Online proceedings - EDA Publishing Association

one of the bonding methods is best adequate for different
ranges of channel dimensions.
Laminating a DFR layer on top of the channel layer was
the most comfortable way of closing since it is the simplest.
As shown in Fig. 4 a), channels were tightly sealed by this
method at an optimum lamination temperature of 75 °C [6].
However, for channels exceeding widths of 250 µm, the lid
DFR layer sagged into the broad chambers and stuck to the
bottom (Fig. 4b)). Therefore, this bonding method is limited
to smaller channel structures. Though, for channels smaller
than 20 µm, unbonded spots occurred at the channel edges.
With respect to biomedical applications, it is desirable for
the biological fluid to be in contact with as few materials as
possible in order to avoid interaction of biological
substances with other materials. On this score, combination
of DFR as the channel layer with lamination of DFR as a lid
forms the simplest way of fabricating a complete
microfluidic system in which the biological fluid is in
contact only with DFR and no other material. Still, as
described above, this proceeding is best applicable for
moderate channel dimensions between 20 µm and 250 µm.
Based on the second bonding approach, the application
range was extended to smaller and larger channel
dimensions. As DFR was laminated onto a PMMA lid plate
at first, sagging of DFR into broad channels or chambers
was obviated entirely. This bonding technique did not
involve any constraints for dimensions of the channels to be
covered. However, if the whole system is supposed to
consist of DFR, channel widths are restrained to aspect
ratios lower than 2:1. Alternatively, smaller channels
fabricated from SU-8 can also be sealed by DFR resulting in
an equally stable bond. Although two materials (SU-8 and
DFR) will be in direct contact with the biological
substances, these materials are chemically very similar.
Yet, when SU-8 is chosen as the channel layer, SU-8 can
also be employed as adhesive bonding layer using the third
bonding approach. This method also revealed stable and
homogenous bonds for any channel dimensions. Fig. 5
depicts a microfluidic system covered by this technique.
Obviously, the channels are open and the bond is
homogenous without any air entrapments or other defects.
Rhodamine was used as a test liquid for checking the leaktightness
of the systems. No bonding defects could be
observed as the liquid filled the channels completely but did
not flow between the resist layers. Admittedly, this process
turned out more sensitive against process parameter
deviations than DFR bonding. For example, marginally
11-13
May 2011, Aix-en-Provence, France
exceeding the optimal temperature for bonding (69 °C)
about 1-2 °C already led to flowing of resist into the
channels, which resulted in clogging. At slightly lower
temperatures, bond defects were found occasionally,
especially in regions of small bonding areas. Totaling, this
fabrication technique is suitable for any channel designs
when process parameters are set accurately.
Generally, this bonding method can be accomplished
with any adhesive material. In comparison with the
technique of Ref. [5], for which only dry film is applicable,
the new fabrication process turns out more flexible with
regard to material choice and combinations. For this reason,
it is adaptive to a broader range of applications.
IV. CONCLUSION
All things considered, the presented manufacturing
options base upon a combination of the polymer materials
PMMA, SU-8 and DFR. The fabrication methods stand out
due to inexpensive materials and manufacturing techniques
compared to conventional silicon and glass assemblies.
Besides the low costs, an eminent benefit is also given by
the transparency of the materials as observability of
processes is a crucial requirement for many biomedical
applications.
Although polymers have poor temperature stability, this
fact is not of disadvantage in the biological field where low
temperature processes are needed to prevent denaturing or
alike damages of biological substances. In comparison with
PDMS systems, the presented techniques are suitable for a
wider range of applications as they allow the fabrication of
smaller channels with higher aspect ratios.
The bonding techniques could also be adapted for CMOScompatible
encapsulation of micromechanical devices such
as switches. For these applications, high temperature
bonding techniques like anodic bonding are often
inappropriate as they can cause thermal distortion
discharging of structural elements.
Summing up we highlighted complete fabrication
techniques for microfluidic systems suitable for a large
variety of biomedical applications. A great advantage is
given by implementing the fluidic interfaces simply by
CNC-assisted mechanical drilling. Very high aspect ratios
(>10:1) were achieved and three different adhesion bonding
techniques for closing of the channels were applied and
compared. These bonding methods cover all dimension
ranges of channels or chambers to be sealed.
a) b)
Fig. 4. Channels on PMMA closed by lamination of DFR. a) Cross-section
of a 220 µm wide channel [6], b) Top view of a 1000 µm wide channel.
Fig. 5. SU-8-channel on PMMA closed by bonding to a SU-8-PMMA lid.
For testing the leak-tightness of channel, rhodamine was flown through.
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