Page 670 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Selected Topics 633

UHMWPE is highly crystalline and highly cross-linked through gamma radiation. Tests have
shown that the UHMWPE wears out at an average of 0.1 mm each year. Since most assemblies
employ about a 10 mm-thick layer of UHMWPE, the lifetime of the hip joint replacement based on
only the wear of the UHMWPE is about 100 years.
Controlled release of drugs can be envisioned as occurring via three major routes. One approach
utilizes diffusion-controlled release through membranes or matrices. Here the rate of release is con-
trolled by the permeability of the membrane or matrix. In the second approach, the drug is captured
within a matrix that undergoes degradation, usually through aqueous-assisted solubilization or deg-
radation (including hydrolysis). Here the rate of drug release is dependent on the break up of the typ-
ically polymeric matrix. For the second approach, a number of polymers have been used including
poly(glycolic acid) (PGA) and polyanhydrides. The third approach involves simple degradation of
a drug-containing polymer where the drug moiety is present as part of the polymer backbone or as
side chains. Degradation of the polymer results in the release of the drug in some fashion.
Controlled release of drugs using polymer intensive materials is becoming more common place.
The release “pack” can be attached externally such as many of the “nicotine patches” that deliver
controlled amounts of nicotine transdermally. The release “pack” can also be introduced beneath
the skin or within the body as is the case with many diabetes treatment assemblies.
A number of siloxane-containing controlled release packs have been devised and are being used.
Glaucoma, motion sickness, and diabetics have been treated using drugs dispersed in a silicon matrix.
This kind of pack needs to be placed near the site of intended activity for greatest effectiveness.
Implant materials can be divided into two general categories dependent on the time require-
ment. Those that are present for release of a drug or to hold a broken bond in place until suffi cient
healing occurs are termed short-term implant materials. The second group includes materials that
are to function over a longer time such as for the life of the patient. In the first case, degradation is

generally required while for the longer-term material inertness and long-term stability are typically
required. There are times when this is not true. For instance, some of the newer biomaterials act as
scaffolds that promote tissue growth by providing a three-dimensional framework with properties
that encourage favorable cell growth. This material may be designed to be either short or long term.
One approach to designing scaffolding material involves placing certain amino acid-containing
units on the polymeric scaffold that encourage cell growth.
Another aspect related to control release of drugs concerns the type of structures that currently
appear to be working. Not unexpectedly, because of compatibility and degradation purposes, most
of the effort on the control release formulations includes polymers that have both a hydrophobic and
hydrophilic portion with the material necessarily containing atoms in addition to carbon. Another
concern is that the products of degradation are not toxic or do not go on to form toxic materials. It
has also been found that amorphous materials appear to be better since they are more fl exible and
permit more ready entrance of potential degradative compounds.
Another area of activity involves the synthesis of supermolecular layers that are connected
through cross-linking giving essentially one molecule thick micelles. Depending on the particular
template and solvents employed, these monolayers can be designed to have almost any combination
of hydrophilic and hydrophobic sites. Again, specific control of release rates, degradation times

and routes, biocompatibility or incompatibility is possible. Many of these micelle-based delivery
systems are based on a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock
or on a polypeptide and poly(ethylene oxide) combination. Drug delivery has also been achieved
using conducting electroactive polymers formed through controlled ionic transport of counterions
(dopants) in and out of membranes.
Hydrogels have been used that shepherd drugs though the stomach and into the more alkaline
intestine. Hydrogels are cross-linked, hydrophilic polymer networks that allow the smaller drugs

access to their interior and that can be designed to inflate, swell at the desired site, to deliver the
drug. These hydrogels have largely been formed from materials with a poly(acrylic acid) backbone.
More about hydrogels in Section 19.13.

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