## 3 Results
### Serpentine Architecture
One purpose of these test chips was to establish the degree of success of the clearance procedure, as a function of the serpentine microchannel width. Single serpentines were chosen for this test to maximize their ability to assess the clearance of channels optically, as the entwining of the channels in a double-serpentine would have complicated the optical measurement unnecessarily. Four chips were printed and each contained 12 serpentine microchannels, three of each of the four cross-sectional widths studied. The cross-sectional widths were XL = 700 \(\upmu\)m; L = 550 \(\upmu\)m; S = 400 \(\upmu\)m; and XS = 250 \(\upmu\)m. The chips are shown in Figure 1C. The cleared channels appear white due to scattering at the rough boundary between the channel surface and air. The uncleared channels appear invisible, since the support material refractive index is close to that of the 3D-printed resin, so interfacing boundaries would not scatter the light significantly. The experimental results showed 100% successful clearance of the XL and L widths, while partial success was noted for the S width, and almost no success for the XS width. The results of the measurements are presented in Table 2.  
These results demonstrate that single serpentines can be cleared, at least for channel widths of \(\geq\)400 \(\upmu\)m. As a result, the fabrication of double-serpentine devices, generally referred to as simply \"serpentine\", was undertaken. The individual channels entwining in each pair are difficult to discern, particularly at smaller scales. Figure 1D shows an example of a double-serpentine device. Note that microchannels in these chips are printed with black resin, are non-functional and are for demonstration purposes only (see Section 2). Functional double-serpentine chips were 3D-printed; however, clearing their microchannels was very challenging. Initially, the usual clearance procedure led to delamination between the soft resin [67], which makes up the body of the chip, and the hard resin, which makes up the ends of the device (Figure 1D). To avoid delamination, the design of the device was updated by extending the soft material nearly all the way to the access ports. As a result, all microchannels were printed entirely from soft material. This design change fixed the material-boundary delamination failure; however, the devices still failed during the clearance procedure. We attribute this failure to two connected factors. First, Polylet technology prints devices in layers that are less strongly connected than the material within the same printing layer. Consequently, layer delamination is possible, particularly if a small region of delamination happens. Once layer delamination begins, it triggers positive feedback in wedge-like processes, where the defect creates increased local stress, which creates a larger defect, resulting in more stress. The result of this wedging is that once layer delamination begins, it will likely grow until the device fails. As fluidic pressure is used in the clearance procedure, it is likely that the pressure generated partial delamination and then helped it spread through the wedging effect. The second factor has to do with the geometry of the serpentine. The serpentine winds back and forth, producing many approximately 90-degree turns. Each turn necessitates a corner, which produces stress concentrations that aid in the initial generation of splits between layers. Rounding off these corners can mitigate this effect at the cost of additional lateral space usage or further plate separation. Since maximal resolution is achieved along the vertical z-axis, the devices are printed \"standing\", i.e., the longitudinal axis is lined up with the z-axis. This means that the corners of the turns would likely end up at the boundaries between printing layers. The combined effect of these two factors is a likely explanation for the observed layer delamination during the clearance procedure. A potential solution to this problem is to switch to stereolithography 3D printers (SLA) [68].