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andβ-glycerophosphate sodium were obtained from Sigma-Aldrich (St. Louis, USA). Cell

Counting Kit-8 (CCK-8) kit and 4',6-diamidino-2-phenylindole (DAPI) were purchased from

Nanjing Keygen Biotech Co., Ltd. (Nanjing, China). Bradford assay kit was purchased from Bio-

Rad (Hercules, CA, USA). Qtracker®525 and 655 Cell Labeling Kits were obtained from Thermo

Fisher Scientific (Waltham, USA). Penicillin-streptomycin, Fetal bovine serum (FBS), and

Dulbecco’s modified Eagle medium (DMEM) were purchased from Gibco (Grand Island, USA).

2.2. Preparation and Characterizations of PMs

Initially, PLGA-based PMs were prepared by a microfluidic technique, as previously

30
reported. Briefly, a customized coaxial microfluidic system was prepared using plastic tubes

(inner diameter, ID: 1 mm), a glass capillary (outer diameter, OD: 1 mm), and a needle (27G, ID:

210 µm, OD: 400 µm). PLGA (180 mg) was dissolved in DCM (4.5 ml), and the aqueous gelatin

(6 % w/v) was then added to form the water-in-oil (W/O) emulsion. The initial emulsion was

stabilized by ultrasonication and then introduced into the microfluidic device as the discontinuous

phase, while PVA solution (1% w/v) was used as the continuous dispersion phase. The flow rates

of the discontinuous and the dispersion phases were set at 0.05 and 2 mL/min, respectively. The

PLGA-based PMs were subsequently collected in a pre-cooled PVA solution (1% w/v). After

stabilization, the PMs were washed with deionized water at 40℃ to eliminate gelatin and achieve

a porous structure. Finally, the PMs were lyophilized and stored until further use.

The morphology of PMs was observed by field emission-scanning electron microscopy (FE-

SEM, S-4800, HITACHI, Tokyo, Japan) after sputter-coated with gold. Fourier transform infrared

(FTIR) spectra were obtained using a Nicolet iS50 model FTIR spectrophotometer (Thermo Nicolet

−1
Corporation, Madison, WI, USA) at a 4 cm resolution using KBr pellets. The degradation of PMs

was investigated by exposing the PMs to PBS (pH 7.4) at 37 ºC, and the changes in the morphology

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