Junshan Liu

Microfluidic platform integrated with worm-counting setup for assessing manganese toxicity

We reported a new microfluidic system integrated with worm responders for evaluating the environmental manganese toxicity. The micro device consists of worm loading units, worm observing chambers, and a radial concentration gradient generator (CGG). Eight T-shape worm loading units of the micro device were used to load the exact number of worms into the corresponding eight chambers with the assistance of worm responders and doorsills. The worm responder, as a key component, was employed for performing automated worm-counting assay through electric impedance sensing. This label-free and non-invasive worm-counting technique was applied to the microsystem for the first time. In addition, the disk-shaped CGG can generate a range of stepwise concentrations of the appointed chemical automatically and simultaneously. Due to the scalable architecture of radial CGG, it has the potential to increase the throughput of the assay. Dopaminergic (DAergic) neurotoxicity of manganese on C. elegans was quantitatively assessed via the observation of green fluorescence protein-tagged DAergic neurons of the strain BZ555 on-chip. In addition, oxidative stress triggered by manganese was evaluated by the quantitative fluorescence intensity of the strain CL2166. By scoring the survival ratio and stroke frequency of worms, we characterized the dose- and time-dependent mobility defects of the manganese-exposed worms. Furthermore, we applied the microsystem to investigate the effect of natural antioxidants to protect manganese-induced toxicity.

Fabrication of PMMA nanofluidic electrochemical chips with integrated microelectrodes

A novel method based on plasma etching was proposed for monolithically integrating planar nanochannels and microelectrodes on a poly (methyl methacrylate) (PMMA) plate, and complete PMMA nanofluidic electrochemical chips with integrated microelectrodes were constructed by bonding with another PMMA plate containing microchannels. The fabrication sequences of nanochannels and microelectrodes were optimized. The oxygen plasma etching rate of PMMA nanochannels was studied, and the average rate was 15 nm/min under optimal conditions. An UV-ozone assisted thermal bonding method was developed to realize a low-temperature chip bonding, and the variations in width and depth of nanochannels before and after bonding were 2% and 5%, respectively. As a demonstration, a nanoparticle crystal (NPC)-based nanofluidic biosensor with integrated Ag microelectrodes was designed and fabricated. Sub-microchannel arrays with a depth of 400 nm and a width of 30 μm on the biosensor functioned as filters, and trapped 540 nm silica nanoparticles modified with streptavidin inside the connected microchannel to assemble the NPC. The interspaces in the NPC formed a three-dimensional nanochannel network with an equivalent diameter of 81 nm. By measuring the conductance across the NPC, a high quality nanofluidic sensing of biotin was achieved. The limit of detection was 1 aM, and the detection range was from 1 aM to 0.1 nM.

Macro-micro modeling design in system-level and experiment for a micromixer

A novel macro-micro modeling method has been presented for designing three-dimensional microfluidic devices in system-level. The macro model was used to solve mass transfer in simple modules of a microfluidic device, such as straight channels. The micro model was used to solve mass transfer in complicated modules of a microfluidic device, such as mixing channels. Then, the mass transfer in a microfluidic device was solved in system-level. The macro-micro modeling method was applied to a micromixer, and the concentration distribution in the micromixer can be evaluated. Compared with numerical simulation, the maximum relative deviation between macro-micro calculation and numerical simulation, results are 1.28% and the computational efficiency of macro-micro model was improved significantly with the numbers of straight channels increasing. The macro-micro modeling method is proven to be an effective way for rapid design of microfluidic devices in system-level.

Effects of Fumed and Mesoporous Silica Nanoparticles on the Properties of Sylgard 184 Polydimethylsiloxane

The effects of silica nanoparticles on the properties of a commonly used Sylgard 184 polydimethylsiloxane (PDMS) in microfluidics were systemically studied. Two kinds of silica nanoparticles, A380 fumed silica nanoparticles and MCM-41 mesoporous silica nanoparticles, were individually doped into PDMS, and the properties of PDMS with these two different silica nanoparticles were separately tested and compared. The thermal and mechanical stabilities of PDMS were significantly enhanced, and the swelling characteristics were also improved by doping these two kinds of nanoparticles. However, the transparency of PDMS was decreased due to the light scattering by nanoparticles. By contrast, PDMS/MCM-41 nanocomposites showed a lower coefficient of thermal expansion (CTE) owing to the mesoporous structure of MCM-41 nanoparticles, while PDMS/A380 nanocomposites showed a larger elastic modulus and better transparency due to the smaller size of A380 nanoparticles. In addition, A380 and MCM-41 nanoparticles had the similar effects on the swelling characteristics of PDMS. The swelling ratio of PDMS in toluene was decreased to 0.68 when the concentration of nanoparticles was 10 wt %.

Fabrication of SU-8 moulds on glass substrates by using a common thin negative photoresist as an adhesive layer

A method for fabricating SU-8 moulds on glass substrates is presented. A common thin negative photoresist was coated on the glass slide as an adhesive layer, and then SU-8 was patterned on the adhesive layer. The presence of the adhesive layer improved the lifetime of a SU-8 mould from a few cycles to over 50 cycles. Moreover, the fabrication of the adhesive layer is quite simple and no additional equipment is required. The effects of the adhesion behavior of the negative photoresist and SU-8 on substrates on the durability of the SU-8 mould were investigated. The work of adhesion of the common thin negative photoresist on glass was 51.2 mJ m−2, which is 22.5% higher than that of SU-8 on silicon and 32.3% higher than that of SU-8 on glass. The abilities of the method for replicating high-aspect-ratio microstructures were also tested. One SU-8 mould with 60 × 60 array micropillars with aspect ratios lower than 3 could be used to cast at least 20 polydimethylsiloxane devices.

Fabrication of a three-layer SU-8 mould with inverted T-shaped cavities based on a sacrificial photoresist layer technique

A novel method for fabricating a three-layer SU-8 mould with inverted T-shaped cavities is presented. The first two SU-8 layers were spin coated and exposed separately, and simultaneously developed to fabricate the bottom and the horizontal part of the inverted T-shaped cavity. Then, a positive photoresist was filled into the cavity, and a wet lapping process was performed to remove the excess photoresist and make a temporary substrate. The third SU-8 layer was spin coated on the temporary substrate to make the vertical part of the inverted T-shaped cavity. The sacrificial photoresist layer can prevent the first two SU-8 layers from being secondly exposed, and make a temporary substrate for the third SU-8 layer at the same time. Moreover, the photoresist can be easily removed with the development of the third SU-8 layer. A polydimethylsiloxane (PDMS) microchip with arrays of T-shaped cantilevers for studying the mechanics of cells was fabricated by using the SU-8 mould.

Nanopore density effect of polyacrylamide gel plug on electrokinetic ion enrichment in a micro-nanofluidic chip

In this paper, the nanopore density effect on ion enrichment is quantitatively described with the ratio between electrophoresis flux and electroosmotic flow flux based on the Poisson-Nernst-Planck equations. A polyacrylamide gel plug is integrated into a microchannel to form a micro-nanofluidic chip. With the chip, electrokinetic ion enrichment is relatively stable and enrichment ratio of fluorescein isothiocyanate can increase to 600-fold within 120 s at the electric voltage of 300 V. Both theoretical research and experiments show that enrichment ratio can be improved through increasing nanopore density. The result will be beneficial to the design of micro-nanofluidic chips.

A Microfluidic Chip with Integrated Microelectrodes for Real-time Dopamine Detection

Abstract A microfluidic chip with integrated microelectrodes for real-time dopamine (DA) detection was designed and fabricated. The chip consisted of a polydimethylsiloxane (PDMS) channel plate and a glass electrode plate. One central channel as the culture chamber of neural stem cells and two lateral channels for the transportation of the culture medium were integrated on the PDMS channel plate. Microelectrodes for real-time dopamine detection were integrated on the glass electrode plate. To solve the problem in demoulding the PDMS channel plate from the silicon mold, a novel demoulding method was developed. An Au-Au-Au three-electrode system was constructed, and it performed well in electrochemical detection. The performance of the microfluidic chip was primarily studied by detecting dopamine dissolved in the medium for the culture of neural stem cells. The detection limit of dopamine was 3.92 μM, the linear detection range was from 10 μM to 500 μM, and the detection reproducibility from different chips was less than 4%.

Research on impact behaviour and silicon insert fracture phenomenon in microinjection moulding

Silicon insert is a promising tool for microinjection moulding (MIM). However, its fracture problem induced by impact in MIM creates a bottleneck for application. The purpose of this paper is to investigate the impact behaviour in MIM and the effect on the fracture of silicon inserts. The finite element method is utilised to calculate the crack propagation of silicon inserts with pressure load and thermal load in the MIM process. The simulation result shows that the crack propagation is more easily induced by the increase of pressure load, while the temperature change has little effect on the crack propagation. An experimental platform, including a novel rotatable insert mould, is developed to investigate the dynamic pressure in the MIM process. The result shows that both the maximum pressure and the maximum loading rate occur in the initial period of MIM process. It indicates that the silicon insert is more prone to fracture at the beginning of the MIM process, and spatial pressure peaks are observed in ...

Cyclic voltammetric determination of glutamic-pyruvic transaminase activity based on transdeamination

Glutamic-pyruvic transaminase (GPT) is one of the most important enzymes in human liver and has a valuable clinical reference for the diagnosis of many liver diseases. Here a method for the determination of GPT activity based on transdeamination is presented. In this method, a three electrode setup was used for the cyclic voltammetric determination of the enzyme. With the electrochemical detection of reduced nicotinamide adenine dinucleotide (NADH) produced from transdeamination, the GPT activity was characterized under optimal circumstances. Firstly, to verify the response of the electrode to NADH, a series of NADH concentrations varying from 39 μM to 2.5 mM were calibrated with cyclic voltammetry (CV). A linear relationship between the NADH concentration and the peak current with R2 0.9999 was obtained. Then the concentration of α-ketoglutarate (α-KG) which can exert great influence on transdeamination was also optimized and the most sensitive response was achieved at the point of 0.75 mM α-KG. Finally, the GPT activity was determined using both the unmodified screen-printed carbon electrode (SPCE) and the electrode modified with CNTs. The results showed that the relationship between the GPT activity and the peak current of the CV curve was linear between 60 U L−1 and 300 U L−1 and the modified electrode exhibited a slightly better linear relationship than the unmodified electrode. This work proposes a new enzymatic reactive system based on transdeamination for the electrochemical detection of GPT activity and combines the electrochemical detection of NADH with the determination of GPT activity.