Silicon nitride (Si3N4) possesses a unique combination of properties that makes it attractive for biomedical applications in orthopedics, but researchers believe that understanding its behavior could lead to even better performance [Bock et al., Acta Biomaterialia (2015), doi: 10.1016/j.actbio.2015.08.014].
Developed 50 years ago as a ceramic able to retain its strength, fracture toughness, and chemical resistance in aggressive environments at high temperature, its biocompatibility indicated additional purposes. The non-oxide ceramic can be produced in the form of fibrous, interlocking grains, strong enough to be used for replacement joints, or as a highly porous material ideal for bone scaffolds. As well as its unique blend of intrinsic attributes, its surface chemistry and roughness (or topography) can be readily altered.
The team from Amedica Corp., University of Missouri, Missouri University of Science and Technology, and Kyoto Institute of Technology in Japan subjected Si3N4 to a range of chemical, mechanical, and thermal treatments and monitored the changes to the material’s wetting and charging behavior.
“We did this because we are developing an understanding and model of how this material interacts with the physiologic medium,” explains first author of the study, Ryan M. Bock of Amedica. “Previous research showed favorable osteointegration and resistance to bacterial colonization [but] we would like to explain these observations and modify the material’s surface to optimize the physiologic response.”
Si3N4 forms a surface passivation layer in air or moist atmospheres, creating a mixture of Si-N, Si-N-O, and Si-O bonds and Si-NH2 and Si-OH functional groups. Thermal and chemical treatments effectively increase (or decrease) the relative proportion of these surface groups. For example, thermally oxidizing in air increases the proportion of Si-O creating a surface that is essentially SiO2. By contrast, etching with HF or chemical mechanical polishing (CMP) increases the ratio of Si-N to Si-O.
These changes in surface oxygen and nitrogen groups lead to markedly different behavior. The researchers found that thermal treatments change Si3N4’s affinity for water, reducing the wetting angle to less than 10°. Thermal treatment in nitrogen also appears to create a new surface phase. Surface charging behavior, meanwhile, indicated by large shifts in zeta potential, can be controlled with chemical and thermal treatments.
“The material’s chemistry offers a unique opportunity for tuning surface properties and implant-physiologic environment interactions, without compromising its desirable bulk properties,” says Bock. “[Si3N4] exhibits excellent mechanical properties that remain stable in the physiologic environment unlike conventional oxide ceramics.”
The researchers now hope that understanding surface treatments, and their effect on surface chemistry, will support the evaluation of Si3N4’s osteointegration, resistance to biofilm formation, and interaction with the physiologic environment, ultimately leading to more sophisticated treatments.