realize high aspect ratio micro-channels, thin wires of copper etched to a diameter
of 50–80μm were inserted between opposing holes in a mold box with a special
tweezers and provided with finite tension using a special jig developed for this
purpose. PDMS commercially available in a 10:1 ratio of a silicone rubber to curing
agent was poured into the plastic box over the wires and replicated the wire array.
This was followed by a release mechanism which comprised of a swelling step
wherein the matrix with embedded wire array was taken out curing the PDMS and
dipped in toluene for a finite amount of time. This swelling of PDMS matrix was
performed mainly to release the grip over the embedded wires. This was followed
by another shrinking step wherein the assembly was heat cured and the whole
PDMS shrinks back to a smaller size. Figure2.5shows the process flow chart of this
technique (Detailed in Fig.2.7).
There has also been utilization of this wire based replication technique for the
development of embedded fluid handling structures like microvalves etc. [ 28 ].
Figure2.8shows an embedded assembly of micro-valving as reported by Singh
et al. In Fig.2.8there are two embedded channel in a piece of PDMS where the
central channel shows a 80μm channel and the solenoidal track around this central
channel shows an embedded valve structure which can be inflated/deflated using
compressed air so that it can squeeze the inner channel and act as a fluid valving
device.
Fig. 2.7 Dimensional micro-channel arrays within PDMS blocks using thin circular copper wires.
(Reproduced from Singh et al. [ 27 ] with permission from the Institute of Electrical and Electronics
Engineers)
2 Microfluidics Overview 43