Our guide to the future of the iPhone continues. The next topic of discussion is:

Screen developments

The screen is an iPhone's centrepiece and crowning glory: the medium via which you interact with your phone and your phone tells you about the world. iPhones don't historically tend to have the best screen resolution (despite the claims made on behalf of its proprietary Retina screen rating, and despite the major step forward seen in the Super Retina iPhone X), but they are solidly sharp and highly responsive - and occasionally Apple evens adds new features, such as 3D Touch and Night Shift.

All of the design changes described above can only be made possible by the screen technologies described below.

Here's where we see the iPhone screen heading in the next few years.

Screen size

The iPhone range presently tops out at the iPhone X's 5.8in screen, but the analyst Chuljoong Kim of Jefferies & Co predicts that in 2018 Apple will roll out an iPhone Xs Plus that stretches to 6.5in (alongside a 5.8in iPhone Xs). Kim expects these two sizes to be offered until 2020, when they will be joined by a 5.2in iPhone XE.

Screen resolution

Again looking to Chuljoong Kim's predictions, we're led to expect further improvements in screen resolution over the coming years.

Apple's late-2017 phones offered resolutions of 2436 x 1125 & 458ppi (X), 1920 x 1080 & 401ppi (8 Plus) and 1334 x 750 & 326ppi (8).

Kim forecasts that in 2018 these will be joined by 2688 x 1242 & 459ppi (Xs Plus); and then in 2019 Apple will go higher still and offer 2860 x 1320 & 491ppi (XI Plus). The iPhone XE in 2020 will, Kim predicts, have a resolution of 2080 x 960 & 441ppi.

(As a side note, it's been noticed on Twitter that eMagin, a microdisplay company which Apple was reported to have invested in early in 2018, has the capability of making displays with vastly higher pixel densities than anything Apple has previously released: higher than 2,500ppi in some case. But these are very small displays designed for ultra-close-up work, and we consequently think this partnership is more likely to bear fruit in Apple's rumoured VR headset project than in the next raft of smartphones.)

Sapphire

iPhone screens are already far tougher than your average piece of glass (they're made of a proprietary material called Gorilla Glass), but they do sometimes crack or even shatter when dropped. Sapphire screens would be more resistant still, and Apple is already using sapphire in the display of the Apple Watch: it's possible that the company is now ready to import this material into its smartphone line-up.

Rumoured plans to rely on an Apple-backed sapphire plant in Arizona (which had the capacity to manufacture 200 million 5in iPhone displays per year) fell through. But more recent reports suggest that long-term Apple supplier Foxconn is gearing up to build its own sapphire plant in Taiwan at a cost of $2.6bn.

Mind you, Apple hasn't given up on Corning's Gorilla Glass - not by a long shot. In May 2017 it announced that $200m of its recently announced Advanced Manufacturing Fund would be invested in Corning, in order to "support Corning's R&D, capital equipment needs and state-of-the-art glass processing".

Wendell P Weeks, Corning's chairman, said: "This investment will ensure our plant in Harrodsburg remains a global centre of excellence for glass technology."

Read more: What is Sapphire glass, and why is it a good idea for the iPhone?

Screens that tell you (and Apple) when they're cracked

An Apple patent published on 16 February 2017 (but applied for back in August 2015 - these things move slowly) presents a means by which future iPhones could detect when their screens have cracked. This might sound pointless - surely you can see when your phone has cracked - but would apply to even hairline fractures, so you can take action before it gets any worse.

The patent, named Coverglass Fracture Detection, "relates to methods and apparatus for detecting and characterising the formation of cracks in a display cover". It covers the basic idea that the device would notify the user when it detects a crack, but a more interesting aspect relates to gradual design improvements: "In some embodiments, crack characterisation data can be sent to a device manufacturer to improve subsequent device models."

iPhone 9 release date rumours: Crack detection patent

Self-healing screens

We don't expect this to appear in consumer tech for some years yet, but researchers at the cutting edge of screen design have come up with a way of building displays that can heal scratches.

Chemists working at the University of California at Riverside have created a stretchable material whose chemical peculiarities - it features a special type of bond called an ion-dipole interaction - cause ions and molecules to be strongly attracted to each other when it splits apart. In tests a piece of the material was ripped in half but had mended itself within 24 hours.

This isn't unprecedented in materials science (some smartphones, in fact, already have self-healing back covers) but this is the first such material to be conductive to electricity. Chao Wang, the leader of the research team, told Business Insider that it was likely to see use in smartphone screens and batteries by 2020.

iPhone 9 release date rumours: Self-healing screen

Project Phire

Corning, the company that makes Gorilla Glass, responded to the looming threat of sapphire glass in early 2015 with the announcement of an ultra-hardened composite material codenamed Project Phire.

James Clappin, president of Corning Glass Technologies, told investors: "We told you last year that sapphire was great for scratch performance but didn't fare well when dropped. So we created a product that offers the same superior damage resistance and drop performance of Gorilla Glass 4 with scratch resistance that approaches sapphire."

Apple never discusses the materials it uses for iPhone screens, but it's great news for consumers that suppliers are jostling to provide the best and most durable screen glass for their smartphones.

Read more about Project Phire here.

Graphene

Sapphire glass is already being used on the non-Sport models of the first-gen Apple Watch and on every model of the Apple Watch Series 2, and Project Phire appears to be in a reasonably advanced state of development, but we're getting closer to the realms of science-fiction.

Graphite, the material used in standard pencils, is made up of stacks of sheets of carbon, each one only a single atom thick. This is why it's so good for writing: the layers naturally slide off on to the paper.

But graphene is a different matter. Graphene is what you get if you're clever enough to isolate one of the layers in graphite, leaving you with a substance that's effectively two-dimensional. It's the thinnest substance known to man, about a million times thinner than a human hair, and for that matter quite possibly the strongest (it's 100 times stronger than steel) and a phenomenally good electrical conductor - 1,000 times better than copper. Oh, and it's virtually transparent, too.

All of which makes graphene an exciting prospect for tech manufacturers. Most obviously, it would make for a tremendously durable coating material for the screen (and would lend itself to bendable displays, too) or indeed any part of the device; but it could really appear in almost any of the sections of this article. Graphene would be a superior replacement for silicon in processor chips, or could be used to make more efficient batteries and solar cells. It's marvellous stuff.

We're also pleased to report that graphene is British - sort of. It was discovered by the Soviet-born physicist Andre Geim at the University of Manchester, where it continues to be studied. (Entertainingly, Geim is the only scientist so far to be awarded both a Nobel and an Ig Nobel prize. Suck it, Planck!)

If you'd like to read more about graphene, I strongly recommend the New Yorker's article on the subject.

Viscoelastic material

Let's move on from the screen and talk about new durable materials for the rest of the iPhone.

How about a bit of drop-resistance? Based on patent activity, Apple is devising a viscoelastic material that would absorb impacts. The material would cover Apple devices and make them survive drops far better. The patent could make sense in all of Apple's mobile devices and laptops, but the iPhone is the obvious area to begin.

...3D display...

An iPhone in the near future could come with a 3D display, according to Economic Daily News, which claims that Apple supply chain partner TPK is working on a project that relates to "naked eye 3D screen" - in other words, a 3D screen that doesn't require glasses to see. Having to pop on a pair of 3D specs every time you use the phone would be a buzzkill.

...or hologram cells

But 3D is very 2009, isn't it? We'd like to see Apple go a step beyond and really capture our imagination with a hologram display, able to project the screen image as a three-dimensional hologram you can view from different angles and even interact with. You might have to wait a while for this one.

At the moment about the best you can manage from a consumer smartphone is a 'holographic effect', based on eye-tracking technology. Not quite what we're looking for, but still fun.

Next: Camera developments