Active Stiffening for Vine Robots via Axially Stacked Pneumatic Pouches

Vine robots enable safe navigation through sensitive environments due to their inherent compliance and their mechanism for growth via tip eversion. However, the compliance of these soft robots limits their ability to withstand significant loads, preventing them from performing tasks that require lifting or manipulating objects. To expand the range of applications for vine robots, they must therefore be able to increase their stiffness to prevent collapse. In this paper, we present a low profile, active stiffening mechanism integrated into the skin of the vine robot. The mechanism consists of a set of axially stacked pneumatic pouches that generate an axial force in the robot's body to mitigate wrinkling in the robot's material, enabling the robot to withstand larger loads without collapsing. We characterized the burst pressure, stiffness, and transverse collapse load of the vine robot with our stiffening mechanism and demonstrated the ability of the robot to simultaneously stiffen and grow. Our active stiffening mechanism is implemented in an 18 mm diameter vine robot and achieves a 680% increase in stiffness.