3-D PRINTING
MATERIALS
▸ Printing with living inks
Guanine crystals lend sight to scallops
Researchers have used functional living inks, which they call Flinks, containing bacteria with useful reactive chemical properties for three-dimensional printing. André R. Studart, Patrick A. Rühs, Manuel Schaffner, and coworkers at ETH Zurich embedded the functional bacteria in hydrogels that can be used in 3-D printers to make a variety of structures for various
What do scallop eyes have in common with iridescent crustaceans and shimmery fish scales? Guanine crystals. The shape of the reflective crystals, it turns out, is an important part of natural mirrors found within scallop eyes. Scallops use mirrors instead of lenses to concentrate light on their retinas. Scientists had suspected these mirrors were composed of alternating layers of guanine and cellular cytoplasm, a strategy used by crustaceans and fish to create glimmering surfaces. However, the delicacy of scallop eye mirrors made elucidating the nanoscale structure challenging. Researchers led by Lia Addadi of This micrograph reveals tightly Weizmann Institute of Science decided tiled plates of reflective guanine to take a closer look by using cryogenic crystals. scanning electron microscopy, which rapidly freezes water in tissue samples to create near-lifelike visualization conditions. “The first time we saw the eye mirror in the electron microscope, our mouths fell open,” says colead author Benjamin A. Palmer. The images revealed a tightly organized mirror structure, with 20 to 30 layers of tiled square plates made of β-guanine, with each layer perfectly aligning the plates in a vertical stack (Science 2017, DOI: 10.1126/science.aam9506). The mirror best reflects blue-green light, the same wavelengths that reach the scallops’ seafloor habitat.—EMMA HIOLSKI
C R E D I T: MA N U EL S C H A FF NE R & PAT R IC K A . RÜH S ( D O LL , S HI RT) ; SC I E N C E (M I CRO G RAP H )
Living inks containing bacteria can be used to prepare 3-D-printed shapes, such as this miniature Tshirt and the film on the surface of this doll’s face. The bacteria are producing a cellulose film, which is fluorescently labeled blue. applications (Sci. Adv. 2017, DOI: 10.1126/sciadv. aao6804). The hydrogel consists of hyaluronic acid, κ-carrageenan, and fumed silica at a 1:1:1 ratio. Increasing the weight percent of those components while maintaining the same ratio allows the researchers to adjust the viscosity and elasticity of the hydrogel. After chemically replacing the hyaluronic acid with glycidyl methacrylate hyaluronic acid after the printing step, the team used ultraviolet light to cross-link the hydrogel to form self-supporting structures. The researchers used a Flink loaded with Pseudomonas putida to make devices that can degrade phenols for bioremediation applications. They demonstrated potential biomedical applications using a Flink embedded with Acetobacter xylinum, a bacterium that produces cellulose. Bacterial cellulose is being developed as a material for skin repair and as a tissue envelope for organ transplantation. The researchers showed that they can controllably use the ink to make complex shapes by printing a scaffold in the shape of a miniature T-shirt and by depositing a film on a doll’s face as a model for a human face. In both cases, the bacteria produced a cellulosic biofilm.—CELIA ARNAUD
SYNTHESIS
▸ Benzene yields to nucleophilic substitution With the aid of a powerful calcium reagent, researchers have achieved a bit of molecular trickery to do what many synthetic organic chemists thought wasn’t possible: They have carried out the first nucleophilic alkylation of benzene (Science 2017, DOI: 10.1126/science.aao5923). Friedel-Crafts
H H
H H
H3C
δ–
Cl AlCl3
Lewis acid-catalyzed electrophilic substitution
R
H
δ+
H2C
then displaces a benzene hydrogen, producing an alkylbenzene. An electron-rich nucleophilic species would normally be repelled by benzene, unless an electron-withdrawing group already on the ring activates it toward nucleophilic attack. Nevertheless, a team led by Michael S. Hill of the University of Bath and Laurent Maron of the University of Toulouse found a way to allow nucleophiles to react with unsubstituted
H
H H
H
H alkylation is a classic example of electrophilic aromatic substitution. A Lewis acid such as AlCl3 binds the halogen of an alkyl halide, creating a positive charge on the alkyl group. This electron-poor species— an electrophile—is attracted to an electron-rich substrate—a nucleophile—such as benzene’s aromatic ring. The alkyl group
CH3
R N δ+ CH N 2 Ca δ–δ– Ca δ+ N N CH2 R CH R 3
R = diisopropylphenyl Calcium-powered nucleophilic substitution
H H
H
H H
H
benzene in spite of the electronic repulsion. The researchers developed a calcium hydride reagent that reacts with alkenes to create an alkylcalcium species, which has nucleophilic alkyl groups by virtue of polarized calcium-carbon bonding. They show that the calcium reagent can readily alkylate benzene.—STEVE RITTER DECEMBER 4, 2017 | CEN.ACS.ORG | C&EN
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