BME 168 - Medical and Biological Polymers - Lecture Notes

Lab Information

• Lab begins this week.
• Attendance is mandatory to pass the class.
• Preparation for lab work is required, including:
  • Close-toed shoes
  • Long pants
  • No food or drink in the lab

BME 168 - Medical and Biological Polymers

• Molecular Origins of Biomaterial Behavior
• Spring 2025
• Instructor: Melinda Simon
• San José State University

Agenda

• Term paper information
• Requirements for biomaterials
  • Biocompatibility
  • Mechanical properties
• Molecular Origins of Biomaterial Properties
  • Types of chemical bonds
  • Crystal structures

Biomaterials Attributes

• Consider three attributes a biomaterial should have from perspectives of:
  • Manufacturer
  • Clinician
  • Patient

Biomaterials: Attributes

• Manufacturer
  • Profitable
  • Manufacturable
  • Reliable
  • Free of defects
  • Sterilizable
  • Modular design
  • Sustainable
• Clinician
  • Biocompatible
  • Insurance-covered
  • Easy to operate/deploy
  • Minimally invasive
  • Reusable
  • Low maintenance
• Patient
  • Insurance-covered
  • Biocompatible
  • Durable
  • Comfortable
  • Convenient
  • Quality of life
  • Effective

Biocompatibility: Definition

• The ability of a material to perform with an appropriate host response in a specific application.
• Examples of “appropriate host response”:
  • Resistance to blood clotting
  • Resistance to bacterial colonization
  • “Uncomplicated” healing
• Must do the patient no harm

Biocompatibility: Key Considerations

• Mechanical properties
• Surface properties
  • Smooth or roughened surface desirable?
  • Hydrophilic or hydrophobic?
• Service life
  • Degradable or non-degradable?
  • Implanted vs. non-implanted device
• Effect of interaction(s) locally and systemically

Biocompatibility: Testing

• As a result of varying requirements, biocompatibility tests need to be application- and tissue-specific
• Dominated by surface characteristics, e.g., roughness, porosity, charge, chemistry, surface tension (of the solid), texture, wettability, etc.

Molecular Origins of Biomaterial Behavior: Chemical Bonds

• Primary bonds (interatomic)
  • Covalent
  • Ionic
  • Metallic
  • Ionic/Covalent mixed character
• Secondary bonds (interatomic/intermolecular)
  • Polar bonds
  • Nonpolar bonds

Covalent Bonds

• Electron sharing to fulfill the octet rule
• Predominantly polymers and other organics
• Relatively strong
• Rotation of atoms is possible
  • Carbon chain “backbone”
  • Crosslinking between chains
• Materials are deformable due to the ability of bonds to rotate
• Electrons are not free – chemical inertness, nonreactivity

Ionic Bonds

• Electron transfer generates ions
• Bonding is due to electrostatic attraction among ions
• Predominantly found in ceramics
• Very strong bonding, equal strength in all directions
• Intolerant to lattice deformation (applying tension, bending)
• Materials are brittle but have a high strength and stiffness

Metallic Bonds

• Interatomic interaction “delocalizes” electrons, creating quasi-free electrons, a.k.a. “electron cloud” or “sea of electrons”
• Predominantly found in metals
• Accounts for good thermal and electrical conductivity
• Bond strength varies - generally weaker than covalent and ionic
• Nondirectional bonds – easily broken but also easily re-formed after deformation (ductility

Electronegativity

• Electronegativity measures how strongly an atom holds onto its electrons.
• The nature of the chemical bond between two atoms depends on the difference in their electronegativity.
• Pauling scale

Electronegativity and Bonding

• Atoms with comparable electronegativity equally share their valence electrons (covalent bond).
  • E.g. C-H or C-O bonds
• Atoms with very different electronegativity form ionic bonds.
  • E.g. Na-Cl bond

Secondary Bonds

• A dipole is a molecule with a spatial separation between the negative and positive charge
  • Permanent dipoles (polar molecules)
  • Fluctuating (instantaneous) dipoles
  • Induced dipoles
• Much weaker than primary bonds

Secondary Bonds - Types

• Fluctuating induced dipole bond (Van der Waals bond)
• Permanent dipole – induced dipole bond
• Permanent dipole bond
  • Hydrogen bond

Types of Intermolecular Forces

Assignments

• Reading
  • Biomaterial Requirements, Molecular Origins of Biomaterial Behavior (1.1.2, I.1.3)

Next Class

• Mechanical properties of biomaterials