A new iPhone case adds a touch-sensitive surface to the rear of the device. It could make a range of activities including games much easier, but faces a common chicken-and-egg situation.

The gadget is designed by Canopy and called Sensus. It’s built in the same way as many cases, with raised rubbed corners that should prevent the surfaces of the phone itself hitting the ground in a fall. However, it adds a plastic panel on the back and side which is billed as registering touch in 10 specific locations.

(It’s not an entirely original idea: Apple registed a patent for a similar system, built directly into the device, before it even launched the first iPhone.)

The touch technology is powered by a connection to the phone’s charging socket, so it will reduce battery life to some degree, though Canopy says the effects are minimal. You’ll also need to choose a specific model depending on whether you have an iPhone 5 with a Lightning connector or a phone with the traditional Apple socket.

At first the device will only be available for iPhone 4 and later, but iPad Mini and iPod Touch versions are being developed. Canopy is also considering a version for the standard iPad.

The idea is that the case would help any app that benefits from using the full screen without your fingers getting in the way. Games are an obvious example, following on from a similar set-up with the PlayStation Vita. It could also work well with simpler applications, such as letting you read a long web page in your browser where you need to be scrolling almost continuously.

Unfortunately there’s a big drawback: the case inputs only work if an app has been specially developed to work with it. Canopy is planning to release its own app which does nothing but let you search for apps that are compatible with the device.

That’s where the quandry comes in. Pricing isn’t yet confirmed but it’s likely to be between $60 and $100. That’s a lot to pay if there aren’t many compatible apps — but there’s little incentive for developers to make apps compatible with a gadget that people are wary about buying.