Kong et al, Nano Letters

Here's a hypothetical question: would you rather have a head-up display on glasses or a contact lens?

If you answered "contact lens", the bad news is that you may be waiting some time. But the good news is that it just got a little more feasible, with the invention of the world's first 3D printer that can print LEDs.

The team, led by Michael McAlpine at Princeton University's McAlpine Research Group, has successfully used its printer to 3D-print quantum dot LEDs -- LEDs that are considered the next step up from OLED. QLEDs shine brighter and with purer colour, at a lower power consumption rate, using cadmium selenide nanocrystals. They're also ultrathin, flexible and transparent -- like, for instance, contact lenses.

"The conventional microelectronics industry is really good at making 2D-electronic gadgets," McAlpine said. "With TVs and phones, the screen is flat. But what 3D printing gives you is a third dimension, and that could be used for things that people haven't imagined yet, like 3D structures that could be used in the body."

McAlpine and his team printed the LED in five layers. A ring made of silver nanoparticles on the bottom layer is the metal conduit for a mechanical circuit. Two polymer layers follow to supply and transfer the electrical current to the next layer, consisting of cadmium selenide nanoparticles (the quantum dots) contained in a zinc sulphide case. The top and final layer is the cathode, made of eutectic gallium indium.

Kong et al, Nano Letters

"What we have presented here is an additional method to integrate electronics that can take into consideration the three-dimensional geometry of an object," said study lead co-author Yong Lin Kong. He also noted that this is the first example of a fully 3D printed, fully functional electronic device.

Potential applications for the technology include wearables, such as the aforementioned contact lens -- if the team can figure out a way to include an on-board power supply. The team is also going to be investigating the inclusion of a 3D-printed transistor for added functionality.

You can find the full study online in the journal Nano Letters.