Rapid self-assembly of robust ultrathin ionogel films for high-performance bioelectronics

Abstract

Ionogels, polymer networks infiltrated with ionic liquids (ILs), are promising for flexible electronics but face trade-offs among mechanical robustness, ultrathin form factors, and scalable fabrication. We present an IL-induced self-assembly strategy enabling the rapid formation of ultrathin polyvinyl alcohol (PVA) ionogel films. Upon contact with ILs, PVA chains spontaneously organize into a robust, noncovalently cross-linked network, achieving ultrafast conversion (<5 s) of viscous precursors into films with tunable thickness (13 to 103 μm). The resulting ionogel films combine high tensile strength (9.69 MPa), toughness (35.93 MJ m^-3), good ionic conductivity (0.2 S m^-1), and excellent environmental stability. This approach allows in situ formation of conformal ionogel coatings on complex, nonplanar surfaces, yielding seamless skin-device interfaces that retain stable functionality under repetitive deformation. Ionogel-based bioelectrodes capture diverse electrophysiological signals with high fidelity while serving as stretchable substrates for printed circuits and electrode arrays. Compatibility with diverse ILs highlights the versatility of this rapid, scalable approach for fabricating ultrathin ionogels with broadly tunable properties.

Fig. 1.. Fabrication of highly robust and ultrathin ionogel films.

(A) Schematic illustration of the rapid preparation process for ultrathin ionogel films via IL-induced self-assembly strategy. (B) Schematic diagrams depicting the formation mechanism and crosslinking structure of PVA/ES ionogel films. (C) Photograph of a large-sized PVA/ES ionogel film (10 cm by 13 cm). (D and E) Cross-sectional SEM image of the PVA/ES ionogel film (D) and corresponding EDS elemental mapping of the cross section (E). (F) Photograph of the PVA/ES ionogel film with a cross section of 10 mm by 13 μm holding up a 100-g weight. (G) Seamless, conformal adhesion of the PVA/ES ionogel film onto human skin via a painting-soaking fabrication process.

Fig. 2.. Mechanism characterization and structural analysis of PVA/ES ionogel films.

(A and B) Raman spectra of [EMIM] [ES], PVA aqueous solution (aq), and PVA/ES ionogel film (A) and corresponding fitting curves of PVA (aq) and PVA/ES ionogel film (B). (C) TGA curves of [EMIM] [ES], PVA, and PVA/ES ionogel film. (D) FTIR spectra of [EMIM] [ES], PVA (aq), and PVA/ES ionogel film. (E and F) XRD curves (E) and SAXS patterns (F) of PVA and PVA/ES ionogel films.

Fig. 3.. Mechanical properties, conductivity, adhesion, and environmental stability of PVA/ES ionogel films.

(A) Photographs showing the high stretchability of the PVA/ES ionogel film. (B) Tensile stress-strain curves and corresponding elastic modulus and toughness (C) of PVA/ES ionogel films fabricated with different induction times (5 s, 60 s, and 6 hours). (D) Radar chart comparing the PVA/ES ionogel film with previously reported ionic conductive gels in terms of gelation time, thickness, tensile strength, and toughness. (E) Conductivity of PVA/ES ionogel films as a function of induction time (5 s, 60 s, and 6 hours). (F) Photograph showing LED luminance using the PVA/ES ionogel film as conductors in series circuits. (G) Microscope and optical images of on-skin PVA/ES ionogel film peeled from skin. (H) Lap-shear strength of PVA/ES ionogel films to different substrates. The insert image shows schematic illustration of lap shear test. (I) Comparison of lap shear strength between the PVA/ES ionogel film and other gels used in bioelectronics. (J) Weight changes of the PVA/ES ionogel film at ambient conditions (25°C, 30% RH) for 10 days. (K and L) Tensile stress-strain curves (K) and corresponding elastic modulus and toughness (L) of the PVA/ES ionogel film stored under ambient conditions for different time periods. Error bars represent the SD of the measure values (n = 3).

Fig. 4.. Application of painted ultrathin PVA/ES ionogels as reliable bioelectrodes for recording electrophysiological signals.

(A) Schematic illustration of the adhesion mechanisms of painted PVA/ES ionogel bioelectrodes on human skin. (B) H&E staining images of healthy tissue sections and tissue sections after 1 week of application with the PVA/ES ionogel film. Scale bar, 200 μm. (C) Skin interfacial impedance as a function of frequency for painted PVA/ES ionogel bioelectrodes, thick PVA/ES ionogels (~0.6 mm), commercial Ag/AgCl electrodes, and Pt electrodes. (D) Comparison of interfacial impedance at 10 and 100 Hz between painted PVA/ES ionogel bioelectrodes and other advanced gel-based bioelectrodes. (E) EMG signals captured by painted PVA/ES ionogel bioelectrodes, thick PVA/ES ionogels, and commercial Ag/AgCl electrodes. (F and G) EMG signals (F) and EMG potential (G) recorded during cyclic operation of a gripper under varying force levels. (H) Photographs depicting EMG testing conducted on hairy skin. (I) Detection of various body movements from EMG signals. The inset images illustrate typical actions: (a) standing on tiptoes, (b) squatting up, and (c) jumping. (J) ECG signals recorded by painted PVA/ES ionogel bioelectrodes, thick PVA/ES ionogels, and commercial Ag/AgCl electrodes (a). Comparison of ECG signals before and after running (b). Continuous ECG monitoring over 250 s using painted PVA/ES ionogel bioelectrodes (c). Error bars represent the SD of the measure values (n = 3). a.u., arbitrary unit.

Fig. 5.. Development of customized patterns on the PVA/ES ionogel film for flexible electronics.

(A) Fabrication process of integrated circuits and electrode arrays including printing EGaIn/PVA ink on the PVA/ES ionogel film, followed by mechanical activation. (B and C) Photographs of printed EGaIn/PVA conductive ink circuits on the PVA/ES ionogel film demonstrating high printing resolution (B) and excellent robustness and conformability (C) including resistance to mechanical deformations such as stretching and puncturing, as well as seamless adhesion to the back of the hand. (D) Images of integrated circuits equiped with LED on the PVA/ES ionogel film under relaxing, laminating on a cylindrical surface, and twisting 180°. (E and F) Three-channel EGaIn/PVA/ES electrode patch positioned onto forearm muscle (E) and face (F) to dectect EMG signals. (G and H) Obtained three-channel EMG signals using the three-channel electrode array during three simple hand movements (G) and facial muscle movements during speaking (H).

Fig. 6.. Universality of the IL-induced assembly strategy for constructing various PVA ionogel films.

(A) Chemical structures of different ILs and corresponding photographs of the obtained PVA ionogel films induced by these ILs. (B and C) Tensile stress-strain curves (B) and corresponding toughness and tensile strength (C) of different PVA ionogel films. (D to F) TGA curves (D), FTIR spectra (E), and XRD profiles (F) of various PVA ionogel films. (G to I) Ashby plots of fracture strain versus fracture stress (G), fracture strain versus toughness (H), and fracture stress versus toughness (I) of PVA ionogel films induced with different ILs and various representative composite ionic gels and elastomers reported in references. Error bars represent the SD of the measure values (n = 3).