Sensing and tactile artificial muscles from reactive materials

Abstract

Films of conducting polymers can be oxidized and reduced in a reversible way. Any intermediate oxidation state determines an electrochemical equilibrium. Chemical or physical variables acting on the film may modify the equilibrium potential, so that the film acts as a sensor of the variable. The working potential of polypyrrole/DBSA (Dodecylbenzenesulfonic acid) films, oxidized or reduced under constant currents, changes as a function of the working conditions: electrolyte concentration, temperature or mechanical stress. During oxidation, the reactive material is a sensor of the ambient, the consumed electrical energy being the sensing magnitude. Devices based on any of the electrochemical properties of conducting polymers must act simultaneously as sensors of the working conditions. Artificial muscles, as electrochemical actuators constituted by reactive materials, respond to the ambient conditions during actuation. In this way, they can be used as actuators, sensing the surrounding conditions during actuation. Actuating and sensing signals are simultaneously included by the same two connecting wires.

Keywords: actuators; artificial muscles; conducting polymers; reactive materials; sensing actuators; sensors; tactile muscles.

Figure 1.

Actuation of natural muscles involves electric signals, chemical reactions, conformational movements, interchange of ions and water and heat production.

Figure 2.

Classification of conducting polymers, (a) Basic conducting polymer, (b) substituted polymer, (c) self-doped polymer, (d) copolymer, (e) blend, (f) hybrid material and (g) composite.

Figure 3.

Transformation of the chemical bonds during oxidation to form polarons (radical cations). When the chain is saturated of polarons, the extraction of new electrons generates bipolarons (dications). Counterions (A⁻) penetrate from the solution for charge balance.

Figure 4.

Schematic representation of the reversible volume change associated with the electrochemical reactions of polypyrrole in electrolytes (adopted from Reference [4] with kind permission from Springer Science and Business Media Media).

Figure 5.

(a) Scheme of the electrochemical cell used to follow the electrochemical behavior of self-supported pPy/DBSA films. (b) Control voltammogram recorded at 6 mV s⁻¹ between −2.0 and 0.80 V in 0.1 M LiClO₄ aqueous solution at room temperature.

Figure 6.

(a) Scheme of the applied current and (b) potential responses to the applied currents during film oxidation (shift to positive potentials) or reduction (shift to negative potentials).

Figure 7.

(a) Chronopotentiograms obtained for different anodic current pulses applied to a self supported pPy/DBSA film, flowing a constant charge of ±180 mC, in 0.1 M LiClO₄ aqueous solution and (b) variation of the consumed electrical energy as a function of the applied current. R² is the correlation coefficient of the linear fit.

Figure 8.

(a) Chronopotentiograms obtained following the scheme form Figure 6a at different temperatures by flow of ±4 mA during 60 s per step through a self-supported film of pPy/DBSA in 0.1 M LiClO₄ aqueous solution. (b) The third oxidation chronopotentiograms.

Figure 9.

Variation of the electrical energy consumed during oxidation or reduction of a pPy/DBSA film during 60 s in 0.1 M LiClO₄ aqueous solution as a function of the temperature. R² is the correlation coefficient of the linear fit.

Figure 10.

(a) Chronopotentiograms obtained from pPy/DBSA self-supported films for different LiClO₄ concentrations. After stabilization of the initial oxidation state by applying a constant current of −0.01 mA for 250 s, square current waves of ±2.25 mA flowing for 40 s per step, are applied to a pPy/DBSA film. (b) Variation of the electrical energy as a function of the electrolyte concentration. R² is the correlation coefficient of the linear fit.

Figure 11.

Three-layer structure for bending polymeric actuators. In electromechanical (EM) actuators, the electroactive polymer constitutes the internal layer, being the two external sputtered metals or electronic conductors. In electrochemomechanical (ECM) actuators, two films of reactive conducting polymers (electroactive polymers) constitute the external layers supported by an internal polymeric, adherent, flexible, non-conducting (or ionic conducting) film.

Figure 12.

Molecular motor: reverse conformational changes (mechanical energy) stimulated by oxidation or reduction of the polymeric chain in an electrolyte. (a) Reduced chain, (b) Oxidized chain (adopted from Reference [3] with kind permission from Springer Science and Business Media Media).

Figure 13.

(a) Polypyrrole/tape bilayer. Induced stress gradients by electrochemical reactions (Reproduced with kind permission from Springer Science and Business Media [2]). (b) Angular movement described by the free end of a bilayer muscle (CP–tape) under a current flow of 15 mA (1, 2 and 3), or of 15 mA (4 and 5), the muscle being immersed in a 0.1 M aqueous electrolyte (adopted from References [2,52] with kind permissions from Springer Science and Business Media Media and Marcel Dekker Inc).

Figure 13.

(a) Polypyrrole/tape bilayer. Induced stress gradients by electrochemical reactions (Reproduced with kind permission from Springer Science and Business Media [2]). (b) Angular movement described by the free end of a bilayer muscle (CP–tape) under a current flow of 15 mA (1, 2 and 3), or of 15 mA (4 and 5), the muscle being immersed in a 0.1 M aqueous electrolyte (adopted from References [2,52] with kind permissions from Springer Science and Business Media Media and Marcel Dekker Inc).

Figure 14.

(a) Linear relationship between the applied current and the angular rate determined from the times required to describe an angular movement of 90 degrees under seven different currents (b) Electrical charge consumed by the triple layer muscle to move through 90 degrees. (c) Electrical charge consumed by the triple layer muscle to describe different angles (30, 45, 60, 90, 120, 135, and 180 degrees) under the different currents studied. Experiments in 1 M LiClO₄ aqueous solution (adopted from Reference [167] with kind permission from The Royal Society of Chemistry).

Figure 15.

Angular rate measured through a movement of 90 degrees using a triple-layer muscle of different dimensions (including different weights of polypyrrole films: 8.3, 7.8, 7.4, 6, 5.5, 5.1, 5, 4, 3.7, 3.5, 3, 2.3 and 2 mg) in 1M LiClO4 aqueous solution under different currents (10, 15, 20, 25 and 30 mA) (adopted from Reference [167] with kind permission from The Royal Society of Chemistry).

Figure 16.

Chronopotentiograms obtained when a triple layer (2 × 1.5 cm², 12 mg of PPy) describes 90° (a) in aqueous solutions of LiClO₄ (3, 1, 0.5, 0.25, 0.1, and 0.05 M) under a constant current of 10 mA, (b) in 0.1 M LiClO₄ at different temperatures: 5, 15, 25, 35 and 45 °C. (c) Under flow of different currents: 5, 10, 15, 20, 25, and 30 mA. (d) Shifting different attached steel weights: 0.6, 1.4, 1.8, 2.04, and 2.16 g with a device of 12 mg of polymer weight (adopted from References [170,171]).

Figure 17.

Consumed electrical energy in Figure 16 by the device as a function of the different studied variables: (a) electrolyte concentration, (b) temperature, (c) current, and (d) shifted weight (adopted from References [170,171]).

Figure 18.

(1) The triple layer muscle initiates its movement under a constant current of 5 mA, in 1 M LiClO₄ aqueous solution; (2) 10 s later; (3), (4) the muscle meets the obstacle weighing 6,000 mg, pushing and sliding it; (5) the angular movement allows the muscle to overcome the border of the obstacle; (6) the free movement continues until the current stops. The original position (1) is recovered by flow of -5mA during the same time (adopted from Reference [173] with kind permission from Wiley Interscience).

Figure 19.

Chronopotentiograms obtained from a triple layer macroscopic muscle containing two polypyrrole films [2 cm × 1.5 cm × 13 μm] weighing 6 mg each, under flow of 5 mA in 1 M LiClO₄. The muscle moves freely, contacting an obstacle after 10 s and sliding it for 3.5 s, overcomes its border and continues with a full angular movement of 108° (from −18° to +90°). The initial position is recovered by applying a current of −5 mA for 57 s. Obstacles weighing 1.2, 2.4, 3.6, 4.8, 6.0, 7.2, 8.4, 9.6 mg were slid, but the muscle was unable to push and slide an obstacle weighing 14.4 g (adopted from Reference [173] with kind permission from Wiley Interscience).