Nanopatterning DOI: 10.1002/smll.200800536 Piezoelectric Inkjet Printing of Biomimetic Inks for Reactive Surfaces** Leila F. Deravi, Jan L. Sumerel, Sarah L. Sewell, and David W. Wright* The intricacies involved in the formation of nanostructured cell walls of marine diatoms have been a source of inspiration for a generation of developmental biologists, chemists, and material scientists. It is understood that the biomineralization of these cell walls is initiated on the surface of an internal valve known as the silica deposition vesicle (SDV). [1–4] The SDV provides a localized environment where cell wall biogenesis is completed as silica condensation is catalyzed by long-chain polyamine moieties or cationic polyeptides. [1,4] A number of biomimetic analogs to the silica precipitating peptides have been developed and characterized in vitro. [5–10] Although they are recognized as excellent examples of bioinspired templates for metal-oxide synthesis, these mimics have yet to success- fully recapitulate spatial and supramolecular control on a two- dimensional (2D) surface. Recent materials-deposition techniques, including sole- noid jet printing, lithography and liftoff patterning, and direct ink write (DIW), have been used to immobilize a variety of silica precipitating precursors. [7,11,12] All methods provided a unique approach towards the advancement of controllable templating for patterned metal oxides; however, each was beset by their own limitations. These included large, non- uniform spots (solenoid jet printing), high temperature reaction conditions (lithography and liftoff), or inherently slow (40 mms À1 ) patterning conditions (DIW), rendering them unfavorable for the rapid production of functional-material patterns under ambient conditions. [11,12] For these reasons, we have employed piezoelectric inkjet printing with the Dimatix Materials Printer (DMP) as an alternative, rapid prototyping (8 m s À1 ) method of deposition for the 2D patterning of templated microstructured silica. The flexibility associated with DMP deposition provided reproducible spot sizes and enabled tunable surface control specific for each reaction environment. A composite ink was developed with a viscosity of 18.5 mPa and was printed onto gold, glass, or sapphire substrates. The components of this ink included 8.6 wt% generation 4 (G4) polyamidoamine (PAMAM) dendrimer in polyethylene glycol (PEG; 25% w v À1 ) and phosphate buffer (100 mM, pH 7.5). The PAMAM dendrimer was used as the defined silica- condensing template, as its primary amine moieties are effective mimics of the propyleneamine units found in silica precipitating peptides. [2] These dendrimer templates have been shown to rapidly precipitate aggregated nanospheres of silica in vitro at neutral pH. [13–15] To investigate the interfacial properties of the composite ink on a gold substrate, the contact angle of a 2 mL sample was determined to be 33.108. This low contact angle (<908) indicates localized stability and confined rheological properties, and that the immobilized ink is loosely packed on the high energy, hydrophobic substrate. [16] The addition of PEG to the ionic ink produces an aqueous composite system that complements its intrinsic self-aggrega- tion. [17] This enhanced aggregation is necessary to keep the printed spots discrete on the surface, as evidenced by both the viscosity and the contact angle of the ink without PEG (9.7 mPa and 16.468, respectively). Once the customized ink has been developed, it is loaded into the print cartridge, where a specific voltage is applied to each nozzle of the print head, releasing a droplet on the order of 10 pL. [18] Because the DMP is a piezoelectric instrument, a user-controlled, pulsed voltage induces an internal stress in the form of a pressure wave inside the nozzle of the print head (16 nozzles total, spaced 256 mm apart). [18,19] In these studies, the maximum voltage pulse was 24 V, and the frequency of the pulses was maintained at 1.0 kHz, ensuring that the same amount of material was deposited during each print cycle. The printed spot sizes were constant and independent of the substrate at 36 Æ 2 mm. Additional properties of the patterned dendrimer were characterized using UV/vis spectrophotome- try, scanning electron microscopy (SEM), and fluorescence microscopy. Varying the spot spacing, the number of print cycles, and the reaction time with monosilicic acid resulted in silicified structures reminiscent of patterned silica on the cell wall of diatoms. The reactivity of the dendrimer ink was studied with the specific aim of controlling the formation of patterned silica on a solid surface. For this reason, initial experiments were aimed at characterizing the silica precipitating reactivity of the dendrimer patterns (Figure 1). Samples were prepared varying only the area of patterned dendrimer (printed eight times with a 46 mm spot spacing) on an R-plane sapphire substrate. Each pattern was then reacted with a monosilicic acid solution (113 mM in phosphate buffer, pH 7.5) for 15 min. The multiple print cycles over larger areas were necessary to produce highly reactive multilayer films, increasing the quantity of accessible surface amines. Arrington et al. have used micro-contact printing in conjunction with atomic force microscopy to show that repetitive dendrimer stamping increases the amount of deposited dendrimer in the form of 3D frameworks of cross- linked amines in air. [20,21] This cross linking enables the amines to act as an adhesive layer on a substrate while providing a highly functionalized surface essential for silica precipitation. As the area of printed dendrimer is increased, the correspond- [ Ã ] Dr. D. W. Wright, L. F. Deravi, Dr. S. L. Sewell Department of Chemistry, Vanderbilt University Nashville, TN 37235 (USA) E-mail: david.wright@vanderbilt.edu Dr. J. L. Sumerel Fujifilm Dimatix 2230 Martin Ave Santa Clara CA 95050 (USA) [ ÃÃ ] This project was funded by the National Institute of Health U54AI057157 Regional Center for Excellence in Biodefense and Emerging Infections. small 2008, 4, No. 12, 2127–2130 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2127