Biomedical Engineering Reference
In-Depth Information
3.2.6 Applications of ''smart
polymers'' as biomaterials
Reversible
precipitation
or
gelation
+ Stimulus
Stimulus
Allan S. Hoffman
+ Stimulus
Reversible
adsorbtion
on a surface
Stimulus
Introduction
+ Stimulus
Stimulus-responsive, ''intelligent'' polymers are polymers
that respond with sharp, large property changes to small
changes in physical or chemical conditions. They are also
known as ''smart'' or ''environmentally sensitive'' poly-
mers. These polymers can take many forms; they may be
dissolved in aqueous solution, adsorbed or grafted on
aqueous-solid interfaces, or cross-linked in the form of
hydrogels.
Many different stimuli have been investigated, and
they are listed in Table 3.2.6-1. Typically, when the
polymer's critical response is stimulated, the behavior
will be as follows ( Fig. 3.2.6-1 ):
The smart polymer that is dissolved in an aqueous
solution will show a sudden onset of turbidity as it
Reversible
collapse of
surface graft
polymer
Stimulus
+ Stimulus
Reversible
collapse of
hydrogel
Stimulus
Fig. 3.2.6-1 Schematic illustration showing the different types of
responses of ''intelligent'' polymer systems to environmental
stimuli. Note that all systems are reversible when the stimulus is
reversed (Hoffman et al., Journal of Biomedical Materials
Research 2000).
phase separates, and if its concentration is high
enough, it will convert from a solution to a gel.
The smart polymer that is chemically grafted to
a surface and is stimulated to phase separate will
collapse, converting that interface from a hydrophilic
to a hydrophobic interface. If the smart polymer is in
solution and it is stimulated to phase separate, it may
physically adsorb to a hydrophobic surface whose
composition has a balance of hydrophobic and polar
groups that is similar to the phase-separated smart
polymer.
The smart polymer that is cross-linked in the form of
a hydrogel will exhibit a sharp collapse, and release
much of its swelling solution.
These phenomena are reversed when the stimulus is re-
versed. Sometimes the rate of reversion is slower when
the polymer has to redissolve or the gel has to reswell in
aqueous media. The rate of collapse or reversal of smart
polymer systems is sensitive to the dimensions of the
smart polymer system, and it will be much more rapid
for systems with nanoscale dimensions.
Smart polymers may be physically mixed with or
chemically conjugated to biomolecules to yield a large
and diverse family of polymerbiomolecule hybrid sys-
tems that can respond to biological as well as to physical
and chemical stimuli. Biomolecules that may be com-
bined with smart polymer systems include proteins and
oligopeptides, sugars and polysaccharides, single and
double-stranded oligonucleotides, RNA and DNA,
simple lipids and phospholipids, and a wide spectrum of
recognition ligands and synthetic drug molecules. In ad-
dition, PEG, which is also a smart polymer, may be
Table 3.2.6-1 Environmental stimuli
Physical
Temperature
Ionic strength
Solvents
Radiation (UV, visible)
Electric fields
Mechanical stress
High pressure
Sonic radiation
Magnetic fields
Chemical
pH
Specific ions
Chemical agents
Biochemical
Enzyme substrates
Affinity ligands
Other biological agents
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