This paper reviews our recent work on vibrating sensors for the physical properties of fluids, particularly viscosity and density. Several device designs and the associated properties, specifically with respect to the sensed rheological domain and the onset of non-Newtonian behavior, are discussed.

This paper describes the working principle of a DC magnetohydrodynamic (MHD) micropump that can be operated at high DC current densities (J) in 75-µm-deep microfluidic channels without introducing gas bubbles into the pumping channel. The main design feature for current generation is a micromachined frit-like structure that connects the pumping channel to side reservoirs, where platinum electrodes are located. Current densities up to 4000 A m⁻² could be obtained without noticeable Joule heating in the system. The pump performance was studied as a function of current density and magnetic field intensity, as well as buffer ionic strength and pH. Bead velocities of up to 1 mm s⁻¹ (0.5 µL min⁻¹) were observed in buffered solutions using a 0.4 T NdFeB permanent magnet, at an applied current density of 4000 A m⁻². This pump is intended for transport of electrolyte solutions having a relatively high ionic strength (0.5–1 M) in a DC magnetic field environment. The application of this pump for the study of biological samples in a miniaturized total analysis system (µTAS) with integrated NMR detection is foreseen. In the 7 T NMR environment, a minimum 16-fold increase in volumetric flow rate for a given applied current density is expected.

In this work, we present a comprehensive theoretical and experimental study of three-dimensional phononic crystals arranged in a simple cubic lattice. The band structure is analytically modeled as a 3D mass spring system and numerically calculated within the corresponding simple cubic Brillouin zone. We report on a design yielding a record bandgap of 166% relative width, validated by simulations and measurements of longitudinal and shear wave transmission in different spatial directions. In the additively fabricated samples, gap suppression reaches −80 dB relative to a solid reference. Comparison of different unit cell geometries showcases approaches to engineer gap width and suppression, as well as transmission bands outside the gap.

The connection of microfluidic devices to the outer world by tubes and wires is an underestimated issue. We present methods based on 3D printing to realize microfluidic chip holders with reliable fluidic and electric connections. The chip holders are constructed by microstereolithography, an additive manufacturing technique with sub-millimeter resolution. The fluidic sealing between the chip and holder is achieved by placing O-rings, partly integrated into the 3D-printed structure. The electric connection of bonding pads located on microfluidic chips is realized by spring-probes fitted within the printed holder. Because there is no gluing or wire bonding necessary, it is easy to change the chip in the measurement setup. The spring probes and O-rings are aligned automatically because of their fixed position within the holder. In the case of bioanalysis applications such as cells, a limitation of 3D-printed objects is the leakage of cytotoxic residues from the printing material, cured resin. This was solved by coating the 3D-printed structures with parylene-C. The combination of silicon/glass microfluidic chips fabricated with highly-reliable clean-room technology and 3D-printed chip holders for the chip-to-world connection is a promising solution for applications where biocompatibility, optical transparency and accurate sample handling must be assured. 3D printing technology for such applications will eventually arise, enabling the fabrication of complete microfluidic devices.

While phononic crystals can be theoretically modeled with a variety of analytical and numerical methods, the practical realization and comprehensive characterization of complex designs is often challenging. This is especially important for the nearly limitless possibilities of periodic, three-dimensional structures. In this contribution, we take a look at these design and fabrication challenges of different 3D phononic elements based on recent research using additive manufacturing. Different fabrication technologies introduce specific limitations in terms of, e.g., material choices, minimum feature size, aspect ratios, or support requirements that have to be taken into account during design and theoretical modeling. We discuss advantages and disadvantages of additive technologies suitable for millimeter and sub-millimeter feature sizes. Furthermore, we present comprehensive experimental characterization of finite, simple cubic lattices in terms of wave polarization and propagation direction to demonstrate the substantial differences between complete phononic band gap and application oriented directional band gaps of selected propagation modes.