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MSE 536- SP2025_Biopolymers_Natural & Synthetic

MSE 536: Advanced Biomaterials

Natural & Synthetic Polymers

Polymeric Biomaterials

  • Polymer-based implants

    • Relatively inexpensive to manufacture.

    • Can be tailored for specific applications.

  • Applications include:

    • Medical disposable supplies

    • Prosthetic materials

    • Dental materials

    • Implants, dressings, extracorporeal devices

    • Encapsulants, polymeric drug delivery systems, and tissue engineered products.

    • Specific Polymers:

      • Ultrahigh molecular weight polyethylene (UHMWPE): extensively used in total joint prostheses.

      • Polymethylmethacrylate (PMMA): used as bone cement.

Requirements for Biomedical Polymers

  • Adapted from Table 3.1, Biomaterials: Principles and Applications.

Advantages & Disadvantages of Polymeric Biomaterials

Advantages

  • Cost-effective: Inexpensive to manufacture.

  • Reactivity: Can be made reactive for functionalization.

  • Biodegradability: Some can degrade in the body, releasing therapeutic agents during decomposition.

Disadvantages

  • Strength: Usually weaker than metals or ceramics.

    • May not support heavy loads effectively.

Commonly Used Synthetic and Naturally Derived Polymers

Examples and Applications

  • Synthetic Polymers:

    • Poly(2-hydroxyethyl methacrylate): Used in contact lenses.

    • Poly(dimethyl siloxane): Used in various medical devices.

    • Poly(ethylene): Used in vascular grafts.

  • Naturally Derived Polymers:

    • Alginate: Used for gel forms and drug delivery.

    • Chitosan: Used in wound dressings and drug carriers.

    • Collagen: Tissue engineering applications due to tensile strength.

Structure of Polymers

  • Most are carbon-based from hydrocarbon molecules.

  • Mers: Smallest building blocks (monomer and oligomer defined).

  • Saturated polymers: All carbons bonded to four atoms.

  • Unsaturated polymers: Contains double bonds.

  • Covalent Bonds: Hold units together; essential for mechanical properties.

  • Secondary Bonds: Determine dissolution and flow properties.

Molecular Weight

  • No fixed molecular weights.

  • Influences polymer properties.

  • Types of Molecular Weight:

  • Number-average molecular weight (Mn): Based on chain size distribution.

  • Weight-average molecular weight (Mw): Based on weight distribution.

Polymer Synthesis (Polymerization)

Types

  1. Addition (Chain-growth) Polymerization:

    • Direct addition of monomers.

    • Requires initiator; three steps: initiation, propagation, termination.

  2. Condensation (Step-growth) Polymerization:

    • Small molecule by-product formed.

    • Involves multiple monomer species without radicals.

Degree of Polymerization

  • Average length represented by repeat units.

  • Polydispersity Index (PI): Measure of molecular weight distribution.

    • Higher PI indicates wider distribution.

  • Higher molecular weight leads to greater viscosity.

Methods of Polymerization

  1. Bulk Polymerization: High yield but challenges with heat dissipation.

  2. Solution Polymerization: Conducted in solvent, requires purification.

  3. Suspension Polymerization: Monomers stirred in water, forms droplets.

Emulsion Polymerization

  • Involves emulsified droplets and surfactant-stabilized particles.

    • Stabilization challenges as particles grow.

Copolymers

  • Involves two or more different monomer types for enhanced properties.

    • Types include random, alternating, block, and grafted copolymers.

Polymer Crystallinity

  • Partially crystalline due to complexity.

  • Factors affecting crystallinity include size and chemical structure.

  • Melting Points: Elevated density and resistance to degradation.

Amorphous Polymers

  • Randomly oriented chains allowing flexibility and deformation.

Glass Transition Temperature (Tg)

  • Critical point at which polymers transition from rigid to rubbery states.

  • Influences application considerations, especially in biodegradable tissues.

Effect of Temperature on Polymers

  • Behavior ranges from liquid to solid depending on molecular structure.

  • Melting Temperature (Tm): Determined by polymer structure and branching.

Thermoplastics and Thermosets

Classification

  1. Thermoplastics: Flexible, recyclable, and soften on heating.

  2. Thermosets: Rigid, non-recyclable, and hard due to cross-linking.

Stress-Strain Behavior

Types of Polymers

  • Brittle Polymers: Fracture during elastic deformation.

  • Plastic Polymers: Exhibit yielding and plastic deformation.

  • Elastomers: High elasticity and low stress recovery.

Fracture of Polymers

  • Generally low fracture strengths. Factors influencing include temperature, strain rate, and specimen structure.

  • Crazing phenomenon helps absorb energy during fracture.

Fatigue in Polymers

  • Occurs under cyclic loading at stress levels below yield strength.

  • Sensitivity to loading frequency and rate of deformation at elevated temperatures.

Synthetic Polymers

  • Various types and applications, including PVC, PE, PMMA, and more.

Biomedical Application of Synthetic Polymers

  • Wide applications across medical fields, including tubing, sutures, and grafts.

Specific Polymers: Polyethylene

  • Five grades: HDPE, LDPE, LLDPE, VLDPE, UHMWPE.

  • UHMWPE: Specifically useful in orthopedic implants, known for its wear resistance and difficulties in processing.

Polymethylmethacrylate (PMMA)

  • Good optical properties and biocompatibility; complications in clinical use due to exothermic polymerization reactions.

Other Acrylic Polymers

  • Varieties like PMA and PHEMA are used in soft contact lenses and as drug delivery systems.

Polylactic Acid (PLA) & Polyglycolic Acid (PGA)

  • Biodegradable polyesters with applications in sutures and drug delivery; mechanisms through hydrolytic scission.

Issues with Hydrolytic Degradation

  • Accumulation of acidic by-products can hinder performance and cell interaction.

Applications of PGA and PLA

  • Use in fixation devices, scaffolding, and dental applications.

Other Synthetic Polymeric Biomaterials

  1. Polycaprolactone (PCL): Slow degradation rates, used for drug delivery.

  2. Polyurethanes: Good mechanical properties for catheters and pacemakers.

  3. Polyanhydrides & Silicones: Used for drug delivery and prosthesis applications.

Hydrogels

  • 3D polymer networks with high water content, important for various biomedical applications.

Synthesis of Hydrogels

  • Involves polymer absorption and cross-linking to achieve desired properties.

Classifications of Hydrogels

  • Based on source, electrical charge, and physical structure.

Properties of Hydrogels

  • Includes porosity, swelling degree, and mechanical strength; depends on preparation methods and structural characteristics.

Stimuli-responsive (Smart) Hydrogels

  • Hydrogels exhibit rapid changes in response to environmental factors like pH and temperature.

Applications of Hydrogels

  • Utilized in contact lenses, tissue engineering, and controlled drug delivery.

Natural Polymers

  • Major sources include proteins and polysaccharides, with significant roles in biomedical applications.

Advantages & Disadvantages of Biopolymers

  • Advantages: Biocompatibility and similarity to natural tissues.

  • Disadvantages: Variable properties and lower mechanical strength.

Collagen

  • Abundant protein with functions in structure and healing; used in various implants and drug delivery systems.

Properties and Applications of Collagen

  • Unique structure lends to many medical applications including hemostats, tissue engineering, and implants.

Structure of Collagen

  • Four-level structure with specific amino acid sequences providing strength and stability.

Types of Collagen

  • Differentiated based on their specific locations and functions in the body.

Applications of Elastin

  • Used for dynamic tissue repair and as a scaffold for tissue engineering.

Chitosan & Alginate

  • Biocompatible polysaccharides used in wound care and cell immobilization systems.

Hyaluronic Acid (HA)

  • Plays a pivotal role in wound healing and tissue regeneration; applications in orthopedics and ophthalmology.

Polymer-Based Nanoparticles

  • Emerging as useful tools in drug delivery systems and targeted therapies.

Polymer Therapeutics

  • Multifunctional, adjustable polymer systems designed to mimic natural proteins for therapy and drug delivery.