Chitosan as a functional interface between biology and microsystems

We report the use of the amino polysaccharide chitosan for the immobilization and patterning of biomolecules on microfabricated surfaces. Chitosan is a biocompatible and biodegradable substance derived from chitin, which is the structural material in the exoskeleton of crustaceans. Chitosan has two key properties of interest to biologists and engineers alike. First, it has an abundance of primary amine groups which can be covalently coupled to various biomolecules. Second, it possesses pH-dependent network-forming properties. Below pH of 6.5, chitosan´s amine groups become protonated and positively charged, making it soluble in acidic conditions. At pH above 6.5 the amines become deprotonated, and chitosan forms an insoluble polymer network. This allows a film of chitosan to be deposited from solution onto a negatively charged electrode due to a localized region of high pH established near the electrode surface. This electrodeposition is a simple yet robust method of patterning biomolecules with significant advantages over traditional patterning techniques such as microcontact printing. We have demonstrated electrodeposited chitosan as an immobilization agent for DNA and several proteins. Amine-labeled DNA are coupled to chitosan by glutaraldehyde crosslinking and proteins are coupled by enzyme-activated genetically engineered tyrosyl residues. These coupling chemistries can be extended to other biomolecules as well. In addition, the biomolecule attachment can be reversed or completely blocked by passivating chitosan. With its electrodeposition property and its easily accessible amine groups, chitosan is an attractive material from both the microfabrication and biology perspective. Chitosan can be used for a wide range of devices as a spatially controllable interface between organic and inorganic components. The possible applications of chitosan include biosensors, drug delivery devices, and labs on a chip. In our work, we have successfully used chitosan for three - - different BioMEMS applications. One is an optical biosensor, in which chitosan is used to immobilize fluorescently labeled biomolecules on the facets of waveguides. Another is a micromechanical biosensor, in which chitosan immobilizes DNA on the surface of a microcantilever. The third one is a microfluidic device, in which chitosan is used to pattern biomolecules inside sealed microchannels. These devices demonstrate the simplicity and flexibility of chitosan-based biomolecular patterning