The next generation of medical implants will require new manufacturing techniques to create customized parts that can last longer, be less intrusive on the body, and that can withstand the changing in situ environment. At Purdue University researchers are using lasers to create composite coatings for bone implants that are subject to less warping and cracking. This may allow for hip implants that would last longer than current ones which typically require replacement about every ten years.
One of the researchers’ techniques works by depositing layers of a powdered mixture of metal and ceramic materials, melting the powder with a laser and then immediately solidifying each layer to form parts. Because the technique enables parts to be formed one layer at a time, it is ideal for coating titanium implants with ceramic materials that mimic the characteristics of natural bone, Shin said. [Yung Shin, a professor of mechanical engineering and director of Purdue’s Center for Laser-Based Manufacturing –ed.]
“Titanium and other metals do not match either the stiffness or the nature of bones, so you have to coat it with something that does,” Shin said. “However, if you deposit ceramic on metal, you don’t want there to be an abrupt change of materials because that causes differences in thermal expansion and chemical composition, which results in cracks. One way to correct this is to change the composition gradually so you don’t have a sharp boundary.”
The gradual layering approach is called a “functionally gradient coating.”
Researchers used their laser deposition processes to create a porous titanium-based surface and also a calcium phosphate outer surface, both designed to better match the stiffness of bone than conventional implants.
The laser deposition process enables researchers to make parts with complex shapes that are customized for the patient.
The laser deposition technique lends itself to the requirement that each implant be designed specifically for each patient.
“These are not like automotive parts,” Shin said. “You can’t make a million that are all the same.”
The process creates a strong bond between the material being deposited and the underlying titanium, steel or chromium. Tests showed the bond was at least seven times as strong as industry standards require, he said.
The researchers use computational modeling to simulate, study and optimize the processes.
The researchers also are developing a technique that uses an “ultra short pulse laser” to create arterial stents, which are metal scaffolds inserted into arteries to keep them open after surgeries to treat clogs. The laser pulses last only a matter of picoseconds, or quadrillionths of a second.
Because the pulses are so fleeting, the laser does not cause heat damage to the foil-thin stainless steel and titanium material used to make the stents. The laser removes material in precise patterns in a process called “cold ablation,” which turns solids into a plasma. The patterns enable the stents to expand properly after being inserted into a blood vessel.