Gradient metallic implant for artificial disc replacement fabricated via freeze-casting densification.Metals such as Ti are widely used as implants in orthopedics and dentistry, but better mechanical properties and bioactivity could reduce subsequent failure and rejection. Researchers think they may have hit upon a solution to these issues using densified porous Ti implants loaded with growth factors [Jung, H.-D., et al., Biomaterials 37 (2015) 49-61, http://dx.doi.org/10.1016/j.biomaterials.2014.10.027].
Porous Ti and biological agents that promote cell growth are well-tried approaches for improving the performance of implants, but the researchers from Seoul National University, Korea University, and Stony Brook University have gone a step further. By using freeze casting to fabricate porous Ti, where a metal powder is dispersed in a liquid and cooled until the liquid solidifies leaving behind a metal powder network, the researchers can create scaffolds ductile enough to undergo further densification. From a starting porosity of more than 50%, the researchers end up with a scaffold of just 7% porosity after compression.
“Our freeze-casting fabrication method allows the densification of porous scaffolds,” explains Song. “This fabrication approach minimizes chemical contamination and structural defects during densification, maintaining the structural integrity of the porous metal without any reduction of mechanical properties.”
Depending on the starting scaffold, the final porosity and mechanical properties can be varied. The new approach enables the mechanical properties to be tuned so that implants for both filling and load-bearing applications can be fabricated.
Next the densified Ti scaffolds are coated with growth factor bone morphogenetic protein-2 (BMP-2), which improves bioactivity and promotes the production of new tissue.
“Due to the unique pore structures [of our metal implants], the coated bioactive molecules are gradually released from the scaffold, maintaining their efficacy for a prolonged period,” explains Juha Song of Seoul National University.
The slow release of growth factors is a significant advantage as high does of some of these agents, such as BMP-2, can be associated with unpleasant side effects. Moreover, the initial porosity and degree of densification can be used to control the release rate.
The new scaffolds show increased strength combined with low stiffness – almost identical to the mechanical properties to real bone. The use of densified Ti could, the researchers believe, mitigate the problem of stress shielding – where the mismatch in stiffness between natural bone and metal implants causes damage to surrounding tissue – and provide excellent long-term stability for implants. The researchers have even devised graded pore structure implants, where the outer later is porous to allow bone ingrowth and the inner core is dense to provide mechanical stability and prolonged bioactivity.