Ceramics and Other Materials

lecture 8

Ceramics, Glasses, and Glass-Ceramics

an inorganic, non-metallic solid prepared from powdered materials and fabricated into products through the application of A LOT of heat (primarily ionic/covalent bonds)

Category of Ceramics

cystallline (bio-resorbable or inert)

Category of Glass

amorphous (no crystalline structures)

Category of Glass-ceramics

starts as glass and then ends up as a polycrystalline ceramic (possibly w/ a residue glassy matrix)

General Properties of Ceramics, Glasses, and Glass-Ceramics

ionic bonds - difficult to shear with very low ductility, high compressive strength, and low tensile strength

low thermal/electrical conductivity (bc ion bonding so no conduction)

refractory (once formed, hard to go back) and high Tm (melting temp.)

high hardness → dental materials

aesthically pleasing appearance

Ionic Bonds - Ceramics, Glasses, and Glass-Ceramics

difficult to shear with very low ductility, high compressive strength, and low tensile strength

Dislocation Glide in Metal vs Ceramics

ceramics - must occur over 2 atomic positions due to electroneutrality requirement (need opposite charges) → less slip but more brittle

metal - one bond at a time (plastic deformation)

Dislocation Glide in Metal vs Ceramics Stress/Strain Curves

Nearly Inert Bioceramics

alumina (Al2O3) which makes sapphires and ruby

Characteristics of Nearly Inert Bioceramics

single crystal or polycrystalline

excellent corrosion resistance and biocompatability (very thin fibrous layer)

high strength → structural support (bone plates/screws)

small grain size with narrow distribution → high hardness and low surface roughness → low friction and wear → joint replacement

Nearly Inert Bioceramics - Small Grain Size/Narrow Distribution

causes high hardness and low surface roughness → low friction and wear so used for joint replacements

reduce grain size = higher strength = smoother (low friction)

increase grain size = lower mech. stremgth = increase roughness/decrease smoothness

Joint Replacement - Ceramics

alumina socket/ball; coefficient of friction decreases w/ time and approaches value of a normal joint (wear is 10 times slower than metal-PE surfaces)

plastic-metal: metal is much harder and grinds against the polymer and will start to degrade it

alumina-alumina: cool-hard surfaces and both smoother so less friction and therefore longer wear

Resorption or Biodegradation in Ceramics

caused by physiological dissolution (depending on solubility of material and local pH bc ionic bonds), physical disintegration into small particles as a result of preferential chemical attack of grain boundaries, and biological factors (phagocytosis)

What does the physiological dissolution of ceramics depend on?

the solubility of the material and the local pH because of ionic bonds

Biological Factors of Resorption or Biodegradation of Ceramics

phagocytosis (engulfs debris) which aslo causes a decrease in local pH

Calcium Phosphate

mineral phase of bone/teeth that is mainly calcium and phosphate ions (already in body so high biocompatilability)

solubility and hydrolysis decrease w/ increasing Ca/P ratio (wanr >= 1 bc if under one than not suitable for bio implantation)

What decreases with increasing Ca/P ratio?

the solubility and hydrolysis of calcium phosphate

Calcium Phosphate Picture

Hydroxyapatite (HA)

Ca₁₀(PO₄)₆(OH)₂

hexagonal rhombic crystal that can be converted from coral/animal bone (naturally) or synthesized and has an elastic modulus of 40-117 GPa

Hard Tissue - Hydroxyapatite (HA)

hard tissue contains 60% HA (mostly carbonate HA), 25% water, and 15% organic materials so HA can be converted from coral or animal bone

Hydroxyapatite (HA) Manufactoring (Synthesize)

Ca(NO₃)₂ + NaH₂PO₄ → precipitate of HA → drying and filtering → furnace 1150 degrees C → grounding (tiny pieces/powder) → sieving → press in a die → sintering (at high temp.)

Hydroxyapatite (HA) Elastic Modulus

40-117 GPA

enamal - 74 GPa

dentin - 21 GPa

compact bone - 12-18 GPA

Hydroxyapatite (HA) - Hexagonal Rhombic Crystal

sub OH- with F- to increase chemical stability

defects/impurities can be characterized by X-ray diffraction (crystallinephase), FTIR (cehm. groups)

Why do some toothpastes have flouride in them?

because in a hexagonal rhombic crystal (ceramic = crystalline) OH- gets subs by F- to increase chemical stability

Hydroxyapatite (HA) Crystal Structure Picture

2 OH (hydroxyl) groups per unit cell because inc roners; very complex

Hydroxyapatite (HA) X-ray Diffraction Picture

shows how crystalline something is

Typical FT-IR Spectrum of HA Picture

used instead of X-ray diffraction (???); uses absorbance to define chemical groups in the sample

Factors that Influence the Degradation Rate of Calcium Phosphate

Ca P degradation rate increases as:

chem. susceptibility to dissolution - increases

surface area - increases

crystallinity - decreases

crystal perfection - dcereases

grain size - decreases

F- substitution - decreases

Clincial Application Advantages of Calcium Phosphate

it is bioactive and osetoconductive (pormotes bone integration → linear component of bone)

Calcium Phosphate Bioactive Bonding Mechanism

differentiated osteoblasts (guided by these) produce a cellular bone matrix of 3-5 um layer at surface → 0.05-0.2 um → normal bone attached through thin epitaxial bonding layer to the bulk implant; has a dense and porous forms and also can be coatings or fillers

Dense Form of Calcium Phosphate

small unloadd implants such as in the middle of an ear implant

Porous Form of Calcium Phosphate

to promote bone regeneration → granuales for filling bony defects in orthopedic and dental surgery

Coatings - Calcium Phosphate

coat outside of metal to promote bone integration → with reinforcing metal posts as in dental materials

Specific Composition of Bioactive Glasses and Glass-ceramics

highly reactive surface in aqueous medium which forms a biologically active carbonated HA layer that provides bonding interface for tissues; interfacial strength >= bulk strength of both implant and tissue; ex. SiO₂ <= 60%, high Na₂O and CaO and CaO/P₂O₅ (all glass has similar components just diff. %’s)

Bioactive Glasses/Glass-Ceramics - Surface

forms a biologically active carbonated HA layer that provides bonding interface w/ tissue

interfacial strength >= bulk strength of both implant and tissue

once binds has a higher interfacial strength than the bulk strength

Bioactive Glasses and Glass-ceramics Clincial Applications

45S5, SiO₂ 45%, Ca/P - 5:1 (bioactive glass)

Ceravital (middle ear surgery to replace ossicles damaged by chronic infection)

Periodontal defect repair (maintenance of alveolar ridge for denture wearers)

Toothpaste ingredient against sensitivity

Compositional Dependence (w%) of Bone and Soft Tissue Bonding Picture

Index of Bioactivity (I_B)

the level of bioactivity of a specific material can be related to the time fro more than 50% of the interface to be bonded (t0.5bb); higher this = higher bioactivity

Sequence of Interfacial Reactions Involved in Forming a Bond Between Tissue and Bioactive Glass Picture

Table of Attachment Types Picture

Dense, Nonpourous, Nearly Inert Attachment Type

ex. Al₂O₃ single crystal or polycrystalline

attachment to host tissue: bone growth into surface irregularities by cementing device into tissues or by press fitting into a defect (morphological fixation), movement → increases thickness of interfacial layer

Porous and Inert Attachment Type

ex. Polycrystalline Al₂O₃, Hydroxyapatite (HA) (coated porous material)

attachment to host tissue: ingrowth occurs that mechanically attaches the bone to material (biological fixation)

limitations: 20/100 um pore size needed for ingrwoth tissue to remain viable/healthy → more focus fror corrosion and strength degradationBio

Dense, Nonporous, and Surface-Reactive Attachment Type

ex. bioactive glass, glass-ceramics, and HA

attachment to host tissue: elicit biological response at the interface causes the formation of chemical bonding with the bone (bioactive fixation)

Dense and Resorbable Attachment Type

ex. calcimum sulphate and several types of calcium phosphates

attachment to host tissue: slowly replaced by bone

complications - 1. maintenance of strength and stability, 2. matching resorption rates to repair rate, and 3. requires metabolically acceptable breakdown product

Fixation Limitations

if not tight enough, there wil be movement and will cause fibrosis which is not desirable

Morphological Fixation

when bone growth into surface irregularities by cementing the device into the tissues or by press fitting into a defect

Biological Fixation

when ingrowth occurs that mechanically attaches the bone to the material

when pores are large enough, they grow into them but if they are too small, no cells so: pores too big = lower mechanical strength

Bioactive Fixation

when elicit biological response at the interface causes the formation of chemical bonding with the bone

forming chemical bond with bone tissue

Carbon Materials

crystalline: diamond, graphite, and fullerene

Fullerene

carbon structure; take a layer of graphite and wrap it in a ball or as a cylindar → electrically conductive bc sp2 C’s

Graphene

singel sheet of graphite = increased conductivity;

Quasicrystalline Carbon Materials

glassy carbon - extremely inert and used as electrodes in electrochem or prosthetics

pyrolytic carbon - as implant surface coating, has higher mechanical strength than glassy/graphite carbon, excellent tissue/blood compatibility, and used in heart valves and finger joint implants

Glassy Carbon

a quasicrystalline carbon material; extremely inert and used as electrodes in electrochem or prosthetics; network of sp2 C’s in a not very well defined structure and bc of sp2 = very electrically conductive

Pyrolytic Carbon

a quasicrystalline carbon material; as implant surface coating, has higher mechanical strength than glassy/graphite carbon, excellent tissue/blood compatibility, and used in heart valves and finger joint implants

Composites

consist of 2 or more chemically distinct parts in the macro-scale and have a distinct interface separating them; properties depend on the properties of each constituent, shape of the heterogeneities, volume fraction, and interface; ex. fiber and particulate composites; real life ex. bone and tendon/vessel

Fiber or Particulate Composites

usually consist of 1 or mre discontinous phases (usually stronger → reinforcing material) that is embedded into a continous phase (matrix)

Composite Properties

depends on properties of each constituent, shape of the heterogeneities, volume fraction, and interface (how tightly bound here they are)

HAPEX

composite of hydroxyapatite (to promote osteoblast attachment since it has teh right mechanical strength for bone implants but not liked my body very much or very osteoblastconductive???) in a polyethylene matrix; stiffnes similar to cortical bone; high toughness; bone bonding in vivo; orbital implant and middle ear implants