The world’s tiniest micro-turbines

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2009

One of the primary functions of these small structures is medical applications involving the brain.

KEY CONCEPTS
• Tiny micro-turbines, believed to be the world’s smallest, were developed with outside diameters of 0.9 and 1.2 millimeters.
• These micro-turbines are used in minimally invasive, non-surgical medical procedures, particularly involving the brain.
• No standard lubricant is required to run the micro-turbines.

Turbines are engines that function by the simple process of extracting energy from a fluid flow. The fluid interacts with blades attached to a rotor assembly. As a result, the blades rotate in response to the fluid flow and the energy captured is transferred to the rotor. The fluid can be a liquid such as water or a gas such as air.

Water turbines used in power plant applications require the action of moving water to turn blades and generate electricity. One of the most attractive alternative energy technologies being developed is wind turbines. They utilize the flow of air to generate mechanical energy that is converted into electricity.

Most of the applications we think about for turbines are for large power-generating uses. But the simple principle of a turbine enables it to be used in very small applications. These applications may not generate the amount of power typically seen in macroscopic uses, but they are very important for specific applications such as medicine.

EFAB TECHNOLOGY
Van Nuys, Calif.-based Microfabrica Inc. has developed tiny micro-turbines that are power sources for medical devices. Adam Cohen, executive vice president, technology and chief technology officer for Microfabrica, says, “We have designed micro-turbines with two diameters. One has an outside diameter of 1.2 millimeters, while the second one has an outside diameter of 0.9 millimeters.” These micro-turbines are believed to be the world’s smallest.

The micro-turbines are produced through a unique process called EFAB.® Starting with a ceramic wafer as the blank substrate, the technology involves growing devices by adding metal in a precise fashion. Cohen adds, “We borrowed similar processing to that used in the semiconductor industry. We build structures by electroplating metal in a layer-by-layer manner. After each layer is prepared, planarization is utilized to polish the metal surface. The geometry of the device is determined by computer-aided design.”

In performing the manufacturing, one metal is used to form the actual device, while a second metal serves as a kind of scaffolding that supports the other metal during fabrication. The device being built is formed within a block of sacrificial metal that is removed in a final step by a highly selective etching process.

Each layer can be as thin as 4 micrometers. Cohen indicates that the accuracy and repeatability of the process is approximately 2 micrometers.

EFAB also enables devices to be manufactured having more than one component without assembly, with all of the components free to move after removal of the sacrificial metal. Cohen notes that Microfabrica makes most devices this way, but an exception is the micro-turbine. He says, “Micro-turbines are not produced fully assembled. We want to use spherical ball bearings, but the layering process does not work well for objects with this geometry. Instead, we assemble the turbine rotor, which rotates independent of the stator, which does not spin, and insert precision ball bearings.”

A representative rotor is shown in Figure 3. The metals used to manufacture the micro-turbines are a proprietary nickel cobalt alloy known as Valloy-120. Another material used in the technology is a proprietary rhodium formulation called Edura-180. Both alloys are biocompatible and can be used in many medical applications. The Edura-180 is employed in applications requiring extreme hardness or wear resistance such as removing calcified deposits, bone or tooth enamel.


Figure 3. One of the components in the tiny micro-turbines is the rotor. These micro-turbines are prepared from biocompatible alloys and can be used in many medical applications. (Courtesy of Microfabrica Inc.)

The micro-turbines can be used with a number of different fluids. Cohen says, “We have used a liquid, such as saline, and gas, such as air, to power the micro-turbines. Speeds up to 120,000 rpm can be realized when using air.”

The fluid impinges on the blades, enabling the rotor to rotate. At the other end of the micro-turbine, the rotor can turn a device such as a saw blade, according to Cohen.

The metals used in the manufacture of the micro-turbines are very corrosion resistant so they will not be affected by a saline environment. Durability is not an issue because the micro-turbines are designed for disposable medical applications. Cohen says, “The micro-turbines are intended for non-surgical procedures, which are minimally invasive. Such processes take tens of minutes and could involve, for example, removing a blood clot.”

One of the most important medical applications for micro-turbines is to facilitate procedures in the brain. Cohen explains, “Actuating devices into the brain through arteries can be difficult. The anatomy is tortuous because the arteries twist and turn, plus they have small radii.”

For example, rotating a power cable through an artery can generate considerable friction. This phenomenon occurs because of the interaction between the rotating drive shaft and its protective sheath. Cohen says, “In contrast, pushing liquid through a tube generates very low friction and is a much better approach to furnishing rotary power at a distance.”

Micro-turbines are termed “building blocks” by Microfabrica. They can be used to provide power to a number of medical devices. No standard lubricant is required to run the micro-turbines. Cohen says, “We are not sure if a fluid such as water might be able to provide lubrication because we have not generated any tribological data on our fabrication materials.”

Cohen is open to the evaluation of micro-turbines in nonmedical applications. Further information can be found at www.microfabrica.com and by sending inquiries to press@microfabrica.com. 

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.