Innovative Apatite Nanoparticles Improve Implant Biocompatibility

Medical implants and biomedical devices often cause inflammatory responses due to poor biocompatibility and while apatite coatings offer a potential solution, limited cell adhesion is often a challenge. To address this, scientists developed advanced apatite nanoparticles for implant coatings with superior cell adhesion. By adjusting pH during synthesis of nanoparticles, they enhanced the surface properties of these nanoparticles, achieving improved water interactions and greater structural stability, paving the way for more effective and biocompatible medical implants.

Medical implants have transformed healthcare, offering innovative solutions with advanced materials and technologies. However, many biomedical devices face challenges like insufficient cell adhesion, leading to inflammatory responses after their implantation in the body. Apatite coatings, particularly hydroxyapatite (HA)—a naturally occurring form of apatite found in bones, have been shown to promote better integration with surrounding tissues. However, the biocompatibility of artificially synthesized apatite nanoparticles often falls short of expectations, primarily due to the nanoparticles’ limited ability to bind effectively with biological tissues.

To overcome this challenge, researchers at Nagaoka University of Technology, Japan, have developed a method for synthesizing surface-modified apatite nanoparticles that results in improved cell adhesion, offering new possibilities for next-generation biocompatible medical implants. Led by Dr. Motohiro Tagaya, associate professor in the Department of Materials Science and Bioengineering at Nagaoka University of Technology, this research aims to enhance apatite coatings’ performance and advance biocompatible materials for medical devices. Members of Dr. Tagaya’s research team included Kazuto Sugimoto and Ryota Akutsu, both from Nagaoka University of Technology; and Dr. Tania Guadalupe Peñaflor Galindo from Sophia University.

Apatites are calcium-phosphorus-based inorganic compounds, with hydroxyapatite—a naturally occurring form found in bones. These compounds are known for their high biocompatibility. Recent studies have found that coating artificial joints and implants with apatite nanoparticles is a plausible solution for improving the biocompatibility of these biodevices. However, the artificially synthesized nanoparticles often show reduced binding affinity to biological tissues in-vitro. According to Dr. Tagaya and his team, this difference could be linked to the nanoscale surface layer of the apatite nanoparticles.

Dr. Tagaya’s research was driven by a desire to unravel the complexities of biocompatible materials, leading his team to develop an interdisciplinary framework that controls the intricate interactions between apatite and biological systems. “The properties of the nanoscale surface layer of apatite nanoparticles are crucial when considered for medical coatings,” Dr. Tagaya said. “In this study, we successfully controlled the nanoscale surface layers of apatite nanoparticles, paving the way for advanced surface coating technologies for biodevices.”

The team synthesized hydroxyapatite nanoparticles by mixing aqueous solutions of calcium and phosphate ions. The pH of the solution was controlled using three different bases, which included tetramethylammonium hydroxide (TMAOH), sodium hydroxide (NaOH), and potassium hydroxide (KOH). The precipitated nanoparticles were then evaluated for their surface layer characteristics and were further used for coating via electrophoretic deposition.

The results revealed that pH was a key factor during synthesis, since it affected the crystalline phases, surface properties, and electrophoretic deposition. Analyzing the nanoparticles’ crystalline phases, researchers observed that the choice of pH influenced the formation of different calcium phosphate phases like calcium-deficient hydroxyapatite (CDHA) and carbonate-containing hydroxyapatite (CHA). Higher pH favored the formation of CHA, leading to better crystallinity, and a higher calcium to phosphorus (Ca/P) molar ratio.

The apatite nanoparticles’ surface shows three different layers. The inner apatite layer/core is characterized by the presence of the crystalline apatite structure. Above the apatite layer is the non-apatitic layer, which is rich in ions like phosphate ions and carbonate ions. This layer reacts with water molecules and forms the hydration layer. Analyzing the surface characteristics of these layers revealed that pH adjustments facilitated the formation of the non-apatitic layer rich in reactive ions, enhancing hydration properties, which was confirmed.

While higher pH facilitates the non-apatitic layer formation, the presence of Na+ ions reduces the phosphate ion concentration, leading to decreased layer reactivity. The introduction of substantial ions by NaOH also affected the uniformity of electrophoretic deposition, as observed in scanning probe microscope studies. This effect was not observed with KOH, indicating that KOH was more suitable than NaOH for forming the non-apatitic layer and ensuring uniform coating.

“This study focuses on the critical interfaces between bioceramics and biological systems and could inspire designs of biocompatible surfaces with preferential cell adhesion,” Dr. Tagaya stated. These findings can be potentially useful for surface coating various biodevices that are implanted in the human body, including artificial joints and implants.

Dr. Tagaya is currently an associate professor at the Nagaoka University of Technology. He earned his doctorate in engineering from the Tokyo Institute of Technology in 2010. A leading biomaterials researcher, he is linked to more than 175 publications and 2,294 citations. His work focuses on nanobioceramics, biomedical engineering, and designing interfaces for cellular therapeutics. Dr. Tagaya’s contributions to bioceramics science and engineering have earned him the Inoue Research Award for Young Scientists of Japan in 2012. His research specializes in nanobiomaterials and bioceramics in an attempt to drive impactful biomedical solutions.