Biomedical Engineering Reference
In-Depth Information
whose fluid motion is generated by electrically induced migration and redistribution
of conjugated ions within its polymeric network of nanoclusters. Every part of the
material can be reached by electric field-induced migration and redistribution of
conjugated ions on a nanoscale for robotic motion action and feedback, as well as
embedded distributed sensing and transduction.
One of the most important characteristics of these nano-IPMNCs as smart mul-
tifunctional polymeric nanocomposites is the ability to allow ionic migration on a
molecular and nanoscale by means of an imposed local intrinsic electric field within
the material. This then causes hydrated or otherwise loaded cations to move and
create a local pressure or fluid motion while carrying additional water as hydrated
water as well.
Such fluid circulatory migration in ionic polymers had been observed in our
laboratories at the Artificial Muscle Research Institute at the University of New
Mexico as early as 1994 in the form of water appearance and disappearance on the
cathode side of IPMNC strips under an imposed sinusoidal or square wave electric
field for actuation. The fact that we could put a pair of electrodes in the middle of
a strip of IPMNC and make it bend and grab objects like a soft parallel jaw robotic
gripper inspired us to think about the rapid nastic sensing and actuation of higher
plants such as the carnivorous or insectivorous plants. The induced spectacular
bending motion and the built-in sensing characteristics of the ionic polymer-metal
nanocomposites led us to think that some higher plants, such as the Venus flytrap
(
Dionaea muscipula
), may use the same ionic migration and water circulation for
sensing and rapid actuation.
In fact, Shahinpoor and Thompson (1994) concluded that what was happening
in ionic polymers in connection with ionic migration and induced bending and
deformation was possibly the mechanism for some amazing nastic movement in
plants such as in the Venus flytrap (fig. 2.34), in which almost digital sensing (the
trapped insect has to disturb the ionoelastic trigger hairs more than two or three
times before the lobes close rapidly to trap the insect) and rapid actuation and
deployment occurs with ionic migration using the plant's sensing and ionic circula-
tory system. Mechanical movement of the trigger hairs (fig. 2.34(d)) puts into motion
ATP-driven changes in water pressure within these cells.
These trigger hairs are located on the trap leaves. If these are stimulated twice
in rapid succession, an electrical signal is generated that causes changes in the water
pressure in different parts of the leaves (Shahinpoor and Thompson, 1994).
The cells are driven to expand by the increasing water pressure, and the trap
closes as the plant tissue relaxes. The mechanisms responsible for such rapid deploy-
ment of lobes in Venus flytrap are acid growth and turgor pressure leaf movement,
which is an osmotic effect in which an ion (in the case of
Dionaea
, K
+
) is released
into the leaf tissues and makes the cells on one surface of the leaves swell. It is
remarkable how these changes are similar to what actually happens in ionic polymer
nanocomposites. One can even have K
+
cations in IPMNCs to cause sensing and
actuation. It is clear that ionic polymer nanocomposites have opened a door to the
mysterious ion engineering world of nastic plant movements and rapid deployments
and this new ionic world now needs to be further explored.
