# A 31.5 GHz Patch Antenna Design for Medical Implants

Authors: Yasir Ahmed; Yang Hao; Clive Parini
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

We have proposed a 31.5 GHz patch antenna for medical implants. The design is based on the transmission line model and is simulated in CST. The patch antenna performs reasonably well in terms of return loss and radiation efficiency. However, the most attractive feature of this design is its form factor. Typical antennas designed for the microwave range are quite large in size, which makes them unsuitable for implants. The proposed design is much smaller in size but still retains the essential characteristics for reliable communication.


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## Body

## 1. Introduction

The design of antennas for communication
with implants inside the human body has received considerable attention from
the research community.  The design of these antennas is quite challenging,  as
there is a limit on the amount on power that can be transmitted and also on the
size of these devices. The limitation on the transmit power is due to the
amount of battery power available as well as due to concerns about the exposure
of the human body to electromagnetic radiation. Typically these devices are
allowed to have a peak transmit power of 25 uW (−16 dBm),
which is quite insufficient at higher frequencies especially when embedded deep
into the tissue. Today these implants are being used to monitor as well as
facilitate the working of various human organs, for example, as cardiac
pacemakers.


## 2. Survey of Antennas for Implants

A number of different antenna designs have
been considered for medical implants. In [1], spiral and serpentine antenna
designs have been considered and the authors have simulated the performance of
these antennas using a single block of muscle and a realistic human shoulder.
The results are also verified experimentally using a tissue simulant material
composed of TX-151, sugar, salt, and water. The effect of shape and size of the
dielectric as well as the location of the feed point on the performance of the
antenna is studied. A similar analysis is performed in [2] for spiral and
planar inverted-F (PIFA) antenna. However, the authors have primarily focused
on the human brain using a six-layer model (brain, CSF, Dura, bone, fat, and
skin). The results are again verified using a human tissue-simulating liquid
made from deionized water, sugar, salt, and cellulose. In both these papers,
the authors have considered a frequency of 402–405 MHz that has
been recommended by the European Radiocommunications Committee (ERC) for
ultra-low-power, active, medical implants. The simulations have been performed
using finite difference time domain (FDTD) method. In addition, an analytic
method using spherical dyadic Green’s function (DGF) has also been considered
[2].In [3], the problem of
antenna detuning and signal loss is studied for a subminiature loop antenna
designed for the 900 MHz ISM and 402 MHz MICS radio bands. It is observed that
with proper encapsulation of areas of high electric field strength antenna
detuning can be considerably reduced. This is in contrast to a monopole that
experiences considerable detuning (as high as 13%) and signal loss at the same
frequencies.The results of an actual
implant operating in the 403 MHz MICS band are presented in [4]. The implant was
placed inside a Perspex body (30 cm diameter cylinder), and the signal strength
was measured as a function of the distance. The error rate of the link was also
evaluated. It was observed that the communication link with the external device
was maintained up to a distance of 3 m when the implant was placed at a depth of
10 cm. The implant used a sleep and wakeup sequence (2.4 GHz, 20 dBm wakeup
signal) to prolong battery life.


## 3. Limitations in the Design Process

Microstrip antenna design is a fairly
mature field, and several designs have been proposed over the years
[5–7]. As with any other antenna design,  the size of the microstrip is
directly proportional to the wavelength at the operating frequency. At 402 MHz,
the wavelength of an electromagnetic wave is approximately 0.75 m. It is obvious
that any antenna with dimensions comparable to this wavelength cannot be used
for an implant. The technique usually used to overcome this problem is to
design a conducting surface that is spiraled along the surface of the
substrate. The resonant frequency of the microstrip is then proportional to the
total length of the spiral and not to the length of any individual element.
Although this results in a size reduction, it is still not quite suitable for
an implant (see Table 2). We have investigated the idea of using a rectangular
patch antenna designed to operate at the millimeter wave frequencies. It is
well known that at these frequencies the electromagnetic wave experiences very
high attenuation, however, the implant does not need to be placed deep into the
tissue and with proper transmit receive combination it is still possible to
maintain reliable communication. The penetration depth of a medium is defined
as(1)δ=1α,where α is the attenuation constant. It is the
distance at which the amplitude of an electromagnetic wave is reduced to e−1=0.3679 of its original value.It has been observed that at millimeter
wave most of the loss occurs within the first layer, that is, skin. There is a relatively
less loss within fat and at the  dielectric boundaries. Figure1 shows the power
loss within the layer of skin as a function of frequency. Also shown are
corresponding half wavelengths which should serve as a guideline to the antenna
size. The actual antenna size would also be dependant on the substrate
material; however, any size reduction would be proportionate across the
frequency range.Figure 1
Power loss within 4 mm of
skin. The corresponding half wavelengths are also shown.


## 4. Design Procedure

A microstrip antenna can be designed using
either the transmission line model or the cavity model (more complex models
also exist that suit a particular design). We have used the transmission line
model since it is fairly simple to implement and results in antenna designs with
reasonably good performance in terms of return loss and efficiency. It is also
quite well suited to the rectangular designs that we have considered  (other
popular designs include circular, elliptical, and disc like). The design starts
with selecting the operating frequencyfr,
selecting a substrate with the required permittivity εr,
and defining the width of the substrate h (Figure 2).
Thick substrates with low permittivity result in antenna designs with high
efficiency and large bandwidths. Thin substrates with high permittivity lead to
a smaller antenna size but with a lower bandwidth and a high-radiation loss
[5]. The tradeoffs between substrate thickness and permittivity and antenna bandwidth
and efficiency have been discussed in [8–10]. We have used Rogers RT6002
in our design with a permittivity of 2.94 and a loss tangent of 0.0012.Figure 2
Patch antenna with a line feed.According to the transmission
line model, the lengthL and width W of the patch are calculated as
(2)W=vo2fr2εr+1,L=vo2frεreff−2ΔL,where vo is the speed of light in free space, εreff is the effective permittivity, and 2ΔL is the extension in length due to fringing
effects:
(3)εreff=εr+12+εr−12[1+12hW]−1/2,ΔL=0.412h(εreff+0.3)(W/h+0.264)(εreff−0.258)(W/h+0.8).
Although the design of the
patch is quite simple, the design of the feeding mechanism is not that
straightforward. There are four possible methods that can be used:
(1)
microstrip-line feed;(2)
probe feed;(3)
aperture-coupled feed;(4)
Proximity-coupled feed.
We have
used a microstrip line feed since it is relatively easy to model, match, and
fabricate. It results in low- antenna bandwidths (2–5%); however,
this should be sufficient for our application. The calculation of the feed
point is based on the principle that maximum power transfer would occur when
the impedance of the line is matched to the patch. At the resonant frequency,
the impedance of the patch is zero at the half point along its length.
Therefore, the feed point is usually a little offset from the centre. We first
calculate G1 and G12 the self and mutual conductance of the patch
terminals (also termed as the radiating slots), respectively:
(4)G1=∫0π[sin⁡((koW/2)cos⁡θ)120π⁢cos⁢⁡θ]2sin⁡3θ⁢dθ,G12=∫0π[sin⁡((koW/2)cos⁢⁡θ)120π⁢cos⁡θ]2Jo(koLsin⁡θ)sin⁡3θ⁢dθ,where
(5)ko=2πεreffλ,
and Jo is the modified Bessel function of the first
kind and order zero. The length of the inset yo can then be calculated based upon the
following relationship:(6)Rin(y=yo)=12(G1±G12)cos⁡(πyoL).Here, Rin is the impedance value required for perfect
matching. Since the inset creates a physical notch in the patch, it also
changes its resonant frequency (due to junction capacitance); however, this
shift is usually a small percentage of the actual frequency and can be ignored.
Alternatively, the length or width of the patch would have to be modified, and
the feed location would have to be recalculated. The width of the inset is
somewhat arbitrary.


## 5. Simulation Results

The designed patch antenna is then
simulated in CST at a frequency of 31.5 GHz. The antenna is placed within a
three-layer body consisting of skin, fat, and air (Figure3) and energized by a waveguide
port with a normalized power of 1⁢WΦ (Φ denotes the 10 gm tissue that we have used.  The transmit
power should be less than 16 mW (1.6 W/kg × 0.010 kg). This would reduce our link
budget by approximately 18 dB).
We used
air as the cavity around the antenna; however, in practice, some other low
permittivity medium might be used to provide a match between the antenna and
the body. The dielectric data used in the simulation is given in Table 1.Table 1
Dielectric properties of human
tissues at 31.5 GHz.
TissuePermittivityConductivity (S/m)Skin14.798027.9710Fat3.59701.8636Muscle22.270036.8130Table 2
Comparison of five
different antennas designed for medical implants. The size of our antenna is
governed by the dimensions of the ground plane.
Antenna typeTrue antenna sizeCharacteristicsSpiral [1]  402 MHzReturn loss>25 dBFrequency detuning ~15%−21 dBW at 1 m(7 mm deep in 2/3 muscle, 1.6 W/kg)Serpentine [1]  475 MHzReturn loss>10 dBFrequency detuning ~20%−25 dBW at 1 m(7 mm deep in 2/3 muscle, 1.6 W/kg)Higher resonant frequency than a
spiralwith the same physical lengthSpiral [2]  402 MHzReturn loss>18 dBReturn loss>5 dB (measurement)Frequency detuning ~5%
(measurement)Front to back ratio ~5 dBRadiation efficiency ~0.16%PIFA [2]  402 MHzReturn loss>16 dBReturn loss>6 dB (measurement)Frequency detuning ~10% (measurement)Front to back ratio ~5 dBRadiation efficiency ~0.25%Higher efficiency than the spiralNew design  31.5 GHzReturn loss>30 dBFrequency detuning ~1%−57.49⁢ dBW/m2 at 1 m(8 mm deep in skin and fat)Front to back ratio ~15 dBFigure 3
Patch implanted inside the model (4 mm Skin, 4 mm Fat, 1 mm cavity).The resultingS-parameters and 3D
radiation patterns are shown below (Figures 4–6). The antenna has a return loss of more than
30 dB and max broadside directivity of 9.32 dBi. The antenna exhibits some
detuning within the body which is removed by adjusting its dimensions. The
P-field patterns in the E-plane and H-plane are shown in Figure 7.   It is
observed that the antenna has a very low efficiency due to the high loss within
the body; however, it has a good front to back ratio. The embedded antenna has
a peak broadside power density of −57.5 dBW/m2 at a distance of 1 m.Figure 4
Return loss of
the antenna with and without the body.Figure 5
Gain of the antenna placed
inside the cavity.Figure 6
Directivity of the antenna
placed inside the cavity.P-field along (a) H-plane
(b) E-plane.
(a)(b)For
an isotropic radiator with a transmit power of 1 W, the power density at a distance of 1 m is(7)S=Pt/4πd2=1/4π=−10.99⁢dBW/m2,and
for the embedded patch with a gain of −46.5⁢dB† the power
density would be(8)S=−10.99−46.50=−57.49⁢dBW/m2,which is equal
to the result obtained through p-field simulation († denotes the computer simulation technology (CST)
that considers the antenna and the body as the radiator. Therefore, the gain is
the gain of the complete structure and not just the designed antenna).Now, if the effective areaAe of the antenna is known then the received
power Pr at a particular distance can be easily
calculated. The effective area [11] of a half wave dipole is given as(9)Ae=Grλ24π=λ28=−49.46⁢dBs,
where Grϒ is the receive antenna gain.    This gives a received power of −106.95 dBW or −76.95 dBm (ϒ denotes the formula given in the text that uses
directivity instead of gain; however, this is only valid if there is no loss in
radiation).In the
absence of the body, the antenna has a power density of−3.63 dBW/m2 at a distance of 1 m. Therefore, there is a
power loss of 53.87 dB within the body. Similar results are obtained for the reverse
link using ray-tracing where a 52.40 dB loss is observed within the layers of
skin and fat (Figure 8). It must be noted that the wireless communication channel between
an implant and an external device is not symmetric and the link budget in one
direction might be quite different from that in the other direction.Figure 8
Relative electric field strength20⁢log10(|Et|/|Eo|). External device to implant
communication.Finally, we have shown the p-field behaviour outside the body as a function of
the distance (Figure9). As expected, in the absence of any external body,   the P-field
follows the 1/d2 rule.Figure 9
P-field from an implant as a
function of distance.


## 6. Conclusion

Although a signal level −76.95 dBm might be
sufficient in theory, it would not be adequate in a realistic scenario where
the signal would experience multipath fading and interference from other
wireless equipment. However, it must be noted that we have considered a simple
dipole antenna at the receiver with a nominal directional gain. Since there is
no limitation on the size of the external equipment, we can make use of a large
antenna with a very high gain along the direction of transmission. In some
scenarios, it might also be possible to place the antenna close to the body
(closer than 1 m) further improving the quality of the received signal.Finally, it
must be noted that the return loss of the antenna increases inside the body
(+11.81 dB) and there is also some frequency detuning (1.14%). Higher return
loss is a desirable characteristic but frequency detuning is not and can be
removed by adjusting the dimensions of the antenna such that the null occurs at
the desired frequency.



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*Source: 167980-2008-09-08.xml*