Definition of Biomaterial Classes of Materials Crystal
Structure Mechanical Behavior Wear Resistance Calcium-phosphate
Bioactive Glasses 2
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A biomaterial is a nonviable material used in medical device,
so its intended to interact with a biological systems. Biomaterials
are manufactured substitutes for natural tissues. They are used in
implants or catheters.artificial organs. drug delivery. wound
dressing. 4
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Biomaterials can be conveniently grouped into three classes:
metals, polymers, and ceramics. Composites, representing another
group of materials, consist of combinations of two or more metals,
polymers, or ceramics. 5
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metallic bond As the electrons can move easily in metals,
making metals easily deformable. The independent electrons in the
metallic bonds can quickly transfer electric charge and thermal
energy. 6
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it is often desirable to coat metallic implants with a
bioactive ceramic film in order to improve implant fixation the
difference in the thermal expansion coefficient between metals and
ceramics results in interfacial shear stresses that can create
microcracks at the interface during cooling subsequent to
plasma-spraying a calcium phosphate coating onto a metallic
implant. http://www.gordonengland.co.uk/img/ps1.gif 7
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As the mechanical properties (and also the chemical and
physical properties) of metals can be improved by alloying, most
metals used in orthopedic surgery are alloyed. Obvious examples are
the alloys based either on titanium or on cobalt. 8
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Ceramics are refractory polycrystalline compounds Usually
inorganic Highly inert Hard and brittle High compressive strength
Generally good electric and thermal insulators Good aesthetic
appearance Applications: orthopaedic implants dental applications 9
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10 Synthetic HABone HA
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covalent bond These large molecules contain many repeating
units, from which comes the word polymers. The polymeric molecular
arrangements can be linear, branched, or cross-linked
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catheters artificial trachea
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Composite materials are solids that contain two or more
distinct constituent materials or phases Most composite
biomaterials have been developed to enhance mechanical and
biocompatibility behavior. The shape of the phases in a composite
material is classified into three categories. 14 platelet fiber
particle
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Some applications of composites in biomaterial applications
are: (1) dental filling composites (2) reinforced methyl
methacrylate bone cement (3) orthopedic implants with porous
surfaces. 15 http://seis.irsm.cas.cz/images/IMG/Bull/Fig_6.jpg
x, y, z x y z a b c r a b c (lattice parameters) 27
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All metals, most ceramics, and some polymers crystallize when
they solidify. A crystalline material is characterized by long-
range order and an infinitely repeating unit cell of atoms/ions.
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TABLE 11-2. Some Materials and Their Representative Equilibrium
Crystal Structure at Body Temperature MaterialStructure
Cobalt-chromium alloyFCC Stainless steel AISI 316LFCC Titanium
(Ti)HCP Tantalum (Ta)BCC Niobium (Nb)BCC Gold (Au)FCC Alumina (Al 2
O 3 )HCP Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ]HCP 32
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Hydroxyapatite (HA), a bone bioactive ceramic. HA structure is
considerably complex, as a result of which, displacement of atoms
within the lattice is difficult. Thus the structure is resistant to
deformation and, when overloaded, fractures rather than deforming
permanently. 33
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As the extent of polymerization increases and the molecular
chains become longer, the relative mobility of the chains in the
structure decreases. As a result, alignment of the chains and
formation of long-range order is difficult. 35
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Factors affecting the strength of polymers further include
chemical composition, side groups, cross-linking, copolymerization,
and blending. 38
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Mechanical properties including elasticity and strength are
important properties to consider in the selection of a material for
a specific implant design. 39
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TABLE 11-3. Typical Values of Tensile Strength for Various
Materials at Room Temperature MaterialTensile Strength (MPa)
Diamond1.05 10 6 Kevlar4,000 High-strength carbon fiber4,500
High-tensile steel2,000 Superalloy1,300 Spider webs (drag
line)1,000 Ti-6Al-4V860 CoCr alloy (F75)655 Aluminum570 Titanium
(grade 4)550 316L SS (F745, annealed)485 (Cold forged)1,351 Bone200
Nylon100 Rubber100 41
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TABLE 11-5. Elastic Properties of Some Typical Materials
MaterialsDirectionality Properties Modulus of Elasticity (MPa)
Cortical boneAnisotropicLongitudinal axis, 17,000 Trabecular
boneAnisotropic Longitudinal axis of femur intertrochanteric 316
293 High-density polyethyleneIsotropic410-1,240
PMMAIsotropic3,000-10,000 Stainless steel AISI 316LIsotropic200,000
Cobalt-chromium alloyIsotropic220,000 TitaniumIsotropic107,000
Ti-6A1-4VIsotropic110,000 Carbon fiber-reinforced graphite
fibersAnisotropicParallel to unidirectional 140,000
AluminaIsotropic Single crystal, 362,700 Polycrystal, 408,900
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TABLE 11-4. Fatigue Properties of Implant Metals MaterialASTM
DesignationCondition Fatigue Endurance Limit (at 10 7 Cycles, R =
-1) (MPa) Stainless steel F745Annealed221-280 F55, F56, F138,
F139Annealed241-276 30% Cold worked310-448 Cold forged820 Co-Cr
alloysF75As-cast/annealed207-310 P/M HIP a 725-950 F799Hot
forged600-896 F90AnnealedNot available 44% Cold worked586 F562Hot
forged500 Cold worked, aged 689-793 (axial tension R = 0.05, 30 Hz)
Ti alloysF67 30% Cold-worked grade 300 F136Forged annealed620
Forged, heat treated620-689 a P/M HIP, Powder metallurgy produced,
hot-isostatically pressed. 52
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Ultrahigh-molecular-weight Polyethylene (UH-MWPE) (MW above 2
10 6 g/mol) The success of UHM-WPE is due to its favorable
properties, including abrasion resistance, impact strength, low
coefficient of friction, chemical inertness, and resistance to
stress cracking. It has long been used for total hip prostheses and
knee prostheses.
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In the majority of contemporary total joint replacements, a
metallic component articulates against UHMWPE. As a patient may be
expected to take an average of 1,200 steps per day, the joint
replacement is expected to withstand millions of loading cycles
during its service lifetime.
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In total hip replacements, where the articulating geometries
consist of conforming spherical surfaces, the wear occurs at a
microscopic length scale (m or less). Wear resistance has been
related to its resistance to multidirectional stresses.
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In total knee replacements, where the articulating surfaces
consist of nonconforming cylindrical, toroidal, or flat surfaces,
the wear process includes various surface damage mechanisms ranging
from pitting and delamination to burnishing and adhesive wear.
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Adhesive wear is a process in which surface asperities of the
polymer adhere to the metal surface and subsequently are torn off.
As a result, either a polymer film is formed on the metal surface
or polymer particles are released and entrapped in the joint. Such
particles as well as remaining cement fragments can generate
abrasive wear, a second mode of wear.
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Abrasive wear can also be generated by asperities on the
gliding metal partner. A third mechanism is fatigue wear; as a
result of creep or plastic flow, folds or cracks are formed that
cause small polymer particles to break off.
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It is generally accepted that particulate debris generated by
mechanical wear of prosthetic components stimulates the generation
of a pseudosynovial membrane at the interface between implant and
bone and the infiltration of fibrocytes and macrophages. In the
presence of debris, these cells release various cytokines and
mediators (such as IL-1 , TNF , collagenase, and prostaglandin E 2
)
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These cytokines have been shown to be involved in bone
resorption by activating osteoclasts. A recent study showed a
positive correlation between cytokine concentration in the
loosening membrane and the degree of underlying osteolysis. Many
reports have investigated the failed total joint arthroplasty and
demonstrated that phagocytosis and cell mortality increase with
particle size and concentration.
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The mechanism that results in this cell death remains unknown,
although a potential role for apoptosis in the pathogenesis of wear
debris-associated osteolysis has recently been suggested. Aseptic
loosening, which is the single most common cause for long-term
failure of TJA, is associated with periprosthetic osteolysis with
the incidence of up to 25% of implant recipients.
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Multiple factors can affect polyethylene wear and the
production of wear debris in vivo after joint arthroplasty. Such
factors include the roughness and material of the femoral head, the
method of polyethylene sterilization, and the mechanical properties
of the polyethylene itself.
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Recent laboratory studies have confirmed that the wear
resistance of UHMWPE can be significantly increased when applying
additives or high-dose irradiation. Many studies reporting
extremely low quantities of wear, typically as low as 2.0 mm 3 per
million cycles for highly cross-linked polyethylenes.
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High-density, high-purity alumina is used in load-bearing hip
prostheses because of its outstanding wear resistance and excellent
corrosion resistance. It has a high Young's modulus and a hardness
second only to that of diamond. These properties have made the
alumina-on- alumina couple for femoral heads and acetabular cups a
materials combination of considerable importance.
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Most alumina devices are very fine-grained polycrystalline -Al
2 O 3 produced by pressing and sintering at high temperature (1600C
to 1700C). A very small amount of MgO (