A composite hydrogel composition

Abstract

A composite hydrogel composition comprising a poloxamer, an alginate, cactus fibres and water. Methods of manufacturing said compositions and uses are also provided. Preferably, the cactus fibres are from a cactus of the Opuntia family and are present at a weight of 1-5 wt%. The alginate may be present at 1-8 wt % of the hydrogel and the poloxamer is present at 0.1-15 wt% of the hydrogel. A cross-linking agent, preferably a calcium-based cross-linking agent, most preferably calcium carbonate may be included. Reinforcing agents, including clays, chitosan and silicas may also be included. The hydrogels may be 3D-printed and / or used for the purpose of vibration damping.

Description

Field of Invention The present invention relates to composite hydrogel compositions comprising a poloxamer, an alginate, cactus fibres and water, methods of manufacturing said composite hydrogel compositions and uses of such composite hydrogel compositions in vibration damping. Background to the Invention Occupational exposure to vibrations caused by machinery and external dynamic loading poses significant health hazards to human operators, the surrounding environment, and the transport and infrastructure capital asset industry as a whole. Passive vibration insulators and acoustic insulators have been developed to dampen unwanted vibrations that are transmitted through mechanical equipment, vehicles, buildings, and other structures. For example, multi-layer insulators, polymeric constrained layer insulators, and free layer dampers are used to stabilise unwanted vibrations in a wide range of applications including cooling systems, aircrafts, helicopters, ground vehicles, buildings, and marine structures. Generally, damping devices utilise fossil-based raw materials, such as mineral oil, petroleum-derived synthetic polymers, and elastomers. For example, polypropylene glycol, polytetrafluoroethylene, poly(styrene-butadiene-styrene), poly(ethylene bis(stearamide)) and polyurethane-based open cell foams may be utilised due to their viscoelastic properties and well-documented mechanical performance. Natural rubber has also been used in vibration damping applications. However, the sourcing, processing, and disposal of these finite materials have negative impacts on the environment, contributing to climate change and deforestation. Therefore, there is an urgent need for alternative and more sustainable materials that can provide comparable or improved properties for vibration damping applications. Summary of the Invention In accordance with a first aspect of the invention, there is provided a composite hydrogel composition comprising about 0.1 wt.% to about 20 wt.% of a poloxamer, about 0.5 wt.% to about 10 wt.% of an alginate, about 0.5 wt.% to about 10 wt.% of cactus fibres, and water. The compositions described herein use sustainable, bio-based, and recyclable materials that are effective for vibration damping, providing a more environmentally friendly alternative to fossil-based materials with a reduced carbon footprint. The composite hydrogels compositions of the present disclosure may be produced using local feedstocks and resources, for example, cactus fibres are derived from endemic species present in the Americas, Africa, Asia, Oceania, and Europe. Utilising bio-based, naturally occurring resources can help to minimize the logistical and supply chain demands that are currently present in the production and application of fossil-based vibration damping materials globally. In addition, some cacti types, in particular Opuntia, are deemed to be invasive plants that spread quickly. This can impact pastures and natural areas and overwhelm native vegetation. The spines of mature Opuntia can also cause injury to grazing livestock and humans which can reduce productivity and grazing activity. Accordingly, providing a use for such invasive plants would be advantageous for both local ecosystems and farmers. The vibration damping performance of the compositions described herein is comparable to soft elastomeric systems and they have been found to outperform natural rubber and existing polydimethylsiloxane-based compositions that are often used in applications such as energy absorbing liners. As such, the compositions described herein not only provide a more sustainable alternative to fossil-based vibration damping materials, they also provide improved mechanical and elastomeric properties, superior biodegradability, biocompatibility, and adjustable viscoelasticity. In accordance with a second aspect of the invention, there is provided a method of forming a composite hydrogel composition comprising the steps of i) mixing together: about 0.1 wt.% to about 20 wt.% of a poloxamer, about 0.5 wt.% to about 10 wt.% of an alginate, 0.5 wt.% to about 10 wt.% of cactus fibres, and water; ii) incubating the mixture to induce sol-gel in the poloxamer; iii) adding a cross-linking agent; and iv) resting the mixture to form the composite hydrogel composition. The method according to the second aspect of the invention can be used to manufacture the composite hydrogel composition according to the first aspect of the invention. The methods of forming composite hydrogel compositions provided herein are simple, efficient, and low cost. In accordance with a third aspect of the invention, there is provided a use of the composite hydrogel composition according to the first aspect of the invention for vibration damping. Brief Description of the Drawings The invention will now be described in detail, by way of example only, with reference to the figures. Figures la to If show the results of quasi-static tests performed on samples of composite hydrogel compositions of the present disclosure with different wt.% of cactus fibre. Figures 2a to 2j show plotted results of vibration testing, specifically dynamic modulus vs. frequency, and loss factor vs. frequency, for samples of composite hydrogel compositions of the present disclosure with different wt.% of cactus fibre. Figures 3a and 3b show the results of vibration testing graphs, specifically dynamic modulus vs. acceleration, and loss factor vs. acceleration for samples of composite hydrogel compositions of the present disclosure with different wt.% of cactus fibre. Description Provided herein is a bio-based composite hydrogel composition that can compete in terms of damping loss factor and stiffness with analogous fossil-based compositions in high energy vibration damping applications. As used herein the term "hydrogel" takes its usual meaning in the art and is used to refer to a three-dimensional hydrophilic or amphiphilic cross-linked network of one or more polymers and a fluid. As used herein, the term "composite hydrogel" is used to refer to a hydrogel material that physically or covalently incorporates particles into a cross-linked network of polymers. The term "composite hydrogel" can be used interchangeably with the term "composite hydrogel composition". As used herein, the term "poloxamer" is used to refer to a triblock copolymer of polyethylene oxide and polypropylene oxide, wherein polypropylene oxide forms a central block flanked by two polyethylene oxide blocks. The hydrophobicity / hydrophilicity properties of the poloxamer may be tuned by varying the molecular weights of each block in the poloxamer. The composite hydrogel composition as described herein comprises about 0.1 wt.% to about 20 wt.% of a poloxamer, based on the total weight of the composite hydrogel composition. The composite hydrogel composition may comprise about 0.1 wt.% to about 15 wt.% of the poloxamer, preferably about 2.5 wt.% to about 13 wt.% of the poloxamer based on the total weight of the composite hydrogel composition. The composite hydrogel composition may comprise about 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, or 13 wt.% of the poloxamer based on the total weight of the composite hydrogel composition. Any suitable poloxamer may be used in the composite hydrogel composition as described herein. Preferably, the poloxamer is Poloxamer 407 or Poloxamer 188. More preferably, the poloxamer is Poloxamer 407. The composite hydrogel composition as described herein comprises about 0.5 wt.% to about 10 wt.% of an alginate based on the total weight of the composite hydrogel composition. The composite hydrogel composition may comprise about 1 wt.% to about 8 wt.% of the alginate, preferably about 3 wt.% to about 6 wt.% of the alginate based on the total weight of the composite hydrogel composition. The composite hydrogel composition may comprise about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, or 8 wt.% of the alginate based on the total weight of the composite hydrogel composition. Alginate is a low-cost natural polymer with low toxicity that may be obtained from algae biomass. Any suitable alginate may be used in the composite hydrogel composition as described herein. Without being bound by theory, it is thought that the molecular weight of the alginate affects the connectivity in the composite hydrogel network. Accordingly, changing the molecular weight of the alginate can result in the ability to tune the dynamic properties of the composite hydrogel composition. Preferably, an alginate with a molecular weight in the range of about 12 kg / mol (kDA) to about 40 kg / mol (kDa) is used, for example sodium alginate which may be derived from brown algae. Alginates with a greater molecular weight can also be used. The composite hydrogel composition as described herein comprises about 0.5 wt.% to about 10 wt.% of cactus fibres based on the total weight of the composite hydrogel composition. Preferably, the composite hydrogel composition may comprise about 1 wt.% to about 5 wt.% of the cactus fibres. The composite hydrogel composition may comprise about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5 wt.% of the cactus fibres. Preferably the composite hydrogel composition may comprise about 1.25 wt.% or 2.5 wt.% of the cactus fibres based on the total weight of the composite hydrogel composition. The cactus fibres may have an average length of about 0.5 pm to about 8 pm, preferably about 1 pm to about 6 pm, and more preferably about 2 pm to about 3 pm. Without being bound by theory, it is thought that the shorter the average length of the cactus fibre, the higher the total surface area of the cactus fibre reinforcement in the composite hydrogel composition. Energy is believed to be dissipated at the gel-fibre interface in the composite hydrogel composition, therefore a shorter average length of cactus fibre provides a higher surface area for energy dissipation. Using cactus fibres with an average length in the preferred range has also been found to be easier for manufacturing. The cactus fibres help to stiffen the composite hydrogel compositions described herein at low dilutions. Preferably the cactus fibres used in the composite hydrogel composition as described herein are Opuntia cactus fibres. More preferably, the cactus fibres may be selected from Opuntia ficus-indica, Opuntia monacantha, Opuntia stricta, Opuntia basilaris, Opuntia engelmannii, Opuntia humifusa or Opuntia phaeacantha. This type of cactus contains racket-shaped fractal fibres which exhibit a slip-stick motion when the fibres move over each other. The synergy between the slip-stick effects of the fractal cactus fibres and the alginate-poloxamer networks generates an increase of the dynamic modulus between about 50 Hz and about 150 Hz under vibration of approximately one order of magnitude. As such, the inclusion of cactus fibres in the hydrogel network leads to a particularly effective vibration damping composition. The composite hydrogel composition as described herein comprises water. The amount of water used makes up the remaining weight of the composite hydrogel composition, such that the total weight of all components is 100 wt.%. Accordingly, the skilled person would be able to calculate the amount of water required based on the total weight of the other components in the composite hydrogel composition. Preferably, the amount of water in the composite hydrogel composition is about 60 wt.% to about 99 wt.%, or about 60 wt.% to about 98.9 wt.% based on the total weight of the composite hydrogel composition. Optionally, the composite hydrogel composition as described herein may comprise about 0.1 wt.% to about 1 wt.% of a reinforcing component based on the total weight of the composite hydrogel composition. As used herein, the term "reinforcing component" relates to an additive, for example in the form of a powder or particle or fibre that provides mechanical reinforcement for the composite hydrogel gel matrix. The reinforcing component may help to enhance the properties of the composite hydrogel composition for example compressive strength and elastic properties, by acting as additional physical cross-linking points. The reinforcing component may be selected from a natural fibre, ceramic, a biopolymer, or a combination thereof. Examples of suitable natural fibres include flax or hemp. Examples of suitable ceramics may include, but are not limited to, nanoclays (such as sepiolite, montmorillonite and palygorskite fibrous clay minerals), microsilica, nanosilica, or combinations thereof. Nanoclays provide good thermal stability and sorption properties. Examples of suitable biopolymers include, but are not limited to, chitosan, collagen, polylactic acid, or combinations thereof. Biopolymers offer advantages such as biodegradability and biocompatibility. Without being bound by theory, it is believed that where a combination of reinforcing components is used, the surfaces of said reinforcing components may need to be treated to prevent them from clustering. For example, surfaces may be subjected to hydrothermal treatment, high-frequency processing or ultrasonic vibrations. The use of naturally occurring ceramics and / or biopolymers such as those described above provide advantages such as greater sustainability due to limited environmental impact during processing, use and disposal, high availability, ease of manufacturing, and low cost compared to synthetic or fossil-based alternatives. Also provided herein is a method of forming a composite hydrogel composition comprising the steps of: i) mixing together: a) about 0.1 wt.% to about 20 wt.% of a poloxamer, b) about 0.5 wt.% to about 10 wt.% of an alginate, c) about 0.5 wt.% to about 10 wt.% of cactus fibres, and d) water; ii) incubating the mixture to induce sol-gel in the poloxamer; iii) adding a cross-linking agent; and iv) resting the mixture to form the composite hydrogel composition. About 0.1 wt.% to about 20 wt.% of a poloxamer may be added, based on the total weight of the composite hydrogel composition. Preferably about 0.1 wt.% to about 15 wt.%, more preferably about 2.5 wt.% to about 13 wt.% of the poloxamer may be added, based on the total weight of the composite hydrogel composition. About 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, or 13 wt.% of the poloxamer may be added based on the total weight of the composite hydrogel composition. As described above, any suitable poloxamer may be used in the composite hydrogel composition as described herein. Preferably, the poloxamer is Poloxamer 407 or Poloxamer 188. Most preferably, the poloxamer is Poloxamer 407. About 0.5 wt.% to about 10 wt.% of an alginate may be added, based on the total weight of the composite hydrogel composition. Preferably, about 1 wt.% to about 8 wt.% of the alginate, and more preferably about 3 wt.% to about 6 wt.% of the alginate may be added, based on the total weight of the composite hydrogel composition. About 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, or 8 wt.% of the alginate may be added based on the total weight of the composite hydrogel composition. As described above, any suitable alginate may be used in the composite hydrogel composition as described herein. Preferably, an alginate with a molecular weight in the range of about 12 kg / mol (12 kDa) to about 40 kg / mol (40 kDa) may be used, for example sodium alginate which may be derived from brown algae. Alginates with a greater molecular weight may also be used. About 0.5 wt.% to about 10 wt.%, preferably about 1 wt.% to about 5 wt.% of cactus fibres may be added, based on the total weight of the composite hydrogel composition. About 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5 wt.% of the cactus fibres may be added, based on the total weight of the composite hydrogel composition. Preferably about 1.25 wt.% or 2.5 wt.% of the cactus fibres may be added, based on the total weight of the composite hydrogel composition. As described above, the cactus fibres may have an average length of about 0.5 pm to about 8 pm, preferably about 1 pm to about 6 pm, and more preferably about 2 pm to about 3 pm. Preferably, the cactus fibres used in the composite hydrogel composition as described herein are Opuntia cactus fibres. The composite hydrogel composition as described herein comprises water. The amount of water used makes up the remaining weight of the composite hydrogel composition, such that the total weight of all components is 100 wt.%. Accordingly, the skilled person would be able to calculate the amount of water required based on the total weight of the other components in the composite hydrogel composition. Preferably, the amount of water in the composite hydrogel composition is about 60 wt.% to about 99 wt.% or about 60 wt.% to about 98.9 wt.% based on the total weight of the composite hydrogel composition. The mixture is incubated to induce sol-gel in the poloxamer. As used herein, the term "sol-gel" is intended to take its usual meaning in the art and refers to a wetchemical process that involves the formation of monomers into a colloidal suspension (sol) and gelation of the sol in a continuous liquid phase (gel) to form a three-dimensional network structure of polymers or discrete particles. The temperature at which the incubation step is performed is dependent on the sol-gel transition temperature of the poloxamer added to the mixture to form the final composite hydrogel composition. As used herein, the term "sol-gel transition temperature" is used to refer to the temperature at which a liquid phase (sol) makes a transition to a gel phase. The sol-gel transition temperature will change depending on the type and molecular weight of the poloxamer. The skilled person will be able to determine the sol-gel transition temperature using standard methods in the art. For example, the sol-gel transition temperature for Poloxamer 407 is about 31 °C to about 36 °C and the sol-gel transition temperature for Poloxamer 188 is about 21 °C to about 30 °C. Preferably, incubation is performed at about 15 °C to about 40 °C, more preferably about 37 °C, for between about 4 hours and 16 hours. Suitable cross-linking agents that may be added to the mixture include, but are not limited to, calcium-based cross-linking agents such as calcium chloride (CaCIz) or calcium carbonate (CaCOs) or calcium sulfate (CaSO4), aluminium-based crosslinking agents, for example aluminium chloride (AlCh), iron-based cross-linking agents, for example iron chloride (FeCh) or iron sulfate (FeSO4), barium-based cross-linking agents, for example barium chloride (BaCIz), manganese-based cross-linking agents, for example manganese chloride (MnCIz), zinc-based crosslinking agents, for example zinc chloride (ZnCIz), or copper-based cross-linking agents, for example copper sulfate (CuSO4). Preferably, the cross-linking agent is calcium-based. More preferably, the crosslinking agent is calcium chloride (CaCIz) or calcium sulfate (CaSO4). Calcium sulfate (CaSO4) can be used to control the rate of cross-linking by slow release, which is particularly useful when the method of forming the composite hydrogel composition is performed on a larger scale. Without being bound by theory, it is believed that changing the cross-linking agent may provide a way to tailor the properties of the composite hydrogel composition, due to variations in the binding strength between different crosslinking agents and the alginate in the polymeric network. Optionally, the gel can be allowed to cool below the sol-gel transition temperature, for example to room temperature, following the incubation step, for example in the cross-linking step or in the resting step. Optionally, about 0.1 wt.% to about 1 wt.% of a reinforcing component based on the total weight of the composite hydrogel composition may be added to the mixture in step i) of the method described herein. As described above, the reinforcing component may be selected from a ceramic or a biopolymer, or a combination thereof. Examples of suitable ceramics may include, but are not limited to, nanoclays (such as sepiolite and montmorillonite), microsilica, nanosilica, or combinations thereof. Examples of suitable biopolymers include, but are not limited to, chitosan, collagen, polylactic acid, or combinations thereof. Finally, the mixture may be rested for about 30 minutes to about 18 hours, preferably about 4 hours to about 18 hours, depending on the thickness and composition of the final composite hydrogel composition. The skilled person would be able to select an appropriate time based on the desired final product. Optionally, the composite hydrogel composition may be formed during manufacture into a desired shape and / or size through the use of techniques such as dialysis or 3D printing. Here, any appropriate apparatus for dialysis or 3D printing may be employed. It will be understood that an appropriate technique may be selected based on the desired shape / size of the composite hydrogel composition. For example, where a dialysis technique is used, the components of the composite hydrogel composition may be mixed together as described above (for example, via dual asymmetrical centrifugation, or DAC) and added to a dialysis membrane containing a mould. The mixture may be incubated in said membrane to induce the sol-gel transition in the poloxamer before the cross-linking agent is added and the mixture is rested. The membrane and mould may then be removed from the final composite hydrogel composition and may be washed and or incubated in water to remove any residual poloxamer. An advantage of using the dialysis technique is that the process is scalable in terms of volume production and so larger batches of composite hydrogel can be manufactured. Alternatively, where a 3D printing technique is used, the components of the composite hydrogel composition may be mixed together, as described above (for example, via dual asymmetrical centrifugation, or DAC). A cross-linking agent may be added to increase the viscosity of the mixture before incubation of the mixture to induce sol-gel in the poloxamer. The mixture may then be 3D printed in a desired shape before being submerged in a solution comprising a crosslinking agent and then rested to form the final composite hydrogel composition. For example, a method of forming a composite hydrogel composition optionally using 3D printing may comprise the steps of: i) Mixing together: a) about 0.1 wt.% to about 20 wt.% of a poloxamer, preferably about 5 wt.% to about 20 wt.%, more preferably 10 wt.% to about 15 wt.%, most preferably about 13 wt.% based on the total weight of the composite hydrogel composition, b) about 0.5 wt.% to about 10 wt.% of an alginate, preferably about 2.5 wt.% to about 8 wt.%, more preferably about 3.5 wt.% to about 7 wt.%, most preferably about 6 wt.% based on the total weight of the composite hydrogel composition, c) about 0.5 wt.% to about 10 wt.% of cactus fibres, preferably about 1 wt.% to about 5 wt.% based on the total weight of the composite hydrogel composition, d) about 0.1 wt.% to about 0.5 wt.% of a cross-linking agent, preferably about 0.3 wt.% based on the total weight of the composite hydrogel composition and e) water ii) Incubating the mixture to induce sol-gel in the poloxamer; iii) 3D printing the gel; and iv) Submerging the gel mixture in a crosslinking agent solution to form the composite hydrogel composition. A cross-linking agent may be added before the 3D printing step to increase the viscosity of the mixture for easier 3D printing. The mixture is not fully crosslinked at this stage. Accordingly, further cross-linking agent is added to the gel mixture, for example the gel mixture may be submerged, soaked, or bathed in a solution of said further cross-linking agent after 3D printing to fully cross-link the composite hydrogel composition, as described above. Preferably, the concentration of the cross-linking solution is about 300 mM to about 700 mM, more preferably 500 mM. The composite hydrogel compositions as described herein may be used for vibration damping in a range of applications. For example, the composite hydrogel compositions may be included in panels on a building or structure or on the frame of a vehicle such as an aircraft, a helicopter, a car, or a ship. Alternatively, the compositions described herein may be included in engines, stators, generators, gearboxes and the like in vehicles, machines, or household appliances such as washing machines. Preferred uses of the composite hydrogel compositions include vibration damping in an aeroplane, a helicopter, a ship, in machinery, or in surface controls of a vehicle. The composite hydrogel compositions may be used as a filler or as part of a damping device or a dashpot, a constrained layer damper, a suspension damper, a tuned viscoelastic damper, or a tubed viscoelastic damper and the like. In the discussion of the present disclosure, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter. In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about" which may be defined as + / - 5%, preferably + / - 2% or more preferably + / - 1%, or + / --5 °C, preferably + / - 2 °C or more preferably + / - 1 °C. Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art. In order that the invention may be more readily understood, it will be described further with reference to the specific examples hereinafter. Examples Example 1 A 50 g gel at 2.5 wt.% cactus fibre, 5 wt.% alginate and 2.5 wt.% Poloxamer was made by adding 1.25 g of cactus fibres, 2.5g of alginate and 1.25 g of Poloxamer with 46.25 g of water. This solution was mixed via dual-asymmetric centrifugation (DAC) then poured into a semi-permeable dialysis membrane containing a plastic mould and incubated in a 2 L beaker of water at 37 °C for 1 hour. The beaker was removed from the incubator and placed onto a magnetic stirrer at 300 rpm, then 147.01 g of CaCh was added to the stirring solution. After 18 hours, the mould was removed from the membrane and the gel cut to liberate the final gel sample. These final shapes were then incubated in water for 24 hours, changing the water once, to removed residual poloxamer. Example 2 A concentrated stock of poloxamer (Stock A) is prepared at 29.95 wt.% poloxamer, for example, by dissolving 29.95 g of poloxamer in 70.05 g water and stored at 4°C. A second stock (Stock B) is made using Stock A and alginate powder. For example, a 50 g Stock B is made by mixing 33 g of Stock A, 4.5 g of alginate powder, and 12.5 g water via dual-asymmetric centrifugation (DAC), giving final concentrations of 19.95 wt.% poloxamer and 9 wt.% alginate. Stock B is stored at 4°C for several weeks. To prepare a final gel solution, 6.67 g of Stock B is mixed with cactus fibre and water (2.33 g minus the mass of cactus fibre) and 1 mL (equal to 1 g) of 200 mM calcium chloride (CaCh), to give final concentrations of 6 wt.% alginate, 13 wt.% poloxamer, 0-5 wt.% cactus fibre, and 20 mM CaCb. The 20 mM CaCh acts as a mild pre-cross-linker to aid in 3D bioprinting. For example, a 10 g gel with 1.25 wt.% cactus fibre is prepared by mixing 6.67 g (6670 mg) Stock B with 125 mg cactus fibre and 2205 mg of water and 1000 mg 200 mM CaCb. The pre-gel is mixed via DAC and transferred to a 10 mL syringe and mixed further by passing back and forth between this and another syringe. The solution is then transferred to 3 cc print cartridge and incubated at 37°C to induce a sol-gel phase transition of the poloxamer. The bioprinter print head and print bed were warmed to 37°C also. After printing, the 3D structure was submerged in 500 mM CaCb to cross-link overnight. Example 3 The Young's modulus, ultimate tensile strength, toughness, and maximum inplane shear stress were measured for composite hydrogel composition samples prepared according to Example 1 or 2 with varying amounts of cactus fibre content (0, 1.25, 2.5, and 5.0 wt.%), as described below. Dog-bone shaped structures with two layers were printed with a gauge length of 15 mm and width 6 mm, and were loaded on to a tensile tester via clamps and stretched at 1 mm / min until the samples failed (e.g. snapped). Stress and strain were recorded. The gradient of the linear region was calculated to contain the Young's Modulus. The ultimate tensile strength was calculated from the peak stress, and toughness calculated from the area under the curve. Maximum inplane shear stress was calculated from 2-layer dog-bone structures printed at ±45° as per ASTM D3518 / D3518M. The gauge was 25 mm long and 5 mm in width. The results of the tests are displayed in the graphs in Figures la to If. Figure la) shows that stiffness in compression increased with the amount of cactus fibre (**, P=0.0021, for all cactus wt.% amounts). Figure lb) shows that printed samples were anisotropic, as they were stiffer in the XY-direction vs the Z-direction, owing to fibre alignment during printing. Figure 1c) shows that for Young's modulus via tensile testing, the sample with 1.25 wt.% cactus fibre stiffened the hydrogel sample the most (*, P=0.0211 for 0 wt.% and 1.25 wt.% cactus fibres), whereas the sample with 5.0 wt.% cactus fibre was more brittle (***, P=0.0002 for all cactus wt.% amounts). Figure Id) shows ultimate tensile strength calculated from the tensile tests. No significant decrease was observed until 2.5 wt.% cactus fibre (**, P=0.001 for 0 wt.%, 1.25 wt.%, 2.5 wt.% cactus fibre) and 5.0 wt.% cactus fibre (****, P<0.0001 for all cactus wt.% amounts) loading, indicating these composite hydrogel samples were more brittle. Figure le) shows a similar trend to Figure Id) but with respect to toughness (*, P=0.243 for 0 wt.% and 1.25 wt.% cactus fibre, ***, P=0.0005 for 0 wt.%, 1.25 wt.% and 2.5 wt.% cactus fibre, ***, P=0.0003 for all cactus wt.% amounts). Figure If) shows that maximum in-plane shear stress significantly increased for the sample containing 1.25 wt.% cactus fibre compared to the control (*, P= 0.0334 for 0 wt.% and 1.25 wt.% cactus fibre). All two-tailed P values calculated with an unpaired T-test with Welch's correction. Overall, it can be seen that compressive stiffness was maximised when the composite hydrogel composition contained 5.0 wt% of cactus fibres, and ultimate tensile strength and shear stress properties were maximized when the composite hydrogel composition contained 1.25 wt% of cactus fibres. Example 4 Vibration transmissibility testing was performed based on ISO 13753 and modified to account for soft and porous materials. Parametric vibration tests against the wt% of the cactus fibres and the effect of the base acceleration over different top masses from 26.7g to 48.9g have been conducted, as described below. Samples were loaded onto a baseplate on an electrodynamic shaker, between the baseplate and a top mass. The top masses (26.7 to 48.9 g) were used to pre-compress the samples, which were subjected to white noise vibration at five acceleration rates (0.6 to 2.8 ms'2). Resonant frequencies were determined and used to calculate dynamic modulus and loss factor. The loss factors for all the gels are between ~20% and 25%. These results are shown in the graphs of Figures 2a to 2j. Figure 2a) shows dynamic modulus vs. frequency at an 0.6 ms-2. Figure 2b) shows dynamic modulus vs. frequency at an 1 ms-2. Figure 2c) shows dynamic modulus vs. frequency at an 1.5 ms'2, Figure 2d) shows dynamic modulus vs. frequency at an 2.2 ms’2. Figure 2e) shows dynamic modulus vs. frequency at an acceleration rate of acceleration rate of acceleration rate of acceleration rate of acceleration rate of 2.8 ms’2. Figure 2f) shows loss factor vs. frequency at an acceleration rate of 0.6 ms’2. Figure 2g) shows loss factor vs. frequency at an acceleration rate of 1 ms’2. Figure 2h) shows loss factor vs. frequency at an acceleration rate of 1.5 ms’2 Figure 2i) shows loss factor vs. frequency at an acceleration rate of 2.2 ms’2. Figure 2j) shows loss factor vs. frequency at an acceleration rate of 2.8 ms’2. It was found that the presence of cactus fibres more than doubles the dynamic modulus of the hydrogels at frequencies above 120 Hz (from ~ 500 kPa to 1 to 1.2 MPa). In addition, the samples containing 1.25 wt.% cactus fibres had the greatest dynamic modulus across all acceleration rates. It was observed that there is a marked improvement in dynamic modulus and loss factors in terms of sensitivity to base acceleration compared to the control sample. All gels with cactus fibre dispersions show relatively constant modulus and loss factors as base acceleration increases from 0.6 ms’2 to 2.8 ms’2. Notably, significant increases in dynamic modulus and loss factor can be achieved with low dilutions of 1.25 wt.% cactus fibres. Larger wt.% of cactus fibres (especially at 5.0 wt.%) lead to a decrease in modulus and loss factor (although this was still better than the 0 wt.% control). Large weight fractions of powder-type reinforcements are known to generate clusters in soft matrix composites, which decrease load transfer capability and worsen mechanical performance. On the other hand, small dilutions that provide a significant improvement compared to the control material are positive for the manufacturing and design of larger scale samples. These results are further illustrated in the graphs of Figures 3a and 3b. 5 It will be appreciated that the compositions and methods of the invention are capable of being implemented in a variety of ways, only a few of which have been illustrated and described above.

Claims

1. A composite hydrogel composition comprising:a) about 0.1 wt.% to about 20 wt.% of a poloxamer;b) about 0.5 wt.% to about 10 wt.% of an alginate;c) about 0.5 wt.% to about 10 wt.% of cactus fibres; andd) water.

2. The composite hydrogel of claim 1 comprising about 0.1 wt.% to about 15 wt.% of the poloxamer, preferably about 2.5 wt.% to about 13 wt.% of the poloxamer.

3. The composite hydrogel composition of claim 1 or 2 comprising about 1 wt.% to about 8 wt.% of the alginate, preferably about 3 wt.% to about 6 wt.% of the alginate.

4. The composite hydrogel composition of any preceding claim comprising about 1 wt.% to about 5 wt.% of the cactus fibres.

5. The composite hydrogel composition of any preceding claim, wherein thecactus fibres have an average length of about 0.5 pm to about 8 pm, preferablyabout 1 pm to about 6 pm, and more preferably about 2 pm to about 3 pm.

6. The composite hydrogel composition of any preceding claim, wherein thecactus fibres are Opuntia cactus fibres.

7. The composite hydrogel of any preceding claim, further comprising about 0.1 wt.% to about 1 wt.% of a reinforcing component.

8. The composite hydrogel of claim 7, wherein the reinforcing component is selected from chitosan, microsilica, nanosilica, montmorillonite, palygorskite, sepiolite or combinations thereof.

9. A method of forming a composite hydrogel composition comprising the steps of:i) mixing together:a) about 0.1 wt.% to about 20 wt.% of a poloxamer,b) about 0.5 wt.% to about 10 wt.% of an alginate,c) about 0.5 wt.% to about 10 wt.% of cactus fibres, andd) water;ii) incubating the mixture to induce sol-gel in the poloxamer;iii) adding a cross-linking agent; andiv) resting the mixture to form the composite hydrogel composition.

10. The method of claim 9, wherein the cross-linking agent is a calcium-based cross-linking agent, preferably calcium chloride.

11. The method of claims 9 or 10, wherein the incubating step is performed at about 15 °C to about 40 °C.

12. The method of claims 9-11, further comprising adding about 0.1 wt.% to about 1 wt.% of a reinforcing component in step i).

13. The method of claims 9-12, further comprising adding a cross-linking agent in step i), and 3D-printing the mixture after step ii).

14. Use of the composite hydrogel composition of claims 1-8 for vibration damping.

15. The use of claim 14 in an aeroengine, a helicopter, a ship hull, in a machinery or in surface controls of a vehicle.

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