Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 282

were found to have a typical depth and spatial profi le and increased with the stimulation
frequency, intensity, and duration [18, 19].
10.4.4 Microelectrodes for continuous recording of pH in vivo
Microelectrodes capable of continuously measuring pH have proven to be useful in
various applications including medical diagnoses and in regulating operative and post-
operative management of patients. Arterial blood gas analyses are essential to monitor
gas exchange in critically ill patients and during anesthesia for major surgery. Usually,
intermittent sampling of arterial blood samples is made and the samples are ana-
lyzed in a central laboratory or by a point-of-care blood gas analyzer. Such intermit-
tently performed measurements may miss some adverse events and delay therapeutic
responses during surgery [126].
Although sensor breakage, drift, and reference electrode failure have limited the
widespread use of continuous pH monitoring, some successful studies have been
reported. Papeschi et al . [105] described an IrOx electrode for in-vivo esophageal and
gastric pH measurement performed on 15 healthy volunteers. The electrode was small
and fl exible, so it was well accepted by patients for 12-24 hours' continuous monitoring
pH changes in the upper gastrointestinal tract. Similarly, blood pH, PO 2 , and PCO 2 are
also continuously monitored using microelectrodes during cardiac surgery [103, 127].
10.4.5 Implantable pH microelectrodes
Some earlier developments and applications of various implantable pH sensors or
measurement systems have been reported [128, 129, 130, 131]. However, reliable pH
sensors for long-term implantations are still not available, and widespread clinical
usage of implantable pH sensors has not been reached. Similar to other implantable
sensors, the development of implantable pH microelectrodes, either fully implanted
in the body or needle type sensors applied through the skin (percutaneous), has faced
serious obstacles including sensor stability deterioration, corrosion, and adverse body
reactions [48, 132, 133]. Among them, encapsulation to prevent corrosion represents a
major challenge for the implantable sensor devices [51]. Failure of encapsulation can
cause corrosion damage on internal components, substrate materials, and electrical
contacts [48]. The dissolution of very thin pH sensitive layers will also limit the stability
and lifetime of implantable micro pH sensors.
Another major challenge for implantable pH sensors is the biological response of
living systems toward sensors when making contact with blood or tissue [134]. The
experiments often showed a progressive loss of function and lack of reliability of the
implanted sensors. This lack of reliability and progressive loss of function, common to
all implantable sensors, is believed to be caused by tissue or blood reactions such as
infl ammatory response, fi brosis, and loss of vasculature and thrombus formation [135,
136]. The delivery of anti-infl ammatory drugs to the sensor site can minimize tissue
reactions and extend the lifetime of the device [50, 137].