M.C. Tracey

Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering

Polydimethylsiloxane (PDMS) elastomers are extensively used for soft lithographic replication of microstructures in microfluidic and micro-engineering applications. Elastomeric microstructures are commonly required to fulfil an explicit mechanical role and accordingly their mechanical properties can critically affect device performance. The mechanical properties of elastomers are known to vary with both curing and operational temperatures. However, even for the elastomer most commonly employed in microfluidic applications, Sylgard 184, only a very limited range of data exists regarding the variation in mechanical properties of bulk PDMS with curing temperature. We report an investigation of the variation in the mechanical properties of bulk Sylgard 184 with curing temperature, over the range 25 ◦ C to 200 ◦ C. PDMS samples for tensile and compressive testing were fabricated according to ASTM standards. Data obtained indicates variation in mechanical properties due to curing temperature for Young’s modulus of 1.32‐2.97 MPa, ultimate tensile strength of 3.51‐7.65 MPa, compressive modulus of 117.8‐186.9 MPa and ultimate compressive strength of 28.4‐51.7 GPa in a range up to 40% strain and hardness of 44‐54 ShA.

A silicon micromachined device for use in blood cell deformability studies

An application of silicon micromachining to the analysis of blood cell rheology is described. The system, based upon a micromachined flow cell, provides a specific measurement of each cell in a statistically significant population in terms of both flow velocity profile and an index of cell volume while the cells flow through an array of microchannels. The rationale, design, and fabrication of the silicon micromachined flow cell is discussed. Interrelated considerations determining the design of the associated fluidic, mechanical, imaging, and real-time image analysis subsystems are examined. Sample data comparing normal and iron deficiency anaemic blood are presented to illustrate the potential of this technique.<<ETX>>

Micro throttle pump employing displacement amplification in an elastomeric substrate

We report a micro throttle pump (MTP) with enhanced throttling resulting from beneficial deformation of its elastomer substrate. In the MTP reported, this has doubled the effective deflection of the piezo electric (PZT) actuator with a consequent five-fold enhancement of throttling ratio. This mode of throttling has been modelled by finite element method and computational fluid dynamic techniques whose predictions agreed well with experimental data from a throttle test structure; providing typical throttling ratios of 8:1 at low pressures. The improved throttles have been incorporated in a prototype, single PZT, MTP, fabricated with double-depth microfluidics, which pumped both water and a suspension of 5 µm polystyrene beads at a maximum flow rate of 630 µl min−1 and a maximum back-pressure of 30 kPa at a pumping frequency of 1.1 kHz. This represents an approximate five-fold enhancement of both performance metrics compared to our previous single PZT device.

The production of precision silicon micromachined non-spherical particles for aerosol studies

Original article can be found at: http://www.sciencedirect.com/science/journal/00218502 Copyright Elsevier Ltd. [Full text of this article is not available in the UHRA]

Increasing pumping efficiency in a micro throttle pump by enhancing displacement amplification in an elastomeric substrate

Fluid transport is accomplished in a micro throttle pump (MTP) by alternating deformation of a micro channel cast into a polydimethylsiloxane (PDMS) elastomeric substrate. The active deformation is achieved using a bimorph PZT piezoelectric disc actuator bonded to a glass diaphragm. The bimorph PZT deflects the diaphragm as well as alternately pushing and pulling the elastomer layer providing displacement amplification in the PDMS directly surrounding the micro channel. In order to improve pumping rates we have embedded a polymethylmethacrylate (PMMA) ring into the PMDS substrate which increases the magnitude of the displacement amplification achieved. FEM simulation of the elastomeric substrate deformation predicts that the inclusion of the PMMA ring should increase the channel deformation. We experimentally demonstrate that inclusion of a PMMA ring, having a diameter equal to that of the circular node of the PZT/glass/PDMS composite, increases in the throttle resistance ratio by 40% and the maximum pumping rate by 90% compared to an MTP with no ring.

A MICROFLUIDICS-BASED MICROCYTOMETER: INTERFACING MICROFLUIDICS WITH MACROFLUIDICS

We have developed an instrument for performing cytomechanical studies of red cell membrane viscoelastic behavior during flow in microfluidic channels of circa (3×3)μm section. This paper discusses interfacing such structures to the necessary supporting ‘millifluidic’ systems that provide operational functionality. Such microfluidic interfacing is more complex in the case of particle-carrying suspensions due to sedimentation and separation effects. Cell-carrying suspensions are additionally characterized by the relative fragility of cells and resulting operational constraints in sample handling. A system and matching microfluid device that seek to address these limitations are presented here. The results of preliminary trials are presented showing broad agreement with design specifications.

Protein droplet actuation on superhydrophobic surfaces: a new approach toward anti-biofouling electrowetting systems

Among Lab-on-a-chip techniques, digital microfluidics (DMF), allowing the precise actuation of discrete droplets, is a highly promising, flexible, biochemical assay platform for biomedical and bio-detection applications. However the durability of DMF systems remains a challenge due to biofouling of the droplet-actuating surface when high concentrations of biomolecules are employed. To address this issue, the use of superhydrophobic materials as the actuating surface in DMF devices is examined. The change in contact angle by electrowetting of deionised water and ovalbumin protein samples is characterised on different surfaces (hydrophobic and superhydrophobic). Ovalbumin droplets at 1 mg ml−1 concentration display better electrowetting reversibility on Neverwet®, a commercial superhydrophobic material, than on Cytop®, a typical DMF hydrophobic material. Biofouling rate, characterised by roll-off angle measurement of ovalbumin loaded droplets and further confirmed by measurements of the mean fluorescence intensity of labelled fibrinogen, appears greatly reduced on Neverwet®. Transportation of protein laden droplets (fibrinogen at concentration 0.1 mg ml−1 and ovalbumin at concentration 1 mg ml−1 and 10 mg ml−1) is successfully demonstrated using electrowetting actuation on both single-plate and parallel-plate configurations with performance comparable to that of DI water actuation. In addition, although droplet splitting requires further attention, merging and efficient mixing are demonstrated.

Electromagnetic stirring in a microbioreactor with non-conventional chamber morphology and implementation of multiplexed mixing

Abstract BACKGROUND Microbioreactors have emerged as novel tools for early bioprocess development. Mixing lies at the heart of bioreactor operation (at all scales). The successful implementation of micro‐stirring methods is thus central to the further advancement of microbioreactor technology. The aim of this study was to develop a micro‐stirring method that aids robust microbioreactor operation and facilitates cost‐effective parallelization. RESULTS A microbioreactor was developed with a novel micro‐stirring method involving the movement of a magnetic bead by sequenced activation of a ring of electromagnets. The micro‐stirring method offers flexibility in chamber designs, and mixing is demonstrated in cylindrical, diamond and triangular shaped reactor chambers. Mixing was analyzed for different electromagnet on/off sequences; mixing times of 4.5 s, 2.9 s, and 2.5 s were achieved for cylindrical, diamond and triangular shaped chambers, respectively. Ease of micro‐bubble free priming, a typical challenge of cylindrical shaped microbioreactor chambers, was obtained with a diamond‐shaped chamber. Consistent mixing behavior was observed between the constituent reactors in a duplex system. CONCLUSION A novel stirring method using electromagnetic actuation offering rapid mixing and easy integration with microbioreactors was characterized. The design flexibility gained enables fabrication of chambers suitable for microfluidic operation, and a duplex demonstrator highlights potential for cost‐effective parallelization. Combined with a previously published cassette‐like fabrication of microbioreactors, these advances will facilitate the development of robust and parallelized microbioreactors.

A microfluidics-based instrument for cytomechanical studies of blood

A complete instrument for the measurement of erythrocyte (red blood cell) flow in microchannels is presented. The instrument measures circa 1500 cells on a cell-by-cell basis in an array of microchannels. Microchannel dimensions and operating pressures are physiologically-analogous. Resulting bi-variate data, obtained by real-time image processing, describes cell flow with a unique volume-velocity space representation which provides detailed insights into both cell flow resistance (independent of cell volume) and the presence of pathological sub-populations. The authors commence by discussing the instruments design, with particular attention to the microfluidics. They proceed to report analyses of the instruments reproducibility, the characterisation of normal blood, and their ability to detect artificial sub-populations of cells with rigid membranes produced by chemically modifying a fraction of the analyte. Having validated the performance of the instrument, the authors then present a clinical result from a transfused thalassaemic displaying highly irregular microrheological metrics.

Micro Fluidics Using Novel Materials

We report two novel microfluidic devices fabricated from PDMS (polydimethylsiloxane). Such devices are indicative of the increasing migration of microfluidics to materials distinct from those of the mainstream Silicon MEMS industry. Specifically, plastics fabrication techniques and materials such as SU8 photostructurable epoxy and microcasting, which are employed in these examples, are proving particularly topical and are discussed here. The devices reported consist of PDMS-glass-piezoelectric hybrids exploiting the compliant nature of elastomer substrates to yield valuable functionality. The first device is a micropump employing novel, non-sealing valves and pumping 300 microlitres per minute and developing a maximum pressure of 6kPa. The second reported device is a micromixer employing temporal interleaving of samples via PDMS-glass microvalves in order to mix effectively within the laminar, microfluidic flow regime.Copyright