Semisolid Dosage Forms

Semisolid dosage forms are medications that are neither solid nor liquid, but somewhere in between. They are typically applied to the skin, nasal mucosa, cornea, rectal or vaginal tissue, buccal tissue, ear, or urethral membrane. Examples include ointments, pastes, creams, emulsions, gels, and rigid foams. They contain one or more active ingredients dissolved or uniformly dispersed in a suitable base and any suitable excipients. They are often used for their therapeutic, protective, or cosmetic functions. In this article we will study their classification, preparation and evaluation methods.

Table of Contents

Classification

Semisolid dosage forms are pharmaceutical preparations that have a consistency between solid and liquid. They are typically applied to the skin, nasal mucosa, cornea, rectal or vaginal tissue, buccal tissue, ear, or urethral membrane. Here are some examples.

Ointments: These are greasy, semi-solid preparations intended for external application to the skin or mucous membranes. They are used for their emollient effects and for drug delivery.
Creams: These are semi-solid emulsions of oil and water. They are used as emollients or for the delivery of medicated substances.
Gels (or Jellies): These are semi-solid systems consisting of dispersions made up of either small inorganic particles or large organic molecules enclosing and interpenetrated by a liquid.
Pastes: These are thicker than ointments and are good for absorbing body fluids. They are used for their protective effects.
Plasters: These are solid or semisolid adhesive masses spread upon a suitable backing material. They are intended for prolonged contact with the skin.
Rigid Foams: These are composed of low-density solids that have a sponge-like texture. In medicine, they are often used to deliver drugs for topical use.

Each of these dosage forms has its own unique properties and uses in various fields such as medicine.

Factor influencing penetrations of drugs

The penetration of drugs in semisolid dosage forms is influenced by several factors.

Concentration: The concentration of the drug in the formulation can affect its penetration.
Molecular Weight: The molecular weight of the drug molecule can influence its ability to penetrate the skin or mucous membranes.
Duration of Contact: The length of time the formulation is in contact with the skin or mucous membrane can affect drug penetration.
Solubility of Medication: The solubility of the drug in the base of the semisolid dosage form can influence its penetration.
Physical Condition of the Skin: The health and integrity of the skin or mucous membrane can affect drug penetration.
Part of the Body Exposed: Different parts of the body have different absorption rates, which can influence drug penetration.
Use of Penetration Enhancers: Certain substances can enhance the penetration of the drug through the skin or mucous membranes.
pH of the Formulation: The pH of the semisolid dosage form can affect the ionization state of the drug, which can influence its penetration.
Temperature: Higher temperatures can increase the permeability of the skin, enhancing drug penetration.
Vehicle Used: The type of vehicle used in the formulation can affect the solubility and hence the penetration of the drug.

These factors need to be carefully considered during the formulation of semisolid dosage forms to ensure optimal drug delivery.

Preparation of ointments

Ointments are prepared by several methods, primarily depending on the nature of the ingredients. Ointments are prepared by using one of the following methods.

Incorporation Method

This method involves grinding together finely divided soluble substances with a small amount of a base or one of its ingredients and then diluting with gradually increasing amounts of the base to evenly disperse the soluble components.

Fusion Method

In this method, all solid ingredients included in ointments, such as beeswax, cetyl alcohol, stearyl alcohol, stearic acid, paraffin, etc., should be melted before application to prevent burning. The melting can be accomplished in two ways,

Method I: Melting the components is done in order of decreasing melting points, which means that the substances with the highest melting points should be melted first, the substances with the next melting points, and so on.
Method II: Substances with low melting points will be protected from overheating in this way.

Emulsification Method

This method is used when the ointment base is an emulsion. The oil phase and aqueous phase are prepared separately, heated to the same temperature, and then mixed until a homogenous product is obtained.

Each method has its own advantages and is chosen based on the nature of the ingredients and the desired properties of the final product.

Evaluation tests for ointments

Ointments are evaluated using several tests to ensure their quality, safety, and efficacy. Here are some of the key evaluation tests.

Uniformity of Weight: Weigh 20 ointments individually and determine their average weight. Not more than two of the individual weights should deviate from the average weight by more than 5%, and none should deviate by more than 10%.
Disintegration Test: This test measures the amount of dosage form that gets dissolved in body fluid in unit time. It’s a measure of the rate of drug release from the ointment.
Content Uniformity Test: Take 10 ointments and determine the active ingredients of each of the 10 ointments using a suitable analytical method.
Melting Point Determination Test: Both macro melting range and micro melting range are determined.
Liquefaction Time (Softening): Softening time is the time for which the ointment melts completely at a definite temperature.
General Appearance Test: The visual appearance of ointments is assessed for uniformity in size, shape, and color.
Assay of Active Contents: The amount of drug present in each ointment is determined to ensure that the correct dosage is being delivered to the patient.
Test of Drug Uptake/Absorption into Bloodstream: This can be done through both in-vitro and in-vivo tests.

Preparation of pastes

Pastes are semisolid preparations containing a high proportion of finely powdered medicaments. They are stiffer than ointments and are usually employed for their protective action and for their ability to absorb serous discharges from skin lesions.

Preparation of Pastes: Pastes are typically prepared by two methods, trituration and fusion. In the trituration method, liquid or semisolid bases are triturated. For bases that are semisolid or solid, the fusion method is used. In the fusion method, all solid ingredients included in pastes, such as beeswax, cetyl alcohol, stearyl alcohol, stearic acid, paraffin, etc., should be melted before application to prevent burning.

Evaluation of Pastes

Pastes are evaluated using several tests to ensure their quality, safety, and efficacy.

Uniformity of Weight: Weigh 20 pastes individually and determine their average weight.
Disintegration Test: This test measures the amount of dosage form that gets dissolved in body fluid in unit time.
Content Uniformity Test: Take 10 pastes and determine the active ingredients of each of the 10 pastes using a suitable analytical method.
Texture Analysis: Used to evaluate the mechanical characteristic where a material is subjected to a controlled force from which a deformation curve of its response is generated.
pH: The pH of the paste is measured.
Viscosity: The viscosity of the paste is measured.
Tube Spreadability: The spreadability of the paste is measured.
Tube Extrudability: The extrudability of the paste is measured.
In vitro Drug Releasing: The drug releasing property of the paste is measured.

Preparation of creams

Creams are semisolid emulsions of oil and water used for their emollient effects or for drug delivery.

Preparation of Creams

Creams are typically prepared by two methods, fusion and emulsification. In the fusion method, all solid ingredients included in creams, such as beeswax, cetyl alcohol, stearyl alcohol, stearic acid, paraffin, etc., should be melted before application to prevent burning. The melting can be accomplished in two ways.

Method I: Melting the components is done in order of decreasing melting points.
Method II: All the components are taken in a subdivided state and melted together.

In the emulsification method, the oil phase and aqueous phase are prepared separately, heated to the same temperature, and then mixed until a homogenous product is obtained.

Evaluation of Creams

Creams are evaluated using several tests to ensure their quality, safety, and efficacy. These include,

Uniformity of Weight: Weigh 20 creams individually and determine their average weight.
Disintegration Test: This test measures the amount of dosage form that gets dissolved in body fluid in unit time.
Content Uniformity Test: Take 10 creams and determine the active ingredients of each of the 10 creams using a suitable analytical method.
Texture Analysis: Used to evaluate the mechanical characteristic where a material is subjected to a controlled force from which a deformation curve of its response is generated.
pH: The pH of the cream is measured.
Viscosity: The viscosity of the cream is measured.
Tube Spreadability: The spreadability of the cream is measured.
Tube Extrudability: The extrudability of the cream is measured.
In vitro Drug Releasing: The drug releasing property of the cream is measured.
Ex vivo Biomucoadhesion: The biomucoadhesion of the cream is measured.

Preparation of Gels

Gels are prepared by mixing a suitable thickening agent with an aqueous vehicle. The drug is dispersed in the aqueous vehicle, and the thickening agent is added by triturating in a mortar. Trituration is carried out until a homogeneous preparation is formed.

Evaluation of Gels

The evaluation of gels involves several tests, including,

Clarity: The gel should be clear and free from any particulate matter.
Homogeneity: The gel should be homogeneous, with the drug uniformly distributed throughout the gel.
Spreadability: This is a measure of how easily the gel spreads on application.
Extrudability: This measures how easily the gel can be squeezed out of its container.
Viscosity: This is a measure of the gel’s resistance to flow.
Surface pH: The pH of the gel should be suitable for application to the skin.
Drug Content Uniformity: The amount of drug in different samples of the gel should be uniform.
In-vitro Drug Diffusion Study: This measures the rate at which the drug diffuses out of the gel.
Ex-vivo Permeation Study: This measures the rate at which the drug permeates through the skin.

Gels have better potential as a vehicle to administer drugs topically in comparison to other topical dosage forms because they are non-sticky and require low energy during formation. However, the formulation and evaluation process must be carefully controlled to ensure the efficacy and safety of the gel product.

Excipients used in semisolid dosage forms

Semisolid dosage forms, such as creams, ointments, gels, and pastes, are formulated using a variety of excipients to provide specific properties and characteristics. The selection and use of excipients in semi-solid dosage forms is important to ensure that the product has the desired properties and characteristics for the intended use and patient population. Here are some common excipients used in semi-solid dosage forms.

Emulsifiers and surfactants: These help to stabilize emulsions and improve the spreadability and uniformity of the product. Examples include polysorbate 80, glyceryl stearate, and lecithin.
Thickening agents: These help to increase the viscosity and provide a smooth and consistent texture to the product. Examples include carbomers, hydroxyethyl cellulose, and xanthan gum.
Solvents: These are used to dissolve or disperse the active ingredient and other excipients. Examples include propylene glycol, ethanol, and polyethylene glycol.
Humectants: These help to retain moisture and prevent the product from drying out. Examples include glycerin, sorbitol, and propylene glycol.
Preservatives: These are added to prevent microbial growth and increase the shelf life of the product. Examples include methylparaben, propylparaben, and benzalkonium chloride.
Antioxidants: These are used to prevent oxidation and degradation of the active ingredient and other excipients. Examples include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and ascorbic acid.
Fragrances and colorants: These are added to improve the aesthetic appeal of the product and enhance patient acceptance.

Comparison of semisolid dosage forms

	Ointments	Creams	Pastes	Gels
Definition	Ointments are semisolid dosage forms that comprise less than 20% water and volatiles, and more than 50% hydrocarbons, waxes, or polyols as the vehicle.	Creams are opaque, viscous, relatively soft, consistently spreadable, semisolid emulsion dosage forms that often comprise more than 20% water and volatiles and normally less than 50% hydrocarbons, waxes, or polyols as the vehicle.	Pastes are thick, stiff preparations that are typically rich in solids. They are less greasy than ointments and are often used for skin protection rather than to deliver medication.	Gels are semisolid systems consisting of dispersions made up of either small inorganic particles or large organic molecules enclosing and interpenetrated by a liquid.
Absorption	Ointments stay on top of your skin rather than being absorbed right away, which offers more protection against moisture loss.	Creams are easily absorbed, so they can cover large areas of skin.	Pastes are not absorbed into the skin. Instead, they provide a protective barrier on the skin surface.	Gels are usually absorbed more quickly than creams and ointments, making them suitable for delivering medications that need to be absorbed rapidly.
Consistency	Ointments have a thicker consistency than creams, but may spread a little farther across your skin.	Creams have a smooth texture with a thicker consistency than lotions.	Pastes are typically thicker and stiffer than creams and ointments.	Gels are typically clear and spread easily, often providing a cooling sensation upon application.
Appearance	Ointments can be opaque or translucent.	Creams tend to be thick enough to maintain their shape whether squeezed from a tube or scooped from a jar.	Pastes are typically white or off-white and maintain their shape well.	Gels are typically clear or translucent and can range from a thin, watery consistency to a thick, jelly-like consistency.
Uses	Ointments are often used for skin conditions that require a high level of hydration, such as eczema or psoriasis.	Creams are often used for skin conditions that require hydration, but not as much as ointments. They are also often used for cosmetic purposes.	Pastes are often used for skin protection, for example in diaper rash ointments or sunscreens.	Gels are often used for acne treatments, anti-aging treatments, or other skincare products that require rapid absorption.

Summary

Semisolid dosage forms include ointments, creams, gels, and pastes applied topically for various uses. Advantages include avoiding first-pass metabolism, reducing side effects, and providing local action. They’re suitable for unconscious patients or those who struggle with oral medications and are more stable than liquids. Disadvantages include potential for oxidation of the base, staining, contamination from application, physicochemical instability, and possible irritation or allergic reactions.

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Semisolid dosage forms