Contents
1. Introduction Page No.
2. Dynamic Surface Antifouling (DSAF) Strategy Page No.
3. Degradable Polymer Page No.
3.1. Degradable Polyurethanes Page No.
3.2 Degradable Polyacrylates Page No.
4. Conclusions Page No.
1. Introduction
Marine biofouling referring to the accumulation of marine microorganisms, animals, and plants has great adverse impacts on marine industries.[1] Biofouling on ship increases its weight and navigation resistance, resulting in additional fuel consumption. It also accelerates the corrosion of metal, thereby shortening the service life of marine facility.[2]Biofouling can block seawater pipelines of nuclear power plants, reducing the cooling efficiency or even slowing down the generators.[3] In aquaculture, biofouling can block the mesh, decreasing the oxygen and nutrients in breeding cages, thereby lowering the production.[4] On the other hand, fouling organisms on vessels can migrate from one sea area to another and affect the ecosystem.[5] Anyhow, marine antifouling is of great economic and environmental importance. However, due to the wide variety of biofouling and complex marine environment, developing efficient and eco-friendly antifouling materials and techniques remains a challenge.
In the past decades, some antifouling approaches including coatings, mechanical treatment, electrolyzing seawater, and ultrasonic cleaning have been developed. Among them, using antifouling coatings is the most convenient, efficient and economic.[6] The so-called self-polishing copolymer (SPC) coating containing tributyltin (TBT) can efficiently prevent the settlement and growth of biofouling. However, due to the toxicity to non-target creatures, it was banned by the International Marine Organization in 2008.[7]Since then, development of eco-friendly and effective antifouling coatings has been an urgent task.
In recent years, cuprous oxide SPC coatings, fouling release coatings,[8] protein-resistant polymers,[9] amphiphilic polymers,[10] and biomimetic polymers[11] have been prepared. While they have some effectiveness in certain scenarios, they exhibit poor antifouling performance on static conditions with short service life. Moreover, they are unable to degrade and form marine microplastics once they are released into the ocean.[12]
Based on a ten-year experience in marine antifouling, we put forward the strategy of Dynamic Surface Antifouling (DSAF), a changeable surface can renew itself in seawater and thus weaken the adhesion of fouling organism.[13] Alone this line, we developed high performance antifouling materials based on biodegradable polymers. Due to the degradation of the main polymer chain, the surface renewing is not driven by external force, so they still have good antifouling ability even under static conditions. They are also eco-friendly since such degradation yields low molecular weight molecules instead of microplastics. In this paper, we present the synthesis, structures and properties of DSAF polymers.
2. Dynamic Surface Antifouling (DSAF) Strategy
Biofouling formation on a submerged surface is a multi-step process involving the initial attachment of fouling organisms, secretion of biological adhesives, and subsequent growth. DSAF polymer can renew its surface and significantly reduce the contact time between microorganisms and coating surface, which is not conducive to the adhesion of biofouling.
We investigated the movement trajectory of bacteria by using digital holographic microscopy (DHM) to understand DSAF mechanism.[14,15] DSAF surface was constructed by biodegradable copolymer (poly(CL-co-HL) of ε-caprolactone (CL) and δ-hexalactone (HL). The degradation rate that characterizes the change of the coating surface was determined by quartz crystal microbalance with dissipation (QCM-D). Clearly, the degradation rate increased with HL content in the range we investigated.
The adhesion of Pseudomonas sp. (P. sp. ) orEscherichia coli (E coli. ) on the coating surface was monitored by recording and reconstructing bacterial behavior with DHM. Subdiffusive motion of the bacteria near the surface was observed before irreversible adhesion.[16] The mean square displacement (MSD) of bacteria indicated that a higher degradation rate diminished the bacterial coverage (10%-20%) and the probability of subdiffusive motion on coating surface (Figure 1a). Accordingly, a dynamic surface with a certain change rate can significantly hinder the adhesion of bacteria.
Atomic force microscopy (AFM) observation showed the adhesion force (γ ) of bacteria on dynamic surface. We can see that γsignificantly decreased as the surface self-renewal rate increased (Figure 1b). Clearly, the self-renewal surface based on degradable polymer can reduce adhesion strength of bacteria, thereby preventing bacteria from irreversible adhesion so that they had to escape from the surface (Figure 1c). DSAF surface can keep dynamic or changeable in seawater with the ongoing hydrolytic and enzymatic degradation.