A Blog by Jonathan Low


May 1, 2019

The Metamaterials Which Could Make Bendable Phones That Actually Work

Form truly follows function. JL

Matt Simon and Arielle Pardes report in Wired:

Metamaterials are artificially structured, man-made materials, where instead of using naturally occurring atoms and molecules, 'we define our own sub-wavelength structures.' Those structures can produce new behaviors. Instead of exploiting glass's larger structure, this lens plays with light within the material itself. You do this by creating structures within the material that are smaller than the light wavelengths you’re trying to manipulate. Metamaterial bends in a folding phone without cracking. It’s made of a super-thin sheet of silver, which is conductive, transparent, and malleable

Just days before its chimeric folding phone was supposed to go on sale, Samsung yanked the Galaxy Fold off the market (for now). Early review units had revealed a number of critical issues—faulty hinges, layers peeling off the display, and screens on the fritz—which turned the $1,980 smartphone dream into a PR nightmare.
Samsung hasn’t yet released a postmortem, but one area of scrutiny will be the screen, which relies on a different combination of materials than standard phones to lend it its folding superpowers. Because, surprise, this stuff is difficult. Bendy phones are only now possible because of fancy new materials, ones that likely don’t have all the kinks worked out yet. Whatever happens with the Galaxy Fold, the desire for bendable screens is leading to some unusual new technologies.
One particularly fascinating approach involves metamaterials. It’s a technology that’s already used, for instance, to create laser-reflecting glasses for airline pilots. And you could see metamaterials popping up in a whole lot more applications in the near future, including phones.
“Metamaterials are essentially artificially structured, man-made materials, where instead of using naturally occurring atoms and molecules, we define our own sub-wavelength structures,” says electrical engineer Jonathan Fan, who studies the stuff at Stanford University.
Those structures can produce a range of new behaviors, for example to manipulate light. Consider the traditional glass lens. Its ability to focus light derives from the lens’s larger structure—its curvature, as well as its changes in thickness from the edges to the center. There’s nothing particularly fancy going on with the material itself, so engineers alter its shape to manipulate light.

A lens made of a metamaterial, by contrast, could actually be flat. Instead of exploiting glass's larger structure, this kind of lens plays with light within the material itself. “You would need a material whose refractive index, which is the factor by which light slows down, needs to be highest in the center of the lens and reduced as you move toward the edges of the lens,” says Steven Cummer, who studies metamaterials at Duke University.
You would do this by creating structures within the material that are smaller than the light wavelengths you’re trying to manipulate. “So light wavelengths are 500 nanometers, plus or minus 25 percent,” says Cummer. “You need a physical structure that is roughly 10 times smaller than that, so 50 nanometers.”
Because you could vary these structures between the edges and the center of the metamaterial lens, you can replicate the effects of a traditional lens on a flat surface. Basically, with a traditional lens, you’re limited by the natural arrangement of glass atoms and molecules. But with metamaterials, you get to create your own structures that interact with light in unique ways.
One metamaterial that has made it into an actual product is in special glasses that can be worn by pilots. You may have heard about laser attacks on airline pilots—in the US alone, pilots report 20 attacks per night—which can lead to blindness. There are special glasses to help with this, but they’re problematic in their own right—they paint the world a demonic red.
“It’s like you’re on a different planet,” says George Palikaras, founder and CEO of Metamaterial Technologies, one of several labs and private companies pursuing metamaterials. “When it comes to aviation, the challenge is not whether you can block a laser—the challenge is blocking a laser while allowing the rest of the light to come in.” After all, pilots need to be able to see runway signage and cockpit instruments without having them look like they’re coated in Martian dust.
Using metamaterials, Palikaras and his team have developed glasses that specifically target the wavelength of green laser light, the color most commonly used in attacks. “It's called a holographic process,” says Palikaras. “The material is flat, and we rearrange and pattern the molecules inside this volume of material.”
They do this with, of all things, lasers: Two beams cross streams, like in Ghostbusters, to create tiny structures where they meet. “If you have a pond and you throw two rocks into the pond, those ripples start to reach each other,” says Gardner Wade, chief product officer of Metamaterial Technologies. “Where they meet, you have this interference pattern. That is the whole principle of holography.” Essentially, they create a multitude of tiny mirrors within the material that reflect only the wavelength of green laser light, as you can see in the GIF below.

So how would all this metamaterial magic work in a bendy phone? As of now, your rigid old phone responds to touch in part because of a layer of material called indium tin oxide, or ITO. “It’s a transparent metal that’s invisible to the human eye,” says Palikaras. When you touch the screen, he says, “it creates a conductive and resisting part that the sensor below it detects.”
On a typical smartphone, ITO works just fine. But on a bendy phone, designed to flex forward and backward again and again, the surface begins to crack. “So when you fold it a few hundred times, you start to see that at that bending area, you lose the sensitivity,” says Palikaras. “Which is totally unacceptable.” When the ITO cracks, its stops being invisible to the human eye and starts clouding the display.
Earlier bendy phones, like those made by phonemakers like ZTE or Royole, have used a number of alternatives to ITO, like metal mesh, silver nanowires, or graphine oxide. “Most companies, including Royole, are using the silver nanowires for now,” says David Hsieh, senior director of IHS Markit Display, which does market research on display technologies. “Samsung is using a different structure which is called Y-OCTA,” which embeds the touch sensors directly onto the AMOLED display. These solutions let you touch your screen, but the materials come at an expense or break down over time.
The problem with silver nanowires is their structure. “Think of silver nanowires like spaghetti,” says Palikaras. “When you lay spaghetti over each other, they are touching on top of each other and they create this web.” At those junction points, the web gets thicker, diminishing transparency, he says.
So ITO is too brittle, and silver nanowires too opaque. What Metamaterial Technologies is developing is an alternative, called NanoWeb, that bends in a folding phone without cracking. It’s made of a super-thin sheet of silver, which is conductive, transparent, and malleable. They etch structures in it with cross-streamed lasers, like the holographic process in the laser glasses. (But because this is working with metal, not plastic, it’s called lithography.)
In this case, the idea isn’t to use metamaterial techniques to manipulate light but to give the silver flexibility so it can bend without cracking. Also, they’re etching on a nice, flat material—meaning, they don’t have spaghetti lying on top of each other, adding thickness.
Palikaras says his company is already looking ahead at how the technology could fit into other parts of the consumer tech space. He imagines bendy screens in cars, or refined displays in home appliances. Metamaterial Technologies also has a contract with a major phone manufacturer, though Palikaras won’t say which one.
The future, it's safe to assume, is looking rather flexible.


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