Lecture 16 - Ceramics Flashcards
Bioceramic Chemistry
Interactions with surrounding tissue depend heavily on initial chemistry and structure (before processing)
Apatites
- Least soluble calcium phosphate
- Hydroxyapatite: Ca10(PO4)6(OH)2
Calcium Phosphates
- “Go-to” for biological interaction
- Stability dependent on temperature and presence of water (body temperature-dicalcium phosphate and hydroxyapatite, high temperature-tricalcium phosphate and tetracalcium phosphate)
- Unhydrated high temp-HA formed at fluid exposed surfaces
Bonding of Calcium Phosphates
- Bone bonds to HA layer
Osteoblasts at junction that deposit/mineralize tissues: - Produces bone matrix with thin layer of collagen
- Bond matures and interface shrinks
- Perfect epitaxial alignment of bone crystallites with apatite crystallites in implant
- Gradient in mechanical properties (prone to fractrue)
Ca/P Ratio
- Must hold chemistries
- Quality assurance (XRD) to ensure making correct ceramic
- Very sensitive
Alumina
- Inert, highly stable oxide
- Low fracture toughness and tensile strength (high compressive strength)
- High hardness (low wear)
Bioactive Glasses
- Bond to bone and/or soft tissue
- Chemistries: SiO2 (>60%), Na2O, CaO, P2O5
Ceramic Material Selection
Based on desired:
- Stability (stay in body with no degradation, remodeled, degradation)
- Tissue response
- Mechanical properties
Raw Materials
- Precipitated in lab (difficult to control)
- Purified
Slip casting
- Prepare slurry or slip (solid particles in liquid)
- Pour slip into mold (porous mold, capillary forces fix particles to wall)
- Condensation of slip onto mold wall (drain then fire)
- Low porosity
- Fewer defects and higher toughness
Rapid Prototyping
- Dropping binder onto bed of powder
- Viscosity of binder, will binder be absorbed
- Shrinks as densifies
- Binder droplet “splats”
4 Types of Implant-Tissue Response
Material Toxic
- Surrounding tissue dies
- Never select option
Material Nontoxic and Biologically Inactive
- Fibrous tissue forms
- Orthopedic implant (articulating surfaces)
Material Nontoxic and Biologically Active
- Interfacial bond forms
- Fully porous bone scaffold, ceramic coating
Material Nontoxic and Dissolves
- Surrounding tissue replaces it
- Scaffold to help replace portion of bone (degrade away when new bone laid down)
Type I
- Bioinert/nearly inert
- No chemical or biological bond at tissue-material interface
- Implant cemented or press-fit into surrounding tissue (morphological fixation)
- Relative movement
- Development of fibrous capsule around implant
- Orthopedic/dental implants
- High resistance to compressive deformation (wear), high hardness
Type II
- Nearly inert microporous
- Ingrowth of tissue into pores on surface or throughout implant
- Increases resistance to motion, decreases fibrous capsule
- Pores provide blood supply (keeps mineralized tissue alive) and space to deposit mineralized tissue
- Decreased mechanical properties with addition of pores
- Often used as coatings
Type IV
- Resorbable
- Degrade gradually over time (time-scales for wound healing differ)
- Balance degradation with mineralization and deposition
Complications: - Maintenance of strength and stability during degradation and replacement period
- Balancing degradation and repair rate (must be near equal)
- Material must be made of metabolically acceptable material (tricalcium phosphate)
