Continuation of a prior problem

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I’m a dentist, working on a medical device. The device mixes a drug (2% Lidocaine) that has been acidified with HCl to pH 3.5 for storage, with NaHCO3 to bring it to physiological pH (about 7.2) just prior to injection.

The current method is by manually mixing an 8.4% NaHCO3 solution with the drug in a 1:9 ratio V/V. In medicine, an 8.4% solution typically means 84mg of drug / ml of solution. So, the current protocol of mixing 100 ml of bicarb solution with 900 ml of drug solution, means the doctors are delivering 8400mg/L of NaHCO3, which is a LOT more than the 27mg that you are pretty sure they need to do the job. You can see why I was skeptical, but I’m going to take your word over theirs and move on with solving the next problem, which is:

If I want to deliver the NaHCO3 in the form of a fluid (whether a solution, slurry, or any other way to make it flow), what is the best way to do that, and how much NaHCO3 can I fit into a vessel with volume of 35 cubic millimeters (pretty small, and all the room I have available in my device)? In other words, what is the maximum concentration of NaHCO3 that will stay in solution at a standard temperature and pressure with no head space in the vessel? I only need to fit 27mg/L x 2ml = 0.054mg of NaHCO3 into the space, and it would be a lot easier to manufacture if it were a liquid rather than a powder.

Also, I’m in need of some physical testing of the device to confirm that mixing has properly occurred at the tip of the needle when the prototype is built in a few weeks. There is some thought that we could use excess NaHCO3 in a chamber, and have bypass mixing occur as the drug passes through the chamber and into the needle, instead of trying to get the whole volume (2ml) mixed prior to moving any drug out of the needle.