Figure 1.
Plant root structures and functions.
Figure (a) illustrates plant root structures and their functions. Four main regions can be identified, each with different roles. New cells are created by mitosis in the meristematic region (MR). These cells are then elongated by osmotic pressure and move to the elongation region (ER). Cell division and cell elongation provide the force to penetrate into the soil; asymmetries with respect to the root axis results in bending. The mature root consists of elongated cells that are stationary and provide a strong anchor to the soil, thus supporting tip penetration. Root cap cells exude mucilage and slough off, which decreases friction during penetration by lubricating the surrounding soil. Figure (b) shows images of an actual maize (Zea mays) root; the images were captured with a digital microscope (HIROX KH-7700). Specifically, image I is a two-week old maize root growing in soil, in which the primary root, the lateral roots and several seminal roots can be observed. Image II shows the mature region of the primary root with visible root hairs. Image III shows the apex of the primary root with a distinguishable root cap. Image IV shows the root cap of the primary root with sloughed cells visible around it.
Figure 2.
Elongation from the tip (EFT) process during three following steps of growth.
The colors indicate the movement of cells from the MR to the mature root and sloughing; mature cells do not move with respect to the soil, whereas the apex is forced into the soil by the new cells originating in MR.
Figure 3.
Figure (a) gives a schematic representation of the cross section of the root-like device that grows from the tip. The functional zones are the growing zone (GZ), the mature zone (MZ), the robotic tip (RT), and the spool (S) with the filament. The growing zone imitates the meristematic region in living plant roots; the GZ exerts an axial force on the robotic tip by adding new material. The mature zone is created by the layer-by-layer addition of material (the filament) and is stationary with respect to the soil. The hollow structure of the MZ allows the passage of the filament (new material) from the spool to the growing zone. Figure (b) shows the 3D design of the growing zone. The filament of new material passes through the filament entrance and the nozzle (located on the deposition head) to the external side of the tubular body between the rim and the previous layer of the mature zone. The rotation of the deposition head with respect to the tubular body, generated by the motor through the transmission mechanism, results in the deposition process that builds the mature zone. The motor receives power through brushes in contact with slip rings. During soil penetration, the growing zone slips inside the mature zone; any rotational movement between the two zones is suppressed by the locking wires.
Figure 4.
Force diagram for overcoming soil resistance.
a) Schematic top view and side view of the growing mechanism. Red arrows represent forces acting on the system during the deposition process: F is the force applied by the motor and W is the vertical resistance during penetration, where M is the torque required for the deposition head (shown in Fig. 3b) to overcome W, d is the thickness of the filament and D is the external diameter of the tubular body b) Equilibrium of forces acting on the filament for one complete turn unwound, where α is the angle made by the helix of the deposited filament with respect to a plane perpendicular to the axis of the tubular body, N is the reaction force, and μ is the friction coefficient between filament and deposition head.
Figure 5.
Figure (a) shows a picture of the root-like growing device. In the deposition head, a decoupled flat tip is assembled in order to eliminate any drilling effect in the penetration process. Figures (b) and (c) show the top and side views, respectively, of the device at the initiation of the mature zone. From the spool (not shown), the filament is drawn by the motor through the filament entrance into the nozzle. The locking wires are spring steel and prevent relative rotational movement between the tubular body and the mature zone.
Figure 6.
Deposition process and growth of the root-like device.
The figure shows the sequence of the growth process. By rotating the deposition head (DH, in white), the filament passes through the nozzle and is deposited around the outer surface of the tubular body (shown in Figs. 3 and 5c). The penetration force is provided by the deposition of the filament at the growing zone (GZ), while the mature zone remains strongly anchored to the soil. The newly deposited filament is always located on the top of the DH, as shown by the colors.
Figure 7.
Energy consumption of the root-like device prototype in EFT and NoEFT penetration trials.
Figure (a) shows the energy required for penetrating 30 mm of artificial soil (POM granules, diameter = 4 mm) in EFT (Red) and NoEFT trials (blue) at various initial depths (100, 150, 200 mm). For each initial depth, 5 tests were performed. Bars are the mean values over 5 repetition for each depth and condition (EFT and NoEFT). Error bars represent the standard error of the mean. Figure (b) shows the penetration speed achieved. The experimental conditions are described in the “Materials and Methods” section and shown in Fig. 8.
Figure 8.
Schematic of the experimental setup for soil penetration with and without elongation from the tip (EFT and NoEFT, respectively).
Figure (a) shows the setup for the evaluation of the energy required in the EFT tests, at the beginning (on the left) and at the end (on the right) of a soil penetration trial. In the initial position, the device is positioned in the soil. The growing zone (GZ) is placed at depth h0 (h0 = 100, 150, 200 mm), and the mature zone (MZ) is fixed to a holding structure (HS). The GZ adds new material at the tip, thus increasing the MZ length until it penetrates a total of Δh (30 mm). Figure (b) shows the setup for the evaluation of the energy required in the NoEFT tests, at the beginning (on the left) and at the end (on the right) of a soil penetration trial. In the initial position, a probe (F, light grey) with the same dimensions and geometric properties as the prototype is positioned at depth h0 in the soil (h0 = 100, 150, 200 mm). The prototype is attached to the HS and pushes the probe from the top. The GZ generated the vertical (penetration) force in both the EFT and NoEFT trials in order to have comparable results.