Photolithography is well established in the fabrication of microfluidic networks; however,
it is difficult to fabricate designs which require multiple depths. Furthermore, the cost/time
to produce photolithographic masks is problematic, particularly when prototyping. Here
we describe fabrication of microfluidic branching networks with multi-depth structures,
ranging from 10s to 100s of microns, using a femtosecond fiber laser with 10 W average
power, followed by chemical etching in a 10:1 solution of 49% HF and 69% HNO3. While
this technique was originally developed using a nanosecond laser, this unique femtosecond
laser enables greater processing precision and faster overall processing speed.
We describe maskless rapid prototyping of a micro-fluidic branching network on a silicon wafer with laser direct writing
(LDW). The branching micro-channel network is designed as a blood oxygenator following Murray's law and satisfying
the necessity of equal path lengths. In development of such micro-fluidic structures, this maskless process will reduce
time and cost compared with the conventional photolithography based technique. The flexibility of laser direct writing
facilitates creating a multi-depth structure of the branching network, ranging from a few microns to a few hundred
microns in depth. In order to create such a wide range of feature sizes, a nanosecond pulsed Nd-YAG laser and a
femtosecond pulsed fiber laser are used together. The femtosecond fiber laser is used to create micro-channels with a
depth of less than 50μm. As post-processing, a chemical etching in a solution of HF and HNO3 is applied to smooth the
laser ablated surface. To realize an optimized design of micro-fluidic structures, influences of operating parameters, such
as the pulse energy, the focal position, the transverse speed, and the number of passes, on the depth of micro-channels
and their surface quality are investigated. Using the laser machined silicon structures as a mold, a
Poly(dimethylsiloxane) (PDMS) replica is created.
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