Plastics can be filled with many materials.  Generally fillers are isotropic particles or nearly isotropic.  Usually they an non-reinforcing depending on the interaction with the matrix.  (RDC 11/4/2011)

Composites
Materials

Recent Journal Articles

Pollen: A Novel, Biorenewable Filler for Polymer Composites
(1055–1062)
 
Macromolecular Materials and Engineering 296 #11 (2011)
Lee et al of the Georgia Institute of Technolog, Georgia, incorporated pollen grains (short ragweed) in two polymers, poly(ε-caprolactone) (PCL) and polystyrene (PS). Under certain solvent and annealing conditions, PS mechanical properties were improved synergistically upon addition of pollen, while those of PCL were always degraded, in strong agreement with wetting behavior of the polymer–pollen interface.  (RDC 3/11/2011) (See Pollen)