Von Frey hair
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Von Frey filaments rely on the principle that an elastic column, in compression, will buckle elastically at a specific force, dependent on the length, diameter and modulus of the material. Once buckled, the force imparted by the column is fairly constant, irrespective of the degree of buckling. The filaments may therefore be used to provide a range of forces to the skin of a test subject, in order to find the force at which the subject reacts because the sensation is painful. This type of test is called a mechanical nociceptive threshold test. The buckling force is inversely proportional to the length of the column (so the shorter the column, the higher the force required to buckle it) and proportional to the cube of the diameter, so that increasing the diameter of a filament by a small amount increases the buckling force considerably. Sets of filaments are normally made of nylon hairs, all the same length, but of various diameters so as to provide a range of forces, typically from 0.008 grams force up to 300 grams force. Von Frey filaments are a diagnostic, research, and screening tool, used both in human and animal medicine. They are readily used to study skin areas with normal responsiveness, as well as hyper- or hyposensitive areas. The determination of a mechanical threshold using von Frey filaments requires a number of discrete tests using filaments with different bucking forces. There are two commonly used algorithms, the up-down method and the percent response method. The up-down method is most commonly used, usually requiring a minimum of four tests after the first response is detected. The first measurement should be made with a filament with a buckling force close to the expected mean of the population. Errors may result if testing is commenced a long way above or below the mean. These errors have been evaluated by a combination of experimentation and mathematical simulation 
- Bradman, Matthew J.G.; Ferrini, Francesco; Salio, Chiara; Merighi, Adalberto (November 2015). "Practical mechanical threshold estimation in rodents using von Frey hairs/Semmes–Weinstein monofilaments: Towards a rational method". Journal of Neuroscience Methods 255: 92–103. doi:10.1016/j.jneumeth.2015.08.010.