3D Printing Technologies - Mitchell Culler
In recent years, 3D printing has become much more accessible and affordable as a way to generate complex prototypes very rapidly compared to conventional manufacturing methods. As 3D printing becomes more “mainstream,” both as a concept and a practice, the technologies available for usage become myriad. While 3D extrusion printing has gained a significant following with consumers, there are many other types of additive manufacturing processes that are available and can be tailored to a wide range of applications.
Fused Deposition (Extrusion) Printing
This is the most common and most mainstream 3D printing method. Consumer-level 3D extrusion printers are available very inexpensively, with many options in the range of a few hundred dollars. This method allows for myriad materials to be used. Theoretically, any material that be melted and cooled to a solid can be used with extrusion 3D printing. One drawback of this method as it applies to microfluidics is that the resolution of these extrusion printers is typically not low enough to be effective for microfluidics channels, although recent improvements have placed resolution down to the micrometer scale . Additionally, during printing in this method, it is very difficult to get the layers to line up perfectly (as shown in Figure 1), making it difficult to use this method to contain fluids. In order to compensate for this drawback, there are several options available for finishing 3D models made using this technique such as exposing the model to a higher temperature so as to re-melt the layers together for a better fit, or using an appropriate chemical gas to partially dissolve the exposed outer layers and smooth them together.
An example technique which has potential to make this method more specific to microfluidic structures is described by Ladd et al. . This research is particularly interesting because this printing occurs at room temperature. A gallium and indium ally with an oxide over layer was able to be extruded through a needle to achieve a resolution of approximately 8 mm. The researchers were able to create wires, arrays of spheres, arches and interconnects.
Selective Laser Melting/Sintering
In contrast to extrusion printing where the printed material is extruded from a moving tip, this method involves using a bed of a powdered material and fusing together a layer of that material in a specific shape using a laser. After adding another layer of the powder, it can then be fused on top of the previous layer, almost like a reverse CNC milling process in that an object is constructed by fusing powder together instead of shaving material off a starting block. Although this allows for a very high resolution, one drawback to this approach is that the powder must be removed after the manufacturing process. For the scale of the small capillaries that microfluidics research necessitates, this makes this method somewhat unsuited to microfluidics channels . Using CO2 laser machining of polymethyl methacrylate (PMMA) Romoli et al. were able to adopt the technique to create 3D structures by printing several PMMA sheets and then bonding them together .
Due to the extremely low cost and availability of ink jet printers, this technique promises to be extremely useful for developing low cost microfluidics solutions. When used in two dimensions, this method typically involves printing hydrophobic boundaries which might be made from wax or polymer onto hydrophilic paper. A recent publication  described a technique wherein this method can be used for 3D printing using two different polymer inks. The first is made from SU-8, which forms the channels, while PMMA supports the structure during the curing process but is subsequently removed by washing. A relatively high vertical resolution of 4.6 um has been reported using this method . The manufacturing steps involve printing an isolation layer. On top is printed a support material using PMMA. The microfluidics channel is then printed on top of the support material, and finally the support material is washed away.
 A. A. Yazdi, A. Popma, W. Wong, T. Nguyen, Y. Pan, and J. Xu, “3D printing: an emerging tool for novel microfluidics and lab-on-a-chip applications,” Microfluid. Nanofluidics, vol. 20, no. 3, p. 50, Mar. 2016.
 C. Ladd, J.-H. So, J. Muth, and M. D. Dickey, “3D Printing of Free Standing Liquid Metal Microstructures,” Adv. Mater., vol. 25, no. 36, pp. 5081–5085, Sep. 2013.
 P. H. King, “Towards rapid 3D direct manufacture of biomechanical microstructures,” 2009.
 L. Romoli, G. Tantussi, and G. Dini, “Experimental approach to the laser machining of PMMA substrates for the fabrication of microfluidic devices,” Opt. Lasers Eng., vol. 49, no. 3, pp. 419–427, Mar. 2011.
 W. Su, B. S. Cook, Y. Fang, and M. M. Tentzeris, “Fully inkjet-printed microfluidics: a solution to low-cost rapid three-dimensional microfluidics fabrication with numerous electrical and sensing applications,” Sci. Rep., vol. 6, no. 1, p. 35111, Dec. 2016.