It has been argued Tassieri, \textit{Soft Matter}, 2015, \textbf{11}, 5792 that linear microrheology with optical tweezers (MOT) of living systems ``\textit{is not an option}'', because of the wide ...gap between the observation time required to collect statistically valid data and the mutational times of the organisms under study. Here, we have taken a first step towards a possible solution of this problem by exploiting modern machine learning (ML) methods to reduce the duration of MOT measurements from several tens of minutes down to one second. This has been achieved by focusing on the analysis of computer simulated trajectories of an optically trapped particle suspended in a set of Newtonian fluids having viscosity values spanning three orders of magnitude, i.e. from \(10^{-3}\) to \(1\) Pa\(\cdot\)s. When the particle trajectory is analysed by means of conventional statistical mechanics principles, we explicate for the first time in literature the relationship between the required duration of MOT experiments (\(T_m\)) and the fluids relative viscosity (\(\eta_r\)) to achieve an uncertainty as low as \(1\%\); i.e., \(T_m\cong 17\eta_r^3\) minutes. This has led to further evidences explaining why conventional MOT measurements commonly underestimate the materials' viscoelastic properties, especially in the case of high viscous fluids or soft-solids such as gels and cells. Finally, we have developed a ML algorithm to determine the viscosity of Newtonian fluids that uses feature extraction on raw trajectories acquired at a kHz and for a duration of only one second, yet capable of returning viscosity values carrying an error as low as \(\sim0.3\%\) at best; hence the opening of a doorway for MOT in living systems.
It is established that the mechanical properties of hydrogels control the fate of (stem) cells. However, despite its importance, a one-to-one correspondence between gels' stiffness and cell behaviour ...is still missing from literature. In this work, the viscoelastic properties of Poly(ethylene-glycol) (PEG)-based hydrogels - broadly used in 3D cell cultures and whose mechanical properties can be tuned to resemble those of different biological tissues - are investigated by means of rheological measurements performed at different length scales. When compared with literature values, the outcomes of this work reveal that conventional bulk rheology measurements may overestimate the stiffness of hydrogels by up to an order of magnitude. It is demonstrated that this apparent stiffening is caused by an induced 'tensional state' of the gel network, due to the application of a compressional normal force during measurements. Moreover, it is shown that the actual stiffness of the hydrogels is instead accurately determined by means of passive-video-particle-tracking (PVPT) microrheology measurements, which are inherently performed at cells length scales and in absence of any externally applied force. These results underpin a methodology for measuring the linear viscoelastic properties of hydrogels that are representative of the mechanical constraints felt by cells in 3D hydrogel cultures.
This work shows a new method that employs surface acoustic waves (SAW) to form liposomes and simultaneously create a narrow distribution of inhalable aerosols that falls within the ideal respirable ...range size. The main advantage of using SAW is that, in one step, we can control the aerosols droplet size distribution and simultaneously create drug carriers of biological and active compounds, which are often degraded in alternative nebulisation methods. To corroborate the effectiveness of the proposed SAW nebulisation platform, nucleic acids were nebulised and deliver to lung cancer cells.
We present a straightforward method for measuring the fluids' relative viscosity via a simple graphical analysis of the normalised position autocorrelation function of an optically trapped bead, ...without the need of embarking on laborious calculations. The advantages of the proposed microrheology method become evident, for instance, when it is adopted for measuring the molecular weight of rare or precious materials by means of their intrinsic viscosity. The proposed method has been validated by direct comparison with conventional bulk rheology methods.
We present a simple and \emph{non-invasive} experimental procedure to measure the linear viscoelastic properties of cells by passive video particle tracking microrheology. In order to do this, a ...generalised Langevin equation is adopted to relate the time-dependent thermal fluctuations of a bead, chemically bound to the cell's \emph{exterior}, to the frequency-dependent viscoelastic moduli of the cell. It is shown that these moduli are related to the cell's cytoskeletal structure, which in this work is changed by varying the solution osmolarity from iso- to hypo-osmotic conditions. At high frequencies, the viscoelastic moduli frequency dependence changes from \(\propto \omega^{3/4}\) found in iso-osmotic solutions to \(\propto \omega^{1/2}\) in hypo--osmotic solutions; the first situation is typical of bending modes in isotropic \textit{in vitro} reconstituted F--actin networks, and the second could indicate that the restructured cytoskeleton behaves as a gel with "\textit{dangling branches}". The insights gained from this form of rheological analysis could prove to be a valuable addition to studies that address cellular physiology and pathology.
We remove the need for Laplace/inverse-Laplace transformations of experimental data, by presenting a direct and straightforward mathematical procedure for obtaining frequency-dependent storage and ...loss moduli (\(G'(\omega)\) and \(G"(\omega)\) respectively), from time-dependent experimental measurements. The procedure is applicable to ordinary rheological creep (stress-step) measurements, as well as all microrheological techniques, whether they access a Brownian mean-square displacement, or a forced compliance. Data can be substituted directly into our simple formula, thus eliminating traditional fitting and smoothing procedures that disguise relevant experimental noise.
Microrheology is a branch of rheology having the same principles as conventional bulk rheology, but working on micron length scales and micro-litre volumes. Optical tweezers have been successfully ...used with Newtonian fluids for rheological purposes such as determining fluid viscosity. Conversely, when optical tweezers are used to measure the viscoelastic properties of complex fluids the results are either limited to the material's high-frequency response, discarding important information related to the low-frequency behavior, or they are supplemented by low-frequency measurements performed with different techniques, often without presenting an overlapping region of clear agreement between the sets of results. We present a simple experimental procedure to perform microrheological measurements over the widest frequency range possible with optical tweezers. A generalised Langevin equation is used to relate the frequency-dependent moduli of the complex fluid to the time-dependent trajectory of a probe particle as it flips between two optical traps that alternately switch on and off.
We present an experimental procedure to perform broadband microrheological measurements with optical tweezers. A generalised Langevin equation is adopted to relate the time-dependent trajectory of a ...particle in an imposed flow to the frequency-dependent moduli of the complex fluid. This procedure allows us to measure the material linear viscoelastic properties across the widest frequency range achievable with optical tweezers.
We present experimental evidence that the effective medium approximation (EMA), developed by D.C. Morse Phys. Rev. E {\bf 63}, 031502, (2001), provides the correct scaling law of the macroscopic ...plateau modulus \(G^{0}\propto\rho^{4/3}L^{-1/3}_{p}\) (where \(\rho\) is the contour length per unit volume and \(L_{p}\) is the persistence length) of semi-flexible polymer solutions, in the highly entangled concentration regime. Competing theories, including a self-consistent binary collision approximation (BCA), have instead predicted \(G^{0}\propto\rho^{7/5}L^{-1/5}_{p}\). We have tested both the EMA and BCA scaling predictions using actin filament (F-actin) solutions which permit experimental control of \(L_p\) independently of other parameters. A combination of passive video particle tracking microrheology and dynamic light scattering yields independent measurements of the elastic modulus \(G\) and \(L_{p}\) respectively. Thus we can distinguish between the two proposed laws, in contrast to previous experimental studies, which focus on the (less discriminating) concentration functionality of \(G\).
