The shapes of unilamellar lipid vesicles are driven out of equilibrium by direct forcing with holographic optical tweezers. Vesicles have been studied extensively due to their relevance as a model for the membrane of cells as well as their potential practical uses e.g. for drug delivery or chemical confinement. We use multipoint laser tweezers formed by a spatial light modulator (holographic optical tweezers) to apply forces to such vesicles in several points simultaneously. To apply forces we utilize an index of refraction difference between the fluid inside the vesicle and the external fluid. Since this higher index of refraction material is fluid, the vesicle shape can changes in response to the optical forces. This shape change reveals the mechanical properties of vesicles subject to multiple stresses. We find that the surface forces on the membrane are localized near the points of forcing. Restoring forces from lipid tethers are used to estimate the total applied optical forces, which are below the pN level. The relaxation of deformations can be decomposed into its Fourier modes. The relaxation of all observable modes can be described well by a third order Landau equation. Ellipsoidal deformations relax more slowly than higher order deformation modes.
The aim of our work is to develop new optical tools to quantify parameters that may enter into models of cell motion in response to chemical gradients (chemotaxis). Dictyostelium discoidium is a well-known model organism for studying chemotaxis. We have developed a technique for manipulating Dictyostelium cells directly using a holographic laser tweezer array. Using this technique we have perturbed crawling Dictyostelium cells by changing their direction of motion. After tens of seconds, the cells generate protrusions perpendicular to the rotated polarization as they reorient in the direction of the local cAMP gradient. Here we describe how such micromanipulation may be used to test proposed biochemical pathways and their connection to mechanical deformations.
Vesicles are phospholipid bilayers that form a surface enclosing a volume of water or solution. They are of importance as model systems to study cells, as well as having practical applications such as containers for performing nanochemistry and facilitating drug delivery. Their properties have been studied for decades. Using a holographic laser tweezer array (LTA), which converts a single laser beam into many laser tweezer points, we stretch the vesicles in controlled ways from several points at once, measuring each force applied. Here, we present data on shape deformations of simple, spherical vesicles and on membrane fracture.