Mosquitos spread potentially fatal diseases affecting humans, including malaria, zika, chikungunya, and yellow fever — making mosquitoes the deadliest animals on Earth. However, treating cellulose with sulfuric acid prompts it to assemble into cellulose nano crystals, which then self-assemble into strong, transparent barrier films. Mixed with water and a small amount of glycerol, the nano crystals can be applied to the skin like a gel. In trials with live Aedes aegypti mosquitos, use of the cellulose nano crystal gel reduced feeding significantly.