Tympanic membrane retraction become eroded. As skin cells die they form a barrier of dry protein called keratin. This layer of keratin is normally pushed out of the ear by migration of skin cells along the ear canal and is turned into wax. Clearance of keratin can be disrupted by tympanic membrane retraction so that keratin accumulates within the retraction pocket. When keratin becomes trapped deep inside the ear and cannot be cleaned out, it is known as cholesteatoma. Growth of bacteria in the trapped keratin causes smelly discharge from the ear and can spread to cause serious infection. Various strategies may be
Semipermeable membrane the rate and the permeability. Many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the thin film on the inside of the egg. Note that a semipermeable membrane is not the same as a selectively permeable membrane. Semipermeable membrane describes a membrane that allows some particles to pass through (by size), whereas the selectively permeable membrane "chooses" what passes through (size is not a factor). An example of a biological semi-permeable membrane is the lipid bilayer, on which is based on the plasma membrane that surrounds all biological cells. A group of