Polyphosphoric acid

The Model 381-20 of cellulose acetate butyrate ester (CAB) was purchased from Eastman Chemical Company(Kingsport, TN, USA). We bought the EVOH Model ET3803 masterbatch from Nippon Synthetic Chemical Industry Co., Ltd.(Osaka, Japan). Glutaraldehyde aqueous solution (GA, 25 wt%), tert-butanol (>98%), acetic acid, polyphosphoric acid (PPA, phosphorus pentoxide content >85 wt%), butane tetracarboxylic acid (BTCA), acetone, phosphate buffer saline (PBS), phosphoric acid (H₃PO₄), NaOH, LiCl, KCl, MgCl₂, NaCl, and NaSO₄ were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Papain, lysozyme, ovalbumin, bromelain, bovine serum albumin, and pepsin were bought from Sangon Biotech Co., Ltd. (Shanghai, China).

The polylactide (PLA 4032D) was obtained from NatureWorks (Minnetonka, MN, USA). Its specific gravity density is 1.24 g/cm³. Microcrystalline cellulose (MCC, product number: 68005761), polyphosphoric acid, phosphoric acid, and acetic anhydride were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The PLA pellets were dried in a vacuum oven at 60 °C for 48 h before use.