What Should We Learn From...Building a Bionic Nervous System?

It ’s an electrifying time to be in neuroscience. Using implanted devices that send pulses of electricity through the nervous system, physicians are learning how to influence the neural systems that control people’s bodies and minds. These devices give neurologists new ways to treat patients with a wide range of disorders, including epilepsy, chronic pain, depression, and Parkinson’s disease. So far, these stimulators have been oneway devices that deliver a steady sequence of pulses to the nervous system but can’t react to changes in the patient’s body. Now, at last, medical device companies are coming out with dynamic neural stimulators that have a bit of “brain” themselves. These smart systems can detect changes in a physiological signal and then respond by delivering a therapy or adjusting the patient’s treatment in real time.

The paper Building a Bionic Nervous System: Smart neural stimulators sense and respond to the body’s fluctuations written by Tim Denison, Milton Morris & Felice Sun is published in IEEE Spectrum 2015 February. It shows very interesting results on this technological frontier, including devices that take advantage of developments in low-power implantable sensors and embedded signal processing. In this article three devices are presented that respond to the flux of biology within the body. Because these devices rely on data related to the processes they influence, the authors call them “closed-loop”systems, but they can also be called the next step in a bionic model of medicine.

Reading the heart: The newest vagus nerve stimulator from Cyberonics looks for an abruptly elevated heart rate, which can herald an epileptic seizure. The device measures the heart’s electrical activity with an electrocardiograph (ECG ) sensor and extracts heartbeat signals. It compares the last few seconds of data with the prior few minutes of data to differentiate sudden spikes from gradual increases. Then it delivers a pulse of electricity through electrodes wrapped around the vagus nerve.

Reading the mind: The responsive neurostimulator from NeuroPace scans for signs of an oncoming epileptic seizure in the brain. Three simple algorithms detect unusual amplitudes or frequencies in recorded brain waves; they quantify the squiggly line’s length, the area within its peaks and valleys, and the number of times the line changes direction. When the device detects the signature of a possible seizure, it emits a stimulating pulse.

Reading the body: Medtronic’s latest spinal cord stimulator adjusts its treatment for chronic pain according to the patient’s body position, which is registered by a three-axis accelerometer. For example, in a patient lying face up, gravity pulls the spinal cord closer to the electrodes; the device then reduces stimulation to avoid unwanted sensations. Conversely, the device increases stimulation for a face-down patient to ensure adequate treatment when the distance between the spinal cord and electrodes is greater.

The goal of all these closedloop systems is to let doctors take their expert knowledge—their ability to evaluate a patient’s condition and adjust therapy accordingly—and embed it in an implanted device. These dynamic systems have a number of potential benefits: They may react faster than current devices, provide more tailored therapy to individuals, and free up clinicians’ time. These smart devices may have something to teach doctors as well. As the stimulators provide therapeutic effects, they also provide data about how physiological states relate to clinical outcomes. From this new information, scientists and engineers hope to learn more about how the nervous system works, how it is affected by disease, and how to design better treatments. Sometimes, it seems, the way to move forward in science is to follow a loop.