Biomaterials

Biomaterials

Prepared by Dr. Alper DUNKI

Biomaterials are synthetic substances, derived from organic or inorganic components, designed to interact with biological systems. Their properties are determined by their structure (elemental composition, atomic bonding, crystalline configuration) and their processing methods (casting, forging, extrusion, sintering, etc.).

 Classes of Biomaterials

• Metals:     Strong, durable, conductive materials; may exist as single elements (Cu,      Ag) or alloys (e.g., stainless steel).
• Ceramics:     Hard, brittle, corrosion-resistant; typically metal oxides (Al₂O₃, ZrO₂).
• Polymers:     Carbon-based chain structures, flexible, corrosion-resistant      (polyethylene, PTFE, silicone, hydrogels).
• Composites:     Mixtures of two or more distinct phases, engineered for specific      properties (fiberglass, concrete).
• Natural      biomaterials: Plant/animal-derived tissues, proteins, polysaccharides.

 Applications and Requirements in Orthopedics

Used in fracture fixation, osteotomy, arthrodesis, wound closure, tissue replacement, and prostheses. They must be biocompatible, corrosion/degradation resistant, and possess adequate mechanical strength and wear resistance.

 Biocompatibility

The ability of a material to elicit an appropriate biological response in vivo.

• Inert:     Minimal tissue response (e.g., stainless steel).
• Bioactive/interactive:     Promote favorable responses (e.g., porous titanium allowing bone      ingrowth).
• Living:     Contain cells and undergo remodeling.
• Reseeding      constructs: Donor tissues re-implanted following culture.
• Biologically      incompatible: Induce undesirable reactions.

 Corrosion and Degradation Resistance

The physiological environment may induce corrosion.

• Types      of corrosion: Pitting, crevice, fatigue, stress cracking, galvanic,      and fretting.
• Polymer      degradation: Depolymerization, oxidation, hydrolysis, additive      leaching, cracking.

  Mechanical Properties

• Basic      concepts: Compression/tension, shear, torsion; stress, strain,      strength, toughness.
• Elastic      modulus: Defines stiffness; yield point marks onset of plastic      deformation.
• Material      types:
  • Brittle:      Fail with minimal deformation (ceramics, glass).
  • Ductile:      Sustain significant deformation (steel, titanium alloys).
• Fatigue      fracture: Failure due to repetitive loading; highly relevant in orthopedics.
• Anisotropy:     Direction-dependent properties (bone, tendon).
• Viscoelastic      behavior: Time-dependent deformation (creep, stress relaxation). 

  Specific Biological and Medical Materials

a. Bone:
Composed of inorganic (calcium phosphate) and organic (type I collagen) phases. Both anisotropic and viscoelastic. Cortical bone density ~1.8 g/cm³; trabecular bone 0.1–1.0 g/cm³. With aging, both mass and elasticity decline.

b. Tendon:
Rich in type I collagen; transmits muscle forces to bone and redirects force. Anisotropic and viscoelastic. Failure often occurs at the bone- or muscle-tendon junction.

c. Ligaments:
Composed primarily of type I collagen; connect bone to bone. Insertional regions play a key role in mechanical strength.

d. Metals:
Crystalline structure with high conductivity; can form alloys.

• Stainless      steel (316L): Low cost, ductile; nickel and chromium may cause      allergic reactions.
• Cobalt      alloys: High strength, long service life.
• Titanium:     Lightweight, highly biocompatible; pure titanium suitable for low-load      applications, alloys for high-load regions.
• Tantalum:     Corrosion resistant, supports osseointegration.

e. Polymers:
Properties determined by monomer composition, molecular weight, and crystallinity.

• PMMA:     Bone cement, may be loaded with antibiotics.
• UHMWPE:     High impact resistance, widely used in joint prostheses.
• Biodegradable      polymers: PLA, PGA; provide controlled degradation and drug delivery.
• Hydrogels:     High water content, low friction, promising in tissue engineering.

f. Ceramics:
Ionic compounds of metals and non-metals; hard, brittle, with high compressive strength.

• Bearing      surfaces: Alumina and zirconia, with low wear rates.
• Bone      substitutes: Hydroxyapatite (slow resorption), tricalcium phosphate      (faster resorption, higher biological activity).

References:

1. Im, G. I., & Lee, Y. (2020). Biomaterials in orthopaedics: the past and future with immune modulation. Biomaterials Research, 24, 10. https://doi.org/10.1186/s40824-020-0185-7

2. Zhang, Y., Lu, H., Wang, S., & He, C. (2024). Advancement in biomedical implant materials — a mini review. Frontiers in Bioengineering and Biotechnology, 12, 1400918. https://doi.org/10.3389/fbioe.2024.1400918

3. Allizond, V., Comini, S., Cuffini, A. M., & Banche, G. (2022). Current knowledge on biomaterials for orthopedic applications modified to reduce bacterial adhesive ability. Antibiotics, 11(4), 529. https://doi.org/10.3390/antibiotics11040529