1. photopatterned SU-8 layers
Low-Cost Injection Molding of Microfluidic Chips using Molding Tools made from Elastomers
ENTWURF 12/04
Norbert Gottschlich 1), Thilo Brenner 2), Günther Knebel 1), Roland Zengerle 2), Jens Ducrée 3)
Greiner Bio-One GmbH, Maybachstr. 2, D Frickenhausen, Germany;
IMTEK – University of Freiburg, Laboratory for MEMS Applications, Georges-Koehler-Allee 103, Freiburg, Germany,
Overview
We present a method of fabricating microstructured masters from silicone, which are then used as molding tools for injection molding and rapid prototyping of plastic microfluidic chips.
Introduction
The trend to use polymeric chips for lab-on-a-chip applications is strongly driven by cost-efficient mass production techniques such as injection molding. However, the lead-time and costs associated with the fabrication of conventional metal masters limit the benefits of this technique for rapid prototyping and small volume production. To overcome this obstacle, we developed a low-cost technique for rapid fabrication of very precisely microstructured masters based on elastomers.
Methods
The masters for replication are generated from elastomers which are cast over photopatterned photoresist (SU-8) structures on silicon wafers. Exposure of sequentially spun resist layers enables the patterning of channels with varying depths. Figure 2 illustrates the fabrication steps comprising SU-8 lithography, casting of silicone rubber and replication by injection moulding.
SU-8 Lithography
Casting of silicone rubber
Replication by Injection Moulding
Results
We realized microfluidic structures with minimum lateral dimensions of 150 µm and depths from 30 µm to 150 µm. The channel structures were replicated in polystyrene. Excellent dimensional conformance was achieved between the elastomeric master and the replicated polymer. In addition, silicone-based molding tools showed excellent demolding properties due to their flexibility.
In addition, silicone-based molding tools showed excellent demolding properties. Even channel geometries featuring slightly negative release angles can be deformed properly due to the flexibility of the silicone elastomer.
The master was used for more than 100 injection cycles. Minor damage of the embossing tool occurred at the edges of the 130 µm high features (figure 4).
The sidewall roughness of the silicone tool and the resulting microchannel shown in figure 3 and 4, respectively, is a result of the photolithographic process. The use of a printer for the mask generation results in a step function rather than a smooth curve.
The SU-8 multilayer technique even enabled the combination of different depths in one single design (figure 5).
Conclusion
Our technique allows the rapid structuring of precisely microstructured elastomeric masters for injection molding within less than two days. Regarding their surface properties which are critical for many microfluidic applications, these replicated structures are “as close as you can get” to mass fabrication quality from metal tools. Furthermore, this approach eases the costs of tooling and polymer replication for rapid prototyping and short-run injection molding.
Figure 4:
SEM Image of a silicone molding tool with 130 µm high features after 100 molding cycles.
Figure 5:
Silicone molding tool with different aspect ratios. The features are 53 µm and 163 µm high, respectively.
1. mask
2. mask Si
SU-8 resist
UV-irr.
Figure 3:
Detail of the replicated structure shown in Figure 1. The channel is 130 µm deep and 155 µm wide. Si
silicone rubber
photopatterned SU-8 layers
(2 depths)
metal frame
Figure 1:
Test Slide fabricated by injection molding using a silicone molding tool. The channels are filled with black ink to enhance contrast. The insert shows the cross sectional view of the channels.
Figure 2:
Schematics of the fabrication process.
LabAutomation 2005; San Jose, CA (USA)
Poster # TP058 (February 1, 2005)