Interfacing of biosystems

Abstract

Functionalization of technical surfaces with native biomembranes is extremely challenging because of their complex physicochemical properties with spatial differences in the nanometer range. The complexity of cellular membranes and their physiological needs are the major challenges for successful interfacing of biosy-stems. While the functionalization of technical devices with living cells is still in its infancy, a wide variety of methods has been developed to engineer surfaces with individual biomolecules. The chapter highlights approaches to functionalize surfaces with biomolecules by employing different interaction mechanisms. These can roughly be divided in physical absorption, chemical bonding and bioaffinity interactions. Well established methods are available to immobilize DNA on various planar substrates as well as on nanoparticles. Covalent binding of DNA is the preferred technique, which combines high binding strength and the option to realize a high uniformity and packing density of oligonucleotides. Various chemistries are available that allow covalent binding of oligonucleotides on gold and silica substrates. Site-specific immobilization can be achieved by the bioaffinity interaction of the streptavidin/biotin recognition system. Besides covalent and bioaffinity binding of the DNA to the substrate, physical interactions based on electrostatic interactions between a positively charged substrate and the negatively charged DNA backbone can be used. The design of functional interfaces based on proteins is more challenging due to the higher structural complexity of this class of biomolecules. Important issues are the maintenance of the structural integrity to avoid denaturation, the stable transfer of the native protein configuration, an optimized spacing between the proteins, and a defined orientation at the interface. The known immobilization routes can be classified in physically, chemically, and bioaffinity-mediated binding. Physical immobilization leads to randomly adsorbed proteins without a preferred orientation. Depending on the exposed amino acids various chemistries can be used for the covalent non-specific or site-specific immobilization of proteins. Interfacing mechanisms of proteins via bioaffinity is characterized by a preferred orientation of the reaction partners and by optional detachment of the bound proteins. The interaction of living cells with artificial microstructures is a topic of rapidly growing interest. The two major goals are (i) the direct immobilization of living cells on the surface of microelectronic devices in a biological environment, and (ii) the immobilization of living cells in a surface layer deposited on the electronic device to generate a quasi three-dimensional natural microenvironment that allows sensing and control by these cells. In the first approach a biologized interface can be engineered which possesses appropriate binding sites allowing a direct interaction with adhesion receptors in the cellular membrane. The second approach, which gets growing relevance for the development of whole-cell biosensors and bioactors, uses hydrogels, silica-based sol-gel techniques or layer-by-layer deposition of polyelectrolytes for embedding the living cells. The recent developments of nanoelectronic platforms will allow communication with living cells/tissues on the cellular and molecular level. The application of nanomaterials such as functionalized carbon nanotubes or silicon nanowires will have a dramatic impact for the detection limit of biosensors and their nanopackaging design.