In this post, my aim is to briefly describe inkjet-printing, its capabilities, as well as its advantages. I will also be showing examples of high performance flexible electronics that can be made using this method.
When scientists working in this area talk about “inkjet printing technology”, they refer to roughly the same technology as the one that is used in your desktop inkjet printer. There are small variations as to whether the nozzles continuously produce drops (Continuous Inkjet) which can be caught, or not, by the a drop catcher to form a pattern on the substrate (the material that is printed on) or whether the drops are only generated when the nozzle is above the point it needs to deposit the ink on (Drop On Demand). To form these drops, the ink can be heated in order to vaporize the solvent and therefore increase the pressure in punctured chamber, as is the case in most desktop inkjet printers, or a piezoelectric actuator can mechanically push the fluid out instead. Apart from these small differences, the fundamentals stay the same: drops of ink are deposited onto a material in very specific locations in order to deposit controlled amounts of specific chemicals.
One of the greatest strengths of this fabrication method is that a wide range of chemicals and particles such as dyes, metal nanoparticles, conductive and non-conductive polymers, nan0materials, etc. can be deposited. Virtually any chemical species or particles smaller than about 1 µm (the size of a small bacterium) and that can be dispersed in a solvent are inkjet printable. Furthermore, these materials can virtually be deposited on any surface, provided the slope of the surface (with regard to the plane perpendicular to the free fall trajectory of the drop) is not too steep. The only constraint on the type of material that can be used as a substrate, is the necessary capacity of withstanding the temperature required to dry and, eventually, process the ink. Fortunately, most materials are compatible with most inks. Therefore, it is possible to print on virtually any material, especially flexible ones like paper, PMMA, polyimide (etc…).
Usually, circuit are produced by selectively removing a conductive metal layer that is at the surface of the clad substrate. Regardless of whether the metal layer is removed by mechanical milling or chemical etching, this process is very wasteful. In contrast, inkjet printing is an additive manufacturing process and therefore only deposits as much material as is requited to make the circuit. Only the solvent, which could even be recovered through a fume recycling system, is usually wasted. As a result, inkjet printing is a very environmentally friendly process and also a very low cost one.
This offers an extraordinary versatility as to what kind of components and system can be fabricated with inkjet printing. The ATHENA group at Georgia Tech, that I am part of, has been working towards utilizing this technology for wireless components and systems fabrication.
A few examples of such components and systems include:
- Multilayer lumped components. Conductive and dielectric inks were used to print these multilayer structures
- Single and multilayer antennas
- Metamaterial structures
- Fully inkjet-printed gas sensors
The fabrication process also has its drawbacks compared to other standard processes.
The biggest issue are the limitation on the resolution of the print and the difficult control of the pattern on non porous substrates. Indeed, once a drop has been deposited onto the substrate, it spreads out over the surface of a disk with a certain radius (as can be seen in the next figure, where each “dot” is one printed drop). The surface covered by the drop is where the chemical that is dispersed or dissolved in the solvent will end up being deposited after drying (dynamic drying phenomena aside). The radius of the drop therefore defines the definition of the printing process.
The radius of each printed drop depends on the interaction between the fluid and the substrate. It is therefore possible to improve the definition through the use of surface modification techniques. Usually this consists in making the surface more hydrophobic, increasing, as a consequence, the contact angle of the drop on the substrate and reducing the size of each drop’s footprint.
Another and very straightforward option is to reduce the size of the drops generated by the nozzles. Less fluid will spread over a smaller area. The nozzles that are typically used for the Dimatix 2830 printer make 10 pL drops. Smaller, 1 pL nozzles are also available but limit the size of the printed particles. With these rather simple methods, definitions down to a few micrometers can be achieved.
More advanced methods are also possible, including local tuning of the hydrophobicity of the surface with a laser or self assembly methods such as the one described in this paper.
Great read on inkjet printing: