Multi-material filaments embed 3D-printable LEDs and sensors

September 13, 2019 //By Julien Happich
Using a custom-engineered fibre-drawing process, MIT researchers have drawn special multi-material filaments whose core embeds either light-emitters or light sensors and which they can use in place of conventional single-material polymer filaments in 3D printers.

The 3D-printer fitted with a special nozzle operates at a lower temperature and pulls the filament through faster than conventional printers do, so that only its outer layer gets partially molten to ensure adhesion to adjacent filaments during the 3D-printing process. The lower temperature also ensures that the embedded microelectronic components stay cool and solid, unaffected by the printing process.

The devices printed with such embedded electronics filaments end-up being either dotted with tiny electroluminescent pixels (about 55µm across each) or can spatially resolve light at micron resolution (when using the light-sensor filament) across their entire centimetre-scale surface.

Described in the paper “Structured multimaterial filaments for 3D printing of optoelectronics“ published in Nature Communications, the specially-made fibres contain multiple interconnected materials at micrometer-level, including metal wires, semiconductors with active functions, and polymer insulators to prevent wires from contacting each other.

The light-emitting filament consists of a metallic BiSn core (laser-heated in the drawn fibre to form discrete microspheres upon capillary breakup), electrically-conducting tungsten (W), and an electroluminescent ZnS layer wrapped around a dielectric-clad copper wire, the whole lot clad into insulating polycarbonate surrounded by a print adhesion tie layer (a cyclic olefin copolymer).

Schematics of the pixelated light-emitting filament (left) and of the light-detecting filament, with an external circuit connected to opposite ends of the different electrodes.

By connecting the tungsten and copper electrodes to an alternating voltage source, the bridging microspheres link the electric potential from the tungsten electrode towards the outer surface of ZnS, enabling sufficient electric field strength to induce light emission from the ZnS layer via electroluminescence, the paper reports.

Using a regular 3D-printing process, the authors note that this 0.6mm-thick filament could yield a maximum pixel density of 107 pixels-per-inch along its length, with a pixel resolution as small as 55µm, on par with the pixel size found in current super-retina high-definition displays.

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