Droplet Ejector Research Articles

Contactless Single Cell Extraction from Monolayer Cell Culture Using A MEMS Acoustic Droplet Ejector.

To achieve a contactless and damage-less extraction of a single (or a few) human retinal pigment epithelial (RPE) cell(s) from a cell monolayer with acoustic droplet ejection. An acoustic droplet ejector based on a self-focusing acoustic transducer (SFAT) is designed, microfabricated, and placed on a precision movable stage that aligns it to the targeted cell(s) in a Petri dish. The device delivers 20.1 MHz focused ultrasound (FUS) with a focal diameter of 100 μm, which ejects droplets capable of extracting and transferring only the targeted cell(s) from a monolayer. The extraction and collection of 1-10 cells are successfully demonstrated. The number of ejected cells can be controlled by the FUS pulse width. As confirmed by fluorescence-based cell viability assays and re-culture experiments, the ejected cell(s) and remaining monolayer cells are intact without damage after cell ejection. Furthermore, real-time reverse transcription polymerase chain reaction (RT-PCR) tests for both housekeeping genes (GAPDH and β -actin) and RPE-specific genes (MITF, PEDF, and PMEL17) show no significant difference between the acoustically ejected cells and those collected manually with a micropipette. The proposed technology realizes a contactless, damage-free extraction of cells with high spatial resolution and precise control of the number of cells ejected, with a simple setup. This powerful technology not only enables efficient, high-precision cell extraction for quality check applications but also opens new avenues for other advanced biotechnologies such as bioprinting.

Longitudinal transducer based membrane resonant ejector for high-viscosity and high-frequency micro-droplet jetting

Aiming at the requirement for high-precision on-demand droplet jetting of high-viscosity industrial inks, a longitudinal-transducer-based membrane resonant droplet ejector (LT-MRDE) is designed and fabricated in this work. The jetting and observing system of LT-MRDE is set up to quantitatively explore the variations of droplet volume and velocity with driving voltage. Moreover, the three-dimensional (3D) multi-relaxation-time (MRT) lattice Boltzmann (LB) model coupled immersed boundary (IB) is employed to elucidate the effects of driving voltage and liquid viscosity on droplet characteristics. Experimental results demonstrate that the LT-MRDE could stably eject the silicone oil with a viscosity of 100 mPa·s from ∼50 μm vibrating orifice. Furthermore, both the volume and velocity of droplet exhibit a proportionality to voltage and an inverse proportionality to viscosity. The optimal droplet volume and velocity of 100 mPa·s silicon oil are 67.27 pL and 9.07 m/s with the driving parameters of 75 Vpp/14.754 kHz. With high jetting frequency and controllable droplet volume, LT-MRDE provides a new alternative for on-demand precision jetting of high-viscosity industrial inks.

Membrane resonant based droplet ejector for micro-droplet jetting

A novel membrane resonant droplet ejector (MRDE) for viscous industrial liquids is proposed in the paper. In MRDE, a vibrating nozzle is built to generate micro-droplets and the nozzle clogging could be reduced. The droplet jetting mechanism is studied by a three-dimensional (3D) lattice Boltzmann (LB) model. The effects of voltage amplitude and driving frequency on droplet volume and velocity are explored. Experimental Results show that low-viscosity water and viscous glycol could be jetted stably in parameters of 30 V/16 kHz and 60 V/20 kHz, respectively. Moreover, the droplet jetting of MRDE and in-situ observation is set up to analysis the droplet jetting characteristics. The critical thresholds for MRDE are9.340≤Re≤17.65, 3.653≤We≤8.846 and 0.014≤oh≤0.411, which has a wider printable range than traditional piezoelectric printheads. With the compact structure, MRDE is easy of integration for mass industrial applications compared with the pneumatic and needle-based ejectors.

Nozzleless Acoustic Droplet Ejector With Electrically Tunable Droplet Size for Picking and Placing Semiconductor Chips

This paper presents a microfabricated nozzleless acoustic droplet ejector with electrically tunable droplet size as a new tool for on-demand picking and placing semiconductor chips for semiconductor packaging. The ejector is based on a 2-mm-thick lead zirconate titanate (PZT) sheet with a planar annular-ring air-cavity acoustic Fresnel lens on top. When driven with sinusoidal pulsed voltage signals of 1.16 MHz, the ejector generates focused ultrasound with 1-mm focal diameter and 5-mm focal depth at 22 mm focal length in sodium polytungstate (SPT) solution. A finite-element-method (FEM) simulation model calculating the acoustic-field-induced fluid motion during the droplet ejection process has been developed, and verified by experiments, in which the device ejects SPT droplets whose diameter is from 850 to 2, 490 μm, controlled by the driving pulse width and voltage. The ejected droplets are able to carry 400- μm-thick square silicon chips with side length from 700 to 3, 100 μm. A polyester channel-embedded guiding cover for semi-automatic loading of silicon chips to the ejection site is designed to avoid manual placement of the silicon chips. With the proof-of-concept system, we demonstrate ejecting silicon chips out of SPT surface onto a nearby paper, assembling them into arrays with good repeatability. [2020-0337].

Microfluidic Injector Simulation With FSAW Sensor for 3-D Integration

This paper presents a possible creation of the optimized liquid sensors for the inkjet nozzles. The proposed focused surface acoustic wave (FSAW) device utilizing aluminum nitride (AlN) single crystal as the piezoelectric substrate is based on the pressure variation due to the continuous droplet ejector. The design, specification, and numerical simulation results are described. Comparisons between the output response of the conventional and concentric structures indicate a more efficient operation of the multiple-segment focused interdigital transducer (FIDT) structure. According to the angular spectrum of the plane wave theory, the amplitude field of FIDTs is calculated through that of straight interdigital transducers. The 3-D integrated model of the FSAW device has a number of advantages, such as the enhancement of the surface displacement amplitudes and an easier fabrication. It is able to detect the breakup appearance of the liquid in the droplet formation process. For the piezoelectric substrate AlN, it is compatible with the CMOS fabrication technology, leading to an inexpensive and reliable system. Moreover, for the proposed FIDTs with multiple straight segments, the acoustic energy is more optimized and focused near the center of the inkjet nozzle. The droplet generation process begins at an output voltage of roughly 0.074 V within 0.25 μs, and the background level of the attenuation of both the mechanical and electrical energy.

Peptide Synthesis on Glass Substrate Using Acoustic Droplet Ejector

This paper describes the synthesis of a 9-mers-long peptide ladder structure of glycine on a modified glass surface using a nanoliter droplet ejector. To synthesize peptide on a glass substrate, SPOT peptide synthesis protocol was followed with a nozzleless acoustic droplet ejector being used to eject about 300 droplets of preactivated amino acid solution to dispense 60 nL of the solution per mer. The coupling efficiency of each mer was measured with FITC fluorescent tag to be 96%, resulting in net 70% efficiency for the whole 9-mer-long peptide of glycine. Usage of a nanoliter droplet ejector for SPOT peptide synthesis increases the density of protein array on a chip.

Nonvolatile Liquid-Film-Embedded Microfluidic Valve for Microscopic Evaporation Control Without Stiction Problem at Liquid–Air Interfaces

A critical problem of quick evaporation speed of fluids in the microscopic scale can cause many unexpected results and failures. Here, we introduce evaporation speed controlling effects of a thin liquid-film-based microfluidic valve in a microfluidic droplet ejector. The thin liquid film with nonvolatility and immiscibility exhibited an excellent microfluidic valve function without any stiction problem between valve components and provided a very effective evaporation protection barrier for the microfluids in the device. We have investigated its effectiveness for evaporation control at a microfluidic port with a liquid-air interface. Successful evaporation control by the liquid-film-embedded microfluidic valve has been demonstrated. We believe that this novel approach has immense potential in many microfluidic devices requiring a high level of evaporation control.

Droplet charging regimes for ultrasonic atomization of a liquid electrolyte in an external electric field

Distinct regimes of droplet charging, determined by the dominant charge transport process, are identified for an ultrasonic droplet ejector using electrohydrodynamic computational simulations, a fundamental scale analysis, and experimental measurements. The regimes of droplet charging are determined by the relative magnitudes of the dimensionless Strouhal and electric Reynolds numbers, which are a function of the process (pressure forcing), advection, and charge relaxation time scales for charge transport. Optimal (net maximum) droplet charging has been identified to exist for conditions in which the electric Reynolds number is of the order of the inverse Strouhal number, i.e., the charge relaxation time is on the order of the pressure forcing (droplet formation) time scale. The conditions necessary for optimal droplet charging have been identified as a function of the dimensionless Debye number (i.e., liquid conductivity), external electric field (magnitude and duration), and atomization drive signal (frequency and amplitude). The specific regime of droplet charging also determines the functional relationship between droplet charge and charging electric field strength. The commonly expected linear relationship between droplet charge and external electric field strength is only found when either the inverse of the Strouhal number is less than the electric Reynolds number, i.e., the charge relaxation is slower than both the advection and external pressure forcing, or in the electrostatic limit, i.e., when charge relaxation is much faster than all other processes. The analysis provides a basic understanding of the dominant physics of droplet charging with implications to many important applications, such as electrospray mass spectrometry, ink jet printing, and drop-on-demand manufacturing.

A numerical and experimental study of a collapsing bubble-induced droplet ejector

This paper aims to study a novel drop-on-demand droplet generation mechanism in which the oscillation and deformation of a non-equilibrium bubble in close proximity to a free surface induce an axisymmetric liquid spike on the free surface. The evolution of the liquid spike and its deformation due to the effect of surface tension force lead to the formation of a droplet. The free surface can be accorded by either a circular hole on a horizontal flat plate or by the top opening/nozzle of a vertical cylinder. A high-speed camera capable of obtaining images at a frame rate of 15,000 fps is utilized to observe the droplet formation process. Numerical simulations corresponding to the experiments are performed using the boundary integral spatial solution coupled with the time integration, i.e., a mixed Eulerian–Lagrangian method. In the experiments the bubble is generated using a very low voltage (only 55 V) in contrast to the relatively much higher voltages usually employed in reported works. This is very attractive from a safety viewpoint and accords great simplification of the setup. A comparison is made between the numerical and experimental results. A reasonable agreement has been found. The influences of the main design parameters, namely, the bubble-free surface distance and the dimension of the hole/nozzle on the bubble dynamics and on the droplet formation process are discussed and the conditions of the bubble dynamics under which a satellite-free droplet can be generated are sought. Furthermore, the effects of different geometries, namely, the horizontal flat plate and the vertical cylinder on the bubble dynamics and on the droplet features are examined. One important feature of the proposed actuation mechanism is the capability of producing droplets much smaller than the nozzle size. The possible applications of this mechanism are those where the accurate direction of the ejected droplet is of great importance such as inkjet printing.

Droplet-Based Microreactions With Oil Encapsulation

This paper reports a microreaction technology for biochemical assay using nanoliter droplets encapsulated inside oil droplets. Microreaction chambers are constructed on a glass substrate by accumulating oil droplets that are dispensed by a directional droplet ejector. Droplets of different aqueous reagents are then directionally ejected (by other directional droplet ejector adjacent to the oil droplet ejector) into the oil microchambers for parallel and combinatory analysis. Because the reagents are encapsulated in oil, the evaporation rate is reduced by several orders of magnitude, and only small amounts of reagents are required for each assay. The microchamber size and the reagent amount are digitally controlled by the number of ejected oil and reagent droplets, respectively. The ejectors for oil and reagents have been integrated on a single chip so that each assay is performed efficiently without any mechanical movement and alignment. We have carried out both physical and chemical microreactions with this method and observed a negligible difference in response from conventional macroreactions.

PZT actuator applied to a femto-liter droplet ejector

Since the rapid advancement of the ink-jet printing technology, microdroplet ejectors have been developed into one of the essential fluid handling devices for delivering miniature liquid droplets in accurate dosage control. The applications of micro-droplet ejectors are increasingly prevalent in various emerging areas, for instance organic/polymer lighting emitting displays, liquid crystal display, optical communication, biochemical sample transport and screening and integrated circuit cooling devices. Among different inkjet printing technologies, piezoelectric driven droplet ejectors have shown their potential in dispensing diverse liquids. In this article, a novel femto-liter droplet ejector is proposed using a shear-type PZT actuator. To assure the workability of the ejector, the characteristics of the actuator were well investigated. Before integrating into the vibration plate of the ejector, the response of the rectangular plate-shaped actuator was evaluated by suspending the actuator on a fixture with both ends fixed. With a pre-specified driven waveform, the actuator vibrates to achieve satisfactory responses of displacement and velocity. Subsequently, the actuator was attached to the vibration plate as an actuating module for the ejector. The vibration plate has the design feature of a pillar on a diaphragm. The actuator pushed the pillar and thus deflected the diaphragm. The dimensions of the actuating module have been optimized by numerical simulations. The properties of diaphragm deformation and actuator transient response of the actuating module were simulated and measured for comparison purpose as well.

Analysis of an annular PZT actuator for a droplet ejector

An analytical solution for deflection of an annular lead zirconate titanate (PZT) actuator with application in droplet ejection is developed. A linear strain distribution across the thickness direction is assumed in the derivation of the analytical solution. The analytical results agree well with finite element analyses. Fabrication process of a droplet ejector actuated by the PZT is described. The PZT is prepared by powder metallurgy and patterned with poling electrodes. The ejector is fabricated by nickel electroforming. Experimental results of static deflection of the fabricated device confirm the effectiveness of the analytical solution. Droplet ejection of the ejector is demonstrated.

Automated Acoustic Matrix Deposition for MALDI Sample Preparation

Novel high-throughput sample preparation strategies for MALDI imaging mass spectrometry (IMS) and profiling are presented. An acoustic reagent multispotter was developed to provide improved reproducibility for depositing matrix onto a sample surface, for example, such as a tissue section. The unique design of the acoustic droplet ejector and its optimization for depositing matrix solution are discussed. Since it does not contain a capillary or nozzle for fluid ejection, issues with clogging of these orifices are avoided. Automated matrix deposition provides better control of conditions affecting protein extraction and matrix crystallization with the ability to deposit matrix accurately onto small surface features. For tissue sections, matrix spots of 180-200 microm in diameter were obtained and a procedure is described for generating coordinate files readable by a mass spectrometer to permit automated profile acquisition. Mass spectral quality and reproducibility was found to be better than that obtained with manual pipet spotting. The instrument can also deposit matrix spots in a dense array pattern so that, after analysis in a mass spectrometer, two-dimensional ion images may be constructed. Example ion images from a mouse brain are presented.

Femtoliter to Picoliter Droplet Generation for Organic Polymer Deposition Using Single Reservoir Ejector Arrays

Direct deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics, has the potential to reduce waste as well as production costs. A new design of acoustically actuated two-dimensional (2-D) micromachined droplet ejector arrays can eject various solvents and other fluids ranging from femtoliter to picoliter droplet volumes. These ejectors do not harm fluids that are heat or pressure sensitive. Moreover, they are chemically compatible with the materials used in integrated circuit manufacturing. Therefore, they can be used for benign deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics. A vibrating circular Si/sub x/N/sub y/ thin-film membrane with an orifice at the center forms the unit cell of a 2-D ejector array. Initially, one side of the membrane is loaded with the ejection fluid. Then, ultrasonic waves generated by a piezoelectric transducer force the membranes to displace at resonance. As a result of this actuation, droplets are ejected through the membrane orifice. We ejected water at 1.06 MHz, isopropanol at 1.14 MHz, ethyl alcohol at 1.06 MHz, and acetone at 1.04 MHz from a 20/spl times/20 single reservoir 2-D micromachined array with 160 /spl mu/m in diameter Si/sub x/N/sub y/ membranes and 10 /spl mu/m in diameter orifices. The performance of single reservoir flextensional membrane-based ejector arrays was compared to flextensional membrane-based ejector arrays with reservoirs. A 50% decrease in the required power per ejected droplet and a reduced design complexity were demonstrated over the 2-D micromachined arrays with individual reservoirs. In addition, we deposited Shipley SPR 3612 photoresist at 1.12 MHz in a dry lab environment. No spinning was done after deposition. We covered a 2/spl times/2-mm area on a wafer with a 5.5-/spl mu/m thick photoresist layer. The maximum thickness variation over the area was 0.4 /spl mu/m. Moreover, we present a directly written 1.6-/spl mu/m thick 900-/spl mu/m wide and 8-mm long homogeneous photoresist line. The photoresist thickness variation along the line was 0.2 and 0.4 /spl mu/m in vertical and horizontal directions, respectively.

Chembio extraction on a chip by nanoliter droplet ejection

This paper describes a novel liquid separation technique for chembio extraction by an ultrasonic nanoliter-liquid-droplet ejector built on a PZT sheet. This technique extracts material from an aqueous two-phase system (ATPS) in a precise amount through digital control of the number of nanoliter droplets, without any mixing between the two liquids in the ATPS. The ultrasonic droplet ejector uses an acoustic streaming effect produced by an acoustic beam focused on the liquid surface, and ejects liquid droplets only from the liquid surface without disturbing most of the liquid below the surface. This unique characteristic of the focused acoustic beam is perfect (1) for separating a top-layer liquid (from the bulk of liquid) that contains particles of interest or (2) for recovering a top-layer liquid that has different phase from a bottom-layer liquid. Three kinds of liquid extraction are demonstrated with the ultrasonic droplet ejector: (1) 16 microl of top layer in Dextran-polyethylene glycol-water ATPS (aqueous two-phase system) is recovered within 20 s; (2) micron sized particles that float on water surface are ejected out with water droplets; and (3) oil layer on top of water is separated out.

Acoustically Actuated Flextensional Si<tex>$_x$</tex>N<tex>$_y$</tex>and Single-Crystal Silicon 2-D Micromachined Ejector Arrays

We propose using two-dimensional (2-D) micromachined droplet ejector arrays for environmentally benign deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics used in IC manufacturing. Direct deposition of these chemicals will reduce waste as well as production cost. The proposed device does not harm heat or pressure sensitive fluids and they are chemically compatible with the materials used in IC manufacturing. Each element of the 2-D ejector array consists of a flexurally vibrating circular membrane on one face of a cylindrical fluid reservoir. The membrane has an orifice at the center. A piezoelectric transducer generating ultrasonic waves, located at the open face of the reservoir, actuates the membranes. As a result of this actuation, droplets are fired through the membrane orifice. Ejector arrays were built with either Si/sub x/N/sub y/ or single-crystal silicon membranes using two different fabrication processes. We show that single-crystal silicon membranes are more uniform in their thickness and material quality than those of Si/sub x/N/sub y/ membranes. The single-crystal silicon membrane-based devices showed thickness and material uniformity across all the membranes of an array. This improvement eliminated nonuniform membrane resonance frequencies across an array as observed with Si/sub x/N/sub y/ membrane-based devices. Therefore, it should be possible to repeatably build devices and to predict their dynamic characteristics. Using the fabricated devices, we demonstrated water ejection at 470 kHz, 1.24 MHz, and 2.26 MHz. The corresponding droplet diameters were 6.5, 5, and 3.5 /spl mu/m, respectively.

Micromachined droplet ejector arrays

In this article we present a micromachined flextensional droplet ejector array used to eject liquids. By placing a fluid behind one face of a vibrating circular plate that has an orifice at its center, we achieve continuous ejection of the fluid. We present results of ejection of water and isopropanol. The ejector is harmless to sensitive fluids and can be used to eject fuels, organic polymers, photoresists, low-k dielectrics, adhesives, and chemical and biological samples. Micromachined two-dimensional array flextensional droplet ejectors were realized using planar silicon micromachining techniques. Typical resonant frequency of the micromachined device ranges from 400 kHz to 4.5 MHz. The ejections of water through a 4 μm diameter orifice at 3.45 MHz and a 10 μm diameter orifice at 2.15 MHz were demonstrated by using the developed micromachined two-dimensional array ejectors.

A New Device for the Simulation of Indoor Air Pollution Sources

A reliable and flexible generation system for volatile organic compounds is a valuable tool for indoor air research ers, for instance, in the context of investigations on interactions between organic vapors and indoor materials. A new VOC vapor generation system has been developed and tested for its applicability in indoor air research. It is based on a modified droplet ejector working on the principle of a piezoceramic ink-jet printer. Different solutions, consist ing of complex mixtures of compounds, were used for performance testing of the system. The compounds covered a wide boiling point range and included lipophilic and hydrophilic substances. Emission rates have been generated in the range from 8 μg/h up to 7.7 mg/h per compound, simulating indoor emission sources from low to high emission potential. Emission rates monitored over >90 h showed relative standard deviations <2% for most compounds, being close to analytical uncertainties. The system shows a very high flexibility with regard to number a...

Acoustic printheads with optical alignment

Acoustic printheads having an optically transparent substrate, at least two optical lenses, which may be part of the substrate, and a first structure, which may also be part of the substrate. The optical lenses focus light which irradiates the substrate into optical focal points at known locations relative to the first structure. The first structure may be part of an acoustic droplet ejector which includes an acoustic lens that is fabricated on the optically transparent substrate. The optical focal points can be used to precisely align the first structure with another structure.

Hot melt ink acoustic printing

To facilitate the use of hot melt inks in acoustic ink printers of the type having a printhead including one or more acoustic droplet ejectors for supplying focused acoustic beams, such a printer comprises a carrier for transporting a generally uniformly thick film of hot melt ink across its printhead, together with a heating means for liquefying the ink as it nears the printhead. The droplet ejector or ejectors are acoustically coupled to the ink via the carrier, and their output focal plane is essentially coplanar with the free surface of the liquefied ink, thereby enabling them to eject individual droplets of ink therefrom on command. The ink, on the other hand, is moved across the printhead at a sufficiently high rate to maintain the free surface which it presents to the printhead at a substantially constant level. A variety of carriers may be employed, including thin plastic and metallic belts and webs, and the free surface of the ink may be completely exposed or it may be partially covered by a mesh or perforated layer. A separate heating element may be provided for liquefying the ink, or the lower surface of the carrier may be coated with a thin layer of electrically resistive material for liquefying the ink by localized resistive heating.