Engineering Implantable Devices for the Brain

Engineering might be difficult, but it’s not exactly brain surgery.

At least, it wasn’t until a joint research effort between a pediatric neurosurgeon and an electrical engineer brought these disparate fields together at last.

Example of a microelectromechanical system (MEMS) chip.

Their ultimate goal is to monitor and stimulate individual human brain cells at room temperature with an microelectromechanical system (MEMS) implanted just above the inner table of the skull for long-term human use. This could make a significant contribution to elimination of brain disease.

Implanting Devices in the Brain: Mad Science or Mad Engineering?

The traditional way to measure and simulate brain activity involved placing electrodes on the brain’s surface. More recently, an approach called deep brain stimulation (DBS) was developed to treat the tremors associated with Parkinson’s disease, as well as depression and a variety of other ailments.

DBS involves implanting a pair of electrodes in a patient’s brain and a generator in the patient’s chest wall. These electrodes then generate a pattern of electrical pulses to simulate portions of the brain. Although the method has proven effective in many cases, the mechanism behind it is not understood, and the risks include bleeding, stroke and infection.

Because the skull is a good insulator, electrical signals from neurons cannot easily pass through it. This means that surgeons need to open the skull in order to place electrodes on or in the brain.

A better alternative would be to avoid penetrating the dura, the covering that protects the brain from infection, and here’s where the engineering comes in.

How Engineering Can Stimulate Your Brain

Srinivas Tadigadapa, a professor of electrical and biomedical engineering at Penn State, has been exploring magnetic MEMS sensors and actuators that can record and stimulate brain activity through non-contact techniques. He and his team are creating tiny coils that can deliver localized stimulation to individual cells.

“Usually what people do to stimulate neurons is to have very big coils that go outside the head,” said Eugene Freeman, a PhD candidate in Tadigadapa’s lab. “They’re not implantable. They’re about the size of your fist, so you have to go in to a lab for the treatment, which is called transcranial magnetic stimulation (TMS).”

Hemispherical microcoils for localized neural stimulation. (Image courtesy of Penn State.)

“These large magnets activate a relatively large part of the brain,” Freeman explained. “You can’t get single neuron specificity. We are experimenting and simulating microcoils in different sizes and shapes, the smallest so far being about 500 microns in diameter (about half the size of a grain of salt). We use microglass structures and pattern 3D copper coils on them.”

Detecting Magnetic Waves in the Brain

There are currently a few devices that are sensitive enough to detect magnetic waves of the brain, however they are not exactly simple to implement for an outpatient program. One needs to be cooled to liquid helium levels in a superconducting quantum interference device (SQUID) while the other must be heated to 180C to vaporize metals in an atomic magnetometer.

A superconducting quantum interference device (SQUID) like this one uses superconducting loops containing Josephson junctions to measure extremely subtle magnetic fields.

In order to make an implantable sensor to detect the brain’s magnetic fields, the noise from the Earth’s magnetic field needs to be cancelled out. Tadigadapa’s proposal is to build active and passive circuits on a CMOS microchip that will cancel out the noise using a simple feedback loop.

The microchip will generate an on-board magnetic field within the MEMS that will compensate for other magnetic fields in the patient’s environment, including the Earth’s. Other circuits in the implantable chip will amplify the brain’s magnetic signals.

“The Earth’s magnetic field is around 60 microTesla,” explained Tadigadapa, “and the magnetic field of the human brain is around a picoTesla (around 60 million times weaker). So there is a need to block the Earth’s huge magnetic field. Currently that is done within an isolated room that costs $10 million to build, plus the cost of the SQUID itself and the high cost of liquid helium to cool the device. We hope to be able to do it with an on-chip circuit.”

“It’s a good technical challenge,” Tadigapada added in a serious understatement.

Brain Implants in Five Years?

Tadigapada and Steven Schiff, pediatric neurosurgeon and professor of engineering at Penn State, are hoping to make rapid progress. Their recent work on magnetoflexoelastic resonators has demonstrated sensitivity in the tens of nanoTesla and on magnetoelectric magnetometers has achieved approximately 300 picoTesla sensitivity, all at room temperature.

“The first chips will probably be on the scale of a centimeter for the part of the chip that just does sensing,” Schiff said. “For implantation the scale we are targeting is 100 micrometers. We won’t be submillimeter for the array the first two years.”

In the meantime, there are numerous technological challenges to overcome. Mathematical filters will need to be developed to distinguish one cell from another. Initial tests will be conducted on rat brain slices in vitro to understand the strength and signals for stimulation. A wireless transmitter and receiver will also need to be developed.

“I think we are looking at a five-year time horizon to the point where we could seriously have the technology ready for application and potential translation testing,” Schiff said.

For more information, visit the website for Penn State’s Millennium Science Complex.