Surface hydration for antifouling and bio-adhesion

Abstract

Antifouling properties of materials play crucial roles in many important applications such as biomedical implants, marine antifouling coatings, biosensing, and membranes for separation. Poly(ethylene glycol) (or PEG) containing polymers and zwitterionic polymers have been shown to be excellent antifouling materials. It is believed that their outstanding antifouling activity comes from their strong surface hydration. On the other hand, it is difficult to develop underwater glues, although adhesives with strong adhesion in a dry environment are widely available. This is related to dehydration, which is important for adhesion for many cases while water is the enemy of adhesion. In this research, we applied sum frequency generation (SFG) vibrational spectroscopy to investigate buried interfaces between mussel adhesive plaques and a variety of materials including antifouling polymers and control samples, supplemented by studies on marine animal (mussel) behavior and adhesion measurements. It was found that PEG containing polymers and zwitterionic polymers have very strong surface hydration in an aqueous environment, which is the key for their excellent antifouling performance. Because of the strong surface hydration, mussels do not settle on these surfaces even after binding to the surfaces with rubber bands. For control samples, SFG results indicate that their surface hydration is much weaker, and therefore mussels can generate adhesives to displace water to cause dehydration at the interface. Because of the dehydration, mussels can foul on the surfaces of these control materials. Our experiments also showed that if mussels were forced to deposit adhesives onto the PEG containing polymers and zwitterionic polymers, interfacial dehydration did not occur. However, even with the strong interfacial hydration, strong adhesion between mussel adhesives and antifouling polymer surfaces was detected, showing that under certain circumstances, interfacial water could enhance the interfacial bio-adhesion.

Fig. 1. Mussel adhesive and animal behavior on different surfaces. (a) Mussels using an adhesive for attaching to each other and the side of an aquarium tank. (b) A mussel bonding to a sheet of polytetrafluoroethylene (Teflon), showing the adhesive plaques and threads. (c) Photograph showing mussels two days after binding to aluminum substrates. Note how all animals remain attached. Top down views of mussels on the substrates are provided in Fig. S1 (ESI†). Each mussel and substrate is resting on a piece of plastic pipe to prevent the animal from placing its adhesive on the more stable aquarium bottom. (d). Mussels after two days on a zwitterionic SBMA surface. Most animals have moved off the substrates.

Fig. 2. Mussel adhesive plaque and SFG sample geometry. (a) Picture of a mussel adhesive plaque on a surface. (b) Schematic showing the SFG sample geometry used to collect SFG spectra from the sample/water interface (position A) and the sample/mussel adhesive plaque interface (position B). The input laser beams penetrate the substrates (silica or silica with polymer thin films) to reach the sample/water and the sample/mussel adhesive interfaces. Aluminum was not studied because it is not transparent for the input laser beams.

Fig. 3. SFG spectra of the substrate/water interface (region A in Fig. 2(b), blue curves), substrate/mussel adhesive interface (region B in Fig. 2(b), black curves), and the substrate/mussel adhesive interface after placing the sample in D₂O for a while (red curves). Note that the y-axes differ between panels in order to better visualize differences in spectra. The SFG intensities in different panels are different because of the different water orientation and ordering at various interfaces due to different interfacial interactions. SFG spectra collected from mussel adhesive plaque/substrate interfaces for the different plaques on the same type of substrate are similar and the results are reproducible. (a) Fused silica. General water and amine peak assignments are shown in green. These same assignments apply to all other spectra shown. (b) PMMA. In the water spectrum, prior to adhesive deposition, a substrate methyl peak was observed. (c) PS. The adhesive spectrum also shows a phenyl peak from the substrate. (d) Zwitterionic SBMA. (e) OEGMA.

Fig. 4. Performance of mussel adhesives on different substrates. Average adhesion of mussel plaques on each surface. The substrates are ordered from the highest to lowest surface energies. Error bars shown are 99% confidence intervals.

Fig. 5. Schematics (not drawn to scale) showing proposed mechanisms of antifouling (a) and strong bio-adhesion (b) of mussels on SBMA or OEGMA surfaces. Because of the strong surface hydration, mussels do not want to stay on the SBMA or OEGMA surface, leading to the excellent antifouling performance of SBMA and OEGMA (a). If mussels are forced to stay on SBMA or OEGMA (e.g., tied with a rubber band), the interfacial water can strongly interact with both surface and mussel adhesive proteins, resulting in strong bio-adhesion (b).