BMED 2420 Midterm 1

What makes materials different:

Composition: atoms/molecules present

Structure: how are they arranged

Processing: how is the material made/treated

What is a biomaterial?

a biomaterial is a material used for medical application to support, enhance, or replace damaged tissue or a biological function (not drugs)

Ionic Bonding

large electronegativity difference

Properties:

• hard and stiffness, but brittle

• high melting points

• electrically insulating

• wear resistant

Ex: hydroxyapatite

Common for CERAMICS

Covalent Bonding

small electronegativity different

single, double or triple bonds

Pros: Flexible, tunable degradation

• strong, directional bonds

• can form long chains

Ex: UHMWPE, PMMA, silicone, PLGA, scaffolds

Common for POLYMERS

Metallic Bonding

“Electron sea” —> valence electrons roam freely

Pros: conductive, strong, ductile (atoms slide without breaking bonds)

Cons: corrosion, stress shielding, ion release

Ex: Ti-6Al-4V, 316 SS, CoCr, Nitinol

Common for METALS

Secondary Bonding

Much weaker than primary bonds

Common in POLYMERS

Controls how polymer chains interact with each other

• NOT the same as crosslinks (permanent)

Controls how proteins stick to material surfaces

Crystal Structures

Two materials can have the same type of bonding but very different properties!

3D arrangement of atoms (crystal structure) controls:

• ductility: how easily atoms slide past e/o

• packing efficiency and density

• directional dependence of properties

Crystalline Structure

atoms in a regular, repeating 3D pattern

*most metals and many ceramics

Amorphous Structure

atoms have local organization, but no regular pattern over long distances

*polymers are amorphous or semicrystalline

Can change how a material dissolves, degrades, or interacts with the body

Ex: PMMA bone cement, bioactive glass (structure helps it react w body fluids and bond to bone)

Body-Centered Cubic (BCC)

(exist within crystalline structures)

Atoms at corners + one in center

Coordination Number: 8

Atoms: 2

APF: 0.68

Atomic Movement: More difficult / temperature dependent

Typical Behavior: Ductility can be temp. dependent

EX: tantalum

Face Centered Cubic

(exist within crystalline structures)

Atoms at corners + one on each face

Coordination Numbers: 12

Atoms: 2

APF: 0.74

Atomic Movement: Many easy slip paths

Typical Behavior: most ductile, isotropic

Ex: CoCr, 316 SS

Hexagonal Close Packed

(exist within crystalline structures)

Hexagonal layers + 3 atoms in middle plane

Coordination Numbers: 12

Atoms: 6

APF: 0.74

Atomic Movement: fewer easy slip paths

Typical Behavior: ductile, anisotropic

Ex: Titanium, magnesium, zinc

Isotropic

having identical physical, mechanical, or optical properties in all directions

Anisotropic

meaning properties vary depending on the direction of measurement (such as wood being stronger along the grain than across it). [1]

Atomic Packing Factor

The fraction of volume in a crystal structure that is occupied by constituent atoms, assuming atoms are hard spheres

(a = edge length)

Polycrystalline Materials

Found in most engineering metals

Grains: regions with different orientation

Impact on Medical Implants:

• Smaller grain: stronger

• Boundaries: corrosion + fatigue crack sites

Specific Heat Capacity

Energy needed to change a material’s temperature

• High Cp: takes more energy to heat up —> temperature changes slowly

• Low Cp: takes less energy to heat up —> temperature changes quickly

q = m(Cp)(Tf - Ti)

q: energy/heat added (J)

m: mass

Thermal Conductivity (k)

How easily heat flows through a material

Metals: high k (free electrons)

Ceramics: vary widely (photons)

Polymers: low k (no free electrons + disordered chains)

Ex: metal fillings are sensitive to cold bc they transfer hot/cold to nerve faster than enamel

Coefficient of Thermal Expansion

Materials expand/contract as temperature changes

Issue: CTE mismatch as at interfaces —> thermal stress —> cracking or debonding

Ex: critical for dental composites, coatings

Thermal Shock Resistance

Rapid temperature change causes uneven expansion and cracking

Vulnerable: Ceramics (brittle)

Resistant: Metals (ductile)

Biomaterial Relevance:

• Autoclaving: 121* —> room temp

Sterilization: Thermal Properties in Action

Every reusable medical device must be sterilized

Autoclaving (steam): 121*, 15-30 min

• Metals and ceramics tolerate it well

• Many polymers can deform or degrade

Alternatives for heat-sensitive material:

• Ethylene oxide (EtO) gas: low temp

• Hydrogen peroxide-based low-temp sterilization

• Gamma or electron-beam irradiation

Electrical Conductivity

Metals: high conductivity

Semiconductors: intermediate and highly tunable

Polymer & many ceramics: generally insulating

Biological tissues: conduct mainly through ions in fluids

Electrical Insulation in Devices

Many electrical implants need insulating coatings

Ex: Pacemaker leads = silicone or polyurethane coating

• Insulates the wire from surrounding tissue

• Only the electrode tip is exposed

Piezoelectric Effect

Some materials generate voltage when squeezed

Direct: stress —> voltage (sensing)

Converse: voltage —> deformation (actuation)

Magnetic Behavior Materials

Diamagnetic: very weakly repelled

• polymers, ceramics, copper, gold

Paramagnetic: weakly attracted

• titanium, aluminum, platinum

Ferromagnetic: strongly attracted (can retain magnetization)

• iron, cobalt, nickel, some alloys

• MRI danger

MRI Compatibility

Concerns: heating, device malfunction, image artifacts

Higher concern with ferromagnetic materials, older/unknown implants

Specific Absorption Rate (SAR)

How quickly radiofrequency energy is absorbed by tissue (W/kg)

Higher SAR —> more potential heating

What affects MRI Heating?

implant size = larger absorb more energy

implant orientation = orientation relative to RF field changes induced currents

geometry = shape and features affect current

Material

Location: tissue types and proximity to other structures influence heating

MRI Fielding Conditions: field strength, RF coil type, scan parameters

Optical Properties

Transparent: light passes through (contact lenses)

Translucent: light partially scattered (dental crows match natural teeth)

Opaque: light blocked (metals)

Imaging Visibility

How materials interact w X-rays and CT

Radiolucent:

• X-rays pass through (low attenuation)

• Appears darker on X-ray/CT

• Ex: polymers (PEEK)

Radiopaque

• X-rays are strongly attenuated

• appears brighter (whiter) on X-ray/CT

• Ex: metals, many ceramics

Metals primary use in implants

Metals dominate load-bearing implants

Pros: strong, fatigue-resistant, ductile, sterilizable, and manufacturable

Cons: Corrosion, ion release from wear, stress shielding

Corrosion

The loss of metal atoms from a metal surface through electrochemical reactions with the surrounding environment

Body fluids are warm, wet, and salty. Chloride ions and moisture can promote corrosion.

Passivation

The spontaneous formation of a thin, stable oxide layer on some metals

• Acts as a barrier between the bulk metal and body fluids, reducing direct contact and slowing electrochemical corrosion reactions.

• Approximately 1-10 nm thick

• Many passive metals can reform the layer, called repassivation

Metallic Alloys

Metals + one or more element

1. Ferrous: iron base metal

2. Non-ferrous: non-iron base metal (less corrsion)

316L Stainless Steel

Ferrous

Pros

• High strength and toughness

• Ductile: deforms before fracture

• Relatively low cost and widely available

Cons

• Lower corrosion resistance than titanium allows and CoCr alloys

• High stiffness contributes to stress shielding

• Potential for metal ion release

Applications: fracture fixation devices (plates, screws, nails), temporary and some permanent implants

Cobalt Chromium (CoCr)

Non-ferrous

Pros

• Excellent corrosion resistance: forms stable, protective oxide layer

• Outstanding wear resistance (ideal for articulating/joint surfaces)

• Very high strength and fatigue resistance

• Well suited for long-term, load-bearing implants

Cons

• very high stiffness: greatest stress shielding concern

• Potential for Co/Cr iron and particle release

• More difficult to machine and fabricate

• Higher cost

Applications: hip and knee bearings, stems, rods, wires

Ti-6Al-4V

Nonferrous

Pros

• Excellent corrosion resistance: forms stable, protective oxide layer

• Highly biocompatible

• Lightweight

• Lower elastic modulus: closer to bone stiffness —> less stress shielding

• Good bone bonding

Cons

• Poor wear resistance: not ideal for highly loaded articulating surfaces

• More expensive

• Can still experience corrosion at interfaces

Application: Widely used in permanent implants due to biocompatibility, corrosion resistance and low density (ie hip implants, knee implants, spinal fixations)

Nitinol (NiTi)

Pros

• Shape-memory and superelasticity: can be deformed at low temps and returns to original shape at body temp

• Enables minimally invasive devices

• Good fatigue resistance: withstands many loading cycles

• Tunable properties: adjusted by composition and processing

Cons

• Nickel ion release

• More expensive

• Lower strength

• Temperature sensitive

Applications: self expanding stents, clamps, clips

Shape Memory in NiTi

Martensite (low temp)

• atoms arranged in a less symmetric structure

• easier to deform

Austenite (high temp)

• atoms in more symmetric structure (cubic)

• More rigid/strong

• “remembered” shape

Biodegradable Metals: Mg Alloys

Implant dissolves as bone heals

Applications: tissue engineering, resorbable screws

Cons: safety concerns about metal-related toxicity, and corrosion could be too fast

Why polymers for biomaterials?

Most versatile material class

Pros

• Tunable properties: soft to rigid, flexible to brittle

• Biocompatibility

• Biodegradable options

• Lightweight: typically low density

• Easy to process: mold, extrude, 3D print, electrospin fibers

• Smart polymers respond to stimuli

Cons

• Weaker than metals

• Properties can change with time, temp, or environment

Number-Average Molecular Weight (M_n)

Given that polymer chains vary in length, this is used to describe average molecular weights

• weights each chain equally,

• more sensitive to shorter chains

influences mechanical properties, degradation rate, and biocompatibility

M = mass of each chain (g/mol)

N = number of chains

Weight-Average Molecular Weight (M_w)

Given that polymer chains vary in length, this is used to describe average molecular weights

• heavier chains count more

• more sensitive to longer chains

influences mechanical properties, degradation rate, and biocompatibility

M = mass of each chain (g/mol)

N = number of chains

Polydispersity Index

Indicates the width of the molecular weight distribution

PDI = 1 —> all chains same length (theoretically)

Most polymers have PDI > 1 (broader distribution)

MW and PDI Affects on Biomaterials

Higher Molecular Weight (longer chains)

• stronger, tougher

• slower degradation

• higher viscosity

• more stable in vivo

Polymer Chain Architecture

(from most to least chain mobility)

1. Linear

2. Branched

3. Crosslinked

4. Network

Polymer Class: Thermoplastic

Linear or Branched Chains

Soften/melt when heated

Can be reshaped

Stress Strain Behavior: Plastic

• yield + neck + draw

Ex: PE, PP, PEEK

Polymer Class: Elastomer

Lightly crosslinked chains

Recover when stretched

Soft and flexible

Stress Strain Behavior: Elastomeric

• huge strain, recoverable

Ex: silicone rubber

Polymer Class: Thermosets

Do NOT melt when heated; eventually degrades

Rigid and permanently set

Stress Strain Behavior: Brittle

• high E, low strain, fractures without yielding

Ex: epoxy

Polymer Crystallinity

Polymers are never 100% crystalline

Amorphous Polymers

• random coils (spaghetti)

• more flexible

• often transparent

Semi-Crystalline Polymers

• ordered regions + amorphous regions

• stronger, stiffer, denser

• often opaque

Ultra High Molecular Weight Polyethylene (UHMWPE)

Pros:

• low friction

• excellent wear resistance

• tough, but flexible

Cons:

• even small amounts of wear at bearing surfaces can cause inflammation and aseptic loosening

Polyether ether ketone