**2. Materials and methods**

### **2.1 3D printing**

A Form 3B printer (Formlabs, Somerville, Massachusetts) is used to print the devices to be studied. This printer features a new technology called Low Force Stereolithography, a step further in SLA printers designed to reduce the manufacturing stress that pieces undergo during printing. In brief, this technology combines a galvanometric system with a series of mirrors to grant an incidence of the laser beam (λ = 405 nm, *P* = 250 mW) perpendicular to the resin tank, whose base will be made of a flexible material capable of deforming when the piece is pushed on it. In this way, the accuracy of the 3D printed structures is improved, as a much more uniform deposition of the laser energy is ensured.

It is well known that the printing orientation will determine the features of the printed devices. Typically, suppliers recommend using 45° as printing orientation in order to optimise the process. Although this recommendation is useful for superficial structures, we realise that for internal channels, the evacuation of uncured resin can produce obstructed lumens [19]. To test the influence of the printing angle on the ability to create internal channels with good quality, a quarter annulus piece was designed (**Figure 2a**) containing seven internal channels oriented at 0°, 15°, 30°, 45°, 60°, 75° and 90° and printed (**Figure 2b**). This study was performed four times for each resin selected, varying the diameter of the internal channels each time. These pieces can be identified in **Figure 2a** as A, B, C and D, corresponding to microchannels with diameter of 250, 500, 1000 and 1500 μm, respectively.

#### **2.2 Data acquisition**

The measurement of the diameter was performed using a Nikon MM 400 metallurgic microscope (Nikon Instruments Europe B.V., Amsterdam, The Netherlands), that performs measurements in real time (**Figure 3a**), and an analysis NIS-Elements Nikon software (Nikon Instruments, Melville, USA), by adjusting a measurement circumference (**Figure 3b**) that the software allows to move and modify over the image. The channels were illuminated in transmission light configuration that allowed us to

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

#### **Figure 2.**

*Picture of some 3D pieces: (a) image of the design used to study the formation of internal channel when varying printing angles and internal diameters, (b) picture of selected annulus printed for different resins, with theoretical internal diameters of 500 μm. From the fore to the ground: Model, Clear, Amber, Dental and Flexible resin. Scaffolding supporting the structures is shown.*

#### **Figure 3.**

*(a) Experimental configuration used to measure the internal channels of the quarter annulus using a microscope. White arrows point channels not fully formed, printed at 0° and 15°. (b) Microscope image of the end of a channel printed using Model resin, at an angle of 75° and a theoretical diameter of 1000 μm. The picture was taken with a 5X microscope objective.*

measure the lumen of each one. Images were acquired using a LU Plan Fluor objective (Nikon Instruments, Melville, USA) with 5X magnification and a CCD camera Nikon DS-FI2 (Nikon Instruments, Melville, USA). Five measures were performed for each channel, obtaining a geometric mean and a standard deviation that will be presented in Section 3. Images of longitudinal sections of the microchannel internal surfaces were obtained with a 3D optical profilometer S neox (Sensofar Metrology, Terrassa, Spain) working in confocal mode.

## **2.3 Materials**

Seven printing resins made by Formlabs for the Form 3B printer were studied: Dental LT V1, BioMed Amber V1, Elastic 50A V1, Clear V4, Model V2, Tough 2000 V1 and Flexible 80A V1. As introduced in Section 1.1, one of the critical factors to obtain high accuracy results with SLA printers is the Z-step of the printing arm allowed by every resin. Thus, the minimum Z-step was selected for each of them, as indicated in **Table 1**. Some of the used resins are even biocompatible (**Table 1**), which increases their potential applications.

After printing, it is necessary to post cure the resin pieces in a two-step process, to improve their mechanical aspects and superficial finishing. This process starts with a wash of the part in isopropanol >90% in the Form Wash tank (Formlabs, Somerville, Massachusetts), in one (Model, Amber and Dental) or two cycles (Clear, Tough, Flexible and Elastic), during times indicated in **Table 1**. The pieces are then left to dry and placed in the UV Form Cure chamber (Formlabs, Somerville, Massachusetts), which allows to control the temperature and is also provided with LEDs emitting at 405 nm. Curing temperatures and curing times can be consulted in **Table 1**.
