 A few wheelchairs attempt to combine the features of both designs by providing a fold-to-rigid mechanism in which the joints are mechanically locked when the wheelchair is in use.Many rigid models are now made with ultralight materials such as aircraft-grade aluminium and titanium, and wheelchairs of composite materials such as carbon-fibre have started to appear.Ultra lightweight rigid wheelchairs are commonly known as 'active user chairs' as they are ideally suited to independent use.