Biomaterials

Clinical Biomaterials
Table – Typical clinical biomaterials  Metals possess high strength, ductility and wear resistance. It’s shortcomings are little bioactivity, high elastic moduli, high density which hinders bony integration, corrosion and subsequent release of metal ions which can simulate an undesired allergic response. Polymers have numerous properties are available in many forms through wide array of processing techniques i.e. solids via injection molding or extrusion, fibers through fiber drawing, films by solvent casting and gels using sol-gel techniques Unfortunately, polymers are often lack strength and rigidity demanded by many orthopedic applications. Ceramics are well known for high biocompatibilities and bioactivities, high compression properties, high wear and corrosion resistance. However, ceramics are limited by low fracture toughness and low mechanical reliability. Composite implants can offer improved properties over any single phase biomaterial mentioned above.
 * Material || First Use || Applications || Ref ||
 * **Metals** ||  ||   ||   ||
 * Stainless steel ||  ||   ||   ||
 * Ti alloys ||  ||   ||   ||
 * Co-Cr alloys ||  ||   ||   ||
 * Ni-Ti alloys ||  ||   ||   ||
 * Gold ||  ||   ||   ||
 * Tantalum ||  ||   ||   ||
 * **Polymers** ||  ||   ||   ||
 * Polyethylene (PE) ||  ||   ||   ||
 * Polyurethane (PU) ||  || Casting, catheters ||   ||
 * Polytetrafluoroethylene (PTFE) ||  ||   ||   ||
 * Polyacetal (PA) ||  ||   ||   ||
 * Polymethymethacrylate (PMMA) ||  ||   ||   ||
 * Polyethylene terephthalate (PET) ||  ||   ||   ||
 * Silcone Rubber (SR) ||  || Joint replacements, catheters ||   ||
 * Polysulfone (PS) ||  ||   ||   ||
 * Polyetherehterketone (PEEK) ||  ||   ||   ||
 * Poly(lactic acid) (PLA) ||  ||   ||   ||
 * Poly(glycolic acid) (PGA) ||  ||   ||   ||
 * **Ceramics** ||  ||   ||   ||
 * Alumina ||  ||   ||   ||
 * Titania ||  ||   ||   ||
 * Zirconia ||  ||   ||   ||
 * Bioactive glasses ||  ||   ||   ||
 * Carbon ||  ||   ||   ||
 * Hydroxyapatite (HA) ||  ||   ||   ||
 * **Polymer Composites** ||  ||   ||   ||
 * HA/PE ||  ||   ||   ||
 * Silica/SR ||  ||   ||   ||
 * Carbon-fiber (CF)/expoxy ||  ||   ||   ||
 * CF/Ultra High Molecular Weight (UHMW)PE ||  ||   ||   ||
 * CF/PEEK ||  ||   ||   ||

 Medical Device Approval

 FDA approval….

 The FDA has recently mandated Johnson & Johnson, Medtronics and Royal Phillips more detailed explanations of safety and efficacy for devices produced. 

Bioactivity
A bioactive material elicits a favorable biological response in which the adjacent tissue forms a bond at the material interface. Bioactive surfaces are described by Hench as an 11 step interfacial //in vivo// reaction process. The first 5 steps are a series of chemical processes on the material surface that lead to the formation of a hydroxycarbonate apatite (HCA) layer. The final steps involve physisorption of biological moieties and that lead to conduction of cellular matter.6

Osteroproductive
Class A bioactive material exhibiting a ‘process whereby a biological surface is colonized by osteogenic stem cells free in the defect environment as result of surgical intervention.’3 Osteoproduction stimulates increased bone matrix formation using existing bone progenitor cells, unlike osteoinduction which generates new osteoprogentior cells. These materials have a very high bioactivity index greater than 8 (IB > 8).

Osteoconductive
Class B bioactivity material in which bone is allowed to grow on a material surface exhibiting an extracellular response.3

Osteoinductive
Requires the presence of BMPs and other proteins natural in bone. BMPs trigger cell division and thus and increasing rate of bone matrix formation by activating cell differentiation of bone progenitor cells into new osteogenic (bone) cells.

Hydroxapatite
Generally, bioceramics are demanded in the medical field because they can stimulate cellular responses at the molecular level. Hydroxyapatite is a type of bioceramic consisting of calcium phosphate with mineral compositions equivalent to bone. The chemical composition is Ca10(PO4)6(OH)2 where the stoiciometric ratio of Ca:P is 1.67. HA is available commercially in powder form from companies that claim achieving this ratio. HA can also be generated by chemical synthesis. A two-step mechanism follows bellow where dibasic calcium phosphate (DCP) degrades into an intermediate form in the presence of calcium carbonate. The intermediate products react with calcium carbonate to form HA evolving water and carbon dioxide.2 The reactions are seen below:

  HA sinters between the 1000°C - 1200°C. Above 1250°C, HA decomposes into tricalcium phosphate (TCP). A potentially insightful paper on the high temperature characteristics of HA can be found by Zhou. 3

<span style="font-family: 'Calibri','sans-serif'; font-size: 14.6667px;"> <span style="font-family: 'Calibri','sans-serif'; font-size: 14.6667px;"> Figure 1: Temperature scale representing microstructural changes for HA

<span style="font-family: 'Calibri','sans-serif'; font-size: 14.6667px;"> HA has bioactive properties lending a great portion of these properties to its structure. XRD analysis for naturally mined HA was evaluated in the 1960s.4 The space group is P63/m with a=b=9.432, c=6.881. Great emphasis was stressed on the convention that the OH bonds are parallel to the c-axis. Since some fluorine impurities are expected in every mined HA sample, it is expected that F causes some disorder to the crystal structure. Interestingly, F positioning within the CaII2+ triangles may lend to the stablility of fluorapatite structures. The stability of F presence has been shown to affect bone growth rates //in vivo//. fluoride containing apatite grew faster and in larger crystal sizes than fluoride-free synthetic HA.