The present invention relates to a method of obtaining a radial pressure for a compression garment material, particularly but not necessarily exclusively for the creation of user-specific compression garments. The invention also relates to a system for characterising a compression garment material in accordance with the method, and to a method for determining an optimised set of material characteristics for a compression garment material.

Compression garments are used to apply pressure to a patient, typically to a limb such as the arm or leg. This pressure encourages circulation of blood, where low pressure could add strain to the heart, or cause swelling in the extremities.

The amount of pressure, known as interface pressure, that a compression garment applies is dependent on the radius of the material at the point of contact with the patient. Low pressure will be applied for a flatter portion of skin, for example, the side of a calf, whereas a higher pressure will be applied for a tighter radius of curvature, for example, around the shin of the patient. The tension in the compression garment material will also affect the interface pressure, which may be significantly altered by the knit characteristics of the fabric material.

As each patient has a unique limb shape, and therefore a unique set of radii of curvature associated with the limb, identical compression garments will provide different interface pressures to different patients.

An optimum compression garment will vary in both size, and therefore strain percentage, and material to deliver the same overall pressure gradient. The optimum garment is therefore bespoke to a patient, creating difficulties for compression garment manufacturers. At present, to characterise a compression garment material, a sleeve of compression garment material is stretched over a number of cylinders of known radius, with an interface pressure being measured directly using a pressure tester, such as a Kikuhime pressure monitor. The sleeves are then provided at set strains, typically 25%, 50%, 75%, 100% and 125%, and the pressure measured using the pressure tester. This results in a plottable curve between radius of curvature and pressure for the specific strains, which can then be fit using a second order polynomial. The strain curves are then linked with a three-dimensional mesh, and data can be extracted at predetermined intervals of radius and strain to provide a data matrix.

There are several difficulties with characterising a compression garment material in this manner.

Firstly, the requisite cylinders for the measurements are difficult to obtain, particularly at larger diameters, since the accuracy of the radius of curvature of each is critical to obtaining useful results. Furthermore, the provision of individual sleeves of specific strain values is very labour intensive; each sleeve effectively only provides a single usable data point.

The measurement process is also performed manually, via reading of the pressure monitor, and furthermore, the pressure monitor is only accurate to +/- 8mmHg, whilst the data readings taken are attempting to record values with a much greater precision. The thickness of the pressure monitor probe also has an interference effect on the compression garment material, warping the measurements compared with a real-world practical application of the compression garment.

The process also results in a very limited data set, from which a large set of information is inferred. In the state of the art, the only data usable is that based on the radius of curvature of cylinders used. As such, database values are limited by the amount of data acquisition undertaken, which may pose difficulties for accurate application of the information to the creation of a compression garment.

It is therefore an object of the present invention to provide an improved method of characterising a compression garment material which eliminates the complex and inaccurate data acquisition process known in the art.

According to a first aspect of the invention, there is provided an enhanced method of obtaining a radial pressure for a compression garment material, the method comprising the steps of: a] obtaining a data set relating to strain and load for the compression garment material; b] determining a characteristic coefficient for the material based on the data set; and c] calculating a radial pressure based on a desired radius of curvature for the compression garment material utilising the characteristic coefficient. The advantage of the present method is that compression garment materials can be characterised using a specific characteristic equation, in which there is a characteristic coefficient which parameterises the relevant compression garment material. This can be used to then calculate any interface pressure for any radius of curvature for the compression garment material, which is otherwise not possible in the current state of the art. This is achieved by the measurement of load on the testing apparatus, rather than interface pressure directly, which results in many of the inaccuracies in the art. The determination of load then allows for a completely different characterisation of the compression garment material than previously possible when using direct pressure monitors. Whilst interface pressure is not directly measured, the present invention demonstrates that load on the actuation assembly can lead to accurate measurements, without interference from the method of measurement, as the geometry of the fabric is unchanged by measurement.

In particular, the equipment used involves existing manufactured or readily available formers, which do not need to be complete cylinders, but can be partial cylinders sufficient to provide the radius of curvature. Furthermore, instead of bespoke sleeves of a material being formed with different strains, a single sleeve of compression garment material can be utilised with all strains and radii of curvature. This vastly improves the speed with which the testing can be completed, and is fully repeatable with different sleeves of different materials.

Optionally, the characteristic coefficient may be or may comprises the Young’s modulus for the compression garment material. Preferably, the characteristic coefficient may be or be directly proportional to the Young’s modulus for the compression garment material for a linear elastic region thereof.

The Young’s modulus is a characteristic value within the elastic limit of the material which enables a mapping of the load to the interface pressure in a largely linear manner.

During step a], the compression garment material may be provided as an endless loop or sleeve.

A sleeve of compression garment material can be readily placed under strain, since the sleeve can be easily hooped over moveable elements which can stretch the compression garment material. This reduces the likelihood of the sleeve being ejected from the apparatus in-use. In one preferred embodiment during step a], strain data may be obtained via linear stretching of the endless loop.

Since the present invention has determined that the compression garment materials obey Hooke’s law in the region of likely use on a patient, a linear stretch up to the elastic limit should yield highly accurate measurements of load, which can then be processed to calculate interface pressure values.

Preferably, step a] may comprise sub steps of: (i) engaging the endless loop with a first pair of formers having a known radius of curvature; (ii) relatively actuating the first pair of formers with respect to one another to induce a strain in the loop; (iii) measuring a load; (iv) calculating an interface pressure for the loop based on the measured load. It may further be possible to include a further step (v), of repeating steps (i) to (iv) for at least one further pair of formers having a different radius of curvature.

This actuation process using formers ensures that well-characterised local radii of curvature can be utilised, ensuring that accurate interpolations can be made for interface pressures at radii which have not been directly measured.

In an alternative embodiment, during step a], the compression garment material may be provided as a strand of yarn.

It is feasible that the present process could be easily adapted to utilise and test other fabric configurations, and thus allow for a priori analysis of materials which could theoretically be used in the creation of bespoke compression garments.

The radial pressure may be calculated based on the characteristic equation: where P is interface pressure, E is the Young’s modulus of the compression garment material, t is the thickness of the compression garment material, e is the strain, and r is the local radius of curvature.

Thickness is here defined as the total thickness of the sleeve of compression garment material when flattened. In other words, this is the thickness of two layers of the compression garment material, hence the “2” in the denominator. If the thickness of a single layer of compression garment material were used, the “2” could be omitted from the above equation.

The characteristic equation is applicable to all elastic compression garment materials, and since load can now be used to calculate interface pressure without interfering with the material properties of the compression garment material, an accurate method of calculating interface pressure for desired radii of curvature becomes achievable.

As a preferred alternative, the radial pressure may be calculated based on the characteristic equation: where P is interface pressure, a is a first fabric coefficient, b is a second fabric coefficient, r is the local radius of curvature, and N is the number of needles used in knitting the course.

This characteristic equation relies on manufacturing parameters for classification, rather than strain, which leads to a more straightforward analysis process. Optionally, the desired radius of curvature may be a patient-specific limb curvature.

The desired implementation of the present invention is to allow for compression garments to be created for specific patients. Ill-fitting compression garments can yield negligible or even negative medical results for a patient, and therefore this will improve patient comfort and mobility. The characteristic coefficient may be a composite coefficient based on a plurality of scalar modifiers associated with a set of material characteristics.

The components which determine the characteristic coefficient may be factorised, based on how the material characteristics of the fabric affect the overall properties.

Optionally, the set of material characteristics may include at least one of: yarn; knit style; thickness; colour; loop size; and needle width. Many material characteristics can affect the Young’s modulus of a fabric. Quantifying these will allow for improved prediction of what aspects of the material will yield specific qualities.

Additionally, or alternatively, the characteristic coefficient may comprise a set of characteristic coefficients.

In purely theoretical terms, the characteristic coefficient should converge on the Young’s modulus, or, for the modified characteristic equation, be directly proportional thereto. However, for the purposes of experimental investigation, the stress-strain relationship of the compression garment material is found to be non-linear at low strain, and therefore the characteristic coefficient may need to be defined by a set of characteristic coefficients to derive the true composite value.

The method may further comprise a step d] of creating a compression garment from the compression garment material.

The ultimate aim is to produce a compression garment from the compression garment material which is bespoke to the needs of a patient, and specifically relating to their pressure requirements at a limb.

According to a second aspect of the invention, there is provided a compression-garment- material-characterising system for characterising a compression garment material, the system comprising: at least one pair of formers having a defined radius of curvature; an actuator assembly, the at least one pair of formers being engagable with the actuator assembly to be actuatable relative to one another; and at least one load cell configured to measure a load imparted at or by the at least one pair of formers.

In order to implement the aforementioned method, there is a requirement to adapt existing systems for load measurement, in order that the strain can be accurately determined. As such, a load cell must be integrated into the apparatus in some manner to measure the load applied by the fabric sample.

Optionally, the actuator assembly may comprise a stepper motor.

A stepper motor provides an effective means of accurately determining the extension distance, and thus strain, of the compression garment material as the actuator assembly is activated. More preferably, the actuator assembly may comprise a pair of stepper motors spaced apart from one another.

A paired set of stepper motors can ensure that there is a consistent extension applied at either end of the compression garment material, so that a correct load is consistently applied.

Optionally, the actuator assembly may be configured to provide a uniform linear actuation.

It is preferred that a uniform strain be applied, since this will lead to the most accurate determination of the characteristic coefficient.

Alternatively, the actuator assembly may be configured to provide a non-uniform actuation.

Non uniform actuation may assist with replicating the dimensions of a user’s limb, which may then allow for more complex assessments of the behaviour of compression garment material.

A plurality of said pairs of formers may be provided, the plurality of said pairs of formers being releasably engagable with the actuator assembly.

Correlating data can be readily achieved for the characteristic coefficient. Since interface pressure is inversely proportional to the radius of curvature, the characteristic coefficient should be identical in spite of the radius of curvature applied. Using different formers can then validate a characteristic coefficient determined using only a single radius of curvature.

A said load cell may be provided for each pair of formers, each load cell being contoured to the radius of curvature of the pair of formers.

The load cell may need to be curved in order to correctly detect the load at the arcuate former, which may be a cylinder, and thus specific shapes of load cell may be necessary.

According to a third aspect of the invention, there is provided a method for determining an optimised set of material characteristics for a compression garment material, the method comprising the steps of: a] determining a desired interface pressure for a compression garment having a known radius of curvature; b] determining a desired characteristic coefficient for a material for the compression garment which would produce the desired interface pressure; and c] determining an optimised set of material characteristics for the compression garment material based on the desired characteristic coefficient.

In order to create a bespoke compression garment for a patient, it is desirable to know what characteristics of different compression garment materials will produce specific desired interface pressures. As such, the present system allows for the characterisation of different compression garments according to a single characteristic coefficient, which greatly simplifies the analysis process for determining an optimum compression garment material to use.

Preferably, the optimised set of material characteristics may include strain and at least one of: yarn; knit style; thickness; colour; and loop size.

The method may further comprise a step d] of determining a most suitable existing compression garment material based on the optimised set of material characteristics.

By characterising a wide variety of different compression garment materials, it becomes possible to more easily select a specific material for a compression garment without needing to manufacture a large number of test garments for the patient to try.

The method may alternatively further comprise a step d] of designing a new compression garment material having or substantially having the optimised set of material characteristics.

A powerful application of the present method is that the characteristic coefficient allows for mapping of a desired radius of curvature to interface pressure, and then automatically determining the characteristic coefficient needed to provide the interface pressure. If a material does not exist having the said characteristic coefficient, then it may be possible to analyse the various material characteristics so as to produce a new compression garment material which will be ideally suited to the patient.

Optionally, each of the optimised set of material characteristics may have a scalar modifier associated therewith which is indicative of a linear relationship with the interface pressure.

It is anticipated that changes in the construction of the compression garment material will lead to linear, either discrete or continuous, scaling of the characteristic coefficient, and the skilled use can then determine how to change existing compression garment materials according to a patient’s needs. More complex scaling is possible, however, with polynomial relationships between stress and strain for non-linearly elastic materials.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a representative stress-strain plot for a material;

Figure 2 shows a graph plotting reciprocal interface pressure against radius of curvature for a compression garment material at different strains;

Figure 3 shows a side view of a first representation of a system for characterising a compression garment material in accordance with the second aspect of the invention;

Figure 4 shows a plurality of formers which may be utilised with the system of Figure 3; and

Figure 5 shows one embodiment of a load cell suitable for use with the system of Figure 3.

The present invention relates to a method of calculating a radial pressure for a compression garment material. The theoretical underpinning of the invention is therefore described, in advance of the detailed description of the implementation of the method.

It is well known that the interface pressure expressed by a fabric garment on a limb is related to the local radius of curvature of the limb and the tension of the fabric. This can be expressed as a modified Laplace equation:

T

P = - r

Where P = interface pressure, T is tension in fabric per unit length, and r is local radius of curvature.

If a desired interface pressure is to be delivered to a limb surface of a known radius of curvature, the tension in the fabric must be controlled, in accordance with the above equation. To generate tension in an elasticated fabric, the fabric can be subjected to strain. As such, a compression garment for a limb can be created as a sleeve, that is, an endless loop of the material rather than a shirt sleeve for instance, which has a circumference which is less than that of the limb, such that the compression garment must be stretched to fit over said limb. This induces strain, and therefore generates an interface pressure. For an elastic material, the force needed to extend a body can be expressed using Hooke’s law.

F = kx

Where F is the force applied, k is stiffness of the material, and x is the extension distance. Hooke’s law states that the force required to extend an elastic material scales linearly with the distance.

Young’s modulus is a mechanical property that communicates the stiffness of a material. It defines the relationship between stress (force per unit area) and strain (proportional deformation). This is commonly expressed by the following equation: s

E = - e Where E is the Young’s modulus of the material, o is the stress, and e is the strain.

Referring to Figure 1, there is shown an indicative relationship between stress and strain for a compression garment material. Up to the elastic limit of the material, Young’s modulus is typically a constant, and is therefore a known mechanical property of the material. Beyond the elastic limit, Young’s modulus varies as the material begins to rupture; however, for the production of a compression garment, extension beyond the elastic limit is exceedingly unlikely, and for the purposes of characterising the material for the compression garment, can be treated as a constant. Atypical compression garment materials, which do not exhibit linear elasticity, could also be interrogated, however. It is therefore possible to characterise the mechanical properties of the compression garment material which is behaving in accordance with Hooke’s law, by deforming the compression garment material by a known amount, to calculate strain, and investigate what tension is induced in the fabric, to thereby calculate stress. By determining this relationship, it becomes possible to determine an interface pressure of a compression garment material of a given material composition for a given strain over a desired radius of curvature, that is, the curvature of a patient’s limb.

This relationship can be provided using a combined characteristic equation:

Ete

P = 2 r Where P is the interface pressure, E is the Young’s modulus, t is the thickness of the fabric, e is the strain, and r is the local radius of curvature.

Thickness is here defined as the total thickness of the sleeve of compression garment material when flattened. In other words, this is the thickness of two layers of the compression garment material, hence the “2” in the denominator. If the thickness of a single layer of compression garment material were used, the “2” could be omitted from the above equation.

An alternative and improved characteristic equation is also feasible.

1 1 b

^{R =} -L where P is interface pressure, a is a first fabric coefficient, b is a second fabric coefficient, r is the local radius of curvature, and N is the number of needles used in knitting the course.

This characteristic equation is a refactoring of the following equation: r

— = aPr + b N

Based on the Laplace equation above, Pr is proportional to tension T in the fabric when a loop of material knitted using N knitting needles is extended by the machine.

It has been determined that, since the garment to be knitted must be physically manufactured, the interface pressure can be categorised in terms of the physical manufacturing parameters. It is only possible to manufacture garments with an integer number of needles at each course, that is, the horizontal yarn of the knit. Given that the pressure exerted on a known radius of curvature is being investigated, the ratio r/N can be used as a surrogate for strain. A plot of r/N against tension thus yields the pair of characteristic coefficients a and b, where a is the gradient and b is the intercept of the plot b can thus be envisioned as the zero-tension radius of curvature for a garment material for a given needle number construction, whilst a is the corollary of the Young’s modulus of the fabric, and will be related to the Young’s modulus by a scalar multiplier.

For the purposes of determining the suitability of a material for use in a compression garment, within the elastic limit of the material, and therefore for a constant value of the Young’s modulus, it is clear that the interface pressure is inversely proportional to the local radius of curvature. The Young’s modulus, or derived characteristic coefficient therefrom, such as coefficient a, can therefore be calculated by measuring stress and strain on the material, which can be performed by measuring the relationship between extension of the fabric material versus the load applied during the extension, which holds within the elastic limit according to Hooke’s law.

Figure 2 shows experimental results demonstrating that this is the case. For a single compression garment material with a total of six different strains applied (20%, 40%, 60%, 80%, 100%, and 120%), a linear relationship between reciprocal pressure and radius of curvature was found.

The challenge therefore lies in the characterisation of compression garment materials to form a database of compression garment materials, so that an optimum compression garment for a desired radius of curvature for a specific patient can be created. This is only achievable since the characteristic equation for the compression garment material has been determined.

Critically, the method requires only a single test to populate the coefficients of the characteristic equation. An apparatus for carrying out the method therefore needs only to be able to measure strain and load, indicative of interface pressure, preferably for a plurality of different radii of curvature for improved accuracy if reliant on the first characteristic equation, in order to be able to derive the characteristic equation above and therefore the Young’s modulus or a modified version thereof. This allows the interface pressure of the material to be calculated for any given radius of curvature, that is, a patient-specific radius of curvature, and a best compression garment material selected.

One exemplary arrangement of an apparatus suited for implementing this method is illustrated in Figure 3, referenced globally at 10. The apparatus 10 comprises a framework 12 to which is mounted an actuation assembly 14. The actuator assembly 14 here comprises two spaced apart elements, formed preferably as cylinders 16a, 16b which are engagable with a sleeve 18 of compression garment material.

The actuator assembly 14 includes at least one actuator 20, and more preferably at least two spaced apart actuators 20 which allow for the cylinders 16a, 16b to be actuated relative to one another, preferably as a fixed cylinder 16a and a movable cylinder 16b. It is preferred that the actuators 20 drive a linear motion, therefore maintaining a uniform separation between the cylinders 16a, 16b, but the provision of paired actuators 20 allows for non-uniform actuations to be performed if required.

Each actuator 20 may be provided as a stepper motor, shown connected to a threaded drive shaft 22, with a mount 24 of a movable cylinder 16b being connected to the threaded drive shaft 22 via a threaded receiver 26. This allows rotational motion of the threaded drive shaft 22 to impart a linear drive to the moveable cylinder 16b. The stepper motors are computer controlled.

An alternative to a stepper motor arrangement might be a rack and pinion system, a belt driven system, or a chain driven system, which is capable of increasing a displacement between the fixed and moveable cylinders 16a, 16b. Linear motors or hydraulic drive systems could also be considered.

The cylinders 16a, 16b may be releasably engagable with the actuation assembly 14 to permit pairs of formers to be attached to the actuator assembly 14. These pairs of formers could just be cylinders 16a, 16b of differing diameters, or alternatively could be a set of specific arced formers 28 such as those shown in detail in Figure 4.

The pairs of formers 28a, 28b, 28c, 28d, 28e, 28f allow for specific radii of curvature to be applied to opposite ends of the sleeve 18 so that, not only is the local radius of curvature known, but also that the radii can be easily altered so that different measurements can be taken. It is noted that this arrangement is not required when using the modified characteristic equation referenced above.

The use of the pairs of formers 28a, 28b, 28c, 28d, 28e, 28f allows for the radii of curvature to be set to a specific value, wherein each pair of formers has a different radius of curvature, whilst the extension distance of the actuator assembly 14 may give an indication of the strain based on Hooke’s law. The apparatus 10 can then utilise a load cell 30 to measure force on the actuator assembly 14, such as that shown in Figure 5, to determine an interface pressure which is being applied to the fixed and moveable cylinders 16a, 16b.

A load cell 30 can therefore be used to measure a radial load imparted onto a cylinder or cylinder section, where the compression garment material is under strain so as to in turn determine the interface pressure. The strain on the compression garment material is determined as a function of the extension distance and is readily determined from the actuator assembly 14.

The measurements can be taken so that they start from a distance between the fixed and moveable cylinders 16a, 16b in which the sleeve 18 of compression garment material has zero strain, and that any increase in separation will result in the sleeve 18 being under strain. Gradual actuation of the moveable cylinder 16b thus creates the strain. One or more limit switches 32 may therefore be provided to establish an origin for the separation between the fixed and moveable cylinders 16a, 16b, which may correlate with the above-mentioned starting distance. This allows the measurements to be readily calibrated.

The measurements from the load cell 30 are taken via the same computer controller 34 as control the stepper motors, such that the strain and force data can be automatically correlated. The test data is stored digitally in a computer-readable format for easy recall against each unique fabric sample, such that a library of test data for various sample types and styles with different material characteristics, such as yarn, knit style, thickness, colour or loop size, can be generated and recalled by both manual and automated systems.

An indicative load cell 30 might include an upper pressure pad 36 which contacts with the cylinder, a bottom plate 38, and a sensor 40 therebetween to measure the load. Pre load springs 42 may be provided to resist the motion of the upper pressure pad 36. A floating pressure pad 44 may also be provided between the upper pressure pad 34 and the sensor 40.

The load cell 30 can be configured to measure the load by mounting inside a cylinder, preferably the fixed cylinder 16a, and may be contoured to the radius of the cylinder. Given that the geometry of the load cell 30 is well known, the interface pressure can be readily determined.

To obtain a radial pressure for the compression garment material, the apparatus 10 can be used to obtain a data set relating to strain, by determination of the extension of the fixed and moveable cylinders 16a, 16b, and load, by measurement at the load cell 30. A sleeve or similar endless loop 18 of compression garment material can be engaged with the fixed and moveable cylinders 16a, 16b and actuated apart from one another in a controlled manner.

From this data set, a stress-strain relationship can be determined, and from a linear portion of this relationship, which is indicative of the elastic capacity of the compression garment material, a characteristic coefficient can be extracted. The characteristic coefficient is or is derived from the Young’s modulus of the material, at the specific thickness measured. Data points taken at or near zero strain may be disregarded, since these may not fully follow the expected linear relationship. The characteristic coefficient may be a composite coefficient, for example, comprising a plurality of scalar modifiers each associated with an individual material characteristic of the compression garment material, such as yarn, knit style, thickness, colour, loop size, or needle width used in the manufacturing process.

Using the characteristic equation outlined above, a radial pressure for any radius of curvature for a compression garment formed from the compression garment material can be gleaned, and a bespoke compression garment formed.

Whilst the theoretical characteristic coefficient should converge on, or be directly proportional to, the Young’s modulus, in practice, the measured stress-strain relationship is not perfectly linear within the apparatus. There is found to be some curvature of the linear relationship at low strain. Indeed, for some elastic materials, the relationship in not linear at all, but is defined by a polynomial relationship. This yields a more complex relationship.

As such, for a practical input, the characteristic coefficient may not necessary be a single value, but may comprise a set of characteristic coefficients which are indicative of the behaviour of the linear portion of the stress-strain relationship. The set of characteristic coefficients may comprise the gradient of the linear portion of the stress-strain curve, as well as a y-axis intercept of the curve, which may be indicative of a zero-strain stress on the system, or in practical terms, a residual material pressure applied by the compression garment material under zero strain. In other words, it could be considered to be indicative of a force applied to the apparatus before the compression garment material has been stretched. Alternatively, the measurements could be used to derive the polynomial relationship between stress and strain.

Indeed, this may lead to a method for determining an optimised set of material characteristics for a compression garment material. Firstly, a desired interface pressure for a compression garment having a known radius of curvature can be determined, and then a desired characteristic coefficient for a material for the compression garment which would produce the desired interface pressure also determined. An optimised set of material characteristics for the compression garment material can then be determined based on the desired characteristic coefficient.

A most suitable existing compression garment material could be determined based on the optimised set of material characteristics, for example, selecting from a set of compression garment materials of different loop size construction. Alternatively, a new compression garment material could be designed having or substantially having the optimised set of material characteristics. For instance, a predicted best loop size construction could be predicted using this method to produce the optimum compression garment for a patient.

To obtain this granular information, it may be necessary to perform the measurements on a single strand of yarn, rather than a sleeve of compression garment material, so that some of the different base properties can be more accurately assessed. No cylinders would be required in the testing apparatus, but instead the yarn would be clamped in position using one or more clamping elements.

It is therefore possible to provide a new mechanism for characterising a compression garment material by testing the load created when the compression garment is under strain, in order to determine a characteristic coefficient of the compression garment material. A corresponding apparatus for testing the material can also be provided, and this leads to a potential mechanism for predictively anticipating which features of a compression garment may be best altered in order to generate a bespoke compression garment for a patient. The words ‘comprises/comprising’ and the words ‘having/including’ when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.