Biomedical Engineering Reference
In-Depth Information
tablets or injections, the drug level in the blood rises after each administration of the drug and then
decreases until the next one. In order to implement such controlled-delivery systems, polymer gels
capable of releasing drugs when placed in appropriate biological environments are currently largely
studied [19-23]. Hydrogels, in particular, offer very interesting properties [20]. When placed in the
body, they are capable of swelling due to absorption of water or other body fl uids (they can comprise
60-90% of fl uid at equilibrium). The swelling process increases the aqueous solvent content within
the polymer network, enabling the drug to diffuse through the swollen mesh into the external envi-
ronment, as represented in Figure 16.5.
More generally, swelling can be achieved by means of different types of chemical-physical
environmental stimuli (Figure 16.2), which are responsible of different mechanisms of activation.
The most relevant examples are reported in Table 16.2.
A specifi c and largely studied application of such materials for controlled drug release con-
sists of the therapy for diabetes. For this disease, the system should be able to deliver insulin upon
Drug
release
Change in pH,
ionic strength,
or temperature
FIGURE 16.5
Schematic drawing of a drug delivery by swelling of an environmental-sensitive polymer gel.
(Adapted from Brannon-Peppas, L.,
Medical Plastics and Biomaterials Magazine
, November issue, 1997.
With permission.)
TABLE 16.2
Polymer Gel Types and Activation Mechanisms for Drug Delivery
Environmental Stimulus
Polymer Gel Type
Mechanism of Activation for Drug Release
Change in pH
Acidic or basic hydrogel
Swelling
Change in ionic strength
Ionic hydrogel
Change in concentration of ions inside the gel, with
related swelling
Change in chemical
species
Hydrogel containing electron-
accepting groups
Formation of charge/transfer complex, with related
swelling
Change in
enzyme-substrate
Hydrogel containing immobilized
enzymes
Enzymatic conversion, with related swelling
Change in temperature
Thermo-responsive hydrogel
poly(
N
-isopropylacrylamide)
Change in polymer-polymer and water-polymer
interactions, with related swelling
Applied electric fi eld
Polymer hydrogel
Electrophoresis of charged drug, with related
swelling
Ultrasound irradiation
Ethylene-vinyl alcohol hydrogel
Ultrasound irradiation, with related temperature
increase
Source
: Brannon-Peppas, L.,
Medical Plastics and Biomaterials Magazine
, November issue, 1997. With permission.
