Method to Improve Ink-Jet Nozzles May Aid Medical Devices

Originally Published MDDI February 2002

R & D DIGEST

A technique developed to significantly reduce the amount of liquid in drops emitted by nozzles such as those used in ink-jet printers is also expected to offer certain advantages in medical device applications, according to Osman Basaran, professor of chemical engineering at Purdue University (West Lafayette, IN). He suggests that in ink-jet printing applications, the ability to make each drop smaller would enable printers to use less ink and produce better quality, higher-resolution documents and images. Working with doctoral student Alvin Un-Teh Chen, Basaran has demonstrated that their technique makes it possible to achieve at least a tenfold reduction in the volume of liquid in each drop while using the same types of nozzles that are now commercially available.

Ink-jet, or "drop-on-demand" technology, is now used in numerous applications outside of printing. For example, it is used by the medical industry in manufacturing diagnostic strips for diabetes, photographic films, adhesive tapes, and analytical devices that operate by applying DNA-laden liquids onto "gene chips." Basaran says the ability to significantly reduce drop size would be advantageous for such applications.

Specifically, Basaran speculates that the method may be immediately beneficial in "creating DNA microarrays; massively parallel drug discovery, where tiny drops are shot into tiny vials by ink-jet heads instead of humans holding pipettes; and printing of diagnostic strips such as those used for diabetes." He adds that the technology's use in pharmaceutical research could mean less fluid use and significantly greater speed. "In these applications, reducing drop size is really a boon because it would reduce costs," Basaran says. "The fluids in these applications are expensive."

Basaran further suggests that the new ink-jet method eventually be used to enhance microencapsulation for controlled release. This application will be "the biggest of all and will be developed over the next several years," the researcher says. "Currently, when other techniques are used to make these time-release capsules, it's hard to make them all of the same size, etc. Ink-jet printing, where you coextrude both the core and the shell material from a concentric set of nozzles, will overcome this problem. Also, the process will be easy to scale up from the lab to actual production."

According to the researchers, the new method is based on using changes in the voltage pulses to control how the nozzles produce each drop of liquid. The nozzles contain piezoceramic elements that move when electricity is applied to them. A positive electrical voltage makes the nozzles contract and a negative voltage makes them expand. Traditionally, each drop has been produced by contracting the nozzle to push out the liquid. "There has been a barrier up until now," Basaran says. "People could make drops that were either the same size as, or much bigger than, the nozzle. It was really hard to make drops that are smaller than the nozzle. This method overcame that barrier."

Un-Teh Chen says, "Probably the most significant challenge we had to overcome in developing the technology was dealing with the very small length and time scales involved in ink-jet drop formation. A typical ink-jet drop is ejected in less than 100 microseconds and is less than 100 micrometers in diameter."

Chen explains, "The key accomplishment with this new technology is that we are now able to dispense drops which are significantly smaller than the diameters of the nozzles from which they are produced (i.e., drops with radii less than half of those of the nozzles). Previously, drops were generally limited to the size of the nozzle or larger. For a given nozzle, this represents a tenfold or greater reduction in the volume of the ejected drop. This translates into increased print resolution, and less reagents required for so-called 'lab-on-a-chip' biomedical devices."

Basaran adds, "People have been trying to make progressively smaller drops since the advent of ink-jet printing, because smaller drops result in higher resolution. But the only way they could get smaller drops was to make the nozzles smaller and smaller, and that's not easy. It's a complicated manufacturing process."

The Purdue researchers have applied for a patent and are working with companies that have expressed interest in the new method.

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