Patent Publication Number: US-2020281529-A1

Title: Wound dressing, patch member and method of sensing one or more wound parameters

Description:
This disclosure relates to a wound dressing, a patch member configured to be secured to a patient&#39;s body proximate to a wound, and a method of sensing at least one parameter associated with a wound or a region of tissue proximate a wound. 
     Wound healing is natural process performed by the human body in response to injury. The amount of time taken for a wound to heal is dependent on many different factors which include the human body&#39;s ability to heal itself and the treatments that may be applied in order to accelerate wound healing. Understanding the healing status of a wound and being able to monitor the healing process helps to inform decisions on further treatment of the wound and can also assist in the development of future wound therapies. 
     One factor that is known to be associated with wound healing is the amount of blood that is supplied to blood vessels, such as capillaries, within tissue at or near a wound. The process of supplying blood to blood vessels within tissue is known as blood perfusion. Oxygen and nutrients carried by blood within wounded tissue are essential for wound healing and so the amount of blood perfusion is known to correlate well with wound healing. The amount of blood perfusion is typically a measure of a parameter associated with a volume of blood that is supplied to tissue during each cardiac cycle or over a predetermined period of time. Various techniques have been developed for determining an amount of blood perfusion in tissue. For example, it is known that oxygen saturation is correlated with blood perfusion and so spectroscopy techniques such as near-infrared imaging have been developed to monitor oxygen saturation in a wound and the surrounding tissue. However, such conventional techniques require specialist equipment that is typically bulky, expensive and must be operated by an experienced operator. Therefore, when used to inspect a wound, a patient must regularly attend a clinic at which the equipment is located for assessment of wound healing. They are also susceptible to movement of a patient, which can lead to erroneous results. The techniques are therefore unsuitable for frequent periodic assessments of a patient or long-term assessments of a patient when at home. 
     Various techniques have been developed to monitor healing status and healing progress of a wound. Such techniques rely on specialist optical equipment that is typically bulky, expensive and must be operated by an experienced clinician. Such techniques are therefore unsuitable for frequent periodic assessments of a patient or long-term assessments of a patient when at home. 
     It is an aim of the present disclosure to at least partly mitigate the above-mentioned problems. 
     It is an aim of certain embodiments of the present disclosure to provide a means to assess wound healing at or proximate a wound that does not interfere with a patient&#39;s daily activities. 
     It is an aim of certain embodiments of the present disclosure to provide a reliable and accurate means to assess wound healing at or proximate a wound which is not adversely affected by motion of a patient. 
     It is an aim of certain embodiments of the present disclosure to provide a small, portable, easy to use, inexpensive and disposable means for monitoring wound healing. 
     It is an aim of certain embodiments of the disclosure to provide a means for remote automated monitoring of wound healing. 
     According to some embodiments, there is provided a wound dressing comprising at least one motion sensor for sensing a motion related parameter associated with motion of the wound dressing; and at least one further sensor for sensing a healing related parameter associated with wound healing at a region of tissue of a wound or proximate a wound covered by the wound dressing. 
     The healing related parameter may be a parameter associated with blood perfusion within the region of tissue. 
     The healing related parameter may be a parameter associated with oxygen saturation of blood within the region of tissue. 
     The wound dressing may further comprise a processing element for processing an output of the motion sensor and/or an output of the further sensor. 
     The processing element may be configured to determine that the output of the motion sensor satisfies a predetermined condition corresponding to a predetermined amount of motion of the wound dressing. 
     The predetermined condition may correspond to a rate of acceleration of the wound dressing which is less than a predetermined rate of acceleration. 
     The wound dressing may comprise a memory element and the processing element is configured to generate data which corresponds to an output of the motion sensor and data which corresponds to an output of the further sensor and to store said data in the memory element. 
     The data which corresponds to an output of the motion sensor and data which corresponds to an output of the further sensor may be generated from contemporaneous outputs from the motion sensor and the further sensor. 
     The processing element may be configured to retain stored data corresponding to an output of the further sensor only when the output of the motion sensor satisfies the predetermined condition corresponding to a predetermined amount of motion of the wound dressing. 
     The processing element may be configured to discard data corresponding to an output of the further sensor when the output of the motion sensor fails to satisfy the predetermined condition corresponding to a predetermined motion of the wound dressing. 
     The stored data may be associated with a trace associated with the output of the motion sensor and a trace associated with the output of the further sensor over a sample period. 
     The sample period may be not less than one second. The sample period may be not less than two seconds. The sample period may be not less than five seconds. The sample period may be not less than ten seconds. 
     The sample period may be not greater than sixty seconds. The sample period may be not greater than thirty seconds. The sample period may be not greater than fifteen seconds. 
     The memory element may be configured to store data representing the predetermined condition. 
     The wound dressing may further comprise a transmitter configured to transmit data stored in the memory element to a remote receiver. 
     The further sensor may be an optical sensor. The further sensor may be a pulse oximeter sensor. The motion sensor may comprise an accelerometer. The accelerometer may be a multiple axes accelerometer. 
     The wound dressing may be an island-type dressing having a central wound protecting portion which, in use, overlies a wound and a border portion. 
     According to some embodiments, there is provided a patch member configured to be secured to a portion of a patient&#39;s body proximate a wound, the patch comprising: 
     at least one motion sensor for sensing a motion related parameter associated with motion of the patch member; and at least one further sensor for sensing a characteristic associated with wound healing at a region of tissue of the portion of the patient&#39;s body to which the patch member is secured. 
     According to some embodiments, there is provided a wound monitoring method comprising the steps: sensing a motion related parameter associated with motion of a patient and a healing related parameter associated with wound healing at a region of tissue of the patient at or proximate to a wound; determining that the sensed motion related parameter satisfies a predetermined condition corresponding to a predetermined amount of motion of the patient; and storing and/or transmitting data which represents the sensed healing related parameter associated with wound healing. 
     The step of sensing the motion related parameter and healing related parameter may comprise the step of monitoring the motion related parameter and the healing related parameter over a sample period. 
     The sample period may be not less than one second. The sample period may be not less than two seconds. The sample period may be not less than five seconds. The sample period may be not less than ten seconds. 
     The sample period may be not greater than sixty seconds. The sample period may be not greater than thirty seconds. The sample period may be not greater than fifteen seconds. 
     The predetermined condition corresponding to a predetermined amount of motion of the patient may be a condition in which the acceleration of the patient or the portion of the patient comprising the region of tissue at or proximate to the wound is below a threshold value. 
     The motion related parameter may be a pulse frequency of pulsatile arterial blood flow through the target region of tissue and the predetermined condition corresponding to a predetermined amount of motion of the patient is a predetermined pulse frequency. 
     The healing related parameter may be associated with an amount of oxygen saturation at the region of tissue at or proximate to the wound. 
     The stored data may be data collected over a sample period in which the sensed motion related parameter satisfies the predetermined condition. 
     The method may further comprise a learning step in which the sample period is set based on attributes of the patient. 
     The predetermined condition may correspond to a predetermined amount of motion of the patient is set based on attributes of the patient. 
     The method may further comprise subsequently repeating the steps of sensing the motion and healing related parameters, determining that the motion related parameter satisfies the predetermined condition and storing data representing the parameter associated with wound healing to compile a plurality of records of data associated with wound healing. 
     The method may further comprise the step of transmitting the data comprising the plurality of records to a remote device for processing. 
     According to some embodiments, there is provided a wound dressing comprising at least one motion sensor for sensing a motion related parameter associated with motion of the wound dressing; at least one further sensor for sensing a healing related parameter associated with wound healing at a region of tissue of a wound or proximate a wound covered by the wound dressing; and a processing element for processing an output of the motion sensor and/or an output of the further sensor, the processing element configured to: determine if the output of the motion sensor satisfies a predetermined condition corresponding to a predetermined amount of motion of the wound dressing; in response to determining that the output of the motion sensor satisfies the predetermined condition, retain data corresponding to the output of the further sensor; and in response to determining that that output of the motion sensor fails to satisfy the predetermined condition, discard the data corresponding to the output of the further sensor. 
     According to some embodiments, there is provided a wound monitoring method comprising the steps: sensing a motion related parameter associated with motion of a patient and a healing related parameter associated with wound healing at a region of tissue of the patient at or proximate to a wound; determining if the sensed motion related parameter satisfies a predetermined condition corresponding to a predetermined amount of motion of the patient; in response to determining that the sensed motion related parameter satisfies the predetermined condition, storing and/or transmitting data which represents the sensed healing related parameter associated with wound healing; and in response to determining that the sensed motion related parameter fails to satisfy the predetermined condition, discarding the data which represents the sensed healing related parameter associated with wound healing. 
     According to some embodiments, there is provided a wound dressing comprising: at least one first sensor for sensing a first parameter associated with a wound or a region of tissue proximate a wound; at least one processing element; at least one memory element, and at least one energy storage device for storing energy and supplying energy to at least one of the first sensor, the processing element and the memory element, wherein the processing element is configured to process an output of the first sensor and to store data associated with the output in the memory element. 
     The energy storage device may comprise a battery. The energy storage device may comprise a capacitor. The energy storage device may comprise a fuel cell. 
     The wound dressing may further comprise an energy generator. The energy generator may be configured to generate energy from movement of the wound dressing. The energy generator may comprise an electromagnetic energy generator arranged to generate energy from movement of the wound dressing and to store energy in the energy storage device. 
     The wound dressing may further comprise an accelerometer configured to sense motion of a body to which the wound dressing is secured. 
     The energy generator may comprise a piezo electric generator. 
     The energy generator may be a thermoelectric generator configured to generate electrical energy from a temperature difference between the temperature of a body to which the wound dressing is applied and ambient temperature. 
     The wound dressing may further comprise a transmitter for transmitting data stored in the memory element. 
     The first sensor may comprise a pulse sensor. The pulse sensor may comprise a pulse oximeter sensor. 
     The first sensor may be one of a plurality of sensors comprising the wound dressing for sensing a first parameter associated with a wound or a region of tissue proximate a wound. 
     According to some embodiments, there is provided a method of sensing at least one parameter associated with a wound or a region of tissue proximate a wound, comprising the steps of: securing a wound dressing comprising a wound protecting portion over a wound whereby the wound protecting portion overlies the wound such that the wound dressing is fixed with respect to the wound; sensing a parameter associated with the wound or a region of tissue proximate the wound using a sensor integral to the wound dressing; processing an output of the sensor using a processing element integral to the wound dressing; and storing data from the processing element in a memory element integral to the wound dressing. 
     The steps of sensing, processing and storing may be repeated to compile a plurality of records of data associated with the wound. 
     The method may further comprise a step of sensing motion of a body to which the wound dressing is secured using an accelerometer integral to the wound dressing. 
     The method may further comprise the step of generating energy using an energy generator configured to generate energy from movement of the wound dressing from movement of the wound dressing. 
     The energy generated by the energy generator may be stored in an energy storage device integral to the wound dressing. 
     According to some embodiments, there is provided a method of sensing at least one parameter associated with a wound or a region of tissue proximate a wound, comprising the steps of: sensing a parameter associated with the wound or a region of tissue proximate the wound using a sensor integral to a wound dressing, the wound dressing comprising a wound protecting portion secured over a wound whereby the wound protecting portion overlies the wound such that the wound dressing is fixed with respect to the wound; processing an output of the sensor using a processing element integral to the wound dressing; and storing data from the processing element in a memory element integral to the wound dressing. 
     The steps of sensing, processing and storing may be repeated to compile a plurality of records of data associated with the wound. 
     The method may further comprise a step of sensing motion of a body to which the wound dressing is secured using an accelerometer integral to the wound dressing. 
     The method may further comprise the step of generating energy using an energy generator configured to generate energy from movement of the wound dressing from movement of the wound dressing. 
     The energy generated by the energy generator may be stored in an energy storage device integral to the wound dressing. 
     According to some embodiments, there is provided a method of sensing at least one parameter associated with a wound or a region of tissue proximate a wound, comprising the steps of: sensing a parameter associated with the wound or a region of tissue proximate the wound using a sensor integral to a wound dressing, the wound dressing comprising a wound protecting portion secured over a wound whereby the wound protecting portion overlies the wound such that the wound dressing is fixed with respect to the wound; processing an output of the sensor using a processing element integral to the wound dressing; and storing data from the processing element in a memory element integral to the wound dressing, wherein the sensor comprises a pulse sensor. 
     According to some embodiments, there is provided a wound dressing comprising: at least one first sensor for sensing a first parameter associated with a wound or a region of tissue proximate a wound; at least one processing element; at least one memory element, and at least one energy storage device for storing energy and supplying energy to at least one of the first sensor, the processing element and the memory element, wherein the processing element is configured to process an output of the first sensor and to store data associated with the output in the memory element, wherein: the wound dressing further comprises an energy generator, the energy generator comprises an electromagnetic energy generator arranged to generate energy from movement of the wound dressing and to store energy in the at least one energy storage device, and the first sensor comprises a pulse sensor. 
     According to some embodiments, there is provided a wound dressing comprising: at least one first sensor for sensing a first parameter associated with a wound or a region of tissue proximate a wound; at least one processing element; at least one memory element, and at least one energy storage device for storing energy and supplying energy to at least one of the first sensor, the processing element and the memory element, wherein the processing element is configured to process an output of the first sensor and to store data associated with the output in the memory element; and an accelerometer configured to sense motion of a body to which the wound dressing is secured, wherein the first sensor comprises a pulse sensor. 
     Certain embodiments of the present disclosure allow for a parameter associated with wound healing to be sensed when it is determined that any motion of a body or portion of a body of a patient to which a wound dressing in accordance with certain embodiments of the disclosure is applied will not adversely affect the measurement obtained. 
     Certain embodiments of the present disclosure allow for wound healing to be assessed when a body or part of a body of a patient to which a wound dressing in accordance with certain embodiments of the disclosure is applied is determined to be motionless. 
     Certain embodiments of the present disclosure allow for wound healing to be monitored without interfering with a patient&#39;s daily activities. 
     Certain embodiments of the present disclosure allow for data associated with a wound or a region of tissue proximate a wound, such as data associated with wound healing, to be collected and stored by a wound dressing for processing or subsequent retrieval. 
     Certain embodiments of the present disclosure allow for data associated with wound or a region of tissue proximate a wound, such as data associated with wound healing, to be collected and stored by a wound dressing over a prolonged period of time or intermittent periods of time without having to be connected to an external power source or external device during the periods of time or between the intermittent periods of time. 
    
    
     
       Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of apparatus comprising a wound dressing and a monitoring device in use; 
         FIG. 2  is a schematic representation of key components of the apparatus shown in  FIG. 1 ; 
         FIG. 3  is a schematic representation of a sensor for sensing a parameter associated with wound healing; 
         FIG. 4  is a flow chart depicting a method of monitoring wound healing at a target region; 
         FIG. 5  is an example of an output produced by the apparatus shown in  FIG. 1 ; 
         FIG. 6  is a further example of an output produced by the apparatus shown in  FIG. 1 ; 
         FIG. 7  shows a further embodiment of an apparatus comprising a wound dressing and a monitoring device; 
         FIG. 8  shows a further embodiment of an apparatus comprising a wound dressing and a monitoring device; 
         FIG. 9  shows a wound dressing; and 
         FIG. 10  is a flow chart showing steps associated with use of a wound dressing. 
     
    
    
       FIG. 1  shows apparatus  2  comprising a wound dressing  4  secured to a patient&#39;s arm  6 , and a monitoring device  8 . 
     The wound dressing  4  includes a wound contact layer which extends across a whole lower surface of the wound dressing and a cover layer that likewise extends across the whole of the wound dressing  4 . The wound dressing  4  comprises a central wound protecting portion  10  spaced away from an edge of the wound dressing  4 , a peripheral securing portion  12  and a sensor module  14  which is formed integrally with the securing portion  12 . The central wound protecting portion  10  may comprise one or multiple internal layers according to use such as, but not limited to wound exudate absorbing layers, fluid transport layers, spacer layers and/or anti-bacterial layers. The securing portion  12  has an adhesive on its lower surface for securing the wound dressing  4  to the arm  6  of the patient. 
     The monitoring device  8  has an integrated display  16  on which information is displayed to a user. The monitoring device  8  may be a hand-held device and may be a smartphone or tablet running a monitoring app. 
       FIG. 2  is a system diagram representing certain components of the apparatus  2  shown in  FIG. 1 . Components of the wound dressing  4  and the monitoring device  8  are enclosed, respectively, by broken lines. 
     The sensor module  14  comprises a first sensor in the form of a motion sensor  18  for sensing motion of the sensor  18  and a second sensor which is an optical sensor  20  for sensing a parameter corresponding to wound healing. In the example described this is a parameter associated with oxygen saturation (SpO 2 ), at a target region of the skin tissue of the arm  6  underneath the optical sensor  20 . The position of the motion sensor  18  is fixed with respect to the optical sensor  20 , wound protecting portion  10 , and peripheral securing portion  12  such that they move together. In the embodiment shown, the motion sensor  18  is a single-axis accelerometer and the optical sensor  20  is a pulse oximeter sensor. The sensor module  14  further comprises signal processing electronics  22  connected to the sensors  18 ,  20 , a first controller or processor  24  configured to process an output from the signal processing electronics  22 , a data storage device in the form of a memory element  26 , and a transmitter  28 . Outputs from the motion sensor  18  and the optical sensor  20  is received by the signal processing electronics  22  before being processed by the first processor  24 . 
     In addition to the display  16 , the monitoring device  8  comprises a second controller or processor  30  and a receiver  32  for receiving signals transmitted by the transmitter  28 . The signals may be transmitted wirelessly via a short-range communication protocol. 
       FIG. 3  is a schematic representation of the optical sensor  20 . The optical sensor  20  comprises two light emitters in the form of a first light emitting diode (first LED)  34  and a second light emitting diode (second LED)  36 , and a light detector in the form of a photodiode  38 . 
     The first LED  34  is configured to emit light in the near-infrared and/or infrared band of the visible spectrum, for example, light having a centre wavelength of 905 nm. The second LED  36  is configured to emit light in the red band of the visible spectrum, for example, light having a centre wavelength of 660 nm. Other numbers of LEDs and other suitable wavelengths could be utilised. 
     The photodiode  38  is configured to detect light at the wavelengths of light emitted by the first and second LEDs  34 ,  36 . 
     The first and second LEDs  34 ,  36  and the photodiode  38  are disposed within a housing  40  and are separated by a shield  42  which is opaque to the wavelength or range of wavelengths of light detectable by the photodiode  38 . The shield  42  prevents emitted light from being transmitted directly to the photodiode  38 . The lower portion of the housing  40  is open or transparent so that light emitted by the first and second LEDs  34 ,  36  can pass through the lower portion to the skin tissue of the arm  6  and light reflected or scattered by the skin tissue of the arm  6  can pass back through the lower portion of the housing  40  to the photodiode  38 . The shield  42  is spaced slightly from the skin tissue so that it does not contact the skin and so does not prevent light from passing underneath the shield  42 . Light received by the photodiode  38  is therefore light which has been emitted by at least one of the LEDs  34 ,  36  and either reflected, scattered or absorbed and reemitted by the skin tissue of the arm  6 . 
     A method of determining a parameter associated with an amount of skin perfusion in tissue surrounding a wound area using the apparatus  1  will now be described with reference to  FIGS. 1 to 6 . 
     In use, the wound dressing  4  is secured to the patient&#39;s arm  6  over a wound, as shown in  FIG. 1 , so that the lower portion of the housing  40  of the optical sensor  20 , which is open or transparent, is adjacent the target area of skin tissue, as shown in  FIGS. 2 and 3 . 
     The first and second LEDs  34 ,  36  emit light towards the skin tissue of the arm  6  at each of their respective wavelengths. The light is then either reflected, scattered or absorbed by the skin tissue depending on the wavelength of the light and the absorption/scattering characteristics of the skin tissue and the blood within the skin tissue. 
     For instance, the skin tissue can be expected to contain both arterial and venous blood. The amount of venous blood within the tissue remains substantially constant throughout the duration of a cardiac cycle (or varies independently of the cardiac cycle). The arterial blood, however, varies in accordance with the cardiac cycle such that a pressure pulse of arterial blood is created each time the heart pumps blood to the tissue. 
     It is this pulsatile arterial blood which delivers oxygen to the wound area and so it is the amount of oxygen saturation of pulsatile arterial blood which provides an indicator of wound healing. 
     At least some of the light which is not absorbed by the skin tissue or the blood within the skin tissue is reflected towards the photodiode  38 . 
     The photodiode  38  produces an output signal which represents the amount of reflected light received by the photodiode  38  from each LED  34 ,  36 . 
     The signal therefore has two components: an infrared/near-infrared component (referred to hereafter as infrared component for clarity) which represents the amount of reflected light received from the first LED  34  and a red component which represents the amount of reflected light received from the second LED  36 . The signal may be a time-multiplexed signal or a combined signal that comprises both components. 
       FIG. 5  shows traces IR, R which represent the components of the output from the optical sensor  20 . Trace IR represents a component of the output from the optical sensor  20  which is indicative of the amount of infrared light emitted by the first LED  34  that is received by the photodiode  38 . Trace R represents a component of the output from the optical sensor  20  which is indicative of the amount of red light emitted by the second LED  36  which is received by the photodiode  38 . Each trace IR, R comprises a series of regular pulses which represent pulsation of arterial blood through the underlying tissue. 
     Each of the traces IR, R has a pulsatile component (which represents the amount of light absorbed by the pulsatile arterial blood) and a non-pulsatile component (which represents the amount of light skin tissue, and non-pulsatile arterial and venous blood). The peaks  500   0,1,2 . . . n  of the trace IR are spaced apart with a frequency dependent upon the physiology of the patient. The peaks  520   0,1,2 . . . n  of the R trace are also spaced apart with a same frequency as the peaks of the IR trace which is dependent on the physiology of the patient. The troughs  510   0,1,2 . . . n  of the IR trace and the troughs  530   0,1,2 . . . n  of the R trace are associated with the respective non-pulsatile components (in the absence of a pulsatile component) and therefore provide an indication of the amount of light absorbed at each frequency by the skin tissue and non-pulsatile blood. 
     Trace M shows a component of the output signal of the motion sensor  18  which represents acceleration of the motion sensor  18 . In this instance, the trace M represents acceleration along a single axis. The components of the outputs from the motion sensor  18  and the optical sensor  20  represented by each trace IR, R, M may are isolated from the respective output signals by the signal processing electronics  22  before being processed by the first processor  24 . In this sense the trace M amplitude, time variation and/or the peak to peak value can be used to determine how still a patient is. 
       FIG. 4  is a flow chart illustrating a process of obtaining a measurement and subsequently processing the measurement. In order to take a measurement in accordance with step S 1010 , the first processor  24  starts recording the outputs of the sensors  18 ,  20  simultaneously at time t 1 . Referring to  FIG. 5 , at time t 2 , after a predetermined period of time T S  has elapsed, the first processor  24  stores the monitored outputs of the sensors  18 ,  20  obtained over the predetermined period of time T S  temporarily in the memory element  26 . The predetermined period of time T S  defines a sampling period over which data is collected and should be set to include at least one pulse of pulsatile arterial blood within the sample data, and may be set to include a plurality of pulses of arterial blood, for example at least 2, 5 or 10 pulses. An adult typically has a heart rate of between 40 and 100 beats per minute. The predetermined period of time T S  may therefore be at least one second, such as at least two, five or ten seconds. It will be appreciated that the longer the predetermined period of time T S , the greater the chance of movement of the patient and hence wound dressing  4 . Consequently, the predetermined period of time T S  should be not greater than sixty seconds, for example not greater than thirty seconds or not greater than fifteen seconds. In the embodiment shown, the predetermined period of time T S  is ten seconds which is sufficiently to obtain a good quality data sample without a high risk of the patient moving to an extent that the quality and accuracy of the data is adversely affected. In order to improve accuracy multiple data samples could be combined. The data sampling rate for each trace IR, R, M is between several Hz and several tens of kHz. 
     As illustrated by step S 1020 , the trace M obtained over the predetermined period of time T S  is analysed by the first processor  24  to determine whether the motion sensor  18 , and hence the wound dressing  4 , has experienced motion during the sampling period which could be expected to result in an erroneous or misrepresentative reading by the optical sensor  20 . Any disturbance, noise or change in the output of the optical sensor leading to an erroneous or inaccurate reading due to the motion that is detected is known as a “motion artefact”. A motion artefact may be caused by a patient standing up, walking or carrying out a task which involves moving a part of the body to which the wound dressing  4  is secured. Different criteria can be used to determine that the motion sensor  18  has moved and hence that a motion artefact is present in the sample. In the embodiment shown, it is determined that a motion artefact is present if the amplitude of the motion trace M exceeds a predetermined threshold amplitude A MAX  during the sampling period. 
     In the example shown in  FIG. 5 , the motion trace M does not contain a motion artefact. The small fluctuations shown in the trace M is associated with signal noise which may be caused sensor noise or electronics noise or by very small movements that would not be expected to adversely affect measurements. The amplitude of the trace M remains below the threshold amplitude A MAX  between t 1  and t 2 . The trace M indicates that no significant movement of the arm  6  has occurred during the sampling period. The components of the output of the optical sensor  20  are therefore processed by the processor  24  to determine the oxygen saturation value (SpO 2 ) of the pulsatile arterial blood, as illustrated by step S 1030  shown in  FIG. 4 . 
     In order to determine the amount of pulsatile blood in the target area, the pulsatile component for each of the infrared IR and red R traces is normalised with respect to the non-pulsatile component. Typically, this can be done by determining a ratio of the pulsatile component to the non-pulsatile component of the signal. Once the first and second components have been normalised, the ratio of the normalised red R component to the normalised infrared IR component is calculated. The ratio can then be used to determine an oxygen saturation value (SpO 2 ) for the pulsatile arterial blood within the tissue at the target area. For example, the ratio can be converted into an oxygen saturation value (SpO 2 ) value in accordance with the Beer-Lambert law, as is known in the art of spectroscopy. The oxygen saturation value is then stored in the memory  26 , as illustrated by step S 1040 . The measurement step S 1010  may then be repeated immediately or after a set period of time has elapsed, as illustrated by step S 1050 . 
       FIG. 6  shows traces IR, R, M comprising a sample period in which a motion artefact is present. A portion  600  of the motion trace M within the predetermined time period T S  comprises multiple oscillations in which the amplitude of the trace M exceeds the predetermined threshold amplitude A MAX . The first processor  24  therefore determines that a motion artefact is present. In this case, the motion artefact corresponds with the patient moving his/her arm relatively rapidly for example when lifting an object. However, a motion artefact may also be identified when a patient moves his/her entire body such as walking up a stairway even when the arm does not move significantly with respect to the head, torso, etc. of the patient. In either case, the quality of the data obtained is likely to be adversely affected due to movement of the wound dressing and hence optical sensor  20  with respect to the skin tissue of the patient or because of an increase in blood flow caused by activity of the patient. The measurement is therefore discarded and the first processor  24  deletes the stored outputs from the memory element  26 , as illustrated by step S 1060 . The process of taking a measurement (steps S 1010  and S 1020 ) may then be repeated, either immediately or after a further predetermined time, until a motion trace M is obtained in which no motion artefact is present. 
     Measurements may be taken periodically, for example on a minute-by-minute, hourly, daily or weekly basis and a record of the oxygen saturation (SpO 2 ) values stored on the memory element  26 . The stored data can then be transmitted by the transmitter  28  to the receiver  32  of the monitoring device  8  for subsequent processing by the second processor  30  and display on the display  16 . For example, the repeated measurements may be used to determine trends that can be subsequently used to determine how well a wound is healing. In the embodiment shown, the monitoring device  8  is a portable handheld device such as a smartphone, tablet or bespoke device having an integrated display in the form of a screen. Alternatively, the stored data may be transmitted via a mobile or wireless network for subsequent processing. Transmission may utilise wireless protocols such as Bluetooth™ Wi-Fi™, Zigbee™, near-field communication or the like may be used. The data or data trends can be visualised, compared to previous readings or incorporated into the records of the patient and an assessment of the wound healing can be carried out. 
     The wound dressing  4  is compact and lightweight and so will not hinder the patient greatly. Furthermore, the wound dressing  4  need not be removed from the patient in order to inspect the wound and assess healing progress. The wound dressing  4  may be applied to other regions of a patient&#39;s body having a wound including another limb such as a leg or a torso or a head or a foot or a hand or other region. 
       FIG. 7  shows a further embodiment of an apparatus  102  comprising a wound dressing  104 , a sensor patch  106  and a monitoring device  108 . The wound dressing  104  comprises a central wound protecting portion  110  and a peripheral securing portion  112  or border. The sensor patch  106  comprises a sensor module  114  and a further securing portion  116 . The securing portion  112  and the sensor patch  106  have an adhesive on respective lower surfaces for securing them to the body of a patient. The sensor module  114  is in accordance with the sensor module  14  shown in  FIGS. 1 and 2 . The monitoring device  108  is a handheld device having an integrated display  118 . The sensor patch  106  can be secured to a patient&#39;s skin adjacent the wound dressing  104 . An advantage of the arrangement is that the wound dressing  104  can be changed periodically without having to remove the sensor patch  106 . In the embodiment show, the sensor module  114  is wireless and is configured to communicate with the monitoring device  108 . The sensor patch  106  can be connected to the wound dressing  104  in a non-removable way or via a releasable connection such as a perforated line. Alternatively, the wound dressing  104  and the sensor patch  106  may be separate. For example, the sensor patch may comprise part of a band or cuff that can be wrapped around a portion of a patient&#39;s body, such as a limb. 
       FIG. 8  shows a further embodiment of an apparatus  202  comprising a wound dressing  204  and a monitoring device  206 . The wound dressing  204  comprises a wound protecting portion  208 , a peripheral securing portion  210  and a sensor module  214  which is formed integrally with the securing portion  210 . The securing portion  210  has an adhesive on its lower surface for securing the wound dressing  204  to the arm of a patient. The monitoring device  206  comprises an integrated display  212 . A lead  216  is connected to the sensor module  214  at one end and to the monitoring device  206  at the other end. The lead  216  provides a means for communication between the sensor module  214  and the monitoring device  206  and may also provide a means of supplying power to the sensor module  214 . In such an embodiment, data may be transmitted directly to the monitoring device and so a processor need not be provided in the wound dressing itself. 
       FIG. 9  shows a wound dressing  302  comprising a wound protecting portion  304 , a peripheral securing portion  306  and four sensor modules  308 A,  308 B,  308 C,  308 D. Each sensor module  308 A,  308 B,  308 C,  308 D is disposed at a respective lobe  310 A,  310 B,  310 C,  310 D of the peripheral portion  306 . Each sensor module  308 A,  308 B,  308 C,  308 D comprises an optical sensor in accordance with the optical sensor shown in  FIG. 3  for sensing an oxygen saturation (SpO 2 ) value at a target region of skin beneath the respective sensor. At least one of the sensor modules  308 A,  308 B,  308 C,  308 D comprises a motion sensor which can be used to determine whether measurements taken from all of the sensors should be stored or rejected. Alternatively, the motion sensor may be separate from the other sensor modules  308 A,  308 B,  308 C,  308 D. 
     In an alternative embodiment, an output from the optical sensor  20  may be analysed to determine whether a patient is active, or has been active, to an extent that an unreliable measurement would be expected. An analysis, such as a Fourier analysis, can be performed on a trace obtained over the predetermined period T S  for at least one of the components of the output from the optical sensor. If signals at certain frequencies not associated with the frequencies of a typical pulse pressure waveform of the patient (for example at lower frequencies) are detected at significant amplitudes which are above expected levels of noise, it is determined that a motion artefact is present. The alternative embodiment therefore does not require a separate motion sensor. 
     A learning process may be performed prior to operation of the apparatus  2  to establish suitable thresholds including a suitable predetermined period T S  and a suitable threshold amplitude A MAX  or whatever predetermined condition is used to determine the presence of a motion artefact. The thresholds can then be stored in the memory element  26  for subsequent recall by the first processor  24 . The learning process may establish typical characteristics for a patient wearing the wound dressing, such as heart rate or amount of motion, for when the patient is motionless and/or resting. A flow chart depicting a method of using a wound dressing that incorporates a learning process is shown in  FIG. 10 . Firstly, at step S 2010  the wound dressing of  FIG. 1  is unpacked and then, at step S 2020 , the wound dressing is applied to a patient over a wound. At step S 2030 , an optional learning process is performed to set or adjust operational parameters such as the sample period and a threshold amplitude A MAX . At step S 2040  a method of determining an amount of skin perfusion in tissue surrounding a wound area, as described above, is then performed using the wound dressing. At step S 2050 , the wound dressing is removed, replaced if necessary by a fresh wound dressing, and discarded. 
     In another embodiment of the disclosure, in order to conserve energy, rather than acquiring data continuously, the sensor module is switched off for a period of time after successfully measuring a pulse rate or SpO 2  and is then woken up again at a future time by a user or a clinician or in accordance with an automated sequence to take another measurement. 
     In the embodiment descried above, a single axis accelerometer was used. However, in other embodiments, at least one multiple axis accelerometer could be used. It will be appreciated that at least one of the following techniques could be used to identify the presence of a motion artefact: 
     detection of whether the acceleration (either linear or angular) in one or more axes or a total acceleration, for example a combined acceleration in two or more axes, has exceeded a pre-determined, derived or adaptive threshold or else has deviated from a baseline by more than a multiple of a baseline noise amplitude;
 
detection of whether the velocity (either linear or angular) in one or more axes or a total velocity, for example a combined velocity in two or more axes, has exceeded a pre-determined, derived or adaptive threshold or else has deviated from a baseline by more than a multiple of a baseline noise amplitude;
 
detection of whether a displacement (either linear or angular) in one or more axes or a total displacement, for example a combined displacement in two or more axes, over the duration of the sample period or a part thereof has exceeded a pre-determined, derived or adaptive threshold or else has deviated from a baseline by more than a multiple of a baseline noise amplitude;
 
comparison of the standard deviation of the acceleration over the sample period or a part thereof against the typical noise level expected; this can be done for one or more axes of the coordinate system or for the total acceleration;
 
comparison of the standard deviation of the velocity over the sample period or a part thereof against the typical noise level expected; this can be done for one or more axes of the coordinate system or for the total acceleration and/or
 
comparison of the amplitude at one or more frequencies or across a band around one or more frequencies against an expected level.
 
     It will be appreciated that other types of motion sensors could be used such as single axis and/or multiple axes gyroscopes, inclinometers or the like. In other embodiments, a non-optical sensor may be used to sense a healing related parameter associated with wound healing. 
     The sensor modules described above may comprise one or more batteries as a power source or energy store. Additionally or alternatively, a range of other power sources may be used to provide power to the sensor module. Such alternatives may include, but are not exclusive to, capacitors, fuel cells or energy generators, which generate energy, for example, from the movement of the wearer, e.g. based on some piezo elements or the like, from temperature differences and heat generated by the user or the environment, using, for example, thermopiles, or from light, using, for example, photovoltaic cells, or other energy generating systems, for example clockwork type mechanisms which can be charged by the user. Any battery used may be non-rechargeable or rechargeable. Recharging can occur in a number of ways known to those trained in the art, including wired or contactless charging techniques. 
     It will be appreciated that throughout this specification reference is made to a wound. It is to be understood that the term wound is to be broadly construed and encompasses open and closed wounds in which skin is torn, cut or punctured or where trauma causes a contusion, or any other superficial or other conditions or imperfections on the skin of a patient or otherwise that benefit from reduced pressure treatment. A wound is thus broadly defined as any damaged region of tissue where fluid may or may not be produced. Examples of such wounds include, but are not limited to, abdominal wounds or other large or incisional wounds, either as a result of surgery, trauma, sterniotomies, fasciotomies, or other conditions, dehisced wounds, acute wounds, chronic wounds, subacute and dehisced wounds, traumatic wounds, flaps and skin grafts, lacerations, abrasions, contusions, burns, diabetic ulcers, pressure ulcers, stoma, surgical wounds, trauma and venous ulcers or the like. 
     It will be understood that embodiments of the present disclosure are generally applicable for use in topical negative pressure (“TNP”) therapy systems, such as be incorporated into a TNP dressing. Briefly, negative pressure wound therapy assists in the closure and healing of many forms of “hard to heal” wounds by reducing tissue oedema; encouraging blood flow and granular tissue formation; removing excess exudate and may reduce bacterial load (and thus infection risk). In addition, the therapy allows for less disturbance of a wound leading to more rapid healing. TNP therapy systems may also assist on the healing of surgically closed wounds by removing fluid and by helping to stabilize the tissue in the apposed position of closure. A further beneficial use of TNP therapy can be found in grafts and flaps where removal of excess fluid is important and close proximity of the graft to tissue is required in order to ensure tissue viability. As is used herein, reduced or negative pressure levels, such as −X mmHg, represent pressure levels relative to normal ambient atmospheric pressure, which can correspond to 760 mmHg (or 1 atm, 29.93 inHg, 101.325 kPa, 14.696 psi, etc.). Accordingly, a negative pressure value of −X mmHg reflects absolute pressure that is X mmHg below 760 mmHg or, in other words, an absolute pressure of (760−X) mmHg. In addition, negative pressure that is “less” or “smaller” than X mmHg corresponds to pressure that is closer to atmospheric pressure (e.g., −40 mmHg is less than −60 mmHg). Negative pressure that is “more” or “greater” than −X mmHg corresponds to pressure that is further from atmospheric pressure (e.g., −80 mmHg is more than −60 mmHg). In some embodiments, local ambient atmospheric pressure is used as a reference point, and such local atmospheric pressure may not necessarily be, for example, 760 mmHg. The negative pressure range for some embodiments of the present disclosure can be approximately −80 mmHg, or between about −20 mmHg and −200 mmHg. Note that these pressures are relative to normal ambient atmospheric pressure, which can be 760 mmHg. Thus, −200 mmHg would be about 560 mmHg in practical terms. In some embodiments, the pressure range can be between about −40 mmHg and −150 mmHg. Alternatively a pressure range of up to −75 mmHg, up to −80 mmHg or over −80 mmHg can be used. Also in other embodiments a pressure range of below −75 mmHg can be used. Alternatively, a pressure range of over approximately −100 mmHg, or even −150 mmHg, can be supplied by the negative pressure apparatus. 
     In the drawings like reference numerals refer to like parts. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to” and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the disclosure are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of the features and/or steps are mutually exclusive. The disclosure is not restricted to any details of any foregoing embodiments. The disclosure extends to any novel one, or novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.