**1. Introduction**

The 3-dimensional (3D) printing has recently become one of the most promising and ground-breaking manufacturing techniques [1–3], allowing to produce highly detailed structures, following simple and systematic steps without the need of the very expensive equipment of traditional technologies that normally require the use of cleaning rooms in large facilities. The 3D printing has facilitated the access to complex processes of manufacturing to a lot of researchers and many and varied industries [4]. Among others, the microfluidics field is a clear beneficiary from the role that 3D printing plays in the microfabrication processes [5], where techniques such as reactive ion etching (RIE) [6] and photolithography [7, 8] that produce a significant polluting chemical waste are predominant.

In addition to its multiple applications in chemistry, engineering or sensing, microfluidics is of great interest in medicine and pharmacology, where one of the challenges is to manufacture complex devices capable of mimicking physiological structures [9–11], such as vessels, veins and arteries, where novel drugs can be tested

#### **Figure 1.**

*Popular 3D printing technologies: (a) fused deposition modelling (FDM) and (b) stereolithography (SLA).*

under static and flow conditions (dynamic regime). These studies, much closer to reality than the studies carried out by traditional methods, involving testing in wells (static regime), could decrease the animal experimentation needed for testing drugs before the patient dispensation. All these devices are made up of different kinds of microchannels, capable of guiding small amounts of liquid samples. To be able to fabricate these devices, new technologies are required to manufacture them in a repeatable and accurate way. The 3D technology emerges as a promising one, since, it allows to achieve in an easy and fast way, microchannels with very high resolutions with simple procedures; to select different geometries for the microchannel profile (circular, rectangular, triangular…) and to create channels on complex surfaces in 3D or even internally.

Currently, two 3D printing technologies outstand above the rest [12, 13]: fused deposition modelling (FDM) [14, 15] and stereolithography (SLA) [16, 17]. FDM printers are based on the extrusion of a heated polymeric filament fused, that forms consecutive layers of a piece (**Figure 1a**). SLA printers use photopolymerisation to selectively cure a liquid resin contained in a tank (**Figure 1b**), manufacturing the model in a precise layer by layer process.

Both technologies are widely used given their versatility and efficiency, but SLA offers the highest accuracies [18]. Given the high quality of the surfaces fabricated by SLA printers, a variety of biocompatible materials suitable for its use with this equipment have emerged, increasing the potential biological applications to be used for [19–22]. There are many examples that show the perspective of SLA printers for complex microfluidic devices fabrication regarding biological applications, thus, making them an option to be used by researchers focused on 3D printing of reliable accurate and biologically solvent microfluidic devices. However, some technical aspects must be considered to optimise the printing results.

#### **1.1 Theoretical fundaments of stereolithography**

The polymerisation of photosensitive resins is mainly governed by two parameters [23]: penetration depth of the curing light and the minimum energy required for polymerisation. The penetration of light follows the well-known Beer-Lambert law of exponential light absorption given by:

*Internal Microchannel Manufacturing Using Stereolithographic 3D Printing DOI: http://dx.doi.org/10.5772/intechopen.102751*

$$P\_x = P\_0 e^{-x/D\_\mathcal{P}} \tag{1}$$

being *P*Z the light power measured at a depth z from the surface; *P*0, the power at the surface; and *D*P, the depth reached when light intensity decreases by a factor 1/*e* of the surface intensity. Note that *D*P is a factor that depends on the resin composition, which determines its absorbance characteristics (dispersion and absorption) [24]. Power terms can be rewritten in terms of energy (so *z* will become the cure depth when the appropriate amount of light is provided) to obtain the working curve equation for SLA 3D printers:

$$C\_D = D\_P \ln\left[\frac{E\_0}{E\_c}\right] \tag{2}$$

where *C*D is the depth/thickness at which the light energy is sufficient to convert the liquid resin into a gel; *E*0 is the energy of light at the surface; and *E*C is the critical energy necessary to initiate photopolymerisation. According to the Beer-Lambert law, the exposed light intensity reaches its maximum value (*E*MAX) at the surface of the resin, and decreases exponentially as light penetrates through the resin due to the attenuation of the absorbing medium.

In the resin, the photopolymerised volume increases with the ultraviolet (UV) irradiation until the resin reaches to the gel point, where it transforms from liquid to solid-state. *D*P and *E*C are parameters that depend on the chemical characteristics of the resins and can be determined by drawing a semi-log plot of *C*D vs. *E*0 obtaining a straight-line curve with slope *D*P and an x-intercept of *E*C [25]. Once *D*P and *E*C are known, it is possible to optimise printing process choosing properly the exposure parameters and achieving the designed piece properties. This is the key for obtaining good results with a high-resolution SLA printing, where minimising the thickness of the deposited and light cured layer to achieve the maximum detail is critical.

In most SLA printers, the light source used to perform photopolymerisation is a laser, so the XY resolution is given by the size of the laser spot on the surface. Knowing the aforementioned parameters, the user or printer manufacturer can choose the proper parameters of light exposure (scan speed, power) to optimise the curing conditions and achieve the best resolution for the final device. Another determining factor is the minimum Z-step allowed by the printing arm, which gradually raises the piece from the bottom of the tank, that determines the corresponding layer thickness for each resin (see **Table 1**).


#### **Table 1.**

*Manufacturer characteristics of the resins used in this chapter.*

Finally, one of the most important aspects to be analysed for obtaining suitable internal channels is the orientation of the designed device, thus, a deep study of the influence of the inclination of the device to be fabricated in the process of photopolymerisation is necessary in order to determine the configurations that provide better results. We have to realise that the printer will slice the piece in a series of layers parallel to the base so that if the original configuration is rotated, these layers will change together with the areas that will be cured. Hence, objects with high surface detail should be printed with an orientation that helps the accurate curing of the layers. It also happens in the case of internal channels, where a proper angle could favour the full evacuation of the wastes of liquid resin from its interior, avoiding clogging.

In this work, a study of the performance of an SLA 3D printer in microfluidic devices is presented. For this purpose, an annular piece with a series of internal channels of different diameters and angles will be designed and manufactured. The dependence on the printing orientation of the device in the results will be evaluated. The study will be made for seven different commercial printing resins.
