Recent Advances in Medical Applications of Spray Systems
Spray system technology is now critical to two medical applications: aerosol sprays and uniform coating of tablets.
Aerosol Medication: Insulin. After ten years of clinical trials and research, inhalation of insulin has been proven to be an effective alternative to direct insulin bloodstream injection. Diabetics currently inject themselves with insulin periodically in a day, making the spray alternative an attractive one. One of the two aerosol therapies currently being developed, Exubera, has just been approved by the FDA. With the Exubera system, the insulin is first spray dried to form solid particles. With the other spray methodology, plastic blisters (sacks) of insulin are punctured with needles and pressurized so that small diameter liquid jets are ejected. The liquid jets break up to generate uniform micro-sized droplets, which are blown by an air stream into a transparent cylindrical chamber. In both cases, the patient inhales the spray and receives the required dose when the cylindrical chamber is clear.
Extensive research at Carnegie Mellon University, in collaboration with the University of Pittsburgh Medical Center (UPMC), has examined the aerodynamics of micron-sized aerosol particles in human respiratory systems. Particles of size greater than 5 microns have sufficient momentum to impact on throat and/or respiratory surfaces, where they lodge or pass down into the stomach. In the stomach, proteins such as insulin are often destroyed by the acids of the digestive track. Particles of 0.5 microns or less (such as cigarette smoke), have high drag forces, which keep them airborne and result in exhalation, followed directly after inhalation.
Particles with diameters between 1 and 4 microns can penetrate through the respiratory tract and reach the lungs; they are then deposited on lung surfaces and the medication is passed directly to the bloodstream. In principle, any medication can be administered by inhalation, provided that the aerosol particles are between 2 and 4 microns in diameter.
Aerosol medication: cyclosporine. At the Lung Transplant Center at UPMC, patients who have undergone lung transplants must be treated with the drug cyclosporine to lower the patient’s immune system, which otherwise treats the transplanted lung as a foreign body and attacks it. Cyclosporine is normally administered orally and passes through the stomach to the bloodstream--with considerable reduction in intensity--before it reaches the lung surfaces. Administering cyclosporine by inhalation of droplets 1-4 microns in size provides topical deposition on lung surfaces, a more effective method for distributing the drug.
Coated medication: tablets. Most medical tablets are coated; coating provides color (“Where are those blue tablets?”) as well as active medication. Delayed release tablets allow the medication to be released slowly over specified periods of time. The release period may be as long as 24 hours, during which time gastric juices in the stomach can break through the coating. The thickness of the coating must be uniform and precise to allow diffusion through the coating, as specified by the physician. An overdose of medication could be dangerous--and possibly lethal.
Pharmaceutical companies produce billions of tablets and pills in large fluidized beds. Thousands of tablets are placed in large chambers where air jets lift the tablets so that they mix in turbulent air flows of fluidized beds. The coating medication is sprayed into the fluidized bed and individual spray droplets impinge on tablet surfaces, where they dry to form a solid coating. Precise coating thickness requires generation of uniform size spray droplets, which must target the tablets that are bouncing in turbulent air flows.
These are some examples of the importance of controlling spray drop size, velocity momentum and trajectory in medical applications.
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