**1.2. Microfluidics as a technology**

**1. Science, technology and advantages of microfluidics**

components: a science, a technology and an applications component.

minted (typically around 20 mm or somewhat more).

**1.1. Microfluidics as a science**

2 Microfluidics and Nanofluidics

As is the case in many fields of scientific research, the field of microfluidics has three main

For microfluidics, a common thread between all of these components is that they are microsized, so size will be briefly discussed first. The dimensions shown in **Figure 1** are approximate because size of naturally-occurring objects (and of some manufactured-things) varies, for example the diameter of a human hair is between 50 and 100 μm; the diameter of the tip of a rollerball pen is between fine, medium and bold (e.g., between 0.5 and 0.7 mm); and of a 1 cent coin with its diameter varying slightly depending on the jurisdiction the penny was

**Microfluidics** has been defined [1–17] as the study of the behavior of fluids (or whatever is in them, e.g., colloids, discrete nanoparticles or individual cells), **in micro or in sub-millimeter channels** or around microstructures. Although microchannels can be relatively long (e.g., several 10's of mm), they are still called microchannels as long as one **critical dimension** (e.g., channel-width or channel-depth or tube radius) is in the micro scale. Microfluidic channels

• **The science of scaling as applied to microfluidics:** a number of physical properties of fluids change as size gets smaller [1–47], to quote *"smaller brings new capability"* [31]. These changes are often non-linear and have been discussed in books [1–17] and in journal papers [18–29]. A non-exhaustive list of size-dependent phenomena and effects is outlined below.

**Figure 1.** Examples of an approximate scale of things. The boundaries between micro and nanofluidics and between micro and millifluidics are fuzzy. In many cases, the strict definition adopted by the National Science Foundation (NSF) of the US for nano as anything with one critical dimension ≤100 nm is not strictly adhered to, thus there is a gap between 100 nm and 1 μm. Similar arguments apply to the NSF definition for micro (defined as one with a critical dimension between 1 and 100 μm). In many cases, the micro-scale is arbitrarily widened to ~1 mm and sometimes slightly more. The term millifluidics has recently been used for channels (or structures) with one critical dimension of a few mm.

can be used for example to confine or to guide or to mix or to manipulate fluids.

**Microfluidics** refers to a variety of approaches that enable exploitation of the phenomena mentioned above by fabricating microfluidic channels on a variety of substrates. For instance, on crystalline Silicon (of c-Si) wafers, on amorphous glass or on polymeric substrates. Due to the advantages of confining flow in microfluidic channels, several fabrication technologies have been developed and tested and will be briefly reviewed. These technologies are often collectively called micro Total Analysis Systems (μTAS) or Lab-on-a-Chip (LoC) or Micro Electro Mechanical Systems (MEMS). Microfluidics or whatever acronym is used to describe it, has attracted significant attention in books [1–17] and in journals [18–29]. While in the topic of publications, older references have been purposely included in this chapter followed by some recent publications. Where possible, the citations in the reference list have been grouped either according to fabrication technology or according to the type of substrate used (e.g., c-Si, amorphous, polymeric) or according to application. Within each technology, the reference list has been sorted out chronologically to help interested readers follow the origin and evolution of ideas and technologies. Despite of the relatively large number of references included, this is not a comprehensive review. The reference list simply offers starting points. Getting back to the main theme, the question still remains: why does microfluidics continue to receive increased attention? What are the advantages of using microfluidics, especially for chemical analysis applications?

For completeness, other methods of microchannel fabrication on inorganic substrates (either crystalline or amorphous) have been described. A short list includes **laser machining** [55– 58]; lithographie galvanoformung adformung (LIGA) or lithography electroplating molding [59–61] which is well suited for fabrication of high aspect ratio channels; deep reactive ion etching (DRIE) [62–65] often used for fabrication of microchannels with a high aspect ratio; and, **SU-8** (an epoxy-based negative photoresist) and its variants such as SU-8 series 2000 and

Microfluidics and Nanofluidics: Science, Fabrication Technology (From Cleanrooms to 3D...

http://dx.doi.org/10.5772/intechopen.74426

5

Technologies involving polymeric substrates [69–83] include replication via **imprinting** [69–73] or **embossing** [74–76]. Polymeric substrates are selected due to their bio-compatibility or to reduce cost of ownership. Examples will be shown later. The terms disposable or recyclable microfluidic devices is often used for microfluidic channels on polymeric substrates. **Soft lithography** [77–83] (defined as a collection of fabrication techniques for replication of microchannels) is a technology that does not require access to a clean room. It is called soft because it uses soft and flexible (primarily) elastomeric materials such as poly di methyl siloxane

There are other techniques that are rather difficult to classify either according to fabrication technology or according substrate. Despite of being brief, the list includes **droplet microfluidics** [84–89], in which discrete droplets or small volumes of immiscible liquids are guided through microchannels. In the early literature, this approach was often called digital microfluidics. As it is known now, **digital microfluidics** [90–95] is an outgrowth of electrowetting [90, 92] and it involves use of discrete droplets on arrays of electrodes, with individual droplets manipulated by electrical means. The list also includes **centrifugal microfluidics** [96–101], a technique that enables micro-flow manipulation by using rotational forces (e.g., Coriolis) obtained by spinning a CD on top of which there are microfluidic channels. This technique is often called *"lab on a CD"*. It also includes **paper microfluidics** [102–108], a technique that uses paper for development of microfluidic approaches intended for use in resource limited situations (e.g., remote geographical areas

**Rapid prototyping via 3D-printing** [109–122] involves both a technology (e.g., a 3D printer) and a materials platform (e.g., a polymer) for formation (primarily) of mill-sized fluidic (and recently) micro-sized channels [115, 117, 120]. An example of 3D printing will be discussed

To highlight substrate-dependence of fabrication, the fabrication steps required for microchannels on c-Si and on amorphous glass or quartz substrates are compared and contrasted in **Figure 2**. It should be noted that depending on crystallographic orientation of the substrate and of the chemical cocktail used in the etching solution, isotropic or anisotropic etching may

**Example 1:** *Planar 2D-chips* **and wet chemical etching for fabrication of microchannels on** 

SU-8 Series 3000) [66–68].

or resource-limited locations).

**2.2. Fabrication technology examples**

**crystalline and amorphous substrates** (**Figure 2**).

later in this chapter.

be obtained [48–51].

(PDMS) and often cyclic olefin copolymer (COC).
