Classes of Biomaterials Used in the Body

Advantages Disadvantages

Metals Corrosion

Density

Processing

Ceramics Brittle – fractures easily in tension

Polymers Not strong

Not rigid

Time-dependent deformation (creep; stress relaxation)

Degradation

(Source: Park; p. 3)

Example

Metals Joint replacement

Dental roots

Orthopedic fixation

Stents

Ceramics Dental implants

Orthopedic implants (some)

Polymers Sutures

Blood vessels (e.g. vascular grafts)

Joint socket (knee, shoulder)

Ear, nose

Soft tissues in general

Biomaterials for Biological Tissue Replacements

Density

(g/cm3)

Elastic modulus

(GPa)

Yield strength

(MPa)

Tensile Strength (MPa)

% Elongation at break

Stainless Steels

SS 316L

Annealed

7.9 172 485 40%

SS 316L

30% Cold Worked

7.9 690 860 12%

Cobalt-Based Alloys

Haynes-Stellite 21 or ASTM F75

Cast Co-Cr-Mo

8.3 450 655 8%

Haynes-Stellite 25 or ASTM F90

Wrought Co-Cr-W-Ni;

Annealed

9.2 445 970 62%

Haynes-Stellite 25 or ASTM F90

Wrought Co-Cr-W-Ni:

44% Cold-worked

9.2 1610 1900 10%

ASTM-F562 or MP35N

Wrought Co-Ni-Cr-Mo-Ti

Hot forged

~9.2 965 1210 50%

ASTM-F562 or MP35N

Wrought Co-Ni-Cr-Mo-Ti

Cold worked

~9.2 1500 1800 8%

Density

(g/cm3)

Elastic modulus

(GPa)

Yield strength

(MPa)

Tensile Strength (MPa)

% Elongation at break

cp-Titanium and Titanium Alloys

cp-Ti (Grade 4) or ASTM F6730 % Cold Worked

4.5 485 760 15%

Ti-6Al-4V or ASTM F136annealed

4.5 830 900 14%

Tantalum

Tantalum

Annealed

16.6 138 345 20-30%

Tantalum

30% Cold Worked

16.6 207 517 2%

Types of Tissues:1. Epithelial tissue: • covers the body surface and forms the lining for most internal cavities. • skin is an organ made up of epithelial tissue.• Function: protect2. Connective tissue:• Bone, cartilage, dense fibrous tissue, loose connective tissue, fat• Function: support, protection, binds tissues together 3. Muscle Tissue:• Skeletal: voluntary; contraction of skeletal parts; attached to bone via tendons• Smooth: involuntary; walls of internal organs and blood vessels• Cardiac: involuntary; walls of the heart4. Nerve Tissue:• Consists of nerve cell or neurons located in the brain, spinal cord, and nerves• Produces and conducts nervous impulses to and from all parts of the body.

Organ: _______________________________________________

skin, bone, heart, lung, brain, eye, stomach, spleen, pancreas, kidneys, liver,intestines, uterus, and bladder.

Tissue: _______________________________________________

Soft Tissues: - Connect, support, or surround other structures and organs of the body- Muscles: support/moves bone- Tendons: connect muscles to bone- Ligaments: connect bone to bone- Synovial tissue: makes up joint capsule- Fascia: sheet or band of fibrous connective tissue enveloping, separating, or binding together muscles, organs, and other soft structures of the body. - Also: nerves, blood vessels, fat

Hard Tissues:- Tissues that _______________________________- Cartilage and bone

Tissues

Biological Tissues: Mechanical Properties

Tissue

Modulus (MPa)

Tensile Strength (MPa)

Strain at Break(%)

Soft Tissues Smooth Muscle, relaxed 0.006 - 300

Smooth Muscle, contracted 0.01 - 300

Carotid Artery 0.084 ± 0.22 - -

Cerebral Artery 15.69 4.34 50

Cerebral Vein 6.85 2.82 83

Pericardium 20.4 ± 1.9 - 34.9 ± 1.1

Patellar Tendon (29-50 yrs. Old) 660 ± 266 64.7±15 14 ± 6

ACL Ligament (21-30 yrs. Old) 345 ± 22.4 36.4 ± 2.5 15 ± 0.8

Hard TissuesModulus

(GPa)Tensile Strength

(MPa)Strain at Break

(%)

Cortical Bone 17 - 24 90 -130 1-3

Cancellous Bone 0.1 - 4.5 10 - 20 5-7

Cartilage 0.001 - 0.01 10 - 40 15-20

Elastic Modulus“Mechanical Mismatch Issues”: A biomaterial that replaces or repairs a tissue but has, for instance, higher stiffness may be problematic (e.g. may cause “stress shielding” or neointimal hyperplasia). Similarly, if it lacks rigidity, it may also be problematic (e.g. recoil of coronary stents, plastic deformation under physiological conditions).

Material E (GPa)

Silicone Elastomer (Rubber) ~0.002

UHMWPE 0.69

PMMA 2.2-3.2

Soft tissue See previous table

Cortical Bone 17-24

Cancellous Bone 0.1 – 4.5

Glass 73

Gold 77

Ti-6Al-4V 114

Stainless Steel 316L 190

Tantalum 190

Haynes-Stellite 21 (Cast Co-Cr-Mo) 210

Aluminum oxide 380

Diamond 700-1200

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Elastic Modulus

DensityA biomaterial that replaces an equivalent volume of tissue may have different weight as a result of the differences in density. In some applications, this can be problematic.

Material Density (g/cm3)

Fat 0.94

UHMWPE 0.94

Water 1.0

Soft tissue 1.01-1.06

Silicone Elastomer (Rubber) 0.99-1.50

PMMA 1.19

Cortical Bone 1.8-2.1

Cancellous Bone ~1

Glass 2.4-2.8

Diamond 3.5

Aluminum oxide 3.98

Ti-6Al-4V 4.5

Stainless Steel 316L 7.9

Haynes-Stellite 21 (Cast Co-Cr-Mo) 8.3

Tantalum 16.6

Gold 19.3

Source: J. Park & R.S. Lakes, Biomaterials: An Introduction (3 rd Ed), p. 93 ISBN 978-0-387-3789-4

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Density

Metallic Biomaterials

Device Annual # of Devices in USA

Intraocular Lenses (2003) 2,500,000

Vascular Grafts 300,000

Breast prostheses 250,000

**Heart Valves (rings, cages) 100,000 (some)

**Pacemakers 400,000

*Coronary Stents 1,500,000

*Hip Prostheses (2002) 250,000

*Knee Prostheses (2002) 250,000

*Dental Implants 910,000

- Source: Ratner, B.D. et al. “Biomaterials Science: An Introduction to Materials in Medicine, 2nd Edition, Elsevier Academic Press, San Diego, CA, 2004.- In 2004 (unless otherwise stated)- *indicates all or predominantly metal- **indicates metal-containing

Categories of Metallic Biomaterials

1. Stainless Steel2. Cobalt-based Alloys3. Titanium Alloys4. Specialty Metallic Alloys

Steel

• A steel alloy that is highly resistant to corrosion• Fe is combined/alloyed with at least 11 wt% chromium (Cr)

1. Stainless Steel (SS)

• An Fe & C alloy- cp-Fe: <0.008 wt% C- steels: 0.008-2.14 wt% C- cast irons: 2.14-6.7 wt% C

• “ferrous alloy”: Fe is main constituent (FYI: Fig. 11.1)• May contain appreciable amounts of other alloying elements

Stainless Steel

Stainless Steel 316L (SS 316L)

• ASTM F138, F139

•Composition:- Fe: 60-65 wt%- Cr: 17-19 wt%- Ni: 12-14 wt%- C: 0.03 wt% max- Minor: Mn, P, Si, Mo

Cr[O2] Cr2O3 (chromium oxide at surface = protective water-barrier)

Low [C] (<0.03 wt%):

Higher [C] (> 0.03 wt%):

Cr + C[O2] Cr23C6 (chromium carbides)

whichppt @ grain boundaries

Depletes Cr at adjacent grain boundariesCr2O3

@ surfacewater penetration

ASTM = American Society for Testing and Materials

• Name:- 316: Contains Mo- L: Low C content

• Rationale for Composition: (1) Corrosion Resistance:

Sensitized SS = SS where carbides have formed

(2) Mechanical Properties of SS 316L

SS316L is “austenitic”• -Fe, FCC• Stabilized by Ni• non-magnetic, generally stronger

“Ferritic” SS • -Fe, BCC• Stabilized by Mo, Si

FYI: Table 11.4

E (GPa) y (Mpa) TS (MPa)

%EL

Cold Worked

190 690 860 12%

Annealed 190 172 485 40%