Success stories

Jeff Tallon: Embedded active nano-stuctures in micro-chip devices

Introduction

Imagine, if you can, a micro-city built on a Silicon chip with microscale roads and buildings where nano-couriers carry nano-parcels through networks of nano-passages, turning, mixing, flowing crowds of nano-ites filling the streets. In terms of technology this would cause a revolutionary increase in information density. Far fetched though it may sound, this is Jeff Tallon's vision for the future of microfluidics. A full scale 'micro-city' will take years to realize but the forces are gathering and first steps are being made to 'lay the foundations'.

What is Microfluidics?

Microfluidics is 'lab on a chip' technology. It involves the transport of nanolitre or picolitre volumes of fluid through microchannels (10-15ｵm in diameter) etched into chips. It is very useful in biotechnology especially in the study of proteins(proteomics) and genes (genomics) and in drug synthesis.

The New Zealand Microfluidic Programme

  • The New Zealand nano-technology microfluidics program involves physicists, inorganic and organic chemists, engineers, mathematicians and biologists from a wide range of organizations, working together to create microstructures imbedded with responsive nanostructure for use in drug synthesis and other applications. This new multidisciplinary approach marks a current trend in science and technology, transcending the boundaries between previously separate fields. Jeff is concerned primarily with creating the microfluidic devices. Other researchers study how fluid flows in the structures, design networks of capilliaries that suit the bio-chemical reactions to be performed, perform chemical reactions and so on. The success of the project relies on the co-operation of all parties involved.

How is a Microfluidic chip made?

  1. A variety of lithographic methods are used to create a network of channels and cavities on a chip. These methods could include traditional methods of etching with electron beams or chemicals or the newly developed soft lithography in which a rubbery polymer is molded to create channels. Thousands of channels can be fitted onto one chip.

  2. This chip is then pressed onto a close fitting plate to create water-tight capillaries (the rubbery polymer used in soft lithography has the advantage of sealing well).

  3. Chemicals can be pumped through capillaries to perform reactions.

What are the advantages of Microfluidics?

There are two major advantages:

  1. Parallel processing: Thousands of reactions can be performed simultaneously. which leads to very high throughput screening. This is especially useful for proteomics. A single cell can produce thousands of different proteins. Each one of these might need to be tested for a particular property. On one microfluidic chip 10,000 proteins could be tested at one time.

  2. Very fast reactions: By sending fluid round the capillaries in tiny droplets rather than in a constant stream, the surface to volume ratio is greatly increased. Fluid circulates in currents through the droplet, which leads to good mixing and therefore very fast reactions.

A major application of this technology is in drug synthesis: The total output of proteins from a healthy cell can be compared to those from a diseased cell (there might be one missing or extra protein). Microfluidics can be used to identify particular 'target' proteins and then find a 'dead' drugs, which attach to the targets to stop them reproducing.

How do you monitor what is happening within the microfluidic chip?

•  If the capillaries are transparent Raman Spectroscopy can be used to detect single molecules in the reaction products (Pablo Etchegoin at VUW is working in this area).

•  Paul Callaghan and his NMR team are using pulsed gradient NMR to provide maps of velocities of liquid flow on microfluidic devices.

How do embedded active nano-structures improve microfluidic chips?

Currently every reactant channel needs to be connected to an external pump. These can be bulky and require moving parts. Mixers and valves are also external to the chip. By embedding active nano-structures Jeff hopes to create on-chip pumps, valves and mixers that respond to external stimuli such as a change in potential or exposure to light.

The prime example of such a nano-structure is a molecule that switches from being hydophillic (water loving) to hydrophobic (water repelling) when a potential is appied. (see the diagram)

Pulses of potential sent along nano-scale tracks of electrodes layed down within the existing micro-structure could be used to send small volumes of liquid down capillaries lined with the switching molecule (a nano-pump). Pulses moving in different directions could mix fluid. The possibilities are endless.

The Nano-Courier:

In certain circumstances if extra potential is applied to the bent hydrophilic/hydrophobic molecule described above, the molecule's feet can lift up and bend over (a nano-flick-flack). The molecule could 'walk' this this way (similar to a slinky's motion) from one place to another along electrode tracks. If another molecule could be attached to this 'courier molecule' at one place (the dispatch platform) then dropped off at another place (the inward goods platform) you could have couriers delivering nano-packages all over the chip. This is the sort of technology we could expect in the future.

Responsive Surfaces:

Another promising application for these switchable molecules. Defouling machinery in large factories is often a time consuming and expensive process, a big problem in many industries. Large milk factories for example have to close down every 10 hours or so to remove protein and bacteria build up inside pipes. Currently used methods such as acid washes create toxic waste and require a large amounts of time and money. Responsive materials could be designed to coat tubes so that they self clean when triggered by an external stimulus such as a voltage.