An introduction to heat transfer

Latest Developments in Micro Total Analysis Systems

This paper looks at the design, fabrication and characterization of stackable microfluidic emulsion generators, with coefficients of variation as low as ~6% and with production rates as high as ~1 L h−1. This work reports the highest throughput reported in the literature for a microfluidic device with simultaneous operation of liquid–liquid droplet generators. The device was achieved by stacking several layers of 128 flow-focusing droplet generators, organized in a circular array. These layers are interconnected via through-holes and fed with designated fractal distribution networks. The proposed layers were milled on poly(methylmethacrylate) (PMMA) sheets and the stack was thermo-compression bonded to create a three-dimensional device with a high density of generators and an integrated hydraulic manifold. The effect of stacking multiple layers was studied and the results show that fabrication accuracy has a greater impact on the dispersity of the emulsion than the addition of more layers to the stack. Particle crystallization of drugs was also demonstrated as a possible application of this technology in industry.

Three-dimensional parallelization of microfluidic droplet generators for a litre per hour volume production of single emulsions

The use of a magnetic field for manipulating the motion of magnetic particles in microchannels has attracted increasing attention in microfluidic applications. Generation of a flexible and controllable magnetic field plays a crucial role in making better use of the particle manipulation technology. Recent advances in the development of magnet systems and magnetic field control methods have shown that it has great potential for effective and accurate manipulation of particles in microfluidic systems. Starting with the analysis of magnetic forces acting on the particles, this review gives the configurations and evaluations of three main types of magnet system proposed in microfluidic applications. The interaction mechanisms of magnetic particles with magnetic fields are also discussed.

Configurations and control of magnetic fields for manipulating magnetic particles in microfluidic applications: magnet systems and manipulation mechanisms.

This paper introduces super absorbent polymer valves and colorimetric sensing reagents as enabling components of soft, skin-mounted microfluidic devices designed to capture, store, and chemically analyze sweat released from eccrine glands. The valving technology enables robust means for guiding the flow of sweat from an inlet location into a collection of isolated reservoirs, in a well-defined sequence. Analysis in these reservoirs involves a color responsive indicator of chloride concentration with a formulation tailored to offer stable operation with sensitivity optimized for the relevant physiological range. Evaluations on human subjects with comparisons against ex situ analysis illustrate the practical utility of these advances.

Super-Absorbent Polymer Valves and Colorimetric Chemistries for Time-Sequenced Discrete Sampling and Chloride Analysis of Sweat via Skin-Mounted Soft Microfluidics

We present a low-cost bonding method for polymethylmethacrylate (PMMA) microfluidics that combines elements of solvent bonding, thermal bonding and microwave bonding. Rather than using specialized equipment, we take household equipment and combine it to produce an effective bonding method that borrows from food packaging technologies for selective heating in a microwave. A poor solvent for PMMA is applied between two halves of a microfluidic system and clamped together using miniature binder clips. Excess solvent from the channels is then drawn out via capillary action and avoids channel clogging during the bonding process. Placing the whole apparatus in a commercial microwave will heat up the thin metal clips and cause the solvent to dissolve and bond the PMMA interface. The whole bonding process takes only a few minutes, and results in high bond strengths.

http://iopscience.iop.org/article/10.1088/0960-1317/20/1/015026/pdf

Microwave-induced, thermally assisted solvent bonding for low-cost PMMA microfluidic devices

Although PMMA can be exposed using a variety of exposure sources, deep-UV at 254 nm is of interest because it is relatively inexpensive. Additionally, deep-UV sources can be readily scaled to large area exposures. Moreover, this paper will show that depths of over 100µm can be created in commercial grade PMMA using an uncollimated source. These depths are sufficient for creating microfluidic channels. This paper will provide measurements of the dissolution depth of commercial grade PMMA as a function of the exposure dose and etch time, using an IPA:H2O developer. Additionally, experiments were run to characterize the dependence of the dissolution rate on temperature and agitation. The patterned substrates were thermally bonded to blank PMMA pieces to enclose the channels and ports were drilled into the reservoirs. The resulting fluidic systems were then tested for leakage. The work herein presents the patterning, development and system behaviour of a complete microfluidics system based on commercial grade PMMA.

http://iopscience.iop.org/article/10.1088/0960-1317/18/11/115029/pdf

Deep-UV patterning of commercial grade PMMA for low-cost, large-scale microfluidics

We present a new micromixer based on highly magnetic, flexible, high aspect-ratio, artificial cilia that are fabricated as individual micromixer elements or in arrays for improved mixing performance. These new cilia enable high efficiency, fast mixing in a microchamber, and are controlled by small electromagnetic fields. The artificial cilia are fabricated using a new micromolding process for nano-composite polymers. Cilia fibers with aspect-ratios as high as 8:0.13 demonstrate the fabrication technique's capability in creating ultra-high aspect-ratio microstructures. Cilia, which are realized in polydimethylsiloxane doped with rare-earth magnetic powder, are magnetized to produce permanent magnetic structures with bidirectional deflection capabilities, making them highly suitable as mixers controlled by electromagnetic fields. Due to the high magnetization level of the polarized nano-composite polymer, we are able to use miniature electromagnets providing relatively small magnetic fields of 1.1 to 7 mT to actuate the cilia microstructures over a very wide motion range. Mixing performances of a single cilium, as well as different arrays of multiple cilia ranging from 2 to 8 per reaction chamber, are characterized and compared with passive diffusion mixing performance. The mixer cilia are actuated at different amplitudes and frequencies to optimize mixing performance. We demonstrate that more than 85% of the total volume of the reaction chamber is fully mixed after 3.5 min using a single cilium mixer at 7 mT compared with only 20% of the total volume mixed with passive diffusion. The time to achieve over 85% mixing is further reduced to 70 s using an array of eight cilia microstructures. The novel microfabrication technique and use of rare-earth permanently-magnetizable nano-composite polymers in mixer applications has not been reported elsewhere by other researchers. We further demonstrate improved mixing over other cilia micromixers as enabled by the high aspect-ratio, high flexibility, and magnetic properties of our cilia micromixer elements.

http://iopscience.iop.org/article/10.1088/0960-1317/24/2/025003/pdf

Microfluidic active mixers employing ultra-high aspect-ratio rare-earth magnetic nano-composite polymer artificial cilia

We demonstrate a new hybrid-soft-lithography micromolding process that results in a mechanically-compliant, magnetically actuated membrane. The new microactuator consists of a thin undoped polydimethylsiloxane (PDMS) membrane with a central magnet feature, which is also micromolded using soft lithography. The central magnet is composed of a magnetic nanocomposite polymer (M-NCP) material, which is achieved by uniformly dispersing rare earth magnetic powder (MQP-12-5) in the PDMS polymer matrix. The hybrid fabrication technique is capable of realizing a highly flexible membrane with the ability of providing bidirectional deflection without sacrificing the transparency of the device, which may be required for many biomedical applications. Furthermore, we show that the hybrid process also yields improved deflection of the membrane using lower magnetic fields than an opaque membrane fabricated entirely in nanocomposite polymer. These lower fields are more suitable to on-chip production of microactuators excited via electronic signals in microcoils.

Fabrication Process for Electromagnetic Actuators Compatible with Polymer Based Microfluidic Devices

We present a new magnetically actuated microfluidic valve that employs a highly magnetic composite polymer (M-CP) containing rare-earth hard-magnetic powder for its actuating element and for its valve seat. The M-CP offers much higher magnetization compared to the soft-magnetic, ferrite-based composite polymers typically used in microfluidic applications. Each valve consists of a permanently magnetized M-CP flap and valve seat mounted on a microfluidic channel system fabricated in poly(dimethylsiloxane) (PDMS). Each valve is actuated under a relatively small external magnetic field of 80 mT provided by a small permanent magnet mounted on a miniature linear actuator. The performance of the valve with different flap thicknesses is characterized. In addition, the effect of the magnetic valve seat on the valve's performance is also characterized. It is experimentally shown that a valve with a 2.3 mm flap thickness, actuated under an 80 mT magnetic field, is capable of completely blocking liquid flow at a flow rate of 1 ml min−1 for pressures up to 9.65 kPa in microfluidic channels 200 μm wide and 200 μm deep. The valve can also be fabricated into an array for flow switching between multiple microfluidic channels under continuous flow conditions. The performance of arrays of valves for flow routing is demonstrated for flow rates up to 5 ml min−1 with larger microfluidic channels of up to 1 mm wide and 500 μm deep. The design of the valves is compatible with other commonly used polymeric microfluidic components, as well as other components that use the same novel permanently magnetic composite polymer, such as our previously reported cilia-based mixing devices.

https://iopscience.iop.org/article/10.1088/0960-1317/26/5/055012/pdf

Mona Rahbar

Papers

Microwave-induced, thermally assisted solvent bonding for low-cost PMMA microfluidic devices

Deep-UV patterning of commercial grade PMMA for low-cost, large-scale microfluidics

Microfluidic active mixers employing ultra-high aspect-ratio rare-earth magnetic nano-composite polymer artificial cilia

Fabrication Process for Electromagnetic Actuators Compatible with Polymer Based Microfluidic Devices

Design, fabrication and characterization of an arrayable all-polymer microfluidic valve employing highly magnetic rare-earth composite polymer