Biomaterials
2011
Difference in wear rate between metal-PE and ZTA-ZTA implants (2 marks):
The wear rate is 350 times less for ZTA-ZTA (0.001mm) than metal-PE (0.35mm)
Two wear mechanisms found in metal-on-PE joint inserts (2 marks):
Adhesive wear (occurs when yield stress values are dissimilar) and abrasive wear (occurs when one
material is softer than another)
Transformation toughening and how it affects the service life of ZTA wear components (5 marks):
Zirconia is the toughest ceramic known due to its property known as transformation toughening. It
can exist in three forms: monoclinic (stable at room temp and has largest specific volume),
tetragonal (metastable and smaller specific volume) and cubic (metastable, smaller specific volume).
When a crack forms on a tetragonal zirconia crystal, the crack energy is consumed in converting
tetragonal to monoclinic, causing the grain to expand (due to a larger specific volume for monoclinic)
and therefore pinch off the crack. This is known as transformation toughening.
Static fatigue; discuss in relation to K1 and explain using diagram. Give one example of bioceramic
implant for which static fatigue is an issue (5 marks):
KI is the stress intensity at the flaw tip. For KI<KIO, stress intensity is too low for any crack
propagation. For KIO<KI<KC, crack propagation occurs slowly, and can be enhanced by corrosion. For
KI=KC, the material instantly shatters. In silicate ceramics, it has been found that moisture enhances
slow crack growth where KIO<KI<KIC. Water molecules assist rack growth in a process known as
static fatigue. Ultra-high purity ceramics (such as alumina, ZTA or zirconia) must be used, because
fatigue strength for alumina for instance is reduced by the presence of water.
Example of bad leaching and example of good leaching (2 marks):
Leaching happens when metals corrode, and thus a metal alloy that contains potentially toxic
elements is safe as long as it does not corrode. An example of bad leaching is when metals used for
implants corrode, and thus these toxic elements are released into the body. An example of good
leaching exists in the case of drug-eluting implants or in the case of bioglass (as the leaching
stimulates osteogenesis).
Vroman effect; describe in terms of two rules for protein adsorption that it unites (3 marks):
Two rules of protein adsorption are:
1. The higher the concentration of a particular protein in the adjacent biofluid, the higher its
adsorption
2. The higher the affinity of the protein for the biomaterial surface, the higher its adsorption
The Vroman effect states that initial adsorption is in accordance with rule 1, while equilibrium
adsorption is in accordance with rule 2.
Two main currently available biodegradable metals and their commercial application (6 marks):
1. Pure iron: Used in bone nails/pins/wires. It is rapidly biodegradable in vivo which gives it a short
service life. Although iron is safe, the high degradation rates present a risk of toxic overload of ions.
2. Magnesium and magnesium-calcium allows: Being considered for stents, including drug-eluting.
Have faster rates of degradation and magnesium-calcium alloys have poor mechanical properties
and are brittle like glass.
Three main methods of sterilisation of biopolymers; one advantage and disadvantage (9 m):
1. Heat sterilisation: can take polymer above its melting or softening points
2. Chemical sterilisation: can cause chemical reactions and induce degradation
3. Radiation (gamma and UV): can break polymer chains and change its mechanical properties
Three key advantages of bioglass over HA for tissue bonding in vivo (3 marks):
1. Specific formulation of bioglass can bond with soft tissue and hard tissue
2. Bioglass is both bioactive and biodegradable (HA is not biodegradable)
3. Can allow for bioactive polymers
Four strategies by which you can minimise or eliminate the fibrous tissue sheath that the body
places around the implant (4 marks)
1. Using a more biocompatible and bioinert the material
2. Minimising movement between the implant and tissues (tightly locking in the implant)
3. Using an implant that does NOT leach toxins (which would otherwise result in a thicker capsule)
4. Using bioactive materials will eliminate the capsule entirely and instead form a direct tissue bond
Simulated body fluid can be used to precisely quantify a specific desirable property of
biomaterials. What is this property, and how is it measured? (3 marks)
It can be used to evaluate bioactivity. SBF precipitates HA onto bioactive surfaces using the in vitro
immersion test. SBF immersion at 37 degrees is the first step in evaluation, and determines the
deposited weight of HA versus time. The next step to evaluate bioactivity is osteoblast cell culturing,
followed by in vivo animal implantation testing and finally clinical human trials.
2010
List stages 1 to 8 of the inflammatory response, and the principal cell involved in each of stages 4-8
(13 marks):
Inflammation is defined as the reaction of vascularised living tissue to local injury. It serves to
contain, neutralise, dilute or wall off the injurious agent process. Injured cells release an alarm
cytokine called histamine, which causes dilation of nearby blood vessels that attracts leukocytes to
the area and stimulates the release of lymphocytes. The area has an increased blood supply and
produces redness and heat.
1. Implantation
2. Blood-biomaterial interactions
3. Provisional matrix formation: occurs within minutes to hours of implantation
4. Acute inflammation: Neutrophils are the predominant leukocyte and arrive immediately
and in vast numbers. They adsorb at the surface of the biomaterial, resulting in frustrated
phagocytosis. In the case of wear debris, phagocytosis can be successful.
5. Chronic inflammation: Macrophages attempt phagocytosis
6. Granulation tissue: Occurs within 1 day after implantation. Fibroblasts and endothelial
cells proliferate and begin to form granulation tissue (connecting tissue)
7. Foreign body reaction: The macrophages form giant cells and wrap around the surface of
the implant (sometimes for life)
8. Fibrosis/fibrous capsule development: When the immune system cannot kill or remove
the foreign body, it walls it off. Fibroblasts create a capsule of connective tissue that wraps
the implant up and isolates it from the adjacent connective tissue.
Reaching stage 8 indicates a biocompatible bioinert material.
List any 4 of the 13 rules of protein adsorption (8 marks):
Protein adsorption is the precursor step to cell attachment. The adsorption of adhesion proteins to
the biomaterial surface turns it into a biologically recognisable material. Within a second of
implantation, proteins are adsorbed onto the surface of the biomaterials. Within seconds to
minutes, a monolayer of protein will be adsorbed. This happens before the cells even arrive at the
surface, so that when they do, they see the protein layer instead of the biomaterial. Can be specific
(antibody to antigen) or non-specific (albumin to a PE implant)
5. The higher the concentration of a particular protein in the adjacent biofluid, the higher its
adsorption
6. The higher the affinity of the protein for the biomaterial surface, the higher its adsorption
7. Soft proteins adsorb more readily than hard proteins
8. The biological activity (folded structure) of a protein is usually different when in the biofluid, as
compared with when it is adsorbed to the biomaterial surface
12. pH will affect adsorption significantly
13. Surface roughness can affect protein adsorption
Three advantages and disadvantages of vitalium (6 marks):
Advantages:
1. Excellent corrosion resistance (unless in a galvanic cell situation)
2. Excellent biocompatibility (better than stainless steel)
3. Excellent wear resistance (only biometal suitable for articulating surfaces)
Disadvantages:
1. Toxic corrosion products
2. Toxic wear particles
3. Extremely high elastic modulus (210-250 GPa, 16 times higher than bone)
What is a bioactive material; give two examples (12C):
Non-toxic, biologically active such as titanium and tantalum (both mildly bioactive)
Bioinert materials; two examples (12C):
Non-toxic, biologically inactive such as gold and ceramics eg zirconia, alumina
Biodegradable material; two examples (one polymer and one metal) (12C):
A biodegradable material is one that degrades in the body over time. An example of a biodegradable
metal is pure iron (used in bone nails/pins/wires) and a biodegradable polymer is polyglycolic acid.
Biodegradable biopolymers are mainly used in sutures and tissue-engineering scaffolds.
Two main advantages and one main disadvantage of HA. As a consequence of this key
disadvantage, what is its principal use in orthopaedic biomaterials? (7C):
HA is bioactive and stable in body for years, biocompatible wear particles. Bonds only to bone and is
brittle, and is therefore used as a coating on metal implants.
What is simulated body fluid and how is it used to evaluate bioactivity? (7C):
It is a protein-free salt solution that mimics the blood serum salt content that can be used to
evaluate bioactivity. SBF precipitates HA onto bioactive surfaces using the in vitro immersion test.
SBF immersion at 37 degrees is the first step in evaluation, and determines the deposited weight of
HA versus time. The next step to evaluate bioactivity is osteoblast cell culturing, followed by in vivo
animal implantation testing and finally clinical human trials.
2009
Discuss each of the four methods of implant fixation in terms of mechanism of fixation, short term
strength and long term strength (12 marks):
(a) Bioactive fixation: Achieved by chemical bonding of surface-reactive ceramics and
glasses directly to bone. Low short term fixation strength, but a long term fixation strength
that matches that of cemented fixation
(b) Biological fixation: Achieved by the ingrowth of tissue into the pore of an implant made
from a bioinert material. Low short-term fixation strength, which gradually increases over
time. Long term fixation strength is lower than cemented and bioactive fixation, and can be
affected by bone-remodelling.
(c) Cemented fixation: Achieved via use of bone cement. High short term fixation strength
(highest of all mechanisms) that decreases slightly over time, long terms strength still higher
than biological and morphological fixation
(d) Mechanical interlock fixation:
Describe the four most important factors of blood-compatibility of a blood-interfacing implant (8
marks):
1. Surface roughness. The rougher the surface, the higher the contact area available for clotting; a
smooth surface is therefore better
2. Surface wettability. Surfaces that repel water were thought to be better, but wettability has been
shown to not strongly correlate with blood clot formation in vivo
3. Electrical properties. A negatively charged surface tends to reduce clotting
4. Chemical properties. Related to electrical properties, but highly reactive chemical elements make
for bad blood compatibility
2008
Key immune cell involved in response to a xenograft, and mode of attack? (6C)
T-lymphocytes- T-cell receptor locks onto the antigen-MHC complex on the surface of an aberrant
cell. Once a T-cell has locked onto an antigen-MHC-1 site, it is activated. Once activated, it multiplies
to make clones which can be effector (killer T-cells) and memory T-cells. Killer T-cells kill by lysis.
Helper T-cells assist in antibody response by secreting cytokines. Suppressor T-cells stop the
response once it is no longer needed.
What is an antigen? (6C)
A marker molecule present on the surface of an object or chemical that identifies it as foreign
What is an antibody? (6C)
The protein produced by the body to recognise and disable the antigen (actual protein is called
immunoglobulin)
There are 3 cells of the body that have a critical role in the immune response to a synthetic
biomaterial: Fibroblasts, macrophages and neutrophils. Each of them is dominant in a different
stage of the inflammatory response. Describe, in correct sequence, the three stages. Identify the
relevant cell and its role when you describe each stage (9 marks)
1. Acute inflammation: Neutrophils are the predominant leukocyte and arrive immediately and in
vast numbers. They adsorb at the surface of the biomaterial, resulting in frustrated phagocytosis. In
the case of wear debris, phagocytosis can be successful.
2. Chronic inflammation: Macrophages attempt phagocytosis
3. Foreign body reaction and fibrous capsule development: The macrophages form giant cells and
wrap around the surface of the implant (sometimes for life), and fibroblasts create a capsule of
connective tissue that wraps the implant up and isolates it from the adjacent connective tissue.
2007
Relevance of Vroman effect to synthetic biomaterial implantation (4 marks)
Why is microscopic wear debris more problematic than the bulk implant itself? (6C)
Which specific cell of the immune system responds to this situation? (6C)
Macrophages, which attempt phagocytosis of the material (in this case, the wear debris)
Describe in detail the four types of in-vivo implant response. Give an example of a biomaterial in
common use in each category (10 marks):
(a) Toxic (no example) Death of the surrounding tissue upon implantation
(b) Bioinert A bioinert material is non-toxic, but biologically inactive. No material implanted
in living tissues is inert, although ceramics do come close to inertness. The surrounding
tissues respond by forming a fibrous capsule of variable thickness around the implant. Highly
bioinert ceramics such as alumina have a thin capsule with a tight fit and a slightly thicker
capsule (100’s of microns) with a loose fit. Weakly bioinert materials such as stainless steel
result in a much thicker capsule.
(c) Bioactive Non-toxic, but biologically active. A chemical bond forms between the implant
and tissue. Titanium is capable of mild bioactivity. Bioactive ceramics eg HA are best
(d) Bioresorbable- Non-toxic and dissolves. The surrounding tissue eventually replaces the
material, e.g. porous calcium phosphate ceramics. Maintaining strength and stability of the
interface during the degradation period can be a problem, as can matching resorption rate
to the repair rate of the tissue. Used in tissue engineering scaffolds
Discuss leaching in relation to (9 marks):
(a) Thermosetting polymers: Are like supercooled liquids, and are usually more bioinert
than thermosetting polymers and therefore do not leach monomer or oligomer. However,
they often contain additives such as placticisers which can leach out, causing inflammation
or toxic outcomes.
(b) Thermoplastic polymers: Usually contain traces of unreacted monomer and oligomer
which can leach out of the polymer and into surrounding tissue. Inflammation or toxic…
PMMA leaching can cause extreme low blood pressure, even death
(c) Metals: Leaching only happens in metals when they corrode. This can result in potentially
toxic elements from the metals entering the body. It is important that metals are strongly
corrosion resistant. Corrosion can be minimised by preventing galvanic cell situations, having
smooth metal surfaces and allowing for passivation.
2006- All repeat
2005
Describe the role of each of the following cells of the immune system, and its activity in relation to
a synthetic or tissue-derived (allogeneric or xenogeneric) implant (8 marks):
(a) Macrophage: most prominent cells found in the foreign body (allograft or xenograft)
response. They attempt phagocytosis of the foreign material via the formulation of a
multinucleated giant cell that tries to engulf the implant. Small wear debris and be
phagocytosed, but large particles lead to frustrated phagocytosis (resulting in a strong
inflammatory response that can damage surrounding tissue).
(b) Neutrophil: also attempt phagocytosis, but arrive before the macrophages and in large
numbers. Involved in the acute inflammatory response stage
(c) B-lymphocyte: Form the basis of the humoral response. When a receptor locks onto an
antigen, a B-cell is activated. They then multiply to form effector B-cells (plasma B-cells) and
memory B-cells. Effector B-cells secrete generic antibodies.
(d) T-lymphocyte: Form the basis of the cell-mediated response (more targeted). Cytotoxic
T-cells identify flagged cells and kill by lysis or apoptosis. Helper T-cells assist B-cells in the
antibody response by secreting cytokines. Suppressor T-cells stop the immune response
when no longer needed.
Three key advantages and disadvantages of these biometals (7 marks):
(a) Titanium
Advantages:
1. Excellent biocompatibility (most biocompatible)
2. Low elastic modulus (closer to bone than CoCr or stainless steel) 120GPa, 7 times higher
3. No toxic corrosion products
Disadvantages:
1. Poor shear strength, not ideal for screws
2. Poor in articulation (not suitable for wear at all)
3. Titanium wear particles may inhibit both osteoblast formation and function
(b) Stainless steel
Advantages:
1. Adequate corrosion resistance
2. Adequate compatibility
3. Low cost
Disadvantages:
1. Toxic corrosion products (and less corrosion resistant than vitalium, nitinol or titanium)
2. Toxic wear articles
3. Extremely high elastic modulus (190GPa)
2004
What is phagocytosis? Explain with use of diagrams (8C)
Phagocytosis is the process of engulfing and ingesting of foreign particles by a phagocyte (mainly
macrophages and neutrophils). The antigen on the foreign body is identified and attracted to the
appropriate antibody on the phagocyte. The antigen binds to the antibody, and the phagocyte slowly
engulfs the foreign body completely, and attempts to internally destroy it.
Significance of phagocytosis in relation to biomaterials (8C):
Phagocytosis is a significant process with respect to biomaterials for two main reasons:
1. It forms part of the inflammatory response in the chronic inflammation stage in response to an
implanted material. When phagocytosis of the implant fails, the body forms a fibrous sheath around
the implant to separate it from surrounding tissue
2. It is the process by which the body deals with wear debris from biomaterials. Macrophages
attempt to engulf the wear particles in an attempt to destroy them (which is sometimes not possible
in the case of PE wear particles, causing inflammation)
Discuss mechanical properties of dental porcelain and their importance to the application (10C)
 A ceramic with a Young’s modulus that matches perfectly with gold
 The white porcelain is therefore used to cover the gold underlay (aesthetic appeal)
 Pure porcelain can be used, but not very strong and can break
 Porcelain is a perfect hardness match for teeth, which makes it better than alumina and
zirconia (which are stronger but can wear down opposing teeth)
 Porcelain crown can be fitted to an abutments implant (usually titanium)
What is moisture-enhanced static fatigue; describe it and significance for dental ceramics (10C)
Moisture-enhanced static fatigue is when moisture in the material enhances slow crack growth is the
ceramic is sustained under static stress. Silica is a common impurity in ceramics, especially alumina
and ZTA, and it concentrates in the grain boundaries. Static fatigue is a problem with dental
porcelain, which is silica based.
Sequence of events in the bioactive fixation tissue response of either HA or bioglass (15C)
Hydroxyapatite:
 Cellular bone matrix from differentiated osteoblasts appears at the surface, producing a
narrow amorphous area
 Between this area and the cells, collagen bundles are seen
 Bone mineral crystals have been identified in this area
 As the site matures, bonding zone shrinks
 The result is normal bone attached through a thin epitaxial bonding layer to the bulk implant
 Process enhanced by silicon doping
Advantages and disadvantages of bioglass and HA, which is why they haven’t been used
widespread commercially. Consider biological response, durability in the body, mech properties
and commercial considerations (15C)
Hydroxyapatite:
 Chemically similar to bone with elastic modulus 20-120GPa (depending on crystal structure)
 Forms direct chemical bonds to hard biological tissue (bioactive)
 Biocompatible wear debris
 Quite brittle, small fraction of toughness of bone and therefore not used extensively
 Solution: HA coatings on metal implants
 Fibre-reinforced HA- research, not clinical
 Lamellar HA- research, not clinical
 Silicon doped HA- enhanced bioactivity, standard clinical
Bioglass:
 Only known material that can form a bond with soft tissue
 Mostly ceramic glass (glass-forming oxides, intermediates and modifiers)
 It’s an amorphous HA
 Largely ignored for several decades when HA was already around
 45s5 only bioglass capable of soft-tissue bonding
 Actively encourage bone growth around an implant
 Strength of healed bone-glass bond is equivalent to healthy bone
 High brittleness makes them not suitable for impact loading
 Bonds well to bone, but not to metals and other implant materials
 Excessive bioactivity encourages rapid bone growth with poor structure
 Used for tissue engineering scaffolds, middle ear prostheses, bone cement
 Bioactive and biodegradable
Advantages and disadvantages of bone cements for hip implant fixation (7 marks)
Advantages:
1. Outstanding bond strength, allows patients to walk the next day
2. Allows surgeons to fill in gaps easily (carpenter)
Disadvantages:
1. Sets rapidly, may cause necrosis
2. Deteriorates over time, therefore leachants and wear particles are a problem
3. Adverse hypotensive reactions in some patients
2003
Leaching for (8C):
(c) Bioactive ceramics
(d) Bioresorbable ceramics
Describe in detail the following factors involved in enhancing or inhibiting leaching from metal
implants (9 marks):
(a) Passivation: Inhibits leaching by preventing corrosion. A stable surface oxide layer on the
outer surface of a metal implant minimises corrosion by not reacting in the body.
(b) Galvanic cells: A galvanic cell situation can enhance leaching via corrosion. Metals used
must not be of the right potential to cause an electron/ion displacement.
(c) Surface finish: Metal surfaces should be smooth on a microscopic scale (polished) to
avoid abrasions and potential corrosion, or damage to the passive layer.
What is stress shielding? Describe in detail, and how can it be minimised? (5 marks)
When a metal with a higher elastic modulus than bone is inserted into bone, it will flex much less
than the bone would have for any given load. The bone will therefore experience reduced strain.
This is known as stress shielding. Since the remodelling of bone is based on its strain experience
(Wolff’s law), the low strain environment causes the bone to resorb, eventually causing it to break.
In order to minimise stress shielding, elastic modulus of the implant must be ideally matched to the
bone. Alternatively, electrical stimulation of osteocytes may enable the prevention of resorption, or
the use of bisphosphonates can help inhibit resorption by poisoning osteoclasts.
Extras
Week 1
Three types of blood cells:
1. Erythrocytes (red blood cells) - about 5,000,000 per mL of blood
2. Platelets (enable blood clotting) – about 300,000 per mL
3. Leukocytes (white blood cells) – about 5,000 to 10,000 per mL of blood
Five type of leukocytes:
1. Basophil
2. Eosinophil
3. Neutrophil (small eater)
4. Monocyte (big eater) - becomes macrophage
5. Lymphocyte: B-cell- makes antibodies, identifies aberrant cells
T-cell- cell mediator, attacks aberrant cells
Types of immune response:
 Humoral (occurs in body fluids) - antibodies, complement. Antibodies specifically target
foreign body and activate blood proteins (complementary immune system) to kill the
invader
 Cell-mediated response (occurs in cell) - neutrophil, macrophages and lymphocytes. Involves
digesting cells (neutrophils and macrophages) and reacting cells (killer T-cells which are
specific and natural killer cells which are general)
Modes of destruction:
 Lysis- attack membrane causing it to burst (done by killer T-cells)
 Phagocytosis- engulfing the antigen (done by macrophages)
 Antibody deactivation- neutralisation of a toxin
Humoral response:
 Responsible for specificity and memory function of the immune system
 Primary cell: B-cell
 Antigen recognised by proteins (Ig) on the B-cell surface
 B-cell binds to it and flags it as MHC-II-antibody complex (MHC-I is body’s marker, major
histocompatibility complex)
 T-helper cells recognise flag and secrete lymphokine, which simulate division of B-cells
 B-cells divide into antibody secreting B-cells (effector B-cells) and memory B-cells
 Antibodies can activate complement system (assists phagocytosis), deactivate antigen or
mark antigen so other cells can attack it
Effect of environment on materials:
Biological: Adsorption of tissue constituents onto implant, enzyme degradation, calcification
Physical-mechanical effects: Wear, fatigue, corrosion, stress-corrosion cracking
Basophils and mast cells also stimulate inflammatory response. Basophils are motile, mast cells are
inert cells of connective tissue. Have the same effect as frustrated phagocytosis (causes local tissue
damage)
Fibrous encapsulation causes problems with load transfer between implant and surrounding
connective tissue, can lead to loosening and device failure.
Adverse systemic complications:
- Embolisation (blood clot, thrombogenesis)
- Hypersensitivity (allergic reaction)
- Elevation of implant elements in blood
- Lymphatic particle transport
- Tumorigenesis
- Migration of debris
Week 2
Inflammatory response (graph):
 Low to high concentration in order of both maximum concentration and time at which they
kick in: macrophages, neovascularisation, foreign body giant cells, fibroblasts, fibrosis
 Neutrophils have high initial intensity, further peaks then decreases in a short period of time
 Mononuclear leucocytes high initial intensity, gradual decrease over longer time, then
sustained at a certain level
Histamine induces inflammatory response. Causes vasodilation, more blood and increased
permeability in the local blood vessels
Week 3
Adsorption is asymptotic. It is initially very rapid, then equilibrium is reached in a matter of min-hrs
Quartz crystal microbalance (QCM) is a ultra-sensitive weighing device. Can be used to measure
mass change and structural properties
Week 4
Nitinol (fourth main candidates of biometals):
Advantages:
1. Excellent corrosion resistance
2. Excellent biocompatibility (better than stainless steel or vitalium)
3. Shape-memory capability
Disadvantages:
1. Shape-memory limits application e.g. cardiovascular catheters
2. Toxic component (nickel)
3. Soft and weak as plastic below transition temperature
Gold: no leaching or corrosion, perfect bioinertness, low elastic modulus (80GPa). Expensive
Platinum: Used for bioelectronic wires, not good mechanically
Tantalum: Not as established, but low elastic modulus (3GPa), highly corrosion resistant, most
biocompatible, porous allows for ingrowth
Zirconium: biocompatible, zirconia film is thin and scratches easily
Titanium nitride: hard ceramic, wear resistant but attacked by hydrogen peroxide
Oxinium: combines toughness and biocompatibility of zirconium with the low wear and friction of
zirconia. Zirconia is thin and scratches easily.
Diamond-like-carbon coatings: Most blood compatible, but high in surface energy and therefore
denatures absorbed proteins. Used in Ventracor LVAD
Dental amalgam: Worst, mercury leaching, stress shielding causing teeth to break after 20-30 years
Hardness factor is NOT a linear relationship
Biopolymers:
 Sutures, catheters, blood bags top 3 applications
 PE (low density- freezer bags, high density- shopping bags, UHMWPE- prosthesis) high
stability, low toxicity, good mech. Properties, no leachants, can fail by fatigue and creep, can
absorb fluids, processing not flexible (has to be compression moulded then machined)
 PE enhanced via carbon fibre, cross-linking and Vitamin E
 Natural e.g. cellulose in dialysis membranes
 Synthetic e.g. Hydron in breast implants
 Hydrogels- soft, difficult to sterilise, used in spinal disc nucleus implants
 Biodegradable biopolymers- degradation products cause inflammatory response, sutures
 Life span affected by- chemical effects (depolymerisation), when wet (e.g. PMMA),
mechanical effects (cyclic loading, fatigue)
Bioceramics:
 Inert and mechanically strong
 Brittle
 Resistant to chemical attack, microbial attack, pH changes, heat
 Clinical devices e.g. thermometers, fibre optics, eyeglasses
 Dental bioceramics e.g. dentures, porcelain crowns
 Orthopaedics e.g. hip replacements, knee replacements
Drug delivery systems
 Exploits principles of leaching and bioresorption
 Release drugs (eg antibiotics, anti-inflammatories bone-growth stuff) at a controlled rate
 Delivery vehicle includes bioresorbable ceramics and polymers, and porous bioinert
ceramics and polymers
 Artificial pancreas (portable insulin delivery)
 Drug-eluting stents
Week 5
Corrosion can be minimised via:
 Careful selection and coupling of metals
 Careful handling to minimise crack initiators
 Modified surface layer leading to a passive alloy
 Minimised tensile stresses which can otherwise enhance corrosion
Diffusion can occur via:
 Into the biomaterial (swelling). Common with polymers, problem in silicon with ball and cage
valves
 Out of the material (leaching). Corrosion of metals or additives leaching from polymers
Friction mechanisms differ depending on system:
 Metal/metal- adhesion & ploughing
 Metal/PE- adhesion, hysteresis
Wear can be categorised by:
 Wear rate
 Abrasive and adhesive wear
 Fatigue and corrosion fatigue wear
Degree of lubrication reduces wear by reducing friction and cooling sliding parts
Failure modes:
 Opening (Mode I)- brittle materials fail here
 Sliding (Model II)
 Out of plan tearing (Mode III)
Theoretical strength of ceramics
 Approximately E/5 to E/10
 Actual strength is E/100 to E/1000 for most ceramics
 Exception of fibrous ceramic, because of polymer film which protects its surface
 Parameter Weibull Modulus is a measure of scatter for strength (which is large)
Theories:
 Griffith crack theory: Stress concentration at the tip of a flaw means that failure will most
likely occur at a notch
 For a ductile material, when local stress concentration at the flaw tip exceeds the yield
point, local plastic deformation will occur and reduce the local stress concentration and
therefore blunt the tip
 In brittle materials, no stress relief occurs and a crack is initiated
 K1 (the stress intensity at flaw tip) is maximised for the deepest flaw and the sharpest
radius. In metals, the flaw coms from casting defects. In ceramics, the flaw is automatically
sharp and caused by sintering defects and surface scratches.
 Therefore fibreglass (surface flaw free) has E/20 and window glass (surface full of scratches)
has E/1000. Bioceramics are E/1000
Zirconia toughened alumina (ZTA) is a very reliable product, with fracture rates being 1000 times
lower, approaching 0.01%. Tetragonal zirconia can run out, but there’s plenty allowing for many
fatigue cycles.
Revision rate of THR:
 Aseptic loosening 10%
 Fracture of early alumina heads 10%
 Fracture of stems 2%
 Septic loosening 1%
Optimising properties of ceramics:
 Avoid silicates
 Maximum purity
 Finest grain size
 High quality surface finish (no surface flaws)
 Fully sintered and flaw free (no internal flaws)
 Transformation toughening
Biomimetics involves taking inspiration from nature in the development of technology.