In oral solid dosage form development, capsules are often dismissed as simple containers, with material selection pushed to the final stages of a project. Yet real-world experience proves that capsule material directly impacts drug stability, dissolution behavior, and even bioavailability—effects that frequently only emerge during accelerated or long-term storage testing.
As an empty capsule manufacturer, we’ve broken down the core characteristics and ideal use cases of different capsule materials from a formulation development lens, to help pharmaceutical companies and brand owners make smarter, risk-mitigated choices.
1. Capsules Are Not Inert: Key Interactions with Contents
Three micro-level interactions have the most profound impact on product performance:
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Moisture MigrationGelatin capsules require 12%–16% moisture to stay flexible. For hygroscopic or hydrolysis-sensitive APIs (e.g., antibiotics, probiotics, enzymes), this moisture can migrate into contents, causing potency loss and impurity buildup. HPMC capsules, by contrast, have only 2%–6% equilibrium moisture, making them far more compatible with moisture-sensitive formulations.
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Dissolution Consistency & Cross-Linking RiskWhile gelatin capsules disintegrate rapidly under normal conditions, they are prone to irreversible cross-linking when exposed to aldehydes (from excipient degradation or packaging). This forms an insoluble film that delays or fails dissolution—even USP enzyme retesting cannot fix severe cross-linking, and clinical data confirms such capsules are not bioequivalent to intact ones. HPMC and pullulan capsules eliminate this risk entirely for consistent in vivo performance.
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Oxygen Barrier ProtectionOxidation is the top degradation pathway for vitamins, probiotics, and unsaturated fatty acids. Pullulan capsules have a uniquely dense molecular structure, delivering oxygen barrier performance far superior to gelatin—making them the gold standard for high-end nutraceuticals and oxygen-sensitive pharmaceuticals.
2. Mainstream Capsule Materials: Core Features & Ideal Application Scenarios
Below are the core features and applicable scenarios for the main capsule types in our product line, fully aligned with the original text:
Standard Gelatin Capsules
This type holds the highest market share, with mature production technology and clear cost advantages. It is suitable for most conventional pharmaceuticals. Two key notes for application: first, confirm whether the API is sensitive to moisture; second, assess the risk of cross-linking caused by aldehydes in the formulation or storage environment. In addition, gelatin capsules are not applicable for markets requiring non-animal sources (such as vegan, halal and other related demands).
PEG-Modified Gelatin Capsules
Polyethylene glycol (PEG) is added as a plasticizer or modifier on the basis of gelatin. The addition of PEG can improve the flexibility of the capsule, reduce its brittleness, and is suitable for filling processes with high requirements for mechanical strength. Meanwhile, PEG can also regulate the dissolution behavior to a certain extent, meeting the needs of specific release profiles.
HPMC Vegetarian Capsules
Using hydroxypropyl methylcellulose as the film-forming material, these capsules feature low moisture content and no cross-linking risk, and naturally comply with vegan, halal, Kosher and other certification requirements. They are especially suitable for APIs with strong hygroscopicity, or for products that aim to cover multiple global markets with a single formulation. The 2025 edition of the Chinese Pharmacopoeia has set up a dedicated quality standard for HPMC empty capsules, providing clear compliance basis for their use in China.
Pullulan Polysaccharide Capsules
Made from pullulan polysaccharide derived from microbial fermentation, these capsules have the characteristics of low oxygen permeability, no cross-linking risk, and low additive content. They have outstanding performance in protecting oxygen-sensitive products such as probiotics, vitamins, and antioxidant ingredients, and are often used in high-end preparations and organic certified products. The cost is relatively high, and they are generally used for products with differentiated positioning.
Enteric Capsules (Gelatin-Based & HPMC-Based)
Enteric capsules are a necessary choice for drugs that need to be released in the intestinal tract (such as proton pump inhibitors, polypeptide drugs, or ingredients that are unstable to gastric acid and irritate the gastric mucosa). We provide two process routes: gelatin-based and HPMC-based. The enteric properties can be achieved through the capsule shell itself or the outer coating, and they can also be used for colon-targeted delivery.
3. Shift From Passive Selection to Active Design
Under the ICH Q8 Quality by Design (QbD) framework, capsule material properties are direct drivers of a formulation’s Critical Quality Attributes (CQAs). We strongly recommend evaluating capsules during pre-formulation development, rather than passively adjusting them at the pilot scale-up or registration submission stage.
The core indicators to be evaluated include: the compatibility between the capsule moisture content and the API, the adaptability of mechanical strength to the filling process, whether the disintegration and dissolution behavior meet the release target, and whether there are special functional requirements such as enteric coating and PEG modification.
4. Actionable Recommendations
- Conduct compatibility studies between capsules and contents during the formulation development stage, carry out accelerated and long-term stability tests in accordance with the ICH Q1A protocol, and guide material selection with data.
- Take the regulatory and certification requirements of the target market into account in advance. For example, the EU has banned the use of titanium dioxide in capsules, the demand for plant-based capsules in the North American market continues to rise, and the Middle East and Southeast Asian markets have clear requirements for halal certification.
- Choose a supplier with a complete product line and stable quality system. A supplier that can provide a full range of products from gelatin, HPMC, pullulan to PEG-modified and enteric capsules can reduce docking costs, and is also conducive to maintaining batch-to-batch consistency and supply chain security.
Capsules are not just packaging—they are functional excipients. Choosing the right capsule is an effective way to find a balance between risk control, quality improvement and product differentiation. For pharmaceutical enterprises, this is essentially a systematic investment in patient safety, regulatory compliance performance and brand value.
Post time:2026-04-24