Microfluidic paper-based analytical devices (µPADs) have gained a lot of attention in recent years because they enable the production of diagnostic devices in a simple and cost-efficient way. To control the fluidic flow, hydrophobic barriers are generated that reach into the fibrous structure of the paper. Popular methods for creating such barriers are wax printing or polymer deposition. These barriers are however very stiff: bending or folding leads to the destruction of the barriers. Another problem is the low resistance of common barrier materials against different solvents, which makes it impossible to execute chemical tests on paper. Destruction of the barriers leads to leakage and causes assay failure. Here we present a method that produces bendable hydrophobic barriers on paper by photolithography. These barriers are based on silanes and withstand solvents such as DMSO. We show that these barriers can also be autoclaved, which is important for conducting biological assays using bacteria or cells on μPADs.
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.
Hydrophobic barriers are one of the key elements of microfluidic paper based analytical devices (μPADs).μPADs are simple and cost efficient and they can be carried out without the need of high standard laboratories. To carry out such a test a method is needed to create stable hydrophobic barriers. Commonly used methods like printing wax or polystyrene have the major drawback that these barriers are stiff and break if bended which means they will no longer be able to retain a liquid sample. Here we present silanes to structure hydrophobic barriers via polycondensation and show a silanization method which combines the advantages of flexible silane/siloxane layers with the short processing times of UV-light based structuring. The barriers are created by using methoxy silanes which are mixed with a photo acid generator (PAG) as photoinitiator. Also a photosensitizer was given to the mixture to increase the effectiveness of the PAG. After the PAG is activated by UV-light the silane is hydrolyzed and coupled to the cellulose via polycondensation. The created hydrophobic barriers are highly stable and do not break if being bended.
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.
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. <p> </p>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. <p> </p>This method enables the creation of complex, highly specific bioactive protein patterns and greatly facilitates the production of μPADs.
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.
There are two main challenges in producing a robust, paper-based analytical device. The first one is to create a hydrophobic barrier which unlike the commonly used wax barriers does not break if the paper is bent. The second one is the creation of the (bio-)specific sensing layer. For this proteins have to be immobilized without diminishing their activity. We solve both problems using light-based fabrication methods that enable fast, efficient manufacturing of paper-based analytical devices. The first technique relies on silanization by which we create a flexible hydrophobic barrier made of dimethoxydimethylsilane. The second technique demonstrated within this paper uses photobleaching to immobilize proteins by means of maskless projection lithography. Both techniques have been tested on a classical lithography setup using printed toner masks and on a lithography system for maskless lithography. Using these setups we could demonstrate that the proposed manufacturing techniques can be carried out at low costs. The resolution of the paper-based analytical devices obtained with static masks was lower due to the lower mask resolution. Better results were obtained using advanced lithography equipment. By doing so we demonstrated, that our technique enables fabrication of effective hydrophobic boundary layers with a thickness of only 342 μm. Furthermore we showed that flourescine-5-biotin can be immobilized on the non-structured paper and be employed for the detection of streptavidinalkaline phosphatase. By carrying out this assay on a paper-based analytical device which had been structured using the silanization technique we proofed biological compatibility of the suggested patterning technique.