  Soft robots have attracted much attention in recent years owing to their high
adaptability. Long articulated soft robots enable diverse operations, and
tip-extending robots that navigate their environment through growth are highly
effective in robotic search applications. Because the robot membrane extends
from the tip, these robots can lengthen without friction from the environment.
However, the flexibility of the membrane inhibits tip retraction. Two methods
have been proposed to resolve this issue; increasing the pressure of the
internal fluid to reinforce rigidity, and mounting an actuator at the tip. The
disadvantage of the former is that the increase is limited by the membrane
pressure resistance, while the second method adds to the robot complexity. In
this paper, we present a tip-retraction mechanism without bending motion that
takes advantage of the friction from the external environment. Water is used as
the internal fluid to increase ground pressure with the environment. We explore
the failure pattern of the retraction motion and propose plausible solutions by
using hydrostatic skeleton robot. Additionally, we develop a prototype robot
that successfully retracts by using the proposed methodology. Our solution can
contribute to the advancement of mechanical design in the soft robotics field
with applications to soft snakes and manipulators.
