Engineers at the Massachusetts Institute of Technology (MIT) have developed an ultra-thin swimming robot that relies on a single layer of living muscle cells to generate movement, in a design that aims to reduce the size of hybrid biorobots and improve their efficiency compared to systems that rely on larger three-dimensional muscle masses.
The robot, which researchers describe as paper-thin, is built on a gel-like structure that is roughly the size of a piece of gum in length and width. The skeleton is divided into two parts that act as fins, each of which is covered by a layer of living muscle cells thinner than a human hair. These cells have been genetically modified so that they contract when exposed to light.
Movement by light instead of motors
When researchers shine light on a fin, the muscle cells on its surface contract; This creates enough movement to propel the robot through the water. The direction and speed of swimming can be changed by controlling which fin is lit, and the timing of the light pulses directed at it.
In tests, the robot was able to swim and turn inside a simple water maze. Its maximum speed was about 4 times its body length per minute, which is a limited speed compared to fast swimmers, but it showed that a very thin muscle layer can produce enough force to move an entire body in the water.
Ritu Raman, associate professor of mechanical engineering at MIT and one of the authors of the study, says that moving in water requires more force than moving in the air. This makes the robot's ability to swim of this size and thickness an indication of the strength that can be produced by the muscular structure used.
Reducing the size of biobots
Hybrid biorobots combine artificial materials with living tissue or cells to generate movement, but many previous designs have relied on three-dimensional blocks of laboratory-grown muscles. This requires large numbers of cells, and leads to thicker and more complex devices.
As for the new design, it relies on a very thin layer of muscle, which can reduce the amount of cells required, and allows the manufacture of robots that are simpler, lighter, and more efficient in movement. The team describes the system as the first example of an ultra-thin 2D muscular robot capable of autonomous movement. The researchers believe that living tissues offer properties that are difficult to achieve using only traditional mechanical components: they are soft, able to respond to their environment, and can have the ability to repair themselves.
Searching for a suitable “structure” for the muscles
The challenge was not limited to growing the cells, but also included developing a base that could convert their small contractions into useful movement. In previous work, the team relied on fibrin, a very soft gelatinous substance, but found that it can contract quickly when exposed to muscle force. Which reduces the efficiency of power transmission; Therefore, the researchers tested different formulations of GelMA, a material used in tissue engineering, by changing its hardness, thickness, and the shape of the grooves on its surface.
Experiments showed that grooves with a shape closer to the square channels helped the cells line up better. The more organized the cells are, the stronger and more coordinated their fusion into muscle fibers is. The team also found that a gelatinous layer about half a millimeter thick provides an appropriate balance between light weight and the ability to support the muscle as it contracts.
“Train” the muscles before use
The researchers also worked on strengthening the muscles themselves through a series of light pulses, in a process similar to training, with the aim of increasing their ability to contract. After optimizing the carrier material and cell arrangement, the team designed the robot with two fins, with grooves on the sides that allow the muscle cells to grow in organized directions. When both fins are operated, the robot moves forward, while operating a single fin allows it to change direction.
Potential applications in sensitive environments
The design is still in an early research stage, and the team says that the next step is to improve the body shape to increase swimming speed and efficiency, but the researchers believe that thin hybrid biorobots may be suitable in the future for tasks that require soft and small devices within environments that traditional robots are not suitable for, such as monitoring some sensitive aquatic environments or exploring fragile areas that cannot tolerate hard or large devices. The study was published in the journal Advanced Functional Materials, while the project received partial support from the US Office of Naval Research.
