Smart Materials with Dynamically Controllable Surfaces
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Smart Materials with
Dynamically Controllable Surfaces Joerg Lahann and Robert Langer
Abstract Recent progress in various biotechnology fields, such as microfluidics, tissue engineering, and cellular biology, has created a great demand for substrates that can undergo defined remodeling with time. As a result, the latest research on materials with dynamically controllable surface properties has led to a variety of novel smart surface designs.
layer consisting of the cyclophane cyclobis(paraquat-p-phenylene) threaded on a diiminobenzene unit was self-assembled onto a gold electrode. In this design, the cyclophane essentially acts as the molecular shuttle undergoing electrochemically driven translocations along the molecular wire (diiminobenzene). Prior to reduction, the shuttles were localized on the molecular string via -donor–acceptor complexes with the diiminobenzene units. Subsequent reduction of the cyclophane to the corresponding biradical dication resulted in the dissociation of the -complex and the movement of the shuttles toward the electrode. Finally, oxidation of the biradical dications resulted in reorganization of the shuttles at the -donor sites. The contact angle of the system reversibly changed from 55 when the cyclophane was in its oxidized state to 105 for the reduced cyclophane.
Photoinduced Switching Keywords: cell adhesion, self-assembled monolayers, stimuli-responsive materials, surface engineering.
Introduction The “right” design of the cellular substrate is critical in biomaterials-based therapies such as ex vivo cell isolation, cell encapsulation, or tissue engineering.1,2 Whereas great progress has been made in the last decade with the development of novel, more biomimetic cell substrates, future research will most likely need to address the problem of overcoming the intrinsically static character of artificial substrates. In this article, we discuss novel materials concepts for smart (dynamically controllable) surfaces.
Turning on Substrate Activity Using Electrochemical Approaches Several groups3,5,6 have developed strategies for substrates that can be turned on based on electrochemical transformations of self-assembled monolayers (SAMs). The electrochemical reactions alter the physicochemical properties of the surface or change the biological activities of discrete ligands. For example, applied electrical potentials have been used in combination with SAMs of alkanethiolates on gold to alter the wettability of a surface.3 After electrochemical desorption of hydrophobic alkanethiolates, a decrease in contact angle from more than 80 to about 0 was witnessed. The fact that surface-confined ferrocene groups can undergo potential-dependent wetting caused by oxidation was used in other SAM-based studies.4,5 Contact angles
MRS BULLETIN • VOLUME 30 • MARCH 2005
changed from 71 to 43 upon oxidation of 15-(ferrocenylcarbonyl)pentadecanethiolates previously assembled on gold. Repeated cycling between oxidation and reduction, however, results in a progressively decreased response as
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