From robotic hand-clapping arms to a device that could show tsunami alerts in the sky, Japanese technology researchers showcased their latest inventions in Tokyo Thursday.

Two pairs of artificial arms welcomed visitors as the Digital Content Expo opened for a three-day run, producing a realistic clapping sound due to the soft palms of the hands.

The arms, named Ondz, are made of white skin-like urethan "flesh" and aluminum "bone". They create what the developer calls the "organic" sound of human hand clapping by the patting of soft palms.

"I want the the audience to enjoy the creepy and surreal feelings this product gives as entertainment," said Masato Takahashi, researcher at the graduate school of media design at Keio University, who molded the design on his own body.

Ondz could be used in musical performances, to enhance the sound of real clapping. Or viewers watching a programme online could click a button to make hands at the broadcast site clap, Takahashi told AFP.

He also said he would like to produce a "spanking machine" to hit comedians, as well as stomping feet to complement the hand-clapping arms.

The expo also displayed more serious technologies that could be used in medical or social situations.

Japan's Burton Inc. showcased its aerial 3D technology, which uses laser beams to create three dimensional images in the air out of tiny bright dots and could be used for advertising -- or for a tsunami alert.

"An obvious use of this is for advertisement. You wouldn't need screens... You can create a huge signboard in the air," said engineer Hayato Watanabe.

Burton chief executive and 3D project director Hideki Kimura said the technology could also be used to show tsunami alerts and other messages in the sky after major disasters.

"If you showed warnings in the sky from the top of fire department buildings, people could see them even after fleeing their homes," he said.

Such images would be more stable than coloured smoke from airplanes, which gets swept away in the wind, and easier to spot than electrical boards hanging from helicopters, he said.

The technology can now show images six metres (20 feet) above the ground and in space the size of a cube measuring three metres squared. But the company is working to realise bigger displays at higher altitude, Kimura said.

Another fun gadget displayed at the fair was a set of dominoes where the pieces can be made to fall without touching each other using radio waves.