CA1121242A - Fiber optic ph probe - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A fiber optic pH probe suitable to be im-planted in tissue for physiological studies is disclosed.
The probe includes an ion permeable membrane envelope which encloses the ends of a pair of optical fibers.
A pH sensitive dye indicator composition is present within the envelope. The probe operates on the concept of optically detecting the change in color of a pH
sensitive dye.

Description

~2~Z~2 dACKGROUND AND 5~MMARY OF THE INVE~TION

The present invention relates to a fiber optic pH
probe which is suitable to be implanted in the body of a human or other animal for physiologlcal studies.
More particularly, the present invention relates to a fiber optic pH probe in the~form of a miniature pH
sensor, constructed with the use of reversible dye indi-cators and single fiber optics, which is small enough to pass through a 22 gauge hypodermlc needle and which can be implanted in tlssue for physiological studies. , -The device lS particularly well sulted for use during exercise of the subject being studled ~ ¦
A determlnation of pH is desirable in a wlde variety of b~ologlcal studies. In partlcular, for studies of blood oxygen content, pH is an important parameter for the oxygen-hemoglobin dissociation curve.
¦ For some diseases, e.g., sickle cell anemia, it is de-sirable tc determine this curve in VlVO during exercis- ¦
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- 2 -Previous pH sensors have included sensors such as the glass electrode pH sensor built into a hypodermic '' needle as described by J.D. Czaban et al., Analytical Chemistry, 47, No. 11, 1787-92 (September 1975) and ibid., 48, No. 2, 277-81 (February 1976). While such electrodes are suitable for some purposes, their rigid needle construction is not desirable in certain physio-logical studies, such as exercise studies, because of irritation. Furthermore, such electrode systems have the inherent risk of electrical hazard.
Other devices for measuring pH are described, for example, in U.S. Patents Nos. 4,033,330 and 4,041,932.
Such prior art devices are designed to measure pH from outside the body and are not constructed so~as to be easily implanted within the body.
8y the present lnvention, there is provided an im-plantable pH measuring device wh~ch is of flexible con- I
struction and which can be placed in muscle or other tissue for in vitro or in VlVO use without a needle ~
20 remaining, The pH probe of the present lnvention is ¦ ~ ;
easily assembled, is potentially very low in~ cost and is thus capable of being readily manufactured in quan-tity for use as a disposable~item. The present pH probe incluaes an icn permeable mem rane envelope which enclo-: ~

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ses the ends of a pair of optical fibers. A pH sensitive dye indicator composition is present within the envelope. The probe operates on the concept of optically detecting the change in color of a pH sensitive dye.
Thus" in accordance with ~he present teachings, a fiber optic pH
probe is provided which is suitable to be implanted in tiss~e for physiological studies and comprises an ion permeable membrane in the form of a hollow, elongated cylinder having a distal end and a proximal end, the membrane is sealed at the distal end thereof. A pair of optical fibers is provided having distal and proximate ends located substantially parallel to each other and ad;acent to each other at their distal ends, the fiber distal ends being located at the proximal end of the hollow membrane. A pH sensitive color-changing dye-containing solid material is provided located within the hollow membrane, the ion permeable 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 material. ; ~`
;' ' ~ `.': : ' BRIEe DESCRIPTION OF THE DRAWINGS ~`

, The advantages and features of the present invention will be more fully understood from the following description of the preferred embodi~
.-.
ments, taken in conjunction with the accompanying drawings, wherein: j-~:: ::.
Figure l~is a perspective view of the distal end of the ~H~probe ``
of the present invention:
Figure 2 is a graph showing indicator dye optical absorption;and Figure 3 is a graph showing an experimental determination of returned light intensity ratio~as a function of pH.

DESCRIPTION OF THE PREFERRED EMBODDMENTS

In the embodiment of the invention as illustrated in Figure 1, there is shown a fiber optic pH probe 10 which includes an ion permeable membrane envelope 11 which encloses the distal ends of a pair of optical ' ' ` '

- 3 -~; ' fibers 12, 13. The envclope 11 is preferably in tubular form, i.e., a hollow, elongated cylinder, which fits closely about the twc fibers 12, 13.
Retained within the membrane 11 and distal to the ends of the fibers 12, 13 is a pH-indicating dye-containing composition 14. A suitable sealing material such as, for example, ultraviolet light-setting optical cement, is employed to seal the distal end 15 of the membrane 11. Such sealing material is also employed where the optical fibers 12, 13 enter the membrane 11, in order to hold the assembly securely together.
The membrane envelope 11 may be of any suitable material which is sturdy enough to provide a protective sheath for the components enclosed therein while being sufficiently flexible or non-brittle so as to be readily positioned within various parts of the body without breaking. An additional requirement for the membrane 11 is that it have pores of a size large enough to allow passage of hydrog-en ions while being sufficiently small so as to pre-clude passage of the dye materials. One membrane 11 material which has been employed with good results is a dialysis tubing known as Cuprophane ~ B4AH
a regenerated cellulose material having the properties as shown in Table I.

TABLE I
Measured Membrane Strength Properties of Cuprophane ~ B4AH

Ultimate Ultimate Tensile Break Strength Elongation greater greater than 300 than gram 20 %
This particular Cuprophan ~tubing which was employed had an inner diameter of 300 microns and a wall thickness of 19 microns.
The pH-indicating dye-containing material 14 may be any of 30 various such materials which, when employed in conjunction with the membrane `
11, will result in the dye being retained in the optical region within the membrane 11 without diffusing therethrough, and which will provide an indi-cation of pH within the range of interest.

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In selecting the dye material 14, it should be kept in mind that the material 14 to be employed may be a dye itself, if the dye molecules of the particular dye are sufficiently large that the dye will not diffuse out of the optical region through the walls of the membrane 11. Examples of such dyes are the "natural" dyes obtained from plants, such as azolitmin. Such dyes are of such molecular size that they will not diffuse readily and can be used as pH indicators. If the dye itself is employed rather than z larger molecular weight composition such as a polymer to which the dye is attached, one problem which may arise is that the corresponding membrane 11 employed with such relatively low molecular weight dye materials will have a pcre size which will be so small as to result in a rate of diffusion of hydrogen ions which is too slow to provide a good speed of response to pH. This is particu-larly true when it is desired to measure rapid changes of p~ during exercise of the patient being studied.
If the dye to be employed does not have sufficient molecular size, an alternative procedure is to employ a dye which is bound to another material, such as a polymer. The dyes which are usually employed include pH indicating dyes such as phthaleins, and sulfonphthaleins. Methods for bonding such dyes with other molecules include such known procedures as the use of a Mannich -type reaction, i.e., formaldehyde condensation cf the dye with an amine group, as in preparation of the complexones. Such reactions are described, for example, in the following publications, incorporated herein by reference RØ Cinneide, Nature, 175, 47 (Jan. 1, 1955~;R. Prible, Analyst, 83, 188-95 (1958); Swiss Patent No. 298, 194 (July 1, 1954);U.S. Patent No. 2,745,720;
and G. Schwartzenbach et al., Complexomet _c Titrations, Methuen Press (Barnes and Noble), 1969.
Another approach which attaches the dye to a polymer is to derivatize the polymer, i.e., attach groups such as an amino group to the polymer so that the dye can be subsequently reacted, such methods being des-cribed for example, by J.K. Inman et al., Biochemistry, 8, 4074-82 (1969).

These techniques require prior formation of a polymer, with subsequent dye attachment.

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Another method of attaching a pH indicating dye to another material is that which employs dyes with the isothiocyanate group attached, such dyes being synthesized by a process as described, for example, in U.S.
Patent ~o. 2,937,186, incorpora~ed herein by reference. Examples of such dyes -include 5(6) Rhodamine B - CNS and Fluorescein - CNS, both of which are avail-able from Eastman Organic Chemical Co. A dye with the - CNS group on it reacts, via this group with active hydrogens and therefore can be attached to many other types of moieties, as described, Eor example, in U.S. Patent No.
3,847,745 and also in J.J. Haaijman et al., Journal of Immunological Methods, 5, 359-74 tl974).
The latter publication describes the attachment of fluorescein and rhodamine isothiocyanates to Sephadex beads, a polysaccharide hydrophilic polymer available from the Pharmacia Corp.
A chemical method of attaching dyes in general involves the use of cyanuric chloride which will react with an amino group on a dye, and this ~`` `
will then react with an amino or hydroxyl group on a support to chemically link the dye to the support. Such methods are described, for example, in C.V. Stead, The Chemistry of Reactive Dyes and Its Relevance to Intracellular Staining Techniques, pp 41~59;and also in S.B. Katen et al., Intracel _lar ~O Staining in Neurobiology, Springer - Verlag (1973).
Dyes for pH indication may also be prepared by microencapsulation, e~ploying techniques as described, for example, in Science, 193, 134-137 (July 9, 1976).
Another method for preparing dye compositions is that ln which `~
free radical polymerizàble gels, e.g., acrylates, are prepared, containing a `i' functional group on the monomer, such as hydroxyl, amine or amide, with the dye being reacted with the functional group either before or after polymeri- ~ `
zation.
A method for bonding the dye to a hydrophilic polymer is still 3Q another alternative procedure which may be employed to produce a dye-contain-ing material suitable for use in the pH probe of the present invention. In this procedure, a suitable dye is copolymerized with an acrylic based monomer 1, '-.z~z~

such as acrylamide, methyl methacrylate or hydroxymethyl methacrylate (Hydron) to form a hydrophilic copolymer in which the pH indicating dye is bound so that a non-diffusible form of the dye is produced. Specific dyes which may be copolymerized in this manner include phenol red, brilliant yellow (CI24890) and rosolic acid. In addi~ion, the dye - polymer combination can be produced in the form of microbeads by emulsion (water-in-oil) polymerization. Specific methods for preparation of such copolymers is described in Canadian application 316,439 filed simultaneously herewlth, in the name of John I. Peterson, said application being commonly assigned.
The optical fibers 12, 13 may be of a length such as about 6 feet, `~
for example, so as to provide adequate flexibility of movement of the device 10 relative to a light source and a light sensor, both of which may be of ~ -conventional construction and thus are not shown in the drawings. One of the fibers 12 is connected at its proximal end to the light source while the other fiber 13 is connected at its proximal end to the light sensor. The fibers 12, 13 are of conventional construction, with one of the fibers employed to con-duct light toward the probe 10 from the light source, while the other fiber is employed to receive and conduct light from the probe 10 to the sensor.
The fibers 12, 13 may be, for example, plastic optic fibers having a diameter of about 1/8 mm. The two fibers 12, 13 may be enclosed along their length exterior to the probe by al mm diameter Teflo~ ~ubing for mechanical protection. ~;
The indicator dye, phenol red, is an organic acid (HA) which dissociates into hydrogen ion (~ ), and an anion, (A-) according to the following equilibrium:

K ~ (H ) (A ) ~constant where the brackets represent solution activities or concentrations. By definition, pH- log ~1 (H~)~, so that - pH = -log K ~log ~ .pK-log tconstant (HA) optical density of A-The (A-) absorbs light at 560nm as a function of its concentration as shown experimentally in Fig. 2, and the (HA) is not independent of (A-). Thus pH
and optical density at 560nm are uniquely related. Since the absorbance at 485nm is nct a function of pH, (see Fig.2), this wavelength can be used as a reference for normalizing the 560nm light. An alternative is to use light at a wavelength greater than 600 nm which is also independent of pH since the dye does not absorb light in that region of the spectru~. In the probe 10, where the light is backscattered th~ough the dye from one fiber into the other fiber, it happens that over the pH range of interest, the relation between pH
and the ratio of intensity at 560 nm to the intensity at 485nm is linear, as shown in the data of Fig. 3.
As previously mentioned, the light source and light sensor are of conventional construction. Thus, for example, excitation light may be pro- S
vided by a 150 watt tungsten high intensity projection lamp (ELV). Forced convection cooling, and an infrared reflecting "heat mirror" are used to pre- ~
vent the plastic fiber from overheating. The large numerical aperture of the ~- -plastic fiber (approximately 1.2 radians total included angle) aids in effi- ~
cient light collection. Measurement of the return light may be performed by ~"'r' a solid state photodiode operational amplifier combination (such as EG&G HAV-4000A) followed by a high gain instrumentation amplifier. Eight narrow band interference filters ~approximately 5nm wide half power pointe) with central wavelengths at 560 and 485nm are alternated on a motor driven wheel placed ~;
between the exit of the return fiber and the light sensor. An alternative is to use a single filter for 560nm light and a single filter for 485nm light (or red light) which are alternately positioned in place by a stepper motor.
The increased dwell time allows more time for electronic signal averaging and subsequent noise suppression. Synchronization pulses generated by two ~-addi~ional photodiodes detecting light shining through appropriately placed holes in the filter wheel control three sample and-hold modules which measure 3Q the sensor output for (a) 560nm light, (b) 485 (or red) nm light, and (c) no input light. The temperature sensitive "dark" output is subtracted from the other two outputs, along with the output due to ambient light (when the :

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excitation light is off) before the light signal ratio is taken in an analog divider. Gain and level adjustments are subsequently made so that pH `
is directly displayed on a digital voltmeter.
In preparing the dye-containing material 14, light scattering particles such as, for example, polystyrene microspheres, of about 1 micron diameter may be added prior to incorporation of the dye material 14 into the hollow membrane 11.
The in vitro test measurements with the basic probe shown in Fig. 3 were performed with a laboratory spectrometer and have a standard deviation of 0.01 pH units. The probe dynamic response can be roughly char-acterized by a 0.7 minute time constant, its temperature coefficient is 0.02 `
pH units/ C, and the ionic strength coefficient at 37C is 0.01 pH units/8%
change in ionic strength.
It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, constructlon and arrangement of the parts without departing from the spirit a~d scope of the invention~or sacrificing its material advantages, the forms hereinbefore described being merely preferred embodiments thereof.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fiber optic pH probe suitable to be implanted in tissue for physiological studies, comprising: an ion permeable membrane in the form of a hollow, elongated cylinder having a distal end and a proximal end, said membrane being sealed at the distal end thereof; a pair of optical fibers having distal and proximal ends located substantially parallel to each other and adjacent to each other at their distal ends, said fiber distal ends being located within the proximal end of said hollow membrane; pH sensitive color-changing dye-containing solid material located within said hollow membrane, said ion permeable membrane having pores of a size large enough to allow passage of hydrogen ions while being sufficiently small so as to to preclude passage of the dye-containing material.

2. The probe of Claim 1, further including a light source connected to the proximal end of one of said optical fibers and light sensor means connected to the proximal end of the other optical fiber.

3. The probe of Claim 1, wherein said dye-containing solid material comprises a hydrophilic copolymer of an acrylic based mononer and a dye.

4. The probe of Claim 3, wherein said acrylic based monomer is selected from the group consisting of acrylamide, methyl methacrylate and hydroxymethyl methacrylate.

5. The probe of Claim 3, wherein said dye is selected from the group consisting of phenol red, rosolic acid or brilliant yellow (CI 24890).

6. The probe of Claim 1, wherein said dye-containing solid material is present in the form of microbeads.

7. The probe of Claim 1, wherein light scattering particles are incorporated with said dye-containing material.

8. The probe of Claim 7, wherein said light scattering particles comprise polystyrene microspheres.