Literature review on laser machining of transparent materials
2.2 Overview of various microchannel fabrication technologies
The fabrication technologies of the microchannels have a strong impact on the ability to implant the microchannels into a small-sized biomedical device. Consequently, the different technologies used for microchannel fabrication on a variety of materials and various applications have been studied by many researchers all over the world and the list is ceaseless. Microchannels are fabricated by both conventional and non-
conventional techniques like micro-milling, dry and wet etching, micro-wire molding, lithography, injection molding, electrospinning, imprinting, laser processing, laser- induced plasma-assisted ablation (LIPAA) etc. A graphical representation of the overview of the various technologies used for the fabrication of microchannels on a variety of materials and their applications is shown in figure 2.2.
Although there are many processes, certain processes are predominantly being used, mainly micro-wire molding, lithography, imprinting, direct laser micromachining and laser-induced plasma-assisted ablation (LIPAA) in the industry. These processes have been discussed in in the following sections.
Figure 2.2 Overview of microchannel fabrication technologies, materials and applications
2.2.1 Micro-wire molding
Electric heater
Metallic tube Micro-wire
Polymer
+ -
Figure 2.3Schematic of wire molding
Micro-wire molding is a simple and flexible technique for microchannel fabrication. It utilizes the concept of embedding a micro wire into a degassed polymer and later, removing the wire after the curing of the polymer. Depending on the strategy of the process, the key resources required in this technique are polymers (on which the microchannel is intended to be fabricated), micro wires, a mold of metallic tube and a heating source (Jia et al., 2008). Here, the mold of metallic tube is taken to be of internal diameter approximately 5 mm. The tube is attached with an electric heater to the bottom of it for heating and temperature control. The temperature is maintained depending on the melting point temperature of the polymer. Wires of varying diameter and shape are fixed to both the end of the mold. The degassed polymer mixture is poured into the molding cavity where the wire acts as the axis of the mold. The mold is then closed under hydraulic pressure and the excess degassed polymer material is removed. The mold is held closed until the polymer is cured. Later, on curing the polymer, the mold is opened and the wire is pulled of the polymer and hence the micro channel is obtained.
Figure 2.3 shows a schematic representation of the wire molding process.
Several literatures have reported the use of this wire molding technique for microchannel fabrication. Effati and Pourabbas (2018) explained a quick method for the design and fabrication of 3D microchannel by the technique of micro-wire molding.
Microchannels on PDMS was fabricated using Nylon and metallic wires of diameter ranging from 150-800 μm. The technique provides the possibility to fabricate microchannels with different cross-sections. The same technique of wire molding has also been utilised by Kumar et al. (2018) for fabricating microchannels in PDMS casting
using copper wire of 0.21mm diameter. Two different types of inlet sections (L and T types) have been constructed by the technique. Further, YueFei et al. (Jia et al., 2008) utilised the conventional molding capability of PDMS for microchannel fabrication by the wire molding process. It was possible to generate different types of topological structures of the microchannels due to the smooth surface, high-intensity, and high flexibility of the wires. Zou et al. (2015) fabricated a new economical microfluidic viscometer to measure the viscosity of biological fluids using the micro-wire molding technique, thereby making the fabrication process easier and cheaper.
2.2.2 Lithography based technology
Lithography is mainly a printing process that is based on the idea of the immiscibility of oil or grease with water. It is simple technique and is the most widely used fabrication technique for microchannels. The key steps involved in the process are coating, baking, exposing and development (Yao et al., 2005). In most of the cases, surface conditioning precedes the step of the surface coating. It prepares the substrate surface to accept the photoresist by providing a clean surface. The substrate is then placed on a vacuum chuck and the resist is applied by the method of spin coating. The speed of the chuck determines the thickness of the resist applied. The thickness of the spin coated layer is usually kept equal to the height of the microchannel. Two types of resists are available, namely positive resist and negative resist. In the positive resist, the portion exposed to the light becomes more soluble, while in the negative resist the portion exposed to the light gets hardened. It is then soft-baked to remove the excess solvents and later cooled to room temperature. The coated substrate is then covered with a mask that has the required pattern on it. Then, it is exposed to a wavelength (light) travelling through the mask, that the resist is designed for. In the region exposed, the resist undergoes a chemical reaction with the light. The substrate is then etched in a suitable solution and thus, the microchannel is finally formed. A simple description of the method is shown in figure 2.4.
Coat and Prebake Light
Exposed to light
Developed and Etched
Microchannel formed Substrate
(Polymer)
Photoresist
Mask
Figure 2.4 Stepwise lithographic process
Literature reports various applications of the lithography technique for microchannel fabrication. Yao et al. (2005) performed the lithography process based on an inconsistent combination of resists and standard lithography facilities. They fabricated microchannels using both one resist layer and multiple resist layers and also demonstrated fluid delivery through the channel. McUsic et al. (2012) also reported the use of lithography process for the fabrication of bio-inspired pattern microchannel scaffolds. The authors used patterned silicon master and PDMS negative mold casting.
The fabricated channels were used for directing the morphogenesis and differentiating the retinal cells derived from mice and human embryonic stem cells. Also, Laviano et al.
(2010) presented the same technique of lithography to produce an inclined array of microchannel. Further, it was demonstrated how it could be utilised to produce a precise vortex flow in a well-controlled environment. Wang et al. (2018) used two cycles of photolithographic and chemical wet etching for the fabrication of micro channel arrays and also capillary connecting channels on glass substrate. The channels after the completion of the fabrication process, were treated with plasma using a plasma cleaner for surface cleaning. The channels were then bonded with a cover plate by dropping ultrapure water at the edge of the substrates. Thus, channels on various materials and of different dimensions can be made utilizing the lithographic technique.
2.2.3 Embossing or imprinting technology
Embossing or imprinting is a technique of creating different structures like microchannel in polymeric matrices, which are complementary to a template in size and shape. It is a simple and cost-effective process that takes into consideration the versatility of the
materials to be imprinted (Chen et al., 2015). In this method, a template is first prepared on a silicon substrate mostly by the photolithography method. They are characterized using an optical microscope to determine the effect of the imprinting procedure. The template is then coated with a solution containing biomolecules. It is one of the simplest methods where the template is brought in contact with the plastic or polymer substrate.
The plastic or the polymer material is however cleaned and dried before bringing to contact with the silicon template. The whole arrangement of this polymer substrate and the silicon template is then sandwiched between two metallic blocks (Pimpin and Srituravanich, 2012). The imprinting time depends on the material to be imprinted and also on the temperature and pressure applied between the two metallic blocks. The process when carried out at room temperature, the pressure and time required are high, while performing the process at a high temperature requires less time and pressure. The metallic blocks are subjected to a hydraulic press, varying the pressure between 450 to 2700 psi at room temperature (Xu et al., 2000). On releasing the pressure, open microchannels are formed on the substrate. The difference in the dimension of the template before and after the imprinting process is generally less than 1%, which indicates the absence of detrimental effect on the fabricated microchannels. Figure 2.5 shows the schematic of the imprinting technique.
Figure 2.5 Schematic of the imprinting technique
Researchers carried out several experiments on fabricating microchannels by the technique of embossing or imprinting. Xu et al. (2000) adopted the room-temperature imprinting method for microchannel fabrication on a variety of plastic substrates. They
showed that this method improved the device yield from approximately ten devices to above 100 devices per template. Indeed, the number of devices produced by the single template showed a variation of less than even 2 %. Zhai et al. (2014) presented the fabrication thin glass/PDMS microfluidic coupled with a monolithic capillary column. In the fabrication process, a copper mold with a microchannel network imprinted on it was utilised. Lei and Tong (2005) also employed the technique of imprinting, however instead of utilizing a conventional mold with microchannel embedded on it as the template, authors used a stainless steel wire mold of outer diameter 105 μm. This makes the technique more flexible.