Papers

• Català, F. et al. “Extending calibration-free force measurements to optically-trapped rod-shaped samples“. Sci. Rep. 7, 42960; doi: 10.1038/srep42960 (2017).

Optical trapping has become an optimal choice for biological research at the microscale due to its noninvasiveperformance and accessibility for quantitative studies, especially on the forces involved inbiological processes. However, reliable force measurements depend on the calibration of the opticaltraps, which is different for each experiment and hence requires high control of the local variables,especially of the trapped object geometry. Many biological samples have an elongated, rod-likeshape, such as chromosomes, intracellular organelles (e.g., peroxisomes), membrane tubules, certainmicroalgae, and a wide variety of bacteria and parasites. This type of samples often requires severaloptical traps to stabilize and orient them in the correct spatial direction, making it more difficult todetermine the total force applied. Here, we manipulate glass microcylinders with holographic opticaltweezers and show the accurate measurement of drag forces by calibration-free direct detection ofbeam momentum.

• R. Bola, F. Català. M. Montes-Usategui, E. Martín-Badosa. “Optical tweezers for force measurements and rheological studies on biological samples”.15th workshop on Information Optics (WIO), 2016.

Measuring forces inside living cells is still a challenge due the characteristics of the trapped organelles (non-spherical, unknown size and index of refraction) and the cell cytoplasm surrounding them heterogeneous and dynamic, non-purely viscous). Here, we show how two very recent methods overcome these limitations: on the one hand, forces can be measured in such environment by the direct detection of changes in the light momentum; on the other hand, an active-passive calibration technique provides both the stiffness of the optical trap as well as the local viscoelastic properties of the cell cytoplasm.

• Martín-Badosa, F. Català, J. Mas, M. Montes-Usategui, A. Farré, F. Marsà. “Force measurement in the manipulation of complex samples with holographic optical tweezers”15th workshop on Information Optics (WIO), 2016.
• Derek Craig, Alison McDonald, Michael Mazilu, Helen Rendall, Frank Gunn-Moore, and Kishan Dholakia. “ Enhanced Optical Manipulation of Cells Using Antireflection Coated Microparticles”.ACS Photonics, 2 (10), pp 1403–1409, (2015).
  

  In molecular studies, an optically trapped bead may be functionalized to attach to a specific molecule, whereas in cell studies, direct manipulation with the optical field is usually employed. Using this approach, several methods may be used to measure forces with an optical trap. However, each has its limitations and requires an accurate knowledge of the sample parameters.6,7 In particular, force measurements can be challenging when working with nonspherical particles or in environments with an inhomogeneous viscosity, such as inside the cell. Recent developments in the field are moving toward obtaining direct force measurements by detecting light momentum changes. For this approach, the calibration factor only comes from the detection instrumentation and negates the requirement to recalibrate for changes in experimental conditions”.

• Xing Ma, Anita Jannasch, Urban-Raphael Albrecht, Kersten Hahn, Albert Miguel-López, Erik Sch?ffer, and Samuel Sánchez. “Enzyme-Powered Hollow Mesoporous Janus Nanomotors”. Nano Lett., 15 (10), pp 7043–7050, (2015).
  

  “Using optical tweezers, we directly measured a holding force of 64 ± 16 fN, which was necessary to counteract the effective self-propulsion force generated by a single nanomotor. The successful demonstration of biocompatible enzyme-powered active nanomotors using biologically benign fuels has a great potential for future biomedical applications.”