Download - Biomaterials
S. Waldman MECH 393
Biomaterials
The objective of these lectures is to review the fundamental requirements for biomaterials used in biomechanical engineering applications.
S. Waldman MECH 393
Material Attributes for Medical Applications
Biocompatibilty Non-carinogenic, non-pyrogenic, non-toxic, non-allergenic, blood
compatible, non-inflammatory
Sterilizability Not destroyed or severely altered by sterilizing techniques such as
autoclaving, dry heat, radiation, ethylene oxide
Physical Characteristics Strength, toughness, elasticity, corrosion-resistance, wear-
resistance, long-term stability
Manufacturability Machinable, moldable, extrudable
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Biocompatibility
Early Definition:
“Lack of interaction between material and tissue”
Implies inert, non-toxic, non-carcinogenic, non-allergenic, non-inflammatory, non-degradable
Thus, material has zero influence…
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Biocompatibility
Contemporary Definition:
“Ability of a material to perform with an appropriate host response, in a specific application”
Refers to a collection of processes and interdependent mechanisms of interaction between material and tissue
“Ability of material to perform” and not just reside in the body “Appropriate host response” must be acceptable given the
desired function “Specific application” must be defined
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Biocompatibility
Specific application must also consider the time scale over which the host is exposed to the material:
Material Contact Time
syringe needle 1-2 s
tongue depressor 10 s
contact lens 12 hr - 30 days
bone screw / plate 3-12 months (or greater)
total hip replacement 10-15 yrs
intraocular lens 30 + yrs
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Host Response
Types of Reactions:
Normal wound healing response
Protein adsorption Acute Inflammation Resolution
Persistent Inflammation
Acute Chronic
Effect of relatively reactive tissue environment on material (i.e. corrosion, degradation products)
Possibility of remote or systemic effects (transient or chronic) if reaction products are transported away from implant site
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Host Response
Types of Reactions:
Infection (early or late onset)
Osteolysis
Neoplasia (cancer)
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Biocompatibility Testing
Considerations: Type of device, principle tissue(s) in contact, period of implantation
Tests for Chronically Implanted Devices: In Vitro: cytotoxicity, carcinogenicity, mutagenicity
In Vivo: pyrogenicity, systemic/acute toxicity
Chronic Animal Implantation Studies
(3 species for 6, 12 and 24 months)
Human Clinical Trials
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Implantable Materials
Metals
Polymers
Ceramics
Composites
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Biomaterials – Metals
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Biomaterials – Metals
Material Applications
316, 316L
Stainless Steel
Fracture fixation
Joint Replacement
Spinal Instruments
Surgical Instruments
Pure Titanium
Ti-6Al-4V
Ti-13Nb-13Zr
Bone and Joint Replacements
Dental Implants
CoCr Alloys Bone and Joint Replacements
Dental Implants
Heart Valves
Gold Alloys Heart Valves
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Biomaterials – Polymers
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Biomaterials – Polymers
Material Applications
Polyethylene (UHMWPE) Joint Replacement Bearings
Polypropylene Sutures, MCP Joints
Polytetrafluoroethylene (Teflon) Vascular Prosthetics
Polyesters Vascular Prosthetics, Drug Delivery, Sutures, Ligament Grafts
Polyurethanes Vascular Prosthetics, Heart Valves, Catheters
Polyvinylchloride (PVC) Catheters
Polymethylmethacrylate (PMMA) Implant Fixation
Silicones Ophthalmology
Hydrogels Ophthalmology
Polylactic and Polyglycolic Acid Resorbable Devices, Drug Delivery
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Biomaterials – Ceramics
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Biomaterials – Ceramics
Material Applications
Alumina Joint Replacements
Zirconia Joint Replacements
Calcium Phosphates Bone Grafting, Surface Coatings for Fixation
Bioactive Glasses Bone Grafting, Surface Coatings for Fixation
Porcelain Dental Implants
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Biomaterial PropertiesTensile
Modulus (GPa)
Yield Strength
(MPa)
UTS
(MPa)
Elongation at Break
(%)
Endurance Limit
(MPa)
Co-Cr-Mo (cast)
200 440 – 570 650 – 750 8 235 – 275
Co-Cr-Mo (forged)
210 650 – 1000 896 35 – 55 400 – 600
Titanium 100 480 – 510 550 – 620 15 – 20 250 – 280
Ti-6Al-4V 100 825 930 10 – 15 400 – 440
316 SS 200 250 – 330 520 – 620 35 – 75 245 – 300
Cortical Bone 18 80 80 – 150 1 – 3 30
Cancellous Bone
0.2 – 0.5 5 – 30 10 – 20 5 – 7 –
UHMWPE 1 20 30 390 16
PMMA 3 – 35 0.25 6
Alumina 350 – 270 0 –
Zirconia 200 – 500 – 650 0 –
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Corrosion
Galvanic Crevice
Stress-Corrosion Cracking Fretting
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Galvanic Corrosion
Electrochemical circuit between two dissimilar metals Anodic material is more basic and oxidizes (corrodes) Cathodic material is more noble and is protected
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Implant Fixation Methods
No such thing as absolute rigidity since both the implant and the underlying bone are deformable.
Some deformation will occur at the bone/implant interface and is only acceptable if:
Magnitude does not progressively increase Does not give rise to pain Does give rise to unacceptable quantities of debris
Biological restrictions: Cortical and cancellous bone are significantly weaker in tension and
compression Fibrous tissue layer that is laid down at the bone/implant interface
during initial healing phase is also weak in tension and shear Therefore, try to avoid tension and shear when condidering fixation
method
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Implant Fixation Methods
Interference Fits Can provide good fixation Bone remodeling can remove interference on which fixation
depends and can lead to loosening
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Implant Fixation Methods
Screws Do not ensure tightness regardless of how many screws are present
and can result in loosening Crevice corrosion is a common problem under screw heads
(observed in fracture fixation plates) and can lead to loosening Locally high contact stresses at bone/screw interface Only suitable for temporary fixation (e.g. fracture fixation)
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Implant Fixation Methods
Bone Cement Gap filling agent Polymethylmethacrylate (PMMA) which is polymerized in situ Distributes load over largest possible area (low contact stresses) Provides mechanically interlocking between implant and cancellous
bone Problems: monomers are toxic, polymerization process is
exothermic (>50°C) and cement is generally brittle
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Implant Fixation Methods
Bone Ingrowth Porous coats, grooves and/or meshes Good for long-term fixation Relative motion must be restricted to ensure bone ingrowth Pore size has a distinct effect on the amount of ingrowth Common approach is to create a layer of partially sintered beads on
the surface of the implant
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Wear
In any system if there is contact and relative motion between two materials, then wear will occur.
The extent of wear is the key issue in biomaterials: Biological response to wear debris Degradation of implant premature failure
Wear is still the major unsolved problem of joint replacements: Early failures (< 7 years for TKRs) Requires revision surgery (typically less effective than primary
surgery)
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Wear
Factors to consider:
Material SelectionSelect more wear resistant materials (e.g. Co-Cr >> Ti)
Develop surface modifications (e.g. TiN)
Materials CombinationsSame (metal-on-metal)
Mixed (metal-on-plastic)
Contact MechanicsLoads (magnitude, static, dynamic)
Mechanical properties of materials
Geometry of contacting bodies (e.g. congruency)
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Wear
Factors to consider:
LubricationLubricant properties
Mechanism of lubrication (e.g. elastohydrodynamic)
Surface Finish2nd body wear, 3rd body wear
Kinematics of ArticulationVelocity, rolling/sliding
Biological Response Bulk versus particulate debris
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Material Combinations (THRs)
Femoral Head Material Acetabular Material Result
Stainless Steel PTFE Wearing out, tissue reaction to wear products
Stainless Steel Silica-filled PTFE Abrasion of femoral head and wear of cup
Co-Cr-Mo Co-Cr-Mo High friction, high levels of metal ions in tissue
Co-Cr-Mo Cartilage Satisfactory
Co-Cr-Mo UHMWPE Low rate of wear
UHMWPE Cartilage Severe wear of UHMPE and cartilage
UHMWPE Co-Cr-Mo Wear of femoral head
Ti-6Al-4V UHMWPE High rate of cup wear
Zirconia UHMWPE Limited experience
Alumina UHMWPE Low rate of wear
Alumina Alumina Low rate of wear
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Mechanisms of Wear
Flat surfaces, even those polished to a mirror finish, are not truly flat on an atomic scale. They are rough, with sharp, rough or rugged outgrowth peaks, termed asperities.
Under compression, the asperities deform, leading to increased contact area (lower stresses) with higher coefficients of friction (µs, µd).
Depending on how the asperities interact under relative motion, different wear mechanisms can occur.
S. Waldman MECH 393
Mechanisms of Wear
Fatigue Primarily related to one material (UHMWPE) Cyclic subsurface tension and compression
Adhesive Related to two materials (metal & UHMWPE) Surface energy between materials in contact
Abrasive Related to three materials (metal, UHMWPE and debris) Hard, rough material removes soft material
Combinations of above can occur
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Wear Testing
Screening tests are typically used to reproduce the mechanisms of wear observed in retrieved implants in a controlled environment.
Stimulators Pros: actual implants used Cons: difficult to model actual biomechanics
Rotating Pin-on-Flat Pros: simpler model than simulator Cons: does not actually model kinematics/dynamics
Reciprocating Pin-on-Flat Pros: sliding motion modeled well (good for THRs) Cons: does not actually model knee kinematics/dynamics
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Consequences of Wear
Excessive wear can lead to premature failure of the component; however, there can also be a biological response to the generated wear debris, such as inflammation and/or osteolysis.
Osteolysis refers to the active resorption of bone tissue as part of a biological reaction to wear particles generated from artificial joint replacements. This process ultimately results in implant loosening and eventually requiring revision surgery.
The magnitude of the osteolytic response is dependent on the nature of the wear particles generated:
chemical composition size (smaller particles have greater effect) shape (shaper particles have greater effect)
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Osteolysis
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Sterilization Methods
Autoclave (Steam): High temperature process (121 – 134°C) Commonly for repeat sterilization (e.g. instruments) Cheap
Ethylene Oxide (EO): Low temperature process (for heat sensitive materials, e.g.
UHMWPE) Residual gas can linger Environmental impact and occupational hazard
Gamma Radiation: Very effective Can cause polymer oxidation and crosslinking