**4. Drug release from chemically cross-linked hydrogels**

Crosslinking of biopolymers provide a stable and non-soluble biomaterial which preserves the properties of the original biopolymer and displays a longer durability. Consequently, the half-life time of the hydrogels is increased when performing its biological application [5].

Usually, biopolymer crosslinking can be accomplished in two ways: by direct addition of a cross-linking agent followed by formation of a the three-dimensional (3D) network, or by chemical modification of the biopolymer chains with functional groups suitable for crosslinking with a compatible cross-linker. The first approach takes advance of the functional groups already present in the biopolymer, typically amine (NH2), hydroxyl (-OH), carboxylic acid (-COOH), amide (-CONH-, -CONH2), thiol (-SH) or sulfate (-SO3H) groups [29]. Examples of cross-linkers are dialdehyde derivatives, NH2-PEG-NH2 molecules, COOH-PEG-COOH derivatives, diglycidyl ether compounds, vinyl sulfone groups, etc. These agents cross-link through Michael-type addition, thiol exchange/disulfide cross-linking or Schiff-base processes among others [30]. In some case the addition of coupling agents such as carbodiimides derivatives, N-hydroxysuccinimide (NHS) or N-hydroxybenzo triazole (HOBt), is required for the cross-linking. In the second approach new active functionalities are created in the biopolymer [31, 32] which are appropriate for a broad range of cross-linking processes such as azide–alkyne cycloadditions, Diels–Alder reactions, ultraviolet (UV) photoinitiated crosslinking, (meth)acrylation reactions [5, 32]. Examples of cross-linkers are oxanorbornadiene, cyclooctyne, maleimide, trans-cyclooctene, norbornene, PEG-di(meth)acrylates among others.

The crosslinking of biopolymers produces hydrogels with elastic and deformable structures and great topochemical accessibility which is able to accommodate different kind of active molecules, such as drugs, for sustained release (**Figure 1**).
