Since 2008, Babak Parviz, an Associate Professor at the University of Washington (Seattle, WA) has been leading a team developing a contact lens-based display. If the project is fully successful, the resulting display should be capable of providing an augmented reality experience, overlaying 3D imagery directly on the user’s field of view.
To convert a conventional contact lens into a display system, the researchers are merging transparent, eye compatible materials with microelectronics and an array of semi-transparent LEDs. Consider some of the issues that must be addressed in producing the display.
First, fitting a contact lens with circuitry is a considerable challenge. Most biocompatible plastic materials cannot withstand the processes used during microfabrication. To address this problem, most of the microcomponents are fabricated independently utilizing silicon on insulator wafers. Each microcomponent includes metal interconnects and is etched into a unique physical shape. The result of this initial processing is a collection of microcomponents that have an almost powder-like appearance. The next step is to embed the microcomponents into the lens “substrate” using a self-assembly process.
The substrate is a 100 µm thick slab of polyethylene terephthalate. The substrate has photolithographically defined metal interconnect lines and binding sites. The binding sites can be pictured as tiny wells about 10 µm deep. The bottom of each well contains a microscopic pool of a low melting point alloy.
The next step is to submerge the lens substrate in a hot carrier liquid that contains the microcomponents. The liquid is made to flow over the substrate. Since the shapes of the wells match the shapes of the microcomponents, the microcomponents will eventually fall into corresponding wells. Each time this occurs, the metal pads on the surface of the microcomponent come into contact with the alloy at the bottom of the well. Capillary force will then act to finish moving the microcomponent into place.
After all the microcomponents have found wells, the temperature is lowered to solidify the alloy. This is a “micro-soldering-like” process in which the alloy bonds the interconnects on the microcomponents to those on the substrate. This process assures mechanical and electrical contact between the microcomponents and the connection matrix of the substrate.
Now, consider the optics. The LEDs are only millimeters from the retina, so how can you focus any image created with these emitters? The solution is to use an array of microlenses on the surface of the contact lens between the eye and the pixels. When the distance between the pixels and the microlenses to be about 360 microns, an image is visible that appears to approximately half a meter from the eye. At this distance, focus should not be a problem.
When the LEDs are off, the contact lens display will be essentially transparent.
Another key design issue is that of power consumption. Not only must the display’s power consumption be very low for the sake of the energy budget, it must also avoid generating eye-damaging heat.
The solution adopted by the U of W group to power the display is to transmit energy wirelessly from a loop antenna located near the user to a resonating antenna in the lens itself. This method is similar to that used in RFID tag technology. Data is transmitted by the same means.
The team has fabricated prototype lenses with an LED, a small radio chip and an antenna. Energy has been successfully transmitted to the lens wirelessly, illuminating an LED. Both red and blue pixels have been developed as has an 8×8 array of LEDs.
To demonstrate that the lens is safe, prototypes have been encapsulated in a biocompatible polymer. These were molded into the shape of a contact lens and successfully tested in trials with live rabbits.
One other difficulty that can be anticipated in placing a display on the eye is the need to prevent it from moving. One potential solution might be to adopt a means similar to that used in normal contact lenses that correct for astigmatism. That is, the lenses are weighted on the bottom to roughly maintain the desired orientation.
The U of W group has yet to combine the optics and the LEDs in the same contact lens. None-the-less, all the basic technologies needed to build an operational contact lens based display have been demonstrated. Next steps include showing that all the subsystems can work together, to further reduce the size of some of the components and to improve RF power collection efficiency and transmission range. I can’t wait to see (through) one of these displays. Art Berman is a consultant for Insight Media. Reach him at firstname.lastname@example.org