Antibacterial Activity of Environmentally Sustainable Polyurethane Based Composites from Castor oil

In this present work, soft and hard antibacterial polyurethane sheets of different composition have been synthesized from castor oil based polyurethane and which was reinforced with natural and synthetic fibres such as jute, sisal, hemp and glass. The activity of polyurethane sheets and selected antibiotics was evaluated against four bacterial pathogens including Staphylococcus aureus, Streptococcus mutans, E.coli, Pseudomonas aeruginosa using modified disc diffusion method. Among the ten samples, hard polyurethanes displayed potential activity against bacterial pathogens E.coli and Streptococcus mutans. Polyurethane sheets showed the highest activity against E.coli which is comparable with zone of inhibition exhibited by streptomycin. Further studies are needed to improve the polyurethane sheets for medical applications.


INTRODUCTION
Increasing world population demands increasing amount of chemicals, materials, polymers for daily usage. Most of the polymers and chemicals are derived from fossil resources but the major problem of using this fossil based material is undesirable environmental problem, frequent oscillation in oil price and limited fossil resources (Wang et al., 2020). Bio-based materials are now gaining considerable attention in the promotion of sustainable chemistry in the manufacturing of materials by its substitution of petroleum-derived raw materials in the production of different products needed in various industries (Corcuera et al., 2010). These bio-based materials are a great substitute to fossil based materials due to renewable feedstock use, easy degradation, easy accessible source and use of low energy for production (van der Meer, 2017). Vegetable oils from both plants and animals (fish oils) are considered to be the most important form of renewable sources. (Meier et al., 2007;Xia, Larock, 2010 (Chen et al.,2013). They cause prosthetic heart valve infections, catheter biofilm infections, hospital-acquired, surgical site and blood stream infections (Paharik et al., 2016;Khan et al., 2008). The main purpose of this study was to develop a antibacterial polyurethane sheets for medical applications.

Materials
Pure castor oil was purchased from a local market in Tholayavattam. 4,4'-methylene diphenyl isocyanate for polyurethane sheet preparation and the catalyst dibutylindilaurate and polyethylene glycol and methyl ethyl ketone were obtained from Madras Scientific Supplies. Synthetic fibres purchased from Siya Chemicals and natural fibres collected from the surroundings and prepared by myself.

Preparation of soft polyurethane sheets
Castor oil based polyurethane was obtained by reacting 4,4'-methylene diphenyl isocyanate with castor oil has dissolved in methyl ethyl ketone at mole ratio 1:4. Here unsaturated fatty acid in castor oil reacted with additional hydroxyl group from methyl ethyl ketone, castor oil could be converted to polyol. Transesterification reaction of castor oil was allowed to carried out in a three-necked round bottomed flask equipped with a reflux condenser at 60 o C for 45 min under nitrogen atmosphere to avoid any oxidation reaction (Ibrahim et al., 2015). Then it was mixed well with few drops of catalyst dibutyltindilaurate which catalyst polymerization reaction. For synthesizing fibre incorporated sheet 0.001m length fibre (sisal, jute, hemp and jute) was mixed separately with polyurethane mixture and poured in a glass mould to cast a neat sheet. The polyurethane sheets were allowed to cure for 2 h in flat surface.

Preparation of hard polyurethane sheets
Hard polyurethane sheet and fibre reinforced hard polyurethane sheets were prepared by the above method but at the time of adding catalyst additionally polyethylene glycol was added.

Antimicrobial assay Preparation innoculum
Antibacterial activity of polyurethane sheet were tested against Gram-positive bacteria

Antibacterial Test
Modified disc diffusion method was used to screen antibacterial activity of polyurethane sheets. Molten cooled Muller hinton agar plates were swabbed with Pathogenic Bacteria culture. Finally, synthesized polyurethane sheets were cut into pieces with the weight of 0.05g under sterile condition. Then the small pieces of sheets were placed on the surface of Mullar-Hinton medium and the plates were kept for incubation at 37°C for 24 hours. At the end of incubation, inhibition zones were examined around the disc and measured with transparent ruler in millimetres (Kohner et al., 1994;Mathabe et al., 2006). here, standard streptomycin disc used as positive control and sterile distilled water used as negative control.

Polyurethane sheet preparation
Soft and hard polyurethane sheets and fibre reinforced sheets were successfully synthesized. During the formation of polyurethane isocyanate was completely converted to amide which is non toxic and improves the polyurethane sheet to become more biocompatible. Addition of fibres to polyurethane composites increase the physical and mechanical property of the polyurethane sheets which makes the sheets more strong and flexible (Sangeetha et al., 2015 andMerlini et al., 2011).

Antibacterial assay
Evaluation of antibacterial activity of polyurethane sheets were determined by modified disc diffusion method against different microorganisms. These organisms are regularly encountered in infectious diseases. All the polyurethane sheets have gd antibacterial activity against Gram-negative bacteria E.coli expect PUSR1G, PUHR1, PUHR1S, all the other samples showed antibacterial activity against Streptococcus mutans. Sample PUHR1 alone showed activity against Staphylococcus aureus. Comparison between soft and hard polyurethane sheets, hard polyurethane sheets showing good antibacterial potential against pathogens. Better antibacterial activity was seen in fibre reinforced polyurethane sheets than non-fibre included sheets. These results were shown in Table 1. Unexpectedly, no decrease in viability in P. aeruginosa was detected for all polyurethane composites (Figure 1a,b,c,d).

CONCLUSION
Fibre reinforced polyurethane sheets were prepared by adding castor oil and 4,4'-methylene diphenyl isocyanate and natural and synthetic fibres. These polyurethane sheets possess good antimicrobial activity, suggesting that such polyurethane sheets could be a good candidate material applied in tissue engineering, wound dressings, biomedical devices and food packing bags, while simultaneously reducing the bacterial colonization. Further studies must be needed to confirm their biocompatibility and cytotoxicity for their uses in biomedical and industrial applications.

ACKNOWLEDGEMENT
We are grateful and thankful to the following organizations to carry out the analysis work. Inbiotics lab for antibacterial studies and SEM Instrumentation Centre, Alagappa University, Karaikudi.