Sintesis dan Karakterisasi Hidrogel Kitosan-Cangkok- Poli(N-Vinil pirrolidon) : Pengaruh Komposisi Monomer NVP, Pengikat Silang, dan Waktu Polimerisasi = Synthesis and Characterization of Hydrogel Chitosan- graft-Poly(N-Vinyl Pirrolidone) : Effect of NVP Monomer Composition, Crosslinker, and Polymerization Time