**3. Cyclodextrins covalently attached to polymeric chains**

This section aims to summarize new research that has emerged in the past few years on materials composed of synthetic or natural polymers to which CD derivatives have been chemically attached. The polymers discussed here were chosen considering the extensive investigations and applications in pharmaceutics and biomedicine as drug excipients, biocompatible alternative materials in tissue engineering, contrast enhancers, molecular recognition models, etc. To achieve novel and/or enhanced properties of these classes of compounds, the functionalization with different CD units is employed. To covalently attach CDs onto polymer chains, multiple types of crosslinking agents (citric acid, epichlorohydrin, aldehydes, carbodiimides, and amines) can be used. In recent years, polymers functionalized with CD units have been studied for the development of a variety of polymeric networks [41, 42]. Moreover, the polymeric material based on CD units can be modulated in such a way to form nano/micro/macroparticles, gels, micelles, coating films, or fibers [43–45]. Many studies in this regard are performed using chitosan, mostly due to its remarkable biological properties, including antimicrobial activity, nontoxicity, biocompatibility, and biodegradability, and at the same time to the possibility to functionalize it [46, 47].

For instance, Campos *et al.* took this advantage and appended β-CD on chitosan nanoparticles in the presence of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS), thus developing a potential carrier for botanical pesticides [48]. A green-assembly strategy to obtain β-CD-chitosan functionalized graphene oxide hydrogels in the presence of sodium ascorbate has been reported, with application as recyclable decontaminants in wastewater treatment [49]. Both simple polymers and polymer mixtures are favorable to the attachment of CD units, as demonstrated by the research of Hardy *et al.* regarding the complex formed between chitosan and alginate for the release of piroxicam [50]. The research revealed that CD appending on chitosan generates a decrease in the number of amino groups, thus modulating the alginate complexation. In a recent study, alginate has also been functionalized with either host units (n-alkyl amine CD derivatives) or guest units (adamantane) [42]. In this particular case, the grafting procedure was

performed in aqueous media in the presence of EDC and N-hydroxysulfosuccinimide (NHSS). The properties of the gel obtained by mixing the two types of alginates in the presence of Ca2+ ions were influenced by the host-guest interaction and the length of the alkyl chain of the β-CD derivatives. Thus, by mixing the decorated alginates with CD and adamantyl units, materials suitable for encapsulating both large molecules and small species can be obtained. In addition, the functionalization of alginate with units of β-CD derivatives by using Cu(I)-catalyzed azide-alkyne cycloaddition click reaction led to materials with good drug release properties [51]. The same procedure was used in another study, showing that the grafting degree of β-CD on alginate can be controlled and modulates the release/uptake of the model molecule methyl orange [52]. In addition, a composite based on sodium alginate grafted with β-CD using epichlorohydrin and NaOH was successfully obtained for the first time and used as a matrix in the immobilization of *Arthrobacter simplex* cells for cortisone acetate biotransformation [53].

Not only natural polymers were grafted with CD derivatives, but also synthetic ones. In this regard, the polyacrylic acid was modified by attaching a CD derivative (2-aminoethyl)amino-deoxy-β-CD) [54]. The study targeted to obtain the selfassembly of β-CD and adamantyl moieties covalently linked to polyacrylate networks for application in controlled complexation and release of ethyl orange, methyl orange, and methyl red. Another study developed a multistimulus responsive supramolecular hydrogel based on host-guest and electrostatic interactions between β-CD dimer and methoxy-azobenzene molecules grafted on polyacrylic acid [55]. The obtained materials showed thermo-, photo-, and pH-responsive behavior determining a reversible sol-gel transition.

Chabalala *et al*. described the grafting of β-CD molecules on polyacrylonitrile using citric and sulfuric acids as crosslinkers [56]. The nanofiber membranes produced by the electrospinning method were used for the adsorption of bromophenol blue and atrazine. Mono-(6-ethylenediamine-6-deoxy)-β-CD was appended in the presence of EDC/NHS crosslinking agents onto the external surface of a plasma separation membrane based on polyvinylidene fluoride [57].
