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Nov/Dec 2000
Vol. 3, No. 9, p. 17.

news in brief

PEG-ing the microsphere

Opening ArtResearchers have long sought a means of delivering injectable-only medications orally. Unfortunately, because acidic conditions in the stomach degrade most drugs, this goal has yet to be reached. It is hoped, however, that hydrogel materials might shepherd the medicines through to the more alkaline intestine.

Hydrogels are cross-linked, hydrophilic polymer networks that are highly permeable to various drug compounds, can withstand acidic environments, and can be tailored to “swell” with increasing pH to release entrapped molecules through their weblike surfaces. During the past two decades, research has focused primarily on networks containing poly(acrylic acid) (PAA) backbones. PAA hydrogels are known for both their superabsorbency and their ability to form extended polymer networks through hydrogen bonding. They are also excellent bioadhesives and can adhere to mucosal linings in the gastrointestinal tract for extended periods, slowly releasing their encapsulated medications.

The major stumbling block to the development of PAA hydrogels for drug delivery purposes has been their synthesis. Traditionally, the gels have been formed using organic solvents that are toxic to humans. Recently, however, Nicholas Peppas and his colleagues at Purdue University in West Lafayette, IN, have developed a novel method for the aqueous synthesis of PAA grafted with poly(ethylene glycol) (PEG) chains.

At the Fall 2000 ACS National Meeting in Washington, DC, graduate researcher Petr Bures presented preliminary results on the molecular dynamics of these nontoxic, pH-sensitive materials. Essentially, the Purdue researchers found that the properties of the hydrogels, which they term “microspheres” in reference to their apparent size and shape, are directly related to the molecular weight (or equivalently, the chain length) of the PEG substituents. Specifically, Peppas’s group found that the greatest hydrogen bonding and most significant swelling occurred in systems incorporating extended PEG chains. The combined results of this investigation add new promise to the development of oral drug delivery systems.


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