One of the major problems in these sensors is that their performance tends to drift or degrade over time and quite often in very unpredictable ways, due to the organic components in the sensors. Therefore, the devices have to be calibrated and configured regularly to prevent errors. This is why there still isn't an artificial pancreas for humans, despite the amount of research, since the blood glucose sensor would only provide reliable readings over a period of a hundred days. After which, the person would have to receive surgery to recalibrate the chemical sensor. Thus most researchers are frustrated at the number of potential uses for these chemical sensors, because they require constant recalibration.

Enzymes are highly specific in the reaction that they catalyze. If an enzyme can be immobilized on a sensing substrate, and the reaction products gathered and analyzed, then one has the basis for a highly selective biosensor. Once again, one of the primary uses for this biosensor is for monitoring glucose levels. For this sensor to operate, the enzyme is placed on a platinum electrode and covered with a polyurethane membrane to protect the enzyme and to prevent the oxygen surrounding the enzyme from mixing with that of the blood. Glucose oxidase, when oxidized, transforms the glucose, that comes in contact with the sensor, into gluconic acid and reducing the enzyme on the sensor into its reduced form. In this state, it interacts with the oxygen in the blood that passes through the membrane. This reaction produces the oxidized form of the enzyme, two hydrogen ions and two oxygen ions. When the sensor is configured to the correct potential, it will reduce one of the oxygen ions so that the products are oxygen and water. The resulting electrical current can be measured and will be related to the concentration of the glucose in the blood. One drawback is that the rate of movement of the glucose in the blood, through the sensor, is quite slow and therefore the sensors will respond slower to any changes in the blood glucose levels.

Another type of chemical sensor is that which can detect electrical signals. This type of sensor is used when studying the nervous system on a cellular (individual neuron) basis. This is accomplished by the use of metal wire microelectrodes. These sensors can detect the signal amplitudes involved (in the region of 100 mV) and high interface impedances (1-10 MW at 1kHz) between the metal and the organic tissue. Their small size enables many of them to be accurately inserted into a small volume of tissue that is under investigation. This is a perfect situation for robotics, since a robotic arm has far more precision than any human. The microelectrodes then operate by detecting the electrical potential generated by the tissue of an active nerve fiber, due to the currents that flow through the fiber membrane.

There are three common types of microelectrodes: the Array-type, Probe-type and the Regeneration electrodes. The Array type microelectrodes are used to form the floor of cell culture dishes. The signals are received from neurons, which are placed or grown over the electrodes. The Probe type microelectrodes have recording sites along its long thin body, which is inserted into the tissue under investigation. Finally, the Regeneration electrodes are placed between the ends of a "severed peripheral nerve trunk," then the nerve fibers re-grow (regenerate) along the microelectrode. (usf9.gif)