A recent paper published in the latest issue of the journal *ACS Applied Materials & Interfaces* has brought the concept of "flexible robots" back into the spotlight. Researchers at the Institute of Physics and Chemistry, Chinese Academy of Sciences, have discovered that gallium-based liquid alloys can change color under the stimulation of sacrificial metals or electric fields. Combined with their excellent thermal and electrical conductivity, low viscosity, good flowability, and biocompatibility, these alloys demonstrate the ability to deform and move under certain conditions. This discovery offers the possibility of optimizing the structure of variable color-changing materials for flexible robots.
As early as 2015, Chinese scientists were the first in the world to demonstrate the possibility of flexible robots made of liquid metal from both theoretical and technical perspectives, based on the deformable properties of liquid metal under an electric field. Previously, Liu Jing, a researcher at the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences and a professor at Tsinghua University, also wrote in her review article on room temperature liquid metals that "the discovery of the deformable machine effect of liquid metal is expected to promote a major breakthrough in the theory and technology of flexible machines."
What are flexible robots?
Professor Zhu Jian of the Department of Mechanical Engineering at the National University of Singapore once gave a simple concept: the characteristics of flexible robots include the softness of materials, excellent environmental adaptability, superior safety, and good human-robot interaction.
The joints of most humanoid robots we commonly see are stiff, and they slam heavily to the ground when they jump and land. Traditional rigid materials make it difficult for robots to be flexible. Future robots should develop in the direction of being lighter, softer, and less dependent on external power. At that time, human-robot collaboration will be safer and more coordinated.
Flexible robots are currently divided into two main categories: industrial and biological. They are primarily developed to address the needs of the manufacturing and medical industries. Interestingly, the definition of flexible robots varies across different fields. From a manufacturing perspective, flexible robots refer to industrial robots with six or more axes that utilize machine vision. From a biological perspective, flexible robots are biomimetic robots created by mimicking the flexibility and agility of living organisms. The latter will be applied in various complex environments to assist or even replace humans in performing specialized and advanced tasks.
While flexible robots offer numerous advantages, most current research remains in the laboratory stage, and many still utilize rigid materials such as metals and plastics. Currently, the scientific community has developed various soft-bodied robots, including worm robots, caterpillar robots, octopus robots, and other similar designs. However, these are mostly structures composed of multiple rigid units, far from possessing the capabilities of advanced machines such as flexibility, universal deformation, and even fusion. They also differ significantly from the soft exteriors and seamless connections of humans and animals in nature. Therefore, this discussion will primarily focus on flexible robots constructed entirely of flexible materials, without any superfluous rigid structures within them.
Those imaginative flexible robots
Stanford University mechanical engineers have drawn inspiration from natural organisms such as grapevines, fungi, and nerve cells, which can grow to cover distances, to develop a self-growing, flexible robot. This vine-like robot can grow over long distances without moving its entire body and can be used in search, rescue operations, and medical applications.
Researchers at Harvard University have created an octopus-shaped, fully flexible robot called "Octobot." This robot is entirely composed of soft, flexible materials and can move on its own without external power. Inspired by the soft bodies and flexible flapping wings of manta rays, marine creatures, the research group of Associate Professor Li Tiefeng and Professor Huang Zhilong at the School of Aeronautics and Astronautics and the Zhejiang Provincial Key Laboratory of Soft Robotics and Intelligent Devices at Zhejiang University utilized dielectric elastomer films as soft artificial muscle actuators.
Researchers from Case Western Reserve University have developed a novel 3D-printed flexible robot based on origami design. Its deformability allows it to deform to absorb additional forces without requiring any additional sensors to detect forces and adjust itself. Therefore, the necessary human intervention is drastically reduced, and the robot is soft and safe.
Materials and actuation of flexible robots
The main technical challenge for flexible robots that require high flexibility and deformability lies in their constituent materials, followed by their actuation. Traditional rigid connectors and shells are no longer suitable, so people are looking for ways to ensure the flexibility of the materials. Currently, a common method is to use 3D printing to create the "shell," such as a gel-like robot made of hydrogel. A research team at MIT conducted a trial experiment, using 3D printing and laser cutting to create a hydrogel shell to achieve the "flexibility" of the "body," and then using hydraulic actuation to drive the robot's movement.
Another approach is to use special materials to create materials similar to artificial muscles. Materials such as electroactive polymers (EAP) and shape memory alloys are good materials for artificial muscles. For example, shape memory alloys can automatically change shape according to temperature and remember these shapes to achieve actions such as bending, shortening, and grasping objects.
The material with the most recent developments is the liquid metal mentioned at the beginning of the article. It can switch between different forms and movement modes at will under the control of external fields such as electricity, magnetism, light, heat, chemicals, and mechanics. It can even exhibit biological-like behaviors such as autonomous movement after consuming "fuel".
From a materials perspective, the primary driving force is electricity. For example, the aforementioned artificial muscle materials and other functional materials require electrical current to generate deformation and thus propulsion. Secondly, power is derived from environmental changes, such as temperature, air, and light. However, these driving methods also have significant drawbacks. Controlling the robot's motion precision is difficult. Furthermore, if the electric field strength required to drive the robot's movement is too high, it can also affect its movement within a certain range.
Therefore, although there are many research institutions studying flexible robots and their research directions are different, there is still a long way to go before they are put into practical use. Flexible robots may enable us humans to develop bio-inspired artificial intelligence and apply it to more different scenarios.
