Apple has introduced the iPhone Duo, its first foldable smartphone, featuring a 7.6-inch glass display designed to withstand repeated bending. Although Google, Samsung, and Motorola already offer foldable devices, Apple is entering a market made possible by advances in materials science and microengineering, tells The New York Times (full article available to subscribers).
The central engineering challenge is making normally brittle glass flexible. Glass is an amorphous solid, meaning its atoms are frozen in a liquid-like arrangement. Thick glass breaks when bent because curvature creates substantial tension along its outer surface. Reducing its thickness dramatically changes this behavior.
The ultrathin glass reportedly used in the foldable iPhone measures less than 30 micrometers thick, compared with roughly 50–100 micrometers for an average human hair. At this scale, bending produces much less strain, allowing the glass to curve instead of fracture. Northwestern University materials scientist John A. Rogers explains that reducing a material’s thickness decreases its stiffness toward bending, much as a thin sheet of paper bends more easily than a piece of wood.
Chemistry provides another route to flexibility. Conventional glass is primarily made from silicon dioxide, but researchers have explored alternative formulations containing boron trioxide. Commonly used in laboratory glassware, this material can offer improved flexibility when produced in extremely thin layers.
Glass alone, however, does not make a foldable display practical. Another ultrathin layer consists of highly transparent, durable plastic engineered to withstand hundreds of thousands of folding cycles without becoming cloudy or distorted.
The display’s multiple layers are joined using optically clear adhesives. These viscous, elastic materials hold the screen together while allowing individual layers to slide slightly past one another as the device bends.
Rather than relying on one breakthrough, foldable displays combine ultrathin glass, specialized chemistry, engineered polymers, and flexible adhesives. Together, these materials create screens that can repeatedly bend while remaining strong enough to withstand the everyday stresses expected of a smartphone.
