Computationally assisted design and selection of maneuverable biological walking machines

ABSTRACTThe intriguing opportunities enabled by the use of living components in biological machines have spurred the development of a variety of muscle-powered bio-hybrid robots in recent years. Among them, several generations of bio-hybrid walkers have been established as reliable platforms to study untethered locomotion. However, despite these advances, such technology is not mature yet, and major challenges remain. This study takes steps to address two of them: the lack of systematic design approaches, common to bio-hybrid robotics in general, and in the case of bio-hybrid walkers specifically, the lack of maneuverability. We then present here a dual-ring biobot, computationally designed and selected to exhibit robust forward motion and rotational steering. This dual-ring biobot consists of two independent muscle actuators and a 4-legged scaffold asymmetric in the fore/aft direction. The integration of multiple muscles within its body architecture, combined with differential electrical stimulation, allows the robot to maneuver. The dual-ring robot design is then fabricated and experimentally tested, confirming computational predictions and turning abilities. Overall, our design approach based on modeling, simulation, and fabrication exemplified in this robot represents a route to efficiently engineer biological machines with adaptive functionalities.

Functional Regulators of Bacterial Flagella

Bacteria move by a variety of mechanisms, but the best understood types of motility are powered by flagella ( 72 ). Flagella are complex machines embedded in the cell envelope that rotate a long extracellular helical filament like a propeller to push cells through the environment. The flagellum is one of relatively few biological machines that experience continuous 360° rotation, and it is driven by one of the most powerful motors, relative to its size, on earth. The rotational force (torque) generated at the base of the flagellum is essential for motility, niche colonization, and pathogenesis. This review describes regulatory proteins that control motility at the level of torque generation.

Molecular machines with bio-inspired mechanisms

The widespread use of molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular machines—which by and large function as switches—and the machines of the macroscopic world, which utilize the synchronized behavior of integrated components to perform more sophisticated tasks than is possible with any individual switch. Should we try to make molecular machines of greater complexity by trying to mimic machines from the macroscopic world or instead apply unfamiliar (and no doubt have to discover or invent currently unknown) mechanisms utilized by biological machines? Here we try to answer that question by exploring some of the advances made to date using bio-inspired machine mechanisms.

Living Machines

This article provides an insight into National Science Foundation (NSF) funded research and development of biological machines. The goal of these research projects is to build living, multicellular machines that sense, move, and solve real-world health problems. One of the Emergent Behaviors of Integrated Cellular Systems (EBICS) group has developed a biobot that walks. Inspired by the structure of human joints, this walker has two short, stubby legs connected by a bridge. The skeleton is constructed from a soft Jell-O-like 3D-printed skeleton called hydrogel, and surrounded by a band of skeletal muscle. The group genetically engineered the cells to produce a protein called channel rhodopsin, a sensory photorecepter that enables the muscle to contract in response to blue light. This provides an easy on–off switch to activate the muscle spurring the bot to move its legs and walk. As biologists understand better how tissues develop, bioengineers will be able to reverse-engineer development to better program cells to self-assemble to create biological machines. Plans for future bots include different cell types and many more functionalities.

Optogenetic skeletal muscle-powered adaptive biological machines

Complex biological systems sense, process, and respond to their surroundings in real time. The ability of such systems to adapt their behavioral response to suit a range of dynamic environmental signals motivates the use of biological materials for other engineering applications. As a step toward forward engineering biological machines (bio-bots) capable of nonnatural functional behaviors, we created a modular light-controlled skeletal muscle-powered bioactuator that can generate up to 300 µN (0.56 kPa) of active tension force in response to a noninvasive optical stimulus. When coupled to a 3D printed flexible bio-bot skeleton, these actuators drive directional locomotion (310 µm/s or 1.3 body lengths/min) and 2D rotational steering (2°/s) in a precisely targeted and controllable manner. The muscle actuators dynamically adapt to their surroundings by adjusting performance in response to “exercise” training stimuli. This demonstration sets the stage for developing multicellular bio-integrated machines and systems for a range of applications.