The shell of the iPhone 4 and 4S is constructed of robust stainless steel sandwiched between two sheets of fragile glass. It's a revolutionary design, but one that has caused a lot of heartache and expense for owners.

According to a rumor that surfaced last week, this could change with the next-generation iPhone, where the vulnerable rear glass panel will be replaced with a robust Liquidmetal shell.

Liquidmetal is the commercial name given to an amorphous metal alloy that is almost twice as strong as the strongest titanium alloys. It was developed by Caltech in 2003 and has been used in a broad range of military, medical, luxury, consumer, industrial, and sporting goods products.

In August 2012, Apple acquired a license to use this material, but has yet to use it for anything more exciting than the iPhone's SIM card eject tool.

I've come across Liquidmetal before: a SanDisk Cruzer Titanium USB flash drive. SanDisk also used Liquidmetal in the construction of the now long defunct Sensa e200 media player.

It's incredibly tough stuff, and I really tested the durability of that flash drive. Short of taking a hammer to it, I could barely put a scratch, let alone a dent, in the thin shell surrounding the drive's delicate electronics.

There's no doubt that Liquidmetal is incredibly tough stuff. You might think that a Liquidmetal iPhone 5 would be a near-indestructible piece of kit?

Wrong.

To begin with, one side of iPhone is still made up of exposed glass, and let's face it, the screen is far more expensive -- not to mention more difficult -- to replace that the rear cover is. But there's also a weird property of Liquidmetal that means a dropped Liquidmetal iPhone could suffer more damage than if the back were made of glass.

Liquidmetal is, as you can see from the video below, excellent at storing elastic energy. This, to you and me, means the material likes to bounce about with Flubber-like vigor when dropped.

I've seen this property in action. That SanDisk Cruzer Titanium flash drive I had would bounce enthusiastically whenever dropped onto a hard surface. A bouncing flash drive is one thing, but a bouncing iPhone is another, and likely to suffer more overall damage than one that just thuds to the ground because each bounce is another chance for gravity to break the screen.

I don't think that it is Liquidmetal's indestructibility that Apple is interested in, because let's face it, Apple doesn't have a track record of building robust devices, but instead the material's high strength-to-weight ratio that interests the Apple engineers.

This property means that you can cast a shell out of Liquidmetal that's much thinner than a shell stamped out of a sheet of metal, as was the case with the early iPhones, or machined out of a block of aluminium, which is how Apple manufactures the shells of devices such as the MacBooks or iPads. A thinner shell means less space taken up per unit volume by the casing, which in turn leaves more space for the important stuff that goes inside the device.

But there's one property of Liquidmetal that no one seems to have considered. How transparent is this material to radio frequencies?

If the material doesn't allow for effective passage of radio frequencies, moving all the iPhone's antennas -- GSM, CDMA, UMTS, GPS, Wi-Fi, and Bluetooth -- inside a Liquidmetal shell doesn't make sense. I've had a look through the tech specs for the material but can't find anything relating to this. I've put a question in to Liquidmetal Technologies about radio frequency transmission, and will update this post if I get an answer.

Image credit: Liquidmetal Technologies.

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