Abstract: BACKGROUND: Poly(L-lactic acid) (PLLA) microparticle (Sculptra®) has been approved by FDA as the dermal filler to regenerate collagen for anti-aging purpose. Because of its irregular particle shape, it is prone to excessive inflammation.
OBJECTIVE: To investigate the effects of irradiation on the molecular weight, size and morphology of the poly(L-lactic acid) microspheres, as well as the foreign body reaction and the neocollagenesis stimulation of the poly(L-lactic acid) microspheres dermal filler implanted subcutaneously into back of rabbits.
METHODS: Microspheres were prepared by the emulsion-solvent evaporation method and utilized as the dermal filler with the same Sculptra’®s formula. The filler was sterilized by irradiation at 25 and 50 kGy, separately and the molecular weight, size and morphology of the microspheres were analyzed. The MTT assay was used to detect the effects of the microspheres at different concentrations (50, 100, 250, and 500 mg/L calculated by the concentration of microspheres) on the proliferation of mouse fibroblasts. The 25 kGy irradiated dermal filler (experimental group) and saline (control group) were injected into the back of the rabbit subcutaneously. Hematoxylin-eosin staining, Masson staining, and immunofluorescence staining of CD68, type I collagen and type III collagen were performed on the back tissue of the injection site at the set time points. 
RESULTS AND CONCLUSION: (1) With the increase of the radiation dose, the viscosity, viscosity-average molecular weight and number-average molecular weight of the microspheres decreased significantly, but the irradiation sterilization did not cause significant changes in the particle size and morphology of the microspheres. (2) MTT assay results demonstrated that when the mass concentration of the filler was lower than 250 mg/L, the cell survival rate was all >90%. Even when the material mass concentration reached the highest level 500 mg/L, the cell survival rate was still higher than 80%. Incubation for 72 hours also did not produce significant cytotoxicity. (3) In animal experiments, hematoxylin-eosin staining and CD68 immunofluorescence staining revealed that the filler caused slight inflammatory response from 0.5 to 4 months after implantation, and the inflammatory response reached the highest level at 6 months after implantation. Some microspheres were degraded to irregular shape or with pores on the surface, and the microspheres were degraded completely at 9 months after implantation. (4) In animal experiments, Masson staining and immunofluorescence staining of type I collagen and type III collagen exhibited that at 4 months after implantation, type I collagen was mainly around the microspheres, and type III collagen was mainly at the periphery. At 6 months after implantation, type I collagen near the microspheres increased, and type III collagen at the periphery increased. At 9 months after implantation, the fibrous encapsulation was mainly composed of type I collagen, and the ratio of type I collagen to type III collagen was close to the same at the periphery. (5) The results indicate that injectable poly(L-lactic acid) microsphere filler can stimulate collagen regeneration and can also reduce the degree of inflammatory response. 

Key words: poly(L-lactic acid), polymeric microspheres, dermal filler, inflammatory response, neocollagenesis, type I collagen, type III collagen, in vivo degradation