Tissue mimicking materials

Abstract

Aspects and embodiments relate to a tissue mimicking material, a clinical phantom including a tissue mimicking material and a method of manufacture of a tissue mimicking material. According to one aspect there is provided: a tissue mimicking material comprising: an alginate matrix; and an ablation-sensitive agent distributed through the alginate matrix. Aspects and embodiments recognise that use of an alginate matrix as a base for a tissue mimicking material provides relatively low cost material having various physical properties and characteristics which can be tuned or refined to simulate the characteristics of a mammalian tissue which the alginate matrix is intended to mimic. In relation to use of an alginate material in the context of thermal and / or pulsed electric field ablation simulation, it has been recognised that an alginate material may be appropriately temperature stable and not degrade upon exposure to high, or low, ablation temperatures, or upon exposure to ablation levels of pulsed electric field. Distribution of an appropriately selected ablation-sensitive agent through the alginate material may allow a change caused by application of an ablation technique to the tissue mimicking material to be observed.

Description

[0001] TISSUE MIMICKING MATERIALS

[0002] TECHNOLOGICAL FIELD

[0003] Various example embodiments relate to a tissue mimicking material, a clinical phantom including a tissue mimicking material and a method of manufacture of a tissue mimicking material.

[0004] BACKGROUND

[0005] Ablation is a clinical procedure used to treat various conditions. Ablation techniques are used to deliberately damage living cells in one or more tissue in order to achieve various outcomes. A range of ablation techniques are available, including: cryoablation, which damages the cells by freezing; radiofrequency ablation (RA), which damages cells via heating; and pulsed electric field (PEF) ablation, which is a nonthermal ablation technique which uses short, high-voltage electrical pulses to damage the cells by creating nanoscale pores in cell membranes.

[0006] The outcome of ablation techniques is determined by a number of factors including, in particular, a shape of a region of damage caused to cells in a tissue by the ablation technique. The region of damage may depend upon parameters such as: duration, power, temperature, pressure, area of contact and type of tissue being ablated.

[0007] It could be beneficial for a clinician to practice use of ablation techniques and assess the outcome of such ablation techniques before using the ablation technique on the tissue of a subject needing treatment.

[0008] BRIEF SUMMARY

[0009] The scope of protection sought for various example embodiments of the invention is set out in the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0010] According to various, but not necessarily all, example embodiments there is provided: a tissue mimicking material comprising: an alginate matrix; and an ablation-sensitive agent distributed through the alginate matrix. Accordingly, use of an alginate matrix as a base for a tissue mimicking material provides relatively low cost material having various physical properties and characteristics which can be tuned or refined to simulate the characteristics of a mammalian tissue which the alginate matrix is intended to mimic. In relation to use of an alginate material in the context of thermal and / or pulsed electric field ablation simulation, it has been recognised that an alginate material may be appropriately temperature stable and not degrade upon exposure to high, or low, ablation temperatures, or upon exposure to ablation levels of pulsed electric field. Distribution of an appropriately selected ablation-sensitive agent through the alginate material may allow a change caused by application of an ablation technique to the tissue mimicking material to be observed. Such a change may comprise a visible change which occurs upon application of an ablation technique to the ablation sensitive agent. The change may, for example, may comprise a visible change. The visible change may be visible with the human eye, and / or may comprise a change which can be determined from one or more image of the tissue mimicking material captured during, and / or after, application of an ablation technique.

[0011] In some arrangements, the ablation sensitive agent is evenly distributed throughout the alginate matrix. Accordingly, the entire tissue mimicking material may be sensitive to an ablation technique.

[0012] In some arrangements, the ablation sensitive agent is evenly distributed through a portion of the alginate matrix. Accordingly, a key region, section, or portion of the alginate matrix may be sensitive to application of an ablation technique.

[0013] In some arrangements, the ablation sensitive agent comprises a layer in the alginate matrix. Accordingly, one or more layer relating to, for example, a depth beneath a surface of the alginate matrix may be sensitive to application of an ablation technique.

[0014] In some arrangements, the alginate matrix comprises: an alginate hydrogel. In some arrangements, the alginate matrix comprises a moulding alginate. In some arrangements, the alginate matrix comprises a cross-linked calcium alginate. In some arrangements, the alginate matrix comprises an alginate hydrogel in the form of an calcium alginate powder gelling agent. In some arrangements the alginate comprises: a calcium alginate gel. In some arrangements the alginate comprises: a sodium alginate gel. In some arrangements the alginate comprises: a ferrous alginate gel. In some arrangements the alginate comprises: a positive metal ion alginate gel. Such arrangements support provision of an ionically cross-linked alginate which is a matrix of such gels. Ionically cross-linked alginate matrices are stable when exposed to ablation techniques, such as thermal or PEF ablation techniques.

[0015] In some arrangements, the alginate hydrogel is doped with salt. In some arrangements, the alginate hydrogel is doped with water. In some arrangements, the alginate hydrogel is doped with saline. Whilst it is envisaged that the dopants comprise water and sodium chloride, it will be appreciated that any soluble salt and ionic solvent could be a candidate for doping the alginate hydrogel. Accordingly, doping the alginate hydrogel allows for adjustment of one or more physical characteristics of the tissue mimicking material.

[0016] In some arrangements, at least one of the following properties of the alginate hydrogel: electrical resistivity; or electrical conductivity; or thermal conductivity; or hardness of the alginate hydrogel is controlled by selection of concentration of a dopant in the alginate hydrogel. It will be appreciated that appropriate ranges of electrical resistivity, thermal conductivity and / or hardness may be selected depending upon the mammalian tissue which is to be mimicked by the alginate-based material. By way of example, mammalian tissue hardness can vary. Generally soft tissue phantoms are made using materials having a shore hardness less than 100A; the more water content that is added to the alginate mix, the softer the resulting gel. Electrical resistivity of mammalian soft tissues are generally in the range 0.5-25.0 Qm. Some tissues, like nerves and cardiac muscles, are more conductive than others so their resistivity is lower. In the case of an alginate-based material, the higher the saline concentration, or salt dopant, the more conductive the resulting material. Similarly, the higher the saline concentration, the lower the electrical resistivity. In terms of thermal conductivity, generally soft tissues range between 0.2-0.8 W / (m K) and if creating an alginate hydrogel, the water content tends to result in a material similarly conductive to water (0.6 W / (m K)).

[0017] In some arrangements, the ablation-sensitive agent comprises: a thermochromic agent. Accordingly, the tissue mimicking material may be used to determine the effect of thermal ablation techniques on the material.

[0018] In some arrangements, the thermochromic agent comprises at least one of: a reversible or irreversible thermochromic pigment. In some arrangements, the thermochromic agent may comprise an agent which temporarily changes colour or permanently changes colour upon exposure of the agent to a change in temperature through a threshold.

[0019] In some arrangements, the thermochromic agent changes colour if it is exposed to a temperature which passes a threshold temperature for a predetermined exposure period. In some arrangements, the threshold temperature is between minus 100 degrees centigrade and 100 degrees centigrade. Accordingly, the threshold temperature may be elected based upon a mammalian tissue of interest and an ablation technique of interest. Accordingly, for example, in relation to heart tissue, for thermal ablation by heating, for example, by radio frequency ablation, the threshold temperature may be between 50 degrees centigrade and 70 degrees centigrade. Similarly, for example, in relation to heart tissue, for thermal ablation by freezing, for example, by cryoablation, the threshold temperature may be between -30 degrees centigrade and -80 degrees centigrade. Furthermore, in the case, for example, of laser ablation, a threshold temperature may be chosen to be greater than, for example, 50 degrees centigrade. In some cases, thermal ablation threshold temperatures may be greater than 100 degrees centigrade. In some cases, the threshold temperature may be as high, in the lase of laser ablation, as 350 degrees centigrade. During laser ablation, thermal energy applied directly to some cells or tissue may transfer and spread to adjacent cells and tissue. Such surrounding tissues can be impacted by the application of an ablation technique and users may have interest in seeing how far- reaching a result of thermal ablation is.

[0020] In some arrangements, the thermochromic agent comprises a pigment in powder or liquid form. In some arrangements, the thermochromic agent changes between white or transparent and a colour upon application of a threshold temperature. Useful thermochromic agents to allow an observable colour change are those that have one state (ablated or default) which is not white to transparent so a colour change of the tissue mimicking material based on an alginate is observable. In other words, pigments that go, for example, from black to white, or red to white, or yellow to red, offer advantages, whilst one which changes from transparent to white may not support an easily visible change when thermally ablated. It will be appreciated that some suitable pigments have colours such as green, blue, red, brown, grey and similar, and transition to white / transparent on application of appropriate thermal energy. Thermochromic agents such as thermochromic liquid crystals or albumin can be used, but work best in a base which is transparent, whereas alginate is typically a milky white colour. In some arrangements, the thermochromic agent comprises at least one of: Kromagen50; or SFXC 60°C Irreversible Thermochromic Pigment; or NNC TM-SL W50-0 Brown.

[0021] In some arrangements, the ablation-sensitive agent comprises: an electrochromic agent. Accordingly, the tissue mimicking material may be used to determine the effect of pulsed electric field (PEF) ablation techniques on the tissue mimicking material.

[0022] In some arrangements, the electrochromic agent comprises at least one of: a reversible or irreversible electrochromic pigment. In some arrangements, the electrochromic agent changes colour if exposed to an applied electrical energy above a threshold. Different PEF ablation device manufacturers vary the voltage output of their devices. Generally an ablation voltage is less than or equal to 2kV and the electrochromic agent is selected to change colour at an equivalent voltage.

[0023] In some arrangements, the electrochromic agent comprises at least one of: Prussian blue (Ci8Fe7Ni8) or another coloured metal oxide of transition, for example, WO3, or MoOs or I rO2, or NiO, or V2O5).

[0024] In some arrangements, the ablation-sensitive agent comprises: an electroluminescent agent. In some arrangements, the electroluminescent agent emits photons if exposed to an applied electrical energy above a threshold. Different PEF ablation device manufacturers vary the voltage output of their devices. Generally an ablation voltage is less than or equal to 2kV and the electroluminescent agent is selected to luminesce at an equivalent voltage. Successful PEF ablation is often achieved between 0.9kV -2kV.

[0025] In some arrangements, the electroluminescent agent comprises at least one of: an electroluminescent pain, for example, Lumilor paint; or an ANEP dye. Different ANEP dyes have different voltage thresholds. In some arrangements, the electroluminescent dye comprises a mix of different dyes of different thresholds. Such an arrangement may support a tissue mimicking material from which a PEF ablation map may be obtained. Alternatively, a single ANEP dye may be used and a resulting map of luminescent response analysed.

[0026] In some arrangements, the tissue being mimicked comprises mammalian tissue. In some arrangements, the tissue being mimicked comprises: mammalian myocardial tissue.

[0027] In some arrangements, the tissue being mimicked comprises: human myocardial tissue.

[0028] A further aspect comprises a clinical phantom comprising a tissue mimicking material according to the first aspect.

[0029] A further aspect provides a method of manufacture of a tissue mimicking material, the method comprising: forming an alginate matrix; and distributing an ablation-sensitive agent through the alginate matrix.

[0030] In some arrangements, forming the alginate matrix comprises: mixing alginate powder and a cross-linking ionic agent with water to form a solution.

[0031] In some arrangements, distributing the ablation-sensitive agent through the alginate matrix comprises mixing the ablation sensitive agent into the solution.

[0032] In some arrangements, the method of manufacture comprises: moulding and curing the solution.

[0033] In some arrangements, the method of manufacture comprises mixing doping agents with the solution.

[0034] In some arrangements, the doping agents comprise: water and / or salt and / or saline.

[0035] In some arrangements, the ablation sensitive agent is evenly distributed throughout the alginate matrix.

[0036] In some arrangements, the alginate matrix comprises: an alginate hydrogel.

[0037] In some arrangements, the alginate hydrogel comprises: a calcium alginate powder gelling agent. In some arrangements, the alginate hydrogel is doped with salt, or wherein the alginate hydrogel is doped with water, or wherein the alginate hydrogel is doped with saline.

[0038] In some arrangements, at least one of the following properties of the alginate hydrogel: electrical resistivity; or thermal conductivity; or hardness of the alginate hydrogel is controlled by selection of concentration of a dopant in the alginate hydrogel.

[0039] In some arrangements, the ablation-sensitive agent comprises: a thermochromic agent.

[0040] In some arrangements, the thermochromic agent comprises at least one of: a reversible or irreversible thermochromic pigment and wherein the thermochromic agent changes colour if it is exposed to a temperature which passes a threshold temperature for a predetermined exposure period.

[0041] In some arrangements, the threshold temperature is between minus 100 degrees centigrade and 100 degrees centigrade.

[0042] In some arrangements, the thermochromic agent comprises at least one of: Kromagen50; or SFXC 60°C Irreversible Thermochromic Pigment; or NNC TM-SL W50-0 Brown.

[0043] In some arrangements, the ablation-sensitive agent comprises: an electrochromic agent.

[0044] In some arrangements, the electrochromic agent comprises at least one of: a reversible or irreversible electrochromic pigment and wherein the electrochromic agent changes colour if exposed to an applied electrical energy above a threshold.

[0045] In some arrangements, the electrochromic agent comprises a coloured metal oxide of transition.

[0046] In some arrangements, the ablation-sensitive agent comprises: an electroluminescent agent.

[0047] In some arrangements, the electroluminescent agent emits photons if exposed to an applied electrical energy above a threshold. In some arrangements, the electroluminescent agent comprises at least one of: Lumilor paint; or an ANEP dye.

[0048] In some arrangements, the tissue being mimicked comprises: mammalian tissue.

[0049] In some arrangements, the tissue being mimicked comprises: myocardial tissue.

[0050] In some arrangements, the tissue being mimicked comprises: human myocardial tissue.

[0051] A further aspect comprises a method of manufacture of a clinical phantom comprising a tissue mimicking material according to the first aspect.

[0052] A further aspect comprises: an ablation simulation apparatus comprising: a device for ablation of tissue; and a tissue mimicking material according to the first aspect.

[0053] In some arrangements, the device for ablation comprises at least one of: a device for thermal ablation; or a device for radio frequency ablation; or a device for cryoablation; or a device for pulsed electric field ablation.

[0054] In some arrangements, the apparatus comprises: an image capture device configured to capture at least one image of the tissue mimicking material during ablation.

[0055] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

[0056] Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

[0057] BRIEF DESCRIPTION

[0058] Some example embodiments will now be described with reference to the accompanying drawings in which: FIGS. 1A to 1E show various alginate-based materials;

[0059] FIG. 1 F

[0060] FIGS. 2A to 2C illustrate schematically test arrangements in relation to physical properties of a tissue mimicking material in accordance with an aspect;

[0061] FIGS. 3A to 3E illustrate test arrangements to determine response of an alginate-based tissue mimicking material according to an aspect to: radiofrequency ablation techniques (FIG. 3A and 3B) and to cryoablation techniques (FIG. 3C and 3D) and a response of an alginate-based tissue mimicking material to ablation techniques (FIG. 3E);

[0062] FIGS. 4A to 4C show cryoablation lesions, both in terms of shape and colour contrast resulting from the following materials in Table 1 : FIG. 4A: Kromagen; FIG. 4B: SFXC and FIG. 4C NNC;

[0063] FIG. 5 illustrates graphically maximum lesion depth for example thermochromic alginate-based materials compared to in-vivo myocardium lesion dimensions after cryoablation.

[0064] DETAILED DESCRIPTION

[0065] Before discussing the example embodiments in any more detail, first an overview will be provided.

[0066] Ablation techniques including: Radiofrequency Ablation (RA), Cryoablation, and Pulsed Electrical Field (PEF) Ablation, offer a mechanism to treat various conditions which occur in living mammalian tissue. Ablation techniques have various applications including, for example, unwanted tissue removal, tumour removal, treatment of muscular tissue and, at a much lesser level, wart removal.

[0067] One example of use of ablation techniques on mammalian tissue relates to use of ablation to treat cardiac arrythmia. Cardiac Arrythmias are pathologies that cause a heart to beat irregularly and asynchronously. Atrial Fibrillation is the most common type of arrythmia effecting about 1.4 million patients in the UK [1],

[0068] Recent studies predict that incidence of cardiac arrythmia in western countries will grow exponentially over the coming decades [2, 3], If or when pharmaceutical treatment of cardiac arrhythmia in a subject does not suffice, a possible clinical treatment comprises identifying defective volumes of a cardiac wall, and using ablation, or surgical, techniques to address those defective volumes. In the case of ablation, the defective volumes are lesioned and turned into scar tissue in a procedure called Cardiac Ablation (CA). By effectively sectioning off the defective volumes from a cardiac wall, a healthy cardiac rhythm may be restored.

[0069] Scarring of the relevant tissue may be induced by killing or damaging target cells. The damage may, for example, be induced by appropriate application of ablative techniques. Killing or damaging cells is caused either by freezing (cryoablation), heating (Radiofrequency Ablation) (RA), or by damaging the cell function Pulsed Electric Field (PEF) ablation.

[0070] The outcome of Cryoablation, RA and PEF techniques is determined by a number of factors including the 3D-shape of lesions / damage caused. This is directly dependant on the portion of tissue that has been exposed to appropriate ablation conditions. In the case of thermal ablation, the appropriate conditions might be, for example, temperatures of 50° - 60°C for RA and -30° - -80°C for Cryoablation [4 - 8], Dimensions of a region exposed to effective ablation conditions may depend on parameters such as, for example, duration, power, temperature, pressure, and / or area of contact.

[0071] In order to test different ablation settings, techniques, and technologies, tests can be performed on animal tissue either in-vivo or ex-vivo. In the case of cryoablation, freezing of tissue initially results in little to no apparent change immediately after ablation. As a result, tests of cryoablation techniques are typically performed on in-vivo samples and results collected by histological analysis weeks later [7-10],

[0072] In addition to the logistical, financial, and ethical limitations posed by the use of cadaver or live samples for testing and practicing of ablative techniques, other factors, such as tissue non-uniformity and effective visualization of ablation lesions can complicate data acquisition.

[0073] Tissue-Mimicking Materials (TMM) can be used to replicate physical properties of tissues in bench tests, providing a homogeneous practical alternative to tissue samples.

[0074] Aspects described in more detail below recognise that a cost-effective TMM offering an observable change if exposed to an effective ablation condition(s) can be utilised to replace in-vivo or ex-vivo tissue for any ablation test. Aspects further recognise that a cost effective TMM may be formulated to replicate one or more physical characteristic of a tissue of interest. By way of example, in the case of cardiac ablation a TMM could be used to create a phantom which replicates one or more physical characteristic of cardiac tissue of relevance to RA, Cryoablation and / or PEF ablation techniques. Similarly, a TMM with different physical characteristics could be used to create a liver phantom suitable for simulation of liver ablation. Aspects may also provide a mechanism to provide tissue mimicking materials suitable for creation of partial or full torso models containing one or more tissues or organs. Such models may be utilised to allow a clinician to practice interventional, for example, laparoscopic techniques.

[0075] Accordingly, an arrangement of one aspect provides a tissue mimicking material comprising: an alginate matrix; and an ablation-sensitive agent distributed through the alginate matrix.

[0076] By way of example, some possible implementations of a tissue mimicking material in accordance with an aspect and use of that tissue mimicking material in simulations and training for ablation techniques are described in detail.

[0077] Aspects described also encompass use of a tissue mimicking material according to one aspect in a test apparatus assembly according to another aspect and to a method of using the tissue mimicking material of one aspect to test the effects of ablation equipment and techniques on mammalian tissue.

[0078] As described generally above, a tissue mimicking material in accordance with one aspect comprises an alginate matrix including an ablation sensitive material distributed through at least a portion of the alginate matrix. In one embodiment, the tissue mimicking material comprises a thermochromic material distributed through an alginate hydrogel. The temperature sensitive material may be selected such that it changes colour at ablation temperatures.

[0079] Accordingly, a further aspect provides a test apparatus according to an arrangement comprises a saline tank containing a sample of a TMM in accordance with a first aspect. The TMM sample may form a part, portion, or whole, of a phantom which may be layered to replicate different features in a body. At least one layer of the phantom may include a tissue mimicking material in accordance with a first aspect. That layer may, in one implementation, be thermochromic. In other implementations, the layer may be electroluminescent. The phantom may be multi-layered. Some or all layers of the phantom may comprise an alginate hydrogel. The alginate hydrogel layers may be doped with compounds such as, for example, salts.

[0080] In the case that the phantom is provided with a thermochromic layer made of a tissue mimicking material in accordance with an aspect, that layer of the phantom may be configured to respond to heating or cooling from an ablation source. The heating or cooling may result from, for example, radiofrequency energy, radiant heat, cooling, microwave energy, and / or electromagnetic energy. The thermochromic layer may include one or more reversible or irreversible thermochromic pigments. In other words, the ablation-sensitive material distributed or dispersed through the alginate matrix of the thermochromic layer may comprise a thermochromic pigment. Pigments and colourants used in the thermochromic layer may be selected to be ones which, in use, change colour at one or more threshold temperature between -100°C and 100°C. The pigment or colourant may permanently change colour upon passing through a threshold temperature, or may temporarily change colour, at least whilst the transition across the threshold temperature is maintained. The pigment or colourant may be reversible and, upon passing through the threshold temperature in the other direction may revert to an original colour.

[0081] Accordingly, exposing a phantom including a thermochromic tissue mimicking material to a thermal ablation technique may result in the causing of an observable change in the tissue mimicking material. That observable change may comprise a visible change in colour in a region which has been exposed to successful ablation conditions. As a result, a clinician may be able to see the result of intended ablation techniques on the tissue mimicking material and infer the likely result if a similar ablation technique is applied to living mammalian tissue.

[0082] Tissue Mimicking Material: Methods of Manufacture

[0083] It will be appreciated that a first aspect provides an alginate tissue mimicking material including an ablation sensitive agent. What follows is a description of possible methods of manufacture of a thermochromic implementation of the first aspect. Methods described relate to formulation and testing of three ablation-sensitive thermochromic coloured mixtures and compare their ablation responses to animal heart tissue. It will be apparent to the person skilled in the art that the described principles may be implemented in a different manner, for example, by adjusting the concentration(s) and / or various components in order to mimic the physical properties of different mammalian tissues.

[0084] Alginate Formulation

[0085] In the described methods of manufacture, white calcium alginate moulding powder (Pebeo, Marseille, France) was utilised as the gelling agent. Saline was prepared at 0.3% and 0.25% mass concentrations using deionised water and NaCI. Thermochromic agents: Kromagen50 (SpotSee, Dallas, Texas); SFXC 60°C Irreversible Thermochromic Pigment (Good Life Innovations Ltd., Seaford, United Kingdom); and NNC TM-SL W50-0 Brown (New Prismatic Enterprise Co., Taipei, Taiwan) were used as the thermochromic pigments for these experiments because they retain their new colour after changing. It will be appreciated that other thermochromic agents and pigments may be selected. Two control mixtures and three coloured mixtures were prepared following the ratios shown in Table 1. Liquids and powders were mixed and measured separately before being combined with a hand blender for 30 seconds. The blender was then opened and the sides were scraped. The mixture was blended for 10 more seconds and poured into appropriate moulds to set for 10 minutes.

[0086] Table 1. Formulations for the mixtures of alginate analysed.

[0087] FIGS. 1A to 1E show the various alginate materials formed from the mixtures set out in Table 1. In particular, FIG. 1 A is the C25 Alginate Mixture, FIG. 1 B is the C30 Alginate Mixture. Those mixtures represent control alginate mixtures. FIG. 1C is the SFXC Alginate Mixture of Table 1 , FIG. 1 D is the NNC Alginate Mixture of Table 1 , and FIG. 1E is the Kromagen Alginate Mixture of Table 1. FIG. 1 F illustrates schematically a cross sectional view of a tissue mimicking material in accordance with an aspect. As shown in FIG. 1 F, there is provided a tissue mimicking material 100 comprising an alginate matrix 200 and an ablation-sensitive agent 300 distributed through the alginate matrix 200.

[0088] In order to assess the suitability of the materials of Table 1 as a myocardial tissue mimicking material, an assessment of relevant physical characteristics of the materials to thermal ablation can be made. FIGS. 2A to 2C illustrate schematically test arrangements in relation to physical properties of a tissue mimicking material in accordance with an aspect.

[0089] Matching Tissue Properties: Electrical Resistivity

[0090] To evaluate the electrical properties of all mixtures, a volume constrained box 45 x 15 x 15mm was 3D-printed in Anycubic Basic Clear Resin (HongKong Anycubic Technology Co., Hong Kong, China). 15mm square copper electrodes were constructed using 0.5mm copper sheets and placed on either side of the volume constrained box. The electrodes were connected to a switch and a multimeter to measure electrical resistance of material placed between them. Internal resistance of the system was measured at 0.00 ± O.OOkQ, using the 20kQ range.

[0091] Ten slices measuring 45 x 15 x 15mm were acquired from left ventricles of porcine hearts acquired 3 days before the test and stored at 4°C.

[0092] Ten slices of each tissue mimicking material as described above were prepared and moulded to size.

[0093] Each slice was placed in the volume constrained box and compressed by the lid, ensuring full contact with the electrodes and consistent volume

[0094] FIG. 2A shows the volume-constrained box 1, electrodes associated with which are connected to a multimeter 3 via wires and a switch 2 to support measurement of electrical resistivity of samples, in the form of slices, contained in the box 1.

[0095] Using the test setup shown in FIG. 2A, the switch 2 was activated so that it was possible to ascertain an initial resistance of the material in the box 1 in kQ. Resistivity of each material within the box was calculated by multiplying the ascertained value of resistance by the cross-sectional area of the sample in the box, divided by its length. The literature value for the electrical conductivity of ex-vivo myocardium was found to be in the range of 0.06S / m to 0.4S / m [1A, 2A]; therefore, the electrical resistivity of ex- vivo myocardium should be between 16.7Qm and 2.5Qm.

[0096] Electrical Resistivity Results: Summary of Materials of Table 1:

[0097] Myocardium: 7.6 ± 0.6Qm

[0098] SFXC: 7.4 ± 0.6Qm (p=0.5131 using a t-test)

[0099] NNC: 7.1 ± 0.2Qm (p=0.0520 using a t-test)

[0100] Kromagen: 7.0 ± 0.5Qm (p=0.0548 using a t-test)

[0101] Electrical Impedance Results

[0102] Thicknesses tested: 1mm, 2.5mm, 4mm, 7mm, 10mm, 13mm 10mm samples were compared to literature 0.075% salinity produced 100-130Q.

[0103] Matching Tissue Properties: Hardness

[0104] FIG. 2B shows a test setup to determine hardness of a material. An HT-651000 OO- Shore Durometer 4 (Landtek Instruments Co., Guangzhou, China) is shown mounted on a remote-controlled linear actuator 5. As shown in FIG. 2B, a 1mm thick cup containing a 20 x 35mm (length x diameter) cylinder of material under test, for example, an alginate tissue mimicking material, is held in place by two M4 screws, such that the material under test is held in a position generally centred with the durometer. The durometer was displaced by 15mm allowing the alginate to come in to contact with the body of the durometer. 10 measurements of hardness were recorded for each alginate mixture under test. For each measurement a new cylinder was used.

[0105] Hydrogels of GO Shore hardness 20 ± 7.5 are considered ideal to recreate cardiac muscle [3A], A 10,000-element vector was created following a normal distribution with mean 20 and standard deviation 7.5 using MATLAB function “normrnd” as reference [4A]

[0106] Hardness Results: Summary of Materials of Table 1 :

[0107] Shore Hardness measured with HT-651000 Durometer

[0108] Myocardium: OO 20±7.5

[0018]

[0109] SFXC: 0027±2 (p<0.0001 using a t-test) NNC: 0021 ±2 (p=0.4004 using a t-test) Kromagen: GO 19±2 (p=0. 4936)

[0110] Matching Tissue Properties: Thermal Conductivity

[0111] FIG. 2C shows a test setup to determine thermal conductivity (TC) of a material. Thermal conductivity of test materials were analysed to compare thermal behaviours of the alginate-based tissue mimicking materials to ex-vivo myocardium. The thermal conductivity of a material subject to thermal ablation techniques directly affects lesion dimensions via the propagation of thermal energy.

[0112] The TC of myocardium falls between 0.58 and 0.75 W / mK [5A, 6A], The paper by D. Koncan et. al. [5A] was chosen as a reference for this investigation, as it analysed the TC of a septum and ventricular wall of a heart, and those represent two common cardiac ablation sites. The study of 5A used applied various heating power to heat porcine myocardium and thus calculate TC of the myocardium. The study reported ventricular TC of 0.58 ± 0.02W / m.K at 0.5W, and 0.62 ± 0.01 W / m.K at 1 ,5W. The average values of TC across all methodologies in the study were reported at 0.60 ± 0.01 W / mK for septal wall, and 0.75 ± 0.03W / m.K for ventricular wall. Using the pooled variance method and averaging means, the overall average TC of myocardium can be calculated to be 0.675 ± 0.02 W / m.K. Normally distributed arrays of 10000 elements were derived on MATLAB as a reference based on: ventricular TC at 1.5W, ventricular TC at 0.5W, and the overall average.

[0113] To measure the thermal conductivity of the alginate based tissue mimicking materials of an aspect, cylindrical 110mm x 37.5mm samples were cast inside polylactic acid 3D- printed moulds. A TSL-100 thermal conductivity analyser 6 (Thermtest Instruments, New Brunswick, Canada) was fully inserted into each cylindrical sample and a value of thermal conductivity was measured. The test process was repeated on 10 different samples of each alginate mixture.

[0114] Like all other measured properties, the TC values acquired during this study were compared against the reference using a 5% confidence level in a two-tailed multivariance t-test.

[0115] Thermal Conductivity Results: Summary of Materials of Table 1: Obtained using TSL-100 Thermal Conductivity Analyser Myocardium: 0.58-0.75 W / m.K

[0010] SFXC: 0.67±0.02W / m.K (p=0.1840) NNC: 0.62±0.02W / m.K (p=0.8686 using a t-test)

[0116] Kromagen: 0.58±0.05W / m.K (p=0.9439 using a t-test)

[0117] Whilst the discussions herein relate primarily to simulation of myocardial tissue, it will be appreciated that other tissues may also be simulated, and that the physical properties of those other tissues would need to be matched by selecting appropriate composition(s) of an alginate material and ablation sensitive material.

[0118] It has been demonstrated that it is possible to match physical properties of relevance to ablation techniques of an alginate-based tissue mimicking material to the physical properties of mammalian tissue which may be ablated. What follows is a consideration of ways in which appropriately designed tissue mimicking material according to an aspect can be used in relation to simulation of ablation for training or testing of ablation techniques.

[0119] Ablation Techniques

[0120] The following discussions and demonstrations relate to thermal ablation, both heating and cooling, but are analogously applicable to PEF ablation techniques, provided an electrical energy absorption response of an alginate-based tissue mimicking material is matched to that of a mammalian tissue of interest.

[0121] FIGS. 3A to 3E illustrate test arrangements to determine response of an alginate-based tissue mimicking material according to an aspect to radiofrequency ablation techniques (FIG. 3A and 3B) and to cryoablation techniques (FIG. 3C and 3D) and a response of an alginate-based tissue mimicking material to ablation techniques (FIG. 3E).

[0122] Testing Alginate-Based Material: Relevance to Radiofrequency Ablation

[0123] To replicate a typical heart wall, 75mm x 55mm x 10mm slices of alginate-based tissue mimicking material according to an aspect were moulded. Those dimensions were selected to simulate the thickness of ventricular walls [7A],

[0124] The alginate-based tissue mimicking materials chosen to test their relevance to radiofrequency ablation included a thermochromic agent.

[0125] As shown in FIG. 3A, slices of an alginate-based tissue mimicking material were mounted on a perforated base 10, together with a 1.5cm sponge layer 20 using a plastic frame. The base 10 is arranged in the centre of a temperature-controlled 55cm x 38cm x 27cm acrylic tank 30 filled with saline 40. The saline concentration could be varied. A Valleylab Return Patient electrode 2a (Covidien, Dublin, Ireland) was placed on the bottom of the tank 30 facing into the saline 40 in the inferior section of the tank to simulate patch placement on the lower back of a human ablation subject.

[0126] In the test setup shown in FIG. 3A the saline 40 was maintained at 37.0°C using a thermostat system 1a, and saline mass concentration was progressively increased (0.05%, 0.10%, 0.20%, 0.30%). A baseline impedance of the material under test was recorded in 5 separate locations on the slice using a Thermocool SmartTouch Unidirectional Navigation Catheter (NAV ST) (Biosense Webster Inc., Irvine, USA) in a CARTO VIZIGO 8.5F (Biosense Webster Inc., Irvine, USA) steerable sheath 50 linked to a SMARTABLATE System (Biosense Webster Inc., Irvine, USA).

[0127] FIG. 3B illustrates an idealised electrical model of the test setup of FIG. 3A. Based on that idealisation, relationship between system impedance (R) versus tank salinity (x) can be derived to be as shown is Equation 1 :

[0128] Equation 1. The derivation of a mathematical model describing the value of System Impedance (R) as a function of Tank Salinity (x), where A, B, C, D are constants and a=1 / A and p=1 / B.

[0129] M. Barkagan et. al. [8A] performed ablation lesions in a similar simulator set-up using ex-vivo porcine ventricular wall. Their low impedance (100-130Q) tests performed lesions at 30W for 20 seconds and averaged a depth of 3.6 ± 0.7 mm and a width of 8.3 ± 1.4 mm. In order to assess alginate-based tissue mimicking materials according to aspects, to this published study, the tank shown in FIG. 3A was filled with 0.075% saline at 37.0°C and 10 ablation lesions were performed on 10mm slices of example thermochromic alginate mixtures.

[0130] The Unidirectional NAV ST catheter was manipulated using the steerable sheath 50 and irrigated with 24°C 0.075% saline of at 8ml / min for each ablation lesion, performed at 30W for 20 seconds. Results of lesion depth and width were measured digitally based on a ruler placed directly next to each sample. The response of one thermochromic alginate-based tissue mimicking material to radiofrequency ablation is shown in FIG. 3E. Results of width and depth of the visible change in the thermochromic material to the radio frequency ablation technique were compared using a t-test against 10,000-element normal distributions of depth and width of mammalian tissue lesions resulting from the same radio frequency ablation technique based on the literature values.

[0131] Radiofrequency Ablation Results:

[0132] Low Impedance (90-130 Q) was achieved at 0.075% tank salinity.

[0133] No statistically significant difference in lesion width / depth of the tissue mimicking materials compared against literature values for in and ex vivo mammalian tissue results.

[0134] To evaluate colour contrast between the “lesions” induced in the alginate-based thermochromic tissue mimicking material and unaffected background alginate material, the RGB values of the alginate mixtures were analysed based on digital images of the material under test acquired under equal light conditions. The RGB values of 125 pixels were acquired in regions of both the “background” and the “lesions” and averaged. The colour contrast can then be calculated using Colour Contrast Analyser (TPGi, Clearwater, USA).

[0135] Testing Alginate-Based Material: Relevance to Cryoablation

[0136] To test the cryoablation properties of alginate-based tissue mimicking materials according to aspects, two 10mm slices were prepared for each material under test.

[0137] For a colour change to occur on exposure to cryoablation techniques, each mixture was prepared by adding a thermochromic pigment(s) in saline and then heating the solutions in plastic constrainers for 10 minutes at 70°C. Each measured solution was cooled then mixed with alginate powder, as described previously, and cast into 75mm x 55mm x 10mm slices.

[0138] As shown in FIG. 3C, a slice under test 60 was held in a 37.0°C heated water bath 70 and ablated using a Medtronic Artic Front Advance 1b (Medtronic Limited, Dublin, Ireland) for 3 minutes using standard equipment. FIG. 3D illustrates that due to the shape of the catheter 4a, the lesions had an annular shape. Two cryoablations were performed for each material under test. The annular ablations were sliced, and depth was digitally measured 16 times.

[0139] F. Bessiere et. al. [8A] performed similar ablation lesions on the ventricular walls of in- vivo dogs, achieving an average depth of 5.1 ± 0.3mm at both 2 and 4 minutes of duration. A 10000-element normal distribution was derived by literature values and compared to the experimental result.

[0140] FIGS. 4A to 4C show cryoablation lesions, both in terms of shape and colour contrast resulting from the following materials in Table 1 : FIG. 4A: Kromagen; FIG. 4B: SFXC and FIG. 4C NNC.

[0141] FIG. 5 illustrates graphically maximum lesion depth for example thermochromic alginate-based materials compared to in-vivo myocardium lesion dimensions after cryoablation.

[0142] Cryoblation Results:

[0143] The NNC material of Table 1 and FIG. 4C showed no statistically significant difference in max lesion depth against literature.

[0144] Test Materials Summary - Table 1 Materials

[0145] The NNC and Kromagen mixtures of Table 1 had physical properties which realistically “match” the relevant physical properties of myocardial tissue. The SFXC material of Table 1 had a hardness of (27.4±2.4 GO) and was significantly harder on the than the literature values (20±7.5OO) expected for myocardial tissue. All pigments in the materials of Table 1 produced realistic lesions during Radiofrequency Ablation. The thermal conductivity of SFXC appeared to balance its lower sensitivity to heat. Kromagen’s colour change faded over time. In relation to cryoablation, NNC and SFXC produced ablation lesions during cryoablation. The higher thermal conductivity caused SFXC cryoablation lesions to appear bigger than an equivalent lesion in myocardial tissue. Only NNC produced realistic cryoablation lesions.

[0146] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements. The ordering of method steps set out above may not be critical or fixed and the exact ordering of the steps may be varied as appropriate.

[0147] Although example embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

[0148] Features described in the preceding description may be used in combinations other than the combinations explicitly described.

[0149] Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

[0150] Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

[0151] Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon.

[0152] REFERENCES

[0153] 1. NHS. (n.d.). Atrial Fibrillation - NHS. NHS. https: / / www.nhs.uk / conditions / atrial- fibrillation / Morillo CA, Banerjee A, Perel P, Wood D, Jouven X. Atrial fibrillation: the current epidemic. J Geriatr Cardiol. 2017; 14(3): 195-203. doi: 10.11909 / j.issn.1671- 5411.2017.03.011

[0154] 2. Lippi G, Sanchis-Gomar F, Cervellin G. Global Epidemiology of Atrial Fibrillation: An increasing epidemic and Public Health Challenge. Int J Stroke. 2020; 16(2) :217-221. doi : 10.1177 / 1747493019897870.

[0155] 3. Iwasawa J, Koruth J, Petru J, Kautzner J, Neuzil P, Peichl P, Wichterle D, Reddy VY. Temperature-Controlled Radiofrequency Ablation for Pulmonary Vein Isolation in Patients With Atrial Fibrillation. J Am Coll Cardiol. 2017 Aug; 70(5):542- 553. doi:10.1016 / j.jacc.2017.06.008

[0156] 4. Wang S, Saija C, Choo J, Ou Z, Birsoan M, Germanos S, Rothwell J, Vakili B, Kotadia I, Xu Z, et al. Cardiac Radiofrequency Ablation Simulation Using a 3D-Printed Bi-Atrial Thermochromic Model. Appl Sci. 2022; 12:6553. doi: 10.3390 / app12136553

[0157] 5. Friedman PL. Catheter Cryoablation of Cardiac Arrhythmias. US Cardiology. 2004; 1(1):126— 8. doi:10.15420 / usc.2004.1.1.126

[0158] 6. Bessiere F, Dubuc M, Andrade J, Mahida S, Zado E, Marchlinski F. Focal Transcatheter Cryoablation: Is a Four-Minute Application Still Required?. J Cardiovasc Electrophysiol. 2017; 28(5):559-563. doi: 10.1111 / jce.13193

[0159] 7. Khairy P, Chauvet P, Lehmann J, Lambert J, Macle L, Tanguay JF, Dubuc M. Lower incidence of thrombus formation with Cryoenergy versus radiofrequency catheter ablation. Circulation. 2003; 107(15): 2045-2050. doi:10.1161 / 01.cir.0000058706.82623.a1

[0160] 8. Thiagalingam A, D’Avila A, Foley L, Houghtaling C, Dukkipati SR, Reddy VY. Importance of catheter contact force during irrigated radiofrequency ablation: evaluation in a porcine ex vivo model using a force-sensing catheter. J Cardiovasc Electrophysiol. 2010; 21(7):806-811. doi:10.1111 / j.1540-8167.2009.01693.x 9. Barkagan M, Rottmann M, Leshem E, Shen C, Buxton AE, Anter E. Effect of Baseline Impedance on Ablation Lesion Dimensions: A Multimodality Concept Validation From Physics to Clinical Experience. Circ Arrhythm Electrophysiol. 2018; 11(10):e006690. doi:10.1161 / CIRCEP.118.006690

[0161] 1A. Miklavcic D, Pavselj N, Hart FX. Electric properties of tissues. Wiley Encyclopedia of Biomedical Engineering. 2006. doi:10.1002 / 9780471740360. ebs0403.

[0162] 2A. Raghavan K, Porterfield JE, Kottam AT, Feldman MD, Escobedo D, Valvano JW, Pearce JA. Electrical conductivity and permittivity of murine myocardium. IEEE Trans Biomed Eng. 2009; 56(8):2044-2053. doi:10.1109 / tbme.2009.2012401.

[0163] 3A. Tejo-Otero A, Fenollosa-Artes F, Achaerandio I, Rey-Vinolas S, Buj-Corral I, Mateos-Timoneda MA, Engel E. Soft-Tissue-Mimicking Using Hydrogels for the Development of Phantoms. Gels. 2022; 8:40. doi:10.3390 / gels8010040.

[0164] 4A. The MathWorks Inc. MATLAB version: 9.13.0 (R2022b). Natick, Massachusetts: The MathWorks Inc. 2022. https: / / www.mathworks.com.

[0165] 5A. Koncan D, Rifel J, Drevensek G, Kocijancic S, Ogorelec S, Budihna MV. Thermal conductivity of the porcine heart tissue. Pflugers Arch - Eur J Physiol. 2000; 440(S 1 ) . doi : 10.1007 / s004240000039

[0166] 6A. Bowman HF, Cravalho EG, Woods M. Theory, measurement, and application of thermal properties of biomaterials. Annu Rev Biophys Bioeng. 1975; 4:43-80. doi:10.1146 / annurev.bb.04.060175.000355.

[0167] 7A. Yalgin F, Abraham TP, Gottdiener JS. Letter regarding article, “Left ventricular wall thickness and the presence of asymmetric hypertrophy in healthy young army recruits: Data from the large heart study.” Circ Cardiovasc Imaging. 2013; 6(5). doi: 10.1161 / circimaging.113.000720.

[0168] 8A. Barkagan M, Rottmann M, Leshem E, Shen C, Buxton AE, Anter E. Effect of Baseline Impedance on Ablation Lesion Dimensions: A Multimodality Concept Validation From Physics to Clinical Experience. Circ Arrhythm Electrophysiol. 2018;

[0169] 11(10):e006690. doi:10.1161 / CIRCEP.118.006690

Claims

CLAIMS1. A tissue mimicking material comprising: an alginate matrix; and an ablation-sensitive agent distributed through the alginate matrix.

2. A tissue mimicking material according to claim 1, wherein the ablation sensitive agent is evenly distributed throughout the alginate matrix.

3. A tissue mimicking material according to claim 1 or claim 2, wherein the alginate matrix comprises: an alginate hydrogel.

4. A tissue mimicking material according to claim 3, wherein the alginate hydrogel comprises: a calcium alginate powder gelling agent.

5. A tissue mimicking material according to claim 3 or claim 4, wherein the alginate hydrogel is doped with salt, or wherein the alginate hydrogel is doped with water, or wherein the alginate hydrogel is doped with saline.

6. A tissue mimicking material according to claim 5, wherein at least one of the following properties of the alginate hydrogel: electrical resistivity; or thermal conductivity; or hardness of the alginate hydrogel is controlled by selection of concentration of a dopant in the alginate hydrogel.

7. A tissue mimicking material according to any preceding claim, wherein the ablation-sensitive agent comprises: a thermochromic agent.

8. A tissue mimicking material according to claim 7, wherein the thermochromic agent comprises at least one of: a reversible or irreversible thermochromic pigment and wherein the thermochromic agent changes colour if it is exposed to a temperature which passes a threshold temperature for a predetermined exposure period.

9. A tissue mimicking material according to claim 7 or claim 8, wherein the threshold temperature is between minus 100 degrees centigrade and 100 degrees centigrade.

10. A tissue mimicking material according to any one of claims 7 to 9, wherein the thermochromic agent comprises at least one of: Kromagen50; or SFXC 60°C Irreversible Thermochromic Pigment; or NNC TM-SL W50-0 Brown.

11. A tissue mimicking material according to any preceding claim, wherein the ablation-sensitive agent comprises: an electrochromic agent.

12. A tissue mimicking material according to claim 11 , wherein the electrochromic agent comprises at least one of: a reversible or irreversible electrochromic pigment and wherein the electrochromic agent changes colour if exposed to an applied electrical energy above a threshold.

13. A tissue mimicking material according to claim 11 or claim 12, wherein the electrochromic agent comprises a coloured metal oxide of transition.

14. A tissue mimicking material according to any preceding claim, wherein the ablation-sensitive agent comprises: an electroluminescent agent.

15. A tissue mimicking material according to claim 14, wherein the electroluminescent agent emits photons if exposed to an applied electrical energy above a threshold.

16. A tissue mimicking material according to claim 14 or claim 15, wherein the electroluminescent agent comprises at least one of: Lumilor paint; or an ANEP dye.

17. A tissue mimicking material according to any preceding claim, wherein the tissue comprises mammalian tissue.

18. A tissue mimicking material according to any preceding claim, wherein the tissue comprises: myocardial tissue.

19. A clinical phantom comprising a tissue mimicking material according to any one of claims 1 to 18.

20. A method of manufacture of a tissue mimicking material, the method comprising: forming an alginate matrix; anddistributing an ablation-sensitive agent through the alginate matrix.

21. A method of manufacture of a tissue mimicking material according to claim 20, wherein forming the alginate matrix comprises: mixing alginate powder and a cross-linking ionic agent with water to form a solution.

22. A method of manufacture of a tissue mimicking material according to claim 21, comprising: distributing the ablation-sensitive agent through the alginate matrix by mixing the ablation sensitive agent into the solution.

23. A method of manufacture of a tissue mimicking material according to claim 21 or claim 22, comprising mixing a doping agent with the solution.

24. A method of manufacture of a tissue mimicking material according to claim 23, wherein the doping agent comprises at least one of: water or salt or saline.

25. An ablation simulation apparatus comprising: a device for ablation of tissue; and a tissue mimicking material according to any one of claims 1 to 18.

Images