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Bioplastic electromechanical actuators based on biodegradable poly(3-hydroxybutyrate) and cluster-assembled gold electrodes
Accepted Manuscript Title: Bioplastic electromechanical actuators based on biodegradable Poly(3-hydroxybutyrate) and cluster-assembled gold electrodes Authors: Lorenzo Migliorini, Tommaso Santaniello, Sandra Rondinini, Paolo Saettone, Mauro Comes Franchini, Cristina Lenardi, Paolo Milani PII: DOI: Reference:
S0925-4005(19)30169-8 https://doi.org/10.1016/j.snb.2019.01.141 SNB 26067
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
5 December 2018 8 January 2019 27 January 2019
Please cite this article as: Migliorini L, Santaniello T, Rondinini S, Saettone P, Comes Franchini M, Lenardi C, Milani P, Bioplastic electromechanical actuators based on biodegradable Poly(3-hydroxybutyrate) and cluster-assembled gold electrodes, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.01.141 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioplastic electromechanical actuators based on biodegradable Poly(3hydroxybutyrate) and cluster-assembled gold electrodes. Lorenzo Migliorini1,2, Tommaso Santaniello2,* [email protected], Sandra Rondinini1, Paolo Saettone3, Mauro Comes Franchini3,4, Cristina Lenardi2, Paolo Milani2,* [email protected] 1
Department of chemistry, University of Milan, Via Golgi 19, 20133, Milan, Italy
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CIMaINa, Department of physics, University of Milan, Via Celoria 16, 20133, Milan, Italy
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Bio-On spa. Via Santa Margherita al Colle 10/3 40136 Bologna Italy
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Department of Industrial Chemistry "Toso Montanari”, University of Bologna, Viale
Risorgimento 4, 40136 Bologna, Italy *
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Corresponding authors:
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- Bioplastic-based electromechanical actuators.
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HIGHLIGHTS
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- Poly(3-hydroxybutirate) and ionic liquid-based ionogels using solvent casting from acetic acid. - Fabrication of compliant cluster-assembled gold electrodes
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- Natural-derived, soft, durable and water-resistant electromechanical actuators
ABSTRACT: We present the fabrication and characterization of electroactive actuators based on natural-
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derived bioplastic able to display reversible bending deformations in response to low intensity electric fields. We used biodegradable poly(3-hydroxybutyrate) (PHB) blended with a suitable ionic liquid to
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produce freestanding electroactive ionogel layers through a solvent casting process employing non-toxic solvents. The ionogels were provided with compliant cluster-assembled gold electrodes fabricated by
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means of supersonic cluster beam deposition (SCBD). These metal layers exhibit tailored electrical properties, large surface areas and mechanical resiliency against deformations. The manufactured actuators were characterized in terms of mechanical, morphological, electrochemical and electromechanical properties. We show that the mechanical properties of the ionogels are similar to those of low density polyethylene, while the actuators demonstrated large bending displacement and stable operational durability. Furthermore, due to the use of PHB in combination with a hydrophobic ionic liquid, the actuators did not exhibit hygroscopic behaviour.
KEYWORDS Polyhydroxybutyrate; bioplastic; biopolymers; soft robotics; smart materials, electromechanical actuators.
ABBREVIATIONS AcOH, acetic acid; BMIM(Tf2N), 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; EIS,
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electrochemical impedance spectroscopy; IEAP, ionic electroactive polymer; IPMC, ionic polymer-metal composite; PET, polyethylene terephthalate; PHB, poly(3-hydroxybutyrate); PMCS, pulsed microplasma cluster source; PVDF, polyvinylidene fluoride; SCBD, supersonic cluster beam deposition; SEM,
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scanning electron microscopy; TBAF, tetrabuthylammonium fluoride.
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1. INTRODUCTION
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Industrial and technological mass production and manufacturing processes consume non-renewable resources and the disposal of a large number of consumer-graded products generates damage for the
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environment. The importance of environmental sustainability leads to a new challenge: to find naturalderived and biodegradable materials to be used as building blocks for the development of innovative green
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devices and technological solutions[1].
In soft robotics traditional rigid materials are replaced by soft and deformable ones to fabricate systems
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with embedded sensing and actuation capabilities, mimicking the behavior of a living organism[2–5]. “Sustainable” soft robots and functional devices that can be produced, used and disposed with minimal
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environmental impact should be manufactured using benign materials and composites derived from renewable resources and safely degradable[6].
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Ionic electroactive polymers (IEAPs)[7–9] can be employed as electro-mechanical actuators and
sensors in soft robotics and in a variety of forefront technological applications, including bioengineering and biotechnology[10]. IEAPs are cross-linked polymeric networks, provided with covalently bound ionic groups, whose electric charge is balanced by inorganic counterions[7]. Thin IEAP films are usually impregnated with an ionic conductive liquid phase (aqueous or organic) and sandwiched between thin, adherent and flexible electrodes[8]. Among these liquid media, ionic liquids are widely employed as a
greener alternative to traditional organic solvents[11,12]. Under the application of an electric field, an asymmetric ionic migration occurs across the polymeric matrix, according to the size and charge of the mobile species. This results in a higher concentration of the liquid phase at one of the electrodes, inducing a localized swelling and a macroscopic bending deformation of the system. These actuators can readily and reversibly convert low power electric signals into mechanical work. Many IEAPs have been studied and produced in the last decades, but very few of them match the
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requirements of environmental sustainability coupled with operational stability in different working conditions. Perfluorinated polymers like Nafion or PVDF represent one of the most performant solutions, but their monomers are toxic and carcinogenic, their fabrication requires high costs, high temperatures and pollutant solvents, and the products are unsafe and not easily degradable[13–19]. Less toxic polymers have also been used[20–23], such as polyvinylic species, but they still come from petroleum derivatives, they are not biodegradable and their mechanical properties are usually weaker. More recently, natural-
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derived polysaccharides have been used, cellulose on top, to fabricate electromechanical resonators at
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high frequencies, but with low displacement [24–26]. Good results have been achieved in particular with the use of bacterial cellulose [27,28]. Nevertheless, all these polymers are water-absorbent or moisture
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sensitive, hindering the actuators to stably operate in unpredictable environmental conditions [29].
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A sustainable alternative to these polymeric matrices may be represented by bioplastics, since they are natural-derived and biodegradable and they present suitable mechanical and waterproof properties[30–
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32]. Although they are not ionic polymers, mobile ions can be supplied by blending with a proper ionic
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medium[26]. Among bioplastics, poly(3-hydroxybutyrate) (PHB) is a plastic polyester produced by bacteria[33–35], usually processed by hot extrusion or casting from chloroform. As reported by Emadian
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et al.[31], PHB possesses an excellent biodegradability both in the soil and in water. In the field of electromechanical actuators, the only use of PHB regards the work of Zhijiang et al. [36], where it was
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used as a reinforcement for a cellulose-based electromechanical resonator. As previously mentioned, every polymeric matrix needs to be coupled with suitable electrodes in order to convert electric signal in mechanical actuation. In this sense, the fabrication of well-adherent, highly
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conductive and thin electrodes able to undergo stable cyclic deformations along with the polymeric substrate is of paramount importance and it still constitutes a critical task. Metallic electrodes fabricated with traditional methods, such as chemical plating or physical vapor deposition, present low adhesion with the underlying polymer and they undergo cracking phenomena after few deformation cycles, due to the metal-polymer mechanical mismatch. A paradigmatic case in this sense is that of ionic polymer metal composites (IPMCs)[37,38]. More compliant electrodes can be fabricated by using carbonaceous species 3
such as graphene, carbon nanotubes, active carbon or conductive polymers[39–41]. However, the manufacturing of carbon-based composite electrodes is time-consuming, it requires high loadings of active material and it is difficult to obtain electrodes with good electrical properties typical of single crystallites[39], while organic conductive polymers are toxic. Moreover, this kind of electrodes are hygroscopic[42,43]. Supersonic cluster beam deposition (SCBD)[44–46] is a manufacturing technique that allows the
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fabrication of thin cluster-assembled metallic electrodes onto polymeric substrates. SCBD relies on the use of highly collimated supersonic beams seeded with neutral metallic nanoparticles directed onto a target. It is a technique that allows the fabrication of cluster-assembled metallic electrodes partially implanted into soft polymeric matrices and characterized by tailored electrical properties, large surface area, robust anchorage to the substrate and mechanical resiliency against deformations, as previously demonstrated[47–49].
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The aim of this work is to use biodegradable PHB as the only polymeric backbone for the fabrication
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of a natural-derived, waterproof and plastic-like electromechanical actuator. To do this, non-ionic PHB
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polymeric chains were blended with a suitable ionic liquid to produce bioplastic-based ionogel thin films through a simple solvent casting process from acetic acid. The resulting samples were coupled with
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cluster-assembled gold electrodes (150 nm-thick) fabricated using the SCBD technique. The obtained PHB-ionic liquid-Au nanocomposites were characterized in terms of mechanical properties, moisture
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absorption, ionic conductivity, double-layer capacitance and electromechanical actuation. The actuators
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exhibited large bending deformation and stable operational durability without showing significant hygroscopic behaviour. This novel bioplastic-based ionogel combines electroactive properties with
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environmental sustainability and constitutes a promising solution for the design and fabrication of green
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soft robotic components and smart bio-inspired devices.
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2. MATERIALS AND METHODS
2.1 Materials and reagents Purified PHB powder was provided by the company Bio-On (Italy). Ionic liquid 1-buthyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide 99% (BMIM(Tf2N)) was purchased from Iolitec. Tetrabuthylammonium fluoride hydrate 98% (TBAF) was purchased from Sigma Aldrich and glacial acetic acid from Riedel-de Haen. Gold rods 99.9% were purchased from 8853 s.p.a. 4
2.2 PHB ionogel synthesis Figure 1(a) reports the chemical structures of the reagents. An ionic mixture was prepared by dissolving TBAF (6.9% w/w, equal to a concentration of 0.375 M) in BMIM(Tf2N) and keeping it under magnetic stirring for 24 hours. PHB powder was dissolved in acetic acid (50 mg/mL) preheated at 110°C with the help an oil bath and a heating plate. After 8 minutes of magnetic stirring, the ionic mixture was added (40
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L/mL of acetic acid) and the stirring was kept for other 7 minutes. Then, the solution was poured into a preheated aluminum mold over a surface area of 70 x 25 mm2 (Figure 1(b)) and casted in an oven at 105°C. After 30 minutes, a solid film formed, with thickness varying from ca. 50 to 200 m, depending on the amount of solution poured (from 1 to 5 mL). The obtained samples were kept in vacuum for at least 16 hours to remove any residual of acetic acid. Each characterization of the material was then conducted on three samples and the relative standard deviation was lower than 5% in all cases (besides
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10% in tensile tests). Figure 1(c) reports a picture of the PHB-IL ionogel.
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2.3 Moisture absorption properties
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The weight of the ionogels (m0) was measured after drying the material in vacuum. To investigate on their moisture absorption, samples were put inside an incubator (Galaxy S, RSBiotech) at 37°C and a
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humidity level of 95% for 24 hours or they were kept outdoor overnight (temperature of about 0°, relative humidity higher than 80%). Their weight was then measured again (mf) and the moisture absorption was
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estimated as the difference between mf and m0 values.
2.4 Tensile tests
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Mechanical tensile tests were carried out on 200 m-thick samples cut in a traditional dog-bone shape, having a central region with length L = 35 mm and a cross-section area A = 0.8 mm2. A 10 N load cell (Sauter FH- 10) was used for the force measure. This was mounted on an automated vertical test stand
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(Sauter TVNM) and both components were provided with clamps. The normal force (FN) on the samples was registered as a function of time using a dedicated software (Sauter AFH—Fast F/D) under a constant traction rate. The stress σ was calculated as FN/A and the strain ε was calculated as the ratio between the samples elongation ( L) and their initial length L.
2.5 Cluster-assembled Au electrodes fabrication 5
150 nm-thick cluster-assembled gold electrodes were fabricated on both sides of the PHB films, using the Supersonic cluster beam deposition (SCBD) technique. Figure 2(a) reports a schematic illustration of its operational principle. SCBD uses a pulsed microplasma cluster source (PMCS) to generate gas-phase neutral nanoparticles of conductive species. Under a pressure gradient, the clusters, seeded in a carrier inert gas, leave the source and enter a second chamber (the expansion chamber) undergoing supersonic expansion. Inside the expansion chamber the clusters are size-selected (3.7 ± 1.7 nm[46]) using an
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aerodynamic focuser. The gas-nanoparticles mixture then passes through a skimmer and forms a highly collimated supersonic beams inside the deposition chamber, where clusters impact and partially implant into a polymeric substrate. Figure 2(b) reports a schematic of the PHB-Il-Au nanocomposite. In this work, vacuum was generated before the beginning of the deposition with values of about 1 × 10-7 Torr in the source and in the expansion chamber and of 1 × 10-5 Torr in the deposition chamber. Then, Argon at a pressure of 40 bar was pulsed inside the PMCS for a time laps of 250 s at a frequency of 5 Hz and it was
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coupled with an electric discharge of 750 V between the gold rod and a copper counter electrode. The
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generated Au clusters were carried by the gas throughout the expansion chamber (average dynamic pressure of about 1 × 10-3 Torr) passing through the 4-lenses aerodynamic focuser and then they reached
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the deposition chamber, were the ionogel samples were mounted on a custom-designed sample holder.
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Figure 2(c) shows a typical sample positioned in the holder before and after the deposition. A coaxial quartz microbalance was also targeted by the cluster beam in order to measure in real time the amount (in
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terms of mass and relative thickness) of deposited gold. The average Au deposition rate was kept at a
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value of ca. 0.5 nm/s. The deposition was performed on both sides of the ionogel samples, until the Au thickness reached a value of 150 ± 5 nm for each side. Figure 2(d) shows a picture of the resulting PHB-
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IL-Au composite cut in a cantilever shape.
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2.6 Optical and electronic microscopy An optical microscope (Axio imager pol, Zeiss) was used to observe the surface of the ionogels before and after the fabrication of the electrodes. Scanning electron microscopy (SEM) imaging was performed on the metallized samples at the MDM laboratories in Agrate Brianza (Milan), using a Zeiss Gemini instrument, equipped with an in-lens detector.
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2.7 Ionic conductivity and double-layer capacitance Electrochemical impedance spectroscopy (EIS) analysis was carried out using a Gamry potentiostat (model Reference 600) on 50 m-thick samples with an electrode area of 0.8 x 0.8 = 0.64 cm2 (on both sides). DC was set at 0 V and AC at ± 5 mV, while the frequency range was explored from 1000 Hz to 0.01 Hz. The obtained frequency-dependent values of the impedance Z (Ω) and of the current phase were used to calculate the bulk resistance Rb (Ωx cm2) and the ionic conductivity
(°)
(S x cm) of the ionogel,
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as well as the double-layer capacitance Cdl (F/cm2) at the ionogel-electrodes interface.
2.8 Electromechanical actuation
The samples used for the electromechanical characterization were cut in a cantilever shape with a free length L = 30 mm, a width w = 3 mm and a thickness t = 50 m and they were clamped at one side. A power supply (EA-PS 2342-10B) was used to apply a V (from 0.1 to 7 V) between the two electrodes
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and a square wave function generator (Thandar TG503) connected to a control circuit was used switch
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the signal bias in a frequency range between 0.1 to 1 Hz. The displacement x (mm) on the X-axis of the
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actuator’s free tip was monitored using a camera and a paper grid. The cantilever curvature k (mm-1) was
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calculated as k = 2x⁄L2 , while the strain was ε = 2tx⁄(L2 + x 2 ), calculated with one of the most used formulas [28]. Cyclic tests have been also conducted by the application of an alternating potential of 2
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and 4 V at a frequency of 0.5 and 0.75 Hz respectively, up to 100 thousand cycles. The displacement retention xret (%) was calculated as xn / x1, where x1 is the displacement at the first cycle and xn is the
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displacement at the nth cycle.
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3. RESULTS AND DISCUSSION
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3.1 Ionogel synthesis and properties PHB is a neutral polymer and to obtain an electroactive composite it is necessary to provide ionic
couples with different size and a liquid medium in which these ions can migrate under the effect of an electric field. Ionic liquids (ILs) are organic salts in the molten state at room temperature and they are an optimal solution to serve both as the active ions and the liquid phase. ILs also possess an extremely low vapor pressure that avoids evaporation of the liquid at ambient conditions, which is a key factor for the 7
design and fabrication of electromechanical actuators meant to operate in air. The ionic liquid BMIM(Tf2N) was chosen to be blended with PHB due to its good miscibility with the polymer, as evaluated on an empirical basis, and the organic salt TBAF was also added since the presence of a quaternary ammonium salt in ionogel-based actuators showed to improve their electromechanical performances[22]. To fabricate ionogel layers, a solvent casting process from acetic acid was followed, similarly to the one proposed by Anbukarasu et al.[50], which is much eco-friendlier than the traditional
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one with chloroform. This fast and simple procedure (ca. 45 minutes from dissolution to casting) allowed to obtain flexible thin films of PHB-IL ionogel.
The moisture absorbance of the samples was checked in two different high humidity conditions: inside an incubator with humidity level of 95% and open-air in the traditional humid climate of the Po Valley[51]. In both cases no water absorption was observed, showing waterproof properties and no moisture absorption typical of traditional plastic materials, which is a unique feature in the field of IEAPs[29]. In
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fact, ionic polymers are always water-soluble or water absorbent and this represent a huge limit for real-
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world applications of electromechanical actuators, since different humidity conditions will affect and change their properties. Instead, PHB-IL ionogels showed to be moisture resistant because of the high
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hydrophobic character of both PHB and BMIM(Tf2N).
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The mechanical properties of the samples were also characterized by means of tensile tests and a representative stress-strain curve is reported in Figure 3(a). It can be seen how an elastic behavior was
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observed up to an elongation of about 1% of its original length. The calculated elastic modulus was 303
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MPa, which is around 5 times lower than the one reported for pure PHB films casted from chloroform (ca. 1.5 GPa[35]). For higher elongations, the relationship between stress and strain is no longer linear
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and a plastic behavior is observed, as expected considering the bioplastic nature of PHB. The overall tensile strength and the elongation at break were also calculated and resulted equal to 10.9 MPa and
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11.1%, respectively, while pure PHB films casted from chloroform possess a tensile strength of about 30 MPa and elongation at break of about 9%. These results highlight how the presence of the ionic liquid is important not only to provide active mobile ions, but also to soften PHB, rendering its elastic modulus
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similar to that of low density PET, rubber and swollen Nafion.
3.2 Electrodes fabrication and actuators characterization Cluster-assembled gold electrodes were fabricated on both sides of the PHB-IL ionogel films using SCBD. This metallization technique was chosen because it operates at ambient temperature and it allows 8
to fabricate highly conductive, stable, well-anchored and very thin (150 nm) conductive layers without damaging the polymeric matrix. The use of gold guaranteed low sheet resistance, which is important to provide the actuators with uniform electric signals across the electrodes. Moreover, since they are deposited onto a soft polymeric matrix, the gold clusters partially penetrate underneath the target surface, leading to a strong adhesion and anchorage of the metallic film to the polymer, and to the formation of a nanostructured metal – ionogel interface. The low thickness of the electrodes (150 nm) was selected to
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both minimize the mechanical mismatch between the metallic layer and the polymer without affecting the bending ability of the material and to achieve low sheet resistance of about 10-12 Ohm. Figure 3(b) reports optical microscope images acquires before and after electrodes fabrication. It can be seen how the thin gold layer does not change the ionogel’s surface morphology at the microscopic level. SEM images are also presented (Figure 3(c)), showing the PHB-IL-Au composite’s cross-section and a detailed view of
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the cluster-assembled electrode, revealing the nanostructured Au morphology.
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Electrochemical impedance spectroscopy (EIS) was then used to study the electrochemical properties
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of the PHB-IL-Au composites. Nyquist plot (Figure 4(a)) was obtained by plotting the inverse value of the impedance’s imaginary component (Zi) against its real component (Zr). In the Bode plots (Figure 4(b))
Cdl and the current phase
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the absolute value (logarithmic scale) of the impedance Z, the logarithm of the double – layer capacitance (°) are reported as a function of the logarithm of the frequency f (Hz) of the
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applied potential. As can be seen from the Bode plot, at high frequencies
= 0°, meaning that the system
has the behavior of a pure resistance. The corresponding Z value can be regarded as an equivalent series
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resistance ESR = 5.32 × 103 Ohm × cm2, that represent the PHB-IL-Au composite’s bulk resistance Rb. Since the electric charge inside the ionogel is transported by mobile ions, Rb value can be used to calculate
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the characteristic ionic conductivity
= 6.58 × 10-7 S/cm. The same values can be obtained by the Nyquist
plot, by considering the intersection between the line and the Zr axis. This relatively high resistance is
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probably due to the insulating nature of the PHB and suggests that the ionic motion across the ionogel is slow. Nevertheless, such a low conductivity implies low values of the ionic current, a feature that usually improves operational durability and stability of the actuator. At low frequencies
reached a value of -75°,
meaning that the system behaves like a real double layer capacitance Cdl, due to the accumulation of the mobile ions at the electrodes. This observation is also confirmed by the vertical profile of the Nyquist plot. More specifically, at the lowest frequency (0.01 Hz), Cdl = 322 F/cm2. Considering the extremely 9
low amount of electrode material (only 150 nm), this value of Cdl proves their high interfacial active area with the underlying ionogel. The formation of an efficient double – layer is a key feature for electroactivity and in this case it demonstrates the possibility to obtain a composite blending an electrically insulating bioplastic and an ionic phase, without losing ionic mobility. For the electromechanical tests, 50 m-thick samples were cut in a cantilever shape with a free length of 30 mm and both DC and AC potential from 0.1 to 7 volts were applied at different frequencies (from
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0.1 to 1 Hz). The actuators showed to be responsive for low voltages down to 0.1 V, bending towards the cathode in a reversible way, as can be seen in Figure 4(d), showing the overlapping of images acquired with the camera during the actuation tests. The tip’s displacement values x (mm) on the X-axis as a function of the applied potentials and frequencies are reported in Supporting material (S.1), as well as the curvature k (cm-1) that takes into account the cantilever’s length and the strain
that considers also the
thickness. The actuator strain values are reported in Figure 4(c). As can be seen, at 0.1 V the actuators
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showed a maximum tip displacement of 0.2 mm and a calculated strain of 24.4 × 10-6. For higher potentials
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the actuation increased as expected. The displacement increased at lower frequencies, but the measured values for frequency down to 0.1 Hz were systematic lower than the maximum amplitude achievable
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applying a DC voltage, further validating the slow ionic migration across the ionogel. However, the
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maximum displacements and associated strains showed to be relatively high: 1 mm ( = 111 × 10-6) at 1 Volts, 5 mm ( = 540 × 10-6) at 3 Volts, 11 mm ( = 1077 × 10-6) at 5 Volts 17 mm ( = 1430 × 10-6) at 7
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Volts.
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Cyclic tests were also conducted by the application of 2 V at 0.5 Hz and 4 V at 0.75 Hz. The results are shown in Figure 4(e). Under the application of 2 V the actuators showed to be stable up to 105 cycles
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and, interestingly, after one thousand cycles the displacement’s amplitude increased up to the 125% of the original value, probably because of a mechanical training of the cantilever. This is a significant result,
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since high stability is a key factor for a durable operation. At higher potentials (4V) the displacement retention (xret) showed to be lower, reaching values of about 80% at 5 × 104 cycles. This behavior is easily explained by considering that a potential of 4 V is at the limit of the window of electrochemical stability
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of the used BMIM(Tf2N) [11], meaning that the ionic liquid gradually reacts and degrades cycle by cycle.
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4. CONCLUSIONS
This work reports the successful use of a natural-derived and biodegradable biopolymer like PHB to develop a bioplastic-based electromechanical soft actuator. Blended with a suitable ionic liquid and casted from acetic acid, the obtained ionogels are water and moisture resistant and they display mechanical properties similar to those of low density PET. SCBD technique was used to fabricate cluster-assembled
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compliant gold electrodes and the resulting PHB-IL-Au actuators displayed slow but large and reversible macroscopic bending for applied potentials from 0.1 to 7 Volt and an excellent operation durability up to 105 cycles at 2 V. Their performances are comparable with those of traditional IPMCs, but with the great advantage of being water and moisture resistant and derived from a biodegradable PHB bioplastic. Thanks to these results, we showed how the natural and biodegradable PHB represents a convenient and performant alternative to traditional synthetic polymers in the context of smart and soft actuation for
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sustainable functional devices. Further studies will be carried out in order to understand how other several
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parameters, such as the thickness of the ionogels and the ionic liquid concentration inside the polymer,
Author Contributions
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AUTHOR INFORMATION
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could affect the electromechanical properties of these composite material.
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All authors have given approval to the final version of the manuscript. Funding Sources
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BIO-ON is gratefully acknowledged for financial support.
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Figure 1. (a) Chemical structures of the reagents used; (b) Photograph of the aluminum mold; (c) Picture
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of a typical PHB-IL ionogel thin film obtained by means of solvent casting.
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Figure 2. (a) Schematic illustration of the supersonic cluster beam deposition (SCBD) technique; (b)
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Schematic illustration of the sandwich structure of PHB-IL-Au composites; (c) Picture of a sample mounted on a sample holder, before and after the Au cluster deposition; (d) Picture of the resulting PHB-
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IL-Au composite cut in a cantilever shape.
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Figure 3. (a) Stress-strain curve of PHB-IL ionogel, obtained through tensile tests; (b) Optical
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microscope images of the PHB-IL ionogel, before and after Au cluster depositions; (c) SEM images
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showing the PHB-IL-Au composite’s cross-section (magnitude 200 X and 1000 X) and the cluster-
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assembled Au nanostructured morphology (magnitude 50000 X).
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Figure 4. Electrochemical and electromechanical properties of PHB-IL-Au composite actuators: (a) Nyquist plot, obtained through EIS and reporting the inverse of the imaginary component of impedance
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(Zi) against the real component (Zr); (b) Bode plots, obtained through EIS and reporting the absolute value
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of the impedance (|Z|), the double – layer capacitance (Cdl) and the current phase (
against the frequency
(f) of the applied potential; (c) Graph reporting the actuator’s strain ( ) as a function of the applied
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potentials at different frequencies; (d) Overlapping images acquired during the actuation tests, showing different displacements according to the applied potential; (e) Graph reporting the displacement retention
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values (xret) against the number of actuation cycles.
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S0925-4005(19)30169-8 https://doi.org/10.1016/j.snb.2019.01.141 SNB 26067
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
5 December 2018 8 January 2019 27 January 2019
Please cite this article as: Migliorini L, Santaniello T, Rondinini S, Saettone P, Comes Franchini M, Lenardi C, Milani P, Bioplastic electromechanical actuators based on biodegradable Poly(3-hydroxybutyrate) and cluster-assembled gold electrodes, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.01.141 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bioplastic electromechanical actuators based on biodegradable Poly(3hydroxybutyrate) and cluster-assembled gold electrodes. Lorenzo Migliorini1,2, Tommaso Santaniello2,* [email protected], Sandra Rondinini1, Paolo Saettone3, Mauro Comes Franchini3,4, Cristina Lenardi2, Paolo Milani2,* [email protected] 1
Department of chemistry, University of Milan, Via Golgi 19, 20133, Milan, Italy
2
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CIMaINa, Department of physics, University of Milan, Via Celoria 16, 20133, Milan, Italy
3
Bio-On spa. Via Santa Margherita al Colle 10/3 40136 Bologna Italy
4
Department of Industrial Chemistry "Toso Montanari”, University of Bologna, Viale
Risorgimento 4, 40136 Bologna, Italy *
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Corresponding authors:
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- Bioplastic-based electromechanical actuators.
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HIGHLIGHTS
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- Poly(3-hydroxybutirate) and ionic liquid-based ionogels using solvent casting from acetic acid. - Fabrication of compliant cluster-assembled gold electrodes
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- Natural-derived, soft, durable and water-resistant electromechanical actuators
ABSTRACT: We present the fabrication and characterization of electroactive actuators based on natural-
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derived bioplastic able to display reversible bending deformations in response to low intensity electric fields. We used biodegradable poly(3-hydroxybutyrate) (PHB) blended with a suitable ionic liquid to
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produce freestanding electroactive ionogel layers through a solvent casting process employing non-toxic solvents. The ionogels were provided with compliant cluster-assembled gold electrodes fabricated by
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means of supersonic cluster beam deposition (SCBD). These metal layers exhibit tailored electrical properties, large surface areas and mechanical resiliency against deformations. The manufactured actuators were characterized in terms of mechanical, morphological, electrochemical and electromechanical properties. We show that the mechanical properties of the ionogels are similar to those of low density polyethylene, while the actuators demonstrated large bending displacement and stable operational durability. Furthermore, due to the use of PHB in combination with a hydrophobic ionic liquid, the actuators did not exhibit hygroscopic behaviour.
KEYWORDS Polyhydroxybutyrate; bioplastic; biopolymers; soft robotics; smart materials, electromechanical actuators.
ABBREVIATIONS AcOH, acetic acid; BMIM(Tf2N), 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; EIS,
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electrochemical impedance spectroscopy; IEAP, ionic electroactive polymer; IPMC, ionic polymer-metal composite; PET, polyethylene terephthalate; PHB, poly(3-hydroxybutyrate); PMCS, pulsed microplasma cluster source; PVDF, polyvinylidene fluoride; SCBD, supersonic cluster beam deposition; SEM,
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scanning electron microscopy; TBAF, tetrabuthylammonium fluoride.
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1. INTRODUCTION
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Industrial and technological mass production and manufacturing processes consume non-renewable resources and the disposal of a large number of consumer-graded products generates damage for the
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environment. The importance of environmental sustainability leads to a new challenge: to find naturalderived and biodegradable materials to be used as building blocks for the development of innovative green
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devices and technological solutions[1].
In soft robotics traditional rigid materials are replaced by soft and deformable ones to fabricate systems
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with embedded sensing and actuation capabilities, mimicking the behavior of a living organism[2–5]. “Sustainable” soft robots and functional devices that can be produced, used and disposed with minimal
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environmental impact should be manufactured using benign materials and composites derived from renewable resources and safely degradable[6].
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Ionic electroactive polymers (IEAPs)[7–9] can be employed as electro-mechanical actuators and
sensors in soft robotics and in a variety of forefront technological applications, including bioengineering and biotechnology[10]. IEAPs are cross-linked polymeric networks, provided with covalently bound ionic groups, whose electric charge is balanced by inorganic counterions[7]. Thin IEAP films are usually impregnated with an ionic conductive liquid phase (aqueous or organic) and sandwiched between thin, adherent and flexible electrodes[8]. Among these liquid media, ionic liquids are widely employed as a
greener alternative to traditional organic solvents[11,12]. Under the application of an electric field, an asymmetric ionic migration occurs across the polymeric matrix, according to the size and charge of the mobile species. This results in a higher concentration of the liquid phase at one of the electrodes, inducing a localized swelling and a macroscopic bending deformation of the system. These actuators can readily and reversibly convert low power electric signals into mechanical work. Many IEAPs have been studied and produced in the last decades, but very few of them match the
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requirements of environmental sustainability coupled with operational stability in different working conditions. Perfluorinated polymers like Nafion or PVDF represent one of the most performant solutions, but their monomers are toxic and carcinogenic, their fabrication requires high costs, high temperatures and pollutant solvents, and the products are unsafe and not easily degradable[13–19]. Less toxic polymers have also been used[20–23], such as polyvinylic species, but they still come from petroleum derivatives, they are not biodegradable and their mechanical properties are usually weaker. More recently, natural-
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derived polysaccharides have been used, cellulose on top, to fabricate electromechanical resonators at
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high frequencies, but with low displacement [24–26]. Good results have been achieved in particular with the use of bacterial cellulose [27,28]. Nevertheless, all these polymers are water-absorbent or moisture
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sensitive, hindering the actuators to stably operate in unpredictable environmental conditions [29].
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A sustainable alternative to these polymeric matrices may be represented by bioplastics, since they are natural-derived and biodegradable and they present suitable mechanical and waterproof properties[30–
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32]. Although they are not ionic polymers, mobile ions can be supplied by blending with a proper ionic
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medium[26]. Among bioplastics, poly(3-hydroxybutyrate) (PHB) is a plastic polyester produced by bacteria[33–35], usually processed by hot extrusion or casting from chloroform. As reported by Emadian
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et al.[31], PHB possesses an excellent biodegradability both in the soil and in water. In the field of electromechanical actuators, the only use of PHB regards the work of Zhijiang et al. [36], where it was
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used as a reinforcement for a cellulose-based electromechanical resonator. As previously mentioned, every polymeric matrix needs to be coupled with suitable electrodes in order to convert electric signal in mechanical actuation. In this sense, the fabrication of well-adherent, highly
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conductive and thin electrodes able to undergo stable cyclic deformations along with the polymeric substrate is of paramount importance and it still constitutes a critical task. Metallic electrodes fabricated with traditional methods, such as chemical plating or physical vapor deposition, present low adhesion with the underlying polymer and they undergo cracking phenomena after few deformation cycles, due to the metal-polymer mechanical mismatch. A paradigmatic case in this sense is that of ionic polymer metal composites (IPMCs)[37,38]. More compliant electrodes can be fabricated by using carbonaceous species 3
such as graphene, carbon nanotubes, active carbon or conductive polymers[39–41]. However, the manufacturing of carbon-based composite electrodes is time-consuming, it requires high loadings of active material and it is difficult to obtain electrodes with good electrical properties typical of single crystallites[39], while organic conductive polymers are toxic. Moreover, this kind of electrodes are hygroscopic[42,43]. Supersonic cluster beam deposition (SCBD)[44–46] is a manufacturing technique that allows the
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fabrication of thin cluster-assembled metallic electrodes onto polymeric substrates. SCBD relies on the use of highly collimated supersonic beams seeded with neutral metallic nanoparticles directed onto a target. It is a technique that allows the fabrication of cluster-assembled metallic electrodes partially implanted into soft polymeric matrices and characterized by tailored electrical properties, large surface area, robust anchorage to the substrate and mechanical resiliency against deformations, as previously demonstrated[47–49].
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The aim of this work is to use biodegradable PHB as the only polymeric backbone for the fabrication
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of a natural-derived, waterproof and plastic-like electromechanical actuator. To do this, non-ionic PHB
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polymeric chains were blended with a suitable ionic liquid to produce bioplastic-based ionogel thin films through a simple solvent casting process from acetic acid. The resulting samples were coupled with
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cluster-assembled gold electrodes (150 nm-thick) fabricated using the SCBD technique. The obtained PHB-ionic liquid-Au nanocomposites were characterized in terms of mechanical properties, moisture
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absorption, ionic conductivity, double-layer capacitance and electromechanical actuation. The actuators
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exhibited large bending deformation and stable operational durability without showing significant hygroscopic behaviour. This novel bioplastic-based ionogel combines electroactive properties with
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environmental sustainability and constitutes a promising solution for the design and fabrication of green
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soft robotic components and smart bio-inspired devices.
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2. MATERIALS AND METHODS
2.1 Materials and reagents Purified PHB powder was provided by the company Bio-On (Italy). Ionic liquid 1-buthyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide 99% (BMIM(Tf2N)) was purchased from Iolitec. Tetrabuthylammonium fluoride hydrate 98% (TBAF) was purchased from Sigma Aldrich and glacial acetic acid from Riedel-de Haen. Gold rods 99.9% were purchased from 8853 s.p.a. 4
2.2 PHB ionogel synthesis Figure 1(a) reports the chemical structures of the reagents. An ionic mixture was prepared by dissolving TBAF (6.9% w/w, equal to a concentration of 0.375 M) in BMIM(Tf2N) and keeping it under magnetic stirring for 24 hours. PHB powder was dissolved in acetic acid (50 mg/mL) preheated at 110°C with the help an oil bath and a heating plate. After 8 minutes of magnetic stirring, the ionic mixture was added (40
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L/mL of acetic acid) and the stirring was kept for other 7 minutes. Then, the solution was poured into a preheated aluminum mold over a surface area of 70 x 25 mm2 (Figure 1(b)) and casted in an oven at 105°C. After 30 minutes, a solid film formed, with thickness varying from ca. 50 to 200 m, depending on the amount of solution poured (from 1 to 5 mL). The obtained samples were kept in vacuum for at least 16 hours to remove any residual of acetic acid. Each characterization of the material was then conducted on three samples and the relative standard deviation was lower than 5% in all cases (besides
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10% in tensile tests). Figure 1(c) reports a picture of the PHB-IL ionogel.
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2.3 Moisture absorption properties
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The weight of the ionogels (m0) was measured after drying the material in vacuum. To investigate on their moisture absorption, samples were put inside an incubator (Galaxy S, RSBiotech) at 37°C and a
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humidity level of 95% for 24 hours or they were kept outdoor overnight (temperature of about 0°, relative humidity higher than 80%). Their weight was then measured again (mf) and the moisture absorption was
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estimated as the difference between mf and m0 values.
2.4 Tensile tests
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Mechanical tensile tests were carried out on 200 m-thick samples cut in a traditional dog-bone shape, having a central region with length L = 35 mm and a cross-section area A = 0.8 mm2. A 10 N load cell (Sauter FH- 10) was used for the force measure. This was mounted on an automated vertical test stand
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(Sauter TVNM) and both components were provided with clamps. The normal force (FN) on the samples was registered as a function of time using a dedicated software (Sauter AFH—Fast F/D) under a constant traction rate. The stress σ was calculated as FN/A and the strain ε was calculated as the ratio between the samples elongation ( L) and their initial length L.
2.5 Cluster-assembled Au electrodes fabrication 5
150 nm-thick cluster-assembled gold electrodes were fabricated on both sides of the PHB films, using the Supersonic cluster beam deposition (SCBD) technique. Figure 2(a) reports a schematic illustration of its operational principle. SCBD uses a pulsed microplasma cluster source (PMCS) to generate gas-phase neutral nanoparticles of conductive species. Under a pressure gradient, the clusters, seeded in a carrier inert gas, leave the source and enter a second chamber (the expansion chamber) undergoing supersonic expansion. Inside the expansion chamber the clusters are size-selected (3.7 ± 1.7 nm[46]) using an
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aerodynamic focuser. The gas-nanoparticles mixture then passes through a skimmer and forms a highly collimated supersonic beams inside the deposition chamber, where clusters impact and partially implant into a polymeric substrate. Figure 2(b) reports a schematic of the PHB-Il-Au nanocomposite. In this work, vacuum was generated before the beginning of the deposition with values of about 1 × 10-7 Torr in the source and in the expansion chamber and of 1 × 10-5 Torr in the deposition chamber. Then, Argon at a pressure of 40 bar was pulsed inside the PMCS for a time laps of 250 s at a frequency of 5 Hz and it was
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coupled with an electric discharge of 750 V between the gold rod and a copper counter electrode. The
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generated Au clusters were carried by the gas throughout the expansion chamber (average dynamic pressure of about 1 × 10-3 Torr) passing through the 4-lenses aerodynamic focuser and then they reached
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the deposition chamber, were the ionogel samples were mounted on a custom-designed sample holder.
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Figure 2(c) shows a typical sample positioned in the holder before and after the deposition. A coaxial quartz microbalance was also targeted by the cluster beam in order to measure in real time the amount (in
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terms of mass and relative thickness) of deposited gold. The average Au deposition rate was kept at a
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value of ca. 0.5 nm/s. The deposition was performed on both sides of the ionogel samples, until the Au thickness reached a value of 150 ± 5 nm for each side. Figure 2(d) shows a picture of the resulting PHB-
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IL-Au composite cut in a cantilever shape.
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2.6 Optical and electronic microscopy An optical microscope (Axio imager pol, Zeiss) was used to observe the surface of the ionogels before and after the fabrication of the electrodes. Scanning electron microscopy (SEM) imaging was performed on the metallized samples at the MDM laboratories in Agrate Brianza (Milan), using a Zeiss Gemini instrument, equipped with an in-lens detector.
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2.7 Ionic conductivity and double-layer capacitance Electrochemical impedance spectroscopy (EIS) analysis was carried out using a Gamry potentiostat (model Reference 600) on 50 m-thick samples with an electrode area of 0.8 x 0.8 = 0.64 cm2 (on both sides). DC was set at 0 V and AC at ± 5 mV, while the frequency range was explored from 1000 Hz to 0.01 Hz. The obtained frequency-dependent values of the impedance Z (Ω) and of the current phase were used to calculate the bulk resistance Rb (Ωx cm2) and the ionic conductivity
(°)
(S x cm) of the ionogel,
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as well as the double-layer capacitance Cdl (F/cm2) at the ionogel-electrodes interface.
2.8 Electromechanical actuation
The samples used for the electromechanical characterization were cut in a cantilever shape with a free length L = 30 mm, a width w = 3 mm and a thickness t = 50 m and they were clamped at one side. A power supply (EA-PS 2342-10B) was used to apply a V (from 0.1 to 7 V) between the two electrodes
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and a square wave function generator (Thandar TG503) connected to a control circuit was used switch
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the signal bias in a frequency range between 0.1 to 1 Hz. The displacement x (mm) on the X-axis of the
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actuator’s free tip was monitored using a camera and a paper grid. The cantilever curvature k (mm-1) was
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calculated as k = 2x⁄L2 , while the strain was ε = 2tx⁄(L2 + x 2 ), calculated with one of the most used formulas [28]. Cyclic tests have been also conducted by the application of an alternating potential of 2
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and 4 V at a frequency of 0.5 and 0.75 Hz respectively, up to 100 thousand cycles. The displacement retention xret (%) was calculated as xn / x1, where x1 is the displacement at the first cycle and xn is the
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displacement at the nth cycle.
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3. RESULTS AND DISCUSSION
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3.1 Ionogel synthesis and properties PHB is a neutral polymer and to obtain an electroactive composite it is necessary to provide ionic
couples with different size and a liquid medium in which these ions can migrate under the effect of an electric field. Ionic liquids (ILs) are organic salts in the molten state at room temperature and they are an optimal solution to serve both as the active ions and the liquid phase. ILs also possess an extremely low vapor pressure that avoids evaporation of the liquid at ambient conditions, which is a key factor for the 7
design and fabrication of electromechanical actuators meant to operate in air. The ionic liquid BMIM(Tf2N) was chosen to be blended with PHB due to its good miscibility with the polymer, as evaluated on an empirical basis, and the organic salt TBAF was also added since the presence of a quaternary ammonium salt in ionogel-based actuators showed to improve their electromechanical performances[22]. To fabricate ionogel layers, a solvent casting process from acetic acid was followed, similarly to the one proposed by Anbukarasu et al.[50], which is much eco-friendlier than the traditional
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one with chloroform. This fast and simple procedure (ca. 45 minutes from dissolution to casting) allowed to obtain flexible thin films of PHB-IL ionogel.
The moisture absorbance of the samples was checked in two different high humidity conditions: inside an incubator with humidity level of 95% and open-air in the traditional humid climate of the Po Valley[51]. In both cases no water absorption was observed, showing waterproof properties and no moisture absorption typical of traditional plastic materials, which is a unique feature in the field of IEAPs[29]. In
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fact, ionic polymers are always water-soluble or water absorbent and this represent a huge limit for real-
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world applications of electromechanical actuators, since different humidity conditions will affect and change their properties. Instead, PHB-IL ionogels showed to be moisture resistant because of the high
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hydrophobic character of both PHB and BMIM(Tf2N).
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The mechanical properties of the samples were also characterized by means of tensile tests and a representative stress-strain curve is reported in Figure 3(a). It can be seen how an elastic behavior was
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observed up to an elongation of about 1% of its original length. The calculated elastic modulus was 303
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MPa, which is around 5 times lower than the one reported for pure PHB films casted from chloroform (ca. 1.5 GPa[35]). For higher elongations, the relationship between stress and strain is no longer linear
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and a plastic behavior is observed, as expected considering the bioplastic nature of PHB. The overall tensile strength and the elongation at break were also calculated and resulted equal to 10.9 MPa and
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11.1%, respectively, while pure PHB films casted from chloroform possess a tensile strength of about 30 MPa and elongation at break of about 9%. These results highlight how the presence of the ionic liquid is important not only to provide active mobile ions, but also to soften PHB, rendering its elastic modulus
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similar to that of low density PET, rubber and swollen Nafion.
3.2 Electrodes fabrication and actuators characterization Cluster-assembled gold electrodes were fabricated on both sides of the PHB-IL ionogel films using SCBD. This metallization technique was chosen because it operates at ambient temperature and it allows 8
to fabricate highly conductive, stable, well-anchored and very thin (150 nm) conductive layers without damaging the polymeric matrix. The use of gold guaranteed low sheet resistance, which is important to provide the actuators with uniform electric signals across the electrodes. Moreover, since they are deposited onto a soft polymeric matrix, the gold clusters partially penetrate underneath the target surface, leading to a strong adhesion and anchorage of the metallic film to the polymer, and to the formation of a nanostructured metal – ionogel interface. The low thickness of the electrodes (150 nm) was selected to
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both minimize the mechanical mismatch between the metallic layer and the polymer without affecting the bending ability of the material and to achieve low sheet resistance of about 10-12 Ohm. Figure 3(b) reports optical microscope images acquires before and after electrodes fabrication. It can be seen how the thin gold layer does not change the ionogel’s surface morphology at the microscopic level. SEM images are also presented (Figure 3(c)), showing the PHB-IL-Au composite’s cross-section and a detailed view of
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the cluster-assembled electrode, revealing the nanostructured Au morphology.
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Electrochemical impedance spectroscopy (EIS) was then used to study the electrochemical properties
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of the PHB-IL-Au composites. Nyquist plot (Figure 4(a)) was obtained by plotting the inverse value of the impedance’s imaginary component (Zi) against its real component (Zr). In the Bode plots (Figure 4(b))
Cdl and the current phase
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the absolute value (logarithmic scale) of the impedance Z, the logarithm of the double – layer capacitance (°) are reported as a function of the logarithm of the frequency f (Hz) of the
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applied potential. As can be seen from the Bode plot, at high frequencies
= 0°, meaning that the system
has the behavior of a pure resistance. The corresponding Z value can be regarded as an equivalent series
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resistance ESR = 5.32 × 103 Ohm × cm2, that represent the PHB-IL-Au composite’s bulk resistance Rb. Since the electric charge inside the ionogel is transported by mobile ions, Rb value can be used to calculate
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the characteristic ionic conductivity
= 6.58 × 10-7 S/cm. The same values can be obtained by the Nyquist
plot, by considering the intersection between the line and the Zr axis. This relatively high resistance is
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probably due to the insulating nature of the PHB and suggests that the ionic motion across the ionogel is slow. Nevertheless, such a low conductivity implies low values of the ionic current, a feature that usually improves operational durability and stability of the actuator. At low frequencies
reached a value of -75°,
meaning that the system behaves like a real double layer capacitance Cdl, due to the accumulation of the mobile ions at the electrodes. This observation is also confirmed by the vertical profile of the Nyquist plot. More specifically, at the lowest frequency (0.01 Hz), Cdl = 322 F/cm2. Considering the extremely 9
low amount of electrode material (only 150 nm), this value of Cdl proves their high interfacial active area with the underlying ionogel. The formation of an efficient double – layer is a key feature for electroactivity and in this case it demonstrates the possibility to obtain a composite blending an electrically insulating bioplastic and an ionic phase, without losing ionic mobility. For the electromechanical tests, 50 m-thick samples were cut in a cantilever shape with a free length of 30 mm and both DC and AC potential from 0.1 to 7 volts were applied at different frequencies (from
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0.1 to 1 Hz). The actuators showed to be responsive for low voltages down to 0.1 V, bending towards the cathode in a reversible way, as can be seen in Figure 4(d), showing the overlapping of images acquired with the camera during the actuation tests. The tip’s displacement values x (mm) on the X-axis as a function of the applied potentials and frequencies are reported in Supporting material (S.1), as well as the curvature k (cm-1) that takes into account the cantilever’s length and the strain
that considers also the
thickness. The actuator strain values are reported in Figure 4(c). As can be seen, at 0.1 V the actuators
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showed a maximum tip displacement of 0.2 mm and a calculated strain of 24.4 × 10-6. For higher potentials
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the actuation increased as expected. The displacement increased at lower frequencies, but the measured values for frequency down to 0.1 Hz were systematic lower than the maximum amplitude achievable
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applying a DC voltage, further validating the slow ionic migration across the ionogel. However, the
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maximum displacements and associated strains showed to be relatively high: 1 mm ( = 111 × 10-6) at 1 Volts, 5 mm ( = 540 × 10-6) at 3 Volts, 11 mm ( = 1077 × 10-6) at 5 Volts 17 mm ( = 1430 × 10-6) at 7
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Volts.
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Cyclic tests were also conducted by the application of 2 V at 0.5 Hz and 4 V at 0.75 Hz. The results are shown in Figure 4(e). Under the application of 2 V the actuators showed to be stable up to 105 cycles
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and, interestingly, after one thousand cycles the displacement’s amplitude increased up to the 125% of the original value, probably because of a mechanical training of the cantilever. This is a significant result,
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since high stability is a key factor for a durable operation. At higher potentials (4V) the displacement retention (xret) showed to be lower, reaching values of about 80% at 5 × 104 cycles. This behavior is easily explained by considering that a potential of 4 V is at the limit of the window of electrochemical stability
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of the used BMIM(Tf2N) [11], meaning that the ionic liquid gradually reacts and degrades cycle by cycle.
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4. CONCLUSIONS
This work reports the successful use of a natural-derived and biodegradable biopolymer like PHB to develop a bioplastic-based electromechanical soft actuator. Blended with a suitable ionic liquid and casted from acetic acid, the obtained ionogels are water and moisture resistant and they display mechanical properties similar to those of low density PET. SCBD technique was used to fabricate cluster-assembled
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compliant gold electrodes and the resulting PHB-IL-Au actuators displayed slow but large and reversible macroscopic bending for applied potentials from 0.1 to 7 Volt and an excellent operation durability up to 105 cycles at 2 V. Their performances are comparable with those of traditional IPMCs, but with the great advantage of being water and moisture resistant and derived from a biodegradable PHB bioplastic. Thanks to these results, we showed how the natural and biodegradable PHB represents a convenient and performant alternative to traditional synthetic polymers in the context of smart and soft actuation for
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sustainable functional devices. Further studies will be carried out in order to understand how other several
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parameters, such as the thickness of the ionogels and the ionic liquid concentration inside the polymer,
Author Contributions
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AUTHOR INFORMATION
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could affect the electromechanical properties of these composite material.
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All authors have given approval to the final version of the manuscript. Funding Sources
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BIO-ON is gratefully acknowledged for financial support.
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Figure 1. (a) Chemical structures of the reagents used; (b) Photograph of the aluminum mold; (c) Picture
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of a typical PHB-IL ionogel thin film obtained by means of solvent casting.
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Figure 2. (a) Schematic illustration of the supersonic cluster beam deposition (SCBD) technique; (b)
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Schematic illustration of the sandwich structure of PHB-IL-Au composites; (c) Picture of a sample mounted on a sample holder, before and after the Au cluster deposition; (d) Picture of the resulting PHB-
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IL-Au composite cut in a cantilever shape.
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Figure 3. (a) Stress-strain curve of PHB-IL ionogel, obtained through tensile tests; (b) Optical
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microscope images of the PHB-IL ionogel, before and after Au cluster depositions; (c) SEM images
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showing the PHB-IL-Au composite’s cross-section (magnitude 200 X and 1000 X) and the cluster-
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assembled Au nanostructured morphology (magnitude 50000 X).
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Figure 4. Electrochemical and electromechanical properties of PHB-IL-Au composite actuators: (a) Nyquist plot, obtained through EIS and reporting the inverse of the imaginary component of impedance
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(Zi) against the real component (Zr); (b) Bode plots, obtained through EIS and reporting the absolute value
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of the impedance (|Z|), the double – layer capacitance (Cdl) and the current phase (
against the frequency
(f) of the applied potential; (c) Graph reporting the actuator’s strain ( ) as a function of the applied
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potentials at different frequencies; (d) Overlapping images acquired during the actuation tests, showing different displacements according to the applied potential; (e) Graph reporting the displacement retention
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values (xret) against the number of actuation cycles.
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