Page 392 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOCERAMICS 369

15.3.4 Critical Properties of Bioactive Ceramics
Important needs in bioactive ceramics research and development include characterization of the
processing-composition-structure-property synergy, characterization of in vivo function, and estab-
lishing predictive relationships between in vitro and in vivo outcomes. Understanding reactions at
the ceramic surface and improving the ceramic/tissue bond depend on (Ducheyne, 1987) (1) charac-
terization of surface activity, including surface analysis, biochemistry, and ion transport; (2) physical
chemistry, pertaining to strength and degradation, stability of the tissue/ceramic interface and tissue
resorption; and (3) biomechanics, as related to strength, stiffness, design, wear, and tissue remodeling.
These properties are time dependent and should be characterized as functions of loading and envi-
ronmental history.
Physical/chemical properties that are important to characterize and relate to biological response
include powder particle size and shape, pore size, shape and distribution, specific surface area,
phases present, crystal structure and size, grain size, density, coating thickness, hardness, and surface
roughness.
Starting powders may be identified for their particle size, shape, and distribution, via sifting tech-
niques or quantitative stereology. Pore size, shape, and distribution, important properties with respect
to strength and bioreactivity, may be quantified via stereology and/or SEM. Specific surface area,
important in understanding the dissolution and precipitation reactions at the ceramic/fluid interface,
may be characterized by B.E.T. Phase identification may be accomplished via XRD and FTIR. Grain
sizes may be determined through optical microscopy, SEM, or TEM. Auger electron spectroscopy
(AES) and x-ray photoelectron spectroscopy (XPS) may also be utilized to determine surface and
interfacial compositions. Chemical stability and surface activity may be analyzed via XPS and mea-
surements of ionic fluxes and zeta potentials.
An additional factor that should be considered in evaluating chemical stability and surface activity
of bioceramics is the aqueous microenvironment and how closely it simulates the in vivo environ-
ment. The type and concentration of electrolytes in solution and the presence of proteins or cells may
influence how the ceramic surface changes when it interacts with a solution. For example, a solution
with constituents, concentrations, and pH equivalent to human plasma most accurately reproduces
surface changes observed in vivo, whereas more standard buffers do not reproduce these changes
(Kokubo et al., 1990b).
The integrity of a biomaterial/tissue interface is dependent on both the implant and tissue.
Therefore, both of these constituents should be well characterized: the implant surface should be
analyzed and the species released into the environment and tissues should also be determined.
Surface analyses can be accomplished with solution chemical methods, such as atomic absorption
spectroscopy; physical methods, such as thin film XRD, electron microprobe analysis (EMP), energy
dispersive x-ray analysis (EDXA), FTIR, and surface-sensitive methods, such as AES, XPS, and sec-
ondary ions mass spectroscopy (SIMS) (Fig. 15.6). The integrity of an implant/tissue interface also
depends on the loading pattern, since loading may alter the chemical and mechanical behavior of the
interface.
The major factors limiting expanded use of bioactive ceramics are their low-tensile strength and
fracture toughness. The use of bioactive ceramics in bulk form is therefore limited to functions in
which only compressive loads are applied. Approaches that may allow ceramics to be used in sites
subjected to tensile stresses include (1) use of the bioactive ceramic as a coating on a metal or ceramic
substrate (Ducheyne et al., 1980), (2) strengthening the ceramic, such as via crystallization of glass
(Gross et al., 1981), (3) use fracture mechanics as a design approach (Ritter et al., 1979), and
(4) reinforcing the ceramic with a second phase (Ioku et al., 1990; Kitsugi et al., 1986; Knowles and
Bonfield, 1993; Li et al., 1995).
No matter which of these strategies is used, the ceramic must be stable, both chemically and
mechanically, until it fulfills its intended function(s). The property requirements depend upon the
application. For example, if a metallic total hip prosthesis is to be fixed to bone by coating the stem
with a Ca-P coating, then the ceramic/metal bond must remain intact throughout the service life of
the prosthesis. However, if the coating will be used on a porous coated prosthesis with the intent of
accelerating ingrowth into the pores of the metal, then the ceramic/metal bond need only be stable
until tissue ingrowth is achieved. In either scenario, mechanical testing of the ceramic/metal bond,