**4. Polymer gels containing cyclodextrins**

Polymers containing CDs have the ability to form supramolecular hydrogels mostly due to host-guest complexation or by the inclusion of linear polymeric chains into host cavities [58]. The latter leads to the formation of pseudorotaxanes. Literature data show that, unlike β-CD, α and γ-CD are able to generate interactions favorable to the obtaining pseudorotaxanes [59]. However, in particular cases, pseudorotaxanes were prepared using β-CD and polylactic acid or polypropylene glycol [60, 61].

Several studies regarding host-guest interactions in hydrogel systems are reported in the literature [60, 62, 63]. By appending host and guest units to the polymer chains, one can modify and control their behavior in solutions and obtain gel systems. This strategy ensures conditions for the creation of topological crosslinks based on host-guest interactions, which have the advantage of being reversible and movable. Host and guest units can be grafted on the chains of numerous polymers (hyaluronic acid [64, 65], carboxymethyl cellulose [66, 67], sodium alginate [42, 68], polyacrylic acid [55, 69, 70], polyvinyl alcohol [71, 72], polymethyl vinyl ether-alt-maleic acid [73, 74], poly-N-isopropylacrylamide [75], and polyethylene glycol [76]). These supramolecular gels find use in the medical field for drug delivery, tissue culture, and

medical treatments [77, 78]. Depending on the desired application, hydrogels formed by host-guest interactions can be generated in several ways. Therefore, CD having the role of guest molecules can be grafted separately on polymer chains and mixed with polymers bearing guest grafts to form supramolecular assemblies [41, 60].

The formation of alginate gels in the presence of divalent cations was monitored by EPR spectroscopy considering the changes in the dynamics of spin-labeled alginate chain [79]. In a recent study, we showed that the functionalization of alginate with CD (as host) or adamantane (as guest) influences the properties of ionotropic generated gel in the presence of Ca2+ ions [42]. As a consequence, polymer functionalization and subsequent interactions between the appended host and guest units change the morphology of the resulting xerogels (**Figure 4**).

In fact, the derivatization, together with the host-guest interactions, has an impact on the rheological properties, i.e., the hydrogels made from a mixture of adamantanefunctionalized and CD-functionalized alginates presented higher storage and elastic

#### **Figure 4.**

*Schematic representation of ionotropic gelation of functionalized alginates. SEM images of alginate xerogels: (a) nonfunctionalized alginate and (b) alginate functionalized with CD and adamantane units [42].*

**Figure 5.**

*(a) Viscoelastic properties and (b) EPR spectra of alginate hydrogels functionalized with β-CD or adamantane units [42].*

moduli values (**Figure 5a**). Appending the paramagnetic moieties to these functionalized alginates allows evidencing the changes at the molecular level of the dynamic of the overall motion (**Figure 5b**). All these findings led to the conclusion that the presence of host-guest interactions can modulate the features of alginate hydrogels.

The polymerization of the inclusion complex is another technique that can be used to obtain CD-based hydrogels. Ikura *et al.* approached the free-radical copolymerization of polyethyl acrylate crosslinked with peracetylated γ-cyclodextrin methylacrylamide monomer and acrylate monomers and investigated the effect of the size of the main chain monomers on the formation of movable crosslinking points [80]. Thus, it was observed that the small polymer main chains penetrated the CD units and acquired the role of movable crosslinking points in the hydrogel, whereas copolymerization with bulky monomers leads to hydrogels without movable crosslinking points.

In aqueous media, host-capped polymers can be mixed with guest-grafted chains to obtain hydrogels. Ioniță *et al.* showed that the reaction of isocyanate end-capped polyethylene glycol with β-CD leads to the formation of a fibrous gel with covalent network [81]. Moreover, by adding spin probes (e.g., TEMPO and adamantane-TEMPO), it was possible to determine the extent by which gel fibers were affected by hydrogen bonding interactions with solvent, crosslinks density, or temperature. The study revealed that, at low temperature, ice crystallization is prevented inside the gels, and this phenomenon is accompanied by the formation of supercooled water.

Another method to obtain hydrogels is the mixing of end-capped guest crosslinkers with certain host-grafted polymers [41, 60]. A recent study has shown that the multivalence effect within a polyethylene glycol-adamantane/β-CD-alginate system can be quantified to create hydrogel-like cell matrices [82]. The complexation of CD functionalized alginate with adamantyl end groups on polyethylene glycol chains changed the valence of the system. Thus, a correlation could be observed between the multivalence generated by the variation in the number of polyethylene glycol arms and the strength of binding affinities inside the hydrogel.

A particular type of gel is formed between guest-grafted polymers that are capable of forming multilayer vesicles and small host molecules grafted on different polymeric chains. A good example is the gel formed by thiolated monolith polymers, in which β-CD vesicles were introduced in order to formulate a hydrogel with pHresponsive properties [83].

Polymeric gel formation can be described using rheological, viscosity, and dynamic light scattering measurements. These methods provide global information *Cyclodextrins as Bricks for Tuning Polymer Properties DOI: http://dx.doi.org/10.5772/intechopen.105688*

on such systems. Spectroscopic methods, such as fluorescence, IR, or UV-Vis spectroscopy, are often used to describe changes in the organization of macromolecules that are usually governed by noncovalent interactions [84]. In the particular case of gel formation, electron microscopy techniques are used to evidence the gel fibers. An interesting, powerful, but still rarely used approach in studying gels involves EPR spectroscopy [34, 79, 81, 84, 85]. The EPR spectroscopy is suitable to study polymer systems and gels as the method can provide insights into local, static, and dynamic properties of these systems. This method can evidence nanoscale inhomogeneities in polymers systems [86, 87].

By using spin-labeled CDs, it was possible to monitor the gel formation process, while the diffusion of various spin probes can evidence the nonuniform properties of covalent gels [81, 88, 89]. Other EPR studies explored the self-assembly of pluronic F127 leading to gel phase as a function of temperature and concentration of CD [34, 35] or the formation of supramolecular gels resulted by the assembly of lowmolecular-weight gelators [85].
