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Biological Electronics and Sensors for Medical Applications

10.23.2007: Electronic and Ionic Neural Interfaces

SPEAKER: Luke Theogarajan, PhD: MIT

MODERATOR: Jay Schnitzer, MD, PhD: MGH

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Experts discuss biological electronics and sensors

Two researchers discussed the topic of “Biological electronics and sensors for medical applications” at the CIMIT Forum on Oct. 23 at Simches Research Center at Massachusetts General Hospital.

Presenting were Luke Theogarajan, PhD, of MIT, and Timothy Swager, PhD, John D. MacArthur Professor and Department Head, Department of Chemistry, MIT.

Dr. Theogarajan’s topic was “Electronic and ionic neural interfaces.” He said that neural prostheses are being developed around the world to alleviate various debilitating conditions. The key component of any neural prosthesis is the biotic-abiotic interface. He discussed the conventional electronic coupling to neurons that has been successful in areas like cochlear prosthesis.

 “The main objective is to enable the design of efficient interfaces so that we can build devices to alleviate some of the debilitating conditions that arise from neural damage like blindness, Parkinson's disease etc. I am not with the clinic at MGH though I work with surgeons at MEEI and in the context of the Boston Retinal Implant project at the Veterans Administration, Boston. I am always looking for collaborations with like-minded people.”

Dr. Swager’s subject was “Electronic polymers and biosensors.” He focused on the applications of conjugated (electronic) polymers to detect molecules of biological origin and develop systems that can be used to measure biological activity. The mechanisms discussed will make use of optical detection and the ability of electronic polymers to amplify.”

He said, “My research program is broad and we are working on many applications in sensors (environmental, national security, and miscellaneous industries).  The road to the clinic for new technologies is a difficult and slow one. I suspect if all goes well, in 5-10 years we could have something out there.”

Many scientists have high hopes for neural prosthetics, devices capable of restoring functions lost as a result of nerve damage.  To date, cochlear implants, which allow otherwise deaf people to hear, are the most refined and most widely used neural prosthetic.  Researchers are attempting to develop retinal prosthetics that may someday provide a useful level of vision to patients with conditions such as age-related macular degeneration or retinitis pigmentosa.  Retinal prosthetics, like all neural prosthetics, will require a robust interface between the device and the patient’s neural circuitry. 

Preliminary trials suggest that retinal prosthetics are not an impossible dream.  Researchers led by Luke Theogarajan of MIT implanted chips of four electrodes beneath the retinas of a few blind volunteers, and upon stimulation, the blind volunteers perceived phosphenes, or the sensation of “seeing stars.”  The electrode chips were powered by an external power supply and received image information from an external sensor.              

An electrical biotic-abiotic interface, however, may not be the best interface for a retinal prosthetic.  Because of the current-siphoning effect of soft tissue and because of certain morphological changes that occur in blind people, a lot of electrical current is needed to stimulate the eye’s nerves.  It is difficult to provide this current, and this current could be large enough to damage tissue. 

Theogarajan’s group is exploring an interface based on ions, instead of electricity.  Ions are naturally abundant in the body, and changing the ion gradient across a neuron’s plasma membrane can trigger an action potential.  So far, it seems that boosting extracellular potassium levels is the most effective way to produce an action potential.  Potassium ions could be sequestered from the device’s environment and would not need to be stockpiled in the device.  Now, researchers must figure out how to reliably deliver potassium ions to a specific area.  Theogarajan’s team is currently investigating an ion-delivery mechanism similar to that found in an inkjet printer. 

Neural prosthetics have the potential to transform medicine, and although electrical interfaces currently dominate the field, biocompatible ionic interfaces are a technically feasible alternative and could provide the way of the future.

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