Наука и технологија (општо)

So far, besides our glucose monitor, we’ve been able to batch-fabricate a few other nanoscale biosensors that respond to a target molecule with an electrical signal; we’ve also made several microscale components, including single-crystal silicon transistors, radio chips, antennas, diffusion resistors, LEDs, and silicon photodetectors. We’ve constructed all the micrometer-scale metal interconnects necessary to form a circuit on a contact lens. We’ve also shown that these microcomponents can be integrated through a self-assembly process onto other unconventional substrates, such as thin, flexible transparent plastics or glass. We’ve fabricated prototype lenses with an LED, a small radio chip, and an antenna, and we’ve transmitted energy to the lens wirelessly, lighting the LED. To demonstrate that the lenses can be safe, we encapsulated them in a biocompatible polymer and successfully tested them in trials with live rabbits.

Seeing the light—LED light—is a reasonable accomplishment. But seeing something useful through the lens is clearly the ultimate goal. Fortunately, the human eye is an extremely sensitive photodetector. At high noon on a cloudless day, lots of light streams through your pupil, and the world appears bright indeed. But the eye doesn’t need all that optical power—it can perceive images with only a few microwatts of optical power passing through its lens. An LCD computer screen is similarly wasteful. It sends out a lot of photons, but only a small fraction of them enter your eye and hit the retina to form an image. But when the display is directly over your cornea, every photon generated by the display helps form the image.

The beauty of this approach is obvious: With the light coming from a lens on your pupil rather than from an external source, you need much less power to form an image. But how to get light from a lens? We’ve considered two basic approaches. One option is to build into the lens a display based on an array of LED pixels; we call this an active display. An alternative is to use passive pixels that merely modulate incoming light rather than producing their own. Basically, they construct an image by changing their color and transparency in reaction to a light source. (They’re similar to LCDs, in which tiny liquid-crystal ”shutters” block or transmit white light through a red, green, or blue filter.) For passive pixels on a functional contact lens, the light source would be the environment. The colors wouldn’t be as precise as with a white-backlit LCD, but the images could be quite sharp and finely resolved.

We’ve mainly pursued the active approach and have produced lenses that can accommodate an 8-by-8 array of LEDs. For now, active pixels are easier to attach to lenses. But using passive pixels would significantly reduce the contact’s overall power needs—if we can figure out how to make the pixels smaller, higher in contrast, and capable of reacting quickly to external signals.

By now you’re probably wondering how a person wearing one of our contact lenses would be able to focus on an image generated on the surface of the eye. After all, a normal and healthy eye cannot focus on objects that are fewer than 10 centimeters from the corneal surface. The LEDs by themselves merely produce a fuzzy splotch of color in the wearer’s field of vision. Somehow the image must be pushed away from the cornea. One way to do that is to employ an array of even smaller lenses placed on the surface of the contact lens. Arrays of such microlenses have been used in the past to focus lasers and, in photolithography, to draw patterns of light on a photoresist. On a contact lens, each pixel or small group of pixels would be assigned to a microlens placed between the eye and the pixels. Spacing a pixel and a microlens 360 micrometers apart would be enough to push back the virtual image and let the eye focus on it easily. To the wearer, the image would seem to hang in space about half a meter away, depending on the microlens.

Another way to make sharp images is to use a scanning microlaser or an array of microlasers. Laser beams diverge much less than LED light does, so they would produce a sharper image. A kind of actuated mirror would scan the beams from a red, a green, and a blue laser to generate an image. The resolution of the image would be limited primarily by the narrowness of the beams, and the lasers would obviously have to be extremely small, which would be a substantial challenge. However, using lasers would ensure that the image is in focus at all times and eliminate the need for microlenses.

Whether we use LEDs or lasers for our display, the area available for optoelectronics on the surface of the contact is really small: roughly 1.2 millimeters in diameter. The display must also be semitransparent, so that wearers can still see their surroundings. Those are tough but not impossible requirements. The LED chips we’ve built so far are 300 µm in diameter, and the light-emitting zone on each chip is a 60-µm-wide ring with a radius of 112 µm. We’re trying to reduce that by an order of magnitude. Our goal is an array of 3600 10-µm-wide pixels spaced 10 µm apart.

One other difficulty in putting a display on the eye is keeping it from moving around relative to the pupil. Normal contact lenses that correct for astigmatism are weighted on the bottom to maintain a specific orientation, give or take a few degrees. I figure the same technique could keep a display from tilting (unless the wearer blinked too often!).

Like all mobile electronics, these lenses must be powered by suitable sources, but among the options, none are particularly attractive. The space constraints are acute. For example, batteries are hard to miniaturize to this extent, require recharging, and raise the specter of, say, lithium ions floating around in the eye after an accident. A better strategy is gathering inertial power from the environment, by converting ambient vibrations into energy or by receiving solar or RF power. Most inertial power scavenging designs have unacceptably low power output, so we have focused on powering our lenses with solar or RF energy.

Let’s assume that 1 square centimeter of lens area is dedicated to power generation, and let’s say we devote the space to solar cells. Almost 300 microwatts of incoming power would be available indoors, with potentially much more available outdoors. At a conversion efficiency of 10 percent, these figures would translate to 30 µW of available electrical power, if all the subsystems of the contact lens were run indoors.

Collecting RF energy from a source in the user’s pocket would improve the numbers slightly. In this setup, the lens area would hold antennas rather than photovoltaic cells. The antennas’ output would be limited by the field strengths permitted at various frequencies. In the microwave bands between 1.5 gigahertz and 100 GHz, the exposure level considered safe for humans is 1 milliwatt per square centimeter. For our prototypes, we have fabricated the first generation of antennas that can transmit in the 900-megahertz to 6-GHz range, and we’re working on higher-efficiency versions. So from that one square centimeter of lens real estate, we should be able to extract at least 100 µW, depending on the efficiency of the antenna and the conversion circuit.

Having made all these subsystems work, the final challenge is making them all fit on the same tiny polymer disc. Recall the pieces that we need to cram onto a lens: metal microstructures to form antennas; compound semiconductors to make optoelectronic devices; advanced complementary metal-oxide-semiconductor silicon circuits for low-power control and RF telecommunication; microelectromechanical system (MEMS) transducers and resonators to tune the frequencies of the RF communication; and surface sensors that are reactive with the biochemical environment.

The semiconductor fabrication processes we’d typically use to make most of these components won’t work because they are both thermally and chemically incompatible with the flexible polymer substrate of the contact lens. To get around this problem, we independently fabricate most of the microcomponents on silicon-on-insulator wafers, and we fabricate the LEDs and some of the biosensors on other substrates. Each part has metal interconnects and is etched into a unique shape. The end yield is a collection of powder-fine parts that we then embed in the lens.

We start by preparing the substrate that will hold the microcomponents, a 100-µm-thick slice of polyethylene terephthalate. The substrate has photolithographically defined metal interconnect lines and binding sites. These binding sites are tiny wells, about 10 µm deep, where electrical connections will be made between components and the template. At the bottom of each well is a minuscule pool of a low-melting-point alloy that will later join together two interconnects in what amounts to micrometer-scale soldering.

We then submerge the plastic lens substrate in a liquid medium and flow the collection of microcomponents over it. The binding sites are cut to match the geometries of the individual parts so that a triangular component finds a triangular well, a circular part falls into a circular well, and so on. When a piece falls into its complementary well, a small metal pad on the surface of the component comes in contact with the alloy at the bottom of the well, causing a capillary force that lodges the component in place. After all the parts have found their slots, we drop the temperature to solidify the alloy. This step locks in the mechanical and electrical contact between the components, the interconnects, and the substrate.

The next step is to ensure that all the potentially harmful components that we’ve just assembled are completely safe and comfortable to wear. The lenses we’ve been developing resemble existing gas-permeable contacts with small patches of a slightly less breathable material that wraps around the electronic components. We’ve been encapsulating the functional parts with poly(methyl methacrylate), the polymer used to make earlier generations of contact lenses. Then there’s the question of the interaction of heat and light with the eye. Not only must the system’s power consumption be very low for the sake of the energy budget, it must also avoid generating enough heat to damage the eye, so the temperature must remain below 45 °C. We have yet to investigate this concern fully, but our preliminary analyses suggest that heat shouldn’t be a big problem.