Field of Invention

The application relates to methods and apparatus, particularly for sensing such as the sensing of a parameter of tissue, and particularly for sensing the oxygenation of target tissue, and in particular the sensing of the oxygenation of target tissue at a plurality of depths below the surface of the tissue, as well as to a method of calibrating the apparatus.

Background

A low level of oxygen in tissue is usually indicative of poor blood circulation or poor lung capabilities. This can be due to or cause heart attack, stroke, ulceration or gangrene (among other conditions), potentially leading to sepsis, amputation and death. Knowing at an early stage that the oxygen level is reducing over time is particularly critical for patients with cardiovascular and respiratory diseases.

Worldwide the number of people living with this condition and related complications, the number of related critically invasive surgeries such as organ repair or amputation, and the cost of managing these conditions are large.

Similarly, monitoring the oxygen level in patients undergoing surgeries where complications due to poor blood circulation can occur is critical to allow the clinical staff to act rapidly to reduce morbidity and mortality.

The current solutions for monitoring blood circulation in tissue are based on regular one-to-one assessments in a clinical setting. The outcomes of these snapshot assessments are often subjective and qualitative. Other commercially available devices, such as imaging examinations using expensive machines requiring trained clinical staff, aim to support the clinical staff during these assessments, but cannot provide non-invasive, continuous, quantitative blood circulation and tissue health monitoring, especially when patients are outside of the clinical setting.

The present application aims to provide non-invasive, continuous, quantitative monitoring of oxygen level in tissue such as skin tissue to deduce the quality of the blood circulation and its composition. This may be used on patients in and outside of the hospital setting, to provide early warning notifications to the clinical staff when degradation is measured.

Statements of Invention

Aspects of the invention are set out in the independent claims. Optional features are set out in the dependent claims.

According to a first embodiment there is provided a sensor for sensing oxygenation of tissue at a plurality of depths below a surface of the tissue. The sensor comprising a light provider configured to direct light at the tissue from a selected emission location, a light receiver configured to receive, at a selected receiving location, the light from said tissue, and an oxygenation signal provider. The oxygenation signal provider is configured to provide a first oxygenation signal based on light received at a first receiving location from a first emission location separated from the first receiving location by a first separation distance, and a second oxygenation signal based on light received at a second receiving location from a second emission location separated from the second receiving location by a second separation distance. Wherein the first separation distance is different from the second separation distance whereby that the first oxygenation signal and the second oxygenation signal correspond to different depths below the surface of the tissue.

Optionally, the said separation distance may be changed by varying the selected emission location; and/or the selected receiving location. Optionally, the light provider may comprise a first light emission element and a second light emission element, the first light emission element positioned closer to the selected light receiving location than the second light emission element. Optionally, the sensor may be configured to provide the first separation distance by operating the first light emission element and may be configured to provide the second separation distance by operating the second light emission element. Optionally, when light is emitted by the first light emission element the first oxygenation signal may correspond to the oxygenation of tissue of less than 5mm below the tissue surface, and when light is emitted by the second light emitting element the second oxygenation signal may correspond to the oxygenation of tissue of more than 15mm below the tissue surface. Optionally, the light receiver may comprise a first light receiving region and a second light receiving region, the first light receiving region being closer to the selected emission location than the second light receiving region.

Optionally, the sensor may be configured to provide the first separation distance by operating the first light receiving region and may be configured to provide the second separation distance by operating the second light receiving region. Optionally, the light provider may be configured to direct light at a wavelength that corresponds to an absorption peak in haemoglobin, for example at a wavelength that corresponds to an absorption peak of oxygenated haemoglobin, or an absorption peak of deoxygenated haemoglobin. Further optionally the wavelength may correspond to an absorption peak to a different molecule found in a different tissue type. Optionally, the light provider may be configured to direct light in the part of the electromagnetic spectrum corresponding to the colour green, and/or in the part of the electromagnetic spectrum corresponding to infrared light, for example wherein the light provider is configured to direct green light at a wavelength of between 500nm and 600nm, for example a wavelength of 568nm, and for example wherein the light provider is configured to direct infrared light at a wavelength of above 750nm, for example 880nm.

Optionally, further comprising a reference light provider, wherein the reference light provider may be configured to direct light at a reference wavelength different to the first light provider, for example where the reference light provider is configured to direct light in the part of the electromagnetic spectrum corresponding to the colour red, for example at a wavelength of between 600nm and 750nm, for example at a wavelength of 640nm. Optionally, the reference light provider may be configured to be used to improve the signal to noise ratio such that the accuracy of said oxygenation signal is increased. Optionally, further comprising a second light provider configured to direct light at a wavelength that corresponds to an absorption peak in haemoglobin, and at a different wavelength to the first light provider, for example if the first light provider may be configured to direct light in the portion of the electromagnetic spectrum corresponding to green light then the second light provider may be configured to direct light in the part of the electromagnetic spectrum corresponding to infrared light. Optionally, the light provider may be configured to emit light having a waves rate, for example wherein the output of the light provider may be modulated to create a waves rate, for example wherein the waves rate is above 200Hz. Optionally, the light provider may be configured to direct light at an intensity between the intensity required for the light receiver to measure a received light signal within a specified range, and the maximum intensity that would enable the sensor to function for at least 24 hours.

According to a further embodiment there is provided a method of operating a device to sense the oxygenation of tissue at a plurality of depths below the surface of the tissue. The method comprising the steps of emitting a first light signal at a first emission location, the first light signal directed at a tissue region, receiving at a first receiving location a first light signal reflected/refracted from the tissue, wherein the first emission location and the first receiving location are separated by a first separation distance, emitting a second light signal at a second emission location, the second light signal directed at the tissue region, receiving at a second receiving location a second light signal reflected/refracted from the tissue, wherein the second emission location and the second receiving location are separated by a second separation distance, thereby to provide an indication of oxygenation level at a plurality of different depths below the surface of the tissue.

Optionally, providing an indication of the oxygenation level may comprise determining a first oxygenation signal based on the light received at the first selected receiving location, the first oxygenation signal corresponding to a first depth below the surface of the tissue, correlating to the first separation distance, and determining a second oxygenation signal based on the light received at the second selected receiving location, the second oxygenation signal corresponding to a second depth below the surface of the tissue, correlating to the second separation distance. Optionally, the first separation distance and the second separation distance are different. Optionally, the first and second oxygenation signals are determined independently of one another. Optionally, further comprising varying the first light emission location and/or the first light receiving location, to provide that the first separation distance and the second separation distance are different. Optionally, further comprising emitting a reference light signal at a different wavelength to the first light signal, receiving a reflected/refracted reference light signal, improving the signal to noise ratio of the first oxygenation signal by using both the received first light signal and the received reference light signal together to determine the indication of the oxygenation level. Optionally, further comprising emitting a third light signal at a different wavelength to the first light signal, and at a wavelength that corresponds to an absorption peak of haemoglobin, receiving at the light receiver a reflected/refracted third light signal, determining a third oxygenation signal based on the received third light signal.

According to a further embodiment there is provided an oxygenation sensor configured to sense the oxygenation level of tissue at two depths below the surface of the tissue. The sensor comprising a light provider and a light receiver, a dressing comprising a cradle adapted to hold the light provider and light receiver with a light transmission window configured to allow light to pass therethrough, and an adhesive surface configured to secure the sensor to the patient.

According to a further embodiment there is provided a dressing which includes a sensor for sensing the oxygenation level of tissue at two depths below the surface of the tissue. The dressing comprising an adhesive surface configured to secure the dressing to a patient, and a cradle configured to hold the sensor, the cradle comprising a light transmission window to allow light to pass therethrough.

According to a further embodiment, there is provided a method of calibration, for an individual user, a device for monitoring the oxygenation of tissue at a series of different depths below the surface of the tissue. The method of calibration comprising the steps of emitting light at a first light intensity, and at a first waves rate, receiving reflected/refracted light and generating a light signal based on the received light, comparing the received light signal to a specified range, and if the received light signal is outside of the specified range, changing the intensity of the emitted light to a second intensity.

Optionally, further comprising continuing to change the intensity of the emitted light through a plurality of different intensity values until the received light signal is within the specified range. Optionally, changing the intensity of the emitted light through a plurality of different intensity values comprises changing the intensity by the smallest increment that will measurably affect the received light signal each time the intensity is changed. Optionally, further comprising changing the waves rate of the emitted light to a second waves rate. Optionally, further comprising continuing to change the waves rate of the emitted light through a plurality of different waves rate values until the received light signal is within the specified range. Optionally, changing the waves rate of the emitted light through a plurality of different waves rate values may comprise changing the waves rate by the smallest increment that will measurably affect the reflected light signal each time the waves rate is changed. Optionally, the waves rate may be changed by changing the time gap between each waves by 2.5ms.

According to a further embodiment there is provided a method of calibration, for an individual user, a device for monitoring the oxygenation of tissue at a series of different depths below the surface of the tissue. The method of calibration comprising the steps of emitting light at a first intensity, and at a first waves rate, receiving reflected/refracted light and generating a received light signal based on the reflected/refracted light, comparing the received light signal to a specified range, and if the received light signal is outside of the specified range changing the waves rate of the emitted light to a second waves rate.

Optionally, further comprising continuing to change the waves rate of the emitted light through a plurality of different waves rate values until the received light signal is within the specified range. Optionally, changing the waves rate of the emitted light through a plurality of different waves rate values comprises changing the waves rate by the smallest increment that will measurably affect the received light signal each time the waves rate is changed. Optionally further comprising changing the intensity of the emitted light to a second intensity. Optionally, further comprising continuing to change the intensity of the emitted light through a plurality of different intensity values until the received light signal is within the specified range. Optionally, changing the intensity of the emitted light through a plurality of different intensity values comprises changing the intensity by the smallest increment that will measurably affect the received light signal each time the intensity is changed. Optionally, the waves rate may be changed by changing the time gap between each wave by 2.5ms.

Aspects of the present disclosure may provide apparatuses and methods for wound tissue monitoring by extrapolation of the tissue oxygen saturation at an area surrounding a wound. For example, in an aspect, there is provided a hypoxic tissue monitoring apparatus for sensing oxygenation of hypoxic tissue of a human or animal body at a plurality of depths below a surface of the body, the apparatus comprising: an optical sensor; a cradle, wherein the cradle is arranged to be coupled to the body so as to hold the optical sensor proximal to the hypoxic tissue; and an oxygenation signal provider coupled to the optical sensor to receive signals therefrom. The optical sensor comprises: a light provider configured to direct light at the hypoxic tissue from a selected emission location when the optical sensor is held in the cradle; and a light receiver configured to receive, at a selected receiving location, the light from said hypoxic tissue when the optical sensor is held in the cradle; wherein the oxygenation signal provider is configured to provide: a first oxygenation signal based on light received at a first receiving location from a first emission location separated from the first receiving location by a first separation distance; and a second oxygenation signal based on light received at a second receiving location from a second emission location separated from the second receiving location by a second separation distance; wherein the first separation distance is different from the second separation distance whereby that the first oxygenation signal and the second oxygenation signal correspond to different depths below the surface of the body.

The hypoxic tissue monitoring apparatus may provide a tissue monitoring apparatus for sensing oxygenation of hypoxic tissue of the human or animal body. The hypoxic tissue may be tissue which is associated with (e.g. affected by or created by) a wound. For example, hypoxic tissue may comprise wound tissue. Wound tissue may comprise tissue of the wound, or tissue which is in the region of the wound (e.g. tissue immediately surrounding the wound). Hypoxic tissue may also comprise tissue which is neither tissue of the wound, nor in the region of wound, but instead this hypoxic tissue may be in a region of the body which is located away from the wound, but still influenced by the wound. For example, the hypoxic tissue may be tissue where that tissue's blood supply which is influenced by the wound, such as tissue which is further away from the heart than the wound. For a wound to a limb, some, or all, of that limb may be deprived of oxygen. For example, hands and feet could have tissue oxygenation levels which are influenced by wounds to the upper arm and leg respectively. Thus, hypoxic tissue may comprise the majority of the tissue of that limb.

Hypoxic tissue monitoring apparatuses of the present disclosure may be arranged for monitoring tissue most susceptible to effects arising from hypoxia of that tissue. For example, an apparatus may be configured to monitor oxygenation levels at the feet (e.g. human, such as adult human, feet). The feet may be considered a weaker part of the body as friction occurring with the feet may cause damage to foot tissue (e.g. by creating ulcers or wounds). As such, hypoxic tissue monitoring apparatuses of the present disclosure may be configured for placement away from a wound to monitor tissue which may nevertheless be influenced by the wound. In other examples, hypoxic tissue monitoring apparatuses of the present disclosure may be configured for placement at, or close to, a wound to monitor wound tissue (e.g. in the immediate vicinity of the wound for monitoring healing of the wound itself).

Optionally, wherein the cradle comprises an adhesive coupling surface for coupling the cradle to the body. Optionally, wherein the cradle comprises a sensor receiving portion arranged to receive the optical sensor and to hold the optical sensor in place proximal to the hypoxic tissue (e.g. tissue in the immediate region of the wound or tissue having oxygenation levels which are still influenced by the wound); and wherein the cradle comprises an optically transparent region arranged to enable: (i) light from the light provider to pass through the optically transparent region towards the hypoxic tissue, and (ii) light from the surrounding area of the hypoxic tissue to pass through the optically transparent region towards the light receiver, when the optical sensor is held in place in the optical sensor receiving portion of the cradle. Optionally, wherein the optical sensor is spaced away from the oxygenation signal provider when the optical sensor is held in the cradle and coupled to the body. Optionally, wherein the optical sensor is coupled to the oxygenation signal provider via a wire connection. Optionally, further comprising a body coupling to couple the oxygenation signal provider to the body, optionally wherein the body coupling comprises attachment means such as a strap or clip. Optionally, wherein the oxygenation signal provider is re-usable, optionally wherein the oxygenation signal provider is housed within a wipe clean and/or steralisable housing.

Brief Description of Drawings

Reference is now directed to the accompanying drawings, in which:

Figure 1 illustrates a cross-section of a sensor according to one implementation. The sensor is shown adjacent target tissue, and with an optional dressing.

Figure 2 illustrates a schematic diagram of a sensor adjacent to target tissue in the foot.

Figure 3 illustrates a sensor according to a first embodiment comprising a light provider, a light receiver and a signal provider. Figure 4 illustrates a sensor according to a second embodiment wherein the light provider comprises a first light emission element and a second light emission element.

Figure 5 illustrates the sensor of the second embodiment in a first mode.

Figure 6 illustrates the sensor of the second embodiment in a second mode.

Figure 7 illustrates the sensor of a third embodiment comprising an additional reference light provider for improving the signal to noise ratio.

Figure 8 illustrates the sensor of the third embodiment wherein the reference light provider comprises a first reference light emission element and a second reference light emission element.

Figure 9 illustrates the sensor of a fourth embodiment comprising a second light provider configured to emit light at a different wavelength to the first light provider.

Figure 10 illustrates the sensor of the fourth embodiment also comprising the reference light provider of the third embodiment.

Figure 11 illustrates the sensor of a fifth embodiment wherein the light receiver comprises a first light receiving region and a second light receiving region.

Figure 12 illustrates the sensor of the fifth embodiment operating in a first mode.

Figure 13 illustrates the sensor of the fifth embodiment operating in a second mode.

Figure 14a illustrates the sensor of the fifth embodiment operating in a first transition state between the first mode and the second mode.

Figure 14b illustrates the sensor of the fifth embodiment operating in a second transition state between the first mode and the second mode.

Figure 14c illustrates the sensor of the fifth embodiment operating in a third transition state between the first mode and the second mode.

Figure 14d illustrates the sensor of the fifth embodiment operating in a fourth transition state between the first mode and the second mode.

Figure 14e illustrates the sensor of the fifth embodiment operating in a fifth transition state between the first mode and the second mode.

Figure 15 illustrates a method of sensing the oxygenation of tissue at a plurality of depths below the surface according to a first embodiment.

Figure 16 illustrates the method of Figure 15, with additional optional steps shown.

Figure 17 illustrates a first embodiment for calibrating a sensor to suit an individual user.

Figure 18 illustrates a second embodiment for calibrating a sensor to suit an individual user.

Figure 19 illustrates a third embodiment for calibrating a sensor to suit an individual user.

Figure 20 illustrates a first embodiment of a dressing for housing the device. Detailed Description of Drawings

Figure 1 shows a cross section of a sensor 2 for sensing a parameter of tissue, for example the oxygenation of tissue. The sensor 2 comprises a light provider 4, a light receiver 3, and a signal provider 10. The sensor 2 is configured to sense the oxygenation of tissue at a plurality of depths below a surface of the tissue. Also shown is an optional dressing 1, and a region of target tissue 7 having a surface 6.

The light provider 4 is configured to direct light at the tissue from a selected emission location. The light receiver 3 is configured to receive, at a selected receiving location, the light from said tissue after it has interacted with said tissue. The signal provider 10 is connected to the light receiver and is configured to provide a signal based on the light received at the receiving location, for example the signal provider 10 may provide an oxygenation signal, indicating a level of oxygenation of the tissue. In particular the signal provider 10 may provide a first oxygenation signal based on light received at a first receiving location from a first emission location separated from the first receiving location by a first separation distance, and also provide a second oxygenation signal based on light received at a second receiving location from a second emission location separated from the second receiving location by a second separation distance. When the first separation distance is different from the second separation distance, the first oxygenation signal and the second oxygenation signal correspond to different depths below the surface 6 of the tissue 7.

As illustrated in Figure 1 the light provider 4, the light receiver 3, and the oxygenation signal provider 10 are provided on a body 5, e.g. on the same face of that body 5. In this implementation, the light provider comprises two LEDs 4a, 4b, the light receiver 3 comprises a photodetector and the body 5 comprises substrate, such as a printed circuit board. The oxygenation signal provider 10 may comprise a processor connected to the light receiver 3. The processor is configured to control the light provider, e.g. by controlling modulation of light intensity produced by the LEDs 4a, 4b. The LEDs are both configure to emit light with the same wavelength.

The first LED 4b is positioned at a first separation distance from the photodetector array 3. The second LED 4a is positioned at a second separation distance from the photodetector array 3. The first separation distance is greater than the second separation distance. Examples of appropriate separation distances include the first separation distance being between 4mm and 7mm, and the second separation distance being between 5.5mm and 8mm. In particular if the LEDs are configured to emit green light the first separation distance may be 4mm, and the second separation distance may be 5.5mm.

The sensor 2 is configured (e.g. sized and arranged) to be positioned within a dressing 1. The dressing 1 comprises a cradle 8 configured to hold the sensor 2 in place, and a window 9. The window 9 is transparent to light emitted from the LEDs 4a, 4b, and is arranged to allow light emitted by the light provider 4a, 4b to penetrate target tissue 7, and for light from the target tissue 7 (e.g. light scattered or reflected by the tissue) to reach the photodetector array 3. The dressing 1 may also comprise adhesive to attach the dressing to a user, for example at the tissue surface 6.

The dressing is arranged so that, when it is secured to the surface of the tissue 6 a sensor 2 held in the cradle 8 is positioned adjacent to target tissue 7. This may be any tissue of the human or animal body, for example skin or muscle tissue. The sensor 2 is positioned such that the LEDs 4a, 4b and the photodetector array 3 face the target tissue 7. This enables the LEDs 4a, 4b to emit light that penetrates the target tissue 7.

In operation, the processor controls the light provider so that the light from each of the different depths within the tissue can be identified separately. For example, the processor can modulate the intensity of the light emitted by the light provider to enable light from the two LEDs to be identified separately at the receiver. To do this the controller may simply switch on the two LEDs at different times. Thus, while the first LED 4a is emitting light, the second LED 4b is switched off and vice versa.

The light emitted by the LEDs penetrates the target tissue 7, and interacts with the target tissue 7 before re-emerging for detection by the light receiver. This interaction may comprise the photons of the emitted light being absorbed by a molecule (such as a protein e.g. haemoglobin, for example oxygenated or deoxygenated haemoglobin) within the target tissue 7, and then being re-emitted, and may further comprise reflection and/or refraction. The intensity of the light that re-emerges from the tissue 7 is dependent on the concentration of the type of haemoglobin in the blood. The intensity of the light received by the photodetector is also based on the wavelength and intensity of light that is initially emitted by the LED. Therefore, by measuring the intensity of the received light at one, or both wavelengths the sensor 2 can determine the oxygenation level of the tissue. In the presently described implementation both LED's 4a, 4b are configured to emit light having a wavelength corresponding to the absorption peak of oxygenated haemoglobin. The first LED 4b is further away from the photodetector array 3 than the second LED 4a. Based on the modified Beer-Lambert law quantifying oxygenation level from multi-spectral analysis, the light follows a banana-shaped path between the light receiver and detector. As such, the light received from the second LED 4a by the photodetector comprises more light intensity received from the tissue 7 close to its surface 6 than from deeper regions of tissue 7. Conversely, because the first LED 4b is further away from the photodetector array 3, the light received from the first LED tends to comprise more light intensity from deeper regions of the tissue than from tissue closer to the surface 6. Thus, the first and second depths are different because the separation distances between the LEDs and the light receiver are different.

In operation, the light signals from the first LED 4b and the second LED 4a are detected by the light receiver. The light receiver provides a signal indicating the light received to the oxygenation signal provider 10. The processor of the oxygenation signal provider 10 determines whether the signal originates with the first LED 4b or the second LED 4a based on its timing (e.g. the light received during intervals in which the first LED 4b is switched on, and the second LED 4a is switched off, provides an indication of light which originates from the first LED 4b). An indication of the light received from the second LED 4a is provided in the same way. The processor then determines the oxygenation at the first depth based on Eq. 1:

Oxygenation Level ~ Q * Concentration of Oxygenated Haemoglobin + e Eq. 1

With Q e R *, e e R. The processor uses the intensity of light received by the light receiver 3 originating from the first LED 4b to deduce the concentration of oxygenated haemoglobin at the first depth. The oxygenation at the second depth is calculated in the same way using the indication of the light received from the second LED 4a. This data may be normalised as shown in equation 2. These indications of oxygenation may be useful, but the use of the sensor using at least two different pairs of LEDs producing two different wavelengths (as described below) may provide more accurate measurements.

The normalisation can be achieved following Eq. 2:

With 8 e l t, e e R,x e M, pa mean function, s the standard deviation function and F a filtering function such as a Gaussian d and e are constant that may be selected for a specific application for example based on patient's anthropometries. In this example d is one, and e is 0. The use of this function allows the data to be normalised independently on patient's anthropometries. Two patients may have different characteristics, but by using this equation to achieve normalisation their data can be compared more effectively.

The oxygenation signal provider may provide a relative oxygenation level, rather than an absolute oxygenation level. If the intensity of the received light originating from the first LED 4b changes then this corresponds to a change in the oxygenation level in the tissue at the first depth. The oxygenation signal provider then provides a relative first oxygenation signal indicating whether the oxygenation level at the first depth has increased or decreased. A relative oxygenation level at the second depth is determined in this manner from a change in the intensity of the received light originating from the second LED 4a.

Having explained one way in which oxygenation at two different depths can be determined, a further example will now be explained. In this second example, the arrangement of the sensor is identical to that described in Figure 1, other than as described below.

The sensor comprises a further pair of LEDs such that there are two pairs of LEDs emitting light. The LEDs are configured to emit light at the same intensity, and are further configured such that the first pair of LEDs 84 emits light in a wavelength band corresponding to an absorption peak of oxygenated haemoglobin, and the second pair of LEDs 87 emits light that in a wavelength band corresponding to an absorption peak of de-oxygenated haemoglobin. Each pair comprises a first LED 84a, 87a and a second LED 84b, 87b, with the first LED closer to the photodetector as described above. The arrangement of these LEDs is shown in Figure 9, whilst in cross section this embodiment is identical to the cross section shown in Figure 1.

The light receiver 83 is configured to receive light at a first wavelength emitted by the first pair of LEDs 84 and at a second wavelength emitted by the second pair of LEDs 87. The intensity of light received in each wavelength band can be measured independently. In this embodiment the oxygenation signal provider also comprises a processor and controls the LEDs so that the light from each of the different depths within the tissue can be identified separately. For example, the processor can modulate the intensity of the light emitted by the LEDs to enable light from the first LED of each pair 84a, 87a to be identified separately from the second LED of each pair 84b, 87b at the receiver. To do this the controller may simply switch on the first LED of each pair 84a, 87a at a different time to the second LED of each pair 84b, 87b. Thus, while the first LED of each pair 84a, 87a is emitting light, the second LED of each pair 84b, 87b is switched off and vice versa.

In operation, the light emitted from the first LED of each pair and the second LED of each pair is detected by the light receiver 83. The light receiver 83 provides a signal indicating the light received to the oxygenation signal provider. The processor of the oxygenation signal provider determines whether the signal originates with the first LED of each pair 84a, 87a or the second LED of each pair 84b, 87b based on its timing (e.g. the light received during intervals in which the first LED of each pair is switched on provides an indication of light which originates from the first LED of each pair). An indication of the light received from the second LED of each pair is provided in the same way. As light from the first LED of the first pair 84a and the first LED of the second pair 87a are received at the same time these signals must be differentiated from each other in another way. As the wavelength of the light emitted by the two pairs of LEDs are different the received light is made up of two wavelength bands. The intensity of light in the first wavelength band corresponds to the intensity of light received from the first LED of the first pair 84a. The intensity of light received in the second band corresponds to the intensity of light received from the first LED of the second pair 87a. The processor then determines the oxygenation at the first depth based on Eq. 3:

With Q e ll », E 6 E. In practice, the intensity of the light received from the first pair of LEDs 84 is proportional to the concentration of oxygenated haemoglobin in the tissue. The intensity of light received from the second pair of LEDs 87 is proportional to the concentration of deoxygenated haemoglobin in the tissue. As the constant of proportionality is the same in both cases this equation can be rearranged following Eq. 4 :

With Q e E *, e e R. The processor uses the intensity of the light received from the first LED of each pair 84a, 87a to determine the oxygenation level at the first depth below the surface. Similarly, the processor uses the intensity of light received from the second LED of each pair 84b, 87b to determine the oxygenation level at the second depth below the tissue surface.

Figure 2 shows a sensor 12 positioned adjacent to the surface of the target tissue 6. In this case the surface of the target tissue may be the skin on the underside of the foot. In particular, the sensor 12 may be configured to sense a parameter of the target tissue, for example the oxygenation level of the target tissue. Further the sensor is configured to sense the parameter at a plurality of depths below the surface of the target tissue. This may give a distribution of the parameter over a range of depths below the surface, which may be useful for a clinician making a clinical decision. The target tissue could be any location on a human or animal body.

The problems described in the background section may be addressed by the features illustrated in Figures 1 and 2 as described above. For example, the provision of a first oxygenation signal and a second oxygenation signal, wherein the signals correspond to different depths below the surface of the tissue enables the device to give information at a plurality of depths. This may give clinicians more information that may be used in diagnostics and may lead to a more informed clinical decision. The Figures described below may provide optional embodiments as to how to implement the sensor described above.

Figure 3 shows a first embodiment of a sensor 22 for sensing a parameter of tissue, for example the oxygenation of tissue. The sensor 22 comprises a light provider 24, a light receiver 23, and a signal provider 21. The light provider and the light receiver are shown positioned on a housing 25. The sensor 22 is configured to sense the oxygenation of tissue at a plurality of depths below a surface of the tissue.

The light provider 24 is configured to direct light at the tissue from a selected emission location. The light receiver 23 is configured to receive, at a selected receiving location, the light from said tissue. The signal provider is configured to provide a signal based on the received light at the receiving location, for example the signal provider may provide an oxygenation signal. In particular the signal provider may provide a first oxygenation signal based on light received at a first receiving location from a first emission location from the first emission location by a first separation distance, and also provide a second oxygenation signal based on light received at a second receiving location from a second emission location from the second emission location by a second separation distance. When the first separation distance is different from the second separation distance the first oxygenation signal and the second oxygenation signal therefore correspond to different depths below the surface of the tissue. The light provider and the light receiver may both be positioned on a single body, or housing, such as a substrate. This may allow the light provider and the light receiver to be positioned adjacent to the target tissue. The light provider may therefore emit light into the target tissue, and the light receiver may receive light from the target tissue. Coupling both of the light provider and the light receiver to the same face of the substrate may allow both to function simultaneously whilst adjacent to the target tissue. Both the light provider and the light receiver are structural elements. The light provider and the light receiver may for example be comprised of suitable materials such as semiconductors, fibre optics, or any other suitable material. For example, the light provider may be a light source such as a laser, or light emitting diode, or it may be a light transportation element such as a fibre optic cable. The light receiver may be any material that could absorb light, or transport light to a suitable other location. For example, the light receiver may be a photodetector, or a fibre optic cable.

Figure 4 shows a sensor 32 according to a second embodiment. This comprises the means of Figure 3 (the signal provider is not shown for simplicity). Figure 4 illustrates that the light provider 34 is comprised of a first light emission element 34a and a second light emission element 34b. The first light emission element 34a is closer to the selected light receiving location than the second light emission element 34b. The first light emission element 34a may also be closer to the light receiver 33 than the second light emission element.

When light is emitted by the first light emission element 34a the first oxygenation signal may correspond to the oxygenation of tissue less than 5mm below the tissue surface, and when the light is emitted by the second light emission element 34b the signal corresponds to the oxygenation of tissue more than 15mm below the tissue surface. The first light emission element 34a may be positioned 4mm from the selected receiving location (for example if it is configured to emit green light), or it may be positioned 7mm from the selected receiving location (for example if it is configured to emit infrared light). The second light emission element 34b may be positioned 5.5mm from the selected receiving location (for example if it is configured to emit green light), or it may be positioned 8mm from the selected light receiving location (for example if is configured to emit infrared light). The light provider 34 may comprise any number of light emission elements. For example, the light provider may comprise 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more light emission elements. The light emission elements may be arranged to be incrementally further from the selected receiving location. This may allow a range of separation distances to be achieved according to the penetration depth below the tissue surface.

The use of two or more light emission elements positioned at different distances from the selected light receiving location enables the separation distance to be changed by modulating which light emission element is emitting light at any one time. This may be advantageous as the change in separation distance may allow the oxygenation level at different depths below the surface of the tissue to be determined.

Figure 5 shows a first mode of operation of the sensor of the second embodiment. In Figure 5 the first light emission element 44a is operating. The first light emission element 44a is the light emission element closest to the selected receiving location, and the light receiver 43. In this mode of operation, the sensor 42 is configured to provide the first separation distance by operating the first light emission element 44a. This mode of operation allows the first oxygenation signal to be provided. This senses the oxygenation level at the first depth below the tissue surface.

Figure 6 shows a second mode of operation of the sensor of the second embodiment. In Figure 6 the second light emission 54b is operating. The second light emission element is the light emission element further from the selected receiving location, and the light receiver 53. In this mode of operation, the sensor is configured to provide the second separation distance by operating the second light emission element. This mode of operation allows the second oxygenation signal to be provided. This senses the oxygenation level at the second depth below the tissue surface.

Figure7 shows a third embodiment of the sensor 62. The sensor comprises a first light provider 64, and a reference light provider 66, as well as a light receiver 63. As in the second embodiment the first light provider 64 comprises a first light emission element 64a, and a second light emission element 64b. These elements are located at different distances from the light receiver 63, such that the first light emission element 64a is closer to the light receiver 63. The reference light provider 66 is configured to direct light at a reference light wavelength different to the wavelength of light directed by the first light provider 64. The reference light directed by the reference light provider 66 may be used to improve the signal to noise ratio of the light received by the light receiver 63 such that the accuracy of the oxygenation signal(s) is increased. In one embodiment the oxygenation level may be determined following Eq. 4 The signal to noise ratio is improved by either lowering the noise in the signal received by the light receiver, or by increasing the amplitude of the signal received by the light receiver. By using the reference light signal as well as the first light signal the overall light signal is increased in amplitude, and so the signal to noise ratio is increased. However, there is no contribution of the reference light to the determination of the oxygenation level. This is because in some embodiments the factor of the reference light is effectively cancelled out from the equation. Therefore, the signal to noise ratio is improved, whilst the value of the oxygenation level provided is unchanged.

The wavelength of light directed by the reference light provider 66 may be configured to be in the part of the electromagnetic spectrum corresponding to the colour red, for example at a wavelength of between 600nm and 750nm, for example of 640nm.

It is noted that the third embodiment may be implemented with only one first light emission element, rather than two. The first light provider may comprise any number of light emission elements.

The inclusion of the reference light provider 66 may be advantageous as it may allow the precision and/or accuracy of the oxygenation signal(s) to be improved. This may be used to improve the consistency of the oxygenation signal measurements. If the oxygenation signal is not consistent across different anthropometries and environments it may be harder to guide clinical decisions, and therefore improving precision and/or accuracy is beneficial.

Figure 8 shows the sensor of the third embodiment (as described with reference to Figure 7). In Figure 8 the reference light provider 76 comprises a first reference light emission element 76a, and a second reference light emission element 76b. The first reference light emission element 76a may be positioned closer to the selected receiving location and the light receiver 73 than the second reference light emission element 76b. The first reference light emission element 76a may be configured to be used to improve the signal to noise ratio of the first oxygenation signal determined from received light originating from the first light emission element 74a. The second reference light emission element 76b may be configured to be used to improve the signal to noise ratio of the second oxygenation signal determined from received light originating from the second light emission element 74b. This may allow the accuracy of both the first oxygenation signal and the second oxygenation signal to be improved. If more light emission elements are provided so to more reference light emission elements may be provided. Optionally, the first reference light emission element 76a may be positioned 6mm from the selected light receiving location. Optionally, the second reference light emission element 76b may be positioned 7mm from the selected light receiving location. Figure 9 shows a fourth embodiment of the sensor 82. The sensor 82 comprises a first light provider 84, a second light provider 87, and a light receiver 83. The first light provider 84 is configured to direct light at a first wavelength. The second light provider 87 is configured to direct light at a second wavelength different to the first wavelength. Both the first and second wavelengths may correspond to an absorption peak of a molecule found in tissue in the body. This molecule may for example be haemoglobin, and in a further example this may be oxygenated haemoglobin, or deoxygenated haemoglobin.

In Figure 9 the first light provider 84 comprises a first light emission element 84a and a second light emission element 84b. These elements are located at different distances from the light receiver, such that the first light emission element 84a is closer to the light receiver 83. Similarly, in Figure 9 the second light provider 87 comprises a third light emission element 87a and a fourth light emission element 87b. These elements are located at different distances from the light receiver 83, such that the third light emission element 87a is closer to the light receiver 83. It is noted that it is not essential that the first and second light providers 84, 87 comprise the light emission elements shown, in some embodiments both the first and second light providers may each comprise a single light emission element. Further in some embodiments a light provider may be configured to emit light at both the first and second wavelength so as to comprise both the first light provider and the second light provider.

The use of both a first and a second light provider may be advantageous as it may allow the concentration of oxygenated haemoglobin to be determined through the use of one wavelength, and for the concentration of deoxygenated haemoglobin to be determined by the use of the other wavelength. The concentration itself may not be determined, rather the light received may be indicative of the concentration, and this may be used to provide the oxygenation signal(s). In other embodiments the first light provider and second light provider may direct light at different wavelengths, but these wavelengths may correspond to separate absorption peaks of the same molecule (for example both may correspond to absorption peaks of oxygenated haemoglobin) so that two measurements may be taken to verify accuracy.

Figure 10 shows the fourth embodiment further comprising the reference light provider 96 (as shown and described with reference to Figures 7 and 8). This embodiment may advantageously improve the signal to noise ratio for the light being received by the light receiver 93, as well as providing the advantages associated with providing the second light provider 97.

Figure 11 shows a fifth embodiment of the sensor 102. The sensor 102 comprises a light provider 104 and a light receiver 103. The light receiver 103 comprises a first receiving region 103a and a second receiving region 103b. The first light receiving region 103a is closer to the selected emission location than the second light receiving region 103b. The first receiving region 103a is also closer to the light provider 104 than the second receiving region 103b.

When light is received by the first light receiving region 103a the first oxygenation signal may correspond to the oxygenation of tissue less than 5mm below the tissue surface, and when the light is received by the second receiving region 103b the signal corresponds to the oxygenation of tissue more than 15mm below the tissue surface. The light receiver 103 may comprise any number of light receiving regions. For example, the light receiver may comprise 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more light receiving regions. The light receiving regions may be arranged to be incrementally further from the selected emission location. This may allow a range of separation distances to be achieved according to the penetration depth below the tissue surface. The use of two or more light receiving regions positioned different distances from the selected light emission location enables the separation distance to be changed by modulating which light receiving region is receiving light at any one time. This may comprise two or more regions receiving light, but only using the light received from the selected receiving region to provide an oxygenation signal. This may be advantageous as the change in separation distance may allow the oxygenation level at different depths below the surface of the tissue to be determined.

Figure 12 shows the sensor of the fifth embodiment in a first mode of operation. The first receiving region 113a is configured receive light, whereas the second receiving region 113b is not. In practice both the first and second receiving regions may receive light, but the signal provider may use only the received light from the first receiving region 113a to provide the oxygenation signal. In this mode of operation, the sensor is configured to provide the first separation distance by operating the first light receiving region 113a. This may allow the first oxygenation signal to be provided.

Figure 13 shows the sensor of the fifth embodiment in a second mode of operation. The second receiving region 123b is configured to receive light, whereas the first receiving region 123a is not. In practice both the first and second receiving regions may receive light, but the signal provider may use only the received light from the second receiving region to provide the oxygenation signal. In this mode of operation, the sensor is configured to provide the second separation distance by operating the second light receiving region. This may allow the second oxygenation signal to be provided.

The sensor of the fifth embodiment may change directly from the first mode of operation to the second mode of operation. Alternatively, there may be a number of transition states between these two operating modes.

Figures 14a-14e illustrate possible transition states between the first and second operating modes. There may be any number of transition states. The transition may therefore be gradual, such that it may involve a substantially continuous movement of a receiving window along a light receiver. Each light receiving region illustrated in Figures 14a (133c), b (133d), c (133e), d (133f) and e (133g) may be a further light receiving region. These may overlap one another.

In the transition between the first mode of operation to the second mode of operation the selected light receiving location may first be shown by Figure 14a, then Figure 14b, then Figure 14c, then Figure 14d, then Figure 14e. This may allow the signal provider to provide oxygenation signals for each selected receiving window. This may allow the oxygenation signals to measure the oxygenation at a range of depths below the tissue surface. It may even give a continuous measure of oxygenation level at a continuum of depths below the tissue surface.

Referring back to Figure 3, or any specific embodiment described above, we note that the following features may optionally be included.

The light provider 24 may be formed by multiple light emission means, and therefore the selected emission location may change dependent upon which light emission means is emitting light at any given time. Moreover, the light provider 24 may be moveable between a plurality of positions, each position being a possible selected emission location. The light provider may be a light source such as an LED, laser, or filament, or may be a light transportation element such as a fibre optic link.

The light receiver 23 may be formed of multiple light receiving regions. These may for example be discrete and separate, or may be joined together into a continuous light receiver. For example, the light receiving regions may form different sections of a single photodetector such as a photodiode, or other two- dimensional light receiver. In some instances, the light receiving regions may overlap one another. The selected receiving location may change dependent upon which light receiving region is receiving light at any given time. The light receiver 23 may also be moveable between a plurality of positions, each position being a possible selected receiving location. The light receiver may be a photodetector, such as a photodiode, or may be a photoplate or other light receiving means.

The light directed by the light provider may interact with the tissue at a depth below the surface of the tissue, and onto the light receiver. For example, light directed by the light provider at the first emission location may follow a first light path such that it is absorbed by molecules in the tissue a first depth below the surface. The photons of the light may also interact (reflect/refract/re-emit) with the molecules in the tissue, and the light then emerge from this tissue such that it is received by the light receiver. This process may be known as absorption and re-emission, or as reflection and refraction. In another example light directed by the light provider at the second emission location may follow a second light path such that it is absorbed by molecules in the tissue at a second depth below the surface. If the light is emitted from the first emission location which is closer to the light receiver, then it may penetrate the tissue to a shallower depth before being absorbed and re-emitted/reflected/refracted by the molecules within the tissue. The first depth may therefore be shallower than the second depth. In one embodiment, light from the first emission location to penetrate at 5mm or less before being absorbed and re emitted/reflected/refracted, whereas light from the second emission location may penetrate only 15mm or more before being absorbed and re-emitted/ reflected/refracted.

The signal provider 21 may be any means such as a converter, a processor, or other suitable means. The signal provider could for example be a digital to analogue converter, an analogue to digital converter, a processor, transistor, or other suitable means. The signal provider 21 may be present in any of the further Figures illustrating embodiment of the sensor. The signal provider is not shown in these Figures for the sake of simplicity.

The separation distance between the selected receiving location and the selected emission location may be changed by varying either the selected emission location, and/or the selected receiving location.

Light emitted by the light provider 24 may be at a wavelength that is a maxima or minima of the spectrum of a molecule found in tissue. For example, this molecule may be haemoglobin, and in particular the molecule may be oxygenated haemoglobin, or deoxygenated haemoglobin. Absorption peaks of haemoglobin include the green portion of the electromagnetic spectrum, as well as the infrared portion of the electromagnetic spectrum. In particular the absorption peak corresponding to green light may have a wavelength of between 500nm too 600nm, and may optionally be 568nm. The absorption peak in the infrared portion of the electromagnetic spectrum may be over 750nm, for example 880nm. The light provider 24 may also direct light with an intensity between the intensity required for the received light to produce a received light signal within a specified range, and the maximum intensity that would enable the sensor to function for at least 24 hours. The light provider 24 may also direct light that has a waves rate of around 200Flz. This means that the light provider modulates light such that in one cycle light is emitted and then stopped, and this cycle is repeated 200 times a second, or every 5ms.

It is noted that the above sensor 22 may also be used to measure the level of other substances in the body. For example, the sensor described above may be used to measure the level of sugar in tissue. The wavelengths may be different to those described above and may instead correspond to the peak of a spectrum of the other substance or molecule(s). The sensor may be used for both people and animals. Alternatively, it may be used on a model of a human or animal for training purposes. It is further noted that the sensor may be powered by a cell or battery such that it is portable. This may ensure that the user does not have to connect the sensor to a power source, such as mains electricity, during use. Moreover, we note that rather than modulating the LEDs by simply switching them on and off the LEDs may be modulated in a different way, for example they may be modulated using time division multiplexing or wavelength division multiplexing.

Figure 15 shows a method of operating the sensor of any of the previous embodiments. This comprises a series of steps. The first step 141 comprises emitting a first light signal at a first emission location, the first light signal directed at a tissue region. The second step 142 comprises receiving at a first receiving location a first light signal reflected from the tissue, wherein the first emission location and the first receiving location are separated by a first separation distance. The third step 143 comprises emitting a second light signal at a second emission location, the second light signal directed at the tissue region. The fourth step 144 comprises receiving at a second receiving location a second light signal reflected, or refracted from the tissue, wherein the second emission location and the second receiving location are separated by a second separation distance. The final step 145 comprises providing an indication of oxygenation level at a plurality of different depths below the surface of the tissue.

It is noted that reflectance encompasses light being absorbed and re-emitted. For example, haemoglobin in the blood may absorb the emitted first and second light signals, and then re-emit them. These re emitted signals may be considered to be reflections or refraction. The plurality of different depths below the surface may for example comprise providing an indication of the oxygenation at a first depth below the surface and a second depth below the surface. The indication of oxygenation at a first depth below the surface may be based on the received first light signal and the first separation distance. The indication of oxygenation at a second depth below the surface may be based on the received second light signal and the second separation distance. It is also noted that the first and second separation distances may initially be the same. The first or second light emission location and/or the first or second light receiving location may then be varied such that the first and second separation distances are different. Alternatively, the first and second separation distances may always be different.

The method of operating a sensor for sensing the oxygenation of tissue may be advantageous. For example, emitting and receiving two light signals in order to provide an indication of oxygenation level at a plurality of different depths below the surface of the tissue may allow the health of the tissue to be monitored effectively. This may give a clinician an indication if the health of the tissue is poor, or changes, or in some way is unexpected, earlier than through other methods. This means that the patient may be treated earlier, potentially reducing the risk of complications. In particular an indication of the oxygenation level at a plurality of levels below the surface of the tissue may enable the health of the tissue to be measured at different depths.

Optionally, the method may further comprise emitting a reference light signal at a different wavelength to the first light signal, and receiving a reflected reference light signal. The received reflected reference light signal may be used to enhance the signal to noise ratio of the first light signal. This may comprise operating a sensor according to the third embodiment of the sensor as described above, and may have the advantages described with reference to the third embodiment as well. The oxygenation signal provider may use both the received reference light signal, and first light signal to determine an oxygenation signal with an improved signal to noise ratio. In particular if the reference light signal is coherent with the first light signal then the sum of the amplitude of the signals is greater, and the noise is the same. Further optionally, the method may comprise emitting a third light signal at a different wavelength to the first light signal and at a wavelength that corresponds to an absorption peak of a molecule such as haemoglobin, and then receiving a reflected third light signal. The method may then comprise determining a third oxygenation signal based on the received reflected third light signal. This may comprise operating a sensor according to the fourth embodiment of the sensor as described above, and may have the advantages described with reference to the fourth embodiment as well.

Figure 16 shows a second embodiment of the method for operating a sensor of any of the embodiments described above. In particular the second embodiment of the method of operating the sensor differs to the first method in that an indication of oxygenation level at a plurality of depth below the surface is not given (instead this feature is optional). Instead in this second embodiment of the method a first oxygenation signal based on the light received at the first selected receiving location is determined, the first oxygenation signal corresponding to a first depth below the surface of the tissue, correlating to the first separation distance 156. The method also comprises determining a second oxygenation signal based on the light received at the second selected receiving location is determined, the second oxygenation signal corresponding to a second depth below the surface of the tissue, correlating to the second separation distance 157.

This embodiment may further comprise the optional feature of providing an indication of oxygenation level at a plurality of depth below the surface.

The second embodiment may be advantageous because the two oxygenation signals may each give separate values relate to the oxygenation at one depth, and so allow a clinician to compare the results of the oxygenation of at least two different depths.

Different users have different anthropometries. For example, some users have higher or lower percentages of fat within their bodies and this may impact the penetration of the light into the tissue. Factors that may affect the penetration of light into tissue include: fat percentage, the location of the tissue being monitored, age, scaring, ethnicity and skin tone, muscular size, as well as other environmental factors. Therefore, it is advantageous for a sensor to adapt itself to accurately and efficiently monitor patients who have different anthropometries without the need for intervention by a trained member of staff. This process may be referred to as calibration.

Figure 17 shows a first embodiment of a method of calibrating a sensor for sensing the oxygenation level of tissue. The method comprises 161 emitting light at a first intensity, and at a first waves rate, and then 162 receiving reflected light and generating a reflected light signal based on the received reflected light. The method then comprises 163 comparing the reflected light signal to a specified range. In the event that the reflected light signal is not within the specified range the method then comprises 164 changing the intensity of the emitted light to a second intensity.

Changing the intensity may allow more packets of photons or energy to penetrate the different layers of tissue. For example if a user has a higher percentage of fat, according to the optical properties of fat and due to the probability distribution of a packet of photons to be re-emitted, the received light level might increase, and so increasing the intensity of the light may advantageously increase penetration of the light and signal to noise ratio to enable the oxygenation level of the tissue to be sensed.

Figure 18 shows a second embodiment of a method of calibrating a sensor for sensing the oxygenation level of tissue. The method comprises 171 emitting light at a first intensity, and at a first waves rate, and then 172 receiving reflected light and generating a light signal based on the received reflected light. The method then comprises 173 comparing the reflected light signal to a specified range. In the event that the reflected light signal is not within the specified range the method then comprises 176 changing the waves rate of the emitted light to a second waves rate.

Changing the waves rate of the light may also increase the amount of light received by the light receiver. The optimal waves rate is linked to the resting heart rate of a user and the optical coherence tissue properties. For example, this may be higher for patients with a higher resting heart rate, and lower for patients with a lower resting heart rate. Therefore by adjusting this parameter the amount of light received by the light receiver may be optimised.

Figure 19 shows a third embodiment of a method of calibrating a sensor for sensing the oxygenation level of tissue. The method comprises 181 emitting light at a first intensity, and at a first waves rate, and then 182 receiving reflected light and generating a reflected light signal based on the received reflected light. The method then comprises 183 comparing the reflected light signal to a specified range. In the event that the reflected light signal is not within the specified range the method then comprises 184 changing the intensity of the emitted light to a second intensity. In the event that the reflected light signal is still not within the specified range the method then comprises continuing 185 to change the intensity of the emitted light through a plurality of different intensity values until the reflected light signal is within the specified range. If despite changing the intensity the reflected light signal is not within the specified range the method then comprises changing the waves rate of the emitted light to a second waves rate 186. In the event this does not work the method then comprises continuing 187 to change the waves rate of the emitted light through a plurality of different waves rate values until the reflected light signal is within the specified range.

Optionally, after continuing to change the intensity, the intensity may revert back to its initial value before the waves rate is changed. Further optionally in the event that after the waves rate is continually changed the reflected light signal is not within the specified range the intensity value is then changed to the second intensity, and then the waves rate is continually changed through a range of values until the reflected light signal is within a specified range. In the event this does not work the intensity may be changed again, and the waves rate changed through a range of values, and this process repeated until a combination of a specified intensity and waves rate lead to the reflected light signal is within the specified range.

Changing both the intensity and then the waves rate (and then optionally the intensity and the waves rate together) will allow a range of combinations to be tested increasing the likelihood of finding a combination of waves rate and intensity that will allow the reflected light signal to be within the specified range.

Changing the intensity of the emitted light may comprise changing the intensity by the minimum amount to reliably change the reflected light signal. This may comprise changing the intensity by an amount that will change the reflected signal by more than the noise of the signal.

Changing the waves rate of the emitted light may comprise changing the waves rate by the minimum amount to reliably change the reflected light signal. This may comprise changing the waves rate by an amount that will change the reflected signal by more than the noise of the signal.

Optionally, the minimum change to waves rate may comprise changing the time gap between each light wave by increments of 2.5ms. Optionally, the range of light intensities may be varied between the intensity required for the received light to produce a received light signal within a specified range, and the maximum intensity that would enable the sensor to function for at least 24 hours. The sensor described above may be positioned directly in contact with the users' skin. Alternatively, it may use a dressing adapted to hold the sensor securely, whilst enabling the sensor to function optimally.

Figure 20 shows a dressing 191 and a sensor 190. The dressing 191 is configured to adhere to a user or patient. The dressing comprises a cradle adapted to hold the sensor, and a light transmission window configured to allow light to pass therethrough. The dressing 191 may also comprise an adhesive surface configured to secure the sensor to the patient. Also shown is a control unit 192. A second cradle 198 is also shown comprising an inner region 199. The light transmission region may be positioned in line with the sensor. This may form part of the cradle and/or the adhesive surface. The control unit may comprise a power source such as a battery or cell, as well as a data store for storing information corresponding to the oxygenation of tissue, and or other parameters of the user. The control unit 192 may also comprise communication means for communicating the data to an external device such as a hospital data network.

The cradle may be shaped to hold the sensor 190, alternatively the cradle may adhere to the sensor 190. There may alternatively be a securing mechanism, such as a snap-fit to connect the control unit 192 to the sensor 190. The dressing 191 may be shaped to have a first adhesive pad and a second adhesive pad, connected by a connecting element, such that the first adhesive pad and second adhesive pad may be attached to opposite sides of a tissue surface (such as the top and bottom of a foot) so that the sensor is securely attached to the user. The dressing may be comprised of a biocompatible material, for example a mesh, and may be comprised of a polymer, or naturally occurring fibres. The dressing may opaque, with the exception of the light transmission window which is transparent. The adhesive may be any suitable biocompatible adhesive, such as those known in the art.

Sensors of the present disclosure are configured to sense oxygenation of tissue at a plurality of depths below a surface of the tissue. Such sensors may find particular utility for monitoring healing of wounds. This may include healing of the wound itself (and its surrounding tissue), as well as monitoring other regions of the body susceptible to have issues associated with wound-induced hypoxia in the region of said tissue. For example, sensors of the present disclosure may be arranged for placement in proximity to a wound. The sensors may be configured to monitor oxygenation of tissue at, or proximal to, the wound. Measurements of this tissue oxygenation in the region of the wound may provide an indication of wound healing progress. Sensor measurements of tissue oxygenation may provide useful indications of healing of the wound itself if the measurements are for an area of body tissue in, or close to, the wound. Tissue oxygenation measurements do not need to be in the wound itself for them to be useful in assessing wound healing (e.g. because properties such as oxygenation levels of both the wound and its surrounding tissue may change over time as the wound heals). In other examples, sensors of the present disclosure may be placed in regions of the body which may suffer consequences of the wound-induced hypoxia of tissue in those regions. In such examples, the sensor may be configured to monitor tissue to check tissue in that region remains healthy.

Embodiments of the present disclosure may provide sensors for monitoring oxygenation of wound tissue. Wound tissue may comprise tissue of the wound itself, as well as tissue surrounding the wound (e.g. surrounding tissue where the oxygenation properties of that tissue will also vary in dependence on the healing of the wound). In an aspect, a wound monitoring apparatus is provided. The wound monitoring apparatus may be provided in combination with a dressing for the wound, or it may not. The wound monitoring apparatus may utilise sensors of the present disclosure to sense an indication of tissue oxygenation for tissue at a plurality of depths below a surface of the tissue. The wound monitoring apparatus may be arranged to enable such a sensor to be secured to a human or animal body in a region proximal to a wound of the body so that the sensor may obtain wound tissue oxygenation measurements for the wound. The wound monitoring apparatus may be arranged to facilitate coupling of the sensor to the body proximal to the wound for extended periods of time. For example, the sensor may be held in position and operated to periodically monitor the wound tissue oxygenation (e.g. from the same position where it is held relative to the wound). Embodiments may enable wound tissue oxygenation measurements to be obtained over time, so that healing of the wound tissue may be monitored (based on the obtained measurements).

For example, a wound monitoring apparatus may comprise an optical sensor, a cradle, and an oxygenation signal provider. These components may comprise the corresponding components described above, e.g. sensor 2, signal provider 10 and/or cradle 8. The signal provider may be coupled to the sensor via a wired connection (so that signals from the sensor may be transmitted to the signal provider via the wired connection). The sensor may be selectively inserted into the cradle. The cradle may be coupled to the body so that the cradle is held in a fixed position relative to the body. When the optical sensor is inserted into the cradle, the cradle may hold the sensor in a fixed position relative to the body. The optical sensor may be inserted so that the light provider and receiver are located on a wound-facing side of the cradle. The cradle may include an optically transparent region in the region between the light provider/receiver and the wound.

The cradle may be arranged to be coupled to the human or animal body in a region proximal to the wound. For example, the cradle may comprise attachment means, such as a surface for attaching to the body (or another material coupled to the body, such as a dressing). The surface of the cradle may be an adhesive surface, such that the cradle may be adhered to the body. The cradle may be configured to extend away from a region in which it couples to the body to an optical sensor holding portion of the cradle. The optical sensor holding portion may be arranged to receive the optical sensor. For example, the holding portion may provide an interference fit with the optical sensor, e.g. so that the optical sensor may be inserted into the holding portion, where it will remain by friction until removed by a user. In other words, the cradle may be configured to provide a secure coupling between the optical sensor and the user (e.g. so that the optical sensor may be held in the cradle proximal to wound tissue of the user so that the optical sensor may obtain relevant light measurements of the tissue for enabling determination of wound tissue oxygenation).

The optical sensor may be configured to fit securely within the optical sensor holding portion of the cradle. Once inserted, the optical sensor may be configured to emit and receive relevant light signals as described above. The optical sensor and cradle may be arranged to position the light transmitting and receiving portions of the optical sensor proximal to wound tissue. For example, the cradle may elevate the optical sensor above the wound tissue. The wound tissue may comprise the wound itself, or tissue which surrounds the wound (e.g. wound tissue may comprise any tissue in the region of the wound where the oxygenation levels for that tissue may provide an indication as to the healing of that wound). The cradle may be arranged to position the optical sensor for obtaining measurements of tissue near to the edge of the wound, e.g. slightly offset from the main opening of the wound itself.

The cradle may be arranged to securely hold the optical sensor in this position for an extended period of time, e.g. so that a time sequence of tissue oxygenation measurements may be obtained for monitoring healing of the wound tissue. The cradle may be adhered to the user directly (e.g. at a location near to the wound) or to a dressing for the wound. The cradle and sensor (and dressing where relevant) may be arranged to provide a clear line of sight between the light transmitting/receiving portions of the sensor and the wound tissue. For example, the cradle may be arranged to have an optically transparent region positioned between the optical sensor and wound tissue. The optically transparent region could comprise a region of the cradle where no material (or an optically transparent material) is present. In other words, the optically transparent region could be an aperture in the cradle. For example, the cradle may have an inner region and an outer region. The outer region may define the surface for coupling the cradle to the body (or to a wound dressing in proximity of the wound). The outer region may also define the optical sensor holding portion (e.g. it may define the surround walls of that portion). The inner region may be the optically transparent region. For example, the walls of the holding portion may have one or more apertures/transparent regions in a tissue facing surface, e.g. for enabling line of sight between the sensor and the wound tissue. For example, the cradle may define an annular adhesive surface for coupling to the body, and the optical sensor may be held in the cradle so that the light transmitting/receiving portions of the sensor are directed through the central region (inside the annulus), where there is no material or transparent material. The optical sensor may have an optical window aligned with the light transmitter/receiver. The apparatus may be configured so that the optical window aligns with the transparent region of the cradle.

The tissue oxygenation signal provider may be provided by a separate component to the optical sensor/cradle. For example, the signal provider may be re-usable, such as by having a wipe clean surface and/or a sterilisable surface (e.g. the signal provider may be designed to withstand application of a sterilising fluid thereto). The optical sensor and/or the cradle may be single use. The optical sensor may be coupleable to the body in a region away from the wound tissue. For example, this may reduce the amount of stress applied to the wound tissue by coupling components to that region of the body. The signal provider may be arranged to couple to the optical sensor to receive signals therefrom. The signals may comprise light signals (e.g. the light received at the receiving portion of the sensor) or they may comprise signals indicative of these light signals (e.g. an electric signal having one or more properties such as current, which are indicative of these light signals). The signal provider may have a wired or wireless coupling to the optical sensor. For instance, a connecting wire may be provided to couple the signal provider to the optical sensor. The connecting wire may also be reusable. Attachment means such as a strap or clip may also be provided for coupling the signal provider to the body. The attachment means may couple the sensor to the body in a region away from the wound tissue.

In operation, the cradle may be coupled to the body, either directly to the body or to a wound dressing at the wound tissue on the body. The optical sensor is then inserted into the sensor receiving portion of the cradle, which is aligned so that the light transmitting/receiving portions of the sensor have a direct line of sight to the wound tissue. The signal provider is also coupled to the body at a region away from the wound tissue (such as by strapping/clipping the signal provider to the body). The signal provider is then coupled to the optical sensor, e.g. using the conductive wire. Tissue oxygenation measurements may then be obtained for the wound tissue proximal to the optical sensor (e.g. in the manner described above). Using these oxygenation measurements, healing of the wound tissue may be monitored.

For example, and with reference to Fig. 20, the control unit 192 may provide the oxygenation signal provider. In the example shown, the control unit may be coupled to the body/dressing 191 via second cradle 198. Flowever, other attachment means may be provided for coupling the control unit to the body, such as a strap or clip. The housing of control unit 192 may be arranged to be sterilised and re-usable. The sensor 190 is separated from the control unit 192. A connecting wire is shown for coupling the control unit 192 to the sensor 190. The cradle may comprise the region of material proximal to the sensor 190. For instance, the cradle may comprise an adhesive surface for attaching the cradle to the body/dressing. The cradle may mechanically support the sensor 190 and hold the sensor in a fixed relation to the wound tissue for obtaining sensor measurements for monitoring healing of the wound tissue. The cradle may be designed to have absence of material, or transparent material, in the region where the light transmitting and receiving portions of the sensor 190 are provided (e.g. to enable line of sight for these components of the sensor with the wound tissue. For example, the cradle may comprise at least partially transparent adhesive tape. This tape may be applied to the body and/or to a wound dressing proximal to the wound tissue. After use, the control unit 192 may be removed and cleaned (e.g. sterilised) for further use. The optical sensor and the cradle (and any other components) may be discarded.

As mentioned above, such sensors need not be for monitoring wound tissue, and instead may be for monitoring other hypoxic tissue. Hypoxic tissue includes wound tissue (e.g. tissue of the wound, and in the immediate region surrounding the wound), as well as tissue which may experience hypoxia due to the wound. For example, distal regions of the body (further away from the heart) may also benefit from tissue oxygenation monitoring, such as the feet. Embodiments may also provide such monitoring in the same manner as described above.

It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. For example, the functionality provided by the signal provider may in whole or in part be provided by the light receiver. In addition, the process functionality described may also be provided by devices which are supported by the sensor. It will be appreciated however that the functionality need not be divided in this way and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and/or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit.

The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims. In some examples, one or more memory elements can store data and/or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and/or claimed herein and/or to provide data processing apparatus as described and/or claimed herein.

The sensor (and any of the activities and apparatus outlined herein) and any of its constituent part may be implemented with fixed logic such as assemblies of logic gates or programmable logic such as software and/or computer program instructions executed by a processor. Other kinds of programmable logic include programmable processors, programmable digital logic (e.g., a field programmable gate array (FPGA), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM)), an application specific integrated circuit, ASIC, or any other kind of digital logic, software, code, electronic instructions, flash memory, optical disks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types of machine-readable mediums suitable for storing electronic instructions, or any suitable combination thereof. Such data storage media may also provide a data storage means for use in conjunction with the sensor to store the oxygenation signals.