Robotic slime: a moving, shape-shifting game changer

10 May 2022

A popular child’s toy that can easily be made at home with a few basic ingredients was the inspiration for a new magnetic slime robot with a host of possible practical applications, according to Dr Zhang Li, a professor from the Chinese University of Hong Kong.

Dr Zhang’s principal area of research is in small-scale robotics, including micro- and nano-robotics and their biomedical applications, as well as functional materials for sensors, actuators and their practical applications.

Zhang is a Professor in the Department of Mechanical and Automation Engineering and a Professor by Courtesy in the Department of Surgery at The Chinese University of Hong Kong. He is also a director of the Shenzhen Institutes of Advanced Technology of the CAS – CUHK Joint Laboratory of Robotics and Intelligent Systems.

Croucher Foundation caught up with Zhang to ask him about his research – the result of a collaboration between researchers at CUHK and the Harbin Institute of Technology. The team initially looked at the simple recipe used in commercial slime – glue, water and a tiny amount of Borax. They then adapted the recipe and refined it to produce the properties they were after – a material capable of deformation and manipulation and a product that is firm when touched quickly, yet soft when pushed gently.

Sun and Zhang’s magnetic slime is made from just three ingredients: polyvinyl alcohol, Borax, and neodymium magnetic particles. It creates a material that is visco-elastic, which can be manipulated by an external magnet. Zhang describes the resulting slime as being a “non-Newtonian fluid-based magnetically actuated slime robot with both the adaptability of elastomer-based robots and significant deformation capabilities”.

The amount of magnetic particles added to the mix is crucial to the slime’s effectiveness. The more particles, the stronger the magnetic attraction and the more easily it responds to manipulation, which is a plus. However, this also makes the slime stiffer and less useful in a biomedical context. For the applications Zhang has in mind, the robotic slime needs to be able to travel easily, squeeze into tight spots, encircle small objects by forming O and C shapes and rotate. He believes he has found the sweet spot between the conflicting properties, allowing him to consider more pragmatically its potential uses.

Overlapped sequential snapshots of videos show that the magnetic slime robot is actuated to navigate through the channel, tube, maze, and on uneven surfaces.

Zhang says the most likely application is in gastrointestinal surgery, particularly in the hard-to-reach and narrow small intestine, which can be up to 7 metres in length and difficult to access with an endoscope.

The robotic slime can be easily swallowed by the patient and manipulated by an external magnet to move down the gastrointestinal tract. Zhang anticipates it would only be in the body for a relatively short period of time, often less than an hour, and explains that the cytotoxicity of the slime is low. To make it even safer for human consumption it would be coated with a naturally occurring material such as silicon dioxide.

Zhang has been working with surgeons to develop a technique for using the slime in the operating theatre. He can manipulate the slime while following it in real time using X-ray fluoroscopy – an imaging technique that produces moving images. Zhang has established that the magnetic particles don’t interfere with the X-rays and he is able to achieve good contrast, making the robotic slime easily visible.

Zhang likens the magnetic slime to an invisible hand for the surgeon. When it reaches the desired destination, it can be manipulated to change shape and “grab” small objects that may have been swallowed, such as batteries and coins.

The team’s co-authored research article was released in Advanced Functional Materials on 1 April 2022. A video produced to illustrate the properties of the slime immediately captured global media and social media interest.

Dr Zhang confessed he was completely unprepared for the surge of interest and quickly found himself fielding a barrage of media requests for interviews.

There is a pleasing circular serendipity to this innovative research by Sun and Zhang. Following a simple idea, they have adapted a children’s toy to help retrieve small objects most likely swallowed by children.

Slime robot grasps a wire by curling behaviour. By controlling the rotation of the magnet, the slime robot can be driven to curl.