Optimization

The above mentioned setup (cf. Sec. 3.2.1) already contains one element to improve the quality of the process: The λ/4-plate leads to a symmetric intensity distribution in the focal volume and, hence, a symmetric voxel. There are more and often more complex methods to further improve the process of DLW that may be required to achieve high resolution or fast processing. Since we want to give a proof of principle of DLW with a new material class in this work these optimizations are not essential. Therefore, this section will only introduce very briefly a selection of methods for optimization.

A first method for optimization is inspired by the stimulated emission depletion (short:

STED) microscopy [102], which was awarded with the Nobel prize in 2014. In STED assisted DLW, a second laser is used to deplete the excited molecules before they reach the

3. Methods

reactive state. To do so, the two laser beams are spatially overlaid. As for the microscopy method, a typical beam profile combination is the excitation with a Gaussian beam and the depletion with a so-called doughnut mode, which has zero intensity in the middle.

By combining these two, the middle, where the maximum excitation takes place, stays excited and is not depleted, while the outer regions of the Gaussian beam are excited by the first laser and depleted by the STED laser.

Also similar to the microscopy method, the two lasers have to use different wavelength ranges, so the STED laser does not further excite the molecules in the photoresist. The STED laser has to work at a wavelength that enables a depletion process. This depends on the material system and the possible states that can be addressed for the depletion process. [82, 103]

The above mentioned beam shapes that can be used for the STED assisted DLW are dis-torted due to aberrations. Therefore, a further optimization can be achieved if these aber-rations are corrected [104]. This already improves the performance without STED [76]

when applied for the excitation laser. The aberration correction can be achieved with a spatial light modulator (SLM). Phase and amplitude patterns loaded onto the SLM dis-play diffract the beam such that the achieved beam profile is optimized. [105].

These are just examples for optimization of the DLW method. There are more possibilities already and research is still in progress to further improve DLW.

Chapter 4

Direct laser writing in a bioinspired material

While conventional photoresists for DLW use mineral oil-based monomers or polymers as their key component [106], the resists presented here use a natural polysaccharide as a raw material. With the focus of Chapter 5 on photonic structures in insects that use chitin as main material, chitin is of course highly interesting. However, substitution reactions of chitin are more complex than for example cellulose substitution reactions.

Hence, to introduce polysaccharide-based photoresists in DLW cellulose is used. The monomers in conventional photoresists are replaced by derivatives of cellulose as the crucial component of the resists. The gained information can be transferred to chitin later on.

Therefore, the first section introduces cellulose as a promising material. The substitu-tion reacsubstitu-tions that are necessary to convert cellulose into a cross-linkable constituent of a photoresist are explained in Section 4.2 as well as the other constituents of a potential re-sist. In the subsequent Section 4.3, the properties of a cellulose-based resist are discussed before in Section 4.4 a first biomimetic structure is fabricated with a such a resist.

4.1 Cellulose - a short profile

Cellulose is the most abundant polysaccharide in the world [107]. It can be extracted out of cell walls in plants and it can be produced by some bacteria [107]. It is a renewable raw material with nearly inexhaustible occurrence in the world. Therefore, it is a cheap and sustainable material, which takes a major role in the development of green chemistry [108].

The use of cellulose is much older than its name (1839) and the knowledge of its molecu-lar formula (1838). Mankind has always used cellulose, for example as building material, energy source or clothing. The use of cellulose, as a chemical raw material, started in the 19th century with the synthesis of Celluloid. [107]

4. Direct laser writing in a bioinspired material

Figure 4.1: The cellobiose is the repeating unit of the cellulose. It consists of two β-D-glucose molecules linked via aβ(1,4)-glucosidic linkage.

Cellulose is biocompatible and, in its pure form, it is hydrophobic and insoluble in most organic solvents [107]. It is part of many natural composite materials in plants. These natural composite materials show a high tensile strength, which increases with an in-creasing amount of cellulose [109]. Concerning the tensile strength, these composites outperform some synthetic polymers. For example, pineapple leaf fibers have a tensile strength of up to 1627MPa [109], while poly(methyl methacrylate) (PMMA), which is used for implants in dentistry [110] or as roofing material, has a tensile strength of less than 100MPa [111].

The above mentioned properties of cellulose arise from the exact molecular structure of the cellulose. Cellulose consists of a chain of glucose molecules that are linked by a β-(1,4)-glycosidic bond. This means that two neighboring β-D-glucose molecules are linked via the first carbon atom of one glucose molecule and the fourth carbon atom of the neighboring molecule (see Fig. 4.1). This fundamental unit cell, the cellobiose, is around 1nm long [112].

Each β-D-glucose molecule of the cellobiose exhibits three hydroxyl groups. These hy-droxyl groups build hydrogen bonds, which determine the physical and chemical prop-erties. Due to intramolecular hydrogen bonds, the cellulose molecules exhibit a linear structure [107] and due to intermolecular hydrogen bonds the cellulose is insoluble in common organic solvents and materials with a high cellulose amount have a high tensile strength.

If the molecular arrangement is changed, as in case of the amylose, an isomer of the cellu-lose and a constituent from starch [113], the chemical, physical and biological properties differ enormously. The amylose also consists of a chain of glucose molecules, which are linked byα-1,4-glucosidic bond [113].

Besides the exact arrangement, the so-called degree of polymerization DP, the total a-mount of fundamental units building a cellulose molecule, influences the exact proper-ties. The degree of polymerization depends on the exact source of the material and varies between aroundDP = 500 toDP = 5000.

The properties of the cellulose can be adapted, if the molecules are for example shortened or if the hydroxyl groups are substituted with other functional groups. These functional groups are changed during substitution reactions and can enable solubility in common

4.2. Components of a bio-resist