4.2. Classes of Drug Delivery Devices
The types of drug delivery device that have been and are being
developed can be classified into four groups, depending on the method
used to modify the drug properties.
4.2.1. Polymeric Systems
A great deal of research has been conducted into the use of
polymers to develop means of controlling delivery of drugs to the
human body. Within this class there are four sub-divisions,
classified by the mechanism which controls the rate of drug release.
- 22.214.171.124. Diffusion Control
- In many cases rate of release of a drug from a device is
controlled by the rate at which the drug diffuses through a
polymer. This class is further subdivided according to how the
drug is held in the polymer. If drug is retained in a reservoir it
is known as a reservoir device. However, when drug is dispersed
throughout the polymer, it is known as a matrix device.
- 126.96.36.199. Chemical control
- Polymeric drug delivery devices have been developed in
which the polymer is susceptible to breakdown by agents present in
the body (usually water, or enzymes). Two scenarios exist, one in
which drug is held inside a biodegradable polymer, and ones in
which the drug is chemically linked to a polymer by a bond that is
biodegradable. The later are known as pendant chain systems, and
the polymer is often soluble.
- 188.8.131.52. Solvent control
- Another mechanism for controlling the release of drug from a
device involves the use of a solvent, which in the case of the
human body is water. In one class the solvent diffuses into the
device and causes swelling, i.e. swelling controlled. In another
case the device contains a high concentration of salt to which the
solvent is attracted by osmosis through a semi permeable membrane,
and the resulting influx of water is harnessed to expel a solution
of the drug out of an orifice in the device.
- 184.108.40.206. Externally activated or modulated
- The last class of polymeric device is being developed in order
to give some form of control to the release that can be triggered
by external means. Two examples include the use of magnetic beads,
implanted in a polymer along with the drug, and activated by
subjecting the beads to an external, oscillating magnetic field,
and the use of externally applied ultrasound energy to increase
drug release from polymer encapsulated drug.
4.2.2. Drug Modification/Protection
An alternative approach is modify the drug so that it is protected
form the environment until it is needed, or the environment is
protected from it, (as would be the case for drugs that are toxic to
- 220.127.116.11. Produg formation
- A prodrug is an in active from of a drug which is converted to
the active form in the body. A classic example of a prodrug is the
drug L-dopa, used in the management of Parkinson's disease.
- 18.104.22.168. Conjugation to a homing molecule
- A great deal of interest has been generated by the prospect of
developing a magic bullet that will home in on the disease area
and deliver drug. This is particularly attractive in the case of
cancer drugs, which are often highly toxic to many normal cells in
the body, as well as the cancer cells. In this way the drug can be
made to avoid areas in which it would cause damage, at the same
time delivering a large payload to the target site.
- 22.214.171.124. Liposomal entrapment
- It is possible to form micro-vesicles that contain a drug.
These bilayered micro-vesicles, known as liposomes, are formed
spontaneously when phospholipids contact an aqueous environment.
Phospholipids are naturally occurring amphipathic molecules
(contain both a hydrophilic and hydrophobic region). If they
contact a solution of drug, they will form a vesicle entrapping
some of the drug solution in the interior.
One can imagine that if drug is in solution it would be very
convenient to pump it into the body through a thin catheter. This is
in fact the method of choice in hospitals, and currently there are
sophisticated syringe pumps which are available to be programmed to
pump a drug solution through an intravenous line. This, however,
requires that the recipient be under constantly supervision by
trained personnel, and also that the protective barrier of the skin
is broken to allow insertion of a needle. This means that intravenous
injection of a drug is an expensive proposition, which can lead to
complications such as infections. One of the first attempts to
regulate insulin in diabetes centered on the development of a
implantable pump containing insulin solution which could be delivered
in a metered doses into the blood stream. For the system to exactly
mimic the delivery system of the body requires a sensor which can
measure blood glucose level and feed this information back to the
pump to administer insulin only when blood glucose levels are high.
This is a closed-loop, feedback mechanism. Early versions were
simpler, and merely pumped a continuous low, or basal dose of
insulin, and the patient was required to administer external insulin
by the usual injection routes after a meal.
4.2.4. Modification of the site of delivery
It is also possible to control how the body takes a drug up by
changing either the properties or the location of the site of
- 126.96.36.199. Use of enhancers
- Many drugs can not pass through barriers such as the skin, or
the nasal mucosa, so one approach to deliver them by these route
is to change the skin permeable either by the use of chemicals
known as enhancers, or by physical methods such as the use of
electrical current (iontophoresis) or ultrasound (sonophoresis).
- 188.8.131.52. Shift in absorption zone
- It is also possible to change the site of delivery by
protecting the drug so that it will pass the first site, and
arrive at a site where it is more likely to be absorbed. This is
the case with drugs that are absorbed through the intestine, but
not throughout the stomach, where they arrive from the mouth. If
the drug is destroyed in the stomach, it must be protected, and
polymeric coatings have been developed that remain in tact in acid
conditions, but break down in the alkaline environment to the
upper intestine. These coatings are known as enteric coatings.