Issue 54 / April - June 2006
Neural Prosthetics Where Man-Made Systems Tap Into the Works of Divine Wisdom
THE DISCIPLINE IN WHICH MEDICINE AND ENGINEERING TRULY MEET AND FACE NEW CHALLENGES IS THE FIELD OF ‚ÄúBIOMEDICAL ENGINEERING,‚ÄĚ AN EMERGING DISCIPLINE THAT IS ONLY A FEW DECADES OLD.
In medical sciences, among many other subjects, we learn about human body parts (anatomy), how the body functions at the cellular (biology) and systemic levels (physiology), its diseases (pathology), and the symptomatic treatment of these diseases through drug therapy (pharmacology). In general the approach taken by medicine is to understand the natural functions of the body in a balanced state (homeostasis) and to try to restore this balance when it is upset by a disease or an invading force. In engineering disciplines, however, the approach taken towards nature is completely different. We study nature, understand the mathematical principles that govern its operations, and use this knowledge to build new systems. The term ‚Äúengineering‚ÄĚ is synonymous with the concept of ‚Äúdesigning‚ÄĚ new things using human experience and intelligence.
The discipline in which medicine and engineering truly meet and face new challenges is the field of ‚Äúbiomedical engineering,‚ÄĚ an emerging discipline that is only a few decades old. In each sub-specialty of biomedical engineering, researchers study the human body, develop new materials and structures using engineering sciences, either as a treatment method for disease (e.g. artificial bone implants, artificial blood, vascular stents, cardiac valves, etc.) or to diagnose them (e.g. imaging methods and other diagnostic instruments in hospitals). Biomedical engineers face the incredible challenge of developing materials and devices that are compatible with biological systems and capable of working inside the human body to substitute bodily functions. Needless to say, the extreme complexity of the human body makes it impossible to mimic the original system or function of the organs in any way. However, even a poor replacement part or a functional improvement provides great benefit to the patients.
One of the most complex systems of the human body is the nervous system, which consists of the central area (the brain and the spinal cord) and the peripheral parts. The branch of biomedical engineering that deals with the nervous system is ‚Äúneural engineering.‚ÄĚ In this article, we will touch upon a specific subject in the broader area of neural engineering, that is, ‚Äúneural prosthetics.‚ÄĚ1 As the name implies, neural prosthetics is an area where engineering knowledge is utilized to treat neural disorders.
The building blocks of the nervous system are called ‚Äúneurons.‚ÄĚ Neurons generate electric pulses to communicate with each other. The fact that these electric pulses can be elicited by artificial means, i.e. by applying small electric currents to the neurons externally, forms the very foundation of the field of neural prosthetics. Neural engineers can input information into the nervous system by taking advantage of this phenomenon, called ‚Äúneural stimulation.‚ÄĚ Likewise, the information content of neuronal activity can be deciphered by recording the electrical pulses from the neurons and interpreting them according to neuronal function. This two way traffic, monitoring and controlling the neural activity, allows researchers in this field to develop methods of treatment for some sensory, motor, and psychological disorders.
Some of the most successful neural prosthetic applications have been in deep brain stimulation in Parkinson‚Äôs disease,
Figure 1: Components of a cochlear implant by Advanced Bionics Corp. (www.bionicear.com). A: The sound processing unit including a microphone, B: the transmitting antenna, C: the implant, which sends the electric signals down to the electrode array through tiny wires, D: the electrode array stimulates the hearing nerve in the inner ear, which carries the sound information to the brain to be heard.
cochlear prosthesis in hearing impairment, bladder emptying and respiration in spinal cord injury, and vagus nerve (10th cranial nerve) stimulation in epilepsy and psychological depression. These are neural prostheses that are readily available as a treatment method for the given ailments. There is a whole host of others that are in the research and development phase. We will review a couple of examples.
In certain diseases of the inner ear hearing is lost as a result of damage to the hair cells inside the cochlea. In normal cochlea the sound information reaches these hair cells after traveling through the ear drum (tympanic membrane) and the structures of the middle ear, causing them to vibrate. This vibration of the hair cells is mechanically transported to the spiral ganglion cells that form the hearing (auditory) nerve. The hearing nerve carries the sound information to the brain in the form of electric pulses. The ganglion cells are healthy and functional even if the entire population of hair cells has been lost as a result of disease. Neural engineers take advantage of the fact that the spiral ganglion cells (which normally accept input from the hair cells) can be electrically stimulated, thus mimicking the function of the hair cells and producing the sensation of sound.2 During a simple surgical operation, the surgeon inserts an electrode into the ear canal which spirals into the lumen of the cochlea so that the sites where the electric current emits from the electrode are adjacent to the spiral ganglion cells (Figure 1). To summarize the principle of the operation; the audio signals are captured by a microphone, processed, converted into electric pulses (A in Figure 1), and transmitted to the implant over a transmitting antenna (B in Figure 1), or headpiece, held in place by magnets. The implant (C in Figure 1) applies the signals to the ganglion cells in the cochlea through tiny electrodes (D in Figure 1). The hearing nerve (auditory nerve) carries the sound information to the brain, where it is ‚Äúheard.‚ÄĚ
Figure 2: Intraocular epiretinal prosthesis conccept. An external video camera would capture an image and a custom microelectronic unit would process the image and transmit data and power to the implant via radio frequency communication. The implant would receive data and power and stimulate the retina with the command pulse pattern (adapted from Weiland and Humayun see note 8).