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Wolfgang Helfrich developed a continuum description of how lipid bilayers store elastic energy when bent. His 1973 theory distinguishes curvature, stretching and tilt, and relates the shapes of closed vesicles to curvature elasticity. In modern notation its bending contribution iswhere $H$ is mean curvature, $K$ is Gaussian curvature, $C_0$ is the preferred sum of principal curvatures associated with spontaneous curvature, and $\kappa ,\bar{\kappa }$ are elastic coefficients. The model explains how a flexible membrane can prefer a shape even without a rigid supporting skeleton.^[ref]