Smartphone Futurology: The science behind smartphone glass

Welcome to Smartphone Futurology. In this new series of science-filled articles, Mobile Nations guest contributor Shen Ye walks through current technologies in use within our phones, as well as the cutting-edge stuff still being developed in the lab. There's quite a bit of science ahead, as a lot of the future discussions are based on scientific papers with a vast amount of technical jargon, but we've tried to keep things as plain and simple as possible. So if you want to dive deeper into just how the guts of your phone function, this is the series for you.

This is the final instalment — for now — in our series on the future of smartphone technology. This week we'll cover the science behind one really important area of smartphone build quality — the glass of the touchscreen. And as we're wrapping up the series, we'll also see how the current state of mobile tech compares to predictions made almost a decade ago. Read on to learn more.

About the author

Shen Ye is an Android developer and MSci graduate in Chemistry from the University of Bristol. Catch him on Twitter @shen and Google+ +ShenYe.

More in this series

Be sure to check out the first three instalments of our Smartphone Futurology series, covering the future of battery technology, smartphone display tech and processors and memory.

Toughened Glass

Billions of dollars are spent on screen repairs every year, with a portion of users deciding to live with their cracked screen instead of spending money on repairs. Nearly all flagship phones of 2014 used Gorilla Glass 3 by Corning, though some opt for generic toughened glass instead. Modern toughened glass is the result of multiple thermal and chemical treatment processes, increasing the strength of the material compared to ordinary glass.

If you look at the surface of a sheet of glass under a microscope, you will find it is filled with tiny flaws and micro cracks. These flaws make glass really susceptible to breakage. If enough stress is applied, these cracks can propagate, fracture and result in a broken sheet of glass. If you imagine 2 sheets of paper, one is perfect and one has a small tear in the centre. If you pulled on the sides of the sheets of paper, the sheet with the small tear will require considerably less force to rip. Now imagine if the small tear was at the edge of the sheet of paper, even less force is required for it to propagate and eventually tip the paper in half. Stress can build up very easily at edges and even more at sharp corners; this is why aircraft are required to have windows with rounded corners.

Regular glass is actually riddled with tiny flaws and cracks — toughened glass closes these up using a variety of different techniques.

Gorilla Glass is a type of toughened glass known as "alkali-aluminosilicate glass". It's the best known brand in toughened glass for smartphones, used in popular Android and Windows phones like the Samsung Galaxy S5, HTC One M8, and many Lumia handsets. The thermal processes temper the glass, which causes a compression force on the outer surface of the glass. This toughens the glass by closing up some of those micro cracks, but also make the glass safer – if the glass breaks it will shatter into small pieces instead of large dangerous shards (similar to a Prince Rupert's drop). Aside from tempering, a chemical process known as "ion exchange" also toughens the material.

The glass contains a lot of sodium from the manufacturing process. As it is dipped into a hot molten potassium bath, the potassium ions move into the glass and displace the sodium ions. Potassium is larger than sodium and this also causes compression force on the surface of the glass — like tempering — which toughens the glass.

Toughened glass is extremely hard. The accepted method of classifying hardness is using "Vicker's hardness test". Gorilla Glass 3 is harder than most metals, and probably the hardest material on the surface of your phone. While putting your phone in the same pocket as your coins and keys might not cause your display to scratch, the chassis probably would pick up some signs of damage. Taking a look at the published specifications of Gorilla Glass, there are a number of ratings describing different kinds of toughness.

  • Young's Modulus – describes the elasticity of a material. Higher number means the material is stiffer, but the side-effect of this is increase in brittleness.
  • Poisson Ratio – the axial stress of the material when it is pulled or pushed. Imagine stretching a piece of bubble gum — the centre of it will become thinner.
  • Shear Modulus – describes the material's response to shearing, a very important factor when it comes to preventing cracks from forming.
  • Fracture Toughness – measurement of the material's resistance to crack propagation.

When comparing the above values between Gorilla Glass 3 and the recently announced Gorilla Glass 4, the big difference is we get a lower Young's modulus, so it should be less brittle. However, the Chemical Strengthening section, reveals more than double the depth layer, from 40 µm to 90 µm. This greatly increases GG4's resistance to cracking and crack propagation, with a thicker compressed surface layer. The image below shows cross sections comparing damage resistance between Gorilla Glass 3 and 4:

Image credit: Corning

However, if you use a screen protector, the differences become less significant. Screen protectors help spread out any impact stress, enough to prevent significant stress build up in one spot to cause a fracture. However much you toughen glass, you can't completely eliminate all these natural defects, which is why some manufacturers are starting to consider more exotic materials like sapphire.

Synthetic Sapphire

Last year there was much hype surrounding reports that the iPhone 6 would have a display made from synthetic sapphire instead of toughened glass. Obviously the entire sheet wouldn't be made from crystalline sapphire (it would be too brittle), but rather a sapphire composite which provides the material some elasticity. Conventional manufacturing methods involve using a thin layer of glass as a substrate onto which aluminium oxide is deposited, forming a thin layer of crystalline sapphire on the surface. The sapphire has a dramatically higher Vicker's hardness than conventional toughened glass, which makes it more resistant to scratching.

Sapphire displays are significantly harder than toughened glass...

However, the cost of manufacturing sapphire displays is enormously higher than that of toughened glass, so they are rarely used for device displays and occasionally used as a lens cover for smartphone cameras, for example in recent iPhone models. However there's reason to be hopeful for cheaper sapphire displays in the future, as the price of sapphire production is gradually decreasing as the processes become more optimized.

Before launch, the iPhone 6 was rumored to be using a sapphire display — in reality, it uses ion-strengthened glass.

... but the manufacturing costs are higher, and there are other technical challenges to solve.

According to Corning's execs, though, sapphire's improved hardness does not outweigh its disadvantages. It has a lower light transmittance which would impact battery life (due to higher backlight levels being required), it's 10x more expensive than glass, takes much longer to manufacture, is 1.6x heavier, and is less resistant to cracking. Corning, of course, is heavily invested in its Gorilla Glass tech, and has reason to pour cold water on this competing material.

With manufacturers including Kyocera and Huawei using sapphire displays, we will get to see how well the device withstands general use. Huawei execs told Android Central at IFA 2014 that the company expected phones with sapphire displays to become an emerging niche in the following year. Meanwhile, Kyocera's Brigadier, a rugged handset using sapphire on its display, was called "near-indestructible" after extensive testing by Android Central.

Once the sapphire manufacturing processes becoming more refined and less expensive, we may see more manufacturers adopting the crystal in their device builds.

Antibacterial Displays

Though we never really think about it, our smartphone touchscreens can carry an incredible quantity of bacteria from numerous environments. And with the smartphone market only growing rapidly in the past few years, there hasn't really been much research into how to combat this.

Your smartphone screen is absolutely filthy — but science can help.

A German university sampled 60 touchscreens1 and discovered an uncleaned touchscreen contained an average 1.37 bacterial colony-forming units per square centimetre. This isn't actually that high, orders of magnitude lower than that of a kitchen sponge, but a few times higher than a hospital toilet seat2. This number was reduced to 0.22 after cleaning with a microfiber cloth, and 0.06 after cleaning with an alcohol wipe – cleaner than a toilet seat after being cleaned with detergent. The researchers identified that the majority of the bacteria were came from human skin, mouth and lungs – not surprising since we keep our devices so close to our face. Most people don't clean their smartphone screens on a regular basis, so touchscreens definitely have the potential to spread germs to others.

In early 2014, Corning unveiled their Antimicrobial Corning Gorilla Glass at CES. It was the first EPA-registered antimicrobial display glass. The display essentially is coated with a thin film of silver ions, which have incredible antimicrobial properties and are reported to kill 90% of bacteria, algae, mould and fungi on the surface. Silver has been widely used in hospitals for its antimicrobial effect, helping to prevent the spread of MRSA, and it was actually used in dressing wounds in World War I to prevent infection.

The amount of silver required for the thin film on smartphone displays is very low, but it will ultimately be up to the manufacturers as to whether they want the added dollars on their device's bill of materials or not. Nevertheless, with health and fitness features becoming central parts of many smartphones, antibacterial displays may present another point of differentiation for phone makers.

Image credit: Tactus

Morphing Displays

Tactus Technologies, a startup in California, has been showing off its innovative morphing touchscreen technology. When at its resting state it looks like an ordinary touchscreen, but when activated it can generate an array of protruding shapes corresponding to what is running on the device. The example they show is a device where keys protrude when the soft keyboard is showing on screen, providing the user some tactile feedback.

Users don't need to press down the individual keys, just touching them will register the key-press. It's an impressive technology which has been developed for several years, but has yet to be implemented in a consumer device. With hardware keyboards being abandoned by manufacturers as they pursue thinner device designs, Tactus may be what hardware keyboard fans are looking for.

Interactive Holograms

At the ACM Symposium on User Interface Software and Technology this year, the University of Tokyo unveiled their prototype display called HaptoMime3. It is a mid-air interaction system which acts like a floating touchscreen which can stimulate your fingertips using ultrasound to provide tactile feedback. Using an imaging plate, an image on a screen is transformed into a floating hologram. When the system detects the user "touching" the hologram, the ultrasonic phased array transducer will create a feeling on the user's fingertip.

The technology not only works with holograms but also 3D displays. It bring us one step closer to Tony Stark-style interactions with our digital devices. This probably will never be fitted into a smartphone, but it's possible it could be crammed into a tablet-like device at some point in the future.

The future of smartphone tech — Are we there yet?

Back in February 2008, 7 months before the initial release of Android, Nokia unveiled a concept phone – the Nokia Morph. Nokia Research Centre and the University of Cambridge's Nanoscience Centre collaborated on this project to produce a concept phone which they believe is the future of smartphones, focussing on nanotechnological applications in portable devices.

How does Nokia's vision of future mobile tech compare to what we have today?

The device featured:

  • Bendable, translucent device
  • Self-cleaning surface
  • 3D protruding surface (like the Tactus display)
  • Solar charging via "nanograss" technology
  • Numerous integrated sensors for sensing factors such as air pollution and hygiene

Nokia predicted that such technologies would be available by 2015, so how far has science progressed to allow such features in a device? In the first two articles in this series, we saw how LG has created a translucent bendable OLED display and there are two candidates for bendable lithium batteries – lithium ceramic and lithium polymer with flexible components. We do not have self-cleaning surfaces yet but there has been a great effort into developing better oleophobic coating for glass, to help keep greasy smudges off our devices. Current "nanofur" prototypes are susceptible to the coatings being rubbed off through general friction in our pockets.

Image credit: University of Massachusetts, Stanford University

A breakthrough in nanograss research was only recently published by a collaboration between two Universities in the US4. Using a sheet of graphene, they were able to densely arrange pillars of highly efficient photovoltaic material – material which converts light to electrical energy. The structure of the nanograss vastly increases the surface area which is in contact with sunlight, improving efficiency by 33% over thin film solar panels.

Image credit: Tzoa

Finally, on to Nokia's predicted pollution and hygiene sensors. In early December a Kickstarter page popped up for a device called a Tzoa, according to the page it is the first wearable that measures the air pollution in the immediate environment. It connects directly to your smartphone, sending over both air pollution data and UV exposure data. The probe does not detect the chemical pollution in the air but in fact detects particulate matter in the air, which also pose a threat to our health.

And we should also mention Samsung's Galaxy Note 4, which in late 2014 became the first mainstream smartphone to ship with a UV light sensor.

Image credit: Caltech

A surprising amount of futuristic stuff is already with us — whether in the lab, or in the devices we use.

Back in 2011, a paper was published on small lens-less platform for analysing microorganisms. It was called the ePetri dish, and was designed to work on a silicon chip5. (It's named after the Petri dish, the conventional method for culturing microbes so they can be analysed.) The ePetri dish doesn't require the large equipment and labour intensive processes, the culture is simply placed on an image chip illuminated by the smartphone display and the assembly is placed into an incubator. The data can be accessed remotely via a laptop or another smartphone, allowing the user to zoom in and analyse individual microbial cells. The technology is very specialised and still a long way from the Nokia Morph concepts, but it is definitely a step closer.

At the moment we have developed a lot of the technology which Nokia and University of Cambridge predicted should be available by 2015. The concept is still very futuristic, but it acts as a good source of inspiration for those developing the smartphone technologies for the future.

Who knows, in another seven years maybe we'll see a device similar to the Nokia Morph, perhaps with technologies we have yet to imagine.

Thanks Eric from Evolutive Labs for teaching me about toughened glass!

  1. M. Egert, K. Späth, K. Weik, H. Kunzelmann, C. Horn, M. Kohl, and F. Blessing, Bacteria on smartphone touchscreens in a German university setting and evaluation of two popular cleaning methods using commercially available cleaning products, Folia Microbiologica, 2014: p. 1-6. 
  2. A. Hambraeus and A.S. Malmborg, Disinfection or cleaning of hospital toilets—an evaluation of different routines, Journal of Hospital Infection, 1980. 1(2): p. 159-163. 
  3. Y. Monnai, K. Hasegawa, M. Fujiwara, K. Yoshino, S. Inoue, and H. Shinoda. 2014, ACM: Honolulu, Hawaii, USA. p. 663-667. 
  4. Y. Zhang, Y. Diao, H. Lee, T.J. Mirabito, R.W. Johnson, E. Puodziukynaite, J. John, K.R. Carter, T. Emrick, S.C.B. Mannsfeld, and A.L. Briseno, Intrinsic and Extrinsic Parameters for Controlling the Growth of Organic Single-Crystalline Nanopillars in Photovoltaics, Nano Letters, 2014. 14(10): p. 5547-5554. 
  5. G. Zheng, S.A. Lee, Y. Antebi, M.B. Elowitz, and C. Yang, The ePetri dish, an on-chip cell imaging platform based on subpixel perspective sweeping microscopy (SPSM), Proceedings of the National Academy of Sciences, 2011. 108(41): p. 16889-16894. 
Shen Ye