Ultrafast, autonomous self-healable iontronic skin exhibiting piezo-ionic dynamics

Abstract

The self-healing properties and ionic sensing capabilities of the human skin offer inspiring groundwork for the designs of stretchable iontronic skins. However, from electronic to ionic mechanosensitive skins, simultaneously achieving autonomously superior self-healing properties, superior elasticity, and effective control of ion dynamics in a homogeneous system is rarely feasible. Here, we report a Cl-functionalized iontronic pressure sensitive material (CLiPS), designed via the introduction of Cl-functionalized groups into a polyurethane matrix, which realizes an ultrafast, autonomous self-healing speed (4.3 µm/min), high self-healing efficiency (91% within 60 min), and mechanosensitive piezo-ionic dynamics. This strategy promotes both an excellent elastic recovery (100%) and effective control of ion dynamics because the Cl groups trap the ions in the system via ion-dipole interactions, resulting in excellent pressure sensitivity (7.36 kPa^-1) for tactile sensors. The skin-like sensor responds to pressure variations, demonstrating its potential for touch modulation in future wearable electronics and human-machine interfaces.

Fig. 1. Molecular structure of the skin and conceptual design mechanism of CLiPS.

a Schematics of the self-healing properties and biological mechanoreceptor of the human skin. The human skin achieves strong autonomous self-healing upon damage owing to the activities of blood cells and platelets. It also perceives external stimuli via the generation of an action potential resulting from ion dynamics. b CLiPS-based device architecture illustrating autonomous self-healing and tractive self-healing of electrodes, as well as the molecular structure emulating the ion dynamics of the human skin based on the trap and release mechanism. c Chemical structure representation of the design rule of CLiPS.

Fig. 2. Autonomous self-healing demonstration of the CLiPS device.

a Autonomous self-healing of a scar within 60 min at room temperature (RH 20–40%) observed via an optical microscope. The experiments (top; scar bar 100 µm and cross-section view; scar bar 200 µm) were conducted independently and produced similar results. b Two cut and spliced individual CLiPS films healed together to withstand stretching. c Demonstration of the tractive self-healing of the electrode’s conductivity using an LED. d Comparison of the self-healing speeds as a function of Cl groups concentration. E3 and E7 have the lowest and highest Cl groups content, respectively. CLPU-IL and CLPU-pristine denote CLPU films with 30 wt% IL and without IL concentration, respectively. All self-healing tests were carried out at room temperature (RH 20–40%) without external stimuli. e Stress–strain curves of the original and self-healed films at various healing times. f Comparison of self-healing speeds between CLiPS and other room-temperature self-healing dielectric elastomers (blue color) and ionic-based materials (red color).

Fig. 3. Molecular characterization of CLiPS.

ATR-FTIR spectra of CLPU@E0-IL (used as reference) and CLiPS films. ATR-FTIR spectra in the spectral regions of a, 1000–1350 cm⁻¹ (pertaining to TFSI⁻ stretching) and b, 1400–1650 cm⁻¹ (pertaining to EMIM⁺ stretching). c, d Raman spectra in the ranges of 200–720 cm⁻¹ and 900–1700 cm⁻¹ corresponding to TFSI^- and EMIM⁺ vibrational bands, respectively. These spectra confirm the trapping of ion pairs to Cl groups via ion–dipole and Coulombic interactions. e (i) Schematic presenting of ion–dipole interaction between Cl groups and the [EMIM]⁺ cation, and Coulomb force between ion pairs, (ii) atomic number scheme of [EMIM]⁺ utilized in (d), and atomic structure of [TFSI]⁻.

Fig. 4. Molecular design and working principle of the trap and release-based CLiPS device.

Design of the piezocapacitive device consisting of the CLiPS film sandwiched between AgNW/CLPU@E5 flexible electrodes (1 mV to 1 V). a Confinement of [EMIM]⁺[TFSI]^- ion pairs to Cl groups (trapped state) at a pre-stimulus condition. b Schematic of CLiPS demonstrating the pumping of ions owing to pressure-impelled breaking of ion–dipole interactions under deformation and EDL formation at the CLiPS/electrode interface. Ion dynamics and free ion density of (c) CLiPS and (d) CLPU@E0-IL films with a stepwise pressure increase. CLiPS exhibits trap and release mechanism as the free ion density increases with pressure. e Charge relaxation time decreases with increased pressure input owing to the release of more free ions in the CLiPS. f Nyquist plot of CLiPS under no pressure (NP), under pressure (UP), and after removing pressure (AP), confirming reversible movement of ions (insert shows ion conductivity under each condition).

Fig. 5. Sensing performance and practical application of the CLiPS device.

a Pressure response of the original and self-healed films under a static pressure condition (1 V applied bias @ 100 Hz). b Pressure sensitivity comparison between CLiPS and CLPU@E0-IL (insert, black) (used as a reference), applied bias of 100 mV at 20 Hz. c Relative change in capacitance plots as a function of the applied pressures (0.08 kPa, 0.2 kPa, 0.5 kPa, and 2.1 kPa) with respect to time (applied bias voltage of 100 V at 100 Hz). d Mechanical durability test results of the CLiPS-based sensor (250 cycles), applied bias at 100 mV at 100 Hz). e Transient response time of the CLiPS piezocapacitive sensor at a loading pressure of 90 Pa. The insert represents a response time of 260 ms and a reset time of 270 ms. f Comparison between CLiPS-based sensor and previously reported autonomous self-healing iontronic pressure sensors in terms of autonomous capability, sensitivity, modulus, self-healing efficiency, and stretchability. Red, black, blue, green, dark-blue, violet, and turquoise colors correspond to this work, ref. , ref. , ref. , ref. , ref. , and ref. , respectively (see detail on Supplementary Table 5). g, h Photographs showing LED brightness before and after applied pressure of the CLiPS-based device (left) and the CLPU@E0-IL-based device (right).