**Conflict of interest**

*Biophysical Chemistry - Advance Applications*

the hydrogel in the swollen state.

**4. Conclusions**

riboflavin.

**Acknowledgements**

PN-III-P2-2.1-PED-2016-0187.

defects, ensuring an interface between osteochondral grafts and host tissues in order to prevent edema, or acting as glue between the graft and the tissue.

properties of dentin and an enhanced stability of the collagen network.

The chitosan/riboflavin pair can be used in filling demineralized dentin substrates, due to the ability of riboflavin to generate, under UV light, radicals that initiate the cross-linking of collagen fibrils from dentin and chitosan. Fawzy et al. [61] have studied in detail the changes in dentin morphology by treating demineralized dentin with chitosan solutions in various concentrations, in the presence of riboflavin, over a period of 6 months. Their results showed improved mechanical

Hyaluronic acid, as biocompatible polymer, is often used in drug formulations. Derivatives of hyaluronic acid such as methacrylate hyaluronic acid and thiolated hyaluronic acid are also biocompatible. In the presence of riboflavin and blue light (458 nm), slow gelation occurs [62], a process that has a different kinetic compared to the system described in Ref. 45. The induced gelation in a solution of these two hyaluronic derivatives occurs in 15 min at body temperature, and this delay in reaching the gel state can be exploited for in situ applications in ophthalmology and stomatology. The systems were characterized by rheological methods and FT-IR, and the ability to encapsulate and release bovine serum albumin and their cyto-compatibility were tested. Interestingly, in the absence of thiolated hyaluronic acid, the gelation process does not occur, while in the absence of riboflavin and light, the transition from sol to gel occurs even slower (in 24 h), and the resulting gel is weaker (the gel moduli have lower values). The gelation process is the result of two reactions: the oxidation of thiol groups leading to the formation of disulfide bonds (evidenced by IR spectroscopy) and the addition of the thiol group to the double bond of the methacrylate group.

A similar gel was obtained by reaction of methacrylated dextran with L-arginine in the presence of riboflavin, L-arginine initiating the photo-cross-linking [63]. The authors found that the riboflavin concentration influences the rate of gelation, an optimum value being in the range 0.2–1.0 wt%. At low riboflavin concentration, the resulting materials are brittle, while at higher concentration, they are compliable. The pH also affects the gelation process, a neutral pH being more favorable. The presence of arginine was essential as cross-linker, since gelation did not occur in its absence. The concentration of arginine was found to influence the properties of the gel. Thus, the weight ratio of arginine/methacrylate precursor in the range 0.8–1.2 led to gels with desirable properties in terms of shape stability, stickiness, and compliability. Microscopic images (SEM) indicated the honeycomb structure of

In conclusion, in this review, we presented the general pathways for the generation of reactive species in the presence of riboflavin and UV-Vis light. The use of riboflavin as photoinitiator for polymerization processes presents the advantage of it being nontoxic and biocompatible. The research papers mentioned in this review also highlight the potential medicinal applications of gels formed in the presence of

This work was supported by the Romanian National Authority for Scientific Research, CNCS–UEFISCDI, project numbers PN-III-P4-IDPCE-2016-0734 and

**20**

The authors declare no conflict of interest.
