Microfluidic devices sacrificial materials

S. Metz, S. Jiquet, A. Bertsch and P. Renaud, Polyimide and SU-8 microfluidic devices manufactured by heat-depolymerizable sacrificial material technique, Lab Chip, 4 (2004) 114-120. 

In the past 15 years, the use of microfluidic devices for chemical analysis has increased tremendously. Indeed, a broad range of chromatographic and electrophoretic separation methods have been implemented in microchips. However, for widespread utilization of microfabricated devices in analysis applications, particularly in the field of proteomics, further efforts are needed to develop simple fabrication techniques that achieve functional integration of multiple tasks in a single device. " In this section, we describe the fabrication of microdevices using sacrificial materials and discuss some of the advantages of this approach over conventional microfabrication methods. 

Sacrificial Layer Fabricated Microfluidic Devices 51.2.2.1 Silica and Glass Materials... 

In summary, sacrificial fabrication of microfluidic devices in glass and silicon overcomes some limitations of conventional manufacturing methods. Important benefits include the ability to fabricate nanometer- to several micrometer-dimension structures and the elimination of a bonding step. Nevertheless, several problems with sacrificial fabrication have limited the implementation of this technique in making microfluidic devices. First, the fabrication protocols are not generally easy to implement and require sophisticated instrumentation. Second, device materials are limited to glass and silicon, the substrates most compatible with the manufacturing methods. Finally, fabrication yield and reproducibility still need improvement. Thus, further work is essential to develop sacrificial fabrication techniques. 

Since then, various methods have been adopted for fabrication of photoresist-based microfluidic devices. The first method shown in Figure 20.9a begins with a spin coating of photoresist onto a substrate and patterning with a photomask." Once the open microchannels are created, a sacrificial material is filled into the space of the microchannel. Subsequently, a second layer of photoresist is spin coated and patterned on top to define the access holes for inlet and outlet. Finally, the sacrificial layer is dissolved to create the closed microchannels. The major disadvantage in this process is the slow dissolution, therefore only short microchannels are applicable. 

Due to various problems, no such depolymerization-based resist has found widespread use in optical lithography. However, depolymerization schemes have been exploited to some degree in e-beam resists as will be discussed later. Such materials have also been utilized as the basis for photodefinable sacrificial materials for microfluidic and MEMS device fabrication,... 

This technique has several limitations. The two polymo- materials must be well matched in terms of chemical compatibihty, adhesion, and processing charactoistics. Heckele and Durand used an upper layer of polyoxymelhylene with a lower layer of cellulose acetate. It is not clear whether suitable material pairs exist that include the preferred materials for microfluidic devices such as polymethylmethacrylate (PMMA), polycarbonate, and Qrclic olefin copolymer. This technique also requires careful control of the penrtration dqith into the sacrificial lower layer. Furthermore, it is not possible to form channels on both sides of the part. 

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