In a paper recently published in the journal Science Robotics, the University of Tokyo researchers presented their new "biohybrid" robot. The robot is a crossover between living tissue and robotics, integrating biohybrid robotics with living muscle tissue grown from the cells of a rat.

The biohybrid robot could perhaps be used to replace missing appendages on humans - should the technology be repeated and replicated with human tissue. But the University of Tokyo suggests their research is laying the foundation for building far more advanced and lifelike robots.

Japanese engineers integrate living muscles into robots.

The robots can mimic actions of human finger https://t.co/r5CeluR0Ss

(Video courtesy of 2018 Shoji Takeuchi, Institute of Industrial Science, the University of Tokyo) pic.twitter.com/gmCizoYwFh — China Xinhua News (@XHNews) May 30, 2018

Building a Biohybrid Robot which Uses Living Tissue

Fortunately, the research did not involve the direct harm of a rodent. The muscle was instead grown from myoblasts - muscle cells from rats. The cells were grown on the surface of a hydrogel which was then attached to a robotic skeleton structure. Over time, the cells grew between two anchor points on the skeletal structure, forming a functional joint. When stimulated with an electrical current, either side of the hybrid muscles can contract or expand, forcing the robot's fingers to bend at the joint - similar to that of a human.

“If we can combine more of these muscles into a single device, we should be able to reproduce the complex muscular interplay that allows hands, arms, and other parts of the body to function,” said Shoji Takeuchi, lead authors of the study and a mechanical engineer at the University of Tokyo. “Although this is just a preliminary result, our approach might be a great step toward the construction of a more complex biohybrid system.”

Naturally, because the robot uses live tissue, it must be kept continuously submerged in water to be kept alive- one of the major limitations of the project. Although, it is not the only problem which has surfaced over the course of its development.

Takeuchi has developed semi-functional biohybrid limbs before, but his previous work struggled to maintain the length of the muscles. Previous techniques involved growing a culture of muscle tissue on top of a flexible layer and controlling it with a current sent through its fibers. The muscles contract, successfully bending the joint, but they would also quickly shrink beyond the point of usefulness.

The problem stems from an inherent trait of skeletal muscle - as they are used, they grow. Growing a single layer of muscle on one side of the substrate can be used temporarily until the muscle becomes too strong for the substrate to snap the joint back to its neutral position. The substrate layer quickly bends to the point where it is no longer useful.

Imitating Life with Biorobotics

It is quite apparent life does not suffer from this limitation. Most vertebrate species - those with a spine or spinal column - circumvent the issue by using muscles in antagonistic pairs.

Antagonistic pairs, such as the bicep and tricep, work collaboratively to cause or inhibit a movement of the arm. As one contracts, the other expands, either enabling or inhibiting the arm from moving - forming an antagonistic pair.

According to Takeuchi, antagonistic pairing prevents wear, significantly increasing the useful lifespan of the muscle tissue. Using this technique, he was able to increase the muscle tissue lifespan to just over a week - significantly longer than previous trials.

In his new research, Takeuchi took advantage of the antagonistic pairing strategy in his new biohybrid robots.

Rather than of growing a single layer of muscle on a substrate, Takeuchi two separate layers of muscles and installed them on opposite sides of a joint. The muscles were lined in parallel to simulate more realistic muscle pairing. Each side was then prodded with gold electrodes which, when induced by an electric current, would cause either side to extend or contract.

The muscles still tighten up, however, they do so equally on both sides, preventing the joint from folding over beyond use as previously failed. The new technique significantly improved the dexterity of the robotic finger movement.

In the chart above, the way each muscle was stimulated to produce a smooth joint rotation is labeled throughout the movement and placement of the ring.

The Problems with Biohybrid Robots

But many problems still remain ahead of researchers. In its current setup, inducing an electric current through the muscle creates bubbles in the surrounding water, a major contributing factor in the degradation of the tissue.

Currently, researchers are investigating other methods to stimulate the movement of the muscles without the need of electricity. These methods may include using a motor neuron to control the stimulation of either muscle - a method which has already proven a success. A motor neuron is a cell capable of directly controlling muscle movement.

Previous research has already proven it is possible to grow a small neural device capable of remotely controlling muscles. Networks can be grown from motor neuron cells and are controlled by stimulating the neurons with lasers.

A motor neuron network could hypothetically be grown to control Takeuchi's biohybrid robotic finger. The device would be artificially stimulated by a laser, inducing the activity in the neural network and forcing the muscles to react accordingly.

Regardless of what the implications may be, such a device would enhance the control of living tissue, enabling scientists to make biohybrid robot movements more accurate, and more lifelike than ever before by using live tissue on robots.