The dividing line between a car interior and a bouncy castle gets ever thinner.

Ford is to offer inflatable rear seatbelts on the 2013 Ford Mondeo, which instantly become a kind of mini-airbag in the event of an accident. Ford says that the inflatable belt disperses the energy of a crash across a body area five times larger than would a conventional seatbelt.

"We've tested the system extensively using our entire crash test dummy family and it offers extra protection over the standard rear seatbelt system," said Joerg Doering, core engineering seatbelts engineer, Ford of Europe. "Bringing together the attributes of an airbag and a seatbelt is a significant development. This technology isn't currently available in Europe."

In everyday use, the inflatable belts operate like conventional seatbelts and are compatible with infant and children car and booster seats. In Ford's research, more than 90 percent of those who tested the inflatable seatbelts found them to be similar to or more comfortable than a conventional belt because they feel padded and softer.

The technology was first offered on the 2011 Ford Explorer in the U.S. and proved immediately popular, with 40 percent of buyers choosing the option.

As with an airbag, Ford's inflatable rear seatbelt is activated when crash sensors detect an accident. This forces compressed gas out of a cylinder housed below the rear seat, through the buckle and into the belt. The inflatable rear seatbelt is fully deployed in less than 40 milliseconds.

"It provides extra support to the head and neck and so is especially effective when worn by young children or the elderly occupants who are more vulnerable in accidents," Doering said.

Unlike airbags, which generate heat when deploying, Ford's inflatable rear seatbelt inflates using cold compressed gas.

"The new Mondeo will be the first vehicle in its class to offer this technology," said Mark Beal, marketing manager, Ford of Europe. "We're confident it will really appeal to customers."