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Why is calcium hydroxyapatite combined with collagen in bone-repair scaffolds?

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Calcium hydroxyapatite and collagen are combined in bone-repair scaffolds to mimic the natural, two-phase composite structure of bone.1

In natural bone, these two components work in tandem to provide essential mechanical properties:
* Hydroxyapatite provides hardness, rigidity, and high compressive strength.1
* Collagen provides elasticity and significant tensile strength.1

By dispersing the rigid, brittle hydroxyapatite within a flexible collagen matrix, the composite prevents the propagation of stress failures that would otherwise occur in the mineral alone. This intimate association allows the material to achieve a high degree of strength and toughness, similar to how fiberglass functions. Furthermore, because hydroxyapatite is the essential mineral component of bone, it is bioactive, meaning it can bond to living bone tissue and support the growth of new bone within the scaffold.123