Brushette bristles are made of US Dupont Tynex Filament, made by extruding a mixture of nylon and abrasive grit.
That's a fancy name to say that we work with the leader of synthetic filament manufacturer. All the bristles around your toothbrush, paint brush or toilet brush or synthetic filament which are most of the time nylon, which is a sort of plastic.
You can read more about the used material here.
Brushette neck adapters are mainly made of ABS or POM plastic and nylon.