138 Chapter 6 or pore size (the distance between fibers) by changing the number of cross-links and / or collagen concentration makes this model interesting for studying conditions which alter the biomechanical properties of surrounding tissue [96]. The stiffness of collagen substrates can be regulated from one hundred Pa up to several kPa, which mimics the biomechanical conditions of fibrotic, normal or emphysematous lungs. Finally, collagen models are compatible with using standard commercially available kits for endpoint measurements such as RNA, DNA, protein isolation and immunokits. All these advantages make collagen gels a good model for studying lung diseases, metastatic growth, wound repair and fibrosis [55, 66, 90, 97]. Collagen models have several limitations that should be recognized. Fibroblasts, and other mesenchymal cells, will contract/rearrange the hydrogel [98]. The structure of a collagen hydrogel, particularly the fiber arrangement within the hydrogel, can be altered by pH, ionic concentration, and temperature through effects on collagen polymerization [96]. In the end, collagen models are limited by their simplicity: cells only interact with one type of matrix protein, there is a lack of broad tunability, and the cells embedded within these hydrogels can elicit undesirable effects on the structural assembly of the scaffold. Figure 2: Schematic representation of different types of hydrogels made with single ECM proteins. Hydrogels of collagen, gelatin and their derivatives present cell-binding sites that facilitate cell attachment. Hyaluronic acid hydrogels in their native form do not contain any cell-binding sites. GelMA and hyaluronic acid hydrogels usually have crosslinks between the protein chains, reinforcing the overall structure and providing mechanical support. Gelatin & Methacrylated Gelatin (GelMA) Gelatin is a naturally occurring, and hydrophilic protein, obtained as a result of permanent hydrolysis of collagen [99]. Depending on the method used for its production, there are two types of gelatin type A and type B. Acidic hydrolysis, which
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