Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 285

With the advancements in the fabrication of miniaturized sensors and analyzers,
pH measurement as part of blood gas analysis using the point-of-care testing (POCT)
[147, 148] or the continuous monitoring devices [126, 149] has been carried out in
laboratory testing and patient care. The i-STAT hand-held clinical analyzer (Abbott
Laboratories), as described earlier, uses a cartridge with multiple thin fi lm-based elec-
trodes as sensors or biosensors for the detection of many parameters, e.g. pH, pCO 2 ,
pO 2 , base excess, bicarbonate, sodium, potassium, ionized calcium, hemoglobin,
hematocrit, and glucose [125].
For pH sensors used in in-vivo applications, especially those in continuous pH mon-
itor or implantable applications, hemocompatibility is a key area of importance [150].
The interaction of plasma proteins with sensor surface will affect sensor functions.
Thrombus formation on the device surface due to accelerated coagulation, promoted by
protein adsorption, provided platelet adhesion and activation. In addition, variation in
the blood fl ow rate due to vasoconstriction (constriction of a blood vessel) and sensor
attachment to vessel walls, known as “wall effect”, can cause signifi cant errors during
blood pH monitoring [50, 126].
In practice, some anticoagulation agents such as heparin or antiplatelet agents, e.g.
nitric oxide (NO) are delivered to sensor sites in order to reduce the risk of thrombus
formation. Nitric oxide (NO), which is a potent inhibitor of platelet adhesion and activa-
tion as well as a promoter of wound healing in tissue, has been incorporated in various
polymer metrics including PVC (poly(vinyl-chloride)), PDMS (poly-dimethyl-siloxane)
and PU (poly-urethanes). Those NO release polymers have been tested in animals as
outer protection coatings and have shown promising effects for the analytical response
characteristics of the sensor devices [137].
10.5.3 Measurement of pH in the brain
Brain tissue acidosis resulting from ischemia can cause brain damage when cerebral
blood fl ow reduction reaches a critical value. Continuous pH monitoring is critical for
the treatment of patients with stroke or severe head injury [15]. A group of research-
ers have measured pH changes in the rabbit brain during cerebral ischemia induced by
bilateral common carotid artery occlusion [132]. They found that there were signifi cant
differences in pH over time between the control and occlusion group. The brain pH in
the control rabbits was between 6.69 and 6.78 throughout the 60-minute observation
period, whereas the brain pH in the group of rabbits with carotid occlusion was found
to decline steadily from 6.82 to 6.46 with increasing occlusion time. In another study,
Doppenberg et al. [15] examined correlations between brain pH, pO 2 , pCO 2 , and cer-
ebral blood fl ow (CBF) in a group of 25 patients with severe traumatic brain injuries.
The pH measurements were made using an optical pH sensor (Paratrend 7, Biomedical
Sensors, Malvern, PA) based on absorbance of phenol red in bicarbonate solution. In a
similar study, the extracellular pH in a rat brain glioma was mapped in vivo by a probe
with pH dependent 1 H resonances detectable by 1 H NMR spectroscopy [3].
Terminal activity causes an increase in local cerebral blood fl ow that can be quanti-
fi ed by measuring the accompanying increase in tissue oxygen. Alkaline pH changes