AC Impedance Spectroscopy (ACIST)

The first application for which ACIST has been developed is in the detection of dental decay. Tooth enamel is a structure made from hydroxyapatite (a crystalline form of calcium phosphate) with a highly ordered morphology. The impedance of a healthy tooth is very high due to relatively low ionic conduction. However, as a tooth demineralises it starts to lose some of its regular structure from beneath the tooth’s surface zone and increasingly larger pores are formed as minerals are eluted out. If the lesion progresses, the pores can connect with each other and the tooth becomes a mix of highly conductive parts (the fluid filled pores) and the low conductive enamel, so impedance decreases. As the decay progresses, dentine becomes involved and cavities are formed, the impedance falls further.

The key to evaluating dental tissue in this way has been to pass a very low current (undetectable to the patient) through a tooth and to measure the impedance of the charge passing through it.

The variance in signals between the different stages of decay is significant. The impedance measured in a healthy tooth is significantly greater than that of a tooth that is demineralising which is itself significantly greater than that of a tooth with a cavity. As a result the sensitivity (ability to find caries when it is present) and specificity (ability to detect healthy tissue, or lack of false positives) of the ACIST system in distinguishing these different stages is very high.

The Measurement Method

In recent years the specific AC measurement technique of ACIST has established itself as a key method in fundamental and applied electrochemistry as well as in materials science. The measurement relies on the application of a small alternating signal through the sample while monitoring the response. In many systems the response varies as the frequency of the applied signal changes. By changing the frequency of the applied signal, an impedance spectrum is obtained that is unique to the sample under investigation, thus providing valuable insights into its physical and chemical properties.

There are generally two approaches for measurement of impedance spectra, either by sequentially sweeping across the frequency range or by pulsing all the frequencies simultaneously in the time domain.

When applied to a clinical application, ACIST can detect and monitor changes in the physical properties of biological material such as teeth, bone and soft tissue without using X-rays.

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