This work is motivated by a demand for inexpensive, robust and reliable biochemical sensors with high signal
reproducibility and long-term-stable sensitivity, especially for medical applications. Micro-fabricated sensors can
provide continuous monitoring and on-line control of analyte concentrations in ambient aqueous solutions. The
piezoresistive biochemical sensor containing a special biocompatible polymer (hydrogel) with a sharp volume phase
transition in the neutral physiological pH range near 7.4 can detect a specific analyte, for example glucose. Thereby the
hydrogel-based biochemical sensors are useful for the diagnosis and monitoring of diabetes. The response of the glucosesensitive
hydrogel was studied at different regimes of the glucose concentration change and of the solution supply.
Sensor response time and accuracy with which a sensor can track gradual changes in glucose was estimated.
Additionally, the influence of various recommended sterilization methods on the gel swelling properties and on the
mechano-electrical transducer of the pH-sensors has been evaluated in order to choose the most optimal sterilization
method for the implantable sensors. It has been shown that there is no negative effect of gamma irradiation with a dose
of 25.7 kGy on the hydrogel sensitivity. In order to achieve an optimum between sensor signal amplitude and sensor
response time, corresponding calibration and measurement procedures have been proposed and evaluated for the
chemical sensors.
The application of stimuli-responsive or smart cross-linked gels in chemical sensors is based on their ability to a phase
transition under the influence of external excitations (temperature, pH, concentration of additives in water). The external
stimulus lowers the energy barrier between two possible gel states: a stable state (shrunk gel) and a metastable state
(swollen gel), and thereby makes possible the gel transition into the swollen state. The amount of the solvent absorbed
due to the external stimulus has been modeled and calculated taking into account the polymer parameters (concentrations
of the hydrophilic, hydrophobic, ionisable and ionised groups as well as polymer cross-linking degree) and the solution
parameters (analyte concentration, ionic strength, viscosity as well as temperature and temperature change rate).
Combining a smart hydrogel and a micro fabricated pressure sensor chip allows to continuously monitor the analytedependent
swelling of a hydrogel and hence the analyte concentration in ambient aqueous solutions. The sensitivity of
hydrogels with regard to the concentration of such additives as H+-ions (pH sensor), transition-metal ions and salts in
water was experimentally and numerically investigated at different temperatures. It has been demonstrated that the
sensor's sensitivity depends on the polymer composition as well as on the polymer cross-linking degree. A higher
sensitivity was observed for polyelectrolyte hydrogels with higher concentrations of ionisable groups. The long-term
measurements have shown that the lifetime of piezoresistive chemical sensors can be prolonged up to several years
provided that specific operation and storage conditions are fulfilled.
Environmentally responsive or smart hydrogels show a volume phase transition due to changes of external stimuli such
as pH or ionic strength of an ambient solution. Thus, they are able to convert reversibly chemical energy into mechanical
energy and therefore they are suitable as sensitive material to be integrated in biochemical microsensors and MEMS
devices. In this work, micro fabricated silicon pressure sensor chips with integrated piezoresistors were used as
transducers for the conversion of mechanical work into an appropriate electrical output signal due to the deflection of a
thin silicon bending plate. Within this work two different sensor designs have been studied. The biocompatible
poly(hydroxypropyl methacrylate-N,N-dimethylaminoethyl methacrylate-tetra-ethyleneglycol dimethacrylate) (HPMADMA-
TEGDMA) was used as an environmental-sensitive element in piezoresistive biochemical sensors. This
polyelectrolytic hydrogel shows a very sharp volume phase transition at pH values below about 7.4 which is in the range
of the physiological pH. The sensor's characteristic response was measured in-vitro for changes in pH of PBS buffer
solution at fixed ionic strength. The experimental data was applied to the Hill equation and the sensor sensitivity as a
function of pH was calculated out of it. The time-dependent sensor response was measured for small changes in pH,
whereas different time constants have been observed. The same sensor principal was used for sensing the ionic strength.
The time-dependent electrical output signal of both sensors was measured for variations in ionic strength at fixed pH
value using PBS buffer solution. Both sensor types showed an asymmetric swelling behavior between the swelling and
the deswelling cycle as well as different time constants, which was attributed to the different nature of mechanical
hydrogel confinement inside the sensor.
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