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Polypyrrole actuator based on electrospun microribbons Mihaela Beregoi, Alexandru Evanghelidis, Victor Diculescu, Horia Iovu, and Ionut Enculescu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13196 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Applied Materials & Interfaces

Polypyrrole actuator based on electrospun microribbons Mihaela Beregoi¶,˥‡, Alexandru Evanghelidis¶,˩‡, Victor C. Diculescu¶, Horia Iovu˥, Ionut Enculescu¶* ¶

National Institute of Materials Physics, Multifunctional Materials and Structures Laboratory,

Functional Nanostructures Group, 405A Atomistilor Street, 077125, Magurele, Ilfov, Romania ˥

University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science,

Department of Bioresources and Polymer Science, Advanced Polymer Materials Group, 1-7 Gh Polizu Street, 011061, Bucharest, Romania ˩

University of Bucharest, Faculty of Physics, 405 Atomistilor Street, 077125, Magurele, Ilfov, Romania

KEYWORDS: polypyrrole, micro-ribbons, actuation, electrospinning, biomimetics

ABSTRACT: The development of soft actuators by using inexpensive raw materials and straightforward fabrication techniques, aiming at creating and developing muscle like micromanipulators, represents an important challenge nowadays. Providing such devices with biomimetic qualities e.g. sensing different external stimuli adds even more complexity to the

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task. We developed electroactive polymer coated micro-ribbons that undergo conformational changes in response to external physical and chemical parameters. These were prepared following three simple steps. During the first step nylon 6/6 micro-ribbons were fabricated by electrospinning. In a second step the micro-ribbons were one side coated with a metallic layer. Finally, a conducting layer of polypyrrole was added by means of electrochemical deposition. Strips of polypyrrole coated aligned micro-ribbon meshes were tested as actuators responding to current, pH and temperature. The electrochemical activity of the micro-structured actuators was investigated by recording cyclic voltammograms. Chronopontentiograms for specific current, pH and temperature values were obtained in electrolytes with different compositions. It was shown that upon variation of the external stimulus, the actuator undergoes conformational changes due to the reduction processes of the polypyrrole layer. The ability of the actuator to hold and release thin wires, and to collect polystyrene micro-spheres from the bottom of the electrochemical cell was also investigated.

1. Introduction Movement is an essential ability of complex life-forms provided and constantly improved over millions of years of trial and error by evolution. The forms of displacement which appear under natural circumstances often result in solutions of uttermost elegance. As a design philosophy, biomimetics aims to draw on such natural solutions to solve engineering problems. In the particular case of actuation, this led to the development of artificial muscles, materials and devices inspired by the mechanisms which propel animals on Earth's surface, underwater, or in the air. Much like their biological counterpart, artificial muscles are expected to contract and relax on command, as well as to be sufficiently flexible, in order to permit a wide range of movement patterns. Electroactive polymers (EAPs) fulfill both these criteria, while offering the


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advantages of being processable at nanoscale and furthermore, possessing sensing capabilities, which expands the realm of their possible applications.1-3 Polypyrrole (PPy) is one such polymer, which has been extensively studied as actuators’ component, but also as a sensor, showing promising characteristics in both cases.4–6 The most common actuator configuration in which EAPs are used is the bi- or multilayer actuator, in which the electroactive film is attached or deposited to a substrate.7–9 The main purpose of the substrate is to mechanically counteract the EAP's changes in volume, thus forming a stress gradient in the whole structure, which then leads to displacement through bending.10,11 When conductive, the substrate can also serve as an electrode. Although theoretical models of various degrees of complexity have been proposed for such actuators, most of them build upon Timoshenko's theory for bimetallic strips, so that the elastic properties of the layers and the ratios between their thicknesses appear to be key factors in describing the bending motion.12-16 Being an ionic EAP, PPy changes its volume as ions enter or leave its structure during a diffusive process strongly dependent on surface area.17,18 As a micro-structured material, electrospun fiber meshes offer a large surface area in a small volume, therefore being ideal candidates for ionic EAP actuator substrates. Electrospinning is an old fiber production method which has received renewed interest over the last two decades, with the continuous advance of nanotechnology. Relying on an intense electrostatic field and on the electrohydrodinamic instability of a charged polymer solution jet, the method can easily produce fibers with submicronic diameters, usually in the form of nonwoven meshes.19,20 Recent adaptations of the method can also reliably produce meshes with anisotropic microstructure as well as different fiber morphologies. Of particular interest in the case of EAP actuators are electrospun meshes of aligned polymeric ribbons. The process by


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which fibers turn to ribbons is believed to be driven by the formation of a thin solid skin from the outer layer of the polymer solution jet which collapses under atmospheric pressure and as the solvent trapped inside escapes, thus creating flattened fibers, i.e. ribbons.21 The phenomenon is not limited to a particular polymer or, indeed, to electrospinning, having been observed in classical fabrication processes as well.22-25 Solution viscosity appears to be a key factor, with higher polymer concentrations being favorable, while relative humidity and temperature also play an important role, due to their large influence on solvent evaporation rate. In this work, the fabrication and characterization of a new actuator configuration based on one side PPy coated electrospun micro-ribbons is presented. Poly(hexamethylene adipamide) (nylon 6/6) electrospun micro-ribbons with moderate alignment and having the width in the 1-1.5 µm range were fabricated. Adding a thin layer of metal on these micro-ribbon meshes turns them conductive and, therefore, usable as working electrodes, both for the electrodeposition of PPy, and further for controlling its electroactivity (ensuring a uniform distribution of the applied potential/current along ribbons). The deposition of PPy was performed using a common electrochemical cell with three electrodes, employing the ribbon nets as micro-structured working electrodes. Afterwards, PPy coated micro-ribbon net strips were morphologically and electrochemically analyzed. It was found that the actuator curls when an anodic current is applied or pH or temperature is increased. The registered chronopotentiograms showed that the actuator also senses the modification of the external stimuli. The ability of these micro-ribbons to manipulate micro-spheres, as well as to hold and release a piece of copper wire was investigated. In the case of micro-sphere manipulation, an adhesive based on mussel protein was used as a sticky connection to the ribbons.


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3. Experimental section 3.1 Materials Nylon 6/6, formic acid (≥ 88 %), LiClO4 (battery grade, dry, 99.99% metals basis), NaCl (≥ 99.5 %), HCl (37 %), NaOH (≥ 98 %), phosphate buffer saline (PBS, which contains NaCl 0.138 M and KCl 0.0027 M with the pH 7.4, for molecular biology) and micro-particles size standard based on polystyrene (PS) monodisperse (with diameters of about 1 µm) from Sigma-Aldrich, pyrrole and acetonitrile from Merck and the adhesive mussel protein from AcroBiosystems were used without further purification. The pH of PBS-based electrolyte was adjusted using 1 M HCl and 1 M NaOH aqueous solutions. The pH was measured using Hanna HI 83141 pH-meter. All solutions were prepared using ultrapure Millipore water.

3.2 Preparation of the PPy coated electrospun micro-ribbons Fabrication of the one side PPy coated micro-ribbons involves three main steps. In the first step, the nylon 6/6 ribbons were obtained using a classical electrospinning setup, feeding a 30% w/v solution of nylon 6/6 in formic acid. Tip-to-collector distance was varied between 15-20 cm, the applied voltage to the spinerette was 20 kV ± 2 kV, the solution feed rate was varied between 0.05–0.10 ml/hr, and deposition time between 30 and 60 minutes. All parameters were adjusted accordingly, in order to ensure continuous spinning under given atmospheric conditions. The micro-ribbons were deposited free-standing on copper wire frames attached to the grounded plate collector. The parallel bars geometry of the frames naturally leading to a moderate alignment of the micro-ribbons. In the second step, partially aligned micro-ribbons attached on copper frames were covered with a gold layer only on one side by DC magnetron sputtering in order to make them conductive


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for depositing PPy (only on the side with gold). As intermediate step, for a better contact and to avoid unwanted reactions, the gold coated micro-ribbons were transferred from copper to overlaying stainless steel frames, onto which the micro-ribbons nets were mechanically gripped. As last step, the gold coated micro-ribbon nets attached on stainless steel frames were covered with a thin PPy layer using a method described by T.F. Otero and M. T. Cortes.26 Briefly, PPy was electrogenerated using a solution consisting of 0.2 M pyrrole and 0.1 M LiClO4 in acetonitrile with 2 % (v/v) water content by applying 0.872 V for 70 s. The working electrode was a free-standing micro-ribbon net attached on stainless steel frames, having the active surface area of about 3 × 2 cm (width × length). The micro-ribbons were immersed in the deposition solution only 1 cm, leaving an uncoated length of around 1 cm which served as contact for the actuator configuration. The counter electrode was centered in front of the gold coated side of the ribbons, in order to maximize the growth of PPy only on the gold covered side. Next, the ribbon nets were dried in air and used by taking samples from them with the size of about 0.1-0.2 × 2 cm (width × length).

3.3 Adhesive mussels’ protein modification The tips of the PPy coated micro-ribbons were immersed in adhesive mussels’ protein and left for about 5 min. to evaporate the solvent.

3.4 Characterization The electrospun micro-ribbons before and after PPy deposition and PS micro-spheres collection process, were morphological characterized using a Ziess EVO 50 scanning electron microscope (SEM).


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The electrochemical investigations by cyclic voltammetry (CV) and chronopontentiometry, as well as the PPy deposition were carried out using a Voltalab PGZ 100 and Parstat 2273 Princeton Applied Research potentiostats, with a three electrode electrochemical cell. The moderately aligned PPy coated micro-ribbons was used as micro-structured working electrode, a platinum plate as counter and a commercial saturated calomel reference electrode (SCE). The uncoated sections of the samples were attached using conductive silver paste to platinum strips in order to provide the electric contact of the working electrode (Figure 1)

Figure 1. Schematic representation of the actuator configuration.

The free-standing PPy coated micro-ribbons were immersed in the electrolyte only with the PPy coated sections (having a geometric area of about 0.1-0.2 cm2). Before any electrochemical experiment, three cyclic voltammograms with a scan rate of 50 mV/s were registered for removing any past structural memory. The pH and current dependences were registered at the temperature of solutions 25 ± 2oC. The weight of the copper wire and ribbons strip was estimated using a Kern 770 microbalance with five decimals. The digital photos and movies were recorded using a Canon DS126201 professional camera.


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2. Results and discussion 2.1 Morphological characterization Digital photos of the ribbon nets corresponding to each fabrication step (as described in 3.2 Preparation of the PPy coated electrospun micro-ribbons) are presented in Figure 2.

Figure 2. Digital photographs of (a) as spun nylon 6/6 micro-ribbons, (b) gold coated mesh and (c) PPy covered metalized net, all attached to stainless steel frames.

The morphological analysis of the gold covered electrospun micro-ribbons before and after coverage with PPy is shown in Figure 3a, aˈ and Figure 3b, bˈ, respectively. The gold coated electrospun micro-ribbons with widths in the range 1-1.5 µm are partially aligned, and show a smooth surface, the gold layer thickness being ~ 100 nm. A high density of micro-ribbons can be observed in SEM images, however this parameter can be modified depending on the application by changing the collection time of micro-ribbons on copper frames during the electrospinning process. Likewise, the PPy coated micro-ribbons present areas with smooth surface. The thickness of the PPy layer is ~ 80 nm. Moreover, it can be seen that these microstructures present large surface-to-volume ratios, which facilitate the ionic exchange during the actuation or sensing processes.


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Figure 3. SEM images at high and low magnifications of (a, a') the gold coated micro-ribbons and (b, b') PPy coated metalized structures.

2.2 Electrochemical characterization Figure 4a shows the CV of free-standing PPy coated micro-ribbons, recorded at different scan rates between -0.60 and +0.60 V in 1 M NaCl. The CV response displays one pair of peaks (at 0.28 and -0.45 V) which are attributed to the oxidation/reduction of the PPy film.27 By increasing the scan rate, the anodic peak moves towards positive potential values, while the cathodic peak moves towards negative potential values. The intensity of the cathodic and anodic peaks has a linear increase with the square root of the scan rate which indicates that the mechanism is controlled by the diffusion processes (Figure 4b). The dependence of the redox process on pH and temperature was investigated in PBS or 1 M NaCl, respectively (Figure 4c, d). The CVs registered in various media with different pH values reveal the characteristic peaks of PPy which increase with the raise of pH and move toward more


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negative potentials. The temperature changes of the electrolyte are also felt by the actuator, thus, when the temperature of the electrolyte increases, the anodic and cathodic current peaks diminish in intensity.

Figure 4. (a) CV of the PPy coated micro-ribbons during actuation, recorded at different scan rates in 1 M NaCl. (b) Representation of the peak currents versus squared root of scan rate where R2 is the correlation coefficient. (c,d) CV of the PPy coated micro-ribbons in: (c) PBS with different pH values at ambient temperature and (d) 1 M NaCl at different temperatures; scan rate of 50 mV/s.


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The electroactivity and sensing capabilities of PPy based micro-ribbons in terms of applied current, pH and temperature were tested by registering the chronopotentiograms in either 1 M NaCl at various temperature or PBS with different pH values.

2.2.1 Electroactivity The chronopotentiograms recorded during oxidation and reduction at different current values are shown in Figure 5a, b. In both cases, the chronopotentiograms show typical aspects with a fast increase during ~ 10 s then reaching constant values for at least 60 s, in agreement with high transition times and high load of electroactive specie. In these conditions, the current is initially used in order to charge the electrolytic double layer whereas in the second step the current leads to oxidation or reduction reactions which proceed at constant rate on the timeframe of the experiment. An exception was observed at high current densities, e.g I > -70 µA, where the electroactive specie is totally reduced/oxidized allowing capacitive contribution. Digital photographs of PPy coated samples with different sizes, taken during oxidation and reduction are presented in Figure 5c, e and 5d, f, respectively. The electroactive micro-ribbons expand and shrink by switching the applied current or potential, following a typical mechanism. First, the conformational changes of the polymer chains are due to the insertion/expulsion of the ions from the electrolyte and electrons from gold. Secondly, the asymmetry and anisotropy of the PPy covered micro-ribbons leads to irregular volume changes inducing the straightening/twisting of the entire piece of net.28,29 Thus, the deposition of the electroactive polymer on only one side of the micro-ribbons means that each individual ribbon behaves as a bilayer actuator. In addition, the good actuation properties observed (i.e. ample movement and high speed) can be attributed to the high surface to volume ratio of the micro-ribbon structures, which facilitates the diffusion


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process and also to the gold layer which allows a homogenous distribution of the applied potential. Moreover, it was found that by increasing the width of the micro-ribbon strips the spiraling motion (Figure 5c, d) is replaced by a rolling movement (Figure 5e, f), possibly because the number of bilayered micro-ribbons integrated in actuator increases. The actuation potentials are in the range ± 0.60 V and currents up to ± 100 µA. The response time is 5 s with the maximum movement, but it can also respond in a much shorter time by diminishing the displacement.

Figure 5. (a,b) Chronopotentiograms at different current values registered in 1 M NaCl during 60 s. (c-f) Photographs of the PPy coated micro-ribbons when (c and e) oxidized and (d and f) reduced showing the (c, d) spiraling motion of narrow and (e, f) rolling movement of wide micro-ribbons.


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No changes in morphology were observed after series of hundreds of cycles of movement, both at macroscopic scale or by SEM observation.

2.2.2 pH sensing The actuation properties of the electroactive microstructures were investigated in media with different pH values. The PPy coated micro-ribbons sense pH changes and the transition from acid to alkaline media leads to the curling of the ribbon strips, without applying any potential or current. The influence of pH on the actuation properties was investigated by recording, chronoapotentiograms in PBS media with different pH values for a current flow of ± 5 µA over 120 s (Figure 6a, b). Thus, the potential turned more electronegative with increasing the pH value. The dependence is linear with slopes of -35 and -15 mV per pH unit for the oxidation and reduction processes, respectively. In Figure 6 c, d are displayed the digital photographs of the micro-structured actuator in acid and basic medium without applying any current. In acid pH medium, the micro-ribbon strip remains straight, but as the pH increases, it starts to curl, this being an almost instantaneous process. This actuation of the micro-ribbons as a function of pH could be associated with the protonation/deprotonation processes and further correlated with the volume changes of the electroactive film. In acid media, the protonation of the PPy occurs through the interaction between the nitrogen and electrolyte species. When the pH is increased, the loss of ions, conductivity and deprotonation take place and, in the presence of hydroxyl anions, the nitrogen loses its availability of conjugation.30-32


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Figure 6 (a, b) Chronopotentiograms at ± 5 µA recorded for 120s at ambient temperature in PBS with different pH values; (c,d) digital photographs of PPy coated micro-ribbons immersed in (c) acid and (d) basic electrolytes, without applied current.

2.2.3 Temperature sensing The behavior of the PPy based actuator during the heating of the electrolyte (1 M NaCl) was investigated. Chronopotentiograms were recorded as a function of temperature, Figure 7a, b. It was found that for a constant current of +5 µA, the applied potential decreases by increasing the temperature. The reverse process is observed when the PPy is reduced by applying -5 µA and the potential decreased with temperature increases. As well, the reaction rate follows an Arrhenius dependence.


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Photographs of the actuator based on PPy coated micro-ribbons in cold and warm 1 M NaCl aqueous solution are shown in Figure 7c, d. A curling motion of the micro-ribbons when no current is applied while increasing the temperature was observed. The mechanism of this behavior could rely on the fact that by raising the electrolyte temperature, a deprotonation process similar with that observed in the case of pH tests occurs. Thus, the permeability of PPy film to water and oxygen is increasing, leading to nitrogen inactivation, the PPy film losing ions and consequently the actuator shrinks.33,34 This temperature dependent actuation should also be related to the multilayer structure of the ribbons.

Figure 7. (a and b) Cronopotentiograms at ± 5 µA recorded during 120 s in 1M NaCl pH = 7.0 at different temperatures. (c and d) Snapshots of the PPy micro-ribbons at (c) 10°C and (d) 30°C, without applied current.


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2.3. Applications The ability of an actuator based on PPy coated micro-ribbons to collect PS micro-spheres from the bottom of an electrochemical cell was tested using a 1 M NaCl aqueous solution as electrolyte. For this purpose, the tip of the micro-ribbons was coated with a thin layer of adhesive mussel protein. It is important to highlight that this kind of ability could be extended to cells, bacteria or other micro- or nanoobjects.35 Figure 8a, a' presents the snapshots taken during the collection process, when the sample is oxidized and reduced by applying successive potential pulses of -0.60 and +0.60 V. It can be noticed that when the PPy film expands, the micro-ribbons unfold, reaching the bottom of the cell where PS micro-sphere agglomerations are placed. Further, the micro-sphere agglomerations stick to the micro-ribbon tips due to the presence of the adhesive. This natural glue is based on proteins extracted from the mussel foot. These proteins produce adhesion due to the presence of L-3,4-dihydroxyphenylalanine amino acids with hydroxyl groups able to interact with solid substrates and generating the sticking.36,37 After several actuation cycles we tested the micro-spheres harvesting processes. Thus, the sample was taken out of the electrolyte, rinsed with water and dried. The SEM images at high and low magnification of the micro-ribbon based collector are presented in Figure 8b ,b'. It can be noticed that the micro-spheres are attached to the micro-ribbons in relatively high densities. As well, it is important to note that no cracks or exfoliation of PPy or gold layer appear after the actuation process, showing that the actuator is stable over many applied potential pulses.


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Figure 8. (a, a') Digital photographs of the PPy coated micro-ribbon mesh strip collecting PS micro-spheres from the bottom of the electrochemical cell; (b, b') SEM images at high and low magnification of the sample after the actuation and harvesting processes.

Further, we tested another ability of the actuator based on PPy coated micro-ribbons, namely to hold a small object (a piece of copper wire) with a weight twenty times higher than its weight (copper wire weight = 880 µg versus micro-ribbon strip (~ 0.2 x 2 cm width x lenght) weight ≤ 40 µg) by applying negative potential (-0.60 V) for the specified time (Figure 9a). When the potential is switched to +0.60 V, the actuator relaxes and releases the piece of metal (Figure 9b), a behavior explained through the shrinking/swelling of the PPy film. After this process, the actuation properties of the material are not affected, no cracks of the micro-ribbons appear,


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proving its capacity to manipulate heavy objects with millimeter size while preserving its motion properties.

Figure 9. Digital photos of a PPy coated micro-ribbon net strip (a) holding and (b) releasing a piece of copper wire in 1 M NaCl when the potential was switched from -0.60 to +0.60 V.

4. Conclusions In conclusion, a novel actuator configuration based on one side PPy coated moderately aligned micro-ribbons was fabricated. The fabrication process involved several reproducible steps, namely the electrospinning of the nylon 6/6 solution, the sputter covering of the as spun ribbons with a gold layer in order to use them as working electrodes for depositing PPy. Further, strips from the PPy coated ribbons meshes were used as a micro-structured actuator and were subjected to different stimuli such as current, pH and temperature. The fabricated actuator responds by curling and straightening when the external stimulus was applied. The actuation mechanism was identified and explained by the insertion/expulsion of the anions and cations in/out of the PPy molecular chains, process facilitated by the high aspect


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surface-to-volume ratio, as well as the anisotropy of the deposited PPy film (only on one side of the ribbons). The ability of the actuator to collect micro-spheres from the bottom of the electrochemical cell and to hold/release a piece of copper was also demonstrated. The described method developed for fabricating actuators could be tailored depending on the desired application, by depositing other metallic layers, conducting polymers or manufacturing other actuator configurations for biomedical applications like micro-tweezers, cell harvesting, artificial muscles or micro-manipulators.

Supporting Information Video 1: Strip of ribbon mat spiraling at 5 second pulses. Video 2: Strip of ribbon mat spiraling at 1 second pulses. Video 3: strip of ribbon mat releasing copper wire. Video 4: Strip of ribbon mat harvesting polystyrene spheres from the bottom of a beaker. Corresponding Author *E-mail: [email protected] Author Contributions ‡Mihaela Beregoi and Alexandru Evanghelidis contributed equally to the present work. Funding Sources Notes


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ACKNOWLEDGMENT The research was financially supported by the Core Founding Programs, Romanian Ministry of Research and Innovation, contract no. PN16-480102 and project PED 128/2017. ABBREVIATIONS

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