Microrheology with Optical Tweezers: Peaks & Troughs

In the ‘70s, a team of researchers led by Arthur Ashkin (the co-winner of the 2018 Nobel Prize in Physics) developed what has revealed to be an invaluable tool for a variety of applications throughout the natural sciences, revolutionising the field of micro-sensing: “Optical Tweezers” (OT). Their success relies on the inherent property that a highly focused laser beam has to weakly trap (in three dimensions) micron-sized dielectric objects suspended into a fluid. OT have proved to be capable of resolving ‘pN’ forces and ‘nm’ displacements with a high temporal resolution; i.e. down to a few ‘μsec’. Therefore, they have been adopted as exceptionally sensitive transducers to study a myriad of biological processes, such as measuring the compliance of bacterial tails, the forces exerted by a single motor protein, the mechanical properties of human red blood cells and those of individual biological molecules; normally inaccessible by conventional methods.
Accessing the time-dependent trajectory of a micron-sized sphere, to a high spatial and temporal resolution, is one of the principles underpinning microrheology techniques. Microrheology is a branch of Rheology (the study of flow of matter) and they share the same principles, but it works at micron length scales and with microliter sample volumes. Therefore, microrheology techniques have revealed to be very useful methods for all those rheological studies where ‘rare’ or ‘precious’ materials are employed, e.g. in biophysical studies. Moreover, microrheology measurements can be performed in environments that couldn’t be reached by conventional rheology experiments, for instance inside a living cell.
A common task of microrheology studies is to correlate the time-dependent trajectories of the tracer particles to the frequency-dependent linear viscoelastic properties of the suspending fluid. In the specific case of OT, methods for performing linear microrheology measurements of complex fluids have been presented and validated against conventional bulk rheology methods. However, in literature there exist some ‘peaks & troughs’ (not to mention inconsistencies) on the modus operandi of microrheology measurements performed with OT that I wish to address and possibly ‘iron them out’. For instance, based on simple statistical mechanics principles, it can be shown that, despite the general trend, microrheology with optical tweezers of living cells ‘is not an option’!