Drop-on-demand Research Articles

Analysis of the operating conditions of pulse electric field–assisted EHD for sodium alginate printing using design of experiment approach

Electrohydrodynamic (EHD) printing is a novel printing technology with high printing resolution, making it suitable for applications such as tissue engineering and micro/nanomanufacturing. Inkjets from EHD fall on the substrate either in the continuous or drop-on-demand (D-O-D) mode. In this research, the D-O-D-based EHD printing process was used to print micro droplets of bioink assisted by a pulse electric field. A 2% (g/mL) sodium alginate solution was used as the bioink. A general full factorial design was used to analyze and observe the relation between the bioink droplet quality and the EHD process parameters. The diameter of the droplet, droplet generation rate, distance between two droplets, and the droplet consistency are the four major responses to evaluate the printing quality. The EHD process parameters, offset voltage, pulse width, and frequency, are the three regressor variables. After conducting the data analysis, it is observed that the higher frequency and the lower voltage produced the smaller droplet diameter, higher droplet generation rate, and nearer distance among the droplets. However, this increased the variance in the droplet patterns thus reducing the consistency of the printing process. It was also found that pulse width had marginal significance on the quality of printed drops. From the data analysis, four regression models were developed to aid in finding the parameter combinations to print 2% sodium alginate at a desired droplet diameter, droplet generation rate, and droplet distance. This proposed model can be used as a guideline for EHD bioprinting experimental setup, and it will significantly reduce the efforts to determine the optimal process parameters.

Establishment of 3D Structuring of Resistant Starch

Inkjet printing technology has been applied to the fabrication of food products with high precision/resolution and minimal material waste. This technique has been applied only to materials with low viscosity because of mechanical limitation. And, it has been challengeable to build 3D structure of food with high resolution for high viscous food materials. In this study, using an ink-jet-type, drop-on-demand (DOD)-based piezo-electric jet printer, RS2 type resistant starch, which is known as “rigid and not easy to build 3D structure”, is laminated and structured to verify the possibility of 3D structuring of resistant starch. Raw potato starch (RPS) was utilized as the RS2-type resistant starch material, and cellulose nanocrystals (CNC) and sodium alginate (AS) were used as the structural materials for RS2-type resistant starch lamination. First, RPS (0%, 1%, 3%, and 5%) was added to the CNC and AS mixture to analyze rheological properties, 3D printing, precision, and physical properties. The mixed ink of CNC, AS and RPS will be named CAP. For rheological properties, a frequency sweep was performed within the range of 0.01 to 16 Hz. The CAP ink according to RPS content exhibited shear-thinning behavior and was easily extruded from the nozzle during 3D printing. The value of G' indicated maintained shape of the structure in the range of 1133.51 to 1326.27 Pa. During 3D printing with the same CAP head parameters, the pressure range of CAP ink extrusion differed depending on RPS content. The CAP with 5% RPS was in contact with the nozzle during printing due to the high pressure. In piezo-electric jet printing, the precision of the RS2-type resistant starch 3D structure was affected by pressure. Hardness increased as RPS content increased. This study showed that 3D lamination of RS2-type resistant starch material was possible up to 3% RPS, demonstrating the possibility of developing 3D resistant starch food products using an inkjet piezo-electric jet printer.

Current Status of Liquid Metal Printing

This review focuses on the current state of the art in liquid metal additive manufacturing (AM), an emerging and growing family of related printing technologies used to fabricate near-net shape or fully free-standing metal objects. The various printing modes and droplet generation techniques as applied to liquid metals are discussed. Two different printing modes, continuous and drop-on-demand (DOD), exist for liquid metal printing and are based on commercial inkjet printing technology. Several techniques are in various stages of development from laboratory testing, prototyping, to full commercialization. Printing techniques include metal droplet generation by piezoelectric actuation or impact-driven, electrostatic, pneumatic, electrohydrodynamic (EHD), magnetohydrodynamic (MHD) ejection, or droplet generation by application of a high-power laser. The impetus for development of liquid metal printing was the precise, and often small scale, jetting of solder alloys for microelectronics applications. The fabrication of higher-melting-point metals and alloys and the printing of free-standing metal objects has provided further motivation for the research and development of liquid metal printing.

Study of the Enzyme Activity Change due to Inkjet Printing for Biosensor Fabrication.

Enzymes, the most commonly used biosensing element, have a great influence on the performance of biosensors. Recently, drop-on-demand (DOD) printing technique has been widely employed for the fabrication of biosensors due to its merits of noncontact, less waste, and rapid deposition. However, enzyme printing studies were rarely conducted on the effect of printing parameters from the aspect of the pressure wave propagation mechanism. This study investigated the effects of pressure wave propagation on enzyme activity from the aspects of wave superposition, wave amplitude, resulting mechanical stress, and protein conformation change using pyruvate oxidase as the model enzyme. We found that the mechanical stress increased the activity of pyruvate oxidase during the inkjet printing process. A shear rate of 3 × 105 s-1 enhanced the activity by 14.10%. The enhancement mechanism was investigated, and the mechanical activation or mild proteolysis was found to change the conformation of pyruvate oxidase and improve its activity. This study is fundamental to understand the effect of both printing mechanism and induced mechanical stress on the properties of biomolecules and plays an important role in modulating the activity of other enzyme-based inks, which is crucial for the development of biosensors.

Direct Droplet Writing – A Novel Droplet-punching Capillary-splitting 3D Printing Method for Highly Viscous Materials

The drop-on-demand (DOD) based three-dimensional (3D) printing methods can fabricate an object with a high level of accuracy and shape complexity using multiple materials. However, a key limitation of the DOD approaches such as ink jetting is only the inks with low viscosity can be used. Such low-viscosity restriction severely limits the material options for the DOD-based 3D printing methods. To address the viscosity issue, we have developed a novel drop-on-demand 3D printing method called direct droplet writing (DDW) for highly viscous material. One main idea of the DDW process is to use direct droplet-punching to enable the printing of materials that may have a viscosity over 190,000 mPa·s; and another main idea of the DDW process is to use capillary-splitting to avoid common issues of various ink-jetting approaches, including splashing, droplet deflection, and satellite droplets. The DDW process can reliably fabricate 3D structures using a wide range of materials that are challenging for the jetting-based and extrusion-based methods. Analytical models to characterize the DDW process are presented. A set of test cases have been conducted using the in-house developed prototype system to characterize the relationship between droplet size and process parameters such as droplet punching speed and dispensing gap. Various materials, including high-loading photocurable tricalcium phosphate (TCP) ink and polyurethane (PU) leather ink, were successfully used in the DDW process. In addition to a much broader range of 3D printable materials, the DDW process is robust, without ink clogging or leaking, and can achieve consistent printing results using digitally controlled droplets.

Experimental Analysis of a Pneumatic Drop-on-Demand (DOD) Injection Technology for 3D Printing Using a Gallium-Indium Alloy

Many liquid metals have a high boiling point, strong electrical conductivity, high thermal conductivity, and non-toxic properties, which make them ideal targets for applications in different fields such as optics, microcircuits, electronic switches, micro-electromechanical System (MEMS) devices and 3D printing manufacturing. However, owing to the generally high surface tension of these liquids, achieving uniform micro-droplets is often a challenge due to the inherent difficulties in controlling their size and shape. In this study, a gallium indium alloy (GaIn24.5) has been used in combination with a pneumatic drop-on-demand (DOD) injection technology to carry out a series of experiments. The micro-droplet forming process has been explored for different pressure and pulse width conditions. Uniform metal droplets (diameter 1080 μm) have been obtained with a 1.5 kPa jet pressure, 100 ms pulse width, and 50% duty ratio. The standard deviation of the measured metal droplets diameter has been found to be approximately 20 μm.

Functionalised PEGs with photo-dimerisable, anthracenyl end-groups: New UV-curable materials for use in inkjet formulations

The synthesis and characterisation of two novel poly(ethylene glycol)-based materials featuring UV-dimerisable anthracenyl end-groups are described. These low molecular weight polymers exhibited good solubility (ca. 100 g/L) in common organic solvents and were deposited successfully using a drop-on-demand (DOD) inkjet printhead to produce clear prints. UV–vis irradiation of a linear bis-anthracene terminated PEG material resulted in polymerisation via photodimerisation of the anthracenyl end-groups, maintaining polymer solubility in common organic solvents and enabling solution state analysis of the polymerisation process using UV–vis and 1H NMR spectroscopies. In contrast, UV irradiation of a three-armed, tris-anthracenyl-terminated PEG resulted in the formation of an insoluble cross-linked network, which was characterised using rheometry, differential scanning calorimetry and lap shear tensiometry. It was found that the incorporation of solvent-soluble dyes into formulations of the tris-anthracenyl PEG resulted in an increase in UV-cure time when compared to the polymer alone, but did not prevent curing. The results highlight the suitability of this latter material as a UV-curable system for inclusion in inkjet formulations.

Inkjet Printing of PEDOT:PSS Based Conductive Patterns for 3D Forming Applications.

This paper presents the formulation, inkjet printing, and vacuum forming of a conductive and stretchable polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), ink on a stretchable and transparent thermoplastic polyurethane (TPU) substrate. The formulation of the conductive and stretchable ink is achieved by combining PEDOT:PSS with additional solvents, to achieve the right inkjet properties for drop-on-demand (DoD) inkjet printing. A conductive pattern can be printed from the 21 µm orifice on a flexible and stretchable TPU substrate, with a linewidth down to 44 µm. The properties of the printed pattern, in terms of sheet resistance, morphology, transparency, impact of weather conditions, and stretching are investigated and show sheet resistances up to 45 Ohm/sq and transparencies as high as 95%, which is comparable to indium tin oxide (ITO). Moreover, in contrast to ITO, one-time stretching up to 40% can be achieved, increasing the sheet resistance up to 214 Ohm/sq only, showing the great potential of this ink for one-time stretching. Finally, as a proof of this one-time stretching, the printed samples are vacuum formed around a 3D object, still showing sufficient conductivity to be applied as a capacitive touch sensor.

Fabrication of 3D Temperature Sensor Using Magnetostrictive Inkjet Printhead

Recently, the three-dimensional (3D) printing technique has received much attention for the shape forming and manufacturing. In this work, for the first time, we present the fabrication of inkjet printed 3-diamentional (3D) low cost temperature sensor on a 3D shaped thermoplastic substrate suitable for packaging, flexible electronics and other printed application. The design, fabrication and testing of a 3D printed temperature sensor is presented. The sensor pattern is designed using computer-aided design (CAD) program and fabricated by inkjet printing with a drop-on-demand (DoD) using magnetostrictive inkjet printhead at room temperature. The sensing pattern is printed using commercially available conductive silver nanoparticles ink (AgNPs). The moving speed of 90 mm/min is chosen to print the sensor pattern. The inkjet printed temperature sensor is demonstrated and characterized with good electrical properties, showing a good sensitivity and linearity. The results indicate that the 3D inkjet printing technology may have a great applied potential in sensor fabrication.

Reliable inkjet printing of chondrocytes and MSCs using reservoir agitation

Drop-on-demand (DoD) inkjet printing has been explored for a range of applications, including those to selectively deposit cellular material, due to the high accuracy and scalability of such systems when compared with alternative bioprinting techniques. Despite this, there remain considerable limitations when handling cell suspensions due to the agglomeration and sedimentation of cells during printing, leading to a deterioration in jetting performance. The objective of this work was to design and assess the effectiveness of a custom agitation system to maintain cellular dispersion within the ink reservoir during printing. The cell printing performance of an inkjet printer was assessed with and without the use of a custom agitation system, with biological characterisation performed to characterise the impact of the agitator on cellular viability and function. Cell printing performance was retained over a 2 h printing period when incorporating an agitated reservoir, with a gradual reduction in performance observed under a non-agitated configuration. Cell assays indicated that the agitation process did not significantly affect the viability, metabolic activity or morphology of the mesenchymal stromal cell (MSC) or chondrocyte cell types. This study therefore provides a new methodology to increase process reliability within DoD printing platforms when jetting cellularised material.

High Speed In-situ X-ray Imaging of 3D Freeze Printing of Aerogels

3D freeze printing (3DFP) combines drop-on-demand (DOD) inkjet printing with freeze casting to fabricate lightweight and multifunctional aerogels with customized geometries. Freeze casting is an efficient and easily implemented method capable of fabricating porous, sponge-like structures for many different applications. This process enables tailoring the microstructure of the final product (i.e., pore morphology, alignment, average size distribution, etc.) by controlling the fabrication conditions and freezing kinetics. Its combination with DOD printing provides the capability of engineering the macrostructure without relying on a mold as reported for 3D freeze-printed aerogels made from graphene, silver nanowires, and other nanocomposites. In this paper, we performed in-situ X-ray imaging to understand the inside process dynamics in 3DFP using a commercially available colloidal silica ink. We investigated the 3DFP process with the following hierarchy: first, single droplets; then, uniform lines obtained from coalescence of droplets; and finally, three consecutive lines deposited layer by layer. With the help of X-ray imaging, the importance of the balance between material deposition and freezing rates was shown in-situ by the observation inside of the freeze front following the tip of the printed line. The effects of the substrate temperature on the elimination of undesired interfacial boundaries were also shown by the observed ice crystals penetrating from lower to upper layer.

Nonaxisymmetric Effects in Drop-On-Demand Piezoacoustic Inkjet Printing

Drop-on-demand (DOD) inkjet printing is well characterized and a well-studied problem, but nonaxisymmetric effects are typically ignored, while these effects can severely reduce the print-head performance and its stability. In this paper we first review nonaxisymmetric droplet formation originating from geometrical effects. We then focus on the possibility that observed nonaxisymmetry arises from surface instabilities of the meniscus by a Rayleigh-Taylor-like (RT) mechanism. It is shown theoretically that the meniscus can become RT unstable beyond a critical acceleration. A comparison with data extracted from high-speed recordings of the meniscus oscillations show that the critical accelerations are exceeded. Using the time duration that the critical acceleration is exceeded and the maximal growth rate, the extent of growth of the unstable wave is estimated.

A Novel 3D Bioprinter Using Direct-Volumetric Drop-On-Demand Technology for Fabricating Micro-Tissues and Drug-Delivery.

Drop-on-demand (DOD) 3D bioprinting technologies currently hold the greatest promise for generating functional tissues for clinical use and for drug development. However, existing DOD 3D bioprinting technologies have three main limitations: (1) droplet volume inconsistency; (2) the ability to print only bioinks with low cell concentrations and low viscosity; and (3) problems with cell viability when dispensed under high pressure. We report our success developing a novel direct-volumetric DOD (DVDOD) 3D bioprinting technology that overcomes each of these limitations. DVDOD can produce droplets of bioink from <10 nL in volume using a direct-volumetric mechanism with <± 5% volumetric percent accuracy in an accurate spatially controlled manner. DVDOD has the capability of dispensing bioinks with high concentrations of cells and/or high viscosity biomaterials in either low- or high-throughput modes. The cells are subjected to a low pressure during the bioprinting process for a very short period of time that does not negatively impact cell viability. We demonstrated the functions of the bioprinter in two distinct manners: (1) by using a high-throughput drug-delivery model; and (2) by bioprinting micro-tissues using a variety of different cell types, including functional micro-tissues of bone, cancer, and induced pluripotent stem cells. Our DVDOD technology demonstrates a promising platform for generating many types of tissues and drug-delivery models.

The effects of coil–stretch transition behavior of polyfluorene inks on single droplet formation during inkjet printing

For drop-on-demand (DOD) inkjet printing, stable and single ink droplet formation without satellite dots is the key to improve the print quality. The formation of stable and single droplet is influenced by filament break up and the polymer chain’s coil–stretch transition behavior. In this paper, the droplet formation behaviors of polyfluorene (PFO) ink at various driving voltages (V), polymer chain’s coil–stretch transition mechanism and its effects on single ink droplet formation are investigated. It indicates that when 58 <V ≤ 63 V, a single and stable droplet is formed with a pulse time of 38.5 μs. At this stage, the Weissenberg number (Wi) < 0.5, the PFO molecular chain is coiled to guarantee stable and single droplets. When V > 63 V, Wi > 0.5, the PFO molecular chain is stretched because of the high hydrodynamic forces, resulting unwanted satellite droplets. When 55 <V ≤ 58 V, the droplet shrinks into the nozzle, which indicates that the kinetic energy supplied by the deformation of the piezoelectric transducer isn't enough to force the droplet to be jetted from the nozzle.

Piezo‐electric based Ultraprecise/Ultratrace 3D Food Printing of Potato Resistant Starch

Recently, compared to traditional food production systems, 3D food printing has gained a lot of interest because of the potential benefits of being able to customized food products fabrication in colour, shape, texture, flavor and even nutrition. Successful printing of food objects in 3D food printing is critical, and high accuracy and precision food printing technology is required to reproduce delicate and complex food 3D structures. In 3D food printing, the properties of food materials, such as rheological properties, chemical properties and thermal stability, are significant for the structure accuracy and precision of printed objects. In this study, we tried to present a 3D structure layering method for potato resistant starch using drop‐on‐demand (DOD) printing method (piezoelectric system type) that can express the fine structure of food by inkjet printing technology. Nanocrystals cellulose (CNC) and sodium alginate (AS) were used to form a food 3D structure, and 3D printing optimal ink formulation ratio of CNC and AS (CNC:AS=80:20) was selected through preliminary experiments. The purpose of this study was to determine the possibility of 3D printing and printability after comparing rheological properties of mixtures with and without annealing by varying the content of potato resistant starch (PRS) in the mixture of CNC and AS. 3D food structure with various combination of potato resistant starch (RS) to a mixture of CNC and AS was printed, and compared the rheological properties of the mixture that had not been temperature‐treated and the one that had been annealed. After that, the optimal material combination for precise 3D printing was explored together with the physical properties of the printed object.Support or Funding InformationThis research was supported in part by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2018R1D1A1B07045349), the High Value‐added Food Technology Development Program, the Ministry of Agriculture, Food and Rural Affairs (MAFRA), Republic of Korea (118059‐2) and the Ewha Womans University Research Grant of 2019.Piezo‐electric based Ultraprecise/Ultratrace 3D Food PrintingFigure 1

Water-matrix interaction at the drop-drop interface during drop-on-demand printing of hydrogels

Hydrogel-based soft materials have been used in numerous applications in healthcare, food, pharmaceutical, and cosmetic industries. Manufacturing hydrogels whose functional properties and compositions are voxelized at superior spatial resolutions can significantly improve current applications as well as will enable a new generation of soft materials. However, it remains challenging to control the structure and composition of soft materials reliably. In this context, the drop-on-demand (DOD) printing of hydrogels shows excellent potential to address this manufacturing challenge. Despite this potential, a lack of mechanistic understanding of the behavior of printed hydrogel drops makes it challenging to design and optimize DOD printing protocols for a wide variety of hydrogels. In particular, the curing of hydrogel drops, which requires dehydration of printed hydrogel drops, is poorly understood. In this study, thus, a hypothesis was postulated and tested that water-matrix interaction at drop-drop interfaces during curing processes determine the quality of hydrogels printed. Both computational and experimental studies were performed to establish a mechanism of the water-matrix interaction within printed hydrogel drops. The results were further discussed to establish a dimensionless similarity parameter that can characterize water transports during the hydrogel dehydration process.

Drop-on-Demand Electrohydrodynamic Jet Printing of Graphene and Its Composite Microelectrode for High Performance Electrochemical Sensing

An effective approach is required to pattern graphene with high spatial resolution and accuracy for advanced graphene-based sensors. In this work, we describe a simple and effective strategy for direct writing micro-scale graphene patterns by drop-on-demand (DoD) electrohydrodynamic jet (E-Jet) printing, using a highly concentrated graphene dispersion. The uniform micro-scale graphene patterns were formed by droplets produced under the electric field “pulling” force, these droplets are far smaller than the inner diameter of nozzle, which can effectively avoid nozzle clogging. With the control of pulse voltage width and frequency, different micro-scale graphene patterns were directly printed using DoD E-Jet printing technique. Graphene lines with a thickness of 5 nm were produced for 1 time printing, which provided a resistivity of 4.2 mΩ·cm. In addition, the graphene layer was directly written on the Pt microelectrodes to form Graphene/Pt (G/Pt) composite microelectrodes. The electrochemical test shows that the peak current of G/Pt composite microelectrodes was more than twice larger than that of bare Pt microelectrodes. The sensing sensitivity was significantly increased, presenting great potential for high performance electrochemical sensing devices.

Inkjet Nozzle Failure by Heterogeneous Nucleation: Bubble Entrainment, Cavitation, and Diffusive Growth

Piezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability.

Ferrofluid droplet formation from a nozzle using alternating magnetic field with different magnetic coil positions

Ferrofluid has been used in many fields, such as microfluidics, droplet formation, and heat transfer, due to its potential to be attracted in the presence of a magnetic field. Droplet formation, itself, has many applications such as emulsions, 3D micro-printers, MEMS, and electro-sprays. In this study, the mechanism of ferrofluid droplet formation from the nozzle in the presence of an alternating magnetic field was investigated. The magnetic coil was fixed at different angles with respect to gravity and the effect of the alternating magnetic field and the angle of the magnetic coil axis with respect to gravity on the produced droplet volume, satellite droplet, and droplet formation frequency were studied for the first time. Also, by using a reservoir instead of syringe pump, a drop-on-demand (DOD) platform was introduced. The results showed that with increasing magnetic force, the volume of droplets in both DC and AC cases decreases, whereas the droplet formation frequency increases. It was also observed that by using a DC magnetic field for all angles, the droplet formation was accompanied by a satellite droplet, whereas in the presence of the AC magnetic field, the satellite droplet was eliminated. So, a new regime of droplet formation was observed. Also, by tuning the alternating magnetic field, a larger droplet with respect to DC magnetic field was detected. Interestingly, it was shown that by increasing the angle of the magnetic coil axis with respect to gravity from zero to 90°, the volume of the produced droplet has a minimum at 45°.

Acoustics of a piezo inkjet channel with an entrained air bubble

Piezo-acoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. However, the stability of piezo-DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. The acoustically driven air bubble modifies the ink channel acoustics, and conversely, the modified ink channel acoustics influences the bubble dynamics. Here, we measure the acoustic eigenfrequency of a MEMS based silicon ink channel as a function of the bubble size. The eigenfrequency was measured using a pulse-echo system and the bubble size using a short-wave infrared imaging setup. We show that the measured eigenfrequency increases when an air bubble is entrained. Surprisingly, the ink channel resonance frequency plateaus at total bubble volumes larger than 10 pl. Moreover, it was found that at a constant total bubble volume, the resonance frequency increases with the number of entrained bubbles. We show that both experimental observations can be quantitatively explained from a simple lumped element model (LEM) comprising the ink channel Helmholtz resonator coupled to the bubble mass-spring system. The results of the LEM model were validated using a full numerical model of the coupled ink channel—bubble system.