Fiber-optic physiological probes - Patent Review 5000901

Description Submit all comments and votes

FIELD OF THE INVENTION

This invention relates to fiber-optic sensors suitable for monitoring physiological pH and blood gas concentrations.

BACKGROUND OF THE INVENTION

In recent years fiber-optic chemical sensors have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, glucose, inorganic ions, and hydrogen ion, in liquids and in gases. Such sensors are based on the recognized phenomenon that the absorbance and in some cases the luminescence of certain indicator molecules is specifically perturbed in the presence of certain analyte molecules. The perturbation of the luminescence and/or absorbance profile can be detected by monitoring radiation that is absorbed, reflected, or emitted by the indicator molecule when illuminated in the presence of a specific analyte. Fiber-optic probes have been developed that position an analyte-sensitive indicator molecule in a light path that is typically made up of a pair of optical fibers. One fiber transmits electromagnetic radiation from a light source to the indicator molecule; the other fiber transmits the return light from the indicator molecule to a light sensor for measurement. The indicator molecule is typically housed in a sealed chamber whose walls are permeable to the analyte.

For example, the fiber-optic pH probe disclosed in U.S. Pat. No. 4,200,110 includes an ion-permeable membrane envelope which encloses the distal ends of a pair of optical fibers. The envelope is a short section of dialysis tubing which fits closely about the two fibers. A pH-indicating dye-containing solid material, e.g., phenol red/methyl methacrylate copolymer, is packed tightly within the membrane distal to the ends of the fibers. Cement is applied to seal the distal end of the membrane and also the proximal end where the optical fibers enter the membrane. The membrane has pores of a size large enough to allow passage of hydrogen ions while being sufficiently small so as to preclude passage of the dye-containing solid material. The probe operates on the concept of optically detecting the change in color of the pH-sensitive dye, e.g., by monitoring the green (570 nm) intensity of phenol red. One of the fibers is connected at its proximal end to a light source, while the other fiber is connected at its proximal end to a light sensor. Light is backscattered through the dye from one fiber into the other fiber. In preparing the dye-containing material, light scattering polystyrene microspheres of about 1 micron diameter may be added prior to incorporation of the dye material into the hollow membrane. A similarly constructed fiber-optic oxygen probe, employing a fluorescent dye sensitive to oxygen quenching, is disclosed in U.S. Pat. No. 4,476,870.

U.S. Pat. No. 4,344,438 is of interest for disclosing a fiber-optic chemical sensor that employs a single optical fiber. Here again, a short section of dialysis tubing is mounted atop the fiber as an analyte-permeable indicator-containing housing.

Such fiber-optic probes are small enough to pass through a hypodermic needle and flexible enough to be threaded through blood vessels for physiological studies. However, promising medical applications, such as continuous blood gas monitoring, have been hindered because experience has shown that such probes are difficult and expensive to manufacture and calibrate. Each probe must be exactingly constructed by hand under a microscope, a process that requires several hours per probe. Considerable unit-to-unit variability in calibration requirements results from the slight variations in the assembled configuration of the components. The unique signal response of each hand-crafted probe must be calibrated at the time of the assay, typically with at least two reference pH or other analyte concentration values to adequately define the calibration curve.

SUMMARY OF THE INVENTION

The invention provides a drift-free fiber-optic sensor suitable for monitoring physiological analyte concentration. An analyte-permeable matrix is disposed in the light path defined by the axial core at one end of an optical fiber segment. The matrix contains an indicator molecule covalently linked to a polymer, preferably methyl methacrylate/methacrylamidopropyltrimethylammonium chloride, N-vinylpyrrolidone/p-aminostyrene, methyl methacrylate/hydroxymethyl methacrylate, methyl methacrylate/N-vinylpyrrolidone, or methyl methacrylate/acrylic acid. Such polymers are preferably formulated in the range of from about 60:40 to about 80:20 wt/wt percent. In representative embodiments, the polymer is approximately 94:6 mole/mole percent of either methyl methacrylate/methacrylamidopropyltrimethylammonium chloride or N-vinylpyrrolidone/p-aminostyrene copolymer. Drift-free performance is obtained with such sensors having analyte-permeable matrices of significantly less than about 70 microns in thickness.

The indicator molecule, which is capable of responding to a targeted analyte in an optically detectable manner, is advantageously covalently linked to the polymer through an aminoarylalkylamine, such as 4-(aminophenyl)-ethylamine or 4-(aminophenyl)-propylamine. The indicator molecule may be an absorptive molecule, such as phenol red or carboxynaphthophthalein (hydrogen ion analyte), in which case the indicator molecule may be covalently linked to the polymer through either an azo-amide or an amidyl-amide linkage. The indicator molecule may be a luminescent molecule, such as carboxynaphthofluorescein (hydrogen ion analyte) or an oxygen-quenchable porphyrin derivative.

The subject sensor may be provided with a reflector disposed in the light path distal with respect to the optical fiber segment to the analyte-permeable matrix. Suitable reflectors include gold foil or films of titanium dioxide, zinc oxide, or barium sulfate.

A pCO.sub.2 sensor is configured with a gas-permeable but ion-impermeable membrane encapsulating an analyte-permeable matrix that includes a base having a pKa ranging from about 6.0 to about 7.8. The outer membrane may be a silicone, polycarbonate, or urethane. The base may be an inorganic salt, such as bicarbonate, in which case the analyte-permeable matrix should include an antioxidant. Alternatively, the base may be a polymeric base containing, e.g., 2-vinylpyridine, 4-vinylpyridine, histamine, 1-vinylimidazole, or 4-vinylimadazole. Gas-permeability is afforded to the matrix by a minor component of hydrophilic polymer such as polyethylene glycol.

A plurality of the foregoing pH, pO.sub.2, and pCO.sub.2 sensors may be disposed together in substantially coterminal array to make a multi-variable probe, which may also include a thermocouple wire. To minimize blood clotting during in vivo use, the multi-variable probe is preferably configured with the pCO.sub.2 sensor distally disposed to pH and pO.sub.2 sensors that are substantially colinearly arrayed.

Also provided is a method of mass producing a fiber-optic sensor suitable for monitoring physiological analyte concentration. A polymer film of substantially uniform thickness is cast, the polymer film being permeable to an analyte in solution and including a covalently-linked or admixed indicator molecule capable of responding to the analyte in an optically detectable manner. From the film are cut or punched a multiplicity of disc-shaped indicator matrices. The individual indicator matrices are affixed to optical fiber segments to produce fiber-optic sensors having substantially uniform performance characteristics.

The subject probes are employed to monitor analyte concentration in a fluid, by contacting the fluid with the analyte-permeable matrix of the fiber-optic sensor, irradiating the matrix through the optical fiber segment at a first wavelength band corresponding to a region of analyte-dependent absorbance by the indicator molecule, measuring absorbance by or emission from the analyte-permeable matrix at a predetermined second wavelength band, and determining the concentration of the analyte in the fluid as a function of the measured absorbance or emission. To minimize blood clotting, the sensor should be inserted through a catheter means into a patient's bloodstream so that the analyte-permeable matrix projects from about 0.5 to about 1.5 mm beyond the end of the catheter means into the bloodstream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a representative pH sensor;

FIG. 2 is a schematic cross section of a representative pO.sub.2 sensor;

FIG. 3 is a schematic cross section of a representative pCO.sub.2 sensor; and

FIG. 4 is a representative multi-variable probe suitable for real-time monitoring of pH, pO.sub.2, pCO.sub.2, and temperature in the bloodstream.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, fiber-optic sensor 10 has an analyte-permeable indicator matrix 12 coated on one end of an optical fiber segment 14. Covalently linked to the indicator matrix 12 is an indicator molecule that responds to the presence of a specific, targeted analyte in an optically detectable manner. The indicator matrix 12 is permeable to the analyte and has a thickness of about 70 microns or less, preferably on the order of 50 to 15 microns.

By optical fiber 14 is meant a fine, transparent filament, a composite of two materials having different refractive indices, that is capable of transmitting electromagnetic radiation. An optical fiber 14, or light guide, suitable for practicing the invention has an axial transmissive core that is coaxially surrounded by a cladding material of lower refractive index. The cladding serves to confine electromagnetic energy to the core region of the fiber by substantially total reflection at the core-cladding interface. An optical fiber segment 14 suitable for monitoring physiological homeostasis may have a diameter of about 250 or 114 microns and a length on the order of 0.5 meter or more. The optical fiber segment 14 may be composed of glasses but is preferably made of plastics.

To manufacture the fiber-optic sensor 10, a clean fiber end is first prepared, that is, optical fiber segment 14 is cleaved and polished to produce a square, smooth fiber end. Such a clean flat fiber end can be prepared by procedures well known in the art by jointing optical fibers. The indicator matrix 12 is then applied to the fiber end. In one embodiment, the fiber end is painted with a resin emulsion containing a resin in a solvent carrier. The resin is selected to render the resulting indicator matrix 12 permeable to the analyte in the test environment, and contains a polymer in which an analyte-sensitive indicator molecule is covalently linked (or, for certain indicator molecules, admixed). The resin emulsion may be deposited on the end of the fiber 14 by dip coating, brushing, spraying, or other conventional coating techniques. The solvent carrier is thereafter drawn off, e.g., by evaporation, to leave an indicator matrix 12 adhering to one end of the optical fiber 14. In an alternative embodiment, a disk-like indicator matrix 12 is preformed and affixed to the fiber end. As discussed below, a reflective foil 16 may also be provided distal to the indicator matrix 12. The resulting fiber-optic sensor 10 can be coupled to conventional instrumentation to detect and monitor ambient analyte concentration in liquid or gaseous test environments.

The choice of materials to be used in fabricating the fiber-optic sensor 10 is influenced by the need to satisfy simultaneously many requirements. Most importantly, the indicator matrix 12 must immobilize the indicator molecule in the light path defined by the axial core of the fiber 14. Otherwise, signal drift will result from leakage of indicator molecules, especially water-soluble molecules like phenol red, from the remarkably thin indicator matrix 12. Water-soluble indicator molecules must therefore be covalently bonded to a component of the resin 12. The resulting sensor 10 is drift-free, that is, there is no detectable leakage or diffusion of indicator molecule from the matrix 12 in the environment of use over the time course of the assay.

The indicator matrix 12 must also permit the free passage in and out of the analyte substance, that is, matrix 12 must be analyte-permeable. For physiological applications in which the analyte is dissolved or dispersed in aqueous solutions, the indicator matrix 12 should be hydrophilic as well as porous to the analyte substance. However, the hydrophilicity of the matrix 12 must be regulated to prevent undue swelling, with attendant risk of dissociation from the fiber end, when the indicator matrix 12 is immersed in aqueous solutions such as blood, lymph, extracellular fluid, and/or serum. For certain applications, the matrix 12 should be semipermeable as well, having minute openings or pores of a size large enough to permit passage of the targeted analyte substance but sufficiently small so as to preclude passage of certain dissolved or colloidal substances, e.g., fluorescent blood proteins, that might chemically or physically interfere with the assay.

The indicator matrix 12 must also be optically clear. In addition, the refractive index of the matrix 12 should be reasonably well matched to that of the optical fiber core, in order to minimize light scattering effects such as Fresnel losses.

The constituent resin must be capable of forming and sustaining the indicator matrix 12 on the fiber end. The resin must produce homogeneous resin emulsions or solutions, and should be readily soluble in conventional solvents, particularly solvents having high vapor pressures. Inexpensive, readily available solvents such as those typically used for painting and coating applications are preferred. The resin emulsion or coating solution should have good film-forming properties, including uniform flow of the solvent casting solution, as physical anomalities such as bubbles in the matrix 12 can cause signal noise.

The matrix 12 should not shrink or crack upon drying and should not swell noticeably in aqueous solutions, as there should be no differential movement of the indicator matrix 12 vis-a-vis the light-transmitting fiber core during the time course of use. The indicator matrix 12 should also retain its rigidity and strength during use, e.g., by having sufficient wet mechanical strength to maintain its integrity while being manipulated through blood vessels. Sufficient wet adhesion strength between the matrix 12 and the fiber is likewise required, and plastic optical fibers 14 are preferred over glass composites for having more available bonding sites for film adhesion. Plastic fibers 14 are also relatively inexpensive and easy to cleave, polish, and bend. The high ductility of plastic fibers 14 is advantageous for in vivo applications. Glass fibers 14 can be conventionally surface treated to increase the adhesion of the indicator matrix 12, such as by the use of silanes. Such surface-treated glass fibers 14 have transmission advantages for operating at short wavelengths below the visible. For in vivo blood sensors 10, plastic fibers 14 having polymethyl methacrylate cores have several advantages over glass fibers, including bendability, thinness, low cost, and ease of cleaving.

The thickness of the indicator matrix 12 over the axial fiber core can vary uniformly in the range of from about five microns to about several hundred microns, the preferable upper limit being about seventy microns, and most preferably 20 to 35 microns, in order to minimize response time and light scattering effects. The indicator matrix 12 must uniformly cover at least the light-transmitting core on the end of the fiber 14. In practice, the matrix 12 can in addition overlap the cladding on the fiber end, as well as adjacent lateral surfaces of the fiber 14.

For physiological sensors such as 10, a resin that satisfies the foregoing requirements is made by copolymerizing a mixture of about 94 mole percent (mole %) methyl methacrylate (MMA) and about 6 mole % methacrylamidopropyltrimethylammonium chloride (MAPTAC; U.S. Pat. No. 4,434,249). Polymethyl methacrylate-based material is an especially appropriate matrix component, because of good refractive index match, when used with plastic optical fibers 14 having methyl methacrylate cores. The above-stated copolymer is highly permeable to water and small ions, especially anions, while retaining all the advantages mentioned above. Methyl methacrylate can alternatively be copolymerized or alloyed with other ionogenous or neutral monomers, such as hydroxymethyl methacrylate, N-vinylpyrrolidone, or acrylic acid, to confer analyte permeability to the resulting matrix 12. N-vinylpyrrolidone/p-aminostyrene copolymer 60:40 to 80:20 wt/wt% is another suitable resin material. Suitable solvents for these resins are known to include alcohols, N,N-dimethylacetamide (DMAC), N,N-dimethylformamide, methyl ethyl ketone, tetrahydrofuran, esters, aromatic and chlorinated hydrocarbons.

The indicator molecule is selected to respond optically in the presence of the targeted analyte substance when immobilized in the indicator matrix 12. The response of the indicator molecule should be highly specific for the targeted analyte in order to minimize interference and background signal noise. For continuous monitoring of analyte concentration, the reaction or response between the indicator molecule and the analyte should be reversible as well as sensitive and specific. Suitable analyte-sensitive indicator molecules are known in the art and can be selected based upon the particular analyte substance whose detection is targeted and upon other factors as described herein.

Covalent bonding functions to immobilize water-soluble indicator molecules within the indicator matrix 12 but otherwise must not significantly impact upon the sensitivity, specificity, and reversibility of its optical response to the analyte. Thus, analyte-sensitive sites on the indicator molecule must not be eliminated or sterically hindered upon covalent binding to the resin. The indicator molecule should therefore be uniformly bound to the resin in a site-specific manner that preserves the optical responsiveness of the indicator molecule to the analyte, using a reaction protocol that prevents or substantially eliminates heterogeneous reaction products.

For this purpose, aminoarylalkylamines are preferably employed to covalently link the indicator molecule to a polymer, which is thereafter admixed in solvent with other resin components to form an emulsion or solution which may be painted on the fiber end. Suitable aminoarylalkylamines have the formula:

NH.sub.2 Ar(CH.sub.2).sub.n NH.sub.2

wherein Ar is nonsubstituted or preferably substituted phenyl and n is an integer. Preferably, n equals 2 or 3, in order to avoid hydrocarbon characteristics associated with longer alkyl chains, which in the case of pH indicator molecules tend to unacceptably displace the pKa of the linked indicator. The aminoarylalkylamine is preferably para-substituted. Exemplary aminoarylalkylamines for practicing the invention are 4-(aminophenyl)-ethylamine and 4-(aminophenyl)-propylamine.

Heterogeneous reaction products are prevented by specifically attaching the alkylamino moiety to the polymer before reacting the arylamino moiety with the indicator molecule. The aminoarylalkylamine is first attached to a polymeric resin component, such as MMA/MAPTAC, by reaction in ethanol at 70.degree. C. with triethylamine as a catalyst. The free arylamino group is then reacted with the indicator molecule of choice, for example by diazotization and coupling with indicator molecules such as phenol red that bear strongly electron-releasing groups, or by formation of an amidyl linkage with carboxylic acid-bearing indicator molecules. The available diazonium binding sites should be saturated with an excess of indicator molecules during this second reaction step, in order to provide a polymeric resin component containing a concentrated amount of the indicator molecule. Suitable protocols are set forth below in Examples 1 to 3.

In an exemplary sensor 10, a luminescent indicator molecule is covalently linked through 4-(aminophenyl)-ethylamine to MMA/MAPTAC copolymer in the indicator matrix 12. For example, the fluorescent indicator molecule carboxynaphthofluorescein is thereby incorporated into sensor 10 for monitoring the pH of physiological fluids, the carboxynaphthofluorescein reacting specifically with hydrogen ion in a fluorescent manner at physiological pH. A resin emulsion can be prepared by admixing one part polymeric component saturated with carboxynaphthofluorescein linked through 4-(aminophenyl)-ethylamine, with about nineteen parts other resin component(s), in a suitable solvent. Suitable protocols are set forth in Examples 4 and 6. To construct the sensor 10, a clean end of an optical fiber 26 is simply dipped into or painted with the admixed resin emulsion so as to coat the fiber end 24 with the emulsion. The solvent is then allowed to evaporate, leaving adhering on the fiber end a proton-permeable matrix 12 containing the linked fluorescent indicator molecule. The thickness of the indicator membrane need be only from about 25 to about 60 .mu.m, preferably about 40 .mu.m or less, when a luminescent indicator molecule is employed in sensor 10.

The resulting fiber-optic sensors 10 can be coupled to instrumentation systems known in the art in order to monitor the pH of physiological fluids as functions of luminescent intensity or lifetime. For example, the sensor 10 can be threaded through a hypodermic needle to contact the indicator matrix 12 with a patient's bloodstream. The other, proximal end of the fiber-optic segment 14 is coupled to instrumentation including a light source and a photodetector. The light source irradiates the indicator matrix 12 through the fiber-optic segment 14 with light at a wavelength band corresponding to a region of analyte-dependent absorbance by the indicator molecule. Luminesc