Biological robot uses rat tissue to ‘walk’

A new ‘bio-bot’ made of a 3D-printed hydrogel skeleton combined with rat muscle and spinal cord tissue could lead to a new field of interactive biological devices with many potential applications, its creators have said.

Known as a ‘spinobot’, the hybrid device was developed at the University of Illinois at Urbana-Champaign.

While previous generations of biological robots, or bio-bots, could move forward by simple muscle contraction, the integration of the spinal cord gives spinobots a more natural walking rhythm, said study leader Martha Gillette, a professor of cell and developmental biology.

“These are the beginnings of a direction toward interactive biological devices that could have applications for neurocomputing and for restorative medicine,” she said.

To make the spinobots, the researchers first printed the tiny skeleton – two posts for ‘legs’, and a flexible ‘backbone’ only a few millimetres across. They seeded it with muscle cells, which grew into muscle tissue. Finally, they integrated a segment of lumbar spinal cord from a rat.

“We specifically selected the lumbar spinal cord because previous work has demonstrated that it houses the circuits that control left-right alternation for lower limbs during walking,” said graduate student Collin Kaufman, the first author of the paper.

“From an engineering perspective, neurons are necessary to drive ever more complex, co-ordinated muscle movements. The most challenging obstacle for innervation was that nobody had ever cultured an intact rodent spinal cord before.”

The researchers had to devise a method not only to extract the intact spinal cord and then culture it, but also to integrate it onto the bio-bot and culture the muscle and nerve tissue together – and do it in a way that the neurons form junctions with the muscle.

The team saw spontaneous muscle contractions in the spinobots, signalling that the desired neuro-muscular junctions had formed and the two cell types were communicating.

To verify that the spinal cord was functioning as it should to promote walking, the researchers added glutamate, a neurotransmitter that prompts nerves to signal muscle to contract. The glutamate caused the muscle to contract and the legs to move in what the team called a “natural walking rhythm”. When the glutamate was rinsed away, the spinobots stopped walking.

The researchers hope to refine the spinobots' movement, making their gaits more natural. They hope the small-scale spinal cord integration is a first step toward creating in vitro models of the peripheral nervous system, which is difficult to study in live patients or animal models.

“The development of an in vitro peripheral nervous system – spinal cord, outgrowths and innervated muscle – could allow researchers to study neurodegenerative diseases such as ALS in real-time, with greater ease of access to all the impacted components,” said Kaufman.

“There are also a variety of ways that this technology could be used as a surgical training tool, from acting as a ‘practice dummy’ made of real biological tissue to actually helping perform the surgery itself. These applications are, for now, in the fairly distant future, but the inclusion of an intact spinal cord circuit is an important step forward.”

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