Biomedical Engineering Reference
In-Depth Information
biotin. In this case the polymer is conjugated to a genetically engineered streptavidin at a
site near its biotin binding site.
7.2.3
Photo-Responsive Materials
Photo-responsive chromophores have been incorporated into polymer chains to produce
photo-responsive polymers. Azobenzene has been one of the more common chro-
mophores utilized. The molecule can undergo changes in shape and polarity upon irradi-
ation. Polymeric chains containing this molecule show reversible photoisomerization (73).
Junge and McGrath (74) produced benzyl aryl ether dendrimers, which underwent
reversible
cis
- or
trans
-isomerization on exposure to UV light.
Desponds and Freitag (75) have prepared biopolymeric conjugated molecules of biotin
with isopropylacrylamide,
N
-acryloxysuccinimide, and (3-minopropyloxy)azobenzene.
These bioconjugates were photosensitive due to the presence of the chromophore.
Irradiation with UV light (330 nm) or visible light (440 nm) caused oscillation of the chro-
mophore between
cis
and
trans
states. The bioconjugates were applied to capture avidin
from solution. The proposed technique may lead to optically switchable biosensors and
bioaffinity ligands.
Bioresponsive hydrogel-based microlenses have recently been reported for protein detec-
tion (76). Stimuli-responsive poly(
N
-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc)
microgels were functionalized using biotin. Silane-modified glass substrate was coated with
arrays containing both pNIPAm-co-AAc microgels (as an internal control) and biotinylated
pNIPAm-co-AAc microgels. The microlens were applied for avidin capture and detection.
The optical properties of the lens changed with the binding events and the process was
reversible. This study is an example of label-free detection of proteins or small molecules.
Analyte-dependent swelling/shrinking properties of ultrathin polymer layers are an
appropriate means for the detection of various analytes (77). Optical metal nanoclusters
can be used to determine the change of the layer's thickness, which is shown by a change
in the color of the chip. By using different cross-linking agents and different polymers (bio-
logical or artificial as well) it was possible to design various sensitive layers showing dif-
ferent swelling or shrinking behaviors. Sensitivity on various analytes could be observed,
since the different types of polymers employed differed in structure, functional groups, or
biorecognitive properties.
Leclerc and Ho (78) describe the thermochromic, ionochromic, and affinity chromic
properties of various neutral conjugated polymers. In particular, the recent development
of cationic, water-soluble, chromic polythiophenes has allowed the easy, rapid, specific,
and ultrasensitive detection of various nucleic acids and proteins in aqueous media.
7.3
Conclusions and Future Prospects
Significant progress has been made in membrane protein engineering over the last 5 years,
based largely on the redesign of existing scaffolds (79). Engineering techniques that have
been employed include direct genetic engineering, both covalent and noncovalent modifi-
cation, unnatural amino acid mutagenesis, and total synthesis aided by chemical ligation of
unprotected fragments. Combinatorial mutagenesis and directed evolution remain, by
contrast, underemployed. Techniques for assembling and purifying heteromeric multisub-
unit pores have been improved. Progress in the de novo design of channels and pores has
