3.3 Biomaterials Science: Properties & Biocompatibility

3.3 Biomaterials Science: Properties & Biocompatibility

Quick Answer: Dental materials succeed or fail by mechanics + chemistry + biology. Know stress, strain, modulus, strength, hardness, creep, fatigue; understand corrosion/galvanism in saliva; master polymerization basics for resins; and judge biocompatibility by leachables, thermal injury, and host response. AFK tests definitions and clinical consequences, not manufacturer codes.

Biomaterials sit in the applied biomedical sciences domain and reappear throughout restorative, prosthodontic, and endodontic chapters. This section is the vocabulary layer: later chapters name specific products; here you learn why a ceramic chips, a polymer shrinks, or a base metal tastes metallic.

Core Mechanical Properties

Stress, strain, and elasticity

• Stress (σ) = force / cross-sectional area (MPa)
• Strain (ε) = change in length / original length (dimensionless)
• Elastic (Young’s) modulus (E) = stress / strain in the linear elastic region

Hooke’s law region: stress ∝ strain while behavior is elastic; remove load → original shape returns.

Strength, hardness, toughness, resilience

Hardness scales (qualitative awareness): enamel ≈ very hard biologic ceramic; dentin softer; restorative hardness should be considered relative to opposing enamel to reduce wear (classic concern with rough porcelain against natural teeth).

Creep, stress relaxation, fatigue, and wear

• Creep: time-dependent plastic deformation under constant load (classic teaching: low-copper amalgam creep → marginal ditching; also relevant to some polymers under sustained load)
• Stress relaxation: stress decreases over time under constant strain (elastomeric impressions, some polymers)
• Fatigue: failure from repeated cyclic loading below ultimate strength (clasps, solder joints, ceramic connectors, implant components conceptually)
• Wear: material loss from abrasion, attrition, erosion, or corrosion-assisted mechanisms; opposing enamel wear is a biocompatibility-of-function issue

Thermal and Dimensional Properties

Percolation concept: repeated thermal cycling with CTE mismatch pumps fluid at margins—contributes to sensitivity and recurrent caries risk if seal fails.

Metals, Corrosion, and Galvanism

The mouth is a warm, wet, oxygenated, chloride-containing electrolyte. Metals can act as electrodes.

Corrosion types (know names + outcomes)

• Uniform attack: general surface loss
• Pitting corrosion: localized deep attack (chloride environments)
• Crevice corrosion: stagnant zones under plaque or at joints
• Stress corrosion: mechanical stress + corrosive medium
• Galvanic corrosion: dissimilar metals electrically coupled in electrolyte—more active (anodic) metal corrodes preferentially

Galvanic shock: patient feels sharp pain when dissimilar metals contact (e.g., foil on amalgam) via pulp nerve stimulation—classic biomaterials vignette.

Passivation: spontaneous oxide film (Cr₂O₃ on stainless/Co-Cr; TiO₂ on titanium) that slows further corrosion—key reason implant titanium and many orthodontic alloys succeed.

Ceramics and Polymers—Property Contrasts

Ceramics (porcelain, glass-ceramics, zirconia—concept level)

• High compressive strength, high hardness, high modulus
• Brittle: poor tensile/flexural tolerance of flaws; cracks propagate with little plastic blunting
• Esthetic and wear-resistant when polished; opposing enamel wear if rough
• Zirconia toughened by transformation toughening (advanced detail optional); still design-sensitive at connectors and margins

Polymers (PMMA, composite resin matrix, elastomers)

• Lower modulus than ceramics/metals (varies widely)
• Viscoelastic: creep and stress relaxation more prominent
• Can absorb water; plasticizers and residual monomer affect properties
• Composite resins = polymer matrix + ceramic/glass fillers; fillers raise hardness, strength, and modulus and reduce shrinkage relative to unfilled resin

Polymerization Science (Resins)

Most dental resins (composites, adhesives, acrylics) polymerize by free-radical addition polymerization:

1. Activation: light (camphorquinone + amine systems common for visible blue light ~470 nm) or chemical (peroxide–amine self-cure) or dual-cure
2. Initiation: free radicals form
3. Propagation: monomers (e.g., Bis-GMA, UDMA, TEGDMA) add to growing chains
4. Termination: radicals combine or disproportionate

| Concept | Clinical importance | |---|---|---| | Degree of conversion | Higher conversion → better mechanical properties, less residual monomer | | Oxygen inhibition layer | Thin uncured surface layer in air—used or removed depending on technique | | Polymerization shrinkage | ~1.5–5% volumetric range depending on material class; causes stress in bonded cavities | | C-factor | Ratio of bonded to unbonded surfaces; high C-factor (e.g., Class I) ↑ shrinkage stress | | Incremental placement / bulk-fill strategies | Manage depth of cure and stress | | Residual monomer | Soft-tissue irritation, allergy (rare), plasticizing effect if high |

Heat-cured vs chemically cured acrylic (denture base): heat-cured generally higher conversion and strength; improper curing increases residual monomer and porosity.

Light curing practical points: adequate energy dose (intensity × time), correct wavelength for photoinitiator, close tip distance, avoid undercure at bottom of deep increments—undercure weakens restoration and increases leachables.

Biocompatibility Principles

Biocompatibility: ability of a material to perform with an appropriate host response in a specific application. No material is universally “inert” in every use.

Levels of biological response

Testing hierarchy (conceptual)

In vitro cytotoxicity → animal usage tests → clinical trials. AFK may not demand ISO numbers but expects you to know that biocompatibility is application-specific (implant titanium vs temporary acrylic monomer exposure).

High-yield biocompatibility clinical rules

1. Seal against bacteria often matters more than minor chemical differences between established materials
2. Heat can injure pulp—cool when cutting, careful curing/polishing
3. Avoid eugenol contamination when bonding to resin (inhibits polymerization)—materials interaction is biocompatibility + chemistry
4. Nickel-containing alloys: screen history of contact dermatitis for jewelry; consider alternatives in sensitive patients
5. Latex: Type I allergy risk—use non-latex options when indicated
6. Mercury hygiene for amalgam: modern encapsulated systems, water spray/high-volume suction, environmental separators—occupational and ecological safety
7. Ceramic dust / silica: lab and adjustment dust control (occupational lung risk)—beyond chairside AFK but shows “materials safety” thinking

Structure–Property–Application Map (Exam Grid)

Integrating Biomaterials for AFK Stems

Work every materials question with three filters:

1. Mechanics: Will it break, bend, wear, or creep in this load environment?
2. Environment: Will oral fluids corrode it, dissolve it, or plasticize it?
3. Biology: What contacts pulp, gingiva, or allergen-sensitized host?

Examples:

• Porcelain fracture of a PFM cusp → brittle ceramic + tensile stress concentration + possible occlusal overload
• Post-cementation sensitivity under composite → polymerization stress, microleakage, or pulpal insult—not always “allergy”
• Metallic taste after new crown opposite dissimilar metal → galvanic cell consideration
• Denture sore mouth with new acrylic → residual monomer, trauma, or candidiasis differential

AFK Study Checklist for This Section

• Define stress, strain, modulus, strength, hardness, toughness, creep, fatigue
• Explain galvanic corrosion and passivation
• Outline free-radical polymerization steps and shrinkage/C-factor ideas
• Contrast ceramic brittleness vs metal ductility vs polymer viscoelasticity
• List three biocompatibility risk pathways (toxicity, allergy, microleakage/heat)

With biochemistry of tissues (3.1–3.2) and materials behavior (3.3), you can reason about both the tooth and what we put in it—the foundation for all restorative science chapters that follow.