Convection enhance delivery (CED) is a local drug delivery technique for the treatment of neurological diseases. With CED, a needle (or cannula) is implanted into tissue and either a constant flow rate or pressure is applied to deliver infusate directly into the extracellular space. Under certain conditions, infusate flows back along the outer cannula wall in the space between the cannula and tissue instead of spreading out into tissue. Higher driving fluid pressures are often needed during CED and this can frequently result in backflow especially at high flow rates or with large needle diameters. Backflow is normally undesirable because specific targeting is not achieved, and drugs can reach regions of the brain where they are not effective, toxic, or result in unintended side effects. Previous models have been developed to predict backflow [1,2] that consider elastic expansion of tissue surrounding the needle due to infusion pressure. However compressive pre-stress between the tissue and the needle wall produced by tissues displaced during needle insertion has not been previously considered.

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