Macrophage isoelectric point, the solution produces moreMacrophage isoelectric point, the solution produces more

Macrophage extracellular matrix is ??mainly
composed of organic and inorganic substances. Organics are predominantly
collagen type I, which is a triple-helical structure13. Its properties include
excellent biocompatibility, biodegradability and ease of in vivo absorption and
antigenic properties. Collagen is the main structural material of vertebrates,
accounting for about 20% to 30% of the total body protein14. The collagen is an
amphipathic protein, because of its structure with-COOH and -NH2, that can
produce the corresponding COO- and NH3+ ions. When the pH of the solution is at the isoelectric point, positive
and negative charges are equal, and when the pH value is above the isoelectric
point, the solution produces more COO-  than NH3+, that results in negatively charged solution. In addition, collagen
and cells can transmit messages to increase the cell attachment effect15, 16. The bone mineral is mainly
composed of bone apatite and calcium phosphate. Calcium phosphate is a mineral
that includes calcium ions (Ca2+), phosphates (PO43-), pyrophosphates
(P2O74-), hydrogen and carboxylate ions. That also
contains a smaller amount of carbonate, citrate, magnesium, fluoride, hydroxyl
and potassium ions. The major mineral phase of bone is hydroxyapatite (HAP).
This component of calcium phosphate has a Ca: P ratio of 1:67 and a hexagonal
structure with the formula Ca10(PO4)6(OH)2
17. Till now no studies have been reported regarding
any local or systemic
toxicity of hydroxyapatite, as it contains only calcium and phosphate ions. HAP
surface supports osteoblastic cell adhesion, growth, and differentiation, and
new bone binds directly to HAP
through a carbonated calcium deficient apatite layer at the bone–implant interface18. HAP scaffolds also
functioning as delivery vehicles for cytokines due to its capacity to
bind  and concentrate bone morphogenetic proteins (BMPs) in vivo19.Due to its ability to mimic the natural
extracellular matrix, gelatin based scaffolds are considered as promising
material for tissue engineering applications. On the other hand,
its poor mechanical properties especially its poor structural consistency in
wet and humid conditions limit its utilization in many areas. Hence structure
modifications are required to enhance its mechanical strength and
water-resistant properties, and thereby to fabricate stable scaffolds