Web of intrigue
An adhesive solution for handling fine pharmaceutical powders could also make an impact in the food and cosmetic industries, reports Tom Shelley
By sticking fine powders to a thin web, they can be handled and dosed out very precisely. The technique, developed for the pharmaceutical industry, is equally applicable to food or even possibly cosmetic industries, by making use of adhesives and/or webs that are edible.
Other potential applications range from generating test strips for diagnostic work, to colour dosing for plastics, to dosing out herbs and spices for exotic domestic cookery.
The ‘Sticky Web’ is the brainchild of 42 Technology (42T), based in St Ives, Cambridgeshire. It started when GlaxoSmithKline asked if the company could come up with a way of precisely metering out powders at high speed for tablet production. They wanted to be able to dose out 0.1mg to 100mg portions of drug at rates of several hundred per minute.
42T first carried out a literature search, but most of the ideas it unearthed were check weighers. “But each would take several seconds,” states chairman Howard Biddle. “Then there were volumetric systems, but I am not sure how accurate they are. Some of the powders are free-running, like castor sugar, while others tend to clog. Whatever they are, we have to ensure they are treated very gently.” Otherwise, some pharmaceutical preparations can be rendered ineffective. Also, adds Biddle, “the FDA [US Food and Drug Administration] is legislating that tablet manufacture must be inspectable.”
Tablets are normally made up of active ingredients, which may be quite small in amount, plus ‘excipients’ - inactive carriers. “You have to make sure the excipients do not react with the actives,” adds Biddle. “This can take many months of development. Sometimes the actives are so delicate that forming them into tablets destroys their activity.”
He showed Eureka how it works, taking a piece of adhesive tape and explaining as he went along. “If you take a sticky web, in this case a piece of Sellotape, dip it into the active powder and tap off the excess, you get what appears to be a consistent coating. When you then cut the powdered tape up into areas, you find that it is consistent”.
In the initial tests, 42T found it was consistent to +/- 10%. “We soon improved this to better than +/- 4 per cent.” The amount of adhering powder is proportional to the available area of tape, about 2mg/cm2.
Biddle showed us a sketch of a design for a machine that has already been prototyped for automating the process. In this, powder is delivered into a vibrating hopper in the centre of a drum with a series of masked apertures in internal cells, partitioned by paddles around the inside of its periphery. The web is fed onto one side of the drum and runs round its outside until it almost reaches the feeding-in point, where it is peeled off. Excess powder lifted by the paddles then falls back into the hopper.
The correct dose may then be obtained when the web is sliced and diced. “Alternatively, if it was a stainless steel web,” says Biddle, “we could doctor blade on the adhesive and powder, which would then become the web.”
The technique allows preparation of doses of pure, active concentrates, without excipients. Both web and adhesive can be made edible, if required. Would something like rice paste and/or gelatine be suitable as a basis for adhesive? Biddle feels that, while both of these would work, say the company has something else in mind that would function even better. The web could also be used as an encapsulant, by adhering another sticky web onto the powder-loaded web, so that, if the web material had a low dissolution rate, one could have a low-release drug, without the need to encapsulate it.
But while GlaxoSmithKline retains worldwide rights to the technology for pharmaceutical dispensing, 42T has the rights for all other markets; and some offer the largest and most interesting possibilities.
For example, it is possible to print gradient doses on increasing areas, which could be used for titration strips.
“We have an idea for a pipette strip for high volume screening,” explains Biddle. “Different antibodies could be deposited on the same strip, for medical diagnostics.”
There are possibilities with regard to adding small quantities of pigments to paint to obtain particular shades of colour – and also to plastic for short-run moulding. It might prove to be a crucial step in manufacturing the dye solar cells to be placed on steel surfaces, described in the June edition of Eureka. There are also opportunities in fine chemistry, both for adding small amounts of ingredients and also by printing two or more chemicals or pharmaceuticals on the same web: having substances that produce something useful when mixed together, but which need to be kept separate until required, because they have a short shelf life in their reacted form.
One really large-scale application being considered is printing combinations of exotic spices, each only required in small amounts for meal servings for small numbers of people. Many Indian spices, some of which cost more than gold and so are used in tiny amounts, keep for months or years, if dry. But these are liable to deteriorate, if mixed in wet curry pastes, and so usually have to be omitted from such mixtures. A particular advantage of the process for consumer products is that it allows powders to be printed on the web, in the form of a company logo. Biddle suggests that different flavours could be printed in succession on to web, which could then be cut into sections and rolled up to form sweets or chewing gum that would change flavour as they were sucked.
* Method allows the accurate and rapid dosing of small quantities of fine powders into consumable form, without having to mix them with non-active diluents or forming them into tablets
* It can be undertaken at high speed to an accuracy of better than +/- 4%
* Developed for the pharmaceutical industry, it also has potential for application in fine chemical, diagnostic and food industries
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