Figure 7.

Cross-linking used for the three most common materials and its combinations. Blue, yellow, and red circles represent chemical, physical and thermal cross-linking, respectively. Superposed circles indicate that two cross-linking classifications have been combined. Each circle contains a classification codification (upper code) and the number of papers for the corresponding code combination (lower number). Chemical cross-linking has been chosen as primary classification, therefore papers not using it have been codified under C7 for graphical representation purposes and no papers are allocated to this code alone, e.g., alginate C2/13 – P4/1 (blue and yellow circles superposed) indicates that 13 papers used C2 ([100–500] mM Ca²⁺ solution) to cross-link alginate and another paper used C2 and P4 (UV light—no data of wavelength) to perform the cross-linking.