Measurement of friction on straight catheters in in vitro brain and phantom material

As part of our studies on the magnetic stereotaxis system (MSS), a means of delivering therapies to the bulk brain, we have measured the frictional forces on a thin, straight tube used to simulate a catheter. Experiments were done with a spring-loaded, stainless steel tube of 1.9-mm diameter which was passed through 5.5 cm of gelatin phantom or, alternatively, through in vitro calf brain. The dynamic response of the tube to sudden displacement of the outer end of the spring yields estimates of the tube's friction per unit length. Twenty-three runs in the two media were analyzed for the static and dynamic frictional forces exhibited. In these series the static frictional forces were found to be (0.0132 +/- 0.0012) N cm-1 [(1.32 +/- 0.12) g cm-1] of length in the gelatin phantom and (0.0079 +/- 0.0008) N cm-1 [(0.79 +/- 0.08) g cm-1] of length in brain. The kinetic friction coefficient, b, was found to be (8.4 +/- 2.1) N s m-1/cm length of catheter in brain and (16.3 +/- 7.6) N s m-1/cm length of catheter in the phantom material. Based on these figures, the MSS will be capable of moving straight catheters of similar friction that are 20-cm long at rates of displacement of 0.02 to 0.05 cm s-1 in the white and grey matter of the brain. Future studies will evaluate the forces arising from curved paths. Unanswered questions remain as to the mechanical difference between in vivo and in vitro brain, between animal and human brain, and the involvement of sulci in practical paths of motion.