Biomedical Engineering Reference
In-Depth Information
1. Drug delivery should be controlled so that only a specific organ or disea-
sed region is targeted, or is subjected to concentrated administration of
the drug.
2. The release or activation of a drug should be able to be specified for a
certain time following administration.
3. A method for observing the distribution state and the drug concentration
should be available.
4. The drug delivery system should applicable to any type of drug.
5. The drug delivery system should be noninvasive.
There are some techniques that satisfy some of these conditions. However,
if any one of these conditions is not satisfied, the DDS cannot be regarded
as a complete method for medication.
To eliminate these problems, a DDS should be a microrobotic system;
that is, controllable DDS system. It should be able to carry any type of
drug and should be measurable and controllable using intelligent diagnostic
decision-making.
Therefore, we have proposed a new concept of an intelligent microrobotic
system in vivo, as a DDS in which drug carriers of air-filled microcapsules
are measured using a novel high-resolution echography system [36-38] and
are noninvasively actuated [39,40] using resonant ultrasound from the skin
surface of a patient.
2.5.2
The Acoustic Characteristics of Microcapsules
as Drug Carriers
In this system, the micro-encapsulated drug carriers are microrobots within
the blood vessels.
Thesubstantialrequisiteforthesemicrorobotsistobemicro-balloonsor
to be microcapsules containing harmless gas. A significant difference between
acoustic impedances of the water and the gas is essential to maintain suitable
ultrasonic backscatter and resonance.
The microrobots - i.e., microcapsules - are visualized and measured using
nonresonant ultrasound. In this regard, it is well known that microbubbles
in the blood are a good contrast agent in the field of ultrasonic diagnosis.
The control of the microrobots is based on the principle that an air-filled
microcapsule in liquid absorbs ultrasound energy at the resonant frequency
[41]. In other words, it can be excited with resonant ultrasound. Fortunately,
the resonant frequency of a microcapsule having the same, or a smaller, dia-
meter as a red blood cell coincides with the frequency of medical ultrasound.
As active microrobots, the micro-encapsulated drug carriers are nonin-
vasively actuated and controlled by resonant ultrasound from outside the
patient's body. Equation (2.1) represents the resonant frequency of a micro-
