Xiaole Mao

Surface acoustic wave microfluidics

The recent introduction of surface acoustic wave (SAW) technology onto lab-on-a-chip platforms has opened a new frontier in microfluidics. The advantages provided by such SAW microfluidics are numerous: simple fabrication, high biocompatibility, fast fluid actuation, versatility, compact and inexpensive devices and accessories, contact-free particle manipulation, and compatibility with other microfluidic components. We believe that these advantages enable SAW microfluidics to play a significant role in a variety of applications in biology, chemistry, engineering and medicine. In this review article, we discuss the theory underpinning SAWs and their interactions with particles and the contacting fluids in which they are suspended. We then review the SAW-enabled microfluidic devices demonstrated to date, starting with devices that accomplish fluid mixing and transport through the use of travelling SAW; we follow that by reviewing the more recent innovations achieved with standing SAW that enable such actions as particle/cell focusing, sorting and patterning. Finally, we look forward and appraise where the discipline of SAW microfluidics could go next.

Surface acoustic wave (SAW) acoustophoresis: now and beyond

On-chip manipulation of micro-objects has long been sought to facilitate fundamental biological studies and point-of-care diagnostic systems. In recent years, research on surface acoustic wave (SAW) based micro-object manipulation (i.e., SAW acoustophoresis) has gained significant momentum due to its many advantages, such as non-invasiveness, versatility, simple fabrication, easy operation, and convenient integration with other on-chip units. SAW acoustophoresis is especially useful for lab-on-a-chip applications where a compact and non-invasive biomanipulation technique is highly desired. In this Focus article, we discuss recent advancements in SAW acoustophoresis and provide some perspectives on the future development of this dynamic field.

Systematic investigation of localized surface plasmon resonance of long-range ordered Au nanodisk arrays

Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance (LSPR) were characterized and compared with results from discrete dipole approximation (DDA) calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated computationally. Furthermore, thermal treatment was found to be essential in improving the nanodisk arrays’ LSPR properties. Using atomic force microscopy and DDA calculations, it was established that the improvements in LSPR properties were due to thermally induced morphologic changes. Finally, microfluidic channels were integrated with the annealed disk arrays to study the sensitivity of LSPR to the change in surroundings’ refractive index. The dependence of LSPR on surroundings’ refractive index was measured and compared with calculated results.

A single-layer, planar, optofluidic Mach–Zehnder interferometer for label-free detection

We have developed a planar, optofluidic Mach-Zehnder interferometer for the label-free detection of liquid samples. In contrast to most on-chip interferometers which require complex fabrication, our design was realized via a simple, single-layer soft lithography fabrication process. In addition, a single-wavelength laser source and a silicon photodetector were the only optical equipment used for data collection. The device was calibrated using published data for the refractive index of calcium chloride (CaCl(2)) in solution, and the biosensing capabilities of the device were tested by detecting bovine serum albumin (BSA). Our design enables a refractometer with a low limit of detection (1.24 × 10(-4) refractive index units (RIU)), low variability (1 × 10(-4) RIU), and high sensitivity (927.88 oscillations per RIU). This performance is comparable to state-of-the-art optofluidic refractometers that involve complex fabrication processes and/or expensive, bulky optics. The advantages of our device (i.e. simple fabrication process, straightforward optical equipment, low cost, and high detection sensitivity) make it a promising candidate for future mass-producible, inexpensive, highly sensitive, label-free optical detection systems.

An in-plane, variable optical attenuator using a fluid-based tunable reflective interface

We introduce an optofluidic based variable optical attenuator with high stability, high reliability, simple and inexpensive fabrication, and an attenuation performance comparable to commercial devices. A standard soft lithography process produces a single-layered polydimethylsiloxane (PDMS) microfluidic device integrated with optical fibers. By altering the refractive index of the fluid within the microchannel, we can control the reflectivity of the fluid/PDMS interface and thus achieve variable attenuation. Theoretical calculations are conducted based on Snell’s law of refraction and the Fresnel equations of reflection, and the calculated attenuation response matches well with experimental data.

Optofluidic tunable microlens by manipulating the liquid meniscus using a flared microfluidic structure

We have designed, demonstrated, and characterized a simple, novel in-plane tunable optofluidic microlens. The microlens is realized by utilizing the interface properties between two different fluids: CaCl(2)solution and air. A constant contact angle of ∼90° is the pivotal factor resulting in the outward bowing and convex shape of the CaCl(2) solution-air interface. The contact angle at the CaCl(2) solution-air interface is maintained by a flared structure in the polydimethylsiloxane channel. The resulting bowing interface, coupled with the refractive index difference between the two fluids, results in effective in-plane focusing. The versatility of such a design is confirmed by characterizing the intensity of a traced beam experimentally and comparing the observed focal points with those obtained via ray-tracing simulations. With the radius of curvature conveniently controlled via fluid injection, the resulting microlens has a readily tunable focal length. This ease of operation, outstandingly low fluid usage, large range tunable focal length, and in-plane focusing ability make this lens suitable for many potential lab-on-a-chip applications such as particle manipulation, flow cytometry, and in-plane optical trapping.

Surface acoustic wave (SAW) induced patterning of micro beads in microfluidic channels

Micro beads were patterned in microfluidic channels by using standing surface acoustic waves (SSAW). The SSAW were formed through two parallel interdigital transducers (IDTs) on a LiNbO3 substrate. A PDMS microchannel was aligned with the IDTs and bonded with the substrate. A solution of fluorescent polystyrene beads (diameter: 1.9 mum) was injected into the channel through a pressure driven flow. When a 25 dBm AC signal (frequency: 23.9 MHz) was applied to the IDTs, the beads were patterned into two straight lines (~5 mum in width) along the pressure nodes inside the channel.

Magnetic bio-nanobeads and nanoelectrode based impedance biosensor for detection of avian influenza virus

A novel impedance biosensor was developed based on the combination of a bio-nanobead separation/concentration procedure and an interdigitated array nanoeletrode and was demonstrated for sensitive and rapid detection of H5 subtype of avian influenza virus (AIV). Magnetic nanobeads with a diameter of 30 nm were coated with H5 subtype-specific monoclonal antibodies to selectively capture the target virus. An interdigitated array nanoeletrode was designed and fabricated for impedance measurement. Changes in the impedance of the antibody coated nanobead-virus complex was measured and correlated to the presence of H5 AIV (e.g., H5N1). The nanobead and nanoeletrode based impedance biosensor was able to detect AIV H5N1 at titer of 0.0128 HA unit/50 μl. Equivalent circuit analysis indicated that the solution resistance was responsible for the impedance change due to the presence of target virus.

High-throughput on-chip flow cytometry system using “microfluidic drifting” based three-dimensional (3D) hydrodynamic focusing

In this work we demonstrate the application of a three-dimensional (3D) hydrodynamic focusing technique, “microfluidic drifting” in the development of a miniaturized on-chip flow cytometry system. “Microfluidic drifting” utilizes viscous drag of Dean flow induced in a curved microfluidic channel to realize 3D hydrodynamic focusing in a single layer planar microfluidic device. Through force scaling analysis, numerical study, and experimental characterization, we show this technique can be successfully applied to focus large microparticles such as biological cells. A laser-induced fluorescence (LIF) detection system was incorporated with the 3D focusing device and a high-throughput (1700 cells/s) cell detection was demonstrated.

In-plane tunable optofluidic microlenses

We introduce two in-plane tunable optofluidic microlens configurations for flexible on-chip light focusing and collimation within a microfluidic device, including (1) an optofluidic cylindrical microlens and (2) a liquid gradient refractive index (L-GRIN) lens.