MONITORING THE BEAT-TENG HEART Implantable medical devices, such as gastric and cardiac pacemakers, cochlear implants, and deep brain stimulators, have become increasingly popular for monitoring, measuring, and soliciting physiological responses in vivo. Recent improvements in these systems have enabled greater in vivo stability, miniaturization, and lower-energy requirements. However, a key challenge remaining for these devices is their battery-based power supplies, which have limited energy density, short lifetimes, chemical side effects, and large volumes. In a recent study, Zheng et al. (DOI: 10.1021/acsnano.6b02693) developed a solution to this conundrum with a triboelectric nanogenerator (TENG) that harvests biomechanical energy in vivo from the motion of a beating heart. The TENG has a multilayer structure, with nanostructured polytetrafluoroethylene (n-PTFE) as the triboelectric layer. This n-PTFE was fixed on a Kapton film as the flexible substrate, with an ultrathin Au layer on the back to form one electrode, while Al foil served as both the second triboelectric layer and another electrode. A titanium strip served as the keel. The entire device was encapsulated twice, first in a polydimethylsiloxane layer and then in a layer of parylene C. Tests showed that this device generated an output voltage and corresponding current in Yorkshire pigs that were 3.5 and 25 times better than reported in vivo output performance of biomechanical energy conversion devices. As proof of principle, the researchers used this TENG to power a wireless transmission system for cardiac monitoring. The authors suggest that this device might be used to power existing implantable devices or for designing a self-powered, wireless healthcare monitoring system.
invasion of cancer cells through pores using an artificial model of interstitial tissue. The researchers used electrohydrodynamic three-dimensional colloidal gold nanoimprinting to print vertically standing gates with different pore sizes on basal gratings that were oriented to maximize cell polarization and directional migration toward the gates. The researchers then allowed cancer cells with different metastatic potential to interact with these pores, monitoring these events with optical microscopy. These observations showed that the ability for the cancer cells to deform depended strongly on the cell cycle phase, with the greatest deformation possible immediately after mitosis. At this peak, the cells had the lowest nuclear volume and rigidity. Artificial chromatin decondensation with trichostatin-A prompted an increase in cell and nuclear deformability and improved pore penetration of cells in the G1 stage. The authors suggest that these results link cell proliferation and pore penetration to promote the interstitial dissemination of metastatic cells.
FINGERPRINTING TWO-DIMENSIONAL MATERIALS Interest in several two-dimensional (2D) van der Waals-bonded materials, including graphene, hexagonal boron nitride, and transition metal dichalcogenides, has grown recently due to their extraordinary electrical, optical, and mechanical properties. Harnessing these properties for high-performance nanoscale device applications will require precise control over the interface to the growth substrate, the layer number, the lateral arrangement of different domains, and the nature of subsurface structure elements. Even slight variations in these characteristics can significantly affect their electrical, optical, and mechanical properties. Thus, it is important to be able to characterize the surface and interface of these materials precisely. However, most existing techniques that accomplish this goal have a number of drawbacks, including destroying materials in the process, requiring restrictive experimental environments, or providing only area integrated information. In a recent study, Tu et al. (DOI: 10.1021/acsnano.6b02402) used contact-resonance atomic force microscopy (CR-AFM), a technique that is extremely sensitive to stiffness changes that arise from even a single atomic layer of a van der Waals-adhered material, to address this challenge. By combining this tool with first-principles atomistic calculations and a continuum mechanics
SEEING HOW CANCER CELLS SQUEEZE THROUGH THE CRACKS Tumor metastasis, the largest contributor to cancer-related mortality, requires cancer cells to migrate through the threedimensional interstitial matrix and proliferate in locations often distant from the primary tumor. To accomplish this feat, they must be able to penetrate layers of healthy cells through interstitial pores. Although some research has shown that the ability to penetrate pores largely depends on the rigidity and deformabilty of the cell and cell nucleus, this process, as well as what factors might affect cellular and nuclear stiffness, is not well understood. In a recent study, Panagiotakopoulou et al. (DOI: 10.1021/ acsnano.5b07406) detail a method they used to monitor the © 2016 American Chemical Society
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DOING THE TWIST WITH CHOLESTERIC LIQUID CRYSTAL DROPLETS The majority of liquid crystal (LC) research has focused on nematic liquid crystals, whose rod-like configurations can be altered by adsorbates or interfacial conditions, giving them promise as chemical sensors or vehicles for nanoparticle selfassembly. Their counterparts, cholesteric liquid crystals (ChLCs), tend to adopt twisted or helical structures in the bulk, but these configurations can differ considerably when ChLCs are confined into spherical droplets. There, the equilibrium free energy depends on a delicate interplay between chirality, bulk elasticity, and surface anchoring. Some studies have suggested that for ChLC droplets with strong planar anchoring, a twisted bipolar structure (TBS) is the most energetically favorable for low chirality, and a radical spherical structure (RSS) is the most stable for high chirality. However, little is known about the transition between these two. In a recent study, Zhou et al. (DOI: 10.1021/acsnano.6b01088) address this gap using a modeling approach. Their work shows that for systems with strong anchoring, ChLCs with intermediate chirality adopt bent structures resulting from competition between elasticity and chirality. Looking next at the effects of anchoring, they found that weakly anchored ChLC droplets have stable structures similar to the bulk helical phase. As anchoring strength increases, this quasi-helical structure gradually evolves into planar bipolar structures and eventually bent structures. Similar to previous work, the researchers’ simulations show that nanoparticles are attracted to defect regions on the surface of the droplets. The authors suggest that this work emphasizes ChLCs’ particular sensitivity to chirality and surface anchoring, factors that could affect their response to different surface-active molecules and nanoparticles.
model, the researchers were able to create a nanomechanical, subsurface-sensitive “fingerprint” of the atomic structure of interfaces for several 2D materials. Using this method, the researchers were able to distinguish between different regions on samples of epitaxial graphene or oxygen-intercalated graphene on SiC, identifying complexities that are not readily apparent using other microscopy methods. The authors suggest that this method offers a versatile way to resolve and to interpret surface and subsurface structure across a wide range of 2D materials and heterostructures.
SUB-10-NANOMETER LITHOGRAPHY: THE HOLE STORY For many years, Moore’s law has accurately predicted the density growth rate on integrated circuits, forecasting the continuing miniaturization of powerful electronics. However, sub-10 nm lithography remains a persistent challenge for the microelectronics industry. Current technologies that approach this limit have several significant process and cost issues. One promising option for reaching these scales is DNA origami, a technique in which long, single-stranded DNA is folded into desired shapes and held with hundreds of short DNA “staples”. DNA origami has already been used to create lithographic masks by metal vapor shadowing or metal film replication of its shape. In a recent study, Diagne et al. (DOI: 10.1021/acsnano.6b00413) demonstrate the utility of this technique by transferring a 9 × 14 nm2 hole from a DNA origami cuboid onto a SiO2 substrate. After creating a DNA origami cuboid with a hole in the center, the researchers randomly attached several of these structures to SiO2. These combined assemblies were then subject to hydrofluoric acid vapor etching for 30−60 s. Atomic force microscopy imaging showed that the DNA origami structures were preserved during etching, serving as masks for pattern formation. Indeed, once they were removed by washing with water and ethanol, nearly every SiO2 pattern displayed a very small cavity present at its center where the DNA origami hole had been. At extended etching times, greater than 600 s, the patterns began to erode and the overall etching reaction was blocked, suggesting that the origami underwent localized destruction of its original shape. The authors suggest that these results offer a process window, fabrication rules, and limits for DNA-based lithography.
THE GENERATOR’S NEW CLOTHES Flexible and wearable electronic devices are the focus of growing scientific and commercial interest, with promising applications in multifunctional intelligent systems. To power these systems, some efforts have concentrated on taking advantage of the enormous amount of ambient mechanical energy generated by the human body through mechanical friction and vibration, most of which goes to waste. Such energy can be harvested by piezoelectric and triboelectric generators. However, although triboelectric generators require a high-performance wearable electrode, such components typically are not foldable, stretchable, or washable, characteristics necessary to incorporate them seamlessly into clothing. Some methods exist to fabricate etextiles, including a bottom-up route in which conducting fibers are woven into textiles and a up-bottom route in which the 6421
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commercial textile is covered with a conducting film coating. However, these methods have numerous drawbacks, including limited conductivity and comfort, lack of optical transparency, or environmentally unfriendly production. In a recent study, Wu et al. (DOI: 10.1021/acsnano.5b08137) detail a method for fabricating e-textiles for wearable triboelectric generators that avoid these problems. Their method involves using a blade-coating technique to apply conductive silver nanowires to commercial polyester fabrics to form conducting textiles, then further coating this material with graphene to improve its electrical and mechanical stability. The resulting material exhibits stable conduction of
