0:00 This September, Apple announced its first foldable iPhone, called the iPhone. Closed, it's a 4-inch phone; opened, it transforms into a 7.6-inch screen. Since phone screens are covered in glass, which cracks when slightly bent, how can this screen fold in half hundreds of times a day without breaking? The evolution of foldable phones over these eight years has focused on three areas: which way the screen folds, the screen's surface material, and how the hinge allows the screen to bend. Eight years ago in the fall, a Shenzhen company called Royole launched the first commercially available foldable phone with a 7.8- inch plastic-surfaced screen that folded outward, displaying images on the front, back, and the corner hinge. However, users found that every time it
0:46 folded, the screen near the hinge would bulge, and it would even make noise. Months later, Samsung gave the first-generation Galaxy Fold to tech journalists for testing, but several units broke after just a few days. Some users saw a film on the screen and, thinking it was a screen protector, peeled it off, but that film was part of the display, and removing it broke the screen. On another unit, no one peeled anything, but a bulge appeared at the crease, and soon the pixels along that line stopped lighting up. It was later suspected that dust entered through the hinge gaps and pushed against the back of the screen, forcing Samsung to delay the launch, extend the film under the bezels, and add caps to the hinge ends to block dust. As for which way to fold, companies initially
1:32 had different answers. Huawei’s Mate X folded outward like Royole's, leaving the screen exposed when closed, making it prone to scratches in a pocket, while Samsung chose to fold inward. Later that same year, Motorola released the Razr, a long phone that folds vertically, shrinking to half its size. Inward folding eventually became the mainstream, and subsequent Huawei foldables followed suit, as the screen is protected between the two halves when closed, preventing scratches and impact, while a secondary outer screen allows for notifications and calls. However, folding inward comes at a price. When folding outward, the screen wraps around the outside with a larger curve, but when folding inward, the
2:19 screen must bend within the narrow gap between the two halves, making that curve much tighter. The smaller the radius, the sharper the bend, and the easier it is for the screen to break. Subsequent changes to screen materials and hinges were made to ensure the screen wouldn't snap despite such sharp bending. The second change concerns the screen's surface material. The first generation of foldable phones used a layer of transparent plastic on top. Plastic is easy to bend, but it's very soft; a fingernail scratch leaves a mark, and over time, a noticeable crease forms in the middle. The following year, Samsung introduced glass into a foldable screen for the first time with the Galaxy Z Flip; that layer of glass was only 30 micrometers thick, thinner than a human hair. This type of glass is made by creating a thin sheet, cutting it to size, and soaking it in molten salt. This replaces the ions on the glass surface
3:05 with larger ones, compressing the surface so it doesn't crack as easily. However, at 30 micrometers, it is still very thin and could break if poked by something sharp, so Samsung added a plastic protective layer on top. Starting from this generation, the screen surface became a combination of plastic and glass layered together. The third change involves the hinge. Early inward-folding phones left a gap between the two halves when closed because the screen was bent into a U-shape within the hinge. Later hinges created more internal space, allowing the screen to fold into a teardrop shape, letting later Samsung foldables close completely flush. With these changes, foldable phones have become increasingly thinner. Samsung's Galaxy Z Fold 6 from last year was only 8.9 millimeters thick when closed, just slightly thicker than a standard phone,
3:51 and Huawei has even created a tri-fold phone that can be folded twice. So, how can glass even bend? And what is the point of the teardrop-shaped space? A foldable screen is made of several thin layers of material stacked together. From top to bottom, the first layer is a protective film, the second is ultra-thin glass, the third is the light-emitting layer, and the fourth is a thin metal support plate. The light-emitting layer uses OLED, which stands for Organic Light-Emitting Diode . "Organic" refers to the material used to emit light, a carbon-based compound that glows when electricity is applied. This material layer sits on a thin plastic film and is inherently flexible . Beneath the light-emitting layer is a circuit layer that controls the brightness of each pixel. This circuit layer and the top glass will crack if stretched too much. When an object is
4:38 bent into an arc, the outer side covers a larger circle and a longer distance, causing it to stretch, while the inner side covers a smaller circle, causing it to be compressed and potentially crushed. Glass can barely be stretched; if the outer side is stretched even slightly, cracks will appear on the surface. How much it stretches depends on two factors: how much it bends and how thick it is. When the bending curvature is the same, the thicker the object, the further the outer surface is from the center, and the more it gets stretched. Standard phone screen glass is about 0.5 millimeters thick, while ultra-thin glass is only 30 micrometers, about 1/16th of that. When bent the same amount, the outer side stretches 1/16th as much. So, ultra-thin glass isn't softer; it’s just thin enough that even at a tight
5:23 angle, the surface stretch remains minimal. You might think, screens have multiple layers; don't they get thicker when stacked? They certainly do. When the whole stack bends, the top is stretched and the bottom is compressed, but at a certain height in the middle, there is a layer neither stretched nor compressed; this is called the neutral axis. "Neutral" means neither pulled nor pushed, so manufacturers adjust the thickness of each layer so the neutral axis falls right on the most fragile circuits. Also, the adhesive between these layers is soft, allowing them to slide slightly when bent. It's like a book where pages slide against each other, making it easy to bend; if the pages were glued shut, it would become a stiff board. Since layers can slide, the amount of stretch depends only on
6:10 the thickness of each individual layer. This makes the amount of stretch even smaller. The hinge is responsible for two things: making both halves of the phone rotate together, and determining how the screen bends in the middle. Inside the hinge are several pivot sets and a row of gears. The gears transfer rotation from one side to the other, so both sides always rotate equally. There are also springs pushing against cams —wavy-shaped parts—where the friction from the spring tension offsets the device's weight, allowing the phone to stay open at any angle. As for how much the screen stretches, besides thickness, it also depends on how sharply it bends. If you fold the screen inward until the two halves touch, the middle section is compressed into a line with a bending radius of
6:56 nearly zero, stretching the outer side significantly. That’s why early designs avoided letting the two halves touch completely, leaving a gap to let the screen form a U-shape. The teardrop hinge creates extra space where the two screens meet, allowing the screen to curve outward before tucking back in. This forms a teardrop shape, letting the two halves sit flush while creating a gentler arc for the bend, which reduces the amount of stretching. Why is there still a crease in the middle after opening it? The protective layers and adhesives are made of plastic-like materials. If kept bent for too long, they gradually deform to match that shape and cannot return to a flat surface; this phenomenon is called " creep." Many foldable phones are packaged in the box while kept in the
7:42 open position. The larger the teardrop space, the gentler the screen curve, and the shallower the crease. However, a larger space requires a thicker hinge and body. Every manufacturer is trying to find an acceptable balance between thickness and the crease. So, what has changed with the iPhone 2? The screen structure Apple announced involves four aspects related to these principles. First, they placed a layer of glass both above and below the light-emitting layer. Most foldables only have a layer on top; by having one on both sides, the thickness is balanced, placing the neutral plane right on the light-emitting layer in the middle. Second, they used a special adhesive between the layers, which Apple says allows each layer to slide against one another like pages in a book. Third, the top protective layer has been replaced with a harder polymer material ; Apple says it’s four layers harder,
8:29 and the surface features a fine matte texture. Creases are visible because light reflecting off an uneven surface distorts the image, but the matte finish diffuses the reflection, making the distortion less obvious. Fourth, they added a titanium plate at the very bottom to make the screen more pressure-resistant, and the hinge casing is also made of titanium alloy, containing over 100 parts. In short, foldable screens work because the glass is ground thinner than a human hair, minimizing stretch during the bend. The most fragile circuits are placed on the neutral plane where there is neither tension nor compression, the layers can slide, and the teardrop-shaped hinge allows for a gentler curve. As for the crease in the middle, it is the result of the materials gradually deforming after being kept in a bent state. That’s all for today’s video. If
9:15 you enjoyed it, please subscribe to help us reach 100,000 subscribers faster. This is
