NO-generating material

Abstract

Amino-functionalised graphene oxide (GO) for use in the generation of nitric oxide (NO) in vivo. A coating for an implantable medical device comprising amino-functionalised GO, a coated implantable me

Description

FIELD OF THE INVENTION The invention relates to an amino-functionalised graphene oxide, for use in the generation of NO in vivo. The invention also relates to coatings comprising an aminofunctionalised graphene oxide; devices coated in an amino-functionalised graphene oxide; coating solutions; methods of coating a device; and an amino-functionalised graphene oxide. BACKGROUND TO THE INVENTION Despite the increasing use and significant advances made in interventional and surgical procedures, cardiovascular disease (CVD) has remained a major cause of morbidity and mortality globally for decades (Roth et al, J. Am. Coll Cardiol 2017). The ageing population in both developed and developing countries has also led to a significant rise in the prevalence of cardiovascular diseases. The endothelium plays a crucial role in maintaining vessel homeostasis, regulating the delicately balanced processes of vascular tone and platelet activation. Vessel homeostasis is maintained by the release of various vasoactive factors from the endothelium. Vasoactive factors can be vasodilatory including nitric oxide (NO), prostacyclin, endothelium-derived hyperpolarising factor, or vasoconstrictive including thromboxane A (Vanhoutte et al, Circ Res 2017), and endothelin-1 (Vanhoutte et al, Circ Res 2017; Vanhoutte et al, Acta Physiol 2017). NO is one of the most important vasodilatory factors in coronary endothelium and is constitutively generated through the conversion of L-arginine and molecular oxygen to NO and L-citrulline catalysed by the enzyme nitric oxide synthase (NOS). The three distinct isoforms of NOS in mammals are neuronal (nNOS), inducible (iNOS) and endothelial NOS (eNOS) (Lundberg et al, Cell, ICIT). In the vasculature, eNOS in endothelial cells is the primary source of NO and is a crucial regulator of blood pressure, cellular proliferation and vascular tone. NO continuously diffuses to vascular smooth muscle cells (VSMCs) where it stimulates soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), thereby activating protein kinase G (PKG), which then phosphorylates multiple target proteins resulting in smooth muscle relaxation and vasodilation (Vanhoutte et al, Circ Res 2017; Lundberg et al, Cell, 2022). nNOS, the main endogenous source of myocardial NO regulates cardiac inotropy and relaxation, modulates intracellular Ca2+ homeostasis, and signalling pathways including nitroso-redox balance (Simon et al, J Mol Cell Cardiol, 2014). Hence it is no surprise that low bioavailability of NO critically drives the progression of CVD. NO down-regulates the expression of the prothrombotic protein tissue factor and inhibits the gene expression of adhesive proteins by endothelial cells, thereby exerting antiinflammatory effects by inhibiting the adhesion of leukocytes. NO also inhibits mast cell activation and mast cell-dependent inflammatory events that contribute to destabilisation of the atherosclerotic plaque (Forsythe etal, Int Immunopharmacol, 2001; Galkina and Ley; Immunol, 2009). NO has antiproliferative effects on SMCs and therefore prevents neointima hyperplasia in response to injury (Tsihlis et al, Cell Biochem Biophys, 2011). Thus, NO has wide ranging vasoprotective, anti-atherosclerotic and antithrombotic roles (Tsihlis et al, Cell Biochem Biophys, 2011). Drugs which may release or produce NO locally represent a significant breakthrough for the control and treatment of C VD However, their overall success is limited by the physicochemical properties of NO, which as a diatomic gaseous molecule with one unpaired electron, is highly reactive with free radical oxygen centred species (Carpenter etal, Chern Soc Rev, 2012). Thus, NO has a high diffusion rate, but a short biological half-life (in the order of 0.5 - 5 seconds), thereby limiting the diffusion distance to around 200 um. NO can be rapidly oxidised to highly toxic nitrogen dioxide in the presence of molecular oxygen. Due to such limitations, there is a growing interest in developing novel formats for the controlled, predictable, and targeted release of NO. Over the last few decades, different NO delivery systems have been devised and investigated in the form of NO donors (such as organic nitrates, nitrites, thionitrites, S'-nitrosothiols (RSNOs), jV-diazeniumdiolates (NONOates), metal-NO complexes (Broniowska and Hogg, Antioxid Redox Signal, 2012; Li and Li, Cell Biochem Biophys, 2011), NO-releasing non-steroidal anti-inflammatory drugs (NO-NSAIDs) (Burgaud et al, Ann N YAcadSci, 2002), small molecule gas-releasing prodrugs (Riccio and Schoenfisch, Chern Soc Rev, 2012), peptides (El-Ferzli et al, ASAIO J, 2015), proteins (Li et al, ACS ApplMater Interfaces, 2017), polymeric membranes (Jin et al, Adv Heathc Mater, 2021), hydrogels (Zhang etal, Acta Biomater, 2020), scaffolds, and nanoparticles (silica, gold, liposomes, dendrimers etc) (Ghalei et al, J Colloid Interface Sci, 2021), to store and release NO in a controlled and therapeutically meaningful manner. Coronary artery disease is characterised by the accumulation of atherosclerotic plaque, which causes the blood vessels supplying the heart to gradually become occluded and accounts for an estimated 17.9 million deaths annually, 31% of all global deaths (Roth et al, J Am Coll Cardiol, 2017). The standard treatment consists of angioplasty to clear the blockage, followed by implantation of a stent, an expandable tubular mesh or scaffold, which keeps the blood vessel patent during the healing process (Giustino et al, J Am Coll Cardiol, 2022). Originally, stents were made from bare metal (BMS), but these were associated with a high incidence of restenosis, a re-narrowing of the injured artery, driven by damaged endothelial cells, uncontrolled smooth muscle cell (SMC) proliferation, and a persistent inflammatory response (Liu et al, Curr Drug Deliv, 2021). This situation led to the introduction of drugeluting stents (DES), whereby a metal stent is coated with biodegradable polymers for the local delivery of antiproliferative drugs (e.g., paclitaxel or sirolimus), which reduce the likelihood of restenosis and have become the gold standard in percutaneous coronary intervention (Gori et al, Life (Basel), 2021). However, the use of antiproliferative drugs can delay the growth and migration of endothelial cells (re-endothelialisation), thereby prolonging chronic inflammation and the vascular healing process, which in turn increases the incidence of late-onset clotting (thrombosis) and neoatherosclerosis (Gori et al, Life (Basel), 2021). As such, an ideal stent would release a compound that inhibits SMC proliferation while simultaneously promoting endothelial cell (EC) survival, preventing thrombosis, and dampening the inflammatory response. Normal healthy vascular ECs continuously generate nitric oxide (NO) via the enzymatic conversion of L-arginine by NO synthase (NOS) (Vanhoutte et al, Circ Res, 2016; Vanhoutte etal, Acta Physiol, 2017). NO has many crucial roles in maintaining vascular homeostasis; for example, it is an important vasodilator and inhibits the activation of platelets and inflammatory cells to prevent thrombosis and local inflammatory responses. Furthermore, it promotes EC survival and inhibits SMC proliferation (Lundberg and Weitzberg, Cell 2022); hence, a NO-eluting stent would theoretically realise all of the ideal properties required to promote vascular healing and prevent restenosis. To this end, several approaches have been investigated to provide controlled and sustained NO delivery (Zhou etal, Adv Drug Deliv Rev, 2021). Non-catalytic approaches involve the attachment of exogenous NO donors such as NO gas, diazeniumdiolates (NONOate), and S-nitrothiols (RSNO), but are de facto constrained by the limited pool of conjugated NO donors to draw upon. Moreover, the short therapeutic half-3 lives of many NO donors limit their use for long-term vascular implants (Lei et al. Nitric Oxide, 2013). In the alternative, catalytic approach, materials are engineered to decompose endogenous NO sources into NO (e.g., RSNOs, such as S-nitrosoglutathione [GSNO], S-nitrosocysteine, and 5-nitrosoalbumin that are naturally replenished in vivo). As such, catalyst materials may provide the advantage of continuous and localised production of NO and show superior catalytic activity by tailoring the appropriate ratios and concentrations of both the catalyst materials and NO sources to achieve physiologically relevant amounts of NO (Luo et al, Small, 2023). Three classes of catalysts have been reported in the literature to generate NO from 2+ endogenous sources. Firstly, metallic ions such as, mercury (Hg ) (Keszler et al, Nitric + 2+ Oxide, 2017), gold (Au ) (Jia et al, J Am Chern Soc, 2009) and particularly copper (Cu ) (Thai et al, ACS Applied Nano Materials, 2022; Zhou et al, Journal of Materials Chemistry, 2021) have been reported to decompose RSNO into NO. For example, the glutamate residue 2+ in GSNO binds with Cu ions to facilitate NO production; however, this reaction is impeded by the accumulation of oxidised glutathione (GSSG), indicating that the catalytic capacity of copper is negatively influenced by the presence of disulfide bonds (Noble et al, Nitric Oxide, 2000). Furthermore, copper leaching results in decreased NO generation by up to 50% over 15 days (Liu et al, jMater Chern, 2012; Garren et al, ACS ApplMater Interfaces, 2021). The second class of catalyst includes group 16 elements, including selenium (Fan et al, Colloids Surf B Biointerfaces, 2016) and tellurium (Hwang and Meyerhoff, Journal of Materials Chemistry 2008); for example, organoselenium immobilised to PEI has been shown to effectively catalyse the decomposition of GSNO to NO in the presence of free thiols such as glutathione (Yang et al, Langmuir, 2008); however, leaching of selenium and deactivation of the catalyst are potential limitations. The third class is small nucleophiles such as hydrogen peroxide (Coupe et al, Perkin Transactions, 1999; van der Vliet et al, J Biol Chern, 1998), hydrazine (Munro et al, Perkin Transactions, 1999) (undesirable due to potential toxicity), and ascorbic acid (Hasanzadeh et al, J Mater Chern B, 2016), which can attack the electrophile nitroso groups of RSNO. Each of these catalysts have drawbacks, particularly in terms of their stability in vivo, biocompatibility, and ease of manufacture. Thus there remains a need for materials which can catalyse the production of NO from endogenous sources, but which are compatible with in vivo usage in terms of their stability and toxicity. SUMMARY OF THE INVENTION The present inventors have found a novel catalytic NO-generating strategy using aminofunctionalised graphene oxide. Graphene oxide (GO) possesses a number of desirable properties, including high hydrophilicity, excellent stability in solvents and biological buffers, and ease of functionalisation owing to the existence of oxygen-containing functional groups on its surface (e.g., carboxyl, hydroxyl, and epoxide). The present inventors have functionalised GO with amino groups and surprisingly found that amino-functionalised GO acts as a very effective catalyst to decompose endogenous NO sources, including S-nitrosothiols, to generate therapeutic amounts of NO. It is thought that the nucleophilic amino groups of the amino-functionalised GO react with the nitroso groups of GSNO and RSNO leading to the release of NO as a leaving group. The present inventors have also found that amino-functionalised GO can be used to uniformly coat articles (such as three-dimensional (3D)-printed bioresorbable stent material, e.g. polylactic acid). Furthermore, the coating may be applied via a simple dip coating method without grossly affecting biodegradability. Thus the present invention provides amino-functionalised graphene oxide, for use in the generation of NO in vivo. The invention also provides a coating for an implantable medical device which coating comprises amino-functionalised graphene oxide. Also provided by the invention is a coated implantable medical device which comprises: (a) an implantable medical device, and (b) a coating on a surface of said implantable medical device, wherein said coating comprises amino-functionalised graphene oxide. The invention also provides a coating solution comprising an organic solvent, a polymer comprising amino groups, and graphene oxide. Also provided by the invention is a process for producing a coated implantable medical device, wherein the process comprises disposing a coating solution as described herein on an implantable medical device. The invention also provides amino-functionalised graphene oxide, which comprises: graphene oxide; a first polymer which is a polymer comprising amino groups; and a second polymer which is a hydrophilic polymer; wherein the percentage by weight of the first polymer is from 8% to 35%, based on the total weight of the amino-functionalised graphene oxide. Also provided by the invention is an implantable medical device functionalised with amino groups, for use in producing NO in vivo. The invention also provides a method of generating NO in vivo wherein said method comprises administering amino-functionalised graphene oxide to a subject. The invention also provides a method of generating NO in vivo wherein said method comprises implanting into a subject an implantable medical device functionalised with amino groups. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a schematic representation of the design and synthesis of functionalised GO conjugate using a N-(3-Dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC) crosslinking approach. Figure 2 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using transmission electron microscopy images. Figure 3 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using x-ray diffraction. Figure 4 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using Raman spectroscopy. Figure 5 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using Fourier-transform infrared spectra. Figure 6 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using thermogravimetric analysis (TGA). Figure 7 shows basic characterisation of GO, PEI@GO and PEI-PEG@GO using water contact angle measurements. Figure 8 shows quantification of NO release from GSNO (10 pM) (top / right hand graph) and SNAP (10 pM) (bottom / left hand graph) with and without different formulations using a NO electrode sensor under physiological conditions. (A) NO release measurements from GSNO alone and GSNO in the presence of different formulations . (n = 3 independent samples). (B) NO release measurements from SNAP alone and SNAP in the presence of different formulations, (n = 3 independent samples). Figure 9 shows catalytic generation of nitric oxide (NO) from GSNO and SNAP using PEI, PEG, and PEI-PEG@GO at different concentrations in PBS buffer (pH 7.4) in the presence of EDTA using chemiluminescence NO analyser (A) Representative NO release profile from PEI and PEG without GSNO and SNAP. (B) Representative NO release profile from PEI@GO and PEI-PEG@GO without GSNO and SNAP. (C) Representative NO release profile from GSNO and PEI-PEG@GO+GSNO and (D) Representative NO release profile from SNAP and PEI-PEG@GO+GSNO. The arrow shows the injection of compounds. Time on the x-axis represents the time since the start of the experiment rather than the rate of NO production; hence, the NO electrode was utilised for real-time NO production as shown in Figure 8. Figure 10 shows characterisation of coating of PEI-PEG@GO on stent material in photographs of 3D printed stents. Figure 11 shows characterisation of coating of PEI-PEG@GO on stent material in SEM micrographs of coated and uncoated stents at different magnifications under different conditions Figure 12 shows characterisation of coating of PEI-PEG@GO on stent material in Raman spectra of uncoated and coated stents using different combinations of coating solutions. Three random fields of view were chosen with ~18 random points mapping per field view along with ~54 points per stent. The optimized coating solution (PEG:PCL 1:1 and 10 wt % GO) was prepared after screening different ratios of PEG and PCL in THF with PEI-PEG@GO. Each spectrum in this panel represents characterization at a spatially distinct sampling point. Figure 13 shows characterisation of coating of PEI-PEG@GO on stent material in FTIR spectra of uncoated and coated stent under different conditions. The characteristic FTIR bands are shown along with region names. Figure 14 shows accelerated hydrolytic degradation of coated and uncoated stent material at elevated temperature, and in particular, weight loss of stents as a function of time in PBS (pH 7.4) at 60 °C for 60 days. All the data are shown as mean ± SD, n = 3. Figure 15 shows accelerated hydrolytic degradation of coated and uncoated stent material at elevated temperature, and in particular, the stability of amine groups on stents over 60 days under hydrolytic degradation conditions is presented in Figure 15. The stability of amine groups was measured using a standard colorimetric assay based on Orange acid II (OAII). All the data are shown as mean ± SD, n = 2. Figure 16 shows accelerated hydrolytic degradation of coated and uncoated stent material at elevated temperature, and in particular, SEM micrographs of the surface of stent material at different time points of the hydrolytic degradation experiments at 60 °C in PBS (pH 7.4). Figure 17 shows shelf-life test to investigate the stability of amine groups on stents for 60 days. All the data shown as mean ± SD, n = 3. One-way ANOVA test was used to calculate statistical significance. DETAILED DESCRIPTION OF THE INVENTION Definitions An amino-functionalised graphene oxide as referred to herein means a graphene oxide functionalised with amino groups wherein each amino group is attached directly or indirectly (e.g. via one or more polymers, or via a linker group) to the graphene oxide. An amino-functionalised graphene oxide as referred to herein comprises a plurality of amino groups. When these amino groups are bonded indirectly to the graphene oxide, this may mean that the amino groups are bonded to, or part of, a further material (e g. a polymer) which itself is bonded to the graphene oxide (GO). The further material may be bonded to the GO directly or indirectly. When the further material is bonded to GO indirectly it may be bonded to a second material, such as a second polymer, which second material is itself attached to GO. Typically the second material is attached directly to GO. Alternatively, an amino group may for instance be bonded indirectly to GO by being attached to a linker group, which linker group is itself attached directly to the GO. When amino groups are bonded directly to GO, they are usually bonded to GO via one of the oxygen-containing functional groups on the surface of the GO, for instance they may be bonded to the carbon atom of a carbonyl group on the surface of GO. Typically, the amino-functionalised GO is GO wherein each of the plurality of amino groups is indirectly bonded to GO. Typically, each amino group is part of a polymer which is itself bonded to GO. The polymer may be bonded to GO directly or indirectly. Typically, the polymer is bonded to GO indirectly. For instance, the polymer is often bonded to a second material, for instance a second polymer, which second material is itself bonded to the GO. The second material is often, but not necessarily, bonded directly to the GO. Generation of NO in vivo as referred to herein means the local generation of NO in vivo, for example from endogenous substrates. An endogenous substrate has its usual meaning in the art. This means an endogenous substrate should be understood as a substrate having originated in vivo, i.e. the substrate should not have to be administered to the body. Examples of endogenous substrates include S-nitrosothiols (SNOs) such as S-nitrosoglutathione (GSNO), S-nitrosocysteine and S-nitrosoalbumin. S-nitroso-N-acetylpenicillamine (SNAP) is an example of non-endogenous S-nitrosothiol. While the amino-functionalised GO of the invention provides a catalyst that is compatible with endogenous sources, in some embodiments the coatings, solutions, devices and materials of the invention may comprises further NO sources (e.g. exogenous NO sources) which may release exogenous NO in vivo. For example, the coatings, solutions, devices and materials of the invention may further comprise NO gas, diazeniumdiolates and S-nitrothiols. In particular, an implantable medical device may comprise a biodegradable material which comprises such sources, and thus releases NO as it degrades. An amino group as referred to herein takes its usual meaning in the art. In particular, an amino group refers to an -NH2 group, and also encompasses -NFh groups. When a structure is displayed herein with an -NH2 group, it should be understood that this may also be an -NHs+ group. For example, each amino group may become protonated (from -NH2 to -NH3 ) or deprotonated (from -NH3+ to -NH2) depending on pH conditions. A polymer, as described herein, is a molecule with at least three repeating units. The polymer may for example have from 3 to 1,000,000 repeating units, for instance from 3 to 100,000 repeating units, or from 3 to 10,000 repeating units, for example from 3 to 1000 repeating units, such as from 3 to 100 repeating units or from 3 to 40 repeating units, for instance from 3 to 10 repeating units. It may for instance have from 20 to 1,000,000 repeating units, for instance from 30 to 100,000 repeating units, or from 50 to 10,000 repeating units, for example from 50 to 1000 repeating units. A hydrophilic polymer as referred to herein is typically a polymer wherein the surface of the polymer has a static water contact angle, 0, which is less than 90°. When an object is described herein as “coated”, this term encompasses partial and complete coverage of the surface of an article. For example, an article which is described as coated may have a coating on up to 100% of the surface, e.g. up to 95%, up to 90% or up to 80%. Typically, a coated article has a coating on greater than 50% of the surface, usually greater than 75%. Typically, therefore, a coated article (e.g. a coated implantable medical device) has a coating covering from 50% to 100% of the surface. Biodegradable as referred to herein refers to any material which breaks down in the body naturally. In particular, the biodegradable polymers used herein are biodegradable as they are reduced to their monomer constituents in an in vitro or in vivo environment in a period of less than 10 years. The term “alkyl”, as used herein, refers to a linear or branched chain saturated hydrocarbon radical. A “Cn-m alkyl” refers to an alkyl having from n to m carbon atoms. Thus, an alkyl group may be a C1-20 alkyl group, a Ci-is alkyl group, a C1-14 alkyl group, a Ci-10 alkyl group, a C1-6 alkyl group or a Cm alkyl group. Examples of a C1-10 alkyl group are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl. Examples of Ci-6 alkyl groups are methyl, ethyl, propyl, butyl, pentyl or hexyl. Examples of Cm alkyl groups are methyl, ethyl, i-propyl, n-propyl, t-butyl, s-butyl or n-butyl. If the term “alkyl” is used without a prefix specifying the number of carbons anywhere herein, it has from 1 to 6 carbons. For the avoidance of doubt, where two alkyl moieties are present in a group, the alkyl moieties may be the same or different. The term “cycloalkyl”, as used herein, refers to a saturated or partially unsaturated cyclic hydrocarbon radical. A "Cn-m cycloalkyl” refers to a cycloalkyl having from n to m carbon atoms. Thus, a cycloalkyl group may be a C3-20 cycloalkyl group, a C3-10 cycloalkyl 10 group, a C3-8 cycloalkyl group or a C3-6 cycloalkyl group. Examples of a C3-8 cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclohexenyl, cyclohex -1,3-dienyl, cycloheptyl and cyclooctyl. Examples of a C3-6 cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. The term “alkenyl”, as used herein, refers to a linear or branched chain hydrocarbon radical containing one or more double bonds. A “Cn-m alkenyl” refers to an alkenyl having from n to m carbon atoms. Thus, an alkenyl group may be a C2-18 alkenyl group, a C2-14 alkenyl group, a C2-10 alkenyl group, a C2-6 alkenyl group or a C2-4 alkenyl group. Examples of a C2-10 alkenyl group are ethenyl (vinyl), propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl. Examples of C2-6 alkenyl groups are ethenyl, propenyl, butenyl, pentenyl or hexenyl. Examples of C2-4 alkenyl groups are ethenyl, i-propenyl, n-propenyl, s-butenyl or n-butenyl. Alkenyl groups typically comprise one or two double bonds. The term “aryl”, as used herein, refers to a monocyclic, bicyclic or polycyclic aromatic ring which contains up to 14 carbon atoms, typically from 6 to 10 carbon atoms, in the ring portion. Examples include phenyl, naphthyl, indenyl and indanyl groups. Phenyl is preferred. The terms “alkylene”, “alkenylene” and “arylene”, as used herein, refer to bivalent groups obtained by removing a hydrogen atom from an alkyl, alkenyl or aryl group, respectively. Such bidentate groups may be substituted or unsubstituted. An alkylene group may be a C1-10 alkylene group, a C1-6 alkylene group or a C1-4 alkylene group. Examples of Ci-6 alkylene groups are methylene, ethylene, propylene, butylene, pentylene and hexylene. An alkenylene group may be a C2-10 alkenylene group, a C2-6 alkenylene group or a C2-4 alkenylene group. Examples of a C2-4 alkenylene group include ethenylene (vinylene), propenylene and butenylene. Examples of arylene groups include phenylene. For alkylene, alkenylene and arylene, these groups may be bonded to other groups at any two positions on the group. Thus, propylene includes -CH2CH2CH2- and -CH2CH(CH3)-, and phenylene includes ortho-, meta- and para-phenylene. The term “substituted”, as used herein, in the context of substituted organic compounds and groups, refers to an organic compound or group (e.g. an alkyl group, an alkylene group, an alkenylene group, an aryl group, an arylene group) which bears one or more substituents selected from C1-10 alkyl, C3-10 cycloalkyl, C3-7 heterocyclyl, aryl, 11 heteroaryl, cyano, amino, nitro, C2-10 alkenyl, C2-10 alkynyl, C1-10 alkylamino, di(Ci-io)alkylamino, arylamino, diarylamino, aryl(Ci-io)alkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C1-10 alkoxy, aryloxy, halo(Ci-io)alkyl, sulfonic acid, thiol, C1-10 alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SOs'. Typically, the one or more substituents are selected from cyano, amino, nitro, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, sulfonic acid, thiol, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid, phosphonate ester and SO3’. When a compound or group is substituted, it typically bears 1, 2, 3 or 4 substituents. For instance, a substituted compound or group may have 1, 2 or 3 substituents, or for example 1 or 2 substituents. However, when a group is halo-substituted, for instance fluoro-substituted, the group may bear 1, 2, 3 or 4 halo substituents, or it may bear more than four halo substituents. In fact, the group may be perhalo-substituted, i.e. all hydrogen atoms of the group may be replaced by halogen atoms. The group may for instance be perfluorosubstituted, i.e. perfluorinated, i.e. all hydrogen atoms of the group may be replaced by fluorine atoms. As used herein the term oxo represents a group of formula: =0. A C(O) group refers to a carbonyl group, i.e. a group of formula C=O. The repeating units described herein can exist in various tautomeric forms and it is to be understood that the invention encompasses all such tautomeric forms. In certain of the repeating units described herein, dependant on the nature of the substituent, there may be chiral carbon atoms and therefore the compounds may exist as stereoisomers. The invention extends to all optical isomers such as stereoisomeric forms of the compounds of the invention, including enantiomers, diastereomers and mixtures thereof, such as racemates. The different stereoisomeric forms may be separated or resolved one from the other by conventional methods or any given isomer may be obtained by conventional stereoselective or sterospecific syntheses. It is also to be understood that any atom present in a compound of the invention may be present in any available naturally-occuring isotopic form. For instance, a carbon atom may be ,2C or 13C. A hydrogen atom may be 'H or 2H (deuterium). As used herein, the terms “treat”, “treating” and “treatment” refer to both therapeutic treatment and prophylactic or preventative measures, wherein the object is to 12 prevent or slow down (lessen) an undesired physiological change or disorder, such as the development or spread of disease. "Treatment" can also mean prolonging survival as compared to expected survival if not receiving treatment. Those in need of treatment include those already with the condition or disorder as well as those prone to have the condition or disorder or those in which the condition or disorder is to be prevented. Amino-functionalised graphene oxide (GO) for use of the generation of NO in vivo The present invention provides amino-functionalized GO for use in the generation of NO in vivo. Amino groups are able to interact with endogenous sources of NO to catalytically release NO in vivo. While the catalytic mechanism has not yet been studied in detail, it is understood that the amino group acts as a nucleophile and the lone pair of the amino group reacts with, for example, the S-N bond of S-nitrosothiols. The amino-functionalised GO for use in the generation of NO in vivo may also interact with an exogenous source (exogenous substrate) to produce NO. For example, GSNO and / or SNAP could be provided (for example in oral or injectable form) to “top-up” endogenous sources by increasing the amount of circulating S-nitrosothiols. This could aid localized production of NO at the site of an amino-functionalized GO as described herein. Accordingly, the use or method of treatment as provided herein may include a further step of administering a source of NO, such as an S-nitrosothiol. Thus, any of the uses or methods of treatment provided herein may further comprise administering a source of NO to the subject. The source of NO may be an S-nitrosothiol. The S-nitrosothiol may be any of the S-nitrosothiols disclosed herein. It may for example be S-nitrosoglutathione (GSNO), S-nitrosocysteine, S-nitrosoalbumin, or S-nitroso-N-acetylpenicillamine (SNAP). Accordingly, the invention also provides a method of generating NO in a subject, wherein said method comprises administering amino-functionalised graphene oxide to the subject. Said method may further comprise implanting into the subject an implantable medical device comprising the amino-functionalised graphene oxide. The amino-functionalised graphene oxide and the implantable medical device may be as further defined anywhere herein. Said method may further comprise administering an exogenous source of NO, such as one or more S-nitrosothiols. Generating NO in vivo may comprise generating NO in a blood vessel in vivo. Similarly, generating NO in a subject may comprise generating NO in a blood vessel of the 13 subject. Generating NO in vivo may comprise generating NO in the wall of a blood vessel in vivo, or in tissue of the blood vessel in vivo. Similarly, generating NO in a subject may comprise generating NO in the wall of a blood vessel of the subject, or in tissue of the blood vessel of the subject. The blood vessel may for instance be a vein or an artery. Therefore, generating NO in a subject may comprise generating NO in an arterial or vascular wall of the subject, for instance in arterial or vascular tissue of the subject. The subject is typically a mammal, more typically a human. The present invention also provides use of an amino-functionalised graphene oxide in the manufacture of a medicament for use in generating NO in vivo. Said use in generating NO in vivo may comprise implanting into a subject an implantable medical device which comprises the medicament. The implantable medical device may comprise the medicament on a surface of the implantable medical device. For instance, the implantable medical device may have a coating on a surface thereof, which coating comprises the medicament. Implanting the implantable medical device into a subject may comprise implanting it into a blood vessel of the subject. Any of the amino-functionalised graphene oxide, the use in generating NO, the implantable medical device, the coating, and the subject, may be as further defined anywhere herein. Typically, the amino-functionalised GO comprises graphene oxide and a polymer comprising amino groups. The amino-functionalised GO for use in generation of NO in vivo may be for use in the treatment or prevention of cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms. Accordingly, the present invention provides a method of treating or preventing cardiovascular disease, cerebrovascular disease, aneurysm and / or bacterial infection in a subject, or preventing the formation of a biofilm on a surface of an implantable medical device, said method comprising administering amino-functionalised graphene oxide to the subject. Said administering amino-functionalised graphene oxide to a subject may comprise implanting into the subject an implantable medical device comprising the amino-functionalised graphene oxide. The present invention also provides use of an amino-functionalised graphene oxide in the manufacture of a medicament for use in the treatment or prevention of cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or 14 formation of biofilms. Said use in the treatment or prevention of cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms may comprise implanting into a subject an implantable medical device which comprises the medicament. The implantable medical device may comprise the medicament on a surface of the implantable medical device. For instance, the implantable medical device may have a coating on a surface thereof, which coating comprises the medicament. Implanting the implantable medical device into a subject may comprise implanting it into a blood vessel of the subject. The amino-functionalised graphene oxide, the implantable medical device, the coating and / or the subject, may be as further defined anywhere herein. Often, the amino-functionalized GO for use in generation of NO in vivo is for use in the treatment or prevention of cardiovascular disease. Said cardiovascular disease may typically be selected from coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease and restenosis (a re-narrowing of arteries following intervention). Generation of NO in vivo can be particularly effective for treatment of cardiovascular disease as NO can act as a vasodilator, causing smooth muscle relaxation in arterial and vascular walls. It can be especially helpful to have generation in vivo, as this can provide a long-term steady supply of NO at specific in vivo sites without regular uncomfortable, inconvenient, and possible painful administration of exogenous NO sources. Furthermore, NO has a very short half-life in vivo, and a very short range of diffusion. Therefore, it is advantageous to be able to generate NO at specific local sites, where NO generation is especially desired. It is also important to have options which can be used long-term as many vascular diseases are chronic conditions which are difficult to completely eradicate and are prone to recurrence, thus long-term treatment is highly desirable. Often, therefore, the method of the invention is a method of treating or preventing cardiovascular disease, typically selected from coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease and restenosis. The invention also provides use of an amino-functionalized graphene oxide in the manufacture of a medicament for use in the treatment or prevention of cardiovascular disease, often selected from coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease and restenosis. The amino-functionalised GO for use in the generation of NO in vivo may be for use in the treatment or prevention of cerebrovascular disease. Cerebrovascular disease as referred to herein may include stroke, carotid stenosis, vertebral stenosis, intercranial stenosis, thrombosis and embolism. The method of the invention may be a method of treating cerebrovascular disease, such as stroke, carotid stenosis, vertebral stenosis, intercranial stenosis, thrombosis and embolism. The invention also provides manufacture of a medicament for the treatment or prevention of cerebrovascular disease, such as stroke, carotid stenosis, vertebral stenosis, intercranial stenosis, thrombosis and embolism. A biofilm as referred to herein is typically a bacterial biofilm. Biofilms can form on the surface of medical devices and increase the risk of infection. NO can be useful in preventing the formation of biofilms on the surface of, for example, indwelling catheters (urinary or vascular), heart valves and stents. This reduces the risk of bacterial infection at the site of the medical device. Often, for the amino-functionalised GO for use according to the invention, the polymer comprising amino groups is a first polymer and the amino-functionalised graphene oxide further comprises a second polymer. Typically the second polymer is a hydrophilic polymer. Typically, the second polymer is bonded to the GO and the first polymer is bonded to the second polymer. In such embodiments, the first polymer is not typically directly bonded to the GO. Such a polymer may be described in shorthand as [First polymer]-[Second polymer]@GO, e g. PEI-PEG@GO. The polymer comprising amino groups as referred to herein may be a polymer with at least three repeating units defined in Formula I: (I) wherein: m is an integer value from 0 to 8; p is an integer value from 0 to 8; each R1 is independently selected from a group consisting of CH2, NH, C(O), or CR5; each R2 is independently selected from a group consisting of CH2, NH, C(O), or CR5; R3 is selected from a group consisting of a bond, unsubstituted or substituted (but typically unsubstituted) C1-6 alkylene, unsubstituted or substituted (but typically unsubstituted) C2-6 alkenylene, and unsubstituted or substituted (but typically unsubstituted) C6-12 arylene; R4 is H, or a unit of Formula (I); and R5 is unsubstituted or substituted (but typically unsubstituted) C1-3 alkyl or phenyl. Often, the polymer comprising amino groups terminates in an amino group, or a Ci-3 alkyl. The polymer may, for instance, have from 3 to 1,000,000 repeating units, for example from 3 to 100,000 or from 3 to 10,000 repeating units, for example from 3 to 1000 repeating units, such as from 3 to 100 repeating units or from 3 to 40 repeating units, or for instance from 3 to 10 repeating units. It may for instance have from 10 to 100,000 repeating units, for instance from 10 to 10,000 repeating units, for example from 20 to 1000 repeating units. Often, the polymer comprising amino groups comprises a number of repeat units of formula (I) wherein for each unit, the definition of m, p, each definition of R1, each definition of R2, R3 and R5 does not vary in the polymer, however the definition of R4 may vary from a further unit of formula (I) or H. Typically, one of R1 is selected from a group consisting of CH2, NH, C(O), or CHR5, and all other R1 are CH2. Often, one of R1 is selected from NH or CH2, and all other R1 are CH2. When m is 5, four R1 may be CH2 and one R1 may be NH, such that (R^m is -(CH2)4NH-. When m is 1, R1 may be CH2. Often, one of R2 is selected from a group consisting of CH2, NH, C(O), or CHR5, and all other R2 are CH2. Often R2 is C(O) or CH2. Therefore, for some repeat units of formula (I) one of R1 is selected from a group consisting of CH2, NH, C(O), or CHR5, and all other R1 are CH2; and, one of R2 is selected from a group consisting of CH2, NH, C(O), or CHR5, and all other R2 are CH2. R2 may be C(O). m is an integer value from 0 to 8. Often, m is an integer value from 0 to 5. m may be 1 or 5. p is an integer value from 0 to 8. Often p is an integer value from 0 to 1. p may be 1. p may also be 0. R3 is typically selected from a group consisting of a bond, unsubstituted C1-6 alkylene, unsubstituted C2-6 alkenylene; or unsubstituted C6-12 arylene. Typically, R3 is selected from a bond, unsubstituted C1-4 alkylene, and unsubstituted phenylene. R3 may be selected from a bond, CH2, (CH2)2 and unsubstituted phenylene. R4 is a further unit of formula (I) or H. Often R4 is H. R5 may be unsubstituted C1-3 alkyl or phenyl Therefore, for some repeat units of formula (I): m is an integer value from 0 to 5; p is an integer value from 0 to 1; one of R1 is selected from NH or CH2, and all other R1 are CH2; R2 is C(O) or CH2; and R3 is selected from a bond, a C1-4 alkyl, and unsubstituted phenylene. Often, m is 1 or 5; p is 0 or 1; R1 is CH2 or NH; optionally wherein when mis 1, R1 is CH2 and when m is 5, four R1 are CH2 and one R1 is NH; R2 is C(O); and R3is selected from a bond, CH2, (CH2)2 and unsubstituted phenylene. When m is 1 and R1 is CH2, p may preferably be 0. When m is 5 and four R1 are CH2 and one R1 is NH, p may preferably be 1, R2 may be C(O), and R3 may be a bond. Typically the weight average molecular weight of said polymers is from about 500 g / mol to about 100,000 g / mol, for instance from about 500 g / mol to about 50,000 g / mol, or from around 20000 g / mol to around 30000 g / mol, typically around 24000 g / mol to 26000 18 g / mol, e.g. around 25000 g / mol. The number average molecular weight is typically from around 300 g / mol to about 20000 g / mol, for instance from about 5000 g / mol to 15000 g / mol, typically around 8000 g / mol to 12000 g / mol, typically around 10000 g / mol. The polymer comprising amino groups may be linear or branched. Typically said polymer is a branched polymer (typically a dendritic polymer). For example, the polymer may be branched poly(ethyleneimine). Typically the branched poly(ethyleneimine) has a molecular weight of from about 500 g / mol to about 50,000 g / mol, for instance from about 20000 g / mol to about 30000 g / mol, typically about 24000 g / mol to about 26000 g / mol, e.g. about 25000 g / mol. The number average molecular weight of the branched poly(ethylenimine) may be from about 300 g / mol to about 20000 g / mol, for instance from about 5000 g / mol to about 15000 g / mol, for instance about 8000 g / mol to about 12000 g / mol, for example about 10000 g / mol. Branched poly(ethyleneimine) is commercially available from Sigma-Aldrich. The polymer may be linear (i.e. each definition of R4is H). The polymer comprising amino groups may be selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(ethyleneimine), poly(propyleneimine) and combinations thereof. Typically the polymer comprising amino groups is poly(ethyleneimine), preferably branched poly(ethyleneimine). The polymer comprising amino groups may comprise, or be, branched poly(ethyleneimine) having a molecular weight (Mw) of from 15 to 35 kDa, for instance from 20 to 30 kDa, or from 22 to 27 kDa, for instance about 25 kDa. The second polymer (typically a hydrophilic polymer) is often selected from poly(ethylene glycol), poly(7V-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine or mixtures thereof. Preferably, the second polymer comprises, or is, poly(ethylene glycol). The poly(ethylene glycol) is preferably branched poly(ethylene glycol). Preferably, the poly(ethylene glycol), which is typically branched, has a molecular weight (Mw) of from 5000 to 15000 g / mol, for instance from 8000 to 12000 g / mol, for instance about 10000 g / mol. 1000 g / mol is equivalent to 1 kDa. Therefore, often the polymer comprising amino groups is selected from polyvinyl amine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and combinations thereof; and the second polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine or mixtures thereof. 19 For example, the polymer comprising amino groups is selected from polyvinyl amine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and combinations thereof; and the second polymer is poly(ethylene) glycol. Preferably, the polymer comprising amino groups is poly(ethyleneimine); and the second polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine or mixtures thereof. Often the polymer comprising amino groups is poly(ethyleneimine) and the second polymer is poly(ethylene) glycol. Typically, for the amino-functionalised GO for use according to the invention, the percentage by weight of the polymer comprising amino groups is from 8% to 35% based on the total weight of the amino-functionalised GO. The inventors have studied the effects of varying the amount of polymer comprising amino groups on the surface of GO and have found that a weight percentage of from around 8% to around 35% is preferable to achieve the highest volume of NO generation. At higher weight percentages, the conjugation to the GO of the polymer comprising amino groups (and the second polymer if present) may be less good, which is thought to be because the polymer needs available active sites for successful conjugation. Furthermore, the present inventors have found that the GO itself plays a role in NO generation. High loading of polymers of the GO does not allow the GO to interact with endogenous sources of NO. Thus, although the total mass of the polymer could be higher, this would not lead to higher generation of NO. At the preferred loadings herein, the inventors have observed good dispersion of polymers, good conjugation and thus improved activation. Often the percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is at least 8%, such as at least 10% or at least 12%. The percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is typically a maximum of (i.e. no more than) 35%, such as a maximum of 30%, 25%, 20% or 18%. Therefore the percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is typically from 10% to 30%, such as from 10% to 25%, from 10% to 20%, or from 12% to 18%. Therefore, the amino-functionalised GO may comprise by weight from 8% to 35% poly(ethyleneimine) based on the total weight of the amino-functionalised GO, typically from 10% to 30%, such as from 10% to 25%, from 10% to 20%, or from 12% to 18%. Typically a second polymer (usually the hydrophilic polymer as described herein) is present in the amino-functionalised GO, such that the percentage by weight of the second polymer is greater than 0%. Typically the percentage by weight of the second polymer based on the total weight of the amino-functionalised GO is greater than 0%, such as from (i.e. equal to or greater than) 1%, from 2%, from 3% or from 4%. The percentage by weight of the second polymer is typically less than or equal to 12%, such as less than or equal to 10% or less than or equal to 8%, based on the total weight of the amino-functionalised GO. Often the percentage by weight of the second polymer is greater than 0% and less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide, preferably wherein said percentage by weight of the second polymer is from 2% to 10%. Typically the percentage by weight of the second polymer is from 2% to 8%, or from 3% to 8%. Therefore, the amino-functionalised GO may comprise by weight from greater than 0% to less than or equal to 12% poly(ethylene) glycol based on the total weight of the amino-functionalised GO, typically from 2% to 10%, 2% to 8%, or from 35 to 8%. Thus the amino-functionalised GO may comprise by weight from 8% to 35% poly(ethyleneimine) based on the total weight of the amino-functionalised GO and from greater than 0% to less than or equal to 12% poly(ethylene) glycol based on the total weight of the amino-functionalised GO. In some embodiments, the remainder of the weight of the amino-functionalised GO is graphene oxide. In some embodiments the amino-functionalised GO consists of graphene oxide, a first polymer comprising amino groups, and a second polymer (often a hydrophilic polymer). The GO, the first polymer and the second polymer may be as further defined anywhere herein. Also provided herein is amino-functionalised GO for use as described herein, wherein said use comprises implanting into a subject an implantable medical device comprising said amino-functionalised GO. The implantable medical device comprising said amino-functionalised GO may be an implantable medical device which is coated with 21 said amino-functionalised GO. Thus, also provided herein is amino-functionalised GO for use as described herein, wherein said use comprises implanting an implantable medical device coated with said amino-functionalised GO in a subject. Implanting the implantable medical device into a subject may comprise implanting it into a blood vessel of the subject, for instance into an artery or vein of the subject. The use may further comprise generating NO in said blood vessel of the subject, or for instance generating NO in a wall of said blood vessel of the subject, or in tissue of the blood vessel of the subject, for instance in arterial or vascular tissue. Coating Also provided herein is a coating for an implantable medical device, wherein the coating comprises amino-functionalised GO. The amino-functionalised GO and constituents thereof (e.g. the GO, the polymer comprising amino groups, and the second polymer, where present) may be as further defined anywhere herein. Often, the coating of the invention further comprises a biodegradable polymer. The polymer can alter the viscosity of the coating to improve ease of application and increase evenness of the coating. The biodegradable polymer is typically selected from polycaprolactone (PCL), polyglycolic acid (PGA), polylactic acid (PLA), poly-p-hydroxybutyrate (PHB), and poly (lactic acid-co-glycolic acid) (PLGA), and mixtures thereof. Often the biodegradable polymer comprises polycaprolactone. The biodegradable polymer may be polycaprolactone. The polycaprolactone may have a molecular weight (Mn) of from 50000 to 110000, for instance from 70000 to 90000, for instance about 80000. Often, the coating of the invention further comprises an additional polymer. The additional polymer may be selected from poly(ethylene glycol), poly(Ar-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof. Usually, however, it is poly(ethylene glycol). Thus, said coating often further comprises said biodegradable polymer which comprises polycaprolactone and an additional polymer which is poly(ethylene glycol). Typically in the coating the weight ratio of the biodegradable polymer and the additional polymer is from 1:5 to 5:1, such as from 1:3 to3:l. Typically the weight ratio of biodegradable polymer to additional polymer is around 1:1. For instance, the coating often comprises 1:1 polycaprolactone to poly(ethylene glycol). Therefore, the coating typically comprises (a) a biodegradable polymer selected from polycaprolactone (PCL), polyglycolic acid (PGA), polylactic acid (PLA), poly-p-hydroxybutyrate (PHB), and poly (lactic acid-co-glycolic acid) (PLGA), and mixtures thereof; and (b) amino-functionalised GO, wherein the amino-functionalised GO comprises: a polymer comprising amino groups selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and combinations thereof; a second polymer selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine or mixtures thereof; and graphene oxide (GO). It often further comprises (c) said additional polymer. Often the coating comprises (a) PLA, and (b) amino-functionalised GO, wherein the amino-functionalised GO comprises: poly(ethyleneimine), poly (ethylene) glycol, and graphene oxide. It often further comprises (c) said additional polymer, which is preferably poly(ethylene glycol). Often the total weight of the amino-functionalised GO based on the total weight of the coating is at least 1 wt %, for instance at least 2 wt %, for instance at least 3 wt %, based on the total weight of the coating. Often the total weight of the amino-functionalised GO based on the total weight of the coating is less than or equal to 15 wt %, for instance less than or equal to 9 wt %, or less than or equal to 8 wt %, for instance less than or equal to 7 wt %, based on the total weight of the coating. Often the total weight of the amino-functionalised GO based on the total weight of the coating is from 1 wt% to 15 wt%, or from 1 wt% to 9 wt%, for instance from 2 wt% to 8 wt%, or for instance 3 wt% to 7 wt%, for example about 5 wt%, based on the total weight of the coating. Often, the total weight of the biodegradable polymer, the additional polymer (when present), and the amino-functionalised GO is 100% the weight of the coating. When the coating comprises both the biodegradable polymer and the additional polymer, the amounts of the additional polymer (typically PEG) and the biodegradable polymer (typically PCL) based on the total weight of the coating are typically from 42 to 53 wt % and from 42 to 53 wt %, respectively, such as from 44 to 51 wt % and from 44 to 51 wt % respectively, typically from 46 to 49 wt% and from 46 to 49 wt% respectively. Usually, the balance of the weight of the coating is the amino-functionalised GO. The weight ratios of the components in the coating described herein typically follow (i.e. they are typically the same as) the weight ratios of those same components in a coating solution, as described herein, that is used to produce the coating. Such coating solutions are described further below. Sometimes, therefore, the coating consists of the amino-functionalised GO and the biodegradable polymer, and, when present, the additional polymer. Any suitable coating thickness may be employed. The coating thickness can usefully be varied to control the duration of action, and therefore to tailor the duration of action to the relevant application. The thickness of the coating may, for instance, be from 200 nm to 50 pm, for instance from 500 nm to 50 pm, for example from 1 pm to 20 pm, or from 1 pm to 10 pm. In some embodiments the coating may further comprise one or more antiproliferative drugs. For example, the coating may further comprise a drug selected from paclitaxel or sirolimus. Implantable Medical Device The invention also provides a coated implantable medical device which comprises: (a) an implantable medical device, and (b) a coating on a surface of said implantable medical device, wherein said coating comprises amino-functionalised graphene oxide. An implantable medical device as used herein refers to an article which can be introduced in vivo to permanently or temporarily support the function or structure of an organ or tissue, monitor physiological activity, and / or deliver therapeutics. Examples of medical devices include cardiovascular implants, such as artificial hearts, artificial heart valves, implantable defibrillators, pacemakers, stents, arterial grafts, and venous grafts; electric implants for use in relieving pain, such as for relieving pain associated with rheumatoid arthritis; catheters, or cannulas. Often the implantable medical device is selected from a stent, an arterial graft, a venous graft, a catheter, and a cannula. Typically, the implantable medical device is a stent. Stents are implantable medical devices used to hold open passages in the body, such as weak or narrowed arteries. One issue with stents is that the insertion of the stent itself, or friction caused by the stent when in position can lead to further damage of arteries that the stent has been implanted to protect. Combining a stent with an NO generating material means that the benefits of a stent can be enjoyed while NO is generated at the site of the stent to prevent restenosis and other conditions caused by the stent, thus preventing recurrence of disease and reducing the need to replace the stent or avoiding the addition of further stents. For the coated implantable medical device of the invention, the aminofunctionalised graphene oxide (and its constituent parts) may be as further defined anywhere herein. The coating on the surface of said implantable medical device may also be as further defined herein, e.g. for the coating of the invention. Many existing implantable medical devices are designed to degrade at a certain speed, such that they support the body for as long as is necessary, but facilitate restoration of function un-aided by a device. In addition, degradation of implantable medical devices can reduce the risk of infection of the foreign body overtime. However, it is important that the device remains in the body, and maintains its form, long enough to be effective. The present inventors have surprisingly found that the present coated implantable medical devices have similar degradation profiles to existing devices, demonstrating that they are suitable for medical use. The implantable medical device may comprise (a) an implantable medical device, and (b) a coating on a surface of said implantable medical device, wherein said coating comprises amino-functionalised graphene oxide, and wherein said implantable medical device comprises a biodegradable material. Said biodegradable material may be, for example, PLGA or PLA, preferably PLA. Said implantable medical device may further comprise one or more antiproliferative drugs, such as sirolimus or paclitaxel. Coating solution The invention also provides a coating solution comprising a solvent, and aminofunctionalised graphene oxide. The solvent is typically an organic solvent. The aminofunctionalised graphene oxide may be as further defined anywhere herein. 25 The amino-functionalised graphene oxide typically comprises graphene oxide and a polymer comprising amino groups. The polymer comprising amino groups is typically a first polymer, and the amino-functionalised graphene oxide often further comprises a second polymer, wherein said second polymer is a hydrophilic polymer. The second polymer may be as described anywhere herein. Said second polymer is typically selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine and mixtures thereof. Typically said hydrophilic polymer (i.e. second polymer) is poly(ethylene) glycol. The polymer comprising amino groups may be as described anywhere herein. For instance, the polymer comprising amino groups is typically selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof. In some preferred coating solutions, the polymer comprising amino groups is poly(ethyleneimine). The coating solution of the invention often further comprises a biodegradable polymer. Said biodegradable polymer typically comprises poly caprolactone, polyglycolic acid (PGA), polylactic acid (PLA), poly-P-hydroxybutyrate (PHB), poly (lactic acid-co-glycolic acid) (PLGA) or a mixture thereof. Preferably the biodegradable polymer is polycaprolactone (PCL). A combination of a hydrophilic polymer and a biodegradable polymer (e.g. a combination of PEG and PCL) can increase the viscosity and reduce the surface tension of a coating solution. This can prevent interaction of a solvent in the coating solution with a medical device substrate. For example, biodegradable stents may be made of biodegradable polymers such as PLA. PLA may dissolve in tetrahydrofuran (THF), so contact with a PLA substrate and THF solvent should be limited. The coating solution may also further comprise an additional polymer. The additional polymer is often a hydrophilic polymer, and may for instance be selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof. It is usually poly(ethylene glycol). Typically in the coating solution the weight ratio of the biodegradable polymer and the additional polymer is from 1:5 to 5:1, such as from 1:3 to 3:1, or from 1:2 to 2:1. Typically the weight ratio of biodegradable polymer to additional polymer is around 1:1. For instance, the coating solution often comprises from 1:2 to 2:1 poly caprolactone to poly(ethylene glycol), for instance about 1:1 polycaprolactone to poly(ethylene glycol). 26 The coating solution according to the invention often also comprises a solvent, which is typically an organic solvent. Any organic solvent may be used. Typically, however, the organic solvent is selected from tetrohydrofuran (THF), acetone, hexane, toluene, xylene and mixtures thereof. The organic solvent is often THF. THF is preferable as a solvent due to its low boiling point, meaning it evaporates rapidly leading to formation of a uniform surface area. Any low boiling point solvent may be preferably used, such as any solvent which evaporates under 100 °C at standard pressure (around 100 000 Pa). In some embodiments, the coating solution comprises amino-functionalised graphene oxide, poly (ethylene) glycol, PLA and THF. To prepare the coating solution of the invention typically the biodegradable polymer and the additional polymer are dissolved in the organic solvent. Often the functionalised GO is then added. Typically the solution is stirred until the material is completely dispersed. Preparation of the coating solution typically occurs at standard atmospheric pressure and temperature, e.g. around 100000 Pa, and around 25 °C. The amount of amino-functionalized GO added (i.e. the amount in the coating solution) is typically from 1 wt% to 15 wt%, for instance from 1 wt% to 9 wt%, based on the total weight of the biodegradable polymer, the additional polymer and the aminofunctionalised GO in the solution, preferably 2 wt% to 8 wt%, most preferably 3 wt% to 7 wt%, or about 5 wt %. When both the biodegradable polymer and the additional polymer are present, the amounts of the biodegradable polymer (typically PCL) and the additional polymer (typically PEG) in the coating solution, based on the total weight of the biodegradable polymer, the additional polymer and the amino-functionalized GO in the solution are typically from 42 to 53 wt % and from 42 to 53 wt %, respectively. Typically, the amounts of the biodegradable polymer (typically PCL) and the additional polymer (typically PEG) in the coating solution, based on the total weight of the biodegradable polymer, the additional polymer and the amino-functionalized GO in the solution are from 44 to 51 wt % and from 44 to 51 wt %, respectively, often from 46 to 49 wt % and from 46 to 49 wt % respectively. Method of coating device The present invention also provides a process for producing a coated implantable medical device, wherein the process comprises disposing a coating solution of the 27 invention as defined herein on an implantable medical device. The implantable medical device may be as further defined anywhere herein. Any suitable method for disposing the solution may be used. For example, the coating solution may be sprayed (i.e. thermal spraying, or plasma spraying), painted, plated (i.e. electroplated) or poured on to the device; or the device may be suspended and removed from a container holding the solution (i.e. dipped in the coating solution). Often the process of the invention further comprises a solvent removal step. Such a step is generally desirable for removing the solvent of the coating solution. Thus, the process of the invention typically further comprises a step of drying the implantable medical device. Any method of drying may be used. For example, the implantable medical device may be air-dried, oven-dried, freeze-dried, dried in an inert gas flow (such as argon or nitrogen), dried in a desiccator, or vacuum dried. Typically, the drying step occurs in a dark area, i.e. an area with limited UV light. UV light may cause composition of the coating later before drying. Amino-functionalised graphene oxide materials The present invention also provides an amino-functionalised graphene oxide, which comprises: graphene oxide; a first polymer which is a polymer comprising amino groups; and a second polymer which is a hydrophilic polymer; wherein the percentage by weight of the first polymer is from 8% to 35%, based on the total weight of the amino-functionalised polymer. Often the percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is at least 8%, such as at least 10% or at least 12%. The percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is usually a maximum of (i.e. it is typically less than or equal to) 35%, such as a maximum of 30%, 25%, 20% or 18%. Therefore, the percentage by weight of the polymer comprising amino groups based on the total weight of the amino-functionalised GO is typically from 10% to 30%, such as from 10% to 25%, from 10% to 20%, or from 12% to 18%. For the ami no-functionalised graphene oxide of the invention, the percentage by weight of the second polymer is typically greater than 0% and typically less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide. Typically, the percentage by weight of the second polymer based on the total weight of the amino-functionalised GO is greater than 0%, such as from (i.e. equal to or greater than) 1%, from 2%, from 3% or from 4%. The percentage by weight of the second polymer is typically less than or equal to 12%, such as less than or equal to 10% or less than or equal to 8%. Often the percentage by weight of the second polymer is greater than 0% and less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide, preferably wherein said percentage by weight of the second polymer is from 2% to 10%. Typically, the percentage by weight of the second polymer is from 2% to 8%, or from 3% to 8%. In some embodiments, the remainder of the weight of the amino-functionalised graphene oxide is graphene oxide. Typically, for the amino-functionalised graphene oxide of the invention the percentage by weight of the first polymer is from 10% to 30%, based on the total weight of the amino-functionalised graphene oxide; and / or the percentage by weight of the second polymer is from 2% to 10%, based on the total weight of the amino-functionalised graphene oxide. Often, the percentage by weight of the first polymer is from 10% to 30% and the percentage by weight of the second polymer is from 2% to 10%, based on the total weight of the amino-functionalised graphene oxide. For instance, the percentage by weight of the first polymer may be from 10% to 20% and the percentage by weight of the second polymer may be from 2% to 8%. The percentage by weight of the first polymer may for instance be from 12% to 18% and the percentage by weight of the second polymer may be from 3% to 8%. The first polymer in the amino-functionalised graphene oxide may be a polymer comprising amino groups as described anywhere herein, and is preferably selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof. Preferably the first polymer is poly(ethyleneimine). The first polymer may also preferably be branched, such as branched poly(ethyleneimine). The second polymer in the amino-functionalise graphene oxide may be a hydrophilic polymer as described anywhere herein. Typically, the second polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof. Often the second polymer is polyethylene glycol). Typically the second polymer (e.g. PEG) is bonded to the graphene oxide, and the first polymer (e.g. branched PEI) is bonded to the second polymer. Usually, the percentage by weight of the branched poly(ethyleneimine) is from 10% to 30% and the percentage by weight of the poly(ethylene glycol) is from 2% to 10%, based on the total weight of the amino-functionalised graphene oxide. Often, the percentage by weight of the branched poly(ethyleneimine) is from 12% to 18% and the percentage by weight of the poly(ethylene glycol) is from 3% to 8%, based on the total weight of the aminofunctionalised graphene oxide. An amino-functionalised graphene oxide as described herein may be prepared using any appropriate method. For example, synthesis may begin using commercially available GO, or by oxidising graphite (e.g. exfoliated graphite) to create graphene oxide, for example using Hummer’s method or a modified Hummer's method as described in Tabish etal, Cancers, 2019; Tabish et al, Sci Rep, 2018; Tabish etal, Nanotechnology, 2017. The GO may then be functionalised using any appropriate method. As the skilled person will appreciate, a wide range of coupling chemistries are available to couple amino groups directly to the functional groups of GO, or to attach organic linker groups to the functional groups of GO which linker groups themselves bear one or more amino groups. Also, a wide range of coupling chemistries known to the skilled person can be employed to attach a polymer to GO, and / or to attach a first polymer to a second polymer. For example, N-(3-dimethylaminopropyl)-N’-ethylcarbodiimine (EDC) coupling could be used, as described further herein, to attach a polymer to graphene oxide (for instance a hydrophilic polymer, or a polymer comprising amino groups). The same EDC coupling can also be used to attach a first polymer to a second polymer, for instance to attach a polymer comprising amino groups to a hydrophilic polymer as defined herein. Further aspects The present invention also provides an implantable medical device functionalised with amino groups for use in producing NO in vivo. The implantable medical device may be as further defined anywhere herein, for instance it may be a stent. The amino groups may be present on an amino-functionalised polymer as further defined herein, which polymer is itself bonded to, or coated on, a surface of the implantable medical device. Alternatively, the amino groups may be bonded to an organic linker which is itself attached to a surface of the implantable medical device, e.g. by coupling to a functional group on a surface of the implantable medical device. Alternatively, the implantable medical device may be made of a polymer comprising amino groups. Alternatively, the implantable medical device may comprise aminofunctionalised graphene oxide, which amino-functionalised graphene oxide may be as further defined anywhere herein. The implantable medical device may comprise a coating on a surface thereof which comprises the amino-functionalised graphene oxide. The use in producing NO in vivo may also be as further defined anywhere herein, for instance it may for treating or preventing cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms in the subject, any of which may be as further defined herein. The invention further provides a method of generating NO in vivo wherein said method comprises implanting an implantable medical device functionalised with amino groups into a subject. The subject is typically a mammal, more typically a human. The implantable medical device functionalised with amino groups may be as further defined above. The device is typically a stent and the implanting is typically into a blood vessel of the subject, for instance an artery or a vein. Stents may be placed in a wide array of blood vessels, both arteries and veins. Representative examples of sites include the iliac, renal, and coronary arteries. Briefly, an angiogram is first performed to determine the appropriate positioning for stent therapy. Angiography is typically accomplished by injecting a radiopaque contrasting agent through a catheter inserted into an artery or vein as an x-ray is taken. A guidewire is then advanced through the lesion or proposed site of treatment. Over the guidewire is passed a delivery catheter, which allows a stent in its collapsed configuration to be inserted into the passageway. The delivery catheter is inserted either percutaneously, or by surgery, into the 31 femoral artery, brachial artery, femoral vein, or brachial vein, and advanced into the appropriate blood vessel by steering the catheter through the vascular system under fluoroscopic guidance. A stent functionalised with amino groups as described herein may then be expanded at the desired area of treatment. A post insertion angiogram may also be utilized to confirm appropriate positioning. Typically, the method of generating NO in vivo is a method of treating or preventing cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms in the subject, and may be as further defined herein. The cardiovascular disease may be coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease, restenosis (a re-narrowing of arteries following intervention). The embodiments of the invention may be further described by the below statements: 1. Amino-functionalised graphene oxide, for use in the generation of NO in vivo. 2. Amino-functionalised graphene oxide for use according to clause 1, wherein said amino-functionalised graphene oxide comprises graphene oxide and a polymer comprising amino groups. 3. Amino-functionalised graphene oxide for use according to clause 1 or clause 2, wherein said amino-functionalised graphene oxide is for use in the treatment or prevention of cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms. 4. Amino-functionalised graphene oxide for use according to clause 3, wherein said cardiovascular disease is selected from coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease and restenosis (a re-narrowing of arteries following intervention). 5. Amino-functionalised graphene oxide for use according to any one of clauses 2 to 4, wherein said polymer comprising amino groups is a first polymer and the amino functionalised graphene oxide further comprises a second polymer, wherein said second polymer is a hydrophilic polymer. 6. Amino-functionalised graphene oxide for use according to clause 5, wherein the second polymer is bonded to the graphene oxide, and the first polymer is bonded to the second polymer. 7. Amino-functionalised graphene oxide for use according to any one of clauses 2 to 6, wherein the polymer comprising amino groups is selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine), and mixtures thereof, preferably wherein the polymer comprising amino groups is poly(ethyleneimine). 8. Amino-functionalised graphene oxide for use according to any one of clauses 5 to 7, wherein the hydrophilic polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, and polysarcosine or mixtures thereof, preferably wherein said hydrophilic polymer is poly(ethylene glycol). 9. Amino-functionalised graphene oxide for use according to any one of clauses 2 to 8, wherein the percentage by weight of the polymer comprising amino groups is from 8% to 35%, based on the total weight of the amino-functionalised graphene oxide, preferably from 10% to 30%. 10. Amino-functionalised graphene oxide for use according to any one of clauses 5 to 9, wherein the percentage by weight of the second polymer is greater than 0% and less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide, preferably wherein said percentage by weight of the second polymer is from 2% to 10%. 11. Amino-functionalised graphene oxide for use according to any one of clauses 1 to 10, wherein said use comprises implanting an implantable medical device coated with said amino-functionalised graphene oxide in a subject. 12. A coating for an implantable medical device which coating comprises aminofunctionalised graphene oxide. 13. A coating according to clause 12, wherein the amino-functionalised graphene oxide is as further defined in any one of clauses 2, and 5 to 10. 14. A coating according to clause 12 or clause 13, wherein the coating further comprises a biodegradable polymer, optionally wherein said biodegradable polymer comprises a polymer selected from polycaprolactone, polyglycolic acid (PGA), polylactic acid (PLA), poly~P-hydroxybutyrate (PHB), poly (lactic acid-co-glycolic acid) (PLGA), and mixtures thereof, preferably wherein said biodegradable polymer comprises polycaprolactone. 15. A coating according to any one of clauses 12 to 14, which further comprises an additional polymer, optionally wherein said additional polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, and preferably wherein the additional polymer is poly(ethylene glycol). 16. A coating according to clause 12 or clause 13 wherein said coating further comprises a biodegradable polymer which comprises polycaprolactone and an additional polymer which is poly(ethylene glycol). 17. A coated implantable medical device which comprises: (a) an implantable medical device, and (b) a coating on a surface of said implantable medical device, wherein said coating comprises amino-functionalised graphene oxide. 18. A coated implantable device according to clause 17, wherein said implantable medical device is selected from a stent, an arterial graft, a venous graft, a catheter, and a cannula. 19. A coated implantable device according to clause 17 or clause 18, wherein said implantable medical device is a stent. 20. A coated implantable device according to any one of clauses 17 to 19, wherein the amino-functionalised graphene oxide is as further defined in any one of clauses 2 and 5 to 10. 21. A coated implantable device according to any one of clauses 17 to 20, wherein the coating is as further defined in any one of clauses 14 to 16. 22. A coating solution comprising an organic solvent and amino-functionalised graphene oxide. 23. A coating solution according to clause 22, wherein said amino-functionalised graphene oxide comprises graphene oxide and a polymer comprising amino groups, optionally wherein the polymer comprising amino groups is selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof; preferably wherein said polymer comprising amino groups is poly(ethyleneimine).. 24. A coating solution according to clause 23, wherein the polymer comprising amino groups is a first polymer, and the amino-functionalised graphene oxide further comprises a second polymer, wherein said second polymer is a hydrophilic polymer, optionally wherein said second polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof; preferably, wherein said second polymer is poly(ethylene glycol). 25. A coating solution according to any of clauses 22 to 24 wherein said coating solution further comprises a biodegradable polymer, preferably wherein said biodegradable polymer comprises a polymer selected from polycaprolactone, polyglycolic acid (PGA), polylactic acid (PLA), poly-p-hydroxybutyrate (PHB), poly (lactic acid-co-glycolic acid) (PLGA), and mixtures thereof; more preferably wherein said biodegradable polymer comprises polycaprolactone. 26. A coating solution according to any of claims 22 to 25 wherein said coating solution further comprises an additional polymer, optionally wherein said additional polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, and preferably wherein the additional polymer is poly(ethylene glycol). 27. A coating solution according to any one of clauses 22 to 26, wherein said organic solvent is tetrohydrofuran, acetone, hexane, toluene, xylene or mixtures thereof. 28. A coating solution according to any one of clauses 22 to 27, wherein said organic solvent is tetrohydrofuran. 29. A process for producing a coated implantable medical device, wherein the process comprises disposing a coating solution as defined in any one of clauses 22 to 28 on an implantable medical device. 30. Amino-functionalised graphene oxide, which comprises: graphene oxide; a first polymer which is a polymer comprising amino groups; and a second polymer which is a hydrophilic polymer; wherein the percentage by weight of the first polymer is from 8% to 35% , based on the total weight of the amino-functionalised graphene oxide. 31. Amino-functionalised graphene oxide according to clause 30, wherein the percentage by weight of the second polymer is greater than 0% and less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide. 32. Amino-functionalised graphene oxide according to clause 30 or clause 31 wherein: the percentage by weight of the first polymer is from 10% to 30%, based on the total weight of the amino-functionalised graphene oxide; and / or the percentage by weight of the second polymer is from 2% to 10%, based of the total weight of the amino-functionalised graphene oxide. 36 33. Amino-functionalised graphene oxide according to any one of clauses 30 to 32, wherein said first polymer is selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof, preferably wherein said first polymer is poly(ethyleneimine). 34. Amino-functionalised graphene oxide according to any one of clauses 30 to 33, wherein said second polymer is selected from poly(ethylene glycol), poly (A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, preferably wherein said second polymer is poly(ethylene glycol). 35. Amino-functionalised graphene oxide according to any one of clauses 30 to 34, wherein the second polymer is bonded to the graphene oxide, and the first polymer is bonded to the second polymer. 36. An implantable medical device functionalised with amino groups, for use in producing NO in vivo. 37. A method of generating NO in vivo wherein said method comprises administering amino-functionalised graphene oxide to a subject. 38. A method of generating NO in vivo wherein said method comprises implanting an implantable medical device functionalised with amino groups into a subject. EXAMPLES Materials and methods Chemicals and reagents Sodium nitrate (NaNOs), sulfuric acid (H2SO4 - 95.0-98.0%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2 - 30 wt%), and hydrochloric acid (HC1 - 36 wt%) were purchased from Thermo Scientific, Fisher Scientific, Acros, Nacalai Tesque, and Alfa Aesar, respectively. Graphite flakes, branched PEI with molecular weight (MW) of 25 kDa, N-(3-dimethylaminopropyl-A'-ethylcarbodiimide) hydrochloride (EDC), branched polyethylene glycol (PEG) with MW of 10 kDa, phosphate-buffered saline (PBS), sodium nitrite (NaNO2 -97%), potassium iodide, vanadium chloride, S- Nitroso-A-Acetyl-D,L-Penicillamine (SNAP), and 5-Nitroso-L-glutathione (GSNO), polycaprolactone (PCL, Mn = 80 000), tetrahydrofuran (THF), EDTA, TEM copper grid, and acid orange II (AO II) were purchased from Sigma Aldrich. All materials were used as received without any further purification unless stated otherwise. Example 1 - Synthesis of graphene oxide Exfoliated graphite was used to create exfoliated graphene oxide (GO) flakes using a modified Hummer's method (Tabish et al, Cancers, 2019; Tabish et al, Sci Rep, 2018; Tabish et al, Nanotechnology, 2017). A 800 niL round-bottom flask containing 2 g of graphite flakes was filled with 1.5 g of NaNOs and 150 mL (98%) H2SO4.The reaction mixture was mixed using magnetic stirring, and the flask was then submerged in an oil bath. The mixture was then heated to 35 °C prior the addition of KM11O4 (9 g) into the flask. The mixture was then stirred for 24 h, followed by the further addition of H2SO4 (280 mL, 5%), the temperature was subsequently increased to 85-95 °C. The mixture was then stirred for additional 2 h before being allowed to cool to 60 °C. Finally, H2O2 (15 mL, 30 wt%) was added, and the mixture was subsequently stirred for another 2 h. The product was washed "-8 times with HCI (3 wt%), and washed 4-5 times with distilled water to remove contaminants. The resulting GO was dispersed by stirring in water. Example 2 Synthesis of amino-functionalised graphene oxide PEG and PEI were covalently conjugated to GO by the formation of an amide bond between PEI, PEG, and GO using N-(3-Dimethylaminopropyl)-N'- 38 ethylcarbodiimide hydrochloride (EDC). EDC coupling involves the conjugation of carboxylic functional groups to primary amines. GO possesses carboxylic functional groups on its surface, which act as anchoring agents for the functionalization of GO with biomolecules through electrostatic interactions. EDC in the presence of carboxylic groups forms O-acylisourea, which then reacts with primary amino groups to form an amide bond. This reaction results in a by-product of urea derivatives, which are purified using centrifugation or filtration methods (Wickramathilaka et al, Journal of Biological Engineering, 2019). The synthesis process has also been schematically illustrated in Figure 1. Specifically GO was functionalized with PEI and PEG by the formation of an amide bond between PEI, PEG, and GO in the presence of EDC. To synthesize PEI-PEG@GO, GO (10 mg) was dispersed in distilled water (10 mL) and was then mixed with 6-armed amine-terminated PEG with a molecular weight of 10 kDA (5 mg dissolved in 10 mL distilled water). The mixture was sonicated for 5 min. EDC (10 mg dissolved in 5 mL distilled water) was added to the reaction mixture and was sonicated for another 5 min. The mixture was stirred at room temperature for 10 min. The mixture was sonicated for 5 min following the addition of PEI (50 mg dissolved in 10 mL distilled water). EDC was added again (10 mg, dissolved in 20 mL distilled water) and was stirred at room temperature overnight. The product was washed according as follows: the resultant product was dissolved in NaCl (1.6 g) and centrifuged at 13000 rpm for 2 h and subsequent filtration using a 100 K ultrafilter. The product received from ultrafiltration was washed 3-4 times with an aqueous solution that contained 10% NaCl in order to remove any unreacted PEI, PEG, and urea. Free PEI and PEG in the ultrafiltrate were detected by orange acid II (OAII), to confirm the complete removal of unreacted PEI and PEG. The step-by-step protocol of OAII is given in the section on stability testing. The product was repeatedly washed with distilled water to remove NaCl and finally dispersed in water to form further concentrations. To synthesize PEI@GO, the same procedure was followed except without the addition of PEG. GO has good dispersibility in water without significant aggregation; however, the attachment of hydrophilic polymers such as PEG significantly improves the dispersibility and stability of GO in biological buffers, which is crucial for biomedical applications (Feng et al, Small, 2013). Moreover, PEI was chosen to introduce primary amine groups on 39 the surface of GO. PEI contains a significant proportion of amino groups, which may serve as building blocks for the catalytic decomposition of endogenous NO sources; however, PEI tends to aggregate over time and becomes unstable (Luo et al, J Mater Sci Mater Med, 2010). The instability of PEI@GO in serum has been attributed to protein adsorption on the surface of GO by electrostatic attractions (Hu et al, ACS Nano, 2011 \ Du et al, J Am Chem Soc, 2011; Luo et al, ACS Appl Mater Interfaces, 2015), thereby neutralizing the surface charge and resulting in GO aggregation. The addition of a hydrophilic protective layer of PEG should prevent undesirable binding of proteins to the surface of GO via the principle of spatial repulsion rejection (Feng et al. Small, 2013). It has previously been shown that PEG significantly improves the stability of PEI conjugated with a diverse range of materials and biomolecules (Conte et al, Nanomaterials, 2019; Sung et al, Biol Pharm Bull, 2003; Wang et al, ACS Sustainable Chemical Engineering, 2019); therefore, both PEI@GO and PEI-PEG@GO were prepared. Example 2 - Physiochemical characterisation of amino-functionalised GO The morphology of GO and PEI-PEG@GO was evaluated using transmission electron microscopy (TEM), with GO exhibiting a distribution of stacked layers in a flat manner. In contrast, the PEI-PEG@GO showed a distribution of stacked layers in an irregular manner due to the amide linkage (Figure 2). This demonstrates that the conjugation of PEI and PEG did not modify the stacked layered-like morphology of GO. X-ray diffraction (XRD) analysis showed the characteristic sharp diffraction peak of GO at 29 = 12.5° corresponding to the (001) plane and indicating the presence of functional groups in the basal planes of graphene (Figure 3). This diffraction peak was shifted to a low angle of 7.5° in both PEI@GO and PEI-PEG@GO due to the incorporation of PEI and PEG within the graphene, which increases the distance between layers. The functionalized PEI@GO showed a broadening of the diffraction peak at 29 = 23° indicating the introduction of amorphous structure to GO, whereas the appearance of a new peak at 21.5° in PEI-PEG@GO reveals the attachment of amorphous polymer. Raman spectroscopy identified the typical Raman bands of graphene (Figure 4), with the D band at 1,359 cm 1 representing the breathing mode of aromatic rings and the introduction of other conjugated molecules, while the G band at 1,699 cm 1 is assigned to the 49 graphitic nature representing the first-order scattering of carbon sp2 atoms (Tabish et al, Nanotechnology, 2017). The reduction in both D and G bands indicates the molecular charge transfer resulting from the covalent conjugation of GO with PEI and PEG. The functionalization interaction of PEI-PEG@GO was further characterized by FTIR, where spectra show the presence of -CH (~2900 cm -COOH (~ 1717 cm '), and C=O (~1110 cm 1) functional groups in GO (Figure 5). The peak at ~ 1400 cm'1 is assigned to the stretching vibrations of the C:::: O band of carbonyl groups, indicating the lack of carboxyl groups on the surface of GO, while the appearance of this peak in the functionalized sample is attributed the ester bonding between PEI, PEG and GO. The FTIR spectra of functionalized samples revealed that all the GO peaks were preserved with a slight shift of peak positions and variation of intensity, confirming the successful conjugation of GO with PEI and PEG. The relative amounts of PEI and PEG on the surface of GO were further estimated by thermal gravimetric analysis (TGA) (Figure 6). Three obvious peaks at ~100, 230, and 500 °C (labelled as 1,2, and 3) are attributed to the evaporation of residual water between sheets, the elimination of oxygen-containing functional groups, and the breakdown and combustion of carbon, respectively (Li etal, ACS ApplMater Interfaces, 2015). In particular, the PEI@GO and PEI-PEG@GO samples exhibited more pronounced weight loss than GO between 250 °C and 400 °C, which can be attributed to the decomposition of covalently bonded PEI and PEI-PEG molecules (Li et al, ACS ApplMater Interfaces, 2015; Qin et al, Front Chem, 2022). The proportion of sample weight for PEI and PEG on PEI-PEG@GO is calculated to be 15% and 4.2%, respectively. Water contact angle profiles of GO, PEI@GO, and PEI-PEG@GO gave values of 20.3°, 14.5°, and 8.6° respectively (Figure 7), indicating the solubility of functionalized GO was better than GO alone and represents further indirect evidence for the successful conjugation of PEI-PEG with GO. For transmission electron microscopy (TEM) analysis, a drop of the graphene samples was deposited on holey carbon copper grids (Protochips, Morrisville, NC, USA). Images were obtained at room temperature using a 300 kV Titan Krios electron microscope that was equipped with a Falcon 3 detector (Thermo Scientific, Waltham, MA, USA) and a Cs corrector (CEOS). X-ray diffraction (XRD) analysis was obtained using Cu Ka radiation; the measurements were performed at a voltage of 40 kV and a current of 40 mA. The spectra were collected at a step size of 0.02° (20) and a step time of 1 s. Fourier-transform infrared (FTIR) spectroscopy measurements were obtained using a Tensor-27 FTIR spectrometer (Bruker Optics, Champs-sur-Marne, France) over the wavenumber range of 4000^500 cm d. FTIR samples were prepared by mixing the samples with potassium bromide (KBr). Raman spectroscopy measurements were performed using laser excitation at 532 nm (Renishaw, Stroud, UK). TGA measurements were carried out on a TGA Q5000 from TA Instruments under synthetic air with a heating ramp of 10 °C min i. A contact angle goniometer was used to calculate the wettability and water solubility of GO, PEI@GO, and PEI-PEG@GO. A digital camera was used to capture the images, and the contact angle was measured using the ImageJ processing program. The contact angle surfaces were developed by dropping a 10 pL drop onto a glass slide. Example 3 - Catalytic generation of NO To investigate the real-time kinetics of NO release from S-nitrosothio1s, an NO electrochemical sensor in phosphate-buffered saline (PBS) (7.4 pH) at 37 °C under continuous stirring, which can selectively detect the generation of NO concentrations above ~1 nM in real-time was used (Xu et al, Electroanalysis, 2014). EDTA was used as a metal ion ligand to block the spontaneous decomposition of both NO sources in the presence of impurities (Bramanti et al, Taianta, 2010). S-nitroso-N-acetylpenicillamine (SNAP) was chosen as a model S-nitrosothiol (RSNO), and S-nitrosoglutathione (GSNO) was chosen as one of the most abundant endogenous NO donors, present at micromolar levels in blood and human tissues. GSNO or SNAP (10 pM) was added to the PBS solution with or without different compounds to detect NO generation over time. Both GSNO and SNAP on their own. were found to spontaneously produce NO up to 19.5 and 11.5 nM / min, respectively. However, the use of PEI-PEG@GO with GSNO or SNAP synergistically increased the rate and quantity of NO production compared to any component alone (Figure 8). It is postulated that the fast rate of NO generation was facilitated by the high density of amine groups on the surface of GO, while the slow rate of NO generation after 20 min (GSNO) and 35 min (SNAP) was facilitated by the amine groups present within the layers of GO. These results demonstrate the possibility of using functionalized GO to catalytically generate NO from endogenous NO sources by tailoring the form of polymers. The total capacity for NO production was then measured by ozone-chemiluminescence using either GSNO or SNAP as substrates. In this assay, the conditions promote rapid decomposition of NO, producing a single narrow peak calibrated for NO concentration. Figure 9A ami B demonstrate that in the absence of the substrate, there is negligible NO production for PEG or PEI on their own or for any formulation of functionalized GO (note y-axis scale relative to panels C and D). Spontaneous NO release was observed with GSNO alone, but total NO production was much higher in the presence of PEI-PEG@GO and in a concentration-dependent manner, which is consistent with a catalytic effect of PEI-PEG-GO (Figure 9C) A similar pattern was observed when using SNAP as a substrate, but with even higher levels of NO production (Figure 9D). PEI-PEG@GO was calculated to release up to 62% of the available NO from GSNO and 91 % from SNAP, indicating a high reaction efficiency. Collectively, these results demonstrate that controlled NO generation can be achieved by tuning the concentrations of PEI-PEG@GO and GSNO or SNAP. The NO generated during the catalytic GSNO and SNAP decomposition was purged from the test solution using helium gas and detected with a chemiluminescence NO analyzer (NOA) (Seivers 280, Boulder, CO) (Kalla el al, The Journal of Physiology; 2016; Bailey et al, Cell Reports: 2019). PBS (pH 7.3) comprising of 0.5 mM EDTA was used as the working solution in order to prepare the required concentrations of GSNO and SNAP. 10 ul aliquot of each sample was added into the purging vessel (with a constant flow / purge of helium gas) using a gas-tight syringe through the injection unit of the vessel. The purging vessel was covered with aluminium foil in order to prevent the light exposure. Each measurement was continued until the NO spectram returned to the baseline levels within several minutes. The calibration curve results were obtained via sodium nitrite reduction in an acidified vanadium chloride solution. The amount of NO generated from the sample was determined based on the calibration curves. Raw data was processed using Microsoft Excel 2010 (Microsoft, Redmond, WA, USA). Real-time NO generation was examined using a free radical analyzer (TBR4100, World Precision Instraments), which was equipped with a NO-sensitive electrode (ISO-NOP, World Precision Instruments) The desired concentrations of GSNO and SNAP were prepared in PBS solution (pH 7.3) containing 0.5 mM EDTA. The reference electrode was polarized 43 and calibrated according to the manufacturer’s instructions. The probe was submerged in 10 mL 0.1 M H2SO4 / 0.1 M potassium iodide solution in a glass vial in order to achieve a stable and flat current baseline. Increasing volumes of 25 pM sodium nitrite solution was added to the mixture in order to generate NO concentrations for the calibration curve. The generated NO concentrations were calculated according to the amount of sodium nitrite input as the conversion of sodium nitrite to NO was stoichiometrically in a 1:1 ratio. To detect the concentration of NO produced from GSNO by PEI, PEG, GO, and PEI-PEG@GO, the NO probe was submerged in a glass vial that was filled with 4 mL of PEI, PEG, GO, and PEI-PEG@GO (125-1000 pg mLN ) in PBS buffer (pH 7.3). 50 pL of GSNO solution, with a final concentration of 10 pM, was introduced to the vial when a stable flat baseline had been achieved. Changes in current response were measured in a real-time manner; the generated NO was analyzed from the calibration curve. Glass vials were shielded by aluminium foil in order to protect the sample solutions from light exposure and maintained at 37 °C using a magnetic stirrer plate with constant stirring. Example 4 - Coating of functionalized graphene on 3D printed stent material The computer-aided design (CAD) of the stent was developed using Blender software (Blender® 2.79b - Open-source 3D editor); it was printed with Ray Ware software (Ray Ware 1.4.5). An STL file was generated and was tested for errors and modified according to the required dimensions. In order to slice the 3D model into multiple thin layers and to generate the coordinates (G-code), the Standard Triangle language (STL) file was processed using a slicer software A Prusa MK3S 3D printer containing a 0.25mm nozzle and Prusament PLA filament was used to prepare the stent. The nozzle and bed temperatures were set to 215 and 65°C. The layer height was set to 0.1 mm to achieve high-quality and accurate prints. Stents of 4 mm inner diameter, 5 mm out diameter, and 8 mm length were printed. The coating solution was prepared by dissolving 100 mg of PEG and 100 mg PCL (El)in 1 mL of THF with continuous stirring at room temperature for 90 minutes. 10 wt % (based on the weight of the PEG or the PCL individually) of functionalized or pristine GO (i.e. 10 mg of functionalized or pristine GO) was added to the solution and stirred for a further 60 minutes until the material was completely dispersed in solution. Stents were dipped in the 44 coating solution for a few seconds and then kept in the dark at room temperature for solvent evaporation. The stents were vacuum-dried and stored at -20 °C. One objective of the present invention is to use a simple and easy-to-apply coating method to provide a uniform layer on both the outer and inner shells of the biodegradable stent material. 3D printed polylactic acid (PLA) stent material of 4 mm inner diameter, 5 mm out diameter, and 8 mm length exhibited well-defined edges (Figure 10). SEM micrographs show that the uncoated stent had a well-defined geometry with a smooth surface finish, while GO and PEI-PEG@GO coatings had a layered-like morphology (Figure 11). The 3D-printed stent material was dip-coated by immersion in a solution of tetrahydrofuran (THF), PCL, and PEG. THF was used as a solvent since it evaporates rapidly, leading to the formation of a uniform surface layer; a combination of PCL and PEG was used to increase the viscosity and reduce the surface tension of the coating solution to avoid the likelihood that THF would dissolve the PLA-based stent. The coating solution was optimized to achieve the highest coating uniformity by varying the ratios of PEG to PCL and using Raman spectroscopy to assess uniformity at multiple points on the stent surface (Figure 12). A 1:1 ratio of PEG and PCL in 10% by weight provided the most uniform coating as demonstrated by the distinct Raman D (1,350 cm ') and G (1,600 cm ' l peaks characteristic of GO (Tabish et al, Nanotechnology, 2017) and the absence of peaks that are typical for PLA, suggesting that PEI-PEG@GO successfully coated the stent both inside and out. Unsuccessful coating formulations had Raman spectra similar to the uncoated stent material with PLA-associated peaks at 2992 cm"1, 1752 cm 1457 cm-1, and 876 cm’1 (Pandele et al, Materials, 2020). Similarly, FTIR spectra suggested complete coating and crosslinking of the stent material, as evidenced by the groups at 2800, 1750, and 1100 cm’1 (Figure 13). The FTIR spectrum of PLA shows a peak at 1100 cm 1 that corresponds to the vibrations of the C=O bond, and a peak at 1750 cm"1 that corresponds to the stretching vibrations of the C=O bond; in the case of coated PLA, the new band at 2800 cm"1 corresponds to the asymmetrical and symmetrical stretching vibrations of the C-H. The spectra of the coated PLA are similar to the PLA spectrum, except for the peak of PEI-PEG@GO, suggesting the coverage of PLA by functionalized GO. The morphologies of the samples were assessed using images that were obtained from a JEOL JSM-7500F neki-ermssion scanning electron microscope (SEM) with 50 mm2 X-MAX detector from Oxford Instruments (Abingdon, United Kingdom). The coated and uncoated materials were cut. using a sharp blade. The specimens were sputter-coated with gold. The samples were also characterized using FTIR. The details of instrument has been given in the previous section. FTIR samples were prepared by cutting the samples using a sharp blade. A confocal Raman microscope (LabRAM HR Evolution, Horiba Scientific, UK) containing an integrated microscope (BX4L Olympus), a motorized XYZ stage, and a 532-nm neodymium-yttrium aluminium garnet laser was used to capture Raman spectra. Spectra were collected using a 50x air objective lens (numerical aperture 0.5), a 0.5-s collection time, and a 300 grooves / mm diffraction grating for an input laser power of 80 mW passing through a 25% neutral density (ND) filter. In each condition, 100-450 spectra were acquired with a spatial resolution at -1.5 urn between each spectrum. The random fields of view were selected for each sample with -18 random points mapping per field view and -54 points per sample. Example 5 - Hydrolytic degradation testing of coated and uncoaled stents The hydrolytic degradation of polymers under physiological conditions is slow, taking months to several years (Miles et al, Polymer degradation and stability, 2021); therefore, accelerated degradation at higher temperatures is widely used to study the decomposition of materials over shorter timescales (Miles et al, Polymer Degradation and Stability, 2021; Bartnikowski et al, Progress in Polymer Science, 2019). The hydrolysis of PLA will result in the release of lactate monomers from the polymer structure, which will be either excreted or metabolized in vivo. However, it is unknown how the degradation rate of PLA will be altered by the presence of a PEI-PEG@GO coating. Therefore degradation rates of coated and uncoated stents were measured at 60 °C to provide accelerated hydrolytic conditions (Dong etal, Journal of Polymer Research, 2011). In the case of uncoated PLA, the weight loss was 20% by day 4, and reached 72.5 % by day 60, while weight loss for PEI-PEG@GO coated PLA was slightly delayed, reaching 17% by day 12 and 61% over 60 days (Figure 14). At day 56, the degradation curve of both coated and uncoated materials is converging; however, the coating remains mostly intact as indicated by the continued presence of amine groups measured by colorimetric assay (Figure 15). The stability of amine groups depends on the nature and strength of bonding (Woods et al, J Proteome Res, 2008) and amine content will typically reduce at higher temperatures (Ge et al, Environ Sei Technol, 2014). Their presence after two months is likely due to the strong intermolecular interaction between GO and amine groups via n-n bonding. The morphology of degraded stents at each time point was evaluated using SEM to further explore the degradation pathways (Figure 16). The stent design was preserved under all conditions and time points, demonstrating that this is a degradation process rather than due to erosion. Taken together, it can be concluded that the PE1-PEG@GO coating alters the degradation kinetics of PLA-based stents, but there is convergence at longer time points, suggesting that the overall life span of the stent will not be significantly altered. Importantly, the PEI-PEG@GO coating remained largely intact over the period tested. Stents were weighed and kept at 60 ± 1 °C in individual vials containing 5 mL of PBS at pH 7.4 (n=3 / group). Every four days, the stent was washed using distilled water at room temperature and then dried at ambient temperature under vacuum until a constant weight was obtained. The dried samples were weighed to determine the average mass loss at each time point (Equation below). The PBS solution was changed weekly. The surface of the coated and uncoated stents at the end of each experiment was analyzed using SEM. Weight loss (%) = 100 x (Final weight - initial weight) / initial weight Example 6 - Shelf-life stability testing of coated stent materials Graphene nanostructures remain remarkably chemically stable under ambient conditions (Liu et al, Angew Chern Int Ed Engl, 2020), and it has been shown previously that coupling of PEG improves the stability of PEI (Sung et al, Biol Pharm Bull, 2003). To investigate the stability of the PEI-PEG@GO coating the stability of conjugated amine content on 3D printed PLA under different conditions was evaluated (dark at room temperature, light at room temperature, and dark at -20 °C) using a standard colorimetric assay based on Orange acid II (OA II) (Noel et al, Biocon jug Chern, 2011; Teixoto et al, Macromolecular Materials and Engineering, 2023). No detectable difference was observed between day 0 and day 60 under any of the tested conditions, indicating that the amine content is highly stable (Figure 17). The stability of amine groups and shelf-storage stability of coated stents were performed by the quantification of amine groups following the methods reported (Peixoto et al, Macromolecular Materials and Engineering, 2023; Qiu et al, Biomaterials, 2019). The samples were initially submerged in acid orange II (AO II) solution (500 mM in HC1 solution; pH 3) in order to measure the surface density of amino groups. After being incubated at 37 °C for 12 h, the samples were washed using HC1 solution (pH 3) to remove weakly attached AO II. The samples were then dissolved in NaOH solution (pH 12) at 37 °C for 15 min in order to release the electrostatically attached AO. The AO concentration, which corresponds to the surface density of amine groups on the samples, was measured in a colorimetric manner using a microplate reader with background subtraction at 485 nm. The samples were tested under different conditions, including under dark stored at -20°C, under dark stored at room temperature, and under light stored at room temperature. Statistical analysis NO generation data were obtained in triplicate (n=3) and are expressed as mean ± standard deviation. The standard deviation measurements are represented by error bars for the hydrolytic degradation (n=4), amine groups stability (n=2), and shelf-storage stability (n=3). The sample size has also been given in figure legends. A one-way ANOVA test was used to calculate statistical significance. Conclusion Multiple desirable functions have been integrated into one vascular stent through 3D printing and amine-catalyzed NO generation. The NO generated from functionalized graphene are 0.7 * 10'10 and 1.1 x 10"10 moj cm-2 min-l frOm 10 pM GSNO and 10 pM SNAP, respectively. Therefore, NO generation from functionalized graphene is physiologically relevant to the healthy endothelium, which continuously produces NO at the rate of 0.5 - 4 xlO'10 mol cm'2 min'1. Although NO generation capacity of graphene-48 coated stent will depend on the levels of GSNO and RSNO present in blood vessels. In comparison to exogenous NO donors, NO-generating nanomaterials may overcome the inherent limited loading capacities of NO donors and associated toxicity. The underlying mechanism of the catalytic role of amine-functionalized GO involves the nucleophilic reaction between the primary amines of PEI on GO and the SNO groups of GSNO and SNAP. These results demonstrate that amine-functionalized GO maintains its structural integrity under physiologically relevant conditions, which is of paramount importance to avoid aggregation / agglomeration and reduce its toxicity. 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Claims

1. Amino-functionalised graphene oxide, for use in the generation of NO in vivo.

2. Amino-functionalised graphene oxide for use according to claim 1, wherein saidamino-functionalised graphene oxide comprises graphene oxide and a polymer comprising amino groups.

3. Amino-functionalised graphene oxide for use according to claim 1 or claim 2, wherein said amino-functionalised graphene oxide is for use in the treatment or prevention of cardiovascular disease, cerebrovascular disease, aneurysm, bacterial infection, and / or formation of biofilms.

4. Amino-functionalised graphene oxide for use according to claim 3, wherein said cardiovascular disease is selected from coronary artery disease, atherosclerosis, thrombosis, arterial stenosis, peripheral vascular disease and restenosis.

5. Amino-functionalised graphene oxide for use according to any one of claims 2 to 4, wherein said polymer comprising amino groups is a first polymer and the amino-functionalised graphene oxide further comprises a second polymer, wherein said second polymer is a hydrophilic polymer.

6. Amino-functionalised graphene oxide for use according to claim 5, wherein the second polymer is bonded to the graphene oxide, and the first polymer is bonded to the second polymer.

7. Amino-functionalised graphene oxide for use according to any one of claims 2 to 6, wherein the polymer comprising amino groups is selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine), and mixtures thereof, preferably wherein the polymer comprising amino groups is poly(ethyleneimine).

8. Amino-functionalised graphene oxide for use according to any one of claims 5 to 7, wherein the hydrophilic polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, preferably wherein said hydrophilic polymer is poly(ethylene glycol).

9. Amino-functionalised graphene oxide for use according to any one of claims 1 to 8, wherein said use comprises implanting an implantable medical device coated with said amino-functionalised graphene oxide in a subject.

10. A coating for an implantable medical device which coating comprises amino-functionalised graphene oxide.

11. A coating according to claim 10, wherein the coating further comprises a biodegradable polymer.

12. A coated implantable medical device which comprises: (a) an implantable medical device, and (b) a coating on a surface of said implantable medical device, wherein said coating comprises amino-functionalised graphene oxide.

13. A coated implantable device according to claim 12, wherein said implantable medical device is selected from a stent, an arterial graft, a venous graft, a catheter, and a cannula.

14. A coating solution comprising an organic solvent and amino-functionalised graphene oxide.

15. A coating solution according to claim 14, wherein said amino-functionalised graphene oxide comprises graphene oxide and a polymer comprising amino groups, optionally wherein the polymer comprising amino groups is selected from polyvinylamine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof; preferably, wherein said polymer comprising amino groups is poly(ethyleneimine).

16. A coating solution according to claim 15, wherein the polymer comprising amino groups is a first polymer, and the amino-functionalised graphene oxide further comprises a second polymer, wherein said second polymer is a hydrophilic polymer, optionally wherein said second polymer is selected from poly(ethylene glycol), polyfV-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof; preferably, wherein said second polymer is poly(ethylene glycol).

17. A coating solution according to any of claims 14 to 16 wherein said coating solution further comprises a biodegradable polymer, preferably wherein said biodegradable polymer comprises polycaprolactone, or polyglycolic acid (PGA), polylactic acid (PLA), poly-P-hydroxybutyrate (PHB), poly (lactic acid-co-glycolic acid) (PLGA), and mixtures thereof, more preferably wherein said biodegradable polymer comprises polycaprolactone.

18. A coating solution according to any of claims 14 to 17 wherein said coating solution further comprises an additional polymer, optionally wherein said additional polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, and preferably wherein the additional polymer is poly(ethylene glycol).

19. A process for producing a coated implantable medical device, wherein the process comprises disposing a coating solution as defined in any one of claims 14 to 18 on an implantable medical device.

20. Amino-functionalised graphene oxide, which comprises:graphene oxide;a first polymer which is a polymer comprising amino groups; anda second polymer which is a hydrophilic polymer;wherein the percentage by weight of the first polymer is from 8% to 35% , based on the total weight of the amino-functionalised graphene oxide.

21. Amino-functionalised graphene oxide according to claim 20, wherein the percentage by weight of the second polymer is greater than 0% and less than or equal to 12%, based on the total weight of the amino-functionalised graphene oxide.

22. Amino-functionalised graphene oxide according to claim 20 or claim 21 wherein: the percentage by weight of the first polymer is from 10% to 30%, based on the total weight of the amino-functionalised graphene oxide; and / orthe percentage by weight of the second polymer is from 2% to 10%, based of the total weight of the amino-functionalised graphene oxide.

23. Amino-functionalised graphene oxide according to any one of claims 20 to 22, wherein said first polymer is selected from polyvinyl amine, poly(allylamine), poly(4-aminostyrene), poly(l-lysine hydrobromide), poly(propyleneimine), poly(ethyleneimine) and mixtures thereof; preferably wherein said first polymer is poly(ethyleneimine).

24. Amino-functionalised graphene oxide according to any one of claims 20 to 23, wherein said second polymer is selected from poly(ethylene glycol), poly(A-vinylpyrrolidone), polyacrylamides, polybetaines, poly(2-oxazoline)s, polysarcosine and mixtures thereof, preferably wherein said second polymer is poly(ethylene glycol).

25. Amino-functionalised graphene oxide according to any one of claims 20 to 24, wherein the second polymer is bonded to the graphene oxide, and the first polymer is bonded to the second polymer.63

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