Karolina Petkovic-Duran

Cavitation microstreaming and stress fields created by microbubbles

Cavitation microstreaming plays a role in the therapeutic action of microbubbles driven by ultrasound, such as the sonoporative and sonothrombolytic phenomena. Microscopic particle-image velocimetry experiments are presented. Results show that many different microstreaming patterns are possible around a microbubble when it is on a surface, albeit for microbubbles much larger than used in clinical practice. Each pattern is associated with a particular oscillation mode of the bubble, and changing between patterns is achieved by changing the sound frequency. Each microstreaming pattern also generates different shear stress and stretch/compression distributions in the vicinity of a bubble on a wall. Analysis of the micro-PIV results also shows that ultrasound-driven microstreaming flows around bubbles are feasible mechanisms for mixing therapeutic agents into the surrounding blood, as well as assisting sonoporative delivery of molecules across cell membranes. Patterns show significant variations around the bubble, suggesting sonoporation may be either enhanced or inhibited in different zones across a cellular surface. Thus, alternating the patterns may result in improved sonoporation and sonothrombolysis. The clear and reproducible delineation of microstreaming patterns based on driving frequency makes frequency-based pattern alternation a feasible alternative to the clinically less desirable practice of increasing sound pressure for equivalent sonoporative or sonothrombolytic effect. Surface divergence is proposed as a measure relevant to sonoporation.

A continuous wavelet transform algorithm for peak detection

Contactless conductivity detector technology has unique advantages for microfluidic applications. However, the low S/N and varying baseline makes the signal analysis difficult. In this paper, a continuous wavelet transform‐based peak detection algorithm was developed for CE signals from microfluidic chips. The Ridger peak detection algorithm is based on the MassSpecWavelet algorithm by Du et al. [Bioinformatics 2006, 22, 2059–2065], and performs a continuous wavelet transform on data, using a wavelet proportional to the first derivative of a Gaussian function. It forms sequences of local maxima and minima in the continuous wavelet transform, before pairing sequences of maxima to minima to define peaks. The peak detection algorithm was tested against the Cromwell, MassSpecWavelet, and Linear Matrix‐assisted laser desorption/ionization‐time‐of‐flight‐mass spectrometer Peak Indication and Classification algorithms using experimental data. Its sensitivity to false discovery rate curve is superior to other techniques tested.

Chaotic micromixing in open wells using audio-frequency acoustic microstreaming

Mixing fluids for biochemical assays is problematic when volumes are very small (on the order of the 10 microL typical of single drops), which has inspired the development of many micromixing devices. In this paper, we show that micromixing is possible in the simple open wells of standard laboratory consumables using appropriate acoustic frequencies that can be applied using cheap, conventional audio components. Earlier work has shown that the phenomenon of acoustic microstreaming can mix fluids, provided that bubbles are introduced into a specially designed microchamber or that high-frequency surface acoustic wave devices are constructed. We demonstrate a key simplification: acoustic micromixing at audio frequencies by ensuring the system has a liquid-air interface with a small radius of curvature. The meniscus of a drop in a small well provided an appropriately small radius, and so an introduced bubble was not necessary. Microstreaming showed improvement over diffusion-based mixing by 1-2 orders of magnitude. Furthermore, significant improvements are attainable through the utilization of chaotic mixing principles, whereby alternating fluid flow patterns are created by applying, in sequence, two different acoustic frequencies to a drop of liquid in an open well.

Acoustic microstreaming applied to batch micromixing

Experiments are presented in which acoustic microstreaming is investigated and applied to a batch micromixing case appropriate to a point-of-care pathology screening test. The flows presented can be created without complex engineering of contacts or surfaces in the microdevice, which could thus be made disposable. Fundamental flow patterns are measured with a micro-Particle-Image Velocimetry (micro-PIV) system, enabling a quantification of the fluiddynamical processes causing the flows. The design of micromixers based on this principle requires a quantification of the mixing. A simple technique based on digital image processing is presented that enables an assessment of the improvement in mixing due to acoustic microstreaming. The digital image processing technique developed was shown to be non-intrusive, convenient and able to generate useful quantitative data. Preliminary indications are that microstreaming can at least halve the time required to mix quantities of liquid typical of a point-of-care test, and significantly greater improvements seem feasible.

Bubble-induced acoustic mixing in a microfluidic device

Homogeneous and fast mixing of samples at microscale is a critical requirement for successful applications of microfluidics in biochemical analysis, chemical synthesis, drug delivery and nanomaterial synthesis. This paper reports the optimisation of bubble-induced mixing in a microfluidic device in terms of voltage, driving frequency, piezo transducer position and PDMS thickness. The microfluidic device consists of a microwell (with the diameter of 1mm and volume of ~95 nL) with two rectangular bubble traps (400×400μm) on both sides of the well. After the injection of liquid, air bubbles were spontaneously trapped in two rectangular traps. When the frequency of a piezo was equal to the resonance frequency of air bubbles, strong liquid recirculation formed (so called acoustic microstreaming) in the vicinity of the interface of air bubbles and water. The acoustic induced flow of microbeads and mixing of water and fluorescence dye were imaged to study the mixing efficiency. For a given voltage and PDMS thickness, when the piezo was placed on top of the well, the mixing was most vigorous. For a given frequency, the mixing efficiency was directly proportional to the voltage (4-20V) and inversely proportional to the PDMS thickness (0.3-2mm). When the frequency driving the piezo was approaching the resonance frequency of air bubbles, the mixing efficiency was maximal, while when it was far away from the resonance frequency of air bubbles, the mixing efficiency was much lower. This work provides guidance to the design and the application of bubble-induced acoustic mixing in microfluidics.

Using acoustic microstreaming to improve detection of gene expression in single cells

Functional heterogeneity among different cells of an organism (brain cell, heart cell etc.) is brought about by expression of different subsets of genes drawn from the same genetic template (DNA). Therefore, to understand the molecular basis of normal (healthy) and abnormal (diseased) cell behavior, one must measure gene expression. The typical procedure is to homogenize large numbers (>1,000) of cells together, isolate the first product of gene expression (RNA), reverse transcribe the RNA to a cDNA copy, then identify and quantify gene-specific cDNAs. Unfortunately, the gene expression profile obtained represents an average across all combined cells and cell behaviors rather than any particular cell. Ideally one would measure gene expression in a single cell; however the very small amount of labile RNA obtainable from a single cell mostly degrades before it can be measured. We have developed an acoustic microstreaming-based device (“micromixer”) which improves mixing of solutions within microliter volume...