A method of forming a composite material includes disposing dried plant material, nanoparticles, and a supercritical fluid in a vessel. A cellular structure of the dried plant material expands when disposed in the supercritical fluid and the nanoparticles migrate into and are embedded within the expanded cellular structure of the disposed dried plant material. The disposed dried plant fibers with embedded nanoparticles are removed from the vessel and mixed with a polymer to form a polymer-nanoparticle mixture. A chemical additive can be added to the supercritical fluid and the chemical additive can remove at least one of hemicellulose, lignin and pectins from the dried plant material.