IEEE Robotics & Automation Magazine - September 2016 - 112

design model perfectly equivalent to the octopus sucker in
size and anatomical proportion. The 3-D information was
used to develop the first passive prototypes of the artificial
suction cups made of silicone [29] that are able to achieve
adhesion in wet conditions. These capabilities demonstrate
the importance of the role of morphology, material compliance, and interaction with the environment.
From a technological perspective, an effort has also been
made to develop the first soft actuation unit integrated into
the artificial suction cup based on dielectric elastomer actuators (DEAs). The actuation unit imitates the role of the acetabular radial muscles in creating suction and moving water
from the infundibulum-substrate interface toward the acetabulum, enhancing attachment. The device works in a wet
environment and is able to produce up to 6 kPa of pressure,
reaching a maximum pressure in less than 300 ms [30].
Figure 3 shows the approach used in designing the artificial
sucker. The two axes represent the level of abstraction of the
solution found, with bioinspiration and functionality increasing in the direction of the arrows.
How Plants and Plant-Inspired Robots Exploit
Morphological Computation
Different from animals, which are determinate in growth and
reach a final size before they are mature, plants exhibit indeterminate growth and continue to add new organs and tissues

Figure 4. A prototype of the PLANTOID robot. The figure shows
two functional roots, a trunk containing a microcontroller main
board and a spool of the material used to grow the robotic root
in polypropylene (nominal diameter d = 2.5 mm), and an aerial
portion with branches that include polymeric artificial leaves
(based on controllable hygromorphic plant-inspired material
moving in response to humidity, see [39]).

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IEEE ROBOTICS & AUTOMATION MAGAZINE

September 2016

for their entire life span. This also implies that they continuously adapt their morphology and physiology in response to
variability within their environment, showing considerable
plasticity, particularly in foraging for resources [8], [31].
These properties are particularly evident in the plant root system, which is able to explore the soil and penetrate the environment with a number of sensorized apexes, resulting in
capillary searching of the entire volume of the medium. This
exploratory ability of plant roots emerges from the complex
and dynamic interaction between their morphology, sensorymotor control, and environment, which represents the basic
principle of morphological computation. The motion of plant
roots is coordinated and efficiently shaped to exploit soil
resources and avoid hazards. In the soil, the roots are exposed
to multiple stimuli, many of which can potentially elicit such
movements. The overall apex bending movement is a combination of both active bending and passive deflection. The
elongation rate of the root apex is determined by both the
root apex growth pressure and by the reaction force of the soil
to its deformation [32], [33]. For example, the mechanical
strength of soil may increase with drying and thereby restrict
root elongation [34]. The mechanical properties of the plant
roots and the morphology of their structure have been considered in developing the first level of control embedded in
the mechanical structure of the first robot inspired by plant
roots, which is named PLANTOID [35], [36] (Figure 4).
An extreme representation of morphological computation
in plants generated by the interaction of the body, materials,
and environment is given by their passive movements. As
stated by Zahedi and Ay [37], "the consensus is that morphological computation is the contribution of the morphology
and the environment to the behavior that cannot be assigned
to a nervous system or a controller." Plant materials are optimized to reduce energy consumption during motion because
of the sedentary nature of plants that obliges them to make
the most of resources available in the environment. Different
from animals, plants cannot move when resources fundamental to survival are not more available. To address these
limitations, they have developed energetically efficient solutions to exploit the interaction with changing environmental
conditions, especially humidity and temperature variations.
Examples of these movements are found in pinecones, which
release their ripe seeds by opening their scales in drying
ambient air conditions and closing their scales in a wet environment [38]. This is possible due to the organization of plant
cell walls, which are composed of a soft matrix (consisting of
hemicelluloses, pectin, structural proteins, and/or lignin,
which are able to swell and desorb humidity) and stiff cellulose fibrils embedded in this pliant medium, which drive the
movement of the plant organ. This actuation principle is
implemented by a wide variety of species in their seed dispersal units so that seeds are able to fly, drill, or bend.
These systems do not require additional control or energy, and this makes them an interesting source of inspiration
in robotics and in actuation technologies that are not necessarily muscle-like. Following this principle, a soft actuator

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016