Lithography based AMT

Laser-scanning based stereolithography (SLA) is the oldest and most widely used AMT.

Figure 7 shows a principal scheme. Typically, a UV laser beam with small diameter passes an acousto-optic modulator (AOM) which turns it on and off. A galvanoscanner containing two rotating mirrors deflects the laser beam before it reaches the surface of a photopolymerisable formulation. Both, the AOM and the galvanoscanner, are computer-controlled. The beam moves on the surface of the specimen according to the cross-sections of the CAD. As the fabrication starts, the first layer cures to a defined depth. It sticks on the computer-driven building platform, which moves away from the surface. The coating system delivers new liquid material and the illumination starts again. The depth of curing is slightly larger than the movement to ensure cross-linking with unreacted functional groups of the previous layer. This ensures good adherence. The procedure repeats until the desired solid object reaches its intended extent. In STL writing speeds of 0.2-0.5 m/s are possible [2].

After fabrication, the operator removes the part from the building platform and cleans it from excess liquid formulation. This requires chemical developers or water (depending on the type of formulation). Afterwards, it is necessary to drain the finished part. In many cases, STL produces parts not fully polymerised. These “green part” are post-cured with UV light to improve their mechanical properties [12]. Subtractive post-processing like machining, grinding, sandblasting, metallizing and/or painting is possible to a certain extent.

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Figure 7 Comparison between a) Laserscanning SLA [6] and b) DLP [13]

Among all commercial AMTs, laser-scanning is still the most accurate. Only the diameter of the laser beam limits the X- and Y- resolution. Hence, today, the laser beam usually passes a focusing device before it reaches the formulation increasing available resolution and providing more energy per unit area. A resolution of 5 µm in the X- and Y- plane and 10 µm in the Z-direction have been reported processing hybrid sol-gel materials without further post-processing [2]. During the fabrication process, the parts have relatively low strength.

Recoating, internal tensions and/or the part’s self-weight can easily deform overhanging structures or cantilevers. These geometries require supporting during the building process (Figure 8). Another disadvantage of laser-scanning and its related bottom-up structuring procedure is the related exposure of the surface to the surrounding environment. Oxygen inhibition might hinder the polymerisation process and reduce the obtainable part quality making it necessary to provide an adequate surrounding (e.g. nitrogen).

Figure 8 CAD part of yarn guide with supports

Figure 7b illustrates a photomask lithography based AMT: Here, the machine fabricates the object top-down rather than bottom-up projecting light on a transparent, non-adhering plate from underneath. The building platform dips into the formulation from above. On one side, this eventually increases the mechanical forces during the separation from the bottom plate after the illumination. On the other, it ensures a smooth surface and prevents oxygen from reaching the polymerising surface. In addition, for low-viscous formulations, recoating is not required. In

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contrast to laserscanning, the dynamic masks can cure the entire cross-section of the part at once. When using high power light sources, this reduces the process time significantly.

Exploiting the full extent of the building platform, the process speed measure is vertical mm/h.

Figure 9 DLP of ceramic materials [13]

One can distinguish between lamp-mask and projector mask processes. The former includes a transparent mask on which the full cross-section of the part is displayed. Strong UV lamps shine through the mask and cure the behind liquid formulation at desired spots. The latter (Figure 7) uses a video beamer to image the cross-section onto the surface to be exposed. In this technology, a digital light projector (DLP), consisting an array of micro-mirrors (digital mirror device), projects a 2D pixel-pattern onto the transparent plate. Depending on the focusing objective and the amount of micro-mirrors, the resolution can be up to 40 µm laterally and 15 µm in Z [13].Our group uses this technique to process formulations filled with ceramic particles (Figure 9). The “green part”, which is obtained after fabrication, is an organic matrix containing ceramic particles. Thermal treatment including drying, debinding and sintering ensures the removal of this matrix. This facilitates the AM fabrication of fully dense ceramic parts. It was possible to fabricate alumina, bioglass and tricalciumphosphate objects [13].

Similar to stereolithography, 2PP is a laser-scanning approach, too. The experimental setups can be similar to that of Figure 7a. However, as fabrication is not limited to the surface of the formulation, the focal point can be moved anywhere in the volume leaving cured polymer along its trace [11][14]. Any arbitrary 3D shape can thus be “recorded” into the volume (see Figure 10). The basic building unit, where the polymerisation takes place (volumetric pixel or voxel) can be regarded as “3D pen”, with which a polymeric line can be created anywhere in the volume of a formulation.

The resolution can be down to 65 nm [15] as the non-linearity of 2PA provides the possibility to reduce the size of the polymerised volume below the diffraction limit [14]. Figure 10 shows a comparison between STL and 2PP fabrication. The former is limited to the surface, whereas in the latter allows to trace the focal volume (volumetric pixel or voxel) through the formulation leaving a complex polymeric structure.

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Figure 10 Right image: 2PP is limited to the focal point of a microscope objective; Left image: STL is limited to the surface and requires layer-by-layer manufacturing [11].

Hence, as first AMT, 2PP offers true 3D polymerisation without the need of a layer-by-layer fabrication procedure. All shortcomings related to the surface formation such as high viscosities of the formulation (leading to high surface tensions), the necessity of recoating, the need of supporting material and oxygen inhibition can be discarded.

For any photopolymerisable formulation it is necessary to define a minimum threshold irradiation (energy and/or time), that is required for starting polymerisation. Likewise, a maximum threshold exists, where high-energy doses lead to bubble formation and subsequent damage to the polymer. Apart from the magnification and the numerical aperture of a microscope objective, also the irradiation dose regulated by laser power and scanning speed determines the voxel size.

The unique features of 2PP are an effect of a nonlinear activation principle, substantially different from one-photon activation in other lithography based AMT. The nonlinearity of this principle places very specific demands to the hardware and chemistry involved in 2PP.

Conventionally, 1PA optimised compounds were used for 2PP, too. However, this resulted in inefficient cross-linking and therefore long process times, a major drawback of 2PP. To face this challenge, it is indispensable to optimise the components of photopolymerisable formulations for 2PP specific needs. Furthermore, we also have to use hardware that is able to rapidly trace the focal point inside the volume of an optimised formulation. Appropriate light sources and fast switching devices are necessary. They facilitate an efficient supply of laser intensity precisely at CAD defined spots inside the volume.

To accommodate 2PP requirements, it is worth to look at this nonlinear two-photon absorption effect from a theoretical side. Its basic physical principle and its history of origin will be topics of the next chapter. The reader will get to know other application than 2PP based on the same two-photon absorption (2PA) effect. Furthermore, we will get a little deeper into 2PP applications in particular.

The structure-property relationship of two-photon absorbing molecules will be part of the next chapter’s second section. Describing the efficiency measure two-photon absorption cross-section (δ), we will explain a two-photon absorbing molecule’s composition and explain in which way different groups of the molecule contribute to this efficiency value. Finally, we will investigate how a molecule’s  is related to its feasibility as efficient two-photon PI.

In the third section of the next chapter, we will explain the basic principle of the light sources used in this work. The reader will get to know femtosecond pulsed lasers and related devices used for beam adjustment.

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