Do you happen to know whether the worry is about the inertness of microplastics, and hence the physical damage of the particles. Or is it in the plastics being chemically interactive with tissue?
The inertness alone means boost in inflamattion, damaged cells, etc. Just consider that asbestos is biologically/chemically inert too - the issues come from inflammation, scarring, dna damage, etc.
But (and varying per microplastic case on lots of factors: composition, dyes used, etc), the chemical interaction can also play a role.