Permittivity of Human Skin in the Millimetre Wave Band

Transcription

1 Pemittivity of Human Skin in the Millimete Wave Band C. M. Alabaste, Canfield Univesity. Abstact: The complex pemittivity of a human skin sample is measued ove the band 60 to 100GHz using a quasi-optical method. The esults ae pesented and compaed with pedictions fom standad models. Thee is a wide ange of modelled esults; each model poviding a patial fit with the measued data. Intoduction: Cuently, vey little measued data is available fo the pemittivity of biological tissues in the millimete wave (MMW) band. This Lette pesents the fist measued pemittivity data fo human skin in the MMW band. Hitheto, the pemittivity of biological tissues in the MMW band has been extapolated fom models of pemittivity as functions of fequency such as Debye and Cole-Cole type equations. Model paametes have been based on measued data at lowe, centimetic, fequencies. With the inceasing use made of the MMW band fo communications, ada and othe passive senso applications it becomes impotant to have measued data available. Since it is anticipated that human skin will be vey lossy within the MMW band it may be assumed that MMW adiation incident on the human body will be almost entiely absobed (o eflected) by the skin laye. Consequently, it is paticulaly impotant to have eliable data on the chaacteistics of human skin. Such data can veify o adjust the standad models and be used as a basis fo detemining the heat deposition and popagation of MMW signals in the body. This Lette descibes the quasi-optical expeimental method and theoy used and pesents the measued esults fo a single, ex-vivo sample of human skin fixed in fomaldehyde togethe with a compaison of data fom standad pemittivity extapolation models. Theoy: A plane wave nomally incident on a slab of dielectic sample of thickness t S is patially eflected, tansmitted and absobed by the dielectic. The eflected and - 1 -

2 tansmitted signals ae compised of an infinite numbe of components due to the multiple eflections between the ai/dielectic intefaces. Thus the total eflected () and tansmitted (t) signals ae given espectively by: exp 2k t 1 1 s s (1) exp 2k st s and exp ksts t (2) 2 1 exp 2k t 1 s s whee k S is the popagation constant in the sample and 1 is the eflection coefficient of the sample/ai inteface. Both ae functions of the elative complex pemittivity,, of the sample given by: 1 O O (3) and k (4) S k O whee k O and o ae the popagation constant and pemittivity in fee space, espectively. Expeimental Method: A slice of sample was held in place on a igid cicula fame placed between and just beyond the fa-field theshold of a pai of standad gain hons connected to a vecto netwok analyse (VNA), simila to the method of [1]. The amplitudes of the tansmission and eflection coefficients wee measued via the VNA S 21 and S 11 paths espectively. The hons wee aligned fo vetical polaisation, paallel with each othe and fo nomal incidence on the sample. The sample was sufficiently lage to subtend an angle of twice the 3dB beamwidth (at least the fist 7-2 -

3 Fesnel zones) of the hons; thus it intecepted the entie main beam and appoximates to an infinitely lage sample. The VNA was used in a time gated mode to isolate the eflection fom the sample o the main though signal fo the eflection and tansmission measuements, espectively. The eflection measuement was calibated with espect to a metal shot cicuiting plate placed in the sample fame wheeas the tansmission measuement was calibated with espect to the path loss with no sample in place. Equations (1) to (4) ae solved using an iteative technique to find solution(s) fo. Solutions fo ae sought ove a use defined seach space and esolution which esult in computed values of tansmission and eflection coefficients which most closely match the measued values. Howeve, due to the multiple eflections pesent in all but vey lossy samples, multiple solutions of may be found. This ambiguity can easily be ovecome based on an initial estimate fom the peak and tough fequencies caused by the multiple eflections beating in and out of phase o by fitting data at seveal neaby fequencies [2]. The skin sample was obtained fom a white, 50 yea old, female dono. Its fat was emoved and it was scapped to a nea unifom thickness (~ 1.5mm) and fixed in 10% fomalin soon afte excision. The skin was lightly stetched ove an annula stainless steel fame and stitched into location aound the fame peiphey. Pio to measuement, the sample was insed in wate and allowed to dy fo 4 hous. A 105mm diamete of the sample was exposed to the measuement. A PTFE sample, whose popeties ae well established, was measued in the same way in ode to veify the method. Results & Discussion: The pemittivity of the PTFE sample was detemined to be: = 2.04 j and 2.00 j at 60GHz and 94GHz, espectively, which ae in close - 3 -

4 ageement with peviously published data. The esults fo the skin sample ae given in Table 1. Table 1 quotes the pemittivity values which epesent the best solutions ove given bands and at cetain spot fequencies of inteest. The esults ae quoted at both the measuement tempeatue and extapolated to 37 0 C. The following tempeatue gadients have been applied as they epesent the mean of those epoted in the liteatue based on data fo wate in the MMW band: eal pat of : +2.35%/ 0 C, imaginay pat of : +1.05%/ 0 C [3], [4], [5] and [6]. Table 2 quotes the values fo dy human skin at 37 0 C pedicted fom standad models used to extapolate the pemittivity of biological mateial. The esults of Table 1 indicate that both eal and imaginay pats of the pemittivity show a eduction with inceasing fequency, as is to be expected fom elaxation phenomena. The sample was slightly moist at the time of measuement. An incease in the pemittivity was obseved when measued wet and is in line with expectations. Indeed, significant vaiation in the pemittivity of skin is also anticipated on a sample by sample basis. The effects of the fomaldehyde fixing wee undetemined. The effects of measuement eo ae not possible to quantify due to the iteative seach technique used. Repeated measuements indicate possible eos of up to ±15%. It is also woth noting that extapolations of the pemittivity data to new tempeatues may be uneliable as thee is a wide vaiety in the tempeatue gadients epoted in the liteatue. The measued values show a consistently highe eal pat of pemittivity than the pedicted values, wheeas the imaginay pats ae in easonable ageement. No one model povides the most consistent fit to the measued data. The likely easons fo the discepancies in the two sets of data include: (i) the unknown moistue content of the sample used hee and also those used in measuements to set the model paametes, (ii) the effects of fixing in fomaldehyde and (iii) natual tissue vaiations

5 Acknowledgments The autho wishes to thank the following: Pof. J. Dahele of Canfield Univesity, my PhD supeviso, fo his help and advice. D P. Zioupos and Ms V. Wise of Canfield Univesity fo the pepaation of the skin sample. The National Disease Reseach Intechange, Philadelphia, PA. fo the povision of the human skin sample. Anitsu Copoation and Bimingham Univesity fo the use of thei VNAs. Refeences 1 MA, Z. and OKAMURA, S.: Pemittivity detemination using amplitudes of tansmission and eflection coefficients at micowave fequency, IEEE Tans. MTT, 1999, 47, pp ALABASTER, C.A., and DAHELE, J.S.: Fee space measuement of pemittivity, Poc. Int. Conf. on Antennas and Popagation, Apil 2003, Exete, Vol. 2, pp STEEL, M.C. and SHEPPARD, R.J.: The dielectic popeties of abbit tissue, pue wate and vaious liquids suitable fo tissue phantoms at 35GHz, Phys. Med. Biol., 1998, 33 (4), pp GRANT, E.H. and SHACK, R.: Complex pemittivity measuements at 8.6 mm wavelength ove the tempeatue ange C, Bit. J. Appl. Phys., 1967, 18, pp GLIBITSKIY, G.M.: Measuement of pemittivity of aqueous solutions at 44 GHz by FM eceive, Int. J. Infaed and millimete waves, 1996, 17 (12), pp SZWARNOWSKI, S. and SHEPPARD, R.J.: Pecision waveguide cells fo the measuement of pemittivity of lossy liquids at 70 GHz, J. Phys. E: Scientific Instuments, 1977, 10, pp KUZNETSOV, A.N.: Biophysics of electomagnetic effects [in Russian], Enegoatomizdat, Moscow, GABRIEL, C. and GABRIEL, S.: Compilation of the dielectic popeties of body tissue at RF and micowave fequencies Final epot fo AFOSR/NL Bolling AFB DC , June Full text of this efeence is available at: Automated calculations based on this model also available at: Autho s affiliation: C. M. Alabaste is with the Depatment of Aeospace, Powe and Sensos, Canfield Univesity, Shivenham, N Swindon, Wiltshie SN11 8SQ, United Kingdom. c.m.alabaste@mcs.canfield.ac.uk - 5 -

6 Table captions: Table 1 Measued Pemittivity Data fo Human Skin Table 2 Pedicted Pemittivity Data fo Human Skin (37 0 C) - 6 -

7 Table 1 Fequency (GHz) Measued Pemittivity T ( 0 C) Pemittivity (Extapolated to37 0 C) j j j j j j j j j j j j j j j j

8 Table 2 Fequency (GHz) Ref [7] Single tem Debye Ref [7] Two tem Debye Ref [8] Fou tem Cole-Cole j j j j j j j j j j j