Joris van Dongen

298 Chapter 12 such as ASC-released CXCL8, play an important role in the different phases of wound healing 33 . During the inflammation phase, macrophages phagocytose pathogens and cellular debris. During the proliferation phase, epithelial proliferation is enhanced by activated macrophages 33 . Furthermore, re-epithelialization by keratinocyte proliferation and migration is also stimulated by CXCL8, which was bound and released by our ECM hydrogels 29 . Besides increased re-epithelialization, CXCL8 is also responsible for the attraction of leukocytes which results in an increased phagocytosis during the inflammation phase 34 . The presence of leukocytes and macrophages are prolonged by the presence of MCP1, as MCP1 is a chemoattractant for macrophages, mast cells and T-cells 29 . A prolonged presence of leukocytes and macrophages result in a sustained pro-inflammatory state of the wound 35 . After the inflammation phase, a decrease in inflammatory signaling is needed to induce formation of granulation tissue and subsequently re-epithelialization. In vitro and in vivo experiments have shown that GAG-binding sites are able to modulate chemokine gradients of MCP1 and IL8 by subduction in excisional wounds. 36 In this way, ECM hydrogels incubated with ASC- CMe can modulate inflammation in wounds by both releasing and attracting growth factors and chemokines. Besides the biological function of our studied hydrogel, its physical properties are also of crucial importance. Physical properties of the hydrogel such as stiffness affect ASC and other cells present in the surrounding tissue of the wound. Low stiffness hydrogels promote adipogenic differentiation of native ASC, while stiff hydrogels promote their osteogenic differentiation 37 . The soft non-diabetic hydrogels support adipogenic differentiation of host tissues ASC, however, the bound factors together with the stiffness of the hydrogel dictate final cell fate 37 . A soft hydrogel is also easier infiltrated by host cells like ASC and endothelial cells than stiff hydrogels. In this way, both ASC and endothelial cells can stimulate angiogenesis inside the hydrogel which might result in increased wound healing rates 38 . It remains to be determined whether the stability and longevity of these hydrogels supports cells’ maintenance, migration and differentiation. Also, it is unknown if the hydrogels with a low stability can resist all mechanical stress during application of the hydrogel in vivo .The enzymatic degradation to convert complex ECM structures into gels has inspired physical chemists to compile predictionmodels 39,40 . However, froma pragmatic point of view, production of (pre)gels from freeze-dried, fine-powdered ECMs, requires careful standardization of enzyme treatment. These should consider e.g. the disease background of the ECM, because our results show differences between diabetic and non-diabetic ECM hydrogels i.e. gels of diabetic origin collapse. Regarding future application, diabetic hydrogels will not suffice due to their limited physical strength. As a matter of fact, viscoelasticity measurements proved impossible because diabetic hydrogels collapsed readily. This indicates the presence of structural differences in matrix between non-diabetic and diabetic donors. This difference could be caused by accumulation of AGEs in ECM