Invasive needle electrodes

Unipolar Bipolar Coax Bipolar + reference Rings 

Hollow needles are used for injection or aspiration of liquid samples. The external shaft may be insulated by, for example, lacquer layer insulation the internal tube is usually 

With such data combined with multivariate analysis, it is possible to determine the type of tissue that surrounds a needle tip and also the position of the needle tip. 

Another example of the use of needle electrodes for tissue characterization is Mishra et al. (2012), who used an impedance-sensing biopsy needle to separate between malignant and benign prostate tissue in a clinical setting. 

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The sum of activities from a nerve bundle may be picked up by skin surface electrodes (e.g., on the arm where the distance from the electrodes are not too many millimeters). Action potentials from single nerve fibers must be measured with invasive needle electrodes. They may be bipolar or unipolar, Section 7.12.3. To find the right position, the signal is monitored during insertion, often guided by sounds in a loudspeaker. 

Often the mechanical strength of the metal is an important property (e.g., in the shaft of a needle electrode). Biocompatibility may be critical if a metal comes in contact with living tissue (e.g., with metals used in sterile invasive electrodes). Flexible large electrodes of nickel may generate allergy in human skin. This can be prevented by using an electrolytic salt bridge between the electrode metal and the tissue. 

Recording activity directly from the muscle fibers themselves can be clinically valuable in identifying neuromuscular disorders [19]. Therefore, invasive electrodes are needed to access the muscle fibers or the neuromuscular junction. Fine-needle electrodes or thin stainless-steel wires are inserted or implanted to obtain local recording from the fibers or neuromuscular junctions [7]. 

Needle electrodes (Figure 26.4e) comprise a small class of invasive electrodes, used when it is absolutely essential to record from the organ itself. The most common application is in recording from muscles or muscle fibers [8]. A metallic, typically steel, wire is delivered via a needle inserted at the site of the muscle fiber. The wire is hooked and, hence, fastens to the muscle fiber, even as the needle is removed. Small signals such as motor-unit potentials can be recorded in this manner [7]. For research applications, similar needle or wire electrodes are sometimes connected directly to the heart muscle. Since such electrodes are invasive, their use is limited to only highly specialized and supervised clinical or research applications. 

The secretion of endorphins is obtained with low frequency (2—4 Hz) stimulation, corresponding to the rhythmic movement of an acupuncture needle. Instead of, or in addition to the mechanical movement, the needle is used as monopolar electrode, pulsed by a low frequency in the same frequency range (1—4 Hz). This is not a TENS method strictly speaking, because the electrode is invasive and the current not transcutaneous. 

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