Page 58 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: GRB Revised Pages
Encyclopedia of Physical Science and Technology EN002J-63 May 18, 2001 14:16

Biomineralization and Biomimetic Materials 199

As revealed by studies of silk, structural proteins in- to form supramolecular clusters, there are many papers on
clude regular and random regions in the chain structure the assembly of charged polymers and particles to form
which would be expected to give rise to crystalline and multilayers and on the assembly of particles or particles
amorphous material. In contrast, the amorphous compo- and films coated with complementary biological recogni-
nent of synthetic polymers arises from entanglements of tion molecules, such as the biotin–streptavidin system.
thecoiledchainsthatcannotberesolvedduringthecrystal- WorkattheMobilCorporationshowedthatmesoporous
lization process. New polymer properties may be achiev- silica could be formed by hydrolysis of tetraethoxysilane
able once we can design polymer chains with such con- entrained in a highly concentrated water/silane/surfactant
trolled sequences. system.Inthisregime,thethree-componentmixtureforms
There has been a report of enzyme-like activity in a ordered structures with a range of symmetries. In one
block copolypeptide, which enhances the rate of hydro- hexagonal phase, rods of water are surrounded by surfac-
lysis of tetraethoxysilane (TEOS, a standard reagent in tant and embedded in a hydrophobic silane matrix. Hy-
sol-gel chemistry) as a suspension in water. If this block drolysis of the silane under suitable conditions, followed
structure is a sequence of units of a hydrophilic amino by drying and sintering, results in a porous silica with
followed by units of a hydrophobic amino acid, it would aligned pores of a few nanometers’ diameter.
be expected to be active at a water–solvent interface. The The growth of the lyotropic liquid crystal precursor is
morphology of the silica that forms is dependent on the very sensitive to the environment. Ozin and co-workers
structure of the copolymer. This system is biomimetic both have shown that complex particle morphologies can result
in the sense of employing a polypeptide catalyst and in the from growth of these mesoporous structures in quiescent
sense of it functioning in a multiphase system, since bio- solutions as diffusion fields and surface forces interact.
logical processes rarely occur in homogeneous solutions. Several workers have shown how the direction of the rods
Many tissues, such as cartilage, arterial wall, and or plates of silica can be controlled. Polymers can be in-
the walls of soft marine organisms, are swollen poly- troduced to form composite structures that are very remi-
mer structures. Swollen polymers, such as plasticized niscent of some biological composites. This does seem to
polyvinylchloride, do occur in artificial structures but they parallel the proposed importance of liquid crystals in the
are usually avoided because loss of plasticizer leads to growth of many biological structures.
shrinkage and cracking. Even the swelling of wood with While one would expect that this approach could be
changes in humidity is a major impediment to its use in extended to many other material combinations, the rules
structures, though here years of experience have taught us are not understood. Efforts to form similar structures other
how to design around it. Skin does change in volume and than oxides, such as titania, or various crystalline materi-
properties as it takes up or loses water. The structure of als,havebeenonlypartlysuccessful.Possibly,anyrapidor
amphibian skin keratin is apparently different from mam- localized conversion process also disrupts the liquid crys-
malian keratin for this reason. talline organization. Stupp and co-workers (2000) have
There have been many suggestions that designers produced a range of amphiphiles that assemble into var-
should make more use of soft structures. These could take ious ribbon and wedge structures, and the authors have
the form of composites of hard fibers with rubbers, in explored their catalytic activity.
which case tires and reinforced plastic tubing could be
considered as examples. We could also envision more use
C. Inorganic Particle Formation
being made of liquid-swollen soft structures. For purely
mechanical systems, this may not make much sense, but in Coccolithophores, single-celled marine algae, assemble
active systems, such as batteries or muscle-like actuators, an external skeleton from single crystals of calcite with
a liquid component is necessary and should probably be very complex shapes. The growing crystal is surrounded
viewed as a soft material rather than simply as a liquid to by a lipid membrane that controls the growth orientation
be contained. in the crystal, but it is not known exactly how this is
achieved. Sponges and diatoms show similar close control
of the shape of silica particles on the micron scale. Mag-
B. Surfactants and Self-Assembly
netotatic bacteria form single-domain iron oxide (mag-
Self-assembly is a hallmark of biological systems, includ- netite) crystals, with a very closely controlled size of a few
ing assembly of protein subunits into holoenzymes, of nanometers, which then aggregate into magnetic chains
proteinsandnucleicacidsintovirusparticles,andoftropo- (Fig. 1). These biological examples all involve growth
collagen into collagen fibers. There has been increasing in- within a compartment surrounded by a membrane. For
terest in synthetic self-assembly. In addition to the assem- sponge spicules there is an organic template on which
bly of molecules with complementary hydrogen bonding the mineral grows. In other cases, there may be specific