Bioceramics

=Bioceramics*=

The replacement of body parts with the synthetic materials began n the 1960s offsetting removal of diseased or damaged tissues. Worldwide, more than 4-5 million devices are implanted each year mitigating pain of millions of patients. A new generation of materials has been proposed in recent decades aimed to restore damaged tissues by triggering quiescent genes designed for regeneration. toc

Types of Ceramics
Biomaterials have progressed throughout the years as three general classes: bioinert, bioresorbable and bioactive. Bioinert and bioactive materials are considered 1st and 2nd generation materials. Currently the third generation comprises of two alternatives: transplants and bioregenerative materials. There are four types of tissue responses related to bioceramic implantation; three of which are classified by name.
 * ~ Type ||~ Class ||~ Tissue-Implant Response ||~ Examples ||
 * = I ||= - || Toxic; causes tissue death ||= - ||
 * = II ||= Bioinert || non-toxic; forms a non-adherent capsule around the implant || Alumina, Zirconia ||
 * = III ||= Bioresorbable || non-toxic; implant resorbs and is replaced by tissue. || Tricalcium Phosphate ||
 * = IV ||= Bioactive || non-toxic; forms a direct bond with tissue || hydroxyapatite, bioglass, A/W glass ceramic ||

Types of Bioceramics
There are 4 types of bioceramics:


 * || Category || Examples ||
 * 1 || hydroxyapatite (HA) || Dense, Si-sbstituted HA ||
 * 2 || bioactive glass || 45S5 Bioglass, 70S30C sol-gel derived foams ||
 * 3 || bioactive glass ceramics || A/W glass ceramics ||
 * 4 || bioactive ceramic composites || HAPEX, Bioglas/polysulfone, Bioglass/PGLA, BG inorgnaic/organic hybrids ||

Bioceramics microstructure range from glasses (bioactive glasses), polycrystalline (HA, alumina), glass-ceramics (AW) and biocomposites of polymer and ceramics (HAPEX). A sub-class of each are porous biocerramics which enable tissue ingrowth to the implant.

A/W Bioactive Glass Ceramic
A/W glass ceramic is a bioactive composite of calium and silica in both an apatibte and wollastonite glassy phase. The strength, toughness and elastic modulus is higher than bone and has been used for bone defect replacement in load bearing circumstances. Thousands of cases over more than 30 years have proven clinical success of this implant with dramatic performance over HA implants. In-vitro studies show HA forming an HCA layer after 30 days, whereas A/W forms a layer after 7 seven. By comparison, Bioglass forms an HCA layer within 24 hours, all indicating the improvement in bioactivity from osteocoductive HA to osteoinductive A/W and Bioglass.

Clinical Requirements
Success of bioceramics applied to replacing muscoskeletal parts of the body simultaneously requires a mechanical and a biological property:
 * 1) Mechanical: A match in mechanical properties
 * 2) Biological: A stable interface between implant and periprosthetic tissue.

Failure to form a stable interface between implant and tissue may cause micromotions and cause the formation of fibrous capsules around the implant over time. Mircomotions lead to loosening which subsequent cause failures in either the tissue or implant. Porous materials prevent loosening by enabling tissue in-growth to the implant. This is termed biological fixation and is typically prefered clinically as opposed to mechanical fixation where parts are fixed and held in place using cements and mechanical forces.

Types of Fixation*

Scaffolding Requirements*
Composite scaffold for bone regeneration. Design involves connecting atomic and macroscopic structures.

__Qualitative Criteria__
 * # || Criteria ||
 * I || Resorbable ||
 * II || Mechanical Stability ||
 * III || Porosity ||
 * IV ||  ||
 * V ||  ||

__Quantitative Criteria__ I - Resorbability: match production of bone II - Mechanical Stability: K_IC > 3 MPa*m^.5 III - Porosity: 100um pores

Resorption*
Suggests the use of synthetics or natural polymers.
 * Polyesters: However, hydrolyze in water easily.
 * Collagen
 * Chitosan

Composites*
There are two types of biocomposites: 1) conventional and 2) nanocomposites

Conventional: bioactive particles in polymer foams Nanocomposite: sol-gel derived, inorganic-organic nanocomposites, inorganic-organic hybrids, star-gels,

Bioactivity Tests*

 * Simulated Body Fluid
 * Bioactivty Index

Ceramic Processing Methods*

 * Melt-derived and sol-gel based bioglass.
 * Sol-gel
 * Hybrids

Importance of Silicon Trace Elements

 * Silicon is important in bone growth.
 * Bioglass upregulates proangiogenic growth factors.
 * The largest concentration of silicon is found in the aorta. Critical silica linkages to provide structural integrity of heart tissue, but the regeneration is not able to be replenished past puberty. [L23 - Clinical Applications (Hench)]

Bio-metals*
In contrast to bioceramic materials are bio-metals. These include metals that show little inflammatory responses in virtro, such as titanium alloys, stainless steel and Cr-Co.


 * Problem:non-adherent, acellular scar tissue formed -> micromotion -> loosening -> failure


 * Long Bone Repair (immobiliation procedure): callus form -> mineralization -> resorption and production, external or internal fixation


 * Spongy bone at the ends of long bones

Modes of Failure*
-Aseptic loosening from osteolysis via polymer wear debris -Pistoning -Dislocation -Stress sheilding

Primary composite failure mode: interfacial attack between phases

Bioactive Glasses: Synthetic Particles Treating Human Cells Outline
Biomaterial Generations [\1st, 2nd, 3rd Generatation biomaterials] [\Discovery of 2nd Gen in 1969 led to bioactive class]

Composition [\most from silica-phosphate...] [\different compositions give different tissue responses] [\bioactive glass and bioactive glass ceramics] [\table of bioactive ceramics and compositions] [\osteo prod, con., induction] [\classes of bioactivity]

Kinetics and Mechanism of Bioactive Glasses [\stages of bioactivity; picture] [\stable release of Ca/P ionic products stimulate gene activation]

Human-side [\Gene activation, cell cycle stimulation] [\picture]

Testing Methods [\SBF Tests] [\Bioraman]

Applications [\forms: monolith, particles, sol-gel derived foams] [\clinical apps: ear-implants, mandible (?) and dental restoration, scaffolds]

[\YAS microspheres -> hepatocellular carcinoma (HCC).

[\non load bearing] [\led to biocomposites]