Joint research

 has developed prototype flexible joints inspired by spiders' legs, which could find their way into future exploratory space missions, such as those planned to Mars over the next 20 years.

The biomimetic research by ESA's Advanced Concepts Team focused on spiders because one of their natural advantages is that they are capable of creating large amounts of force starting from negligible mass. The reason for this is the unique method they use when extending their legs.

While spiders — like humans — use flexor muscles to bend their limbs they do not have the corresponding extensors that most animals have to then re-extend their legs.

Instead of muscle, spiders use a peculiar hydraulic system that manipulates the spider's blood to pressurise its joints by pumping the fluid into the spaces between muscles and the skeleton, forcing the leg to straighten. It was this characteristic that ESA researchers thought could be of use during space missions.

Dr Carlo Menon works in the agency's biomimetic research group and developed the idea for the joints along with colleagues at the Centre for Mechatronics at Lithuania's University of Technology.

'We kept only the principle of the spider's legs and then looked at what technology could be used to make this principle work, beginning with the cheapest implementation,' said Menon.

The result of the research was a prototype called Smart Stick. The device is a miniaturised tube with a 1mm in diameter that can be embedded into flexible structures to act as a simple hydraulic joint. The tube is fitted with a series of fluidic actuators, within which — upon receiving a command — the fluid is compressed to deform the tube between the joints. The increased pressure and inflation in turn forces the joint to move, and this rotation is fed back to a sensor which measures how far the joint has moved and controls the actuators accordingly.

Menon said: 'We knew there were some compelling applications for such a system so we focused on it. We could use this type of technology with integrated actuators on micro-systems, and apply it to many different types of mechanisms in space.'

A functioning hydraulic system that could operate in a wide range of functions in space could be created using a set of individual Smart Stick modules, said Menon. However, one of the obstacles to using hydraulic systems in space is that the liquid used can often be lost as a gas.

To solve this, Menon suggests that the spider-inspired system would have to operate in a closed loop to rule out the possibility of losing pressurised gas during operation. Future incarnations of the device could use a large number of mini-tubes operating together which would offer even more flexibility — but it is the development of a closed-loop hydraulic system which is key to the system's future development, according to Menon.

The use of more advanced materials could also improve the technology's efficiency. Electro-active polymers or shape-memory alloys could be introduced into the design to improve the actuation while the use of magneto-restrictive fluids could also help boost the system's performance.

According to Menon, the biggest advantage of such a system is that it would be very light, as the folding and deforming of the tube rules out the need for heavy mechanical connectors.

'It is important that you save on mass because when you are launching into space the cost is based on weight,' he said.

As the results of the project have only just been published it is too early to tell which missions the Smart Stick may be used in the future. However, Menon suggested that it could be used onboard for long-term exploratory missions, such as those that will form part of ESA's Aurora programme to explore the solar system.

'This work has only really just started and the results are still very fresh,' he said. 'There is much more that is needed to be done before it can be a full, working system but we are already looking to work with other teams within ESA who could carry on the project.'

He added: 'This is still a transition phase and as our group only works at concepts level it is difficult to know what might happen next, but we are hopeful it could have a role to play.'