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Fiber push-in nanoindentation is conducted on a unidirectional carbon fiber reinforced bismaleimide resin composite (IM7/BMI) after thermal oxidation to determine the interfacial shear strength. A unidirectional IM7/BMI laminated plate is isothermally oxidized under various conditions: in air for 2 months at 195 °C and 245 °C, and immersed in water for 2 years at room temperature to reach a moisture-saturated state. The water-immersed specimens are subsequently placed in a preheated environment at 260 °C to receive sudden heating, or are gradually heated at a rate of approximately 6 °C/min. A flat punch tip of 3 μm in diameter is used to push the fiber into the matrix while the resulting load-displacement data is recorded. From the load-displacement data, the interfacial shear strength is determined using a shear-lag model, which is verified by finite element method simulations. It is found that thermal oxidation at 245 °C in air leads to a significant reduction in interfacial shear strength of the IM7/BMI unidirectional composite, while thermal oxidation at 195 °C and moisture concentration have a negligible effect on the interfacial shear strength. For moisture-saturated specimens under a slow heating rate, there is no detectable reduction in the interfacial shear strength. In contrast, the moisture-saturated specimens under sudden heating show a significant reduction in interfacial shear strength. Scanning electron micrographs of IM7/BMI composite reveal that both thermal oxidation at 245 °C in air and sudden heating induced microcracks and debonding along the fiber/matrix interface, thereby weakening the interface, which is the origin of failure mechanism. [ABSTRACT FROM AUTHOR]

The metallic interconnects are essential components of energy devices such as fuel cells and electrolysis cells, batteries, as well as electronics and optoelectronic devices. In recent years, 3D printing processes have offered complementary routes to the conventional photolithography- and vacuum-based processes for interconnect fabrication. Among these methods, the confined electrodeposition (CED) process has enabled a great control over the microstructure of the printed metal, direct printing of high electrical conductivity (close to the bulk values) metals on flexible substrates without a need to sintering, printing alloys with controlled composition, printing functional metals for various applications including magnetic applications, and for in situ scanning electron microscope (SEM) nanomechanical experiments. However, the metal deposition rate (or the overall printing speed) of this process is reasonably slow because of the chemical nature of the process. Here, we propose using the CED process to print a single layer of a metallic trace as the seed layer for the subsequent selected-area electroless plating. By controlling the activation sites through printing by the CED process, we control, where the metal grows by electroless plating, and demonstrate the fabrication of complex thin-film patterns. Our results show that this combined process improves the processing time by more than 2 orders of magnitude compared to the layer-by-layer printing process by CED. Additionally, we obtained Cu and Ni films with an electrical resistivity as low as ∼1.3 and ∼2 times of the bulk Cu and Ni, respectively, without any thermal annealing. Furthermore, our quantitative experiments show that the obtained films exhibit mechanical properties close to the bulk metals with an excellent adhesion to the substrate. We demonstrate potential applications for radio frequency identification (RFID) tags, for complex printed circuit board patterns, and resistive sensors in a Petri dish for potential biological applications.

The democratization of thermoplastic 3D printing is rooted in the ease of processing enabled by economical melting and shaping. Thermosetting polymers, on the other hand, have not enjoyed this advantage given that thermosetting resins cannot hold their shape without cross-linking or excessive fillers, and once cross-linked, they cannot be extruded for printing. Due to this formidable challenge, thus far, 3D printing of thermosetting polymers has been limited to the photopolymerization of specialized photosensitive resins or extrusion of resins loaded with large fractions (as high as 20 wt %) of rheology modifiers. Here, we report a rheology-modifier- and photoinitiator-free process for the 3D printing of a pure commercial epoxy polymer, without any resin modification and using a conventional 3D printer. A low-cost non-Newtonian support material that switches between solid-fluid states under a nozzle shear stress enables the printing of complex 3D structures and the subsequent and ″one-step″ curing. Our results show that the one-step curing eliminates the often-compromised interlayer adhesion common in layer-by-layer 3D printing processes and results in unprecedented isotropic mechanical properties (strength, elastic modulus, tensile toughness, and strain to failure). This in-bath print and cure (IBPC) 3D printing process for thermosetting polymers is low-cost, scalable, high-speed (nozzle speeds exceeding 720 cm/min), and high-resolution (down to 220 μm filament size). We demonstrate potential applications for hobbyists, structural and aerospace components, and fiber-reinforced composites, among others.

Achieving a viable process for three-dimensional (3D) printing of ceramics is a sought-after goal in a wide range of fields including electronics and sensors for harsh environments, microelectromechanical devices, energy storage materials, and structural materials, among others. Low laser absorption of ceramic powders renders available additive manufacturing (AM) technologies for metals not suitable for ceramics. Polymer solutions that can be converted to ceramics (preceramic polymers) offer a unique opportunity to 3D-print ceramics; however, due to the low viscosity of these polymers, so far, their 3D printing has only been possible by combining them with specialized light-sensitive agents and subsequently cross-linking them layer by layer by rastering an optical beam. The slow rate, lack of scalability to large specimens, and specialized chemistry requirements of this optical process are fundamental limitations. Here, we demonstrate 3D printing of ceramics enabled by dispensing the preceramic polymer at the tip of a moving nozzle into a gel that can reversibly switch between fluid and solid states, and subsequently thermally cross-linking the entire printed part "at-once" while still inside the same gel. The solid gel, which is composed of mineral oil and silica nanoparticles, converts to fluid at the tip of the moving nozzle, allows the polymer solution to be dispensed, and quickly returns to a solid state to maintain the geometry of the printed polymer both during printing and the subsequent high-temperature (160 °C) cross-linking. We retrieve the cross-linked part from the gel and convert it to ceramic by high-temperature pyrolysis. This scalable process opens up new opportunities for low-cost and high-speed production of complex three-dimensional ceramic parts and will be widely used for high temperature and corrosive environment applications, including electronics and sensors, microelectromechanical systems, energy and structural applications.

The noncollagenous interfibrillar interface in bone provides the critical function of transferring loads among collagen fibrils and their bundles, with adhesive mechanisms at this site thus significantly contributing to the mechanical properties of bone. Motivated by the experimental observations and hypotheses, a computational study is presented to elucidate the critical roles of two major proteins at the nanoscale interfibrillar interface, that is, osteopontin (OPN) and osteocalcin (OC) in bone. This study reveals the extremely high interfacial toughness of the OPN/OC composite. The previously proposed hypothesis of sacrificial bonds in the extracellular organic matrix is tested, and the remarkable mechanical properties of the nanoscale bone interface are attributed to the collaborative interactions between the OPN and OC proteins.

Although various processes for metal printing at the micro- and mesoscale have been demonstrated, printing functional devices such as thermocouples, thermopiles, and heat flux sensors that function based on interfaces between an alloy and another alloy/metal demands processes for printing alloys. Furthermore, a high-quality and crystalline alloy is required for acceptable function of these devices. This article reports for the first time co-electrodeposition-based printing of single-phase solid solution nanocrystalline copper/nickel (Cu/Ni) alloy with various controllable compositions (Cu100Ni0 to Cu19Ni81) from a single electrolyte. The printed alloy is nanocrystalline (<35 nm), continuous, and dense with no apparent porosity, with remarkable mechanical and magnetic properties, without any postprocessing annealing such as heat treatment. In addition, a functional thermocouple fabricated using this process is demonstrated. Such a process can not only be used for fabrication of functional devices, it may also facilitate fundamental studies on alloys by printing a continuous library of alloy composition for material characterization.

Given its high temperature stability, oxidation-, corrosion- and wear-resistance, and ferromagnetic properties, Nickel (Ni) is one of the most technologically important metals. This article reports that pure and nanocrystalline (Ni) films with excellent mechanical and magnetic properties can be additively printed at room environment without any high-temperature post-processing. The printing process is based on a nozzle-based electrochemical deposition from the classical Watt's bath. The printed Ni film showed a preferred (220) and (111) texture based on x-ray diffraction spectra. The printed Ni film had close to bulk electrical conductivity; its indentation elastic modulus and hardness was measured to be 203 ± 6.7 GPa and 6.27 ± 0.34 GPa, respectively. Magnetoresistance, magnetic hysteresis loop, and magnetic domain imaging showed promising results of the printed Ni for functional applications.

Printing functional devices on flexible substrates requires printing of high conductivity metallic patterns. To prevent deformation and damage of the polymeric substrate, the processing (printing) and post-processing (annealing) temperature of the metal patterns must be lower than the glass transition temperature of the substrate. Here, a hybrid process including deposition of a sacrificial blanket thin film, followed by room environment nozzle-based electrodeposition, and subsequent etching of the blanket film is demonstrated to print pure and nanocrystalline metallic (Ni and Cu) patterns on flexible substrates (PI and PET). Microscopy and spectroscopy showed that the printed metal is nanocrystalline, solid with no porosity and with low impurities. Electrical resistivity close to the bulk (~2-time) was obtained without any thermal annealing. Mechanical characterization confirmed excellent cyclic strength of the deposited metal, with limited degradation under high cyclic flexure. Several devices including radio frequency identification (RFID) tag, heater, strain gauge, and temperature sensor are demonstrated.

Noncollagenous proteins at nanoscale interfaces in bone are less than 2-3% of bone content by weight, while they contribute more than 30% to fracture toughness. Major gaps in quantitative understanding of noncollagenous proteins' role in the interfibrillar interfaces, largely because of the limitation of probing their nanoscale dimension, have resulted in ongoing controversies and several outstanding hypotheses on their role and function, arguably going back to centuries ago to the original work from Galileo. Our results from the first detailed computational model of the nano-interface in the bone reveal "synergistic" deformation mechanism of a "double-part" natural glue, that is, noncollagenous osteopontin and osteocalcin at the interfibrillar interface. Specifically, through strong anchoring and formation of dynamic binding sites on mineral nanoplatelets, the nano-interface can sustain a large nonlinear deformation with ductility approaching 5000%. This large deformation results in an outstanding specific energy to failure exceeding ∼350 J/g, which is larger than the most known tough materials (such as Kevlar, spider silk, and so forth.).

DNA has long been viewed as a promising material for nanoscale electronics, in part due to its well-ordered arrangement of stacked, pi-conjugated base pairs. Within this context, a number of studies have investigated how structural changes, backbone modifications, or artificial base substitutions affect the conductivity of DNA. Herein, we present a comparative study of the electrical properties of both well-matched and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI)-containing DNA molecular wires that bridge nanoscale gold electrodes. By performing current-voltage measurements for such devices, we find that the incorporation of PTCDI DNA base surrogates within our macromolecular constructs leads to an approximately 6-fold enhancement in the observed current levels. Together, these findings suggest that PTCDI DNA base surrogates may enable the preparation of designer DNA-based nanoscale electronic components., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Infiltration of a molten metal phase into a ceramic scaffold to manufacture metal-ceramic composites often involves high temperature, high pressure, and expensive processes. Low-cost processes for fabrication of metal-ceramic composites can substantially increase their applications in various industries. In this article, electroplating (electrodeposition) as a low-cost, room-temperature process is demonstrated for infiltration of metal (copper) into a lamellar ceramic (alumina) scaffold. Estimation shows that this is a low energy consumption process. Characterization of mechanical properties showed that metal infiltration enhanced the flexural modulus and strength by more than 50% and 140%, respectively, compared to the pure lamellar ceramic. More importantly, metal infiltration remarkably enhanced the crack initiation and crack growth resistance by more than 230% and 510% compared to the lamellar ceramic. The electrodeposition process for development of metal-ceramic composites can be extended to other metals and alloys that can be electrochemically deposited, as a low-cost and versatile process.

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability. Herein, we report the fabrication of a family of low-density, ambient-dried and hydrophobic poly(isocyanurate-urethane) aerogels derived from a triisocyanate precursor. The bulk densities ranged from 0.28 to 0.37 g cm -3 with porosities above 70% v/v. The aerogels exhibit a highly stretchable behavior with a rapid increase in the Young's modulus with bulk density (slope of log-log plot > 6.0). In addition, the aerogels are very compressible (more than 80% compressive strain) with high shape recovery rate (more than 80% recovery in 30 s). Under tension even at high strains ( e.g. , more than 100% tensile strain), the aerogels at lower densities do not display a significant lateral contraction and have a Poisson's ratio of only 0.22. Under dynamic conditions, the properties ( e.g. , complex moduli and dynamic stress-strain curves) are highly frequency- and rate-dependent, particularly in the Hopkinson pressure bar experiment where in comparison with quasi-static compression results, the properties such as mechanical strength were three orders of magnitude stiffer. The attained outcome of this work supports a basis on the understanding of the fundamental mechanical behavior of a scalable organic aerogel with potential in engineering applications including damping, energy absorption, and substrates for flexible devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Nanotwinned-metals (nt-metals) offer superior mechanical (high ductility and strength) and electrical (low electromigration) properties compared to their nanocrystalline (nc) counterparts. These properties are advantageous in particular for applications in nanoscale devices. However, fabrication of nt-metals has been limited to films (two-dimensional) or template-based (one-dimensional) geometries, using various chemical and physical processes. In this Letter, we demonstrate the ambient environment localized pulsed electrodeposition process for direct printing of three-dimensional (3D) freestanding nanotwinned-Copper (nt-Cu) nanostructures. 3D nt-Cu structures were additively manufactured using pulsed electrodeposition at the tip of an electrolyte-containing nozzle. Focused ion beam (FIB) and transmission electron microscopy (TEM) analysis revealed that the printed metal was fully dense, and was mostly devoid of impurities and microstructural defects. FIB and TEM images also revealed nanocrystalline-nanotwinned-microstructure (nc-nt-microstructure), and confirmed the formation of coherent twin boundaries in the 3D-printed Cu. Mechanical properties of the 3D-printed nc-nt-Cu were characterized by direct printing (FIB-less) of micropillars for in situ SEM microcompression experiments. The 3D-printed nc-nt-Cu exhibited a flow stress of over 960 MPa, among the highest ever reported, which is remarkable for a 3D-printed material. The microstructure and mechanical properties of the nc-nt-Cu were compared to those of nc-Cu printed using the same process under direct current (DC) voltage.

Nanotwinned (nt)-metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nanograined counterparts. nt-metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (additive manufacturing) of nt-metals. Microscale 3D printing of nt-Cu is demonstrated with high density of coherent twin boundaries using a new room temperature process based on localized PED (L-PED). The 3D printed nt-Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps. The L-PED process enables direct 3D printing of layer-by-layer and complex 3D microscale nt-Cu structures, which may find applications for fabrication of metamaterials, sensors, plasmonics, and micro/nanoelectromechanical systems., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

This study was performed to investigate the changes over time in polypropylene (PP) mesh explants from women with stress urinary incontinence originally treated with a midurethral PP sling. Following Institutional Review Board (IRB) approval, 10 PP explants removed for pain or obstructive symptoms between January and June 2016 were analyzed through various techniques to determine the degradation of the material in vivo. Exclusion criteria were exposed or infected mesh sling or sling in place for less than six months. One pristine control was studied for comparison. The explant samples were analyzed with scanning electron microscopy to visualize the surface defects as well as infrared spectroscopy and energy dispersive X-ray spectroscopy to determine if the degradation was oxidative in nature. The results show qualitative and quantitative bioerosion over the surface of the explant samples and an increase in the content of oxygen pointing toward oxidative degradation occurring in vivo.

Smart textiles are envisioned to make a paradigm shift in wearable technologies to directly impart functionality into the fibers rather than integrating sensors and electronics onto conformal substrates or skin in wearable devices. Among smart materials, piezoelectric fabrics have not been widely reported, yet. Piezoelectric smart fabrics can be used for mechanical energy harvesting, for thermal energy harvesting through the pyroelectric effect, for ferroelectric applications, as pressure and force sensors, for motion detection, and for ultrasonic sensing. We report on mechanical and material properties of the plied nanofibrous piezoelectric yarns as a function of postprocessing conditions including thermal annealing and drawing (stretching). In addition, we used a continuous electrospinning setup to directly produce P(VDF-TrFE) nanofibers and convert them into twisted plied yarns, and demonstrated application of these plied yarns in woven piezoelectric fabrics. The results of this work can be an early step toward realization of piezoelectric smart fabrics.

It is well-known that nucleation and growth of the mineral phase in bone are intimately linked to the interaction between the apatite phase and the collagen matrix at the molecular scale. The exact mechanism of this interaction, however, is not clear due to the challenges involved in experimental characterization at the small size-scale. Herein, we employed molecular dynamics (MD) simulations to investigate the early state of nucleation (i.e. clustering) and growth of apatite clusters on a super-twisted collagen microfibril under mechanical tension in an aqueous solution. The results reveal that mechanical tension (force) facilitates the clustering and growth of the mineral phase on collagen. These results contribute to the understanding of hydroxyapatite (HAP)-collagen interaction and bone biomechanics at the microfibril level.

Light-emitting electrochemical cells (LEECs) utilizing small molecule emitters such as iridium complexes have great potential as low-cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single layer devices. Recent work has shown that the luminance, efficiency, and responsiveness of iridium-based LEECs are greatly enhanced by the inclusion of small amounts of lithium salts (≤0.5%/wt) into the active layer. However, the origin of this enhancement has yet to be demonstrated experimentally. Furthermore, although iridium-based devices have been the longstanding leader among small molecule LEECs, fundamental understanding of the ionic distribution in these devices under operation is lacking. Herein, we use scanning Kelvin probe microscopy to measure the in situ potential profiles and electric field distributions of planar iridium-based LEECs and clarify the role of ionic lithium additives. In pristine devices, it is found that ions do not pack densely at the cathode, and ionic redistribution is slow. Inclusion of small amounts of Li[PF6] greatly increases ionic space charge near the cathode that doubles the peak electric fields and enhances electronic injection relative to pristine devices. This study confirms and clarifies a number of longstanding hypotheses regarding iridium LEECs and recent postulates concerning optimization of their operation.

A scalable and catalyst-free method to deposit stoichiometric molybdenum disulfide (MoS2) films over large areas is reported, with the maximum area limited by the size of the substrate holder. The method allows deposition of MoS2 layers on a wide range of substrates without any additional surface preparation, including single-crystal (sapphire and quartz), polycrystalline (HfO2), and amorphous (SiO2) substrates. The films are deposited using carefully designed MoS2 targets fabricated with excess sulfur and variable MoS2 and sulfur particle size. Uniform and layered MoS2 films as thin as two monolayers, with an electrical resistivity of 1.54 × 10(4) Ω cm(-1), were achieved. The MoS2 stoichiometry was confirmed by high-resolution Rutherford backscattering spectrometry. With the method reported here, in situ graded MoS2 films ranging from ∼1 to 10 monolayers can be deposited.

It has been known for decades that bone exhibits piezoelectric behavior. In recent years, it was directly proved that this effect stems from a polymeric matrix in bone, i.e., collagen fibrils. This effect in collagen is distinctly different from organic piezoelectric crystals, given the semicrystalline molecular structure of the collagen biopolymer. As such, the molecular mechanism of this electromechanical coupling effect in a realistic "super-twisted" model of collagen has been elusive. Herein, we present an investigation on the molecular mechanism of piezoelectric effect in collagen using full atomistic simulation based on the experimentally verified "super-twisted" microstructure of collagen. Our results reveal that collagen exhibits a uniaxial polarization along the long axis of the collagen fibril. In addition, the piezoelectric effect in collagen originates at the collagen molecule level and is due to the mechanical stress-induced reorientation and magnitude change of the permanent dipoles of individual charged and polar residues. A piezoelectric constant in the range of 1-2 pm/V (pC/N) is obtained from the simulation, which agrees well with the experimental data.

High performance piezoelectric devices based on arrays of PVDF-TrFE nanofibers have been introduced in the literature for a variety of applications including energy harvesting and sensing. In this Research Article, we utilize uniaxial tensile test on arrays of nanofibers, microtensile, and nanoindentation and piezo-response force microscopy (PFM) on individual nanofibers, as wells as DSC, XRD, and FTIR spectroscopy to investigate the effect of annealing on microstructure, mechanical, and piezoelectric properties of arrays and individual electrospun nanofibers. For PVDF-TrFE nanofibers annealing in a temperature between the Curie and melting temperature (in paraelectric phase) results in ∼70% increase in crystallinity of the nanofibers. The findings of our multiscale experiments reveal that this improvement in crystallinity results in ∼3-fold increase in elastic modulus, and ∼55% improvement in piezoelectric constant. Meanwhile, the ductility and tensile toughness of the nanofibers drop by ∼1 order of magnitude. In addition, nanoscale cracks were observed on the surface of the annealed nanofibers; however, they did not result in significant change in the strength of the nanofibers. The results of this work may have important implications for applications of PVDF-TrFE in energy harvesting, biomedical, and sensor areas.

Nanomechanics of individual collagen fibrils govern the mechanical behavior of the majority of connective tissues, yet the current models lack significant details. Majority of the current models assume a rod-shape molecule with homogenous mechanical properties. Recent X-ray crystallography revealed significantly different microstructures in the D-period of collagen microfibrils, markedly different from the conventionally assumed rod-shaped molecule. Motivated by this recent microstructure, the nanomechanics of hydrated collagen molecules are investigated through molecular dynamics simulations. The results reveal significant mechanical heterogeneity in individual collagen molecules, which is expected to significantly impact the biomechanics of collagen fibrils in healthy and diseased tissues.

Atomic force microscopy (AFM), as an indispensable tool for nanoscale characterization, presents major drawbacks for operation in a liquid environment arising from the large hydrodynamic drag on the vibrating cantilever. The newly introduced 'Trolling mode' (TR-mode) AFM resolves this complication by using a specialized nanoneedle cantilever that keeps the cantilever outside of the liquid. Herein, a mechanical model with a lumped mass was developed to capture the dynamics of such a cantilever with a nanoneedle tip. This new developed model was applied to investigate the effects of the needle-liquid interface on the performance of the AFM, including the imaging capability in liquid.

We report on highly stretchable piezoelectric structures of electrospun PVDF-TrFE nanofibers. We fabricated nanofibrous PVDF-TrFE yarns via twisting their electrospun ribbons. Our results show that the twisting process not only increases the failure strain but also increases overall strength and toughness. The nanofibrous yarns achieved a remarkable energy to failure of up to 98 J/g. Through overtwisting process, we fabricated polymeric coils out of twisted yarns that stretched up to ∼740% strain. This enhancement in mechanical properties is likely induced by increased interactions between nanofibers, contributed by friction and van der Waals interactions, as well as favorable surface charge (Columbic) interactions as a result of piezoelectric effect, for which we present a theoretical model. The fabricated yarns and coils show great promise for applications in high-performance lightweight structural materials and superstretchable piezoelectric devices and flexible energy harvesting applications.

We report on the synthesis and the mechanical characterization of an alginate-collagen fibril composite hydrogel. Native type I collagen fibrils were used to synthesize the fibrous composite hydrogel. We characterized the mechanical properties of the fabricated fibrous hydrogel using tensile testing; rheometry and atomic force microscope (AFM)-based nanoindentation experiments. The results show that addition of type I collagen fibrils improves the rheological and indentation properties of the hydrogel.

This brief report describes a novel tool for microfluidic patterning of biomolecules and delivery of molecules into cells. The microdevice is based on integration of nanofountain probe (NFP) chips with packaging that creates a closed system and enables operation in liquid. The packaged NFP can be easily coupled to a micro/nano manipulator or atomic force microscope for precise position and force control. We demonstrate here the functionality of the device for continuous direct-write parallel patterning on a surface in air and in liquid. Because of the small volume of the probes (~3 pL), we can achieve flow rates as low as 1 fL/s and have dispensed liquid drops with submicron to 10 µm diameters in a liquid environment. Furthermore, we demonstrate that this microdevice can be used for delivery of molecules into single cells by transient permeabilization of the cell membrane (i.e., electroporation). The significant advantage of NFP-based electroporation compared with bulk electroporation and other transfection techniques is that it allows for precise and targeted delivery while minimizing stress to the cell. We discuss the ongoing development of the tool toward automated operation and its potential as a multifunctional device for microarray applications and time-dependent single-cell studies.

The ability to precisely deliver molecules into single cells is of great interest to biotechnology researchers for advancing applications in therapeutics, diagnostics, and drug delivery toward the promise of personalized medicine. The use of bulk electroporation techniques for cell transfection has increased significantly in the past decade, but the technique is nonspecific and requires high voltage, resulting in variable efficiency and low cell viability. We have developed a new tool for electroporation using nanofountain probe (NFP) technology, which can deliver molecules into cells in a manner that is highly efficient and gentler to cells than bulk electroporation or microinjection. Here we demonstrate NFP electroporation (NFP-E) of single HeLa cells within a population by transfecting them with fluorescently labeled dextran and imaging the cells to evaluate the transfection efficiency and cell viability. Our theoretical analysis of the mechanism of NFP-E reveals that application of the voltage creates a localized electric field between the NFP cantilever tip and the region of the cell membrane in contact with the tip. Therefore, NFP-E can deliver molecules to a target cell with minimal effect of the electric potential on the cell. Our experiments on HeLa cells confirm that NFP-E offers single cell selectivity, high transfection efficiency (>95%), qualitative dosage control, and very high viability (92%) of transfected cells.

Piezoelectric nanowires are promising building blocks in nanoelectronic, sensing, actuation and nanogenerator systems. In spite of great progress in synthesis methods, quantitative mechanical and electromechanical characterization of these nanostructures is still limited. In this article, the state-of-the art in experimental and computational studies of mechanical and electromechanical properties of piezoelectric nanowires is reviewed with an emphasis on size effects. The review covers existing characterization and analysis methods and summarizes data reported in the literature. It also provides an assessment of research needs and opportunities. Throughout the discussion, the importance of coupling experimental and computational studies is highlighted. This is crucial for obtaining unambiguous size effects of nanowire properties, which truly reflect the effect of scaling rather than a particular synthesis route. We show that such a combined approach is critical to establish synthesis-structure-property relations that will pave the way for optimal usage of piezoelectric nanowires., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Atomic force microscope (AFM) probe with a long and rigid needle tip was fabricated and studied for high Q factor dynamic (tapping mode) AFM imaging of samples submersed in liquid. The extended needle tip over a regular commercially available tapping-mode AFM cantilever was sufficiently long to keep the AFM cantilever from submersed in liquid, which significantly minimized the hydrodynamic damping involved in dynamic AFM imaging of samples in liquid. Dynamic AFM imaging of samples in liquid at an intrinsic Q factor of over 100 and an operational frequency of over 200 kHz was demonstrated. The method has the potential to be extended to acquire viscoelastic material properties and provide truly gentle imaging of soft biological samples in physiological environments.

Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d(33), d(13), and d(15). Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. Based on finite element modeling, this finding has major implication on the design of energy harvesting systems exhibiting unprecedented levels of power density production. The presented method is applicable to other piezoelectric NW materials as well as wires manufactured along different crystallographic orientations., (© 2011 American Chemical Society)

The microstructure of type I collagen, consisting of alternating gap and overlap regions with a characteristic D period of approximately 67 nm, enables multifunctionalities of collagen fibrils in different tissues. Implementing near-surface dynamic and static nanoindentation techniques with atomic force microscope, we reveal mechanical heterogeneity along the axial direction of a single isolated collagen fibril from tendon and show that, within the D period, the gap and overlap regions have significantly different elastic and energy dissipation properties, correlating the significantly different molecular structures in these two regions. We further show that such subfibrillar heterogeneity holds in collagen fibrils inside bone and might be intrinsically related to the excellent energy dissipation performance of bone.

Understanding piezoelectricity, the linear electromechanical transduction, in bone and tendon and its potential role in mechanoelectric transduction leading to their growth and remodeling remains a challenging subject. With high-resolution piezoresponse force microscopy, we probed piezoelectric behavior in relevant biological samples at different scale levels: from the subfibrillar structures of single isolated collagen fibrils to bone. We revealed that, beyond the general understanding of collagen fibril being a piezoelectric material, there existed an intrinsic piezoelectric heterogeneity within a collagen fibril coinciding with the periodic variation of its gap and overlap regions. This piezoelectric heterogeneity persisted even for the collagen fibrils embedded in bone, bringing about new implications for its possible roles in structural formation and remodeling of bone.

Piezoresponse force microscopy was applied to directly study individual type I collagen fibrils with diameters of approximately 100 nm isolated from bovine Achilles tendon. It was revealed that single collagen fibrils behave predominantly as shear piezoelectric materials with a piezoelectric coefficient on the order of 1 pm V(-1), and have unipolar axial polarization throughout their entire length. It was estimated that, under reasonable shear load conditions, the fibrils were capable of generating an electric potential up to tens of millivolts. The result substantiates the nanoscale origin of piezoelectricity in bone and tendons, and implies also the potential importance of the shear load-transfer mechanism, which has been the principle basis of the nanoscale mechanics model of collagen, in mechanoelectric transduction in bone.

An improved approach is introduced in damping measurement with atomic force microscope (AFM) for the in situ measurement of the offset phase shift needed for determining the intrinsic mechanical damping in nanoscale materials. The offset phase shift is defined and measured at a point of zero contact force according to the deflection part of the AFM force plot. It is shown that such defined offset phase shift is independent of the type of sample material, varied from hard to relatively soft materials in this study. This improved approach allows the self-calibrated and quantitative damping measurement with AFM. The ability of dynamic mechanical analysis for the measurement of damping in isolated one-dimensional nanostructures, e.g. individual multiwalled carbon nanotubes, was demonstrated.