Surface erosion

poly(lactic acid-co-glycolic acid) and PCL are biodegradable. However, these materials typically biodegrade by bulk erosion. This may result in large particles breaking away from the degrading stent. Such particles, when released into the bloodstream, may cause emboli or other complications. Therefore, materials are desirable that degrade by surface erosion rather than bulk erosion. 

Degradation by surface erosion is more likely with hydrophobic polymers. The overall hydrophobic nature of the pol mier precludes or at least inhibits the incursion of water into the interior of the polymer, while water-labile linkages exposed on the surface of the polymer hydrolyze resulting in the degradation of the pol mier from the outermost surface of the bulk pol mier. Therefore, hydrophobic polymers are more suitable for degradable medical implants than hydrophilic pol m ers (74). 

Biodegradable hydrophobic polymers useful for the fabrication or coating of implantable medical devices are summarized in Table 8.12. 

Poly(ethylene glycol-co-butylene terephthalate) Poly(4-hydroxy-L-proline ester) Poly(l,10-decanediol-co-L-lactic acid) 

Therapeutic agents may be optionally added to the polymers to create a composition useful for localized sustained delivery of the agents to a patient at the site of implantation of a medical device. The therapeutic agent may be incorporated during the polymerization process or it may be blended with the polymer after it is formed (74). 

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Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

Figure 13-9a shows the relative separation of the full-film, mixed-film, and boundary. If a full-film exists, the bearing life is almost infinite. The limitation in the case of full-film is due to lubricant breakdown, shock load, bearing surface erosion, and fretting of bearing components. Figures 13-9b and 13-9c are cross sections showing the various contamination types. Oil additives are contaminants that form beneficial surface films. 

Cavitation wear, which occurs when a solid such as ship propeller moves at high speed in a liquid, leading to the formation of bubbles, which are a maj or cause of surface erosion. 

More definitive evidence of enzymatic attack was obtained with 1 1 copolymers of e-caprolactone and 6-valerolactone crosslinked with varying amounts of a dilactone (98,99). The use of a 1 1 mixture of comonomers suppressed crystallization and, together with the crosslinks, resulted in a low-modulus elastomer. Under in vitro conditions, random hydrolytic chain cleavage, measured by the change in tensile properties, occurred throughout the bulk of the samples at a rate comparable to that experienced by the other polyesters no weight loss was observed. However, when these elastomers were implanted in rabbits, the bulk hydrolytic process was accompanied by very rapid surface erosion. Weight loss was continuous, confined to the... 

FIGURE 23 Rate of enzymatic surface erosion of a 1 1 copolymer of e-caprolactone and 6-valerolactone, crosslinked with a dilactone to form an elastomer. The effect of substitution of the e-caprolactone nucleus is also shown. (From Ref. 98). 

FIGURE 24 Rate of enz3nnatic surface erosion of e-caprolactone crosslinked with 12 mol % of the dilactone, 2,2-bis( e-caprolactone-4-yl)propane. (From Ref. 98.)... 

When a hydrophobic polymer with a physically dispersed acidic excipient is placed into an aqueous environment, water will diffuse into the polymer, dissolving the acidic excipient, and consequently the lowered pH will accelerate hydrolysis of the ortho ester bonds. The process is shown schematically in Fig. 6 (18). It is clear that the erosional behavior of the device will be determined by the relative movements of the hydration front Vj and that of the erosion front V2- If Vj > V2, the thickness of the reaction zone will gradually increase and at some point the matrix will be completely permeated with water, thus leading to an eventual bulk erosion process. On the other hand, if V2 = Vj, a surface erosion process wiU take place, and the rate of polymer erosion will be completely determined by the rate at which water intrudes into the matrix. 

Convincing evidence for a surface erosion process is shown in Fig. 8, which shows the concomitant release of the incorporated marker, methylene blue, release of the anhydride excipient hydrolysis product, succinic acid, and total weight loss of the device. According to these data, the release of an incorporated drug from an anhydride-catalyzed erosion of poly (ortho esters) can be unambiguously described by a polymer surface erosion mechanism. 

Surface erosion not only leads to zero-order drug release from devices that maintain a constant surface area, but has other important consequences. Among these are the following (1) the rate of drug release is directly proportional to drug loading, (2) the lifetime... 

Although it was possible to achieve constant in vitro release of levonorgestrel for up to 410 days at which point the experiment was discontinued, release of the drug was not controlled by surface erosion of the polymer but instead the device underwent bulk erosion and release of levonorgestrel was completely controlled by its rate of dissolution. Bulk erosion of the rod-shaped device was evident by scanning electron microscopy as shown in Fig. 17. 

Heller, J., Control of polymer surface erosion by the use of excipients, in Polymers in Medicine II (E. ChieUni, P. C. Migliaresi, Giusti, and L. Nicolais, eds.). Plenum Press, New York, 1986, pp. 357-368. 

Figure 4. Close-up view of the GG-loaded gel in the pyloric antrum of the stomach. Surface erosion is indicated by arrowheads.

Figure 5. Lateral radiographs of the abdomen made at 36 h (A), 48 h (B), and 60 h (C). The size of a hydrogel (arrow) gradually reduced in the presence of food mainly due to the surface erosion.

Various factors that influence the release of drugs from particulate carriers are listed in Table 10. Drugs can be released by diffusion or by surface erosion, disintegration, hydration, or breakdown (by a chemical or an enzymatic reaction) of the particles. The release of drugs from microspheres follows a biphasic pattern that is, an initial fast release followed by a slower... 

Surface erosion particle diffusion and leaching Total disintegration of particles Environment... 

One effect of the electrochemical reactions in an aqueous system is a local pH change around the electrodes. By water electrolysis, hydronium ions (H30+) are generated at the anode, while hydroxyl ions (OH ) are produced at the cathode. These changes have been utilized for controlling the permeability of polyelectrolyte gel membrane or on-off solute release via ion exchange or surface erosion of interpolymer complex gels. 

Another mechanism for modulated drug release is local pH-induced surface erosion of interpolymer complex gels with entrapped solutes, as shown in... 

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