Implantable wireless biosensor monitors cerebral blood flow

Researchers have created a new medical sensor that promises to revolutionize the ability of doctors to treat brain aneurysms. The device, which is battery-less, is a capacitive sensor with an inductor. It can be implanted directly in patients’ brains and, oddly enough, that’s significantly less invasive than the most common treatment the medical profession uses today.

Robert Herbert and Woon-Hong Yeo, both from the Georgia Institute of Technology, described the device in a recent edition of Advanced Science . The work was done with colleagues at Georgia Tech and at Hanyang University in South Korea.

The miniaturization of biomedical sensors coupled with efficient wireless protocols have led to a new generation of medical sensors with a profound impact on the healthcare sector. Even stethoscopes are getting revolutionary updates to better help manage health information.

The latest microelectronics technology has been particularly beneficial for implantable solutions, helping them to become extensible biosensors. The industry is rising to meet technological challenges involving wireless monitoring capabilities, reliability, and production scalability. They are also being manufactured using new materials more compatible with tissues and blood vessels.

The Georgie Tech sensor benefits from these advances. The sensor is implanted in the blood vessels of the human brain, thus helping doctors to assess any abnormalities that can cause patient death. The sensor is composed of biocompatible polyimide, two layers created by silver nanoparticles, a dielectric and a soft polymeric encapsulating material. A coil is powered through electromagnetic energy harvested by another device located outside the body. The capacity of the implanted sensor changes according to blood flowing, which alters the output signals of a third coil located outside the body. The device can detect changes in blood flow of just 0.05 milliliters per second (m/s).

In the laboratory, capacity changes were measured at 6 cm distance from a sensor implanted in the flesh to stimulate brain tissue.

The sensor uses inductive coupling to allow wireless detection of the biomimetic hemodynamics of the cerebral aneurysm. The researchers said the sensor is easily constructed with 3D aerosol jet printing to create conductive silver traces on elastomeric substrates, which reduce costs and accelerate production. This technique allows the creation of very small electronic functions without using traditional lithographic processes, thus obtaining a microelectronic structure suitable for implants in the field of micro-medicine.

Currently, the healing of cerebral aneurysms is monitored by repeated angiographic imaging, which requires injecting the patient with imaging dyes; the process is considered highly risky for the patient. The use of the new sensor could allow more efficient evaluations without the use of imaging dyes.

Yeo, an assistant professor at Georgia Tech’s George W. Woodruff School of Mechanical Engineering, said, “The beauty of our sensor is that it can be seamlessly integrated into medical stents or flow divisions that clinicians are already using to treat aneurysms.”

The sensor combines an LC circuit (capacitance C and coil L of the sensor), thus forming a resonant circuit with a frequency proportional to f = 1 / (2π√LC). This method of reading, with respect to the frequency domain method for observing impedance variations, has been shown to reach longer reading distances (Figure 1).

 

Figure 1: Schematic and equipment overview of the battery-less wireless system with an implantable flow sensor and two external antenna coils [Source: Advance Science]

The analytical and experimental studies of the team have identified electrical characteristics for the correct optimization of the measurement. A correlation between the quality factor of the sensor, the coupling coefficient, and the transfer coefficient define a critical value for the number of winding of the coil with a diameter of the order of 100 µm to reach high efficiency. Coils with 44-53 winding and 30 mm in length allowed longer reading distances. The resulting coils have inductances between 1.5 and 1.9 µH, and the average induced power is of the order of 1.2 mW for about 40/50 winding.

These tests use capacities from 150 to 220 pF to obtain low resonance frequencies between 10.2 and 12.2 MHz. Changing the sensor capacity does not affect the induced power, but reduces the quality factor, producing a less pronounced resonance peak.

Transdermal sensors of this type can be applied to the skin for monitoring various medical parameters which, if properly controlled, can help cure diseases. Medical devices generally combine microelectronics and micromechanics, representing, in fact, a perfect MEMS. They can also monitor glucose and blood pressure, treat chronic pain, or even administer medication.

>> This article was originally published on our sister site, EE Times: “Wireless, battery-free sensor for brain aneurysm treatment.”

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