Polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT:PSS) is a well-known conducting polymer that possesses high air stability, high electrical conductivity, and biocompatibility. Nevertheless, the healability of PEDOT:PSS has not been explored much. Researchers at Department of Chemical Engineering, Polytechnique Montréal, Canada have demonstrated that damages in PEDOT:PSS films could be electrically healed by simply wetting the damaged areas with a few drops of water or by wetting the films with water, which enables a self-healing nature for the PEDOT:PSS films, without the need for any external stimulation.
The PEDOT:PSS films were prepared by drop-casting PEDOT:PSS suspension onto glass, uniform spreading and sequential baking at 80 °C for 1 h, 110 °C for 1 h, and 140 °C for 4 h to eliminate bubble formation. In electrically-assisted healing experiments, the PEDOT:PSS film is biased at 0.2 V. Damage of the film with a razor blade leads to an interruption in the current flow. Wetting the damaged area of the film with a drop of DI water has lead to complete recovery of the current to its initial value within 150 ms (Fig.1(a) and inset of Fig. 1(a)).
The current-time characteristics of wet PEDOT:PSS films (soaked in DI water for 5 s) under an electrical bias of 0.2 V indicate no significant change in current for repeated cuts at different regions of the film (Fig. 1(b)). The thickness of the PEDOT:PSS films should be at least 1 μm to trigger electrically-assisted healing of a 40 μm damage within 150 ms. For 1 to 10 μm thick films, no significant dependence of healing time could be observed with film thickness. Repeated damage and repair at different regions of the same film has produced a similar effect in terms of current recovery and response time, thus substantiating the high reproducibility and reliability of the process.
Fig. 1 Current-time transients of PEDOT:PSS films biased at 0.2 V: (a) effect of damage and healing with a drop of DI water (inset: surge in current response due to rapid healing of the damage); (b) effect of wet film to damage
The PEDOT:PSS films prepared using glycerol (conductivity enhancer) and Capstone FS-30 (plasticizer) exhibit water-induced healing behavior without any electrical bias. The film was cut using a razor blade to create a gap of about 40 μm and the damaged area was healed after addition of water (Fig. 2). The damaging and repairing process of the PEDOT:PSS film was demonstrated by connecting it in a simple circuit with light-emitting diode (LED) bulb (Fig. 2). The healing effect is also ascertained after exposure of damaged PEDOT:PSS film to water vapor in a humidity chamber. At RH between 50% and 70%, no significant recovery in current is observed even after 30 min. The current is recovered in ~5 min at 80% RH whereas complete recovery of current to the initial value is observed only at ≥ 90% RH. The healing of damages in the PEDOT:PSS film in water vapor is much slower than in liquid.
Fig. 2 SEM images of the damaged area of PEDOT:PSS film (a) before; and (b) after healing with a 10 μL drop of DI water; (c) schematic representation of water-induced mechanical and electrical healing; and (d) demonstration of damage and healing effect on PEDOT:PSS film connected in a circuit with a LED bulb at 3 V: (i) intact film; (ii) damaged film; and (iii) after dropping DI water on the damage that enables repair of the circuit within 150 ms.
The exact mechanism of water-assisted healing of the damages in PEDOT:PSS films is not clear. It is presumed that the healing effect is due to the swelling of PSS− chains upon water exposure, which increases the viscoelasticity and softness of the film. The swelling of PSS− chains simultaneously enables the PEDOT+ chains to shift, thus allowing formation of PEDOT+-PEDOT+ conducting paths across the damage, leading to healing of the damage with a total restoration of electrical conductivity of the film. The slower current recovery upon exposure to water vapor is due to the lower water absorption rate. Water-induced reversible hydrogen bond breaking and restoring could have also contributed to the separation and propagation of PSS− and PEDOT+ grains to the damaged area. The inability of other solvents such as a fluorinated solvent, glycerol and Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) in place of water, to swell PSS− or break the hydrogen bonding between PSS− chains excludes the possibility of healing and current recovery either by mechanical movements of the film or by transport of conducting debris to the damaged area.
It is also possible to obtain free-standing PEDOT:PSS films by using a water-assisted wedging method, which exhibits excellent conformability on various surfaces. Moreover, the detached wet free-standing films can be easily shaped on objects even with irregular shapes.
Fig. 3 (a) Illustration of water-assisted wedging method to obtain free-standing PEDOT:PSS films; (b, c) the film is not deteriorated during its detachment from the glass substrate; (d, e) excellent conformability of PEDOT:PSS free-standing films (10 μm thickness) on finger and fingertip.
The ultrafast electrically-assisted healing of wet PEDOT:PSS films will be useful in application such as electronic skin, self-healable large-scale electronics, and epidermal electronics. The free-standing PEDOT:PSS films can be effectively used as healable electrodes or electronic welding patches.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Shiming Zhang and Fabio Cicoira, Water-Enabled Healing of Conducting Polymer Films, Adv. Mater. 2017, 1703098, DOI: 10.1002/adma.201703098
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