Metered dose inhalers (MDI’s) are one of the most commonly used drug delivery devices, with millions relying on them for relief of asthma symptoms and for treatment of other respiratory diseases. But the technology remains fraught with problems that medical device designers have been struggling to overcome.
This excellent article from Today’s Medical Developments (onlinetmd.com) explains some of the challenges of current MDI technology:
Inhalers use a complicated interaction of chemistry, physics, and thermodynamics to deliver drugs into the lungs of patients. Inside the canister, the drug is suspended in a tiny chamber with a cosolvent and a liquid propellant. When the canister is depressed, the chamber opens to the outside, the propellant flash boils, and the pressure shoots a precisely controlled volume of mixture out of the chamber through a nozzle and into the patient’s mouth. As the spray expands, the cosolvent evaporates, until only droplets of the medication remain suspended in air to move through the trachea and into the lungs.
The challenge is to get those droplets exactly the right size and with the right distribution. Too small, and the droplets hang in the air, milling around in the lungs until being exhaled without settling on the surface of the alveoli (air sacs) and delivering the medication. Too large, and the droplets won’t remain suspended in air long enough to make it deep into the lungs, instead coating the mouth and trachea where the drug is not effectively absorbed.
Drug makers currently rely on a trial-and-error process that uses simplified predictions to change input variables, such as the type and amounts of propellants or cosolvents, to adjust the output droplet size. Without a fundamental understanding of what goes on inside the spray, creating a better inhaler relies on too much guesswork.
Work being conducted at the Argonne National Laboratory might provide a way past the MDI technological impasse. Researchers at Argonne are using “brilliant x-rays” to image the complex thermodynamics that occurs inside an MDI nozzle (brilliance is a measure of the quality of an x-ray light source. The greater the brilliance, the more light photons can be concentrated on a spot – Wikipedia). Device developers have been able to see inside the spray cone once it has dispersed enough to become translucent (several inches from the nozzle). But until now, there has been no way of seeing what occurs inside the MDI nozzle and the first few millimeters of spray – critical in understanding how to better control the spray.
Hopefully, the research being done at Argonne will provide medical device developers with new insights that will lead to better inhaler designs.