microorganisms, plants, and animals on artificial surfaces immersed in water. significant buildup of biofouling on marine vessels can be problematic. traditionally, biocides, a chemical substance or microorganism that can control the growth of harmful organisms by chemical or biological means, are used in order to prevent marine biofouling. biocides can be either synthetic, such as tributyltin (tbt), or natural, which are derived from bacteria or plants. tbt was historically the main biocide used for anti-fouling coatings, but more recently tbt compounds have been considered toxic chemicals which have negative effects on human and environment, and have been banned by the international maritime organization. the early design of anti-fouling coatings consisted of the active ingredients (e.g. tbt) dispersed in the coating in which they "leached" into the sea water, killing any microbes or other marine life that had attached to the ship. the release rate for the biocide however tended to be uncontrolled and often rapid, leaving the coating only effective for 18 to 24 months before all the biocide leached out of the coating. figure 3: biocide release over time this problem however was resolved with the use of so-called self-polishing paints, in which the biocide was released at a slower rate as the seawater reacted with the surface layer of the paint. more recently, copper-based anti-fouling paints have been used because they are less toxic than tbt in aquatic environment, but are only effective against marine animal life, and not so much weed growth. non-stick coatings contain no biocide, but have extremely slippery surfaces which prevents most fouling and makes it easier to clean the little fouling that does occur. natural biocides are found on marine organisms such as coral and sponges and also prevent fouling if applied to a vessel. creating a difference in electrical charge between the hull and sea water is a common practice in the prevention of fouling. this technology has proven to be effective, but is easily damaged and can be expensive. finally, microscopic prickles can be added to a coating, and depending on length and distribution have shown the ability to prevent the attachment of most biofouling.