## 2 Demineralized Dentin Matrix (DDM) Microstructure  
Dentin consists of about 65% (by weight) inorganic substances, mainly hydroxyapatite (HA), tricalcium phosphate (TCP), octac calcium phosphate, amorphous calcium phosphate and dicalcium phosphate dehydrate [8]. The minerals consist of intrafibrillar mineral which is secreted by odontoblasts in a well-orchestrated order. Dentin contains about 35% (by weight) organic matrix and water [24]. The organic matrix includes collagen and non-collagenous components. The diameter of dentin collagen is approximately 80-100 nm [25]. Dentin collagen fibrils contain laterally organized microfibrillar units [26; 27], which may be self-organized in a twisted arrangement of bundles, each measuring about 20 nm in diameter [28]. The next grade of structure in dentin collagen relates to each individual collagen microfibril, with 5-6 nm in diameter. Laterally, individual microfibrils contain five stranded triple-helical collagen molecules [26]. Besides minerals, collagen and water, dentin also contains about 5% non-collagenous components. The non-collagenous proteins, proteoglycans and enzymes play an important role in the development of dentin [29]. The small integrin-binding ligand N-linked glycoprotein (SIBLING) family includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP) and matrix extracellular phosphoglycoprotein (MEPE) [30]. The SIBLING family has key non-collagenous components, and they participate in the process of mineralization in dentin development [30]. The components of DDM are similar to those of the dentin matrix; however, they have different organic/inorganic ratios. DDM in powder form has a mineral content of about 5%-10%, while DDM in block form has a mineral content of about 10%-30% [31]. The demineralization agents and time used to make DDM are different in different reports, and they affect the demineralization extent and mineral percentage in the resulting DDM. Growth factors are present in DDM, while they are trapped and hidden in UDM. The demineralization process opens the dentin tubules and frees up the growth factors, which can stimulate tissue repair and regeneration. Previous studies demonstrated transforming growth factor \(\beta\) (TGF-\(\beta\)1), bone morphogenetic proteins ( BMPs), vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), platelet-derived growth factor (PDGF) and insulin-like growth factor-1 (IGF-1) in DDM [32; 33; 34; 35; 36]. The amount of growth factors was detected by enzyme-linked immunosorbent assay (ELISA). Results showed that TGF-\(\beta\)1 was the most abundant growth factor; BMP-2, FGF-2, VEGF, PDGF and IGF-1 demonstrated intermediate abundance; while BMP-4 and BMP-7 had the lowest concentrations, as shown in Table 1[32]. DDM proposed osteogenic and dentingenic differentiation of bone mesenchymal stem cells (BMSCs), however, after fractionation of growth factors from DDM by heparin affinity chromatography, the osteogenic responses of BMSCs diminished, indicated that growth factors play significant role in the osteogenesis and dentingenesis of DDM. The growth factors are present at pg levels, have no toxicity issues and no potential for ectopic mineral deposit. These results demonstrated that DDM could deliver growth factors at physiological levels for bone regeneration [32]. Dentin contains micrometer-sized tubules [37]. The mineral surrounding the dentin tubules is highly mineralized and collagen-free, and is called peritubular dentin (PTD) (Figure 1A, black arrow). The mineral in the intertubular matrix is embedded in collagen-rich matrix (Figure 1A, white arrow), which is termed intertubular dentin (ITD) [37]. In making the DDM, the demineralization process mainly dissolves the mineral in PTD, producing opened and wider tubules and a meshwork protruding towards the tubule lumen (Figure 1B) [37]. Higher magnification images of demineralized dentin (Figure 1C) reveal a complex network of filaments protruding from