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.

Thermochemical decomposition of organic materials under heat-treatment in the absence of oxygen, known as the pyrolysis process, is often employed to convert micro and nano patterned polymers into carbon structures, which are subsequently used as device components. Pyrolysis is performed at ≥900 °C, which entails substrate materials with a high thermal stability that excludes flexible, polymeric substrates. We use optimized laser radiation to pattern graphitic carbon structures onto commercially available polyimide (Kapton) sheets in the micrometer to millimeter scale by inducing a localized, rapid pyrolysis, for the fabrication of flexible devices. Resulting laser carbon films are electrically conductive and exhibit a high-surface area with a hierarchical porosity distribution along their cross-section. The material is obtained using various combinations of laser parameters and pyrolysis environment (oxygen-containing and inert). Extensive characterization of laser carbon is performed to understand the correlation between the material properties and laser parameters, primarily fluence and power. A photothermal carbonization mechanism based on the plume formation is proposed. Further, laser carbon is used for the fabrication of enzymatic, pH-based urea sensors using two approaches: (i) direct urease enzyme immobilization onto carbon and (ii) electrodeposition of an intermediate chitosan layer prior to urease immobilization. This flexible sensor is tested for quantitative urea detection down to 10⁻⁴ M concentrations, while a qualitative, color-indicative test is performed on a folded sensor placed inside a tube to demonstrate its compatibility with catheters. Laser carbon is suitable for a variety of other flexible electronics and sensors, can be conveniently integrated with an external circuitry, heating elements, and with other microfabrication techniques such as fluidic platforms.

Bulk acoustic wave (BAW) and surface acoustic wave (SAW) sensor devices have successfully been used in a wide variety of gas sensing, liquid sensing, and biosensing applications. Devices include BAW sensors using thickness shear modes and SAW sensors using Rayleigh waves or horizontally polarized shear waves (HPSWs). Analyte specificity and selectivity of the sensors are determined by the sensor coatings. If a group of analytes is to be detected or if only selective coatings (i.e., coatings responding to more than one analyte) are available, the use of multi-sensor arrays is advantageous, as the evaluation of the resulting signal patterns allows qualitative and quantitative characterization of the sample. Virtual sensor arrays utilize only one sensor but combine it with enhanced signal evaluation methods or preceding sample separation, which results in similar results as obtained with multi-sensor arrays. Both array types have shown to be promising with regard to system integration and low costs. This review discusses principles and design considerations for acoustic multi-sensor and virtual sensor arrays and outlines the use of these arrays in multi-analyte detection applications, focusing mainly on developments of the past decade.