Shahrzad Yazdi

Hydrodynamic mechanisms of cell and particle trapping in microfluidics

Focusing and sorting cells and particles utilizing microfluidic phenomena have been flourishing areas of development in recent years. These processes are largely beneficial in biomedical applications and fundamental studies of cell biology as they provide cost-effective and point-of-care miniaturized diagnostic devices and rare cell enrichment techniques. Due to inherent problems of isolation methods based on the biomarkers and antigens, separation approaches exploiting physical characteristics of cells of interest, such as size, deformability, and electric and magnetic properties, have gained currency in many medical assays. Here, we present an overview of the cell/particle sorting techniques by harnessing intrinsic hydrodynamic effects in microchannels. Our emphasis is on the underlying fluid dynamical mechanisms causing cross stream migration of objects in shear and vortical flows. We also highlight the advantages and drawbacks of each method in terms of throughput, separation efficiency, and cell viability. Finally, we discuss the future research areas for extending the scope of hydrodynamic mechanisms and exploring new physical directions for microfluidic applications.

Three-dimensional continuous particle focusing in a microfluidic channel via standing surface acoustic waves (SSAW)†

Three-dimensional (3D) continuous microparticle focusing has been achieved in a single-layer polydimethylsiloxane (PDMS) microfluidic channel using a standing surface acoustic wave (SSAW). The SSAW was generated by the interference of two identical surface acoustic waves (SAWs) created by two parallel interdigital transducers (IDTs) on a piezoelectric substrate with a microchannel precisely bonded between them. To understand the working principle of the SSAW-based 3D focusing and investigate the position of the focal point, we computed longitudinal waves, generated by the SAWs and radiated into the fluid media from opposite sides of the microchannel, and the resultant pressure and velocity fields due to the interference and reflection of the longitudinal waves. Simulation results predict the existence of a focusing point which is in good agreement with our experimental observations. Compared with other 3D focusing techniques, this method is non-invasive, robust, energy-efficient, easy to implement, and applicable to nearly all types of microparticles.

Tunable patterning of microparticles and cells using standing surface acoustic waves

We have developed an acoustic-based tunable patterning technique by which microparticles or cells can be arranged into reconfigurable patterns in microfluidic channels. In our approach, we use pairs of slanted-finger interdigital transducers (SFITs) to generate a tunable standing surface acoustic wave field, which in turn patterns microparticles or cells in one- or two-dimensional arrays inside the microfluidic channels--all without the assistance of fluidic flow. By tuning the frequency of the input signal applied to the SFITs, we have shown that the cell pattern can be controlled with tunability of up to 72%. This acoustic-based tunable patterning technique has the advantages of wide tunability, non-invasiveness, and ease of integration to lab-on-a-chip systems, and shall be valuable in many biological and colloidal studies.

Effect of a planar interface on time-averaged locomotion of a spherical squirmer in a viscoelastic fluid

We examine the time-averaged locomotion of a spherical squirmer with reciprocal surface motion near a planar interface in a viscoelastic fluid. The system dynamics is investigated through a phase portrait in the swimming orientation and distance from the interface for three types of swimming gaits, namely, pullers, pushers, and neutral swimmers. To examine the kinematics of locomotion near different types of boundaries, the ratio of viscosities of the two phases adjacent to the planar interface is varied. Our results show that the near-wall attraction layer previously reported for a two-dimensional squirmer does not exist for spherical pullers and pushers. However, the presence of a stable node can attract the swimmer to the vicinity of the interface, depending on the initial swimming direction. In contrast to a two-dimensional neutral squirmer that always swims towards a no-slip boundary, a spherical neutral swimmer moves away from the interface, but the direction of time-averaged rotational velocity fav...

Acoustic tweezers: Achieving quasi-dynamic micropartcile patterning using tunable surface acoustic waves

Here we develop a method for quasi-dynamically patterning micron-scale particles in solutions utilizing standing surface acoustic waves (SSAWs). Slanted finger interdigital transducers (SFITs), instead of commonly used uniform interdigital transducers (IDTs), are used to generate SAWs based on the property that the position and wavelength of the excited SAWs is dependent on the frequency of the input AC signal. Since the location and period of particle patterning are dependent upon the position and wavelength of excited SAWs, respectively, they can be dynamically adjusted by tuning the input frequency. In this work, dynamic one-dimension (1D) and two-dimension (2D) micro-particle patterns were obtained using pairs of SFITs. This quasi-dynamic patterning method has advantages such as miniaturization, low power intensity, non-invasiveness, and high speed.

Experimental Demonstration of Localized Flow Control in a Microchannel Using Induced-Charge Electroosmosis

In this paper we investigate the use of induced charged electroosmosis (ICEO) as a means of providing localized flow control within bulk pressure-driven flow. Conductive posts are positioned in a microchannel in such a way that an AC electric field can be applied across them. This AC field induces an electric double layer (EDL), leading to ICEO flow around the conductive object. A pressure gradient is applied across the length of the channel to drive a background flow past the ICEO region. The combination of AC and pressure-driven flow fields is expected to create recirculation regions around the posts which could be useful for trapping particles or focusing the flow, e.g. for lab-on-a-chip applications. Numerical models of ICEO flow were developed and used to provide guidance for the design of microfluidic devices. These numerical models were also used to explore the number, position and shape of the conducting posts to create useful flow patterns. However, this paper focuses on the fabrication of and experiments within a prototypical microdevice. The device was fabricated from silicon dioxide and conducting gold pillars positioned in the glass channel. Experimental results obtained from this device have demonstrated localized ICEO-based flow control. Specifically, wake regions devoid of particles are created behind the posts.Copyright

Microfluidic Flow Control Using Induced-Charge Electroosmosis

Work with dc electrokinetics has demonstrated that is works well for bulk transport of fluid an particles. However, it is difficult to achieve control of individual or groups of particles. This paper investigate the use of induced-charge electroosmosis (ICEO) as a means of providing control over particles within bulk dc electroosmotic flow. ICEO flow develops when an electric double layer is induced by an applied electric field at the surface of a conducting object. Here conducting posts are positioned in a microfluidic channel and ICEO flow develops around them due to an applied ac electric field. A dc electric field is applied across the length of the channel to induce electroosmotic flow past the ICEO region. Around one arrangement of posts the ac and dc flow fields combine to produce a region of recirculation which could be useful for holding a particle or particles within a fixed region of the channel. An alternative arrangement of posts functions to focus the flow into the center of the channel. A numerical model of the system is developed and used to explore means of adapting the ICEO flows to many situations. A method for fabricating a microfluidic system for ICEO flows is presented.Copyright