A thin solid that allows certain substances to diffuse through them is called a semipermeable membrane. Semipermeable membranes are typically made of materials such as cellulose, cellulose acetate, or polycarbonate. They have pores that are small enough to block the passage of larger molecules, such as proteins and starch, but allow smaller molecules, such as water and ions, to pass through. Semipermeable membranes are used in a variety of applications, including water purification, dialysis, and the production of biofuels.