An Innovative
Actuation Method
based on the
Osmotic Principle
Fabio Mattioli, Lucia Beccai and Barbara
Mazzolai
IIT – Istituto Italiano di Tecnologia
Center for Micro-BioRobotics IIT@SSSA, Viale Rinaldo
Piaggio, 34 - 56025 - Pontedera (PI), Italy
email: [email protected]
http://mbr.iit.it
Motivation, Problem Statement
and Related Works
Motivation
We propose an innovative, bio-inspired
actuator based on forward osmosis principle
to obtain a pressure suitable for actuation. The
final aim is to conceive an actuation system
characterized by energy efficiency, simple and
robust structure, and easy to miniaturize in
order to work in the milli-micro domain.
Osmosis represents one of the fundamental
natural processes, e.g. cellular chemical
exchanges, and it is at the basis of some highperformances natural movements (e.g. Venus
flytrap leaves closure, cnidocysts exocytosis
in Cnidarian, etc.).
Problem statement
Osmosis is the spontaneous flow of a solvent
across a selectively permeable membrane,
from a region of higher water chemical
potential (i.e. lower solute concentration) to a
region of lower water chemical potential (i.e.
higher solute concentration). Forward
Osmosis (FO) is a powerless process, based
on the chemical potential non-equilibrium of
two chemicals separated by an osmotic
membrane. The core idea is to induce FO to
obtain actuation; to this aim, the major
technological challenge is to vary the solute
concentrations between two solutions. The
performances are directly correlated with the
solute used (because of its solubility in the
selected solvent) and the dissociation
coefficient. In addition, the efficiency of the
osmotic actuation system strongly depends on
the features of the osmotic membrane, which
must be: a) a high mechanical strength, in
order to resist to the high pressure obtained
and limit its deflection; b) a high water
permeability coefficient, which is directly
correlated to the actuation velocity; c) a very
low solute permeability, to avoid the loss of
actuation reversibility.
Related work
Osmosis has been applied in many fields,
from desalination methods (e.g. reverse
osmosis [2]) to power production as
renewable source of energy [3]. Interesting
works in which osmosis is used for actuation
are: i) an osmotic pump for drug delivery,
described in [4], in which authors obtain a
long time functioning at a requested drug
release rate; ii) the chemo-mechanical
actuator described in [5], based on protein
transporters extracted from plant cell
membranes, which can exert a force
comparable with conducting polymer actuator
(up to 1mN [5]); iii) an electro-osmotic
actuator embedded in a robotic plant root
inspired artefact [6], used as steering system.
Technical Approach
The actuator design is based on Organic
Electronic Ion Pumps (OEIP) [7], modified to
have a higher positive ion transport and faster
response (in [7] the researchers obtain the
transport of about 200 nmol in 10 minutes).
To maintain electro-neutrality, an anionic
membrane will be employed. The osmotic
membrane will be the FO membrane
developed
by
Hydration
Technology
Innovations (HTI), in which a polyester mesh
is embedded within the polymeric osmotic
Figure 1: A simple-shape two chambers osmotic
actuator; the chambers are indicated in figure as “1”
and “2”
layer for mechanical support. The membrane
thickness is less than 50 μm.
designed experimental setup.
Results
A preliminary evaluation of the osmotic
actuator performances were performed, based
on a simple-shape architecture (Fig. 1), in
which the lateral area is rigid while the top
and down faces are flexible. Chamber 1 will
contain a CaCl2-water solution (because its
high osmolarity), whereas chamber 2 will
include pure water. The performances were
calculated
at
4
different
solution
concentration levels. For an evaluation of the
scale effect, three actuator diameters have
been hypothesized. Table 1 shows the design
data.
Table 1: Actuator evaluation parameters
Parameter
Value
geometry 1: 1
Diameter and height [mm]
geometry 2: 5
geometry 3: 10
Molarity [mol l-1]
0.05, 0.1, 1, 6.5
Temperature [K]
293
5.00 · 10-6 (typical value taken
Volumetric membrane water
from [8])
permeability constant Av [m3 m-2
s-1 MPa-1]
0.98 (typical value taken from
σ
[8])
Stroke
diameter/3
The results are presented in Table 2 and Fig.
2. The osmotic actuation shows high specific
performances, especially for the small
dimension geometries.
Table 2: Results of performances evaluation
Chamber 2
0,05
0,1 mol/l
1 mol/l
molar
mol/l
concentration
Osmotic
0,365
0,73
7,3
Pressure
[MPa]
Actuation
32,2
16,1
1,61
Time [s]
Force [N]
0,287
0,573
5,73
6,5 mol/l
Figure 2: Comparative analysis with other actuation
technologies (adapted from [1])
47,5
0,24
37,2
Experimental set-up
An experimental set-up to study and validate
the expected performances was developed.
This set-up is characterized by its reduced
dimensions (Φ = 26 mm, h = 55 mm), the full
sensorization for an online control and
monitoring, simple architecture and easiness
of assembling. The structure has been
designed to resist to up to 30 MPa of internal
pressure. Fig. 3 shows some views of the
Figure 1: Osmotic actuator experimental set-up
Main Experimental Insights
The insight of this work is to employ the
osmotic pressure, coming from the
spontaneous solvent flow derived by the
difference in chemical potential, in a well
controlled way. Technical advancements are
coming from the study of biological examples
that adopt osmosis as actuation strategies.
Several applicative scenarios can be
envisaged by developing the described
osmotic actuator, such as, for example, a drug
delivery pump for drug controlled release rate
in real time, as well as an osmotic actuator
system embedded in a microrobotic system
inspired to plant apex root for elongation and
steering movements during soil exploration
tasks. More in general, this technology could
be potentially applied in the design and
construction of optimal nastic actuation
models.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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