Biomedical Engineering Reference
In-Depth Information
materials prepared from collagen solutions have appeared (Schoof et al. 1998,
2000, 2001). Thereafter, the freeze-casting of ceramic-based biomaterials has
received a great deal of attention during the past few years. Current research
focuses on both the fundamentals of the freeze-casting process, drawing on
the well-established field of directional solidification of alloys, and the prop-
erties of the materials created (Deville 2008). This simple process, where a
material suspension is simply frozen and then sublimated, provides mate-
rials with outstanding mechanical properties and unique porous archi-
tectures, where the porosity is almost a direct replica of the frozen solvent
crystals. These remarkable properties are related to the fine-scale structure
of the pore struts, a laminate of thin inorganic crystallite layers, and tough
biopolymers, arranged in an energy-absorbing hierarchical microstructure,
as recently reviewed by Deville (2010). It is thus a process with great prom-
ise for the development of biomimetic ceramics that emulate several design
principles of the natural tissue that they are designed to replace.
Deville, Saiz, Nalla, et al. (2006) have shown how to replicate the nacre struc-
ture of shells using controlled freezing of mixtures of water and HAp ceramic
powder. The composites made by this route have remarkably improved
mechanical properties (e.g., up to 145 MPa for 47% of porosity and 65 MPa
for 56% of porosity). The researchers achieve this by combining conventional
manufacturing approaches as follows: (1) fine powder processing; (2) con-
trolled solidification; and (3) freeze-drying. Their nacre-like composites start
with a lamellar template assembled by ice crystals. Water freezes as lamellar
dendrites, and the ice dendrites push the HAp particles into the interdendritic
regions, making layers on the same scale as the ice. After the ice is removed
by freeze-drying, the HAp ceramic keeps the shape of the interdendritic lay-
ers, forming a template for subsequent injection of tough polymer.
Fu et al. (2008b) and Rahaman and Fu (2008) investigated the relationship
between HAp particle concentration, cold temperature, and solvent compo-
sition and porous microstructure. Freeze casting of aqueous suspension of
HAp particles on a cold substrate produced porous constructs with a uni-
form, lamellar-type microstructure in which platelike HAp lamellas were
oriented in the direction of freezing. Changes in the particle concentration of
the suspension and temperature of the cold substrate did not drastically alter
the lamellar-type microstructure but affected the porosity, the pore width,
and thickness of the HAp lamellas. A decrease in the temperature of the cold
substrate (-20°C to -196°C) caused a reduction in the size of the HAp lamel-
las and the pore cross-section. Addition of glycerol (~20 wt%) or dioxane (~60
wt%) to the aqueous solvent used in the suspensions produced finer pores
and a larger number of dendritic structures, or much larger pores of cellular
microstructure connecting the HAp lamellas. They also extended this study
and investigated the effect of sintering conditions on the microstructure and
mechanical behavior of freeze-cast HAp. Sintering (1250°C~1375°C) leads to
a decrease in porosity (<5%) but an increase in strength of nearly 50%. The
mechanical response shows high strain tolerance (5%-10% at the maximum
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