Nico Keller

Three-dimensional printing of transparent fused silica glass

Glass is one of the most important high-performance materials used for scientific research, in industry and in society, mainly owing to its unmatched optical transparency, outstanding mechanical, chemical and thermal resistance as well as its thermal and electrical insulating properties. However, glasses and especially high-purity glasses such as fused silica glass are notoriously difficult to shape, requiring high-temperature melting and casting processes for macroscopic objects or hazardous chemicals for microscopic features. These drawbacks have made glasses inaccessible to modern manufacturing technologies such as three-dimensional printing (3D printing). Using a casting nanocomposite, here we create transparent fused silica glass components using stereolithography 3D printers at resolutions of a few tens of micrometres. The process uses a photocurable silica nanocomposite that is 3D printed and converted to high-quality fused silica glass via heat treatment. The printed fused silica glass is non-porous, with the optical transparency of commercial fused silica glass, and has a smooth surface with a roughness of a few nanometres. By doping with metal salts, coloured glasses can be created. This work widens the choice of materials for 3D printing, enabling the creation of arbitrary macro- and microstructures in fused silica glass for many applications in both industry and academia.

Liquid Glass: A Facile Soft Replication Method for Structuring Glass

Liquid glass is a photocurable amorphous silica nanocomposite that can be structured using soft replication molds and turned into glass via thermal debinding and sintering. Simple polymer bonding techniques allow the fabrication of complex microsystems in glass like microfluidic chips. Liquid glass is a step toward prototyping of glass microstructures at low cost without requiring cleanroom facilities or hazardous chemicals.

Transparent, abrasion-insensitive superhydrophobic coatings for real-world applications

Superhydrophobic surfaces and surface coatings are of high interest for many applications in everyday life including non-wetting and low-friction coatings as well as functional clothing. Manufacturing of these surfaces is intricate since superhydrophobicity requires structuring of surfaces on a nano- to microscale. This delicate surface structuring makes most superhydrophobic surfaces very sensitive to abrasion and renders them impractical for real-life applications. In this paper we present a transparent fluorinated polymer foam that is synthesized by a simple one-step photoinitiated radical polymerization. We term this material “Fluoropor”. It possesses an inherent nano-/microstructure throughout the whole bulk material and is thus insensitive to abrasion as its superhydrophobic properties are not merely due to a thin-layer surface-effect. Due to its foam-like structure with pore sizes below the wavelength of visible light Fluoropor appears optically transparent. We determined contact angles, surface energy, wear resistance and Vickers hardness to highlight Fluoropor’s applicability for real-word applications.

Glassomer-Processing Fused Silica Glass Like a Polymer

Fused silica glass is one of the most important high-performance materials for scientific research, industry, and society. However due to its high chemical and thermal resistance as well as high hardness, fused silica glass is notoriously difficult to structure. This work introduces Glassomer, a solid nanocomposite, which can be structured using polymer molding and subtractive technologies at submicrometer resolution. After polymer processing Glassomer is turned into optical grade fused silica glass during a final heat treatment. The resulting glass has the same optical transparency as commercial fused silica and a smooth surface with a roughness of a few nanometers. This work makes high-performance fused silica glass components accessible to high-throughput fabrication technologies and will enable numerous optical, photonic and medical applications in science and industry.

Suspended Liquid Subtractive Lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices

The miniaturization of synthesis, analysis and screening experiments is an important step towards more environmentally friendly chemistry, statistically significant biology and fast and cost-effective medicinal assays. The facile generation of arbitrary 3D channel structures in polymers is pivotal to these techniques. Here we present a method for printing microchannels directly into viscous curable polymer matrices by injecting a surfactant into the uncured material via a steel capillary attached to a 3D printer. We demonstrate this technique using polydimethylsiloxane (PDMS) one of the most widely used polymers for the fabrication of, e. g. microfluidic chips. We show that this technique which we term Suspended Liquid Subtractive Lithography (SLSL) is well suited for printing actuators, T-junctions and complex three dimensional structures. The formation of truly arbitrary channels in 3D could revolutionize the fabrication of miniaturized chips and will find broad application in biology, chemistry and medicine.

Suspended liquid subtractive lithography: printing three dimensional channels directly into uncured PDMS

Polydimethylsiloxane (PDMS) is one of the most widely used polymers for the generation of microfluidic chips. The standard procedures of soft lithography require the formation of a new master structure for every design which is timeconsuming and expensive. All channel generated by soft lithography need to be consecutively sealed by bonding which is a process that can proof to be hard to control. Channel cross-sections are largely restricted to squares or flat-topped designs and the generation of truly three-dimensional designs is not straightforward. Here we present Suspended Liquid Subtractive Lithography (SLSL) a method for generating microfluidic channels of nearly arbitrary three-dimensional structures in PDMS that do not require master formation or bonding and give circular channel cross sections which are especially interesting for mimicking in vivo environments. In SLSL, an immiscible liquid is introduced into the uncured PDMS by a capillary mounted on a 3D printer head. The liquid forms continuous “threads” inside the matrix thus creating void suspended channel structures.

Photolithographic structuring of soft, extremely foldable and autoclavable hydrophobic barriers in paper

Microfluidic paper-based analytical devices (μPADs) offer the possibility to carry out laboratory test on a piece of paper. This enables on-site monitoring in regions with scarce laboratory infrastructure but also promises cost savings for health care systems in highly-developed regions. One key element of all μPADs are hydrophobic barriers which control the liquid flow during the analysis. There are different approaches to generating hydrophobic barriers such as, e.g., wax or polymer printing as well as lithographic techniques. However, all of these introduce stiff barriers into the otherwise soft and foldable paper which significantly limits its handling. In almost all cases, once the paper is folded strongly the barriers break and are no longer able to retain a liquid sample. In this paper, we present a method for structuring hydrophobic barriers by a light-based approach making use of a light-controlled locally confined silanization. This method combines the advantages of photolithography and 3D printing in terms of process speed and flexibility with a chemical modification technique which locally modifies the wetting behaviour of the paper instead of applying a physical bulk barrier. This allows generating hydrophobic barriers which retain the flexibility of the paper and can be freely folded without losing their liquid-retaining properties even after as many as 50 fold cycles. The structures produced in this way are highly chemically stable and can even be autoclaved. We demonstrate the suitability of this method in bioanalytics using an enzymatic assay demonstrating that the silanization chemistry does not impair the biocompatibility of the substrate.

Next generation 3D printing of glass: The emergence of enabling materials (Conference Presentation)

Additive manufacturing and 3D printing have seen significant improvements in terms of processing and instrumentation with the aim of increasing the complexity of the objects constructible, increasing resolution and lateral dimensions as well as speed of manufacturing. Interestingly, the choice of materials has not been increasing significantly. One of the oldest materials mankind has used was, until now, missing: Glass. Account of man-made objects in glass date back to 5000 BC which makes it the oldest artificial material used by mankind. Glass has numerous advantageous properties including unmatched optical properties, mechanical, thermal as well as chemical stability to name but a few. However, due to the fact that class can almost exclusively processed by etching using hazardous chemicals or from the melt (i.e., at temperature in the range above 1500 °C) glass has remained, until now, a material inaccessible for modern manufacturing methods including 3D printing. Our group has recently introduced a major paradigm shift in the processing of glass with the introduction of a “Liquid Glass” nanocomposite which can be shaped at room temperature using methods known from polymer replication as well as modern 3D printing techniques. The nanocomposite is a honey-like transparent syrup which can be cured by light and, after thermal debinding and sintering, yields three-dimensional components with transparency, as well as chemical and mechanical properties identical to pure fused silica glass. The surface quality of these components meets the demand of (micro)optics and allows the manufacturing of diffractive and refractive optical elements as well as lenses.

Rapid structuring of proteins on filter paper using lithography

Microfluidic paper based analytical devices (μPADs) are simple and cost efficient and can be used everywhere without the need for a high standard laboratory for obtaining a readout. These devices are thus especially suited for the developing world or crisis regions. To fabricate a bioanalytical test, certain biomolecules like proteins or antibodies have to be attached to paper strips. Common immobilization methods often rely on non-covalent, unoriented attachment which leads to reduced bioactivity of the immobilized species. Specific Immobilization of biomolecules on surfaces still poses a great challenge to biochemical research and applications. We propose a method for the specific immobilization of biomolecules on functionalized filter paper using a maskless projection lithography setup. The paper was functionalized either by applying an adhesive protein coating or by covalent attachment of methacrylate groups. Fluorescently labelled biomolecules were attached by exploiting the formation of radical species upon bleaching of the fluorophore. A custom made maskless photo-lithography setup and a low cost approach were used to produce microscale biomolecule greyscale patterns. Protein patterns were visualized by fluorescence, enzyme patterns were tested for bioactivity by substrate conversion with colorimetric readout. This method enables the creation of complex, highly specific bioactive protein patterns and greatly facilitates the production of μPADs.

Tacky COC: a solvent bonding technique for fabrication of microfluidic systems

The academic community knows cyclic olefin copolymer (COC) as a well suited material for microfluidic applications because COC has numerous interesting properties such as high transmittance, good chemical resistance and good biocompatibility. Here we present a fast and cost-effective method for bonding of two COC substrates: exposure to appropriate solvents gives a tacky COC surface which when brought in contact with untreated COC forms a strong and optical clear bond. The bonding process is carried out at room temperature and takes less than three minutes which makes it significantly faster than currently described methods: This method does not require special lab equipment such as hot plates or hydraulic presses. The mild conditions of the bond process also allow for such “tacky COC” lids to be used for sealing of microfluidic chips containing immobilized protein patterns which is of high interest for immunodiagnostic testing inside microfluidic chips.