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Summary Biomaterials
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-Biomat def:“A biomaterial is a nonviable material used in a medical device, intended to interact withbiological systems” (European Society of Biomaterials-ESB, 1987) ,“A biomaterial is a material intended to
interface with biological systems to evaluate, treat, augment or replace any tissue, organ, or function of the
body” (ESB, 1992) Example: porous titanium dental implant
-Biomaterials science – the study of materials and their interactions with living systems
-Biomaterials engineering – the application of the principles of biomaterials science and its foundation
disciplines to find solutions for technical problems of human health
“Biocompatibility - the ability of a biomaterial to perform with an appropriate host response in a specific
application” (ESB, 1987)
Biocompatibility - functionality + safety + application
-The introduction of a foreign material will determine a host response with local, systemic, remote effects
-Medical devices - instrument, apparatus, machine including any component or accessory that is intended for
use in the diagnosis of disease, or in the mitigation, treatment or prevention of disease in humans
-FDA: Ensures the safety and efficacy of: medical devices and radiation emitting products
(Centre for device and radiological health-CDRH)
FDA-CDRH: evaluates devices for marketing, monitors devices after approval, acts against firms when the law
is violated, performs research, develop standard methods, educates professionals and consumers on safe use of
devices
Medical device classification system (1976) – use, invasiveness and risk to the user
• Class I (ca 40%)
minimum invasiveness, well-established safety and effectiveness (e.g., elastic bandages)
• Class II (ca 50%)
higher degree of invasiveness but not long durations (e.g., contact lenses, hearing aids)
• Class III (< 10%)
highest degree of invasiveness, life sustaining, high risk of illness or injury (e.g., heart valves, cardiac
pacemakers, orthopedic implants)
Ethical and legal aspects in biomaterials:animal tests, clinical trials, industrial support for research -
regulation
Bio osteo-conductive: induce making new bone
Bone tissue: Cortical (compact) bone, porous (trabecular) bone. Cortical bone is anisotropic and viscoelastic.
Structural components of bone:collagen matrix – protein with a high tensile strength and viscoelastic properties
• hydroxyapatite crystals – calcium phosphate based compound with properties similar to ceramics
• hierarchical structure (nano-, micro- & macro-scale)
Bone cells:
• osteoblasts – bone forming cells
• osteoclasts – bone removing cells
• osteocytes – receptors of mechanical and chemical changes in the
environment, communication with osteoblasts and osteoclasts to elicit the required cellular response
Cartilage:
Physiological role:
• low-friction bearing surface of the articular joints (hip, knee, shoulders)
Components: extracellular matrix, proteoglycans (PGs) ,collagen, cells: chondrocytes, water (60 – 80 wt%)
-articular cartilage: avascular tissue (limited ability to heal once damaged) , loaded mostly in compression
(water is released and partially resorbed during the cyclic loading), low friction
-Osteoarthritis: Degradation of the articular cartilage due to aging, injury, obesity, heredity
-Polymer structure: monomers linked together by covalent bonds (can be linear, crosslinked or branched)
Attractive features of polymer:
greatest versatility in chemistry and processing, lighter than metals, used as composite materials with ceramics
Concerns of polymer:
, long-term chemical biocompatibility, wear debris
-cartilage replacement:
• UHMWPE - articulating bearing surface in TJR
• silicone rubber - hand joints
• implant fixation (PMMA)
-fixation parts for bone fracture: polymeric materials for fracture fixation parts (plates and screws)
• controlled biodegradability (avoid second surgery) - e.g. PLGA
Polymeric biomat: polymethyl methacrylate (PMMA), ultrahigh molecular weight polyethylene (UHMWPE)
PMMA(Plexiglass,Lucite): bioinert polymer, linear chain, amorphous structure, hard. Bone filler, immediate
fixation of a total joint implant within the medullary canal, minimizes the need for perfect fit between bone and
the implant, can be loaded with antibacterial agents. Side effects: produces debris through fatigue and biological
processes and cause osteolysis (bone loss), third-body wear of the acetabular cup and/or femoral head. extra
interface (bone-cement-implant) - reduce the life span of the implant by loosening
Alternative: modification of implant surface to promote osseointegration.Osseointegration creates secondary
stability.
UHMWPE: bioinert, linear chain, semicrystalline, strength, ductility, wear resistance (Increased regions of
crystallinity - increased strength). Used as bearing material in TJRs such as hip, knee, shoulder, wrist, finger,
toe joints . Used as a liner in acetabular cups.Tibial spacer for a total knee replicement. It has sterilization and
wear debris issue: Abrasive wear - wear debris and implant failure
Wear debris (osteolysis):
bone loss mediated by cellular processes in response to wear debris released around the implants (‘small particle
disease’)
- Compounds of metallic and non-metallic elements – ionic bonds
-Ceramic biomat: Inert: Al2O3 , ZrO2 Bioactive: Ca-based, Bioglass
Advantage: dense and hard materials (scratch resistant), ability to be polished to an ultra smooth finish, wear
resistance and low friction,inert/bioactive
Limitations: brittle, low tensile and bending strength, low fracture strength, difficult processing control
Alumina (Al2O3): total hip replacements (THR) – head, acetabular cup
Zirconia (ZrO2): alternative material to alumina, lower Young’s modulus, lower wear rates of ZrO2-
UHMWPE vs Al2O3-UHMWPE (grain size, roughness, residual compressive stresses), increased fracture
toughness (stabilized with Y2O3)
Composite of alümina zirconia is better for stress bearing.
Ca based Ceramics: Ca3(PO4)2 – tricalcium phosphates (TCP) , Ca10(PO4)6(OH)2 – hydroxyapatite (HA,
OHAp). Applications: synthetic bone substitutes (bulk), coatings on metallic devices (HA on Ti femoral stems)
– improved fixation, composites with bioglasses, polymers
Bioglasses: mineral-rich structures designed to bond to the bone
Metallic degredation: corrosion, wear, combination of the two. Corrosion effects: reduces structural integrity
of the implants, potential toxic corrosion products
Factors influencing corrosion of metallic biomaterials: composition & microstructure of the metallic
implants (variations within/between the implants), surface variables (surface microstructure, oxide composition
and structure), geometric variables (taper crevices in modular hip prostheses), mechanical variables (stress
levels, loading conditions, relative motion), handling in manufacture, delivery and insertion, solution variables
(pH, proteins, enzymes, pathology, cells, bacteria)
-corrosion + wear = particulate debris (this can cause inflammation, aseptic loosening, metallosis)
-metallic biomat. Has high thoughness and usually high strength to weight ratio but in ceramics this ratio
is higher
-Metallic biomat: Stainless steels (SS),Cobalt based alloys, Titanium (Ti) & its alloys, Porous tantalum (Ta),
Zirconium-Niobium (Zr-Nb), Magnesium, Iron, zinc (last three degredable metals under development)
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