# Innovative Antibacterial Polymer Coatings
## 2 Antibacterial Coatings
### Coatings That Kill Bacteria upon Contact
#### 2.2.2 Contact-Active Surfaces Containing Synthetic Polymers  
Typical contact-active antibacterial coatings contain quaternary ammonium compounds (QACs), which form electrostatic bonds with the negatively charged cell membrane, thereby disrupting the integrity and function of the entire cell. At lethal concentrations, they displace cytoplasmic components and dissociate phospholipids. The direct incorporation of QACs into coatings can lead to high release rates, rapid depletion of the biocide, and high ecotoxicity. A more accepted method is to graft the active compound onto a polymer, thus binding QAC to various surfaces. For instance, the biocide 3-(trimethoxysil)-propyldimethyloctadecyl ammonium chloride (organosilicon quaternary ammonium compound (Si-QAC)) is attached via the silane group of Si-QAC, which condenses with free hydroxyl groups on the surface and is stabilized by intermolecular siloxane bonds (Si-O-Si). The antibacterial mechanism relies on the quaternary amine (N+ atom), which attracts negatively charged bacteria to the needle-like C18 structure of the hydrophobic chain, leading to the piercing of the bacterial cell envelope and causing cell death [48].  
In another development aimed at producing antibacterial surfaces, nanogels based on (P(NIPAM-co-DMAPMA) were used [52]. Through the tertiary amine in DMAPMA, the nanogels were quaternized, introducing antibacterial activity due to binding with the bacterial membrane and the intercalating capability of the aliphatic quantized ammo  
Figure 3: LbL process for imparting long-lasting antibacterial properties to stainless steel. Chemical structures of **(1)** P(mDOPA), **(2)** Fox(mDOPA), and **(3)** P(mDOPA)-co-P(DMAEMA\({}^{+}\)). Adapted from ref. [51] with permission from Royal Society of Chemistry, Copyright 2011.  
nium compound (QAC). Subsequently, the quaternized nanogels enable the formation of intraparticle hydrophobic domains that allow the incorporation of triclosan, a commonly used antimicrobial agent (Figure 4). Coating with nanogels loaded with triclosan provides excellent bacteria-killing properties, with an efficiency of up to 99.99% against bacteria adhered to the surface, while also possessing anti-fouling activity, as described in the next section.  
Other effective contact-active surface materials are polyaniline (PANI) and polypyrrole (PPY). Their surfaces are conductive and attract negatively charged bacteria through electrostatic force. However, changes in the chemical structure of PANI with pH (the emeraldine salt form being more active than the emeraldine base form) may present significant obstacles for sustained antimicrobial activity [48]. Recently, these conductive polymers showed superior antibacterial properties by forming polymer composites with metal/metal oxide nanoparticles, which are well-known antimicrobial agents that react with the negatively charged bacterial membranes due to their positive zeta potentials and nanosized dimensions [53]. Silver, copper oxide, and titanium oxide are among the most studied antibacterial agents used to form such conductive polymer composites and could be developed for therapeutic applications and coatings of medical devices [54]. Polymeric conductive composites containing carbon-based nanomaterials also offer superior antimicrobial activities, and could be applied in the food packaging industry. PANI is one of them, which forms carbon-based composites by using graphitic carbon nitride (g-C3N4) and multiwalled carbon nanotubes (MWCNT), which demonstrated very good bactericidal properties against various numbers of Gram-positive and Gram-negative bacteria. Moreover, the incorporation of silver nanoparticles (AgNPs) into PANI/MWCNT nanocomposites significantly enhances their antibacterial activity against _E. coli_ and _S. aureus_[55; 56].