Crosslinking of Polylactide by High Energy Irradiation and Photo-Curing

Abstract

Polylactide (PLA) is presently the most studied bioderived polymer because, in addition to its established position as a material for biomedical applications, it can replace mass production plastics from petroleum. However, some drawbacks of polylactide such as insufficient mechanical properties at a higher temperature and poor shape stability have to be overcome. One of the methods of mechanical and thermal properties modification is crosslinking which can be achieved by different approaches, both at the stage of PLA-based materials synthesis and by physical modification of neat polylactide. This review covers PLA crosslinking by applying different types of irradiation, i.e., high energy electron beam or gamma irradiation and UV light which enables curing at mild conditions. In the last section, selected examples of biomedical applications as well as applications for packaging and daily-use items are presented in order to visualize how a variety of materials can be obtained using specific methods.

Keywords: crosslinking; electron-beam; gamma rays; irradiation; photo-crosslinking; poly(lactic acid); polylactide.

Figure 1

Possible processes involving radicals on polylactide (PLA) chains formed by irradiation with an electron beam (on the basis of Reference [27]).

Figure 2

Multifunctional crosslinking agents applied in radiation-induced crosslinking [30]. (Adapted with permission from Elsevier, 2005).

Figure 3

The proposed mechanism of photo-crosslinking of not functionalized PLA in the presence of benzophenone [78]. (Reproduced with permission from Wiley, 2013).

Figure 4

Cyclodimerization of cinnamoyl groups.

Figure 5

Compounds used for polycondensation with PLA diols.

Figure 6

Mechanism of the formation of covalent bond between species bearing azide group and the compound with reactive hydrogen.

Figure 7

PLLA crosslinked by electron beam irradiation (50 kGy). (A) shrinkable tube (a); possible use (b). (B) Appearance of cups after using for hot water: (a) the unirradiated product, (b) the product crosslinked by irradiation [31]. (Adapted with permission from Elsevier, 2005).

Figure 8

SEM images of preparation methods on elastomer scaffold structure: (a) scaffold made using only paraffin microbeads, (b) scaffold prepared using only water emulsified in the polymer solution, (c) scaffold prepared with combined emulsion and paraffin microbeads [91]. (Adapted with permission from Elsevier, 2009).

Figure 9

Images of PDLLA network scaffolds with a gyroid architecture prepared by stereolithography: (A) photograph, (B) microcomputed tomography (µCT) visualization and (C) SEM image. In (D) a light microscopy image is shown for a scaffold seeded with mouse pre-osteoblasts after 1 d of culturing. Scale bars represent 500 µm [57]. (With permission from Elsevier, 2009).

Figure 10

Topography images of crossing lines generated by direct laser writing using a formulation comprising the macromonomer: (a) linear-YNE and (b) star-YNE PLAs, both with a stoichiometric amount of the thiol (stoichiometry alkyne/thiol 1:2) and 3 wt % of photoinitiator. Images were obtained using a confocal microscope [107]. (Adapted with permission from Elsevier, 2017).

Figure 11

SEM images of porous structures with Schwarz primitive pore network architecture prepared by stereolithography from PDLLA and nano-HAP composite resins containing 5 wt % nano-HAP. Scale bars 200 µm [108]. (Adapted with permission from Elsevier, 2013).

Figure 12

(a) Micrographs of a 2PP-fabricated PLA scaffold, (b) fluorescence of PLA scaffolds after implantation into mice; MSC—mesenchymal stem cell [71]. (Adapted from Future Medicine, 2016).