Auxetic Fibers
Auxetic fibers are materials that have a negative or effectively negative Poisson's ratio. In contrast to most conventional materials, auxetic fibers possess the property that under a tensile load the material expands perpendicularly to the axis along which the tensile load is applied. In other words, auxetic materials expand as they are stretched. Conversely, materials are also auxetic if a compressive load applied along an axis results in a reduction in the dimension of the material along an axis perpendicular to the axis along which the compressive load is applied. Most materials exhibit a positive Poisson's ratio, this ratio being defined by the ratio of the contractile transverse strain relative to the tensile longitudinal strain.” [Lee, Lee, Koh and Jin, US Patent 7,858,055 (12/28/2010)]
Recent US Patents
12/28/2010
7,858,055
Moisture sensitive auxetic material
Lee et al of Kimberly-Clark, Wisconsin, developed an auxetic fiber that not only responds to an external force, but also responds to moisture. The auxetic fiber is made in part from a moisture activated shrinking filament. Even if no external force is applied to the fiber, a pseudo tensile force is created by wetting the auxetic fiber. This fiber has two components where component 1 is wrapped around component 2. Component 1 is moisture sensitive with high stffness. The second component is less stiff. The moisture sensitive component shortens when wetted and includes cellulose and acrylic fibers. The second component is a rubber. The first component shrinks when wet and creating porosity in the second component.