Frequency-Chirped Actuation of Chiral Magnetic Microbots for Viscosity Mapping in Heterogenous Media: From Model...

Fast, accurate, and location-specific measurement of mechanical parameters of complex, heterogeneous environments at the microscale can impact multiple scientific domains. This is a nontrivial technological challenge since the measurement scheme needs to be simultaneously sensitive and fast, such as to obtain meaningful measurements with spatial resolution comparable to the probe dimensions. As we show here, a frequency-chirped external drive with chiral magnetic microbots allows micron-scale spatial resolution with sensitivities better than a few cP/$\sqrt{Hz}$. Through simultaneous observation of two degrees of freedom, one can measure the local viscosity even when the motion of microbot is severely constrained. We demonstrate this measurement technique to be applicable in model systems and make spatial maps of viscosity in biologically relevant, inherently heterogeneous, and crowded media, like the cell cytoplasm. The techniques demonstrated in this manuscript establish chiral microbots as a next-generation active, mechanical measurement tool, especially suitable for probing crowded and heterogeneous environments that are ubiquitous in the natural world.