Temperature sensor

Abstract

A temperature sensor comprising a light emitter, an electrical circuit for applying a reverse bias voltage across the light emitter and for measuring a reverse current, and means for calculating a temperature from the measured reverse current.

Claims

1. An implantable biomedical device for optogenetic stimulus of nervous function, the implantable biomedical device incorporating a temperature sensor, the temperature sensor comprising: a microphotonic light emitter comprising a junction between two materials and having a light emission mode for optogenetic stimulation; an electrical circuit for applying a reverse bias voltage across the light emitter and for measuring a reverse current; and means for calculating a temperature from the measured reverse current; wherein the electrical circuit comprises a second generation current conveyer configured for maintaining the reverse bias voltage within +/−5% of a target bias.

2. An implantable biomedical device according to claim 1, wherein the means for calculating a temperature from the measured reverse current includes a predefined correlation of reverse current versus temperature for the light emitter.

3. An implantable biomedical device according to claim 1, wherein the temperature that is calculated is a junction temperature of the light emitter.

4. An implantable biomedical device according to claim 1, wherein the temperature that is calculated is a surface temperature of the light emitter.

5. An implantable biomedical device according to claim 4, wherein the surface temperature is calculated using a model that relates surface temperature to a junction temperature of the light emitter and/or to the measured reverse current.

6. An implantable biomedical device according to claim 4, wherein the surface temperature is calculated using a transfer function to determine the surface temperature based on a current/voltage measurement from the light emitter.

7. An implantable biomedical device according to claim 1, wherein said electrical circuit is configured to maintain the bias voltage at a constant value between 0 and 5V in reverse bias across the optical device.

8. An implantable biomedical device according to claim 1, wherein the light emitter is a microphotonic component.

9. An implantable biomedical device according to claim 8, wherein the microphotonic component is a light emitting diode.

10. An implantable biomedical device according to claim 8, wherein the microphotonic component is a stimulated emission device.

11. An implantable biomedical device according to claim 10, wherein the stimulated emission device is a semiconductor laser diode.

12. An implantable biomedical device according to claim 8, wherein the microphotonic component is formed in Gallium Nitride, Silicon Carbide, Aluminum Nitride, Indium Nitride, Gallium Phosphide, Aluminium Phosphide, Zinc Sulphide, Magnesium sulphide, Magnesium selenide, or Zn Selenide.

13. An implantable biomedical device according to claim 1, wherein the electrical circuit comprises one or more low pass filters to filter switching noise, whereby a photonic device temperature can be accurately determined despite variations in power supply noise.

14. An implantable biomedical device according to claim 1, wherein the light emitter has two modes of operation, a temperature sensing mode when the reverse bias voltage is applied across the light emitter and a light emission mode when a forward bias voltage is applied across the light emitter.

15. An implantable biomedical device according to claim 14, wherein the temperature sensor is controlled to switch back and forth between the temperature sensing mode and the light emitting mode at a predefined frequency.

16. An implantable biomedical device according to claim 15, wherein said predefined frequency is from 10 Hz to 1 GHz.

17. An implantable biomedical device according to claim 14, wherein the temperature sensor can additionally be switched to an off mode in which no voltage is applied across the light emitter.

18. An implantable biomedical device according to claim 14, further comprising a controller for switching the temperature sensor between modes.

19. An implantable biomedical device according to claim 18, wherein the controller is configured to switch the temperature sensor between modes at defined time points, pulse widths and/or repetition frequencies.

20. An implantable biomedical device according to claim 18, wherein the controller is configured to stop subsequent operation in the light emission mode if the calculated temperature exceeds a predefined threshold.

21. An implantable biomedical device according to claim 20, wherein the predefined threshold is in the range of 0.1 to 3 C above an ambient temperature of the tissue.

22. An implantable biomedical device according to claim 14, wherein the forward bias voltage is in the range of 1.5V 10V.

23. An implantable biomedical device according to claim 14, wherein a current in the forward bias is between 1 uA to 50 mA.

24. An implantable biomedical device according to claim 1, wherein in the light emission mode the light emitter of the temperature sensor performs optogenetic stimulus of electrical activity or chemical activity indirectly via opto-electrical stimulus in non-nervous cells and/or tissue.

25. An implantable biomedical device according to claim 24, further comprising means operable to record cellular electrical or calcium potential in nervous and/or non-nervous tissue.

26. An implantable biomedical device for optogenetic stimulus of nervous function, the implantable biomedical device incorporating a temperature sensor, the temperature sensor comprising: a microphotonic light emitter comprising a junction between two materials and having a light emission mode for optogenetic stimulation; an electrical circuit for applying a reverse bias voltage across the light emitter and for measuring a reverse current; and a model or transfer function for calculating a temperature from the measured reverse current; wherein the electrical circuit comprises a second generation current conveyer configured for maintaining the reverse bias voltage within +/−5% of a target bias.

Description

BRIEF DESCRIPTION OF FIGURES

(1) An embodiment of the invention is described below with reference to the accompanying figures, in which:

(2) FIG. 1 shows hot spot formation in implantable optrodes. Left an implantable optrode which is capable of delivering both optical emission and electrical recording. Centre a testing methodology in air utilising photodiode and infra-red camera. Right the temperature increase if two of the incorporated the micro-LEDs are illuminated continuously;

(3) FIG. 2 shows Formats of optical interrogation of neural tissue (a) cuff optrode (b) planar surface emitting optrode array (c) penetrating singular optrode (d) penetrating optrode array;

(4) FIG. 3 shows Measurement results achieved for GaN LED (a) absolute reverse current versus temperature in different bias voltages and (b) revers current versus voltage in different temperatures. Note how the reverse current increases much more significantly with reverse bias;

(5) FIG. 4 shows Multiple quantum well architecture of LEDs and VCSEL lasers (a) band diagram without doping (b) band diagram with doping;

(6) FIG. 5 shows (a) Active optrode including temperature sensor and (b) Exemplar penetrating biomedical device for applications in central nervous tissue;

(7) FIG. 6 is a Block diagram of the proposed temperature sensor;

(8) FIG. 7 is an Exemplar embodiment of the sensing architecture at a transistor level;

(9) FIG. 8(a) is a block diagram of the LED employment for light emission and temperature sensing functions; and

(10) FIG. 8(b) is timing diagram of the applied signals to the LED for light emission and temperature sensing;

(11) FIGS. 9(a) and 9(b) schematically show the approach to sensor control and FIG. 9c show a temperature model;

(12) FIG. 10 shows Current gain of the designed CCII (Iz/Ix). The designed CCII should deliver the received current to its output with a linear gain in a wide range of input current (a) simulation results (b) experimental results

(13) FIG. 10 shows Current gain of the designed CCII (Iz/Ix). The designed CCII should deliver the received current to its output with a linear gain in a wide range of input current (a) simulation results (b) experimental results;

(14) FIG. 11 shows Voltage of terminal X in CCII (Vx) which is used as reference voltage, 3.3V to bias the LED at its anode (a) simulation results (b) experimental results;

(15) FIG. 12 shows Frequency analysis response for the amplifier in the last stage of the sensor with high gain;

(16) FIG. 13 shows amplifier output for a specific CCII load versus input current of CCII; and

(17) FIG. 14 Layout of the designed temperature sensor including bias circuit, CCII and amplifier.

DETAILED DESCRIPTION

(18) Utilising Microphotonics to Measure Own Junction Temperature

(19) There are two primary forms of microphotonic emitters that could be used for useful optical interrogation of biological tissue: 1. Micro-sized light emitting diodes, primarily those utilising multi-quantum well (MQW) structures to ensure high efficiency optical emission. 2. Micro-sized stimulated emission devices i.e. lasers, with the most promising candidates being vertical cavity surface emitting lasers.

(20) The optically active core material of these emitters could comprise of Gallium Nitride, Silicon Carbide, Aluminum Nitride, Indium Nitride, Gallium Phosphide, Aluminium Phosphide, Zinc Sulphide, Magnesium sulphide, Magnesium Selenide, or Zn Selenide depending on the desired optical properties. An example of the use of Gallium Nitride diodes for optical interrogation of astrocyte tissue can be seen in Berlinguer et al [13].

(21) Vertical cavity surface emitting lasers are still in their infancy. Recent examples include a recent VCSEL developed by Wen-Jie Liu at Xiamen University [14]. Such devices are still in their infancy, but at their core is a fundamentally similar semiconductor structure to the light emitting diode. As the charge generation and current flow in all diodes is temperature dependent they may be used to detect the junction temperature, and from there infer the surface temperature of a given device.

(22) In forward bias the LED current will exponentially increase with voltage. The current levels are thus high and will result in light emission (and thermal emission). Thus, from the perspective of diagnostics switching the diode into reverse bias allows interrogation of current level within a specific voltage domain. Currents in the reverse bias case are low and do not lead to perceptible light emission under normal voltages.

(23) FIG. 3 shows the results achieved from a standard Gallium Nitride micro-LED incorporating a multiple quantum well architecture. The reverse current of such LEDs change linearly with temperature for a fixed bias voltage (FIG. 3a), but exponentially with reverse bias (FIG. 3b).

(24) Theoretical Investigation

(25) At a first approximation, an LED per definition is a light emitting diode. The diode structure consists of p and n doped layers which are combined in a p-n junction structure. The Shockley equation for a p-n junction given by (1) is used to derive the temperature dependency of the reverse current.

(26) $\begin{array}{cc}I=I_s\left[\exp \left(\frac{\mathrm{eV}}{\mathrm{nkT}}\right)-1\right]I=I_S\left[\exp \left(\frac{\mathrm{eV}}{\mathrm{nkT}}\right)-1\right]& \left(1\right)\end{array}$
where I is the junction current, I.sub.S is the saturation current, e is the elementary charge, V is the junction voltage, n is the ideality factor which is about 1-2, k is the Boltzmann constant, and T is the absolute temperature in kelvins [10-12]. The LED saturation I.sub.S current can be expressed by

(27) $\begin{array}{cc}I=\mathrm{eA}\left[\sqrt{\frac{D_n}{{\tau }_n}}\frac{1}{N_D}+\sqrt{\frac{D_p}{{\tau }_p}}\frac{1}{N_A}\right]N_C{N}_V\exp \left(\frac{-E_g}{\mathrm{hT}}\right)\left[\exp \left(\frac{\mathrm{eV}}{\mathrm{nkT}}\right)-1\right]& \left(2\right)\end{array}$

(28) Here, A is the cross-sectional area of the p-n junction; D.sub.n and D.sub.p are diffusion constants for electrons and holes exhibiting a T.sup.−1/2 temperature dependency; τ.sub.m and τ.sub.p are the minority carrier lifetimes for electrons and holes supposed to be temperature independent; ND and NA are doping concentration of donors and acceptors which are independent of temperature; N.sub.C and N.sub.V are effective densities of states at the conduction-band and valence-band with temperature dependency of T.sup.3/2. E.sub.g is the energy bandgap given by:

(29) $\begin{array}{cc}{E}_g\left(T\right)=E_g\left(0\right)-\frac{\alpha T^2}{T+\beta }& \left(3\right)\end{array}$
where E.sub.g(0) is the energy bandgap in T=0 K and α and β are the Varshni parameters [15, 16]. The modelling results based on Eq (3) shows a linear relation for I.sub.R-T in pn junctions when a constant reverse voltage is applied.

(30) Multiple Quantum Well Structures

(31) In practice the theoretical framework of LEDs is more complex. Fundamentally, LEDs generate light by electron-hole recombination. Thus, the efficiency of light generation is a result of the percent of injected current (electrons and holes) which recombine to give our light compared to those that continue to the opposite terminal or recombine non-radiatevely. As a result, LED designers create a multiple quantum well structure with multiple layers of materials of different bandgaps to create trapping centres for recombination.

(32) A quantum well is a heterostructure with one thin well layer surrounded by two barrier layers. Both electrons and holes are confined in the well layer which is so thin about 40 atomic layers. The quantum wells can be grown using molecular beam epitaxy (MBE) [17], and metal-organic chemical vapor deposition (MOCVD) [18]. The typical materials to be grown can be any sequence of GaAs, AlAs, and AlGaAs. The multiple quantum well layers act as trapping centres for charge mobility. Escape from such traps is therefore mediated primarily by thermionic emission and quantum tunneling (FIG. 4) over the traps. Both effects are temperature dependent [15, 19] resulting in a linear dependency within the temperature range interesting to this invention.

(33) To summarize: Based on FIG. 3, it is very challenging to determine the junction temperature of an LED in reverse bias because the current rises considerably faster due to variations in bias than variations in temperature. Thus to achieve effective functionality, the LED must be interrogated at a fixed reverse bias which does not deviate in time or due to noise. Furthermore, although the light emitter will typically be close to the surface, what is being measured is the junction temperature rather than the surface temperature itself. A thermal model of how the LED sits in the host system must therefore be used to provide further accuracy in determining the surface temperature.

(34) CMOS Temperature Sensor

(35) FIG. 5 shows a single penetrating optrode with inbuilt stimulation and recording circuits along its shaft. The LED based temperature sensor circuitry is placed close to the μLEDs to be easily operating in antiphase with optical emission.

(36) Experimental Verification

(37) As the reverse current of LED is employed as temperature sensitive parameter to design the sensor we have experimentally explored GaN LEDs performance to investigate the linear relationship between junction temperature (Tj) and reverse current [15].

(38) An LED test setup is implemented using GaN LED to extract the needed IR-T curve at different bias voltages. The LED under test is placed in an isolated dark box to guarantee that the measured current is only due to the temperature change. The box is also temperature isolated to ensure about the accuracy of the measured temperature. A hot plate is placed under the LED to increase the temperature which can be measured and recorded by the IR Optris PI camera and the camera interface software, respectively.

(39) In a fixed reverse bias voltage across the LED, temperature is increased using the hot plat from 28° C. to 60° C. and the reverse current is measured. The measurement has been repeated for different bias voltages from −1.0V to −2.2V.

(40) The results achieved from the LED testing experiment are shown in FIG. 3. The reverse current of LED is changing linearly with temperature for a fixed bias voltage. On the other hand, the reverse current changes with voltage in a fixed temperature. In other word, a variant or temperature dependent bias voltage can cause a large reverse current variation which is not purely related to temperature variation. As a result the designed temperature sensor should provide a fixed temperature independent bias voltage for the LED to avoid current variation because of bias voltage variation.

(41) Sensor Structure

(42) FIG. 6 depicts the block diagram of the proposed temperature sensing system based on LED. The designed sensing system measures the reverse current of the LED which is reversely biased using a bias voltage. Temperature variation linearly changes this reverse current if a fixed bias voltage is applied across the LED. The bias voltage must be precise and temperature independent not to contribute in reverse current changing because the reverse current can change with bias voltage significantly. Therefore, a circuit is needed to provide the bias voltage across the LED and receive its current in same time. For this purpose, a second generation current conveyor (CCII) is used at the first stage of the sensor which is capable of providing a precise bias voltage at the input (X) while receiving current using the same input terminal. In other word, CCII conveys the received current from LED to the output (Z). The CCII receives an external bias voltage in other input (Y) and copies this voltage on the input (X) which is supposed to be connected to LED and receive the reverse current. In this design VX is match to VY and the output current should follow the current at the input (X).

(43) A unity current gain for a range of frequencies is needed for the CCII to accurately translate the current changes to temperature variation in the next stages. Also, linearity is important for a specific input range of reverse current. Therefore, a robust bias voltage, unity current gain, linearity and low power are the most important specifications in designing the CCII.

(44) A resistor is used to convert the CCII out current to voltage to be amplified using the amplifier in the last stage. The designed amplifier is a single input single output high gain amplifier. This amplifier receives the current variation as voltage in the input and amplifies that to a large signal suitable to digitize using ADC.

(45) The most important parameters in the design of this amplifier are high gain, low power, high dynamic range, high CMRR and PSRR. The common mode rejection properties are determined by matching of transistors and in particular, matching at the input differential stage. This is achieved through enlarging transistors and performing Monte Carlo modelling at the design stage to ensure a probabilistic match. Similarly, the use of larger transistors supports the low pass filtering to reject high frequency power supply noise. A negative feedback loop is utilised to support high gain and thermal noise rejection.

(46) An exemplar of our designed sensor was created using a standard CMOS (Complementary Metal Oxide Semiconductor) process from Austra Microsystems. The technology node was 0.35 μm and standard CAD/EDA tools were utilised for its design. The schematic of the design is shown in FIG. 7. The CCII at first stage receives the LED current in terminal X which has the same vdd voltage as input Y. The received current is copied to the output Z connected to a resistive load [20]. The second stage includes a two-stage high gain amplifier.

(47) Operation

(48) The sensor circuitry is placed close to the LED which is easily operating in antiphase with optical emission done using stimulation circuits. Temperature sensing is supposed to be performed after LED light emission. In other word, forward biased LED emits light where its intensity can be controlled using pulse width modulation. Then, switching the LED in to the sensor path provides reverse bias for the LED using a specific voltage reference (FIG. 8).

(49) Table 1 lists different examples for LED timing operation. PW1 is used to stimulate the LED by providing the forward bias. After this operation, temperature sensing will be performed to sense the temperature around the LED and its variation by operating in different time slots after LED stimulation.

(50) Stimulation duration (PW1) should be enough to do the LED stimulation and provide the required light intensity. Therefore, PW1 is chosen based on the applied signal as bias to the LED, its specification and the required light intensity. Sensor operation time (PW2) should be enough to do temperature sensing operation and in same time small enough to decrease the overall power consumption in the system as the amplifier of the sensor is the most power consuming part of the design.

(51) TABLE-US-00001 TABLE 1 exemplar timing operation for stimulating the LED with PW1 and then sensing the temperature with PW2 Frequency T PW1 Duty PW2 Duty (Hz) (msec) (msec) cycle1 (msec) cycle2 5 200 10  5% 2 1% 10 100 10 10% 2 2%

(52) Creating a Transfer Function for the Control Unit

(53) The proposed sensor measures the LED junction temperature. A thermal model is needed to be developed to perform final optimisation of the results to accurately determine the surface temperature of the device as opposed to the junction temperature of the light emitter. This modelling can be performed using finite element tools such as COMSOL Multiphysics. A model can be built up using the LED layer materials, implant host materials and passivation cladding materials. Provided the layers are appropriately determined, the host implant should act as a heat sink to distribute heat along its bulk and then surface to prevent hot-spot development. Nevertheless this will still occur at sufficient intensities and pulse durations, necessitating this invention. The model itself can be built up of thermal resistive elements for the materials and their dimensions. Once created a transfer function can be developed to determine what a specific junction temperature means in terms of surface temperature.

(54) Once implemented on the control unit. The controller will define pulse periods for stimulus and tissue sensing. During these periods, there will be intermittent forward and reverse voltage polarities to determine the temperature during optical stimulus. Should the temperature not exceed a threshold, this will continue until the end of the tissue stimulus or sensing period. Alternatively, should the temperature exceed a defined threshold, then the control unit will stop the stimulus and a negative event logged for long term monitoring of the implant operation.

(55) The value of the thermal resistance for the device can be used to calculate the device surface temperature based on the measured junction temperature. FIG. 9 shows the all steps needed to measure the junction temperature and surface temperatures and their variation.

(56) Results

(57) FIG. 10 shows the simulation and experimental results for the output current of the designed CCII versus input current. FIG. 11 shows the results for the voltage at terminal X of CCII which is expected to be 3.3V and act as reference voltage for the LED. The specifications achieved for CCII from simulation and experiments are almost matched and listed in Table 2.

(58) TABLE-US-00002 TABLE 2 Design specifications for CCII Specifications @ Simulation Experimental Ix = 0~200 nA results results Current gain 1 1 Vx/Vy 3.299/3.3 3.275/3.3

(59) FIG. 12 shows the frequency analysis response for the designed high gain amplifier. FIG. 13 shows the output voltage of the amplifier versus input current of CCII where a specific load resistor is placed at the output of CCII. This voltage is linear up to 200 nA for the LED reverse current.

(60) The specifications of the designed amplifier and overall system are listed in Table 3 and Table 4, respectively. The overall gain of the sensing system is about 107V/A without taking amplifier in to saturation region. For small range of temperature variations and consequently small reverse current variations a higher gain can be chosen. The layout of the design is depicted in FIG. 14.

(61) TABLE-US-00003 TABLE 3 Amplifier specifications Specifications Simulation results Gain 86 dB PM 52° BW 1.5 kHz

(62) TABLE-US-00004 TABLE 4 Design specifications Specifications Simulation Results Supply voltage 5 V Power dissipation 52 μA × 5 V   Overall gain 10.sup.7 (V/A) Size 133 μm × 64 μm

CONCLUSION

(63) An LED-based temperature sensor in AMS 0.35 μm CMOS technology has been designed to detect temperature variations around the implanted micro-LEDs in biomedical applications like optogenetics. The modeling and measurements results for GaN LEDs show that the reverse current of the LED can be considered as a reliable temperature sensitive parameter to sense thermal variations-around the LED. The designed CMOS temperature sensor consisting of a second generation current conveyor and a high gain amplifier is capable to receive and amplify the small current variations up to 200 nA. These current variations will then be translated into temperature variations based on the applied reverse voltage. This proposed method of temperature sensing is area efficient by eliminating area consuming sensing blocks which are usually used for temperature sensing in implantable systems. Also, the danger of failure because of the other devices can be decreased.

(64) Embodiments of the invention are described above by way of example only. The skilled person will appreciate that various modification can be made without departing from the invention.

REFERENCES

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