{
    "protocol": "To synthesize hydrophilic polymers with functional groups, first treat the polymer substrates with a 10 wt% solution of hydrophobic photo- or thermoinitiators (e.g., benzophenone, 4-methyl benzophenone, benzoyl peroxide, azobisisobutyronitrile) in an organic solvent (e.g., ethanol, isopropanol, acetone) via swelling-driven surface absorption to form a surface-bound diffusion layer of hydrophobic initiators. Next, immerse the treated polymer substrates in a hydrogel pregel solution bath containing hydrophilic photo- or thermoinitiators (e.g., Irgacure-2959, α-ketoglutaric acid, ammonium persulfate, potassium persulfate) and hydrogel monomers (e.g., acrylamide (AAm), acrylic acid (AA), N,N-dimethylacrylamide (DMAA), N-vinylpyrrolidone (VP), hydroxyethyl methacrylate (HEMA)) in aqueous solution. Polymerize the hydrogel pregel solution using UV light or heat, allowing the hydrophobic initiators to act as grafting agents and oxygen scavengers, while the hydrophilic initiators facilitate the polymerization of the hydrogel monomers. After polymerization, wash the non-crosslinked hydrogel polymer solution away with water to obtain the polymer substrates with hydrogel skins.The polymer substrates were first cleaned with isopropanol and deionized water followed by drying under nitrogen flow. To enhance wettability of the polymer substrates, the substrates were treated by atmospheric plasma by a plasma cleaner for 3 min. The plasma treated polymer substrates were then immersed into a hydrophobic initiator organic solution for 3-5 min. After gently rinsing with isopropanol followed by drying under nitrogen flow, the substrates were immersed into a hydrogel monomer aqueous solution containing hydrophilic initiator. To form hydrogel skins, the monomer solution bath was subjected to either UV irradiation or an 80°C oven for 30–90 min. After formation of hydrogel skins, unreacted regents were thoroughly rinsed with a large amount of deionized water for 24 h."
}