Plasma-mediated polymer grafting methodology

Abstract

The present disclosure relates to methods of forming a graft of a second polymer on the surface of a material comprised of a first polymer, and methods of preventing the deterioration of said graft.

Description

Plasma-mediated polymer grafting methodologyTechnical field

[0001] The present disclosure broadly relates to methods for grafting polymers to polymeric surfaces.Background

[0002] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Blood-contacting polymeric medical devices are highly susceptible to adverse biological responses during clinical use, compromising patient health and limiting device durability. Without the protective mechanisms that the endothelium imbues to native vasculature, polymeric materials regularly succumb to thrombosis, inflammation, the foreign body reaction, calcification, or infection at their interface with biological systems. In particular, thrombosis frequently eventuates in many blood-contacting devices, such as central venous catheters (CVCs), mechanical heart valve replacements, ventricular assist devices, and extracorporeal membrane oxygenation circuits. Furthermore, blood-contacting devices are prone to infection, such as cases of infective endocarditis after transcatheter heart valve implantation, or bloodstream infections after CVC placement.

[0004] Many biological responses are fundamentally initiated by the nonspecific adsorption of blood proteins onto the biomaterial surface, herein termed biofouling, which occurs within seconds of contact with blood. To this end, polymeric materials with antibiofouling properties may limit adverse downstream pathologies. Polymers commonly used in blood-contacting devices typically aim to replicate the mechanical properties of the surrounding tissues, such as polyurethanes (PUs), which possess a unique combination of strength and elasticity that closely imitates the mechanics of soft biological tissues.

[0005] The suboptimal biocompatibility of such polymers can be addressed by grafting biocompatible moieties onto the surface of a mechanically suitable substrate, to improve biocompatibility without affecting bulk mechanics. Passive surface modifications may facilitate antifouling properties by pre-emptively mitigating interactions between the substrate surface and the biological environment, which is achieved by minimising the thermodynamic and kinetic incentives for protein adsorption. A range of chemistries may allow this, including, in particular zwitterions.

[0006] Zwitterions contain positively and negatively charged groups, which both facilitate strong electrostatic interactions with water molecule dipoles. Moreover, the number of anions and cations within the zwitterion is balanced, equating to overall electrical neutrality. This charged but neutral chemistry permits a tightly associated but loosely structured shell of water molecules to form around the zwitterion, allowing associated water to exist in a structurally similar state to bulk water. This removes the entropic incentive for water to dissociate in the event of protein encroachment, thus maintaining an energetically stable hydration barrier that imbues the underlying substrate with antifouling properties. Zwitterionic surface modifications considerably reduce protein adsorption and cellular adhesion on underlying substrates, resulting in lower incidences of thrombosis, fibrosis, calcification, and infection. The development of a suitable zwitterion grafting process is required to ensure effective, versatile and scalable modifications of polymeric blood-contacting devices to improve long-term biocompatibility.

[0007] Most of these processes require chemical linkers such as those that rely on dopamine or azide moieties. Instead, a linker-free process may prove superior in its scalability and versatility. One outstanding linker-free grafting strategy involves the functionalisation of the underlying polymer with plasma, providing surface-anchored reactive sites that can participate in zwitterion grafting.

[0008] Such zwitterion-grafted polymers should be resistant to biofouling over the entire period of their use. However, the long-term (over several months) durability of polymeric grafts, particularly those prepared using a plasma-based methodology, has not previously been studied.Summary

[0009] In a first aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of (optionally consisting essentially of) a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days, or at least about 1 day or about 1 to about 20 days.

[0010] In another aspect, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of (optionally consisting essentially of) a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer, said solution containing no free radical initiator.

[0011] The contacting may be for a duration of at least about 10 days, or at least about 1 day or about 1 to about 20 days.

[0012] In a second aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of (optionally consisting essentially of) a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days, or at least about 1 day or about 1 to about 20 days.

[0013] The following options may be used in conjunction with any of the above aspects of the disclosure, either individually or in any combination.

[0014] The contacting the surface with a solution comprising monomers may be for a duration of between about 10 days and about 20 days, or at least about 1 day or about 1 to about 20 days. The contacting the surface with a solution comprising monomers may be for a duration of about 14 days.

[0015] The method of the first and / or second aspect of the disclosure may further comprise covering the surface of the material with a polar medium.

[0016] In a third aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment,contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0017] In a fourth aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0018] In a fifth aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, wherein the graft of the second polymer has been prepared by: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; the method comprising covering the surface with a polar medium.

[0019] The following options may be used in conjunction with the third, fourth, and / or fifth aspects of the disclosure, either individually or in any combination.

[0020] Covering the surface of the material with a polar medium may comprise immersing the material in the polar medium.

[0021] The polar medium may be selected from water, methanol, ethanol, and a mixture of water with methanol and / or ethanol.

[0022] In a sixth aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, andcontacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M, or at least about 0.1 M.

[0023] In a seventh aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer; wherein the solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M, or at least about 0.1 M.

[0024] The following options may be used in conjunction with the sixth and / or seventh aspects of the disclosure, either individually or in any combination.

[0025] The concentration of the solution comprising monomers may be between about 0.5 M and about 5 M or between about 0.1 M and 5 M. The concentration of the solution comprising monomers may be between about 0.7 M and about 2 M.

[0026] The contacting the surface with a solution comprising monomers may be for a duration of at least about 10 days. The contacting the surface with a solution comprising monomers may be for a duration of between about 10 days and about 20 days. The contacting the surface with a solution comprising monomers may be for a duration of about 14 days.

[0027] The method of the sixth and / or seventh aspect of the disclosure may further comprise covering the surface of the material with a polar medium. The polar medium may be selected from water, methanol, ethanol, and a mixture of water with methanol and / or ethanol.

[0028] The following options may be used in conjunction with the first, second, third, fourth, fifth, sixth and / or seventh aspects of the disclosure, either individually or in any combination.

[0029] The first polymer may have a glass transition temperature below about body temperature.

[0030] The first polymer may be selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, and polytetrafluoroethylene and mixtures / blends of any two or more of these.

[0031] The monomer may be a zwitterionic compound comprising a polymerizable functional group. The second polymer may be a zwitterionic polymer. The polymerizable functional group may be a polymerizable double bond. The monomer may be selected from the group consisting of sulfobetaine methacrylate, 2-methacryloyloxyethyl phosphorylcholine, 2-[[2- (methacryloyloxy)ethyl]dimethylammonio]acetate, 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propionate, 3-[(3- acrylamidopropyl)dimethylammonio]propanoate, 4-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[bis[2-(methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, 3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3- methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and 3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate. The monomer may be sulfobetaine methacrylate and the second polymer may be poly(sulfobetaine methacrylate).

[0032] The step of contacting the surface with a solution comprising monomers may comprise immersing the material in the solution comprising monomers. The step of contacting the surface with a solution comprising monomers may be carried out at a temperature between about 20 °C and about 80 °C, or about 0 °C and 80 °C.

[0033] The plasma treatment may be selected from the group consisting of plasma-immersion ion implantation, plasma cleaning, and plasma treated media. The plasma treatment may be plasma-immersion ion implantation.

[0034] The water contact angle of the surface of the material 60 days after being subjected to the plasma treatment is increased by less than 100%, compared to the water contact angle of the surface of the material one day following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer.

[0035] The deterioration of the graft of the second polymer on the surface of the material comprised of the first polymer may be reduced. The deterioration of the graft of the second polymer on the surface of the material comprised of the first polymer may be reduced whenthe water contact angle of the surface of the material 60 days after being subjected to the plasma treatment is increased by less than 100%, compared to the water contact angle of the surface of the material one day following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer.

[0036] In a further aspect, there is provided a method for forming a coating on a surface, said surface comprising a first polymer, said method comprising subjecting the surface to a plasma treatment, contacting the surface with a monomer so as to graft a layer of a second polymer on the first polymer; and applying a treatment to the second polymer so as to retain, or at least partially retain, one or more properties of the second polymer on said surface.

[0037] The following options may be used with this aspect, either individually or in any suitable combination.

[0038] The surface may form at least a part of a surface of a medical device.

[0039] The one or more properties may include hydrophilicity.

[0040] The first polymer may be selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, polytetrafluoroethylene, polymethylpentene. It may be some other carbon-based material such as graphite, graphene, or pyrolyzed graphene.

[0041] The second polymer is a polymer of the monomer. It may be a homopolymer thereof or a copolymer thereof. It may be linear or it may be crosslinked. The second polymer may be a polar polymer. The monomer may be a zwitterionic compound comprising a polymerisable functional group. The polymerizable group may be polymerizable by a free radical polymerisation mechanism. The polymerizable group may be an acryloyloxy group or a methacryloyloxy group or an acrylamido group or a methacrylamido group. The monomer may be selected from the group consisting of sulfobetaine methacrylate, 2-methacryloyloxyethyl phosphorylcholine, 2-[[2-(methacryloyloxy)ethyl]dimethylammonio]acetate, 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propionate, 3-[(3- acrylamidopropyl)dimethylammonio]propanoate, 4-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[bis[2-(methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, 3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3- methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and 3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate.

[0042] The plasma treatment may comprise plasma-immersion ion implantation.

[0043] The step of contacting may comprise exposing the surface to a solution of the monomer.

[0044] The treatment may comprise covering the second polymer with a polar material, for example an aqueous liquid. The treatment may comprise crosslinking the second polymer. The treatment may be applied concurrently with the contacting or it may be applied subsequently or a portion of the treatment may be applied concurrently and a portion may be applied subsequently. For example, crosslinking may be applied concurrently and covering with a polar material may be conducted subsequently. The one or more properties may comprise hydrophilicity, e.g. super-hydrophilicity.

[0045] The monomer may be contacted with the surface in the absence of a free radical initiator.

[0046] In yet another aspect there is provided an item having a surface comprising a first polymer and a second polymer grafted to the first polymer, wherein there is no linker group between the first polymer and the second polymer.

[0047] The following options may be used in conjunction with this aspect, either individually or in any suitable combination.

[0048] The item may form at least a part of a medical device.

[0049] The first polymer may be selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, and polytetrafluoroethylene, polymethylpentene. It may be some other carbon-based material such as graphite, graphene, or pyrolyzed graphene.

[0050] The second polymer may be polar. It may be a polymer of a zwitterionic compound comprising a polymerisable functional group. It may be crosslinked. The surface may be super-hydrophilic. It may retain a contact angle of less than about 25° when stored in water at 25°C for 12 weeks.

[0051] In still another aspect there is provided a method for rendering a surface hydrophilic, optionally super-hydrophilic, said method comprising subjecting the surface to a plasma treatment, and contacting the surface with a monomer so as to form a graft of a second polymer on the first polymer, said monomer being a zwitterionic compound comprising a polymerisable functional group; and applying a treatment so as to retain hydrophilicity of thesecond polymer on said surface, said treatment comprising either crosslinking the second polymer or covering the second polymer with a polar liquid or both.

[0052] In this aspect, hydrophilicity may be reflected by a water contact angle of less than about 50°, or less than about 45, 40, 35, 30 or 25°. The retaining may be for a time of at least about 1 week, or at least about 2, 3, 4, 5, 6, 7, 8, 9, 11 or 12 weeks.

[0053] In the above summary, certain aspects have options detailed. It should be understood that these options apply not only to the designated aspects but to any aspect to which they may be relevant. Thus, for example, method options may be applied to any method aspect unless they introduce an inconsistency.Brief Description of the Drawings

[0054] Figure 1. Morphology and elemental analysis of zwitterion-grafted polyurethanes and controls using SEM and XPS, respectively. The surface morphology of (a) PU, (b) C-PU, (c) PIII-PU, and (d) ZG-PU were imaged using scanning electron microscopy, demonstrating a distinct morphology formed due to Pill-mediated zwitterion grafting, (e) The XPS survey spectrum exhibited two new peaks at the S1 s and S2p orbitals for ZG-PU, which was absent in PU, C-PU, and PIII-PU. (f) Detailed analysis of the S2p spectrum demonstrated a significant peak for ZG-PU.

[0055] Figure 2. Surface and bulk polymer properties of ZG-PU. (a, b) ZG-PU was significantly more hydrophilic than controls, and (c) retained their hydrophilicity after an extensive mechanical bending program. The (d) Young’s modulus and the (e) flexural rigidity of ZG-PU were not significantly different to the control samples.

[0056] Figure 3. Hydrophobic recovery of ZG-PU and PIII-PU stored in air.

[0057] Figure 4. Minimising hydrophobic recovery by modifying the plasma functionalisation process, (a, b, c) The effect of changing PHI treatment time on the sample hydrophilicity and hydrophobic recovery, (d, e, f) The effect of changing PHI voltage on initial hydrophilicity and hydrophobic recovery, (g, h, i) The effect of changing plasma type on initial hydrophilicity and hydrophobic recovery. Note for each study, the first graph ( / .e., a, d, g) indicates the hydrophilicity of the material after fabrication, while the third graph ( / .e., c, f, i) indicates the hydrophilicity of the same respective materials three months after fabrication.

[0058] Figure 5. Minimising hydrophobic recovery: modifying polymerisation process and storage, (a, b) The effect of changing zwitterion concentration on the sample hydrophilicity and hydrophobic recovery, (c, d) The effect of changing zwitterion immersion time on initialhydrophilicity and hydrophobic recovery, (e, f) The effect of changing sample storage conditions on hydrophobic recovery, highlighting the difference in hydrophilicity after two months.

[0059] Figure 6. A graft-from strategy was explored to enhance the scalability of the zwitterion grafting process. ZG-PU was successfully fabricated, demonstrating (a) hydrophilicity, (b) sulphur content, and (c) morphology, (d) When using high concentration monomer solution and a fourteen-day grafting period, a graft-from strategy considerably minimised hydrophobic recovery (GT = graft-to; GF = graft-from, 2d = two day grafting period, 14d = fourteen day grafting period, 100 mg / mL = lower concentration SBMA solution, 500 mg / mL = higher concentration SBMA solution).

[0060] Figure 7. Minimising hydrophobic recovery: modifying grafting mechanism, (a) A graft- to mechanism (GT) was initially used; however, the utility of a graft-from (GF) mechanism was explored, (b, c) The effect of the graft-from mechanism and changing concentrations using this method on the initial hydrophilicity and hydrophobic recovery profile, (d, e) The initial hydrophilicity and hydrophobic recovery profile of zwitterion-grafted polyurethanes fabricated via the graft-from method but keeping samples in zwitterion solution for two weeks.

[0061] Figure 8. Evaluation of the antifouling performance of ZG-PU using BSA as a model protein; (a) comparison of fluorescence absorbance after material immersion in BSA solution, and (b) the correlating BSA concentrations left in solution. The difference between solution concentration was assumed to be due to adsorption to the immersed samples, with a correction included for adsorption to the well plate; thus (c) demonstrates the adsorption density of the materials relative to the control well.

[0062] Figure 9. Water contact angle of ZG-PMMA and controls, (b) Hydrophobic recovery of ZG-PMMA and controls.

[0063] Figure 10. The versatility of this process was demonstrated by performing the process on different substrates: (a) PCL, (b) PVC, and (c) PTFE. For each material, the water contact angle of the (i) pristine material and the (ii) zwitterion-grafted material were (iii) significantly different, with (iv) the zwitterion -grafted materials (purple lines) possessing sulphur as identified by XPS analysis of the S2p spectrum, (d) Furthermore, a different zwitterionic modification (i) (2-methacryloyloxyethyl phosphorylcholine) was successfully grafted to (ii) PU, resulting in (iii) an MPC-grafted ZG-PU (MPC-PU) that (iv) was much more hydrophilic than PU and similar to SBMA-grated ZG-PU (SBMA-PU), and (v) possessed phosphorus as identified by XPS analysis of the P2p spectrum.

[0064] Figure 11 : Water contact angle of crosslinked zwitterion-grafted polyurethanes, (a) Contact angle of unmodified polyurethane and zwitterion -grafted groups, and representative images for materials made with (b) a low concentration of zwitterion monomer, and (c) a high concentration of zwitterion monomer.

[0065] Figure 12: Elemental composition of crosslinked zwitterion-grafted polyurethanes, (a) X-ray photoelectron spectroscopy was performed for each sample, including the survey spectra, spectra for S2p and N1 s orbitals, and their respective peak areas, (b) Atomic sulphur and nitrogen concentrations for main samples, obtained from analysis of the survey spectrum, (c) Sulphur content on samples fabricated by introducing crosslinkers when using a 500mg / mL zwitterion monomer concentration.

[0066] Figure 13: Morphological and mechanical analysis of crosslinked zwitterion-grafted polyurethanes, (a) Imaging of the surface morphology via scanning electron microscopy and atomic force microscopy (AFM); (b) surface roughness, determined via AFM; (c) Young’s modulus of the surface, determined via Tip Force AFM; and (d) estimated layer thickness, determined via AFM analysis of a border region between unmodified and modified materials.

[0067] Figure 14: Determining layer thickness via AFM. (a) A schematic of the process used to make samples with a boundary between non-modified and zwitterion-grafted regions. The profiles obtained from this approach are shown for (b) 100-0, (c) 100-0.1 , (d) 100-0.5, (e) 100- 1 , and (f) 500-0

[0068] Figure 15: Thrombosis of zwitterion-grafted polyurethanes with varying brush or network architectures.

[0069] Figure 16: Characterisation of zwitterion-grafted polyurethanes with brush and network architectures over a twelve-week aging period in air. Analyses included (a) water contact angle (demonstrating representative images after twelve weeks), (b) sulphur content, (c) ammonium-bound nitrogen content, (d) surface roughness, and (e) thrombosis. All panels contain a graph plotting these metrics against time, and a column graph to visualise these metrics at the twelve-week point

[0070] Figure 17: Representative images for all samples after twelve weeks of aging, generated via scanning electron microscopy and atomic force microscopy.

[0071] Figure 18: Thrombogenesis of samples after twelve weeks aging, (a) Images of clot formation on all samples, (b) PIII-PU became more thrombogenic over time, which importantlyindicates that the antithrombogenic properties emerge from zwitterion grafting, (c) Thrombus formation on samples generated with 500mg / mL zwitterion monomer concentrations.

[0072] Figure 19: Water contact angle of samples stored in water over twelve weeks, demonstrating a mitigation of hydrophobic recovery due to the polar storage environment

[0073] Figure 20: Surface roughness of samples stored in water over twelve weeks, demonstrating a mitigation of hydrophobic recovery in 100-0 due to the polar storage environment.

[0074] Figure 21 : Thrombosis of an optimal brush-based zwitterion-grafted polyurethane and an optimal network-based zwitterion-grafted polyurethane under shear blood flow conditionsDescription of embodiments

[0075] The methods of the disclosure generally relate to a system where a polymer (the second polymer) is grafted onto the surface of a material comprising another polymer (the first polymer) by subjecting the surface of the material to a plasma treatment, then exposing it to a solution of monomers of the polymer which is to be grafted. In such systems the graft may deteriorate over time. In this context, the term "deteriorate" refers to the hydrophilicity of the graft. Thus the hydrophilicity of the grafted surface may reduce over time. The water contact angle of the grafted surface may increase over time.

[0076] The inventors have surprisingly found that when grafting a polymer to a polymeric surface using a plasma treatment, the deterioration of the graft over time may be reduced by contacting the surface with monomers of the graft polymer for a prolonged time, storing the grafted surface in a polar medium, and / or performing the grafting in the absence of a radical initiator and with a high concentration of monomers of the graft polymer. The deterioration of the graft over time may be reduced by increasing the time over which the grafting is conducted.

[0077] In a first aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days.

[0078] In a second aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days.

[0079] In a third aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0080] In a fourth aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0081] In a fifth aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, wherein the graft of the second polymer has been prepared by: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer;the method comprising covering the surface with a polar medium.

[0082] In a sixth aspect of the disclosure, there is provided a method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein the solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M.

[0083] In a seventh aspect of the disclosure, there is provided a method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer; wherein the solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M.

[0084] In the methods of the first to seventh aspects of the disclosure, a second polymer is grafted onto the surface of a material comprised of a first polymer. The first and second polymers may be the same. The first and second polymers may be different. The first polymer may be a polymer suitable for the fabrication of medical devices. The first polymer may be selected from thermoplastic elastomers and elastomeric polyolefins. The first polymer may be selected from the group consisting of polyurethane (PU), polymethylmethacrylate (PMMA), polycaprolactone (PCL), polyvinylchloride (PVC), polytetrafluoroethylene (PTFE), acetal copolymer (POM-C), polycarbonate (PC), polyether ether ketone (PEEK), polypropylene (PP), polyphenylsulfone (PPSU), polyethylene (PE), polysulfone (PSU), polyphenylen sulfide (PPS), polyvinylidene fluoride (PVDF), polyetherimide (PEI), polyisobutylene, polystyrene, polyacetate, epoxy resin, polydimethylsiloxane (PDMS), silicones, and polyphenylene oxide (PPO). The first polymer may be selected from the group consisting of polyurethane (PU), polymethylmethacrylate (PMMA), polycaprolactone (PCL), polyvinylchloride (PVC), andpolytetrafluoroethylene (PTFE), polymethylpentene (PMP). It may be some other carbonbased material such as graphite, graphene, or pyrolyzed graphene. The first polymer may be polyurethane. It may be a polyesterurethane or a polyetherurethane or a po lycarbon ateu reth an e .

[0085] The first polymer may have a glass transition temperature below about body temperature. The first polymer may have a glass transition temperature below about 40 °C, or below about 35, 30, or below about 25 °C. Without wishing to be bound by theory, the inventors believe that graft deterioration, particularly where the graft contains a polar motif, may involve the reconfiguration of mobile chains of the first polymer to embed the polar motifs of the graft of the second polymer under the surface, to minimise free energy at the air-polymer interface. This may therefore necessitate additional treatment in order to maintain the polar nature of the graft. Where the first polymer has a high glass transition temperature, the polymer chains may not be sufficiently mobile for this process to occur when the material is in use, for example as a medical device in the body, or during storage at room temperature.

[0086] The material comprised of the first polymer generally refers to a solid comprised of the first polymer. The material may be comprised of a single polymer, or may be comprised of a mixture of two or more polymers. The single polymer, or any one or more of the two or more polymers, may be a homopolymer or may be a copolymer. The material may be comprised of a mixture of two or more first polymers listed above. The material may further comprise other components such as a plasticiser, an antioxidant, an anti-microbial additive, and / or inorganic fibres or other fillers / reinforcing agents. In some embodiments, the material may consist of the first polymer, or consist essentially of the first polymer.

[0087] The material may form a solid item such as a medical device or a portion thereof. The medical device may be a medical device which, when in use, is in contact with the blood of a user. The medical device may be a catheter, particularly a central venous catheter or a peripheral venous catheter, a heart valve replacement, a ventricular assist device, an element in an extracorporeal membrane oxygenation circuit, a blood bag, a vascular stent, a component of a pacemaker, a cardiac patch, or a cardiac graft.

[0088] The second polymer may be any polymer that imparts beneficial properties to the material when grafted to its surface. For example, where the material forms a medical device, the second polymer may impart beneficial properties to the medical device when grafted to its surface, such as resistance to biofouling, hydrophilicity, antithrombogenic properties, reduced adhesion and / or increased lubrication. The second polymer may be a hydrophilic polymer. In some embodiments, the second polymer is a zwitterionic polymer, a super-hydrophilicpolymer, or a super-hydrophobic polymer. The material, when grafted with a super-hydrophilic polymer, may have a low water contact angle. It may have a water contact angle of less than about 10°, or less than about 9, 8, 7, 6 or 5°. The material, when grafted with a superhydrophobic polymer, may have a high water contact angle. It may have a water contact angle of greater than about 150°, or greater than about 155, 160, 165 or 170°. In both instances, the material may have this contact angle after at least about one day, or after at least about 2, 3,4, 5, 6, 7, 8, 9, 10, 11 , 12, 13 or 14 days. In some embodiments, the second polymer is a zwitterionic polymer. The zwitterionic polymer may be selected from the group consisting of polysulfobetaine methacrylate (SBMA), poly-2-methacryloyloxyethyl phosphorylcholine(MPC), poly-2-methacryloyloxyethyl phosphorylcholine, poly-2-[[2-(methacryloyloxy)ethyl]dimethylammonio]acetate, poly-3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propionate, poly-3-[(3- acrylamidopropyl)dimethylammonio]propanoate, poly-4-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, poly-3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, poly-3-[bis[2-(methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, poly-3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, poly-4-[(3- methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and poly-3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate. The zwitterionic polymer may be selected from the group consisting of polysulfobetaine methacrylate (SBMA) and poly-2- methacryloyloxyethyl phosphorylcholine (MPC). The zwitterionic polymer may be polysulfobetaine methacrylate (SBMA). Mixtures of any two or more of these polymers may also be used. These specific polymers may confer superhydrophilicity on the material.

[0089] In a first step of the method of the first to fourth, sixth and seventh aspects of the disclosure, the surface of the material is subjected to a plasma treatment. In the method of fifth aspect of the disclosure, the graft of the second polymer has been prepared by subjecting the surface to a plasma treatment. The plasma treatment may be any suitable plasma treatment which results in the generation of free radicals on the surface of the material. The plasma treatment may be selected from the group consisting of plasma-immersion ion implantation (PHI), plasma cleaning, plasma treated media such as plasma water, and plasma tube. The plasma treatment may be PHI. PHI may be performed using nitrogen as the working gas. Subjecting the surface to a plasma treatment may comprise applying PHI for a duration of between about 0.1 minutes and about 10 minutes, or between about 0.1 minutes and about 5 minutes, or between about 5 minutes and about 10 minutes, for example about 0.1 , 0.2, 0.3, 0.4, 0.5, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or 10 minutes. Subjecting the surface to a plasma treatment may comprise applying PHI for a duration of between about 5 minutes andabout 10 minutes. Subjecting the surface to a plasma treatment may comprise applying PHI at a voltage of between about 1 kV and about 25 kV, or between about 1 -5, 1 -10, 1 -15, 1 -20, 5-10, 5-15, 5-20, 5-25, 10-15, 10-20, 10-25, 15-20 or between about 20 kV and about 25 kV, for example about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24 or 25kV. Subjecting the surface to a plasma treatment may comprise applying Pill at a voltage of between about 15 kV and about 25 kV. Subjecting the surface to a plasma treatment may comprise applying PHI at a voltage of between about 15 kV and about 25 kV for a duration of between about 5 minutes and about 10 minutes.

[0090] In the methods of the first to fourth, sixth and seventh aspects of the disclosure, following the step of subjecting the surface to a plasma treatment, the surface is contacted with a solution comprising monomers to form a graft of the second polymer. In the method of fifth aspect of the disclosure, the graft of the second polymer has been prepared by contacting the surface with a solution comprising monomers to form the graft of the second polymer, following the plasma treatment. That is, the monomers are monomers of the second polymer. In this step, free radicals generated on or near the surface of the material during the plasma treatment react with the monomers in the monomer solution, causing the monomers to become covalently linked with the surface and to polymerise, resulting in the formation of a graft of the second polymer.

[0091] The step of contacting the surface with a solution comprising monomers may comprise immersing the material in the solution comprising monomers. Alternatively, the solution may be sprayed or drip coated onto the material. The step of contacting the surface with a solution comprising monomers may take place at between about 20 °C and about 80 °C, or between about 20 °C and about 60 °C, e.g. at about 20, 25, 30, 35, 40, 45, 50, 55 or 60 °C. In some instances it may take place below 20°C. It may take place at a temperature of at least about 0°C, or at least about 5, 10, 15 or 20°, or from about 0 to about 80°C. The step of contacting the surface with a solution comprising monomers may take place at between about 40 °C and about 80 °C, or about 60 °C, for about 48 hours, followed by at between about 20 °C and about 30 °C, or room temperature, or about 25 °C, for the remainder of the duration. The step of contacting the surface with a solution comprising monomers may take place at between about 20 °C and about 30 °C, or room temperature, or about 25 °C, for the entire duration. In some embodiments, where the solution comprising monomers does not comprise an initiator, the step of contacting the surface with a solution comprising monomers may take place at between about 20 °C and about 30 °C, or room temperature, or about 25 °C, for the entire duration. The surface may in some instances be retained in an oxygen-free, or substantially oxygen-free atmosphere following the plasma treatment up until the exposure to the solutionof monomers. This is thought to reduce the deterioration of plasma-generated free radicals on the surface of the material, since oxygen is a diradical and can therefore react with surface free radicals. The time between completion of the plasma treatment and the exposure of the plasma-treated surface to monomer(s) may be kept short. This may also serve to reduce the deterioration of plasma-generated surface free-radicals. The time may be less than about 10 minutes, or less than about 9, 8, 7, 6, 5, 4, 3, 2, 1 minute, or less than about 30, 20 or 10 seconds.

[0092] In the methods of the first to fifth aspects of the disclosure, the concentration of monomers in the solution comprising monomers may be between about 0.1 M and about 5 M, or between about 0.1 -0.5, 0.1 -1 , 0.1 -2, 0.1 -3, 0.1 -4, 0.5-1 , 0.5-2, 0.5-3, 0.5-4, 0.5-5, 1 -2, 1 -3, 1 -4, 1 -5, 2-3, 2-4, 2-5, 3-4, 3-5, or between about 4 M and about 5 M. The concentration of monomers in the solution comprising monomers may be between about 0.1 M and about 0.5 M. The concentration of monomers in the solution comprising monomers may be between about 0.1 M and about 1 M. The concentration of monomers may be for example about 0.1 , 0.2, 0.3, 0.4, 0.5, 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5 or 5M.

[0093] In the methods of the sixth and seventh aspects of the disclosure, the concentration of monomers in the solution comprising monomers is at least about 0.5 M, or at least about 0.7, 1 , 1 .2, 1 .5, 1 .7 or at least about 2 M. The concentration of monomers in the solution comprising monomers may be between about 0.5 M and about 5 M, or between about 0.5-1 , 0.5-2, 0.5- 3, 0.5-4, 1 -2, 1 -3, 1 -4, 1 -5, 2-3, 2-4, 2-5, 3-4, 3-5 or between about 4 M and about 5 M. The concentration of monomers in the solution comprising monomers may be between about 0.5 M and about 2 M. The concentration of monomers in the solution comprising monomers may be between about 0.7 M and about 2 M. It may be about 0.5, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5M.

[0094] In the methods of the first to seventh aspects of the disclosure, the solvent of the solution comprising monomers may be any solvent which is suitable for the polymerisation of the monomers and formation of the graft of the second polymer to take place. The solvent may be water. The water may comprise dissolved salts such that the solvent may be saline, such as buffered saline, for example phosphate buffered saline, or isotonic saline. In some instances, the surface may be exposed to neat monomer, i.e. no solvent may be present. This may be convenient when the monomer or mixture of monomers is liquid at the temperature of the exposure.

[0095] In the methods of the first and second aspects of the disclosure, and optionally in the methods of the third to seventh aspects of the disclosure, the contacting the surface with asolution comprising monomers may be for a duration of at least about 10 days, or at least about 11 , 12, 13, or at least about 14 days. The contacting may be for a duration of between about 10 days and about 20 days, or between about 10-12, 10-14, 10-16, 10-18, 12-14, 12- 16, 12-18, 12-20, 14-16, 14-18, 14-20, 16-18, 16-20, or between about 18 days and about 20 days. The contacting may be for a duration of about 10 days, or about 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, or about 20 days. In some instances shorter times may suffice. The contacting may therefore be for at least about 1 day, or from about 1 to about 20 days, or about 1 to 10, 1 to 5 or 5 to 10 days. Without wishing to be bound by theory, the inventors believe that over a prolonged duration continuous interactions occur between the material surface and monomers or growing polymer chains, as more high energy and unstable functional groups migrate to the surface of the material over time.

[0096] The solution comprising monomers is a solution comprising monomers of the second polymer. The monomers may be hydrophilic or nay be hydrophobic. They may be zwitterionic monomers, super-hydrophilic monomer, or super-hydrophobic monomers. The monomers may be zwitterionic monomers. The monomers may be a zwitterionic compound comprising a polymerisable functional group. That is, the monomers may comprise a positively charged functional group, a negatively charged functional group, and a polymerisable functional group. The polymerisable functional group may be a functional group which is polymerisable by a free radical mechanism. The polymerisable functional group may be a double bond. It may be an acryl group, or a methacryl group, or a styryl group or some other polymerizable olefinic group. The positively charged functional group may be selected from the group consisting of a quaternary ammonium group, a phosphonium group, and a sulfonium group. The negatively charged functional group may be selected from the group consisting of a carboxylate group, a sulfate group, a phosphate group, a phosphonate group, and a sulfonate group. The monomer may be selected from the group consisting of sulfobetaine methacrylate (SBMA), 2- methacryloyloxyethyl phosphorylcholine (MPC), 2-[[2-(methacryloyloxy)ethyl]dimethylammonio]acetate, 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propionate, 3-[(3- acrylamidopropyl)dimethylammonio]propanoate, 4-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[bis[2-(methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, 3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3- methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and 3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate. The monomer may be SBMA. The solution comprising monomers may comprise a single type of monomer, or may comprise amixture of two or more monomers, e.g. any two or more of the abovementioned monomers. Where the solution comprising monomers comprises two or more monomers, the second polymer will be a copolymer. It may be a binary copolymer, a ternary copolymer, a quaternary copolymer etc.

[0097] In the methods of the first to fifth aspects of the disclosure, the solution comprising monomers may also comprise a radical initiator. The radical initiator may be any suitable radical initiator, for example ammonium persulfate, potassium persulfate, 4,4'-azobis(4- cyanovaleric acid, azobisisobutyronitrile, or benzoyl peroxide. It may be a water-soluble radical initiator. Alternatively, the solution comprising monomers may not comprise a radical initiator. Where the solution comprises a radical initiator, the monomers may polymerise in solution before becoming covalently linked to the surface of the material (referred to as a ‘graft- to’ process). Where the solution does not comprise a radical initiator, the monomers first become covalently linked to the surface of the material, then the chains of the second polymer propagate from the monomer which is covalently linked to the surface of the material (referred to as a ‘graft-from’ process). In the event that a radical initiator is used, the polymerisation may be conducted at a temperature which is no more than 20 Celsius degrees less than the 10- hour half-life temperature of the radical initiator (i.e. at a temperature of at least (T-20)°C, where T is the 10-hour half-life temperature of the radical initiator).

[0098] In the methods of the sixth and seventh aspects of the disclosure, the solution comprising monomers does not comprise a radical initiator. As discussed above, in this case the concentration of monomers in the solution comprising monomers is at least about 0.5 M. Without wishing to be bound by theory, the inventors believe that, whereas in the presence of a radical initiator increased monomer concentration tends to impede graft formation due to the steric hinderance of polymer chains formed in solution, in the absence of a radical initiator this does not occur and the increased concentration of monomers can readily access the material surface, leading to improved graft formation. In this context, "improved" may refer to a greater rate of grafting, or a greater mean length of the graft or to a longer lasting graft.

[0099] In the methods of the third to fifth aspects of the disclosure, and optionally in the methods of the first, second, sixth and seventh aspects of the disclosure, following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer, the surface is covered with a polar medium. Covering the surface with a polar medium may comprise, for example, immersing and / or storing the material in the polar medium such that the surface is covered. It may comprise spraying the polar medium onto the surface. The surface may be completely covered by the polar medium, i.e. such that none of the surface is exposed to the atmosphere. The polar medium may be selected from the groupconsisting of water, methanol, ethanol, and mixtures of water with methanol and / or ethanol. The polar medium may be, or may comprise, water. The polar medium may comprise at least 50% w / w, or at least 60, 70, 80, or at least 90% w / w water. The water may comprise dissolved salts such that the solvent may be saline, such as buffered saline, for example phosphate buffered saline, or isotonic saline. Alternatively or additionally the polar medium may be, or may comprise, an ionic liquid, e.g. an imidazolium or pyridinium liquid salt. Covering the surface with a polar medium may be for a duration of at least about 28 days, or at least 42, 63, 84, or at least about 105 days. It may be for a period between shortly after the production of the coated surface and shortly before the use of an item comprising the coated surface. In this context "shortly before" may refer to a period of less than about 1 day, or less than about 12, 6, 3 or 1 hour. Covering the surface with a polar medium may be for a duration of between about 28 days and about 105 days, or between about 28-42, 28-63, 28-84, 42-63, 42-84, 42- 105, 63-84, 63-105, or between about 84 days and about 105 days. Covering the surface with a polar medium may be for a duration of up to about 28 days, or up to about 42, 63, 84, or up to about 105 days. In some instances it may be for greater than 105 days. For example, once the graft is formed, the material may be removed from the solution comprising monomers and immersed in a storage solution, which comprises a polar medium. It may be stored in the storage solution until it is used. Without wishing to be bound by theory, the inventors believe that when the second polymer is polar, the polymer chains may embed in the material to minimise free energy at the surface, i.e. the air-material interface, if exposed to air for an extended period, which may result in effective deterioration of the graft by the reconfiguration of the functional chemical groups away from the surface and into the bulk. Storage in a polar medium may mitigate this effect by minimising the free energy at the surface. In some instances the storage is at a temperature below the glass transition temperature of the first polymer. It is thought that this may further inhibit the embedding of grafted chains in the surface of the material.

[0100] When the second polymer is a zwitterionic polymer, the formation of a graft of the second polymer on the surface of the material may result in the hydrophilicity of the surface being increased. It may result in a highly hydrophilic surface, e.g. a superhydrophilic surface. This can be advantageous, for example to impart resistance to biofouling in a medical device. In this case, deterioration of the graft may result in the hydrophilicity of the surface decreasing, relative to its value immediately following formation of the graft. This can be conveniently measured by determining the water contact angle of the surface of the material, as water contact angle decreases with increasing hydrophilicity. Methods of measuring water contact angle are well-known, such as the static sessile drop technique using a contact angle goniometer. Contact angles referenced in this specification may be as measured by thismethod. The method may be according to ASTM D5946 2017 or as described in the Material characterisation section of the Examples provided in this specification.

[0101] In some embodiments, the water contact angle of the surface of the material 60 days after being subjected to the plasma treatment may be increased by less than 100%, compared to the water contact angle of the surface of the material one day following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer. The water contact angle of the surface of the material 60 days after being subjected to the plasma treatment may be increased by less than about 100%, or less than about 90, 80, 70, 60, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, or less than about 2%. The water contact angle of the surface of the material 60 days after being subjected to the plasma treatment may be increased by less than about 50%.

[0102] In an eighth aspect of the disclosure there is provided a material comprised of a first polymer having a graft of a second polymer on a surface thereof, wherein the graft of the second polymer is prepared according to the method of the first, third, or sixth aspect of the disclosure.

[0103] The inventors have observed gradual reductions in the hydrophilicity of a zwitterion- grafted polyurethane, which naturally had functional consequences on its antithrombogenic performance. Hydrophobic recovery is particularly apparent in materials with low glass transition temperatures, such as polyurethanes, due to the increased mobility of the polymeric chains. Problematically, polyurethanes are an especially promising group of materials for many biomechanical applications precisely due to this mobility, as they offer a unique combination of strength and elasticity that makes them desirable polymers for many cardiovascular applications. Consequently, the inventors have sought to minimise hydrophobic recovery by modifying processing parameters. For example, it was observed that storing materials in water after fabrication helped to limit recovery due to dipole-dipole interactions between the polar groups in the liquid and on the surface. Additionally, the molecular weight of the grafted zwitterion chains was increased by increasing the concentration of the zwitterionic monomer: as the same Pill-treated surface was utilised, monomer concentration was proportional to the rate and degree of polymerisation. This successfully limited hydrophobic recovery, attributed to the understanding that higher molecular weight chains cannot easily diffuse into the bulk, although an excessive quantity of zwitterionic monomer was required.

[0104] The inventors postulated that modifying the architecture of the zwitterion graft to introduce molecular rigidity might also help to limit hydrophobic recovery by reducing themolecular mobility of the surface layers. The generation of crosslinked zwitterionic grafts was thought to be a promising strategy for developing durable, antithrombogenic materials.

[0105] The inventors therefore explored crosslinking a directly grafted (i.e. linker free) polymer surface in order to inhibit the deterioration of beneficial properties such as hydrophilicity. In particular, they explored zwitterionic surface modification in the context of a linker-free plasma-mediated grafting process. By integrating a low molecular weight crosslinker into the previously developed zwitterion-grafting protocol, the architecture of the grafted polymer layer could be modified from a brush to a network. The effect of graft chain length was also explored by varying monomer concentration. The successful synthesis of this surface architecture was characterised by examining the elemental composition, wettability, morphology, mechanics, and thrombogenicity of a range of groups to demonstrate that developing network topologies could aid in the retention of antithrombogenic zwitterion grafts without the requirement for excessive reagent. The stability of these metrics was assessed over a twelve-week period to demonstrate the longevity of crosslinked surface architectures against hydrophobic recovery. Finally, the susceptibility of the crosslinked surface to thrombosis under dynamic conditions, demonstrating a remarkably biocompatible surface modification. Details of these explorations are described in Example 2 vide infra).

[0106] The present invention employs a treatment in order to prevent loss of beneficial properties imparted by grafting the second polymer onto the surface comprising the first polymer.

[0107] One such treatment, described in detail elsewhere herein, comprises covering the second polymer, i.e. the graft polymer, with a polar material. In this instance, the property to be retained is hydrophilicity. The covering may comprise immersing the second polymer, or the surface having the grafted second polymer, in a polar liquid, commonly an at least partially aqueous liquid. This liquid may be an aqueous solution. Thus storing the surface (or a device comprising the surface) in such a liquid was found to retain the hydrophilicity of the surface.

[0108] A second such treatment is to crosslink the second polymer. Conveniently, the crosslinking is conducted simultaneously with the formation of the second polymer. This may be achieved by including a crosslinking monomer with the monomer for the second polymer. In order to retain a certain flexibility in the second polymer, the level of crosslinking should be kept fairly low. To this end, the proportion of crosslinking monomer in the monomer for the second polymer should be less than about 5 mol%, or less than about 4, 3, 2, 1 , 0.5, 0.1 , 0.05 or 0.01 mol%, or between about 0.01 and about 5 mol%, or between about 0.05 and 5, 0.1 and 5, 0.5 and 5, 1 and 5, 2 and 5, 0.1 and 2, 0.1 and 1 , 0.1 and 0.5, 0.5 and 2, 0.01 and 1 ,0.01 and 0.1 or 1 and 2 mol%, e.g. about 0.01 , 0.05, 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 mol%. In the simultaneous crosslinking, the crosslinking monomer is simply combined with the monomer for the second polymer before these are exposed to the plasma-treated surface comprising the first polymer. The monomers may be deoxygenated prior to this exposure. This may restrict the inhibitory effect that may otherwise be caused by the presence of oxygen. The deoxygenation may be by bubbling an oxygen-free gas such as nitrogen, helium, argon or a mixture of any two or more of these through the monomers, or may be accomplished by applying a vacuum to the monomers, for example using one or more freeze-pump-thaw cycles.

[0109] The crosslinking monomer may be a monomer comprising two polymerizable olefinic groups. It may be a polar monomer. It may be a bisacrylamide, or a bismethacrylamide, or a bisacrylate or a bismethacrylate or may be some other type of polar monomer. The two polymerizable olefinic groups, or two groups comprising these, may be joined by an alkylene linker, e.g. a methylene, ethylene or n-propylene linker, or may be joined by a polar linker such as a polyether linker. Thus, for example, they may be joined by an ethylene glycol linker, a diethylene glycol linker or similar. Suitable crosslinking monomers therefore include N,N’- methylenebis(acrylamide), ethylene glycol diacrylate, ethylene glycol dimethacrylate, N,N'- methylenebis(methacrylamide), diethylene glycol diacrylate and diethylene glycol dimethacrylate. The crosslinking monomer may be a water-soluble crosslinking monomer.[001 10] In an alternative process, the crosslinking may be conducted subsequently to the grafting. In this process, a mixture of the crosslinking monomer and a free radical initiator is infused into the grafted polymer, and the resulting mixture heated so as to initiate grafting.[001 11 ] The monomers may be used in a solution, commonly a solution in a polar solvent such as an aqueous and / or alcoholic solution. The concentration of the monomers in the solution may be from about 5 to about 70%w / v, or from about 10 to 70, 200 to 70, 50 to 70, 5 to 50, 5 to 30, 5 to 10, 10 to 50, 20 to 50 or 10 to 30%, e.g. about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70%. Alternatively, the monomers may be used neat, i.e. without solvent.[001 12] The coated surfaces produced by the method described herein may have a surface roughness of from about 30 to about 70nm, or from about 30 to 50, 50 to 70 or 40 to 60nm, e.g. about 30, 35, 40, 45, 50, 55, 60, 65 or 70nm. The surface layer of grafted second polymer may have a thickness from about 2 to about 50nm, or about 5 to 50, 10 to 50, 20 to 50, 2 to 20, 2 to 20, 2 to 5, 5 to 20, 5 to 10 or 10 to 20nm, e.g. about 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50nm. It may have a Young's modulus of less than about 5MPa, or less than about 4, 3, or 2MPa, or from about 1 to about 5MPa, or about 1 to 4, 1 to 3, 1 to 2, 2 to 3 or 2 to5MPa, e.g. about 1 , 1.1 , 1.2, 1.3, 1.4, 1 .5, 2, 2.5, 3, 3.5, 4, 4.5 or 5MPa. The coated surface may be less thrombogenic than the uncoated surface. It may be less than 80% as thrombogenic, or less than about 70, 60 or 50% as thrombogenic. It may have a dynamic thrombogenicity less than the uncoated surface. It may have a dynamic thrombogenicity at least 20% less than the uncoated surface, or at least 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80% less than the uncoated surface.[001 13] The coated surfaces of the invention may have an initial water contact angle of less than about 20°, or less than about 15 or 10°, or of about 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19 or 20°. After storage in air at 25° for 12 weeks, they may have a water contact angle of less than 50°, or less than about 45, 40, 35 or 30°. After storage in water at 25° for 12 weeks, they may have a water contact angle of less than about 30°, or less than about 25, 20 or 15°.Definitions[001 14] The following are some definitions that may be helpful in understanding the description of the present disclosure. These are intended as general definitions and should in no way limit the scope of the present disclosure to those terms alone, but are put forth for a better understanding of the following description.[001 15] The term "polymer" as used herein is defined by IUPAC (International Union of Pure and Applied Chemistry) as "[a] molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass." This includes natural polymers and synthetic polymers and also extended networks of repeating units such as those in graphene, graphite and pyrolytic carbon (in which the repeating unit is a benzene ring). Polymers may be linear, branched, crosslinked or networked. It includes homopolymers, in which there is a single repeating unit and copolymers, in which there is more than one repeating unit.[001 16] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term "consist" or variations thereof will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step, or group of elements, integers or steps. The term "consist essentially" or variations thereof will be understood to imply the inclusion of a stated element,integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step, or group of elements, integers or steps other than those which have negligible effect on the operation of the invention or which are not intentionally included.[001 17] The terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.[001 18] In the context of this specification the term "about" is understood to refer to ±10% of the recited value or to a variation which has only a minor effect on the operation of the invention.[001 19] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of 1 .0 to 5.0 is intended to include all sub-ranges between (and including) the recited minimum value of 1 .0 and the recited maximum value of 5.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 5.0, such as 2.1 to 4.5. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein.

[0120] The term "may" is intended to convey both positive and negative alternatives. For example, the statement "A may be B" should be taken to include both the possibility "A is B" and the possibility "A is not B".

[0121] Any description of prior art documents herein, or statements herein derived from or based on those documents, is not an admission that the documents or derived statements are part of the common general knowledge of the relevant art.

[0122] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise statedExamples

[0123] The present disclosure is further described below by reference to the following nonlimiting examples.Example 1

[0124] A carbonate-based polyurethane (ChronoFlex C 80A) was purchased from AdvanSource Biomaterials. Tetrahydrofuran (THF), sulfobetaine methacrylate (SBMA), ammonium persulfate (APS), poly(caprolactone) (PCL), poly(vinyl chloride) (PVC), poly(methyl methyacrylate) (PMMA), and bovine serum albumin (BSA) were purchased from Sigma Aldrich.Polyurethane film preparation

[0125] PU (polyurethane) films were prepared by casting a 10 w / v% PU / THF solution in a glass Petri dish. These casts were evaporated for 24 hours in a fume cupboard to produce PU films.Zwitterion solution preparation

[0126] Graft-to zwitterion solutions. SBMA was dissolved in MQ water at a concentration of 100 mg / mL. APS was then added at a concentration of 0.1 mol% based on the stoichiometry of SBMA in the solution. A stir bar was placed in the solution, and the solution placed into an ice bath on top of a stir plate to facilitate solution mixing while maintaining a cold environment. At the same time, the solution was degassed by bubbling nitrogen through the solution for 30 minutes. Different concentrations of SBMA solution were made via the same method.

[0127] Graft-from zwitterion solutions. These solutions were made in a similar manner to the graft-to solutions but without the addition of APS.PHI treatment

[0128] PHI treatment was carried out in a PI3 platform. 6 x 5 cm PU films were attached onto the sample holder and a mesh was fixed over the sample surface. This holder system was mounted into the PI3 chamber and electrically connected to the high voltage PI3 unit. The chamber was vacuumed to 2 x 10-5Torr, before supplying nitrogen gas to the chamber, bringing the chamber pressure to 2 x 10-3Torr. Nitrogen plasma was generated via a radiofrequency power of 100 W, which was then accelerated towards the PU substate via 20 kV pulses, each of 20 ps and with a frequency of 50 Hz. PHI treatment was carried out for 360 seconds. During process optimisation, the voltage of the plasma pulses was modified to 15 kV and 10 kV, and the treatment time was modified to 300, 240, 180, 120 and 60 seconds. After treatment, the materials were carefully removed from the PI3 system and placed in polystyrene dishes for zwitterion grafting.Zwitterion grafting

[0129] Graft-to zwitterion. Plasma functionalised PU films were fixed to the bottom of polystyrene dishes in zwitterion solution prepared for the graft-to process. The dishes were sealed with paraffin and aluminium foil and placed in the oven at 60 °C for 48 hours. After this time, the samples were removed from solution and washed thoroughly at least three times with MQ water. The films were carefully cleaned with a lint-free wipe to remove macroscopic polymer synthesised in solution. The materials were then immersed in MQ water for at least six hours before washing three times with water. For graft-to samples that extended polymerisation time, the samples remained in solution for their respective time periods, however, were still removed from heat after 48 hours.

[0130] Graft-from zwitterion. Plasma functionalised PU films were placed in polystyrene dishes in zwitterion solution prepared for the graft-from process. These dishes were sealed with paraffin and stored at room temperature for 48 hours. Then, the samples were removed from solution and washed thoroughly at least three times with water. Although no macroscopic polymer was formed in solution, the surface was still carefully cleaned with a lint-free wipe. The materials were then immersed in MQ water for at least six hours before washing three times with water. For graft-from samples that extended polymerisation time, the samples were simply kept in solution for that length of time before undergoing the described washing procedure.

[0131] The resulting material comprising the zwitterionic graft is referred to as ZG-PU.Air plasma treatment

[0132] Air plasma treatment was performed using a plasma cleaner under slight vacuum with a compressed air feed. PU samples were fixed inside the treatment chamber, the chamber was vacuumed and then plasma treatment was performed for 360 seconds. After plasma treatment, materials were grafted with zwitterions via identical protocols as described above.Material characterisation

[0133] Surface imaging. The materials were imaged using Scanning Electron Microscopy (Phenom™ XL G2 Desktop SEM, Thermo Fisher Scientific) operating with a back scatter electron detector with a 15 kV acceleration voltage. Before loading into the microscope, materials were sputter coated with a 16 nm thick gold / palladium layer via a plasma sputter coater.

[0134] Water contact angle. Material hydrophilicity was analysed by measuring the water contact angle of the surface (DSA25B Drop Shape Analyser, KRUSS). 5 pL droplets were dispensed at a rate of 200 pL / min and carefully touched to the surface of the loaded material to transfer from the syringe to the material. The droplet on the material surface was imaged using a high-resolution IEEE 1394 camera, and the water contact angle was measured using the instrument software. Six measurements were recorded for each sample.

[0135] Elemental analysis. The surface chemistry of the samples was analysed using X-ray Photoelectron Spectroscopy (K-alpha+ XPS, Thermo Fisher Scientific). Samples were mounted on to a standard sample stage and then loaded into the XPS machine. The XPS experiment was constructed using the connected software (Avantage Data System, Thermo Fisher Scientific). The X-ray gun spot size was set to 400 pm, and the flood gun was turned on. For each material, XPS survey, C1 s, O1 s, N1 s and S2p spectra were obtained (ten times each). As sulfur-containing compounds may be damaged by X-ray radiation, the S2p spectra were obtained first, followed by the XPS survey, C1 s, O1 s, and N1 s spectra. The resulting data was analysed using the Avantage software.

[0136] Tensile properties. The tensile properties of the materials were characterised by a tensile test using a mechanical testing machine (Instron BioPlus). Samples were cut into 30 x 5 mm strips (n=8) and loaded into the machine to allow a material length of 20 mm in between the clamps. A tensile test method was run with a strain rate of 50 mm / min. Force (F) and displacement (AZ) were converted to tensile stress (a) and tensile strain (5) by normalising by the samples’ cross-sectional area and initial length, respectively. Young’s modulus (E) was calculated as the gradient in the elastic region of the resulting stress-strain graphs, where:

[0137] Flexural properties. The flexural properties of the materials were calculated using a uniformly loaded cantilever beam test, allowing calculation of the flexural rigidity via a derivation of Euler-Beam theory. Samples were prepared into strips (n=8) that allowed a 15 mm length (L) of material for testing, plus enough to anchor the sample at one end. Sample mass were measured, and the uniform load on the strip (q) was calculated by dividing sample mass by testing length. The strips were anchored at one end and allowed to flex under the uniform load of gravity. The flexed samples were imaged, and the angle of deflection (< >) calculated manually. Flexural rigidity (EZ) was then calculated using the following equation:

[0138] Cyclic bending. 50 x 40 mm ZG-PU materials were prepared using a graft-to strategy and the hydrophilicity determined using water contact angle, as previously described. Samples were then subject to a cyclic bending protocol under hydrated conditions to determine the effect of mechanical manipulation on graft attachment. Samples were loaded into a mechanical testing machine (Instron BioPlus), allowing for 30 mm of material in between the clamps. The entire sample and clamp system was then immersed in a water bath. A cyclic compression method with a strain rate of 15 mm / min and a compression displacement of 20 mm was performed for 3000 cycles. The compression method allowed for the sample (a film) to bend upon machine compression. After completion of the method, samples were removed from the water bath, air dried for two hours and then the water contact angle measured again.Process versatility

[0139] The same PHI modification and graft-from zwitterion grafting steps were performed on PMMA, PTFE, PCL and PVC materials using the same parameters as described above. Similarly, PU samples were functionalised via identical PHI treatment protocols but then grafted with 2-methacryloyloxyethyl phosphorylcholine (MPC) instead of sulfobetaine methacrylate, however using the same parameters as described above. Characterisation via water contact angle and XPS was also performed similarly. For XPS, specific spectra were analysed for each material; fluorine for PTFE, chlorine for PVC, and phosphorus for MPC- g rafted PU.Antifouling analysis

[0140] Bovine serum albumin (BSA) adsorption was evaluated via a fluorescent assay. Six 1 cm2squares of PU and ZG-PU materials were fixed in the wells of a BSA-blocked 24 well plate. 1 mL of a 40 mg / mL solution of BSA in PBS was added for 18 hours, after which point 150 pL of the BSA solution from each well was pipetted into a fluorescence-specific 96-well plate. 50 pL of a fluorescamine was pipetted into each well plate and the plate was immediately loaded into a fluorescent spectrometer and scanned between 400 and 500 nm wavelengths. The resulting absorbance (A) was correlated to a BSA concentration (CBSA) via a predetermined calibration curve:A = (870.07 X CBSA) + 15,759

[0141] The difference between this concentration and the initial concentration was used to determine the mass of BSA absent from solution (based on the initial volume used), with this quantity assuming to have adsorbed to the material or the fluorescent well plate. To correct for this latter occurrence, six samples of a 40 mg / mL BSA solution without any materialimmersion was prepared in the initial 24-well plate, and then fluorescence was measured similarly to the test samples. The difference between the initial concentration and the concentration of the control solution was assumed to be due to adsorption to either the initial well plate or the fluorescent well plate and was then subtracted from the test results. The remaining difference was used to calculate the BSA mass adsorbed to the immersed materials, resulting in an estimation for BSA adsorption density.Results and discussionFabrication

[0142] A zwitterion layer was successfully grafted on to the surface of a commercial, medicalgrade polyurethane via Pill functionalisation followed by immersion in a zwitterion solution. As a control, identical polyurethanes were immersed in zwitterion solution without prior Pill functionalisation. Both the pristine commercial polyurethane (PU) and the control polyurethane (C-PU) exhibited a uniform, featureless surface when analysed by scanning electron microscopy (Figure 1 a, 1 b). PI Il-functionalised substrates without zwitterion grafting (PIII-PU) exhibited a similar surface morphology but with extensive cracks (white arrows) (Figure 1 c). In contrast, the zwitterion-grafted polyurethane (ZG-PU) possessed a sprawling wave-like morphology, suggesting the presence of a zwitterion layer (Figure 1 d). The processes involved in the fabrication of these four groups are summarised in Table 1 .Table 1

[0143] Elemental analysis of PU, C-PU, PIII-PU, and ZG-PU were performed using X-ray photoelectron spectroscopy (XPS). Analysis of the survey spectra defined peaks corresponding to S1 s and S2p orbitals for ZG-PU but not for the other three materials, suggesting the unique presence of sulfur in ZG-PU materials (Figure 1 e). Closer analysis of the S2p orbital identified a strong peak for ZG-PU, a trivial peak for C-PU and negligible peaks for PU and PIII-PU (Figure 1 f). Normalising the sulfur content of each material against their respective carbon counts, ZG-PU reported a S:C ratio of 0.08, whereas PIII-PU, C-PU and PU reported ratios of 0.00, 0.01 and 0.00, respectively. Sulfur is only present in the sulfonate anion of SBMA, but not in the PU backbone, thus confirming the zwitterionic graft in ZG-PU. Slight sulfur presence in C-PU may be attributed to incomplete washing, however these remainingmoieties were believed to be negligible and non-functional due to the comparable morphology of this sample to pristine PU. Thus, zwitterionic grafting was readily achieved via Pill-mediated substrate functionalisation.

[0144] During Pill treatment, nitrogen ions embed within the surface layers of the polyurethane substrate, generating free radicals that migrate to the surface. Upon immersion in zwitterion solution, these surface-anchored radicals readily facilitate covalent bonding between the methacrylate groups of zwitterion moieties and the polyurethane, resulting in a ZG-PU. It must be noted that ZG-PU was fabricated using a “graft-to” mechanism, wherein polyzwitterion chains were propagated in solution via radical polymerisation and then grafted to the polyurethane substrate. Initiation of zwitterion polymerisation was achieved with a thermal initiator; exposure to heat would further increase the dissociation of unstable bonds at the surface, generating more free radicals. An alternative strategy involves a “graft-from” mechanism, wherein these surface radicals provide reactive sites to initiate radical polymerisation of zwitterion monomers to propagate polyzwitterion chains from the surface, thus precluding the requirement for a chemical initiator. The utility of a graft-from strategy is discussed in detail in later sections, however the following analyses utilise a graft-to approach.Surface and bulk properties

[0145] The hydrophilicity of each material was analysed by water contact angle (Figure 2a, 3b). While PU was borderline hydrophobic with a contact angle of 83.85 ± 2.75°, ZG-PU was extremely hydrophilic (12.98 ± 1.43°). Notably, this hydrophilicity approaches the realm of so- called “superhydrophilicity”, which occurs under a water contact angle of 10° and has been associated with resistance to biofouling. C-PU remained hydrophobic (85.75 ± 3.35°), confirming that residual zwitterion moieties had negligible functional effect. While PIII-PU was also significantly more hydrophilic than PU (45.83 ± 1.59°), it remained significantly less hydrophilic than ZG-PU, thus indicating that the remarkable hydrophilicity of ZG-PU can be attributed to zwitterion grafting rather than PHI treatment alone. This was an important distinction as plasma treatment is a well-established platform for generating hydrophilic surfaces. Furthermore, the durability of the zwitterion graft under mechanical stress was tested by subjecting ZG-PU to extensive bending cycles under hydrated conditions. ZG-PU hydrophilicity did not change after 3000 bending cycles, endorsing a covalent interaction between the zwitterion and PU components (Figure 2c). Furthermore, mechanical analysis was performed to determine whether the grafting process affects the bulk polymer. Tensile and flexural properties were analysed via uniaxial tensile testing and cantilever flexural testing, respectively. Both the Young’s modulus (Figure 2d) and flexural rigidity (Figure 2e) of ZG-PU were not significantly different to PU, C-PU, or PIII-PU, demonstrating that the grafting processdid not affect the mechanical properties of the underlying substrate. Crucially, this implies that this process may enhance the biocompatibility of ready-made polymers and medical devices without impairing their intended mechanical function.Hydrophobic recovery

[0146] Despite successful fabrication, ZG-PU gradually recovered hydrophobicity over time; taking water contact angle as a measure for hydrophilicity, ZG-PU shifted from 12.98° to around 70° four months after fabrication (Figure 3). Similarly, the PIII-PU samples became increasingly hydrophobic over the same time, almost returning to the hydrophobicity of the pristine PU material. There was a steep decrease in ZG-PU hydrophilicity during the first two weeks (1.557day), followed by a shallower decrease thereafter (0.387day). Throughout this period, ZG-PU remained more hydrophilic than PIII-PU, suggesting the continued presence of the zwitterion layer, albeit with gradually diminishing functionality.

[0147] The contribution of potential graft detachment to this observation was considered negligible due to the demonstrated resilience under mechanical deformation (Figure 2c), and the similar hydrophobic recovery observed in PIII-PU, and in other Pill-treated materials. Thus, this observation may be due to hydrophobic recovery, wherein the mobile PU chains reconfigure to embed the polar zwitterionic motifs under the surface to minimise free energy at the air-polymer interface. To explore this theory, the same grafting process was successfully performed on poly(methyl methacrylate) (PMMA). The glass transition temperature (Tg) is much higher for PMMA than PU, and well above room temperature (-105 °C), with reduced PMMA chain mobility at room temperature significantly reducing the capacity for zwitterion- grafted PMMA (ZG-PMMA) to undergo hydrophobic recovery. While ZG-PMMA hydrophilicity decreased in the first few days, it plateaued thereafter; when compared to the continuous hydrophilic decrease of ZG-PU, this confirmed the occurrence of hydrophobic recovery in ZG- PU.

[0148] A secondary mechanism may be attributed to the continued generation and migration of surface free radicals due to the PHI functionalisation process. This may explain the substantial early reductions in hydrophilicity associated with not only highly mobile polymers such as PU (Figure 3), but less mobile polymers such as PMMA after PHI treatment. Thus, surface changes after PHI-mediated zwitterion grafting may be primarily attributed to the hydrophobic recovery phenomenon, with delayed free radical migration potentially contributing to the steep change during the initial weeks after PHI functionalisation.

[0149] Importantly, this zwitterion grafting process fundamentally strives to extend the durability of the underlying polymer or device by enhancing their biocompatibility; as this functionally is rooted in hydrophilicity, the observed hydrophobic recovery is detrimental to material performance, and by extension, the clinical viability of any resulting devices. Interestingly, no long-term studies of zwitterionic-grafted polymers have been reported beyond one month, with the limited literature offering contrasting findings. Accordingly, further studies were pursued to explore whether hydrophobic recovery could be minimised via process optimisation.Process optimisation studiesPlasma functionalisation

[0150] Various aspects of the plasma functionalisation process were manipulated, including the PHI treatment time, the system voltage, and the utility of milder plasma platforms. The treatment time refers to the time during which the substrate is subjected to plasma treatment, while voltage influences the energy of the plasma particles. Therefore, the former predominantly determines the ion fluence and thus the concentration of high-energy groups formed in the substrate, while the latter prescribes the ion penetration depth and the types of bonds affected by the plasma treatment.

[0151] As PHI treatment time increased, initial ZG-PU hydrophilicity slightly improved, although this trend was only significant when comparing extremes (Figure 4a). Nonetheless, this suggests that increasing PHI time may slightly improve zwitterion grafting, which agrees with known correlations between treatment time, ion fluence and free radical concentration; longer treatment times translate to increases in ion fluence, leading to increased free radical concentration, thus increasing the capacity of the functionalised surface to graft zwitterions. However, while changing treatment time may improve grafting efficiency, it did not significantly affect the profile of the hydrophobic recovery trends (Figure 4b, 4c). Alternatively, the PHI voltage was decreased to reduce the penetration depth of the nitrogen ions. Lower voltages resulted in less hydrophilic ZG-PU surfaces, suggesting poorer zwitterion grafting (Figure 4d), however hydrophilic loss became notably flatter during the first two weeks (Figure 4e). At a voltage of 20 kV, the rate of hydrophilic loss in this period was 1 .557day, with lower voltages of 15 kV and 10 kV correlating with flatter gradients of 0.997day and 0.877day, respectively. However, decreasing voltage likely just decreased substrate functionalisation.

[0152] Meanwhile, the use of milder plasma technologies for polyurethane functionalisation was also investigated. To this end, a plasma cleaner using compressed air under a slight vacuum, herein referred to as air plasma, was able to successfully fabricate ZG-PU, albeit witha less hydrophilic surface compared to the ZG-PU developed via Pill treatment, suggesting less zwitterion graft coverage (Figure 4g). The initial loss in hydrophilicity of air-mediated ZG- PU was much steeper than Pill-mediated ZG-PU (Figure 4h), which can be attributed to the mechanism of plasma functionalisation. As the air plasma did not implant ions into the substrate, there was no reservoir for sustained free radical migration to the surface; this allowed reactions with oxygen molecules to occur rapidly after removal from zwitterion solution, extinguishing any plasma-mediated surface changes within days of functionalisation. This is reflected in the identical long-term hydrophobic recovery trends that both air- and Pill- mediated ZG-PU exhibit after the rapid initial shift (Figure 4h, 4i). Nonetheless, PHI seems to offer a more robust functionalisation platform for zwitterion grafting due to superior grafting efficiency and slight mitigation of short-term hydrophilic losses.

[0153] These studies highlight the ability to optimise substrate functionalisation (and thus the efficiency of the following zwitterion graft) via modifying plasma functionalisation parameters, but also highlights the inability to address long-term hydrophobic recovery by manipulating the plasma functionalisation process itself. Furthermore, while alternative plasma platforms can also be used to functionalise substrates for zwitterion grafting, the robust nature of PHI functionalisation validates its use in this process. To this end, further optimisation studies were performed on the zwitterion grafting step.Zwitterion grafting

[0154] Increasing zwitterion concentration trended towards improved hydrophilicity, until a threshold where hydrophilicity reduced (Figure 5a). While the increasing trend was only significant when comparing the extremes (i.e., 100 mg / mL and 300 mg / mL), this may be due to the water contact angle being very small at the lowest concentration, thus only allowing trivial improvements with higher concentrations. The reduced hydrophilicity (i.e., higher contact angles) at the highest concentration may be attributed to sterically limited access of the polyzwitterion chains to the substrate surface. As this very high monomer concentration lent no improvement to the material, this sample was removed from future analysis. Despite slight improvements in hydrophilicity, increasing the concentration did not affect hydrophobic recovery trends, with all samples exhibiting similar trends over time (Figure 5b). Taken together with the negligible effects of plasma treatment time on hydrophobic recovery trends, it was apparent that modifying the nature of the initial graft could not contribute to hydrophilic maintenance.

[0155] The zwitterion grafting process was then modified by altering the time which the plasma-functionalised substrate was immersed in zwitterion solution, herein termed thegrafting period; considering that zwitterion grafting to plasma-functionalised substrates occurs via radical polymerisation, extending the grafting period was expected to increase the possibility of zwitterion reaction with the surface. Initially, ZG-PU was fabricated using a two- day grafting period; increasing this to five or seven days seemed to have a minimal effect on the resulting graft, as their hydrophilicity upon removal from solution was similar to their respective time points in ZG-PU with two-day grafting periods (ZG-PU-2d) (Figure 5c, d). However, extending the grafting period to fourteen days was found to increase ZG-PU hydrophilicity compared to the fourteen-day time point of ZG-PU-2d (Figure 5c, d). Moreover, while ZG-PU with five- or seven-day grafting periods followed similar hydrophobic recovery trends (albeit slightly mitigated), a fourteen-day grafting period maintained a very low hydrophilicity (Figure 5d), suggesting that extending the grafting period mitigates ZG-PU hydrophobic recovery. Importantly, these findings remain valid when correcting these trends to the time after removal from zwitterion solution. This is likely due to continuous interactions between the functionalised substrate surface and zwitterion chains, as more high energy and unstable functional groups migrate to the substrate surface over this period. A fourteen-day grafting period was tested to cover the period of rapid surface changes observed after substrate functionalisation (Figure 3), with the successful mitigation of hydrophobic recovery indicating that safeguards must be implemented in this period to maintain the quality of the zwitterion graft.Storage conditions

[0156] Hydrophobic recovery fundamentally describes the process by which polymer chains embed their polar elements to minimise free energy at the interface of air-substrate; thus, it was theorised that maintaining a polar storage environment could also mitigate hydrophobic recovery. To this end, ZG-PU stored in water maintained a low water contact angle of -25° up to three months after fabrication, indicating a much more hydrophilic surface than ZG-PU stored in air (Figure 5e, f).Graft-from strategy

[0157] A graft-to strategy was utilised throughout this report for ZG-PU fabrication; however, a graft-from strategy may prove superior for process scalability and commercial translation. Generally, graft-to strategies involve the initiation of polymerisation adjacent to the surface, followed by chain propagation, and terminating by surface binding. On the other hand, a graft- from strategy involves surface-initiated polymer chain propagation; in the context of this process, this involves directly immersing PI Il-functionalised PU substrates in a zwitterionic monomer solution without a chemical initiator, resulting in zwitterion monomers binding to thefree radicals on the functionalised PU surface, facilitating surface-anchored polyzwitterion propagation via free radical polymerisation. This protocol successfully fabricated ZG-PU (GF- ZG-PU), as demonstrated by similar hydrophilicity (Figure 6a), elemental composition (Figure 6b), and morphology (Figure 6c) as previous graft-to ZG-PU (GT-ZG-PU).

[0158] This graft-to method was found to substantially mitigate hydrophobic recovery when combined with a fourteen-day grafting period and a higher zwitterionic monomer concentration (Figure 6d, Figure 7). Notably, this contrasted with the limited utility of high zwitterion concentrations which was observed using the graft-to approach, importantly suggesting that steric limitations are overcome when using a monomer solution instead of a polymer solution. These parameters facilitate the most stable zwitterion grafts, with the lowest hydrophilicity consistently recorded for one and two months after fabrication, relative to graft-to ZG-PU samples, and graft-from samples with lower SBMA concentrations and grafting periods (Figure 6d). Using a graft-from methodology, the surface migration of high-energy unstable moieties and delayed free radical generation of plasma-treated substrates becomes advantageous as they offer a continuous supply of reactive sites to graft zwitterion monomers and initiate polymerisation. Moreover, this approach does not require the addition of an external initiator, nor does it necessitate an initiating step, e.g., heat or UV, thus reducing the reagents and infrastructure required for the process, as well as minimising waste generation. Furthermore, as no external initiators are present in the monomer solution, the solution may be reused in the style of a continuous batch process, given that its only constituents are unreacted zwitterion monomers and water.Anti-fouling performance

[0159] The adsorption of BSA was investigated due to the albumin being a predominant blood protein that is fundamentally implicated in the early stages of biofouling. BSA adsorption was evaluated by a fluorescence assay, wherein BSA solutions exposed to either PU or ZG- PU materials were conjugated with fluorophores via amine reactions and the resulting absorbance used to define the concentration of BSA in solution; less BSA in solution implied more BSA that had been removed from solution via adsorption to the immersed material. To correct for adsorption to the well plate, the fluorescence of a control solution without any material immersion was also tested. The fluorescence absorbance of PU, ZG-PU and control samples were 36,291.16 ± 265.1 1 , 37,541.03 ± 182.82 and 37,598.12 ± 169.30 (arbitrary units), respectively (Figure 8a). Using a pre-determined calibration curve, these translated to BSA solution concentrations of 23.60 ± 0.46, 26.19 ± 0.21 , and 25.10 ± 0.23 mg / mL (Figure 8b). Based on these solution concentrations, and after correcting for adsorption to the well plate, PU adsorbed 1 ,201 ± 429.98 pg / cm2more than the control, while ZG-PU adsorbed1 ,083 ± 210.12 pg / cm2less than the control; that is, the BSA adsorption density on PU was approximately 2,285 pg / cm2greater than the BSA adsorption density on ZG-PU (Figure 8c). While a proportional comparison between BSA adsorption on PU and ZG-PU was not possible due to ZG-PU recording a lower adsorption than the control well, this observation itself clearly demonstrates the antifouling nature of ZG-PU. This importantly indicates the antifouling performance of ZG-PU, which can be attributed to the robust zwitterion graft. Furthermore, these tests were performed almost two months after material fabrication, thus further demonstrating the stability of the optimised zwitterion grafting process described in this report.Process versatility

[0160] The versatility of this process to other polymer substrates and zwitterions was investigated. A range of polymeric substrates were successfully modified, including PMMA (Figure 9), PCL (Figure 10a), PVC (Figure 10b), and PTFE (Figure 10c), suggesting that this process may be applicable to a range of materials that have well-established medical utility. Furthermore, an alternative zwitterion, 2-methacryloyloxyethyl phosphorylcholine (MPC) was successfully grafted to PU (Figure 10d), resulting in similar hydrophilicity to SBMA-grafted ZG- PU (Figure 10d.iv) and the development of a phosphorus peak in XPS analysis (Figure 10d.v).

[0161] Taken together, these results demonstrate that this process can readily modify the surface of a range of ready-made polymeric materials with commercial methacrylated zwitterions, imbuing antifouling and antithrombogenic properties that minimise adverse biological responses. Much work has been reported recently in the design and development of superior zwitterion chemistries, with newer generations of these zwitterionic grafts expected to be able to be readily integrated into this process, provided that a carbon double bond is available to react with the functionalised substrate.Example 2Materials

[0162] A carbonate-based polyurethane (ChronoFlex C, Shore Hardness 75A) was purchased from Mitsubishi Chemical Group. Tetrahydrofuran (THF), (sulfobetaine methacrylate, SBMA), and N-N’-methylenebis(acrylamide) (bisA) were purchased from Sigma Aldrich.ZG-PU preparationPolyurethane preparation

[0163] Polyurethane was dissolved in THF at a concentration of 10 w / v% and casted into glass Petri dishes to prepare polyurethane films. Dishes were covered with aluminium foil with a few small holes to control the rate of THF evaporation and placed in a fume cupboard overnight to produce polyurethane films of approximately 150 pm thickness.Zwitterion solution preparation

[0164] SBMA was dissolved in deionised water at concentrations of 10Omg / mL or 500mg / mL. Then, bisA was added to the solution at concentrations of 0.1 , 0.5, or 1 mol%, with respect to the molar concentration of SBMA. Solutions were stirred to dissolve, and then degassed by nitrogen bubbling for at least 30 minutes.Pill treatment

[0165] Plasma immersion ion implantation (PHI) treatment was carried out using a custom- made system. Polyurethane films (6x5cm) were fixed to a sample holder and a mesh attached over the top of the sample surface. This holder was mounted into the Pill chamber and electrically connected to a high voltage PHI unit. The chamber was vacuumed to 5x10-5Torr and nitrogen gas was introduced to bring the chamber pressure to 2x10-3Torr. Nitrogen plasma was generated via a radiofrequency power of 100W and pulsed towards the PU substrate at a pulse time of 20 ps, a frequency of 50 Hz, and with a voltage of 20kV. Pill treatment lasted for 400 seconds.Zwitterion grafting

[0166] Immediately after PHI treatment, the films were transferred to a petri dish and immersed in zwitterion solution. This was to establish surface-initiated free radical polymerisation of the zwitterion monomers, using the free radicals generated at the PHI-treated surface as initiators. Dishes were sealed with paraffin film and placed on an orbital shaker agitating at 60rpm to limit sedimentation occurring within the zwitterion solution. Grafting was performed for fourteen days to counteract any residual migration of implanted radicals to the surface. Samples were removed from solution and washed at least three times in deionised water, and then immersed in deionised water for at least six hours to swell and delaminate any residual non-grafted zwitterion.Chemical characterisationElemental analysis

[0167] Surface chemistry was analysed using X-ray Photoelectron Spectroscopy (K-alpha+XPS, Thermo Fisher Scientific). Samples (n = 5) were mounted on a standard sample stage and loaded into the machine. The experiment was created using the Avantage Data System software (Thermo Fisher Scientific), setting the X-ray gun spot size to 400 pm. The flood gun was turned on to discharge. For each sample, ten survey, C1 s, O1 s, N1 s and S2p spectra were obtained. S2p was placed first due to the potential for X-rays to damage sulphur- containing molecules. The resulting data was analysed using the Avantage software. Each spectrum was deconvoluted to identify the individual peaks, and the area under these peaks were used to quantify the elemental composition of the sample. Survey spectra were used to determine atomic concentrations, and S2p and N1 s spectra were used to quantity sulphur and nitrogen content, respectively. To allow comparison between samples, peak areas were normalised by the peak area of C-C from the C1 s spectrum.Water contact angle

[0168] Hydrophilicity was analysed by measuring the water contact angle using a drop shape analyser (Theta Flex Auto 2, Biolin Scientific). A 5 pL static sessile droplet was introduced to the material surface at room temperature, and the contact angle recorded ten seconds after initial deposition. The software was used to analyse the contact angle for each droplet. Eight contact angle measurements were recorded per sample.Morphological characterisationSurface imaging and analysis

[0169] Materials were firstly imaged using Scanning Electron Microscopy (Phenom™ XL G2 Desktop SEM, Thermo Fisher Scientific) using a back scatter electron detector applying a 15kV voltage. Samples were sputter coated using a plasma coater to deposit a 16nm thick gold / palladium layer. Various images were captured across the surface of the sample, with representative images presented in the main text. For each image, the focus, brightness, and contrast were manually configured. To obtain a three-dimensional visualisation of the surface, Atomic Force Microscopy (NeaSNOM, Neaspec) was used, with the resulting maps processed in Gwyddion to visualise the morphology, and obtain the surface roughness. Surface roughness was taken from multiple points across the same sample. Separate AFM measurements were taken using a Tip Force mode, wherein an AFM cantilever with a 2.7 N / m spring constant was used to measure the Young’s modulus of the surface. The resulting maps were then analysed using Gwyddion, taking the average Young’s modulus of multiple points across the same sample. Measurements were corrected using the known Young’s modulusof the unmodified polyurethane (16.21 ± 0.27 MPa). This value was obtained by performing tensile tests on 30mmx10mm films of the unmodified polyurethane (n=6) at a strain rate of 10mm / min (Table 2). The ratio between the known and recorded moduli for this polyurethane was then applied to all other measurements.Table 2. Young’s Modulus of ChronoFlex C 75A determined by tensile tests.1 15.9982 15.5323 17.284 15.7585 16.7176 15.955Average 16.21Estimating layer thickness

[0170] Layer thickness was estimated using AFM by preparing specific zwitterion-grafted polyurethanes that featured a distinct boundary between grafted and non-grafted regions. A mask was adhered over half of a polyurethane film, and plasma treatment and zwitterion grafting performed as previously described. After this time, the mask was removed and washed thoroughly to remove any residual adhesive. These samples were mounted in the AFM, and this boundary between non-modified and modified regions was found using the optical microscope. A 100 pm region normal to this boundary was analysed using the AFM protocol previously described (i.e., the cantilever probed 50 pm into each region from the boundary). A schematic of this can be found in Figure 14a. The maps were analysed using Gyddion and the mean height of the most extreme 20 pm of both regions were recorded and the difference between these heights on the same sample taken as the graft height. Three samples were measured for each group.ThrombogenesisStatic thrombosis assay

[0171] Zwitterion-grafted polyurethanes were cut into 1 cm2samples (n = 4), weighed and attached to the bottom of a BSA-coated 24 well plate. Fresh whole blood was withdrawn under sterile conditions can immediately combined with 3x10-4v / v% heparin. Heparinised blood wasthen placed on ice and 750 piL pipetted into each well to completely cover each sample. Any samples that detached from the bottom of the well plate were removed from analysis. Samples were agitated at 65 rpm while being incubated at 37°C for 40 minutes. Each sample was then removed from the well plate and carefully washed with PBS solution. After washing, samples were weighed again, and the difference in mass from before incubation was calculated as the mass of the developed thrombus.Dynamic thrombosis assay

[0172] Dynamic thrombosis tests were performed using a Chandler Loop system. Zwitterion- grafted polyurethanes, non-modified polyurethanes, and plasma-treated polyurethanes were cut into 0.6 cm x 1 .2 cm samples and weighed. PDMS tubing (0.25 cm internal diameter) was cut to fit in a loop around the rotating component of the Chandler Loop, and a sample was placed inside the lumen, with each tube only holding one sample (n = 5). Fresh whole blood was withdrawn under sterile conditions and immediately combined with 2x10-4v / v% heparin. 2.5mL of heparinised blood was immediately injected into each tube, and the Chandler Loop rotated at 20 rpm for 50 minutes at 37°C. Each tube was carefully emptied, and the sample, any adhered thrombus, and any free thrombus were collected for weighing. The difference in mass from before incubation was calculated as the mass of the developed thrombus.Results and DiscussionFabrication of crosslinked zwitterion-grafted polyurethanes

[0173] A commercially available polycarbonate urethane) (PU) was modified via plasma treatment and zwitterion grafting using a variation of a protocol previously developed. Briefly, the polyurethane film was irradiated with nitrogen plasma under vacuum using a Pill platform to generate surface-anchored free radicals. The plasma-activated polyurethane was then immersed in an aqueous solution of [2-(methacryloyloxy)ethyl]dimethyl-(3- sulfopropyl)ammonium hydroxide, herein referred to as sulfobetaine methacrylate (SBMA). A low molecular weight crosslinker, N,N’-methylenebis(acrylamide) (bisA), was added to this solution at different concentrations to explore the impact of modulating the crosslinking density within the surface-grafted zwitterion architecture on the surface’s capacity to limit hydrophobic recovery. Given that lower molecular weight zwitterionic brushes may be more susceptible to hydrophobic recovery due to their capacity to either diffuse or more easily reorientate, a range of crosslinked zwitterion-grafted polyurethanes were fabricated with a lower zwitterion monomer concentration than previously employed. Specifically, whereas 500mg / mL of SBMA monomer was previously found necessary to limit hydrophobic recovery (attributed to thelimited diffusion and reorientation of longer, packed chains), in this study 100mg / mL of SBMA monomer was used to fabricate materials, allowing the impact of crosslinking on the mobility of the grafted surface to be more clearly elucidated. To this end, bisA crosslinker was added to 10Omg / mL at concentrations of 0, 0.1 , 0.5, and 1 mol% (based on SBMA), which alongside a control group of 500mg / mL with no crosslinker (as the conclusion of our previous study), formed the main sample groups tested throughout this study (Table 3).Table 3. Description of main sample groupsSample Substrate concentration concentration Expected polymer architectureNamePolycarbonate100-0 100 0 Brush (low molecular weight) urethane)Poly(carbonate100-0.1 100 0.1 Network (low density) urethane)Poly(carbonate100-0.5 100 0.5 Network (medium density) urethane)Poly(carbonate 100-1 100 1 Network (high density) urethane)Poly(carbonate500-0 500 0 Brush (high molecular weight) urethane)Chemical characterisation of crosslinked zwitterion-grafted polyurethanes

[0174] As expected, all grafting strategies increased the wettability of the material, transforming the hydrophobic PU (water contact angle of 81.35° ± 4.53°) into extremely hydrophilic surfaces (Figure 11 ). Among the two brush architectures, 500-0 was superhydrophilic (9.12° ± 1.27°), while 100-0 was nearly superhydrophilic (11.75° ± 2.59°). Generating network architectures was able to push this latter sample into the superhydrophilic range, with 100-0.1 , 100-0.5, and 100-1 demonstrating contact angles of 7.12° ± 1 .40°, 7.70° ± 0.82°, and 7.60° ± 1.27°, respectively (Figure 11a). Notably, there were no significant differences in hydrophilicity when adding crosslinkers to samples fabricated with 500mg / mL SBMA (Figure 11c), contributing to the decision to establish lower SBMA chains to explore the impact of crosslinkers.

[0175] X-ray photoelectron spectroscopy was then used to evaluate the elemental composition of the surface, specifically surveying for sulphur and nitrogen content as indicators for SBMA presence (Figure 12). Whereas PU demonstrated no peak at sulphur (S2p) orbitals, all fabricated materials saw the development of well-defined sulphur peaks (Figure 12a.i, Figure 12a.ii). Furthermore, deconvolution of the nitrogen (N1 s) revealed the development of a predominant peak at 402eV in all zwitterion-grafted samples, but not in pristine PU. Meanwhile, a peak at 399eV was present in all samples but strongest in PU Figure 12a. Hi. The former was associated with the ammonium cation of the zwitterionic side chains, while the latter was associated with the amine groups in the urethane linkages (and to a smaller extent the amines in the crosslinker molecules). Together with the hydrophilicity analyses, these observations confirm the successful development of a zwitterion graft, regardless of zwitterion or crosslinker concentrations. When relying on brush architectures alone, longer chain lengths correlated with higher readings for sulphur and ammonium-bound nitrogen, which were associated with the sulfonate anion and the ammonium cation of SBMA, respectively. For example, when normalising sulphur content by sp3 carbon, 500-0 reported around twice as much sulphur as 100-0 (0.23 ± 0.06 vs. 0.12 ± 0.07, respectively) (Figure 12a.iv). Similarly, normalised ammonium-bound nitrogen was 0.15 ± 0.01 on 500-0, compared to 0.08 ± 0.03 on 100-0 (Figure 12a.v). Importantly, generating a network architecture via the introduction of crosslinkers also resulted in increases in both sulphur (Figure 12a.iv, Figure 12b) and ammonium-bound nitrogen content (Figure 12b.v). Both elements were present in similar quantities to 500-0 samples, despite these materials being fabricated with a fifth of the zwitterion. As the same monomer concentration was used for the brush and network groups, this observation can be readily explained by crosslinkers facilitating the retention of zwitterionic moieties at the surface. By linking adjacent chains together, this approach effectively increased the molecular weight of the graft and reduced its mobility, thus establishing kinetic barriers to the process of hydrophobic recovery. Whereas previously the brushes were mobile enough to either diffuse or reorientate substantial numbers of polar groups away from the surface, the generation of a network sterically prevents this from happening, thus retaining the zwitterionic chemistries at the surface. This conclusion is corroborated by considering the surface behaviour when introducing crosslinkers to higher zwitterion concentrations; when modulating the 500-0 (brush) group to contain crosslinks, there was no change in the sulphur content (via the survey spectrum, or via the S2p spectrum), indicating that there was no benefit in adding crosslinkers to such surface architectures (i.e., the long zwitterionic brushes of 500- 0) as they are already difficult to move away from the surface (Figure 12c). As hydrophobic recovery after plasma treatment is most dramatic during the first few days, the fact that there were observable differences between brush and network architectures at this early stage inthe study (i.e., before aging) highlights the potential utility in developing network architectures to mitigate hydrophobic recovery.Morphological and mechanical characterisation of crosslinked zwitterion-grafted polyurethanes

[0176] Using both scanning electron microscopy (SEM) and atomic force microscopy (AFM), the morphology of the surface was altered with zwitterion grafting, with the bare and nondescript polyurethane surface developing a distinct wave-like topography with the grafting of a zwitterion layer, regardless of polymer architecture (Figure 13a). Increased roughness was associated with the wave-like morphology of the zwitterion graft. While that the pristine polyurethane exhibited a very low roughness (16.7nm ± 3.15nm), all grafted samples were significantly rougher (>39nm) (Figure 13b). Moreover, the roughness of the surface tended to increase as crosslinking concentration increased, going from 41.34nm ± 7.67nm on 100-0 (brush) to 51.47nm ± 6.06nm on 100-1 (dense network). AFM was then employed to obtain more detailed information about the surface by evaluating the stiffness of the surface and estimating the thickness of the grafted layer.

[0177] The Young’s modulus (E) of the surface was evaluated using AFM in Tip Force mode (Figure 13c). All grafted samples demonstrated a significantly lower E than pristine PU; while the E of PU was 16.15 ± 0.43MPa, all grafted groups had moduli below 1.3MPa. This was expected as bulk zwitterions and other hydrogels typically have very low Young’s moduli, especially when compared to thermoplastic polymers such as polyurethane. For example, polyurethanes embedded with zwitterionic moieties in the bulk decrease the tensile strength of the overall polymer. Moreover, a U-curve trend was observed when progressing from brush architectures (100-0) to increasingly dense network architectures; i.e., 100-0.1 and 100-0.5 were slightly softer (1 ,08MPa ± 0.03MPa and 1 ,02MPa ± 0.03MPa, respectively) than 100-0 (1 ,32MPa ± 0.05MPa), but 100-1 was then stiffer than 100-0.5 at 1 ,32MPa ± 0.07MPa (Figure 13c). This can be explained by two competing factors: 1 ) the contribution of crosslinkers to limiting hydrophobic recovery due to reduced molecular mobility, and 2) the stiffness naturally imbued into a crosslinked network due to the reduction in molecular mobility. Specifically, low- density (100-0.1 ) and medium-density (100-0.5) networks exhibit sufficient crosslinker concentrations to retain the zwitterionic graft, but further increases in crosslinker density bear no benefit on graft retention (based on a lack of significance amongst these groups in water contact angle and elemental concentration), and only serve to stiffen the zwitterion network. This theory is also corroborated by considering the even softer measurement for 500-0, which was expected to have a low modulus due to hydrophobic recovery being limited by the length of the zwitterionic chains themselves, thus not requiring crosslinkers to generate stiff networks.Taken together, these results indicate that while introducing crosslinkers to create networked zwitterionic architectures limits hydrophobic recovery, they may also be detrimental to the surface properties by unnecessarily increasing the surface stiffness. This indicates that an optimal crosslinker concentration must be found that balances graft retention with desirable surface mechanics. Additionally, this brief discussion on material mechanics encapsulates the current challenges facing the development of polymers for medical devices: mechanically suitable polymers lack biocompatibility, while biologically suited polymers lack mechanical integrity. As such, this field either focusses on modifying mechanically-suitable polymers with biocompatible components, or mechanically reinforcing biologically-suitable polymers. Notably, the present study posits that modifying just the surface of mechanically-suitable polymers (i.e. rather than the entire bulk) is the most promising approach to this challenge as this facilitates the development of interfacial biocompatibility without compromising mechanical properties.

[0178] Finally, the thickness of the zwitterion layer was approximated by fabricating specific samples that were only plasma treated and grafted on half of the surface by using a mask during both processes (Figure 14a). This generated a boundary between modified and nonmodified regions that was analysed using AFM to determine differences in height between the two regions (Figure 14b-f). Considering just the brush architectures, samples made to have high chain length (500-0) were much thicker than samples with low chain length (100-0); the former had a thickness of 30.70nm ± 6.30nm, while the latter had a thickness of just 4.27nm ± 2.37nm (Figure 13d). This confirms the approach of using higher monomer concentrations to generate longer zwitterionic brushes, given that given that monomer concentration is proportional to the rate and degree of polymerisation, albeit disallowing control over exact molecular weights and distribution via the described free radical approach. This finding helps to explain the conclusions of our previous study, where we found 500mg / mL to be an optimal zwitterion monomer concentration. By generating longer chains at the surface, especially when using a graft-from strategy where chain density is likely maximised, steric limitations against molecular reconfiguration would naturally emerge due to the compact and extensive nature of the zwitterion brushes. Furthermore, graft thickness increased as crosslinked concentration increased, progressing from 4.27nm ± 2.37nm in the 100-0 brush architecture to as thick as 38.33nm ± 4.71 nm in the 100-1 dense network architecture (Figure 13d). Thus, the thickness of the surface layer decreases in the absence of crosslinking, attributable to highly mobile molecules diffusing or embedding underneath the surface layers during the fabrication process. It must be noted that this approach only offered an approximation due to the large height variation on each surface; the average height for each sample was used, despite the maximum and minimum heights for each sample varying substantially.Nonetheless, this approach allowed the comparison between different groups as the same analysis was universally applied.

[0179] Taken together, these findings demonstrate that modulating zwitterionic surface architectures by generating crosslinked networks enables the development of thicker surface layers by limiting early-stage hydrophobic recovery.Thrombogenicity of crosslinked zwitterion -grafted polyurethanes

[0180] The thrombogenicity of these samples was assessed using an in vitro static coagulation assay (Figure 15). Pill treatment of the PU surface resulted in significantly greater thrombus generation (thrombus mass: 19.9mg ± 7.50mg) than PU alone (6.8mg ± 2.48mg), attributable to its charge-based hydrophilicity, which thermodynamically favours water displacement from the surface and biomolecule adhesion. With a thrombus mass of 10.85mg ± 4.46mg, 100-0 induced less thrombus formation than PIII-PU but remained more thrombogenic than pristine PU. Introducing crosslinkers to generate networked architectures caused the surface to become remarkably antithrombogenic, with all crosslinked groups resulting in thrombus masses of less than 3mg, which approached the thrombus mass on 500- 0 (1.13mg ± 0.92mg). This correlates well with prior characterisations, with hydrophobic recovery on the low molecular weight brush architecture of 100-0 depleting zwitterionic moieties from the surface, thus generating a more thrombogenic interface. The generation of a network architecture overcomes these issues by facilitating graft retention, thus retaining the antithrombogenicity of the zwitterionic surface.

[0181] Until this point, all tests have been on freshly prepared samples, which entails a brief plasma-processing step, followed by two weeks immersion in zwitterion solution before washing and immediate characterisation. As was observed in the differences between brush and network architectures at low zwitterion monomer concentrations, the inclusion of crosslinkers already helped to limit hydrophobic recovery, resulting in thicker zwitterionic grafts and more antithrombogenic surfaces. The next progression was to age these samples and evaluate their properties over time to draw conclusions about the utility of generating zwitterionic networks on the longevity of these polymers as blood-contacting materials.Monitoring the durability of crosslinked zwitterion-grafted polyurethanes

[0182] Samples were aged for twelve weeks in air to elucidate the impact of hydrophobic recovery on the surface graft. Based on the previous characterisation results, superior graft retention was evaluated as materials with the (1 ) lowest hydrophilicity, measured via water contact angle (Figure 16a); (2) highest elemental composition of sulphur (Figure 16b) andammonium-bound nitrogen (Figure 16c), via XPS; (3) maintenance of a wave-like morphology, determined via surface roughness (Figure 16d) and scanning electron microscopy (Figure 17); and the least thrombogenic behaviour, measured via a static coagulation assay (Figure 16e, Figure 18).

[0183] All samples exhibited indicators of hydrophobic recovery as they aged, however the magnitude of changes varied considerably between samples. Firstly, the water contact angle of the 100-0 group (72.78° ± 9.37°) increased substantially more than all crosslinked groups and the 500-0 group, which were all under 50° (Figure 16a). Moreover, there was a U-shaped trend among the groups with network architectures, with a slightly greater hydrophilicity occurring at a medium crosslinking density. This can be explained by the medium crosslinking density being sufficient to limit hydrophobic recovery without substantially reducing the porosity of the surface. Notably, the hydrophilicity of all network-based samples was slightly lower than 500-0, indicating that network architectures generally demonstrated greater resistance to hydrophobic recovery than brush architectures. Supporting this assumption, when monitoring the hydrophilicity of an identical catalogue of samples that were stored in water over the twelve weeks, water contact angles remained around 20° for all groups (Figure 19), which can be attributed to hydrophobic recovery being limited due to dipole-dipole interactions with the polar water environment. Thus, the differences between samples stored in air can be attributed to varying resistance to hydrophobic recovery, thus indicating that developing crosslinked zwitterionic surfaces offers a promising route for the development of long-lasting biocompatible polymers.

[0184] These findings are further supported by chemical analysis via XPS, wherein the content of both sulphur and ammonium-bound nitrogen after twelve weeks was greater on samples with network architectures than samples with brush architectures (Figure 16b, Figure 16c). As before, this indicates that these samples exhibited the highest concentration of zwitterionic moieties due to their ability to be retained at the surface via their integration into a less mobile molecular network. Surface roughness was also retained in all crosslinked samples but dropped substantially to 16.7nm ± 3.15nm in 100-0, quantifying a loss in the distinct wave-like morphology of the zwitterion-grafted polyurethane (Figure 16d). Importantly, when an identical 100-0 material was stored in water, this roughness did not reduce over time (Figure 20), reinforcing the notion that hydrophobic recovery may be slowed via crosslinking. Moreover, the trend towards increased crosslinker correlating with increased roughness was again reflected after twelve weeks, going from 41.34nm ± 1.41 nm to 48.46nm ± 1.44nm to 51 ,47nm ± 3.71 nm for 100-0.1 , 100-0.5, and 100-1 , respectively.

[0185] Finally, static thrombosis assays were repeated after twelve weeks aging, with 100-0 exhibiting the most thrombogenic surface with a clot mass of 17.53mg ± 2.31 mg, reiterating how the hydrophobic recovery of this low molecular weight brush architecture directly impacts the functional performance of the material (Figure 16e). Importantly, Pill-treated samples with no zwitterion grafting gradually increased in thrombogenicity over the same time, confirming that the zwitterions indeed imbue the antithrombogenic properties (Figure 18b). Interestingly, networked grafts with the greatest crosslinking density (100-1 ) demonstrated the second-most thrombogenic surface (11.08mg ± 1.99mg), while the least thrombogenic surfaces were associated with long brush architectures (500-0) and medium-density networks (100-0.5), at 4.90mg ± 1 ,62mg and 5.7mg ± 1 ,88mg, respectively. The U-shaped trend with increasing crosslinker density can be attributed to a combination of different mechanisms. While raising the crosslinker concentration helps to avoid hydrophobic recovery by increasing the molecular rigidity of the zwitterionic graft, this is only beneficial until a certain threshold of crosslinker concentration. Beyond this threshold, sufficient zwitterionic graft can be retained to impart antithrombogenic properties, and the increasing crosslinking density only serves to further stiffen the network, as previously observed in the initial mechanical characterisations (Figure 13c). The stiffness of these high-density zwitterion networks may become (relatively) detrimental to their antithrombogenic performance. For example, the effect of network stiffness on thrombogenicity was recently elucidated via a surface-grafted double-network hydrogel of poly(2-acrylamido-2-methylpropanesulfonic acid and chitosan, wherein increasing the hydrogel stiffness (by modulating the ratio of the two networks) correlated to an increase in platelet adhesion and extension of clotting time. In fact, when adding crosslinker at a 500mg / mL SBMA concentration, where the length of the brushes alone contribute to limiting hydrophobic recovery, increasing the crosslinker concentration correlated with an increase in thrombus formation (Figure 18c). Alongside other evidence demonstrating the importance of low-friction surfaces on protein repulsion, these findings indicate that crosslinking the zwitterionic surface graft benefits the antithrombogenic performance of the modified material due to chemical mechanisms (zwitterion retention due to hindered hydrophobic recovery) until a certain threshold of crosslinker concentration, beyond which the network architecture begins to contribute to thrombogenesis due to mechanical mechanisms (increased stiffness and friction).

[0186] Importantly, these findings correlate well with similar reports wherein the longevity of zwitterionic grafts could be extended by crosslinking the surface layers, and thus developing more rigid network architectures to resist hydrophobic recovery. Notably, both these works pursued a photoreactive strategy requiring chemical linkers to promote grafting, however this work demonstrates that zwitterionic network architectures with resistance to hydrophobicrecovery can also be generated via a linker-free plasma-mediated approach. Considering these findings, generating zwitterionic network architectures with a medium crosslinker density (which in this report correlates to a crosslinker concentration of 0.5 mol%) offers a promising strategy for limiting hydrophobic recovery in blood-contacting materials, without generating unnecessarily stiff networks which contribute to thrombogenesis. However, in parallel to this, these findings have further demonstrated that developing brush architectures with high molecular weights (using 500mg / mL SBMA monomer in this report) also demonstrate resistance to hydrophobic recovery, thus presenting two possible polymer architectures for the development of long-lasting blood-contacting surfaces. Accordingly, the following section examines these two groups (100-0.5 and 500-0) under a more physiologically representative thrombosis assay.Dynamic thrombogenicity assessment

[0187] The static thrombogenicity assessments performed previously offered a convenient tool for screening the catalogue of generated materials but does not accurately represent the physiological requirements for blood contacting materials. Typically, surfaces implanted in the cardiovascular system are subject to some degree of shear due to the flowing blood, which is not properly captured in static tests. Coagulation occurs differently depending on the shear conditions, with proteins such as von Willebrand’s factor being more involved in adhering platelets under high shear, and fibrinogen under low shear. To this end, thrombosis during dynamic interaction between the surface and blood was tested via a Chandler Loop system (Figure 21 ). Pristine polyurethane induced a clot mass of 42.02mg ± 6.06mg, and PHI treatment made the surface more thrombogenic, with a clot mass of 91.35mg ± 16.35mg on PIII-PU. Both 100-0.5 and 500-0 became substantially antithrombogenic, with respective clot masses of 8.27mg ± 1 ,79mg and 10.23mg ± 3.36mg. This validates the findings from the static thrombosis assays, but also demonstrates that these zwitterionic surfaces exhibit antithrombogenic properties under both static and dynamic conditions.

[0188] These observations must be explained from a thermodynamic perspective. The chemical inertness of hydrophobic surfaces such as PU precludes any capacity for bond formation with adjacent water molecules, thus placing them in a high energy state due to disruption of the interfacial water shell. Naturally, this energy may be minimised by protein adsorption, which can occur via hydrophobic interactions between the PU surface and hydrophobic domains of the protein structure, denaturing the protein and triggering thrombosis. On the other hand, the hydrophilicity of PIII-PU is based on charged moieties, wherein water molecules can form strong bonds with the surface but have little thermodynamic incentive to be retained at the surface due to their highly structured state. Thus, the adsorptionof proteins onto the PIII-PU surfaces can occur due to water dissociation, again triggering thrombosis. Both zwitterion-grafted polyurethanes exhibit an entirely different form of hydrophilicity than PIII-PU, with the presence of equal numbers of oppositely charged groups at the zwitterionic surface associating water molecules in a state similar to that of bulk water molecules, thus presenting little entropic drive for water dissociation in favour of protein adsorption.Conclusions

[0189] The surface of medical-grade polyurethanes was modified using a plasma-mediated strategy to develop interfacial zwitterionic brushes and networks. The architecture of these grafts was explored to elucidate the impact of crosslinking on mitigating hydrophobic recovery while retaining desirable biological properties for use in blood-contacting applications. Low molecular weight brush architectures alone were not suitable for long-term performance due to high molecular mobility, however introducing crosslinkers could readily resist hydrophobic recovery by reducing that mobility, However, beyond a certain threshold of crosslinker density, the benefits of graft retention were contested by the detrimental effects of stiffening the surface, revealing an optimal surface architecture to be a medium density zwitterionic network. Concurrently, extending the chain length themselves, but maintaining a brush architecture, was also found to limit hydrophobic recovery. These surfaces demonstrated promising resistance to thrombosis, presenting viable candidates for use in blood-contacting devices. Therefore, by modulating the architecture of zwitterionic interfaces grafted to a commercial polyurethane via plasma treatment, this report presents a platform for generating durable blood-compatible polymers.

[0190] Those skilled in the art will appreciate that the disclosure described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of an two or more of said steps, features, compositions and compounds.

[0191] A number of forms of the invention are set out below.

[0192] Form 1 : A method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, andcontacting the surface with a solution comprising monomers to form a graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days.

[0193] Form 2. A method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days.

[0194] Form 3. The method of form 1 or form 2, wherein the contacting the surface with a solution comprising monomers is for a duration of between about 10 days and about 20 days.

[0195] Form 4. The method of any one of forms 1 to 3, wherein the contacting the surface with a solution comprising monomers is for a duration of about 14 days.

[0196] Form 5. The method of any one of forms 1 to 4, further comprising covering the surface of the material with a polar medium.

[0197] Form 6. A method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0198] Form 7. A method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment,contacting the surface with a solution comprising monomers to form the graft of the second polymer, and covering the surface with a polar medium.

[0199] Form 8. A method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, wherein the graft of the second polymer has been prepared by: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; the method comprising covering the surface with a polar medium.

[0200] Form 9. The method of any one of forms 6 to 8, wherein covering the surface of the material with a polar medium comprises immersing the material in the polar medium.

[0201] Form 10. The method of any one of forms 6 to 9, wherein the polar medium is selected from water, methanol, ethanol, and a mixture of water with methanol and / or ethanol.

[0202] Form 1 1. A method of forming a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form the graft of the second polymer; wherein solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M.

[0203] Form 12. A method of reducing deterioration of a graft of a second polymer on a surface of a material comprised of a first polymer, the method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a solution comprising monomers to form a graft of the second polymer;wherein the solution comprising monomers does not comprise a radical initiator, and wherein the concentration of the solution comprising monomers is at least about 0.5 M.

[0204] Form 13. The method of form 11 or form 12, wherein the concentration of the solution comprising monomers is between about 0.5 M and about 5 M.

[0205] Form 14. The method of any one of forms 11 to 13, wherein the concentration of the solution comprising monomers is between about 0.7 M and about 2 M.

[0206] Form 15. The method of any one of forms 11 to 14, wherein the contacting the surface with a solution comprising monomers is for a duration of at least about 10 days.

[0207] Form 16. The method of any one of forms 11 to 15, wherein the contacting the surface with a solution comprising monomers is for a duration of between about 10 days and about 20 days.

[0208] Form 17. The method of any one of forms 11 to 16, wherein the contacting the surface with a solution comprising monomers is for a duration of about 14 days.

[0209] Form 18. The method of any one of forms 11 to 17, further comprising covering the surface of the material with a polar medium.

[0210] Form 19. The method of any one of forms 11 to 18, wherein the polar medium is selected from water, methanol, ethanol, and a mixture of water with methanol and / or ethanol.

[0211] Form 20. The method of any one of forms 1 to 19, wherein the first polymer has a glass transition temperature below about body temperature.

[0212] Form 21. The method of any one of forms 1 to 20, wherein the first polymer is selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, and polytetrafluoroethylene.

[0213] Form 22. The method of any one of forms 1 to 21 , wherein the monomer is a zwitterionic compound comprising a polymerisable functional group and the second polymer is a zwitterionic polymer.

[0214] Form 23. The method of form 22, wherein the polymerizable functional group is a polymerisable double bond.

[0215] Form 24. The method of any one of forms 1 to 23, wherein the monomer is selected from the group consisting of sulfobetaine methacrylate, 2-methacryloyloxyethyl phosphorylcholine, 2-[[2-(methacryloyloxy)ethyl]dimethylammonio]acetate, 3-[[2- (methacryloyloxy)ethyl]dimethylammonio]propionate, 3-[(3- acrylamidopropyl)dimethylammonio]propanoate, 4-[[2-(methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[bis[2-(methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, 3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3- methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and 3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate.

[0216] Form 25. The method of any one of forms 1 to 24, wherein the monomer is sulfobetaine methacrylate and the second polymer is poly(sulfobetaine methacrylate).

[0217] Form 26. The method of any one of forms 1 to 25, wherein the step of contacting the surface with a solution comprising monomers comprises immersing the material in the solution comprising monomers.

[0218] Form 27. The method of any one of forms 1 to 26, wherein the step of contacting the surface with a solution comprising monomers is carried out at a temperature between about 20 °C and about 80 °C.

[0219] Form 28. The method of any one of forms 1 to 27, wherein the plasma treatment is selected from the group consisting of plasma-immersion ion implantation, plasma cleaning, and plasma treated media.

[0220] Form 29. The method of any one of forms 1 to 28, wherein the plasma treatment is plasma-immersion ion implantation.

[0221] Form 30. The method of any one of forms 1 to 29, wherein the water contact angle of the surface of the material 60 days after being subjected to the plasma treatment is increased by less than 100%, compared to the water contact angle of the surface of the material one day following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer.

[0222] Form 31 . The method of any one of forms 1 to 30, wherein the deterioration of the graft of the second polymer on the surface of the material comprised of the first polymer is reduced.

[0223] Form 32. The method of form 31 wherein the deterioration of the graft of the second polymer on the surface of the material comprised of the first polymer is reduced when the water contact angle of the surface of the material 60 days after being subjected to the plasma treatment is increased by less than 100%, compared to the water contact angle of the surface of the material one day following the step of contacting the surface with a solution comprising monomers to form a graft of the second polymer.

Claims

CLAIMS1. A method for forming a coating on a surface, said surface comprising a first polymer, said method comprising: subjecting the surface to a plasma treatment, contacting the surface with a monomer so as to graft a layer of a second polymer onto the first polymer; and applying a treatment to the second polymer so as to at least partially retain one or more properties of the second polymer on said surface.

2. The method of claim 1 wherein the surface forms at least a part of a surface of a medical device.

3. The method of claim 1 or claim 2 wherein the first polymer is selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, and polytetrafluoroethylene, polymethylpentene, graphite, graphene, and pyrolyzed graphene.

4. The method of any one of claims 1 to 3 wherein the one or more properties includes hydrophilicity.

5. The method of any one of claims 1 to 4 wherein the second polymer is a polar polymer.

6. The method of any one of claims 1 to 5 wherein monomer is a zwitterionic compound comprising a polymerisable functional group.

7. The method of claim 6 wherein the polymerizable group is polymerizable by a free radical polymerisation mechanism.

8. The method of any one of claims 1 to 7 wherein the monomer is selected from the group consisting of sulfobetaine methacrylate, 2-methacryloyloxyethyl phosphorylcholine, 2- [[2-(methacryloyloxy)ethyl]dimethylammonio]acetate, 3-[[2- (methacryloyloxy)ethyl]dimethylammonio]propionate, 3-[(3- acrylamidopropyl)dimethylammonio]propanoate, 4-[[2- (methacryloyloxy)ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2- (acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[bis[2- (methacryloyloxy)ethyl](methyl)ammonio]propane-1 -sulfonate, 3-[(3- methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3-methacrylamidopropyl)dimethylammonio]butane-1 -sulfonate, and 3-[(3- acrylamidopropyl)dimethylammonio]propane-1 -sulfonate.

9. The method of any one of claims 1 to 8 wherein the plasma treatment comprises plasma-immersion ion implantation.

10. The method of any one of claims 1 to 9 wherein the step of contacting comprises exposing the surface to a solution of the monomer.

11. The method of any one of claims 1 to 10 wherein said treatment comprises covering the second polymer with a polar material.

12. The method of claim 11 wherein the polar material is an aqueous liquid.

13. The method of claim 12 wherein the treatment comprises storing an item having the surface with the second polymer grafted thereon in the aqueous liquid until use of said item.

14. The method of any one of claims 1 to 13 wherein the treatment comprises crosslinking the second polymer.

15. The method of any one of claims 1 to 14 wherein the treatment is applied concurrently with the contacting.

16. The method of any one of claims 1 to 15 wherein the monomer is contacted with the surface in the absence of a free radical initiator.

17. An item having a surface comprising a first polymer and a second polymer grafted to the first polymer, wherein there is no linker group between the first polymer and the second polymer.

18. The item of claim 17 which forms at least a part of a medical device.

19. The item of claim 17 or claim 18 wherein the first polymer is selected from the group consisting of polyurethane, polymethylmethacrylate, polycaprolactone, polyvinylchloride, and polytetrafluoroethylene.

20. The item of any one of claims 17 to 19 wherein the second polymer is polar.

21. The item of claim 19 or claim 20 wherein the second polymer is a polymer of a zwitterionic compound comprising a polymerisable functional group.

22. The item of any one of claims 17 to 21 wherein the second polymer is crosslinked.

23. The item of any one of claims 17 to 22 which is super-hydrophilic.

24. The item of claim 23 wherein the surface retains a contact angle of less than about 25° when stored in water at 25°C for 12 weeks.

25. A method for rendering a surface super-hydrophilic, said method comprising: subjecting the surface to a plasma treatment, and contacting the surface with a monomer so as to form a graft of a second polymer on the first polymer, said monomer being a zwitterionic compound comprising a polymerisable functional group; and applying a treatment so as to retain hydrophilicity of the second polymer on said surface; said treatment comprising either crosslinking the second polymer or covering the second polymer with a polar liquid or both.

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