Biomedical Engineering Reference
In-Depth Information
The hydrogel sensing layer responded to the pH of the solution in which it is immersed
leading to changes in ionic mobility across the gel. The sensor could operate in high-ionic
strength solutions, which was a big advantage. The response time was 5 min and was
operated for more than 6 weeks with a 50% loss of sensitivity over 3 weeks.
Simonian et al. (61) have produced biosensor platforms for detection of pesticides.
Organophosphate hydrolase enzymes were integrated with several different transduction
platforms including conventional pH electrodes, fluoride ion-sensitive electrodes, and
pH-responsive fluorescent dyes. Detection limit for most systems was found to be in the
low parts per million concentration range.
7.2.2
Thermosensitive Materials
Several investigations have been carried out in the literature on the temperature-respon-
sive polymer poly(
N
-isopropylacrylamide) (PNIPAAm). Park and Hoffman were among
the first to report on the temperature-dependent swelling and collapse of PNIPAAm (62).
Following this several copolymers were investigated with
N
-isopropylacrylamide
(NIPAAm) including acrylic acid and methacrylic acid, 2-methyl-2-acrylamidopropane
sulfonic acid, trimethyl-acrylamidopropyl ammonium 3-methyl-1-vinylimidazolium
iodide, sodium acrylate, sodium methacrylate, and 1-(3-sulphopropyl)-2-vinyl-pyri-
dinium betaine (63-65). The NIPAAm-methacrylic acid polymer is both pH and tempera-
ture responsive (55). Bioactive “smart” protein-polymer conjugates were also synthesized
by polymerizing from defined initiation sites on proteins (66). Cysteine 34 of bovine serum
albumin (BSA) and Cys-131 of T4 lysozyme V131C were modified. Polymerization of
NIPAAm from the protein macroinitiators resulted in thermosensitive BSA-PNIPAAm
and lysozyme-PNIPAAm in greater than 65% yield. The resultant conjugates were char-
acterized by gel electrophoresis and size exclusion chromatography (SEC) and easily
purified by preparative SEC. Lytic activities of the lysozyme conjugates were determined
by two standard assays and compared to that of the unmodified enzyme prior to poly-
merization; no statistical differences in bioactivity were observed (66).
In another study, NIPAAm gels seeded with ferromagnetic materials were prepared,
which showed magnetic-field-sensitive swelling-deswelling transition. The likely reason
was the heating of magnets in a magnetic field. This led to the collapse of temperature-
sensitive NIPAAm (67). Similarly, Yoshida et al. (68) demonstrated swelling-deswelling
transition controlled by redox reaction using NIPAAm seeded with tris (2,2
-bipyridyl)
ruthenium (II). The gel was found to undergo contraction and shrinking repeatedly as
ruthenium changed from
3 oxidation states.
Thermal-responsive polymer gels have been applied in immunoassays (69). Miyata et
al. (70) first attached rabbit IgG to
N
-succinimidylacrylate and subsequently copolymer-
ized this with acrylamide, along with
N
,
N
2 to
-methylene bisacrylamide as a cross-linker. The
resulting hydrogel was labeled antigen-antibody semiIPN. When an antigen to IgG was
introduced in the network, the gel swelled. Removal of the antigen resulted in gel shrink-
age. Thus, a shape memory response occurred on exposure to the antigen, which was
found to be reversible.
Hoffman (71) has designed “intelligent” polymer gels by conjugating them near a
protein's ligand binding site. On exposure to an appropriate stimulus or trigger, the pro-
tein-gel network collapses and blocks the ligand binding. This was demonstrated using
PNIPAAm and streptavidin. Temperature was applied as the trigger to prevent the
binding of biotin. Similarly, Ding et al. (72) have conjugated PNIPAAm to a site near a
genetically engineered streptavidin. They demonstrated that the temperature-induced
collapse of the polymer released bound biotin. Similarly, using NIPAAm-acrylic acid
copolymers pH can also be utilized for biotin release. This brings about the release of
