Materials and Fabrication Techniques for Nano- and Microfluidic Devices

KIN FONG LEI

1.1 Introduction

Microfluidic technology has enabled the realisation of a vast range of miniaturised analytical devices. Microfluidic devices are commonly associated with lab-on-chip (LOC) systems or micrototal-analysis system ((μTAS), when scaled-down operations are performed on miniaturised versions of conventional laboratory bench top instruments. One of the main objectives of microfluidic technologies is to provide a total solution, from sample input to display of the analysed results. Complete analytical protocols, from sample pretreatment through to sample/reagent manipulation, separation, reaction, and detection, can be performed automatically on well-designed and integrated miniaturised devices.

Historically, developmental advances of microfluidic devices originated from the microelectronics manufacturing sector. Silicon has been used as the base substrate material for fabricating microfluidic devices for various applications. Well-established silicon processing and extensive studies of silicon properties have contributed to the rapid evolution of microfluidic technologies. The fabrication process for silicon-based microfluidic devices involve substrate cleaning, photolithography, metal deposition, and wet/dry etching. However, silicon substrate is relatively expensive and optically opaque to certain electromagnetic wavelengths, limiting its applications in optical detection. To combat these shortcomings, glass and polymeric materials have been used to fabricate microfluidic devices. Compared with silicon, glass and polymer materials are inexpensive and optically transparent. Polymer materials include polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), and polydimethylsiloxane (PDMS). Amongst these polymer materials, PDMS has been one of the most widely used materials for fabricating microfluidic devices in recent years due to its flexibility in moulding and stamping, optical transparency, and biocompati-bility. Recently, paper has been proposed to be an alternative material used as a substrate of microfluidic devices. Paper is inexpensive, lightweight, available in a wide range of thickness, and is disposable. Aqueous solutions can be transported by wicking, thus realising passive pumping. In addition, well-defined pore sizes in paper can be manufactured and suspended solids within samples can be separated based on size exclusion before an assay is performed. Paper is biocompatible with various biological samples and can thus be modified with a wide range of functional groups to enable covalent bonding of proteins, DNA, or small molecules creating bespoke biochemical sensing systems.

In this chapter, materials used in the fabrication of microfluidic devices are grouped for discussing microfabrication techniques and applications. Moreover, the ability of system integration, cost of processing, and suitability for specific applications will be highlighted. An up-to-date and systematic approach for fabricating nano- and microfluidic devices will be presented.

1.2 Traditional Silicon-Based Microfluidic Devices

From the beginning of the 20th century, continual rapid development of microelectronic technologies made computing processors fast and inexpensive. In 1965, Gordon Moore observed that the number of transistors per unit area would double every two years. This extraordinary growth rate led to the realisation of current personal computers that run on the computing power of millions of transistors within a centimetre scale environment. In the 1980s, microelectromechanical systems (MEMS) were inspired by microelectronic technologies and were developed from microelectronic fabrication processes to build machines on the order of micrometres. The majority of MEMS devices are made from single-crystal silicon wafers and their fabrication processes include deposition of poly-crystalline silicon for resistive elements, metal deposition for conductors, silicon oxide for insulation and as a sacrificial layer, and silicon nitride and titanium nitride for electrical insulation and passivation. Sensors, actuators, and control functions can also be cofabricated on standard silicon wafers. There has since been remarkable progress in research in MEMS technologies, under strong capital promotions from both national governments and industry.

Microfluidic technology is one of the branches of MEMS that handles fluids within submillimetre environments, i.e. typically microlitres, nano-litres, or even picolitres. Fluids are manipulated, mixed, or separated on a compact platform for various biomedical, biochemical and chemical analytical applications. One of the objectives of the development of microfluidic devices is to provide a total solution (i.e. sample-to-answer) in low-cost and rapid systems. For instance, point-of-care (POC) diagnostic applications can be realised based on the advantages of miniaturisation, integration, and automation of the microfluidic system. Microfluidic devices can, and are often modelled as miniaturised versions of conventional laboratory devices, with early developments of microfluidic technologies being based predominantly on silicon as the substrate of choice for many microfluidic devices.

1.2.1 Microfabrication with Silicon

Silicon microfabrication is the process for the production of devices on silicon wafers in the submicrometre to millimetre range. Normally, structures in microfluidic devices have relatively high aspect ratios compared to those in microelectronic devices, which are fabricated to within the top few micrometres of the substrate material. Microfluidic devices may require the whole substrate thickness, utilise both sides of the substrate, or require bonding multiple substrates together. Besides the conventional microelectronic fabrication techniques, such as photolithography, thin-film deposition, and etching, some newer processes were introduced to fulfill the fabrication requirement of microfluidic devices. Since there is a plethora of silicon microfabrication techniques, only the important processes in fabricating microfluidic devices are discussed. For a more comprehensive over-view of further techniques refer to ref....