Fusion of Seashell Nacre and Marine Bioadhesive Analogs: High-Strength Nanocomposite by Layer-by-Layer Assembly of Clay and L-3,4-Dihydroxyphenylalanine Polymer

Nature has evolved materials that possess mechanical properties surpassing many man-made composites. Bones, teeth, spider silk, or nacre, are just a few well-known examples of biomaterials that exhibit exceptionally high tensile strengths, hardness, or toughness. These remarkable properties have driven scientists to study and model their architectures and compositions, from microto nanoscales, in the hope of developing analogous synthetic materials. Of these, probably the most studied is nacre. It is composed of 95 % brittle CaCO3 plates with just a few percent of organic “glue”, yet it is twice as hard and more than ca. 1000 times as tough as its constituent phases. These exceptional mechanical properties together with the macroscopic beauty and elegance of its nanoscale hierarchy serve as a model for design of high-performance materials. Preparation of artificial analogs of nacre has been approached by using several different methods and the resulting materials capture some of the characteristics of the natural composite. In our own work, we have used a layerby-layer (LBL) assembly technique to prepare a nanostructured analogue of nacre from inorganic nanometer-sized sheets of Na-Montmorillonite clay (C) and a polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The structure, deformation mechanism, and mechanical properties of this material were found to be comparable with those of natural nacre and lamellar bones (tensile strength, r= (100± 10) MPa, and Young’s modulus, Y = (11± 2) GPa). Contrary to other preparation techniques the LBL method is relatively simple and highly versatile in merging different functionalities into a single composite. At the same time, a vast array of available assembly components allows us to generate alternative designs as a means of understanding the different interactions necessary for preparation of nacrelike composites with application-tailored mechanical responses. LBL technique has proven to be an ideal method for preparation of multifunctional, nanostructured materials. Since its inception in 1990s, there has been a virtual explosion in the amount of scientific literature in this subject. Similarly, LBL assembly of clays was also pioneered and further studied in the 1990s by Ferguson’s group. Since then, the LBL technique has been found to be applicable for the preparation of superhydrophobic surfaces, sensors and semipermeable membranes, drug and biomolecules delivery, optically active and responsive films, fuel cells and photovoltaic materials, biomimetic and bioresponsive coatings, semiconductors, catalysts, and magnetic devices, to name a few. All of the potential applications mentioned above also require both control and improvement of mechanical properties. Using the mix-and-match approach to LBL films, that is, stratified multilayers, the mechanical properties can be incorporated in virtually any LBL functionality, if a convenient pair of LBL partners is available. Similarly to our work on nanostructured nacre, we have also previously shown that preparation of high-strength LBL composites from singleand multiwalled carbon nanotubes (CNTs). They demonstrated mechanical properties as high as: r= 220 MPa, and Y = 5 GPa, and are particularly suited for multifunctional stratified coatings with electrical conductivity. Having at hand versatility of the LBL technique and potential for use in a wide array of applications, we have set out to improve the mechanical properties of our composite further. Clay nanosheets possess exceptionally high mechanical properties, with Y calculated at ca. 250–260 GPa, which is two orders of magnitude greater than the mechanical properties of most clay nanocomposites achieved thus far. We have hypothesized that improving load transfer from the weak polymeric component to the inorganic nanosheets in our artificial nacre should increase the composite’s mechanical properties. This required a polymer that would have a potentially stronger interaction with the clay than the ionic bonds in PDDA/C. For inspiration we have turned to another exceptional biomaterial, the unusual protein adhesive secreted by mussels. C O M M U N IC A IO N