Plant cells expand and elongate. Their cumulative actuation defines organ morphing. Inspired by this modular transformability, this study proposes a modular concept for growing robots that will be able to grow by adding at their tip Transformable Modules (TMs). We provide a two-module implementation to evaluate the concept viability. We designed and characterized Shape-Retention Bellows (SRBs) that constitute the TM and are used to maintain the shape once the extension force is relaxed. We demonstrate module radial expansion and axial elongation in a straight and bent configuration (up to ~4°). This is the first concept of growing robots to enact the robot's modularity and transformability for future deployment in distributed growing systems capable of acting in various scenarios.

This paper presents a new material deposition strategy for developing a growing robot capable of building its own body. The growing robot is inspired by plant growth and is based on a 3D printer-like mechanism. The plotting of a filament near the tip allows the forward movement of the robot and results in building a tubular body. A material deposition process is introduced to perform a straight continuous growth as well as a turning behavior in order to permit the navigation of the robot in the environment. Bending is achieved by controlling the filament height in each position of the plotting, lowering or increasing plotting velocity with a position PID control algorithm. We demonstrate that the continuous deposition of the filament allows to obtain homogeneous and robust structures, with a significant improvement of the robot's performance compared to our previous version of the system (i.e., more than 100 N pulling force and 200 N shear force). The current version of the robot can sustain its weight, move efficiently by growing in the environment - both air and soil - and penetrate hard medium (up to 60kPa).

In this paper, we present a study on peculiar plant root movements, named circumnutations, adopted by plants as an efficient strategy to penetrate the soil and implemented in robotic roots to improve their performance in exploration tasks. The robotic root is able to bend and interact with soil owing to a soft actuation mechanism based on the translation of rotary to linear motion by means of helical compression springs. The robotic root integrates a stimulus-oriented behavior combined with circumnutation movements, which leads to an extraordinarily large reduction (more than 80 times) of the force needed for penetration in artificial soil (sawdust) while performing circumnutations with respect to a straightforward movement.