Michigan Tech Research Magazine 2011

Graduate Research

Nanotube-coated implants speed healing

Titanium and its alloys have a leg up on all other materials used to make the orthopedic implants used by surgeons to repair damaged bones and joints. They are light, super-strong, and virtually inert inside the body. But whether the implants are destined for your knee, your hip, your spine, or your jaw, the silvery metal has one big drawback.

"Titanium has a mirror surface," says Tolou Shokufar, a PhD candidate in mechanical engineering–engineering mechanics. Cells don't adhere to it very well, so implants are often roughened up before they are placed in the body.

A good way to roughen titanium is to etch nanotubes into it, since they provide a superb surface for bone cells to grasp onto as part of the healing process. But doing that is neither cheap nor easy. Conventional techniques require platinum, which costs over $1,700 an ounce.

Through her PhD work with Professor Craig Friedrich, Shokufar has developed a cheaper way to etch nanotubes into the titanium alloy. In a weak solution of ammonium fluoride, she immerses two rods, one of the alloy, another of copper, and hooks them up to a power source. An electrical current flows into the copper, through the solution, and out the titanium.

"It corrodes the titanium dioxide layer on the titanium in the form of a nanotube," Shokufar says. Growing the ideal tube takes about two hours.

Then she applies heat and pressure to the titanium alloy, annealing the nanotubes to give them a crystalline structure. Tests show the surface provides a friendly place for cells; Shokufar has conducted experiments with bone-forming osteoblasts, and the results are encouraging. The cells grow more quickly and adhere better to the surface of nanotube-coated titanium.

"This is promising for reducing the healing period after implant surgery, as well as reducing the risk of implant failure," she said.


Sharing the spectrum: the polite world of cognitive radio

Suryabh Sharma is working on the wireless communication system of the future, where there are no dropped calls and you can download your favorite movie in no time at all.

"The fancy name for these is cognitive radio networks," says Sharma, who is earning his master's degree in electrical engineering. Up until now, wireless systems have been, well, non-cognitive.

Sharma explains. "For all our wireless communications, we have a radio spectrum in a range of frequencies. The Federal Communications Commission (FCC) has divided these into small bands, one for radio, one for TV, for amateur radio, etc.

If you monitor each of these bands, you can see we are not using them to their full capacity."

Sometimes, however, they are completely full, and that's when the system breaks down. "If there is high traffic on the band I'm using to watch a video on my cell phone, then it will be fragmented," says Sharma.

For the moment, there's nothing that can be done about that fragmented video; all WiFi transmissions must use the 2.4 gigahertz band. Now, the FCC is looking at changing the rules to allow so-called cognitive radios to transmit on different bands, such as those used for television, on a space-available basis. For example, if the band used by cell phones is experiencing high traffic, a phone could stream video by selecting an underused band.

With his advisor, Zhi Tian, an associate professor of electrical and computer engineering, Sharma is investigating ways for cell phones, iPads, and whatever new wireless devices arise to share the radio spectrum efficiently, a field known as resource management. Devices must identify the radio frequencies that have room for more traffic, switch their own transmission frequencies, and somehow manage to negotiate for space with thousands of other users.

Cognitive radio networks will have to take into account multiple variables, including the location of users and their patterns of use, issues that Sharma is investigating in his research.

It's a complicated problem, Sharma says, but one that can't be ignored. "With the number of users growing, we'll have to come up with some sort of intelligent allocation," he says. "It's all about optimization."


Researchers pair clot-busting technology with blood-gas sensor

Implantable blood-gas sensors, which can detect serious medical problems, came on the scene thirty years ago but are no longer on the market. The problem: as artificial implants in an artery, they promote clotting, which can cause heart attacks or strokes. As well, clotting compromises the sensor itself; it can monitor only the environment of the actual blood clot on the sensor—not the "bulk blood" in the body.

Matthew Nielsen, a PhD student in biomedical engineering, is working on fiber-optic technology that could resurrect implantable blood-gas sensors, which continuously monitor levels of oxygen and carbon dioxide in the blood, as well as pH. All of these can be early indicators of other problems, such as lack of blood flow, inadequate breathing, kidney failure, and poor metabolism.

Yet there is still the persistent problem of clotting, which is where Nielson and his advisor, Assistant Professor Megan Frost, are focusing their research. They are copying nature, harnessing the properties of nitric oxide, which the body routinely releases to dilate blood vessels, increase blood flow, and prevent clots—think of the nitroglycerin pills some heart patients take at the first sign of chest pain.

"We're using what the body is already doing," Nielsen says. "Nobody else has a method to release nitric oxide in the body in a controlled way to deal with the clotting."

Their method relies on the same fiber-optic technology as the sensor, but with a different purpose. Their blood-gas sensor, two inches long and wire-thin, is paired with an equally small fiber-optic device that releases nitric oxide to the sensor to preclude clotting. "Light is the trigger," Nielsen says. "Increase or decrease light—increase or decrease the amount of nitric oxide." Controlled release of nitric oxide is critical; researchers need to know when to use it, how long to use it, and how much to use.

The effort has broader implications. "Nitric oxide," Frost says, "could make more sensing devices biocompatible, and the technology should ultimately improve the performance of any device that contacts the body, such as a subcutaneous glucose sensor for diabetes or implants for kidney dialysis."

"I'm very excited," Nielsen says. "This work has the potential to change modern medicine."