Recent Advances on Polypyrrole Electroactuators

Abstract

Featuring controllable electrochemomechanical deformation and excellent biocompatibility, polypyrrole electroactuators used as artificial muscles play a vital role in the design of biomimetic robots and biomedical devices. In the past decade, tremendous efforts have been devoted to their optimization on electroactivity, electrochemical stability, and actuation speed, thereby gradually filling the gaps between desired capabilities and practical performances. This review summarizes recent advances on polypyrrole electroactuators, with particular emphases on novel counterions and conformation-reinforcing skeletons. Progress and challenges are comparatively demonstrated and critically analyzed, to enlighten future developments of advanced electroactuators based on polypyrrole and other conducting polymers.

Keywords: conducting polymer; counterion; delamination; electroactivity; high-speed.

Figure 1

Output stain and stress of PPy doped with typical counterions.

Figure 2

Electrochemomechanical deformation of polyethylene glycol (PEG)-modified PPy films. (a) Schematic illustration of the mechanism of PEG-retarded overoxidation of PPy, where I, II, III and IV refer to immersion time of 0.5 h, 2 h, 48 h, and 120 h in the testing solution, sequentially. Adapted with permission from Ref. [68]. Copyright (2011) Elsevier. (b) Simulation of voltage drop along the monolithic film (left), nonuniform bending of a PPy/PEG-borate strip in phosphate buffered saline (PBS) (middle), and degradation of strain and capacity when cycled by square waves of ±1 V amplitudes (right). Adapted with permission from Ref. [72]. Copyright (2017) IEEE.

Figure 3

Actuation of PPy/IC electroactuators based on Au/Ti/Si substrates. (a) Schematic illustration of the redox-induced molecular conformation change of ICs in low-pH solvents. Adapted with permission from Ref. [44]. Copyright (2016) American Chemical Society. (b) Current density, stress and mass changes of PPy/IC during consecutive square-wave potential cycling (−0.25–0.25 V at 10 mV/s in 0.2 M LiCl solution). Adapted with permission from Ref. [45]. Copyright (2016) American Chemical Society. (c) Stress response as a function of active IC content in PPy; data collected from a PPy/IC film cycled from −0.5 V to 0.5 V. Adapted with permission from Ref. [44]. Copyright (2016) American Chemical Society.

Figure 4

Delamination of bilayered PPy electroactuators. (a) U-shaped edge enclosures preventing delamination from a strip substrate; (b) Interfacial stress as a function of distance to edges. Adapted with permission from Ref. [108]. Copyright (2015) IEEE. (c) Simulation and test on delamination for substrates of various geometries. The color bar from blue to red indicates increasing interfacial stress. Adapted with permission from Ref. [108]. Copyright (2015) IEEE.

Figure 5

PPy electroactuators involving conductive skeletons. (a) Scanning electron microscope (SEM) morphology of nanoporous gold with PPy coating (a1), cyclic voltammograms (a2), strain response during triangular potential stimulation (a3), and square-wave potential stimuli (green curve in (a4)) cycled from −0.1 V to 0.4 V in 1M HClO₄. Adapted with permission from Ref. [117]. Copyright (2017) Elsevier. (b) Schematic of the synthesis procedure (b1), two types of orientation for multi-walled carbon nanotubes (MWCNTs) (b2), cross-sectional SEM of a PPy-MWCNTs composite film (b3), and electroactive response of the composite electroactuator in a potential cycle of −1 V to 1 V, tested in 0.5 M KCl solution (b4). Adapted with permission from Ref. [125]. Copyright (2015) John Wiley and Sons. (c) Operation of an interpenetrating polymer network (IPN)-based PPy electroactuator (c1), adapted with permission from Ref. [126]. Copyright (2011) AIP Publishing LLC, and a cross-sectional view of the layered structure (c2), adapted with permission from Ref. [127]. Copyright (2006) Elsevier.

Figure 5

PPy electroactuators involving conductive skeletons. (a) Scanning electron microscope (SEM) morphology of nanoporous gold with PPy coating (a1), cyclic voltammograms (a2), strain response during triangular potential stimulation (a3), and square-wave potential stimuli (green curve in (a4)) cycled from −0.1 V to 0.4 V in 1M HClO₄. Adapted with permission from Ref. [117]. Copyright (2017) Elsevier. (b) Schematic of the synthesis procedure (b1), two types of orientation for multi-walled carbon nanotubes (MWCNTs) (b2), cross-sectional SEM of a PPy-MWCNTs composite film (b3), and electroactive response of the composite electroactuator in a potential cycle of −1 V to 1 V, tested in 0.5 M KCl solution (b4). Adapted with permission from Ref. [125]. Copyright (2015) John Wiley and Sons. (c) Operation of an interpenetrating polymer network (IPN)-based PPy electroactuator (c1), adapted with permission from Ref. [126]. Copyright (2011) AIP Publishing LLC, and a cross-sectional view of the layered structure (c2), adapted with permission from Ref. [127]. Copyright (2006) Elsevier.

Figure 6

PPy/graphene oxide (GO) electroactuators. (a) Fabrication process (left top), cross-sectional view (right top), schematic illustration of dual-responses to water and electrochemical stimuli (left bottom), and tensile test showing the strain-stress curve of the composite film (right bottom). Adapted with permission from Ref. [134]. Copyright (2016) American Chemical Society. (b) Bending test of a PPy/GO strip electroactuator with the holding tip immersed (upper) or above (lower) the electrolyte solution. Adapted with permission from Ref. [132]. Copyright (2012) The Royal Society of Chemistry.

Figure 7

Radar diagrams of various substrates featuring different combinations of properties.