Princeton's LED 3D printer has taken the next step beyond printing quantum dot LEDs: it has managed to integrate them with a standard contact lens, creating a device that can project beams of coloured light.
It's not wearable yet: The lens itself is made of hard plastic, which is unsuitable to be worn on the eye, and the QLEDs require an external power supply. However, it does demonstrate the feasibility of 3D-printed electronics, integrated with complex shapes to create fully realised devices.
"This shows that we can use 3D printing to create complex electronics including semiconductors. We were able to 3D print an entire device, in this case an LED," said Assistant Professor of Mechanical and Aerospace Engineering Michael McAlpine, at Princeton University's McAlpine Research Group.
"We used the quantum dots [also known as nanoparticles] as an ink. We were able to generate two different colours, orange and green."
The 3D printer prints the QLEDs, revealed last month, in a series of 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.
"The conventional microelectronics industry is really good at making 2D-electronic gadgets," McAlpine said at the time. "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."
The lens is a continuation of the team's work in this area: last year, the team 3D printed an ear out of living tissue embedded with an antenna for a cochlear implant. The challenges involved in the lens, however, were quite different: 3D printing in diverse materials that have inherent incompatibilities, such as different temperature sensitivities.
"For example, it is not trivial to pattern a thin and uniform coating of nanoparticles and polymers without the involvement of conventional microfabrication techniques, yet the thickness and uniformity of the printed films are two of the critical parameters that determine the performance and yield of the printed active device," said study lead co-author Yong Lin Kong.
This is what the $20,000 printer, built with the assistance of chemistry graduate Ian Tamargo, fluid dynamics expert Hyoungsoo Kim and Assistant Professor of electrical engineering Barry Rand, was specifically designed to circumvent.
To create the device, the team first 3D scanned the shape of the lens. This data was programmed into the 3D printer, which was then able to adapt its QLED printing process to a curved surface.
As for getting a contact lens HUD? Well, there's a lot of work to do before that can happen; such as internalising the power source, and integrating it with a soft lens, rather than hard plastic -- never mind the actual usability.
In the future, the team will be researching the feasibility of other 3D-printed electronics, such as transistors.