Powder (pharmacy)

In pharmaceutical technology , powders are dispersed systems of the solid to gaseous category , in which the particles of the inner phase touch one another. The powder particles can differ in mass, shape, size and in their physicochemical properties. They can be present individually in a crystalline or amorphous structure or as a composite of particles . A special form of the pharmaceutical powder is the powder , which is a very fine powder with a particle diameter <100 μm.

Powders (from Middle High German powder "the / the powder, powdered drug") are used pharmaceutically both directly for oral , cutaneous or inhalative use and for the production of other medicinal forms, such as solutions and suspensions , as starting material for the production of granules and tablets or for Filling of capsules .

Classification according to the European Pharmacopoeia

According to the European Pharmacopoeia, powders are divided according to the way they are used.

Oral powder

Oral powders are preparations that consist of solid, loose, dry and more or less fine particles. They can be filled into single-dose or multi-dose containers, a suitable metering device being required for the latter. Single-dose containers are sachets or vials.

Ingredients:

One or more active ingredients
if necessary auxiliary materials
required dyes
Taste corrections

Powder for cutaneous use

Powders for cutaneous use are preparations that consist of solid, loose, dry and more or less fine particles. Powders must be sterile when used on large, open wounds or damaged skin. They can be filled into single-dose or multi-dose containers. The dosing device for the latter can be a scattering device or a mechanical spray device, or the preparation is present in a pressurized container.

Ingredients:

One or more active ingredients
if necessary auxiliary materials
required dyes

Further dosage forms in which powders are used

Powders also play a role in the following monographs:

liquid oral preparations
Parenterals
Inhalation preparations
Preparations for nasal use
Preparations for rectal use
Preparations for vaginal use
Preparations for use on the eye

Basics of powders

Powders or powders are preparations that should be as free from irritation as possible when they are applied. To ensure this, a sufficiently small particle size and, if necessary, the addition of suitable raw materials are necessary.

When it comes to the raw materials of powders, a distinction can be made between inorganic and organic bases. Some basics are listed below as examples.

	Fabrics
inorganic basics	talc
	zinc oxide
	White clay
	Titanium dioxide
	Magnesium oxide
	Magnesium carbonate
	fumed silica
organic foundations	Strength
	Stearates
	Lactose
	glucose

Properties of powders

Various parameters that can be determined experimentally are used to characterize powders.

Particle size and particle size distribution

The particle size is of outstanding importance for a large number of physicochemical properties that are of pharmaceutical importance. This includes, among other things, the rate of dissolution and flow properties as well as adsorption and adherence to surfaces. According to the Noyes-Whitney equation for describing the rate of dissolution of substances in media, a small particle size is essential for poorly soluble medicinal substances in order to achieve adequate bioavailability .

Particle size determination

The method with the greatest historical importance in pharmaceutical technology is analytical sieving . This enables both the particle sizes of the particles and their distribution within a powder to be determined. The sieving methods used include wet sieving , hand sieving and machine sieving with sieve towers or air jet sieves . With these methods, the bulk material is sieved one after the other through a series of sieves with decreasing mesh size and thus the grain size is classified based on the respective sieve residues and the sieve passage. A graphical evaluation can then be carried out by plotting in an RRSB diagram or a histogram.

In addition, the following methods can also be used to determine the particle size:

microscopy
Scattered light method
Light Blockade (Single Particle Optical Sensing)
sedimentation
Impulse method
Sighting

Flow properties

The flow behavior resulting from the interparticle friction or the resistance during movement is called flow properties .

The flow behavior of powders is important for its dosing accuracy. This is important both for direct application and for further processing into granules or tablets, for example.

Tests according to the European Pharmacopoeia:

Flow rate: The flow rate is defined as the time it takes a certain amount of powder to pass through a funnel device with a defined diameter. It is given in time per mass. According to the European Pharmacopoeia, funnel openings with a diameter of 10, 15 or 25 mm are to be used. The smallest funnel opening at which the powder is currently flowing is used.

Angle of repose : The angle of repose is the angle between the surface of a powder cone and its base. It is calculated from the height of the powder cone h , the radius of the cone r and tan α

${\ displaystyle \ tan \ alpha = h / r}$

Powder density

The density of a powder depends on its shape, particle size and grain size distribution. Powders that consist of particles with a needle-like to rod-like shape or whose particles repel each other due to electrical charges tend to have a lower density, whereas powders with spherical or platelet-shaped particles have a higher density.

The Hausner factor is used to describe the compressibility of a powder. After determining the bulk and tamped volume with a defined mass using appropriate methods, the Hausner factor can be represented mathematically as the quotient of bulk to tamped volume or tamped density to bulk density. In general, the flow behavior of a powder is evaluated using the table below. The greater the compression tendency of a powder under mechanical stress, such as vibration, the worse the flow behavior. The resulting compacting can therefore lead to problems with volume metering.

Hausner factor	Flow behavior
1.00 - 1.11	excellent
1.12 - 1.18	Well
1.19 - 1.25	satisfactory
1.26-1.34	moderate
1.35-1.45	bad
1.46-1.59	very bad
> 1.60	insufficient

Manufacture of powders

There are different ways to make powder. Various processes are used to generate the desired requirements for the preparation in terms of fineness and grain size distribution.

In pharmaceutical technology, three processes are connected in series to produce powders. The starting material is first brought to the desired particle size by grinding processes using various milling systems, before these can in turn be divided into fractions of different grain sizes by sieving processes. Finally, the same fractions are mixed to ensure the homogeneity of the powder preparation.

Crushing

Various milling systems are used to shred the raw materials. The most common are shown in the following table.

Mill	principle	Degree of shredding
Roll crusher	Pressure, friction, shear	1 - 2 mm
Hammer mill	Impact, impact	0.1 - 5 mm
Pin mill	Impact, impact	20-200 µm
Ball mill	Impact, shear	20 µm
Air jet mill	Bulging	1 - 30 µm
Colloid mill	Shear	1 - 30 µm

Spray drying

In spray drying, solutions or suspensions of the dispersed starting materials are sprayed into a stream of hot air in the form of fine droplets. In this air flow, the droplets are dried within seconds due to the increased surface area achieved by the spraying. What remains is a fine powder with a narrow grain size distribution.

Freeze drying

In rare cases, freeze-drying is also used. This is a process that is used for substances that are particularly thermally unstable and sensitive to hydrolysis.

literature

Alfred Fahr : Voigt Pharmaceutical Technology: For study and practice . 12th edition, Deutscher Apotheker Verlag, Stuttgart 2015, ISBN 978-3-7692-6194-3 .
European Pharmacopoeia 9th edition, basic work 2017. Deutscher Apotheker Verlag, Stuttgart 2017, ISBN 978-3-7692-6641-2 .
Petra Hille, Annes Rappert, Gundolf Keil : The powder dosage form in the surgical specialist literature of the High and Late Middle Ages. In: István Gazda u. a. (Ed.): Ditor ut ditem. Tanulmányok Schultheisz Emil professzor 80th születésnapjára. Budapest 2003 (= Magyar tudomanytörténeti szemle Könyvtára. Volume 36), pp. 54-104.