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 45^o angle can provide highest mixing efficiency, and a 95% mixing efficiency can be achieved within 8 stage within a 2.38 mm long microchannel.

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.

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.

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 (F_E), inertial lift force (F_L) and Dean drag force (F_D). 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 (X_eq) 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.

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.