Erodible polymer

Heller, J., Controlled drug release from poly(ortho esters) A surface eroding polymer, J. Control. Rel., 2, 167-177, 1985. 

Figure 21 Schematic presentation of erodible polymer matrix by an electric current.

IC Kwon, YH Bae, SW Kim. Electrically erodible polymer gel for controlled release of drugs. Nature 354 291-293, 1991. 

Concerning drug delivery, electrically erodible polymer gels for controlled release of drugs have been prepared, and a measured release rate of insulin has been observed under electrical stimulus [69]. A suspension of zinc insulin in a mixed solution of poly(ethyloxazoline) and PMAA was formed into a gel by decreasing the pH of the suspension. The obtained complex gel with 0.5 wt% of insulin was attached to a woven platinum wire cathode which was 1 cm away from the anode and immersed in 0.9% saline solution. When a stepped function of electrical current of 5 mA was applied to the insulin-loaded gel matrix, insulin was released in a stepwise manner up to a release of 70%. The insulin rate measured was 0.10 mg/h. 

When describing erosion of and drug release from surface erodible polymers, it is often implicitly assumed that the matrix erodes uniformly, thus resulting in a uniform release profile for a homogenously dispersed drug. While this may be a valid assumption for some homopolymer systems, neglecting the effects of crystallinity, some multicomponent... 

Figure 7 Schematic illustration of a self-polishing antifouling paint with soluble CU2O particles exposed to seawater (no insoluble pigments present for simplicity). Notice the pigment-leached layer and the two moving fronts (eroding polymer front, zE, and dissolving pigment front, zP). After Kiil et al. (2002c).

Which of the following products is made of a surface-eroding polymer (a) Lupron Depot (b) Zoladex (c) Gliadel and/or (d) Vitrasert. 

High-molecular-weight drug in bulk-eroding polymer (limited release by diffusion)... 

Nevertheless, monolithic microspheres can be made to release drug at an approximately constant rate. " The core loading of these microspheres may be increased to create structures similar to those of reservoir microcapsules. An optimum combination of particle sizes (a size distribution), may be prepared to achieve a constant rate of drug release. Preparing microspheres with an erodible polymer in such a way that maximum erosion occurs in conjunction with minimum diffusion may establish a constant release rate. Although the principles described here appear simple, they are difficult to utilize because of their dependence on a number of factors, each of which can complicate the process. 

It was decided that one class of surface-erodible polymers, poly(ortho esters), could potentially fulfill these design criteria, and prehminary results [56] using polyfortho ester) microspheres to deliver model DNA vaccines in mice are discussed in this chapter. 

The first example of a surface eroding polymer was a partially esterified copolymer of methyl vinyl ether and maleic anhydride, published in 1978 [1], These polymers solubilize by ionization of carboxylic acid groups as shown in Scheme 1 and because at low pH the carboxyl groups are unionized and hence the polymer is insoluble, these materials are useful as enteric coatings to protect oral dosage forms from dissolution in the stomach [4-6]. 

Because drug release in a surface-eroding polymer occurs as a consequence of an eroding front that moves through the device, rate of drug release depends... 

The erosion in polyanhydride matrices has been reported to be purely surface of the heterogeneous type, and the results of this study support the past observation [82]. No lag time for drug release was found, as observed also for poly-lactic acid and poly-lactic acid-glycolic acid, which have been known to undergo bulk erosion [79]. In addition, fit of the data to Eqs. (1) and (2) (which have been derived for a surface eroding polymer), confirm the surface eroding nature of the polyanhydrides. 

---