Functionalized Surface

Abstract

A method of treating a polymer surface using conventional plasma nitriding, and a nitrided polymer surface obtained thereby. The method comprises introducing nitrogen into the polymer surface using conventional plasma nitriding, and optionally functionalizing the nitrided polymer surface with a molecule, such as an antimicrobial moiety, which is capable of forming a covalent bond with the nitrogen atoms within the polymer surface.

Claims

1: Use of conventional plasma nitriding for treating a polymer surface.

2: A method of treating a polymer surface, the method comprising introducing nitrogen into the polymer surface using conventional plasma nitriding, thereby producing a nitrided polymer surface.

3: The method of claim 2, wherein the conventional plasma nitriding comprises: placing an article comprising the polymer surface into a vessel; and applying an electrical current between the article and a wall of the vessel in the presence of a gas comprising nitrogen.

4: The method of claim 3, wherein the pressure of the gas inside the vessel is no more than 5 mbar.

5: The method of claim 3, wherein the gas comprises at least 20% nitrogen.

6: The method of claim 3, wherein the plasma nitriding process is carried out at a temperature of no more than 100° C.

7: The method of claim 3, wherein the plasma is generated by applying a voltage of from 300 V to 375 V between the polymer surface and the vessel wall.

8: The method of claim 3, wherein the current is from 5 to 10 A.

9: The method of claim 3, wherein the plasma nitriding process is carried out for a period of from 1 hour to 25 hours.

10: The method of claim 3, further comprising a step of purging the vessel prior to carrying out the nitriding process.

11: The method of claim 2, wherein the polymer is polyamide, polycarbonate, polyester, polyethylene, polyethylene terephthalate, polypropylene, polystyrene, polyurethane, polyvinyl chloride, polyvinylidene chloride, acrylonitrile butadiene styrene, polyepoxide, polymethyl methacrylate, polytetrafluoroethylene, phenolics, melamine formaldehyde, urea formaldehyde, polyetheretherketone, maleimide, bismaleimide, polyetherimide, polyimide, plastarch material, polylactic acid, nylon, silicone, polysulfone, or a combination thereof.

12: The method of claim 2, further comprising the step of functionalizing the nitrided polymer surface with a molecule which is capable of forming a covalent bond with the nitrogen atoms within the polymer surface.

13: The method of claim 12, wherein the molecule comprises an antimicrobial moiety.

14: The method of claim 13, wherein the antimicrobial moiety is antibacterial.

15: The method of claim 13, wherein the molecule or the antimicrobial moiety is a peptide, optionally wherein the peptide comprises or consists of a defensin peptide sequence, or a fragment or variant thereof.

16: A nitrided polymer surface producible by the method of claim 1.

17: A nitrided polymer surface, wherein the surface is functionalized with a molecule comprising an antimicrobial moiety.

18: The nitrided polymer surface of claim 17, wherein the surface is functionalized with a first molecule comprising an antimicrobial moiety, and a second molecule which is different to the first molecule.

19: An article comprising the nitrided polymer surface of claim 16, wherein the nitrided polymer surface constitutes a portion of or the whole of a surface of the article.

Description

[0135] Embodiments of the invention will now be described by way of example and with reference to the accompanying Figures, in which:

[0136] FIG. 1 is a schematic diagram of a nitriding vessel for use in a method of the present invention;

[0137] FIG. 2 is a graph showing the bacterial growth on a melamine formaldehyde plug which has been nitrided and functionalized with an antimicrobial, compared to an untreated plug;

[0138] FIG. 3 is a graph showing the bacterial growth on a PVC light switch which has been nitrided and functionalized with an antimicrobial, compared to an untreated light switch;

[0139] FIG. 4 is a graph showing the bacterial growth on a PVC bath mat which has been nitrided and functionalized with an antimicrobial, compared to an untreated bath mat;

[0140] FIG. 5 is a graph showing the bacterial growth on polystyrene fittings which has been nitrided and functionalized with an antimicrobial, compared to untreated fittings;

[0141] FIG. 6 is a graph showing the bacterial growth on the outer lumen of a silicone catheter which has been nitrided and functionalized with an antimicrobial, compared to an untreated silicone catheter;

[0142] FIG. 7 is a graph showing the bacterial growth on the port of a silicone catheter which has been nitrided and functionalized with an antimicrobial, compared to an untreated catheter port;

[0143] FIG. 8 is a graph showing the bacterial growth on plastic surfaces which have been nitrided and functionalized with an antimicrobial, compared to an untreated plastic surface, after 15 minutes and 30 minutes;

[0144] FIG. 9 is a graph showing the bacterial growth on aged plastic samples which have been nitrided and functionalized with an antimicrobial, compared to an untreated aged plastic sample; and

[0145] FIG. 10 is a graph showing the bacterial growth on a silicone catheter which has been nitrided using a Henniker apparatus and functionalized with an antimicrobial, compared to an untreated silicone catheter.

EXAMPLE 1

[0146] Methodology

[0147] Nitriding

[0148] A polymer article (the workload) was loaded into a Rubig DC plasma nitriding vessel normally used for the treatment of steel and metal materials. A schematic diagram of a typical nitriding vessel 10 is shown in FIG. 1. The vessel 10 is connected to a gas supply 12, a vacuum pump 14, a power supply 16 and temperature controllers 18. Inside the vessel 10, a load plate 20 is positioned on supports 22. The load plate 20 receives the parts 24 (the work load) to be nitrided.

[0149] In the trials carried out, the work load was connected to the internal structure of the vessel by means of metal wire, although other materials could be used to locate the work load in the vessel. The pressure within the vessel was reduced using a vacuum pump to 1.5 mBar over a period of 5 minutes. The vessel was then backfilled with nitrogen (preferably high purity (99.9%) nitrogen) to a pressure of 50 mBar to purge the vessel before being pumped down again to a pressure of 1.5 mBar.

[0150] In a first stage external heaters were used to achieve a pre-set temperature value, prior to developing the plasma to create the ionisation phase. The vessel and the workload were heated to a temperature of 55° C.±10° C. using external wall heaters. The time taken to reach the pre-set temperature was typically in the range of 10 to 15 minutes. An atmosphere of 5% argon, 50% nitrogen, 45% hydrogen was used. Hydrogen is used in the mixture to remove any oxygen from the system prior to plasma nitriding. During this pre-heating phase the pressure is typically in the range of 100-200 mbar.

[0151] The vessel was pumped down again to 1.5 mBar to 2 mBar and then backfilled to a pressure of 2 mBar to 4 mBar with 75% H2:25% N.sub.2. The pressure was maintained at the required level by a regulating valve on the vacuum pump.

[0152] A DC plasma voltage of 350V was applied between the cathodically charged polymer workload and the anodic vessel wall to generate the plasma in the gas mixture and embed the positively charged nitrogen ions in the polymer surface. For the purpose of the experimental runs the following parameters were employed: [0153] Pulse duration (− and +) 100 μs [0154] Pulse pause 150 μs.

[0155] The voltage was maintained for a period of 20 hours before the DC plasma voltage was turned off. The vessel was then backfilled with high purity nitrogen to atmospheric pressure so that the vessel could be opened to remove the workload.

[0156] Functionalizing Nitrided Surfaces with Antimicrobial Molecules

[0157] A variety of plastic articles were nitrided using the methods described above, including a melamine formaldehyde plug, a polyvinyl chloride (PVC) light switch, a PVC bath mat, polystyrene fittings, and a silicone catheter. The articles were functionalized by immersing them in a solution comprising acetonitrile (10 L), HBTU (100 g), DIEA (0.6 L) and chlorhexidine (0.1 L, 20% v/v stock solution) and agitated for 5 hours at room temperature before washing with water.

[0158] Testing of Antimicrobial Function

[0159] The nitrided functionalized surfaces were tested according to the ‘simulated splash model’, which compares coated and uncoated surfaces. An overnight subculture of MRSA in LB broth was prepared. The MRSA culture was applied to each surface in 1 uL drops in a grid pattern. The surfaces were incubated at room temperature for 10 minutes. The bacteria were then removed from the surface, plated out onto LB agar plates and incubated overnight at 37° C., and the colonies were counted. Each sample was tested in triplicate and the results averaged.

[0160] Results

[0161] FIGS. 2-7 show the effect of the nitrided functionalized surfaces on bacterial growth. As shown in FIG. 2, functionalizing the nitrided melamine formaldehyde plug with chlorhexidine resulted in a reduction in colony forming units (CFU) from over 500 to less than 30. Negligible bacterial growth was observed on the functionalized PVC light switch (FIG. 3). For the PVC bath mat, bacterial growth was reduced from over 500 to less than 100 CFUs (FIG. 4). Similarly, for the polystyrene fittings growth was reduced from over 500 to less than 30 CFUs (FIG. 5). For the silicone catheter, bacterial growth was reduced from 100 to less than 5 CFUs on the outer lumen (FIG. 6) and from 160 to less than 115 CFUs on the outside of the port (FIG. 7). These results show that a significant reduction in bacterial growth on polymer surfaces can be achieved by nitrided the surfaces and functionalizing the nitrided surface with an antimicrobial molecule.

EXAMPLE 2

[0162] In a second example, plastic surfaces were nitrided and functionalized as described for Example 1 above. The plastic surfaces were tested with MRSA as described for Example 1 above, with one group of surfaces being incubated for 15 minutes and another group of surfaces being incubated for 30 minutes. FIG. 8 shows the effect of the nitrided functionalized surfaces on bacterial growth after 15 and 30 minutes, compared with a control sample. Sample 1 was a polypropylene surface, while samples 2 and 3 were uPVC surfaces. The graph in FIG. 8 shows that the bacterial growth was significantly reduced from over 30 CFUs to less than 10 CFUs after incubating for 30 minutes.

EXAMPLE 3

[0163] In a third example, polypropylene and uPVC plastic pieces were nitrided and functionalized as described for Example 1 above and then aged to simulate 10 years of service. The aging was performed by tumbling the plastic pieces with zirconium oxide beads for 48 hours. The antimicrobial function was then tested with MRSA as described for Example 1 above, with the plastic pieces being incubated for 30 minutes.

[0164] FIG. 9 shows that, in the treated samples, bacterial growth was reduced from around 50 CFUs to less than 10 CFUs. This demonstrates the longevity of the antimicrobial effect that can be achieved by nitriding and functionalizing the nitrided surface with an antimicrobial molecule.

EXAMPLE 4

[0165] In a fourth example, a silicone catheter was nitrided and functionalized as described for Example 1 above, but using a Henniker plasma nitriding unit instead of a Rubig plasma nitriding unit. The operating parameters of the Henniker apparatus were kept exactly the same as for the Rubig plasma nitriding unit. FIG. 10 shows that bacterial growth on the treated catheter was significantly reduced compared with an untreated catheter.