BIOMATERIALS sensu stricto

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BIOMATERIALS sensu stricto
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• Titanium and titanium based alloys
• bioceramics
• Alumina,
• Zirconia
• Carbon
• Hydroxyapatite
• glasses (vetroceramics, bioglasses)

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Titanium and Titanium-based alloys
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Titanium a light (AW 47,9), non-magnetic
metal. The ninth most common element in the
earth crust (after oxygen, silicon, aluminum,
iron, magnesium, calcium, sodium, potassium),
constituting the 0,6 and also the fourth most
abundant structural metal after aluminum, iron
and magnesium. As the ionic radius of titanium
is close to that of other common elements (Al3,
Fe3, Mg2), most minerals, stones and soil
contain small amounts of titanium, although
proper titanium minerals, containing more than 1
of titanium are only found in few places.
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Mineral sources of titanium are rutile (TiO2)
and ilmenite (FeTiO3), more common.
Rutile contains 90-97 of TiO2,
together with impurities of silicon, iron,
vanadium, niobium and tantalium.
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Two treatments of the minerals. One, the Kroll
process, uses chlorine to yield TiCl4, then
reduced with metallic Na and Mg to spongy
metallic Ti. TiCl4 4 Na ? Ti 4 NaCl The
other in the electrochemical reduction of TiO2 in
a CaCl2 bath. By successive melting processes
metallic titanium is purified (at high
temperature Ti combines with O2, N2, H2, C, Fe,
therefore its melting is delicate). Titanium in
standard conditions is a silvery metal with low
density (4,54 g/cm3), high mechanical strength
(660 MPa) and marked resistance to corrosion.
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Spongy metallic Titanium produced through the
Kroll process
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Electrolytic Process of reduction of Titanium
from Rutile
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• Most used alloys in the biomedical field are
• ASTM F67 (nearly-pure Titanium 98,9 - 99,6 )
  used in dentistry or as coating (poor mechanical
  properties)
• ASTM F136 (Ti-6Al-4V), used in orthopedics
  (where high loads are applied), because of good
  mechanical properties, resistance to corrosion,
  biocompatibility, and elastic modulus closer to
  that of the bone than other alloys.

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ASTM F67 As commercially pure Ti actually
contains impurities, ASTM (American Society for
Testing and Materials) uses a ranking into 4
groups (grade 1, 2, 3 and 4), for each of which
the highest tolerable content in N, C, O and Fe,
as well as the minimum values of some mechanical
properties.
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The four grades of commercially pure (CP) Ti
differ in oxygen content and mechanical
properties Grade 1 CP Ti with a low oxygen
content low tensile strength , high
ductility.   Grade 2 CP Ti with a higher oxygen
content the most used species with best
compromise between the various mechanical
requiements.   Grado 3 CP Ti used for pressure
vessels.   Grado 4 Ti commercially pure for
aeronautics
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Properties of pure Ti and related phase diagrams
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Densità a 25C   4,5 g/cm3 (alta purezza) 4,51 g/cm3 (purezza commerciale)
coefficiente di espansione lineare a 25 8,5?10-6 K-1
calore latente di fusione 20,9 kJmol-1
conduttività termica a 20-25C 0,221Wcm-1K-1 (alta purezza) 0,226-0,201Wcm-1K-1(purezza com)
modulo di elasticità a 25C 100-110 GPa
modulo di rigidezza a 25C 411,8-431,5 GPa
Bulk modulus a 25C 122,6 GPa
resistività elettrica a 25C a 600C 42 ??cm 140-150 ??cm
suscettibilità magnetica di ?-Ti a 25C 3,2?10-6cm3/g
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Pure Ti has a phase transition di fase at 882C
from Ti-a (HCP) to Ti-ß (BCC), stable up to the
melting point (1668 C). Alloying with other
elements stabilizes either Ti-a or Ti-ß.
Aluminum stabilizes Ti-a (increased temperature
of transition) V has the opposite effect
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882ºC
      
Content in Al of F136
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Aim of alloying is to increase mechanical
properties.
Alloys are termed as Ti-a, Ti-(aß) and Ti-ß
according to the phase present at room
temperature. As the BCC structure of Ti-ß has
more slide planes than the HCP Ti-a, Ti-ß is
more easily workable. Alloys ß and (a ß) are
formed at high temperatures
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Microstructure and properties of Ti alloys
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• ASTM F67
• Being nearly pure Ti, it is monophasic of type
  a (HCP)
• grain size 10- 150 mm.
• interstitial atoms (C, N, O) causes
  strengthening.
• The surface TiO2 layer brings about resistance
  to corrosion and to good osteointegration

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• ASTM F136
• An a-b alloy at ambient temperature
• its microstructure depends heavily on thermal
  history.
• If heated between 700 and 950C (below the
  transition to b) tiny grains of a (3-10 micron)
  with crystals of Ti-b at the boundaries of the
  primary a phase.
• This type of structure is recommended for
  surgical implants

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If heated above 975C and then slowly cooled to
ambient temperature, a biphasic structure is
obtained, where Ti.a HCP (rich in Al and poor in
V) precipitates within the grains of the Ti-b BCC
matrix, as sotto forma di lamellae or oriented
needles. If cooling is quick, a microstructure
is obtained due to solid-state non-diffusive
transformations (martensitic).
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Metallographic micrographies concerning different
microstructures A lamellar grains of HCP Ti-?
plus BCC Ti-? B granular structure C
globular structure.
A
B
C
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• Cracks induced by cyclic loads (fatigue tests)
  originate from grain boundaries between Ti-a and
  Ti-b.
• Microstructures with a grains small (lt20 micron)
  and round (not needle-like or lamellar) and a
  low interfacial area a/b show higher fatigue
  limit (500-700 MPa), because they stand better
  crack formation.
• Lamellar structures with high a/b interfacial
  area have fatigue limit 300-500 MPa.
• Treatments with hydrogen further improve the
  mechanical properties
• Yield Strength 974-1119 MPa,
• UTS 1025-1152 MPa,
• Fatigue limit 643-669 MPa

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Aspects of Titanium as a biomaterial
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• BIOCOMPATIBILITY
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• Very high, due to the formation of a surface
  layer of Titanium oxide, chemically inert
  (passivation)
• ? Extensive use in implants.
• (Dental alloys may cause allergies)
• for the same reason
• i) good resistance to corrosion
• ii) lack of taste (no metallic aroma)
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Further favorable features RADIOTRASPARENCY
A prosthesis in Ti may be X-rayed to control the
integrity of a piece THERMAL CONDUCTIVITY 14
times less than Au. No gun pulp irritation of
thermal origin as with Au alloys
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Note the better mechanical properties of F136 as
compared to F67
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Use of Ti for implants (all main joints hip,
knee, elbow, shoulder)
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The end