Name: David Jones

Email: d.jones@sheffield.ac.uk

CompanyName: Sheffield University


Country: United Kingdom

Abstract: This paper considers the measurement of the impedance spectrum of epithelial tissue using
surface probes. Epithelial tissue provides a particular challenge because one of its
functions is to act as barrier to the flow of ions, and hence to the flow of low frequency
electrical current. This makes it difficult to force current to flow in the tissue itself rather
than being short-circuited by any conductive fluid, such as blood or mucus, which might
be present on the surface of the epithelium. This acts as a confounding factor to the
investigation of two attributes of interest of the epithelium, which are seen at different
frequencies:

Attribute 1 (<1kHz): the d.c. resistance across the surface cell layer, which gives an
indication of the integrity of the epithelial barrier.
Attribute 2 (1-100kHz): the amount of lamina propria underlying the surface cell layer.
The lamina propria provides a low resistance path in which the current does not have to
continually cross cell membranes.

Two impedance probes have been constructed to measure these epithelial attributes:

Probe A: consists of 8 electrodes of diameter 0.4mm spaced equally about a circle of
diameter 1.8mm, allowing several different tetrapolar measurements to be made. This
probe has been used to make measurements on ex-vivo samples of oesophageal tissue,
including normal squamous tissue and a range of abnormalities, such as columnar
metaplasia, dysplasia and carcinoma. There is a clear difference between squamous and
columnar tissue. However, the differences between other grades of tissue (the
metaplasia/dysplasia classification being of particular clinical interest) are much less
pronounced. The impedance at 2kHz was seen to increase by a factor >2 after washing the
tissue surface with low conductivity tap water. Finite element analysis reveals a complex
inter-relationship between the epithelial attributes, the surface fluid, and the measured
impedance spectrum, which gives rise to problems in the interpretation of experimental
results.
Probe B: consists of a circular electrode concentric with a series of ring electrodes and
makes a tripolar measurement. The central drive electrode is surrounded by two receive
electrodes, then a guard electrode, and then a large outer ground electrode. The guard
electrode prevents lateral current flow through the surface fluid, and current is forced
down into the tissue. Finite element modelling shows that the surface fluid has little effect
on the measurement, and it is possible to relate the impedance spectra to the epithelial
attributes.

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