Kai Sachsenheimer

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.

Online monitoring of biofilm growth and activity using a combined multi-channel impedimetric and amperometric sensor

Biofilms are ubiquitous in water interfaces and therefore influence our daily lives in an ambivalent manner. In medicine, infections can be attributed to biofilm formation. In technical systems, biofilms are causative agents for biocorrosion, contamination, and clogging processes and are responsible for shear force modification in marine systems. To control and manipulate biofilm formation advanced technologies are needed. This paper reports on a novel real-time biofilm monitoring system using custom-made electronics. The system is able to monitor four electrochemical impedance spectroscopy (EIS) electrodes and three amperometric sensors in two microfluidic channels assessing biofilm growth and activity in parallel using Pseudomonas aeruginosa as a model system. The biofilm was characterized during its seeding and growth stages as well as during different injection intervals of a biocide (sodium azide) which allowed monitoring biofilm destabilization and deactivation effects in real time. The results obtained were confirmed by fluorescence microscopy after live/dead cell staining of the bacteria in the measured biofilm.

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.

Multi-channel microfluidic biosensor platform applied for online monitoring and screening of biofilm formation and activity.

Bacterial colonization of surfaces and interfaces has a major impact on various areas including biotechnology, medicine, food industries, and water technologies. In most of these areas biofilm development has a strong impact on hygiene situations, product quality, and process efficacies. In consequence, biofilm manipulation and prevention is a fundamental issue to avoid adverse impacts. For such scenario online, non-destructive biofilm monitoring systems become important in many technical and industrial applications. This study reports such a system in form of a microfluidic sensor platform based on the combination of electrical impedance spectroscopy and amperometric current measurement, which allows sensitive online measurement of biofilm formation and activity. A total number of 12 parallel fluidic channels enable real-time online screening of various biofilms formed by different Pseudomonas aeruginosa and Stenotrophomonas maltophilia strains and complex mixed population biofilms. Experiments using disinfectant and antibiofilm reagents demonstrate that the biofilm sensor is able to discriminate between inactivation/killing of bacteria and destabilization of biofilm structures. The impedance and amperometric sensor data demonstrated the high dynamics of biofilms as a consequence of distinct responses to chemical treatment strategies. Gene expression of flagellar and fimbrial genes of biofilms grown inside the microfluidic system supported the detected biofilm growth kinetics. Thus, the presented biosensor platform is a qualified tool for assessing biofilm formation in specific environments and for evaluating the effectiveness of antibiofilm treatment strategies.

Rapid bonding of polydimethylsiloxane to stereolithographically manufactured epoxy components using a photogenerated intermediary layer.

We describe a low cost, photo-induced, room-temperature bonding technique for bonding epoxy components to flexible PDMS membranes in less than half an hour. Bond strengths (~350 kPa) were characterized by ISO-conform tensile testing for a popular stereolithography resin and found comparable bond strengths as reported for PDMS/PDMS bonds.

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.

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.

Rapid bonding of polydimethylsiloxane (PDMS) to various stereolithographically (STL) structurable epoxy resins using photochemically cross-linked intermediary siloxane layers

In this paper we present a fast, low cost bonding technology for combining rigid epoxy components with soft membranes made out of polydimethylsiloxane (PDMS). Both materials are commonly used for microfluidic prototyping. Epoxy resins are often applied when rigid channels are required, that will not deform if exposed to high pressure. PDMS, on the other hand, is a flexible material, which allows integration of membrane valves on the chip. However, the integration of pressure driven components, such as membrane valves and pumps, into a completely flexible device leads to pressure losses. In order to build up pressure driven components with maximum energy efficiency a combination of rigid guiding channels and flexible membranes would be advisable. Stereolithographic (STL) structuring would be an ideal fabrication technique for this purpose, because complex 3D-channels structures can easily be fabricated using this technology. Unfortunately, the STL epoxies cannot be bonded using common bonding techniques. For this reason we propose two UV-light based silanization techniques that enable plasma induced bonding of epoxy components. The entire process including silanization and corona discharge bonding can be carried out within half an hour. Average bond strengths up to 350 kPa (depending on the silane) were determined in ISO-conform tensile testing. The applicability of both techniques for microfluidic applications was proven by hydrolytic stability testing lasting more than 40 hours.

Development of a Graphic and Braille Optimized Plotter for the Blind and Visually Impaired

With the development of this plotter, a cost-effective and easy assembly was created to allow a better accessibility to well-recognizable braille text and tactile graphics on standard DIN-A4 paper for blind and visually impaired users. In comparison to currently employed systems, the developed device consists of a commercially available X/Y-plotter, which enables a movement in two orthogonal directions at once. Thus, not only the embossing of single dots, but also of continuous lines on a flat surface is possible. The plotter was extended by a solenoid that can change the force on a convertible embossing pen in Z-direction. Further parts were constructed and added to enable material feeding, material removal and paper fixation while printing. In a survey with six blind and visually impaired people, the results of the plotter were tested for good readability and recognition and compared to a conventional needle-based-system. Most of the participants would use the results produced by the plotter at hand in their daily life.

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.