Article Focus

• Ti DRIE

• Gas nitriding

• Process tradeoffs

Driven to develop techniques for strengthening microscale and nanoscale medical devices, a University of California researcher is exploring the use of a near-century-old technique used by car-part manufacturers to increase the wear resistance of macroscale metal parts.

Masaru Rao, an assistant professor of mechanical engineering at UC Riverside, received a 5-year, $400,000 award from the National Science Foundation for his work, which began more than a decade ago when he helped develop the Titanium Deep Reactive Ion Etching (Ti DRIE) technique for micromachining titanium. Rao’s process relies on plasma to sculpt titanium-based devices with microscale to nanoscale dimensions and opens the door to miniaturized medical devices made of biocompatible material for implantation applications.

There’s a catch, though. Increasing the strength of small parts is a big challenge due to the limitations of micromachining. Rao’s own technique only works with pure titanium, due to the highly chemical nature of the material removal mechanism upon which it is based. Other high-strength biomedical alloys (e.g. stainless steel), or even high-strength titanium alloys, cannot be used since they contain additional metallic elements that adversely affect the micromachining process. Moreover, coating machined parts to increase their hardness is severely constrained at microscale and nanoscale by challenges associated with maintaining coating uniformity and quality over complex device structures.

But wait. Or, if you prefer, hit the brakes. Gas nitriding, the near-century-old technique for increasing the wear resistance of macro-scale metal parts for automotive (e.g. hardened engine camshafts) and other applications (e.g. rifle firing pins) could hold the key to overcoming coating challenges at microscale and nanoscale. With gas nitriding, the surfaces of machined parts are strengthened by heating them in a nitrogen atmosphere, which causes nitrogen to diffuse into the metal. When applied to titanium parts, the nitrogen atoms tend to squeeze into the spaces between the titanium atoms, thus forming a nitrogen-based titanium alloy. The presence of the nitrogen atoms makes it more difficult for the titanium atoms to rearrange themselves in response to mechanical loading, which leads to significant strengthening.

However, the relatively slow diffusion of nitrogen within the titanium means that strengthening is typically limited to the near-surface region, and the concentration of nitrogen decreases with depth.Nevertheless, Rao thinks that this technique holds significant potential for miniaturized medical devices. “While pure titanium has adequate strength for many applications, device performance and reliability could be enhanced significantly if we can increase strength,” says Rao. “Gas nitriding may provide a means for doing so, since it can be applied to our devices after they have been fabricated; it won’t suffer from the limitations of coating-based processes; and the diminutive dimensions of our devices will make it easy to diffuse nitrogen throughout the entire structure, thus allowing through-thickness strengthening.”

However, Rao indicates that the translation of this technique to his devices won’t be as simple dropping them into a conventional nitriding furnace. For example, while diffusion of a small amount of nitrogen into titanium can yield significant increases in strength, it doesn’t take much to overdo it, due to the limited amount of space available between the titanium atoms. When this concentration threshold is surpassed, titanium nitride intermetallic compound begins to form. Although titanium nitride is far harder than titanium, its presence is often undesirable, because its brittleness can adversely affect strength and fatigue resistance.

At the macroscale, this is typically dealt with by grinding the surface of nitrided parts down to a sufficient depth to remove the brittle titanium nitride layer, thus revealing a surface that contains just enough nitrogen for strengthening. This is not an option for tiny titanium devices, however, due to the lack of such material removal capability at microscale and nanoscale.

“Consequently,” says Rao, “the key here will be to precisely control the diffusion of nitrogen into the titanium surface, to ensure that its concentration is kept below the threshold for titanium nitride formation. This will be a challenge, particularly for structures with such small dimensions.”

To illustrate the need for such precision, Rao points to a separate project in his lab that is focused on developing titanium-based microneedle devices for minimally-invasive drug delivery.

“Since these devices are intended for penetration of relatively robust but also highly sensitive tissues, such as cornea and sclera, strength is crucial,” says Rao.“Increased strength will allow us to make smaller devices, which will reduce tissue trauma and insertion force, both of which are important performance metrics.”

While gas nitriding may provide means for achieving such strengthening, precise control of nitrogen diffusion into the devices will be required to ensure that titanium nitride formation does not occur, since it will be impossible to remove this material from the surfaces of these highly complex devices after nitriding.

Another potential tradeoff of the gas nitriding process is the adverse effect this will have on ductility. This is because the same constraint of titanium atom movement that produces strengthening also reduces the extent to which the material can be stretched prior to failure. For some applications, addressing this issue will be paramount.

As an example, Rao points to another project in his lab, which is seeking to develop vascular stents that use nanoscale surface topography to enhance healing after implantation. While stents have been in widespread use for more than a decade, a small percentage of patients receiving such devices still suffer from serious complications that arise from incomplete healing of the vessel around the stent.

In collaboration with Victor Rodgers, a professor in the Department of Bioengineering at UC Rioverside, Rao has been exploring the potential for reducing such complications through the creation of nanoscale structures on the surface of titanium-based stents that may actually accelerate the healing process.

The surface structures we’re investigating,” says Rao, “consist of periodic arrays of precisely defined grooves with widths as small as a few hundred nanometers, which is well-beyond the capability of other metal micromachining techniques. Thus far, we’ve shown that these diminutive structures can significantly enhance the response of endothelial cells in vitro, thus suggesting the promise for enhancing healing response in vivo.”

Enabling platform

Looking toward the future, Rao says he believes the Ti DRIE process will serve as an enabling platform for miniaturized medical devices.

“As such,” says Rao, “we will continue to seek opportunities to work with clinicians, researchers, and medical device manufacturers to help identify and address compelling unmet or underserved needs.”