Realistic facial expressions may be coming to a robot near you, thanks to work from engineers at the National University of Singapore.

Unless they’re a master poker player, a person’s facial expressions can be more revealing than words. They also been shown to influence how we interact with each other... there is a lot of science behind the phrase ‘smile and the whole world smiles with you’. For those developing humanoid robots, the challenge of recognising and reproducing human facial expressions sits high on the research agenda. A paper to be published in the March issue of Extreme Mechanics Letters [DOI: 10.1016/j.eml.2015.12.007] reports on artificial jaw muscles that can achieve movements similar to those in a human jaw.

Humans have 42 facial individual muscles, so the team from the National University of Singapore focused on just one – the masseter muscle, which is important in chewing. To reproduce realistic movements, roboticists look to everything from bulky electric motors to temperature-sensitive shape memory alloys. In this paper, the muscles were made from a dielectric elastomer membrane, a soft, flexible material that can deform in response to a small voltage.

The team applied a pair of their muscles to an artificial skull, between the cheek and the lower jaw. 3D-printed clamps were used to hold them in place, and a layer of conductive carbon grease added to both sides of each, to act as the electrodes. Two plastic fibres were embedded lengthwise into to each muscle, to ‘pre-stretch’ it. When a voltage was applied to the muscles, the jaw opened and closed as expected. But the fibre-embedded muscles could displace the jaw by 20 mm, nearly four times as much as those without the fibres. They were also found to achieve a linear strain of 48%, comparable to the performance of human muscles.

By applying AC voltage to the dielectric elastomer muscles, they were shown to be capable of mimicking natural muscles’ dynamic behaviour, as seen in chewing, talking and singing. A video of a Luciano Pavarotti performance was used as a benchmark for this round of tests (see Supplementary materials), and the results showed reasonable agreement between the artificial muscles and the movement of the singer’s jaw.

The researchers believe that “facial expressions in humanoid robots play an important role in improving the interactions between human beings and machines”, so perhaps this work brings the vision of soft robotics just that little bit closer.


Y. Wang and J. Zhu, “Artificial muscles for jaw movements”, Extreme Mechanics Letters (2016) 6, 88–95. DOI: 10.1016/j.eml.2015.12.007