Hydroxyapatite is an inorganic mineral, a calcium phosphate, that serves as a fundamental building block in biological structures. The formula \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\) shows its primary components: calcium, phosphate, and hydroxyl ions. This compound is the most thermodynamically stable form of calcium phosphate under physiological conditions, making it suitable for long-term structural integrity. Synthetic versions are now used as highly compatible biomaterials in medical and technological applications.
Hydroxyapatite in Biological Systems
Hydroxyapatite is the primary structural component of mineralized tissues, providing rigidity and strength. In bone tissue, it makes up about 65% to 70% of the dry weight, forming a nanocomposite with collagen. The HA crystals are organized within the collagen fibrils, creating a material that combines the flexibility of the protein matrix and the hardness of the mineral phase to withstand mechanical stress.
The human tooth contains the highest concentration of hydroxyapatite, particularly within the enamel layer. Dental enamel is the hardest substance produced by the body, consisting of approximately 96% inorganic material, which is almost entirely HA. These crystals are larger and more perfectly aligned than those in bone, contributing to exceptional wear resistance. Dentin, the tissue underlying the enamel, is about 70% HA by weight but incorporates a higher organic matrix content, making it less brittle.
Biological apatite, the mineral phase in biological tissues, is a carbonated and substituted form of HA. It incorporates trace elements like magnesium, strontium, sodium, and carbonate ions, which alter the crystal lattice structure. This non-stoichiometric composition allows for constant ion exchange, tissue remodeling, and repair. The substitutions make biological apatite more soluble and reactive than its pure synthetic counterpart.
Unique Structure and Chemical Properties
The chemical formula \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\) represents the stoichiometric composition of hydroxyapatite. Structurally, the mineral adopts a hexagonal crystalline lattice, where ten calcium ions, six phosphate groups, and two hydroxyl ions are precisely arranged within the unit cell. This highly ordered atomic structure is responsible for the material’s mechanical strength and resistance to dissolution under neutral conditions.
A defining characteristic of hydroxyapatite is its exceptional biocompatibility, meaning it is non-toxic and does not provoke an adverse immune response. This property stems directly from its chemical identity as the mineral component of native human bone and teeth. The HA crystal surface is also highly bioactive, capable of interacting directly with surrounding biological fluids and cells.
The mechanism of ion exchange is an important chemical property that allows HA to integrate with living tissue. The hydroxyl (\(\text{OH}^-\)) ions can be partially substituted by other ions, such as fluoride (\(\text{F}^-\)). Similarly, the calcium ions (\(\text{Ca}^{2+}\)) can be replaced by ions like strontium (\(\text{Sr}^{2+}\)). This ability is fundamental to remineralization, restoring lost mineral content by drawing calcium and phosphate from the surrounding environment.
Applications in Medical and Dental Fields
Synthetic hydroxyapatite is suitable for restorative and regenerative applications due to its chemical resemblance to the body’s natural mineral. In dentistry, nano-hydroxyapatite (nano-HA) is widely used because its ultra-fine particle size mimics natural enamel crystals. These nanoparticles penetrate and fill microscopic defects, effectively remineralizing early-stage enamel decay.
Nano-HA is incorporated into toothpastes and mouthwashes to reduce tooth sensitivity by sealing exposed dentinal tubules. The particles block the channels leading to the tooth’s nerve, preventing external stimuli from reaching the pulp. This provides relief from hypersensitivity without relying on chemical nerve desensitizers.
In medical and orthopedic surgery, synthetic HA is a prominent material for bone graft substitutes and fillers used to repair defects. Its osteoconductive property provides a scaffold, or physical framework, that guides the growth of new bone cells and blood vessels into the damaged area. The HA material is gradually resorbed and replaced by the patient’s own native bone tissue over time.
HA is also used as a coating material for metallic orthopedic and dental implants, such as hip and knee replacements. Applying a thin layer of HA enhances osseointegration—the direct connection between living bone and the implant surface. This leads to increased stability and long-term success.
Safety and Regulatory Considerations
Hydroxyapatite is a safe and highly biocompatible material due to its natural presence within the human body. Safety focus has shifted to nano-HA, specifically concerning particle size and shape in consumer products. Global regulatory bodies review nanoparticles to ensure they pose no health risk.
The European Union’s Scientific Committee on Consumer Safety (SCCS) has deemed nano-HA safe for use in oral care products under specific criteria. The safety evaluation applies only to non-needle-shaped particles and sets maximum concentration limits, such as 10% in toothpaste and 0.465% in mouthwash. Nano-HA is restricted in applications that could lead to inhalation exposure, such as sprayable products, due to a lack of data on lung safety.
In the United States, traditional HA is a well-established biomaterial for medical devices. Synthetic HA is considered an inert ingredient in dental products, and its nano-form is currently being evaluated. When manufactured to specific, non-toxic standards, hydroxyapatite is a well-tolerated and effective material.