*2.1. Chemicals*

Gelatin (GA, Bloom 250 g) was obtained from Aladdin, Inc. (Shanghai, China). Nylon 66 (PA66, pellets) was obtained from Sigma Aldrich (St. Louis, MO, USA). 88% (*v*/*v*) Formic acid and glacial acetic acid (analytical reagen<sup>t</sup> grade) were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
