**5. Conclusions**

Different types of radiation were successfully used to crosslink polylactide which appeared very sensitive to irradiation mainly due to the presence of methine hydrogen atoms along the polymer chain as well as the possibility of introducing functional end-groups. However, not only crosslinking but also other reaction as branching and chain scission proceeds upon irradiation. The proportions of these processes depend on many factors and the specified goal can be achieved by manipulating the irradiation conditions, by PLA functionalization and by the application of reactive additives. The choice of the irradiation type and its parameters depends on these specified goals.

Polylactide-based networks with high gel fraction were synthesized from both high molecular weight PLA and its copolymers as well as from low molecular weight PLAs functionalized at the chain ends by chain linking (coupling) methods.

The first of these approaches was mainly used in crosslinking by irradiation with high energy rays (electron beam and γ-rays). However, to have any control over crosslinking under irradiation, which can generate many radicals, multifunctional crosslinking agents containing unsaturated groups have to be added. As crosslinking by electron beam and gamma irradiation is always accompanied by chain scission, this method cannot be applied for all applications. Chain scission may be limited by adjusting the irradiation dose. The evident advantage of ionizing irradiation is the possibility to

perform crosslinking at low temperatures and excellent penetration. One should, however, have in mind that the radiation a ffects many characteristics of the material, i.e., causes the decrease of glass transition, cold crystallization, and melting temperatures.

Crosslinking by exposure to less energetic UV (or visible) light undergoes in a more controlled manner, although undesired radical reactions may also proceed. Photo-curing is considered as the best method for crosslinking of PLA-based polymers designed for tissue engineering. For crosslinking by UV light, medium molecular weight polylactides (linear and star) functionalized with unsaturated end groups should be prepared. Polylactide macromers are mixed with photosensitive compounds being a source of radicals that initiate polymerization of unsaturated groups. Among di fferent photo-initiators, the compounds which are reported to be not harmful in the context of biomedical applications can be chosen [110]. Because prepolymers used for crosslinking are characterized by low viscosity, it is often possible to mix polymer with the photoactive compound in bulk without using a solvent. This, in turn, enables in-situ crosslinking at low temperature, e.g., after placing liquid oligomers in a body. The weakness of this crosslinking method is the necessity of e fficient PLA (meth)acrylation or another functionalization introducing reactive end groups. Moreover, radical chain-growth polymerization generates non-biodegradable high molecular weight acrylic chains. These acrylic chains become the major drawback as they are di fficult to eliminate from the body. To avoid acrylic chains some authors prepared polylactide networks from prepolymers which were linked via di fferent "click" type reactions; for biomedical application photo-induced, "thiol-ene" addition is well suited. Although photo-crosslinking seems to be a very suitable method for biomedical PLA applications, it also has a limitation, namely limited depth of penetration.

The methods presented here of radiation and photo-induced crosslinking of PLA-based materials could find an application at present (and possibly in the future) in di fferent areas. The still developing sophisticated techniques in tissue engineering (e.g., photolithographic techniques) and in complicated drug delivery systems but also predictions for PLA mass production of durable bioplastics sugges<sup>t</sup> also development of crosslinking methods for this biocompatible and bioderived polyester.

**Author Contributions:** Conceptualization, original draft preparation, supervision of the manuscript, M.B.; participation in writing, classification and preparation of the material for tables, K.B.; preparation of references, revisions at the stages of manuscript preparation, P.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Science Centre in Poland under Grant no. 2018/31/B/ST8/01969. The funder has no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, nor in the decision to publish the results.

**Conflicts of Interest:** There are no conflicts to declare.
