Marine biofouling is being taken into account recently with great importance. In brief, biofouling is defined as the undesirable accumulation of microorganisms, plants, algae, and/or animals on wetted structures.
Biofouling is divided into microfouling, which is the formation of biofilms and bacterial adhesion, and macrofouling, which is the attachment of larger organisms, of which the main culprits are barnacles, mussels, polychaete worms, bryozoans, and seaweed. Together, these organisms form a fouling community. Individually, small, accumulated biofoulers can form enormous masses that severely diminish the maneuverability and carrying capacity of ships, which could result in the reduction of vessel performance and consequently increase fuel requirements. Fouling leads to huge material and economic costs in the maintenance of mariculture, shipping industries, naval vessels, and seawater pipelines. Biofouling is also observed in almost all circumstances where water-based liquids are in contact with other materials. Industrially important examples include membrane systems, where membrane bioreactors and reverse osmosis spiral wound membranes are used to cool water cycles of large industrial equipment and power stations. The concept of antifouling has been developed to address the issue of biofouling. Antifouling is the process of removing or preventing the accumulation of microorganisms. In industrial processes, bio-dispersants can be used to control biofouling. In the past forty to fifty years, scientists have been working in search of an effective surface to be used as an antifoulant against barnacles. In this regard, most of the researchers worked with tributyltin moiety (TBT) and polydimethylsiloxane (PDMS) to discover effective anti-fouling agents. Although TBT was found to be an excellent antifouling agent, it is banned by the International Maritime Organization from 2008 because of its high endocrine-disruption effect.
These days, scientists are concentrating on the search for suitable materials as antifoulants, which will not only have an excellent capacity to inhibit biofouling, but will also be environmentally benign. Very few scientists have worked with hydrogels to establish them as effective and environmentally friendly materials to be used as antifouling agents. The main reason for choosing hydrogels as antifouling agents is its softness, which inhibits barnacles from settling on them because barnacles are usually found to settle on hard surfaces like wood, plastic, bamboo, glass, blue mussel shells, oyster shells, etc. Moreover, the hydrophilic nature of the hydrogels may be the key factor to render them excellent candidates against biofouling because the glycoproteinous substance in barnacle cement has the characteristic of hydrophobicity. Therefore, barnacles dislike settling on hydrogel surfaces. So far, in vitro as well as in vivo tests used to investigate the anti-fouling capacity of hydrogels against barnacles have been effective. Not only the settlement but also the growth of barnacles on hydrogels will be examined in the near future.
A barnacle is a type of arthropod belonging to the infraclass Cirripedia in the subphylum Crustacea; hence, it is related to crabs and lobsters. Barnacles are exclusively marine and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders and have two nektonic larval stages. Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, "acorn barnacles" (Sessilia) are sessile, with their shells growing directly onto the substrate. Members of the order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.
Although they have been found at water depths of up to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of all species living in water depths of less than 100 m (300 ft) and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacles live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.
Barnacles have two distinct larval stages. In the first stage, a fertilized egg hatches into a nauplius, a one-eyed larva having a head and a telson, without a thorax or abdomen. It undergoes six moults before transforming into the bivalved cyprid stage. Nauplii are typically initially brooded by the parent, and are released as free-swimming larvae after the first moult. The second stage (the cyprid stage) lasts from days to weeks . During this part of the life cycle, the barnacle searches for a place to settle. It explores potential surfaces with modified antennule structures. Once it has found a potentially suitable spot, it attaches headfirst with its antennules by using a secreted glycoproteinous substance. As the larva exhausts its finite energy reserves, it becomes less selective in the settlement sites. If the spot is to its liking, the larva cements down permanently with another proteinaceous compound. As the larva accomplishes this process, it undergoes metamorphosis into a juvenile barnacle before developing into a mature adult.
Barnacles are biofoulants, like many other organisms. A member of the phylum Arthropoda, barnacles have been considered obnoxious creatures in the marine environment. They usually adhere on wood, plastic, bamboo, glass, blue mussel shells, oyster shells, i.e., any type of hard surfaces. Barnacles damage water-submerged materials such as ship hulls, fishing nets, and cooling water intake channels of power plants by “wall-to-wall” adhesion on these surfaces, causing serious economic and environmental problems. In the past forty to fifty years, scientists have been working in search of an effective surface to be used as an antifoulant.
In the laboratory, a number of researchers have carried out experiments to examine the settlement of different species of marine organisms on hard surfaces, such as glass, PS, PDMS, PE . James F. Schumacher et al. (2007)  studied the settlement behavior of the barnacle Balanus amphitrite on PDMS with different engineered topographies. Kenji Mori et al. (2006)  investigated the anti-fouling activity of linear polymer/silica hybrid materials, such as PVAc/silica and PMMA/silica against blue mussels. The former composite showed a comparatively higher repellent activity towards blue mussels. However, none of the materials have been observed to be very durable in marine environments for long term usage. Previously, no studies have been carried out using hydrogels against barnacles. Hydrogels have been chosen as an antifoulant because of their softness, non-toxicity, and long-term durability.
In laboratory testing (in vitro), Murosaki et al. (2009)  selected a number of hydrogels to investigate their antifouling characteristics against the barnacle Balanus amphitrite. In this test, hydrogels were categorized into two groups in terms of their antifouling behavior against barnacles. One group (Group-1: PHEMA, PHEA, PNaMPS, PAMPS, PNaSS, PDMAEA-Q, PAAc/PAAm DN , agarose, and PVA gels) showed strong antifouling performance irrespective of the elasticity (E) or swelling degree (q). On the other hand, the second group (Group-2: PAAm, PDMAAmm, PDMAPAA-Q, and PAMPS/PAAm DN gels) showed relatively weaker antifouling performance that was E- or q-dependent. In the latter case, a relatively high cyprid settlement was observed with increase in the elastic modulus of the gel. The number of barnacles that settled on the gels of Group-2 decreased with an increase in the swelling degree of the gels.
The adhesion mechanism of barnacles, named “Easy Release” on hydrogels, has already been established. In this case, the “Easy Release” of initially attached cyprids is the main determinant of the conversion from random cyprid contact and surface exploration to settlement and metamorphosis. It should be noted that the “settlement” defined in this work required sufficiently strong and persistent attachment of the surface-contracting cyprid antennule tips (with cement) to trigger metamorphosis of the organism to the acorn barnacle form. When the adhesion strength is less than the contractile force of the retracting cyprid antennules, cyprids do not “settle” but wander aimlessly and eventually die without undergoing metamorphosis.
Hydrogels were also treated in marine environments  to observe their long term (about one year) anti-fouling behavior against barnacles on agarose, κ-carrageenan, PAAm, PAMPS, PAAc, PAMPS /PAAm DN gel, PAAc/PAAm DN gel, PAAc/PAAm/PAAc TN gel, and PVA gel. In this research, two main obstacles in carrying out the experiment in marine environments were overcome. One was the weak mechanical strength of the gel and the other was the method of fixation of gel samples in marine environments. Although three different methods of fixation of the above gels were implemented in marine environments, the stainless frame method performed with mechanically strong PVA and the PAMPS/PAAm DN gel was found to be the most successful. In this case, the species of barnacles settled on gel surfaces were Megabalanus rosa, Fistulobalanus kondakovi, and Amphibalanus amphitrite and the other sessile organisms found were sponges, algae, and sea squirts.
In the marine environment test, the degree of destruction of the surfaces PE, PAMPS/PAAm DN gel, and PVA gel, created by sessile barnacles, and the growth of these barnacles on different surfaces were investigated. The behaviors of barnacles and other sessile organisms were practically observed. The adjacent figure shows that a large number of barnacles adhered onto PE control surfaces compared with the gels. The red dashed line refers to the substratum area. Moreover, from the figure, it is evident how deformation occurred in the case of gels. However, the PE surface remained undistorted.
Moreover, deformation was also found in the case of barnacle basal plates. This deformation at the basal plates also agrees with that which occurred in the case of gel surfaces, meaning that a flat basal surface was observed on hard PE (E = 151 MPa). On the other hand, a very rough and concave basal surface was observed on the soft PVA gel surface (E = 0.09 MPa). On the PAMPS/PAAm DN gel, which was relatively rigid (E = 1.25 MPa) in comparison with the PVA gel, a slightly concave surface was observed.
This observation indicates a dependence of the morphology of the barnacle basal surface on the substratum on which it settled. In the case of soft surfaces, the barnacle was found to embed into the surface. This observation was also investigated by Berglin et al.  who used rigid PMMA and relatively soft PDMS surfaces. A concave shape of the barnacle base surface was observed in the case of PDMS surfaces.
The growth of barnacles on PHCL PVA (physically cross-linked PVA) gels, PDMS, PAMPS/PAAm DN gels, and glass surfaces is being investigated in vitro. In this case, the observation will be carried out in three phases: observation of first, the settlement of cyprid larvae, second, the long-term growth progress of acorn barnacles, and finally, the relative size and morphology of barnacles.
Copyright © Hokkaido University All Rights Reserved.