Applying natural rubber latex in a film falling reactor, Schlögl et al.47 described a crosslinking reaction of the double bonds via a thiol-ene reaction with the aim to generate an allergen free surgical glove.
47a
Schlögl, S.; Aust, N.; Schaller, R.; Holzner, A.; Kern W. Monatsh Chem 2010, 141, 1365-1372. b Schlögl, S.; Temel, A.; Holzner, A.; Kern, W. J Appl Polym Sci 2012, 124, 3478-3486.
Background
A
NTIBACTERIALP
OLYMERS1.2
1.2.1 INTRODUCTION
Since the French scientist Pasteur demonstrated that certain bacteria strains are crucial to fermentation and moreover the awareness that microbes are responsible for a high number of diseases took hold, microbiological hygiene started to be of interest. With the discovery of penicillin by Fleming, the successful fight against bacterial infections was possible.
Nevertheless, antimicrobial contaminations are still an issue today, not only in hospital environment but also for water purification, food processing as well as sanitary and household equipment.
Conventional disinfectants are small molecules such as halogens, alcohols, phenols, acids or antibiotics.48 However, those active agents are generally not covalently immobilized;
therefore they suffer from disadvantageous leaching and accumulation effects with probable worst impact on the environment.49 Additionally, bacteria cells are able to form resistances against low-molecular weight disinfectants.50 Furthermore, because of the constant release of biocides, the antimicrobial activity of these materials will expire within time.51 In that way, the alternative use of water insoluble macromolecular biocides is an attractive option.
Besides, the increase in molecular weight and the accumulation of charges have a positive impact on the performance of the material.52 Bacterial resistances are not known.
48a
Paulus, W. Microbicides for the protection of materials, Chapman & Hall: London- Glasgow- New York- Melbourne- Madras, 1993. b Block, S.S. Desinfection, Sterilization and Preservation, Lea &
Felbiger, Philadelphia, 1983.
49 Kenawy, E.-R.; Mahmoud, Y. A.-G. Macromol Biosci 2003, 3, 107–116.
50 Tegos, G.; Stermitz, F. R.; Lomovskaya, O.; Lewis, K. Antimicrob Agents Chemother 2002, 46, 3133-3141.
51 Cheng, G.; Xue, H.; Zhang, Z.; Chen, S.; Jiang, S. Angew Chem 2008, 120, 8963–8966.
52 Kenawy, E.-R.; Worley, S. D.; Broughton, R. Biomacromolecules 2007, 8, 1359-1384.
Background 1.2.2 THE BACTERIA CELL
Bacteria can be classified as prokaryotes. Contrary to eukaryotes, their cell nucleus is not separated from the rest of the cell by a cell wall, but is located in the cytoplasm in an irregular shape as a single circular chromosome, called nucleoid.
Further intracellular constituents are the cytoplasm and the ribosome, surrounded by a cell membrane, which serves as a barrier to hold nutrients. This lipid bilayer consists of two layers of phospholipids, featuring a hydrophilic head and two hydrophobic tails. The hydrophilic head groups consist of negatively charged phosphate groups, whereas the hydrophobic tail is composed of fatty acid hydrocarbon chains. This structure allows arrangement in water into a two-layered sheet (bilayer) with all the hydrophobic tails being orientated to the center.
Figure 1-1 Structure and content of a typical Gram-positive bacteria cell53
The cytoplasmic membrane is surrounded by a bacteria cell wall, consisting of peptidoglycan, which is made from polysaccharide chains cross-linked by peptides. Generalized, there are two types of cell walls in bacteria, which allow a classification in negative and
53 source: http://en.wikipedia.org/wiki/Bacteria, 04th December 2012
Background positive cells. Gram-positive bacteria (depicted in Figure 1-1) possess a thick cell wall (up to 50% of dry mass) containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall (approximately 10% of dry mass) consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins as shown in Figure 1-2. The naming results from a different behavior against staining with crystal violet and a subsequent decolorization of Gram-positive and negative cells which allows a very fast assignment. The differences in the composition of the cell wall lead to drastic differences in their stability against antimicrobial agents. Typical representatives for Gram-positive cells are Staphylococcus or Listeria. The proteobacteria are a major group of Gram-negative bacteria, including, amongst others, Escherichia coli, Salmonella or Pseudomonas. Moreover, the groups of cyanobacteria or green sulfur and non-sulfur bacteria belong to Gram-negative bacteria.54
Figure 1-2 Gram-positive and -negative cell wall structure55
These structural features determine the main strategy for designing antibacterial polymers.
The teichoic acid molecules of Gram-positive bacteria cell wall, the liposaccharides and phospholipids of Gram-negative outer membrane and the cytoplasm membrane itself, composed of a phospholipid bilayer with embedded essential functional proteins, provide a net negative charge of the bacteria cell, stabilized by the presence of cations such as Mg2+ or Ca2+. The cytoplasmic membrane regulates the transfer of metabolites and nutrients in and
54 source: http://en.wikipedia.org/wiki/Gram-negative_bacteria, 04th December 2012
55 source: http://en.wikipedia.org/wiki/Gram-positive_bacteria; 04th December 2012
Background out the cell. Targeting this sensible system, most antibacterial polymers are designed as hydrophilic-hydrophobic macromolecules.66
1.2.3 ANTIBACTERIAL POLYMERS
What makes polymers preferred candidates for usage in hygienic applications is their higher activity than exhibited by their molecular counterparts.56 Additionally, they are generally non-volatile, do not penetrate skins and are therefore less vulnerable towards losses due to decomposition or transport.
Worley and Sun57 demand several characteristics, that should be fulfilled by antibacterial polymers: first, an easy and inexpensive synthesis, stability for long-term applications, non-volatile and insolubility in water in case of application in aqueous media, no decomposition and release of toxic residues, non-toxicity towards those handling it and of course, high potency against a range of microbes in brief contact times.
In order to design antimicrobial polymers, several general approaches are possible. One method to obtain antibacterial activity is to add an organic or inorganic biocide to the polymers during or after processing.57 Alternatively, the preparation of monomers equipped with antibacterial groups and the subsequent homo- or copolymerization of those is an often chosen approach.58 Especially Ikeda and co-workers performed a lot of pioneering work on this topic in the early 80s.
56 Ikeda, T.; Tazuke, S. Makromol Chem 1984, 185, 869-876.
57 Worley, S. D.; Sun, G. Trends Polym Sci 1996, 4, 364-370.
58 a
Ikeda, T.; Tazuke, S. Makromol Chem, Rapid Comm 1993, 4, 459-461. b Kreutzwiesner, E.;
Noormofidi, N.; Wiesbrock, F.; Kern, W.; Rametsteiner, K.; Stelzer F., Slugovc, C. J Poly Sci, Part A:
Polym Chem 2010, 8, 4504-4515.
Background 1.2.3.1 MODE OF ACTION
The major part of antimicrobial polymers is designed as so-called membrane-active agents.
Thereby, hydrophobic and hydrophilic structural elements make up a macromolecular system, targeting the cytoplasmic membrane.
The cationic polyelectrolyte salt poly(hexamethylene biguanide) chloride PHMB was the first polycation, whose mechanism of interaction with Gram-negative bacteria cell E. Coli and model phospholipid membranes was studied by Broxton et al.59 as well as Ikeda et al.60 Generally, six elemental steps have been defined by Ikeda et al.60 describing the elementary events leading to the lethal action: (i) adsorption onto the bacterial cell surface, (ii) diffusion through the cell wall, (iii) adsorption onto the cytoplasmic membrane, (iv) disruption of the cytoplasmic membrane, (v) leakage of the cytoplasmic constituents and (vi) death of the cell.
Due to the high charge density along the polymer chains, step (i) is especially effective in case of macromolecular biocides. Figure 1-3 illustrates this mode schematically.