Microaspiration: Development & Applications - A Tool to Study the Mechanical Properties of Soft Particles

Abstract

This Thesis describes the development and application of microaspiration, a new technique to study the mechanical properties of individual soft microparticles. A microaspiration experiment uses suction pressure to deform and completely draw a soft microparticle into and through the conical tip of a micropipette, a process known as aspiration. Optical microscopy images are captured during microaspiration, as in the previously established micropipette aspiration (MA) technique. However, unlike MA, when using microaspiration an ion current passing through the pipette tip is recorded and synchronized with the optical microscopy. Aided by the optical images, mechanical properties of whole individual microparticles are quantified using ion current measurements for the first time to the best of the author's knowledge. Such current recordings have the advantages of high particle throughput and high temporal resolution, and they are not limited by optical resolution. The microaspiration apparatus is also capable of non-invasively identifying particles on a surface prior to aspiration, using the scanning ion conductance microscopy (SICM) technique. The Thesis details the development an SICM system and its adaptation to enable microaspiration. This includes the hardware construction and software design of the apparatus. The methodologies for processing, analysis and synchronisation of the recorded data are discussed. The SICM system is employed to scan the topography of tunable pores in elastomeric membranes. The effects of the variation of current threshold, step size and pore size modulation on topography mapping are investigated. It is found that lowering the current threshold increases the sensitivity of the pipette while scanning, up to the point where the tip contacts the surface. Additionally, an increase in the pore area is observed with increased pore size modulation. To the best of the author's knowledge this is the first report of electrochemical spatial mapping of these elastomeric pores, which are used for resistive pulse sensing. This work also demonstrates the ability of the developed SICM system to identify microscale features on a surface. The first application of the microaspiration technique is demonstrated using bovine red blood cells (erythrocytes). Effective mechanical properties are deduced by using the ionic current response to measure the dwell time and velocity of erythrocytes aspirated with a micropipette. The ionic resistance through the tip provides information regarding the progress of particle deformation. Synchronised optical microscopy images are used to verify the stages of aspiration. Values obtained for the effective viscosity represent liquid-like deformation of the whole cell, and show a consistent shear thinning effect. The effective elasticity also varies with applied force, and is higher for erythrocytes stored for several weeks. These findings are consistent with literature reports. The data also show that wall friction is an important factor for the uptake dynamics, and that the erythrocytes are damaged or ruptured when passing through pipettes of opening diameter 0.7 m, the smallest used. The second application of microaspiration presented concerns measurement of polymer gel microparticles. Image analysis of the optical microscopy data is used to refine elastic and viscoelastic MA models in order to deduce the mechanical properties of polyacrylamide (PA) and polydimethylsiloxane (PDMS) particles from electrical current data. Ion current signatures for the poroelastic PA particles are found to be qualitatively different from those of the viscoelastic PDMS particles. For PA particles it is found that the maximum change in current during aspiration increases with particle size. The measured effective shear modulus of the PA particles and the effective elastic modulus of the PDMS particles are in excellent agreement with previously reported values. This study provides further validation of the microaspiration method. Overall, microaspiration represents important steps towards high precision mechanical sensing of micro- and nanoparticles based on an ion current signal in isolation.