Measurements of biomechanical properties inside living cells require minimally invasive methods, such as optical tweezers, a technique that won the Nobel Prize in Physics in 2018.
A team of researchers led by Cornelia Denz from the University of Münster (Germany) has now developed a simplified method to perform the necessary calibration of the optical tweezers in the system under investigation. Scientists from the University of Pavia in Italy were also involved. The results of the study have been published in the journal Scientific Reports.
The calibration ensures that measurements of different samples and with different devices are comparable. In simplified terms, the underlying procedure to perform the calibration works as follows: the micro- or nanometer-sized particles are embedded in a viscoelastic sample held on the stage of a microscope. Rapid and precise nanometer-scale displacements of the specimen stage cause the optically trapped particle to oscillate. By measuring the refracted laser light, changes in the sample's position can be recorded, and in this way, conclusions can be drawn about its properties, such as stiffness. This is usually done sequentially at different oscillation frequencies.
The team led by Denz and Randhir Kumar, a doctoral student in the Münster research group, now performed the measurement at several frequencies simultaneously for a wide frequency range. This multi-frequency method leads to a shortened measurement time of a few seconds. The scientists used solutions of methyl cellulose in water at different concentrations as samples. These have a similar viscoelasticity to living cells.
Photo: A microparticle held with optical tweezers in the microscope. Inset: Illustration of the held particle (magnified); shown in red is the light of the infrared laser used. Credit: Pascal Runde