The newly designed three-dimensional micro-interdigital structures are used in the context of several projects to firstly measure the conductivity of liquids and suspensions in order to analyse their composition with the combination of other analytical methods. This is interesting for use in biogas plants, for example. Secondly, these switchable micro-interdigital structures offer the possibility to distinguish between suspensions and the cells in them (joint project MASTITIS with the Institut für Bioprozess- und Analysenmesstechnik e.V. Heiligenstadt). Thirdly, such an arrangement can be used to monitor the cell growth of cell cultures. This is interesting, for example, for biological monitoring to carry out toxicity tests.
The spacing of the realised comb structures ranges from a few μm down to slightly over one μm and the height of the combs is between 4 and 5 μm. They were made of highly doped polysilicon. The equipment works perfectly up to a frequency of one MHz without any detectable disturbing influences on the measurement by the electronic components.
Firstly, the arrangement can optionally be connected directly to an LCR meter via four SMA sockets; alternatively, the data evaluation can also take place with a chip of the type AD5933, a 12-bit impedance converter of the company Analog Devices, which is located directly on the board. With this chip, the measurements are currently limited to 100 kHz. Therefore, in the near future, a chip is to be used for this purpose that works in a frequency range of up to 1 MHz. The chips with the interdigital structures on the board are easy to replace and can also be replaced by other technologies. They are located under the PEEK flow-through clip visible in the picture above, but this can also be omitted as an option.
Alternatively, structures made of gold on glass substrate or interdigital fingers made of MoSi2, which have also been provided with an ALD layer made of TiO2, can be used. Interdigital structures made of gold are a standard technology. However, the other variants have the advantage over gold that they are mechanically more robust. In addition, the 3-D structures operate in a much lower frequency range.