Biomedical Engineering Reference
In-Depth Information
Delayed dissolution
Drug molecules retained until
further polymer erosion
Polymer dissolves or
degrades slowly
Released drug molecules
can dissolve
Diffusion controlled
Molecules slowly diffuse
through voids in polymeric
device
Drug molecules unable to freely diffuse
due to insoluble polymer matrix
Drug solution flow control
H
2
O
Drug solution flow
into environment
Water molecules cross semipermeable
membrane due to high osmotic gradient
Drug molecules dissolve in water and
flow through pore at controlled rate
FIGURE 8.1
Examples of mechanisms of temporal-controlled drug release. (Reprinted from Uhrich, K.E.
et al.,
Chem. Rev.
, 99, 3181, 1999.)
Biodegradable polymers undergo a chemical degradation process in the human body that involves
bond cleavage to form smaller molecules. These molecules can then be resorbed or excreted. Natural
polymers such as starch, proteins, and chitosan are biodegradable materials with minimum toxicity
(this concept of naturally degradable biomaterials was fi rst applied successfully in the biomedical
fi eld with catgut sutures).
14
Inspired by these natural materials and realizing their physicochemical
limitations, chemists have developed other synthetic biodegradable polymers such as poly(esters),
poly(saccharide), poly(amides), poly(anhydride), poly(ortho esters), and poly(saccharide-amide).
4,14
These synthetic biopolymers combine both the biocompatibility and the biodegradability of natural
materials and the versatility of synthetic structural design, readily tailorable with mechanical prop-
erties to meet specifi c drug delivery requirements, for instance, the degradation time of synthetic
biopolymers can be easily adjusted. Guan and coworkers have demonstrated of a class of hybrid
copolymers with good biodegradability and low cytotoxicity. This class of hybrid copolymers was
synthesized using natural saccharide and amino acid building blocks, and the potential application
of this class in gene-delivery has been demonstrated.
15
