Walking 'bio-bots' powered by muscle

Known as ‘bio-bots’, the tiny machines made of flexible 3D printed hydrogels and living muscle cells are less than a centimetre long and could one day be used a variety of applications such as drug delivery, surgical robotics, 'smart' implants, or mobile environmental analysers.

The team from University of Illinois at Urbana-Champaign had previously demonstrated bio-bots able to ‘walk’ on their own, powered by beating heart cells from rats, but the heart cells constantly contract meaning the researchers had no control over the bot’s motion.

Now the group has created a new generation of bio-bots powered by a strip of skeletal muscle cells that can be triggered by an electric pulse, providing a simple way to control the bio-bot so that it can be turned on and off, sped up or slowed down – a crucial step to customising bio-bots for specific applications.

“Skeletal muscles cells are very attractive because you can pace them using external signals,” said study leader Professor Rashid Bashir, head of bioengineering at the university.

“For example, you would use skeletal muscle when designing a device that you wanted to start functioning when it senses a chemical or when it received a certain signal. To us, it’s part of a design toolbox. We want to have different options that could be used by engineers to design these things.”

In a paper published in the online early edition of journal Proceedings of the National Academy of Science, the team explains how a bot’s speed can be controlled by adjusting the frequency of the electric pulses – a higher frequency causes the muscle to contract faster, thus speeding up its progress.

Next, the researchers will work to gain even greater control over the bio-bots’ motion, by doing things like integrating neurons so the bio-bots can be steered in different directions with light or chemical gradients.

“Biological actuation driven by cells is a fundamental need for any kind of biological machine you want to build,” said Bashir.

“Our goal is for these devices to be used as autonomous sensors. We want it to sense a specific chemical and move towards it, then release agents to neutralize the toxin, for example. Being in control of the actuation is a big step forward toward that goal.”

The design is inspired by the muscle-tendon-bone complex found in nature. There is a backbone of 3-D printed hydrogel, strong enough to give the bio-bot structure but flexible enough to bend like a joint. Two posts serve to anchor a strip of muscle to the backbone, like tendons attach muscle to bone, but the posts also act as feet for the bio-bot.

“The goal of 'building with biology' is not a new one – tissue engineering researchers have been working for many years to reverse engineer native tissue and organs, and this is very promising for medical applications,” said graduate student Ritu Raman, co-first author of the paper.

“But why stop there? We can go beyond this by using the dynamic abilities of cells to self-organize and respond to environmental cues to forward engineer non-natural biological machines and systems.”

In future work, the team hopes to design a hydrogel backbone that allows the bio-bot to move in different directions based on different signals using 3D printing to explore different shapes and designs quickly.

Bashir and colleagues even plan to integrate a unit into undergraduate lab curriculum so that students can design different kinds of bio-bots.