A flow sensor is shown here on a stent backbone.
A flow sensor is shown here on a stent backbone.
Source: John Toon, Georgia Tech

Integrated sensor could monitor brain aneurysm treatment

A highly flexible and stretchable sensor that can be integrated with the flow diverter in order to monitor hemodynamics in a blood vessel without costly diagnostic procedures has been developed by the engineers at Georgia Institute of Technology.

This sensor uses capacitance changes to measure blood flow, and it is able to decrease the need for testing to monitor the flow through the diverter. The team has demonstrated that the sensor accurately measures fluid flow in animal blood vessels in vitro. Currently, the researchers are developing wireless operation that could allow in vivo testing. “The nanostructured sensor system could provide advantages for patients, including a less invasive aneurysm treatment and an active monitoring capability,” said Woon-Hong Yeo, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering and Wallace H. Coulter Department of Biomedical Engineering. “The integrated system could provide active monitoring of hemodynamics after surgery, allowing the doctor to follow up with quantitative measurement of how well the flow diverter is working in the treatment.”

Cerebrаl anеurysms оccur in about tо fivе percеnt of thе pоpulation, with eаch anеurysm cаrrying a оne pеrcent risk pеr yеar оf rupturing. Anеurysm rupturе will cаuse dеath in up tо hаlf of affеcted patiеnts. “We have developed a highly stretchable, hyper-elastic flow diverter using a highly-porous thin film nitinol,” Youngjae Chun, an associate professor in the Swanson School of Engineering at the University of Pittsburgh, explained. “None of the existing flow diverters, however, provide quantitative, real-time monitoring of hemodynamics within the sac of cerebral aneurysm. Through the collaboration with Dr. Yeo’s group at Georgia Tech, we have developed a smart flow-diverter system that can actively monitor the flow alterations during and after surgery.”

With gloved fingers for scale, a flow sensor is shown here on a stent backbone.
With gloved fingers for scale, a flow sensor is shown here on a stent backbone.
Source: Woon-Hong Yeo, Georgia Tech

Sоft elastоmeric matеrial

“We are trying to develop a batteryless, wireless device that is extremely stretchable and flexible that can be miniaturized enough to be routed through the tiny and complex blood vessels of the brain and then deployed without damage,” said Yeo. “It’s a very challenging to insert such electronic system into the brain’s narrow and contoured blood vessels.”

The sensor uses a micro-membrane made of two metal layers surrounding a dielectric material, and wraps around the flow diverter. The device is just a few hundred nanometers thick, and is produced using nanofabrication and material transfer printing techniques, encapsulated in a soft elastomeric material. “The membrane is deflected by the flow through the diverter, and depending on the strength of the flow, the velocity difference, the amount of deflection changes,” Yeo explained. “We measure the amount of deflection based on the capacitance change, because the capacitance is inversely proportional to the distance between two metal layers.”

Because the brain’s blood vessels are so small, the flow diverters can be no more than five to ten millimeters long and a few millimeters in diameter. That rules out the use of conventional sensors with rigid and bulky electronic circuits. “Putting functional materials and circuits into something that size is pretty much impossible right now,” Yeo said. “What we are doing is very challenging based on conventional materials and design strategies.”

The researchers tested three materials for their sensors: gold, magnesium and the nickel-titanium alloy known as nitinol. All can be safely used in the body, but magnesium offers the potential to be dissolved into the bloodstream after it is no longer needed.The proof-of-principle sensor was connected to a guide wire in the in vitro testing, but Yeo and his colleagues are now working on a wireless version that could be implanted in a living animal model.

While implantable sensors are being used clinically to monitor abdominal blood vessels, application in the brain creates significant challenges. “The sensor has to be completely compressed for placement, so it must be capable of stretching 300 or 400 percent,” said Yeo. “The sensor structure has to be able to endure that kind of handling while being conformable and bending to fit inside the blood vessel.”

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