Rheology and Liquid Dosage Form Preparation in Pharmacy

Introduction to Rheology and Liquid Dosage Forms

Rheology is the study of the deformation and flow of matter, focusing on "the viscosity characteristics of powders, fluids, and semisolids" (Adesina "Rheology" 2). It is this branch of physics that enables pharmacists to determine the proper mix and flow of materials used in the treatment of patients. Everything from the way in which a drug is packaged to how it is poured, squeezed, or injected must be considered by the pharmacist trained in rheology prior to the drug's serving as an effective intervention. This paper discusses the factors that would be considered when preparing a liquid dosage form and explains the relationships between these factors and the desired effects or outcomes that the drug form should possess.

There are various types of liquid dosage forms — pharmaceutical solutions (liquid preparations containing chemical substances) and sterile dosage forms. Types of the former include syrups, elixirs, spirits, aromatic waters, tinctures, injections, and fluidextracts (Adesina "Oral Solutions" 2). When preparing a liquid dosage form, numerous factors must be considered — for example, the nature of the form's flow (Newtonian or non-Newtonian), the purpose of the dosage, and the intended application.

Types of Flow in Pharmaceutical Solutions

Newtonian flow forms consist of simple liquids — water, alcohol, and organic solvents (Adesina "Rheology" 18). Non-Newtonian flow forms comprise the majority of pharmaceutical solutions and consist of three different classes: pseudoplastic flow, dilatant flow, and plastic flow.

Pseudoplastic flows are used in liquid dosage forms when colloidal systems are required — typically polymer-based solutions that become more fluid the more they are stirred or shaken. This behavior is known as shear thinning (Adesina "Rheology" 21). Such solutions are necessary when the pharmaceutical solution is desired to retain its integrity over time.

Dilatant flow is the opposite of pseudoplastic flow: these solutions are fluid when free of stress but become more viscous when shaken. They are, in other words, shear-thickening solutions. The deflocculated particles within the mixture bunch together when the solution is stirred or stressed.

Plastic flows contain a concentrated amount of flocculated particles in suspension; the particles are bunched together, and flow cannot occur until this bunching effect is overcome. Such flows are also desirable for pharmaceutical applications that have limited shelf-life periods or in solutions that require aqueous additives — because the solution alone may be too unstable — in order to facilitate ingestion (Adesina "Oral Solutions" 13).

These three types of flow describe the various pharmaceutical solutions used by consumers for diverse interventions, from the easing of cold or flu symptoms to healing burns, cuts, and rashes. Depending on the type of form preferred by the consumer — spray (aerosol), cream, or ingestible — manufacturers of the pharmaceutical solution must consider which flow types are conducive to the end product and then formulate the product's ingredients accordingly.

Key Formulation Factors for Liquid Dosage Forms

To prepare a liquid dosage form, one of the key factors to consider is whether an antimicrobial agent such as alcohol will need to be added to the solution. Topical solutions or tinctures often utilize alcohol for this reason (Adesina "Oral Solutions" 35). Other considerations depend on the type of solution in question.

For instance, co-solvents are used when needed to support stability and/or solubility. Sometimes preservatives are added if the solution is not self-preserving. For a liquid dosage that is to be sprayed, the solution must be broken up into small particles conducive to a Newtonian flow. Antiseptics, local anesthetics, and skin protectants are all types of liquid solutions that would benefit from a spray-type application. Fungal infections such as athlete's foot or ringworm can be treated in a similar manner.

The manner in which the liquid form is to be applied is also a consideration for the drug manufacturer. If the solution is to be administered in medicinal drops, a small bottle with an applicator would be the preferred packaging. If the liquid dosage is to be sprayed, a spray bottle or medicinal atomizer would be the desired packaging.

Works Cited

Adesina, Simeon. "Oral Solutions, Topical and Nasal Solutions." PowerPoint Presentation.

Adesina, Simeon. "Rheology." PowerPoint Presentation.

Adesina, Simeon. "Sterile Dosage Forms." PowerPoint Presentation.