Bastian E. Rapp

Let there be chip—towards rapid prototyping of microfluidic devices: one-step manufacturing processes

Microfluidics is an evolving scientific field with immense commercial potential: analytical applications, such as biochemical assay development, biochemical analysis and biosensors as well as chemical synthesis applications essentially require microfluidics for sample handling, treatment or readout. A number of techniques are available to create microfluidic structures today. On industrial scale replication techniques such as injection molding are the gold standard whereas academic research mostly focuses on replication by casting of soft elastomers such as polydimethylsiloxane (PDMS). Both of these techniques require the creation of a replication master thus creating the microfluidic structure only in the second process step—they can therefore be termed two-(or multi-)step manufacturing techniques. However, very often the number of pieces to be created of one specific microfluidic design is low, sometimes even as low as one. This raises the question if two-step manufacturing is an appropriate choice, particularly if short concept-to-chip times are required. In this case one-step manufacturing techniques that allow the direct creation of microfluidic structures from digital three-dimensional models are preferable. For these processes the number of parts per design is low (sometimes as low as one), but quick adaptation is possible by simply changing digital data. Suitable techniques include, among others, maskless or mask based stereolithography, fused deposition molding and 3D printing. This work intends to discuss the potential and application examples of such processes with a detailed view on applicable materials. It will also point out the advantages and the disadvantages of the respective technique. Furthermore this paper also includes a discussion about non-conventional manufacturing equipment and community projects that can be used in the production of microfluidic devices.

Biosensors with label-free detection designed for diagnostic applications

AbstractSince the first biosensor was introduced in 1962 by Clark and Lyons, there has been increasing demand for such analytical devices in diagnostic applications. Research initially focussed mainly on detector principles and recognition elements, whereas the packaging of the biosensors and the microfluidic integration has been discussed only more recently. However, to obtain a user-friendly and well-performing analytical device, those components have to be considered all together. This review outlines the requirements and the solutions suggested for the integration of suitable biosensors in packaging and the integration of those encapsulated biosensors into a microfluidic surrounding resulting in a complete and efficient analytical device for diagnostic applications. The components required for a complete biosensor instrument are described and the latest developments which meet the requirements for diagnostic applications, such as single-use components and arrays for multiparameter detection, are discussed. The current state and the future of biosensors in the field of clinical diagnostics are outlined, particularly on the basis of label-free assay formats and the detection of prominent biomarkers for cancer and autoimmune disorders. Figure CaptionComponents to be considered in an efficient biosensor system

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.

Advances in DNA-directed immobilization

DNA-directed immobilization (DDI) of proteins is a chemically mild and highly efficient method for generating (micro)structured patterns of proteins on surfaces. Twenty years after its initial description, the DDI method has proven its robustness and versatility in numerous applications, ranging from biosensing and biomedical diagnostics, to fundamental studies in biology and medicine on the single-cell level. This review gives a brief summary on technical aspects of the DDI method and illustrates its scope for applications with an emphasis on studies in cell biology.

Maskless Projection Lithography for the Fast and Flexible Generation of Grayscale Protein Patterns

Protein patterns of different shapes and densities are useful tools for studies of cell behavior and to create biomaterials that induce specific cellular responses. Up to now the dominant techniques for creating protein patterns are mostly based on serial writing processes or require templates such as photomasks or elastomer stamps. Only a few of these techniques permit the creation of grayscale patterns. Herein, the development of a lithography system using a digital mirror device which allows fast patterning of proteins by immobilizing fluorescently labeled molecules via photobleaching is reported. Grayscale patterns of biotin with pixel sizes in the range of 2.5 μm are generated within 10 s of exposure on an area of about 5 mm(2) . This maskless projection lithography method permits the rapid and inexpensive generation of protein patterns definable by any user-defined grayscale digital image on substrate areas in the mm(2) to cm(2) range.

Bioinspired air-retaining nanofur for drag reduction.

Bioinspired nanofur, covered by a dense layer of randomly distributed high aspect ratio nano- and microhairs, possesses superhydrophobic and air-retaining properties. Nanofur is fabricated using a highly scalable hot pulling method in which softened polymer is elongated with a heated sandblasted plate. Here we investigate the stability of the underwater air layer retained by the irregular nanofur topography by applying hydraulic pressure to the nanofur kept underwater, and evaluate the gradual changes in the air-covered area. Furthermore, the drag reduction resulting from the nanofur air retention is characterized by measuring the pressure drop across channels with and without nanofur.

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.

Synthetic enzyme supercomplexes: co-immobilization of enzyme cascades

A sustainable alternative to traditional chemical synthesis is the use of enzymes as biocatalysts. Using enzymes, different advantages such as mild reaction conditions and high turnover rates are combined. However, the approach of using soluble enzymes suffers from the fact that enzymes have to be separated from the product post-synthesis and can be inactivated by this process. Therefore, enzymes are often immobilized to solid carriers to enable easy separation from the product as well as stabilization of the enzyme structure. In order to mimic the metabolic pathways of living cells and thus to create more complex bioproducts in a cell-free manner, a series of consecutive reactions can be realized by applying whole enzyme cascades. As enzymes from different host organisms can be combined, this offers enormous opportunities for creating advanced metabolic pathways that do not occur in nature. When immobilizing this enzyme cascades in a co-localized pattern a further advantage emerges: as the product of the previous enzyme is directly transferred to its co-immobilized subsequent catalyst, the overall performance of the cascade can be enhanced. Furthermore when enzymes are in close proximity to each other, the generation of by-products is reduced and obstructive effects like product inhibition and unfavorable kinetics can be disabled. This review gives an overview of the current state of the art in the application of enzyme cascades in immobilized forms. Furthermore it focuses on different immobilization techniques for structured immobilizates and the use of enzyme cascade in specially designed (microfluidic) reactor devices.

Connecting microfluidic chips using a chemically inert, reversible, multichannel chip-to-world-interface

In this paper we present a reusable, chemically inert, multichannel Chip-to-World-Interface (CWI). The concept of this interface is based on a force fit connection similar to the hollow screw connectors known from high-performance liquid chromatography (HPLC) instruments. It allows contamination free connection of up to 100 thermoplastic tubes to microfluidic chips made from various materials e.g., epoxy polymers, glass and polydimethylsiloxane (PDMS). The spacing of the tubes is fixed whereas the outer dimensions of the CWI can be adapted to the microfluidic chip it should be used with. We demonstrate that such a CWI with 100 tubes is pressure-tight up to (at least) 630 kPa (6.3 bar) pressure and the connection easily sustains flow rates above 4 ml min(-1). The presented CWI is designed such that the fluid probed in the microfluidic chip is in direct contact only with the tube material and the material from which the microfluidic chip is made. This not only enables fluid transport without dead volume, it also ensures that CWI itself will not be contaminated or contaminate the samples being probed. Using polytetrafluoroethylene (PTFE, Teflon®) tubing we demonstrate that the CWI can even be used with harsh organic solvents such as dichloromethane or dimethylformamide during continuous solvent probing over several hours without damage to the CWI or leakage. This CWI therefore effectively allows using almost all types of organic solvents in microfluidic applications.

Microfluidics on liquid handling stations (μF-on-LHS): an industry compatible chip interface between microfluidics and automated liquid handling stations

We describe a generic microfluidic interface design that allows the connection of microfluidic chips to established industrial liquid handling stations (LHS). A molding tool has been designed that allows fabrication of low-cost disposable polydimethylsiloxane (PDMS) chips with interfaces that provide convenient and reversible connection of the microfluidic chip to industrial LHS. The concept allows complete freedom of design for the microfluidic chip itself. In this setup all peripheral fluidic components (such as valves and pumps) usually required for microfluidic experiments are provided by the LHS. Experiments (including readout) can be carried out fully automated using the hardware and software provided by LHS manufacturer. Our approach uses a chip interface that is compatible with widely used and industrially established LHS which is a significant advancement towards near-industrial experimental design in microfluidics and will greatly facilitate the acceptance and translation of microfluidics technology in industry.