Droplet formation at megahertz frequency

Droplet formation has been a fascinating subject to scientists for centuries due to its natural beauty and importance to both scientific and industrial applications, such as inkjet printing, reagent deposition, and spray cooling. However, the droplet generation frequency of common drop‐on‐demand (DOD) jetting techniques is mostly limited to ～10 kHz. This article presents an investigation of the possibility of jetting at megahertz frequencies to boost the productivity of DOD material deposition by ～100 times. The focus of this article is to understand the limitations of generating droplets at a megahertz frequency and to explore possible solutions for overcoming these limitations. A numerical model is first developed for the simulation of droplet formation dynamics. The numerical model is validated against available experimental data from the literature. Aided by insights gained from scaling analysis, the validated model is then used to study the effects of different parameters on high frequency jetting. The study finds energy density input to the nozzle is the key to achieve megahertz frequency droplet breakup. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2367–2377, 2017     

工艺参数对聚合物熔体喷射成滴的影响

介绍了一种机械冲击式聚合物熔体微滴喷射装置,并采用实验分析了不同螺杆转速与冲击频率、不同喷嘴直径、不同基板到喷嘴距离3组工艺参数对聚丙烯（PP）熔体微滴喷射成滴的影响。结果表明,熔体喷射成滴的质量受以上各参数的共同作用影响,只有精确控制以上各工艺参数才能实现均匀微小的聚合物熔体微滴的喷射,从而保证其用于三维打印（3D打印）制造技术时可以实现高精度成型。     

Effects of polymer properties on jetting performance of electrohydrodynamic printing

We perform systematic study on the jetting performance of electrohydrodynamic (EHD) with an insulating polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). EHD printing applies electrostatic field to ink droplet hang on nozzle tip, which causes the deformation of the meniscus to generate discrete droplets or continuous jet stream. Although EHD jetting mechanism has been frequently investigated with conducting or semiconducting materials, there still needs to elucidate EHD jetting of insulating polymer materials for producing controllable droplets. In the present study, we focused on how the physical/chemical properties (conductivity, dielectric constant, and molecular weight) of an insulating polymer affect jetting behavior of EHD printing (especially, the deformation of the meniscus and the corresponding morphology of the printed one). The relationship between the printing parameters and applied voltage is also investigated, thereby allowing the optimization of EHD printings for PS and PMMA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45044.     

The forming process of polymer melt droplet deposition three-dimensional printing

The polymer droplet deposition technology is a rapidly developing three-dimensional (3D) printing technology, and is becoming a research hotspot. In this study, the qualitative relationship between the temperature-field distribution and the forming quality was determined by using the finite-element analysis, that is, the “birth and death of element” technology, the ANSYS parametric design language command. We obtained the relationship between process parameters and droplet size and droplet spacing through orthogonal experiments, and the theoretically optimal process parameters for droplet formation and arrangement were determined. We studied the relationship between the 3D spacing (W_x, W_y, and W_z) in the droplets and the product forming quality. We further optimized the theoretically optimal process parameters in the actual droplet deposition experiment, and finally determined the actual optimal droplet deposition process parameters as follows: the jet frequency is 14 Hz, the screw rotation speed is 10 rpm, the printing speed is 30 mm/s, and the printing distance is 1 mm. This article theoretically analyzed the influence of the polymer droplet deposition process on the forming quality from thermodynamic simulation and experiments that provide a theoretical basis for further improving the forming quality of the process.     

Personalized Single‐Cell Encapsulation Using E‐Jet 3D Printing with AC‐Pulsed Modulation

With the growing therapeutic importance of cell microcarriers, there has been a rise in the need to develop technologies that facilitate efficient microencapsulation of cells, currently limited by a lack of straightforward and low‐cost strategies for single‐cell isolation and printing. Thus, the aim of this study is to develop a gentle and cell‐compatible electro‐hydrodynamic jet 3D printing technique to facilitate the efficient microencapsulation of cells in hydrogel microspheres, and investigate the effects of parameters (flow rate, voltage frequency, nozzle diameter, working distance, and substrate velocity) on the printing process. Stable microspheres are obtained by regulating these parameters to balance various forces, with control of their diameters in the range of 100–600 µm. The study demonstrates that under optimized conditions, the technique is able to successfully encapsulate cells within hydrogel microspheres with high viability over a wide range of diameters. This 3D printing technique expands the potential utility of microspheres into additional biological applications, such as cancer biology and drug screening. It can also be used as an effective platform for the production of tumor spheroids, generating multicellular spheroid models in vitro or for injectable cell delivery.     

Performance modulation and 3D printing parameters optimization of implantable medical tricalcium-silicate/polyetherimide composite

Tricalcium silicate (C3S)/polyetherimide (PEI) stents are manufactured through an additive manufacturing process using binder jetting. The key issues of C3S/PEI composite ceramic slurry and additive manufacturing process parameters are discussed in detail. Firstly, the low-temperature auxiliary sintering temperature of the sample was determined, and the influence of PEI content on the compressive strength and bending strength before and after sintering was studied. The sintering temperature and optimal PEI content are 340 °C and 10 wt%. Under this PEI content, the flow rate change during the printing process of the slurry was measured, and a C3S/PEI composite slurry suitable for binder jetting additive manufacturing was obtained, and it had excellent mechanical properties. The effect of the parameters of the binder jetting additive manufacturing process on the molding quality of the C3S/10PEI composite ceramic slurry was studied. The effect of the printed layer height on the deposition line width and height was explored, resulting in a selection rule for the printing layer height using nozzle diameters. The influence of the number of layers of the printed sample on the height and line width of the sample is studied. Under the condition that the height of the printing layer is 80% of the nozzle diameter and the hot air assisted drying, the maximum error of the forming size is only 3.13%. Finally, the biocompatibility and cell adsorption effect of the scaffold were studied, and it was found that the C3S/PEI scaffold, which was additively manufactured by binder jetting and sintered at low temperature, had good biological properties.     

UV‐assisted three‐dimensional printing of polymer nanocomposites based on inorganic fillers

In this work, nanocomposites based on a UV‐curable polymeric resin and different inorganic fillers were developed for use in UV‐assisted three‐dimensional (UV‐3D) printing. This technology consists in the additive multilayer deposition of a UV‐curable resin for the fabrication of 3D macro structures and microstructures of arbitrary shapes. A systematic investigation on the effect of filler concentration on the rheological properties of the polymer‐based nanocomposites was performed. In particular, the rheological characterization of these nanocomposites allowed to identify the optimal printability parameters for these systems based on the shear rate of the materials at the extrusion nozzle. In addition, photocalorimetric measurements were used to assess the effect of the presence of the inorganic fillers on the thermodynamics and kinetics of the photocuring process of the resins. By direct deposition of homogeneous solvent‐free nanocomposite dispersions of different fillers in a UV‐curable polymeric resin, the effect of UV‐3D printing direction, fill density, and fill pattern on the mechanical properties of UV‐3D printed specimens was investigated by means of uniaxial tensile tests. Finally, examples of 3D macroarchitectures and microarchitectures, spanning features, and planar transparent structures directly formed upon UV‐3D printing of such nanocomposite dispersions were reproducibly obtained and demonstrated, clearly highlighting the suitability of these nanocomposite formulations for advanced UV‐3D printing applications. POLYM. COMPOS., 38:1662–1670, 2017. © 2015 Society of Plastics Engineers     

Photorealistic ink‐jet digital printing – factors influencing image quality,image stability and print durability

Digital cameras have now replaced film‐based cameras as the most popular method of still image capture. As a consequence, there has been a rise in growth of the photo‐realistic digital‐printer market as many amateur and professional photographers choose to produce their own hard copy images. The most popular digital printing technology for producing photo‐realistic images is currently drop‐on‐demand ink jet. There has been much research into key factors influencing the quality, stability and durability of images produced using this, and other, digital printing technologies. A key area of study in achieving photo‐realistic images from digital printing systems has been ink‐receivable layers and dye/pigment colorants, and most importantly compatibility between the two. As with any new technology it is important to achieve an acceptable standard of performance and, to this end, research work has been instigated by the International Standards Institute since the mid‐1990s, to achieve a set of standards appertaining to areas such as light fastness, water fastness, thermal stability, humidity fastness and pollution susceptibility. This paper reviews the current state regarding the aforementioned areas with respect to their influence on print quality, stability and durability.     

3D打印技术的应用

本文介绍了3D打印在小批量物件制造方面的优势及应用。3D打印是一场制造技术的革命,是中国制造业升级的重要一环。国内北航、华中科大、西安交大、清华四大研发中心在3D打印方面积累了国际一流的技术储备,在航空结构件锻造等领域甚至实现了世界首次突破。从需求端看,未来几年内航天军工、民用消费、模具设计、医疗领域将驱动3D打印需求超越式增长。     

Nanogrooved carbon microtubes for wet three‐dimensional printing of conductive composite structures

Recent advances in three‐dimensional (3D) printing have enabled the fabrication of interesting structures which are not achievable using traditional fabrication approaches. The 3D printing of carbon microtube composite inks allows fabrication of conductive structures for practical applications in soft robotics and tissue engineering. However, it is challenging to achieve 3D printed structures from solution‐based composite inks, which requires an additional process to solidify the ink. Here, we introduce a wet 3D printing technique which uses a coagulation bath to fabricate carbon microtube composite structures. We show that through a facile nanogrooving approach which introduces cavitation and channels on carbon microtubes, enhanced interfacial interactions with a chitosan polymer matrix are achieved. Consequently, the mechanical properties of the 3D printed composites improve when nanogrooved carbon microtubes are used, compared to untreated microtubes. We show that by carefully controlling the coagulation bath, extrusion pressure, printing distance and printed line distance, we can 3D print composite lattices which are composed of well‐defined and separated printed lines. The conductive composite 3D structures with highly customised design presented in this work provide a suitable platform for applications ranging from soft robotics to smart tissue engineering scaffolds. © 2019 Society of Chemical Industry     

An Electrospray Method Using a Multi‐Capillary Nozzle Emitter

A multi‐capillary nozzle emitter consisting of one metal plate with capillary nozzles and a ring type counter electrode was used as a multi‐electrospray atomizer. The number of capillary nozzles, flow rate of the liquid and the interval between the capillary nozzles were changed, and the droplet diameter and the voltage required for a steady cone‐jet mode were measured. For the multi‐capillary nozzle emitter, the interaction between the capillary nozzles is the important factor for obtaining fine droplets of uniform size. These fine droplets are obtained when there is only a small interaction between the capillary nozzles, and the equations obtained from the single capillary nozzle case are also applicable for the multi‐capillary nozzle emitter. When the number of capillary nozzles decreases (a situation which is not good for obtaining a large amount of droplets) or the interval between the capillary nozzles increases, the interaction between the capillary nozzles can be reduced. As the number of capillary nozzles increase, a higher voltage is required to obtain a fine droplet of uniform size.     

Mechanical Assembly of Thermo‐Responsive Polymer‐Based Untethered Shape‐Morphing Structures

Shape‐morphing robotic structures can provide innovative approaches for various applications ranging from soft robotics to flexible electronics. However, the programmed deformation of direct‐3D printed polymer‐based structures cannot be separated from their subsequent conventional shape‐programming process. This work aims to simplify the fabrication process and demonstrates a rapid and adaptable approach for building stimulus‐responsive polymer‐based shape‐morphing structures of any shape. This is accomplished through mechanically assembling a set of identical self‐bending units in different patterns to form morphing structures using auxiliary hard connectors. A self‐bending unit fabricated by a 3D printing method can be actuated upon heating without the need for tethered power sources and is able to transform from a flat shape to a bending shape. This enables the assembled morphing‐structure to achieve the programmed integral shape without the need for a shape‐programming process. Differently assembled morphing structures used as independent robotic mechanisms are sequentially demonstrated with applications in biomimetic morphing structures, grasping mechanisms, and responsive electrical devices. This proposed approach based on a mechanical assembling method paves the way for rapid and simple prototyping of stimulus‐responsive polymer‐based shape‐morphing structures with arbitrary architectures for a variety of applications in deployable structures, bionic mechanisms, robotics, and flexible electronics.     

A descriptive force‐balance model for droplet formation at microfluidic Y‐junctions

In a previous article, we studied the basics of emulsification in microfluidic Y‐junctions, however, without considering the effect of viscosity of the disperse phase. As it is known from investigations on many different microstructures that viscosity and viscosity ratio are governing parameters for droplet size, we here investigate whether this is also the case for microfluidic Y‐junctions and do so for a wide range of process conditions. The investigated Y‐junctions have a width of 19.9 or 12.8 μm and a depth of 5.0 μm, and the formed monodisperse droplets (CV < 1%) are between 3 and 20 μm. We varied the disperse‐phase viscosity using different oils (1–105 mPa s), and continuous‐phase viscosity using glycerol–water and ethanol–water mixtures (1.0–6.2 mPa s), which corresponds to disperse‐to‐continuous‐phase viscosity ratios from 0.4 to 105.0. Through the variation of the liquids, also a range in interfacial tensions (12–55 mN m^?1) is assessed. The disperse‐phase flow rate is varied from 0.039 to 18.0 μL h^?1, the continuous‐phase flow rate from 1.39 μL h^?1 to 0.41 mL h^?1, and this corresponds to flow rate ratios from 1.1 × 10^?3 to 0.14, which is once again based on wide range of conditions. For all these conditions, in which droplets are formed in the dripping and jetting regime, the droplet size could be described with a model based on the existing force‐balance model, but now extended to incorporate the cross‐sectional area of the droplet and the resistance with the wall. Surprisingly enough, it was found that the droplet size is not influenced by the disperse‐phase viscosity, or the viscosity ratio, but it is dominated by the resistance with the wall and the continuous‐phase properties. Because of this, emulsification with Y‐junctions is intrinsically simpler than any other shear‐based method as droplet size is only determined by the continuous phase. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Experimental and computational study of microfluidic flow‐focusing generation of gelatin methacrylate hydrogel droplets

This work presents the experimental and computational study of droplet generation for hydrogel prepolymer solution in oil using a flow‐focusing device. Effects of different parameters on hydrogel droplet generation and droplet sizes in a flow‐focusing device were investigated experimentally and computationally. First, three dimensional (3D) computational simulations were conducted to describe the physics of droplet formation in each regime and mechanism of three different regimes: squeezing, dripping, and jetting regime of hydrogel were investigated. Subsequently, the effects of viscosity, inertia force, and surface tension force on droplet generation, and droplet size were studied through these experiments. The experiments were carried out using different concentration of gelatin methacrylate (GelMA) hydrogel (5 wt % and 8 wt %) as the dispersed phase and two different continuous phase liquids (light mineral oil and hexadecane) with various concentrations of surfactant (0 wt %, 3 wt %, and 20 wt %). All experimental data was summarized by capillary number of dispersed phases and the continuous phases to characterize the different regimes of droplet generation and to predict the transition of dripping to a jetting regime for GelMA solution in flow‐focusing devices. It is shown that the transition of dripping to a jetting regime for GelMA happens at lower capillary numbers compared to aqueous solutions. Moreover, by increasing the viscous force of continuous phase or decreasing the interfacial force, the size of GelMA droplets was decreased. By controlling these parameters, the droplet sizes can be controlled between 30 μm and 200 μm, which are very suitable for cell encapsulation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43701.     

Solvent-Cast 3D Printing of Biodegradable Polymer Scaffolds

3D printing is a popular fabrication technique because of its ability to produce complex architectures. Melt-based 3D printing is widely used for thermoplastic polymers like poly(caprolactone) (PCL), poly(lactic acid) (PLA), and poly(lactic-co-glycolic acid) (PLGA) because of their low processing temperatures. However, traditional melt-based techniques require processing temperatures and pressures high enough to achieve continuous flow, limiting the type of polymer that can be printed. Solvent-cast printing (SCP) offers an alternative approach to print a wider range of polymers. Polymers are dissolved in a volatile solvent that evaporates during deposition to produce a solid polymer filament. SCP, therefore, requires optimizing polymer concentration in the ink, print pressure, and print speed to achieve desired print fidelity. Here, capillary flow analysis shows how print pressure affects the process-apparent viscosity of PCL, PLA, and PLGA inks. Ink viscosity is also measured using rheology, which is used to link a specific ink viscosity to a predicted set of print pressure and print speed for all three polymers. These results demonstrate how this approach can be used to accelerate optimization by significantly reducing the number of parameter combinations. This strategy can be applied to other polymers to expand the library of polymers printable with SCP.     

Design and fabrication of transdermal drug delivery patch with milliprojections using material extrusion 3D printing

This paper investigates the effect of various 3D printing parameters on a transdermal drug delivery system with milliprojections printed using poly(lactic acid) (PLA). The parameters studied involve printing temperature, layer thickness, extrusion width, infill width, and nozzle orifice diameter. Their effects on the final print quality were evaluated based on the surface finish and dimensional accuracy of the milliprojections. The change in the molecular weights of the polymers after extrusion and printing suggests thermal degradation. Further thermal analysis also showed that 3D printing decreases polymer crystallinity. The parameters studied showed varying effects on print quality with respect to PLA types and the dimensions involved. In general, it is sensible to process at temperature close to the melting point of a semicrystalline polymer or at the lowest temperature for which the polymer flows for an amorphous polymer. Although the layer thickness and extrusion width affect each dimension differently, it was found to be a reasonable approach to choose a thinner layer for a more accurate tip, and a thinner infill width for more accurate part diameters. Lastly, it was found that the smaller nozzle orifice and increased spacing between milliprojections produced better surface finish but had no significant effect on part accuracy. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48777.     

4D printed stereolithography printed plant-based sustainable polymers: Preliminary investigation and optimization

The increasing demand for applying shape memory polymer to tissue culture and biomedical engineering has opened up research opportunities in the field of 4D Printing. The biocompatibility of the scaffolds as a culture medium resulted in the use of plant-based polymers to provide an ambient environment for the growth of cells. This research investigates the 4D printing of acrylated epoxidized soybean oil (AESO), a plant-based shape polymer. The objective of the present work is to establish the relationship between the 4D printing parameters (laser power frequency and print speed) and different properties of the printed material viz. tensile stress, surface roughness, wettability, recovery time, strain fixity and glass transition temperature. The maximum fixity was about 85%, while the recovery time as low as 1.6 s. The print parameters are optimized using regression modeling and multi-objective optimization techniques. The shape memory effect of the polymer is demonstrated by printing samples at the optimized conditions. Dynamic mechanical analysis is performed to evaluate the variation in the glass transition temperature of AESO at specific print parameters. The adoption of an optimal set of laser frequency and print speed is found to improve the properties of AESO, while built by micro stereolithography (micro-SLA).     

Stepwise Optimized 3D Printing of Arbitrary 3D Structures at Millimeter Scale with High Precision Surface

3D printing based on additive manufacturing has attracted widespread attention in the fields of microbiology and microelectronics due to its advantages of waste reduction, arbitrary manufacturing, and rapid prototyping in potential applications. These techniques can create structures at the centimeter scale, however, there are some limitations in terms of resolution and geometric constraints. Here, a micro–nano 3D printing protocol based on additive manufacturing to achieve the 3D structure (3DS) not only possessing millimeter scale structural dimensions but also nanometer features are proposed. A theory is verified to assist the design and fabrication of the 3DS with millimeter scale and nanometer precision. The structures are predesigned and the scanning strategy is optimized before 3D printing to improve the manufacturing efficiency and precision. A customized 3DS with a height of 2.2 mm is obtained, which is a challenge for the conventional two‐photon polymerization fabrication. Furthermore, a 1.2 mm 3DS with inside scaffold and smooth surface is efficiently achieved within 2.7 h with a nanometer surface roughness by using the proposed stepwise optimized 3D printing process. This study offers a flexible and low‐cost technology to generate highly customizable, precisely controllable 3DS for potential applications in microelectronics and microdevices.     

Self Assembly–Assisted Additive Manufacturing: Direct Ink Write 3D Printing of Epoxy–Amine Thermosets

The use of self‐assembling, pre‐polymer materials in 3D printing is rare, due to difficulties of facilitating printing with low molecular weight species and preserving their reactivity and/or functions on the macroscale. Akin to 3D printing of small molecules, examples of extrusion‐based printing of pre‐polymer thermosets are uncommon, arising from their limited rheological tuneability and slow reactions kinetics. The direct ink write (DIW) 3D printing of a two‐part resin, Epon 828 and Jeffamine D230, using a self‐assembly approach is reported. Through the addition of self‐assembling, ureidopyrimidinone‐modified Jeffamine D230 and nanoclay filler, suitable viscoelastic properties are obtained, enabling 3D printing of the epoxy–amine pre‐polymer resin. A significant increase in viscosity is observed, with an infinite shear rate viscosity of approximately two orders of magnitude higher than control resins, in addition to, an increase in yield strength and thixotropic behavior. Printing of simple geometries is demonstrated with parts showing excellent interlayer adhesion, unachievable using control resins.     

Biodegradable poly(ester‐ether)s: ring‐opening polymerization of D,L‐3‐methyl‐1,4‐dioxan‐2‐one using various initiator systems

The synthesis and detailed characterization of racemic 3‐methyl‐1,4‐dioxan‐2‐one (3‐MeDX) are reported. The bulk ring‐opening polymerization of 3‐MeDX, to yield a poly(ester‐ether) meant for biomedical applications, in the presence of various initiators such as tin(II) octanoate, tin(II) octanoate/n‐butyl alcohol, aluminium tris‐isopropoxide and an aluminium Schiff base complex (HAPENAlOⁱPr) under varying experimental conditions is here detailed for the first time. Polymerization kinetics were investigated and compared with those of 1,4‐dioxan‐2‐one. The studies reveal that the rate of polymerization of 3‐MeDX is less than that of 1,4‐dioxan‐2‐one. Experimental conditions to achieve relatively high molar masses have been established. Thermodynamic parameters such as enthalpy and entropy of 3‐MeDX polymerization as well as ceiling temperature have been determined. Poly(D ,L ‐3‐MeDX) is found to possess a much lower ceiling temperature than poly(1,4‐dioxan‐2‐one). Poly(D ,L ‐3‐MeDX) was characterized using NMR spectroscopy, matrix‐assisted laser desorption ionization mass spectrometry, size exclusion chromatography and differential scanning calorimetry. This polymer is an amorphous material with a glass transition temperature of about ?20 °C. Copyright © 2010 Society of Chemical Industry