Biodegradable polymer erosion

Different horticultural items made of biodegradable polymers, such as plant pots, seed/fertilizer tape and binding materials, foams and nets for erosion control, offer reduction of system complexity by reducing the number of work steps. 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Bittner, B., Witt, C., Mader, K., and Kissel, T. (1999), Degradation and protein release properties of microspheres prepared from biodegradable poly(lactide-co-glycolide) and ABA triblock copolymers Influence of buffer media on polymer erosion and bovine serum albumin release, J. Controlled Release, 60, 297-309. 

Kipper, M. Narasimhan, B. Molecular description of erosion phenomena in biodegradable polymers. Macromolecules 2005, 38 (5), 1989-1999. 

Mader K, Cremmilleux Y, Domb AJ, et al. In vitro in vivo comparison of drug release and polymer erosion from biodegradable P(FAD-SA) polyanhydrides - a noninvasive approach by the combined use of electron paramagnetic resonance spectroscopy and nuclear magnetic resonance imaging. Pharm Res 1997 14(6) 820-826. 

A much more desirable erosion mechanism is surface erosion, where hydrolysis is confined to a narrow zone at the periphery of the device. Then, if the drug is weU-immobihzed in the matrix so that drug release due to diffusion is minimal, the release rate is completely controlled by polymer erosion, and an ability to control erosion rate would translate into an ability to control dmg delivery rate. For a polymer matrix that is very hydrophobic so that water penetration is limited to the surface (thus Hmiting bulk erosion), and at the same time, allowing polymer hydrolysis to proceed rapidly, it should be possible to achieve a drug release rate that is controlled by the rate of surface erosion. Two classes of biodegradable polymers successfully developed based on this rationale are the polyanhydrides [31] and poly (ortho esters) [32], the latter of which is the subject of this chapter. 

Aliphatic polycarbonates are a class of surface erosion biodegradable polymers attracting great interests due to their good biocompatibility, favorable mechanical properties and low toxicity (18,19), The polymerization of aliphatic cyclic carbonates such as trimethylene carbonate (TMC) have been extensively studied (20,21). 

M. Deng, et al.. In situ porous structures a unique polymer erosion mechanism in biodegradable dipeptide-based polyphosphazene and polyester blends producing matrices for regenerative engineering, Adv. Funct. Mater. 20 (17) (2010) 2794-2806. 

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