In a lipid bilayer, molecules are oriented in such a way that their polar fragments are turned towards the aqueous phase and form two hydrophilic surfaces, and non-polar tails form a hydrophobic region within the bilayer (see Fig.). A bilayer is a thermodynamically favourable structure of polar lipids in aqueous solution. Virtually all biological membranes are made mostly of phospholipid (phosphatidylcholine, phosphatidylserine, etc.) bilayers. A bilayer is usually 4-5 nm thick. Phospholipids are used to produce liposomes and nanosomes for the encapsulation and delivery of biologically active substances in medicine and cosmetology.