As robots get more sophisticated, there is increasing demand to make them capable of working alongside humans, but as traditional robots are known for their health and safety-risking hard frames, there has been considerable focus on soft robotics.

With soft outer shells, these robots are ideal for working alongside humans in schools, homes and offices, but there is an issue: the sections that hold their mechanics do have to be hard, and the area where the soft and hard materials meet is prone to higher stresses and thus structural failure.

However, researchers at Harvard’s Wyss Institute for Biologically Inspired Engineering may have the answer, in the form of a 3D printing process that grades between hard and soft materials.

“We leveraged additive manufacturing to holistically create, in one uninterrupted 3D printing session, a single body fabricated with nine sequential layers of material, increasing in stiffness from rigid to soft towards the outer body,” said Dr Robert Wood, study co–senior author co–leader of the Bioinspired Robotics Platform at Wyss.

“By employing a gradient material strategy, we have greatly reduced stress concentrations typically found at the interfaces of soft and rigid components which has resulted in an extremely durable robot.”

The researchers designed and printed a specific robot to test the process, which is the first to be completely printed using this layering technique.

Printed in a single session, the robot is designed to jump without the need for tethers or wires.

It is powered by an explosive actuator running on butane and oxygen, with three pneumatic legs that tilt to control which direction it jumps in.

The soft, squishy finish obviously protects nearby humans from damage if they get in the way, but it also helps keep the robot intact, by minimising damage on landing both from impact and by more evenly distributing the stress on its structure.

It’s an impressive achievement, made far more achievable thanks to the speed of 3D printing.

“Traditional molding–based manufacturing would be impractical to achieve a functionally–graded robot, you would need a new mold every time you change the robot’s design,” said study co-first author and Wyss graduate researcher Nicholas Bartlett.

“3D printing manufacturing is ideal for fabricating the complex and layered body exhibited by our jumping robot.”

While in itself not a functionally remarkable robot, the researchers believe their work, which is published today in the journal Science, is extemely promising for the advancement of soft robotics, and thus the use of robots alongside humans.

“This ability to fabricate unitary soft robots composed of gradient materials that emulate natural stiffness gradients of living structures paves the way for mass fabrication of robots that can integrate seamlessly with people, whether in our homes, at work or in operating rooms in the future,” said Dr Donald Ingber, founding director of Wyss.

“This new breakthrough demonstrates the power of combining insights into nature’s innovations with the most advanced man–made technological advances – in this case 3D printing technologies – when trying to overcome technical limitations that currently hold back a field.”