A chameleon-inspired stretchable electronic skin with interactive colour changing controlled by tactile sensing

Abstract

Some animals, such as the chameleon and cephalopod, have the remarkable capability to change their skin colour. This unique characteristic has long inspired scientists to develop materials and devices to mimic such a function. However, it requires the complex integration of stretchability, colour-changing and tactile sensing. Here we show an all-solution processed chameleon-inspired stretchable electronic skin (e-skin), in which the e-skin colour can easily be controlled through varying the applied pressure along with the applied pressure duration. As such, the e-skin's colour change can also be in turn utilized to distinguish the pressure applied. The integration of the stretchable, highly tunable resistive pressure sensor and the fully stretchable organic electrochromic device enables the demonstration of a stretchable electrochromically active e-skin with tactile-sensing control. This system will have wide range applications such as interactive wearable devices, artificial prosthetics and smart robots.

Figure 1. Illustration of the concept of a chameleon-inspired e-skin.

Also shown are (bottom left) the structures of both the neutral and oxidized states of the electrochromic polymer in poly(3-hexylthiophene-2,5-diyl, P3HT), and (bottom right) a schematic of the circuit layout (PS, pressure sensor; ECD, electrochromic device). (Ambanja panther chameleon and young hand images from www.123rf.com).

Figure 2. A schematic of the fabrication process and their SEM images.

(a) A schematic for the fabrication of PSs PS-1, PS-10 and PS-30. (b) The SEM image of PS-1. (c) The SEM image of PS-10. (d) The SEM image of PS-30. Higher magnification SEM images of individual pyramid are provided in the inset.

Figure 3. Pressure-sensing characterization of PS.

The pressure response for four consecutive measurements of (a) PS-1, (b) PS-10 and (c) PS-30. (d) The comparison of the resistance response and the threshold of resistance switching range of PSs (second forward). (e) The comparison of the resistance response of PS-10 at different strains. (f) A schematic diagram of single SWNT-coated pyramidal-microstructured PDMS (inset: each slope of the resistance response of PS-10).

Figure 4. The properties of the stretchable polymer-based ECDs.

(a) The ultraviolet-visible spectra of the neutral and oxidized P3HT. (b) The colour switching behaviour of the ECDs at 0% strain. (c) The enlarged spectra of the single switching cycle at 0% strain. (d) The enlarged spectra of the single switching cycle at 20% strain. (e) The cyclic switching at 0% strain. (f) The cyclic switching at 20% strain.

Figure 5. Mechanical and absorption characteraization of ECDs.

(a) The resistance comparsion of neutral and oxidized P3HT for strains ranging from 0 to 100%. (b) The absorption spectra of P3HT ECD at different strains.

Figure 6. The schematic layout and measurement of the integrated systems.

(a) The schematic layout of interactive colour-changeable e-skin, the layout of the circuit and the photos of PSEC. (b) The absorption spectra related to the pressures. (c) The colour retention properties of ECDs. (d) The real-time absorption changes corresponding to the various applied pressures, in which each pressure was mainatined for 10 s. (e) The speed of colour saturation for different pressures, each pressure was applied till colour saturation for all cases. (f) The absorption and time versus the pressure for both the low- and high-pressure regimes.

Figure 7. An interactive colour-changing and tactile-sensing e-skin.

Sequential images of a teddy bear show the expression of tactile sensing into visible colour changes. In specific, the original colour of the ECD is changed from dark red to blue grey upon a weak applied pressure (∼50 kPa), reverts back to dark red upon pressure release and changes to pale blue upon a strong applied pressure (∼200 kPa; left to right images). The number at the bottom right corner of each image indicates the elapsed time.