Spray drying is a drying method that was firstly described more than 140 years ago as an improvement in drying and concentrating liquids1. But it was not until the beginning of the 20th century that the level of sophistication and knowledge of the process allowed its industrial use. The production of milk powder was the first commercial application and still remains one of the most important uses of the technology.
Spray drying involves the atomisation of a liquid feed into very small droplets within a hot drying gas leading to flash drying of the droplets into solid particles. The particles are then separated from the drying gas, using a cyclone and/or a filter bag, as a final spraydried product. The feed can be a solution, a suspension or an emulsion and the resulting product can be classified as a powder, granules or agglomerates.
In a single continuous step, spray drying therefore converts a liquid feedstock into a powder with well-defined properties. Properties such as level of moisture or residual solvent in the powder, particle morphology or size and powder density can be manipulated to a great extent to target levels. The remarkable flexibility in tailoring the properties of the final powder, the gentleness of the process and its economics when compared with competing technologies such as freeze drying led to its proliferation in multiple industrial applications including cosmetics, fine chemicals, detergents, polymers, excipients and pharmaceuticals.
Advantages of spray drying
1. Increased bioavailability
Many modern therapeutic compounds are stable in a crystalline form but often display poor aqueous solubility, and with this, low dissolution rates. This reduces the bioavailability of the API, sometimes to the point of nullifying the therapeutic effect.
With spray drying, it is possible to co-precipitate an API with a polymer into a stable amorphous solid dispersion, thereby greatly improving the dissolution rate. Specifically, it is the unparalleled drying rate that enables the API to be captured in amorphous form.
An interesting technique for improving the dissolution rate is to create nanoparticles that are isolated in larger composite particles and then recovered by the spray drying process. By enhancing the dissolution rate in this way, spray drying has the potential to make treatments possible that are currently unfeasible due to low bioavailability.
2. Modified release and taste masking
Encapsulation offers a number of commercial and medical advantages. It allows the sustained release of, e.g., antibiotics, reducing dosage requirements. By preventing drug concentration peaks, encapsulation is also an effective way to treat chronic illnesses with reduced side effects. Taste masking and the physical protection of the API are other common applications.
Spray drying makes it possible to engineer particles to create specific release patterns and other desired properties. For encapsulation, the API and biodegradable excipients are dissolved and/or suspended. Subsequently, the feed is atomized and dried into a powder.
An interesting alternative approach is spray congealing. Here, the API is melted or mixed with molten excipient and the powder particles produced by atomization and cooling.
3. Aseptic production
Aseptic spray drying offers a number of advantages over traditional methods of aseptic drying like lyophilization. Spray drying provides more control over the drying process and, as a result, over the shape, density and morphology of the final product. Lower running and capital costs also mean reduced overhead.
Production of dry sterile dosage forms often involves mixing the API with one or more excipients. To achieve a homogeneous mixture, the particle size distribution of the excipient(s) must match that of the API. In a one-step operation, spray drying can turn a sterile solution into sterile particles of the required size without any risk of introducing impurities – a well-known problem with milling.
4. Powders for inhalation
The number of medications taken by deeply inhaling them into the lungs is steadily increasing. For diseases directly affecting the lungs, this method of administration has long been an obvious choice. However, pulmonary administration is equally suited for other types of drugs, especially large molecules biotherapeutics, such as hormones, peptides and proteins, where tablet administration is challenging due to degradation risks during ingestion and where injection is often unpractical and disliked by many patients.
The key to success of pulmonary administration is that the drug is airborne all the way to the target point. Droplets delivered with nebulizers or particles of micronized powders produced by milling, and for example mixed with lactose, have high density. In contrast, lower density powders produced from a multitude of liquid feed mixtures and spray-dried at optimum process conditions can form particles with morphologies especially adapted to inhalation.
5. Direct compressibility
Solid dosage pharmaceuticals often require a separate granulation step in the production cycle to avoid segregation and to produce a powder with flow properties that can accommodate a high-speed tablet press.With the Fluidized Spray Dryer concept, the granulation step can be made an integral part of the continuous drying process .
