Hydroxyapatite is a naturally occurring mineral of calcium and phosphate with applications in medicine and dentistry. When formulated into particles between 20 and 80 nanometers, they are known as hydroxyapatite nanoparticles. Their small size allows them to interact with biological structures on a molecular level. Research in the 1970s, aimed at helping astronauts maintain bone health in space, significantly advanced their biomedical development.
The Natural Role in the Human Body
Hydroxyapatite is a fundamental component of the human body, serving as the primary mineral in bone and teeth. In bone, it makes up 65% to 70% of its weight, with its crystals embedded in a flexible collagen matrix. This combination of a hard mineral and pliable protein gives bone its unique strength and resilience, allowing the skeleton to support the body.
In teeth, hydroxyapatite is more concentrated, comprising 70% to 80% of dentin and up to 97% of enamel. Tooth enamel is the hardest substance in the human body due to its high density of hydroxyapatite crystals. Unlike bone, enamel lacks collagen, with other proteins guiding mineral formation. This dense mineral layer protects the inner tooth from physical damage and acid produced by oral bacteria.
Applications in Dentistry
The most common application of hydroxyapatite nanoparticles is in oral care products like toothpaste. Their function centers on remineralization, which restores minerals lost from tooth enamel. Acidic foods and bacteria can create microscopic pores in the enamel, weakening the tooth. Since the nanoparticles are chemically similar to enamel, they deposit into these areas, filling gaps and rebuilding the structure.
This process also reduces tooth sensitivity. Hypersensitivity occurs when worn enamel exposes the underlying dentin and its microscopic channels, called dentinal tubules, which lead to the tooth’s nerve. Open tubules allow stimuli like hot or cold substances to trigger pain. Hydroxyapatite nanoparticles physically block these exposed tubules, forming a protective layer that prevents stimuli from reaching the nerve.
This biomimetic approach, using a material that mimics natural tooth structure, is effective for repairing early enamel damage and relieving sensitivity. The nanoparticles integrate with the tooth surface, creating a smoother, more resilient enamel layer. This helps protect against plaque attachment and contributes to overall oral health.
Biomedical and Orthopedic Uses
Beyond dentistry, hydroxyapatite nanoparticles have applications in orthopedics. They are used as a material for bone grafts and fillers to encourage bone tissue regeneration. This property, known as osteoconduction, provides a supportive scaffold for the body’s bone-forming cells to grow upon, facilitating the healing of fractures or bone defects.
Another use is as a coating for orthopedic and dental implants, like those in hip replacements. Metallic implants are coated with a thin layer of the nanoparticles to improve integration with the surrounding bone, a process called osseointegration. This bioactive surface encourages bone to bond directly to the implant, increasing stability and reducing the risk of loosening over time. The coating’s nanoscale texture provides an ideal surface for bone cells to attach and grow.
In tissue engineering, these nanoparticles are investigated for creating advanced scaffolds to support new tissue growth. The scaffolds can be combined with materials like polymers to create composites that mimic natural bone. Researchers are also exploring loading these nanoparticles with therapeutic agents, such as growth factors or antibiotics, for localized delivery to a surgical site to enhance healing and prevent infection.
Safety and Biocompatibility
The widespread use of hydroxyapatite nanoparticles is due to their high degree of biocompatibility. Biocompatibility is the ability of a material to contact living tissues without causing a harmful response. Since hydroxyapatite is a natural component of bone and teeth, the body does not recognize it as a foreign substance. This reduces the likelihood of inflammation or rejection when used in implants or oral care products.
The safety evaluation of these nanoparticles depends on their intended use. In toothpaste, ingested particles are expected to dissolve in the stomach’s acidic environment, minimizing systemic exposure. For surgical applications like bone grafts, the material is resorbed and replaced by natural bone over time. Studies show the nanoparticles are internalized by cells in the cytoplasm without entering the nucleus, supporting their safety at a cellular level.
The specific characteristics of the nanoparticles are a factor in their safety assessment. Factors such as particle size, shape, and concentration are considered by regulatory bodies. Different forms may require separate evaluation to ensure safe and effective use across all applications.