While listening in on the conversation between neurons is a challenge, learning how to inject our thoughts into the conversation is also difficult. We must manipulate voltages or inject currents to make ourselves heard without damaging the cells or their surroundings and ensuring that our message reaches the intended cell or group of cells. The challenge then is to find the most effective and safest way to communicate with neurons.
Extensive research continues to focus on how to best communicate with cells. While impaling a cell with an electrode is the most direct approach, the cell inevitably dies from the wound, and the approach is not practical for a functional neuroprosthesis. Many of the methods used to listen in on cells also function well as signal transmitters. Regardless of the method employed, the key issues that must be addressed are the amplitude of the stimulating signal (voltage or current), the duration and polarity of the signal, and the spatial selectivity. To be successful, the biologist must investigate how the natural processes of a cell are altered by a foreign stimulus and must determine the limits of this response before damage occurs. For example, a biphasic (two-polarity), charge-balanced signal best replicates the natural ebb and flow of ionic currents when a current stimulus is used (see Chapter four). Engineers, material scientists, and physicists must then find the best way to generate such a signal and deliver it to the cell. Sophisticated modeling techniques, such as those described in Chapters five and seven, estimate the voltage or current generated by competing electrode designs as a function of time and space within adjacent tissue. By adjusting the properties (surface area, geometry, and conductivity), researchers attempt to target specific cell groups with a sufficiently large stimulus. Different implant materials and architectures influence the electrical power required to deliver a desired density of charge to the cell, which in turn affects the electrical efficiency and heat generation of the implant. As the available technology continues to evolve, researchers continue to refine their techniques.
In addition to creating a safe and effective electrical connection with the cell, the implant must not physically endanger the cell and its surroundings. Many implant materials, such as semiconductors and most metals, are poisonous to the human body. Insulating materials such as silicone prevent any interaction between the poisons and the tissue without acting as a barrier to the electrical signals. An implant must not cause tearing or other physical damage to the tissue at the point of connection. The human body experiences significant amounts of movement and jarring impacts in even a normal day, and the implant must move in concert with the surrounding tissue to avoid injury. Implants must also allow the exchange of nutrients and waste to proceed naturally so that the surrounding tissue remains healthy. If the cells that communicate with the implant perish, the implant becomes ineffective. These issues are treated with greater detail in Chapter eleven.
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