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

S. Waldman MECH 393

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

S. Waldman MECH 393

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

S. Waldman MECH 393

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

S. Waldman MECH 393

Biomaterials – Polymers

S. Waldman MECH 393

Biomaterials – Polymers


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

S. Waldman MECH 393

Biomaterials – Ceramics


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

S. Waldman MECH 393

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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Interference Fits Can provide good fixation Bone remodeling can remove interference on which fixation

depends and can lead to loosening

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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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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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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

S. Waldman MECH 393

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)

S. Waldman MECH 393

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)

S. Waldman MECH 393

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

S. Waldman MECH 393

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

S. Waldman MECH 393

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

S. Waldman MECH 393

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)

S. Waldman MECH 393

Osteolysis

S. Waldman MECH 393

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

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Biomaterials