Helen Berney

Application of magnetohydrodynamic actuation to continuous flow chemistryElectronic supplementary information (ESI) available: figures depicting a silicon MHD microreactor, finite element solution for velocity profile in the silicon microreactor annulus, and the effect of MHD actuation conditions on the PCR product previously generated by conventional amplification methods and on the PCR reagents prior to thermocycling by conventional methods. See http://www.rsc.org/suppdata/lc/b2/b206756k/

Continuous flow microreactors with an annular microchannel for cyclical chemical reactions were fabricated by either bulk micromachining in silicon or by rapid prototyping using EPON SU-8. Fluid propulsion in these unusual microchannels was achieved using AC magnetohydrodynamic (MHD) actuation. This integrated micropumping mechanism obviates the use of moving parts by acting locally on the electrolyte, exploiting its inherent conductive nature. Both silicon and SU-8 microreactors were capable of MHD actuation, attaining fluid velocities of the order of 300 microm s(-1) when using a 500 mM KCl electrolyte. The polymerase chain reaction (PCR), a thermocycling process, was chosen as an illustrative example of a cyclical chemistry. Accordingly, temperature zones were provided to enable a thermal cycle during each revolution. With this approach, fluid velocity determines cycle duration. Here, we report device fabrication and performance, a model to accurately describe fluid circulation by MHD actuation, and compatibility issues relating to this approach to chemistry.

Structuring laminar flows using annular magnetohydrodynamic actuation

Laminar flow behaviour is typically observed when transporting fluids in micron-scale channels. Here, cross channel mass transport occurs only by molecular diffusion and mixing adjacent fluid streams becomes problematic. A parabolic velocity profile is also observed with pressure-driven laminar flow in a conduit. This property can be exploited by circulating fluid in an annulus such that two initially separated liquids are forced to pass through each other resulting in massive increases in the interfacial area to promote conditions for mass transfer. Miniature machined and micromachined prototypes with an integrated magnetohydrodynamic (MHD) micropump for fluid circulation were fabricated and tested. Annular MHD micromixing was characterised using fluorescein, bromophenol blue and hydrogen ion solutes for a range of velocities and modelled to include both diffusive and convective components. Furthermore, a lateral partitioning mechanism was identified and examined.

Investigation into immobilisation of lactate oxidase to improve stability

Abstract Lactate oxidase (LOx) is an unstable enzyme. In this work, a variety of immobilisation techniques are investigated in an effort to improve the long-term stability of the enzyme. These include covalent linkage to two membrane types, encapsulation in a BSA gel and four different sol–gel matrices. The enzyme glucose oxidase (GOx) was also immobilised in the same sol–gel matrices. The methods were assessed for both activity and stability of the enzyme and the mechanical rigidity of the matrix. The BSA and sol–gels both formed physically robust enzyme layers. The enzyme retained its activity in the BSA gel for 20 days. Activity of the enzyme was much higher in the sol–gel matrices and remained stable for at least 55 days. Sol–gel processing conditions were also investigated.

Thermal modelling of Ohmic heating microreactors

This paper describes the static and transient thermal modelling of an Ohmic heating microreactor for biological sample processing for the purpose of genetic analysis. Precise thermal management can be used for the effective preparation of analyte DNA molecules prior to detection. Due to the small dimensions of the microreactor, the direct measurement and monitoring of the temperature distribution presents a challenge. To overcome this, thermal modelling has been used to accurately predict the thermal behaviour of the microreactor and sample component. It is further possible to calculate the required input power levels and provide design criteria to optimise the design of the microreactor.

Development of PCR conditions in a silicon microreactor DNA-amplification device

A silicon microsystem was developed which functions as a miniaturised DNA-amplification device. The system represents a technology platform for performing a polymerase chain reaction (PCR) with reduced volumes of 7 µL. The silicon microreactor was fabricated using silicon bulk micromachining, and a platinum heater was fabricated on a Pyrex substrate. A miniaturised DNA-amplification system permitted rapid heating and cooling, and shorter reaction times of 30 min were achieved. In this work, biocompatibility issues are addressed; conditions for efficient PCR in a silicon-based microreactor are established for the amplification of 500 bp DNA from the Escherichia coli bacteriophage Lambda; and the conditions are verified by amplifying a 255 bp region from the Mycobacterium tuberculosis rpoB gene. This work describes the PCR volume scale down experiments that were conducted and concentrations of the reactants; Taq polymerase, oligonucleotides, MgCl2 and template DNA were determined for DNA-amplification reactions with this novel device.

The CAP-FET, a scaleable MEMS sensor technology on CMOS with programmable floating gate

A new MEMS sensor architecture is presented that converts mechanical displacement of a conductive diaphragm directly to a current. The electrical bias on the mechanical element is capacitively coupled to an electrically floating MOS gate that controls the sensor output current. The sensor is manufactured using a process module that slots directly in to a CMOS process. Both the sensor architecture and process module will scale with shrinking CMOS generations. Injection of charge onto the floating gate can be used to program the sensor threshold voltage. The sensor architecture has been demonstrated as a pressure sensor on a CMOS process.

Wafer level inspection for determination of yield of surface-micromachined pressure sensors

Development of a surface micromachining process for commercial scale production of absolute pressure sensors necessitates the definition of inspection tests at each stage of the process and for the completed packaged product. The yield measurement described in this paper is for a Field Effect Transistor pressure sensor integrated into a CMOS process. This measurement can be divided into verification of the electrical and mechanical properties of the pressure device. This paper describes the development of a simple method for mechanical yield determination. The method is based on a visual inspection procedure where the interference rings observed under a conventional 5X/20X inspection microscope, are correlated with white light interferometry (wafer level and packaged devices) and Atomic Force Microscopy (AFM) (wafer level) measurements. The application of these two profiling methods is also compared in the paper. Based on this work a simple, low cost, automatic system for yield determination using standard equipment can be developed. Initial results from a software system for inspection automation are presented.