Ink-Jet printing technology
Ink-Jet printing technology can reproducibly dispense spheres of fluid with diameters of 25-100µm (10pl to 0.5nl) at rates of 0-4,000 per second for single droplets on-demand, and up to 1MHz for continuous droplets. Piezoelectric dispensing technology is adaptable to a wide range of material dispensing applications, such as biomedical reagents, liquid metals, and optical polymers. Ink-jet microdispensing has been used in the manufacturing of immunoassay diagnostics and is being developed for manufacturing DNA diagnostic arrays. Rapid, accurate fluid microdispensing using ink-jet technology has the potential to increase throughput and lower cost in combinatorial drug synthesis, screening, and testing. MicroFab has developed demand mode printheads with up to 120 addressable channels in less than an inch.
Miniaturization has enabled the electronics/computer age by driving down the cost and increasing the function of electronic and photonic devices. Biomedical processes, such as medical diagnostics, drug synthesis, functional screening of drugs, can utilize high density, low cost electronic and photonic devices to increase productivity. However, the fundamental physics of fluid dispensing significantly limit the degree to which these biochemical processes can miniaturize. The various technologies that are generally referred to as "ink-jet printing" have the potential to remove many of these fluid dispensing limitations.
The phenomena of uniform drop formation from a stream of liquid issuing from an orifice were noted as early as 1833 by Savart (1) and described mathematically by Lord Rayleigh (2) and Weber (3). In the type of system that is based on their observations, fluid under pressure issues from an orifice, typically 50-80µm in diameter, and breaks up into uniform drops by the amplification of capillary waves induced onto the jet, usually by an electromechanical device that causes pressure oscillations to propagate through the fluid. The drops break off from the jet in the presence of an electrostatic field, referred to as the charging field, and thus acquire an electrostatic charge. The charged drops are directed to their desired location, either the catcher or one of several locations on the substrate, by another electrostatic field, the deflection field. This type of system is generally referred to as "continuous" because drops are continuously produced and their trajectories are varied by the amount of charge applied. Theoretical and experimental analysis of continuous type devices, particularly the process of disturbance growth on the jet that leads to drop formation, has been fairly extensive. (4,5). Continuous mode ink-jet printing systems produce droplets that are approximately twice the orifice diameter of the droplet generator. Droplet generation rates for commercially available continuous mode ink-jet systems are usually in the 80-100kHz range, but systems with operating frequencies up to 1MHz are in use. Droplet sizes can be as small as 25µm in a continuous system, but 100µm is typical. MicroFab has built systems that produce droplets as large as 1mm (~0.5µl).
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1
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2Figure 1 shows a schematic of this type of ink-jet printing system, and Figure 2 shows a photomicrograph of a 50µm diameter jet of water issuing from a MicroFab droplet generator device and breaking up due to Rayleigh instability (continuous mode) into 100µm diameter droplets at 20,000 per second.
In the 1950's, the production of drops by electromechanically induced pressure waves was observed by Hansell (6). In this type of system, a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric material that is coupled, directly or indirectly, to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a drop that issues from an orifice (7,8). Since the voltage is applied only when a drop is desired, these types of systems are referred to as drop-on-demand, or "demand mode."
In most commercially available ink-jet printing systems today, a thin film resistor is substituted for the piezoelectric drive transducer. When a high current is passed through this resistor, the ink in contact with it is vaporized, forming a vapor bubble over the resistor. This vapor bubble serves the same functional purpose as the piezoelectric transducer (9). This type of printer is usually referred to as a thermal ink-jet printer.
Figure 3 
Figure 4Figure 3 shows a schematic of a drop-on-demand type ink-jet system, and figure 4 shows a MicroFab drop-on-demand type ink-jet device generating 50µm diameter drops of ethylene glycol from a device with a 50µm orifice at 2,000 per second. Demand mode ink-jet printing systems produce droplets that are approximately equal to the orifice diameter of the droplet generator (10). As Figure 3 indicates, demand mode systems are conceptually far less complex than continuous mode systems. On the other hand, demand mode droplet generation requires the transducer to deliver three or more orders of magnitude greater energy to produce a droplet, compared to continuous mode, and there are many "elegant' (i.e., complex) array demand mode printhead designs (11).
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5One of the characteristics of ink-jet printing technology that makes it attractive as a precision fluid microdispensing technology is the repeatability of process. The images of droplets shown in Figure 2 and figure 4 were made by illuminating the droplets with an LED that was pulsed at the droplet generation frequency. The exposure time of the camera was ~1 second, so that the images represent thousands of events superimposed on each other. The repeatability of the process results in an extremely clear image of the droplets, making it appear to be a high speed photograph. To further illustrate this point, Figure 5 shows two 60µm diameter jets of water breaking up into 120µm diameter droplets streams at 20,000 per second, and being caused to merge into a single droplet stream. Again, this image was created using a "strobed" LED and a ~1 second exposure time. Not only is the droplet formation process so repeatable that the image of the droplets is sharp, but when the droplets are caused to merge, the formation of the highly contorted merged droplets is seen to be just as repeatable.
Continuous mode ink-jet systems are currently in widespread use in the industrial market, principally for product labeling of food and medicines. They have high throughput capabilities, especially array continuous mode systems, and are best suited for high duty cycle applications. Few continuous mode ink-jet systems are multicolor, but two color systems are in use. Because they require unused drops to be recirculated or wasted, the potential for using continuous mode ink-jet technology in biochemical processes is limited, particularly in processes where many fluids need to be dispensed. However, continuous mode ink-jet technology has been used for almost a decade to "print" immunoassay reagents onto substrates for medical diagnostics.
Drop-on-demand ink-jet systems have been used primarily in the office printer market and have come to dominate the low-end printer market (HP's DeskJets, Cannon's Bubble Jets, and Epson's Stylus). Over 95% of these systems are thermal ink-jet printers, although piezoelectric systems are expanding their market share. Demand mode ink-jet systems have no fluid recirculation requirement, and this makes their use as a general fluid microdispensing technology more straightforward than continuous mode technology. Thermal demand mode ink-jet technology systems can achieve extremely high fluid dispensing performance at a very low cost. However, this performance/cost has been achieved by highly tailoring the ink: thermal ink-jet systems are restricted to fluids that can be vaporized (without igniting the fluid) by the heater element and their performance/life can be degraded drastically if other fluids are used.
Since piezoelectric demand mode ink-jet technology does not require recirculation of the working fluid, does not create thermal stress on the fluid, and does not depend on a thermal process to impart acoustic energy to the working fluid, it is the most adaptable of the ink-jet printing technologies to fluid microdispensing in general, and biochemical process in particular. All of the results and potential applications discussed in this chapter will be based on piezoelectric demand mode ink-jet technology.