Biomaterials - Introduction to Biomedical Engineering - page 224

TABLE 5.2 Mechanical Properties of Materials with Literature Values or Minimum Values from Standards—

Cont'd

Yield MPa UTS MPa Deform % Modulus GPa

PEEK, 61% C fiber, long 2,130 1.4 125

PEEK, 61% C fiber, þ 45 300 17.2 47

PEEK, 30% C fiber, chopped 208 1.3 17

Biologic tissues

Hydroxyapatite (HA) mineral 100 0.001 114-130

Bone (cortical) 80-150 1.5 18-20

Collagen 50 1.2

1

F138, wrought stainless steel: 17-19 Cr, 13-15.5 Ni, 2-3 Mo, < 2 Mn, < 0.08 or < 0.03 C.

2

F75, cast cobalt-chromium-molybdenum alloy: 27-30 Cr, < 1.0 Ni, 5-7 Mo, < 1 Mn.

3

F799, wrought Co-Cr-Mo alloy: 26-30 Cr, < 1.0 Ni, 5-7 Mo, < 1.0 Mn, < 1.5 Fe, < 1.5 C.

4

F136 Titanium 6Al-4V alloy: 5.5-5.5 Al, 3.5-4.5 V, < 0.015 N, < 0.13 O, < 0.08 C.

5.2.1 Metals
Metallic biomaterials represent the most highly used class of biomaterials. Metals have
high strength and resistance to fracture and are designed to resist corrosion. The main
metallic biomaterials in use today can be categorized into three groups: iron-base alloys
(stainless steels), cobalt-base alloys, and titanium-base alloys. Many orthopedic devices
are made in part of metal, such as hip and knee joint replacements (Figures 5.2 and 5.3)
due to its high strength and ability to resist failure even after many cycles of loading.
The implants provide relief from pain and restore function to joints in which the natural
cartilage has been worn down or damaged. Plates and screws that hold fractured bone
together during healing also are made of metal and are shown in Figure 5.4. Sometimes
the metallic plates and screws are retrieved after successful healing, but in other cases
they are left in place. Dental root prosthetic implants are also made of metal (Figure 5.5).
Otherexamplesofmetalsusedinmedicaldevicesandtheirmechanicalpropertiesare
shown in Tables 5.1 and 5.2.
Materials selection for a medical device is complicated. The selection depends on a num-
ber of factors, including the mechanical loading requirements, chemical and structural
properties of the material itself, and the biological requirements. The longstanding use of
metals for knee and hip joints, bone plates, and spinal fusion devices is due to the high
mechanical strength requirements of these applications and proven biocompatibility in
these settings. The advantages of metals over other materials such as ceramics and poly-
mers are that they are strong, tough, and ductile (or deformable, particularly as compared
to ceramics). The metal atoms are arranged in a highly ordered crystalline manner, yet have
nondirectional metallic bonding that allows for the propagation of energy-absorbing dislo-
cations of some of the atoms rather than abrupt, catastrophic cracking. Disadvantages
include susceptibility to corrosion, again due to the nature of the metallic bond (free
electrons). In fact, the steels that were used in the early 1900s for hip implants corroded
rapidly in the body and caused adverse effects on the healing process. This has led to the