Biomedical Engineering Reference
In-Depth Information
polymers, as well as the effect of different cations such as K+, Na+, Li+, and Cs+ and
some organometalic cations on the actuation and sensing performance of IPMNCs.
Tadokoro (2000) and Tadokoro et al. (2000, 2001) have presented an actuator
model of IPMNC for robotic applications on the basis of physicochemical phenomena.
On the characterization front, to understand the underlying mechanisms of sens-
ing and actuation, Mojarrad and Shahinpoor (1997a) presented plots of blocked force
versus time for an ionic polymer bender subjected to sinusoidal, triangular, square,
and sawtooth waveforms. Shahinpoor and Mojarrad (1997b) showed displacement
versus frequency for a 2-V periodic input.
The first report of ionic polymer transducer sensing was published by Sade-
ghipour and colleagues (1992), who created a Nafion
-based accelerometer. They
fabricated a wafer-like cell that applied pressure across Nafion the thickness of a
piece of Pt-plated Nafion (an ion-exchange membrane product of DuPont), and they
measured the voltage output. The cell was approximately 2 in. in diameter, and its
sensitivity was on the order of 10 mV/g. An interesting feature of their work is that
the Nafion was not hydrated. Prior to use, it was saturated with hydrogen under high
pressure. Also, the load was applied across the polymer's thickness, while most other
ionic polymer transducer research has been performed using cantilevered benders.
Shahinpoor (1995a, 1996c) and Shahinpoor and Mojarrad (1997, 2002) reported
the discovery of a new effect in ionic polymeric gels—namely, the ionic flexogelectric
effect in which flexing or loading of IPMNC strips created an output voltage like a
dynamic sensor or a transducer converting mechanical energy to electrical energy.
Consequently, Mojarrad and Shahinpoor (1997b) investigated displacement sensing by
measuring the output voltage versus the applied tip displacement for a cantilevered ionic
polymer transducer and observed that the output was dependent on the orientation of
the transducer with respect to the electrodes.
Motivated by the idea of measuring pressure in the human spine, Ferrara et al. (1999)
applied pressure across the thickness of an IPMNC strip while measuring the output
voltage. Their experiment was repeated with a maximum stress of almost 900 kPa, and
the results were similar. More recently, Henderson and colleagues (2001) performed an
experimental frequency-domain analysis of the output voltage with a tip displacement
input for a cantilevered bender. Their purpose was to evaluate the suitability of ionic
polymer transducers for use in near-DC accelerometers. The transducer used in the
experiment was allowed to dry in typical atmospheric conditions for approximately one
month before testing. They observed a sensitivity of approximately 50 mV/m for an 11-
×
29-mm cantilever and concluded that ionic polymer transducers might be a useful
technology for low-frequency accelerometer applications. Recent studies of Aluru and
coworkers on ionic polymer gels (De et al., 2002) are also of relevance to this topic.
With regard to the modeling of IPMNC biomimetic sensing and actuation, it must
be emphasized that most of the models proposed for IPMNCs can be placed in one
of three categories: physical models, black box models, and gray box models. For the
physical models, researchers select and model the set of underlying mechanisms they
believe to be responsible for the electromechanical response and subsequent deforma-
tion (actuation) or electrical output (sensing). For the black box models (also called
empirical models and phenomenological models), the physics are only a minor con-
sideration, and the model parameters are based solely on system identification. The
