**2.3. Interpenetrated networks (IPN) based on collagen and other polymers**

IPN hydrogels are based on the physicochemical interactions between the collagen polymeric chains and chains of another type of polymer, as shown in **Figure 4**. The hydrophobic, ionic or hydrogen bonding inside the IPN is responsible for the improved mechanics and degradation behavior. Two examples are the IPNs formed between collagen and chitosan [29], and collagen and polyethylene oxide (PEG) [30]. In these approaches, the ECM extracted collagen is combined with different mass concentrations of polymers, and later this mixture is incubated at 37°C to induce the collagen polymerization. The polymerization process is influenced by the presence of the exogenous polymeric chains altering the collagen fiber size and the physical cross-linking. The IPN hydrogels show poor stability with the change of the temperature and pH [31]; but the enhanced mechanical properties of these biomaterials are adequate for the cell and drug encapsulation [32].
