The history of coating goes back to the early 1950s when the first pills were coated with sugar to conceal the unpleasant taste. More and more attention was paid to processes that were not only faster and more efficient, but also had a decisive influence on the release of active ingredients. Conventional processes based on water and organic solvents were developed for this purpose. These processes are the most widely used today and are constantly being further developed. A new field is trying to apply the coating substances directly in dry form, without first dissolving them in water or organic solvents. These solvent-free processes could displace conventional methods in the future, as they are efficient, resource-saving, quick to implement and energy-saving.
Coating is a broad field even in pharmaceuticals, as the question arises: when, how, for how long, and where should the active ingredient be released? Even the released amount in each case can be controlled. All parts of a capsule or tablet can be coated: if, for example, the drug itself, the granules or the resulting capsule/tablet are coated differently, the release of the active ingredient can be controlled variably.
The coating process is used for mainly three reasons: Improvement of compliance/drug safety, protective function and improvement of bio pharmacy.
Improving compliance and drug safety is a main reason why tablets are coated: The applied coating makes the tablet look nicer, hides bad taste and facilitates identification (most active ingredients and excipients are white). The patient or customer can not only better identify his product, but also swallow it more easily: due to selected substances, there is no absorption of water from the oral mucosa (into the tablet). The result is pleasant swallowing without possible sticking to the inside of the mouth-esophagus.
The coating also protects the active ingredient from atmospheric oxygen, moisture, light and gastric acid. It also prevents mechanical abrasion, for example during transport: the resulting dust can be harmful to health if inhaled. The coating is not only a protection for the patient and from the active ingredient, but also a protection from other active ingredients: if several active ingredients are pressed into one tablet, chemical reactions can occur among the active ingredients. A protective film can prevent this and increase patient compliance by eliminating the need to take multiple tablets.
The biopharmaceutical setting is also advantageous: the applied coating not only protects the gastric mucosa from active ingredients, but also safeguards the drug against aggressive gastric acid. If the right coating materials are chosen, the drug delivery can be specifically modified or controlled (e.g. enteric-coated, small intestine-soluble coating).
In pharmacy, coating is one of the coated solid dosage forms with the main forms being dragées and conventional dragées. Dragées are further subdivided into conventional dragées and quick dragées.
In conventional dragées, the actual core is coated with sugar so that the resulting shape & volume are completely different from the original core. This also makes desired embossing impossible, as the sugar layer is too thick. In order to be able to apply these quantities of sugar, the core must have a certain web height and curvature. In this case, the sugar solution is applied in several layers of different composition (difference to fast dragées; see below) until the weight of the shell corresponds to the weight of the core. During the process, the liquid sprayed onto the tablet changes from a sticky liquid to a semi-solid and finally to a non-sticky dry surface. The disadvantage of conventional dragées is the poor shelf life due to the high sugar content and the long production times.
The actual process takes place in dragées vats, which resemble a large laundry drum. The rotation keeps the kernels moving and mixing evenly. In order to absorb the large amount of aqueous sugar solution, the core or blank is first sprayed with a cover layer of coating syrup with binder, anti-adhesive and structuring filler. The core is thus mechanically stable and protected from moisture. The masking layer is followed by the application layer, which consists only of coating syrup. Once several layers have been applied until the desired size has been reached, coloring can begin. Finally, another layer of smoothing syrup with clear pure syrups is applied to protect the coloring. To make the outer layer look more beautiful, another polishing layer with hard waxes is applied.
A similar type of dragées are the quick dragées, which are similar to the dragées in the principle of construction. Here also is a sugar solution applied several times until the shell is approximately 20-50% of the core weight. This enables shorter production times with less material input. The difference with conventional dragées is also the composition of the sugar layers: these are identical, unlike conventional dragées. Fast dragées are manufactured in the same way as conventional dragées. Both dragées are microbially susceptible due to their sugar content.
Both forms of sugar-coated dosage forms are produced in rotating kettles or non-rotating coaters. In the case of rotating kettles, a further distinction is made between conventional inclined and horizontal coating kettles. Non-rotating coaters are divided into fluidized bed processes (smooth, Wurster, ball coaters) and fluidized bed processes (see below).
In coating of film-coated tablets, the core is coated only with a polymer that does not change its shape & volume. The polymer, unlike dragée, consists of only a single layer and occupies only about 5-15% of the core. There are no requirements for the core shape, as the polymer solution dries more quickly than the sugar solution. During the process, the liquid sprayed onto the tablet changes from a sticky liquid to a semi-solid to finally a non-tacky dry surface. Advantages result from shorter manufacturing times, better shelf life and more differentiated dissolution properties depending on the type of film former.
The polymer solution usually consists of several substances contained in the polymer solution to be applied. These include:
Film former (see below) Plasticizer, to prevent curing polymer layer from becoming brittle. (e.g. citric acid ester) Anti-adhesive agents to prevent tablets from sticking together (e.g. talcum) Dispersing or smoothing agents (e.g. sorbitan monolaurate) Coating agent to protect the tablet or to enhance the subsequent coloring (e.g. titanium oxide) Dye
Solvent to dissolve the above substances. There is usually the possibility to work with aqueous and organic solvents.
The raw tablets are still sharp-edged due to the pressing process. These edges still have to be removed so that the polymer layer does not tear later. Once the tablets have been deburred, they are cleaned with air to remove dust residues. The tablets are placed in the coating kettles or fluid bed equipment.
Coating of tablets in the coating kettle differs from coating (see above) only in the liquid sprayed. There are several experimental setups for fluid bed processes (smooth, Wurster, spherical coater and fluidized bed processes), although they do not differ in principle: instead of mechanical rotation, the tablets are kept in motion by means of an air stream. These air streams are usually arranged in a circular pattern, so that the tablets always fall back to their starting position. Advantages result from the simple application of the polymer solution and the rapid drying of this.
If an aqueous or organic solvent is used, this must also be removed again. In all processes (including coating), this is done with the aid of air streams. Today, the eddy current process is increasingly being used.
The chemical substances of film formers are divided into chemical or functional groups. The chemical group is divided into: Cellulose derivatives, polyacrylates, polymethacrylates (known as eudragites), vinyl polymers (such as polyvinylpyrrolidone (PVP) and its derivatives) and the so-called shellac (this is a secretion of the insect Kerria lacca).
Although the chemical classification provides a good overview, the functional classification is more practical:
Fast-soluble film formers, such as PVP and its derivatives, and methacrylates.
Gastric juice-soluble and small intestine-soluble film formers, such as hydroxypropyl methylcellulose phthalate (HPMC), carboxymethylcellulose (Na-CMC), polyvinyl acetate phthalate, methacrylic acid ester and shellac
Insoluble film formers such as ethyl cellulose and methacrylic acid esters.
In general, there are no limits to the areas of application. Even frozen, meat and fish products can be coated accordingly. Large quantities are coated in the production of raw materials. But special coatings also give fruit and vegetables a "second protective skin". The processes used for coating are no different from those used in the pharmaceutical industry. If the requirements for the applied coating are not very high, simple spraying by means of nozzles can be used.
The coating substances must be suitable for foodstuffs and must not form toxic degradation products. Depending on the product, fats, waxes and water-soluble coating substances are thus used. If end products, such as fruit, are coated, there is no obligation to label the substances used.
Among the most common pharmaceutical dosage forms used today are tablets, dragées, pellets and pills. To make these dosage forms usable by the user, the odor, taste and color of the drug must be concealed. It is also advantageous to be able to release the active ingredient specifically at a desired location. Coating, or overcoating, is a process that applies a layer of coating material to the surface of the dosage form. This layer alone can fulfill all the requirements mentioned.
New research is being conducted in the direction of solvent-free coating. At the same time, this process enables resource-saving application (with all the other advantages). Several research projects have already been carried out to put this process into practice, two of which should be highlighted. Firstly, the core can be placed directly in a matrix, which then encases the core via a pressing process. This process would then be a hybrid of coating and tablet pressing. On the other hand, the polymer used can be applied to the cores in powder form and then converted into liquid form using external energy sources. If the liquid form is present, a film is automatically formed around the tablets. The form of energy introduced distinguishes the current processes and ranges from supercritical gases to UV light.
The manufacturing process is not only used in the pharmaceutical industry, but also in the food industry: here, too, attempts are made to modify the surface properties of raw materials.
Often, substances are needed in a free-flowing state to ensure improved dosing capability of substances. In this case, the substances already have the specified size, but cannot flow due to substance-specific properties (e.g. excessive adhesive forces between the individual particles). This results in inaccurate metering and consequently errors in the end product. If these fine-grained or crystalline materials are coated with appropriate substances, the materials can flow freely.
If grains are moved by bulk processes (e.g. during loading), friction occurs among the grains. The resulting dust can again cause flow problems or form explosive gases. A coating can reduce this effect and thus lead to higher productivity and safety in the manufacturing process.
As in the pharmaceutical sector, undesirable tastes and odors must also be masked in the food industry. A coating prevents these substances from being released onto the surface and thus leads to greater acceptance by the customer.
A coating also protects the substances from moisture, oxidation and undesirable chemical reactions in substance mixtures (see above).
A similar advantage arises from substances that are to be deliberately introduced into the coating: if these also meet the criteria of coating substances, they can be distributed more finely. These substances, which usually have an intensive effect, can thus be applied in a targeted and finely distributed manner.
Many foodstuffs contain substances that are initially dissolved on contact with water (e.g. sugar). Slow release (retarded delivery) is advantageous here, so that the end product does not dissolve too quickly (and lose its flavor, for example). With selected means, a coating material can meet these requirements.
Through uniform application, a coating can also be used for coloring (visual appeal to the customer). In the food industry, this is done with food coloring or, for example, paprika powder.
Coatings in the food sector are becoming more and more important. New natural materials can seal the products airtight and thus make secondary packaging (plastic) superfluous. Used plastic material can thus be avoided, which significantly improves the eco-balance of the goods. One current project is trying to spray a tincture of liquid silk (liquidseal) onto vegetables.