Biomedical Engineering Reference
In-Depth Information
bubble in water [42]:
3
kP
P
1
πd
f
=
,
(2.1)
where
f
is the resonant frequency;
d
is the diameter of the microbubble or
microcapsule;
k
is the specific heat ratio of gas, a constant;
P
is the pressure
of the water; and
p
is the density of the water, a constant.
The resonant frequency depends mainly on the bubble diameter and the
pressure of the surrounding water.
Although this formula does not describe the structure of a microcapsule
shell, it is valid for the order estimation of microcapsule resonance with thin
shell walls.
Equation (2.1) assumes that the microcapsule is a perfect sphere. Micros-
copic photographs show perfect spherical microcapsules made of polymethyl-
methacrylate (PMMA). Newton rings, which interfere between the microcap-
sule membrane and the stage glass, demonstrate the perfect smoothness of
the microcapsule surface. They all contain harmless gas. These data confirm
the validity of (2.1).
In addition, an ultrasonically excited drug matrix releases the drug com-
ponent into the surrounding liquid [43]. If the shell of a microcapsule was
made of drug matrix, then the release of the drug component from the
microrobotic DDS could be controlled by resonant ultrasound.
To confirm the physical possibility of resonance control based on shell
design, three series of microcapsules were made from one series of PMMA
and two series of vinylidenedichloride VCl, with diameters ranging from 5 to
90
m. The experimental results showed that the resonant frequencies of the
microcapsules made of hard VCl with mean diameters of 82, 54, and 26
μ
μ
m
were 110, 220, and 330 kHz, respectively.
Thus, the smaller the diameter, the higher is the resonant frequency. The
resonant frequencies of the microcapsules almost coincided with the theore-
tical values calculated by (2.1) [44].
In the case of microcapsules having the same diameter, the resonant fre-
quency depends on the material. The harder the material, the higher is the
resonant frequency. The resonance of the soft VCl capsules was lower than
that of the hard VCl capsules. Rigid PMMA microcapsules showed the hig-
hest resonance.
These findings show that we can design microcapsules of sucient reso-
nance to be controlled by ultrasound. Microcapsules smaller than red blood
cells can provide a feasible resonant frequency for clinical use through ad-
justment of their diameter and material.
2.5.3 A Noninvasive Measurement System for DDS
For adequate control of the microrobotic system, a measurement method
is necessary. Information concerning the accurate location and velocity of a
