This disclosure teaches the concept, and method of creating, a dual use device intended for persons who take 
insulin. In one embodiment, the novel device is an 
insulin delivery cannula, the outer wall of which contains electrodes, chemical compounds and electrical interconnects that allow continuous 
glucose sensing and delivery of data to a remote device. Heretofore, the main problem in attempting to sense glucose at the site of 
insulin delivery has been the 
high current resulting from oxidation by the sensor of the preservatives in the insulin formulations. One means of eliminating these interferences is to 
poise the indicating 
electrode(s) of the sensor at a bias sufficiently low to avoid the 
signal from oxidation of the preservatives. One way of obtaining a glucose 
signal at a low bias is to use an 
osmium-ligand-
polymer complex instead of conventional 
hydrogen peroxide sensing. Another is to use a size exclusion filter located in line with the 
insulin delivery tubing in order to remove the smaller phenolic 
preservative molecules while allowing the larger insulin molecules to pass unimpeded. These 
filtration concepts can also be more broadly applied, that is, the general concept of removal of unwanted 
drug formulation excipients from a 
drug delivery system.