Evaluating object and region of concentric electrode in bio-electrical impedance measurement

Engineering

Electrical Engineering fields

Okayama University Year 2003

Evaluating object and region of

concentric electrode in bio-electrical

impedance measurement

Yoshitake Yamamoto Kiyoko Shirai Noriko Goda

Okayama University Okayama University Okayama Univeristy

Takao Nakamura Toshimasa Kusuhara Hiroyuki Okuda

Okayama University Okayama University Okayama University

This paper is posted at eScholarship@OUDIR : Okayama University Digital Information Repository.

EVALUATING OBJECT AND REGION OF CONCENTRIC ELECTRODE IN

BIO-ELECTRICAL IMPEDANCE MEASUREMENT

Yoshitake YAMAMOTO, Kiyoko

SHIRAI,

Noriko GODA, Takao NAKAMURA,

Toshimasa KUSUHARA, Hiroyuki OKUDA

Faculty of Health Sciences, Okayama University Medical School. JAPAN

Ahsfracf-Concentric electrode is easy to use and used widely for measuring b i d e d d impedance B$ b evaluating region was

notinvestigatedinderiuZThe~mecharaaerirtia~mneentric~

were studied h m v+ pints ofview. In c a ~ e of use witfmut dRbode

we,

dedmde and skin wfiace M e r all h.pquenry ' ~ " g e In c a ~ e of use with electrode past, impedance is composed of sh-mm corneuminthe h-qunry range of20E-lkHz and is mainly mmpafied ofsubcutaneous

ating q i o n ofmnmmic dfftrode is a m khan the radius or the gap o f m t e r elecrmde

Keywords -Bio-eleetrieal impedance, skin impedance, COOCPO-

tric electrode, evaluating region

1,lNTRODUCTlON

~ h m e - o f 2 0 0 1 i H z - l l \ ~ I n m e h i g h f r e q " ~ - R l t l u -

The method measuring bio-electrical impedance using concentric electrode in local part non-invasively is widely applied. Although detecting bio-impedance varies by elec- trode size, frequency, and types of electrode paste, the de- tail results have not been cleared yet. Then these points were investigated through some experiments such as adhe- sive tape stripping. This paper shows some concrete yard- sticks as evaluating object and region based on the results.

11. METHOD AND MATERIALS

Gold concentric electrode as shown in Fig.1 which is used for admittance skin moisturization meter[l][2] was investigated about evaluating region toward depth. Measurements were camed to capacitance C, at frequency lMHz with various depth d of tank filled with distilled water. The results are shown in Fig.2 as the ratio to CO for large value of d. The electrode gaps were gl=2.5mm, g 2 4 m m and g3=30mrn. Next, skin admittance of foream under various conditions were measured at frequency from IOHz to 3.5MHz, whose results shown in Fig.3. Finally, vector impedances of foream with cream electrode paste were measured from 20Hz to lMHz in normal skin and 15 times stripping skin with adhesive tape. Gap length was

IOmm. The results are shown in Fig.4.

b = 10

r n m 6

c = 2 0 m r n $

g = 2.5 rnm

Fiz. I The ~ o n ~ i r n ~ t i o n of concentric elecnode

lot 100 -

-

-E

0"

-

-

Fi9.l Influences of test material thiehnpss. .stripping 9%NaCl .- E normal (dry)

P

Fig.: Frequency characteristics of skin impedance.

111. RESULTS AND DISCUSSIONS

In the case of standard electrode as shown in Fig.1, ca- pacitance was 8.92pF for enough size of depth and the capacitance for limited size water field was decreased shown in Fig.2. But, the change of capacitance depend on

different gap are small. In case of 3mm of material thick- ness, the decrease of capacitance is about 4%. Then, the

evaluation region of homogeneous material is about 3mm depth. The depth of current pass become deeper with lar- ger gap length. However, the contribution of wide spread- ing current area to total impedance is not so large. In the high frequency range, evaluating region of concentric elec- trode is the area less than the radius or the gap of center electrode.

From the skin impedances of normal condition and 15 times stripping the ratio o f contribution of stratum corneum to total skin impedance were evaluated in every frequency point as shown in Fig.5. From 20Hz to lkHz stratum corneum impedance is dominant. At higher fre- quency the contribution of subcutaneous tissue increases and at over 200kHz subcutaneous tissue impedance is al- most whole. It depends that the subcutaneous tissue im- pedance is almost same level over all frequency hand and stratum corneum impedance decreases largely with the

0-7803-7944-6/03/$17.0002003 IEEE

20

-

r

f

6

r

Fig.4 Vecior impedance loci ainormal skin impedance and IS times s i p p i n g .

-

=:

-

Rg.5 Contribution from stratum comeom impedance ioor total skin impedance.

increasing frequency[3][4]. Almost same result was re- ported as the result of finite elemental method[5].

In skin inoisturization meter measuring the admittance without paste, the impedance in low frequency band should be used on theory. Howewr, the imrledance in high frequency band is used paradoxically because of the influ- ence of ion in low frequency band. In low frequency hand, the ion influences not only to the resistance but also to the capacitance by the reason of a dispersiori[h]. Then, it

shows in incorrect moisturization data. The principle of skin moisturization measurement is application of not elec- trical properties but mechanical properties of skin where

skin moisturization varies the contactness between elec- trode and skin surface. For the measurerlent of stratum corneum impedance low frequency 20Hz-1 kHz is desired. And at high frequency band, it needs rejection of subcuta- neous tissue impedance.

In the measurenient of the characteristic:j of subcutane- ous tissue, the electrode must be contact perfectly to skin, and the two electrode system needs the frequency band of 200kHz-1MHz. Funher, the stripping of stratum corneum is very effective for precision measurement.

IV. CONCLUSION

Concentric electrode is easy tO use and is used widely

for measuring bio-electrical impe83ance.

( I ) From 20Hz to lkHz stratum corneum impedance is

the majority in the skin. At higher frequency the contribu- tion of subcutaneous tissue increases and ;a1 oYer 2OOkHz subcutaneous tissue impedance is almost whole.

(2) In skin moisturization meter measuring the admit-

tance without paste, the impedance in low frequency band should be used in theory. Howzever, the impedance in high frequency band is used paradoxically because of the influ- ence of ion in low frequency band.

(3) For the measurement of stratum corneum impedance low frequency 2OHz-lkHz is desired. And at high fre- quency band, it needs rejection of subcutarieous tissue im- pedance.

(4) In the measurement of the ,characteristics of subcu- taneous tissue, the electrode must be contact perfectly to skin, and the two electrode system needs: the frequency band of 200kHz-1MHz. The eva.luating rgjm ofconentic

elearodeisthearealessthantheradi~orthe~cdcenterelfftrode.

REFERENCES

[I] Y . Yamamoio. T. Yamamoto. and T. Ozawa. Characteristics o i

Skin Admittance for Dry Electrodes and the M~asuremenr of Skin Moin- tunration. Med, & B i d Ens. 6. Comp,r.. vol. 24, pp.71-77> 1986.

[?I Y. Yamamoto. Measurement and Analysis of Skin Electrical h- pedmce. Acm D e m r o Veneredogica. vol. 185. pp.3'1-38, 1994. (31 T. Yamamoto and Y. Yamamoto. Ekctrical Properlies of the Epi-

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5 0 0 , 1976.

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