Polymers have assumed the leading role as substrate materials for microfluidic devices in recent years. They offer a broad range of material parameters as well as material and surface chemical properties which enable microscopic design features that cannot be realised by any other class of materials. A similar range of fabrication technologies exist to generate microfluidic devices from these materials. This review will introduce the currently relevant microfabrication technologies such as replication methods like hot embossing, injection molding, microthermoforming and casting as well as photodefining methods like lithography and laser ablation for microfluidic systems and discuss academic and industrial considerations for their use. A section on back-end processing completes the overview.

Polymer microfabrication technologies for microfluidic systems

SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

https://iopscience.iop.org/article/10.1088/0960-1317/17/6/R01/pdf

SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography

Microfluidics is an emerging field that has given rise to a large number of scientific and technological developments over the last few years. This review reports on the use of various materials, such as silicon, glass and polymers, and their related technologies for the manufacturing of simple microchannels and complex systems. It also presents the main application fields concerned with the different technologies and the most significant results reported by academic and industrial teams. Finally, it demonstrates the advantage of developing approaches for associating polymer technologies for manufacturing of fluidic elements with integration of active or sensitive elements, particularly silicon devices.

https://iopscience.iop.org/article/10.1088/0960-1317/17/5/R01/pdf

Lab-on-chip technologies: making a microfluidic network and coupling it into a complete microsystem : a review

Since its introduction in the nineties, the negative resist SU-8 has been increasingly used in micro- and nanotechnologies. SU-8 has made the fabrication of high-aspect ratio structures accessible to labs with no high-end facilities such as X-ray lithography systems or deep reactive ion etching systems. These low-cost techniques have been applied not only in the fabrication of metallic parts or molds, but also in numerous other micromachining processes. Its ease of use has made SU-8 to be used in many applications, even when high-aspect ratios are not required. Beyond these pattern transfer applications, SU-8 has been used directly as a structural material for microelectromechanical systems and microfluidics due to its properties such as its excellent chemical resistance or the low Young modulus. In contrast to conventional resists, which are used temporally, SU-8 has been used as a permanent building material to fabricate microcomponents such as cantilevers, membranes, and microchannels. SU-8-based techniques have led to new low-temperature processes suitable for the fabrication of a wide range of objects, from the single component to the complete lab-on-chip. First, this article aims to review the different techniques and provides guidelines to the use of SU-8 as a structural material. Second, practical examples from our respective labs are presented.

SU-8 as a structural material for labs-on-chips and microelectromechanical systems.

The fabrication of multi-level SU-8 microstructures using multiple coating and exposure steps and a single developing step has been achieved for up to six layers of SU-8. Alternating layers of SU-8 2010 (thin) and SU-8 2100 (thick) photoresist films were spin coated, followed by soft-bake, ultraviolet (UV) exposure and post-exposure bake steps. The multiple SU-8 layers were simultaneously developed to create patterned microstructures with overall thicknesses of up to 500 µm and minimum lateral feature size of 10 µm. The use of a single developing step facilitated fabrication of complex multi-level SU-8 microstructures that might be difficult, or even impossible, to achieve by sequential processing of multiple SU-8 layers that are individually coated, baked, exposed and developed.

https://iopscience.iop.org/article/10.1088/0960-1317/16/2/012/pdf

Fabrication of multi-layer SU-8 microstructures

This paper proposes a novel method to fabricate multi-layers, embedded micro fluidic structures by simply employing dosage-controlled UV exposure on thick SU-8 resist and anti-reflection coating on the bottom surface to prevent the reflection UV-light from inducing exposure. Experimental results show the top wall thickness of the embedded micro-channels can be well controlled in a resolution of 2 μm for the UV dosage from 120 to 190 mJ/cm2. Stacked micro-channels have also been successfully realized and showed no interference on the bottom structures when the top structures are being exposed. Numerical simulation of the top wall thickness by UV exposure dosage control has also been conducted, and the comparison between the calculated and experimental results showed similarity in trend. This simple and inexpensive method can be applied to fabricate multi-layers of complex fluidic systems for the applications of μTAS (MicroTotal Analysis System), inkjet printhead, capillary electrophoresis, and micro PCR (Polymerase Chain Reaction).

Jen-Hau Cheng

Papers

A novel fabrication method of embedded micro-channels by using SU-8 thick-film photoresists

Self-driven flow loop in microscale