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Planar (bio)sensors for critical care diagnostics.

Advances in planar (bio)sensors have allowed whole-blood diagnostics to be applied in testing close to the patient, resulting in rapid turnaround times, which are especially desirable in critical care settings. Several new technologies and custom chemicals had to be integrated to allow high performance, small sample size, fast response time, and cost-effective devices. (Bio)sensors described below are used for measurement of blood gases, blood electrolytes, glucose, and lactate in point-of-care environments.

Manufacturing of planar thick-film electrodes on ceramic wafers is now done with standard processes yielding precise patterns through the use of ultrapure metals for prolonged use-life under constant polarization. A platinized carbon paste ink has been developed to screenprint the active electrode of the glucose and lactate biosensor (Fig. 1, top).

In the amperometric sensor for [Po.sub.2], Nafion (polymeric perfluorinated ionomer; Aldrich) is used as an internal electrolyte and is spin-coated with a custom-made, patented polymer [1]. This polymer has a relatively low permeability for oxygen but is permeable to water vapor to allow fast wetting and a stable steady-state response signal (Fig. 1, middle).

For ion-selective sensors, a copolymer of methacrylamidopropyltrimethylammonium chloride and methyl methacrylate (MAPTAC/MMA) is used as a solid internal contact, resulting in minimal shifts in offset potential over >30 days [2] (Fig. 1, bottom)

For enzyme sensors, we applied a combination of an interference rejection cover membrane (FC 61 from Dow Corning) [3] and a correcting electrode to cope with known interferences [4]. The glucose and lactate sensor are virtually free of interference at maximum expected values of the individual substances being tested (see [5, 6] for examples). FC 61, a silicone material spun-cast from an aqueous emulsion, rejects interferences and has a restricted permeability for the substrates (glucose and lactate) but a high permeability for oxygen, making the sensor oxygen-independent over the P02 range of 25-700 mmHg.

For biosensors we also were required to apply enzyme-stabilizing agents, such as polyvinylalcohol, to achieve extended lifetime in a multianalyte, multiuse application. The sensors are polarized at ~+400 mV (vs Ag/AgCl). The use of platinum-activated carbon as the working electrode material permits these lower potentials to be used for the oxidation of hydrogen peroxide. The advantages of the lower operating potentials include a reduced interference from oxidizable substances in blood, which may permeate the FC 61 membrane, as well as the added benefit of extending sensor use-life (Fig. 1).

Except for lactate, all sensors show a use-life of >30 days for measuring 30 whole-blood samples per day. Accuracy is checked with NIST standards, where available, and values reported agree with those of accepted NCCLS Reference Methods, when such exist (see Table 1) [7, 8].



[1.] Foos JS, Edelman PG, Flaherty JE, Berger J. Extended use planar sensors. US Patent 5,595,646; 1997.

[2.] Chan AC. Material for establishing solid state contact for ion selective electrodes. Eur Patent Application 0 643 299 A1; 1994.

[3.] McCaffrey RR, D'Orazio P, Mason RW, Maley TC, Edelman PG. Clinically useful biosensor membrane development. In: Butterfield DA, ed. Biofunctional membranes. New York: Plenum Publishing, 1996:45-69.

[4.] NCCLS Document EP7-P. Interference testing in clinical chemistry; proposed guideline. Wayne, PA: NCCLS, 1986.

[5.] D'Orazio P, Parker B. Interference by the oxidizable pharmaceuticals acetaminophen and dopamine at electrochemical biosensors for blood glucose [Abstract]. Clin Chem 1995;41:S156.

[6.] D'Orazio P. Interference by thiocyanate on electrochemical biosensors for blood glucose [Letter]. Clin Chem 1996;42:1124.

[7.] Foos J, Blake D, Degen B, Taggliaferro D. New generation of solid-state sensors for electrochemical measurements: [Na.sup.+], [K.sup.+], [Ca.sup.++], Cl [Abstract]. Clin Chem 1996;42:S281.

[8.] Orvedahl D, Chan ADC, Murphy C, Fennyl S, Krouwer J. New generation of solid-state sensors for electrochemical measurements: p[O.sub.2] [Abstract]. Clin Chem 1996;42:S282.

Paul A. D'Orazio, Thomas C. Maley, Robert R. McCaffrey, Andy C. Chan, Donna Orvedahl, Joe Foos, David Blake, Sue Degnan, John Benco, Chris Murphy, Peter G. Edelman, and Hans Ludi* (Chiron Diagnostics, 63 North St., Medfield, MA 02052; * author for correspondence: fax 508-359-3955, e-mail
Table 1. Sensor performance summary for whole-blood
 samples. (a)

Sensor Measurement range Use-life, days (b) cv, % (c)

Glucose 0.5-55 mmol/L 60 6
Lactate 0-30 mmol/L 12 10
[PO.sub.2] 0-600 mmHg >30 1.5
[PCO.sub.2 (HC03) 10-150 mmHg >30 2.5
pH 6.5-7.8 >30 0.008
Na 100-200 mmol/L >30 1
K+ 0.5-15 mmol/L >30 1
[Ca.sup.2+] 0.2-5.0 mmol/L >30 1.5
[CI.sub.-] 60-140 mmol/L >30 1.5

(a) Performed with whole blood supplemented at three concentrations.
A 9-sample, multifactorial experiment was run daily to determine
precision, carryover, and dynamic range. The sensors were tested
initially and at 30 days for selectivity and interferences.

(b) Determined by exposure to >1000 protein samples (serum or blood).
Use-life was defined as meeting all performance specifications with
respect to accuracy, precision, and selectivity. This was assessed by
comparing performance with commercial systems (e.g., Chiron
Diagnostics 860). Planar sensors performed at least equal to the
commercial systems for whole blood, serum, and quality-control

(c) Within-run precision in reference interval values in whole blood.
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Title Annotation:Poster Session
Author:D'Orazio, Paul A.; Maley, Thomas C.; McCaffrey, Robert R.; Chan, Andy C.; Orvedahl, Donna; Foos, Joe
Publication:Clinical Chemistry
Date:Sep 1, 1997
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