Journal: PNAS Nexus
Article Title: Active and probe-free intracellular rheology via phase-sensitive thermoviscous flows
doi: 10.1093/pnasnexus/pgag190
Figure Lengend Snippet: Viscoelastic model system, data extraction pipeline and sensitivity to the time-resolved solidification of fluids. a) Schematic representation of tunable PAAm gels employed as model viscoelastic systems: independent tuning of the elastic and viscous contributions is achieved through varying the concentrations of cross-linked (gray) and linear (red) PAAm, respectively. Oscillations are flow-induced, independently of mechanical probes. Small tracers merely report on the oscillations and are not required in systems, where resolvable image features already enable tracking. In this study, we used 1-µm carboxylated polystyrene particles due to the full transparency of the gels. b) Fourier filtering analysis is used to convert a noisy raw trajectory into low-noise canonical response of the material, where the clean signal is reconstructed by filtering for a given oscillation frequency. The laser driving (implicit stressing) begins at the start of the acquisition (reference time 0). This reconstructed signal provides access to the phase delay between the driving stimulus and the sample response, which reveals the mechanical properties of the test sample (data show 3 vol% cross-linked PAAm and 1 vol% linear PAAm probed at 1 Hz). The uncertainty in the phase lag originates from the relative magnitude of the amplitude compared to the Brownian noise floor. c) Real-time tracking of gelation in cross-linked PAAm gels (left) and entanglement in liquid suspensions of linear polyacrylamide chains (right). The dashed curves represent exponential fits with matching relaxation times for gelation (mixtures of cross-linked and linear PAAm), while solid curves confirm the relatively constant repeated measurements in viscoelastic fluids. Each data point of the same color represents a separate successive measurement on the same system. The errors in the individual phase angles were estimated based on the amplitude of each oscillation compared to a Brownian noise floor. The errors in the mean phase angles were estimated based on the standard deviation around the plateau (steady-state) region in each curve.
Article Snippet: The probe-particles we used in the flow-based microrheology experiments on PAAm gels were fluorescent (Dragon Green) 1,037-μm carboxylated polystyrene beads (PS-COOH uniform dyed microspheres, Bangs Laboratories, Inc., FC04F).
Techniques: Extraction, Standard Deviation
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