Characterisation of Antimicrobial and Leaching Properties of TPU for Portable Hydration Reservoirs

Characterisation of antimicrobial and leaching properties of TPU for portable hydration reservoirs

Abstract

This paper aims to characterise silver ion antimicrobial coating and to determine the effects of water exposure on TPU. The silver particles were characterised using scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR). The use of SEM was inadequate for the observation of the silver ions in the material. FTIR indicated that the silver ions interacted directly with the polyurethane as several chemical groups were modified.  The presence of inorganic and organic compounds was confirmed using gas chromatography and liquid chromatography linked to a mass spectrometer through the detection of three compounds known to give taste to water: Sodium, Zinc and Copper. The relative quantities of each compound were insufficient to provide any modification to water flavour. On the other hand, the presence of dibutyl phthalate and diaminopimelic acid, therefore bacteria, provides an explanation to the bitter and musty taste of the water.

1. INTRODUCTION

Over the last few decades, athletes have been pushing the boundaries of the impossible. To achieve these performances, key aspects of a physical effort need to be respected; keeping the body hydrated is the most important of these aspects and can become complicated when running long distances and/or in rough terrains. The need for a compact and efficient method of hydration is therefore required. In addition, the environment and product the material used is subject to renders the hydration system vulnerable to microbial colonization. Companies, such as CamelBak, have been continuously innovating in terms of technology to accommodate athletes in the best possible way. Throughout the years of research, CamelBak has proposed various methods of hydration including the CamelBak Antidote reservoir. The alliance of two technologies has allowed CamelBak to provide an ideal hydration method through the combination of a thermoplastic polyurethane and an efficient silver ion antimicrobial coating (1){Gary, 8 October 2008}.

Thermoplastic polyurethane (TPU) is an important and widely-used polymer due to the numerous possible physical property combinations. To start, polyurethane is a polymer which is composed of a repeating urethane group and a combination of other moieties (e.g. urea, ester or ether) (2). The linkage between urethanes is the result of the reaction between an isocyanate and an alcohol during which the hydrogen atom of the hydroxyl group is transferred to the nitrogen atom of the isocyanate (3, 4). Thermoplastic Polyurethane is a linear segmented block copolymer composed of a hard and a soft segment. Depending on the type of segments used, TPU obtains different properties. The hard segment can either be aromatic or aliphatic. In general, aromatic hard segments are used but when colour and clarity retention in sunlight exposure are required, aliphatic hard segments are used. The soft segment is changed depending on the application and can either be a polyether or a polyester type. Polyether-based TPUs are used when the material will be exposed to wet environments. On the other hand, polyester-based TPUs are used when oil and hydrocarbon resistance is required. In addition to the multiple properties possible, TPU offers flexibility without the use of plasticizers allowing the removal of phthalates, a toxic chemical previously used in hydration equipment (5). The CamelBak reservoir uses a polyether-based TPU as it resists well in wet environments.

Silver has been used as an anti-microbe for thousands of years, especially in medieval France where hospital wards were plated with silver to prevent infection from harmful bacteria. The biocidal effect of silver has a broad spectrum of activity including bacterial, fungal and viral agents and is termed “oligodynamic activity” (6). The antimicrobial activity results from the inactivation of sulfhydryl groups in the cell wall through the formation of insoluble compounds with silver ions. The sulfhydryl groups are essential components of enzymes as they allow the transmembranous energy metabolism and electrolyte transport. Without these mechanisms, the microorganisms dry out and shrink due to the loss of fluid and electrolytes (7). After blocking the respiratory chain of bacteria, the silver ions bind to bacterial DNA and bacterial spores (8). Antimicrobial activity of silver ions also depends on the concentration of the latter over time. Low concentrations of silver ions are bacteriostatic, preventing the proliferation of microbes, which then resumes after a period of a few hours. On the other hand, high concentrations are bactericidal, killing the microbes completely (6). The former concentration is used in the CamelBak Hydration Reservoir.

This research paper will investigate various properties of the hydration pack as well as the impact of short and long term exposure of the material to water. The overall integrity of the hydration pack and the efficiency of the antimicrobial coating will be tested by submerging samples of TPU in deionized water and exposing them to UV light and darkness over various periods of time. In addition, the effect of long term use of the hydration pack will be investigated by exposing it to water at room temperature over a period of 4 months and testing the organic compound concentration in the water.

2. EXPERIMENTAL PROCEDURE

A.   Technology

SEM

Scanning Electron Microscopy (SEM) is a technique that produces a high-quality image of a sample by scanning its surface with a focused beam of electrons. A sample is placed in a chamber situated below an electron gun. The electron gun emits electrons via the heating of a tungsten filament cathode. The electrons are focused into a small beam as they pass through a series of electromagnetic fields and lenses. The beam follows a vertical path through the microscope, which is held within a vacuum. Scanning coils are positioned near the end of the column which deflect the beam in the x and y axes, allowing the beam to scan in a raster fashion over a rectangular are of the sample surface. Once the beam hits the sample, electrons and X-rays are ejected from the sample. These X-rays, backscattered electrons, and secondary electrons are collected by detectors, which then convert them into a signal. The signal is converted into an image using a monitor.

Images of each sample were obtained to assess the effect of the water on the material and antimicrobial agent.

FTIR

Fourier Transform InfraRed is a technique which uses the interaction between infrared light and chemical bonds composing materials to determine the various chemical groups within the material (9). The spectrometer scans the sample producing a plot of measured infrared radiation intensity versus wavenumber (10). The spectrometer can scan either in transmittance or absorbance. For this paper, the spectrometer is set to absorbance.

The spectrometer is used in combination with a software called “Spectrum V6” which enables the visualization the infrared spectra obtained from each sample. Before analysing the samples, the crystal is cleaned with ethanol to remove any unwanted substances that could cause error in the analysis. A reference test without a sample is conducted, providing the software with a background information of the sample holder. A sample is then placed on the crystal and held down by lowering the pressure arm and screwing it to the wanted pressure. The scan is then initiated, producing an IR spectra of the sample tested which is then saved onto the desktop. The procedure is repeated for all the samples. When all tests are completed, an .asc file of each spectra is collected and converted to excel. It is then possible to analyse the IR spectra and allocate chemical bonds to the peaks observed on the graphs.

ICPMS

Inductively Coupled Plasma Mass Spectrometry is a type of mass spectrometry used to detect metals and several non-metal compounds. This technique consists in ionizing the sample with inductively coupled plasma and using a mass spectrometer to separate and quantify the compounds (11, 12).

The samples were sent to TATA Steel to proceed with the identification of the inorganic compounds.

GCMS

Gas Chromatography (GC) and Mass Spectrometry (MS) are two instrumental methods of analysis which can be combined to identify the organic composition of a sample.

Gas chromatography enables us to separate a mixture of compounds by passing it through a tubbed glass coil packed with a powdered solid metal fitted into an oven. The sample is first dissolved in a solvent and then injected into one end of the column. The sample is carried through the column by the means of an unreactive gas (usually nitrogen). As the different compounds of the sample travel through the column, they become separated and travel at different speeds. At the end of the column, the substances leave separately and are detected by a detector determining the retention time, the time they take to travel through the column. A graph is produced where the peaks represent each substance according to the retention time (13).

A mass spectrometer is used to identify substances very quickly and accurately, even in small amounts. GCMS is therefore obtained by linking a gas chromatography machine to a mass spectrometer. The substances leaving the column can then be detected and identified (14, 15).

In this study, the GCMS was run using Electron Ionisation mode.

LCMS

Liquid Chromatography (LC) Mass Spectroscopy (MS) is a technique that combines the separation capabilities of liquid chromatography and the analytical capabilities of a mass spectrometer to determine the organic composition of a sample.

Liquid chromatography consists in inserting the samples of interest into a column of stainless steel packed with fine, chemically modified silica particles (16). LCMS has the ability to identify a broader range of compounds with minimal sample preparation (17).

In this study, the LCMS was run using Electrospray ionization. The LCMS and GCMS were both done by the National Mass Spectrometry Facility at Swansea University. GC shows less polar and more volatile material, while LC is good for more polar and less volatile samples.

B.   Samples

Organic/Inorganic compound content

A 1.5L CamelBak Antidote Hydration Reservoir is cut into ten samples of 2 cm wide by 2 cm in length providing an active area of 4

cm2. The samples are then exposed, in a closed vial, to 30ml of deionized water for various amounts time (24 hours to 6 weeks), as shown in Table 1. One sample is maintained in a dark environment for 6 weeks to demonstrate the effect of photo degradation on the polyurethane. The samples are then taken out of the vial using a pair of sterile tweezers to protect the water taken from the vial from any contamination. The organic compound content in the water is then determined using liquid chromatography mass spectrometry (LCMS) and the inorganic compound content is determined using inductively coupled plasma mass spectrometry (ICPMS).

Table 1. Samples used in the organic and inorganic compound content test

Antimicrobial coating

Once the samples cut out of the Hydration Reservoir are removed from the deionized water, they are stored in separate plastic bags. Using FTIR, the chemical groups in each sample are determined as well as their quantity. The spectrometer produces a spectra for each sample, which are then compared to determine the variations in chemical group composition.

The samples are also examined using a Zeiss EVO LS25 SEM with the SmartSEM software. The SEM is set to variable pressure secondary electron mode rather than the high vacuum mode. This allows the machine to use the gas present in the chamber to charge the surface of the material rather than send the electrons through it, therefore preventing damage to the material.

CamelBak long term test

A CamelBak antidote reservoir was filled with 900 ml of deionized water and left to sit for 120 days (17/11/2016 – 15/03/2017). The reservoir was exposed to temperatures between 18 °C and 23°C and shaken every few weeks to mimic the movement produced by walking or running. Two samples of 30 milliliters of water are then removed and placed into separate vials. The organic and inorganic compound content is determined respectfully using LCMS and ICPMS.

3. RESULTS AND DISCUSSION

1. Antimicrobial coating characterisation

The scanning electron microscopy (SEM) micrographs of the CamelBak reservoir are shown in Figure 1. The microstructure in the 100x magnification micrograph (Figure 1a) shows a rough and wavy surface. The rough surface might be used to help reinforce the material and increase abrasion resistance.  By increasing the magnification to 20000x (Figure 1b), the wavy surface disappears and gives place to a flat surface. At this magnification, the microstructure of the silver ions in the polyurethane begins to be seen. The resolution of the SEM was insufficient to properly observe the silver ions due to the small size of the latter (8nm to 15nm). Other methods of observation would have to be used to observe the silver ions present in the TPU. The field emission scanning electron microscopy (FESEM) micrograph of silver ion in polyurethane are shown in Figure 1c where the silver ions clearly stand out in the polyurethane.

Figure 2 and 3 respectively show the fourier transform infra-red (FTIR) spectra of the CamelBak Reservoir and of pure polyurethane between 400 and 4000

cm-1. Carbonyl groups are responsible for the absorption peaks seen at 1703 and 1731

cm-1. The formation of TPU is responsible for the distinction between the two peaks as the former peak is due to the hydrogen bonding between the carbonyl group and the –NH groups of neighbouring hard segments whereas the latter is due to non-hydrogen bonding of the carbonyl with a soft segment. The addition of silver ions to the polyurethane has modified the structure of the material. This can clearly be observed as the peaks at 1310 and 1537

cm-1, respectively due to NH- and C-H stretches, are enhanced in the loaded TPU. Generally, a broadening of the peak at 3326

cm-1can be observed, as shown in the research done by Paul Deepen (18).

2. Analysis of inorganic compound content

To assess the leaching of the polyurethane into the water, the inorganic and organic compound contents of the latter were tested. By using ICPMS, many compounds were found in the samples. The main compounds detected were Potassium, Sodium, Selenium and Zinc. Several other elements were found in the samples, although in too small quantities to affect the water. Overall, the inorganic compound contents of samples do not seem to behave in a proportional manner as the samples are exposed for longer periods to the deionized water. Potassium (K) levels vary from 13.75 to 218.77 ppb. The Sodium (Na) levels vary from 30.91 to 111.56 ppb. Sodium is an element known to give a salty taste to water. Drinking water usually contains about 50 mg/L of sodium whereas the maximum sodium level in the tested water equates to 0.1 mg/L. Selenium (Se) levels vary between 0 and 59.74 ppb. Although Selenium salts are toxic in large amounts, they do not give any taste to water and the quantities observed in the water would not harm the human body. In trace amounts, Selenium salts are beneficial for cellular function and are used in many multivitamins and dietary supplements. The Zinc (Zn) levels vary from 2.33 to 16.05 ppb. There is also a slight presence of copper in the water. These two compounds are known to give a metallic taste to water above about. The water begins to have a metallic taste when the copper and zinc levels go above 2-5 mg/l and 4-9 mg/l respectively. As the zinc levels do not go above 0.016 mg/L, the zinc would not have a metallic taste.

Although these compounds are known to influence the taste of water, the content levels of each of them are too small to produce any variation in water taste. The quantity of each element being found in parts per billion also are almost below limits of quantification, therefore rendering the results from the chemical analysis limited by machine accuracy.

A significant difference in composition can be noted between the two-week sample and the remaining samples. Two sets of samples were analysed using ICPMS. The first set was composed of a blank sample and a two-week sample. The second set was composed of 7 samples ranging from one day to six weeks. This set of samples seems to have been contaminated during the ICPMS testing. Although the ICPMS equipment is kept in a contamination free environment at TATA Steel, the presence of metallic particulates in the air would explain the contamination observed. The comparison of the blank samples reinforces this point as they do not contain the same elements.

Table 2. Comparison of inorganic compound content using ICPMS

Figure 4. Variation of inorganic compound content (*parts per billion)

3. Analysis of organic compound content

The organic compound content of the samples was determined using GCMS and LCMS. Gas chromatography showed two quite weak components at retention time 2.69 and 5.66 minutes as seen in Figure 5. The peak at 2.69 minutes is due to the presence of dibutyl phthalate, a plasticiser used in many polymers. This phthalate ester is known to leach from finished products into the food or water it is contact with and therefore into the human body (19). Dibutyl phthalate is a compound that has a strong and bitter taste, which, although present in small quantities, could explain the water taste after long term exposure. The peak at 5.66 minutes is due to the presence of bacteria in the water. By looking at the mass spectrum for this peak and comparing it to a database of compounds, the presence of diaminopimelic acid is noted. DAP is an amino acid characteristic of certain cell walls of bacteria such as Escherichia coli. The silver ion antimicrobial coating helps reduce the proliferation of the bacteria. The presence of this latter can lead to a musty odour of the water.

Figure 6 shows the liquid chromatography (LC) spectra of the long-term exposure sample. The background for the LC spectra has been plotted from the blank, showing a peak at 413 (Figure 7). This peak corresponds to a phthalate with composition C24 H38 O4 Na1. This is the sodiated version of C24 H38 O4 or Bis(2-ethylhexyl) phthalate, a very common background ion. This compound is present due to the exposure of the water to the material in which the samples were kept. The water sample tested was found to contain several compounds. The peak at retention time 16.226 minutes corresponds to dibutyl phthalate, reinforcing the data obtained with gas chromatography.

Figure 5. Comparison of Blank sample and Long term sample using GCMS

Figure 6. Comparison of Blank sample and Long term sample using LCMS

Figure 7. Identification of Bis(2-ethylhexyl) phthalate + Na using GCM

4. CONCLUSIONS

Although ICPMS detected several compounds responsible for various water flavours, the quantities present in the water are insufficient to alter water taste or affect the human body in any way.

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