2.2. Fabrication of DBS and double-network nanocomposite hydrogels (Gels)

The debranched starch was obtained from normal corn starch by enzymatic modification according to the method (Sun, Li, Dai, Ji, & Xiong, 2014). Briefly, the gelatinized starch slurry (20% wt) was cooled to 58 °C, debranched with pullulanase (4, 8, and 12 h), and then heated in boiling water for pullulanase deactivation. Finally, DBS powders were obtained by freeze-dried (-80℃ for 72 h) and the corresponding samples were coded as DBS₄, DBS₈, and DBS₁₂, respectively.

The fabrication of Gels was according to our previous method (Qin et al., 2020). Firstly, PVA powder (4% wt) was dispersed in deionized water at 90 °C and stirred for 1 h until PVA was dissolved completely. Subsequently, the DBS and PVA homogeneous solution was prepared by the addition of DBS powders (10% wt) into cooled PVA solution (25 °C) and stirred at 135 °C for 30 min in an oil-bath. To avoid the evaporation of water, the heating and stirring process was performed in a hermetically sealed flask. Furthermore, the solution was sonicated for 5 min to remove residual bubbles. The resultant solution was quickly poured into the cylindrical molds (Φ = 25 and 4 mm), and then retrograded (4 °C for 24 h) to form a starch network. After that, the specimen was treated with a frozen and thawed cycle (-40 °C for 8 h and 25 °C for 4 h) to further obtain PVA network. Note that the freeze–thaw cycle was applied two times. Finally, the as-prepared hydrogels were coded as Gel₄, Gel₈, and Gel₁₂, respectively.