Patients with brain aneurysms – bulging blood vessels that could burst at any time – often have stent-like blood diverters implanted in the damaged blood vessel to protect the bulging and weakened section. This approach is less invasive than some alternatives, but it requires frequent monitoring and costly visits to the cardiologist while the vessels heal.
Researchers at The Georgia Institute of Technology have come up with a solution to this dilemma: a highly flexible and stretchable sensor that could be combined with the flow diverter to monitor flow and pressure in a blood vessel without costly diagnostics.
The sensor relies on capacitance changes to measure blood flow and it could reduce the need for testing to monitor flow through the diverter as the patient heals. The sensor has been shown to accurately measures fluid flow in animal blood vessels in vitro.
The Georgia Tech team had developed a stretchable, hyper-elastic flow diverter out of a thin, porous thin film of nitinol. But no existing flow diverters, however, provide quantitative, real-time monitoring of hemodynamics within the aneurysm. And repairing the damaged artery takes months or even years, during which the diverter must be monitored using MRI and angiograms, which is costly and involves magnetic dye injected into the bloodstream. The new sensor could provide simpler monitoring in a doctor’s office using a wireless inductive coil to send electromagnetic energy through the sensor. By measuring how the energy’s resonant frequency changes as it passes through the sensor, the sensor could measure blood-flow changes in the sac.
The sensor uses a micro-membrane made of two metal layers surrounding a dielectric material that wraps around the flow diverter and is encapsulated in a soft elastomer. It is a few hundred nanometers thick and made using nanofabrication and material-transfer printing techniques.
The membrane gets deflected by blood flow through the diverter, and depending on the strength of the flow, the amount of deflection changes. Measuring the amount of deflection is based on the capacitance change because the capacitance is inversely proportional to the distance between the two metal layers.
The brain’s blood vessels are small, so the diverters can be no more than five to ten millimeters long and a few millimeters in diameter. That rules out conventional sensors with rigid and bulky electronic circuits.
The researchers tested three materials for their sensors: gold, magnesium, and a nickel-titanium alloy known as nitinol. All can be safely used in the body, but magnesium offers the potential to dissolve 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 the team is now working on a wireless version that could be implanted in a living animal. Although implantable sensors are being used clinically to monitor abdominal blood vessels, application in the brain creates significant challenges. For example, the sensor must be completely compressed for placement, so it must stretch 300 to 400%. The sensor must also be conformable and bend to fit inside blood vessels.