Technology is transporting us to a world that only used to exist in the human imagination, where organisms are no longer just biological. Nevertheless, nowadays we still think of robots as machines made from hard parts; the jointed monoliths in the film Interstellar are a good recent example. But as Bristol University robotics engineer Jonathan Rossiter explained to OpenMind, a promising line of research in the impending robotic revolution is that of soft robots, built with intelligent, flexible and adaptable materials to become more like organic creatures. And along this path there is still a step beyond: so-called biohybrid robots, which incorporate biological tissues.
The fusion between living organisms and artificial devices has become familiar to us through the concept of the cyborg (cybernetic organism). This approach consists of restoring or improving the capacities of the organic being, usually a human being, by means of technological devices. On the other hand, biohybrid robots are in some ways the opposite idea: using living tissues or cells to provide the machine with functions that would be difficult to achieve otherwise. The idea is that if soft robots seek to achieve this through synthetic materials, why not do so directly with living materials?
In the case of soft robots in general, their advantages are substantial. Their greater flexibility enables them to move more like living organisms than machines, allowing them to “interact with the human body safely,” in Rossiter’s words. This also includes the interior of the body: medical applications are one of the aspirations of these technologies, which are based on the use of biocompatible and biodegradable materials. Devices such as the ROWBOT developed by Rossiter also have the advantage of not needing batteries, as they are able to generate their own electricity from compounds in their environment. Currently, even NASA is investigating the development of soft robots to explore other worlds in the Solar System.
Machines made of living tissues
Similarly, “biohybrid robots have the potential of being fully autonomous, intelligent and self-assembled,” says Taher Saif, a robotics engineer at the University of Illinois, to OpenMind. “They might be able to learn from prior experience and heal from damage or injuries.” Here we begin to see the added advantages offered by biological materials: regenerative capacity, one of the properties that engineers try to implement in the robots of the future, is a peculiarity of living tissues. So is the fact that cells do not need electricity to function, but nutrients; these robots will not be plugged in, but will eat.
Shoji Takeuchi and his team create biohybrid robots that combine living tissue with 3D printed structures. Credit: University of Tokyo
An immediate first approach is to use muscle cells to move the robots, replacing motors, gears and cables. Developing this idea, a team from the University of Tokyo has created a robotic finger with a joint that rotates 90° thanks to its rat muscle cells, which researchers have cultivated in the laboratory and placed aligned with one another as in natural fibres, in a pair of muscles that relax and contract antagonistically, as also occurs in living beings. According to the first author of the study, Yuya Morimoto, “using this antagonistic arrangement of muscles, these robots can mimic the actions of a human finger,” something the researchers have demonstrated by picking up and putting down objects with their robotic finger.
Muscle cells can also allow biohybrid robots to walk or swim. A team from Harvard University and Caltech created in 2012 a tiny silicone jellyfish coated with rat cardiomyocytes —heart muscle cells— that were contracted to propel it through water. More recently, Harvard researchers have developed a swimming robot in the shape of a ray fish, with a micro-fabricated gold skeleton and a 16-millimetre rubber body that swims by undulating its body by means of 200,000 live rat cardiomyocytes.
How to power artificial muscles
Obviously, biohybrid robots will still have to overcome major hurdles. Living cells are delicate; the organism provides them with the protection and environmental conditions they require, something difficult to achieve in a robot. Additionally, muscles need electrical stimulation to contract, which in the case of the robotic finger and the Harvard and Caltech jellyfish is supplied by electrodes. As an alternative, the researchers who made the rubber ray fish modified the muscle cells through genetic engineering in order for them to be activated with light.
Artificial jellyfish swims in a heartbeat. Credit: Wyss Institute
However, there is another, more desirable option, which is to activate the muscles by means of the natural electrical stimulation provided in the body by motor neurons, which are specialised in controlling muscle fibres. Researchers have already succeeded in creating in vitro cultures of combined muscle cells and motor neurons similar to the systems of living beings.
They are now applying them to the manufacture of biohybrid robots. Previously, the team led by Saif had created tiny sperm-like robots, propelled by muscle cells that beat autonomously. More recently, the engineer has succeeded in improving the system by introducing motor neurons that power the muscles. The robot, the size of a pinhead, self assembles when researchers cultivate muscle cells and neuronal precursors with the double-tailed synthetic soft body, designed by engineer Mattia Gazzola.
This soft-robotic ray swims using fin motions and turns following externally applied light cues. Credit: Disease Biophysics Group (Harvard)
For now, Saif has used neurons that are genetically modified to be activated by pulses of light, but a further step will be to add to the sensory neuron system neurons that are capable of detecting external stimuli and sending signals to motor neurons to activate the muscle; in other words, living machines that respond to their environment and make decisions. “We are developing biohybrid swimmers that can swim along x and y direction and turn, and sense the presence of a mechanical obstacle and avoid it during swimming. They will also be trained for memory and logic,” says the engineer.
“These are the first generation of biohybrid robots,” says Saif. In the future, he says, “they may be used both for medical and environmental purposes for drug delivery in vivo, and for testing drug efficacy. They may be deployed for the detection of toxins in the environment as well as their clean-up.” And on the horizon there is still one more advance on which some researchers are already working: replacing the synthetic materials in the bodies and skeletons of biohybrid robots with others of biological origin, such as collagen. Then we will no longer be talking about robotic engineering, but rather about the engineering of organisms specifically designed for particular missions. As Rossiter predicts, “we are at the start of a new revolution.”