Top-Down Particle Size Reduction
Milling and grinding are ancient techniques that are working their way into high-tech markets.
Kyle James, VP of sales at Retsch USA (Newtown, PA) notes that demand for particle size reduction systems has grown significantly from “alternative” industries. “Demand from the energy sector has been substantial, particularly for processing biomass materials such as wood, refuse, and even garbage. We’ve recently sold six of our SM 300 cutting mills to universities and companies for grinding up biomass,” Mr. James says.
Solar energy R&D is another emerging venue for Retsch. Earlier this year the company released a new “jaw crusher,” the BB 300, which provides rapid but gentle crushing and pre-crushing of medium-hard, hard, brittle, and otherwise tough materials. The device works in both batch and continuous modes.
Retsch advertises the BB 300 for use with alloys, basalt, cement clinker, ceramics, chamotte, coal, coke, construction materials, feldspar, glass, and granite. According to Mr. James, “emerging industry” users use it for silicon semiconductor materials for advanced solar cells.
The breadth of life science applications has given rise to fierce competition between homogenization and milling/grinding. Both have utility in this area, but the tug of war between competing methods goes on.
Fatty acid sample preparation from animal or plant cells is usually conducted with a high-speed emulsifier (e.g., the Ultra-Turrax from IKA Works). The technique, according to literature from Spex Sample Prep (Metuchen, NJ), may be unsuitable for high-throughput labs. Until recently, high-throughput, mechanical cell disruption based on grinding employed traditional ball or swing mills adapted to microtiter plate formats.
A technique based on ball milling designed at Sanofi Aventis, using a SPEX Geno/Grinder® 2000, changes the picture significantly. Originally designed for grinding plant seeds, the Geno/Grinder® 2000 has been repositioned for lysing animal and human tissue samples.
In one such application, freeze-dried rat muscle or liver tissue was applied to deep-well (1.5 to 2 mL) plate wells, and spiked with 100 mM potassium phosphate buffer (pH 2) or trichloroacetic acid. Two grinding balls were added to each well, and the plate was covered with a sealing mat and shaken for one minute at 1,700 strokes per minute.
The result was a homogeneous sample whose quality was a function of the grinding accessories used. For example, 2-mm zirconium oxide beads did not provide shear force sufficient to grind the frozen tissue, but 4-mm stainless steel balls did.
Another SPEX customer, RiceTec (Alvin, TX), devised a high-throughput “quick and dirty” technique, also based on the Geno/Grinder, for extracting DNA from rice for subsequent polymerase chain reaction (PCR) analysis. One rice seed was dispensed into each 1-mL well of a 96-well assay block, followed by the addition of one 4-mm stainless steel bead and extraction buffer. Samples were ground at 500 strokes per minute for two minutes, centrifuged for one minute, then incubated at 95ºC for 20 minutes. After cooling, re-centrifugation, and the addition of neutralizing extraction buffer, the block was sealed and centrifuged for 10 minutes at 300 rpm. The final DNA concentration of between 3 and 7 ng/μL was sufficient for expansion and analysis by PCR.
A salve for flagging innovation?
The pharmaceutical industry has been hit hard by patent expirations and a dearth of new drug approvals. With chemical innovation lagging, drug developers increasingly turn to novel formulations to improve prospects for both old and new compounds. Nanotechnology— the creation of submicron-sized particles of drugs and ingredients—has played a significant role in these efforts.
Nanoformulated drugs dissolve and enter tissues more easily than large crystals, and may even be used to create oral dosage forms and injectible suspensions. Bill Henry, executive VP at Jet Pulverizer (Moorstown, NJ), notes that ball and jet mills have traditionally been used to reduce particle sizes down to just below 1 micron, normally the upper-size domain limit for all things “nanotech.” “That’s the focus for these mills, particularly for electronics and battery applications,” says Mr. Henry. In pharmaceuticals, he adds, milling may be thought of as a “pre-grind,” or precursor to nanoparticle- generating milling processes.
Yet for some pharmaceuticals, the higher end of the nano-size domain might be just what the doctor ordered. Ball milling is being investigated extensively for drug particle size reduction; the manufacturing processes for five approved drugs already use milling.
Jan Möschwitzer, Ph.D., who heads early pharmaceutical development at Abbott Healthcare Products (Weesp, The Netherlands), has published extensively on the production of ultrafine pharmaceutical powders and their significance for the drug industry.
Writing in the journal American Pharmaceutical Review, Dr. Möschwitzer demonstrated that under test conditions the uptake of milled formulations is somewhat attenuated, delayed, or accelerated relative to particles produced by high-pressure homogenization (HPH) and simple micronization. This is not always a bad thing.
One could envision formulations consisting of precisely fractionated particle sizes achieving any desired uptake profile in humans, which is the basis of controlled-release formulations.
Two basic strategies exist for creating submicron-sized drug particles. The “bottom up” approach involves precipitation or chemical synthesis; the “top down” approach uses jet milling, wet ball milling, and HPH.
These methods may be combined; for example, bottom-up plus top-down (precipitation to supra-micron particles followed by milling) or top-down plus top-down (micronization followed by nanonization).
HPH, which produces the finest particles of the three techniques, employs a piston-gap homogenizer in water at room temperature. As drug suspensions pass through a “homogenization gap,” the particles are ripped apart during the collision of two fluid streams as a result of their impact with neighboring particles plus shear and cavitational forces.
Dr. Möschwitzer suggests that in selecting a particle size reduction technology, drug developers first consider the bioavailability and pharmacokinetic parameters desired for a specific therapeutic.
Perhaps equally important is how easily the size reduction method scales to manufacturing. Producing a small batch of a highly active nanosized drug in the lab is one thing; manufacturing it at tons per year is quite another.
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at email@example.com.
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