Emerging Technologies Inspired by Plants - Bioinspired Approaches for Human-Centric Technologies - page 126

Fig. 4.6 (a) The drilling motion of Erodium awns (Evangelista et al. 2011 ). (b) The opening
motion of conifer cones as a function of wetting and drying transients ( left ) [adapted from Dumais
and Forterre ( 2012 )]. Schematic of a pine cone cut along its longitudinal axis ( right ): the cellulose
fibril orientations in the cell walls of fibers on the upper ( white ) portion and on the lower ( gray )
portion are indicated by the inclined lines in each rectangle. The relative fibril orientation is
responsible for the opening of the pine cone, which is driven by the absorption of moisture
[Adapted from Burgert and Fratzl ( 2009 )]. (c) A schematic of the structure and function of
wheat awns. (A) The structure of a wheat awn dispersal unit. The actuating portion of the awn
(magnified in the circle ) is composed of an outer active portion (ridge) and a passive portion (cap)
that faces inwards. The cellulose fibril orientations are indicated by the inclined lines in each
rectangle. (B) The daily cycle of dispersal unit movement, which results in a soil penetration:
(i) day, (ii) night, and (iii) day [from Burgert and Fratzl ( 2009 )]
fibrils (Fahn and Werker 1972 ; Burgert and Fratzl 2009 ). Asymmetry in the
orientation of the fibrils at the organ level then converts this local swelling or
shrinking to a global bending motion.
Some interesting examples of this movement include the opening and closing of
a pine cone (Dawson et al. 1997 ; Reyssat and Mahadevan 2009 ), the drilling motion
of Erodium awns (Evangelista et al. 2011 ), and the swelling and drying mechanisms
of the seed dispersal units of wild wheat (Elbaum et al. 2007 ) (see Fig. 4.6 ).
4.4.2.1 Smart Materials for Passive Mechanisms
In materials science, a large amount of research is being conducted to develop
polymeric materials that can react to external stimuli or change in environmental
conditions by modifying their shape or other properties. As a result, these materials
are commonly called smart polymers. Among the numerous classes of polymeric
materials and composites, some candidates that are inspired by plants exhibit a
potential to be used in biomimetic applications.
For example, hydrogels, which are three-dimensional polymer networks filled
with aqueous solutions, have some potential in this regard because they can mimic
the swelling and shrinking behavior of plant cells and produce macroscopic