Impedance-based detection of extracellular DNA in wounds

Wound infection status is a relevant diagnostic parameter to enhance wound treatment towards better healing rate. Impedance evaluation is a powerful tool to measure the inflammatory response like the released DNA of neutrophils. In our research we investigated the dielectric behaviour of neutrophils settled on electrodes in vitro. The cells have been stimulated to react in the same way as in a wound infection. The result is a significant impedance deviation of about 50 % with comparable amount of cells like in an infected wound. Microscopic fluorescence verifications acknowledge these findings.

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Infection Monitoring in Wounds

Wound infection monitoring is a challenging task. It is only solvable by designing an integrable and cost-efficient sensor which measures a relevant set of parameters. One viable parameter is the formation of neutrophil extracellular traps (NETs). Their task is trapping pathogens in the wound. A wound infection results in massive release of them which can be detected with impedimetric methods. Our investigations focused on the characterization of the biological process with an in vitro model. The model environment is a cell culture with neutrophil granulocytes cultured on interdigitated electrodes which represent the sensor surface. Detected impedance changes caused by NET-formation were in the range of 35% and even higher. This implies that impedance measurements are suitable for NET detection. We derived a measurement and evaluated it by differing conditions like changing stimulation agent and varying the cell number. For both conditions the results of impedance and phase angle deviation can be confirmed. In combination with other parameters a sensor can be designed for specific detection of wound infections. These aspects are integrated in our sensor concept.

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Miniaturized wound sensors based on detection of extracellular chromatin

Sensors for monitoring wound infections are important to improve care management especially for chronic wounds. As detection parameter the formation of extracellular chromatin was chosen which has characteristic dielectric properties in ionic solvents due to its bound negative charges. Experiments with planar electrodes resulted in a high impedance increase of nearly 450%. The analysis of the relative permittivity revealed a cut-off frequency at 5 kHz. It is shown for the very first time that the changing electrical medium properties during Neutrophil Extracellular Traps (NET) formation are relevant for the occurring dispersion. A textile sensor set-up is proposed to fulfill the requirements of miniaturization and bio-compatibility. With these experimental results it is possible to design a fiber-based sensor based on an impedance detection principle.

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Impedance Model of Immune Reaction Leading to NETosis

The so-called NETs (neutrophil extracellular traps) are found in vast amounts in inflamed wounds and are therefore a good marker for wound infections. They are a product of an immune reaction. Their integrant DNA has a certain dielectric behavior due to its charge. This allows a direct electric determination without the need of a transducer. Human neutrophils were used to measure the release of NETs in vitro. However, the structural changes of the cells during this process have to be taken into account. In this work a model was developed which reflects these changes. This model was compared with impedance measurements. We found that changes in the medium composition strongly modify the dielectric behavior of the system. The most obvious change here is caused by the appearance of the NETs. These changes remain also stable after the cells died and did not undergo more structural changes. The measurement of NETs is a very promising approach to support the diagnosis of inflammation processes especially in wounds.

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