**3. Results and discussion**

The manufacturing of internal channels with a continuous and unobstructed lumen is one of the main challenges for actual SLA printers, because of their many applications in microfluidics [16, 17]. The fabrication of cavities in a bulk with a proper lumen is a very difficult process, since the photopolymerisation of each layer is sustained by the previous one, so the evacuation of the non-polymerised resin can be tedious.

In many cases, the goal of obtaining unobstructed channels goes against the need for the printer to introduce scaffolds in the largest cavities, causing that internal channel collapse if some supports are not used during the printing. In addition, the own resolution of the printer can act as a limiter for very small channels, which do not have a structural challenge. In order to properly establish the dimensional limit between small channels and large cavities and to study the dependence of the internal channel performance on the diameter and angle of the printer, quarter annulus crossed by internal channels (**Figure 2**) were printed for each resin and the experimental diameters were measured as indicated in Section 2.2.

From the obtained results, three printing regimes can be defined. In the case of channels with small diameters (250 μm), no channel was fabricated for any resin at any angle, so no data can be presented. It can be concluded that, for these sizes, the formation of internal cavities in this range is not possible due to its small size, which prevents the correct evacuation of the resin. This implies that, the resolution for structures inside the printed piece is lower than the resolution for external ones, as structures of this size could be formed if they were made on the surface [19].

Next, for 500–1000 μm in diameter (medium diameters), channels begin to be formed (see **Figure 4a** and **b**) as will be detailed below. The bottom of these channels has been measured using the experimental configuration presented in **Figure 3**. We defined the accuracy as the ratio between the printed and theoretical designed diameter, in percentage. The tendency observed is an increase of experimental diameters as the printing angle increases, for a theoretical fixed value. For channels of 500 μm in diameter (**Figure 4a**), Amber and Dental resins provide the best results, almost reaching a 100% accuracy for an angle of 90°. In addition, for angles greater than 60°, they are all above 80% accuracy, together with Model resin. For lower values of the angles, the channels are narrower than those designed and are more incomplete (longitudinally) as the angle decreases, so for 15°, only Amber

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

#### **Figure 4.**

*Accuracy of the printing for the internal channels with diameters of (a) 500 μm, (b) 1000 μm and (c) 1500 μm in diameter, respectively. The error bars represent the standard deviation of the accuracies, and the area between the errors has been filled to facilitate the interpretation of the graphs.*

Clear and Model resins form channels and for 0°, none. Longitudinally, Clear and Dental only form complete channels for 90° while Amber resin enables the formation of complete channels for 60°, 75° and 90°. For other values, the channels are not completely formed, although the unobstructed length of the channel increases as the angle increases (see **Figure 3a**).

When channels of 1000 μm in diameter (**Figure 4b**) are fabricated, the printing accuracy suffers a global increase, being always above 70% for every studied angle. As the angle increase, an improvement in the precision is observed, and from 45°, all resins show an accuracy of more than 80% (except for Model, which shows a more irregular trend). The best results are obtained for 90°, where all the resins are above 90%, being the Model resin the exception, reaching an 88%.

In the case of channels with 1500 μm in diameter (wide channels), an 85% on accuracy is achieved for all channels at every studied angle (**Figure 4c**). The length of the channels increases until they form completely (unobstructed) at 45° for Clear resin and at 15° for Amber and Dental resin. For greater angles, complete channels are formed for these resins. For these diameters, results are particularly suitable for angles greater than 60° degrees, where all resins show a printing accuracy greater than 95%, being the exception again the accuracy of Model resin, which is much closer to 90%. Therefore, internal channel with wide diameters allows to fabricate internal cavities for any angle and do not need scaffolding inside. Note that, in the case of the Tough and Model resin, the length of the channels cannot be evaluated by naked eye due to their opacity.

Channels fabricated at 45° and 500 μm in diameter were chosen for inspecting the internal surface of unobstructed channels. In particular, Tough, Clear and Model resins were selected to be analysed because of their different properties (Z-step, biocompatibility, transparency…). **Figure 5** shows confocal images of longitudinal sections of the channels, where it can be observed that semi-circular designed profile is properly translated to the printed pieces.

By comparing the confocal images of the Tough (**Figure 5a**) vs. Clear and Model resins (**Figure 5b** and **c**), a decrease of the surface waviness with the Z-step is observed. The smoothest profile was achieved using the Model resin (**Figure 5c**).

From the previous analysis, we realise that the angle of impression is critical and has a major influence in preventing (**Figure 6a**) or favouring (**Figure 6b**) the formation of internal channels, so a larger angle (closer to 90°) is observed as the most suitable for channels to form properly and to have dimensions closer to those designed. The other key parameter found in this study is the diameter. As we have seen, a larger diameter allows results to be obtained with a higher resolution.

**Figure 5.**

*Confocal images of sections of channels designed with 500 μm in diameter and printed at 45° using: (a) Tough, (b) Clear and (c) Model resin.*

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

#### **Figure 6.**

*Microscope images of (a) obstructed and (b) unobstructed channels, with 500 μm of theoretical diameter, printed with amber resin at 0° and 75°, respectively. The images were taken with a 5X microscope objective.*

The fact that orientation and diameter are so critical in the manufacture of channels is rooted in the way SLA printers operate and is intimately related to the evacuation of uncured resin, which will be more likely to occur the larger the channel and the more perpendicular the channel to the base (so gravity can enhance evacuation).
