Sensors lifetime

High thermal, chemical, and photochemical stability, which simplifies sterilization and extends sensor lifetime. 

From the medical point of view, in contrast to the analytical which refers to the mechanism of sensor signal formation, a direct measurement is defined as a measurement carried out directly in undiluted sample (whole blood, plasma, serum, urine, etc.), whereas indirect measurement employ sample dilution. For an analytical chemist, a direct measurement is more challenging because of small sample volume, interferences and matrix effects, diffusion potential and carry-over effects, and the influence on the sensor lifetime due to high extracta-bility of active components by undiluted samples such as serum or urine. However, this measurement method has one important advantage it allows the measurement of the activity of analytes as-they-are . For... 

An indirect measurement with an ISS has several advantages less sample volume is required, the effects of the interferences and matrix are reduced, diffusion potential and carry-over effects become easier to control and the sensor lifetime is extended. The application of diluent itself offers the possibility of imposing the physical and chemical properties of the diluent on the sample, such as pH, ionic strength and even undesirable interferences and influences suppression. 

Flow-Based Systems Needle-type sensors with a fluid flowing over the sensor tip seem to resist biofouling and extend sensor lifetime.31 There are numerous methods that have been investigated for flow-based sensors, such as microperfusion systems,75 microdialysis,76 77 and ultrafiltration.78 Reduced fouling was found with an open microflow system where slow flow of protein-free fluid over the sensor surface at the implant site is effected.73 Different from the other flow-based sensors, the open microflow is controlled by the subcutaneous tissue hydrostatic pressure and does not require a pump. 

Polyurethanes have also been employed as outer sensor membranes. Yu et al. evaluated the biocompatibility and analytical performance of a subcutaneous glucose sensor with an epoxy-enhanced polyurethane outer membrane.15 The membrane was mechanically durable and the resulting sensors were functional for up to 56 days when implanted in the subcutaneous tissue of rats. Despite the improved sensor lifetime, all of the polyurethane-coated sensors were surrounded by a fibrous capsule, indicating an enduring inflammatory response that is undesirable due to the aforementioned effects on analytical sensor performance. To date, the clinical success of most passive approaches has been rather limited. It is doubtful that one passive material alone will be capable of imparting long-term (i.e., weeks to months) biocompatibility for in vivo use due to the extremely dynamic nature of the wound environment. 

TABLE 11.2 Calculated Sensor Lifetimes Based on the Fluorophore Photobleaching Rate Constants from the Literature... 

Fluorescent probe Photobleaching rate constant (h-1) Fluence (mW/cm2) Sensor lifetime... 

Use of conventional reference electrodes is a limiting factor in reducing the size of the various CHEMFETs. This could be solved by incorporating the reference electrode into the CHEMFET chip. An example of this is the on-chip fabrication of an Ag/AgCl electrode containing a gel-filled cavity sealed with a porous silicon plug [84]. Unfortunately, sensor lifetime can be limited by leakage of the reference solution. 

Problems such as peeling of the polymer films and leaching of the membranes need to be overcome to increase sensor lifetime. New membranes such as those based on siloxanes show possible increases in performance and stability. 

The simplest way to deal with the problem of sensor lifetime is to circumvent it completely by using each sensor once. This requires the production of inexpensive, disposable sensor strips for a variety of anions, possibly by a method such as screen printing. An alternative method may be to further develop photoprocessable polymers, which can then be assembled into the final sensor by a process such as photolithography. 

When constructing biosensors, which are to be used continuously in vivo or in situ, maintaining sensor efficiency while increasing sensor lifetime are major issues to be addressed. Researchers have attempted various methods to prevent enzyme inactivation and maintain a high density of redox mediators at the sensor surface. Use of hydrogels, sol-gel systems, PEI and carbon paste matrices to stabilize enzymes and redox polymers was mentioned in previous sections. Another alternative is to use conductive polymers such as polypyrrole [123-127], polythiophene [78,79] or polyaniline [128] to immobilize enzymes and mediators through either covalent bonding or entrapment in the polymer matrix. Application to various enzyme biosensors has been tested. 

During the last 25 years, the plug-type gas sensors have shown a greater mechanical integrity than other types of zirconia-based sensors of similar construction, and thus have lower leak rates and greater resistance to thermal and/or mechanical shock, factors which produce long sensor lifetimes and enable in-situ oxygen probes to be used in applications found to be beyond the capability of the other zirconia-based probes. 

An essential goal is therefore to consider reliability aspects from the very beginning of a development project and to include it already in the concept and design phases ( design for reliability ). This holistic approach to increasing sensor lifetime is based on the idea of considering reliability aspects in all phases of product development (Fig. 5.9.1). 

In addition to fast response, the O2 sensors demonstrated good reproducibility and reliability. This sensor design also allows incorporation of a water reservoir on the front side (the electrode side) of the sensor to increase sensor lifetime without degrading the sensor s response time. This reservoir provides suflScient moisture to the Nation electrolyte which requires water to be functional. 

Lifetime studies with protected sensor fdms indicate functional sensor lifetime has been extended to 12 months. 

Demonstrate an extrapolated sensor lifetime greater than 3 years... 

An experiment was carried out where a sensor was exposed to H2 over a three-month period. The temporal sequence of the gas stream included a 24-hour dry air purge followed by a 10-minute exposure to 1% H2. Figure 5 shows the long-term test results. The sensor remains fully functional after approximately three months of repeated exposure to H2. This observation, while it does not directly provide a measure of sensor lifetime, is encouraging. 

The coming decade will undoubtedly see an enormous expansion in the area of intelligent chemical instrumentation. Instrumental specifications will include automated features such as fault detection, calibration and temperature compensation, with the more advanced versions having the capacity to interpret sample composition. Increasing confidence in the results obtained from these instruments should generate more applications, which will in turn stimulate more progress. A movement towards solid state sensors will be required in order to obtain the vital extended sensor lifetimes and long-term stability. 

The coupling of sensors with flow injection analysis (FIA) is already a very popular option. The flow regime offers important advantages over discrete manual measurements that include (1) Sample preparation processes such as reagent mixing, selectivity enhancement (e.g., removal of large molecular mass interferents such as protein by dialysis in clinical assays), and solvent extraction can all be carried out online. The improved sample preparation and more reproducible sample delivery result in improved measurement precision and accuracy. Drift is less of a problem as measurements are made of peak heights relative to a baseline. (2) Improved sensor lifetime in flow analysis, the sensor may be exposed to the sample for only a short period of time, and maintained in a friendlier matrix between measurements that can help counteract or delay the deleterious effects of the sample. (3) Automation the entire analysis can be... 

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