Underwater adhesion in nature
The answers to overcome all the challenges for underwater adhesion can be found in nature. In the sea, there is a diversity of organisms that specialize in sticking to all types of wet surfaces. Barnacles, oysters, octopi, limpets and mussels are a few familiar examples. Recently, extensive efforts have been made towards understanding these natural adhesive systems.6–11 Briefly, these organisms are able to bond materials underwater using protein-based adhesives: barnacles use secretions to glue calcareous base plates to rocks,11,12 mussels use a network of threads to attach their soft invertebrate body to hard surfaces,13,14 and both sandcastle worm and caddisfly larvae assemble a protective tubular shell by gluing together mineral particles underwater.11,15 In each case, nature has created effective adhesives that exhibit the following traits: (i) minimal H2OSwelling,plasticizationH2OAdhesiveH2OAdherend AAdherend BErosion,hydrolysisWicking, absorbingWater at the interface preparation of the resident surface either by displacing competing ions or forming coordination complexes, (ii) repulsion of surface bound water, (iii) just-in-time and in situ setting reaction, (iv) strong cohesive and elastic properties to withstand significant shear forces, (v) ability to remain insoluble in water due to crosslinking.11,16 In this section we present the most widely studied natural adhesive systems; the marine mussel (Mytulis edulis) and the sandcastle worm (Phragmatopoma californica) (Figure 1-2). Between these two species, there are several differences, yet, the adhesive chemistries show several similarities.
Adhesives of Mussels and Sandcastle Worms
Mussel byssus formation is a tightly coupled choreography of chemistry and processing steps.17 Briefly, mussels adhere to surfaces by using structures referred to as byssal threads, which terminate at the substrate surface in a plaque in which it secretes a protein-based adhesive.4,19 The strong attachment of this adhesive enables the mussel to survive in the turbulent, wet and saline habitat of intertidal zones and is attributed to the post-translational modification of tyrosine to 3,4-dihydroxypolyalanine (DOPA). To date, 25–30 different mussel foot proteins have been identified to contribute to the water-resistant adhesive and coating properties of the byssus.20 However, all proteins contain DOPA, which is a distinctive functionality of the mussel byssus. DOPA is highly repeated within the amino acid sequence of the adhesive proteins found in the plaque.6,21 The protein is secreted in a liquid form, which then solidifies to form a byssal thread and an adhesive plaque. The byssal threads are engineered to withstand elevated mechanical loads applied by waves and currents. A byssal thread connects a mussel to the adhesive plaque that is anchored to a foreign surface.
On the other side, the sandcastle worm secretes small quantities of adhesive to join together mineral grains, such as sand, to form a protective tube underwater, which it uses in a similar fashion to a shell.11,18 After an initial curing period of less than 30 s, the adhesive is strong enough to hold the particles in place. In the next hours, a second curing step follows which darkens the color. The resulting cement is a porous solid where the pores are filled with liquid. The adhesive compounds of the sandcastle worms are rich in nonpolar and ionic groups. Enhanced by nonpolar amino acids, complex coacervates and metal ion–polyelectrolyte complexes are formed from oppositely charged compounds. As a result, of complexation, which is a cohesive feature, the adhesive material is concentrated and insoluble in water (Figure 1-3).
Both organisms co-secrete catechol oxidase with their proteins, resulting in the conversion of DOPA into DOPA-quinone. Consequently, covalent bonds are formed between DOPA-quinone groups or other amino acids that promote cohesion, such as cysteine or lysine.6,15,23 A variety of such interactions and possible chemical reactions have been used (either separately Underwater adhesion: context and objectives. or in combination) in the development of several successful materials with promising underwater adhesion properties.
Adhesive systems based on well crosslinked hydrogels
As introduced before, the design of a strong adhesive requires the optimization of intermolecular interactions at the interface and maximizing the energy dissipation in the bulk OH+-+OHOHOHOHOHHOOHOHNH2Electrostaticπ-πCation-πH-bonds M3+OOOOMetal coordinationHydrophobic interactionCovalent crosslinkSSdisulfide bonds of the adhesive. To address this challenge, highly stretchable and tough double network hydrogels (DN), as first reported by Suo and coworkers,34 serve as good candidates to tackle this problem. Combining the ideas of DN hydrogels and molecular electrostatic interactions, several groups have designed hydrogels with a bulk dissipative mechanisms and interactions at the interface that can adhere to soft wet human tissues. For instance, Karami et al,.35 reported a DN gel composed of covalently cross-linked poly(ethylene glycol)dimethacrylate and ionically crosslinked alginate reinforced with nanofibrillated cellulose. No tissue surface modification was needed to obtain high adhesion properties underwater with the developed hydrogel. Additionally, Li et al.25 reported a tough hydrogel with adhesive properties consisting of two layers: a positively charged adhesive surface based on either chitosan or poly(acrylic acid) and a dissipative matrix based on the tough double network hydrogel of acrylamide-alginate of Suo and coworkers.34 The former adheres to the substrate mainly by electrostatic interactions, as wet tissues are mostly negatively charged.36 The two layers synergistically lead to higher adhesion energies on wet surfaces containing blood such as porcine skin. For short contact times (~1 min) they reported values in the order of ~250 J/m2, however, for contact times higher than 20 min, they extraordinarily reached values in the order of 103 J/m2.25 Therefore, this novel hydrogel adhesives may be useful in many areas of application, including tissue adhesives, wound dressings, and tissue repair.
Adhesive systems based on complex coacervates
As introduced before, one of the phenomena, which is believed to play a fundamental role in the adhesive delivery of natural adhesive systems, is complex coacervation. In practice, it is an associative liquid-liquid phase separation of oppositely charged polyelectrolyte solutions.15,23 Complex coacervates are particularly suitable for underwater adhesion, because of their fluid-like, yet water immiscible properties33,37 and good wettability.38 In natural systems, after establishing molecular contact upon delivery, the complex coacervate liquid transforms into a solid-like material by the introduction of covalent or non-covalent but strong interactions activated by a change in environmental conditions (e.g. higher pH in seawater, metal ions coordination).
This principle has been mimicked in synthetic systems by designing polyelectrolyte material systems either responsive to a particular trigger (pH,29,39,40 ionic strength,41,42) or reinforced via a crosslinking reaction.29,31,43,44 For instance, Zhao et al. designed a fully synthetic underwater adhesive that was applied to a water immersed surface via solvent exchange.19 The adhesive consisted of oppositely charged polymers: a random copolyanion containing anionic acrylic acid and catechol-functionalized acrylic acid (7:3), and a polycation composed of quaternized chitosan ion-paired with bis(trifluoromethane)sulfonamide (Tf2N−). The use of Tf2N−counterions allowed chitosan to dissolve in dimethyl-sulfoxide (DMSO). Without complex formation taking place, the polymers were combined in a single DMSO solution and subsequently applied onto a water-immersed glass slide. Miscibility of DMSO and water enabled solvent exchange, which resulted in deprotonation of acrylic acid by water, followed by complexation of acrylic acid and chitosan. After 25 seconds, the complex turns into a viscoelastic material with strong underwater adhesive properties. This polyelectrolyte complex adhesive attached to a wide variety of surfaces, ranging from glass to hydrophobic plastics and as well as metals and wood, making it a multifunctional underwater glue.
This adhesive systems are very encouraging, however, our understanding is still very poor in terms of the specific role played by one single type of molecular interactions in the macroscopic underwater adhesion of these very complex systems (natural or synthetic). Therefore, the central topic of the following section will be to briefly describe the types of molecular interactions that can occur within the adhesives system. We specifically focus on H-bonds, charge-charge interactions, and cation-π interactions.
Molecular interactions involved in underwater adhesion
Molecular interactions (also known as non-covalent interactions) are attractive or repulsive forces between molecules and between non-bonded atoms. Molecular interactions were first described by the Dutch scientist Johannes Diderik van der Waals.45,46 They are important in diverse fields of protein folding, drug design, material science, sensors, nanotechnology, separations, and even origins of life.2 All molecular interactions are fundamentally electrostatic in nature and can be described by some variation of Coulomb’s laws. However, we reserve the term ‘electrostatic interaction’ to describe interactions between formally charged species. Interactions between partial charges are given by other names.
The Surface Force Apparatus (SFA)
One of the most used technique is the surface force apparatus (SFA) developed by D. Tabor together with R. Winterton in the late 60s58 and with J. Israelachvili57 in 1972. Later, J. Israelachvili and G. E. Adams adapted this technique to measure nanoscale forces in aqueous environments,65 which opened up a whole new field of science.2 Several modified versions of the instrument have been developed since the 80s keeping the same measurement principle.66,67 The SFA contains two curved molecularly smooth surfaces of mica (of radius ~1 cm) between which the interaction forces are measured. The two surfaces are in a crossed cylinder configuration, which is locally equivalent to a sphere near a flat surface or to two spheres close together. A typical SFA has a normal and a lateral distance resolution of 0.1 nm and 1 μm, respectively. The force sensitivity is about 10 nN and given the geometry of this technique, the sensitivity in measuring adhesion and interfacial energies is approximately 10-3 mJ/m2.2.
Over the past few years. SFAs have identified and quantified most of the fundamental interactions occurring between surfaces in air and under aqueous environments. Remarkably, repulsions resulting from electrostatic “double-layer” forces have been extensively studied with the surface force apparatus.14,68,69 and more recently, with the inclusion of the Electrochemical Surface Forces Apparatus (EC-SFA),70 attractive forces due to electrostatic interactions have been measured between an atomically smooth gold electrode (with controllable surface potential by the EC-SFA) and a mica surface covered with a self-assembled amino monolayer. Remarkably, with this new set-up, they were able to measure precisely attractive and repulsive electrostatic forces validating experimentally the DLVO theory (which will be explained more in detail in the Chapter 3) for electrostatic forces. A limitation of the SFA technique is the need for an extremely particle free condition of the medium near the surfaces, and its low lateral resolution (which is no better than that of an ordinary optical microscope). However, it has a significant advantage in the direct visualization of the contact region with the Multiple Beam Interferometry (MBI). Another limitation, and one currently shared with most other techniques at this scale, is that there is not direct link between the measured forces and the molecular composition and structure (e.g., molecular orientations) of the tested materials.
Synthesis of negatively charged hydrogel thin films
Surface-attached thin hydrogel films were prepared by simultaneously crosslinking and grafting pre-functionalized poly(acrylic acid) (PAA) onto thiol-modified silicon wafers. The crosslinking and grafting took place through a thiol−ene click reaction following a previously published procedure, which will be described in the following paragraphs.
Figure 2-4. (Top) Ene-functionalization reaction of acrylic acid with allylamine by peptide bond with EDC and NHS. (Down) 1H NMR spectrum of ene-functionalized PAA. Regions between 1.1 – 1.2 ppm (residual ethanol CH2), 3.55-.3.65 ppm (residual ethanol CH3) and 4.55- 4.85 (D2O) omitted in the interest of clarity.
Characterization of the elastic properties of the bulk hydrogels
Cylindrical samples (8 mm diameter, 12 mm height) were prepared in a silicone mold and tested 24 h after the polymerization, to measure the shear modulus in the preparation state (𝐺0), and at swelling equilibrium (𝐺𝑒). Compression tests were carried out using a custom-built setup with a uniaxial testing machine (Instron, model 3343) and a 10 N load cell. Each sample was preloaded with a compression force of 50 mN followed by a compressive loading at a constant displacement rate of 50 μm/s until a maximum load of 5 N was achieved. Before the test, all specimens were coated with paraffin oil to avoid friction forces between hydrogels and the testing plates during the uniaxial compression. The compressive modulus (𝐸) was calculated as the slope of the linear regression line for data between 5% and 20% of strain. Assuming that hydrogels are incompressible for these relatively high rates compare to their geometry (Poisson’s ratio = 0.5), the shear modulus (𝐺) was estimated as 𝐺=𝐸3.
Streaming Potential Measurements on Charged Surfaces
Electrokinetic measurements are a versatile tool for investigating charge formation at interfaces between polymers and aqueous solutions.35,36 By using the GSGC model described before, it is possible to relate the measured electrokinetic quantity (i.e. streaming potential or streaming current) with the degree of dissociation and the interfacial charge density of the PAA thin films. In this project, the surface potentials of the PAA hydrogel thin films were determined by streaming potential measurements using an Electrokinetic Analyzer (EKA) (Anton Paar GmbH, Austria).
For the streaming potential measurements, two pieces of silicon wafers (10 mm x 20 mm each) grafted with PAA (dry thickness: ~30 nm) were attached to the rectangular cell with adhesive tape so that they were facing each other and formed a streaming channel where the measuring fluid flows through. During the experiment, the pressure inside the fluid channel ( 0) was continuously varied and the streaming potential (Δ𝑈) at zero net current conditions was measured for each value of Δ (Figure 2-7). The zeta potential 𝜁 was then calculated using the expression developed by Smoluchowski37 𝜁=Δ𝑈Δ 𝜂𝜀𝑟𝜀0𝑘𝑐(19).
where 𝜀0 is the vacuum permittivity constant, 𝜀𝑟, 𝜂 and 𝑘𝑐 are the dielectric constant, viscosity and the specific conductivity of the measuring fluid respectively. These last 3 variables are measured independently for each specific pH. The dielectric constant 𝜀𝑟 of the medium is highly dependent on the ionic strength and, therefore, 1 mM is used to be close to pure water. The pH-dependence of the zeta potential (𝜁) for PAA thins films was determined in a KCl solution (1 mM) for a pH range from 2.5 to 10.5. Measurements started at pH ~ 6 followed by stepwise addition of HCl or KOH (0.1 M) to sweep between more acidic and more basic pH values, respectively. One pair of PAA films was used for the acidic environment and a different pair was used for the basic environment. Four measurements were conducted at each specific pH.
Table of contents :
1. UNDERWATER ADHESION: CONTEXT AND OBJECTIVES.
1.2. UNDERWATER ADHESION IN NATURE
1.3. BIO-INSPIRED ADHESIVE SYSTEMS
1.4. MOLECULAR INTERACTIONS INVOLVED IN UNDERWATER ADHESION
1.5. MEASUREMENT OF ADHESION UNDERWATER
1.6. OBJECTIVES OF THE THESIS
1.7. OUTLINE OF THIS MANUSCRIPT
2. FROM MOLECULAR ELECTROSTATIC INTERACTIONS TO MACROSCOPIC UNDERWATER ADHERENCE.
3. UNDERWATER ADHERENCE SCREENED BY SALT. A PROBE-TACK TEST AND AFM COLLOIDAL PROBE STUDY.
4. UNDERWATER ADHESION BETWEEN OPPOSITELY CHARGED GELATIN-BASED HYDROGELS.
4.6. PERSPECTIVES OF BIO-BASED SYSTEMS FOR UNDERWATER ADHESION
5. UNDERWATER ADHESION OF COMPLEX COACERVATES.
5.2. CHARACTERIZATION OF COMPLEX COARCERVATES
5.3. UNDERWATER ADHESION
6. GENERAL CONCLUSION AND FURTHER REMARKS
PART I. MODEL SYNTHETIC SYSTEM
PART II. BIO-BASED SYSTEM
PART III. UNDERWATER ADHESION OF COMPLEX-COACERVATES
SYNTHESIS OF COMPLEX COARCERVATES BASED POLYELECTROLYTES POLYMERS GRAFTED WITH PNIPAM.
SMALL ANGLE X-RAY SCATTERING (SAXS)