A new strategy for ultra-thin ion gel, only 16 microns thick!

Background

Soft electronic skin (e-skin) can be attached to biological tissue for a long time, enabling continuous and uninterrupted biological monitoring. These properties make e-skin very suitable for applications in different fields such as human motion state analysis, visual medical diagnosis, human-machine interface, soft robotics, and biomedical engineering. Conductive gels (including conductive hydrogels, conductive organic gels, and ion gels) are a class of soft electronic materials characterized by porous structures and unique conductive properties. Conductive gels are very similar to the physiological and mechanical properties of biological tissues, making them promising candidates for electronic skin sensors. However, the considerable thickness of these gels often leads to poor breathability, and long-term use may lead to adverse health effects such as skin irritation, inflammation, and allergies.

Ultrathin gels refer to materials with a thickness of less than 20 μm, which generally have excellent mechanical compliance. This property not only reduces contact impedance and improves the accuracy of biomonitoring, but also ensures a close fit with the skin and promotes seamless connection between human and machine interfaces. Thin gels usually reduce mechanical properties such as strength, toughness and stretching range, which poses great challenges for practical health monitoring applications. To address these issues, fiber reinforcement technology has been introduced as a strategy to build strong and durable ultrathin gels. For example, Gao et al. developed an ultrasoft fiber-composite ultrathin hydrogel (less than 5 μm thick) with a tensile stress of about 6 MPa and enhanced tear resistance. Similarly, Zhang et al. designed a 10-μm-thick nanomesh reinforced hydrogel for continuous, high-quality electrophysiological monitoring. However, these studies mainly focused on conductive hydrogels, which have inherent limitations such as dehydration at high temperatures and solidification at low temperatures, which hinder their applicability for long-term monitoring.

Ionogel is a conductive gel that combines the high conductivity and thermal stability of ionic liquids (ILs) with excellent sensing performance and stable mechanical durability, and is therefore expected to be a substitute for hydrogels. However, the development of ultrathin ionogel materials (thickness below 20 μm) with strong mechanical properties faces several challenges: i) the porous structure of the nanomesh complicates the polymerization of ionogels; ii) it is difficult to achieve uniform thickness of ultrathin ionogels; iii) it is challenging to transfer ionogel materials to a predetermined substrate without loss. To date, only a few studies have successfully addressed these challenges encountered in the preparation of ultrathin ionogels.

Highlights of this article

1. This work describes the design and fabrication of nanomesh-supported ion gels with a thickness of approximately 16 µm, which is the thinnest ion gel reported to date. 

2. This ultrathin ion gel has excellent toughness (5.51 MJ m ⁻³ ), a wide strain range (0-375%) and excellent fatigue resistance, capable of withstanding more than 3,000 cycles.

3. It also has excellent water resistance and high water vapor transmission rate (WVTR), supporting perspiration and improving skin breathability.

4. This ultrathin ion gel also has the potential to be used as a carrier in transdermal drug delivery systems (TDDS), highlighting its applicability in biomedical applications.

Graphical analysis

Figure 1. An ultrathin ion gel with a thickness of 16.7 μm and supported by a thermoplastic polyurethane nanomesh was developed for continuous physiological monitoring. a) Schematic diagram describing the design concept and various functional properties of the ultrathin ion gel supported by the nanomesh. b) Cross-sectional SEM image of the ultrathin ion gel showing its structural features.

Figure 2. Characterization of the ultrathin ion gels included the following evaluations: a) SEM imaging of the internal structure of the ultrathin ion gel after freeze drying. b) Fourier transform infrared spectra of thermoplastic polyurethane, ion gel, and nanomesh-supported ion gel, showing the characteristic peaks of each material. c) Light transmittance of different nanomesh-supported ion gels. d) Comparison of water vapor transmission rate (WVTR) of the blank group (open bottle), control group (polyethylene film), and test group (ultrathin ion gel), with illustrations of each group from left to right. e) Water loss measurements of the blank group, control group, and test group over 15 days. f) Contact angle measurements of different samples, showing hydrophobicity. g) Schematic diagram of the tear resistance of the nanomesh-supported ion gel. h) Image showing the stretching behavior of the ultrathin ion gel when cracks appear. i) Photograph of the suspended ultrathin ion gel supporting 245 times its own weight in water before the first drop of water begins to fall.

Figure 3. Mechanical and adhesion properties of ultrathin ion gels: a) Stress-strain curves of thermoplastic polyurethane nanomesh, ion gel, and nanomesh-supported ion gel. b) Toughness measurements of thermoplastic polyurethane nanomesh, ion gel, and nanomesh-supported ion gel show their ability to absorb energy before failure. c) Photograph of ultrathin ion gel clamped by a tensile tester. d) Single-cycle stress-strain curves of ultrathin ion gel in the strain range from 10% to 300%. f) Photograph depicting the separation process of ultrathin ion gel from artificial skin. g) Force-displacement curves showing the adhesion of ultrathin ion gel. h) Areal adhesion energy of ultrathin ion gel on various substrates showing its adhesion properties. i) Ashby plots revealing the toughness of ultrathin ion gel as a function of strain range compared to other conductive gels. j) Ashby plots of toughness versus thickness of TPU nanomesh-supported ion gel and other reported conductive gels. k) Mechanical strength comparison of ultrathin ion gel with other ion gel materials in the literature.

Figure 4. Environmental tolerance of ultrathin ion gel. a) TGA curve of ultrathin ion gel; b) DSC curve; c) mass change; d) stress-strain curve; e) Bode plot; f) ionic conductivity of ultrathin ion gel stored at 70°C, -25°C and deionized water for 15 days.

Figure 5. Electromechanical and sensing properties of ultrathin ion gel. a) Measurement factor curve. b) Response signals of ultrathin ion gel at different strain rates, c) 10%-300% strain, and d). e) Retention curve of the response value of ultrathin ion gel at different strains. f) Response recovery time of ultrathin ion gel at 100% strain. g) 3000 cycles of ultrathin ion gel at 50% strain.

Figure 6. Ultrathin ion gel for physiological signal monitoring. a) Schematic diagram of the wireless monitoring system. b) Schematic diagram of measuring electrooculogram signals. c) Electrooculogram signals generated by the tester rotating the eyeball to the left and right. d) Schematic diagram of measuring EEG signals. f) Schematic diagram of measuring EEG signals. h) ECG signals in different states: walking, meditating, playing table tennis, and doing experiments. i) Schematic diagram of EMG signal measurement. j) Comparison of EMG signals generated by clenching fists ten times using commercial hydrogel and ultrathin ion gel.

Figure 7. Biosafety evaluation of ultrathin ion gel. a) In vitro drug release curve b) In vitro skin permeation curve c) H&E staining photos and digital photos of each group at 0 and 24 hours after application d) ΔEI value of each group at 24 hours e) Relative cell survival rate of L929 mouse fibroblasts inoculated at 24 hours.

 
Original link: First author: Jiale Zhang, Zhuangzhuang Ma

Corresponding author: Lichao Jia, Hongqiang Wang

Corresponding Units: Shaanxi Normal University, Northwestern Polytechnical University

DOI: 10.1002/adfm.202415694

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