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Title:
A MULTIFUNCTIONAL SENSOR APPARATUS AND ASSOCIATED METHODS
Document Type and Number:
WIPO Patent Application WO/2017/081361
Kind Code:
A1
Abstract:
An apparatus comprising: a primary sensor and one or more secondary sensors; a memristor; and a power supply, wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply, and wherein the primary sensor and one or more secondary sensors are coupled to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli.

Inventors:
BESSONOV, Alexander (110 Brooks Road, Cambridge CB1 3HR, CB1 3HR, GB)
ALLEN, Mark (7 Chervil Way, Great Cambourne CB23 6BA, CB23 6BA, GB)
Application Number:
FI2016/050748
Publication Date:
May 18, 2017
Filing Date:
October 25, 2016
Export Citation:
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Assignee:
NOKIA TECHNOLOGIES OY (Karaportti 3, Espoo, 02610, FI)
International Classes:
G01D5/16; G06F3/00; H01G7/06
Domestic Patent References:
WO2015000009A12015-01-08
WO2012156126A22012-11-22
Other References:
None
Attorney, Agent or Firm:
NOKIA TECHNOLOGIES OY et al. (Ari Aarnio, IPR DepartmentKarakaari 7, Espoo, 02610, FI)
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Claims:
Claims

1 . An apparatus comprising:

a primary sensor and one or more secondary sensors;

a memristor; and

a power supply,

wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply, and

wherein the primary sensor and one or more secondary sensors are coupled to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli. 2. The apparatus of claim 1 , wherein the apparatus is configured such that the power supply to the one or more secondary sensors also provides power to the primary sensor to enable the sensing of its respective stimulus.

3. The apparatus of claim 1 or 2, wherein the apparatus comprises two or more secondary sensors configured to sense different physical stimuli.

4. The apparatus of any preceding claim, wherein the primary sensor is a resistive sensor configured to exhibit a decrease in resistance on exposure to its respective stimulus, the decrease in resistance allowing a voltage from the power supply to be applied to the memristor to switch the memristor from the high resistance state to the low resistance state.

5. The apparatus of any preceding claim, wherein the primary sensor, the one or more secondary sensors and the memristor share a common channel member, and the apparatus comprises source and drain electrodes configured to enable a flow of electrical current through the channel member between the source and drain electrodes, and wherein the memristor is configured to inhibit or allow the flow of electrical current depending on whether the memristor is in the high resistance state or the low resistance state, respectively.

6. The apparatus of claim 5, wherein at least one of the primary and one or more secondary sensors comprises a respective sensitizer configured to facilitate the sensing of their respective stimulus. 7. The apparatus of claim 6, wherein the sensitizer of one or more of the sensors comprises a quantum dot material configured to generate electron-hole pairs on exposure to incident electromagnetic radiation to produce a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the incident electromagnetic radiation.

8. The apparatus of claim 6 or 7, wherein the sensitizer of one or more of the sensors comprises graphene oxide having one or more functional groups attached thereto, the one or more functional groups configured to react with water in the surrounding environment to generate protons to produce a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the environmental humidity.

9. The apparatus of any of claims 6 to 8, wherein the sensitizer of one or more of the sensors comprises a receptor species configured to bind to a charged chemical or biological species in the surrounding environment such that the charged chemical or biological species is in sufficient proximity to the channel member to cause a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the chemical or biological species. 10. The apparatus of any preceding claim, wherein the apparatus comprises a respective switch connected between the power supply and each of the primary and secondary sensors to enable electrical readout of each sensor independently.

1 1 . The apparatus of any of claims 1 to 9, wherein the primary sensor and one or more of the secondary sensors are connected to one or more resistors such that each respective sensor forms part of a resistive bridge circuit configured to enable electrical readout of each sensor simultaneously.

12. The apparatus of claim 1 1 , wherein the primary and secondary sensors are sensitive to light, and the apparatus comprises a pulsed light source configured to illuminate the sensors at a predefined frequency and duty cycle such that they can be read out using a lock-in technique.

13. The apparatus of any preceding claim, wherein at least one of the primary and one or more secondary sensors comprises a protective coating configured to prevent exposure of the sensor to the physical stimuli sensed by the other sensors.

14. A method of using an apparatus,

the apparatus comprising a primary sensor and one or more secondary sensors; a memristor; and a power supply, wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply, and wherein the primary sensor and one or more secondary sensors are coupled to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli,

the method comprising sensing one or more physical stimuli using the secondary sensor(s) on exposure of the primary sensor to its respective stimulus. 15. A method of making an apparatus,

the apparatus comprising a primary sensor and one or more secondary sensors; a memristor; and a power supply, wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply,

the method comprising coupling the primary sensor and one or more secondary sensors to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli.

Description:
A multifunctional sensor apparatus and associated methods Technical Field The present disclosure relates particularly to sensors, associated methods and apparatus. Certain embodiments specifically concern an apparatus comprising a primary sensor and one or more secondary sensors coupled to a common memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state thereby allowing the one or more secondary sensors to sense their respective stimuli. Some embodiments may relate to portable electronic devices, in particular, so-called hand-portable electronic devices which may be hand-held in use (although they may be placed in a cradle in use). Such hand-portable electronic devices include so-called Personal Digital Assistants (PDAs) and tablet PCs. The portable electronic devices/apparatus according to one or more disclosed example aspects/embodiments may provide one or more audio/text/video communication functions (e.g. tele-communication, video-communication, and/or text transmission, Short Message Service (SMS)/ Multimedia Message Service (MMS)/emailing functions, interactive/non- interactive viewing functions (e.g. web-browsing, navigation, TV/program viewing functions), music recording/playing functions (e.g. MP3 or other format and/or (FM/AM) radio broadcast recording/playing), downloading/sending of data functions, image capture function (e.g. using a (e.g. in-built) digital camera), and gaming functions.

Background

Research is currently being done to develop new sensor devices.

The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge.

Summary

According to a first aspect, there is provided an apparatus comprising:

a primary sensor and one or more secondary sensors;

a memristor; and a power supply,

wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply, and

wherein the primary sensor and one or more secondary sensors are coupled to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli. The apparatus may be configured such that the power supply to the one or more secondary sensors also provides power to the primary sensor to enable the sensing of its respective stimulus.

The apparatus may comprise two or more secondary sensors configured to sense different physical stimuli.

The primary sensor may be a resistive sensor configured to exhibit a decrease in resistance on exposure to its respective stimulus, the decrease in resistance allowing a voltage from the power supply to be applied to the memristor to switch the memristor from the high resistance state to the low resistance state.

The primary sensor, the one or more secondary sensors and the memristor may share a common channel member, and the apparatus may comprise source and drain electrodes configured to enable a flow of electrical current through the channel member between the source and drain electrodes, and wherein the memristor may be configured to inhibit or allow the flow of electrical current depending on whether the memristor is in the high resistance state or the low resistance state, respectively.

At least one of the primary and one or more secondary sensors may comprise a respective sensitizer configured to facilitate the sensing of their respective stimulus.

The sensitizer of one or more of the sensors may comprise a quantum dot material configured to generate electron-hole pairs on exposure to incident electromagnetic radiation to produce a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the incident electromagnetic radiation. The sensitizer of one or more of the sensors may comprise graphene oxide having one or more functional groups attached thereto, the one or more functional groups configured to react with water in the surrounding environment to generate protons to produce a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the environmental humidity.

The sensitizer of one or more of the sensors may comprise a receptor species configured to bind to a charged chemical or biological species in the surrounding environment such that the charged chemical or biological species is in sufficient proximity to the channel member to cause a detectable change in the electrical current flowing through the channel member which is indicative of one or more of the presence and magnitude of the chemical or biological species. The apparatus may comprise a respective switch connected between the power supply and each of the primary and secondary sensors to enable electrical readout of each sensor independently.

The primary sensor and one or more of the secondary sensors may be connected to one or more resistors such that each respective sensor forms part of a resistive bridge circuit configured to enable electrical readout of each sensor simultaneously.

The primary and secondary sensors may be sensitive to light, and the apparatus may comprise a pulsed light source configured to illuminate the sensors at a predefined frequency and duty cycle such that they can be read out using a lock-in technique.

At least one of the primary and one or more secondary sensors may comprise a protective coating configured to prevent exposure of the sensor to the physical stimuli sensed by the other sensors.

The apparatus may comprise a gate electrode configured to enable the electrical conductance of the channel member to be controlled by a voltage applied to the gate electrode. At least one of the primary and one or more secondary sensors may be configured to sense at least one of the following physical stimuli: electromagnetic radiation, humidity, a chemical species, a biological species, strain, temperature and pressure. The memristor may be configured to have a resistance of 10 6 -10 8 Ω in the high resistance state and a resistance of 10 2 -10 3 Ω in the low resistance state.

The memristor may be configured to switch from the high resistance state to the low resistance state when a voltage of 0.1 -5V is applied thereto.

One or more of the memristor materials, dimensions and fabrication conditions may be configured to define the voltage at which the memristor switches from the high resistance state to the low resistance state. The memristor may be configured to switch back from the low resistance state to the high resistance state when a reverse voltage above a predefined threshold is applied thereto.

The memristor may be configured to switch back from the low resistance state to the high resistance state automatically after a predefined period of time following removal of the applied voltage.

The channel member may be formed from one or more two-dimensional materials.

The one or more two-dimensional materials may comprise at least one of a transition metal dichalcogenide and a graphene-like material.

The memristor may comprise a layer of transition metal oxide in contact with a layer of transition metal chalcogenide. A variety of other memristive materials from the group of metal oxides, metal chalcogenides, metal halides and organic polymers may be used instead.

The apparatus may be one or more of an electronic device, a portable electronic device, a portable telecommunications device, a mobile phone, a personal digital assistant, a tablet, a phablet, a desktop computer, a laptop computer, a server, a smartphone, a smartwatch, smart eyewear, a multifunctional sensor, and a module for one or more of the same. According to a further aspect, there is provided a method of using an apparatus, the apparatus comprising a primary sensor and one or more secondary sensors; a memristor; and a power supply, wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply, and wherein the primary sensor and one or more secondary sensors are coupled to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli,

the method comprising sensing one or more physical stimuli using the secondary sensor(s) on exposure of the primary sensor to its respective stimulus.

According to a further aspect, there is provided a method of making an apparatus,

the apparatus comprising a primary sensor and one or more secondary sensors; a memristor; and a power supply, wherein the primary sensor and one or more secondary sensors are configured to sense different physical stimuli when connected to a power supply,

the method comprising coupling the primary sensor and one or more secondary sensors to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state resulting in connection of the power supply to the one or more secondary sensors to enable the sensing of their respective stimuli.

The method may comprise forming at least one of the primary sensor, one or more secondary sensors and memristor.

The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated or understood by the skilled person. Corresponding computer programs for implementing one or more steps of the methods disclosed herein are also within the present disclosure and are encompassed by one or more of the described example embodiments.

One or more of the computer programs may, when run on a computer, cause the computer to configure any apparatus, including a battery, circuit, controller, or device disclosed herein or perform any method disclosed herein. One or more of the computer programs may be software implementations, and the computer may be considered as any appropriate hardware, including a digital signal processor, a microcontroller, and an implementation in read only memory (ROM), erasable programmable read only memory (EPROM) or electronically erasable programmable read only memory (EEPROM), as non- limiting examples. The software may be an assembly program.

One or more of the computer programs may be provided on a computer readable medium, which may be a physical computer readable medium such as a disc or a memory device, or may be embodied as a transient signal. Such a transient signal may be a network download, including an internet download.

The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure.

The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 a shows one example of the present apparatus (schematic);

Figure 1 b shows how the resistances of the first and second circuit portions of Figure 1 a vary on exposure to the respective stimuli;

Figure 2a shows another example of the present apparatus (plan view);

Figure 2b shows the apparatus of Figure 2a in cross-section;

Figure 3 shows another example of the present apparatus in which each sensor forms part of a resistive bridge circuit (schematic);

Figure 4 shows a method of making the present apparatus (perspective);

Figure 5 shows another example of the present apparatus (schematic);

Figure 6a shows a method of using the present apparatus;

Figure 6b shows a method of making the present apparatus; and

Figure 7 shows a computer-readable medium comprising a computer program configured to perform, control or enable a method described herein (schematic). Description of Specific Aspects/Embodiments

Existing sensors typically comprise an active layer and two or three electrodes dedicated to the sensing of a particular stimulus. When a situation requires the sensing of multiple stimuli, a plurality of these dedicated sensors would normally need to be used, each with its own respective circuitry (e.g. microprocessor, analogue-to-digital converter and electrical connections). This approach is energy consuming and leads to high circuit complexity caused by the number of components and amount of wiring. Such circuity also tends to be relatively bulky which inhibits device miniaturisation.

There will now be described an apparatus and associated methods that may address this issue. Figure 1 a shows one example of the present apparatus 101 . The apparatus 101 comprises a primary sensor 102 and one or more secondary sensors 103 (in this case only one secondary sensor 103 is used) each configured to sense different physical stimuli when connected to a power supply, a memristor 104, and a power supply 105. The primary sensor 102 and one or more secondary sensors 103 are coupled to the memristor 104 such that exposure of the primary sensor 102 to its respective stimulus causes the memristor 104 to switch from a high resistance state to a low resistance state resulting in connection of the power supply 105 to the one or more secondary sensors 103 to enable the sensing of their respective stimuli. In this example, the primary sensor 102 is a light sensor configured to sense one or more of the presence and magnitude of incident electromagnetic radiation, and the secondary sensor 103 is an analyte sensor configured to sense one or more of the presence and magnitude of a chemical or biological species. Furthermore, the primary sensor 102 is a resistive sensor configured to exhibit a decrease in resistance on exposure to the incident electromagnetic radiation, the decrease in resistance allowing a voltage from the power supply 105 to be applied to the memristor 104 to switch the memristor 104 from the high resistance state to the low resistance state (like a voltage divider).

As can be seen in Figure 1 , the primary sensor 102 forms part of a first circuit portion 106 in combination with the memristor 104, the power supply 105, a first switch 1 10 (Switch 1 ) and a first ammeter 108, whilst the secondary sensor 103 forms part of a second circuit portion 107 in combination with the memristor 104, the power supply 105, a second switch 1 1 1 (Switch 2) and a second ammeter 109. The first 1 10 and second 1 1 1 switches are used to connect the power supply 105 to one circuit portion 106, 107 at a time so that the output of the primary sensor 102 (e.g. current or resistance) does not affect the output of the secondary sensor 103 (e.g. current or resistance) and vice versa. In this way, each sensor 102, 103 can be read out independently.

Figure 1 b shows how the respective resistances R1 , R2 of the first 106 and second 107 circuit portions of Figure 1 a vary on exposure to the respective stimuli. Initially, the memristor 104 is in the high resistance state (e.g. having a resistance of 10 6 -10 8 Ω). Voltage pulses V pu ise are alternately applied from the power supply 105 to the first 106 and second 107 circuit portions by opening and closing the first 1 10 and second 1 1 1 switches. In the absence of incident electromagnetic radiation, the resistance of the primary sensor 102 is relatively high and the voltage applied to the memristor 104 when the first switch 1 10 is closed is below the threshold voltage of the memristor 104 (0.1 -5V depending on one or more of the memristor materials, dimensions and fabrication conditions). As a result, the memristor 104 remains in the high resistance state which prevents readout of the primary sensor 102 (i.e. the memristor 104 effectively acts as a break in the first circuit portion 106). Furthermore, because the memristor 104 is in the high resistance state, the secondary sensor 103 cannot be read out when the second switch 1 1 1 is closed (i.e. the memristor 104 effectively acts as a break in the second circuit portion 107), regardless of whether or not the secondary sensor 103 is exposed to the chemical or biological species. The resistances R1 and R2 of the first 106 and second 107 circuit portions are therefore relatively high at this moment in time.

Once the primary sensor 102 is exposed to incident electromagnetic radiation, however, the resistance of the primary sensor 102 decreases causing a voltage to be applied from the power supply 105 to the memristor 104 which exceeds the threshold voltage of the memristor 104. This causes the memristor 104 to switch from the high resistance state to the low resistance state (e.g. having a resistance of 10 2 -10 3 Ω) allowing readout of the primary sensor 102 when the first switch 1 10 is closed (since the effective break in the first circuit portion 106 has now been removed). When the second switch 1 1 1 is closed, the low resistance state of the memristor 104 also allows the secondary sensor 103 to be read out (since the effective break in the second circuit portion 107 has also been removed). The resistances R1 , R2 of the first 106 and second 107 circuit portions are therefore lower than they were before exposure of the primary sensor 102 to the incident electromagnetic radiation. Furthermore, detectable changes in the resistances R1 , R2 of the first 106 and second 107 circuit portions now occur each time the respective switches 1 10, 1 1 1 are closed in response to the physical stimuli. The output signal from each sensor 102, 103 may comprise one or more of the resistance value R1 , R2 itself and the current flowing through the respective circuit portion 106, 107 as measured by the first 108 or second 109 ammeter.

Once the primary 102 and secondary 103 sensors have been read out, the memristor 104 may be switched back from the low resistance state to the high resistance state (i.e. reset) ready for future sensing operations. This may be achieved by applying a reverse voltage above a predefined threshold to the memristor 104. Alternatively, the memristor 104 may be configured to switch back from the low resistance state to the high resistance state automatically after a predefined period of time following removal of the applied voltage. Since the memristor 104 is initially in the high resistance state, little or no current flows through either circuit portion 106, 107 until the primary sensor 102 is exposed to the incident electromagnetic radiation. This helps to preserve the charge of the power supply 105 (which may comprise one or more of a battery, supercapacitor and battery-capacitor hybrid) until the sensors 102, 103 actually need to be read out. Examples of suitable applications include lab-on-a-chip platforms for measuring skin chemistry during photoplethysmograph (PPG) or SpC>2 measurements, detection of by-products of light- induced chemical reactions, and safety devices for indicating toxic gas exposure synchronised with the presence of a certain wavelength of light. The sharing of the memristor 104 and power supply 105 between the primary 102 and secondary 103 sensors also serves to reduce the size and complexity of the circuitry.

Figures 2a and 2b show another example of the present apparatus 201 in plan view and cross-section, respectively. In this example, the apparatus 201 comprises two secondary sensors 203a, b connected in parallel to the memristor 204, but it could be extended to include any number of secondary sensors 203 configured to sense different physical stimuli. As per the example of Figures 1 a and 1 b, exposure of the primary sensor 202 to its respective stimulus enables detection of the physical stimuli associated with the secondary sensors 203a, b (with respective switches 210, 21 1 being used to allow readout of each of the sensors 202, 203a, b independently). As shown on Figures 2a and 2b, the primary sensor 202, the one or more secondary sensors 203a, b and the memristor 204 share a common channel member 212, and the apparatus 201 comprises source 213a-c and drain 214 electrodes configured to enable a flow of electrical current through the channel member 212 between the source 213a-c and drain 214 electrodes. In this case, electrodes BE1 , BE2 and BE3 serve as the source electrodes 213a-c for each of the first, second and third circuit portions, and electrode TE serves as a common drain electrode 214 for all three circuit portions. The channel member and electrodes are supported on a substrate. Furthermore, the memristor material 217 is configured to inhibit or allow the flow of electrical current through the channel member depending on whether the memristor 204 is in the high resistance state or the low resistance state, respectively. In some cases, the apparatus 201 may further comprise a gate electrode (e.g. separated from the lower surface of the channel member 212 by a layer of dielectric material) configured to enable the electrical conductance of the channel member 212 to be controlled by a voltage applied to the gate electrode (not shown). This feature allows the sensitivity of the apparatus 201 to be tuned.

Integration of the primary sensor 202, secondary sensors 203a, b and memristor 204 as a single monolithic device results in a more compact apparatus 201 than existing multifunctional sensors, decreases the number of manufacturing steps and reduces the overall circuit complexity.

The channel member 212 may be formed from one or more two-dimensional materials, such as a transition metal dichalcogenide (TMD) and a graphene-like material. The TMD may comprise one or more of WX 2 , M0X2, SCX2, TiX 2 , HfX 2 , ZrX 2 , VX 2 , CrX 2 , MnX 2 , FeX 2 , CoX 2 , NiX 2 , NbX 2 , TcX 2 , ReX 2 , PdX 2 , PtX 2 , where X = S, Se or Te; and the graphene-like material may comprise one or more of graphene oxide, phosphorene, silicene, germanene, stanene, h-BN, AIN, GaN, InN, InP, InAs, BP, BAs and GaP. The memristor 204 may be formed from a layer of transition metal oxide (TMO) in contact with a layer of TMD. The TMD may be part of the channel 212 (comprising one or more of the materials listed above); and the TMO may comprise one or more of WO n , MoO n , ScO n , TiO n , HfO n , ZrOn, VOn, CrOn, MnOn, FeOn, CoO n , NiO n , NbO n , where n = 2-3. The electrodes 213a-c, 214 may be formed from a metal or a metal nanoparticle ink comprising one or more of silver, gold, copper, nickel, cobalt. Additionally or alternatively, the electrodes 213a-c, 214 may comprise one or more of a conductive metal oxide, carbon-based material (e.g. graphene), organic material and polymer. In this example, each of the primary 202 and secondary 203a, b sensors comprise a respective sensitizer 215a,b configured to facilitate the sensing of their respective stimuli. The sensitizer 215a of the primary sensor 202 comprises a quantum dot material configured to generate electron-hole pairs on exposure to incident electromagnetic radiation to produce a detectable change in the electrical current flowing through the channel member 212 which is indicative of one or more of the presence and magnitude of the incident electromagnetic radiation.

On the other hand, the sensitizer 215b of one of the secondary sensors 203a comprises a receptor species configured to bind to a charged chemical or biological species in the surrounding environment such that the charged chemical or biological species is in sufficient proximity to the channel member 212 to cause a detectable change in the electrical current flowing through the channel member 212 which is indicative of one or more of the presence and magnitude of the chemical or biological species.

Although not shown in the figure, the sensitizer of the other secondary sensor 203b may comprise graphene oxide having one or more functional groups attached thereto. The one or more functional groups may be configured to react with water in the surrounding environment to generate protons to produce a detectable change in the electrical current flowing through the channel member 212 which is indicative of one or more of the presence and magnitude of the environmental humidity.

In practice, the primary 202 and secondary 203a, b sensors may be configured to detect any physical stimuli (other examples of which include strain, temperature and pressure). For example, the primary sensor 202 may be a humidity sensor and one of the secondary sensors 203a, b may be a chemical or biological analyte sensor. In this scenario, an increase in humidity may be used to indicate the presence of an aqueous fluid (which may or may not contain the analyte species) and trigger the operation of the secondary sensor 203a,b.

Each sensor 202, 203a, b may have a positive response (i.e. increase in current or decrease in resistance) or negative response (i.e. a decrease in current or increase in resistance) to its respective stimulus depending on the stimulus and sensor materials. Although Figure 2b shows the use of sensitizers 215a,b to facilitate the sensing of the respective stimuli, the channel member 212 may be inherently sensitive to these stimuli. In this scenario, one or more of the sensitizers 215a,b may be omitted, but each "sensor region" 212a-c of the channel member 212 may require a protective coating (not shown) configured to prevent exposure of that region 212a-c to the physical stimuli sensed by the other sensor regions 212a-c. This feature may be particularly beneficial in examples which do not use respective switches 210, 21 1 to enable electrical readout of each sensor 202, 203a, b independently (see below).

Figure 3 shows another example of the present apparatus 301 . In this example, the primary RSENS.I and secondary RSENS,2 sensors are connected to one or more resistors such that each respective sensor RSENS,I,2 forms part of a resistive bridge circuit configured to enable electrical readout of each sensor RSENS,I,2 simultaneously. This configuration allows the sensors RSENS,I,2 to be read out by measuring the voltage output of the bridges rather than the current or resistance of the circuit portions.

As shown in Figure 3, the primary sensor RSENS.I, secondary sensor RSENS,2, memristor RMEM and a reference resistor RREF form separate branches of the bridge circuit. The reference resistor RREF is monolithically integrated with the sensors RSENS,I,2 and memristor RMEM (i.e. formed using the same channel material 312) so that any changes in the resistance state of the memristor RMEM affect the sensors RSENS,I,2 and reference resistor RREF equally. The reference resistor RREF may, however, comprise a protective coating (not shown) so that its resistance remains as stable as possible under the various stimuli. The bridge circuit also comprises three external resistors REXT,I-3 which each form part of a branch of the bridge circuit together with the corresponding sensor RSENS,I,2 or reference resistor RREF. Although only one secondary sensor RSENS,2 and reference resistor RREF is used in this example, the bridge circuit could be extended to include multiple secondary sensors RSENS,2 and reference resistors RREF.

The apparatus 301 of Figure 3 does not require switches to alternately connect the primary RSENS.I and secondary RSENS,2 sensors to the power supply 305. Instead, a voltage VD is coupled across the various branches of the circuit at the same time. Initially, the memristor RMEM is in the high resistance state and most of the applied voltage VD is coupled over the memristor branch. When the primary sensor RSENS.I is exposed to its respective stimulus, however, its resistance decreases and the memristor RMEM is switched from the high resistance state to the low resistance state. As a result, the majority of the applied voltage VD becomes coupled over the sensor and resistor branches which are in parallel. The branch with the reference resistor RREF is unaffected by the external stimuli and therefore provides a reference potential defined by the voltage division between the reference resistor RREF and its associated external resistor REXT,2. Similarly, the applied voltage VD is divided for the two sensor branches (RSENS,I/REXT,I and RSENS,2/REXT,3). The primary RSENS.I and secondary RSENS,2 sensors can then be read out simultaneously by measuring the difference between the potential at the midpoint of the corresponding sensor branch and the potential at the midpoint of the resistor branch as indicated by the readout voltages VOUT.I and VOUT,2, respectively. Any changes in the resistance of the primary RSENS.I or secondary RSENS,2 sensor can therefore be detected as a change in the corresponding output voltage Vom-,1,2. The external resistors REXT,I-3 of the bridge circuit do not need to be monolithically integrated with the sensors RSENS,I,2, memristor RMEM and reference resistor RREF, and may therefore be discrete components. The use of discrete resistors REXT,I-3 advantageously allows the output voltages VOUT,I,2 to be zeroed in the absence of physical stimuli even if the current flowing through each branch is different due to differing sensor materials. This helps to improve the sensitivity of the circuit.

The primary RSENS.I and secondary RSENS,2 sensors of Figure 3 could alternatively be read out using a lock-in technique. In this scenario, the primary RSENS.I and secondary RSENS,2 sensors are both sensitive to light (e.g. the primary sensor RSENS.I may be a dedicated light sensor and the secondary sensor RSENS,2 may be a chemical sensor with a light sensitivity that is dependent upon the chemical environment), and the apparatus 301 comprises a pulsed light source (not shown) configured to illuminate the sensors RSENS,I,2 at a predefined frequency and duty cycle to repeatedly switch their resistances. The pulse rate should be sufficiently high (e.g. >100Hz) that the memristor is not switched during the pulsing, but instead should switch only when the light is first applied and remain in that resistance state throughout the entire measurement (i.e. the memristor effectively only experiences the cumulative effect of the pulsing). The resulting output voltages VOUT,I,2 will reflect this pulsing signal. As a result, it is then possible to lock each of the output voltages VOUT,I,2 to the pulsing signal to detect relatively small variations in the sensor resistances.

Figure 4 shows one method of making the present apparatus 401 . First, a substrate 416 is provided as a structural support for the various components of the apparatus 401 . The source electrodes 413a,b are then deposited on top of the supporting substrate 416 using one or more vacuum (e.g. sputtering, atomic layer deposition, chemical vapour deposition or physical vapour deposition) or printing (e.g. dispensing, screen printing, offset printing, gravure printing, flexography, aerosol jet printing or inkjet printing) techniques. A layer of material 412 (e.g. a transition metal dichalcogenide) is then deposited on top of the source electrodes 413a,b (e.g. using one or more of spin-coating, slot die-coating, spray coating, soft lithography, transfer printing, drop-casting, aerosol jet printing, inkjet printing and thin film transfer) to form the channel member, and subsequently patterned to define the respective sensor regions 412a, b. Next, a portion 417 of the channel member is oxidised (e.g. a transition metal oxide) to form the memristor in combination with the channel material 412. The drain electrode 414 is then deposited on top of the supporting substrate 416 such that it overlies the memristor. The primary sensor region 412a of the channel member is then coated with a sensitizer 415a (in this case a quantum dot material) to form the primary sensor. A sensitizer 415b may also be deposited on top of the secondary sensor region 412b to form the secondary sensor. Rather than depositing a sensitizer 415a,b on top of the primary or secondary sensors, the sensor regions 412a,b of the channel member 412 could be modified by chemical (e.g. doping or redox reactions) or physical (e.g. plasma, e-beam, ultraviolet ozone or laser) treatment. As mentioned previously, however, the primary and secondary sensor regions 412a, b may be intrinsically sensitive to the respective stimuli without the need for modification.

Figure 5 shows another example of the present apparatus 501. The apparatus 501 may be one or more of an electronic device, a portable electronic device, a portable telecommunications device, a mobile phone, a personal digital assistant, a tablet, a phablet, a desktop computer, a laptop computer, a server, a smartphone, a smartwatch, smart eyewear, a multifunctional sensor, and a module for one or more of the same. In the example shown, the apparatus 501 comprises the various components described previously (denoted collectively by reference numeral 518), a processor 519 and a storage medium 520, which are electrically connected to one another by a data bus 521 .

The processor 519 is configured for general operation of the apparatus 501 by providing signalling to, and receiving signalling from, the other components to manage their operation. The storage medium 520 is configured to store computer code configured to perform, control or enable operation of the apparatus 501 . The storage medium 520 may also be configured to store settings for the other components. The processor 519 may access the storage medium 520 to retrieve the component settings in order to manage the operation of the other components. Based on the voltages applied by the power supply and the currents measured by the ammeters, the resistance of each circuit portion can be calculated by the processor 519 and used to determine one or more of the presence and magnitude of the respective stimuli. In some embodiments, the processor 519 may just use the current measurements directly rather than calculating the resistance of each circuit portion. Furthermore, if the apparatus 501 comprises a resistive bridge circuit, the processor 519 may be configured to determine the presence/magnitude of the respective stimuli based on the output voltages associated with the sensors. In order to determine the presence/magnitude of the respective stimuli, the processor 519 may use predetermined calibration data saved in the storage medium 520 which correlates the magnitude of the stimuli with the resistance, current or voltage of the corresponding circuit portion/branch.

The processor 519 may be a microprocessor, including an Application Specific Integrated Circuit (ASIC). The storage medium 520 may be a temporary storage medium such as a volatile random access memory. On the other hand, the storage medium 520 may be a permanent storage medium such as a hard disk drive, a flash memory, or a non-volatile random access memory.

Figure 6a shows schematically the main steps 622-623 of a method of using the present apparatus. The method generally comprises: exposing the primary sensor to its respective stimulus 622; and sensing one or more physical stimuli using the secondary sensor(s) 623.

Figure 6b shows schematically the main steps 624-625 of a method of making the present apparatus. The method generally comprises: forming the primary sensor, one or more secondary sensors and memristor 624; and coupling the primary sensor and one or more secondary sensors to the memristor such that exposure of the primary sensor to its respective stimulus causes the memristor to switch from a high resistance state to a low resistance state 625. As indicated by the dashed box, step 624 may be performed separately from step 625 and is therefore optional.

Figure 7 illustrates schematically a computer/processor readable medium 726 providing a computer program according to one embodiment. The computer program may comprise computer code configured to perform, control or enable one or more of the method steps 622-625 of Figure 6a or 6b. In this example, the computer/processor readable medium 726 is a disc such as a digital versatile disc (DVD) or a compact disc (CD). In other embodiments, the computer/processor readable medium 726 may be any medium that has been programmed in such a way as to carry out an inventive function. The computer/processor readable medium 726 may be a removable memory device such as a memory stick or memory card (SD, mini SD, micro SD or nano SD).

Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101 , 201 , 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.

It will be appreciated to the skilled reader that any mentioned apparatus/device and/or other features of particular mentioned apparatus/device may be provided by apparatus arranged such that they become configured to carry out the desired operations only when enabled, e.g. switched on, or the like. In such cases, they may not necessarily have the appropriate software loaded into the active memory in the non-enabled (e.g. switched off state) and only load the appropriate software in the enabled (e.g. on state). The apparatus may comprise hardware circuitry and/or firmware. The apparatus may comprise software loaded onto memory. Such software/computer programs may be recorded on the same memory/processor/functional units and/or on one or more memories/processors/functional units. In some embodiments, a particular mentioned apparatus/device may be pre-programmed with the appropriate software to carry out desired operations, and wherein the appropriate software can be enabled for use by a user downloading a "key", for example, to unlock/enable the software and its associated functionality. Advantages associated with such embodiments can include a reduced requirement to download data when further functionality is required for a device, and this can be useful in examples where a device is perceived to have sufficient capacity to store such pre-programmed software for functionality that may not be enabled by a user.

It will be appreciated that any mentioned apparatus/circuitry/elements/processor may have other functions in addition to the mentioned functions, and that these functions may be performed by the same apparatus/circuitry/elements/processor. One or more disclosed aspects may encompass the electronic distribution of associated computer programs and computer programs (which may be source/transport encoded) recorded on an appropriate carrier (e.g. memory, signal). It will be appreciated that any "computer" described herein can comprise a collection of one or more individual processors/processing elements that may or may not be located on the same circuit board, or the same region/position of a circuit board or even the same device. In some embodiments one or more of any mentioned processors may be distributed over a plurality of devices. The same or different processor/processing elements may perform one or more functions described herein.

It will be appreciated that the term "signalling" may refer to one or more signals transmitted as a series of transmitted and/or received signals. The series of signals may comprise one, two, three, four or even more individual signal components or distinct signals to make up said signalling. Some or all of these individual signals may be transmitted/received simultaneously, in sequence, and/or such that they temporally overlap one another.

With reference to any discussion of any mentioned computer and/or processor and memory (e.g. including ROM, CD-ROM etc), these may comprise a computer processor, Application Specific Integrated Circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out the inventive function. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure. While there have been shown and described and pointed out fundamental novel features as applied to different embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.