actuators , or artificial muscles , based on conducting polymers ( cps ) , and other electrochemically active organic and carbonbased materials , constitute the base for the development of promising polymeric sensing motors , tools for surgeons , or zoomorphic and anthropomorphic soft robots.1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 they are electrochemical devices ; the cp oxidation / reduction drives the exchange of ions and solvent with the electrolyte causing reversible volume variations and device actuation .
consequently , they are very reliable faradaic motors ; the rate of the movement is a linear function of the reaction driving current and the amplitude of the bending movement is under linear control of the consumed charge.11 , 12 , 13 , 14 in addition , the fabrication methodology is very robust ; by using different devices ( different shapes , including different masses of the cp and fabricated during several years ) , the same actuation charge per unit of polymer mass always generates the same displacement.11 , 13 
  the exchange of ions is controlled by the driven electrochemical reactions .
the exchange of solvent is driven by physical osmotic or electroosmotic effect following , or driven by , respectively , the ionic exchange.15 , 16 , 17 as a consequence of the delay between faradaic and osmotic effects , the coulodynamic responses show some hysteresis ; for the same charge , the muscle position during the forward ( oxidation ) and return ( reduction ) is different ( actuation hysteresis cycle or dynamical hysteresis ) .
in addition , some devices present creeping effects ; after an oxidation / reduction cycle some devices do not recover the initial position.18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 recently , irreversible reactions taking place in some cps were identified as one of the creepingeffect origins ( chemical creeping).28 
  by changing the solvent , some cps move from a reactiondriven exchange of cations to reactiondriven exchange of anions.29 , 30 , 31 by changing the salt ( nabr and nacl ) in aqueous solutions , the ppy  hpps / tape ( ppy  hpps : polypyrrole  paraphenolsulfonic acid blend ) , the muscle moves from reactiondriven exchange of br to a consecutive exchange of cations ( na ) and anions ( cl ) in nacl.33 those results should indicate that the muscle actuation , and the reactiondriven ionic exchange in cps , is conditioned by the variation of the intra and intermolecular forces inside the polymer under the influence of different physical or chemical variables . by changing the solvent or the salt , the dielectric constant , permittivity , dipolar moments , polymerion interactions , or salt dissociation degrees in both the dense polymer gel and the electrolyte should change , altering the ionic exchanges and the concomitant actuation mechanism .
herein , we present the attained results from a different salt , that is , napf6 aqueous solutions , in which the anion is closer to br from a chemical point of view . a comparative study between an asymmetric bilayer muscle ( comprised of two different cps ) and
the three studied bilayer muscles are represented here as : tape / ppy  dbs , ppy 
hpps / tape , and ppy  hpps / ppy  dbs , indicating the relative ( left / right ) position of each layer component during experiments .
thus , clockwise / anticlockwise bending movements will be attained under the following reactiondriven volume changes : 
 from the tape / ppy  dbs bilayer by ppy 
hpps / tape bilayer by ppy  hpps shrinking / swelling , respectively.from the ppy 
hpps shrinking or ppy  dbs swelling ( clockwise ) versus preponderant ppy  hpps swelling or ppy  dbs shrinking ( anticlockwise ) . 
  from the tape / ppy  dbs bilayer by ppy 
hpps / tape bilayer by ppy  hpps shrinking / swelling , respectively . from the ppy 
hpps shrinking or ppy  dbs swelling ( clockwise ) versus preponderant ppy  hpps swelling or ppy  dbs shrinking ( anticlockwise ) .
figures  1  a  c show stationary coulovoltammetric ( q / e ) responses ( each closed loop minimum was taken as the charge zero origin ) from the tape / ppy  dbs , ppy 
hpps / tape , and ppy  hpps / ppy  dbs bilayer muscles , respectively , in 0.5  m napf6 aqueous solutions .
we used the specific charge ( charge consumed by reaction of the mass unit of cp working inside the electrolyte ) because , for artificial muscles exchanging anions11 or cations,34 both the rate of the movement and the angular position are linear functions of the driving current or specific charge , respectively , and , therefore , independent of the film thickness or muscle dimensions .
each bilayer q / e stationary response was obtained after consecutive cycles up to the same anodic potential limit of 0.7  v from a different cathodic potential limit , ranging between 0.6 and 2.0  v. positive charge increases describe oxidations , whereas negative charge increases are related to reduction reactions.35 each q / e response is comprised of two different parts : a closed coulovoltammetric loop , for which the charge ( maximum minus minimum ) increase gives the charge consumed by reversible oxidation / reduction of the ppy films , and an open part on the left side , for which a charge increase from the initial potential of the cycle to the final potential is the charge consumed by irreversible and slow hydrogen evolution from the organic acid ( hdbs or the hpps ) taking part in the cp.36 
  a )  coulovoltammetric responses from ppy 
dbs / tape bilayer artificial muscle in napf6 aqueous solutions obtained by increasing the cathodic potential limit ( from 0.6 to 2.0  v ) to the same anodic potential limit of 0.7  v. the same experiments were repeated for b )  ppy 
from the tape / ppy  dbs bilayer ( figure  1  a ) , both reversible ( closed loop ) and irreversible ( open part ) charges increase for rising cathodic potential limits . the reversible charge increase from 1 to 11  mc when the cathodic potential limit moves from 0.6 to 2.0  v , indicating that the polymer reduction occurs in the full studied potential range , up to high cathodic potential limits .
irrespective of the cathodic potential limit , most of the voltammetric charge is consumed by the reversible film oxidation and reduction reactions.35 
  in terms of the q / e response from the ppy 
hpps / tape bilayer muscles ( figure  1  b ) , the charge of the closed loop ( reversible charge ) increases ( from 4 to 6  mc ) when the cathodic potential limit shifts from 0.6 to 1.2  v. then , the reversible charge drops to 3  mc when the potential limit rises to 2.0  v. the irreversible charge always increases with the cathodic potential limit . the asymmetric bilayer shows ( figure  1  c ) an intermediate behavior ; the redox activity of the bilayer increases from 10 to 13  mc and then drops to 12  mc when the cathodic potential limit moves from 0.6 to 1.5 and then to 2.0  v , respectively .
the irreversible charge increase is very fast when the cathodic potential limit moves from 1.5 to 2  v. from the video frames recorded during the bilayers bending movement in parallel to the applied potential cycle , between 1.0 and 0.7  v at 10  mv  s , the dynamovoltammetric ( angle  potential , /e ) plots were attained ( figure  2  a from the tape / ppy  dbs bilayer , figure  2  b from the ppy  hpps / tape bilayer , and figure  2  c from the ppy 
the images in figure  2  d show the asymmetric bilayer muscle position at each potential point , as indicated on the q / e response in figure  2  c , and the described angular displacements between the consecutive potential ( points on the q / e response ) . 
 
evolution of the described angles from the different bilayer artificial muscles : a )  ppy  hpps / ppydbs , b )  ppy  hpps / tape , and c )  ppy 
dbs / tape artificial muscles in 0.5  m napf6 aqueous solution during voltammetric experiments between 1.0 and 0.7  v at 10  mv  s at room temperature .
d )  images of the ppy  hpps / ppy  dbs artificial muscles in 0.5  m napf6 aqueous solution .
the three muscles present a similar /e behavior , that is , an anticlockwise bending movement during oxidation and clockwise bending movement during reduction .
this means that the ppy  dbs film from the tape / ppy  dbs bilayer shrinks during oxidation and swells during reduction ; the electrochemical reaction drives the exchange of na [ eq . 
( 1)]:(1)ppy0dbs - nc+nsmgelppyn+dbs - n+nc++ms+ne- 
  where ppy represents the neutral ppy chains , c are the cations exchanged to keep the electroneutrality , s are solvent molecules to keep the osmotic pressure balance inside the polymeric gel ( indicated by subindex gel ) , and ppy represents every oxidized ppy chain after removal of n electrons ( e).37 
  the amplitude of the described angle is only 4. in a previous paper , an angle of 50 was attained in nabr aqueous solutions under analogous potential cycling.32 this indicates a strong influence of the anion , although , according with equation  ( 1 ) , this influence could be neglected . for the ppy 
hpps / tape bilayer , the oxidationdriven clockwise angular displacement means that the ppy  hpps film swells during oxidation , shrinking during reduction to give anticlockwise movement .
therefore , the reversible electrochemical reaction drives the exchange of anions , a , ( pf6 
  here ) [ eq . 
( 2)]:33 
 ( 2)ppy0hpps+na-+msppyn+hppsa - nsmgel 
  again , a strong influence of the anions emerges , when compared with the results attained from nacl aqueous solutions , whereby cations were exchanged at the beginning of the anodic potential sweep and anions at the end of the sweep ; for the results obtained from nabr solutions , only the anion is exchanged in the full potential range . as a consequence ,
the amplitude of the angle described here per cycle was 24 and only 14 in the previous work ( cation exchange followed by anion exchanges gives opposed bending movements during the anodic sweep ) . as a partial conclusion , in napf6 aqueous solution during oxidation , the ppy  dbs film shrinks and the ppy  hpps film swells .
thus , when the asymmetric ppy  hpps / ppy  dbs bilayer was checked in napf6 aqueous solution , both films developed a cooperative dynamic effect ; when one of the layers swells and pushes the bending bilayer under oxidation , the second layer shrinks through oxidation and trails the muscle giving larger angular displacements .
structural changes and resulting forces reverse during the bilayer reduction ( figure  3 ) . as a consequence of this cooperative actuation of the two constitutive layers ,
the angle described by the asymmetric bilayer per voltammetric cycle is 133 , which is more than six times the amplitude described per cycle for the ppy 
hpps / tape muscle ( 22 ) and over 33  times that described by the tape / ppy  dbs muscle ( 4 ) when cycling in the same potential range .
a second consequence of the cooperative actuation of the two layers in napf6 is a very low creeping effect of 0.3 per cycle . 
 
points  ( 2 ) and ( 3 ) correlate with both the dynamovoltammograms shown in figure  2 and the coulodynamic evolution of the described angles shown in figure  4 . in conclusion , owing to the cooperative actuation , when both layers follow complementary volume changes ( swelling / shrinking or shrinking / swelling ) driven by the same electrochemical reaction , the angular displacement per potential cycle is greater than those attained from each of the ppy  hpps / tape or tape / ppy  dbs bilayer muscles . by combining figures  1 and 2 ,
the evolution of each muscle position ( angle ) as a function of the specific consumed charge ( coulodynamic evolution ) in napf6 aqueous solutions is attained , as depicted in figures  4  a 
hpps / ppy  dbs muscle angular displacement per cycle is 133 ( figure  4  c ) , which is several times higher than those described by the ppy 
b ) or by the ppy  dbs / tape bilayer ( 4 per cycle ; figure  4  a ) in the same electrolyte .
the coulodynamic efficiencies ( slopes ) result in 8.5 , 3.0 , and 1.0  mc for the asymmetric bilayer , the ppy 
hpps / tape bilayer , and the ppy  dbs / tape bilayer , respectively .
the magnitudes , described angle per cycle , and coulodynamic efficiency quantify the cooperative dynamical effect of the two layers in the asymmetric bilayer muscle . 
 
relationship between the bending angle described for the muscles and the electrical charges consumed during voltammetric experiments studying reactions of different artificial muscles between 1.0 and 0.7  v at 10  mv  s at room temperature : a )  ppy  dbs / tape , b )  ppy  hpps / tape , and c )  ppyhpps / ppy  dbs artificial muscles in napf6 aqueous solution .
both anodic and cathodic displacements show a linear dependence on the charge , with a similar slope , corresponding to any faradaic process driven by the reactions given in equations  ( 1 ) and ( 2 ) .
the charge spent at the beginning and at the end of the oxidation shows a very low efficiency toward bending the muscle .
these charges can be related to hydrogen evolution from the film , producing acids , at the most cathodic potentials and to the elimination of protons at the most anodic potentials.36 , 38 both reactions result in very low volume variations . in napf6 aqueous solution ,
all of the studied cp / tape bilayers behave as faradaic motors ; the amplitude of the described angle ,  , is a linear function of the consumed charge , q [ eq .  (
3)]:(3)=0+kq 
  where  
 0 is the initial angle and k (   mc ) is a constant dependent on the system ( cp , electrolyte , tape , or second cp film ) .
nevertheless , the actuation of the asymmetric bilayer presents an important hysteresis effect ; for the same charge , the muscle positions during the anodic and the cathodic sweeps are quite different .
smaller hysteresis effects were attributed in previous systems to osmotic and electroosmotic physical effects.33 , 39 here , important fractions of the involved charge are consumed at the end of the anodic and cathodic sweeps , giving very low angular displacements ( figure  4  c ) , pointing to an important contribution of the irreversible charge fractions detected in figure  ( 1 ) to the observed hysteresis .
the cooperative actuation in the asymmetric bilayer leads to a higher efficiency of the consumed charge describing larger angular displacements per unit of consumed charge , which is almost one order of magnitude higher than those described by each of the cp / tape bilayers . nevertheless , the use of cp blends including anions from large organic acids results in important hysteresis effects , which means complex control of the motor movement . in
the two studied aqueous electrolytes ( nacl and napf6 ) , the expansion and contraction of the ppy  hpps film dominates the reactiondriven bending movement of the asymmetric bilayer .
the ppy  dbs oxidation / reduction has a minor influence on the described amplitude of the angular displacement .
the resulting cooperative actuation , when both layers follow complementary volume changes ( swelling / shrinking or shrinking / swelling ) driven by the same reaction , leads to larger described angles per potential cycle than those attained from each of the ppy 
in addition to the electrochemical chemodynamic results , it is remarkable that , in napf6 aqueous solutions , the ppy  hpps film exchanges , under oxidation / reduction , pf6 
  with the solution for charge balance . in a previous paper , the same bilayer muscle in nacl aqueous solutions exchanged cations at the beginning of the oxidation sweep and anions at the end of the oxidation sweep .
we can conclude that only by changing the anion of the dissolved salt ( nacl vs. napf6 ) in ppy 
this is equivalent to saying that the presence of pf6 
  in solution modifies , at least , the ppy 
pps , and pf6 
 na interaction forces inside the reacting film , moving from weak na  pps interactions to strong ones ; na balancing the pps anion is not expelled during oxidation , forcing the entrance of pf6 
 , which is required to balance the positive charges generated on the ppy chains [ eq  ( 2 ) ] .
cps have been considered , during their electrochemical reactions in liquid electrolytes , as the simplest material models of the intracellular matrix ( icm ) dense gel of living cells.40 recently , it has also been discovered that by changing the solvent , but keeping the same dissolved salt , also changes the reactiondriven exchange of cations by exchange of anions.29 these results show that cps and bending artificial muscles constitute a good model for the study and quantification of intermolecular forces evolved during reactions in biomimetic reacting dense gels .
the three studied bilayer muscles are represented here as : tape / ppy  dbs , ppy 
hpps / tape , and ppy  hpps / ppy  dbs , indicating the relative ( left / right ) position of each layer component during experiments .
thus , clockwise / anticlockwise bending movements will be attained under the following reactiondriven volume changes : 
 from the tape / ppy  dbs bilayer by ppy 
hpps / tape bilayer by ppy  hpps shrinking / swelling , respectively.from the ppy 
hpps shrinking or ppy  dbs swelling ( clockwise ) versus preponderant ppy  hpps swelling or ppy  dbs shrinking ( anticlockwise ) . 
  from the tape / ppy  dbs bilayer by ppy 
hpps / tape bilayer by ppy  hpps shrinking / swelling , respectively . from the ppy 
hpps shrinking or ppy  dbs swelling ( clockwise ) versus preponderant ppy  hpps swelling or ppy  dbs shrinking ( anticlockwise ) .
figures  1  a  c show stationary coulovoltammetric ( q / e ) responses ( each closed loop minimum was taken as the charge zero origin ) from the tape / ppy  dbs , ppy 
hpps / tape , and ppy  hpps / ppy  dbs bilayer muscles , respectively , in 0.5  m napf6 aqueous solutions .
we used the specific charge ( charge consumed by reaction of the mass unit of cp working inside the electrolyte ) because , for artificial muscles exchanging anions11 or cations,34 both the rate of the movement and the angular position are linear functions of the driving current or specific charge , respectively , and , therefore , independent of the film thickness or muscle dimensions .
each bilayer q / e stationary response was obtained after consecutive cycles up to the same anodic potential limit of 0.7  v from a different cathodic potential limit , ranging between 0.6 and 2.0  v. positive charge increases describe oxidations , whereas negative charge increases are related to reduction reactions.35 each q / e response is comprised of two different parts : a closed coulovoltammetric loop , for which the charge ( maximum minus minimum ) increase gives the charge consumed by reversible oxidation / reduction of the ppy films , and an open part on the left side , for which a charge increase from the initial potential of the cycle to the final potential is the charge consumed by irreversible and slow hydrogen evolution from the organic acid ( hdbs or the hpps ) taking part in the cp.36 
  a )  coulovoltammetric responses from ppy 
dbs / tape bilayer artificial muscle in napf6 aqueous solutions obtained by increasing the cathodic potential limit ( from 0.6 to 2.0  v ) to the same anodic potential limit of 0.7  v. the same experiments were repeated for b )  ppy  hpps / tape muscle and c )  ppyhpps / ppy  dbs muscle . from the tape / ppy 
dbs bilayer ( figure  1  a ) , both reversible ( closed loop ) and irreversible ( open part ) charges increase for rising cathodic potential limits .
the reversible charge increase from 1 to 11  mc when the cathodic potential limit moves from 0.6 to 2.0  v , indicating that the polymer reduction occurs in the full studied potential range , up to high cathodic potential limits .
irrespective of the cathodic potential limit , most of the voltammetric charge is consumed by the reversible film oxidation and reduction reactions.35 
  in terms of the q / e response from the ppy 
hpps / tape bilayer muscles ( figure  1  b ) , the charge of the closed loop ( reversible charge ) increases ( from 4 to 6  mc ) when the cathodic potential limit shifts from 0.6 to 1.2  v. then , the reversible charge drops to 3  mc when the potential limit rises to 2.0  v. the irreversible charge always increases with the cathodic potential limit . the asymmetric bilayer shows
( figure  1  c ) an intermediate behavior ; the redox activity of the bilayer increases from 10 to 13  mc and then drops to 12  mc when the cathodic potential limit moves from 0.6 to 1.5 and then to 2.0  v , respectively .
the irreversible charge increase is very fast when the cathodic potential limit moves from 1.5 to 2  v. from the video frames recorded during the bilayers bending movement in parallel to the applied potential cycle , between 1.0 and 0.7  v at 10  mv  s , the dynamovoltammetric ( angle  potential , /e ) plots were attained ( figure  2  a from the tape / ppy  dbs bilayer , figure  2  b from the ppy  hpps / tape bilayer , and figure  2  c from the ppy  hpps / ppy  dbs asymmetric bilayer ) .
the images in figure  2  d show the asymmetric bilayer muscle position at each potential point , as indicated on the q / e response in figure  2  c , and the described angular displacements between the consecutive potential ( points on the q / e response ) . 
 
evolution of the described angles from the different bilayer artificial muscles : a )  ppy 
dbs / tape artificial muscles in 0.5  m napf6 aqueous solution during voltammetric experiments between 1.0 and 0.7  v at 10  mv  s at room temperature .
d )  images of the ppy  hpps / ppy  dbs artificial muscles in 0.5  m napf6 aqueous solution .
the three muscles present a similar /e behavior , that is , an anticlockwise bending movement during oxidation and clockwise bending movement during reduction .
this means that the ppy  dbs film from the tape / ppy  dbs bilayer shrinks during oxidation and swells during reduction ; the electrochemical reaction drives the exchange of na [ eq . 
( 1)]:(1)ppy0dbs - nc+nsmgelppyn+dbs - n+nc++ms+ne- 
  where ppy represents the neutral ppy chains , c are the cations exchanged to keep the electroneutrality , s are solvent molecules to keep the osmotic pressure balance inside the polymeric gel ( indicated by subindex gel ) , and ppy represents every oxidized ppy chain after removal of n electrons ( e).37 
  the amplitude of the described angle is only 4. in a previous paper , an angle of 50 was attained in nabr aqueous solutions under analogous potential cycling.32 this indicates a strong influence of the anion , although , according with equation  ( 1 ) , this influence could be neglected .
hpps / tape bilayer , the oxidationdriven clockwise angular displacement means that the ppy  hpps film swells during oxidation , shrinking during reduction to give anticlockwise movement .
therefore , the reversible electrochemical reaction drives the exchange of anions , a , ( pf6 
  here ) [ eq . 
( 2)]:33 
 ( 2)ppy0hpps+na-+msppyn+hppsa - nsmgel 
  again , a strong influence of the anions emerges , when compared with the results attained from nacl aqueous solutions , whereby cations were exchanged at the beginning of the anodic potential sweep and anions at the end of the sweep ; for the results obtained from nabr solutions , only the anion is exchanged in the full potential range . as a consequence ,
the amplitude of the angle described here per cycle was 24 and only 14 in the previous work ( cation exchange followed by anion exchanges gives opposed bending movements during the anodic sweep ) .
as a partial conclusion , in napf6 aqueous solution during oxidation , the ppy  dbs film shrinks and the ppy  hpps film swells .
thus , when the asymmetric ppy  hpps / ppy  dbs bilayer was checked in napf6 aqueous solution , both films developed a cooperative dynamic effect ; when one of the layers swells and pushes the bending bilayer under oxidation , the second layer shrinks through oxidation and trails the muscle giving larger angular displacements .
structural changes and resulting forces reverse during the bilayer reduction ( figure  3 ) . as a consequence of this cooperative actuation of the two constitutive layers ,
the angle described by the asymmetric bilayer per voltammetric cycle is 133 , which is more than six times the amplitude described per cycle for the ppy 
hpps / tape muscle ( 22 ) and over 33  times that described by the tape / ppy  dbs muscle ( 4 ) when cycling in the same potential range .
a second consequence of the cooperative actuation of the two layers in napf6 is a very low creeping effect of 0.3 per cycle . 
 
scheme showing the cooperative actuation of the bending ppy  hpps / ppy  dbs asymmetric bilayer muscle .
points  ( 2 ) and ( 3 ) correlate with both the dynamovoltammograms shown in figure  2 and the coulodynamic evolution of the described angles shown in figure  4 . in conclusion ,
owing to the cooperative actuation , when both layers follow complementary volume changes ( swelling / shrinking or shrinking / swelling ) driven by the same electrochemical reaction , the angular displacement per potential cycle is greater than those attained from each of the ppy 
by combining figures  1 and 2 , the evolution of each muscle position ( angle ) as a function of the specific consumed charge ( coulodynamic evolution ) in napf6 aqueous solutions is attained , as depicted in figures  4  a  c . the ppy 
hpps / ppy  dbs muscle angular displacement per cycle is 133 ( figure  4  c ) , which is several times higher than those described by the ppy  hpps / tape bilayer ( 22 per cycle ; figure  4 
b ) or by the ppy  dbs / tape bilayer ( 4 per cycle ; figure  4  a ) in the same electrolyte .
the coulodynamic efficiencies ( slopes ) result in 8.5 , 3.0 , and 1.0  mc for the asymmetric bilayer , the ppy  hpps / tape bilayer , and the ppy 
the magnitudes , described angle per cycle , and coulodynamic efficiency quantify the cooperative dynamical effect of the two layers in the asymmetric bilayer muscle . 
 
relationship between the bending angle described for the muscles and the electrical charges consumed during voltammetric experiments studying reactions of different artificial muscles between 1.0 and 0.7  v at 10  mv  s at room temperature : a )  ppy 
hpps / tape , and c )  ppyhpps / ppy  dbs artificial muscles in napf6 aqueous solution . both anodic and cathodic displacements show a linear dependence on the charge , with a similar slope , corresponding to any faradaic process driven by the reactions given in equations  ( 1 ) and ( 2 ) .
the charge spent at the beginning and at the end of the oxidation shows a very low efficiency toward bending the muscle .
these charges can be related to hydrogen evolution from the film , producing acids , at the most cathodic potentials and to the elimination of protons at the most anodic potentials.36 , 38 both reactions result in very low volume variations . in napf6 aqueous solution ,
all of the studied cp / tape bilayers behave as faradaic motors ; the amplitude of the described angle ,  , is a linear function of the consumed charge , q [ eq .  (
3)]:(3)=0+kq 
  where  
 0 is the initial angle and k (   mc ) is a constant dependent on the system ( cp , electrolyte , tape , or second cp film ) .
nevertheless , the actuation of the asymmetric bilayer presents an important hysteresis effect ; for the same charge , the muscle positions during the anodic and the cathodic sweeps are quite different .
smaller hysteresis effects were attributed in previous systems to osmotic and electroosmotic physical effects.33 , 39 here , important fractions of the involved charge are consumed at the end of the anodic and cathodic sweeps , giving very low angular displacements ( figure  4  c ) , pointing to an important contribution of the irreversible charge fractions detected in figure  ( 1 ) to the observed hysteresis .
the cooperative actuation in the asymmetric bilayer leads to a higher efficiency of the consumed charge describing larger angular displacements per unit of consumed charge , which is almost one order of magnitude higher than those described by each of the cp / tape bilayers . nevertheless , the use of cp blends including anions from large organic acids results in important hysteresis effects , which means complex control of the motor movement . in
the two studied aqueous electrolytes ( nacl and napf6 ) , the expansion and contraction of the ppy  hpps film dominates the reactiondriven bending movement of the asymmetric bilayer .
the ppy  dbs oxidation / reduction has a minor influence on the described amplitude of the angular displacement .
the resulting cooperative actuation , when both layers follow complementary volume changes ( swelling / shrinking or shrinking / swelling ) driven by the same reaction , leads to larger described angles per potential cycle than those attained from each of the ppy 
in addition to the electrochemical chemodynamic results , it is remarkable that , in napf6 aqueous solutions , the ppy  hpps film exchanges , under oxidation / reduction , pf6 
  with the solution for charge balance . in a previous paper , the same bilayer muscle in nacl aqueous solutions exchanged cations at the beginning of the oxidation sweep and anions at the end of the oxidation sweep .
we can conclude that only by changing the anion of the dissolved salt ( nacl vs. napf6 ) in ppy 
this is equivalent to saying that the presence of pf6 
  in solution modifies , at least , the ppy 
pps , and pf6 
 na interaction forces inside the reacting film , moving from weak na  pps interactions to strong ones ; na balancing the pps anion is not expelled during oxidation , forcing the entrance of pf6 
 , which is required to balance the positive charges generated on the ppy chains [ eq  ( 2 ) ] .
cps have been considered , during their electrochemical reactions in liquid electrolytes , as the simplest material models of the intracellular matrix ( icm ) dense gel of living cells.40 recently , it has also been discovered that by changing the solvent , but keeping the same dissolved salt , also changes the reactiondriven exchange of cations by exchange of anions.29 these results show that cps and bending artificial muscles constitute a good model for the study and quantification of intermolecular forces evolved during reactions in biomimetic reacting dense gels .
we have presented a comparative coulovoltammetric ( q / e ) , dynamovoltammetric ( /e ) , and coulodynamic ( q/ ) study in napf6 aqueous solution of the asymmetric polypyrrole bilayer muscle ( ppy  hpps / ppy  dbs ) and the two bilayer muscles tape / ppy  hpps and ppy  dbs / tape .
the q/ responses indicate that , in the tape / ppy  hpps bilayer muscle , the actuation originates from reactiondriven exchange anions ( pf6 
 ) , swelling upon oxidation and shrinking upon reduction .
the ppy  dbs / tape bilayer actuation in napf6 aqueous solution drives the exchange of the cation ( na ) , shrinking upon oxidation and swelling upon reduction .
hpps / ppy  dbs asymmetric bilayer muscle , with a cooperative actuation ( dynamic synergy ) increasing the angle described per unit of charge .
high cooperative actuation effects allow the development of more efficient polymeric motors for different medical tools or for the development of zoomorphic and anthropomorphic soft robots .
the use of polymeric blends with organic anions or organic salts leads to large hysteresis actuating effects in aqueous solutions ; for the same charge , the muscles position is different during the anodic potential sweep compared to that during the cathodic sweep .
the presence of hysteresis effects during actuation results in complex control of the motor movement .
new families of cps13 , 37 , 41 , 42 must be explored to find the most important cooperative actuations in the absence of these important hysteresis effects , with minimum creeping effects .
reactants , equipment , used electrochemical methodologies , electrogeneration of the cp monolayer and bilayer films , as well as the construction of the cp / tape bilayers were described in previous papers.32 , 33 
  ppy  dbs films were cut into 20  mm1  mm strips , each one was 20  m thick and had a mass of 0.532010  mg .
a paint ( max effect , maxfactor ) strip , 6.012.0  mm from the upper border , prevents direct contact between the electrolyte ( by capillarity ) and the metallic clamp that allows the electrical contact.32 , 33 the ppy  dbs / tape bilayer muscle inside the solutions was 8  mm long .
hpps films were also cut into 20  mm1  mm strips , each one was 20  m thick and had a mass of 0.137015  mg .
hpps / ppy  dbs bilayer films were cut into 15  mm1  mm strips and were 20  m thick and had a mass of 0.3493  mg .
each film was painted with a transversal paint strip on both film sides , 2.07.0  mm of the upper border .
 abstractthree bilayer muscles [ polypyrrole  paraphenolsulfonic acid / polypyrrole  dodecylbenzensulfonic acid ( ppy  hpps / ppy  dbs ) asymmetric bilayer , ppy  hpps / tape , and ppy  dbs / tape ] were characterized during potential cycling in napf6 aqueous solutions . in parallel , 
 the angular displacement of the muscle was videorecorded . 
 the dynamovoltammetric ( angle  potential ) and coulodynamic ( charge  potential ) results give the reactiondriven ionic exchanges in each ppy film . 
 electrochemical reactions drive the exchange of anions from the ppy  hpps layer and cations from the ppy  dbs layer . 
 this means that both layers from the asymmetric bilayer follow complementary volume changes ( swelling / shrinking or shrinking / swelling ) , owing to complementary ionic exchanges ( entrance / expulsion ) driven by the bilayer oxidation or reduction . 
 the result is a cooperative actuation ; the bending amplitude described by the asymmetric bilayer muscle is one order of magnitude larger than those attained from each of the conducting polymer / tape muscles . 
 the cooperative actuation almost eliminates creeping effects . 
 a large dynamical hysteresis persists , which can be attributed to an irreversible reaction of the organic acid components at high overpotentials . 