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  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04141

In this paper, we present our study on a new type of passive micromixer based on a washboard microfluidic configuration. Periodic geometrical barriers like washboard are built inside a microfluidic channel that alters the flow patterns transversely and vertically. The advantages of this type of mixer is its mixing barriers are at the bottom of the microfluidic channel, and it does not need a complex 2-D or 3-D configurations to perform mixing process. This micromixer can easily be fabricated by one step SU-8 photolithographic process and one molding process. Solutions to be squeezed vertically and laterally while encounter the periodic barriers. Thus, the laminar flow pattern is distorted to create mixing process. To study the mixing mechanism of the skew corrugated micromixer, we study the mixing efficiency of washboard structures at three different angles, including 30, 45, and 60 degrees. Finite element simulations are conducted to study the mixing pattern and efficiency. Simulation results suggested that a skew corrugated microchannel with 45o angle can provide highest mixing efficiency, and a 95% mixing efficiency can be achieved within 8 stage within a 2.38 mm long microchannel.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04142

Microalgae have been studied intensively in the past decade because they have great potential in simultaneous production of biofuels and other high-value products [1]. For example, microalgae extracts have shown great antioxidant and anti-cancer effects [2] and many of the antioxidant pigments have already been commercialized. However, the production of microalgae biomass and their cellular contents strongly depends on the kind of microalgae, the cultivation condition, and the stress for inducing the accumulation of specific molecules. Conventional analyses for the cellular components of microalgae are multi-step and time-consuming, making the optimization of cultivation strategy challenging and prolonged. Therefore, a rapid and high-throughput platform for assessing the quality of microalgae culture is in great need.   

 

To rapidly investigate the effects of cultivation conditions and stresses on microalgae, micro-bioreactors have been developed and applied in enhancing the production of lipids [3] and astaxanthin [4]. The accumulation of lipids and antioxidant pigments is induced by nutrient starvation, high irradiation, high temperature, or extreme pH values. However, nutrient starvation creates a changing stress that is challenge to track and control. Oxidative stresses created by adverse environment can arrest the growth of microalgae. On the other hand, a weak electric field is reported to enhance the production of both chlorophyll and carotenoids in microalgae. Therefore, we design a micro-bioreactor integrated with microelectrodes to investigate the improvement of production of microalgal biomass and pigments by the electrical stimulus.  

 

The micro-bioreactor is composed of a glass slide containing microelectrode and multiple layers of PDMS, including the inlet layer (containing inlet microchannel), the bioreactor layer, the microelectrode layer, the outlet layer (containing outlet microchannel, and the cover layer. Two microalgae, C. vulgaris and S. abundans, are inoculated in the micro-bioreactor and fresh medium is supplied into the micro-bioreactor using a syringe pump. Twelve micro-bioreactors are operated simultaneously for the same combination of nutrient compositions and electric field to obtain statistically reliable outcomes. The biomass and total pigments are quantified by the optical density at 682 nm and 440 nm, respectively.

 

The effect of electric field on the production of microalgal biomass is first investigated and the results are shown in Fig. 1. An electric field higher than 5 V/cm promotes the production of biomass for both microalgae. The increment of biomass is most significant in 10 V/cm and the biomass of C. vulgaris and S. abundans increases to more than 200% of the untreated culture. The combined effect of nutrient supply and electric field is also investigated. The electric field has the highest promoting effects on the biomass of C. vulgaris cultured in glucose and sucrose and S. abundans cultured in glucose (Fig. 2). Finally, the ratio of pigment per cell (OD440/OD682) is investigated and Fig. 3 shows that electric field increases the ratio of pigment per cell to higher than 150% for S. abundans cultured in sucrose. In conclusion, the micro-bioreactor can serve as an effective tool in searching suitable nutrient compositions and stresses for the improved production of microalgal biomass and pigments.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04143

We present an innovative concentration generator with on chip vacuum pump. The concentration generator is contained the microfluidic channel and self-priming microfluidic device. The microfluidic channel structure is made up from polypropylene which characteristic are ultra-thin (under than 100 um), cost effective, mass producible, rapid fabrication process.  The self-priming microfluidic device also has many feature, like thin residual layer (under than 100 um), on chip new pumping method, standing alone device, minimal manual operation, commercialize. In this paper, we propose the design of microchannel which generating concentration gradient, and introduce the fabrication process, simulation, and experimental result. Due to all the advantages, we think this concentration generator system can be used on biochip.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04144

The elasto-inertial focusing has been widely employed for various biomedical applications such as cell sorting, monitoring and stretching measurement [1]. However, the widely-employed channel geometries have been limited to simple straight channels which commonly occupy a large footprint. The spiral channel, which can roll up a long channel (up to the order of 10 cm) in a small footprint (e.g., 1 cm2), has been regarded as a classical channel design in inertial microfluidics [2], but is rarely employed in elasto-inertial focusing due to the coexistence of inertial migration, Dean flow and viscoelastic effect. In spiral microchannels, the three above-mentioned effects may simultaneously affect the particle focusing at finite Reynolds numbers. As illustrated in figure 1(a), particles randomly-dispensed near the inlet would equilibrate at a specific lateral position under the competition of elastic force (FE), inertial lift force (FL) and Dean drag force (FD). Our previous work [3] explored the complex dependent of particle focusing patterns on flow rate and channel structure under the coupling of these three effects. However, to our best knowledge, the flexible control of particle focusing in spiral channels has not been reported.

In this work, we realized the control of particle focusing position in a compact spiral channel through adjusting the polymer concentration of viscoelastic fluids. It is found that the lateral focusing position away from the inner channel wall (Xeq) could be flexibly controlled via adjusting the concentrations of the selected Poly(vinyl pyrrolidone) (PVP) solutions (see figure 1(b, c)). At the high polymer concentration (8.0 wt%), the particles could be prefect single-line focused at the channel centerline at specific flow rates (see figure 2) due to the dominance of elastic force over inertial lift force and Dean drag force. The particles in previously-reported spiral inertial microfluidics equilibrate very close to the channel wall [2], which prevents the application of this technique for traditional optical interrogations due to the unavoidable scattering of optical signals at the wall interface. Therefore, this center-line focusing may serve as a potential pretreatment for microflow cytometry detection. At low PVP concentrations (i.e., 5.2 wt%, 3.6 wt% and 2.0 wt%), the particles were found to shift towards the outer channel wall (see figure 3), which enables the continuous particle concentration at a low energy consumption to be possible. The physics behind the controllable particle shifting via adjusting polymer concentration is the comparable competition between the involved three forces (elastic force, inertial lift force and Dean drag force). To better understand the effect of polymer concentration on particle focusing, we quantitatively measured the focusing widths (normalized with particle diameter) and the lateral focusing positions of particles. The measured results were plotted as a function of flow rate (see figure 4). It is obviously to found that the particle focusing would shift towards the channel centerline with increasing polymer concentration.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04155

This paper reports the design, fabrication and preliminary characterization of optofluidic Fabry-Perot micro coupled cavities enabling on-chip refractometry with large dynamic range.

 

Large dynamic range refractometry is needed in several applications such as in portable food analyzers, where the quality of fruits and beverages are classified based on their Brix number, which indicates the sugar content. The Brix numbers is obtained from the refractive index using standard conversion tables [1-2], where the refractive index varies from 1.333 (0 oB) to 1.49 (80 oB). The conventional design of integrated refractometer is based on Fabry-Perot cavity, in which the wavelength of longitudinal modes shifts with the change in the refractive index of the filling medium [3-4]. Their dynamic range is usually in the order of 10-3, limited by the free spectral range (FSR) of the micro cavity. Therefore, the food analyzers are usually based on volume optics components and the change of the refraction angle with the change in the refractive index that allows for the large dynamic range of sensing.

 

In this work, we report a novel design based on cascading two coupled Fabry-Perot micro cavities allowing for orders of magnitude increase in the FSR besides the decrease in the linewidth which enhances the resolution as well. Fig. 1 shows the schematic diagram of the new design and the idea of operation. The lengths of the two cavities are slightly different and adjusted such that some modes are suppressed after each allowed mode. The use of Si/Air layers to form the Bragg mirror with thickness of 3.8/3.6 mm allowed us to achieve designs with mode separation of 40 nm around a wavelength of 1550 nm.

 

Fig. 2 shows a SEM photo of part of the fabricated structures. The fabrication was done using standard MEMS technology in which DRIE process is used to make a 150 mm deep etching in Si to form the Bragg mirrors, fiber grooves, and the microfluidic channels and ports. Test structures in the form of single cavities and mirrors were also fabricated on the same chip.

 

Fig. 3 shows the measured transmittance of one of the fabricated coupled-cavities together with the measured transmittance of the single cavity corresponding to one of them. The single cavity has a length of 128 mm and the coupled-cavities have lengths L1 = 160 mm and L2 = 128 mm as referred to Fig. 1. The mirrors are composed of two Si layers in both cases. We can see the increase in the FSR achieved in the coupled cavity which is about 3 times as the single cavity and also the reduction in the line-width by less than half. In fact the technology tolerance, especially the over-etching, had a great effect on the fabricated structures. The fabricated mirrors had a wider bandwidth than expected and we could obtain a much larger FSR in the coupled-cavity. In some designs we obtained only one peak in the whole measured band of 140 nm.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04156

In this presentation, an efficient sample preparation process utilizing an ion concentration polarization (ICP) phenomenon would be introduced, especially the simultaneous operation of separation and preconcentration and detailed dynamics of the preconcentrated analytes. ICP is traditional electrochemical ion transportation process and appears as a steep concentration gradient near nanoporous membrane under dc bias [1, 2, 3]. The major function of ICP is an active ion control by an external electric field so that it is significantly useful to study the new ion transportation through nanoporous junction (or membrane) and develop novel engineering applications [4, 5, 6].

 

Here, we would like to introduce a device as shown in Figure 1 performing selective preconcentration and online collection of charged molecules with different physicochemical properties based on ICP. The device allows subsequent processes of the highly preconcentrated and separated molecules on-chip or off-chip in a single solution. The molecules were highly preconcentrated at each equilibrium position balanced between electroosmotic drag force and electrophoretic force. By the repeated chamber geometry, the ion depletion zone was stabilized and the plugs were well-defined. For subsequent on-chip or off-chip application, pneumatic micro-valve system was integrated. The successive operation of selective preconcentration and valve operation would recover target molecules at the preconcentration factor more than 30 as shown in Figure 2.

 

Furthermore, we investigated the detailed spatiotemporal dynamics of preconcentrated analytes for multiple analyte mixture. In the case of single-analyte preconcentration, there were two distinct regimes: staking and propagating regime as shown in Figure 3. Meanwhile, the equilibrium position of the plug was shifted in the case of multiple analyte preconcentration due to electrokinetic intermolecular interactions. A critical mobility was extracted for determining the types of preconcentration and it was confirmed both by experiment and numerical simulations as shown in Figure 4.

 

These results would play a key role in enhancing accuracy of practical ICP applications through elucidating comprehensive preconcentration mechanisms which are critical in analytical systems such as diagnostics, biology researches, and point of care systems.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04157

In this study, we demonstrated a high-throughput light-directed assembly as a printing technology by introducing gold nanorods to induce thermal convection flows that move microparticles and cell-laden hydrogel microparticles (diameter = 40 µm to several hundreds of micrometers) to specific light-guided locations, forming desired patterns.

     Light-directed technologies, such as optical tweezers [1], optoelectronics tweezers [2], and fiber devices [3], offer precise control of the movement of nano/microscale objects to manipulate functional units. However, high laser intensities are usually required to generate sufficient optical forces to pattern the objects, which might damage micro-bio-objects, such as tissues. To enhance optical forces, researchers developed arrays of plasmonic nanoantennas [4], plasmonic nanostructures and electrokinetics [5] to convert light energy into local heat-generating convection flows for object assembly. Nevertheless, with plasmonic methods, arbitrary control over the configuration was not easily achieved because the assembled structures were predetermined by the patterned nanoarray. To date, a lack of light-directed manipulation technologies exists for the microscale, especially for bottom-up biofabrication using low-power lasers to effectively assemble micro-bio-objects with precise control.

    Here, a novel high throughput light-directed assembly method on the microscale was investigated by suspending gold nanorods (GNRs), as photothermal transducers, in a fluidic medium to induce thermoplasmonic convections for the assembly of building blocks fabricated through microfluidics (Figure 1). Because significant local thermoplasmonic convections (Figure 2) were generated by precisely controlling the low-power infrared laser spot size and direction, effective building block assembly with high resolution enabled the desired patterns (Figure 3). By using an automatic motorized stage with optical source integration, the assemblies with desirable patterns were approached with programmable manner (Figure 4). This method was used as an advanced printing technology to form centimeter-scale functional units in ~10 min by integrating various hydrogel building blocks, which were fabricated using droplet-based microfluidics (Figure 5). To illustrate the application of light-directed assembly in bottom-up  tissue engineering, we demonstrated that tissue patterns (scaffold-free) with high cell viability and proliferation after long-term culture can be assembled and printed using mesenchymal stem cell (MSC)-seeded microgel as building blocks to form bioinspired microtissues containing an extracellular matrix (ECM) surrounded by MSCs. Excellent cell viability and proliferation were characterized in these microtissues after the long-term culturing of the MSC seeded inside (Figure 6 & 7).

    The presented high-throughput, bottom-up method of building block assembly through precise thermoplasmonic convective flow controls can revolutionize current biofabrication processes This method is suitable for biomedical engineering, including surgical operations and in vivo applications such as the elimination of a preshaping scaffold and prevention of contamination, as well as reagent delivery and precise cell deposition. With the capability of precisely controlled high-throughput building-block assembly, a broad array of applications is expected, ranging from 3-D bio-printing to regenerative medicine, tissue engineering, bottom-up manufacturing and biofabrication.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04158

Electroosmotic flow (EOF) is an electrokinetic phenomenon. The fluid motion originates from the electrical body force acting on the excess counterions in the electrical double layer (EDL) when an external electric field is applied across a microchannel. It can be employed in numerous microfluidic applications, ranging from pumping to chemical and biomedical analyses. Nanoscale networks/structures are often integrated within microchannels for a broad range of applications, such as sieving matrices for electrophoretic separation of biomolecules, and its introduction has been known to reduce EOF [1, 2]. Hitherto, the mechanics for EOF reduction due to nanostructured surfaces is still not well understood. To better elucidate the mechanics, we develop a novel fabrication method to produce microchannel with large-area nanostructures for investigation. The micro-/nanostructures produced demonstrate good regularity over a relatively large area and can be mass-produced cost-effectively. Despite the availability of various micro-/nanofabrication techniques, the existing techniques do not satisfy the aforementioned criteria.

Microchannels with/without nanostructures on the bottom wall (see Figure 1) were fabricated by a series of steps which can be divided into four phases: fabrication of master structures on a silicon (Si) wafer, creation of negative mold insert via electroplating, injection molding with Topas 5013L-10 Cyclic Olefin Copolymer (COC), and thermal bonding and integration of practical inlet/outlet ports. The type of nanostructures employed in our investigation was black silicon nanostructures (prolate hemispheroid-like structures with diameter of 270.0 ± 73.2 nm, height of 175.2 ± 22.3 nm and spatial distance of 350.6 ± 89.2 nm). The fabrication scheme involves a two-step reactive ion etching (RIE) process for (1) construction of the microchannel and (2) production of the black silicon structures. This maskless method provides an easy and fast alternative for large-area nanopatterning within the microchannel as compared to other conventional techniques, such as electron beam lithography which is prohibitively costly and time consuming for large-area structuring.

The effect of black silicon nanostructures on EOF behavior was studied experimentally by current monitoring method (see Figure 2) and numerically by finite element simulation (COMSOL Multiphysics) (see Figure 3). The average EOF velocity of 1mM sodium bicarbonate (NaHCO3) in microchannel with black silicon structures was calculated from the displacement time measured by current monitoring experiment, in comparison to the smooth microchannel (see Figure 4). It can be observed that the average EOF velocity is reduced by approximately 10 ± 5 % with the introduction of black silicon nanostructures. This is in good agreement with the simulation results as shown in Figure 4, which indicates a reduction in average EOF velocity by approximately 7%. EOF originates from the interaction between the EDL and the applied electric field. The numerical simulation reveals that the nanostructures distort the local electric field distribution at the wall, resulting in the decrease of average electric field, and hence reduce the overall flow velocity.

The outcomes of this investigation enhance the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.

REFERENCES:

[1] Y. Koga, R. Kuriyama, Y. Sato, K. Hishida, and N. Miki, “Effects of micromachining processes on electro-osmotic flow mobility of glass surfaces,” Micromachines 2013, 4, 67-79.

[2] T. Yasui, N. Kaji, M. R. Mohamadi, Y. Okamoto, M. Tokeshi, Y. Horiike, and Y. Baba, “Electroosmotic flow in microchannels with nanostructures,” ACS Nano 2011, 5, 7775-7780.

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04159

Cancer is a highly heterogeneous disease, thus a “one-size-fits-all” treatment approach has not been effective in cancer management. Interactions between tumor and their environment at the cellular and systemic levels have been shown to contribute to variability in therapy outcomes.  Tumor cells interact at the cellular level with immune cells and tumor associated fibroblasts to modulate tumor cell susceptibility to various treatment modalities. At the systemic level, anti-cancer drugs can cause differential toxic side effects to other tissues, which limits the maximum tolerable dose for patients. While patient derived xenografts (PDx) are being explored as mouse avatars for pre-clinical screening of an optimal personalized therapeutic regime, animal models are expansive and not scalable for screening applications. To fill this technological gap, we have developed Patient-Derived Micro-Avatar Chips (PD-MAC), which integrate the biological diverse characteristics of patient-derived tumor cells and high configurability of microfluidic systems, to study tumor-environment interactions at the cellular and systemic level. A 3D microenvironment can be engineered using micropillar structures to support the formation and remodeling of patient-derived parental and metastatic oral squamous cell carcinoma (OSCC) into 3D micro-tumors (PD-mTs). We have developed a modular approach to achieve system integration with various microfluidic components such as micro-pumps and valves, and a second tissue chip to facilitate the scaling of the PD-MAC to study systemic effects of chemotherapeutic drugs. Finally, we demonstrate the manipulation of parental and metastatic OSCC tumor and immune (NK) cells using hydrodynamic trapping arrays to investigate differential immune-mediated cytotoxicity.

 

  • Sciforum conference paper
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In Proceedings of the Optofluidics 2017 25-28 July 2017; MDPI AG , 2017,
doi: 10.3390/optofluidics2017-04160

There is a pressing need for simple, rapid and effective ways of concentrating and capturing bacteria and bacterial spores for detection and identification [1]. Acoustofluidic approaches can act on a large number of cells simultaneously over a relatively wide area, allowing cells to be imaged, concentrated, or captured in a flow-through system. This paper discusses the use of planar ultrasonic resonator systems for high throughput 2D cytometry, for cell concentration and finally for cell capture on functionalized surfaces. Recent results demonstrate the capture of bacterial spores on antibody functionalized surfaces and it is shown how the same technology can be used to capture cells from more complex fluids such as whole blood.

 Acoustophoretic forces tend to move cells to regions of low acoustic pressure and within planar resonator systems [2] it is generally simplest to generate forces towards the centre of a flow-through chamber as shown in Fig. 1 (a) (a half wave resonance). Such a half wave mode can be used for particle concentration in low aspect ratio channels [3] but in the wide channels of these resonators the fluid behaviour makes the extraction of a high concentration challenging [4]. Such a half-wave mode can successfully be used to move particles into the focal plane of an imaging system [5]. This approach not only places particles or cells at the imaging focus, but has the additional significant advantage that within a highly laminar flow, all the particles in this optical focal plane will travel through the imaging field at identical velocities, allowing particle tracking during imaging without motion blur. This has allowed 2D imaging of beads and cells at very high throughputs – up to 200,000 beads per second [5].

 A better approach to concentrating bacteria is to force them to a surface rather than to the centre of a channel. Early acoustic designs for moving particles to a surface were very sensitive to precise layer thicknesses [6], but by using just a thin layer to form the reflector of the device so that the reflection is effectively from the low acoustic impedance air boundary, produces a more robust field [7]. Such an approach for concentrating bacteria, as shown in Fig. 1 (b), can be used to increase the concentration of E. coli by a factor of 60 and at a flow rate of 20 ml/hr [4].

This flow-through concentration requires a further identification stage but a more efficient approach is to detect bacteria of interest within the flow-through chamber itself. This can be achieved using the approach shown schematically in Fig. 2 in which the machined stainless steel device shown in Fig. 3 incorporates a “thin reflector” that comprises a cover-slip with antibody functionalization for bacterial capture. Fig. 4 shows an image of a slide with spots functionalized to capture Bacillus globigii (BG) spores using a spore concentration of 105 per ml and a flow rate of 10 ml/hr. The same approach has also been used successfully to isolate Basophils from diluted whole blood.

REFERENCES:

[1] C. Paez-Aviles, E. Juanola-Feliu, J. Punter-Villagrasa, B. Del Moral Zamora, A. Homs-Corbera, J. Colomer-Farrarons, et al., "Combined Dielectrophoresis and Impedance Systems for Bacteria Analysis in Microfluidic On-Chip Platforms," Sensors (Basel), vol. 16, Sep 16 2016.

[2] P. Glynne-Jones, R. J. Boltryk, and M. Hill, "Acoustofluidics 9: Modelling and applications of planar resonant devices for acoustic particle manipulation," Lab on a Chip, vol. 12, pp. 1417-26, 2012.

[3] M. Evander, A. Lenshof, T. Laurell, and J. Nilsson, "Acoustophoresis in wet-etched glass chips," Analytical Chemistry, vol. 80, pp. 5178-5185, Jul 2008.

[4] D. Carugo, T. Octon, W. Messaoudi, A. Fisher, M. Carboni, N. R. Harris, et al., "A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics," Lab on a Chip, vol. 14, pp. 3830-3842, 2014.

[5] R. Zmijan, U. S. Jonnalagadda, D. Carugo, Y. Kochi, E. Lemm, G. Packham, et al., "High throughput imaging cytometer with acoustic focussing," RSC Advances, vol. 5, pp. 83206 – 83216, 2015.

[6] S. P. Martin, R. J. Townsend, L. A. Kuznetsova, K. A. J. Borthwick, M. Hill, M. B. McDonnell, et al., "Spore and micro-particle capture on an immunosensor surface in an ultrasound standing wave system," Bio-sensors and Bioelectronics, vol. 21, pp. 758-767, 2005.

[7] P. Glynne-Jones, R. J. Boltryk, M. Hill, N. R. Harris, and P. Baclet, "Robust acoustic particle manipulation: A thin-reflector design for moving particles to a surface," The Journal of the Acoustical Society of America, vol. 126, pp. EL75-EL79, 2009.

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