Iron Man star Robert Downey Jr has given a seven-year-old boy a bionic arm.

Video footage of the moment shows the actor dressed as Tony Stark, who becomes Iron Man in the popular Hollywood films.

Downey Jr then opens a pair of metal cases marked Stark Industries.

In the cases are an Iron Man arm and a red-and-gold prosthetic limb for the boy, named Alex, who was born with a partially developed right arm.

The bionic arm was created by engineering student Albert Manero, who makes 3D-printed bionic limbs for children around the world.