Joris van Dongen

281 Adipose ECM hydrogels to release paracrine factors INTRODUCTION Autologous fat grafting has been widely investigated and used for soft-tissue defects and regenerative purposes such as the treatment of open wounds as well as anti- scarring treatments 1-4 . Diabetes is associated with dysfunctional wound healing and occurrence of non-healing ulcers. A significant proportion of diabetics do not respond to revascularization therapies which regularly results in amputation of the affected limb. Few non-controlled clinical trials and case reports have shown that diabetic wound healing is augmented by lipografting 1,2,5,6 . One of the key components of this regenerative potential of adipose tissue that dictate tissue regeneration, is the stromal vascular fraction (SVF), which is rich in adipose tissue-derived stromal cells (ASC). In the SVF, ASC are present as pericytes or periadventital vascular cells 7,8 . The clinical efficacy of SVF is often ascribed to these resident ASC or other precursor cells, but definitive proof still lacks. Another important but neglected contributor to regeneration is the extracellular matrix (ECM) which provides the tissue architecture and structural support as well as key biochemical signaling cues. The role of ECM in tissue regeneration is but poorly studied, if at all. ASC secrete a plethora of growth factors, cytokines, lipid-based mediators and matrix molecules such as structural components and matrix metalloproteases (MMPs) that remodel ECM. For instance, MMPs can degrade the excessively deposited ECM by myofibroblasts in scars, while vascular endothelial growth factors (VEGF) and fibroblast growth factors (FGF) promote wound healing 9,10 . The ECM in SVF supports the proliferation and survival as well as differentiation of cells by binding of their surface-expressed integrins to ECM molecules. Moreover, paracrine factors bind and are retained by proteoglycans and glycoaminoglycans of ECM. In this way, ECM stores paracrine factors and functions as a physiological controlled ‘on-demand’ release system 11,12 . Concentration of the SVF of adipose tissue i.e. to get rid it of the large amounts of adipocytes and thereby reducing volume, might augment the clinical observed regenerative potential compared to standard autologous fat grafting. To concentrate SVF, the SVF can be isolated enzymatically or mechanically 13 . Enzymatic isolation destructs the ECM and its interaction with cells and thus yields a suspension of single SVF cells (cellular SVFor cSVF).Via centrifugation, adipocytes are readily removedwith enzymatic isolation 13 . When a non-enzymatic isolation procedure is used, adipocytes are mechanically destructed and the ECM is still intact as are its interactions with bound cells while it also acts for sustained release of bound factors (tissue-like SVF or tSVF) 12,13 . Therefore, we anticipate a non-enzymatically obtained SVF to have a higher therapeutic benefit than enzymatic SVF or unprocessed lipografts.