As this novel printing approach can fabricate electronics and microfluidics, the team believe it could advance devices such as optical sensors used for health monitoring and lab-on-a-chip devices that integrate and automate multiple laboratory functions onto a small circuit, or chip.
"The positions and sizes of features can easily be modified and there is virtually no material waste with inkjet printing," contends Fabian Lütolf of CSEM."But the surface tension of the inks makes it difficult to print lines with a specific height – necessary to create a waveguide."
The researchers says that depositing the ink in two steps, rather than one, enabled printing of lines with a specific height and with much smoother features.
The technique can be used to print 2.5 dimensional optical waveguides and tapers made of acrylic polymer, and can also be used with other materials such as metallic inks to make electronics or sucrose mixtures for biodegradable applications.
"The fact that our approach could allow components with multiple functionalities to be fabricated with a single printer paves the way toward additive manufacturing of entire integrated circuits on chips," claims Lütolf. "This means that optical components could be added to flexible hybrid electronics and that optoelectronic components such as light sources or detectors could be integrated into printed optical circuits."
Due to surface tension, the inks deposited on a substrate tend to bulge or split, Lütolf explains, but the two-step technique enabled them to turn this into an advantage.
After depositing a series of droplets, the ink printed in the second step seeks to minimise its surface energy by self-aligning between the droplets from the first print. Unlike previous inkjet printing approaches, the researchers didn’t have to pre-pattern the substrate, which increases the available design space and simplifies fabrication.
The team printed a series of droplets called pinning caps. These spherical caps pin liquid bridges formed by the ink from the second print, creating a configuration that immobilised the ink and prevented the formation of bulges in the printed line. Along with creating straight lines between two dots, the researchers say the technique can be used to connect three or more junctions to make corners or sharp edges.
"Inkjet printing doesn't require a physical mask like photolithography and it is easier to connect components," says Lütolf. "Also, if you just want to quickly test an idea or vary a parameter, additive manufacturing methods such as inkjet printing only require adaption of the digital design."
To evaluate the new printing method, the researchers created a polymer waveguide that was 120microns wide and 31microns tall with a taper that allowed light from an external laser source to enter the waveguide. They measured the optical loss within the waveguide to be 0.19dB/cm, only an order of magnitude higher than state of the art waveguides created using photolithography.
The first inkjet-printed waveguides had loss characterisation. "For the applications we envision,” Lütolf says, “the waveguides would carry light for short distances, and not across entire networks. The current level of losses can be tolerated for such applications."
According to the researchers, the smallest possible waveguides consists of a single droplet of ink – limited by the nozzle of the inkjet printer. For the printer used in the study, the narrowest waveguides would be in the 40-micron range with a height of around 10micrometers.
"With our current combination of materials and hardware, it's not possible to make waveguides below 10micrometers, as typically required for single mode operation. But we are close," says Lütolf. "There is, however, no fundamental physical limit that would prevent us from printing single mode waveguides."
He adds that several groups have demonstrated printing capabilities in the submicron range with techniques such as electrohydrodynamic printing and believes it will be possible to combine these with the inkjet printing technique to create single mode waveguides.
The team is now looking to optimise the printing method and the ink to further lower the amount of light lost by the waveguide. They are also working to make the inkjet process more applicable for large-scale fabrication and, eventually, commercial implementation.
