semiflexible

Semiflexible describes materials whose stiffness lies between that of flexible polymers, which readily form random coils, and rigid rods, which resist bending. In polymer physics, semiflexible polymers or filaments exhibit bending fluctuations that are intermediate in scale, leading to conformations that are neither fully coiled nor straight. The concept is central to the worm-like chain model, which describes a polymer as a continuous, locally stiff filament.

A key parameter is the persistence length, Lp, a measure of the length scale over which directional correlations along the polymer backbone persist. If Lp is much smaller than the contour length L, the polymer behaves flexibly; if Lp is comparable to or larger than L, the polymer behaves semiflexibly or rigidly. The bending rigidity kappa relates to Lp by kappa = Lp · kB T, where kB is the Boltzmann constant and T is temperature. In aqueous environments at room temperature, Lp and kappa determine the extent of thermal bending and the statistical shape of the polymer.

Common examples of semiflexible polymers include actin filaments in the cytoskeleton, which have an estimated persistence length of about 10–20 micrometers, and DNA, with a persistence length of roughly 50 nanometers under standard conditions. Collagen fibers and certain biopolymer networks also exhibit semiflexible behavior. Semiflexibility influences mechanical properties such as viscoelasticity, strain stiffening, and network architecture, making it a key consideration in cellular mechanics and the design of biomimetic materials.