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Figure 2. Interfacial chemical and structural asymmetry of the AP-HC bilayer hydrogel. (A) ATR-FTIR spectra of the AP-layer and HC-layer surfaces of AP-HC; (B) XPS survey spectra of the AP-layer and HC-layer surfaces of AP-HC; (C) High-resolution XPS C 1s and N 1s spectra collected from the AP-layer and HC-layer regions of AP-HC; (D) Cross-sectional EDS elemental mappings of C and Cl in the AP-HC gel; (E) SEM images of the cross-sectional microstructures in the AP-layer and HC-layer regions of AP-HC; (F) Optical microscopy images of the AP-layer and HC-layer sides of the bilayer hydrogel; (G) Three-dimensional surface profilometry maps of the AP-layer and HC-layer surfaces of AP-HC. ATR-FTIR: Attenuated total reflectance Fourier-transform infrared; XPS: X-ray photoelectron spectroscopy; EDS: energy-dispersive X-ray spectroscopy; SEM: scanning electron microscope.