Frank Bunge

The observation of a cell culture and its growth is a very important part of diagnostics in medical applications as well as biological applications. Especially medical applications benefit from a fast knowledge of the existence of bacteria growth. It can be used for the development of antibiotics as well as for the diagnostics of diseases. The recovered time can be vital for the success of a treatment. We present an approach and device to monitor on-chip bacteria growth based on the absorption measurement with a low limit of detection. A low limit of detection, high sensitivity and a small sample size define the capability of sensing the initial growth rate of small bacteria samples within a few hours. Because of the small height of microfluidic channels, a standard optical density method is not applicable. In our approach, the optical path is 20 times longer compared to an out-of-plane approach by performing an in-plane optical density measurement so that the sensitivity is improved and thereby the limit of detection is lowered. An LED (580 nm centre wavelength) is used to propagate light through a sample in the measurement channel. The passing light intensity is measured at the outlet by a photo diode. The relation between the absorbed light and the bacteria concentration agrees well with the theory and a growth curve can be recorded. A particular focus is laid on a reproducible setup based on 3D-printed holders where external disturbances such as ambient light are minimized. Because of the increased sensitivity, the limit of detection is only 1.5×106 bac/ml. By applying the method of this contribution, additional standard laboratory operations can be integrated into chips.

An additively manufactured three-dimensional mixing element (3DME) integrated in a microchannel is presented. The 3DME increases the interfacial contact-surface by swapping the positions of the incoming streaming liquids. The novel design incorporates important design alterations to a prior prototype. These alterations significantly improve the integration of microelements written by two-photon polymerization (2pp) into photolithographically defined microchannels. Anchor-structures for increased strength as well as a roof to compensate for height-discrepancies between the lithographically defined microchannel and the 2pp-written 3DME are addressed. These allow for a better positioning as well as an increased sealing reliability between the top of the 3DME and a PDMS-lid which in our case is used to close the microchannel.

Microfluidic cell cultures are often used in academic research but only rarely in pharmaceutical research because of unsuitable designs, inappropriate choice of materials or incompatibility with standard equipment. In particular, microfluidic cell cultures to control the gaseous microenvironment rely on PDMS despite its disadvantages. We present a novel concept for such a cell culture device that addresses these issues and is made out of hard materials instead of PDMS. Our device contains two microfluidic chambers that are separated by a porous membrane of anodized aluminum oxide. Because of the small pore sizes but high porosity, this design allows a gas supply from one chamber to the other while leakage of the medium is avoided. Furthermore, the cells can be cultured directly on the membrane which induces the same advantageous cell response as cultivation on very soft materials. Furthermore, the chip, made out of silicon and glass, is fabricated with clean-room technologies and thus allows mass production. The interfaces to the outer world are small reservoirs which are accessible with conventional pipettes so that the setup does not require any pump. The fabricated chip is characterized regarding its diffusion characteristics. HaCaT-cells are cultivated successfully up to 14 days inside the chip but can be also removed for further processes. The presented chip is a step to bring cell cultivation with controlled gas supply from academic to industrial applications.

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.

Long-term measurements of dissolved oxygen in microfluidic systems are challenging because of the photo-oxidation of the required sensing film which consumes oxygen and because of the fluctuation of temperature that increases the uncertainty. We present a microfluidic chip made of glass and silicon with integrated phosphorescent elements composed from a matrix out of silica gel and an oxygen sensitive dye (PtTFPP). Our device has a 36 times lower oxygen consumption rate compared to conventional films, which uses an organic matrix. Furthermore, the influence of the temperature is eliminated by integrating a temperature sensor and heating elements on the chip.

A new method to monitor the relative flow rates of two different liquids in a microchannel is presented. By using a simple out-of-plane interferometer setup, shifts in the position of the interface between the two liquids can be determined. This label-free and real-time approach to observe the interface position yields the ratio of the flow rates. Applying an out-of-plane geometry allows the use of a flexible setup of chip-external optical components such as a laser and an image sensor, which is an advantage over fixed in-plane optics. With this measurement method we are able to monitor a change of about 1% of the relative flow rate between two fluidic inlets.

This paper reports a method to integrate silica aerogels monolithically in microfluidic chips. Silica Aerogel is a highly porous bulk material. The gel was synthesized from tetraethyl orthosilicate by a sol-gel process. Polyethylene glycol and an extended aging period were used to strengthen the matrix minimizing gel shrinkage. This technique allows alcogel structures with high strength and stiffness that withstand high pressure during the subcritical drying process. Hexamethyldisilazane provides for hydrophobizing and prevents the formation of siloxane bonds during the drying process. The resulting transparent aerogels reach porosities of 85%, pore diameters around 50 nm and contact angles of 136°.

Lab-on-a-Chip (LoC) applications for the long-term analysis of mammalian cells are still very rare due to the lack of convenient cell cultivation devices. The difficulties are the integration of suitable supply structures, the need of expensive equipment like an incubator and sophisticated pumps as well as the choice of material. The presented device is made out of hard, but non-cytotoxic materials (silicon and glass) and contains two vertical arranged membranes out of hydrogel. The porous membranes are used to separate the culture chamber from two supply channels for gases and nutrients. The cells are fed continuously by diffusion through the membranes without the need of an incubator and low requirements on the supply of medium to the assembly. The diffusion of oxygen is modelled in order to find the optimal dimensions of the chamber. The chip is connected via 3D-printed holders to the macroscopic world. The holders are coated with Parlyene C to ensure that only biocompatible materials are in contact with the culture medium. The experiments with MDCK-cells show the successful seeding inside the chip, culturing and passaging. Consequently, the presented platform is a step towards Lab-on-a-Chip applications that require long-term cultivation of mammalian cells.

In this contribution, we focus on the analysis of complete omnidirectional acoustic band gaps in additively manufactured three-dimensional (3D) phononic crystals. We present a numerical analysis of band structure and phononic band gaps of different cubic unit cell geometries. For validation, we report experimental results for transmission of acoustic waves in different characteristic spatial directions through various phononic crystal samples. These results are supplemented by numerical transmission analysis. The elements form the building blocks of wideband, high-resolution phononicfluidic systems for measuring physical properties such as fluid density, speed of sound, and concentration.