The Neurovascular Approach

One of the most attractive possibilities that come to mind in trying to solve the hardware problem concerns the development of a vascular approach. The fact that the nervous system parenchyma is totally permeated by a very rich vascular bed that supplies blood gas exchange and nurturing to the brain mass makes this space a very attractive candidate for our interface. The capillary bed consists of 25,000 meters of arterio-venous capillary connections with a gage of approximately 10 microns. At distances more proximal to the heart, the vessels increase rapidly in diameter, with a final dimension of over 20 millimeters. Concerning the acquisition of brain activity through the vascular system, the use of n-wire technology coupled with n-technology electronics seems very attractive. It would allow the nervous system to be addressed by an extraordinarily large number of isolated n-probes via the vascular bed, utilizing the catheter-type technology used extensively in medicine and in particular in interventional neuro-radiology.

The basic idea consists of a set of n-wires tethered to electronics in the main catheter such that they will spread out in a "bouquet" arrangement into a particular portion of the brain's vascular system. Such arrangement could support a very large number of probes (in the millions). Each n-wire would be used to record, very securely, electrical activity of a single or small group of neurons without invading the brain parenchyma. Obviously, the advantage of such system is that it would not interfere with either the blood flow exchange of gases or produce any type of disruption of brain activity, due to the tiny space occupied in the vascular bed.

In order to give a more precise description of the proposed interface, an illustration of the procedure is shown in Figure C.10. A catheter is introduced into the femoral carotid or the sub-clavial artery and is pushed up to one of the vascular territories to be addressed. Such procedure is, in principle, similar to interventional neuro-radiology techniques where catheters are guided to any portion of the central nervous system. The number of 0.5 micron diameter wires (recording points) that could be introduced in a one-millimeter catheter is staggeringly large (in the range of few million). Once the areas to be recorded or stimulated are reached, a set of leads held inside the catheter head would be allowed to be extended and randomly distributed into the brain's circulatory system. Since a catheter can be placed in any major brain vessels, the maximum length of n-wire electrodes required to reach any capillary bed is of the order 2 to 3 cm. Hence, a large number of electrodes would cover any region of the central nervous system from the parent vessels harboring the stem catheters.

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