Biomedical Engineering Reference
In-Depth Information
Fig. 4.10
The single-cell
radio
cell
radio
DNA
The nanoradio is based on nanotubes or nanoparticles. For example, a nanoradio
can consist of a single CNT cantilever that emits electrons via the field-emission
effect, its emission being modulated by a remote radio transmitter signal and
detected by a cathode located in vacuum near the vibrating CNT resonator (Jensen
et al. 2008). Thus, a single vibrating CNT resonator is acting as a rudimentary radio
able to sense the signal of a remote radio transmitter that is emitting at a frequency
equal to the mechanical resonance frequency of the CNT. The minimum detectable
field is 60 dBmV m
1
Hz
1=2
, and the resonance frequency can be tuned in the 50-
400 MHz range by controlling the length of the cantilever. The detection of the
radio signal is performed by collecting the field-emission current of the CNT in the
presence of applied dc field E. This current is expressed by the Fowler-Nordheim
equation
I
D
b
1
A.˛E/
2
exp.
b
2
=˛E/;
(4.1)
where b
1
and b
2
are constants, and ˛ is the local field enhancement factor. The CNT
is vibrating in response to a wireless signal, superimposed on the dc field, so that
the enhancement factor becomes
˛.t/
D
˛
0
C
˛.t/:
(4.2)
As a result, the current has a nonstationary part
I.t/
D
I
0
.1
C
a
C
a
2
=2/.˛.t/=˛
0
/
2
;
(4.3)
where a
D
b
2
=˛
0
E. The first two terms in (
4.3
) describe the amplification of the
emitted current, and the last term expresses the demodulation effect in the same
device. The CNT radio is displayed in Fig.
4.11
. It is important to note that this
CNT radio is 4-5 orders of magnitude smaller than contemporary radio devices
implemented with the most advanced semiconducting technologies. In its present
form, the CNT radio has some severe drawbacks. It must be biased around 200 V,
which is a value that is too large for an autonomous capsule. In addition, the
tunability achieved by changes in the bias is small, of only 4 MHz, which is five
times lower than the FM bandwidth.
Therefore, an improved type of nanoradio—the tunneling nanoradio—was pro-
posed in
Dragoman and Dragoman
(
2008
) to alleviate the abovementioned draw-
backs. In the tunneling nanoradio, the detection method based on field emission
from a vibrating CNT was replaced by a tunneling detection mechanism. The
tunneling nanoradio is displayed in Fig.
4.12
.
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