L929 cells were cultured in DMEM containing 10% fetal bovine serum (FBS, Gibco, Thermo Fisher Scientific, USA) and 1% penicillin-streptomycin (FBS, Gibco, Thermo Fisher Scientific, USA). The cells were maintained on humidified incubator (Panasonic MCO-18AC, Japan) with 5% CO2 at 37 °C.

CCK-8 assay: Samples were placed in a 48-well plate and sterilized by ultraviolet irradiation for 60 min. L929 cells were then seeded on the sample surface at a density of 5 × 104 cells cm−2. After culturing for 1, 3 and 7 days, respectively, samples were taken out and placed in culture dishes, and 100 μL of serum-free DMEM medium and 10 μL of CCK-8 reagent were added to the culture dishes. After incubation at 37 °C for 4 h, the absorbance of the supernatant was measured at 450 nm on a microplate reader (MULTISKAN MK3, Thermo, USA).

Live/Dead staining: Samples were put in a 48-well plate, and a suspension of L929 cells in the fibroblast medium was added to each well at a density of 5 × 104 cells cm−2. The plate was incubated at 37 °C in 5% CO2. On day 1, 3 and 7 after incubation, the fibroblast medium was replaced by PBS solution containing 1.5 μL propidium iodide and 1 μL calcein-AM (Calcein-AM/PI Double Staining Kit, Japan) and incubated for another 30 min. Then the samples were gently washed with PBS and then observed under a fluorescence microscope.