## Conducting polymer soft actuators based on polypyrrole films 

## -Energy conversion efficiency-

Keiichi Kaneto, Hisashi Fujisue, Masakastu Kunifusa and Wataru Takashima

Graduate School of Life Science and Systems Engineering, Kyushu
Institute of Technology,

2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan

**ABSTRACT**

The electrochemomechanical deformation (ECMD) of conducting polymers can
be utilized to create soft actuators or transducers of electric power to
mechanical work. Polypyrrole (PPy) films, which were electrodeposited
from a methyl benzoate solution of tetrabutylammonium (TBA)
trifluoromethansulfonate, TBACF~3~SO~3~ were employed to investigate the
energy conversion efficiency. The films are known to have high tensile
strength and produce large strain at high stress in ECMD. The current as
a function of load stress under constant applied potentials vs. a
reference electrode was studied in various electrolytes. Reduction
currents increased with increasing load stress for contraction of the
film (lifting weights) as well as for the oxidation current (expansion),
indicating that the electrical input was sensing the load hung on the
actuators. During the contraction, the conversion efficiency was
estimated from the mechanical work energy. The maximum work
energy-per-cycle was 140 kJ/m^3^. It has been found that the energy
conversion efficiencies are very small (less than 0.25 %). Most of the
input electrical energy is stored electrochemically, but a significant
fraction is also dissipated.

Keywords: soft actuators, conducting polymers, polypyrrole,
electrochemomechanical deformation, energy conversion efficiency.

**1. INTRODUCTION**

Upon oxidation the electrical conductivity of conducting polymers
increases to a level of metals, while keeping the flexibility of
polymeric materials. It is also known that conducting polymers swell and
shrink upon electrochemical oxidation and reduction \[1-5\],
respectively. The swelling is induced by some combination of the
insertion of bulky ions and the change of polymer conformation due to
the delocalization of π-electron upon oxidation. We have named this
process electrochemomechanical deformation (ECMD)\[2\]. If the
magnitudes of strain and stress are comparable to or larger than those
of skeletal muscles, then the ECMD can be utilized as artificial muscles
or soft actuators.

Among various materials for soft actuators \[1,6-10\], conducting
polymers are the most promising materials from the view point of low
operating voltage and large stress, though the strain and the response
times were original poor, making practical application difficult
\[2-5\]. Amongst conducting polymers, polypyrrole (PPy) is the most
investigated since PPy is electrochemically active over a wide range of
pH from acid to base and easy to prepare. In the development of
conducting polymer actuators, efforts have been mostly paid to improving
the strain and response time\[11-18\].The peak strain, stress and strain
rate of PPy films so far are 39 %, 22-34 MPa and 11%/s\[11-18\],
respectively, which can be compared with those of skeleton muscles being
approximately 30%, 0.4MPa and 0.1 s, respectively. High voltage
application to a PPy film \[20\] had improved the response time,
avoiding the serious degradation of the film by shaping voltage forms.
Other important factors for the better performance of actuators are the
energy conversion and durability (cycle life).

Spinks and Truong \[21\] calculated actuation behaviours based on
working against isotonic loads and working against restoring springs.
They discussed how the actuator strain and work-per-cycle depended on
the elastic modulus, which was varied by electrical stimulus. Madden et
al. explored the load and time dependence of strain and charge in ECMD
of PPy \[19\]. They found that the impedance dropped as load was
increased, indicating that the electrical response changes upon changing
of load and history of actuation. Otero and Cortes \[22\] reported that
a triple layered bending PPy actuator with two electrical terminals had
the sensing ability of temperature, concentration of electrolyte and
load. The variation of these factors was monitored by the change of
applied potential under the operation of galvanostatic mode. Madden et
al. \[23\] examined also the creep and cycle life of PPy films for both
passive mechanical stress and active ECMD under load to elucidate the
degradation mechanisms. However, little has been studied on the problem
of energy conversion efficiency from electrical input to mechanical
output, apart from a brief modeling discussion and frequency response
results \[24\].

In this paper, preliminary results on the energy conversion efficiency
of PPy actuators are mentioned. The electrical responses of PPy
actuators operated under load stress of weights in various electrolytes
were monitored. In our case, when a stepwise constant voltage vs. a
reference electrode was applied to the actuator, the magnitude of
observed reduction (contraction) currents strictly depended on the
loads. By assuming that an increased current was due to the increased
load stress, then the fraction of the increased electrical energy was
assumed to be consumed to lift the load weights. Mechanical output
energy was estimated from the lifted distance and mass of weights under
gravity. As pointed out by Madden et al. \[19\] the strain to charge
ratio is almost constant over wide rage of loads, though the voltage
depended strongly on loads as to drop of impedance. The drop of
impedance should result in the increase of current under the constant
voltage mode operation of the actuator. We will discuss the electrical
energy consumption and gain of mechanical work from the present
experimental results.

**2. ECMD IN CONDUCTING POLYMERS**

By the electrochemical oxidation of conducting polymers, electrons are
extracted from the π-electron systems of conducting polymers. The
positive charge delocalizes along the backbone of the polymer chain,
resulting in extension of polymer due to the conformation change of
polymer backbone. An equal number of bulky anions in electrolyte
solution is inserted in the polymer matrix, resulting in the swelling,
since the volume of electron is negligible compared with anions. Upon
electrochemical reduction of the film, the anions are released to the
electrolyte solution and the film shrinks to the original volume. The
electrochemical cycles are reversibly controlled by changing the
polarity of applied voltage between the conducting polymer and counter
electrode. Thus expansion of film upon oxidation is named as anion
driven ECMD \[10, 11\], where anions are involved in doping and dedoping
processes. It is believed that employment of larger anions results in
larger strain, however, the larger the anion gives slower ECMD response.

If anions are entangled and immobilized in the conducting polymer matrix
during electrochemical reduction, cations are doped to compensate anions
instead of anions being released \[25\]. Then the film swells. This
scheme occurs when extremely large anions like dodecylbenzene sulfonic
(DBS) acid are employed for the electrodeposition of PPy film \[12, 13,
26\]. Doping and dedoping of cations during reduction and oxidation,
respectively, are named as cation driven ECMD \[12,13,26\].

Conducting polymer films are stiff in the oxidized state compared with
that of neutral state \[21,27,28\], likely because of the strong
interaction between ions and the delocalized π-electron systems in the
oxidized state. When soft and flexible materials are utilized, the
actuator does work to pull up weight during the contraction, like
natural muscles, and does not work to push up. As stated by Spinks et
al. \[21\], it is desirable to employ materials that are compliant in
the expanded state and stiff in the contracted state to maximize strain
and work density. However, most of previous reports on ECMD behaviours
as the function of load stress were done for anion driven PPy films,
since the film shows the larger strain and stress in ECMD and are easy
to analyze. In the present work, an anion driven PPy films was employed
also. The work property in cation driven films will be reported
separately in the future \[29\].

**3. EXPERIMENTAL**

Polypyrrole films were electrodeposited on a metal electrode of Ti in an
electrolyte solution of 0.25 M pyrrole and 0.2 M of TBACF~3~SO~3~ in
Methylbenzoate (MB) at a constant current of 0.2 mA/cm^2^ for 4h vs. a
platinum counter electrode \[15\]. The resulting film is named as
PPy/CF~3~SO~3~ and the thickness of film was approximately 13 μm. The
pyrrole and MB were obtained from Wako chemical Co. Ltd., and
TBACF~3~SO~3~ was from Toyama Chemical Co. Ltd. They were used without
further purification. The electrolyte solution were bubbled with a dried
nitrogen gas to remove oxygen and followed by electrochemical
polymerization. Visco-elastic and tensile properties of PPy films were
measured using Perkin Elmer Dynamic Mechanical Analizer, DMA 7e.

Figure 1 shows the schematic diagram of experimental setup for the
direct measurement of expansion and contraction of films. A PPy film was
hung on the working electrode (WE) of Pt wire and the bottom end of the
film was connected to a thin Pt wire with the diameter of 25 μm. The Pt
wire suspended a tray or reflecting plate through the pinhole of the
cell. The up-down of the plate due to the contraction and expansion of
the film was measured by a laser displacement meter directly with the
accuracy of 10 nm. Reference electrode (RE) and counter electrode (CE)
were Ag wire with diameter of 1 mm and 2 x 20 mm^2^ Pt foil,
respectively. Typical width of films was 2 mm and the length (*l*~0~)
was 7 mm and thickness of 13 μm was employed for the experiment. The
density of film was approximately 1.4 g/cm^3^ at dried state. The ECDM
of films were operated in aqueous electrolyte of 0.1 M NH~4~PF~6~, 0.1 M
NH~4~Cl and NH~4~PF~6~ in water/acetonitrile (4:1). The electrolyte
solution of 0.1 M NH~4~PF~6~ showed the best ECMD response among various
electrolytes tested \[17,18\]. The mixed electrolyte, namely denoted as
PF~6~/Mix gave rise to quick response, since organic solvent makes films
swell and easy for diffusion \[16\]. The films were cycled several times
without load to reach equilibrium before measurement.

Figure 2 shows the wave forms of applied voltage (*v*) vs. the reference
electrode, which was a rectangular wave for PPy/CF~3~SO~3~ film. There
might be some difference in the estimated energy conversion efficiency
by wave forms, as suggested Otero \[22\]. However, at this stage of
experiment we have not yet measured the wave form dependence of
conversion efficiency. Putting various weights (*m*) on the tray, the
ECMD strain-stress curves of the films were measured. A personal
computer was used to control the galvano/potentiostat and to collect the
data. The electrical input energy was estimated from *E*~Ein~ = ∫*vidt*,
where *i* is current and *t* is time. The voltage for this calculation
is tentatively given from the reference electrode (actually it should be
measured against the counter electrode). The mechanical output energy
was estimated from *E*~Mout~ = *mg∆l*, where *g* is gravity and *∆l* is
the length lifted with the weight upon contraction of film. The maximum
weights (*m*) were 50 g.

**4. RESULTS AND DISCUSSION**

Curves in Fig.3 show typical time response of ECMD under application of
weights. The deformation was measured by % elongation of the PPy film.
Under the load weights of less than 1g the current responses were
repeatable within several cycles, however, creep was significant under
weights of larger than 10 g. Therefore the measurement of current
responses under the weight larger than 10 g was done by a single shot of
rectangular wave form, namely 300 s for oxidation followed by 300 s for
reduction. To see the reproducibility of data and to avoid the film
fatigue, weights of load were applied to the film by the sequence of 20,
1, 30, 10, and 40 g. Time for putting heavy weights was minimized in
order to shorten the natural creep. The voltage was applied immediately
from the rest potential after the load was applied to the film. The
curves in Fig.3 were adjusted at the beginning of oxidation by t = 0 and
0% elongation.

It is interesting to note that creeping was obvious for reduction cycle
at heavy load as shown in Fig.3, but not for oxidation. The similar
result that creeping was significant in the reduced state was reported
by Madden et al. \[19\] in PPy film doped with PF~6~. The result is
likely related to the fact that the stiffness of film in the oxidized
state is larger than that of reduced state. In fact Young's modulus of
the wet film with water was 0.29 and 0.48 GPa for the reduced and
oxidized states, respectively.

The typical current responses of oxidation and reduction are shown in
Fig.4(a) and (b) respectively, indicating that the current increased
with larger load stress. It is noted that the current response for
reduction lasted longer and was plotted vs. log*t*. As seen in Fig.4 (a)
the peak current of oxidation was apparently larger for heavier weights,
however, the total charges for various weights were not different much.
On the other hand, the reduction current apparently increased with
increasing stress as shown in Fig.4 (b). There are many possible
mechanisms, namely, (1) a load tension to expand the film makes the
charge migration (ionic conductance) easier in the film. This is clearly
observed for oxidation cycle, but the transferred charges were not
changed much. (2) The redox history changes the ionic conductance with a
tendency of the current to increase with cycling. The observed current
recovery, for example, ECMD at lighter load of 3.8 MPa exhibited a
smaller current after measurements of 11.4 and 15.2 MPa, may exclude
this possibility. (3) The increased current or transferred charge is
consumed by lifting weights. However, this result may be inconsistent
with a prediction of Madden et al. \[19\] that the transferred charges
were basically constant for the same degree of deformation. Net charges
evaluated from one cycle of charging and discharging should be zero, if
a side reaction does not take place. The increased transferred charges
during the reduction, despite of the oxidation charge being constant
with increasing load stresses, is one of the key subjects to be
discussed. We discuss the conversion efficiency with the results of
reduction current later, and show how the current increase relates to
mechanical output work.

Madden \[19\] observed the potential variation during oxidation at
galvanostat mode under load stress, which indicated the lower potential,
namely, the decrease of impedance at larger loads. This result may be
mainly related to the mechanism (1) as well as (2) and (3) to some
extent. On the other hand, Otero \[22\] obtained the larger potential at
larger loads, though the actuator structure of trilayer was different
from the present case of linear actuator. Trilayer actuator was actually
working and pushing up a weight.

Figure 5 shows *E*~Ein~ for the film during reduction as a function of
load stress in various electrolytes. The *E*~Ein~ in the unloaded film
was 16-22 mJ, which is attributed to charging of a battery (*E*~charge~)
based on oxidation of the PPy film. The *E*~charge~ includes also the
energy to expand or contract mechanically the film itself. Most of
electrical input energy stored as in a rechargeable battery may be able
to be harvested again. It should be noted that the *E*~Ein~ for the
electrolyte of NH~4~Cl was nearly constant. The result contrasts to the
case of PF~6~ and PF~6~/Mix, in which the strain of ECMD was much larger
as seen in Fig.6.

Curves in Fig.6 show the strains of ECMD against the load stress of the
films in various electrolytes. The strain of the films operated in
aqueous NH~4~PF~6~ under no load was approximately 6 %, decreasing
monotonically to zero at approximately 15 MPa (as a blocking force
\[21\]). The operation of the film in NH~4~Cl solution showed small
strain, likely due to the transfer of Cl^-^ ions, whose small size
produces little strain. The film operated in mixed solvent of
water/acetonitrile shows large strain of 9% under no load and smaller
blocking force of approximately 9 MPa. This result is consistent with
the small modulus of film wetted with the mixed solvent being 0.12 and
0.17 GPa for the reduced and oxidized states, respectively.

According to analysis by Spinks et al. \[21\] for an electrochemical
actuator, for the first order approximation, the strain (*∆l*) can be
expressed by,

*∆l* = *∆l~0~* --* f* *l~0~* /(*SE'*), *    
                                                           *(1)

where *∆l* ~0~, *f*, *S* and *E' * are the strain at zero applied force,
force applied to the film, cross section and the modulus of the film at
the reduced state, respectively. The dotted line in Fig.6 shows a fitted
line to the curves of the film operated in NH~4~PF~6~, giving the
parameters of *∆l* ~0~/*l*~0~= 7 % and *E'*= 0.21 GPa. The *E'* can be
compared with the measured value of 0.29 GPa, indicating fairly good
agreement within an experimental error.

The half cycle mechanical output energies (*E*~Mout~) as a function of
load stress are shown in Fig.7. For instance, when 20 g was lifted up by
a film with a length of 7.0 mm it strained by approximately 4 %, and the
corresponding energy was approximately 52 μJ or 140 kJ/m^3^. It is noted
that *E*~Mout~ showed maximum (*f*~max~) at approximately several MPa,
which is obvious from the mechanical out put energy is derived from the
well known relationship \[21\] of

*E*~Mout~ = *f∆l* = *f∆l~0~* --* f* ^2^*l~0~* /(*SE'*) (2),

*f*~max~ =   *∆l~0~* *SE'* /2*l~0~* . (3)* *

The curves in Fig.7 show good agreement with the analytical prediction
being a parabolic curve for various electrolyte solutions.

The conversion energy efficiencies of the films as a function of load
stress are shown in Fig.8. The conversion energy efficiency showed a
maximum at several MPa, and was 0.25% operated in NH~4~PF~6~ electrolyte
at best. The conversion efficiency was of course similar to the curves
of work done shown by Fig.7, since the electrical input energy increased
slightly.

It is interesting to note that the net consumed electrical energy of
*E*~Ein~ - *E*~charge~ for half cycle is tremendously large compared
with the energy worked by lifting weights. This is clear by taking the
electrical energy, *E*~Ein~ - *E*~charge~, which is order of mJ as seen
from Fig.5, and comparing it with the mechanical energy, *E*~Mout~,
which is on the order of several tens of μJ. The large unknown consumed
electrical input energy is puzzling, in spite of the careful
recalculation. By comparison of curve shapes in Fig. 5 and Fig.7 at
higher stresses, however, one may aware that *E*~Ein~ - *E*~charge~ is
not directly related to the mechanical work. In the case of ECMD
operated in NH~4~Cl solution, a small *E*~Ein~ - *E*~charge~ and a small
strain compared with the case of PF~6~ and PF~6~/Mix were obtained,
though the same magnitude of mechanical work energy was observed. These
results may suggest that the large consumed electrical input energy
under large load stress may be used to enhance the creeping or
degradation of films. Further investigations are needed to elucidate the
details of conducting polymer actuators as an energy transducer.

**5. Summary**

The conversion energy efficiency from electrical input to mechanical
output in PPy soft actuators has been studied. It was found the
conversion efficiency without recovering stored electrical energy was
quite low, with 0.25 % of the input electrical energy at most being
converted to work. The maximum efficiency was obtained under loads of
several MPa. The mechanism of the electrical energy loss (not including
charging or oxidizing PPy) is not known at the present state and a
problem to be solved. A simple calculation from the mechanical work of
the PPy actuator operated in aqueous NH~4~PF~6~ showed the volumetric
work-per-cycle was 140 kJ/m^3^ at best.

**Acknowledgements**

The authors express their sincere thanks Drs. S. Hara and T. Zama of
Eamex Co. Ltd for the financial support and collaboration. This work is
supported by Priority Area of the Ministry of Education, Sport, Culture
and Science. The authors also appreciate to Prof. John Madden for
critical reading of the manuscript.

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**Figure captions**

Fig.1 Experimental setup.

Fig.2 Rectangular wave forms of applied voltage to PPy/CF~3~SO~3~ film
at -0.7 to 0.3 V vs. Ag/AgCl with the period of 600s.

Fig.3. Load dependence of ECMD in PPy/CF~3~SO~3~ film operated in
aqueous 0.1M NH~4~PF~6~. Numerical in circle is sequential of
measurement.

Fig.4. Current responses in ECMD of the film at various load stresses,
(a) Oxidation (elongation) and (b) Reduction (contraction).

Fig.5 Load stress dependence of electrical input energies *E*~Ein~.

Fig.6 Electrochemomechanical strain --stress curves in PPy/CF~3~SO3
film. Dotted line is obtained by equation (1), with the parameter of
*∆l~0~/l*~0~ = 7.0 % and *E'=* 0.21GPa.

Fig.7 Stress dependence of mechanical output energy, *E*~Mout~ of PPy
film operated in various electrolytes.

Fig.8 Energy conversion efficiency against load stress in various
electrolytes.