A multifunctional structural coloured electronic skin monitoring body motion and temperature

Multifunctional e-skins provide information on physiological and environmental parameters. However, the development and fabrication of such devices is challenging. Here, structural coloured electronic skins are presented, which are prepared via scalable methods that can simultaneously monitor the skin temperature and body motion when patched onto the human skin.


Fabrication of the AgNW/PET substrates
The procedure to fabricate the AgNW/PET substrates with gravure printing is identical to earlier reported work. 1 A silver nanowire containing ink (TranDuctive N15, Genesink) was applied on top of a biaxially oriented transparent PET substrate having a thickness of 100 µm (Melinex 506, DuPont Teijin Films) via gravure printing (IGT F1 Printability Tester). The AgNW/PET foils were prepared by using a 'gravure printing with pre-inking' mode with a printing speed of 0.5 m/s, an anilox force of 250 N, 50 % anilox speed, and three pre-ink revolutions for the anilox before the AgNW-based ink was applied on top of the PET substrate. Afterward, the transparent foils (24 x 5 cm 2 ) were cured for 5 min at 90 °C in an oven to induce film formation. After curing, a fourprobe method was used to determine the sheet resistance (R s ) of the printed AgNW/PET substrates: R s = 25.97 ± 0.41 Ω sq -1 .

Preparation of the temperature-responsive photonic emulsion
Polyvinyl alcohol (15 wt %; M w = 9.000-10.000 g/mol, 80 % hydrolysed) and glycerol (2 wt %) were dissolved in demineralized water while stirring at 70 °C until a homogeneous mixture was obtained. A temperature-responsive CLC mixture consisting of a nematic liquid crystalline mixture E7 (70 wt %) and a chiral dopant S811 (30 wt %) was obtained by blending these components at 60 °C to yield a homogeneous mixture. The CLC mixture was added to the aqueous PVA solution in a 20/80 weight ratio and subsequently emulsified at 35 °C for 0.5 h using an overhead stirrer (IKA T 18 digital ULTRA-TURRAX) at a stirring speed of 3000 rpm.

Fabrication of the multifunctional structural coloured e-skins (PDCLC/AgNW films)
The above-described photonic emulsion, consisting of CLC microdroplets dispersed in an aqueous PVA solution, was diluted by adding a PVA/glycerol solution (15 wt % PVA; Mw = 31.000-50.000 g/mol, 87-89 % hydrolysed, and 2 wt % glycerol in H 2 O) in a 10/90 ratio, respectively. The mixture was shaken by hand at room temperature for 2 min to ensure homogeneous mixing and was degassed under vacuum before usage. The diluted emulsion was manually drop cast on top of the gravure printed AgNW/PET substrates after which the system was left to dry for at least 12 hours. After drying, smaller pieces were cut from the PDCLC/AgNW/PET foils to ensure proper contact with the human finger. The dried film was peeled off the substrate using a razor blade, yielding the temperature-responsive PDCLC/AgNW films having an average thickness of 80 µm.
Before recording the resistance changes of the PDCLC/AgNW wearable, wires were attached to the photonic wearable using a conductive epoxy glue (Chemtronics, CW2400) spread out over the width of the films and cured at room temperature for 4 h to establish proper electrical contact. The wires were connected to a DC power supply (Keithley 2400 SourceMeter), which monitored the resistance of the PDCLC/AgNW film over time.

Characterization of the PDCLC/AgNW films
The temperature-responsive reflection band shifting was analysed by heating the free-standing PDCLC/AgNW films with an external hot plate while recording the reflection spectra with a Perkin Elmer Lambda 750 UV/vis/NIR spectrophotometer. Air was used as a baseline for the reflection measurements. Polarized optical microscopy (Leica DM2700M microscope) images were collected in reflection mode under crossed polarizers.   Figure S3. Temperature-responsive reflective color of the photonic wearable at different environmental temperatures. It can be observed that in this case a small color shift was observed for an environmental change of ΔT = 5°C due to heat convection between the user and the environment. Fig. S4. a) Images of the structural coloured e-skin (T = 33 °C) captured from different viewing angles, displaying the angular-independent reflective colour. b) Uniform colouration of the structural coloured eskin was observed upon finger bending regardless of the viewing direction (T = 33 °C). Fig. S5. Reflection spectra of a PDCLC/AgNW film (T= 33 °C) measured in the unstrained state (ε 1 = 0, ε 2 = 0) and under unidirectional (ε 1 = 0.5, ε 2 = 0) and bidirectional stretching (ε 1 = 0.5, ε 2 = 0.5). Figure S6. Real-time resistance monitoring at room temperature of the multifunctional structural coloured e-skin that experienced cyclic finger bending (green), unbending (red), and steady-state periods (grey) corresponding to time intervals for which the finger was in the rest state. Figure S7. Real-time resistance monitoring of an e-skin that experienced tactile sensing by rapidly pressing the fixed film.