WO2004090510A1

WO2004090510A1 - Optical fibre needle for spectroscopic analysis of liquids

Info

Publication number: WO2004090510A1
Application number: PCT/DK2004/000267
Authority: WO
Inventors: John Erland ØSTERGAARD
Original assignee: Alight Technologies A/S
Priority date: 2003-04-14
Filing date: 2004-04-14
Publication date: 2004-10-21

Abstract

The present invention relates to an optical fibre needle for handling small liquid samples for spectroscopy purposes. The needle is formed by an optical fibre having one or more through holes, preferably a crystal fibre or a photonic band gap fibre. The needle can collect the liquid by drawing the liquid into its through holes by capillary action. When held in the through holes, the liquid is easily illuminated by guiding light in the optical fibre needle, and emitted or scattered light from the liquid is easily collected and guided to a photo detector such as a spectrometer. The optical fibre may be used as a hypodermic needle for collecting blood samples or as a pipette for collecting samples from a microtitherplate in the screening of chemical compositions for drugs.

Description

OPTICAL FIBRE NEEDLE FOR SPECTROSCOPIC ANALYSIS OF LIQUIDS

FIELD OF THE INVENTION The present invention relates to handling of and spectroscopy on liquids. More specifically, the invention relates to collecting and performing spectroscopy on extremely small amounts of liquid, such as blood samples or in the screening of chemical compositions for drugs.

BACKGROUND OF THE INVENTION

Concentrations of different chemical compounds have been detected in blood using Raman spectroscopy with a laser diode at 830 nm (Enejder et al. Optics Letters 27, p. 2004 (2002)). These compounds are e.g. glucose, protein, urea, cholesterol, albumin, hemoglobin, bilirubin, hematocrit. The main limitation in the technique has been the detection of the Raman signal, which is wavelength shifted relative to the radiation exposing the blood sample. US 5,615,673 relates to Raman spectroscopy of dissolved gas in blood.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a needle for sampling a liquid to undergo spectroscopy.

It is another object to provide a method for performing emission spectroscopy on a liquid sample, wherein a large ratio of the radiation emitted or scattered from the sampled liquid is detected.

It is still another object to provide a needle which can collect and hold the liquid sample in a manner suited for performing spectroscopic measurements on the liquid sample.

It is a further object to provide a needle which can hold a liquid sample an efficiently guide light to illuminate the sample.

It is a further object to provide a needle which can hold a liquid sample and collect radiation emitted or scattered from the sampled liquid and guide it to a spectrograph.

In a first aspect, the invention provides a method for measuring an optical spectrum of a liquid, the method comprising the steps of providing an optical fibre having one or more through holes, - providing a light source, providing a light detector, - contacting the liquid with a first end of the optical fibre, drawing liquid into at least one of the one or more through holes, - exposing liquid held in the through hole(s) by guiding light from the light source in the optical fibre,

- guiding, in the optical fibre, scattered and/or emitted light from liquid held in the through hole(s), and - detecting the guided scattered and/or emitted light with the light detector to obtain an optical spectrum.

Preferably, the method according to the first aspect further comprises the steps of connecting the first or a second end of the optical fibre to an output of the light source and to an input of the light detector.

In a second aspect, the invention provides a system for measuring optical spectra of liquids, the system comprising an optical fibre having at least a first through hole for holding liquid and a core for guiding electromagnetic radiation, the core and the first through hole being formed so that an evanescent field of radiation to be guided in the core extends into the first through hole, the system further comprising a laser source to be connected to a first end of the optical fibre and a radiation detector to be connected to the first or a second end of the optical fibre.

Preferably, the measured optical spectrum is an emission spectrum, as the optical fibre efficiently collects light emitted or scattered from the liquid and guides it to the light detector. However, other types of spectroscopy, such as absorption spectroscopy, mat be performed according to the present invention.

In the present context, an optical fibre is a structure which efficiently guides electromagnetic (EM) radiation. Typically, an optical fibre has a core surrounded by a cladding, wherein some relationship between the core and the cladding confines EM radiation in the core so as to form a waveguide. This relationship may e.g. be a decrease in the refractive index when going from the core to the cladding - such as in index guiding fibres. The refractive index step may be obtained by applying different materials or by e.g. having through holes in the cladding thereby decreasing the average refractive index of the cladding. In another alternative, the relationship may be that the cladding does not allow propagation of light. This may be obtained by photonic bandgap effects resulting from a periodic modulation of the cladding region, e.g. formed by through holes. In such photonic bandgap fibres, the core may have a lower refractive index that the cladding, such as a hollow core.

From the above definition, it is to be understood that e.g. a hollow glass tube does not constitute an optical fibre.

Preferably, a first end of the optical fibre is adapted to penetrate a skin of a human or animal in that the first end is sharp or pointed. Optionally, the optical fibre is adapted to be used as a hypodermic needle in that the optical fibre is provided with a metal coating for mechanical strength and rigidity.

In a third aspect, the invention provides a system for measuring optical spectra of blood, the system comprising a metal coated crystal fibre having one or more through holes, a laser source to be connected to a first end of the crystal fibre, and a spectrograph to be connected to the first or a second end of the crystal fibre.

Preferably, the laser source is a VCSEL (Vertical Cavity Surface Emitting Laser) or a fibre laser.

In a fourth aspect, the invention is the use of an optical fibre having one or more through holes as a hypodermic needle.

In a fifth aspect, the invention is the use of a metal coated crystal fibre having one or more through holes as a hypodermic needle.

In a sixth aspect, the invention is the use of an optical fibre having one or more through holes as a sample collector for collecting a liquid, as a means for exposing liquid held in the one or more through holes to electromagnetic radiation, and as a means for collecting emitted and/or scattered radiation from liquid held in the one or more through holes.

It is an advantage of the invention that radiation emitted or scattered from a sample held in the through holes cannot escape the fibre except through the end of the fibre. This greatly improves the detecting efficiency of radiation emitted or scattered from the blood sample.

Radiation emitted and/or scattered by the liquid held in the interstitial holes of the crystal fibre will be emitted isotropically. Some of the light will be emitted substantially along the axis of the fibre and will naturally couple to the modes supported by the fibre. Light emitted in directions substantially along the long axis of the fibre will also be guided by the fibre, but here the efficiency will to a large degree depend on the type and design of the fibre as well as on the wavelength of the light.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the components of a system for sampling and analysing blood samples.

Figure 2 illustrates the use of optical fibre needle as a pipette for sampling from a microtitherplate.

Figures 3A and B show cross sectional views of different optical fibre needles. Figures 4-6 show different configurations for connecting an optical fibre needle to a light source and a light analyser.

Figure 7 shows a automated docking station for receiving a sampling device containing an optical fibre needle according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment illustrated in Figure 1, the invention is a hypodermic needle 10 formed by a mechanically stabilised crystal fibre 12. A first end 13 of the fibre 12 is sharpened so that it may penetrate the skin 11 of the patient to make contact with the blood. The blood extraction is done very similarly to how blood samples are extracted using conventional hypodermic needles, i.e. by entering the tip into the area or vein from which a blood sample must be extracted. Crystal fibres 12 have through-going interstitial holes 14 surrounding a core part 15 shaping a refractive index contour confining radiation in its core part 15. Upon making contact with the blood, capillary forces can draw blood into the through holes 14 and hold it there, also when the needle 10 is pulled out. Having the blood inside a crystal fibre 12 makes it directly accessible to optical radiation under controllable conditions. By connecting a second end 16 to a laser source 17, the blood can be irradiated to perform spectroscopy. By connecting the first end 13 to a detector 18 such as a spectrograph, an emission spectrum 19 can be recorded.

In some cases, the hypodermic needle 10 may be produced with a small portion of anti- coagulation liquid such as heparin into the holes inhibiting the blood from coagulating too fast for the extraction process and the subsequent measurement process. Also, to obtain a good optical input/output in first end 13 of the fibre 12 after having received the sample, the fibre 12 can be cleaved near the first end 13 to remove blood drops or other unwanted material before connecting it to laser source 17 or detector 18. In another embodiment illustrated in Figure 2, the crystal fibre 12 forms a pipette 21 for collecting liquid samples 22 from a microtitherplate 23. Upon making contact with the liquid samples 22, through-going interstitial holes 14 draws up liquid by capillary forces. With the small diameter of the holes (typically in the micrometer range) the capillary forces will be so strong that liquid can be extracted millimetres or centimetres or longer into the holes. As for the embodiment described in relation to Figure 1, an emission spectrum of the liquid held in the through holes of the fibre can be made.

The optical fibre needle can be any crystal fibre having through-going interstitial holes. Crystal fibres are typically rather thin with a cladding of 125 micrometer and therefore too fragile to be used as a hypodermic needle. A metal coating is therefore evaporated on the fibre providing it with the mechanical stability enabling the use as a hypodermic needle and penetrate skin without bending or breaking. In the production stage of the hypodermic fibre it may prove advantageous to cleave the fibre after the metal coating to open the interstitial holes in the fibre tip. Such metal coated fibre can have an outer diameter of e.g. around 0.2 millimetre, comparable to the thinnest hypodermic needles available today. Such thin needles provides pain-free use similar to needles used in acupuncture, i.e. the patient will not feel the hypodermic needle entering the skin. However, some applications may require a thicker metal coating resulting in a larger diameter than 0.2 millimetre. By removing the metal coating in the first end, the fibre tip can be made sharp so that it can be stuck through the patient's skin. The tip of the fibre can also be sharpened further by polishing.

A number of parameters determine the quality of the emission spectrum:

- The power and the spectrum of the laser, a high power monochromatic laser source is preferred.

- The overlap between the fibre mode(s) with the blood filled interstitial holes, the crystal fibre can be designed to give as large an overlap as possible.

- The length over which the light interacts with the blood, i.e. the length of the blood sample in the optical fibre needle. The holes should preferably be filled along as long a length of the optical fibre needle as possible.

- The collection efficiency of radiation scattered and/or emitted from the sample upon exposure. If there is total internal reflection in the fibre at the wavelength of the scattered and/or emitted radiation, they can only escape though the ends of the fibre.

Figure 3A and B shows cross-sections of a crystal fibre 12 with and without liquid filling the interstitial holes. In Figure 3A, the evanescent field 30 of the fibre mode extends into the region holding the air-filled interstitial holes 14 surrounding the core 15. When the interstitial holes is filled with liquid as in Figure 3B, the refractive index in the holes is increased which enlarges the transverse mode(s) of the fibre giving a larger overlap between the radiation in the evanescent field 30 and the liquid-filled interstitial holes 31. Thus, the evanescent field of radiation guided in the core has a large overlap with liquid held in the interstitial holes of the crystal fibre, thereby providing an efficient exposure of the liquid.

Having the liquid sample in the through holes of the crystal fibre 12, the fibre is connected to a laser 17 and the detector 18. Figures 3-5 shows three schemes for connecting the laser 17 and the detector 18 to the ends of the fibre 12, the fibre being a hypodermic needle 10 or a pipette 21. In Figure 4, the laser 17 and the detector 18 are connected through fibre sections 42 to each end of the fibre 12 using fibre connectors 40. In Figure 5, the laser 17 and the detector 18 are both connected to a first end of the fibre 12 using a fibre connector 40 and a splitter section 44. The second end of the fibre 12 is connected to a reflecting element 43, e.g. a fibre section terminated by a metal coating. In Figure 6, both ends of the fibre 10 are connected to a ring-shaped fibre section 45, which is again connected to the laser 17 and the detector 18 through splitter sections 44. In the schemes illustrated in Figure 5 and 6, radiation from the liquid in the through holes propagating in both directions is collected.

The detector 18 can be a spectrograph being a lens for collimating the output of the fibre into a beam illuminating a grating diffracting the incident radiation onto a CCD recording the intensity of the spectrum from the grating. This will in some cases involve optical imaging components such as lenses and mirrors.

Figure 7 shows another embodiment of the invention. Here, the optical fibre needle 10 is placed in a housing 70 for mechanical handling, having only the tip 13 accessible to be entered into the skin to extract a blood sample. The housing 70 can fit to a docking station 72 which can be closed with a lid 73. The docking station 72 and the lid 73 have fibre connectors which will ensure good optical connections with both ends of the optical fibre needle 10 when the housing 70 is positioned in the docking station 72. The docking station holds the laser and the detector as well as a computer of similarly for collecting and analysing data. After having collected the sample from the patient, the housing is placed in the docking station, and the result of the analysis, e.g. concentrations of compounds, will automatically determined and stored.

In a preferred embodiment, the emission spectroscopy performed in the optical fibre needle is Raman spectroscopy. As described by Enejder et al. (Optics Letters 27, p. 2004 (2002)), Raman spectroscopy can be used for quantitative measurements of biomolecular contents in highly light-scattering and absorbing media such as whole blood. Using the detection and analysis described in Enejder et al., the optical fibre needle of the invention can :

• act as a hypodermic needle

• act as a pump, because the capillary forces will draw liquid into the fibre.

• guide laser radiation to the liquid for exposure of the liquid

• collect the Raman signal efficiently • guide the Raman signal efficiently to the spectrograph

• provide easy connection to the laser and the detection/read-out system using fibre connectors (such as SMA connectors or equivalent)

The optical fibre needle according to the invention can be fabricated cheaply and act as a throw-away product for one-time use. Also, an optical fibre needle can be used for long term storage, such as freezing, of blood samples.

In the following, an example of a typical measurement of a Raman spectrum after collection of a sample is described. The optical fibre needle is connected to an optical mount such as a SMA or other fibre coupler enabling efficient coupling of laser light into the fibre mode composed of the core and the blood filled interstitial holes. The wavelength of the laser light is typically in the infrared range of the spectrum, however, other wavelengths can be used too. The light propagating in the optical fibre needle (by total- internal reflection like in normal optical fibres) will interact with various molecular compounds in the blood, resulting in the generation of a number of Raman peaks in the spectrum of the transmitted light.

The fibre tip is then mounted by a simple mechanical mount close to the entrance of the detector of the blood analysing apparatus. Before starting the measurement, a cover may be closed over the mounted fibre in order to reduce light noise from other light sources. The actual optical measurement is done over a period of time resulting in enough sensitivity to provide a certain accuracy in the measurement. The longer time the more accurate. Typically a measurement will take around one minute of measurement time thus allowing an accurate detection of known specimens in the blood sample resulting in distinct Raman peaks as described by Enejder et al. This measurement time may be different dependent on which specimens in the blood that is preferentially measured with a desired measurement accuracy. The apparatus integrates the optical signal at the various CCD pixels, possibly having the Raman signals and the laser light transmitted through an optical filter reducing, maybe significantly, the laser light power and transmitting only the Raman signals. An electronic read-out system will provide information about the amplitude of the various Raman signals and therefore of the concentrations of the various specimens given an initial calibration of the system. The system may be connected to a computer collecting and analysing the data. Alternatively the system may be connected to a wireless communication system such as a mobile phone transmitting the result of the blood analysis to a database or other storage system for later or immediate use in journalising or professional evaluation of the results.

After the measurement the hypodermic fibre may be stored to keep the blood sample for later use or it may simply be disposed. Typically, the optical fibre hypodermic needle can only be used once, removing any risk of transferring blood or any other material from one human being or animal to another.

In a preferred embodiment, the spectroscopy performed in the optical fibre needle is absorption spectroscopy. A typical example is photometric determination of the haemoglobin content in a blood sample. This test involves lysing the red blood cell (erythrocytes), thus producing an evenly distributed solution of haemoglobin in the sample. Lysing may be performed by having a lysing reagent in the optical fibre needle which is brought into contact with the blood upon sampling. Alternatively, the lysing may be carried out using an external electromagnetic field such as electrical discharges, microwaves or RF. The haemoglobin is typically chemically converted to the more stable and easily measured methemoglobintriazole-complex, which is a coloured compound that can be measured colorimetrically. This chemical conversion is preferably performed by having the reagent in the optical fibre needle, e.g. as a coating of the inside of the through holes. The concentration can being calculated from the amount of light absorption using Beer's Law. The method requires measurement of haemoglobin at approx. 540 nm where the absorption is high with a turbidity correction measurement at 880 nm where the absorption is low.

When applied in absorption spectroscopy, it is preferred that the through holes cover a large section of the light guiding part of the cross sectional area of the fibre.

Claims

1. A method for measuring an optical spectrum of a liquid, the method comprising the steps of providing an optical fibre having one or more through holes,

- providing a light source,

- providing a light detector,

- contacting the liquid with a first end of the optical fibre, - drawing liquid into at least one of the one or more through holes,

- exposing liquid held in the through hole(s) by guiding light from the light source in the optical fibre,

2. A method according to claim 1 wherein the step of contacting the liquid with a first end of the optical fibre, comprises the step of inserting the first end of the optical fibre into a skin of a human or animal.

3. A method according to claim 1 wherein the step of drawing liquid into at least one of the one or more through holes, comprises the step of drawing liquid by the use of capillary forces.

4. A method according to claim 1 further comprising the step of connecting the first or a second end of the optical fibre to an output of the light source.

5. A method according to claim 1 further comprising the step of connecting the first or a second end of the optical fibre to an input of the light detector.

6. A system for measuring spectra of liquids, the system comprising an optical fibre having at least a first through hole for holding liquid and a core for guiding electromagnetic radiation, the core and the first through hole being formed so that an evanescent field of radiation to be guided in the core extends into the first through hole, the system further comprising a laser source to be connected to a first end of the optical fibre and a radiation detector to be connected to the first or a second end of the optical fibre.

7. A system according to claim 6, wherein a first end of the optical fibre is adapted to penetrate a skin of a human or animal in that the first end is sharp or pointed.

8. A system according to claim 6, wherein the optical fibre is adapted to be used as a hypodermic needle in that the optical fibre is provided with a metal coating for mechanical strength and rigidity.

9. A system according to claim 6, the system further comprising a splitter to be connected to the optical fibre for providing at least one further input and/or output end to the optical fibre.

10. A system for measuring spectra of blood, the system comprising a metal coated crystal fibre having one or more through holes, a laser source to be connected to a first end of the crystal fibre, and a spectrograph to be connected to the first or a second end of the crystal fibre.

11. A system according to claim 10, wherein the laser source is a VCSEL (Vertical Cavity Surface Emitting Laser).

12. A system according to claim 10, wherein the laser source is a fibre laser.

13. The use of an optical fibre having one or more through holes as a hypodermic needle.

14. The use of a metal coated crystal fibre having one or more through holes as a hypodermic needle.