Microscale Components and Features

Although the parts produced at macroscale are impressive, the Perfactory Mini Multi-Lens system does not have the build volume to produce parts of a useful size. It has been specifically set up in order to maximise resolution at the cost of build area, and in this section a number of microscale components will be discussed.

5.3.1 Meshes

An early test component fabricated using the Perfactory Mini Multi-Lens SLA system was a fine horizontal mesh, supported by a circular frame. The part was fabricated using the standard build parameters, and shown in Figure 5.7. As can be seen, the fine detail afforded by the SLA system can be used to create structures that are both complex and robust. The meshes required a series of prototype builds, of differing hole and mesh sizes, until the composite 50 and 100 μm mesh shown was found to give good fabrication yields. The mesh was flexible, and could be bent with the end of a finger before returning to its original shape.

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Figure 5.7 – Photographs of a circular mesh test piece fabricated using the Perfactory Mini Multi-Lens SLA system. (a) Mesh with UK 20p piece for scale. (b) Close-up of the mesh, which is 100 μm thick, built in 25 μm layers, with 100 μm wide holes in a 50 μm wide mesh. It was found that a 100 μm wide “superstructure” mesh increased the mesh yields to close to 100%, resisting the fabrication forces.

Figure 5.8 – SEM electron micrographs of a 100 μm mesh with 100 μm holes. Although the structure itself was reasonably regular, the mesh can be seen to have delaminated.

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Further structural details can be seen in Figure 5.8. This mesh was a 100 μm wide structure surrounding 100 μm square holes. As can be seen, the holes are not square at high magnification, and take the appearance of a micro-sized plastic fencing. Although the front layer is uniform, the back layers have delaminated. This was found to be a particular problem with uniform meshes, whilst those with an overarching superstructure, as shown in Figure 5.7 did not suffer from this problem. The holes size did not seem to have a bearing, although if the hole size to mesh size ratio got too high then often the mesh would be weak due to the resultant lack of material in the mesh itself.

The mesh structures were seen to have potential as scaffolds for tissue engineering applications. In collaboration with Dr. Judith Hoyland of Manchester University, the structure seen in Figure 5.9a was designed using SolidWorks. The structure was created as a scaffold for replacement back disks, and was separated into two concentric structures: a central mesh, surrounded by a number of linked concentric rings. The inner mesh structure was actually composed of 3 parallel mesh elements, identical to those seen in Figure 5.7, joined by a series of interconnected struts, as shown in the cut-away view in Figure 5.9b. The struts were designed to aid the migration of cells between the meshes, and were 200 μm wide square structures.

The part was fabricated horizontally on the build platform, using standard fabrication parameters. The resulting component can be seen in Figure 5.10. The structure was surprisingly strong and flexible, with pressure applied laterally to the concentric rings causing elastic deformation of the structure, but no breakages or cracks in the surface. Opening up the structure (not shown) allowed confirmation that the internal cross-beam structure had indeed fabricated, although a few struts were not fully in contact with the upper meshes.

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Figure 5.9 – Structure designed in SolidWorks as a potential scaffold for tissue engineering, in collaboration with Dr. Judith Hoyland of the School of Medicine at Manchester University.

Figure 5.10 – Resultant structure fabricated using the Perfactory Mini Multi-Lens SLA system, from the CAD data shown in Figure 5.10. The structure is 30 mm across its widest point.

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The structures shown are a good example of geometries that would simply have been impossible using traditional machining techniques. This is taken further by the mesh sphere show in Figure 5.11. This delicate looking, yet surprisingly robust component measures 600 μm across, and is composed of a hexagonal lattice of 70 μm thick struts. The ball is not entirely perfect, as there is a small break in the structure that corresponds to where it was built off the platform, it maintains its shape, and can even be bounced off hard surfaces without damage.

Figure 5.11 – Mesh sphere fabricated using the Perfactory Mini Multi-Lens SLA system. It is composed of a hexagonal lattice of 70 μm-thick struts.

Although Figure 5.8 shows that complex vertical structures are possible, it was found that thin vertical features such as meshes and membranes did not build well. This can be seen in Figure 5.12. Although these meshes shown did resolve, these were two of only a few of these 100 µm thick structures that did build to completion. Many others ripped during fabrication, seemingly independently of the hole size. Therefore builds containing thin features such as meshes or membranes are generally constrained in their positioning on the build platform, to avoid such aberrations.

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Figure 5.12 – 100 µm thin mesh structures built on a vertical plane, with (a) 200 µm and (b) 100 µm diameter holes. It is noticeable that the horizontal struts of the mesh are wider than the vertical features. The struts in the CAD file were of the same size.

5.3.2 Microscale Test Build

In order to push the envelope of the SLA system, a test build was designed to probe the minimum feature sizes and the ability of the system to produce complex micro-scale structures. The fabricated components can be seen in Figure 5.13. The microstructure shown in Figure 5.13a was reproduced well from the CAD drawing, with only a slight enlargement of the base of the structure legs visible. The bridge structure in Figure 5.13b also fabricated well, although damage is visible on the upper edge where some material has peeled away during fabrication. Perhaps the most impressive feature is the wall structure in Figure 5.13c, which at 20 µm wide is constructed from the exposure from only a single pixel in the projector. However, less successful was the tree- like structure shown in Figure 5.13d. This was designed with 3 offset branches, 100 µm wide with 20 µm sidewalls. As can be seen, these have nearly entirely failed to build. It is assumed that although extremely thin features, such as those seen in Figure 5.13c are possible when built from a large bulk of cured material, the relatively small amount of material present in the “trunk” of the structure in Figure 5.10d means that the whole structure will move around during fabrication. This may cause misalignments of the pixel grid, with each layer not being directly on top of the last, causing structural weakness and the resultant build failure.

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Figure 5.13 – SEM electron micrographs of test build designed to probe the minimum feature sizes of the Perfactory Mini Multi-Lens system. (a) A microstructure, with 100 µm wide beams meshed together. The structure stands 645 µm tall, and is 700 µm wide. (b) A bridge structure, once again with 100 µm wide struts. The structure is 300 µm tall and 900 µm long. (c) 20 µm wide, 200 µm tall wall structure, a single pixel width wide. (d) Failed tree-like structure, standing around 700 µm tall (was designed as 900 µm). The central post was designed to have 3 offset “branches”, which were hollow with 20 µm wide outer walls.

5.3.3 Single Pixel Structures

It was decided to try and produce a microstructure using the Perfactory Mini Multi-Lens consisting of just the exposure of the resin by a single pixel. Pillars 20 µm wide were attempted as part of the build shown in Figure 5.13, but no features were found on the component. Later experiments relating to MSL-produced microneedles (see following chapters) did however produce a result. A pyramidal microstructure was designed as part of a larger array of microneedle shapes. The structure in question was just 125 µm tall, and 100 µm wide, and is shown in Figure 5.14. At the tip of the structure, which can be seen to be composed of just 5

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times 25 µm thick layers, was a single 20 × 20 µm blob of cured resin, representing the light output from a single pixel.

It is assumed that the previous columns had failed as they were built at 20 µm width directly from a flat bulk base. This may have caused extra stress during the peeling process, as the breaking surface tension between the resin tray and the bulk part may have ripped off the microstructures. However, the pyramid being built in steps increases the distance from the single pixel to the bulk material, and may have reduced this stress, allowing the fabrication to complete successfully.

Figure 5.14 – Pyramidal microstructure fabricated using the Perfactory Mini Multi-Lens system. The structure is 100 µm wide and 125 µm tall, and has a point of 20 × 20 µm area that is the output curing of a single projector pixel.