From plant root's sloughing and radial expansion mechanisms to a soft probe for soil exploration

In this paper, we present an innovative probe for soil penetration inspired by the strategies of radial expansion and sloughing mechanisms exploited by plants at the level of their root apical regions (i.e., apexes). These phenomena help roots in reducing friction with soil during their movements and pressure needed for penetration. We imitated these natural features developing four different probes endowed with spheres and ball bearings in their tips. These solutions produce a sliding effect of the probe while it moves into the soil with an improvement in terms of penetration energy consumption by 13.0% and penetration force by 13.4% respect to the probe without sloughing strategy. The prototype that got the best performance required 0.4 J energy consumption, 7.1 N penetration force for 150 mm penetration depth. Additionally, we mimicked the root apex radial expansion strategy via multi-chamber soft actuators and we observed a reduction of soil impedance by 91% at 120 mm depth. Moreover, we measured a total energy saved by the probe with radial expansion by 11% in comparison with the same system without a radial expansion. We tested the bioinspired probes in a granular soil (POM, polyoxymethylene beads), in controlled environmental conditions. Our results can be useful both for improving current soil diggers and conceiving innovative tools for different application fields, such as earth and planetary exploration and geotechnical studies.