Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 22

∑ bulk polymerization;

∑ polymerization in organic solvents;

∑ emulsion polymerization (suitable for nanoparticles); and

∑ interfacial polymerization (suitable for nanocapsules).

in particular, PaCa nanoparticles have been widely investigated (Dossi
et al. , 2010); they have been used as delivery systems for anticancer drugs,
proteins and peptides, and oligonucleotides (lanza et al. , 2007). PaCa
nanospheres have also been used for ocular therapies, in order to bypass the
issue of the short half-life time of eye drops (Vauthier and Couvreur, 2005).
Polycyanoacrylates are also employed as tissue adhesives for the closure of
skin wounds, and as embolytic material in endovascular surgery. as regards
degradation, two mechanisms have been proposed. In the first, the hydrolysis
products formaldehyde and alkylcyanoacetic esters are formed, an inverse
Knoevenagel reaction; the reaction is subjected to equilibrium which depends
on pH, temperature, and alkylcyanoacrylate ester involved. This mechanism
is slow and competes with other more rapid processes; moreover, degradation
rate depends on the physical form of the polymer. The second kinetic path
comprises hydrolysis of the alkyl side chains at basic pH, with alcohol and
poly(cyanoacrylic acid) as degradation products (Vauthier and Couvreur,
2005). This mechanism is catalyzed by various esterases, and thus it is
believed to be the main degradation process for in vivo applications. even
if formaldehyde has been found to be a possible hydrolysis product, PaCa
shows low toxicity, both in form of nanoparticles or implantation.

1.8 Poly(propylene fumarate) (PPF), poloxamers,
poly( p -dioxanone) (PPDO), polyvinyl alcohol
(PVA)
Poly(propylene fumarate) (PPF) is an unsaturated linear polyester and
degrades into fumaric acid and propylene glycol, which are both biocompatible
compounds (Fig. 1.28). PPF can be subjected to crosslinking with a vinyl
monomer, because of the presence of a double bond of the fumarate. This
material has been tested for bone repair surgery (Domb et al. , 1997).
Poloxamers are copolymers composed of two polyoxyethylene blocks
separated by a polyoxypropylene block (Fig. 1.29). Polyoxyethylene content
can vary from 10 to 90% in weight terms. Poloxamers are soluble in water
and in polar solvents; they can be solid, liquid, or paste depending on POE/
POP ratio and molecular weight. This material has been investigated for drug
delivery applications. However, since poloxamers contain ether linkages,
they are not readily metabolized and thus are not truly bioresorbable (Domb
et al. , 1997).

￿ ￿ ￿ ￿ ￿ ￿

Poly( p -dioxanone) (PPDO) is an aliphatic poly(ether ester) (Domb et al. ,