Yesterday, the World Intellectual Property Organisation published a Google patent dealing with a “foldable computing device”. The recently published patent, however, has less to do with foldable displays and instead focuses entirely on the single biggest hurdle preventing the practical implementation of the technology – that is, the inability of foldable displays and associated flexible PCB underpinnings to survive the numerous bending and straightening cycles experienced during the course of their lifespan.
Interestingly, the published document doesn’t read like the average foldable smartphone patent. Google hasn’t necessarily patented a technology that could be applied to a singular product as such, but has instead laid down virtually all possible means to improve the durability of foldable displays. In other words, even if the company doesn’t launch a foldable smartphone of its own, it can potentially collect royalty from any brand that employs this crucial piece of technology.
Using Traditional Engineering to Improve Durability
Google’s idea to improve the durability of foldable displays warrants deeper dissection since it is steeped in physics and traditional engineering. When a foldable display is bent, the inner and outer layers comprising the display are subjected to compressive and tensile stresses respectively. Repeated enough times, these tensile and compressive forces damage the polymer display components as well as the flexible printed circuit boards; thereby reducing the lifespan of flexible displays.
However, this isn’t the first time this very engineering challenge was faced by mankind. The same tensile and compressive forces affecting a typical longbow made it impossible to miniaturise it enough for horseback archery. The Mongols cracked this engineering challenge with the composite bow. Comprised of wood and sinew laminated together, the compact Mongol composite bows made horseback archery possible by combining the compression generated by wood with the elasticity of animal hide. Not surprisingly, the Mongols went on to dominate Central Asia under Genghis Khan circa 13th century.
By employing physics and engineering principles exhibiting similar ingenuity and impact as the Mongol composite bow, Google’s patent can potentially change the very way in which foldable displays are implemented in smartphones and other mobile devices bearing a display. Although Google’s patent doesn’t deal with exotic flexible displays and PCBs with improved resistance to tensile and compressive stresses, it nevertheless employs a number of clever engineering solutions to prevent the aforementioned components from ever being subjected to damage.
Every flexible PCB and display component is rated to bend to a certain degree without experiencing permanent damage. DuPont, for example, specifies that single and double-layered flexible PCBs can be bent to a radius that is no smaller than six times the overall thickness of the material. Multi-layered implementations, however, are more fragile and require the minimum bend radius to be no smaller than 12 times the overall thickness. Multi-layered flexible displays present a similar challenge, but Google’s patent features multiple implementations that leverage simple and cost-effective mechanical solutions to elegantly prevent bending damage.
Google Patent Could Make Foldable Devices Thinner
Google’s solution involves adding a bend limit layer. This mechanical layer is attached parallel to the flexible display and designed to protect the TFT and OLED components from damage by preventing them from being bent beyond the minimum bend radius.
Google specifies three distinct implementations – a simple one involving low stretch fibres, another more mechanically complex implementation involving nickel-titanium alloy segments, and the final implementation that employs advanced foam or gel materials to prevent foldable displays from being bent beyond a certain degree.
The first implementation involving low stretch fibres is fairly simple. It involves arranging the fibres in the bend limit layer in a fashion that ensures that the fibres reach their limit of elasticity well before the display is bent beyond the minimum bend radius. In simpler words, the low stretch fibres only extend enough to allow the foldable display to be bent to a safe degree. These fibres can be comprised of anything from DuPont’s Kevlar branded aramid variety to composite materials comprised of graphite, carbon fibre, ceramic, and glass. These fibres are laser soldered to the bend limit layer in a serpentine configuration to allow the bendability of the display to be precisely controlled.
The document also details further implementations where the bend limit layer is ribbed and the ribs themselves comprised of materials whose bulk modulus varies in relation to the compressive forces experienced during bending. This article will gloss over this implementation to focus on the next one that seems a lot more exciting.
Foldable Device That Unfolds Flat by Itself and…
The second mechanically complex implementation features a bend limit layer comprised of segmented nickel-titanium alloy blocks (such as Nitinol) attached to DuPont’s Kapton HN polyimide film that is widely used in flexible PCBs. The document also specifies that the segments can be formed from alternative materials such as polymers, glasses, and ceramics, along with detailed information on how these segments can be manufactured.
These alloy segments are distinct and physically separated from each other and shaped such that they continue to remain physically separated from one another as long as the radius of curvature of the foldable display layer is greater than the minimum bend radius. The segments physically touch each other the moment the display is bent at or beyond the minimum bend radius; thereby rigidly locking the entire assembly and preventing the display from being folded further.
It must be noted that Nitinol is a shape memory alloy that exhibits the unique property of being able to remember (or rather revert to) its original form on application of external stimuli such as heat. The fact that Google plans to use Nitinol in the bend limit layer alludes to the possibility of future foldable smartphones that can automatically revert to their original shape. In other words, foldable devices that can become perfectly flat without requiring the user to meticulously bend them back to shape. That’s perfect for those afflicted with a mild OCD when it comes to these things.
…Open, Close, Assume Specific Shapes Automatically
The final implementation of the bend limit layer employs foam or gel materials whose stiffness or bulk modulus (resistance to compression) changes in a non-linear fashion. Think of these as non-Newtonian fluids that become rock hard on impact. Instead, in this case, we are dealing with a foam or gel equivalent that becomes thicker and resistant to compression when bent. The most interesting aspect of the patent document involves the way in which Google intends to change the stiffness or the bulk modulus of these foams and gels.
The gel or foam-based bend limit layers can be infused with electro-active materials such as ferro-electric polymers, polymer-metal composites such as DuPont’s Nafion, and even carbon nanotube impregnated polymer materials. In other words, electric current and/or voltage can be applied to the aforementioned gel/foam-based electroactive bend limit layer in order to make it rigid or flexible electronically at a moment’s notice.
More interestingly, the same principle can be employed to bend, twist, expand, contract, or shrink the bend limit layer. In layman’s terms, this could allow the foldable device to be programmed to bend, open flat, or even assume specific shapes without physically requiring the user to bend or manipulate the device.
Next Pixel Could Make A Fine Foldable Smartphone
No matter how you look at it, this patent contains some fine examples of engineering ingenuity. Google’s idea of employing the thin and invisible bend limiting layer to prevent foldable displays from being damaged is a lot more elegant and compact compared to bulky implementations seen recently in Samsung’s foldable display patents.
Here’s hoping Google actually intends to employ these great ideas in its future Pixel smartphones, and not just to seek royalty or, worse yet, prevent the same from being implemented in other foldable devices.
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