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Materials Science and Engineering C 78 (2017) 1179–1186

Contents lists available at ScienceDirect

Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Characterisation of polyamide 11/copper antimicrobial composites for medical device applications Nikhil Thokala a, Carmel Kealey b, James Kennedy c, Damien B. Brady b,⁎, Joseph B. Farrell a a b c

Dept. of Mechanical and Polymer Engineering, Ireland Dept. of Life & Physical Science, Ireland Kastus, Grangegorman, Dublin 7, Ireland

a r t i c l e

i n f o

Article history: Received 29 November 2016 Received in revised form 8 March 2017 Accepted 13 March 2017 Available online 18 March 2017 Keywords: Device related infections Antimicrobial polymers Inorganic antimicrobial additives Antimicrobial efficacy Polymer morphology Ion release

a b s t r a c t Direct incorporation of antimicrobial additive into the polymer matrix is a cost effective approach for the development of polymer/metal antimicrobial composites. Application of these antimicrobial composite systems for manufacturing medical devices addresses the issue of device related infections. In the present study, commercially available inorganic copper based additive, Plasticopper, was incorporated into a Polyamide 11(PA 11) matrix during the polymer processing stage. These polymer composites were evaluated for their morphological, mechanical, antimicrobial and ion release properties. Isothermal crystallisation studies showed that the copper additive acted as a nucleating agent and promoted faster crystallisation. Short term mechanical studies confirmed that the incorporation of copper has reinforcing effect on the composites with 5 and 10% copper loadings and did not adversely affect the short-term mechanical performance of the polymer composites. These composite systems were shown to be active against Escherichia coli ATCC 8739 with N99.99% reduction in bacterial population. Corresponding ion release profiles for these composites indicated long term antimicrobial activity. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Since the first pacemaker implant in 1958, in-dwelling medical device development has overcome multiple challenges with regard to material functionality. New classes of modified biomaterials used in the design and manufacture of medical devices include metals, ceramics, glass and polymers. Polymer materials are increasingly used in the manufacture of medical device components because of the wide range of desirable properties they offer, including biocompatibility, low cost, design flexibility and their particular balance of physico-mechanical properties. However, many polymer surfaces are easily colonised by microorganisms, particularly in-dwelling devices such as urinary Foley catheters, especially when they are used for extended periods [1–3]. Device-related infections are a major cause of morbidity and mortality of patients in intensive care units and can cause major medical and economic sequela [4–5]. These infections are often not detected at an early stage and pose a substantial health risk to patients and cause considerable costs to the healthcare system [6]. Late stage infections are not associated with surgical procedures but by planktonic bacteria circulating in the blood stream and their attachment to the implant surface, where

⁎ Corresponding author. E-mail address: [email protected] (D.B. Brady).

http://dx.doi.org/10.1016/j.msec.2017.03.149 0928-4931/© 2017 Elsevier B.V. All rights reserved.

they multiply and ultimately form a biofilm that eventually leads to infection [7,8]. The increasing awareness of hospital acquired antibiotic resistant infections has led to increased use of alternative antimicrobials in medical devices, equipment housings and hygienic surface applications. For medical devices, silver and copper have emerged as popular candidates due to their wide range of activity against Gram-positive, Gram-negative bacteria and also pathogenic fungi. Both silver and copper are commercially available as antimicrobial additives for medical applications in elemental, ionic forms, as oxides and associated with inorganic carriers [9–11]. These metal ions disable bacterial cells by acting on them in several ways and this multiplicity of action results in a strong biocidal effect. Silver and copper ions act as antimicrobial agents by strongly binding to critical biological molecules like proteins, DNA, RNA and disrupting their functions [12]. Due to the similar electron configurations of copper and silver (Cu: [Ar] 3d10 4s1; Ag: [Kr] 4d10 5s1), the two elements behave as mimetics in biological systems and their microbiocidal mechanisms are similar. Copper exerts its effect on microorganisms by the displacement of essential ions, thereby obstructing functional groups of proteins, inactivating enzymes, producing hydroperoxide free radicals and altering membrane integrity [13]. The low cost of copper and its alloys when compared to silver has resulted in its extensive use in the medical devices and food processing industries to curb antibiotic resistant microorganisms [14,15]. Strategies like coating these antimicrobial agents to

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the polymer surfaces have been employed [16]. However, coatings are not ideal for long term applications, where the biocidal activity of these coatings cease after these coatings wear off [17]. Polyamide 11, is an aliphatic partly crystalline homopolymer regarded as highly versatile and easy to process which makes it suitable for use in a wide range of applications. It is characterised by mechanical properties ideally suited for medical tubing, where pushability and torque performance are primary considerations. Therefore, it has been extensively used for medical applications such as sutures, catheters, laproscopy devices and blood sets [18]. Direct incorporation of metallic antimicrobial agents into the polymer matrix is a better alternative to coating techniques for the development of antimicrobial polymers [19,20]. An advantage of this approach is that the processing parameters, as well as the polymer technologies involved in device manufacture, do not require significant modification. As a result, the cost of metallic antimicrobial additives has minimal impact on overall manufacturing costs. Furthermore, antimicrobial composites prepared using this method will render long-term antimicrobial efficacy when compared to coating techniques [21]. It is important to understand which polymer characteristics could impact on the efficacy of the polymer/antimicrobial composite. Chemical structure, morphology and polarity could influence the rate of metal ion release. In this context polyamides represent an important class of biomaterial as it may be possible to exploit their hydrophilic nature to increase the release of metal ions at a concentration level that is capable of ensuring antimicrobial action. Polyamides and polyamide block copolymers dominate the percutaneous transluminal coronary angioplasty area of the catheter market, in addition to being the polymer of choice for balloon catheters and for stent delivery catheters. The primary focus of this present study is to evaluate the efficacy of micro-scale copper particles in controlling clinically important bacterial species using a medical grade Polyamide 11 as the host matrix. Polyamide 11/Cu composite samples of suitable dimensions were prepared by injection and compression moulding and evaluated for their physical, mechanical, copper elution and antimicrobial characteristics. 2. Materials and powdered masterbatch preparation

press at 200 °C. A force of 2 kN was applied for 9 min and a final pressure of 38 bar was then applied for 1 min. 2.2. Scanning electron microscopy (SEM) Morphology and weight (% w/w) composition of the test samples were determined using a Tescan Mira XMU variable pressure scanning electron microscope (SEM) in high vacuum mode. Elemental analysis was also performed simultaneously using Oxford EDS/ WDX (energy/wave dispersive X-ray). The specimen setup was scanned between 10 and 20 kV at different ranges of magnification. Additional sample treatment such as surface etching or coating with a conductive layer (Gold, ~15 nm thick) was applied before surface scanning to provide a path for the incident electrons to flow to ground. 2.3. Melting and crystallisation studies Differential scanning calorimetric (DSC) studies were carried out using a Perkin Elmer Pyris 6 DSC unit in accordance with ASTM D3417. The instrument was calibrated using indium as standard. Samples weighing approximately 10 mg were used for analysis. A first scan was carried out to eliminate previous thermal history. The result of the second scan in the range of 30 °C to 220 °C at a heating rate of 10 °C/min was recorded to determine the heat of fusion and percent crystallinity. Isothermal crystallisation studies were performed to study the crystallisation characteristics of the PA11/Cu composite materials. The optimum crystallisation temperature (Tc) was determined as 176 °C by trial and error through a series of experiments conducted at various temperatures. Initial scans were carried out to remove previous thermal effects and a second scan was then carried out from 30 °C to 220 °C at a rate of 10 °C/min. The samples were held for 1 min at 210 °C to allow complete melting, then rapidly cooled from 220 °C to the predetermined crystallisation temperature (Tc), and held until the crystallisation process was complete. Cumulative integration of the exothermic curve recorded allowed the rate of crystallisation to be evaluated.

Polyamide 11(Rilsan BMNO) a medical grade semi-crystalline polymer with a melt index value of 11.0 g/10 min. (Arkema, France) and ionic copper antimicrobial additive (Plasticopper) having average particle size of 20–30 μm (Plasticopper, Chile) were mixed in a blender at room temperature to make up a 50% w/w powder masterbatch. This helps copper particles to disperse in a finer and uniform manner in the PA powder. The powdered masterbatch was then compression moulded and injection moulded at 2, 5 and 10% w/w final copper additive loading levels for antimicrobial and short-term mechanical tests respectively.

2.4. Dynamic mechanical analysis (DMA)

2.1. Processing of antimicrobial formulations

2.5. Tensile testing

2.1.1. Injection moulding Test specimens of dimensions 40 mm × 3 mm × 6 mm, suitable for short-term mechanical and thermal studies were prepared using an Auburg 221 K injection moulding machine. The temperature profile from the feeding zone to the nozzle was 180/190/200/210/215 °C, while the mould temperature was maintained at 40 °C. Low injection pressures and injection speeds of 400 bar and 20 mm/s respectively were used during the moulding process. An ASTM D638 Type IV mould was used to prepare the tensile test samples.

Tensile testing was performed on an Instron 3365 universal testing machine using a 5 kN load cell and a crosshead speed of 10 mm min−1. An injection moulded type IV dumbbell specimen was used in all cases. The test was carried out in accordance with ASTM 638 at ambient temperature. A minimum of 5 test specimens from each of the PA 11/Cu formulations and virgin PA 11 were tested as individual sets. The mean values and standard deviation values recorded are presented in Table 5.

Dynamic mechanical properties of the virgin PA 11 and PA 11/Cu composited were identified using a TA DMQ-800 instrument in accordance with ASTM D4065-06. The test was carried out in single cantilever mode using samples of dimension 17 mm × 6 mm × 3 mm. The test sample was anchored at one end by a stationary clamp and by the drive shaft on the other. The test frequency used was 1 Hz and a heating rate of 3 °C/min was used. Storage modulus and loss tangent data were recorded for the virgin PA 11 and PA 11/Cu composites over the temperature range from −150 °C to 150 °C.

2.6. Antimicrobial studies 2.1.2. Compression moulding Test samples (virgin PA 11 and PA 11/Cu composites), of dimensions 35 mm × 35 mm and thickness ca. 0.4–0.5 mm, for antimicrobial efficacy studies were prepared using a Servitec Polystat 200 T compression

Escherichia coli strain ATCC 8739, recommended for ISO 22196 test method, was obtained from MicroBioLogics Inc., USA. The test organism, stored at −80 °C on beads, was grown overnight in Mueller Hinton and

Nutrient broths at 37 °C for MIC and antimicrobial efficiency studies respectively. The resulting bacterial suspensions were centrifuged at 10000 rpm for 10 min and the pellet resuspended in sterile phosphate buffered saline (PBS) to give working bacterial populations of 1 × 108 CFU/mL and 1 × 106 CFU/mL for MIC and efficiency studies respectively. 2.6.1. Minimum inhibitory concentration The MIC of Plasticopper additive against E. coli ATCC 8739 was evaluated using the broth dilution method in accordance with Clinical Laboratory Standards Institute (CLSI). 100 μL of Plasticopper suspensions ranging from 0 μg/mL to 100 μg/mL, were aseptically transferred into the wells of the microtiter plate and supplied with 100 μL of the test bacteria. The microtitre plate was then incubated for 16–18 h at 37 °C at a speed of 125 rpm. 100 μL of the resulting mixtures were then inoculated on to the MH agar plates and incubated for 24 h at 37 °C. The additive concentrations showing complete reduction in bacterial colonies was recorded as the MIC as presented in Table 1. 2.6.2. Evaluation of antibacterial efficiency of the PA 11/Cu composites The antimicrobial efficacy of the PA 11/Cu composites was evaluated according to the ISO 22196 standard. In brief, the square test specimens (6 untreated and 3 treated with plasticopper) prepared in Section 2.1.2 were placed in Petri dishes and inoculated with 200 μL of the test organism. The inoculum was covered with a sterile coverslip (22 mm × 22 mm), incubated for 24 h at 37 °C and 95% relative humidity for 3 of the 6 untreated specimens and 3 specimens treated with copper additive. Bacterial cell suspension was then recovered from the surface of the specimens with 10 ml of neutralising solution. The procedure was repeated for the 3 untreated specimens prior to incubation to provide comparative baseline data. Subsequently the serial dilutions of recovered bacterial cell suspension in physiological PBS (800:1 PBS and NaCl mixture) was spread on to the plate count agar plates in duplicate. These plates were then incubated for 40–48 h at 37 °C, after which colony forming units were determined. 2.7. Preliminary copper ion release and long term antimicrobial efficacy studies Preliminary Ion release studies were carried out using Varian Atomic Absorption Spectrometer. Initially a standard curve was drawn for the 1000 mg/L copper standard solution obtained from Sigma Aldrich, Ireland. Copper ion release kinetics were measured for 2, 5 and 10% w/w PA 11/Cu composites by immersing 1 g of the test composite in 100 ml aqueous mixture (95 ml d. H2O and 5 ml 0.1 N HNO3) at 37 °C. The same procedure was followed for control sample, virgin PA 11. The immersion liquid was then recovered and replaced at different time intervals (48 h to 5 weeks) and quantitatively analysed for the copper ions released by the test specimen. Selected PA 11/Cu composites

Weight percentage (%w/w)

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50 45 40 35 30 25 20 15 10 5 0 C

O

Al

Si

Cl

Cu

Zn

Elements Fig. 1. Energy/wave dispersive X-ray analysis using Tescan Mira XMU SEM showing the elemental composition of Plasticopper. The weight percentage of copper was found to be 16.07% w/w. Results are the mean (±S.D) of the weight percentages of elements present in the antimicrobial for a sample size of 3.

were analysed for long term antimicrobial activity, using the recovered immersion liquids, similar to the method used for determining the MIC. 3. Results and discussion 3.1. Determination of MIC When tested against E. coli ATCC 8739 at a working bacterial population of 1 × 108 CFU/mL, it was observed that Plasticopper additive suspensions with concentrations N10 μg/mL showed complete reduction in the working bacterial population, indicating the MIC's for Plasticopper was 10 μg/mL as displayed in Table 1. 3.2. Elemental analysis by EDS (energy/wave dispersive X-ray spectrum)/ SEM Plasticopper, an inorganic glass carrier based antimicrobial was found to be composed of aluminium, silicon, along with minute quantities of zinc and chlorine. The higher concentrations of oxygen and carbon can be attributed to the aluminium and silicon oxide, silicon carbide components of the carrier system. Copper oxide and zinc oxide parts of the additive might also influence the antimicrobial properties of the additive. The active element, copper weight percentage was identified as 16.07% (w/w) as shown in Fig. 1. An average particle size of ca. 20–40 μm was observed for the copper additive as shown in Fig. 2.

Table 1 The bacterial colonies observed for different concentrations of Plasticopper suspension, when tested against 1 × 106 CFU/mL E. coli ATCC 8739. The MIC was identified as 10 μg/ml. Concentration of the additive suspension (μg/mL)

CFU/mL recorded for Plasticopper

0.0 2.5 5.0 6.0 7.0 8.0 9.0 10 20 30 40 50 100

1.96 × 107 (±8.50) 1.78 × 103 (±2.08) 7.83 × 102 (±2.88) 1.12 × 102 (±2.08) 9.5 × 101 (±1.52) 3.86 × 101 (±1.15) 2.06 × 101 (±1.52) 0 (±0.0) 0 (±0.0) 0 (±0.0) 0 (±0.0) 0 (±0.0) 0 (±0.0)

Fig. 2. Scanning electron micrograph of the copper additive powder coated with the gold layer showing that the average particle size of the plasticopper additive is ca. 20–40 μm. Spectrum 1, 2 and 3 indicate the regions of the scan analysed for the average weight percentage of elements detected in additive.

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Fig. 3. Heat flow as a function of temperature for PA 11 and PA 11/Cu composites.

Table 2 Thermal properties of PA11/Cu composites. Materials

Melting point Tm (°C)

Enthalpy of fusion, ΔH (J/g)

Crystallinity, Xc (%)

Polyamide 11 2% w/w PA 11/Cu 5% w/w PA 11/Cu 10% w/w PA 11/Cu

193.03 191.16 191.23 191.57

63.08 40.52 34.93 37.84

30.62 19.67 16.95 18.36

Xðt Þ ¼ 1− exp −Kt n

3.3. Melting and crystallisation studies With the removal of thermal history effects via an initial DSC run the samples were subjected to a full thermal study; the curves presented in Fig. 3 are typical of those recorded for PA 11 and the PA 11/Cu composite materials. A single endothermic peak was observed for PA 11 at 193 °C which corresponds to the crystalline melting temperature of this partly crystalline matrix. No significant changes in the melting points were recorded for the PA 11 composites loaded with 2, 5 and 10% w/w copper additive. The enthalpies of melting (Δ H) values recorded are presented in Table 2. The degree of crystallinity (Xc) of the PA11 and PA11/Cu composites was calculated using Eq. (1) and the values obtained are presented in Table 2. It was observed that the degree of crystallinity decreased with the incorporation of copper additive; a change in the PA 11 morphology of this nature is likely to impact the copper ion release characteristics of these PA 11/Cu composites. Xc ¼

ΔH  100% ð1−wÞ: ΔH 

where ΔH is the enthalpy of fusion, w is the concentration of silver and ΔH* is the enthalpy of fusion of 100% crystalline polymer phase; ΔH* quoted for PA11 is 206 J/g. The isothermal crystallisation behaviour of PA 11 can be described by the Avrami equation which generally written as [22]:

ð1Þ



ð2Þ

where, X(t) is the fraction of crystallisable polymer crystallised at time t, ‘n’ is a parameter that describes the nucleation type, homogeneous or heterogeneous and crystal growth process, and ‘K’ is the overall crystallisation rate constant with nucleation and growth rates. The parameters n and K were obtained by fitting isothermal crystallisation data at X(t) b 0:8 to a linearised form of the Avrami equation. log ½– ln ð1−XðtÞÞ ¼ log K þ n log t

ð3Þ

Linear regression was used to obtain n and K, from plots of the data in the form suggested by Eq. (3). Initially, isothermal crystallisation studies of virgin PA 11 were carried out at a number of predetermined temperatures as illustrated in Fig. 4; clearly the crystallisation peak becomes broader and shifted to a longer crystallisation time as the isothermal temperatures was increased from 172 °C to 178 °C. It was observed that the optimum isothermal crystallisation temperature was 176 °C. To establish the effect of the antimicrobial additives on the crystallisation behaviour of PA 11, isothermal scans were performed at 176 °C on selected PA11/Cu formulations. The incorporation of copper particles in PA 11 matrix resulted in a reduction in the overall crystallisation time. The experimental trends recording the relative

Fig. 4. Isothermal crystallisation scans of virgin PA 11 at selected isothermal temperatures.

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Fig. 5. Isothermal crystallisation scans of virgin PA 11 (BMNO) and PA 11/Cu formulations at 176 °C.

crystallinity as a function of time for the PA 11/Cu formulations are presented in Fig. 5. Clearly there is evidence to suggest that copper additive was acting as heterogeneous nucleating site, promoting a faster nucleating rate. It was observed that time for completion of the crystallisation process for the PA-11/Cu composites with 2, 5 and 10% antimicrobial loadings was lower, when compared to the virgin PA 11. This reduction in crystallisation time observed was dependent of the antimicrobial loading used in the preparation of the PA 11/Cu composites as shown in Fig. 5.

Fig. 6 shows the relative crystallinity as a function of crystallisation time for the virgin PA 11 and PA11/Cu composites. All of the curves exhibit a sigmoidal shape, crystallinity increasing slowly with the formation of nuclei in the early stages, rapidly in the mid stage and then slow again during the final secondary crystallisation stage. Secondary crystallisation is caused by spherulite impingement and perfection of internal spherulite crystallisation in the late stages of the crystallisation process [23,24]. Fig. 7 show the plots of log [−ln(1 − X(t))] versus lot T for the PA 11 and PA11/Cu composites. The Avrami exponents were determined from the slope of these plots; these values are presented in Table 3. The Avrami constant for virgin PA 11 (n = 3.17) is consistent with values reported in previous studies [25]. The incorporation of silver in the PA 11 matrix resulted in significant changes of the Avrami constant; the values recorded for the PA 11/Cu composites were in the range 2.26–2.51. These values suggest that the growth mechanism here was two dimensional and copper additive has an influence on the overall crystallisation growth mechanism.

3.4. Dynamic mechanical analysis

Fig. 6. Relative crystallinity as a function of time for virgin PA 11 and PA 11/Cu composites at 176 °C.

The dynamic mechanical properties of unmodified PA 11 and PA 11/ Cu composites were evaluated using a TA DMQ-800 instrument. Loss tangent and storage modulus (E′) as a function of temperature data for these materials are presented in Figs. 8 and 9 respectively. Selected storage modulus and loss tangent data are also presented in Table 4. Fig. 8 illustrates that PA 11 has exhibits three distinct relaxation peaks, defined at decreasing temperatures as α, β and γ. In the glassy state γ relaxation at −138 °C corresponds to the local motions of methylene groups between the amide groups, whereas the β relaxation at −64 °C can be assigned to the localised movement of chain segments, including amide groups not involved in hydrogen bonding. At higher temperatures the alpha relaxation at 52 °C corresponds to the temperature at which mobility of the main-chain segments within amorphous regions of the polymer occurs; it is thus directly related to the glassy transition temperature. The magnitude of the loss tangent peaks at Tg for the PA 11 composites was, as expected, lower than that observed

Table 3 Avrami exponent values recorded for virgin PA 11 and PA 11/Cu composites.

Fig. 7. Log [−ln(1 − X(t))] as a function of log T for virgin PA-11 and for PA 11/Cu composites at 176 °C.

SAMPLE

Avrami exponent

Virgin PA 11 2.0% w/w PA 11/Cu 5.0% w/w PA 11/Cu 10% w/w PA 11/Cu

3.17 2.26 2.47 2.51

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Fig. 10. Typical stress strain curve for PA 11 and PA 11/Cu composites.

Fig. 8. Temperature dependence of tan δ for unmodified PA 11 and PA 11/Cu composites.

With reference to Fig. 9 it is clear that the only significant increase in storage modulus observed for all composites systems was at temperatures below −50 °C, in other words in the glassy region. No significant change in E′ was observed at temperatures above ambient for composites containing 2% and 5% copper additive loadings; a small increase in E′ above ambient was recorded for 10% copper loading, increasing by 19.8% at 25 °C. This increase in the modulus observed for 10% w/w PA11/Cu composite probably results from strong interfacial interactions between the polymer and the inorganic antimicrobial systems and the reduced mobility of the chains confined between the antimicrobial particles. 3.5. Short-term mechanical properties

Fig. 9. Temperature dependence of E′ for unmodified PA 11 and PA 11/Cu composites.

Table 4 Thermal properties recorded for PA 11 and PA 11/Cu composites. Materials

Polyamide 11 2% w/w PA 11/Cu 5% w/w PA 11/Cu 10% w/w PA 11/Cu

Relaxation γ (°C)

β (°C)

α (°C)

−137.94 −140.49 −144.49 −142.89

−64.28 −65.63 −65.90 −67.50

52.88 55.04 55.31 54.00

Storage modulus at 25 °C (MPa) 904 895 916 1083

for the virgin PA 11; however, the peak position did not change significantly, indicating little change in the glass transition temperature for the PA 11 composite materials.

To assess the effect of silver on the PA 11 short term mechanical properties tensile testing was carried out on the antimicrobial composites prepared. The responses measured were Young's modulus, yield stress, strain at break and energy at break; the mean values recorded are presented in Table 5. Stress-strain responses, typical of those recorded for virgin PA 11 selected PA 11/Cu composites tested is presented in Fig. 10. A change in the stress-strain response after the incorporation of copper additive was observed. Clearly the virgin PA 11 exhibits behaviour typical of a tough thermoplastic with a modulus of ~ 943.19 MPa, yielding at low strain and significant cold drawing prior to fracture. This type of behaviour is not unexpected since PA 11 is linear partly crystalline polar polymer with amide groups spaced out at regular intervals along its backbone, allowing significant dipole interaction between adjacent backbone chains. The polymer chains also have aliphatic segments which give flexibility in the amorphous region that contributes to the toughness of the material. The yield strength and strain at break values recorded for PA 11 were 45.74 MPa and N 250% respectively. While no significant change in Young's modulus was observed it is noteworthy that adding 2% to 10% of copper additive gives rise to a reinforcing effect as reflected in the increase recorded in yield stress values. The increase observed is probably brought about by the copper particles exerting restraints on the gross deformability of the polymer matrix during mechanical loading. The magnitude of these restraints depends primarily on the development of frictional forces arising at the polymer particle interface and become greater as the copper additive concentration increases. Given that PA 11 is a polar polymer and

Table 5 Tensile test data for PA 11 and PA 11/Cu composites. Parameter

Virgin PA 11

2.0% w/w PA/Cu

5.0% w/w PA/Cu

10% w/w PA/Cu

Young's modulus (MPa) Yield stress (MPa) Tensile strain at break (%) Energy at break (J)

943.19(±24.27) 45.74 (±2.73) 254(± 5.30) 30.28(±1.70)

931.06 (±18.90) 51.81 (±1.36) 242.83 (±15.09) 28.70 (±1.43)

1056.71 (±12.94) 52.55(± 2.06) 89.94 (±11.01) 15.98 (±7.21)

1110.98 (±11.35) 54.08 (±0.54) 39.62 (±13.01) 4.46 (±1.82)

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Table 6 Preliminary long term antimicrobial activity data for selected PA 11/Cu composites, tested against 1 × 106 CFU/mL E. coli ATCC 8739 strain, in accordance with the CLSI test method. Sample

Virgin PA 11 5.0% w/w PA 11/Cu 10% w/w PA 11/Cu Fig. 11. Log10 CFU/mL for the untreated PA 11 and PA11/Cu composites after 24 h contact with 1 × 106 CFU/mL E. coli. Results are the mean (±S.D) of the Log colony forming units with a sample size of 6 for test specimens.

is capable of strong, specific interactions with groups on filler surfaces a change in its morphology at the interface may also be a contributing factor. Significant cold drawing was observed for PA 11 and 2% PA 11/Cu composite. The average elongation at break values for PA 11/Cu composites containing 2% w/w was equivalent to that recorded for PA 11. Significantly less cold drawing was observed for the 10% w/w PA 11/Cu composites. The reduction in elongation at break observed is due primarily to the constraining effects on matrix flow caused by the interaction between neighbouring copper particles.

3.6. Antimicrobial efficacy of PA 11/Cu composites Test specimens with copper additive loadings of 2, 5 and 10% w/w were examined for their antimicrobial efficacies, against 1 × 106 CFU/mL E. coli ATCC 8739 strain. Untreated PA 11 was used as the reference sample. Fig. 11 depicts that the reference sample had no antimicrobial effect, after 24 h exposure the number of viable bacteria increased from1 × 106 CFU/mL to 1.6 × 107 CFU/mL. According to the test standard (ISO 22196) in order to consider an antimicrobial system to be effective in eliminating the test bacteria, it must generate log reduction values in the range of ≥2 log values [26]. As presented in Fig. 11 the composite systems tested were found to be active against the test bacteria with the bacterial reductions of 99.9% and N 99.99% at 2 and 5% w/w Plasticopper loadings respectively. A complete reduction in the viable bacteria was observed at 10% w/w loading. The electrostatic attraction between the positively charged copper ions and the negatively charged cell membrane of the microorganism results in the bacterial cell wall disruption. Several studies have suggested that the possible formation of pits on the bacterial cell on interaction with the metal cations, leading to cell wall disruption. Within 24 h the PA 11/ Cu composites showed antimicrobial activity on the test bacteria, and the antimicrobial efficacies were magnified with the increase in the copper additive concentrations. This behaviour is evident from the burst release of copper ions within 48 h for 5% and 10% w/w PA 11/Cu test specimens.

Bacterial population recorded for the samples as a function of immersion time (days) 2

7

14

21

28

35

+++ −−− −−−

+++ −−− −−−

+++ −−− −−−

+++ −−− −−−

+++ −−− −−−

+++ +++ −−−

+ + + Bacterial population in excess of 1 × 105 CFU/mL. − − − Zero bacterial population.

3.7. Preliminary copper ion release and long term antimicrobial efficacy for PA 11/Cu composites The copper ion release rates for the PA 11/Cu composites were determined for the extended durations of 48 h–35 days as mentioned in Section 2.7. Fig. 12 depicts a burst release in the copper ions was observed for 5% and 10% w/w PA 11/Cu composites within 48 h. The polar components of the semi-crystalline polyamide help in the diffusion of the water molecules into the polyamide matrix and resulted in the burst release effect within the 48 h. Kumar et al., compared the ion release properties of PA6 composites with elementary silver and ionic silver in a carrier and concluded that some of the ionic silver additives showed the burst release effect initially and gradually became inefficient [27] Delgado et al., also observed similar burst ion release effect for non-polar PP/CuO nanocomposites [28]. Though the ion release rates were gradually reduced over the time for the PA 11/Cu composites analysed; In contrast to Kumar's observation, composites with 5 and 10% copper loadings continued to show efficient copper ion release rates to act against bacteria for approximately for 4–5 weeks. This ion release behaviour of composites with higher copper loadings will be beneficial for long term medical tubing applications. Preliminary long term antimicrobial studies also confirm this activity for composites with 5% and 10% copper as presented in Table 6. 4. Conclusion In this work antimicrobial efficacy, ion release rates, morphology and mechanical characteristics of PA 11/Cu composites were examined as a function of copper additive loading levels. All the PA 11/Cu composites tested were active against E. coli ATCC 8739 with bacterial reduction rates N99.9% within 24 h. Preliminary ion release studies suggest that the composites with 5 and 10% additive loading levels were prominent in efficient ion release rates for prolonged immersion times. From isothermal crystallisation studies it was observed that copper particles accelerate the PA 11 crystallisation process, which is in accordance with other studies that showed the nucleation effect of inorganic particles on the crystallisation process of aliphatic type homopolymers. The retention of mechanical properties suggests that the incorporation of plasticopper is feasible, without eroding the optimised properties of the virgin polymer.

Copper ion release (µgl-1g-1)

1400 1200

Acknowledgment Virgin PA 11

1000 800

2.0%w/w PA 11/Cu

600

Authors would like to acknowledge the kind donation of materials from Innovative Polymer Compounds Ltd, Ireland.

5.0%w/w PA 11/Cu

400

10%w/w PA 11/Cu

References

200 0 0

10

20

30

40

Immersion time (Days) Fig. 12. Cu ions released from 1 g of selected PA 11/Cu composites as a function of immersion time.

[1] L.E. Fisher, A.L. Hook, W. Ashraf, A. Yousef, D.A. Barrett, D.J. Scurr, R. Bayston, Biomaterial modification of urinary catheters with antimicrobials to give long-term broadspectrum antibiofilm activity, J. Control. Release 202 (2015) 57–64. [2] P. Tenke, B. Kovacs, T.E. Bjerklund Johansen, T. Matsumoto, P.A. Tambyah, K.G. Naber, European and Asian guidelines on management and prevention of catheter-associated urinary tract infections, Int. J. Antimicrob. Agents 31 (Suppl. 1) (2008) S68–S78.

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[3] R.O. Darouiche, Device-associated infections: a macroproblem that starts with microadherence, Clin. Infect. Dis. 33 (9) (2001) 1567–1572. [4] I.R. Cooper, M. Pollini, F. Paladini, The potential of photo-deposited silver coatings on Foley catheters to prevent urinary tract infections, Mater. Sci. Eng., C 69 (2016) 414–420. [5] S. Saint, R.H. Savel, M.A. Matthay, Enhancing the safety of critically ill patients by reducing urinary and central venous catheter-related infections, Am. J. Respir. Crit. Care Med. 165 (11) (2002) 1475–1479. [6] N. Daneshpour, R. Collighan, Y. Perrie, P. Lambert, D. Rathbone, D. Lowry, M. Griffin, Indwelling catheters and medical implants with FXIIIa inhibitors: a novel approach to the treatment of catheter and medical device-related infections, Eur. J. Pharm. Biopharm. 83 (1) (2013) 106–113. [7] Y.G. Kwak, S.O. Lee, H.Y. Kim, Y.K. Kim, E.S. Park, H.Y. Jin, J.M. Kim, Risk factors for device-associated infection related to organisational characteristics of intensive care units: findings from the Korean Nosocomial Infections Surveillance System, J. Hosp. Infect. 75 (3) (2010) 195–199. [8] S. Tamilvanan, N. Venkateshan, A. Ludwig, The potential of lipid- and polymer-based drug delivery carriers for eradicating biofilm consortia on device-related nosocomial infections, J. Control. Release 128 (1) (2008) 2–22. [9] D.R. Monteiro, L.F. Gorup, A.S. Takamiya, A.C. Ruvollo-Filho, E.R. de Camargo, D.B. Barbosa, The growing importance of materials that prevent microbial adhesion: antimicrobial effect of medical devices containing silver, Int. J. Antimicrob. Agents 34 (2) (2009) 103–110. [10] S. Saengmee-Anupharb, T. Srikhirin, B. Thaweboon, S. Thaweboon, T. Amornsakchai, S. Dechkunakorn, A. Kamaguchi, Antimicrobial effects of silver zeolite, silver zirconium phosphate silicate and silver zirconium phosphate against oral microorganisms, Asian Pac. J. Trop. Biomed. 3 (1) (2013) 47–52. [11] R. Tekin, N. Bac, Antimicrobial behavior of ion-exchanged zeolite X containing fragrance, Microporous Mesoporous Mater. 234 (2016) 55–60. [12] T. Kruk, K. Szczepanowicz, J. Stefańska, R.P. Socha, P. Warszyński, Synthesis and antimicrobial activity of monodisperse copper nanoparticles, Colloids Surf. B Biointerfaces 128 (2015) 17–22. [13] M.C. Sportelli, R.A. Picca, N. Cioffi, Recent advances in the synthesis and characterization of nano-antimicrobials, TrAC Trends Anal. Chem. 84 (2016) 131–138.

[14] U. Bogdanović, V. Lazić, V. Vodnik, M. Budimir, Z. Marković, S. Dimitrijević, Copper nanoparticles with high antimicrobial activity, Mater. Lett. 128 (2014) 75–78. [15] M. Vincent, P. Hartemann, M. Engels-Deutsch, Antimicrobial applications of copper, Int. J. Hyg. Environ. Health 219 (7 Pt A) (2016) 585–591. [16] A.M. Nandkumar, M.C. Ranjit, S.S.P. Kumar, P.R. Hari, P. Ramesh, Antimicrobial silver oxide incorporated urinary catheters for infection, Resistance 24 (3) (2010) 156–164. [17] K. Vasilev, J. Cook, H.J. Griesser, Antibacterial surfaces for biomedical devices, Expert Rev. Med. Devices 6 (5) (2009) 553–567. [18] C. Damm, H. Münstedt, A. Rösch, The antimicrobial efficacy of polyamide 6/silvernano- and microcomposites, Mater. Chem. Phys. 108 (1) (2008) 61–66. [19] H. Palza, Antimicrobial polymers with metal nanoparticles, Int. J. Mol. Sci. 16 (1) (2015) 2099–2116. [20] M. Jokar, R. Abdul Rahman, N.A. Ibrahim, L.C. Abdullah, C.P. Tan, Melt production and antimicrobial efficiency of low-density polyethylene (LDPE)-silver nanocomposite film, Food Bioprocess Technol. 5 (2) (2010) 719–728. [21] R. Kumar, H. Münstedt, Polyamide/silver antimicrobials: effect of crystallinity on the silver ion release, Polym. Int. 54 (8) (2005) 1180–1186. [22] T.D. Fornes, D.R. Paul, Crystallization behaviour of nylon 6 nanocomposites, Polymer 44 (2003) 3945–3961. [23] B. Wunderlich, Macromolecular Physics, 2, Academic Press, New York, 1977. [24] X. Zhang, T. Xie, G. Yang, Isothermal crystallization and melting behaviours of nylon 11/nylon 66 alloys by in situ polymerization, Polymer 47 (2006) 2116–2126. [25] B. Wang, Z. Dingand, G. Hu, Melting behaviour and isothermal crystallization kinetics of nylon 11/EVOH/dicumyl peroxide blends, Polym. Eng. Sci. 48 (2008) 2354–2361. [26] M. Altan, H. Yildirim, Mechanical and antibacterial properties of injection molded effects of surface modification, J. Mater. Sci. Technol. 28 (8) (2012) 686–692. [27] C. Radheshkumar, H. Münstedt, Antimicrobial polymers from polypropylene/silver composites—Ag+ release measured by anode stripping voltammetry, React. Funct. Polym. 66 (7) (2006) 780–788. [28] K. Delgado, R. Quijada, R. Palma, H. Palza, Polypropylene with embedded copper metal or copper oxide nanoparticles as a novel plastic antimicrobial agent, Lett. Appl. Microbiol. 53 (1) (2011) 50–54.