Biomechanical Characterization of Hook-Climber Stems for Soft Robotic Applications
99
2.3 Statistics
We compared tensile test results on the different stem segments with one-way analysis
of variance (ANOVA) followed from Tukey’s post hoc tests. In all, 19 stem segments
were tested. Error bars in bar graphs reported the standard deviation.
3 Main Results
3.1 Variation of Structural and Mechanical Properties Along the Stem
The morphology and anatomy of G. aparine stem are summarized in Fig. 1. The lower
(i.e., basal) and upper (i.e., middle and apical) parts of the stem are analyzed (see Fig. 1A).
Anatomical observations of the lower and upper parts of G. aparine stem are reported
in [7]. There are evident structural differences between the lower and the upper parts:
the lower part is formed from “cable-like” arrangement with a central core of lignified
vascular tissue (see Fig. 1B), while the upper part is formed from a “four pointed starlike” arrangement with mechanical tissue and not lignified sclerenchyma in the angles
(see Fig. 1C) [7]. The cross-sectional area of the basal parts are ≈0.43 mm 2 , while the
cross-sectional area of the middle and apical parts are ≈1.44 mm 2 .
Tensile tests highlighted significant differences between the three stem parts (Fig. 2).
An example of a stress-strain plot from which we extracted the main mechanical
parameters is reported in Fig. 2A. As expected, the basal part is much more extensible
than the middle and apical parts, with significant differences in the breaking strain values
(p < 0.01); in contrast, the comparison between the measurements on the middle and the
apical part have shown no significant differences (p > 0.01). Particularly, the ε max ranged
from 29 ± 8% in the basal part to 8 ± 2% in the middle part and 9 ± 2% in the apical
part (Fig. 2B). Similarly, the stiffness of the stem significantly decreases from basal (E
= 112 ± 23 MPa) to upper parts (p < 0.01), but there are no significant differences
between the middle (E = 42 ± 19 MPa) and apical parts (E = 33 ± 14 MPa) (p > 0.01)
(Fig. 2C). In contrast, the tensile strength decrease between all three parts (σ max from
8 ± 2 MPa to 1.4 ± 0.4 MPa; p < 0.01 between BP and MP, and BP and AP; p < 0.1
between MP and AP) (Fig. 2D).
99
2.3 Statistics
We compared tensile test results on the different stem segments with one-way analysis
of variance (ANOVA) followed from Tukey’s post hoc tests. In all, 19 stem segments
were tested. Error bars in bar graphs reported the standard deviation.
3 Main Results
3.1 Variation of Structural and Mechanical Properties Along the Stem
The morphology and anatomy of G. aparine stem are summarized in Fig. 1. The lower
(i.e., basal) and upper (i.e., middle and apical) parts of the stem are analyzed (see Fig. 1A).
Anatomical observations of the lower and upper parts of G. aparine stem are reported
in [7]. There are evident structural differences between the lower and the upper parts:
the lower part is formed from “cable-like” arrangement with a central core of lignified
vascular tissue (see Fig. 1B), while the upper part is formed from a “four pointed starlike” arrangement with mechanical tissue and not lignified sclerenchyma in the angles
(see Fig. 1C) [7]. The cross-sectional area of the basal parts are ≈0.43 mm 2 , while the
cross-sectional area of the middle and apical parts are ≈1.44 mm 2 .
Tensile tests highlighted significant differences between the three stem parts (Fig. 2).
An example of a stress-strain plot from which we extracted the main mechanical
parameters is reported in Fig. 2A. As expected, the basal part is much more extensible
than the middle and apical parts, with significant differences in the breaking strain values
(p < 0.01); in contrast, the comparison between the measurements on the middle and the
apical part have shown no significant differences (p > 0.01). Particularly, the ε max ranged
from 29 ± 8% in the basal part to 8 ± 2% in the middle part and 9 ± 2% in the apical
part (Fig. 2B). Similarly, the stiffness of the stem significantly decreases from basal (E
= 112 ± 23 MPa) to upper parts (p < 0.01), but there are no significant differences
between the middle (E = 42 ± 19 MPa) and apical parts (E = 33 ± 14 MPa) (p > 0.01)
(Fig. 2C). In contrast, the tensile strength decrease between all three parts (σ max from
8 ± 2 MPa to 1.4 ± 0.4 MPa; p < 0.01 between BP and MP, and BP and AP; p < 0.1
between MP and AP) (Fig. 2D).