Introduction to Microencapsulation, Mucosal & Implantable Systems
This unit delves into specific advanced delivery technologies. It explores Microencapsulation (definition, core and coating materials, microspheres vs. microcapsules, methods, and applications). It covers Mucosal Drug Delivery, emphasizing the principles of bioadhesion/mucoadhesion and the specifics of buccal delivery. Finally, it introduces Implantable Drug Delivery Systems, highlighting their advantages and the functioning of the osmotic pump system.
Syllabus & Topics
- 1Microencapsulation – Definition & Materials: Microencapsulation: Technology of enclosing solid, liquid, or gas cores within a thin polymeric coating to produce microparticles (size range: 1 µm to 1000 µm). Terminology: Core material (active ingredient) and Coating/Wall material (polymer matrix). Microspheres vs Microcapsules: Microcapsules: The core is completely surrounded by a distinct continuous polymer shell (reservoir type). Microspheres: The drug is dispersed/dissolved uniformly throughout the polymer matrix (matrix type). Advantages: Tastes/odor masking, protection from environmental degradation (light, moisture, oxidation), sustained/controlled release, conversion of liquids to free-flowing solids, reducing GI irritation. Disadvantages: Expensive process, difficulty in achieving reproducible continuous coating.
- 2Methods of Microencapsulation: (1) Air suspension (Wurster process): Solid core drug particles are suspended in an upward moving air stream while a coating solution is sprayed onto them. Best for solid particles. (2) Coacervation-Phase Separation: Three steps – formation of three immiscible chemical phases (liquid manufacturing vehicle, core material, and coating material); deposition of the coating on the core; rigidization of the coating (cross-linking, heat, or desolvation). (3) Spray Drying/Congealing: Core dispersed in liquefied coating material and sprayed into an environment (hot air for drying, cold air for congealing) causing coating to solidify. (4) Pan Coating: Oldest method, used for larger particles; core particles tumbled in a pan while coating is applied. (5) Solvent Evaporation: Drug + polymer dissolved in an organic solvent, emulsified into an aqueous phase, solvent evaporates leaving solid microparticles. Applications: Taste masking, sustained release formulations.
- 3Mucosal Drug Delivery & Bioadhesion: Mucosal delivery refers to drug administration via mucous membranes (buccal, sublingual, nasal, vaginal, rectal). Advantages: Avoids first-pass hepatic metabolism, rapid absorption due to rich blood supply, non-invasive compared to injections. Bioadhesion/Mucoadhesion: State in which two materials, at least one being biological, are held together for extended periods by interfacial forces. When the biological surface is a mucous membrane, it’s called mucoadhesion. Mechanisms: (1) Wetting/Hydration theory: Polymer spreads and hydrates over the mucus layer. (2) Interpenetration theory: Polymer chains and mucin glycoprotein chains interpenetrate and entangle. (3) Electronic theory: Electron transfer between mucin and polymer creating attractive forces. Common mucoadhesive polymers: Carbopol, Chitosan, HPMC, Sodium alginate.
- 4Buccal Drug Delivery Systems: Buccal delivery implies drug administration through the mucosal lining of the cheek. Advantages: Easy access, avoids first-pass metabolism, rapid onset of action, dosage form can be easily removed if toxicity occurs. Disadvantages: Small surface area compared to GI tract, continuous secretion of saliva can wash away the drug (swallowing leads to GI tract/first-pass), taste is a significant consideration. Transmucosal Permeability: Drug must pass through epithelial barrier. Facilitated by permeation enhancers (surfactants, bile salts, cyclodextrins). Formulation Considerations: Mucoadhesive tablets/patches/films are used to ensure the dosage form stays in place against the cheek. Must not dissolve too quickly in saliva, should have unidirectional release (towards mucosa, not into oral cavity).
- 5Implantable Drug Delivery Systems: Implants: Sterile, solid pharmaceutical devices designed for subcutaneous or intramuscular insertion (often via minor surgery or special syringe) to provide prolonged, continuous drug release (months to years). Advantages: Excellent for delivering drugs that require strict compliance (contraceptives) or have very short half-lives; zero-order release is possible; avoids first-pass. Disadvantages: Invasive (surgery required for insertion and often removal), pain/discomfort, risk of infection at the insertion site, ‘burst effect’ (initial rapid release) can cause toxicity, highly expensive. Example: Norplant (levonorgestrel implant for 5-year contraception, now mostly superseded by single-rod systems like Nexplanon).
- 6Osmotic Pump: A highly reliable controlled delivery system operating on the principle of osmosis. Osmosis: Movement of solvent across a semipermeable membrane from low solute concentration to high solute concentration. The elementary osmotic pump (OROS technology): A tablet core containing the drug and an osmogen (e.g., NaCl, to draw water) is coated with a rigid semipermeable membrane (e.g., cellulose acetate). A small delivery orifice is drilled via laser. In the body, water is drawn into the core by the osmogen. The increased volume/pressure forces the dissolved/suspended drug out through the orifice at a highly predictable, constant (zero-order) rate. Advantage: Release rate is truly independent of physiological factors like GI pH, food presence, or GI motility. Limitation: If the orifice blocks or the membrane cracks, dose dumping occurs. Example: Procardia XL (nifedipine).
Learning Objectives
Exam Prep Questions
Q1. How does coacervation-phase separation work in microencapsulation?
It involves three key steps:
Three immiscible phases are formed: the core material, the liquid coating polymer phase (coacervate), and the manufacturing solvent.
A change in conditions (e.g., changing temperature, pH, or adding a salt) causes the coating polymer phase to separate from the solvent and deposit itself around the core particles.
The deposited coating is then rigidified or hardened (using cross-linking agents like glutaraldehyde or cooling) to form stable microcapsules.
Q2. Why is swallowing an issue in buccal drug delivery?
The main advantage of buccal delivery is avoiding first-pass metabolism by absorbing the drug directly into the systemic circulation via the cheek mucosa. However, the oral cavity continuously secretes saliva. If the drug dissolves into the saliva and the patient swallows it, the drug enters the gastrointestinal tract and will be subjected to first-pass liver metabolism, defeating the primary purpose of buccal administration. Thus, mucoadhesive polymers that stick firmly to the mucosa and release drug unidirectionally are used.
Q3. What makes the osmotic pump superior to standard diffusion-based controlled release tablets?
Standard diffusion-based matrix tablets can be significantly affected by environmental factors in the GI tract, such as pH changes, the presence of food, and gastrointestinal motility. The osmotic pump (e.g., OROS), relies solely on osmotic pressure. The semipermeable membrane only allows water in. Since the osmotic pressure difference across the membrane remains constant as long as solid osmogen exists, the drug release rate is strictly zero-order and nearly independent of patient-specific physiological GI variations.