Biohybrid robotics combines engineered artificial structures and living bio-systems. The main motivation for biohybrid robots is to provide actuation using bio-hybrid actuators. Biohybrid robots may use bacteria or other motile cells. Actuators have also been based on explanted whole-muscle tissues, cardiomyocytes, optogenetically modified cardiomyocytes, insect self-contractile tissues and engineered skeletal muscle.
Proposed applications for biohybrid actuators include miniaturized therapeutic robots and biohybrid medical devices, in vitro muscle models for drug testing and platforms for investigating muscle contractions. In the longer term biohybrid actuators have applications in the interaction of soft robotic artifacts with humans, microscale devices that perform medical procedures, biobased surveillance systems, manufacturing systems that self-assemble and self-repair and environmental monitoring with swarm biorobots.
Biobots that could be injected into a patient’s bloodstream and destroy a blood clot or cancer would require sensing, computation and actuation. Living cells have an advantage over synthetic microswimmers in their ability to detect and respond to environmental stimuli without additional components. Bacteria and algae can be steered using their attraction to light, which is not possible in deep tissues of the body. Magnetic control is one possibility for medical applications since magnetic fields can safely penetrate the body. Magnetotactic bacteria naturally have magnetic nanocrystals and other living cells could be artificially magnetized if embedded with iron oxide nanoparticles or by attaching them to magnetic substrates. Chemicals can also control cell behavior. Biohybrid devices could respond to biochemical signals released from tumor cells. Thermotaxis and aerotaxis are also possible ways to steer microorganism-based biohybrid robots.
Researchers at Polytechnique Montréal demonstrated magnetic guiding of a magnetotactic bacteria bound to drug-containing nanoliposomes. These Magentococcus marinus bacteria also use aerotaxis, to migrate towards low oxygen in their natural environment. Since tumors are also low oxygen environments, the researchers demonstrated in mice that this aerotaxis can be exploited for delivering drugs to tumors.
Beating cells from a rat (ventricular cardiomyocytes) were first placed on poly(dimethylsiloxane) (PDMS) thin films in 2007 by Whitesides and Parker. This work defined a biohybrid soft robotic device as a hybrid device which has a soft body and where the actuation is performed by a living biological part. Soft biohybrid robots go beyond replication of nature, but seek to fulfill tasks that neither nature nor physical robots can achieve.
Professors Anne Staubitz and Christine Selhuber-Unkel at University of Bremen and University of Kiel proposed a taxonomic scheme for the concept of “living materials” used in biohybrid soft robotics. Their taxonomy builds on the taxonomy scheme put forth by Webster-Wood.
Materials taxonomy key from Appiah et al. Adv. Mater. 2019, 31, 1807747
The following are examples of biohybrid soft robots described by the taxonomy scheme. The walker is a biobot actuated by a skeletal muscle ring modified for optogenetic control with light. The pump is a bio-micropump on a chip powered by earthworm muscle.
Figure from Appiah et al. Adv. Mater. 2019, 31, 1807747
Researchers at University of Illinois developed soft robotic devices driven by neuromuscular tissue that triggers when stimulated by light using optogenetics, which was published in 2019 in PNAS. In 2014, the team developed self-propelled biohybrid swimming and walking biobots that uses cardiac muscle cells from rats, which beat on their own, as motors. Their swimming biobots were modeled after sperm, with a single tail. In the 2019 version, optogenetic neuron cells, derived from mouse stem cells, were added to their device with two tails. The biobots self assemble since neurons advanced towards the muscle and formed neuromuscular junctions. The resulting neuromuscular tissue worked with their synthetic biobot skeletons. Muscle activity is controlled by neuron activity, which is controlled by exposure to light using optogenetics.
