Inkjet Technologies and Their Applications

Dear Colleagues,

During the last five to six decades, inkjet technology has emerged as a proven technology to dose and position small amounts of fluids on a substrate. The leading technologies are piezo jet and bubble jet. An emerging technology is electrospraying. The standard arrangement is a print head jetting downwards onto a flat substrate.

The main application of the technology is the printing of documents and graphics. The overall trend is to move to smaller droplets, down to picoliter size and sometimes to tents of femtoliters and to design print heads that can be driven at very high frequency in order to reduce printing time. The size of the print head (made out of a number of stacked print heads) and the number of nozzles are increased to make it possible to print documents page wide.

Inkjet printing is fundamentally concerned with the non-contact dosing and positioning of precisely metered small amounts of fluid on well-defined spots on a substrate.

Regarding biomedical applications, a number of applications have evolved. Examples are the printing of microarrays for genotyping with a large number of different probe spots, the printing of biosensors on paper to combine the biosensor area with all the microfluidic channels, patterning for the in situ synthesis of DNA, printing for cell manipulation, printing for automated bioassaying, and the manufacturing of 3D-printed structures for scaffolds for tissues/organs in vitro.

Another very interesting application is the printing of functional fluids for the manufacture of electronic circuits and displays. Often the inks are viscoelastic and may contain small particles. Particles may clog the nozzle, and means and methods must be applied to avoid this at all times.

Inkjet printing has entered the 3D arena. By stacking multiple droplets on top of each other, 3D structures can be made. Applications range from stereolithography to the manufacturing of spare parts for the aviation industry. The drying time and/or the solidification of the printed dots are now extra process parameters that determine how fast the process can be as well as its ultimate precision. Another application of 3D printing is printing direct to shape. The position of the print head is manipulated such that is possible to print directly onto complicated non-flat substrates. Although designed to spit downwards, by careful handling of the nozzle pressure it appears to be possible to print in all directions, even upside down. High-speed jetting (droplet speed above 50 m/s) allows for needleless injection.

Electrospraying is a method to jet small amounts of fluid out of a nozzle with a relatively large diameter by switching on and off an electrical field between the fluid in the nozzle and substrate. The advantage of this method is that nozzle clogging is less of a problem. By making the switching on–off time short, electrospraying can be used as an on-demand printing technology.

Key for these applications are reliability, dot placement precision, and droplet size. Decreasing the droplet size while maintaining placement accuracy allows for further miniaturization and higher droplet velocity. Inkjet printing is a feed-forward process. There is one direction of information flow—it is about sending print instructions to the print head. There is no feedback on whether the operation asked for (namely, the placement of a drop on a predefined spot in space) has actually taken place. In the document and graphics printing world, the reliability issue has been solved by increasing the number of dots per inch and relying on the limited resolution of the human eye. For biomedical and electronic applications, every drop counts. Therefore, means and methods have been developed and will be extended in the future to make the mantra “every drop counts” reality.

High-speed inkjet printing is now the main technology for decorating tiles. Here, all circumstances are present to make the process less reliable: erosive dust, heavy substrates, enormous amounts of ink, very high throughput speed, and a large gap between print head and rough substrate.

Prof. Dr. Frits Dijksman
Guest Editor

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