Japanese scientists have used human cells to develop an equivalent to living skin that can be attached to robotic surfaces to flash a realistic -- if creepy -- smile.

The University of Tokyo researchers published their findings this week along with a video of the gooey-looking pink material being stretched into an unsettling grin.

They used a "skin-forming cell-laden gel" to create a "robot covered with living skin", their study in the journal Cell Reports Physical Science said.

The biohybrid robot specialists hope the technology will one day play a role in the invention of androids with human-like appearances and abilities.

"We also hope this will help shed better light on wrinkle formations and the physiology of facial expressions," and help to develop transplant materials and cosmetics, the team led by professor Shoji Takeuchi said.

The new material could signal a departure from traditional humanoid robots covered with genuine-looking skin often made of silicone rubber, which cannot sweat or heal itself.

The scientists' goal is "to endow robots with the self-healing capabilities inherent in biological skin", but they are not there yet.

In previous studies they grafted collagen onto a cut on lab-grown skin covering a robotic finger to demonstrate how it could be repaired.

But they said conducting similar repair tests on their smiling robotic skin "is a future challenge".

To create what they described as a "natural smile" that moves fluidly, they gelatinised the skin-like tissue and fixed it inside the robot's holes, a method inspired by real human skin ligaments.