Apple on Sept 9 unveiled the iPhone Duo, its first foldable smartphone with a 7.6-inch glass screen that is capable of bending countless times without breaking.
The Duo is not the first foldable phone; Google, Samsung and Motorola have all released devices with displays that bend. But in his pitch for the Duo, Apple’s new CEO, John Ternus, said the company’s latest offering “will redesign the experience of using a foldable phone.”
The screens of foldable phones are a triumph of materials science and microengineering. No single breakthrough or industrial secret makes them possible; rather, a series of complementary feats makes a display flexible enough to bend many thousands of times without snapping yet strong enough to survive drops and other abuse.
Glass – the brittle, breakable kind – is what scientists call an “amorphous solid.” It looks and acts like a solid, but glass atoms are actually frozen in a liquid-like state. Everyday glass shatters when bent because those atoms lie in a rigid matrix that, when thick, leads to extreme tension on the outer edge of the curved surface.
But glass becomes flexible if it’s thin. The average human hair is between 50 and 100 micrometres thick. The superthin layer of glass in a foldable iPhone is reportedly less than 30 micrometres thick, or much finer than a human hair. At this step of miniaturisation, the strain on the outer edge becomes so low that the glass tends to curve rather than break.
“The thickness of a material determines its stiffness toward bending,” said John A. Rogers, a professor of materials science and engineering at Northwestern University. “If you make something very thin, it becomes easier to bend.”
Rogers compared breakable and bendable glass to wood and paper. “If you think about a piece of wood, it’s not very bendable,” he said. “But a sheet of paper, which is also wood, is flexible because it’s much thinner. The same applies to glass.”
In addition to making glass thinner, chemists found that alternative glass formulas could enhance the flexibility. Everyday glass is made of silicon dioxide, which comes from melted sand. But boron trioxide, used mainly in laboratory glassware that expands and contracts very little when heated and cooled, has an atomic matrix that’s amorphous rather than rigid and crystalline. Lab testing showed that, when thin, it bent more easily than regular glass.
Another extremely thin layer in a foldable screen is made of ultra-durable, highly transparent plastic that’s engineered to endure hundreds of thousands of fold cycles without clouding or warping.
Finally, the many layers of a foldable screen are bonded together with what the industry calls optically clear adhesives. Viscous and elastic, they act like a gel, holding the layers together firmly but, under the stress of a bend, still allow the layers to glide past each other. – ©2026 The New York Times Company
This article originally appeared in The New York Times.
