, and high reagent consumption. In addition, it is difficult to achieve complex drug combination screening. Therefore, there is an urgent need for rapid and low-cost drug screening methods to reduce the time and cost of drug development. Microfluidic techniques, which can manipulate and control microfluids in microscale channels, have the advantages of low consumption, high efficiency, high throughput, and automation. It can overcome the shortcomings of screening systems based on multi-well plates and provide an efficient and reliable technical solution for establishing high-throughput cell-based screening systems. Moreover, microfluidic systems can be flexibly changed in terms of cell culture materials, chip structure design, and fluid control methods to enable better control and simulation of cell growth microenvironment. Operations such as cell seeding, culture medium replacement or addition, drug addition and cleaning, and cell staining reagent addition are usually involved in cell-based microfluidic screening systems. These operations are all based on the manipulation of microfluids. This paper reviews the research advances in cell-based microfluidic screening systems using different microfluidic manipulation modes, namely perfusion flow mode, droplet mode, and microarray mode. In addition, the advantages and disadvantages of these systems are summarized. Moreover, the development prospects of high-throughput screening systems based on microfluidic techniques has been looked forward. Furthermore, the current problems in this field and the directions to overcome these problems are discussed.

3D printing of multi-scalable structures via high penetration near-infrared photopolymerization

3D printing consisted of in-situ UV-curing module can build complex 3D structures, in which direct ink writing can handle versatile materials. However, UV-based direct ink writing (DIW) is facing a trade-off between required curing intensity and effectiveness range, and it cannot implement multiscale parallelization at ease. We overcome these difficulties by ink design and introducing near-infrared (NIR) laser assisted module, and this increases the scalability of direct ink writing to solidify the deposited filament with diameter up to 4 mm, which is much beyond any of existing UV-assisted DIW. The NIR effectiveness range can expand to tens of centimeters and deliver the embedded writing capability. We also demonstrate its parallel manufacturing capability for simultaneous curing of multi-color filaments and freestanding objects. The strategy owns further advantages to be integrated with other types of ink-based 3D printing technologies for extensive applications.

Currently UV-based direct ink writing (DIW) is facing a trade-off between required curing intensity and effectiveness range. Here the authors overcome this problem by introducing near-infrared photopolymerization into DIW

Soft Actuators Based on Liquid-Vapor Phase Change Composites

Liquid-vapor phase change materials (PCMs), capable of significant volume change, are emerging as attractive actuating components in forming advanced soft composites for robotic applications. However, the novel and functional design of these PCM composites is significantly limited due to the lacking of the fundamental understanding of the mechanical properties, which further inhibits the broad applications of PCM based materials in the engineering structures requiring large deformation and high loading capacity. In this study we fabricate PCM-elastomer composites exhibiting large deformation and high output stress. Thermomechanical properties of these composites are experimentally and theoretically investigated, demonstrating enhanced deformation and loading capacity due to the induced vapor pressure. By controlling the distribution and content of the PCM inclusions, structures with tunable deformability under a relatively small strain in comparison with traditional soft materials are fabricated. Accompanying with the asymmetrical friction and deformation, complex locomotion and adaptable grabbing function are achieved with excellent performance.

On-demand modulation of 3D-printed elastomers using programmable droplet inclusions

One of the key thrusts in three-dimensional (3D) printing and direct writing is to seamlessly vary composition and functional properties in printed constructs. Most inks used for extrusion-based printing, however, are compositionally static and available approaches for dynamic tuning of ink composition remain few. Here, we present an approach to modulate extruded inks at the point of print, using droplet inclusions. Using a glass capillary microfluidic device as the printhead, we dispersed droplets in a polydimethylsiloxane (PDMS) continuous phase and subsequently 3D printed the resulting emulsion into a variety of structures. The mechanical characteristics of the 3D-printed constructs can be tuned in situ by varying the spatial distribution of droplets, including aqueous and liquid metal droplets. In particular, we report the use of poly(ethylene glycol) diacrylate (PEGDA) aqueous droplets for local PDMS chemistry alteration resulting in significant softening (85% reduced elastic modulus) of the 3D-printed constructs. Furthermore, we imparted magnetic functionality in PDMS by dispersing ferrofluid droplets and rationally designed and printed a rudimentary magnetically responsive soft robotic actuator as a functional demonstration of our droplet-based strategy. Our approach represents a continuing trend of adapting microfluidic technology and principles for developing the next generation of additive manufacturing technology.

Multi-Material 3D and 4D Printing: A Survey

Recent advances in multi-material 3D and 4D printing (time as the fourth dimension) show that the technology has the potential to extend the design space beyond complex geometries. The potential of these additive manufacturing (AM) technologies allows for functional inclusion in a low-cost single-step manufacturing process. Different composite materials and various AM technologies can be used and combined to create customized multi-functional objects to suit many needs. In this work, several types of 3D and 4D printing technologies are compared and the advantages and disadvantages of each technology are discussed. The various features and applications of 3D and 4D printing technologies used in the fabrication of multi-material objects are reviewed. Finally, new avenues for the development of multi-material 3D and 4D printed objects are proposed, which reflect the current deficiencies and future opportunities for inclusion by AM.

Acoustic-Controlled Bubble Generation and Fabrication of 3D Polymer Porous Materials

Porous materials have a variety of applications such as catalysis, gas separation, sensing, tissue engineering, sewage treatment, and so on. However, there are still challenges in the synthesis of porous materials with light weight, high porosity, and superhydrophobicity. Herein, we demonstrate one acoustic-controlled microbubble generation method, which is used to synthesize 3D polymer porous materials. The acoustic-controlled microbubble generation based on focused surface acoustic wave (FSAW) is suitable for not only the generation of gas-in-oil microbubbles but also the gas-in-water microbubbles. The size of microbubbles can be real-time controlled by adjusting the frequency or the driving voltage of the FSAW. The as-prepared poly(vinyl alcohol) (PVA) foams composed of microbubbles can be used as a template to fabricate the PVA-based porous gel materials through freezing-thawing cyclic processing, and the various sized bubbles result in different porosity of the PVA-based porous gel materials. Moreover, excellent properties like oleophilicity and superhydrophobicity of the PVA-based porous gel materials can be obtained through a further hydrophobic modification treatment. The oil/water separation experiments have been done to demonstrate the good absorption and reliability of the modified porous gel materials, which are capable of multiple uses.

Development of a Disposable Single-Nozzle Printhead for 3D Bioprinting of Continuous Multi-Material Constructs

Fabricating multi-cell constructs in complex geometries is essential in the field of tissue engineering, and three-dimensional (3D) bioprinting is widely used for this purpose. To enhance the biological and mechanical integrity of the printed constructs, continuous single-nozzle printing is required. In this paper, a novel single-nozzle printhead for 3D bioprinting of multi-material constructs was developed and characterized. The single-nozzle multi-material bioprinting was achieved via a disposable, inexpensive, multi-fuse IV extension set; the printhead can print up to four different biomaterials. The transition distance of the developed printhead was characterized over a range of pressures and needle inner diameters. Finally, the transition distance was decreased by applying a silicon coating to the inner channels of the printhead.

Mosaic Immunoassays Integrated with Microfluidic Channels for High-Throughput Parallel Detection

Using the ice-printing technique, we have integrated micromosaic immunoassays (μMIAs) with microfluidic channels, which reduces the sample consumption and response time and allows high-throughput parallel detection. The ice-printing method is a low-temperature and contaminant-free process, which is more convenient, precise, and biofriendly than the traditional fabrication method. Meanwhile, based on the ice-drying process, this method can obtain a uniform distribution of the residue protein patterns, which leads to a uniform fluorescence result. As a proof of concept, the test of stability, sensitivity, and specificity of μMIA based on one-step ELISA are demonstrated. In this device, immobilized antigens surrounded with ice could remain biological at -20 °C for months.

Rebound of self-lubricating compound drops

Drop impact on solid surfaces is encountered in numerous natural and technological processes. Although the impact of single-phase drops has been widely explored, the impact of compound drops has received little attention. Here, we demonstrate a self-lubrication mechanism for water-in-oil compound drops impacting on a solid surface. Unexpectedly, the core water drop rebounds from the surface below a threshold impact velocity, irrespective of the substrate wettability. This is interpreted as the result of lubrication from the oil shell that prevents contact between the water core and the solid surface. We combine side and bottom view high-speed imaging to demonstrate the correlation between the water core rebound and the oil layer stability. A theoretical model is developed to explain the observed effect of compound drop geometry. This work sets the ground for precise complex drop deposition, with a strong impact on two- and three-dimensional printing technologies and liquid separation.

Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

Wearable flexible sensors based on integrated microfluidic networks with multiplex analysis capability are emerging as a new paradigm to assess human health status and show great potential in application fields such as clinical medicine and athletic monitoring. Well-designed microfluidic sensors can be attached to the skin surface to acquire various pieces of physiological information with high precision, such as sweat loss, information regarding metabolites, and electrolyte balance. Herein, the recent progress of wearable microfluidic sensors for applications in healthcare monitoring is summarized, including analysis principles and microfabrication methods. Finally, the challenges and opportunities for wearable microfluidic sensors in practical applications are discussed.

Pressure-Driven Two-Input 3D Microfluidic Logic Gates

Microfluidics is a continuously growing field with potential not only in the fields of medical, chemical, and bioanalysis, but also in the domains of optics and information technology. Here, a pressure-driven 3D microfluidic chip is demonstrated with multiple logic Boolean functions. The presence and absence of fluid at the output of the gates represent the binary signals 1 and 0, respectively. Therefore, the logic gates do not require a specially functionalized liquid to operate. The chip is based on a multilevel of poly(methyl methacrylate) (PMMA)-based polymeric sheets with aligned microchannels while a flexible polyimide-based sheet with a cantilever-like structure is embedded to enable a one-directional flow of the liquid. Several Boolean logic functions are realized (AND, OR, and XOR) using different fluids in addition to a half adder digital microfluidic circuit. The outputs of the logic gates are designed to be at different heights within the 3D chip to enable different pressure drops. The results show that the logic gates are operational for a specific range of flow rates, which is dependent on the microchannel dimensions, surface roughness, and fluid viscosity and therefore on their hydraulic resistance. The demonstrated approach enables simple cascading of logic gates for large-scale microfluidic computing systems.

Tunable Energy Absorption Characteristics of Architected Honeycombs Enabled via Additive Manufacturing

Tailoring of material architectures in three-dimensions enabled by additive manufacturing (AM) offers the potential to realize bulk materials with unprecedented properties optimized for location-specific structural and/or functional requirements. Here we report tunable energy absorption characteristics of architected honeycombs enabled via material jetting AM. We realize spatially tailored 3D printed honeycombs (guided by FE studies) by varying the cell wall thickness gradient and evaluate experimentally and numerically the energy absorption characteristics. The measured response of architected honeycombs characterized by local buckling (wrinkling) and progressive failure reveals over 110% increase in specific energy absorption (SEA) with a concomitant energy absorption efficiency of 65%. Design maps are presented that demarcate the regime over which geometric tailoring mitigates deleterious global buckling and collapse. Our analysis indicates that an energy absorption efficiency as high as 90% can be achieved for architected honeycombs, whereas the efficiency of competing microarchitected metamaterials rarely exceeds 50%. The tailoring strategy introduced here is easily realizable in a broad array of AM techniques, making it a viable candidate for developing practical mechanical metamaterials.

Multimaterial 3D Printing for Arbitrary Distribution with Nanoscale Resolution

At the core of additive manufacturing (3D printing) is the ability to rapidly print with multiple materials for arbitrary distribution with high resolution, which can remove challenges and limits of traditional assembly and enable us to make increasingly complex objects, especially exciting meta-materials. Here we demonstrate a simple and effective strategy to achieve nano-resolution printing of multiple materials for arbitrary distribution via layer-by-layer deposition on a special deposition surface. The established physical model reveals that complex distribution on a section can be achieved by vertical deformation of simple lamination of multiple materials. The deformation is controlled by a special surface of the mold and a contour-by-contour (instead of point-by-point) printing mode is revealed in the actual process. A large-scale concentric ring array with a minimum feature size below 50 nm is printed within less than two hours, verifying the capacity of high-throughput, high-resolution and rapidity of printing. The proposed printing method opens the way towards the programming of internal compositions of object (such as functional microdevices with multiple materials).

Three-dimensional-printing for microfluidics or the other way around?

As microfluidic devices are designed to tackle more intricate tasks, the architecture of microfluidic devices becomes more complex, and more sophisticated fabrication techniques are in demand. Therefore, it is sensible to fabricate microfluidic devices by three-dimensional (3D)-printing, which is well-recognized for its unique ability to monolithically fabricate complex structures using a near-net-shape additive manufacturing process. Many 3D-printed microfluidic platforms have been demonstrated but can 3D-printed microfluidics meet the demanding requirements in today's context, and has microfluidics truly benefited from 3D-printing? In contrast to 3D-printed microfluidics, some go the other way around and exploit microfluidics for 3D-printing. Many innovative printing strategies have been made possible with microfluidics-enabled 3D-printing, although the limitations are also largely evident. In this perspective article, we take a look at the current development in 3D-printed microfluidics and microfluidics-enabled 3D printing with a strong focus on the limitations of the two technologies. More importantly, we attempt to identify the innovations required to overcome these limitations and to develop new high-value applications that would make a scientific and social impact in the future.