Robust Soil Water Potential Sensor to Optimize Irrigation in Agriculture.

Extreme weather phenomena are on the rise due to ongoing climate change. Therefore, the need for irrigation in agriculture will increase, although it is already the largest consumer of water, a valuable resource. Soil moisture sensors can help to use water efficiently and economically. For this reason, we have recently presented a novel soil moisture sensor with a high sensitivity and broad measuring range. This device does not measure the moisture in the soil but the water available to plants, i.e., the soil water potential (SWP). The sensor consists of two highly porous (>69%) ceramic discs with a broad pore size distribution (0.5 to 200 µm) and a new circuit board system using a transmission line within a time-domain transmission (TDT) circuit. This detects the change in the dielectric response of the ceramic discs with changing water uptake. To prove the concept, a large number of field tests were carried out and comparisons were made with commercial soil water potential sensors. The experiments confirm that the sensor signal is correlated to the soil water potential irrespective of soil composition and is thus suitable for the optimization of irrigation systems.

Online tissue discrimination for transcutaneous needle guidance applications using broadband impedance spectroscopy.

This paper reports on a novel system architecture for measuring impedance spectra of a biological tissue close to the tip of a hollow needle. The measurement is performed online using fast broadband chirp signals. The time domain measurement raw data are transformed into the transfer function of the tissue in frequency domain. Correlation technique is used to analyze the characteristic shape of the derived tissue transfer function with respect to known "library functions" for different types of tissue derived in earlier experiments. Based on the resulting correlation coefficients the exact type of tissue is determined. A bipolar coaxial needle is constructed, simulated by finite element method and tested during various in vitro and in vivo experiments. The results show a good spatial resolution of approximately 1.0 mm for a needle with a diameter of 2.0 mm. The correlation coefficients for the three tested tissue types muscle, fat, and blood allow for a clear tissue classification. Best results have been obtained using the characteristic phase diagrams for each tissue. Correlated to the corresponding library transfer function the coefficients are in the range of +0.96 to +0.99 for the matching tissue. In return, the resulting coefficients for correlation with nonmatching tissues are in the range of -0.93 to +0.81.

Real-time cannula navigation in biological tissue with high temporal and spatial resolution based on impedance spectroscopy.

In many medical applications a well-directed positioning of a cannula in body tissue is mandatory. Especially the accurate placing of the cannula tip in the tissue is important for efficient drug delivery or for accessing blood vessels and nerves. This paper presents a new approach for a universal cannula navigation system based on tissue classification on the cannula tip by impedance spectroscopy. The cannula serves as coaxial, open ended waveguide which is connected to remote measurement equipment. Objective of the new system is to reach a high spatial and temporal resolution for dynamic cannula guidance. Therefore the proposed coaxial cannula design has been analyzed by Finite Element Simulation to investigate the sensitivity of the cannula tip. For fast tissue impedance spectrum measurement the Time-Domain-Reflectometry method is used in order to achieve a high temporal resolution. Measurement data derived in the laboratory is analyzed and interpreted using the general Cole-Cole model for tissue. Based on the results we propose to use a chirp signal for impedance measurement in order to improve the sensitivity of the system towards specific tissue properties.

Capacitive on-line hematocrit sensor design based on impedance spectroscopy for use in hemodialysis machines.

This paper presents a new design for an on-line and in-line hematocrit (HCT) sensor. Special feature of the sensor is the capability to measure the hematocrit of a blood sample inside standard plastic tubing widely used in medical equipment. No blood sample has to be extracted out of existing extracorporeal blood circulation systems such as hemodialysis machines or heart-lung machines. The sensor principle is based on electrical impedance spectroscopy. Dielectric properties of the blood and the plastic tubing are measured at various frequencies. In order to optimize the sensitivity, a unique electrode configuration is developed and optimized by Finite Element Simulation. The new electrode design optimizes the overall sensitivity of the sensor towards a change in dielectric properties of the blood caused by the HCT value and therefore decreases the sensitivity to side effects caused by temperature drift and component tolerances. As a result of the optimized overall sensor performance the complexity of a sensor readout circuitry can be reduced to a minimum which leads to an unmatched price-performance ratio for a complete measurement system.