Sensor system, method and cross-linked hydrogel for detecting the presence or concentration of analytes

Abstract

A sensor system detects a presence or concentration of an analyte in a medium. The sensor system contains a sensor having a sensor head with a chamber. The sensor head has a permeable area through which the analyte can pass into the chamber when the sensor head contacts the medium. A cross-linked hydrogel fills the chamber, the hydrogel is configured to undergo a change in volume when contacting the analyte passed into the chamber which leads to a change in pressure in the chamber. A pressure sensor is configured to measure the pressure in the chamber for detecting the presence or concentration of the analyte.

Claims

1. A sensor system for detecting a presence or concentration of an analyte in a medium, the sensor system comprising: a sensor having a sensor head with a chamber, said sensor head having a permeable area through which the analyte can pass into said chamber when said sensor head contacts the medium; a cross-linked hydrogel filling said chamber, said cross-linked hydrogel being configured to undergo a change in volume when contacting the analyte passed into said chamber which leads to a change in pressure in said chamber, said cross-linked hydrogel having a plurality of a responsive component, said responsive component being responsive to the analyte such that the volume of said cross-linked hydrogel decreases when said responsive component binds to the analyte such that the pressure in said chamber decreases, and the volume of said cross-linked hydrogel increases when the analyte is released from said responsive component such that the pressure in said chamber increases, said responsive component comprising a crown ether; and a pressure sensor configured to measure the pressure in said chamber for detecting the presence or the concentration of the analyte, wherein said cross-linked hydrogel is in isochoric condition in said chamber and has a generally linear dependency of a measured pressure for temperatures of said cross-linked hydrogel below a lower critical solution temperature, wherein molecules of said crown ether are attached to said cross-linked hydrogel such that they form pairs of said crown ether molecules, wherein two of said crown ethers of a respective pair of molecules are configured to bind the analyte simultaneously, and wherein the analyte is disposed between said crown ethers of the respective pair of molecules in a bound state.

2. The sensor system according to claim 1, wherein said cross-linked hydrogel comprises at least one of: a temperature coefficient KT=Δp/ΔT being a slope of a Pressure-Temperature-curve of said cross-linked hydrogel, the temperature coefficient KT lying within a range of 200 mbar/K to 600 mbar/K; and a hydrogel coefficient KHG=KK+/KT in a range from 0.001 to 0.2.

3. The sensor system according to claim 1, wherein said cross-linked hydrogel comprises a polymer of one of a monomer selected from the group consisting of N-isopropylmethacrylamide, N-ethyl-N-methyl acrylamide, N,N-diethyl acrylamide, N,N-dimethylaminoethyl methacrylate, and (ethylenglycol)methacrylate.

4. The sensor system according to claim 1, wherein said cross-linked hydrogel has a network with a plurality of cross-linked polymer chains and a plurality of non-cross-linked polymer chains bound to said network, and only said non-cross-linked polymer chains contain the responsive component.

5. The sensor system according to claim 1, wherein said sensor contains an analyzer configured to determine the presence or the concentration of the analyte using a measured pressure.

6. The sensor system according to claim 5, wherein said sensor contains a telemeter configured to transfer the measured pressure to said analyzer.

7. The sensor system according to claim 1, further comprising: at least one reference sensor having a reference hydrogel for reducing cross-sensitivity of said sensor, said reference sensor further having a reference sensor head with a reference chamber filled with said reference hydrogel and a reference pressure sensor for measuring a reference pressure in said reference chamber of said reference sensor; and an analyzer configured to correct a pressure measured in said chamber of said sensor with or by subtracting the reference pressure.

8. The sensor system according to claim 1, wherein said sensor contains a component which mechanically decouples a change in the pressure of said cross-linked hydrogel in said chamber resulting from a change in temperature of said cross-linked hydrogel from an analyte concentration dependent pressure measured by said pressure sensor of said sensor.

9. The sensor system according to claim 1, wherein said responsive component is configured to detect K+ as the analyte.

10. The sensor system according to claim 1, wherein said responsive component achieves a linear dependency between temperature and pressure irrespective if the analyte is present or not.

11. The sensor system according to claim 1, wherein said cross-linked hydrogel contains a Poly(N-isopropylacrylamid) having at least 20 mol-% of said crown ether chemically bonded to polymer chains.

12. The sensor system according to claim 1, wherein said crown ether is 15-crown-5.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a graph showing a volume transition of a hydrogel in isobaric conditions;

(2) FIG. 2 is a schematic illustration of a sensor system according to the invention;

(3) FIG. 3 is a graph showing a 2:1 crown ether complex (responsive compound for capturing metal ions, particularly K.sup.+);

(4) FIG. 4 is a graph showing different polymer structures of a hydrogel that can be used with the present invention: (A), (B), (C), (D), (E);

(5) FIG. 5 is a graph showing a P-T-curve (e.g. measured pressure over temperature) of an isochoric hydrogel according to the present invention (without analyte);

(6) FIG. 6 is a graph showing hydrogel specific parameters as derived from the P-T-curve of the isochoric hydrogel;

(7) FIG. 7 is a graph showing a is a graph showing hydrogel coefficient for a cross-linked hydrogel (squares) in comparison to linear hydrogels (diamonds);

(8) FIG. 8 is an illustration of a preferred configuration of crown ether in a cross-linked hydrogel according to the present invention for capturing metal ions, particularly K.sup.+; and

(9) FIGS. 9A-9E are illustrations showing different possible structures ((A) to (E)) of a hydrogel according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(10) Referring now to the figures of the drawings in detail and first, particularly to FIG. 2 thereof, there is shown a schematic representation of a sensor system 1 according to the present invention. As indicated in FIG. 2, the sensor system 1 comprises for detecting the presence or concentration of an analyte A in a medium M at least an (e.g. implantable) sensor 10 having an sensor head 100 enclosing a chamber 101, which sensor head 100 further comprises a permeable area 105 through which the analyte A can pass into the chamber 101 when the sensor head 100 contacts the medium M.

(11) Further, the sensor 10 comprises a cross-linked hydrogel H filling the chamber 101, which hydrogel H is configured to undergo a change in volume when contacting the analyte A passed into the chamber 101 which leads to a change in pressure in the chamber (101), and a pressure sensor 102 configured to measure the pressure P in the chamber 101 for detecting the presence or concentration of the analyte A.

(12) Particularly, as indicated in FIG. 1, the hydrogel H comprises a lower critical solution temperature LCST, which decreases (from LCSTb to LCSTa) when the analyte A, here K.sup.+, bonds to the hydrogel H and which increases when the analyte A is released from the hydrogel H.

(13) However, due to the fact that the hydrogel H according to the present invention is kept in an isochoric state in the chamber 101, the pressure measured in the chamber by a pressure sensor 102 surprisingly depends essentially linearly (and/or mildly quadratically) on the temperature of the hydrogel H in the chamber 101 as indicated in FIG. 4, which leads to a constant sensitivity of the sensor 10 for the analyte A, here K+, over a broad range of temperatures. In contrast thereto, FIG. 3 shows the sigmoidal curves (swelling degree vs. temperature for free swelling which result in case of isobaric measurements that disadvantageously comprise a rather small measuring window of only a few degrees in temperature.

(14) The hydrogel H according to the invention can be characterized by the following parameters that can be derived from the pressure-temperature curve (P-T curve) as shown in FIG. 6.

(15) The first of these parameters is the temperature coefficient K.sub.T=Δp/ΔT which is the slope of the (isochoric) P-T-curve of the hydrogel H measured by pressure sensor 102 that preferably lies within the range of 200 mbar/K to 600 mbar/K.

(16) A further characteristic parameter is the sensitivity for the respective analyte (here e.g. K+) which is denoted as K.sub.K+. As indicated in FIG. 6, the latter quantity can also be easily derived from the P-T-curve. Due to the potassium-dependent shift of the P-T-curve, a pressure change occurs at each given temperature. For instance, in FIG. 6 a pressure reduction of about −400 mbar occurs in the chamber 101 at 28° C. due to adding of 20 mM KNO.sub.3 to the hydrogel H. Thus, the corresponding sensitivity is K.sub.K+=−400 mbar/20 mM=−20 mbar/mM for the hydrogel N residing in chamber 101.

(17) A third parameter is the so called hydrogel coefficient, which is defined as K.sub.HG=K.sub.K+/K.sub.T and described the concentration dependent (left) shift of the p-T-curve. Preferably K.sub.HG=K.sub.K+/KT lies in the range from 0.001 to 0.2 and most preferably in the range from 0.004 to 0.1.

(18) In contrast to a free (isobaric) swelling of the hydrogel (see FIG. 3), the sensitivity in the isochoric process according to the present invention is essentially constant over a broad range of temperatures as can be seen from FIGS. 4 and 6.

(19) Furthermore, so far, only linear 2:1 hydrogels have been discussed which comprise depending on potassium concentration a high sensitivity in a narrow temperature region. However, until now, for the cross-linked variants high sensitivities have not been observed. The high sensitivities of the linear polymers are possible due to a high fraction of crown ether in the respective polymer, particularly—depending on the crown ether fraction—the hydrogel coefficient K.sub.HG can be increased by increasing the crown ether fraction in the linear hydrogel, i.e., in case of a fraction of 15 mol-% a coefficient K.sub.HG being larger than 0.3 K/mM is observed (see FIG. 7), which corresponds to a shift of the swelling degree temperature curve of more than 6 K when adding 20 mM KNO.sub.3 in water.

(20) Surprisingly, the hydrogel coefficients KHG for the linear and cross-linked hydrogels correlate very well (see FIG. 7), wherein the sole parameter seems to be the crown ether fraction. This is intriguing since the detection of potassium in a 2:1 complex two crown ethers are required. The original assumption according to which linear hydrogels can fold more easily than 3D cross-linked polymers so that the crown ethers can come more easily in close proximity can thus not be verified. However, in case of cross-linked hydrogels an increase of the crown ether fraction is not easily possible without further ado, since the relatively large crown ether monomers disturb the cross-linking sterically.

(21) Adapting the synthesis accordingly, cross-linked hydrogels having a crown ether fraction of 10% could be generated.

(22) Suitable hydrogels can be prepared according to the following exemplary procedure.

(23) The monomers of N-isopropylacrylamid, acrylamide and 4-acrylamidobenzo-15-crown-5 (B15C5Am) as well as the cross-linking agent N,N′-methylen-bis(acrylic acid amide) (MBAAM) were solved in KNO.sub.3 solution in a vial equipped septum and magnetic stir bar, degassed for 15 minutes upon argon supply and cooled to 15° C. Subsequently a enhancing agent tetramethylethylenediamine (TEMED) and an initiator (ammonium persulfate, APS) were added to the KNO.sub.3 solution and the resulting solution is degassed for about 90 sec. Thereafter the reaction solution is transferred quickly to argon flooded reactor. After reacting for 24 hours at 15° C. the resulting hydrogel is washed with water (MilliQ water) up to 6 times.

(24) In order to significantly increase the fraction of crown ether, the following strategies can be applied.

(25) As indicated in FIG. 8, here in case of the analyte A in form of K.sup.+, the H of the sensor system 1 comprises a plurality of a responsive component R, here crown ether, wherein the respective responsive component R is responsive to the analyte A such that the volume of the hydrogel H tends to decrease when the respective responsive component R binds to the analyte A such that the pressure P in the chamber 101 decreases, and wherein the volume of the hydrogel H tends to increase when an analyte A is released from the respective responsive component R. Particularly, as indicated in FIG. 8, the crown ether molecules R are attached such to the hydrogel H that they form pairs, particularly in a chelation configuration, wherein the two crown ethers of the respective pair are configured to bind the analyte A simultaneously, wherein particularly the analyte A is arranged between the crown ethers of the respective pair in its bound state, as shown in FIG. 8. Particularly, the two crown ethers can be generated in the chelation configuration before the actual hydrogel synthesis. The configuration of crown ethers shown in FIG. 8 is also denoted as pincer structure.

(26) For this, the two crown ethers are spatially and covalently bonded in the form of the pincer by using suitable synthesis parameters. This allows an easy increase of the crown ether amount by a factor of two. Furthermore, the crown ethers are already positioned in close proximity for the potassium detection, so that a reaction due a changing potassium concentration can take place much faster than using a conventional approach, since polymer chains do not have to be re-oriented for finding potassium due to the crown ether sandwich.

(27) Further, according to an embodiment indicated in FIG. 9A, the crown ether fraction may be enhanced by providing a hydrogel H that comprises a network N, e.g. a PNIPAAm network, comprising a plurality of cross-linked polymer chains wherein a plurality of non-cross linked polymer chains C is bound to the network N (and distributed over the network), wherein only the non-cross-linked (e.g. linear) polymer chains C comprise the responsive component R (e.g. the crown ether structure). The molar fraction of crown ether in the non-cross linked (e.g. linear) chains preferably lies in the range about 40 to 60% and is preferably approximately 50%. Particularly, a free radical polymerization can be used for producing this special hydrogel, in which the individual components are distributed inhomogeneously in the network in a defined manner. Particularly a PNIPAAm network without crown ethers can be provided, in which linear (non-cross-linked) areas with high crown ether fraction are incorporated by polymerization.

(28) Alternatively, the PNIPAAm network N may comprise a high fraction of crown ether. The linear (non-cross linked) areas can be configured as before.

(29) The generation of the linear regions C with a high crown ether fraction in the hydrogel H can also be achieved by configuring the polymer of the hydrogel H as a star polymer (see FIG. 9B), as a dendrimer (see FIG. 9D), as a blend (see FIG. 9 C), or as a copolymer, particularly as a graft polymer or toothbrush-like polymer (see FIG. 9E).

(30) As an alternative to the free radical polymerization also strategies for a controlled radical polymerization can be used (e.g. NMP, ATRP, RAFT).

(31) As also indicated in FIG. 2, the sensor system 1 may comprise further components.

(32) Particularly, in an embodiment, the sensor system 1 further comprises an analyzing unit 2 that is configured to determine the presence or concentration of the respective analyte A using the measured pressure P.

(33) Preferably, for transmitting the measured pressures P to the unit 2, preferably wirelessly, the sensor 10 may comprises a telemetry unit 104 that communicates with the analyzing unit 2.

(34) This allows to implant the sensor 10 into a patient, while the analyzing unit can be arranged outside the patient.

(35) Furthermore, for reducing cross-sensitivity of the sensor 10, the sensor system 1 may comprise at least one reference sensor 11 that comprises a reference hydrogel H′. The reference hydrogel H′ can be identical to the hydrogel H wherein now the analyte A is not allowed to enter the chamber 101 of the sensor head 100 of the reference sensor 11. For this the reference sensor 11 may comprise a suitable area 105 or no such area at all. Alternatively, the reference hydrogel H may be configured as the hydrogel H, but does not comprise the responsive component R so that a change in analyte concentration in the chamber 101 of the reference sensor 11 does not affect the reference pressure measured by the pressure sensor 102 of the reference sensor.

(36) Preferably, the analyzing unit 2 is configured to offset the pressure measured in the chamber 101 of the sensor 10 against the reference pressure measured by the pressure sensor 102 of the reference sensor 11.

(37) Particularly, in an embodiment, the sensor system 1 may comprise multiple sensors 10, which may be identically configured, with the exception that each sensor 10 comprises a hydrogel that is sensitive to a different analyte. Further, such a system 1 may comprise a corresponding number of reference sensors 11 to reduce/eliminate cross-sensitivities and other disturbances.

(38) For further reduction of such cross-sensitivities and disturbances, the sensor system may further comprise a separate temperature sensor 20 and/or pressure sensor 30 that may also communicate with the analyzing unit 2.

(39) Furthermore, the sensor system may comprise an external pressure sensor 40 for reducing the influence of external barometric pressure changes on the pressure measurements in the sensors 10 and/or 11.

(40) Finally, as indicated in FIG. 2, each sensor 10, 11 or individual sensors of the system 1 may optionally comprise a component 103, O which mechanically decouples a change in the pressure of the hydrogel H, H′ in the respective chamber 101 resulting from a change in temperature of the hydrogel H from the concentration dependent pressure P measured by the respective pressure sensor 102. Particularly, the respective pressure sensor 102 may be arranged in compartment 101a separated from the chamber 101 by a flexible pressure membrane (e.g. bellows, polymer film) 103 to which the hydrogel H residing in the respective chamber 101 is mechanically coupled (e.g. contacts the pressure membrane 103). Particularly, the respective compartment 101a may be filled with a material (e.g. a fluid), such as an oil O, which material O comprises a positive temperature coefficient and particularly functions as a compensation component. In contrast thereto, the hydrogel H (H′) comprises a negative temperature coefficient. In case the hydrogel's H temperature increases, the hydrogel H shrinks, whereas the material O arranged in the compartment 101a expands. Particularly, the amount of the material O is selected such that the increase in volume of the material O with temperature exactly or at least partly compensates the temperature induced decreasing volume of the hydrogel.