Page 60 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002J-63 May 18, 2001 14:16

Biomineralization and Biomimetic Materials 201

FIGURE 2 Elongated titania particles formed by impregnation of a stretched two-phase polymer with titanium
alkoxide, followed by hydrolysis.

nucleation sites on the membrane surface. This suggests ever, most simple minerals are not sufficiently anisotropic
synthetic approaches where inorganic particles are grown to form such elongated particles. The shape of biologi-
within a micromold, a predefined space. cal crystals is probably controlled by selective inhibition
There have been many efforts to grow particles in lipo- of growth on specific crystal faces. Very elongated sil-
somes, spherical shells with a lipid bilayer wall. One solu- ica particles are also formed in lipid vesicles but we do
tion is trapped when the liposome is formed and precipita- not know how important these are for controlling shape
tion occurs when a second reagent, often base, diffuses in in crystals. While synthetic methods offer many ways of
through the wall. Generally, the trapped solution must be controlling particle size, we lack good methods for con-
quitediluteinordertoavoiddestabilizingtheliposome.As trolling shape. One route is to form long micromolds by
a result, the precipitate therefore only occupies a small part phase separation in a two-phase polymer, which is cold-
of the internal volume. A method is needed to introduce a drawn to elongate the included phase. The included phase
continuous feed of both reagents through the membrane. is then swollen with a metal alkoxide, which is hydrolyzed
There have also been many efforts to grow particles in to oxide (Fig. 2).
multiphase polymer systems, such as block copolymers.
For instance, a two-phase polymer can be soaked in one
D. Tough-Layered Structures
reagent, which selectively absorbs into one phase. A cad-
mium salt could be taken up by a polyether phase. Subse- Large organisms operate in an environment that subjects
quent treatment with hydrogen sulfide results in precipi- them to fluctuating forces, from the action of wind and wa-
tation of cadmium sulfide within the polyether. While the ter on plants and from the locomotion of animals. These
volume fraction of sulfide formed is quite small, repeated fluctuating forces and collisions will often result in local
cyclescangiverisetohigherfractionsofthemineral.Work damage that should not lead to catastrophic failure. As a
on precipitation in lyotropic liquid crystalline amphiphile result, many biological tissues contain structural features
solutions has led to composites structures. that add toughness without severely compromising stiff-
One characteristic of biological minerals is their elab- ness or strength.
orate shapes. For mechanical reinforcement of soft ma- In mollusk shells and teeth, toughness may arise from
trices, a filler should be a high-aspect ratio rod or plate added polymeric layers or from very fibrous structures,
particles. These may grow as a natural outcome of differ- as discussed by Heuer. The addition of polymer could
ing crystal growth rates along different crystal axes. How- be especially effective if the polymer structure is capable