Characterization of Silver Nanoparticles on Hydroxyapatite Coatings

Hydroxyapatite (HAP), Ca5(PO4)3(OH), given its similarity to human bone, shows great promise as a material with which to coat titanium orthopedic implants. In addition, silver nanoparticles can be deposited onto the HAP giving the coated implant antibacterial qualities to fight infection (metal nanoparticles upon a HAP structure also have potential as a catalyst support). This project characterized and compared an electrochemically deposited HAP coating bare, the HAP coating with electrochemically reduced Ag nanoparticles, and the HAP coating with sputter coated Ag nanoparticles. The techniques used for characterization included secondary electron imaging (SEI), backscattered electron imaging (BSD), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and image analysis/colorization software. It was found that the 1 um thick HAP coating readily accepted nanoparticles from both methods, with the electrochemically reduced Ag particles being significantly larger and more pervasive than the sputtered particles. EDS confirmed the presence of Ag in both, but in higher relative abundances using the electrochemical technique. AFM found the roughness of these nanoparticles to be 25-30 nm. The nanostructures from this deposition technique were found to be simpler to image, quantify, and characterize, than the sputtering technique, but both are viable options for producing Ag nanoparticle covered HAP coatings. Future work should focus upon studying the electrochemical processes themselves to ensure fewer abnormalities and defects.

Hydroxyapatite (HAP), Ca5(PO4)3(OH), is an incredibly versatile material with innumerable applications. Its proton conductivity gives it the potential to be used as a fuel cell membrane, as well as become polarized. Its structural strength makes it an ideal catalyst support if the metal can be deposited in the form of nanoparticles onto the HAP; in addition, given its chemical properties, it has the potential to be involved in the catalysis itself. One of HAP’s current, most popular applications is in the field of biomedical devices.

Titanium bone implants have become standard in many orthopedic surgeries. To improve the speed of bone integration and the adhesion to the implant, many implants are coated with a hydroxyapatite layer. The body recognizes and responds more quickly to the calcium phosphate compound as it closely resembles natural bone. Several different methods of coating have been developed, but this project focuses on one technique, laid out by the Yates research group at the University of Rochester, electrochemical deposition, producing micro-sized or smaller, oriented crystals.

Infection still proves the largest risk to orthopedic surgery, especially when a foreign object, like an implant, has been placed within a body. In an effort to reduce these rates of infection, incorporating localized antibiotics into implants has gained interest. Some research has been done to incorporate small amounts of silver metal into the coating surface for its antibacterial properties. The Yates group is similarly pursuing integrating silver, in the form of nanoparticles, onto the surface of its coatings.

This project seeks to characterize, analyze, and compare the morphology and composition of two separate techniques of placing silver nanoparticles upon HAP coatings. Electrochemical deposition and sputter coating both will produce nanostructures of silver without interfering with the qualities of the HAP coating itself. The qualities of each technique’s output will be explored through various microscopy techniques.

The structure and appearance of the silver and HAP will be collected through secondary electron imaging (SEI) in a scanning electron microscope (SEM). Backscattered electron’s (BSD) sensitivity to elemental differences will provide a good technique for imaging the size and location of the nanoparticles. More morphological information and estimated roughness parameters will be gathered through atomic force microscopy (AFM). To verify the composition of the HAP coating, presence of Ag, and to analyze the relative elemental abundances, an X-ray microanalysis using energy dispersive spectra (EDS) will be undertaken. The collected images will be further analyzed quantitatively and colorized for presentation purposes.