Physical characterization of neurocatheter performance in a brain phantom gelatinwith nanoscale porosity: steady-state and oscillatory flows

An agarose gelatin having nanoscale transport properties similar to those of in vivo mammalian brain was employed as a Surrogate for living brain tissue in the evaluation of infusion therapy protocols and neurocatheters to be used in the treatment of brain tumours. The catheters under study were a polyimide tube of 950 mum outer diameter (OD) and 750 mum inner diameter (1D), and a silicone tube of 2.5 mm OD and 1.25 mm ID. From the pressure profiles that were measured during infusions of a solution of Bromphenol Blue dye into this gel, we infer that forces on the order of 0.1 fN were driving the solute molecules through the approximate to200 nm intramatrix voids in the gel at rates of approximate to10 mum s(-1), which is within a factor of 5 of the value derived from the time rate of change of the volume of distribution of the infusate. In some of the infusions, a column of trapped air was left purposely in the catheter prior to delivery of dye into the gel in order to mimic the clinical situation where the neurosurgeon must keep the catheter unprimed because of the need to use a stylet to guide it into the brain. (In those cases, the catheter is attached to the infusion pump only following withdrawal of the stylet, and there is thus a column of air inside the catheter.) In our series of such 'unprimed infusions', an oscillatory behaviour in the pressure profiles was observed, with a centre frequency of approximate to35 mHz. Surface tension effects in the resulting air bubbles within the gel likely limited to <3 nm the range of pore sizes into which flows could be coupled in that case. Circumstances associated with backflow of the dye along the catheter insertion path are also discussed.