Ultra‐Conformable Ionic Skin with Multi‐Modal Sensing, Broad‐Spectrum Antimicrobial and Regenerative Capabilities for Smart and Expedited Wound Care

Abstract While rapid wound healing is essential yet challenging, there is also an unmet need for functional restoration of sensation. Inspired by natural skin, an ultra‐conformable, adhesive multi‐functional ionic skin (MiS) with multi‐modal sensing capability is devised for smart and expedited wound care. The base of MiS is a unique skin‐like, conductive and self‐adaptive adhesive polyacrylamide/starch double‐network hydrogel (PSH) and self‐powered, flexible, triboelectric sensor(s) is integrated on top of PSH for multi‐tactile sensing. MiS could enhance wound contraction, collagen deposition, angiogenesis, and epidermis formation in a full‐thickness skin defect wound model in vivo, while significantly inhibiting the biofilm formation of a wide range of microorganisms. MiS also exhibits multi‐modal sensing capability for smart and instant therapeutics and diagnostics, including skin displacement or joint motion, temperature, pressure and tissue exudate changes of wound bed, and locally releasing drugs in a pH‐responsive manner. More importantly, MiS could restore the skin‐mimicking tactile sensing function of both touch location and intensity, and thus could be used as a human‐machine interface for accurate external robotic control. MiS demonstrates a new comprehensive paradigm of combining wound diagnosis and healing, broad‐spectrum anti‐microbial capability and restoration of multi‐tactile sensing for the reparation of severe wound.


Fourier Transform Infrared (FTIR) and scanning electron microscopy (SEM) analyses.
FTIR spectra of the PAM, GPAH, and PSH were recorded in a range of 500 cm −1 to 3800 cm −1 using FTIR spectroscopy (Nicolet 6700, Thermo Fisher Scientific, USA). The microstructure of the surface of freeze-dried PAM and PSH was observed by SEM (JSM-7100F, JEOL, Japan).
The SEM samples were sputter-coated for 60 s using an ion-sputter coating instrument (SC7620, Quorum, England).
Measurements of the adhesiveness of PSH partially swelled with saline. In order to evaluate the effect of normal perspiration on the adhesiveness of PSH to skin, partially swelled PSH was prepared by absorbing 50 wt % of saline. The interfacial strength and interfacial toughness of partially swelled PSH to fresh porcine skin was measured with the same experimental protocol of test on as-prepared PSH.
Measurements of the water absorption of PSH at different pH value. PSH samples were immersed in PBS at various pH values (pH = 4, 7, and 10) at 37 °C. At 12 h, the samples were retrieved and weighed after wipe of water from the sample surface. The amount of water absorbed in the materials was defined using the following equation: Water absorption ratio % 100% where Wi and W0 represent the sample weight after immersed in PBS for 12 h and the sample theoretical dry weight, respectively. 3

Measurements
To evaluated the self-healing capability of conductivity, the PSH conductor was cut in half and re-connected. The variation in the current before cutting and after re-connecting were measured by a multimeter and also demonstrated by the brightness of a LED light that was connected in series with PSH. The change in the current was expressed by the I/I0 ratio, where I and I0 are the current through damaged (either separated or re-connected) and intact hydrogel samples, respectively.
In order to evaluated the effect of perspiration-caused NaCl retention on the electrical property of PSH, the conductivity of PSH containing 0.9 wt % of NaCl was also measured. Gauge factor (7) where R0 is the resistance when the strain is 0%; ΔR is the changes in the resistance when the strain of MiS is 100, 200, 300 and 400%, respectively; ε is the strain of MiS. Statistical analysis: All the results were reported as a mean with standard deviation. Sample size (n) of independent repeated experiments for each statistical analysis was given in the figure legends. Statistical differences between two groups were determined by two-tailed unpaired Student's t-test using GraphPad Prism 6 (GraphPad Software, USA). Differences were considered significant at p < 0.05.

Cross-linking mechanism of PSH
The formation of covalent bonds between acrylamide monomers is the result of free radical polymerization. [88,89] The physical cross-linking of starch molecules in GPAH is likely mediated through ligand binding between amylopectin hydroxyl groups and positively charged Ca 2+ . [90][91][92] The FTIR of PSH ( Figure S1) revealed a peak at 3220 cm −1 corresponding to an -          Demonstration of the MiS, as a flexible human-machine interface, accurately controlling a robotic arm by sensing tactile signals on its top surface. The robotic system includes the MiS, a signal-acquisition system, a computer, a drive system, and the associated robotic arm.