Comprehensive Insight into Tablet Formulations: Types, Manufacturing Techniques, and Evaluation Standards

Oral solid dosage forms are a vital part of the pharmaceutical industry, recognised for their simplicity in administration and efficiency in drug delivery. Tablets stand out as one of the most widely utilised forms, accounting for nearly 70% of pharmaceutical products. These can be broadly categorised into compressed tablets and moulded tablets, with subtypes including chewable tablets, directly compressible tablets, and triturates. Defined as solid pharmaceutical units, tablets consist of active drug ingredients and suitable excipients, manufactured using compression or moulding techniques. They are designed to ensure ease of use and accurate drug dosing for therapeutic purposes.

What Is a Tablet?

A tablet, also known as a pill, is an oral pharmaceutical form categorised as a solid dosage unit. It consists of a mixture of active ingredients and excipients, compressed into a solid state. Its primary advantages include reliable dosing, ease of use, and efficient drug delivery into the systemic circulation.

Properties Of Tablets:

1) The product must possess an elegant design that reflects its unique identity while being devoid of any defects, including chips, cracks, discolouration, and contamination.

2) It should exhibit sufficient strength to endure the shocks experienced during production, packaging, shipping, and dispensing processes.

3) The product must demonstrate physical stability, ensuring that its attributes remain consistent over time.

4) It is essential that the product can deliver the medicinal agent(s) into the body reliably and consistently.

5) Additionally, the product must maintain appropriate chemical stability over time to prevent any alteration of the medicinal agents.

Benefits Of Tablets:

- As unit dosage forms, tablets provide the highest level of dosing accuracy and the least variability in content among oral dosage forms.

- They allow for precise dosing with minimal effort.

- The rate at which the drug is released from tablets can be adjusted to fulfil pharmacological requirements.

- Tablets are the simplest and most economical to package and strip.

- They are easy to swallow, minimising the risk of obstruction.

- The use of enteric coating enables the creation of sustained-release formulations.

- Coating methods can conceal undesirable odours and bitter tastes.

- Tablets offer superior chemical and microbial stability compared to other oral dosage forms.

Drawbacks of Tablets:

- Swallowing tablets can pose difficulties for children and patients who are unconscious.

- Some pharmaceuticals cannot be compressed into solid tablets due to their amorphous nature and low density.

- Drugs that have poor wetting properties, slow dissolution rates, or require high absorption in the gastrointestinal tract may be challenging to formulate as tablets while ensuring sufficient bioavailability.

- Bitter Medications, have an undesirable smell, or are oxygen-sensitive, may need to be encapsulated or coated. In such scenarios, capsules may be the most effective and economical solution.

Tablet Ingredients: In addition to active ingredients, the tablet contains several inert materials known as additives or excipients. Different excipients are:

Types of Tablets:

1. Oral Tablets for Ingestion

1) Standard Compressed Tablets: These tablets are the simplest and most widely used type, created through a single compression process. Their compact and durable nature makes them ideal for mass production. Once ingested, they disintegrate in the gastrointestinal tract to release the active ingredient, making them effective for systemic treatments.

2)Multiple Compressed Tablets: Produced using multiple compression cycles, these tablets allow for complex formulations such as:

i)Compression Coated Tablets:

a) Sugar-Coated Tablets: Encased in a sugar layer to mask unpleasant tastes, protect against moisture, and improve the tablet's appearance.

b) Film-Coated Tablets: Wrapped in a thin, tasteless film that resists environmental damage and dissolves quickly in the stomach.

c)Gelatine-Coated Tablets: Coated with gelatine for easier swallowing and better protection of the active ingredient.

d)Enteric-Coated Tablets: Designed to resist stomach acid, these dissolve in the intestines, protecting either the drug from degradation or the stomach lining from irritation.

ii)Layered Tablets: Comprising multiple layers, each designed for a specific function. For instance, one layer may dissolve rapidly, providing an immediate drug release, while the other sustains release over time.

iii)Inlay Tablets: A smaller core tablet is embedded within a larger one to separate incompatible ingredients or achieve targeted delivery.

3) Targeted Tablets:

a) Floating Tablets: Formulated to remain buoyant in gastric fluid, these tablets extend gastric retention time, improving bioavailability.

b) Colon-Targeting Tablets: Engineered to release their contents specifically in the colon, beneficial for conditions like Crohn’s disease or ulcerative colitis.

4) Chewable Tablets: These tablets are chewed rather than swallowed whole, making them suitable for individuals who find swallowing difficult. They dissolve quickly in the mouth, ensuring faster action.

5) Dispersible Tablets: Dissolvable in water before ingestion, these tablets are ideal for children and elderly patients who struggle with traditional tablets.

2. Used Tablets in the Oral Cavity

2) Sublingual Tablets: Placed under the tongue, these dissolve rapidly and are absorbed directly into the bloodstream, bypassing the digestive system. They are often used for quick-acting medications like nitro-glycerine.

3) Buccal Tablets: Dissolve slowly when placed between the cheek and gum, providing extended drug release and avoiding gastrointestinal degradation.

4) Dental Cones: Inserted into tooth sockets after dental surgery, these tablets deliver localised antimicrobial or pain-relief effects.

5) Mouth-Dissolving Tablets: Quickly disintegrating on contact with saliva, these tablets are effective for rapid relief in conditions such as migraines.

3. Tablets Administered by Other Routes   

2) Rectal Tablets: Administered via the rectum, these are beneficial for localised or systemic effects when oral administration isn’t viable.

3) Implants: Solid tablets designed for subcutaneous implantation, offering controlled drug release over extended periods, sometimes lasting months.

4. Tablets Used to Prepare Solutions

2) Moulded Tablets: Soft and prepared using moulds, these dissolve quickly to ensure fast action. 

a) Hypodermic Tablets: Sterile tablets designed to dissolve in water for use in injection solutions, although their usage has declined with prefilled syringes.

3) Tablet Triturates: Small, easily dissolvable tablets used to prepare solutions or for direct sublingual use.

5. Structure-Based Classifications

2) Aperture Tablets: Incorporate a hole through the tablet's core to facilitate regulated drug release.

3) Concave Convex Tablets: Their unique shape makes them easier to swallow and enhances their mechanical strength.

4) Core Tablets: These feature an active core surrounded by inert layers for delayed or controlled drug release.

6. Action-Based Classification

1)Modified-Release Tablets: Designed to alter the drug's release rate, timing, or location, these include:

2)Extended-Release Tablets: Gradually release the drug over a prolonged period, reducing dosing frequency.

3)Delayed-Release Tablets: Prevent release until a specific time or location (e.g., in the intestines).

4)Controlled-Release Tablets: Maintain a consistent drug level in the bloodstream for an extended duration.  

Method of Preparation:

1)Wet granulation method:

Wet granulation is one of the most commonly employed techniques in tablet manufacturing due to its versatility and reliability. This method involves several carefully controlled stages, starting from the accurate measurement of ingredients to the final compression of tablets. The process not only enhances the compressibility of powders but also improves the uniformity of drug distribution, making it ideal for complex formulations. The first step in wet granulation is the weighing and mixing of raw materials, which typically include the active pharmaceutical ingredient (API), diluents, and disintegrants. These components are blended thoroughly to ensure even distribution. Afterwards, the mixture is passed through a sieve to break down any clumps and obtain a uniform powder consistency. A binder solution, such as starch paste or polyvinylpyrrolidone (PVP), is gradually added while stirring to form a cohesive wet mass. The choice of binder is critical as it determines the granule's strength and stability. Adequate binder quantity is essential to prevent over-wetting, which could lead to sticky granules that are difficult to process, or under-wetting, which results in fragile granules prone to crumbling during handling. The moist mass is then granulated by forcing it through a screen to form small, uniform granules. The granules are dried to remove excess moisture, ensuring they have sufficient mechanical strength. Tray drying is the traditional drying method; however, modern fluidised bed dryers are increasingly being used due to their efficiency in reducing drying time and providing uniform heat distribution. After drying, the granules are passed through fine sieves, such as 60-100 mesh, to eliminate oversized particles and obtain a consistent granule size. A lubricant such as magnesium stearate is then added to improve the flow properties of the granules and to reduce friction during compression. This ensures smooth tablet production in high-speed machines. Finally, the prepared granules are compressed into tablets using advanced tablet presses, which produce tablets with uniform weight, size, and hardness.

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2)Dry Granulation Method:

Dry granulation is a method employed to manufacture tablets when the ingredients are moisture-sensitive or incapable of withstanding high temperatures during drying. It is also ideal for drugs that possess poor flow or compressibility properties and need modification before compression. Unlike wet granulation, this process bypasses the use of liquid binders and heat-based drying methods, making it particularly suited for fragile compounds. The procedure begins with blending the active pharmaceutical ingredient (API) with excipients, including diluents, disintegrants, and lubricants. The mixture is then compacted into slugs or compressed sheets using heavy-duty tablet presses or roller compactors, a modern alternative to slugging. Roller compaction offers better control over particle size distribution and granule density compared to slugging, which can lead to variability. Once the slugs or compacted material are formed, they are milled or passed through sieves to create uniform granules. These granules undergo further blending with additional lubricants to enhance their flow properties and ensure a smooth compression process. Finally, the granules are compressed into tablets using high-speed tablet machines.

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 3)Direct compression method:

The direct compression method involves the production of tablets directly from powdered raw materials, bypassing the need for granulation or other intermediate processes. In this method, the active pharmaceutical ingredient (API) and excipients are mixed thoroughly and then compressed into tablets without altering their physical properties. It is regarded as one of the simplest and most cost-effective approaches for tablet formulation, making it particularly suitable for formulations where minimal processing is required. This technique is ideal for compounds with excellent compressibility and flow properties, such as crystalline substances. It is also well-suited for APIS that are sensitive to heat or moisture, as it avoids the use of high temperatures or liquid binders. The simplicity of the process not only reduces production time but also minimises the risk of contamination, making it a popular choice for bulk tablet production.

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 Evaluation Parameter of Tablets: (Indian Pharmacopoeia, 2010)

To fabricate tablets and subsequently oversee their production quality, quantitative evaluations and assessments of a tablet’s chemical, physical, and bioavailability features must be conducted.

i) Appearance: The physical appearance of a tablet is an important indicator of its quality. For uncoated tablets, observation under a lens reveals their structural integrity. A single-layer tablet should display a uniform, mono-layer structure, while a coated, multi-layered tablet will have visibly separate layers beneath the coating. Any defects such as discolouration, cracks, or surface irregularities must be identified and rectified during the inspection process. Advanced imaging technologies, like scanning electron microscopy (SEM), are now commonly used for precise visual analysis of tablet surfaces.

ii) Size and Shape: The size and shape of a tablet are crucial for ensuring consistent dosing and patient compliance. Measurements of thickness, diameter, and length are typically conducted using micrometres or digital callipers. In production control, multiple tablets (usually 5-10) may be placed on a holding tray, and their total crown thickness is measured collectively with a sliding calliper. A deviation of no more than 5% from the standard value is acceptable for uniformity. Tablets are often designed with specific shapes, such as round, oval, or scored, to cater to therapeutic requirements or to facilitate easy splitting.

iii) Organoleptic Properties: Organoleptic evaluation involves examining sensory characteristics like odour, taste, and colour. Unusual odours, such as the acetic acid scent from degraded aspirin tablets, can indicate underlying stability problems. These odours might also arise from vitamins with inherent characteristics or from flavouring agents added to improve patient compliance. Tablets designed for pediatric or geriatric populations are often flavoured to mask bitterness, while their appearance is enhanced with colourants for better acceptability.

iv) Uniformity of Weight: Weight uniformity ensures consistent drug content in each tablet, a critical parameter for therapeutic effectiveness. For testing, 20 tablets are randomly selected and weighed individually. The average weight is calculated, and the deviation of individual weights from this average is evaluated against pharmacopoeial standards. Typically, no more than two tablets should deviate by more than the percentage specified in the standard. This test is particularly important for low-dose formulations, where minor variations can significantly impact efficacy.

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 vi) Hardness Assessment: Tablet hardness refers to the mechanical strength required to fracture the tablet under diametric compression. This parameter is evaluated by placing the tablet between two anvils, followed by applying a force until the tablet breaks. The maximum crushing strength recorded during this process indicates the tablet's hardness. This assessment ensures the tablets are durable enough to withstand the stress encountered during packaging, transport, and handling.

Commonly utilised hardness testers include:        

? Pfizer hardness tester

? Monsanto hardness tester                         

? Erweka hardness tester

? Schleuniger hardness tester

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vii) Friability Test: The friability test evaluates the tablet's ability to resist abrasion and mechanical stress during handling. Using a device called the Roche friabilator, tablets are rotated at 25 revolutions per minute, dropping repeatedly from a height of approximately six inches. A pre-measured sample of tablets undergoes 100 revolutions, after which the tablets are cleaned to remove dust and weighed again. Tablets demonstrating a weight loss of no more than 0.5% to 1.0% are deemed acceptable.

viii) Disintegration Test: Disintegration is defined as the breakdown of a tablet into smaller fragments or a soft mass without retaining its firm core when exposed to a liquid medium. This test ensures that uncoated and coated tablets disintegrate within a specified timeframe under controlled experimental conditions. It is particularly important for immediate-release tablets, though sustained-release tablets are exempt from this test. The disintegration apparatus features immersion baskets that hold tablets while simulating the environment of the gastrointestinal tract, ensuring they meet pharmacopoeial standards for disintegration time.