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An integration of bio-medical system and wireless power transmission (WPT) technology offers a great potential to revolutionize energy harvesting for current medical devices. The system integration of microwave power transmission devices with smart microsensors is crucial for replacing a power storage devices and miniaturizing wireless biomedical systems. This research focus is to replace battery power supply with an implantable millimeter-wave rectenna. In this paper, performances of various rectennas developed for x-bands will be presented, and discussed various influences features and safety issues on humans.
Microwave Energy Harvest, Biological
Effects, Wireless Power Transmission, and Rectenna,
Coupling Efficiency
Wireless power transmission (WPT) technology has widely been developed in many applications such as solar power satellite (SPS) [1, 2], smart actuators [3-
6], medical applications [7-10], and charging vehicles
[11-13]. This development mainly focused on how effectively coupled a rectified circuit with incoming
RF waves in terms of materials involved and distance between the source and receiving devices. Therefore, a major concerning factor for this WPT research is how to improve the transmission efficiency of the devices. At the same time, this RF wave will also be coupled with human body parts if this device is engaged in or around of human such as medical devices or cellular phones. Recently, there has been an increasing public concerning about health hazards resulted from exposing of RF or microwaves by a mean of wireless communications. Even more uses of microwave coupling technology that has been engaged for treatment or removing unwanted parts in human body such as Transurethral microwave therapy (TUMT) [14-16], it is important to understanding how does this treatment affects on other adjacent part of human body. Therefore, it is important to evaluate these influences on human body.
The most of researches known so far have mainly focused on how to effectively couple a rectified circuit with incident microwave. However, for medical applications, the absorption of microwave through human tissues also needs to be characterized and clarified for setting up the exposure threshold with microwave power and its frequency. Recently, there has been an increasing public concern about health hazards resulted from exposure of RF or microwave. Therefore, it is important to evaluate the effect and impact of microwave power coupling on human body.
Microwave/RF energy has been used for wireless power transmission including many therapeutic applications, such as transurethral microwave therapy
(TUMT). For safe uses of RF power, it is important to know how to deliver microwave energy on focused area and control temperature changes not to
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drastically increase on adjacent areas. Graphical analysis of thermal loading factor is important to understand how to achieve effective transmission of microwave through the tissue. The loss mechanism while transmission often appears as thermal effects due to absorption of microwave, especially for materials such as human skin, muscles, and other organic parts including brain. In this paper, microwave thermal effects are investigated to measure temperatures, penetration depth through animal skins in terms of input power and various frequencies. This result will be compare with the case of human applications.
The interactions of microwave with matters are, in general, described by various coupling mechanisms, such as dielectric, resonant, and inductive or capacitive couplings. On the other hand, the penetration of wave through skin or tissues can be defined by skin depth calculation.
The skin depth of the incoming microwave can be calculated as
s
=
f
o
R
Where,
ρ
= resistivity (ohm-meters) f= frequency (Hz)
µ
ο
= permeability constant,
(Henries/meter)
µ
R
= relative permeability
4
π x10
-7
Based on the electrical properties of humans [17], the skin depths for various humans’ parts are calculated as shown in Fig. 1. The depth of penetration is lower in tissues of high water content such as brain, blood, and skin while the absorption is an order of magnitude lower in tissues of low water content such as tooth in the range of below 1 GHZ. As the frequency is reached to 10 GHZ or higher, the skin depth of various parts is not much different.
The second part of this study shows how the entering wave energy is absorbed through the organic tissues.
The patterns of microwave absorption are dictated by the frequency, dielectric constant of tissues, source configuration, and geometry of the tissues. As mentioned above, any additional amount of thermal energy in the tissues can be readily dissipated away by circulatory systems as a cooling mechanism of the tissue.
Figure 1. Penetration depth of various humans’ parts
Figure 2. Dielectric constants of human body parts in terms of frequencies
The complex permittivity of a material can be written as
"
=
d
"
+
σ
"
σ
"
=
0
Where,
ε d
is a loss due to dipole rotation and
ε
σ ‘’ is due to ionic conduction which are dominant loss mechanism in the system;
σ
is the ionic conductivity in Siemens/m of a material, free space, and
ε
ο is the permittivity of
ω
is the angular frequency of incident microwave. The ionic conductivity of the skin is
2

50
40
30
70
60
20
10
0 increased as frequency is increased due to active ionic conduction by higher frequency. However, the dielectric constant of the skin, for an example, is decreased as the frequency is increased as shown in the figure 3.
Dielectric Constant
Conductivity [S/m]
10 2 10 3
Frequency (MHz)
10 4 10 5
This dielectric loss will contribute into heat in the skin. The increase in temperature of the material can be calculated [18] from mC p
∆
T
∆ t
=
5 .
563 x 10
−
11 fE
2
"
Where, C in Jkg
-1o
C p
-1
is the specific heat capacity of the material
, m is the density of the material in kg/m
3
, E is the electric field intensity in V/m, f is the frequency in Hz,
∆Τ is the temperature changes in the material in
0
C, and
∆ t is the time duration in seconds. According to this equation, the temperature of the materials will increase as to the dielectric loss factor is increased.
Ultra Wideband (UWB) have been pursued on biomedical sectors, because of high data rate communication in low power operation [19]. A broad frequency range of 3.1 GHz to 10.7 GHz is allowed for wireless communication spectrum. Not to interfere with governmental radio networks, transmission power is limited mostly for indoor communication. With the high data rate advantages,
UWB based medical devices have been introduced on the market of vital signs monitoring of patients, home care or low acuity care. Also, rapid advances in the use of UWB have been reported especially for medical imaging applications and early warning
3 system [20, 21]. It is expected that UWB may soon become one of the most cost efficient and highly functional medical technologies.
In addition, WPT system for biomedical applications as a power source has been investigated by various researches [22-24], such as neural recording and measurement of glucose level in human body. With the strong advantages removing the battery, wireless power transmission technology is considered as one of strong candidates in implantable electrical power source of medical electronics. Even though there have been remarkable advances in the field of wireless power transmission, practical issues in exposure of microwave with biological substances in wireless biomedical sensing applications are not fully addressed. In this paper, we will present transmission rates electromagnetic wave interactions with the sample skins. Specifically, the absorption or attenuation of electromagnetic power at the near field region within the UWB frequency spectrum of 6 – 12
GHz is discussed for targeting the wireless communication system integration with this WTP module.
Flexible Rectenna Design
A patch rectenna was designed for 8.4 GHz center frequency operation and fabricated with microstrip feed structures, a Schottky diode and capacitors as shown in Fig. 4. This configuration will mitigate some of the polarization effect of microstrip antenna.
The patch and microstrip patterns were made on both sides of the substrate by a wet etching technique. The
Schottky diode was MA2054 series of MA-COM, which has 0.25~0.35 V
Forward
@ I
F
=1mA,
I
Reverse
=100nA@1V
R
.

Figure 5. Experimental setup
An experimental test-bed for the rectenna array was set up as shown in Fig. 5 and the irradiance of the microwave along the distances was measured. A combination of signal generator and amplifier provides either a 2 W or 20 W of microwave power to the horn antenna at a frequency range set at 6 - 12
GHz. The microwave power was delivered through a waveguide connected with a rectangular horn. All other items listed in the equipment and material section were externally placed and connected to the horn antenna and the rectenna via a feedthrough backplate at the rear of the chamber as shown in the figure. The injection horn antenna was connected to the amplifiers through a waveguide which is in turn connected to the signal generator via a coaxial cable.
The polyimide and cellulose rectenna arrays shown in the fig. 4 was placed inside of the chamber and measured the output power. Microwave power absorption with the skin has been measured by monitoring of power absorption with and without covering the rectenna with a mouse or swine skin sample. The trend of power attenuation through a rat skin (t= 3.4 mm) and a swine skin sample (2.65 mm) is shown in figure 6. The center frequency shift is attributed to electromagnetic coupling of the skin with the patch antenna. At 8.8 GHz, the signal attenuation through the rat skin is the maximum value of 77%, which may be related to the resonance frequency shift. Along with given frequency ranges, the attenuation rate varies 0 to 77 % for the rate skin while the attenuation rate of the swine skin varies 0 to
50 %. The rat skin has more significant effects on incident frequencies.
On the other side, the attenuation mainly occurred through absorption due to dielectric properties of the skins and possibly due to scattering and reflection from the skins.
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Figure 6. Power loss through rat and swine skin
The receiving power was measured from a Narda
Detector (Model: 8723D) with and without the samples. With the values measured, attenuation of the skins is calculated by where P
0
is a power measured without the skin and P w is a power through the skin. It was assumed that the reflection from the skin is negligible.
For temperature measurement of the skins, a thermal couples (J-type) and a FLIR SC 325 IR camera are used as complementally. The resolution of the camera is 320 x 240 pixels, and the temperatures were verified with a thermal couple and Keithey Nano
Voltmeter. The irradiance level to be exposure was approximately 40 mW/cm
2
.
Figure 7 shows a time history temperature of swine and bovine skins measured by the thermal couple described as an example, in the highest absorption frequency (9 GHz). The trends of temperature history of the swine and bovine are similar, and changed approximately 2 degree of Celsius. The temperature of the skin reached and stabilized at 7 min, and when the microwave power is turned off and the cooling progress is begun. Both samples have to be preserved at a refrigerator, so that initial temperatures are different by two degree. The samples were wetted, but they are quickly dried during the experiment. The temperature reading from the IR camera and the

thermal coupled was matched within 10 %. This difference may from the conditions of thermal couple contact on the skin sample.
Figure 7. Temperature history of the swine and bovine skin
Conclusion
A mechanism of microwaves interaction with various humans’ materials is reviewed. Based on the calculation with available materials’ properties, it is significant influence of frequency of incident microwave radiation may be predicted. With x-bands radiation, the temperature of swine skin does not significantly changed as long as the irradiance is maintained below of Threshold Limit Value and
Specific Absorption Limits. With an approximately
40 mW/cm
2
of irradiance, there is a maximum 3 o
C change in the swine and bovine skins observed in the highest absorption frequency. It is important to understand all adjunct materials’ property of human body when microwave radiation is exposed on specific area for medical treatment.
This research work was partially supported by a
NASA grant (NNX11AC11G).
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