A star‐shaped cholic acid‐core poly(CL‐ran‐LA)‐b‐PEG copolymer act as a promising drug‐loaded biomaterial for liver cancer chemotherapy [302]. Further the protein fouling by enzymes was lowered by block copolymer nanoparticles through self‐assembly of PEO‐b‐PLA [303]. Modified thiolated chitosan greatly increases its mucoadhesiveness and permeation properties, thus increasing the chances of nanoparticle uptake by the gastrointestinal mucosa and improving drug absorption for chemotherapy of lung cancer [304]. Linear and star‐shaped amphiphilic PEG‐b‐PLA with or without β‐cyclodextrin (β‐CD) conjugation were synthesized. Oil‐based formulations are formed by emulsion method in organic solvent with a defined core‐shell structure and a particle size of ~150–300 nm. Such reverse micelles (RMs), consisting of a hydrophilic core surrounded by hydrophobic surface, were constructed using PEG‐b‐PLA‐β‐CD in nonpolar solvents and used to sequester hydrophilic guest molecules. They have attracted much attention as drug delivery cargos, as they can form a continuum with other lipid barriers in the body, such as skin lipids and cell membranes. This oil‐based formulation fabricated from above‐mentioned copolymer allowed a high percentage of protein loading, which is prudential for cellular delivery [305].
4.6.2 Radiation Effects
The effect of radiation (γ‐ and electron‐beam) on the degradation of PLA and its copolymers received considerable attention in the past [274, 306, 307]. Irradiation of polymers generates free radicals that induce chemical changes such as chain scission and cross‐linking. The atmosphere of the surroundings, irradiation dose, chemical composition, and morphology of the polymer influence the degradation mechanism. The type of end groups, pendant units, and copolymer structure (such as aromatic or aliphatic units) showed a significant effect on the stability of the polymers toward irradiation. In aliphatic polyesters, the ester linkage and the tertiary carbons in the branched polyesters are the preferred site for the degradation. Mechanical properties and molar mass are significantly affected by radiation. Therefore, the sterilization method should be carefully chosen. Ethylene oxide is mainly used when sterilizing degradable devices, by tuning the method can implants be sterilized without influencing the polymer. Electron beams and γ‐rays are also used for the sterilization of implants [306]. Generally, a dose of 25 kGy is used for such purposes [308]. Albertsson and coworkers [309] reported that copolymerization of LLA with a small amount of CL or DXO increased the stability in comparison to PLLA. The most abundant low molar mass degradation product was identified as DXO.
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