Elastic-plastic properties of mesoscale electrodeposited LIGA nickel alloy films: microscopy and mechanics.

The elastic-plastic properties of mesoscale electrodeposited LIGA Ni alloy specimens are investigated as a function of specimen size, strain rate, and material composition. Two material compositions are studied: a high-strength fine-grained Ni-Fe alloy and a high-ductility coarse-grained Ni-Co alloy. The specimens have thicknesses of approximately 200 µm and gauge widths ranging from 75 µm to 700 µm. Tensile tests are conducted at strain rates of 0.001/s and 1/s using tabletop loading apparatuses and digital image correlation (DIC) for strain measurement. For each test condition, the apparent Young's modulus, yield strength, ultimate tensile strength, and strain hardening exponent and strength coefficient are extracted from the tensile tests. The true strains to failure are also assessed from fractography. Size, rate, and composition effects are discussed. For most properties, the statistical scatter represented by the standard deviation exceeds the measurement uncertainty; the notable exceptions to these observations are the apparent Young's modulus and yield strength, where large measurement uncertainties are ascribed to common experimental factors and material microplasticity.

Photo-tunable hydrogel mechanical heterogeneity informed by predictive transport kinetics model.

Understanding the three-dimensional (3D) mechanical and chemical properties of distinctly different, adjacent biological tissues is crucial to mimicking their complex properties with materials. 3D printing is a technique often employed to spatially control the distribution of the biomaterials, such as hydrogels, of interest, but it is difficult to print both mechanically robust (high modulus and toughness) and biocompatible (low modulus) hydrogels in a single structure. Moreover, due to the fast diffusion of mobile species during printing and nonequilibrium swelling conditions of low-solids-content hydrogels, it is challenging to form the high-fidelity structures required to mimic tissues. Here a predictive transport and swelling model is presented to model these effects and then is used to compensate for these effects during printing. This model is validated experimentally by photopatterning spatially distinct hydrogel elastic moduli using a single photo-tunable poly(ethylene glycol) (PEG) pre-polymer solution by sequentially patterning and in-diffusing fresh pre-polymer for further polymerization.

Dominant factors for fracture at the micro-scale in electrodeposited nickel alloys.

Two different LIGA electrodeposited nickel alloys displayed distinct fracture modes after meso-scale tensile testing. The Ni-Co alloy failed in a ductile manner, while the Ni-Fe alloy failed in a more brittle-appearing manner. Various factors affecting the fracture are discussed; it was determined that the fracture mode did not depend upon the strain rate but did depend upon the sample geometry. The difference in the microstructure is likely the cause of the difference in fracture mode, as the Ni-Co alloy is fine-grained, while the Ni-Fe alloy is nano-grained and likely failed by a creep-like mechanism.

US-Czech conference strengthens bilateral and multidisciplinary collaborations in nanotechnology and chemistry.

There are several methods to encourage bilateral and multidisciplinary collaborations-the most oft-used and effective include technical conferences, workshops, and seminars with the optimal combination of people, agenda, and length. However, this balance is difficult to attain, especially for meetings across academic disciplines and geographic borders. For nearly two decades, the US National Academies have developed a series of bilateral meetings to bring together scientists in different countries, which appear to have addressed such challenges via concise meetings with select emerging leaders from disparate fields and sectors. The authors utilized a similar approach to promote US and Czech collaborations in nanotechnology, given their mutual investments in the area and interests in building relationships. The bilateral meeting was held in Prague and designed to focus on nanotechnology and chemistry. The goals were four-fold: (1) evaluate US and Czech capabilities and challenges in nanotechnology and chemistry, (2) identify likely research projects and funding sources for collaborations, (3) celebrate 100 years of US and Czech diplomatic relations, and (4) increase government and public support for bilateral scientific research. The two-day meeting was named the US-Czech Conference on Advanced Nanotechnology and Chemistry. A summary of the research presentations, funding opportunities, panel discussions, and potential joint projects are given herein. It is our hope that this perspective will facilitate similar bilateral meetings in other fields to promote more multidisciplinary research.

Layer-by-layer thinning of MoS2 via laser irradiation.

Layer-by-layer thinning of molybdenum disulfide (MoS2) via laser irradiation was examined using Raman spectroscopy and atomic force microscopy. In particular, the effects of number of layers, laser conditions, and substrate were systematically identified. The results demonstrated the presence of nanoparticles on the MoS2 at sufficient laser treatment conditions prior to layer-by-layer thinning. The volume of nanoparticles was found to increase and then decrease as the number of MoS2 layers increased; the non-monotonic trend was ascribed to changes in the thermal conductivity of the film and interfacial thermal conductance between the film and substrate with number of layers. Moreover, the volume of nanoparticles was found to increase as the magnification of the objective lens decreased and as laser power and exposure time increased, which was attributed to changes in the power density with laser conditions. The effect of substrate on nanoparticle formation and layer-by-layer thinning was investigated through a comparison of freestanding and substrate-supported MoS2 subjected to laser irradiation; it was illustrated that freestanding films were thinned at lower laser powers than substrate-supported films, which highlighted the function of the substrate as a heat sink. For conditions that elicited thinning, it was shown that the thinned areas exhibited triangular shapes, which suggested anisotropic etching behavior where the lattice of the basal plane was preferentially thinned along the zigzag direction terminated by an Mo- or S-edge. High-resolution transmission electron microscopy of freestanding MoS2 revealed the presence of a 2 nm thick amorphous region around the laser-treated region, which suggested that the crystalline structure of laser-treated MoS2 remained largely intact after the thinning process. In all, the conclusions from this work provide useful insight into the progression of laser thinning of MoS2, thereby enabling more effective methods for the development of MoS2 devices via laser irradiation.

Spatially patterned matrix elasticity directs stem cell fate.

There is a growing appreciation for the functional role of matrix mechanics in regulating stem cell self-renewal and differentiation processes. However, it is largely unknown how subcellular, spatial mechanical variations in the local extracellular environment mediate intracellular signal transduction and direct cell fate. Here, the effect of spatial distribution, magnitude, and organization of subcellular matrix mechanical properties on human mesenchymal stem cell (hMSCs) function was investigated. Exploiting a photodegradation reaction, a hydrogel cell culture substrate was fabricated with regions of spatially varied and distinct mechanical properties, which were subsequently mapped and quantified by atomic force microscopy (AFM). The variations in the underlying matrix mechanics were found to regulate cellular adhesion and transcriptional events. Highly spread, elongated morphologies and higher Yes-associated protein (YAP) activation were observed in hMSCs seeded on hydrogels with higher concentrations of stiff regions in a dose-dependent manner. However, when the spatial organization of the mechanically stiff regions was altered from a regular to randomized pattern, lower levels of YAP activation with smaller and more rounded cell morphologies were induced in hMSCs. We infer from these results that irregular, disorganized variations in matrix mechanics, compared with regular patterns, appear to disrupt actin organization, and lead to different cell fates; this was verified by observations of lower alkaline phosphatase (ALP) activity and higher expression of CD105, a stem cell marker, in hMSCs in random versus regular patterns of mechanical properties. Collectively, this material platform has allowed innovative experiments to elucidate a novel spatial mechanical dosing mechanism that correlates to both the magnitude and organization of spatial stiffness.

Operational and environmental conditions regulate the frictional behavior of two-dimensional materials.

The friction characteristics of single-layer h-BN, MoS2, and graphene were systematically investigated via friction force microscopy measurements at various operational (e.g., normal force and sliding speed) and environmental (e.g., relative humidity and thermal annealing) conditions. The low friction characteristics of these single-layer materials were clearly observed from the normal force-dependent friction results, and their interfacial shear strengths were further estimated using a Hertz-plus-offset model. In addition, speed-dependent friction characteristics clearly demonstrated two regimes of friction as a function of sliding speed - the first is the logarithmic increase in friction with sliding speed regime at sliding speeds smaller than the critical speed and the second is the friction plateau regime at sliding speeds greater than the critical speed. Fundamental parameters such as effective shape of the interaction potential and its corrugation amplitude for these single-layer materials were characterized using the thermally-activated Prandtl-Tomlinson model. Moreover, friction of single-layer h-BN, MoS2, and graphene was found to increase with relative humidity and decrease with thermal annealing; these trends were attributed to the diffusion of water molecules to the interface between the single-layer materials and their substrates, which leads to an increase in the puckering effect at the tip-material interface and interaction potential corrugation. The enhanced puckering effect was verified via molecular dynamics simulations. Overall, the findings enable a comprehensive understanding of friction characteristics for several classes of two-dimensional materials, which is important to elucidate the feasibility of using these materials as protective and solid-lubricant coating layers for nanoscale devices.

Determination of ceramic flaw populations from component strengths.

A procedure is outlined for determining the population of flaws in manufactured ceramics from strength measurements of sampled components. The broad applicability of the procedure is demonstrated in a quantitative manner, using strength measurements from a range of ceramic materials (eg, glass, glass-ceramic, single crystal, and polycrystal) with different flaw types (eg, bulk, surface, and edge). The deconvoluted flaw populations are mostly dominated by small flaws with extended large flaw tails and are all in domains of tens of micrometers. The procedure greatly extends the useful information to be gained by ceramics manufacturers and designers from strength distribution measurements and emphasizes the importance of identifying strength-limiting characteristics within a flaw population.

Electronic and Morphological Inhomogeneities in Pristine and Deteriorated Perovskite Photovoltaic Films.

We perform scanning microwave microscopy (SMM) to study the spatially varying electronic properties and related morphology of pristine and degraded methylammonium lead-halide (MAPI) perovskite films fabricated under different ambient humidity. We find that higher processing humidity leads to the emergence of increased conductivity at the grain boundaries but also correlates with the appearance of resistive grains that contain PbI2. Deteriorated films show larger and increasingly insulating grain boundaries as well as spatially localized regions of reduced conductivity within grains. These results suggest that while humidity during film fabrication primarily benefits device properties due to the passivation of traps at the grain boundaries and self-doping, it also results in the emergence of PbI2-containing grains. We further establish that MAPI film deterioration under ambient conditions proceeds via the spatially localized breakdown of film conductivity, both at grain boundaries and within grains, due to local variations in susceptibility to deterioration. These results confirm that PbI2 has both beneficial and adverse effects on device performance and provide new means for device optimization by revealing spatial variations in sample conductivity as well as morphological differences in resistance to sample deterioration.

Hydrogels with Reversible Mechanics to Probe Dynamic Cell Microenvironments.

The relationship between ECM mechanics and cell behavior is dynamic, as cells remodel and respond to changes in their local environment. Most in vitro substrates are static and supraphysiologically stiff; thus, platforms with dynamic and reversible mechanical changes are needed. Herein, we developed hyaluronic acid-based substrates capable of sequential photodegradation and photoinitiated crosslinking reactions to soften and then stiffen the hydrogels over a physiologically relevant range of moduli. Reversible mechanical signaling to adhered cells was demonstrated with human mesenchymal stem cells. In situ hydrogel softening (from ca. 14 to 3.5 kPa) led to a decrease in the cell area and nuclear localization of YAP/TAZ, and subsequent stiffening (from ca. 3.5 to 28 kPa) increased the cell area and nuclear localization of YAP/TAZ. Each photoreaction was cytocompatible and tunable, rendering this platform amenable to studies of dynamic mechanics on cell behavior across many cell types and contexts.

Ullmann-like reactions for the synthesis of complex two-dimensional materials.

Engineering two-dimensional materials through surface-confined synthetic techniques is a promising avenue for designing new materials with tailored properties. Developing and understanding reaction mechanisms for surface-confined synthesis of two-dimensional materials requires atomic-level characterization and chemical analysis. Beggan et al (2015 Nanotechnology 26 365602) used scanning tunneling microscopy and x-ray photoelectron spectroscopy to elucidate the formation mechanism of surface-confined Ullmann-like coupling of thiophene substituted porphyrins on Ag(111). Upon surface deposition, bromine is dissociated and the porphyrins couple with surface adatoms to create linear strands and hexagonally packed molecules. Annealing the sample results in covalently-bonded networks of thienylporphyrin derivatives. A deeper understanding of surface-confined Ullmann-like coupling has the potential to lead to precision-engineered nano-structures through synthetic techniques.

Near-theoretical fracture strengths in native and oxidized silicon nanowires.

In this letter, fracture strengths σ f of native and oxidized silicon nanowires (SiNWs) were determined via atomic force microscopy bending experiments and nonlinear finite element analysis. In the native SiNWs, σ f in the Si was comparable to the theoretical strength of Si〈111〉, ≈22 GPa. In the oxidized SiNWs, σ f in the SiO2 was comparable to the theoretical strength of SiO2, ≈6 to 12 GPa. The results indicate a change in the failure mechanism between native SiNWs, in which fracture originated via inter-atomic bond breaking or atomic-scale defects in the Si, and oxidized SiNWs, in which fracture initiated from surface roughness or nano-scale defects in the SiO2.

Green's function modeling of response of two-dimensional materials to point probes for scanning probe microscopy.

A Green's function (GF) method is developed for interpreting scanning probe microscopy (SPM) measurements on new two-dimensional (2D) materials. GFs for the Laplace/Poisson equations are calculated by using a virtual source method for two separate cases of a finite material containing a rectangular defect and a hexagonal defect. The prescribed boundary values are reproduced almost exactly by the calculated GFs. It is suggested that the GF is not just a mathematical artefact but a basic physical characteristic of material systems, which can be measured directly by SPM for 2D solids. This should make SPM an even more powerful technique for characterization of 2D materials.

Mechanical measurements of heterogeneity and length scale effects in PEG-based hydrogels.

Colloidal-probe spherical indentation load-relaxation experiments with a probe radius of 3 µm are conducted on poly(ethylene glycol) (PEG) hydrogel materials to quantify their steady-state mechanical properties and time-dependent transport properties via a single experiment. PEG-based hydrogels are shown to be heterogeneous in both morphology and mechanical stiffness at this scale; a linear-harmonic interpolation of hyperelastic Mooney-Rivlin and Boussinesq flat-punch indentation models was used to describe the steady-state response of the hydrogels and determine upper and lower bounds for indentation moduli. Analysis of the transient load-relaxation response during displacement-controlled hold periods provides a means of extracting two time constants τ1 and τ2, where τ1 and τ2 are assigned to the viscoelastic and poroelastic properties, respectively. Large τ2 values at small indentation depths provide evidence of a non-equilibrium state characterized by a phenomenon that restricts poroelastic fluid flow through the material; for larger indentations, the variability in τ2 values decreases and pore sizes estimated from τ2via indentation approach those measured via macroscopic swelling experiments. The contact probe methodology developed here provides a means of assessing hydrogel heterogeneity, including time-dependent mechanical and transport properties, and has potential implications in hydrogel biomedical and engineering applications.

Orthogonal analysis of functional gold nanoparticles for biomedical applications.

We report a comprehensive strategy based on implementation of orthogonal measurement techniques to provide critical and verifiable material characteristics for functionalized gold nanoparticles (AuNPs) used in biomedical applications. Samples were analyzed before and after ≈50 months of cold storage (≈4 °C). Biomedical applications require long-term storage at cold temperatures, which could have an impact on AuNP therapeutics. Thiolated polyethylene glycol (SH-PEG)-conjugated AuNPs with different terminal groups (methyl-, carboxylic-, and amine-) were chosen as a model system due to their high relevancy in biomedical applications. Electrospray-differential mobility analysis, asymmetric-flow field flow fractionation, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, inductively coupled plasma mass spectrometry, and small-angle X-ray scattering were employed to provide both complementary and orthogonal information on (1) particle size and size distribution, (2) particle concentrations, (3) molecular conjugation properties (i.e., conformation and surface packing density), and (4) colloidal stability. Results show that SH-PEGs were conjugated on the surface of AuNPs to form a brush-like polymer corona. The surface packing density of SH-PEG was ≈0.42 nm(-2) for the methyl-PEG-SH AuNPs, ≈0.26 nm(-2) for the amine-SH-PEG AuNPs, and ≈0.18 nm(-2) for the carboxylic-PEG-SH AuNPs before cold storage, approximately 10 % of its theoretical maximum value. The conformation of surface-bound SH-PEGs was then estimated to be in an intermediate state between brush-like and random-coiled, based on the measured thicknesses in liquid and in dry states. By analyzing the change in particle size distribution and number concentration in suspension following cold storage, the long term colloidal stability of AuNPs was shown to be significantly improved via functionalization with SH-PEG, especially in the case of methyl-PEG-SH and carboxylic-PEG-SH (i.e., we estimate that >80 % of SH-PEG5K remained on the surface of AuNPs during storage). The work described here provides a generic strategy to track and analyze the material properties of functional AuNPs intended for biomedical applications, and highlights the importance of a multi-technique analysis. The effects of long term storage on the physical state of the particles, and on the stability of the ligand-AuNP conjugates, are employed to demonstrate the capacity of this approach to address critical issues relevant to clinical applications.

Controlled formation and characterization of dithiothreitol-conjugated gold nanoparticle clusters.

We report a systematic study of the controlled formation of discrete-sized gold nanoparticle clusters (GNCs) by interaction with the reducing agent dithiothreitol (DTT). Asymmetric-flow field flow fractionation and electrospray differential mobility analysis were employed complementarily to determine the particle size distributions of DTT-conjugated GNCs (DTT-GNCs). Transmission electron microscopy was used to provide visualization of DTT-GNCs at different states of aggregation. Surface packing density of DTT and the corresponding molecular conformation on the Au surface were characterized by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. Results show that DTT increases the aggregation rate of gold nanoparticles (AuNPs) up to ≈100 times. A mixed conformation (i.e., combining vertically aligned, horizontally aligned, and cross-linking modes) exists for DTT on the Au surface for all conditions examined. The primary size of AuNPs, concentration of DTT, and the starting concentration of AuNPs influence the degree of aggregation for DTT-GNCs, indicating that the collision frequency, energy barrier, and surface density of DTT are the key factors that control the aggregation rate. DTT-GNCs exhibit improved structural stability compared to the citrate-stabilized GNCs (i.e., unconjugated) following reaction with thiolated polyethylene glycol (SH-PEG), indicating that cross-linking and surface protection by DTT suppresses disaggregation normally induced by the steric repulsion of SH-PEG. This work describes a prototype methodology to form ligand-conjugated GNCs with high-quality and well-controlled material properties.

Probing the Mechanical Properties of 2D Materials via Atomic-Force-Microscopy-Based Modulated Nanoindentation.

As the field of low-dimensional materials (1D or 2D) grows and more complex and intriguing structures are continuing to be found, there is an emerging need for techniques to characterize the nanoscale mechanical properties of all kinds of 1D/2D materials, in particular in their most practical state: sitting on an underlying substrate. While traditional nanoindentation techniques cannot accurately determine the transverse Young's modulus at the necessary scale without large indentations depths and effects to and from the substrate, herein an atomic-force-microscopy-based modulated nanomechanical measurement technique with Angstrom-level resolution (MoNI/ÅI) is presented. This technique enables non-destructive measurements of the out-of-plane elasticity of ultra-thin materials with resolution sufficient to eliminate any contributions from the substrate. This method is used to elucidate the multi-layer stiffness dependence of graphene deposited via chemical vapor deposition and discover a peak transverse modulus in two-layer graphene. While MoNI/ÅI has been used toward great findings in the recent past, here all aspects of the implementation of the technique as well as the unique challenges in performing measurements at such small resolutions are encompassed.

Stiffness anisotropy coordinates supracellular contractility driving long-range myotube-ECM alignment.

The ability of cells to organize into tissues with proper structure and function requires the effective coordination of proliferation, migration, polarization, and differentiation across length scales. Skeletal muscle is innately anisotropic; however, few biomaterials can emulate mechanical anisotropy to determine its influence on tissue patterning without introducing confounding topography. Here, we demonstrate that substrate stiffness anisotropy coordinates contractility-driven collective cellular dynamics resulting in C2C12 myotube alignment over millimeter-scale distances. When cultured on mechanically anisotropic liquid crystalline polymer networks (LCNs) lacking topography, C2C12 myoblasts collectively polarize in the stiffest direction. Cellular coordination is amplified through reciprocal cell-ECM dynamics that emerge during fusion, driving global myotube-ECM ordering. Conversely, myotube alignment was restricted to small local domains with no directional preference on mechanically isotropic LCNs of the same chemical formulation. These findings provide valuable insights for designing biomaterials that mimic anisotropic microenvironments and underscore the importance of stiffness anisotropy in orchestrating tissue morphogenesis.

Cell spreading and proliferation in response to the composition and mechanics of engineered fibrillar extracellular matrices.

The extracellular matrix (ECM) consists of a complex mixture of biochemical and physical stimuli that together regulate cell behavior. In this study, we engineer a model ECM consisting of fibrillar Type-1 collagen plus fibronectin that allows systematic examination of the effects of matrix composition and mechanics on cells. On this combined protein matrix, cells exhibit intermediate degrees of spreading and proliferation compared to their responses on collagen or fibronectin alone. Adhesion to the combination matrix could be blocked by peptides containing the sequence arginine-glycine-aspartic acid (RGD) and by antibodies against α1 integrin, suggesting cell-matrix engagement was mediated by a combination of integrin receptors that recognize fibronectin and collagen. Regardless of integrin engagement, cells were sensitive to the mechanical properties of the combination ECM, suggesting that cells could process biochemical and mechanical cues simultaneously and independently.

Temperature-programmed electrospray-differential mobility analysis for characterization of ligated nanoparticles in complex media.

An electrospray-differential mobility analyzer (ES-DMA) was operated with an aerosol flow-mode, temperature-programmed approach to enhance its ability to characterize the particle size distributions (PSDs) of nanoscale particles (NPs) in the presence of adsorbed and free ligands. Titanium dioxide NPs (TiO2-NPs) stabilized by citric acid (CA) or bovine serum albumin (BSA) were utilized as representative systems. Transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry were used to provide visual information and elemental-based PSDs, respectively. Results show that the interference resulting from electrospray-dried nonvolatile salt residual nanoscale particles (S-NPs) could be effectively reduced using the thermal treatment process: PSDs were accurately measured at temperatures above 200 °C for CA-stabilized TiO2-NPs and above 400 °C for BSA-stabilized TiO2-NPs. Moreover, TEM confirmed the volumetric shrinkage of S-NPs due to thermal treatment and also showed that the primary structure of TiO2-NPs was relatively stable over the temperature range studied (i.e., below 700 °C). Conversely, the shape factor for TiO2-NPs decreased after treatment above 500 °C, possibly due to a change in the secondary (aggregate) structure. S-NPs from BSA-stabilized TiO2-NPs exhibited higher global activation energies toward induced volumetric shrinkage than those of CA-stabilized TiO2-NPs, suggesting that activation energy is dependent on ligand size. This prototype study demonstrates the efficacy of using ES-DMA coupled with thermal treatment for characterizing the physical state of NPs, even in a complex medium (e.g., containing plasma proteins) and in the presence of particle agglomerates induced by interaction with binding ligands.