Exploring granulation in pharmaceuticals: the role of cellulose ethers

Solid pharmaceutical dosage forms, such as tablets and capsules, are the most used drug delivery systems, favored for their ease of production and patient preference. These forms combine active pharmaceutical ingredients (APIs) with excipients that are carefully selected and processed to ensure stability, efficacy, and safety. A key process in manufacturing these forms is granulation, which improves crucial powder properties such as flowability, compactability, and compressibility. These factors are essential in ensuring uniform content and proper dissolution of the final product. Granulation transforms fine powders into granules, making the material easier to handle for processes like tablet compression or capsule filling. [1]

Understanding the granulation process in more depth reveals its importance in pharmaceutical manufacturing. Different granulation methods, such as wet and dry granulation, offer distinct advantages that optimize product performance and address formulation challenges. [2,3] Binders play a crucial role in both methods, helping to create strong and uniform granules. Among the most versatile and widely used binders are cellulose ethers, which improve granule strength and flow properties. In wet granulation, cellulose ethers act as binding agents that facilitate particle adhesion, [4] while in dry granulation, some cellulose derivatives enhance compressibility and tablet integrity. [5] This article explores how granulation works and examines why cellulose ethers are useful for this process.

Which characteristics should have the granules?

Granules are formed when primary powder particles adhere together, typically ranging in size from 0.2 to 4.0 mm, depending on their intended application. In the case of tablet and capsule production, granules serve as an essential intermediate, with an optimal size range of 0.2 to 0.5 mm. [6] Achieving granules with the desired properties requires careful attention to several factors. Enhancing granule density improves their strength and stability, while a spherical shape significantly enhances flowability, facilitating smoother processing. Maintaining a narrow particle size distribution is equally important, as it optimizes packing density and further improves performance. Moisture content also plays a pivotal role, with ideal levels typically kept between 1% and 2%, ensuring the granules remain cohesive without becoming overly wet. Together, these factors contribute to the consistent and efficient performance of granules in their intended applications. [7]

Granulation processes can be divided into two primary types:

1. Wet Granulation

Wet granulation is a technique where a liquid solution (normally water or a solvent) is added to powdered ingredients to create bonds between particles. This process involves mixing the powder with a binder solution, forming agglomerates or even a paste or dough that are later broken down into granules and dried. Wet granulation provides strong, uniform granules, making it a preferred choice for formulations that demand consistency and cohesion. In wet granulation processes, the choice of equipment plays an important role in achieving desired particle properties. Among the most widely used technologies are high-shear mixers (Figure 1a), fluid bed (Figure 1b) and twin screw granulators (Figure 1c). [8,9]

Key Benefits of Wet Granulation:

• Enhanced granule strength: The added moisture creates solid bonds, resulting in durable granules that hold up well during tablet compression.
• Improved drug release: Granules created via wet granulation often dissolve more uniformly, supporting controlled drug release.
• Compatibility with multiple binders: Wet granulation allows for a range of binders, making it suitable for various formulations

The Role of Cellulose Ethers in Wet Granulation

Materials commonly used as wet binders in pharmaceutical practices include synthetic and semi-synthetic compounds, such as cellulose derivatives, vinyl pyrrolidone derivatives, and modified starches.[10] Among these, cellulose ethers are especially popular in wet granulation for their effective binding properties. When dissolved in water or other solvents, cellulose ethers form a viscous solution that coats particles and promotes cohesion, enhancing granule strength and uniformity. [11]

The following cellulose ethers help achieve granules in wet granulation with ideal flow and compression properties, making them particularly useful for tablets requiring precise dissolution rates and consistent API distribution.

Hydroxypropyl Methylcellulose, Hypromellose (HPMC): This polymer is often used in formulations requiring controlled-release effects. Its gel-forming properties make it ideal for sustained-release tablets, as it can slow down the drug dissolution rate.  Typically, low viscosity grades of HPMC such as 3 to 6 mPas are mostly used as binder either in dry powder form or solubilized in the granulation solution. Shin-Etsu offers low viscosity grades of Hypromellose 2910 from 3 to 15 mPas under the brand names PHARMACOAT® and TYLOPUR®.

Hydroxypropyl Cellulose (HPC): This cellulose ether is highly effective in forming a gel matrix, which promotes binding during granule formation. It also enhances the stability of the granules, making them less likely to break apart.

Low-substituted Hydroxypropyl Cellulose (L-HPC): is a low-substituted form of hydroxypropyl cellulose that performs well in applications requiring swelling and disintegration, making it ideal for wet granulation. Its good  water retention and broad compatibility with drugs and excipients ensure its versatility in tablet formulations. Typical grades used are  LH-11, LH-21 or LH-B1.

2. Dry granulation

Dry granulation is an alternative technique for compressing and compacting powder particles without using liquids. This method is ideal for moisture-sensitive drugs that may degrade if exposed to water or other solvents. Dry granulation involves either roller compaction (Figure 2) or slugging to compress the powder mixture into larger, dense compacts, which are then milled into granules. This process eliminates the need for drying, making it simpler and faster than wet granulation. [12]

Key benefits of dry granulation:

• No solvent requirement: Perfect for solvent-sensitive APIs and excipients.
• Reduced processing steps: With no need for drying, dry granulation is a simplified process, saving time and energy.
• Enhanced stability: Since no solvents are used, there is less risk of chemical degradation or instability in sensitive formulations.

The role of cellulose ethers in dry granulation

Although cellulose ethers are traditionally used as binders in wet granulation, they also offer significant advantages in dry granulation, primarily for their compressibility-enhancing properties.[13] In dry granulation, the most used cellulose derivatives improve the structural integrity of granules and tablets without additional moisture, making them crucial for producing stable, high-quality tablets. Examples of these include:

Microcrystalline Cellulose (MCC): although not technically an ether, MCC is a widely used cellulose derivative that provides excellent compressibility having dry binding properties. It adds mechanical strength to the granules, ensuring uniform tablet formation during compression.

Hydroxypropyl Cellulose (HPC): Used in low concentrations, HPC can improve the mechanical strength and compressibility of granules, contributing to tablet durability while still allowing quick disintegration.

Low-substituted hydroxypropyl Cellulose (L-HPC): It provides cohesion in the compressed granules, improving flowability and enhancing mechanical strength. Its ability to function without moisture, while offering excellent compression properties, makes it ideal for efficient granule formation and stable tablet production. Suitable L-HPC grades are LH-31-32 or the NBD range.

Wet vs. dry granulation: choosing the right method
Both wet and dry granulation methods offer unique benefits,[14] and the choice between them depends on the specific needs of the formulation:

Formulation stability: If the drug or excipients are sensitive to moisture, dry granulation is often preferred to avoid potential degradation.

Dissolution requirements: Wet granulation is commonly chosen for formulations that require controlled or sustained release, as it allows for greater manipulation of dissolution rates with binders like HPMC.

Process simplicity: Dry granulation involves fewer steps and no drying, making it a quicker and more cost-effective option for stable drugs. Summary Both wet and dry granulation play indispensable roles in modern pharmaceutical manufacturing, each with its advantages and ideal applications. Cellulose ethers are essential in both processes, acting as versatile binders and functional additives that enhance granule formation, compressibility, and even drug release. Whether in the cohesive structure of wet granulation or the compressible nature of dry granulation, cellulose ethers are essential in ensuring the quality, consistency, and effectiveness of pharmaceutical tablets and capsules.

Summary

Both wet and dry granulation play indispensable roles in modern pharmaceutical manufacturing, each with its advantages and ideal applications. Cellulose ethers are essential in both processes, acting as versatile binders and functional additives that enhance granule formation, compressibility, and even drug release. Whether in the cohesive structure of wet granulation or the compressible nature of dry granulation, cellulose ethers are essential in ensuring the quality, consistency, and effectiveness of pharmaceutical tablets and capsules.

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Source: Shin-Etsu new blog article, website Exploring granulation in pharmaceuticals: the role of cellulose ethers, https://www.setylose.com/en/blog/exploring-granulation-in-pharmaceuticals-the-role-of-cellulose-ethers

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