Softer, stronger robots

Materials World magazine
1 Nov 2017

Ellis Davies reports on an actuator material that could act as a muscle in soft robots.

Untethered soft robots’ actions and movements can help mimic human movement, but have a long-standing problem – the lack of a soft, bulk material capable of functioning as a soft muscle.

A team at Columbia University, USA, has tackled this issue by developing a 3D-printable synthetic soft muscle that is able to expand and does not require a compressor or electrical equipment, such as pumps or valves, to function. The material is made up of a silicone rubber matrix and ethanol, distributed throughout in micro-scale bubbles, and is cheap, simple and user- and environment-friendly. 

Aslan Miriyev, post-doctoral researcher at Columbia, told Materials World, ‘The material that we have developed is an artificial tissue.’ He claims it is ‘the first and only equivalent to natural muscles that exists today’. Miriyev said it is a self-contained material-actuator, in which expansion as a response to a stimulus is an inherent property. ‘Electrically activated using as little as 8V, the material may actuate in various manners and provide soft-bodied bending, twisting, pushing, pulling, lifting weight and any other ways of manipulation.’ 

This material is entirely soft and has a density of 0.84 g/cm3, a low cost (lab cost of three cents per gram) and can be easily prepared by untrained personnel. It can be cast or 3D-printed into a desired shape. As well as being electrically triggered, the muscle can be heated by exposing it to hot water or air, or warmed up in a microwave oven.

‘Upon heating the composite to a temperature of 78.4°C, ethanol boils and the local pressure inside the bubbles grows, forcing the elastic silicone elastomer matrix to comply by expanding to reduce the pressure,’ said Miriyev. To further expand the muscle, higher temperatures are required because with growth in local pressure, the boiling temperature increases. 

Miriyev claims that the artificial muscle is three times stronger than natural ones. ‘It exhibits 15 times higher strains (when comparing specific strain), and can lift 1,000 times its own weight.’

3D-printed to go

Once printed or cast, the material has only to cure before it is ready to go. The team is also developing a 3D-printer and printing process that will allow immediate curing of each layer of material. This means that at the end of the printing process the entire robot could potentially walk out of the printer.  

Miriyev described the process. ‘We ran several preliminary experiments for 3D printing of our material. This allowed us to empirically find the ideal printing conditions, so we could 3D print the material to a desired shape. Now, we are conducting an in-depth rheology-based study, to develop a fully controlled 3D printing process for our material. Simultaneously, we are developing a unique open-source 3D printer, which will allow us to synthesise and 3D print our material in a desired composition and design in one step.’

The team has also engineered a bulk material, with the same properties, that can be scaled to any imaginable size, with the only requirement being adequate heating. ‘Our next big goal is to add sensing capabilities to the artificial muscle, and to use artificial intelligence (AI) to learn how to control it. In order to create self-aware independent soft or hard-soft robots, we must be able to predict the actuation behavior with time,’ said Miriyev. The team hopes that succeeding in this will possibly bridge one of the last gaps towards replicating natural muscle.