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Sprayable display brings 3D printing to life

Technology News |
By Nick Flaherty


Researchers at the Universities of Bristol and Bath in the UK have developed a technique to create a sprayable display on a 3D printed object. Working with researchers at MIT in the US, the team aims to combine plastic 3D printing and sprayable touchscreen technique in a single machine.

The ProtoSpray sprayable display technique uses conductive channels built into a 3D printed design to create the base electrodes for the display. These electrodes are then sprayed with dielectric and active materials to produce illumination and even touchscreens. The team looked at six different topologies, analysing the spray orientations, surface topologies and printer resolutions, to see how spray nozzles can be integrated into traditional 3D printers.

A video of the technique is here

The technique is based around electroluminescent (EL) ink which requires uniform deposition to avoid unpredictable electrical behaviour and short circuits and needs to be in thinner layers, for energy efficiency, than commodity 3D printer resolution allow. Conductive electrodes for the display traditionally also have to be made from optically transparent materials to allow light through from the EL layer, but 3D printers can’t currently process these materials and running the material through a filament or high temperature extrusion risk the loss of the electrical properties.

Instead, a coating of EL ink and a dielectric layer is sprayed on interconnected electrodes created by 3D printing conductive filaments to supply electrical power to displays.

“3D printers have enabled personal fabrication of objects but our work takes this even further to where we print not only plastic but also other materials that are essential for creating displays,” said researcher Ollie Hanton at the University of Bristol. “Using 3D printing of plastics and spraying of materials that light up when electricity is applied, we can support makers to produce objects of all shapes that can display information and detect touch. Our vision is to make screen/display a fundamental expressive medium in the same way people currently use ink, paint, or clay.”

The spray coating allows the precise, thin and cohesive application required to create thin displays on irregular surfaces without having to use a masking technique.

For an EL display to emit light it requires four layers: a conductive bottom electrode, an insulating dielectric layer, an insulating light-emitting layer, and a conductive top electrode. The bottom electrode is often made from a highly conductive metal, such as copper or silver ink. The dielectric layer is an electrical insulator, and must spread beyond the electrode layers to prevent short circuits between the top and bottom of the structure. The light-emitting layer is EL phosphor suspended in solvent. The top conductive layer must be transparent as with some polymers or metal oxides. To light up, an alternating current of around 200V operates between the electrodes, across the dielectric layer, energizing the light-emitting layer.

Using conductive channels rather than ‘on surface’ electrodes has a number of advantages to object design. Using 3D printed channels allows digitisation of the process for defining cell shape, increasing the potential for fully automating the process. Routing a conductive pathway inside the object also allows the only points on the surface to be the EL cells and the electrode attachment points, without the need for an on-surface conductive trace between the two. As a result, base electrode channels can cross each other in ways they wouldn’t be able to in 2D.

This gives a wider range of possible cell placement and opens design options and gives a potential for higher resolution of display due to denser cell placement, since space on the surface is no longer required for base electrodes merely for the attachment sites. It also allows easier electrode attachment points can be 3D printed in a wider range of potential locations, being less dependent on segment sizes/shape/location.

Automating the printed base electrodes through 3D printed conductive channels, means that the EL fabrication approach is significantly less sensitive to user experience level and opens potential for a combined fabrication process. However, this would need the design tools to include knowledge of the sprayable display process.

The team used an Ultimaker S5 3D printer on default settings with a 0.4mm nozzle/print core, printing at a 0.15mm layer height with conductive filament printed with a 100 per cent infill to maximise conductivity. For spray coating they used an Iwata Eclipse hp-cs airbrush with a 0.35mm nozzle and an AS186 air compressor at a distance of around 30cm. The displays are illuminated with two 9V batteries with a standard EL driver. 

As well as powering the EL sprayable display, the electrodes can also serve as capacitance sensors to enable touch sensing. Lacquer layers can be added to the top to create a protective insulating layer and separate the AC current powering the display from the user’s touch. Experiments showed comparable capacitive sensing capabilities to channels with no coating.

The paper is at https://doi.org/10.1145/3313831.3376543

www.bristol.ac.uk

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