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Chapter 6: Biocompatible Materials
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[Table of Contents]

Materials must ensure biocompatibility, protection, and safe passage or
degradation - critical for patient safety and device performance.

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6.1 Capsule Materials
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Requirements:
  Biocompatibility: Non-toxic, non-immunogenic (ISO 10993, USP Class VI)
  Mechanical: Withstand GI pressures (up to 200 mmHg)
  Barrier: Moisture (< 1 g/m²/day WVTR), chemical (pH 1-8), gas protection
  Degradation: Controlled dissolution or biodegradation

Capsule Materials:
  Gelatin (Animal Collagen):
    - Properties: Water-soluble, disintegration < 30 min @ 37°C
    - Types: Hard (13-16% moisture) vs. soft (higher plasticizer)
    - Advantages: Inexpensive, established safety, fast dissolution
    - Limitations: Moisture sensitive, religious/dietary concerns

  HPMC (Hydroxypropyl Methylcellulose):
    - Source: Plant cellulose (vegetarian/vegan friendly)
    - Moisture: Lower (< 5%), more stable
    - Advantages: No crosslinking, religious-neutral, allergy rare
    - Cost: 20-30% more expensive

  Enteric Polymers (pH-Responsive):
    - Eudragit L100 (pH > 6.0), S100 (pH > 7.0), FS30D (pH > 7.0)
    - HPMCP (pH > 5.5), Shellac (pH > 7.0), CAP (pH > 6.0)
    - Application: Gastric protection, intestinal targeting

  Biodegradable Polymers:
    - PLGA (Poly-lactic-co-glycolic acid): Hydrolysis → lactic + glycolic acid
    - Ratios: 50:50 (fast, 1-2 mo), 75:25 (medium, 4-5 mo), 85:15 (slow, 5-6 mo)
    - FDA-approved for drug delivery, tissue engineering

Smart Materials:
  - PNIPAM: Temperature-responsive (LCST ~32°C)
  - pH-responsive: Eudragit, chitosan, alginate
  - Enzyme-responsive: Pectin, azo-polymers (bacterial degradation)

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6.2 Sensor Coatings
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Purposes: Protection (chemical, mechanical, biological), selectivity, sensitivity

Coating Materials:
  Nafion (Ionomer):
    - Charge: Negative, repels anions (ascorbic acid, uric acid)
    - Passes: Neutral molecules (glucose, oxygen)
    - Thickness: 100-500 nm, Applications: Glucose sensors

  Polyurethane:
    - Biocompatibility: Excellent
    - Permeability: Gas and small molecules
    - Flexibility: High, Applications: Outer protection, glucose membranes

  Cellulose Acetate:
    - Biocompatibility: Medical-grade established
    - Molecular weight cutoff: 1-100 kDa (excludes proteins)

  Chitosan:
    - Source: Chitin deacetylation (crustacean shells)
    - Properties: Biocompatible, biodegradable, antimicrobial
    - pH-dependent: Dissolves in acidic (< 6.5)

  Anti-Fouling Coatings:
    - PEG (Polyethylene Glycol): 70-90% protein adsorption reduction
    - Zwitterionic: Super-low fouling (> 95% reduction)
    - Superhydrophobic: Contact angle > 150°, repels proteins/cells

Coating Methods:
  - Dip coating: Simple, 100 nm - 10 μm/dip
  - Spin coating: Very uniform, 10 nm - 1 μm (flat surfaces only)
  - Spray coating: Complex shapes, thicker coatings
  - Vapor deposition (CVD/PVD): High-density, pinhole-free
  - Electrodeposition: Room temperature, precise thickness
  - Layer-by-layer (LbL): Nanometer precision, multi-functional

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6.3 Safety Testing
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ISO 10993 (Medical Device Biological Evaluation):
  - Part 5: Cytotoxicity (cell viability > 70% to pass)
  - Part 10: Irritation and sensitization (skin, guinea pig tests)
  - Part 11: Systemic toxicity (acute, subacute, subchronic)
  - Part 4: Blood compatibility (hemolysis < 5%)
  - Part 13: Material characterization (FTIR, NMR, GC-MS, ICP-MS)

USP Class VI:
  - Systemic injection (mouse IV + IP): Weight > 90%, no death
  - Intracutaneous (rabbit): Erythema/edema score < 1.0
  - Implantation (rabbit muscle): Minimal tissue reaction

GI-Specific Testing:
  - Mucosal irritation: Histology, inflammation markers (IL-6, TNF-α)
  - Transit safety: In vivo animal studies (dogs, pigs), X-ray tracking
  - Enzyme stability: Pepsin, trypsin, colonic bacterial enzymes

Leachables/Extractables:
  - Extractables: Harsh conditions (60°C, organic solvents)
  - Leachables: Physiological conditions (actual release)
  - Analysis: GC-MS, LC-MS, ICP-MS for toxic substances

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6.4 Degradation and Excretion
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Intact Excretion (Non-Degradable):
  - Size: < 13-15 mm (ileocecal valve limit), typical 8-12 mm
  - Shape: Capsule/spherical, rounded edges (no sharp protrusions)
  - Timeline: 24-72 hours (most < 48 hrs)
  - Confirmation: Patient visual, signal termination

Biodegradation (In-Body Breakdown):
  - Hydrolysis: PLGA, PLA (ester bonds) → lactic/glycolic acid
  - Enzymatic: Gelatin, chitosan, pectin (digestive/bacterial enzymes)
  - Timeline: Hours (gelatin) to weeks (high MW PLGA)
  - Safety: Non-toxic, metabolizable products

Safety Considerations:
  Obstruction Risk (< 0.01% incidence):
    - Risk factors: Strictures (Crohn's, surgery), diverticula, tumors
    - Prevention: Screening, patency capsule (dissolves in 30 hrs)
    - Management: Conservative, endoscopic, surgical (last resort)

  Perforation (< 0.001%): Prevented by design (rounded, smooth, appropriate size)

  Aspiration: Prevented by conscious swallowing, upright position

  Battery Safety:
    - Encapsulation: Double sealing
    - Materials: Non-reactive (gold, titanium)
    - Types: Silver-oxide safe, lithium requires stricter sealing

  MRI Compatibility:
    - Assessment: Heating (SAR), magnetic force, image artifact
    - Labeling: Unsafe, conditional, or safe
    - Requirement: Confirm excretion before MRI

Key Principle: All materials and components are designed and tested with
patient safety as the highest priority. Extensive biological evaluation is
mandatory before regulatory approval.

[Next: Chapter 7]
[Table of Contents]

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