**4. Conclusions**

Microfluidics is a multidisciplinary field that needs versatile technologies capable for manufacturing structures with high accuracy in a precise and reliable way. 3D printing seems to be a promising technology to researchers and industries through easy procedures and a low pollution process. In particular, stereolithographic 3D printers become very attractive due to the developments achieved in lasers, making them one of the most promising choices with greater accuracy and finishing within the existing manufacturing technologies.

The performance in internal channel manufacturing of an SLA 3D printer is tested, since this is one very important piece in several microfluidic devices. Several resins (Clear, Dental, Tough, Amber, Flexible, Elastic and Model) was used for printing the internal channels in terms of accuracy (from hundreds to thousands of micrometres). For this, an annular piece containing several internal channels with different diameters and at different angles was designed and printed for each resin, to analyse the achievable range of dimensions and accuracy.

In light of the results, resin accumulation was found to be the key element behind the correct formation of the channels. This has its origin in the operation principle of SLA printers, based on the layer by layer photopolymerisation of a liquid resin contained in a tank. Thus, the uncured resin must be properly evacuated from the successive layers if a suitable cavity without obstructions and malformations wants to be obtained. It was found that there are two critical parameters: the diameter of the channels and the printing orientation of the device.

While no channel formation was observed for diameters of 250 μm for any of the fabrication angles neither the studied resins, from 500 μm onwards, open lumens

began to form. This was the case of Dental and Amber resin, which form channels with printing accuracy (ratio between the printed and theoretical designed diameter) over 80% for values of the angles above 60° and diameters above 500 μm.

In the case of larger diameters (around 1000 μm), the measured accuracies were greater than 70% for every studied resin and grew with the angle. For channels with a diameter of 1500 μm, it was found that all the resins achieved higher accuracy than 90%, so this range can be considered the optimum for the manufacture of complete and fully functional internal channels.

In conclusion, SLA 3D printers are one of the promising technologies in the fabrication of internal channels, showing interesting and promising results for channels of hundreds of micrometres in dimension, very suitable for the growing field of microfluidics. However, the formation of complete internal channels is difficult below 250 μm due to the incomplete evacuation of the uncured resin. There is still room for improvement, and it will be necessary to find both light sources and printing resins that allow higher accuracies, of the order of several tens of micrometres.
