Droplet Printing Research Articles

Heat accumulation effect in metal droplet-based 3D printing: Evolution mechanism and elimination Strategy

Uniform morphology and good metallurgical bonding between neighboring droplets are crucial in metal droplet-based 3D printing. However, this technique is usually accompanied by a heat accumulation effect during the printing process, leading to poor forming accuracy and non-uniform microstructure and properties of the formed parts. Since the heat accumulation effect is an intrinsic physical property in metal droplet-based 3D printing technique under fixed-parameter conditions, morphological deterioration or even instability phenomena are still common defects in this technique. Herein, a set of pillar printing experiments based on the 7075 aluminum alloy were conducted to demonstrate the influence of the heat accumulation effect on the forming accuracy in metal droplet-based 3D printing. A semi-quantitative analysis of the heat accumulation effect was also carried out based on the one-dimensional thermal conductivity theory. To further reveal the evolution mechanism of the heat accumulation effect during the aluminum micro-droplet printing, this paper develops a 3D numerical model of aluminum micro-droplet printing based on the volume of fluid (VOF) method. The evolution of the temperature field, heat flux, solid fraction, velocity field, and solidified morphology, during the continuous deposition of aluminum micro-droplets, were systematically investigated. The evolution mechanism of the heat accumulation effect and its influence on the forming accuracy was revealed. Finally, based on the thermal equilibrium theory, this paper proposed a variable frequency printing strategy to effectively eliminate the heat accumulation effect. This study provides a foundation for further development of metal droplet-based 3D printing technology. • Heat accumulation effect is verified to be a nature of metal droplet printing. • Adverse effects of heat accumulation on metal droplet printing are revealed. • Thermodynamics evolution mechanism during metal droplet printing is investigated. • A strategy is proposed to eliminate heat accumulation in metal droplet printing.

Acoustic Droplet Printing Tumor Organoids for Modeling Bladder Tumor Immune Microenvironment within a Week.

Current organoid models are limited by the incapability of rapidly fabricating organoids that can mimic the immune microenvironment for a short term. Here, an acoustic droplet-based platform is presented to facilitate the rapid formation of tumor organoids, which retains the original tumor immune microenvironment and establishes a personalized bladder cancer tumor immunotherapy model. In combination with a hydrophobic substrate, the acoustic droplet printer can yield a large number of homogeneous and highly viable bladder tumor organoids in vitro within a week. The generated organoids consist of all components of bladder tumor, including diverse immune elements and tumor cells. By coculturing tumor organoids with autologous immune cells for 2 days, tumor reactive T cells are induced in vitro. Furthermore, it is also demonstrated that these tumor-reactive T cells can also enhance the killing efficiency of matched organoids. Because of the easy operation, repeatability, and stability, the proposed acoustic droplet platform will provide a reliable approach for personalized tumor immunotherapy.

Fabricating patterned microstructures by embedded droplet printing on immiscible deformable surfaces

The deposition of droplets inside the deformable surfaces has attracted researchers for decades due to its application in direct-writing microporous polymer architectures, printing embedded flexible wires, patterning functional nanoparticles, etc. Herein, a patterned microstructure method based on droplets, named as the embedded droplet printing (EDP), is proposed. The experiments were conducted at a Weber number of 5.49–17.7 to explore the impact outcome, spreading laws and embedded morphology of the droplets. The rebound of droplets impacting the viscous surface was suppressed under appropriate conditions. The spreading factor of the droplet impacting on the high viscous surface followed the power law d*∝ t α. However, the exponent α was observed to be in the range 0.042–0.031, much smaller than the reported values (0.1–1), which could be explained by the Oh number. The diameters of droplets wrapped with viscous PDMS were only about 1/6th as compared to the spreading on the surface of PDMS precured for 30 min. In particular, a domain map was plotted in which patterns of solid bracts, plates and coffee rings were printed. Overall, EDP is a promising candidate to tailor the size, depth and morphology of droplets for preparing the patterned microstructures inside the soft materials.

Strip formation mechanisms and characteristics models in 3D printing of viscous polymer inks

The strip formation process on a glass surface in 3D printing of viscous droplets is presented with the relevant strip characteristics modelled. PEDOT:PSS polymer inks at different viscosities are used. It is found that unlike the symmetric circulation flow developed by a single droplet deposition, strip-printing by multiple droplets is achieved by the coalescence of droplets spreading unequally in different directions. Whereas the liquid viscous cohesion retards the flow in the direction aligning with the centre line of the coalesced droplets, the side spreading driven by liquid surface tension fills up the space on the target surface along the bridge between two coalesced droplets. When the deposition distance or space of two consecutive droplets is set at greater than a critical value, this flow inequality results in strips of scallop shape. With a deposition distance below the critical value, the strips formed tend to have parallel edges and the strip width increases with a decrease in the deposition distance. However, if the distance is set below a saturate level at which the liquid surface tension cannot withstand the deposited liquid volume, bulges are detrimentally formed randomly along the printed strips. Because of the viscous retardation to the spreading of drop liquid, the strips printed using a higher viscosity ink is narrower than that of a lower viscosity. Based on the strip formation process, the relevant characteristics of the printed strips are mathematically modelled using dimensional analysis. The developed models are found in good agreement with the experimental data and can be used to guide the selection of the operating parameters for strips with desired characteristics.

A low-cost, programmable, and multi-functional droplet printing system for low copy number SARS-CoV-2 digital PCR determination

Droplet digital polymerase chain reaction (ddPCR) is a rapidly developing technology used for accurate, quantitative analysis of rare samples. However, ddPCR has only been implemented in research field but rarely in clinical trials due to its relatively high cost and negative user experiences compared with qPCR. We developed a novel programmable on-demand droplet generator based on a microfluidic adaptive printing system (MAP-ddPCR) to create a cost-effective ddPCR process. This process easily produces low-volume, spot-on-demand droplet dispensing and data analysis using simple equipment and workflow. Compared with the existing droplet generation systems that rely on surface-assistant, the proposed MAP system generates a variety of droplet arrays on regular non-surface-treated glass slides. This system directly processes PCR and performs data analysis without requiring a secondary droplets transfer. The static and independent properties of each droplet dramatically avoid cross-contamination during PCR, provide the opportunity to trace droplets in real-time and simplify the analysis. We demonstrated that the MAP-ddPCR produces reliable data using gradient concentrations of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in human genomic cDNA. These concentrations were further verified by quantitative PCR (qPCR). In addition, a very low viral load of SARS-CoV-2 was precisely detected and quantified by the MAP-ddPCR system. Finally, this system is affordable and simpler to integrate compared to other more expensive commercial digital PCR methods. Therefore, the proposed MAP-ddPCR system is expected to have a significant impact on market applications.

Emulsion Designer Using Microfluidic Three-Dimensional Droplet Printing in Droplet.

Hierarchical emulsions are interesting for both scientific researches and practical applications. Hierarchical emulsions prepared by microfluidics require complicated device geometry and delicate control of flow rates. Here, a versatile method is developed to design hierarchical emulsions using microfluidic 3D droplet printing in droplet. The process of droplet printing in droplet mimics the dragonfly laying eggs and has advantages of easy processing and flexible design. To demonstrate the capability of the method, double emulsions and triple emulsions with tunable core number, core size, and core composition are prepared. The hierarchical emulsions are excellent templates for the developments of functional materials. Flattened crescent-moon-shaped particles are then fabricated using double emulsions printed in confined 2D space as templates. The particles are excellent delivery vehicles for 2D interfaces, which can load and transport cargos through a well-defined trajectory under external magnetic steering. Microfluidic 3D droplet printing in droplet provides a powerful platform with improved simplicity and flexibility for the design of hierarchical emulsions and functional materials.

Material jet printing of transparent ceramic Yb:YAG planar waveguides.

A new, to the best of our knowledge, 3D additive manufacturing technique utilizing particle-loaded ink jet printing to fabricate transparent ceramic Yb:YAG planar waveguides for laser gain media was demonstrated. Rheological optimization of YAG particle-loaded inks resulted in successful droplet formation and printing resolution. Planar waveguides composed of a Yb:YAG guide encased in undoped YAG cladding were printed with guide thicknesses ranging between 25 and 350 µm and consolidated to high optical quality via solid-state sintering. Sufficiently low optical (1-3%/cm) and intermodal scatter allowed single-mode propagation with a core/clad index difference of $\Delta {n}\sim{5.0} \times {{10}^{- 4}}$ (corresponding to 3 at.% Yb). The waveguides were cladding-pumped longitudinally with a 940 nm diode array resulting in 23.6% slope efficiency in 2 ms pulsed operation.

Continuous Embedded Droplet Printing in Yield‐Stress Fluids for Pharmaceutical Drug Particle Synthesis

AbstractEmbedded droplet printing is a recent mode of generating and processing droplets within yield‐stress fluids. This technique has shown promise for performing sensitive processes like pharmaceutical crystallization, as well as chemical synthesis and biological experimentation due to the unique ability to process droplets that are quiescently suspended. Despite improving on conventional microfluidic technologies in numerous ways, current embedded droplet printing methods are limited to batch processes, severely hampering their overall utility. A new platform that enables continuous production of embedded droplets is presented and characterized. This platform expands the capabilities of embedded droplet printing and allows for its application to areas of continuous materials discovery, screening, and manufacturing. Here, the platform is used for the rapid production of pharmaceutical particles that are highly spherical and uniform, key targets for flowability, and ultimately manufacturability of pharmaceutical drug products. The presented platform achieves a maximum throughput of over 100 g per day, enabling characterization of the superior powder flow properties. The available operating space of this platform is demonstrated for an antisolvent crystallization process with an anti‐malarial drug. This understanding provides design guidelines for how similar platforms may be engineered for precise, rapid, customized, and distributed manufacturing of drug particles with superior flowability.

Laser-induced forward transfer of graphene oxide

Achieving excellent quality printing of graphene oxide is highly demanded in superior performance device fabrication. Laser-induced forward transfer (LIFT) is a promising method for printing low viscosity liquids, which alleviates the restrictions regarding the rheological properties and the size of the particles suspended in the printing ink. However, the investigation of GO printing is very limited, and this procedure still suffers from low quality and reproducibility. Herein, an effective approach of printing reproducible and well-defined GO droplets through LIFT was presented. Laser ablation was used to create a few microns deep cavity on the microscope glass, which led to the superhydrophilicity of the ablated surface and helped in the formation of a uniform GO film. Subsequently, droplet printing using LIFT method with the modified donor substrate and various pulse energies was investigated. Droplets with a relatively circular shape and limited debris were successfully printed. The droplet diameter grew linearly from 40 to 165 μm with the increasing pulse energy from 4 to 11 μJ. Finally, LIFT of GO with the cleaned donor substrate was studied, and the repetitive printing results showed the feasibility of reusing the modified donor substrate.

Droplet printing reveals the importance of micron-scale structure for bacterial ecology

Bacteria often live in diverse communities where the spatial arrangement of strains and species is considered critical for their ecology. However, a test of this hypothesis requires manipulation at the fine scales at which spatial structure naturally occurs. Here we develop a droplet-based printing method to arrange bacterial genotypes across a sub-millimetre array. We print strains of the gut bacterium Escherichia coli that naturally compete with one another using protein toxins. Our experiments reveal that toxin-producing strains largely eliminate susceptible non-producers when genotypes are well-mixed. However, printing strains side-by-side creates an ecological refuge where susceptible strains can persist in large numbers. Moving to competitions between toxin producers reveals that spatial structure can make the difference between one strain winning and mutual destruction. Finally, we print different potential barriers between competing strains to understand how ecological refuges form, which shows that cells closest to a toxin producer mop up the toxin and protect their clonemates. Our work provides a method to generate customised bacterial communities with defined spatial distributions, and reveals that micron-scale changes in these distributions can drive major shifts in ecology.

Drop-on-demand assessment of microdrops of dilute ZnO–water nanofluids

Shrinking device dimensions demand a high level of control and manipulation of materials at microscale and nanoscale. Microfluidics has a diverse application spectrum including thermal management of chips, point-of-care diagnostics, and biomedical analysis, to name a few. Inkjet printing (IJP) is a manufacturing method used for micro-/nanofabrication and surface restructuring, and liquid inks are characterized based on their density, surface tension, and viscosity for their printability. Nanofluids as colloidal dispersions of nanoparticles hold potential in various heating, cooling, lubricating, and biomedical applications with the premise of nanoparticles’ size and concentration effects and interactions between nanoparticle–nanoparticle and nanoparticle–base fluid. In order to explore the microfluidic behavior of nanofluids, using micro-volumes of nanofluids and/or confining them in a micro-system is essential. With this motivation, we present a printability assessment on the potential of low concentration ZnO–water nanofluids by utilizing a combined theoretical and experimental approach. For 0.05 vol. %–0.4 vol. % of ZnO–water nanofluids, results showed that for a nozzle diameter of 25 μm, the samples do not exhibit the energy necessary for drop formation, while for 50 μm and 100 μm nozzle diameters, the samples behave as satellite droplets. Although satellite droplets were generally not desirable for IJP, the recently introduced satellite droplet printing concept may be applicable to the printing of aqueous nano-ZnO dispersions considered in this work.

The acoustic droplet printing of functional tumor microenvironments.

The fabrication of functional tissue is important for tissue engineering, regenerative medicine, and biological research. However, current 3D bioprinting technologies mean it is hard to precisely arrange bioinks (composed of cells and materials) in a high-fidelity 3D structure and print cells of multiple types with sufficient concentrations and superior viabilities; this can severely constrain cell growth, interactions, and functions. Here, an acoustic droplet printing method is introduced to solve these problems in 3D bioprinting. Being nozzle-free, the acoustic printer stably enables high-concentration cells, or even cell spheroids, to be printed without clogging. Cell viability (>94%) using post acoustic printing is higher than those obtained with currently used inkjet-based (>85%) and extrusion-based (40-80%) bioprinting methods. Also, this method involves a small printer that can be flexibly integrated, allowing different kinds of bioinks to be printed. Moreover, the limited printability of low-concentration gelatin methacryloyl (5% (w/v) GelMA) materials is overcome by determining the positioning, fluidity (e.g., spreading), and 3D morphology of the GelMA droplets; therefore, high-fidelity 3D constructs can be fabricated. As a proof of concept, a tumor microenvironment consisting of one tumor spheroid surrounded by a high concentration of cancer-associated fibroblasts (CAFs) was constructed; this was able to establish a dynamic tumor invasion function modulated by reciprocal tumor cell-CAF interactions. The nozzle-free, contact-free, and low cell-damage merits of this method will advance bioprinting, allowing the creation of more functional native tissues, organoids, or disease models.

A novel method to improve the line resolution of stretchable graphene-based line by embedded uniform droplet printing

Solution-phase printing of nanomaterials is a low-cost and effective way to build functional devices. While a multitude of industries have witnessed steady progress in it, the low line resolution remains a challenge for inkjet printing, since the droplet spreading on a substrate inevitably widens the printing tracks. In the present work, a novel embedded printing method based on a liquid viscous film to produce high-resolution lines without a small nozzle (<20 μm in diameter) or modified substrates was demonstrated. The influences of pre-cured time, substrate velocity and solvent ratio on the diameter of a single droplet and the continuity of multiple droplets were explored. A cylindrical reduced graphene oxide (rGO) line with a diameter of 10 μm had been deposited on the surface of the viscous liquid substrate and slowly wrapped into polydimethylsiloxane (PDMS) thin film by using a nozzle of 110 μm in diameter after optimizing the experimental parameters. The rGO lines encapsulated into PDMS could be stretched to 180% of its original length, during which no shedding or breaking occurred. This work presents a facile way to improve the line resolution of flexible/stretchable graphene-based lines, which probably contribute to further application of inkjet printing technology in the field of flexible and stretchable devices.

Embedded droplet printing in yield-stress fluids

Microfluidic tools and techniques for manipulating fluid droplets have become core to many scientific and technological fields. Despite the plethora of existing approaches to fluidic manipulation, non-Newtonian fluid phenomena are rarely taken advantage of. Here we introduce embedded droplet printing-a system and methods for the generation, trapping, and processing of fluid droplets within yield-stress fluids, materials that exhibit extreme shear thinning. This technique allows for the manipulation of droplets under conditions that are simply unattainable with conventional microfluidic methods, namely the elimination of exterior influences including convection and solid boundaries. Because of this, we believe embedded droplet printing approaches an ideal for the experimentation, processing, or observation of many samples in an "absolutely quiescent" state, while also removing some troublesome aspects of microfluidics including the use of surfactants and the complexity of device manufacturing. We characterize a model material system to understand the process of droplet generation inside yield-stress fluids and develop a nascent set of archetypal operations that can be performed with embedded droplet printing. With these principles and tools, we demonstrate the benefits and versatility of our method, applying it toward the diverse applications of pharmaceutical crystallization, microbatch chemical reactions, and biological assays.

Subnanoliter precision piezo pipette for single-cell isolation and droplet printing

Although microliter-scale liquid handling with a handheld pipette is a routine task, pipetting nanoliter-scale volumes is challenging due to several technical difficulties including surface tension, adhesion and evaporation effects. We developed a fully automated piezoelectric micropipette with a precision of < 1 nanoliter, improving the efficiency of imaging-based single-cell isolation to above 90%. This improvement is crucial when sorting rare or precious cells, especially in medical applications. The compact piezoelectric micropipette can be integrated into various (bio)chemical workflows. It eliminates plastic tubes, valves, syringes, and pressure tanks. For high-quality phase-contrast illumination of the sample, e.g., cells or tiny droplets, we constructed rings of LEDs arranged concentrically to the micropipette. The same device can be readily used for single-cell printing and nanoliter-scale droplet printing of reagents using either fluorescent or transparent illumination on a microscope. We envision that this new technology will shortly become a standard tool for single-cell manipulations in medical diagnostics, e.g., circulating tumor cell isolation.

Embedded printing trace planning for aluminum droplets depositing on dissolvable supports with varying section

• The formation of hole-defects is revealed to be an orientation-dependent feature. • The printing strategy for the corners of dissolvable support is first proposed. • Droplet scanning spacings on surfaces with preset dimensions are established. • A dense aluminum horn with high-quality inner surfaces is successfully printed. Droplet-based 3D printing is a promising method to manufacture thin-walled parts with high-quality cavity surfaces of a complex geometry by virtue of dissolvable supports. However, conventional equispaced droplet scanning strategies cannot be used for dissolvable support assisted droplet printing that involves more complex impingement conditions, particularly when the dissolvable supports possess varying sections. This paper investigates the printing strategies for aluminum droplets depositing on dissolvable support that possesses preset dimensions, inclined surfaces and corners. Experimental and numerical results show that: (1) printing orientation on the oblique surfaces of dissolvable supports significantly influences the formation of hole-defects; (2) the printing strategy for corners should be specially adjusted to minimize defects. Moreover, the droplet scanning spacings on the support surfaces with preset shape and dimensions are elaborately regulated according to four different cases. Finally, the fabrication experiments of aluminum horns are performed to further examine the formability of the established droplet scanning strategies. The density of the printed horn is measured to be ~98% using the standard Archimedes method, and the corresponding industrial CT scanning results also verify a porosity-free inner structure. The cavity surfaces of the horn are smooth without obvious defects, and the average surface roughness of the inner surface is ~Ra 4.2 μm (only 0.53% of the droplet diameter). These measurement results indicate that the proposed strategies can effectively eliminate hole-defects that are commonly seen in equispaced printing.

Laser-induced forward transfer of silver nanoparticle ink using burst technique

Laser-induced forward transfer (LIFT) is an effective approach to print materials in liquid state with high resolution. This procedure, however, suffers from bulging problem when printing continuous lines or patterns. In this study, LIFT of silver nanoparticle ink using burst technique was developed to mitigate this issue during printing continuous lines. Firstly, a set of droplet printing experiments were conducted to investigate the influences of the pulse energy and burst mode on the morphologies of the printed features. It was found that the resolution was enhanced and the phenomenon of splashing was improved with the introduction of burst technique at the same energy level. Thereafter, a group of lines were printed by changing the scanning speed in bursts of 1, 2 or 3 pulses. The results showed that the bulging of the printed line was effectively mitigated and the resolution was significantly improved in burst-3 mode.

Mass printing of colored natural patterns on Al plate by roll imprinting and thin film deposition

Abstract Metal surfaces finished in color patterns are useful and attractive for both practical and artistic purposes. Although the surface of metals can be patterned by diverse methods including computerized-numerical-control machining, laser-direct surface structuring, and droplet printing, these serial processes are not suitable for the massive manufacturing of complex patterns with variant feature sizes. In this work, we present a continuous roll-imprinting process as a large-area, high-throughput method for producing arbitrary surface patterns on Al. The surface patterns of natural objects such as silk, leather, and paper are replicated via Ni electroplating, and the electroplated Ni replica mold is used as the imprinting roller to transfer the original natural pattern onto an Al plate. The conformity of the imprinted pattern to the original pattern is 80―85 %. Surface-patterned Al plates are also colorized by coating a metal-dielectric double layer, where the top metal layer adjusts the amount of light incident into the underlying dielectric film, strengthening the interference effect. Diverse colors are generated by varying the thicknesses of the coating layers. The colors obtained via experiments agree well with the results predicted using finite-difference time-domain simulation. This study provides a fast and scalable route for producing colored surface patterns on metals and may find various applications ranging from surface decoration and visual arts to product identification and anti-counterfeiting.

Physical processes affecting the survival of microbiological systems in laser printing of gel droplets

We consider laser printing of gel microdroplets – a promising method for microbiology, biotechnology and medicine. In the printing process, small volumes of gel containing living microorganisms are transferred as a result of cavitation caused by the absorption of a short laser pulse in a metal film. However, in such a transfer, certain physical factors arise that can lead to damage and death of biological material. These factors include elevated temperature and pressure, high radiation intensity and some others. Experimental estimates of these parameters are conducted, based on measurements of the acoustic response of laser printing, electron microscopy of the affected areas and the results of high-speed imaging of the transfer process. It is shown that these factors are not a significant limitation for the technology being developed. Laser printing is performed by exposing a metal film to laser pulses with an energy of 5 – 30 μJ and a duration of 8 – 14 ns, the laser beam diameter being 30 μm.

Role of Surfactant in Evaporation and Deposition of Bisolvent Biopolymer Droplets.

Inkjet printing of biopolymer droplets is gaining popularity because of its potential applications in regenerative medicine, particularly the fabrication of tissue-regenerative scaffolds. The quality of bioprinting, which affects cellular behaviors and the subsequent tissue formation, is determined by the solvent evaporation and deposition processes of biopolymer droplets, during which instantaneous local viscosity and surface tension changes occur because of the redistribution of the biopolymer inside the drop. Such dynamics is complex and not well understood. Most biopolymer inks also contain multiple solvents of distinct evaporation rates, further complicating the system dynamics. Using high-speed interferometry, we directly observe in real time the instantaneous drop shape of inkjet-printed picoliter gelatin drops containing glycerol and water. It is observed that, for bisolvent gelatin drops with surfactants, highly viscous gelatin and glycerol accumulated near the pinned contact line at an early stage suppress the evaporation-driven outward flow and create a stagnation zone near the contact line region. Lower surface tension at the contact line, because of its high local surfactant concentration, as compared to the drop apex induces a strong Marangoni recirculation, which in conjunction with a stagnation zone in the contact line region causes the instantaneous drop shape to transition from a spherical cap to a volcano shape during evaporation and resulting in a volcano-like deposition profile. In contrast, the suppressed evaporation outward flow together with a weak Marangoni flow leads to a domelike deposition for the case without surfactant. The role of surfactant in polymer drop deposition with water-only solvent is also investigated and compared against that of bisolvent drops. For the single-solvent case, the deposition profile is found to shift from a coffee-eye shape in the presence of surfactant to a uniform deposition without surfactant. The results reveal new insight into the complex role surfactant plays during polymer drop evaporation and deposition processes.