Direct manipulation of malaria parasites with optical tweezers reveals distinct functions of Plasmodium surface proteins.

Plasmodium sporozoite motility is essential for establishing malaria infections. It depends on initial adhesion to a substrate as well as the continuous turnover of discrete adhesion sites. Adhesion and motility are mediated by a dynamic actin cytoskeleton and surface proteins.

Measuring forces between two single actin filaments during bundle formation.

Bundles of filamentous actin are dominant cytoskeletal structures, which play a crucial role in various cellular processes. As yet quantifying the fundamental interaction between two individual actin filaments forming the smallest possible bundle has not been realized. Applying holographic optical tweezers integrated with a microfluidic platform, we were able to measure the forces between two actin filaments during bundle formation.

Optical force sensor array in a microfluidic device based on holographic optical tweezers.

Holographic optical tweezers (HOT) are a versatile technology, with which complex arrays and movements of optical traps can be realized to manipulate multiple microparticles in parallel and to measure the forces affecting them in the piconewton range. We report on the combination of HOT with a fluorescence microscope and a stop-flow, multi-channel microfluidic device. The integration of a high-speed camera into the setup allows for the calibration of all the traps simultaneously both using Boltzmann statistics or the power spectrum density of the particle diffusion within the optical traps.

Biomimetic models of the actin cytoskeleton.

The cytoskeleton is a complex polymer network that plays an essential role in the functionality of eukaryotic cells. It endows cells with mechanical stability, adaptability, and motility. To identify and understand the mechanisms underlying this large variety of capabilities and to possibly transfer them to engineered networks makes it necessary to have in vitro and in silico model systems of the cytoskeleton.

Tuning the orbital angular momentum in optical vortex beams.

We introduce a method to tune the local orbital angular momentum density in an optical vortex beam without changing its topological charge or geometric intensity distribution. We show that adjusting the relative amplitudes a and b of two interfering collinear vortex beams of equal but opposite helicity provides the smooth variation of the orbital angular momentum density in the resultant vortex beam. Despite the azimuthal intensity modulations that arise from the interference, the local orbital angular momentum remains constant on the vortex annulus and scales with the modulation parameter, c = (a-b)/(a+b).