Determined by the Molecular Diffusion CoefficientDwhich can range consid-
erably between bio-molecules; for small molecules, the diffusion coefficient would
be10^3 cm^2 s^1 whereas for large molecules (e.g., proteins), the coefficient could be
in the range of 10^7 cm^2 s^1. Even though the dimensions are quite small, diffusive
mixing is rather slow as the time scales needed to diffuse half the width of a
microchannel can range from 10 s for small molecules to 16 min for large proteins.
2.4 Diagnostic Applications
A full-fledged Point of Care (POC) Diagnostic device requires a scope of attributes;
first and foremost, POC systems allow patient diagnosis in a more decentralised
manor away from the infrastructure of primary health care facilities. With this in
mind, a POC device must include a method of sample preparation, biorecognition
of target molecule, signal transduction and signal conversion to a readout or
display, which can be interpreted qualitatively or quantitatively.
Using this general schematic, a number of biological analyses have been
implemented on-disc including DNA hybridisation Microarrays [ 6 ], Real Time
PCR (qPCR) Amplification [ 7 ], Protein Microarrays [ 8 ], Immunoassays [ 9 , 10 ]
biochemical analysis [ 11 , 12 ], cell based screening bioassays [ 13 ] (Burger
et al. 2012) and biochemical screening of (individualized) cells [ 14 ].
2.4.1 Nucleic Acid Amplification
The most established method of nucleic acid amplification is Polymerase Chain
Reaction (PCR) which uses enzymes to duplicate strands of DNA as the reaction
mix alternates between high ( 95 C) and low temperatures ( 65 C) in the
prescience of excess nucleotide bases [ 15 ]. This thermocycling requires a method
of heating and cooling designated locations of a rotating disc; previously described
methods for achieving this have ranged from including battery packs on the disc,
slip rings to transfer power to resistors and infrared radiation [ 15 ]. Each of these
methods has allowed amplification of a targeted DNA sequence which has great
relevance in the field of POC molecular diagnostics. PCR can be used to identify the
presence of pathogenic microorganisms by including primers specific to the micro-
organism’s DNA, if the DNA is present the amplification will occur whereas no
amplification occurs in the absence of the pathogens DNA.
2.4.2 Immunoassays
Automation of immunoassays on disc is an area of increasing interest in
microfluidics as they are one of the most widely used biological tests in patient
diagnostics, food safety and environmental monitoring. The most frequently used
120 B. Henderson et al.