Inspired by arthropod insects and spiders, Harvard professor George Whitesides and Alex Nemiroski, a former postdoctoral fellow in Whitesides’ Harvard lab, used ordinary plastic drinking straws to create a type of semi-soft robot capable of standing and walking. The team also created a robotic water strider capable of pushing itself along the liquid surface.
Earlier generations of soft robots could stand and awkwardly walk by inflating air chambers in their bodies. However, these new robots are designed to be far nimbler. The researchers settled on using plastic drinking straws, and began by cutting a notch in the straws to allow them to bend. They inserted short lengths of tubing which, when inflated, would force the joints to extend. A rubber tendon attached on either side would then cause the joint to retract when the tubing deflated.
Using the simple joint concept, the team built a one-legged robot capable of crawling, and moved up in complexity as they added a second and then a third leg, allowing the robot to stand on its own. “With every new level of system complexity, we would have to go back to the original joint and make modifications to make it capable of exerting more force or to be able to support the weight of larger robots,” Nemiroski said. They eventually graduated to six- or eight-legged arthrobots.
The researchers could control the simpler robots by hand using syringes, but they turned to computers to control the sequencing of the limbs as the designs increased in complexity. “We put together a microcontroller run by Arduino that uses valves and a central compressor,” Nemiroski said. “That allowed us the freedom to evolve their gait rapidly.”
Although the researchers could replicate ants’ distinctive “triangle” gait using their six-legged robot, duplicating a spider-like gait was trickier. “A spider has the ability to modulate the speed at which it extends and contracts its joints to carefully time which limbs are moving forward or backward at any moment,” Nemiroski said. “But in our case, the joints’ motion is binary due to the simplicity of our valving system. Either you switch the valve to the pressure source to inflate the balloon in the joint, and thus extend the limb, or you switch the valve to atmosphere to deflate the joint and thus retract the limb.”
In the case of the eight-legged robot, the team had to develop a gait that was compatible with the binary motion of the joints. Developing a system that can fine-tune the speed of leg actuation needs further research, and would require programmable control over the flow rate supplied to each joint. Though it may be years before the robots find their way into any real-world applications, the researchers hope the robots could eventually be used in search operations following natural disasters or in conflict zones.