Creeping and structural effects in Faradaic artificial musclesby Laura Valero, Jose G. Martinez, Toribio F. Otero

J Solid State Electrochem

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ORIGINAL PAPER

Creeping and structural effects in Faradaic artificial muscles

Laura Valero & Jose G. Martinez & Toribio F. Otero

Received: 23 January 2015 /Revised: 26 January 2015 /Accepted: 28 January 2015 # The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract Reliable polymeric motors are required for the construction of rising accurate robots for surgeon assistance. Artificial muscles based on the electrochemistry of conducting polymers fulfil most of the required characteristics, except the presence of creeping effects during actuation. To avoid it, or to control it, a deeper knowledge of its physicochemical origin is required. With this aim here bending bilayer tape/PPy-DBSH (Polypyrrole-dodecylbenzylsulphonic acid) full polymeric artificial muscles were cycled between −2.5 and 1 V in aqueous solutions with parallel video recording of the described angular movement. Coulo-voltammetric (charge-potential,

QE), dynamo-voltammetric (angle-potential, αE), and coulo-dynamic (charge-angle, Qα) muscular responses corroborate that 10 % of the charge is consumed by irreversible reactions overlapping the polymer reduction at the most cathodic potentials. In parallel, the range of the bending angular movement (145°) shifts by 15° per cycle (creeping effect) pointing to the irreversible charge as possible origin of the irreversible swelling of the PPy-DBS film. Different slopes in the closed loop part of theQE identify the different reaction driven structural processes in the film: oxidation-shrinking, oxidation-compaction, reduction-relaxation, reduction-swelling, and reduction-vesicle’s formation. Despite the irreversible charge fraction, the muscle motor keeps a Faradaic behaviour: described angles are linear functions of the consumed charge in the full potential range.

Keywords Polypyrrole . Artificial muscle . Creeping effect .

Coulo-dynamics . Irreversible reaction . Structural electrochemistry

Introduction

The design and construction of robots for surgeon’s assistance is becoming a critical issue for clinical advances. Reducing operator fatigue, improving accuracy and increasing repeatability consume most of the efforts in the area of computerintegrated surgery and robotic assistants [1, 2].

The simultaneous use of magnetic resonance images for the visual control of the surgeon area imposes limitations to the use of ferromagnetic materials [3]. Polymers are becoming the most suitable materials for developing two basic robotic components: actuators [4–13] and mechanical sensors [14–23].

Indeed artificial muscles based on conducting polymers work as haptic motors [24–27]: one physically uniform device fulfils both actuating and sensing requirements. From an engineering point of view, they are very robust motors due to its

Faradaic nature: the position of the motor is under linear control of the consumed charge, the rate of the movement is under linear control of the applied current, and different devices with different geometry or including different masses of conducting polymers produce the same angular displacement under flow of the same charge per unit of conducting polymer mass and per unit of time [28–31].

Different families of materials constitute the named conducting polymers [32–37]. One of the remaining problems to get confidential products from artificial muscles based on conducting polymers is that some of those conducting polymer families use to develop active creeping effects. After submitting the device to a potential cycle, or to a charge cycle, the

L. Valero : J. G. Martinez : T. F. Otero (*)

Center for Electrochemistry and Intelligent Materials (CEMI),

Universidad Politécnica de Cartagena, ETSII, Paseo Alfonso XIII.

Aulario II, 30203 Cartagena, Spain e-mail: toribio.fotero@upct.es

L. Valero

EngineeringSchool, Universidad Autónoma del Estado de México,

Toluca 50000, Mexico

J Solid State Electrochem

DOI 10.1007/s10008-015-2775-1 device doesn’t recover its initial position originating a continuous displacement of the movement range on consecutive actuation cycles [38–43]. Creeping effects require quite complex self-compensation control processes to get confident products [44–50]. A more precise knowledge of the creeping effect origin may allow its elimination or an easier theoretical compensation. Not many efforts have been done in this direction. Recently, we have discovered from the voltammetric and coulo-voltammetric responses of self-supported electrodes of polypyrrole (PPy) blends with organic macro-anions (MA−) in aqueous solutions cycled up to −2.5 V that most of the involved charge is consumed by the reversible film oxidation/ reduction with exchange of cations (C+) and water (S) with the electrolyte following the general reaction (1) [28]:

Pol0  

MA‐ð Þn Cþð Þn Sð Þm   gel ⇄ Polnþð Þ MA‐ð Þn   s þ n Cþð Þ þ m Sð Þ þ n e‐ð Þmetal ð1Þ

In addition, a minor fraction of the consumed charge gives irreversible reactions at high cathodic potentials, overlapping reaction 1 backwards. Coulo-voltammetric responses allow a quantitative determination of this irreversible charge from self-supported electrodes of polypyrrole blends with dodecyl-bencylsulphonic acid (DBSH) [51] or parabenzolsulfonic acid (PBSH) [52] or from polypyrrole electrodes coating metals [53]. This irreversible open coulovoltammetric fraction is not present in responses from PPy electrodes exchanging small anions [51, 54]. Irreversible charges were attributed to the hydrogen evolution from the organic acid constituent. Here, we will investigate the evolution under potential cycling of the angular movement described by a bilayer PPy-DBS bending artificial muscles with the consumed charge from the coulo-dynamic (charge-angle) responses quantifying, simultaneously, charges consumed per cycle by reversible and irreversible reactions trying to find some correlation between the irreversible charge and creeping effects.