Recent work


Microrheology With an Anisotropic Optical Trap

Andrew B. Matheson, Tania Mendonca, Graham M. Gibson, Paul A. Dalgarno, Amanda J. Wright, Lynn Paterson and Manlio Tassieri

Front. Phys. 9 (April, 2021)

doi: 10.3389/fphy.2021.621512

Abstract: Microrheology with optical tweezers (MOT) measurements are usually performed using optical traps that are close to isotropic across the plane being imaged, but little is known about what happens when this is not the case. In this work, we investigate the effect of anisotropic optical traps on microrheology measurements. This is an interesting problem from a fundamental physics perspective, but it also has practical ramifications because in 3D all optical traps are anisotropic due to the difference in the intensity distribution of the trapping laser along axes parallel and perpendicular to the direction of beam propagation. We find that attempting viscosity measurements with highly anisotropic optical traps will return spurious results, unless the axis with maximum variance in bead position is identified. However, for anisotropic traps with two axes of symmetry such as traps with an elliptical cross section, the analytical approach introduced in this work allows us to explore a wider range of time scales than those accessible with symmetric traps. We have also identified a threshold level of anisotropy in optical trap strength of ~30%, below which conventional methods using a single arbitrary axis can still be used to extract valuable microrheological results. We envisage that the outcomes of this study will have important practical ramifications on how all MOT measurements should be conducted and analyzed in future applications.

Optical Tweezers with Integrated Multiplane Microscopy (OpTIMuM): a new tool for 3D microrheology

Sci Rep 11, 5614 (Feb, 2021)

doi: 10.1038/s41598-021-85013-y

Abstract: We introduce a novel 3D microrheology system that combines for the first time Optical Tweezers with Integrated Multiplane Microscopy (OpTIMuM). The system allows the 3D tracking of an optically trapped bead, with ~ 20 nm accuracy along the optical axis. This is achieved without the need for a high precision z-stage, separate calibration sample, nor a priori knowledge of either the bead size or the optical properties of the suspending medium. Instead, we have developed a simple yet effective in situ spatial calibration method using image sharpness and exploiting the fact we image at multiple planes simultaneously. These features make OpTIMuM an ideal system for microrheology measurements, and we corroborate the effectiveness of this novel microrheology tool by measuring the viscosity of water in three dimensions, simultaneously.


Microrheology reveals microscale viscosity gradients in planktonic systems

Òscar Guadayol, Tania Mendonca, Mariona Segura-Noguera, Amanda J. Wright, Manlio Tassieri and Stuart Humphries

PNAS 118 (1) (Jan, 2021)

doi: 10.1073/pnas.2011389118


Abstract: Microbial activity in planktonic systems creates a dynamic and heterogeneous microscale seascape that harbours a diverse community of microorganisms and ecological interactions of global significance. Over recent decades a great deal of effort has been put into understanding this complex system, particularly focusing on the role of chemical patchiness, while overlooking a governing feature of microbial life affected by biological activity: viscosity. Here we use microrheological techniques to measure viscosity at length scales relevant to microorganisms. Our results reveal the viscous nature and the spatial extent of the phycosphere and show heterogeneity in viscosity at the microscale. Such heterogeneity affects the distribution of chemicals and microorganisms, with pervasive and profound implications for the functioning of the entire planktonic ecosystem.



More publications from group


i-Rheo: determining the linear viscoelastic moduli of colloidal dispersions from step-stress measurements

R. Rivas-Barbosa, M. A. Escobedo-Sánchez, M. Tassieri, and M. Laurati

Phys. Chem. Chem. Phys., Jan. 2020.

doi: 10.1039/C9CP06191F

Abstract: We report on the application of a Fourier transform-based method, ‘i-Rheo’, to evaluate the linear viscoelastic moduli of hard-sphere colloidal dispersions, both in the fluid and glass states, from a direct analysis of raw step-stress (creep) experimental data. We corroborate the efficacy of i-Rheo by comparing the outputs of creep tests performed on homogenous complex fluids to conventional dynamic frequency sweeps. A similar approach is adopted for a number of colloidal suspensions over a broad range of volume fractions. For these systems, we test the limits of the method by varying the applied stress across the materials' linear and non-linear viscoelastic regimes, and we show that the best results are achieved for stress values close to the upper limit of the materials' linear viscoelastic regime, where the signal-to-noise ratio is at its highest and the non-linear phenomena have not appeared yet. We record that, the range of accessible frequencies is controlled at the higher end by the relative weight between the inertia of the instrument and the elasticity of the complex material under investigation; whereas, the lowest accessible frequency is dictated by the extent of the materials' linear viscoelastic regime. Nonetheless, despite these constrains, we confirm the effectiveness of i-Rheo for gaining valuable information on the materials' linear viscoelastic properties even from ‘creep ringing’ data, confirming its potency and general validity as an accurate method for determining the material's rheological behaviour for a variety of complex systems.

Microrheology with optical tweezers: peaks & troughs

M. Tassieri

Current Opinion in Colloid and Interface Science, vol. 43. Elsevier Ltd, pp. 39–51, 01-Oct-2019.

doi: 10.1016/j.cocis.2019.02.006

Abstract: Since their first appearance in the 1970s, optical tweezers have been successfully exploited for a variety of applications throughout the natural sciences, revolutionising the field of microsensing. However, when adopted for microrheology studies, there exist some peaks and troughs on their modus operandi and data analysis that I wish to address and possibly iron out, providing a guide to future rheological studies from a microscopic perspective.


Model‐Free Rheo‐AFM Probes the Viscoelasticity of Tunable DNA Soft Colloids

J. A. Moreno‐Guerra, I. C. Romero‐Sánchez, A. Martinez‐Borquez, M. Tassieri, E. Stiakakis, and M. Laurati

Small, vol. 15, no. 42, p. 1904136, Oct. 2019.

doi: 10.1002/smll.201904136 

Abstract: Atomic force microscopy rheological measurements (Rheo‐AFM) of the linear viscoelastic properties of single, charged colloids having a star‐like architecture with a hard core and an extended, deformable double‐stranded DNA (dsDNA) corona dispersed in aqueous saline solutions are reported. This is achieved by analyzing indentation and relaxation experiments performed on individual colloidal particles by means of a novel model‐free Fourier transform method that allows a direct evaluation of the frequency‐dependent linear viscoelastic moduli of the system under investigation. The method provides results that are consistent with those obtained via a conventional fitting procedure of the force‐relaxation curves based on a modified Maxwell model. The outcomes show a pronounced softening of the dsDNA colloids, which is described by an exponential decay of both the Young's and the storage modulus as a function of the salt concentration within the dispersing medium. The strong softening is related to a critical reduction of the size of the dsDNA corona, down to ≈70% of its size in a salt‐free solution. This can be correlated to significant topological changes of the dense star‐like polyelectrolyte forming the corona, which are induced by variations in the density profile of the counterions. Similarly, a significant reduction of the stiffness is obtained by increasing the length of the dsDNA chains, which we attribute to a reduction of the DNA density in the outer region of the corona.

Peptide gels of fully-defined composition and mechanics for probing cell-cell and cell-matrix interactions in vitro.

Ashworth JC, Thompson JL, James JR, Slater CE, Pijuan-Galitó S, Lis-Slimak K, Holley RJ, Meade KA, Thompson A, Arkill KP, Tassieri M, Wright AJ, Farnie G, Merry CLR.

Matrix Biol. 2019 Jul 8. pii: S0945-053X(19)30120-9.
doi: 10.1016/j.matbio.2019.06.009.

Abstract: Current materials used for in vitro 3D cell culture are often limited by their poor similarity to human tissue, batch-to-batch variability and complexity of composition and manufacture. Here, we present a “blank slate” culture environment based on a self-assembling peptide gel free from matrix motifs. The gel can be customised by incorporating matrix components selected to match the target tissue, with independent control of mechanical properties. Therefore the matrix components are restricted to those specifically added, or those synthesised by encapsulated cells. The flexible 3D culture platform provides full control over biochemical and physical properties, allowing the impact of biochemical composition and tissue mechanics to be separately evaluated in vitro. Here, we demonstrate that the peptide gels support the growth of a range of cells including human induced pluripotent stem cells and human cancer cell lines. Furthermore, we present proof-of-concept that the peptide gels can be used to build disease-relevant models. Controlling the peptide gelator concentration allows peptide gel stiffness to be matched to normal breast (<1 kPa) or breast tumour tissue (>1 kPa), with higher stiffness favouring the viability of breast cancer cells over normal breast cells. In parallel, the peptide gels may be modified with matrix components relevant to human breast, such as collagen I and hyaluronan. The choice and concentration of these additions affect the size, shape and organisation of breast epithelial cell structures formed in co-culture with fibroblasts. This system therefore provides a means of unravelling the individual influences of matrix, mechanical properties and cell-cell interactions in cancer and other diseases.

Computational Image Analysis of Guided Acoustic Waves Enables Rheological Assessment of Sub-nanoliter Volumes

Muhammad Arslan Khalid, Aniruddha Ray, Steve Cohen, Manlio Tassieri, Andriejus Demcenko, Derek Tseng, Julien Reboud, Aydogan Ozcan and Jonathan M. Cooper

ACS Nano 2019, 13, 11062−11069
doi: 10.1021/acsnano.9b03219

Abstract: We present a method for the computational image analysis of high frequency guided sound waves based upon the measurement of optical interference fringes, produced at the air interface of a thin film of liquid. These acoustic actuations induce an affine deformation of the liquid, creating a lensing effect that can be readily observed using a simple imaging system. We exploit this effect to measure and analyze the spatiotemporal behavior of the thin liquid film as the acoustic wave interacts with it. We also show that, by investigating the dynamics of the relaxation processes of these deformations when actuation ceases, we are able to determine the liquid’s viscosity using just a lens-free imaging system and a simple disposable biochip. Contrary to all other acoustic-based techniques in rheology, our measurements do not require monitoring of the wave parameters to obtain quantitative values for fluid viscosities, for sample volumes as low as 200 pL. We envisage that the proposed methods could enable high throughput, chip-based, reagent-free rheological studies within very small samples.

Detection of a common odd aberration in confocal reflection microscopy by means of an edge scan

Pieter Smid, Chung W See and Amanda J Wright

Journal of Optics, Volume 21, Number 12
doi: 10.1088/2040-8986/ab4b33

Abstract: In reflection laser scanning microscopes, detection of odd aberrations is challenging because aberration cancellation can occur after the second passage of the light beam through the system. A method is proposed that uses a sample containing high spatial frequencies, such as an edge scan, to detect and measure the presence of odd aberrations. The new approach is demonstrated by scanning the focal spot over an edge in a confocal reflection microscope when coma is present in the imaging system (a common odd aberration). It is shown that the edge response displays characteristic distortions which are typical of coma. Detection of amplitude, sign and orientation of the coma aberration is made possible by comparison of the measured edge responses with theoretical curves.