*4.2. The Preparation of PVA Hydrogel*

First, 8.0 g PVA powder was added to 92 g deionized water to obtain 8 wt% PVA solution. Secondly, the mixtures were put into in a three-mouth flask and then stirred at 95 ◦C until the PVA powder was completely dissolved. Subsequently, the PVA solution was cooled down to room temperature, until the air bubbles completely disappeared. Lastly, 8 wt% PVA solution was poured into a watch glass and placed in a −20 ◦C refrigerator for 2, 4, 6 and 8 h respectively to obtain PVA hydrogels with different freezing durations.
