Building biomedical materials layer-by-layer

One of the most rapidly growing means of generating thin films in the field of materials science has been the alternating adsorption based layer-by-layer (LbL) assembly method. This method is based on the simple alternating adsorption of complementary multivalent species on a substrate via electrostatic interactions, hydrogen bonding, or other secondary interactions(1-3). LbL assembly has particular potential in the area of biomaterials(4-9) because it enables nanometer level control of the composition of a thin film, and the generation of highly complex, tailor-made coating compositions(10,11); however, unlike other molecular level deposition techniques, thin films are constructed in water at room temperature, preserving the activity of sensitive proteins, nucleic acids, and other functional biomacromolecules. It has been demonstrated that LbL multilayers can beautifully and conformally coat structures as small as 10 nm diameter gold nanoparticles(12,13), and surfaces with nanoscale complexity, and yet can also be used to coat large scale macroscopic three-dimensional objects. Finally, multilayer thin films can be designed to exhibit high levels of biocompatibility(14-18) both in vitro and in vivo, and can be used to incorporate both large and small molecules, as well as a number of organic and inorganic nanomaterials. A particularly unique aspect of this approach is the ability to build drugs in separate sets of layers, with the potential for sequential delivery(19,20). These properties enable us to explore LbL as a means of achieving controlled drug release from thin films and coatings with high drug density (Fig. 1).