**5. Comparison between different 3D micro-fabrication technologies**

**2. Micro-injection molding**

4 Lab-on-a-Chip Fabrication and Application

**3. Micro-stereolithography**

**4. Graytone lithography**

Micro-injection molding is currently the technology most widely used for mass-fabrication of three-dimensional (3D) hollow micro-structures [3,4,11–13]. In micro-injection molding, thermoplastic granules are first melted in the plastifying unit of a micro-injection machine. Then, the molten plastic is injected at high pressure into the hollow space of an injection molding tool. After a cooling process, the injection tool is disassembled and the molded object removed. Although micro-injection molding is an established technology that supports mass production of hollow objects, the process is not without limitations. To date, the sizes of hollow objects made in conventional micro-injection molding technology are typically in millimeter range, not in micron range. Injection molding of an object at millimeter resolution is not possible without a special injection machine and auxiliary equipment. The mold to be used for formation of object at millimeter resolution has to be equipped with inlets and outlets in order to allow high-speed injection, gas evacuation, and the expulsion. More importantly, the process of micro-injection molding involves many energy intensive steps which are eco-unfriendly.

Another most widely used 3D micro-fabrication technology is micro-stereolithography [1,2,8]. It works by scanning an UV laser on a liquid monomer, curing the monomer into solid polymeric slices layer by layer, and stacking together all these polymeric slices with various contours. This UV-induced photo-polymerization repeats in a layer-by-layer fashion until the desired 3D object is fully formed. This technology has made it possible to fabricate any form of 3D micro-structures. The surface profile of a fabricated micro-structure can be as compli‐ cated as a human face. However, micro-stereolithography is a time-intensive process. The typical scanning speed of a micro-stereolithography machine is about 200–300 layers per hour [8], depending on the geometry and the resolution of the 2D slice to be formed on each layer. Fabricating a simple 3D object of 1 mm in height can take more than 30 minutes. Fabricating a small array of micro-needles can take anywhere between 50 minutes and several hours. Micro-stereolithography technology is currently being pushed developed aggressively focusing on for improvements in both resolutions, speed and flexibility in choice of photocurable materials. However, due to the use of a laser and a scanner system, the initial invest‐

ment costs of a micro-stereolithography-based process are unavoidably high.

Graytone lithography [9,10] is another inexpensive 3D micro-fabrication technology. This technology has been evolved to one step mask-less fabrication using SU-89. For the applica‐ tions not requiring or not suited with no access to using an expensive micro-stereolithography machine, gray tone lithography is an interesting alternative to micro-stereolithography. Graytone lithography is a modification of conventional 2D optical lithography. It works by exposing a positive photoresist to a UV light through a grayscale mask which defines the

To the authors' knowledge, to date, there has not been any cost-effective approach dedicated to mass production of hollow micro-structures at micron or sub-micron resolution. In this paper, the proposed fabrication process presented is a fundamentally different approach based on an improved version of another process published in references [5,6]. The proposed process can be carried out photo-lithographically with conventional photo-lithographic equipment. It provides a convenient alternative for researchers without no access to a micro-stereolithogra‐ phy machine or other expensive fabrication facilities. It is not intended as a replacement of other already established and accessible 3D micro-fabrication technologies. Instead, we believe that the proposed process should be used in conjunction with other 3D micro-fabrication technologies to optimizing maximize the advantages in speed, precision, repeatability, and costs of manufacture. Unlike micro-stereolithography, which is only suitable for fabricating small objects in the micron range, the proposed technology can be used to fabricate larger objects with dimension in excess of 1 mm with no sacrifice of speed. The fabricated structures or micro-structures are optically smooth. **Table 1** summarizes the advantages and disadvan‐ tages in comparison with other competing technologies.



**Table 1.** Comparison between different 3D micro-fabrication technologies.
