Mechanical Strain Distribution Research Articles

Regional distribution of mechanical strain and macrophage-associated lung inflammation after ventilator-induced lung injury: an experimental study

BackgroundAlveolar macrophages activation to the pro-inflammatory phenotype M1 is pivotal in the pathophysiology of Ventilator-Induced Lung Injury (VILI). Increased lung strain is a known determinant of VILI, but a direct correspondence between regional lung strain and macrophagic activation remains unestablished. [68Ga]Ga-DOTA-TATE is a Positron Emission Tomography (PET) radiopharmaceutical with a high affinity for somatostatin receptor subtype 2 (SSTR2), which is overexpressed by pro-inflammatory-activated macrophages. Aim of the study was to determine, in a porcine model of VILI, whether mechanical strain correlates topographically with distribution of activated macrophages detected by [68Ga]Ga-DOTA-TATE uptake.MethodsSeven anesthetized pigs underwent VILI, while three served as control. Lung CT scans were acquired at incremental tidal volumes, simultaneously recording lung mechanics. [68Ga]Ga-DOTA-TATE was administered, followed by dynamic PET scans. Custom MatLab scripts generated voxel-by-voxel gas volume and strain maps from CT slices at para-diaphragmatic (Para-D) and mid-thoracic (Mid-T) levels. Analysis of regional Voxel-associated Normal Strain (VoStrain) and [68Ga]Ga-DOTA-TATE uptake was performed and a measure of the statistical correlation between these two variables was quantified using the linear mutual information (LMI) method.ResultsCompared to controls, the VILI group exhibited statistically significant higher VoStrain and Standardized Uptake Value Ratios (SUVR) both at Para-D and Mid-T levels. Both VoStrain and SUVR increased along the gravitational axis with an increment described by statistically different regression lines between VILI and healthy controls and reaching the peak in the dependent regions of the lung (for strain in VILI vs. control was at Para-D: 760 ± 210 vs. 449 ± 106; at Mid-T level 497 ± 373 vs. 193 ± 160; for SUVR, in VILI vs. control was at Para-D: 2.2 ± 1.3 vs. 1.3 ± 0.1; at Mid-T level 1.3 ± 1.0 vs. 0.6 ± 0.03). LMI in both Para-D and Mid-T was statistically significantly higher in VILI than in controls.ConclusionsIn this porcine model of VILI, we found a topographical correlation between lung strain and [68Ga]Ga-DOTA-TATE uptake at voxel level, suggesting that mechanical alteration and specific activation of inflammatory cells are strongly linked in VILI. This study represents the first voxel-by-voxel examination of this relationship in a multi-modal imaging analysis.

Advanced Mechanical Transfer of Micro-LEDs Enabled by Structurally Modified Wide Sapphire Nanomembranes through Thermal Reflow of Photoresist.

Micro light-emitting diodes (micro-LEDs) are pivotal in next-generation display technologies, driven by the need for high pixel density. This study introduces a novel methodology utilizing wide sapphire nanomembranes (W-SNM) as a dual-purpose template for high-quality epitaxial growth and the mechanical lift-off of individual micro-LEDs. Micro-LEDs grow individually on W-SNM, obviating the chip singulation process. By employing mechanical fracturing of the thin W-SNM, our method facilitates the transfer of micro-LEDs without the conventional laser lift-off (LLO) process. Previously introduced sapphire nanomembranes (SNM) have shown promise in enhancing epitaxial layer quality; however, they encountered challenges in managing micro-LED size variation and achieving efficient mechanical transfer. Here, we apply simple yet effective adjustments to the SNM structure, specifically, its elevation and widening. This strategic modification allows micro-LEDs to endure applied forces without incurring cracks or defects, ensuring that only the targeted W-SNM are selectively fractured. The mechanically transferred vertical 15 × 15 μm2 micro-LED device operates at an optimal turn-on voltage of 3.3 V. Finite element simulations validate the mechanical strain distribution between the W-SNM and GaN when pressure is applied, confirming the efficacy of our design approach. This pioneering methodology offers a streamlined, efficient pathway for the production and mechanical transfer of micro-LEDs, presenting new avenues for their integration into next-generation, high-performance displays.

JEDI: A versatile code for strain analysis of molecular and periodic systems under deformation.

Stretching or compression can induce significant energetic, geometric, and spectroscopic changes in materials. To fully exploit these effects in the design of mechano- or piezo-chromic materials, self-healing polymers, and other mechanoresponsive devices, a detailed knowledge about the distribution of mechanical strain in the material is essential. Within the past decade, Judgement of Energy DIstribution (JEDI) analysis has emerged as a useful tool for this purpose. Based on the harmonic approximation, the strain energy in each bond length, bond angle, and dihedral angle of the deformed system is calculated using quantum chemical methods. This allows the identification of the force-bearing scaffold of the system, leading to an understanding of mechanochemical processes at the most fundamental level. Here, we present a publicly available code that generalizes the JEDI analysis, which has previously only been available for isolated molecules. Now, the code has been extended to two- and three-dimensional periodic systems, supramolecular clusters, and substructures of chemical systems under various types of deformation. Due to the implementation of JEDI into the Atomic Simulation Environment, the JEDI analysis can be interfaced with a plethora of program packages that allow the calculation of electronic energies for molecular systems and systems with periodic boundary conditions. The automated generation of a color-coded three-dimensional structure via the Visual Molecular Dynamics program allows insightful visual analyses of the force-bearing scaffold of the strained system.

Evolutionary Conservation of Mechanical Strain Distributions in Functional Transitions of Protein Structures

One of the tenets of molecular biology is that dynamic transitions between three-dimensional structures determine the function of proteins. Therefore, it seems only natural that evolutionary analysis of proteins, presently based mainly on their primary sequence, needs to shift its focus toward their function as assessed by corresponding structural transitions. This can be facilitated by recent progress in cryogenic electron microscopy that provides atomic structures of multiple conformational states for proteins and protein assemblies isolated from evolutionarily related species. In this work, we study evolutionary conservation of multiprotein assembly function by using mechanical strain as a quantitative footprint of structural transitions. We adopt the formalism of finite strain theory, developed in condensed matter physics, and apply it, as a case study, to a classical multiprotein assembly, the ATP synthase. Protein strain analysis provides a precise characterization of rotation domains that agrees with the present biophysical knowledge. In addition, we obtain a strain distribution on the protein structure associated with functional transitions. By analyzing in detail the strain patterns of the chains responsible for ATP synthesis across distinct species, we show that they are evolutionarily conserved for the same functional transition. Such conservation is not revealed by displacement or rotation patterns. Furthermore, within each functional transition, we can identify conserved strain patterns for ATP synthases isolated from different organisms. The observed strain conservation across evolutionary distant species indicates that strain should be essential in future structure-based evolutionary studies of protein function. Published by the American Physical Society 2024

Mathematical modelling of thermal stresses of induction surface hardening in axi-symmetric formulation

The paper presents the original general model of induction surface hardening based upon analysis of coupled electromagnetic, thermal, mechanical and metallurgical phenomena. The distribution of mechanical strains and stresses in surface layers of steel materials subjected to induction hardening is determined. This distribution is not caused only by the thermoelastic processes, but often, it is also influenced by plastic deformations of the exposed layers and transformation of particular levels of steel. An illustrative example shows the methodology of solution for an axi-symmetric configuration.

Smart monitoring of RC T beams strengthened by external bonded FRP

This study aimed to investigate the cracking behaviour of RC beams strengthened by external smart bonded FRP. A smart CFRP consists of two textile fabrics inside which a distributed fibre-optic sensor (DFOS) is inserted. During the test, the sensor measured the mechanical strain distribution of the composite with a millimetric spatial resolution. Simultaneously crack openings along the beams were detected and measured using digital image correlation (DIC). The results of the strengthened beams were analyzed in comparison to the reference beam, and the crack width are compared. Subsequently a smart CFRP was described and implemented in the tested beams and was used to measure the cracks location. The measurement of strain profile allow then to obtain the level of beam damaged during the loading.

Fatigue crack initiation life prediction using different plasticity models in cold expanded AL-alloy plates utilized in double shear lap joints

Cold expansion of fastener holes is a recognized technique for enhancing the fatigue life of holed plates utilized in detachable joints. This method involves introducing compressive residual stress around the holes, which has proven to be effective in improving the structural integrity and longevity of such components. In this research, the role of using different plasticity models in finite-element simulations of cold expansion (CE) in determining residual stress distribution and predicting the fatigue life of lap joints was investigated. Finite-element simulations were conducted to analyze the distribution of residual stress in cold-expanded Al-alloy 2024-T3 plates utilized in double-shear lap joints. Three time-independent plasticity models, specifically multilinear kinematic hardening, multilinear isotropic hardening (M.L.I.H) based on monotonic tensile tests, and nonlinear combined hardening models derived from saturated hysteresis stress and strain loops, were employed in the simulations. These models allowed for an accurate determination of the residual stress distribution in the plates. In finite-element simulations, two CE sizes of 1.5% and 4.7% were employed to create residual stresses. In the simulations, after creating residual stress by CE, remote sinusoidal loads are applied to the joints corresponding to the previously conducted fatigue tests in order to obtain stress and mechanical strain distributions. In order to predict the fatigue life, four different multiaxial fatigue criteria (Smith–Watson–Topper, Glinka, Kandil–Brown–Miller, and Fatemi–Socie) were employed, using the stress and strain distributions obtained from the finite element simulations with the different plasticity models. The simulations yielded varying stress and strain distribution results for the multilinear kinematic and M.L.I.H models, while the results of the nonlinear combined hardening model fell between the other two models. Notably, the fatigue life prediction based on the nonlinear combined hardening plasticity model closely matched the experimentally observed fatigue lives, with an absolute percentage deviation of 24.8%.

Inverse analysis based on DFOS technology for the study of the bonding behaviour of FRP to concrete beams

This paper proposes a methodology for the inverse analysis to predict the interface shear stress in reinforced concrete (RC) structures strengthened using externally bonded carbon fibre-reinforced (CFRP) polymers. The model is calibrated employing experimental results obtained by testing two beam configurations with two steel ratios under 4-point bending and three specimens in a double-lap shear test. The interface information is obtained by adopting a smart CFRP fabric bonded on the bottom side of the beams. A smart CFRP consists of two texture fabrics inside which a distributed fibre-optic sensor (DFOS) is inserted. During the test, the sensor measured the mechanical strain distribution of the composite with a millimetric spatial resolution. The recorded strains were used to deduce the evolution of bond shear stresses at the CFRP/concrete interface. Simultaneously crack openings along the beams were detected and measured using digital image correlation (DIC).The obtained model from inverse analysis and the experimental results allows calculating the concrete-CFRP interface stress value, using the measured crack width as input data. The scientific interest of this model lies in its ability to monitor the evolution of a key structural performance parameter of CFRP-reinforced concrete beams: the crack induced debonding. This debonding can be predicted by comparing at each point the shear stresses obtained by the proposed model with the maximum debonding stress.

Spatial mapping of a low-frequency combination Raman mode in twisted bilayer graphene

The exotic properties of twisted bilayer graphene (tBLG) are profoundly affected by interlayer interactions, which can be sensitively probed by the low-frequency Raman modes. However, conventional Raman spectroscopy lacks spatial resolution and, hence, can hardly disclose spatial-dependent inhomogeneous properties of interest. Herein, we demonstrate spatial mapping of low-frequency Raman modes in tBLG via micro-Raman spectroscopy (mRS) imaging. In our constructed tBLG, the shear (C) and out-of-plane breathing (ZO′) modes are directly mapped out, and, more importantly, a regional-dependent, highly localized awakening of the combination C+ZO′ mode is clearly visualized. Through a set of mRS-based analyses, including correlation of Raman G- and 2D-mode frequencies, G-band splitting, and polarization-dependent Raman intensity distributions, we reveal that its turning on/off correlates with the mechanical strain distributions in a sensitive fashion. The spatial mapping of this unique combination mode offers an indicator for scrutinizing the elusive strain in the tBLG system or the like.

Enlarged tensile strain at edge of flexible substrate due to anticlastic curvature

Understanding the mechanical strain imposed on flexible electronics during bending is an essential consideration for the robust design of devices. In general, the bending behavior of flexible electronics is analyzed such that the cross-sectional mechanical strain distribution is the same along the width direction. However, even though a single curvature is applied to the cross section, the anticlastic curvature occurs along the width owing to the Poisson's ratio effect along the width. In this study, we analyze the effect of the anticlastic curvature on the maximum tensile strain of a flexible substrate because it is critical to the fracture of brittle films on the substrate. The strain distributions at the center and edge of a bent 75 μm-thick polyethylene terephthalate (PET) substrate are measured using the microscale digital image correlation (DIC) method. The anticlastic curvature effect on the maximum tensile strain of the bent substrate is analyzed through a three-dimensional finite element method (FEM) simulation. Results of DIC analysis and FEM simulation show that the maximum tensile strain at the edge of the substrate is larger than that at the center of the substrate owing to the anticlastic curvature. The enlarged tensile strain at the edge of a flexible substrate is experimentally validated by bending a laser patterned hard-coated PET substrate.

Effects of premature contact in maxillary alveolar bone in rats: relationship between experimental analyses and a micro scale FEA computational simulation study.

The aim of the investigation was to evaluate the maxillary alveolar bone morphology, bone architecture, and bone turnover in relation to the mechanical strain distribution in rats with dental premature contact. Fifty 2-month-old male Wistar rats were used. The premature contact group (N=40) received a unilateral (right side) resin cementation on the occlusal surface of the upper first molar. The animals were distributed in 4 subgroups according to the periods of euthanasia: 7, 14, 21, and 28 days after cementation (N=10, for each period). For the control group (N=10), the teeth were kept without resin, featuring a normal occlusion. The pieces including the upper first molars, alveolar bone, and periodontal tissue were processed to histological and immunohistochemical evaluation of RANK-L and TRAP protein expression. A three-dimensional bone microarchitecture analysis was performed, where the heads of animals were scanned using microtomography and analyzed using CT-Analyser software (Bruker, Kontich, Belgium). In the computer simulation by finite element analysis, two micro-scaled three-dimensional finite element models of first molar and dentoalveolar tissues were constructed, in representation of control and premature contact groups, using Materialise MIMICS Academic Research v18 (Materialise, Leuven, Belgium). The analysis was set to simulate a maxillary molar biting during the power stroke phase. The total deformation, equivalent strain, and minimum principal strain distribution were calculated. The expression of RANK-L and TRAP presented higher positive ratio in the 7-day period compared to the control group. The three-dimensional morphometry showed decrease of bone volume in the premature contact, with significant values between the control and the 7-day and 14-day groups (P = 0.007). In FEA, the premature contact model presented a uniform compressive strain distribution in the alveolar bone crest compared to a non-uniform compressive strain distribution in the control model. The results from FEA, 3D bone microarchitecture, and histological and immunohistochemical analyses showed that a model with dental traumatic occlusion resulted in changes of alveolar bone mechanobiology and, consequently, its morphology. These results could be applied in dental treatment planning bringing biological and mechanical feedback to provide an effective mechanism to obtain physiological bone loss responses. Furthermore, this association between experimental and computational analyses will be important to figure out the alveolar bone response to mechanical stimulation in different clinical conditions.

Effect of load-induced local mechanical strain on peri-implant bone cell activity related to bone resorption and formation in mice: An analysis of histology and strain distributions

The purpose of this study was to investigate the effect of load-induced local mechanical strain on bone cell activity of peri-implant bone in mice. Titanium implants were placed in the maxillae of 13-week-old male C57BL/6J mice and subjected to intermittent 0.15 N, 0.3 N, or 0.6 N loads for 30 min/day for 6 days. The animals were sacrificed 2 days after the final loading. Unloaded mice were used as controls. An animal-specific three-dimensional finite element model was constructed based on morphological data retrieved from in vivo microfocus computed tomography for each mouse to calculate the mechanical strain distribution. Strain distribution images were overlaid on corresponding histological images of the same site in the same animal. The buccal cervical region of the peri-implant bone was predetermined as the region of interest (ROI). Each ROI was divided by four strain intensity levels: 0–20 με, 20–60 με, 60–100 με, and ≥100 με, and the bone histomorphometric parameters were analyzed by the total area of each strain range for all loaded samples. The distance between the calcified front and calcein labeling as a parameter representing the mineral apposition rate was significantly greater in the areas with strain intensity ≥100 με than in the area with strain intensity <100 με, suggesting that the bone formation activity of osteoblasts was locally enhanced by a higher mechanical strain. However, the shrunken osteocytes and the empty osteocyte lacunae were significantly lower in the highest strain area, suggesting that osteoclastogenesis was more retarded in higher strain areas than in lower strain areas. The histomorphometric parameters were not affected geometrically in the unloaded animals, suggesting that the load-induced mechanical strain caused differences in the histomorphometric parameters. Our findings support the hypothesis that bone cell activity related to bone resorption and formation is local strain-dependent on implant loading.

The Application of Digital Volume Correlation (DVC) to Evaluate Strain Predictions Generated by Finite Element Models of the Osteoarthritic Humeral Head

Continuum-level finite element models (FEMs) of the humerus offer the ability to evaluate joint replacement designs preclinically; however, experimental validation of these models is critical to ensure accuracy. The objective of the current study was to quantify experimental full-field strain magnitudes within osteoarthritic (OA) humeral heads by combining mechanical loading with volumetric microCT imaging and digital volume correlation (DVC). The experimental data was used to evaluate the accuracy of corresponding FEMs. Six OA humeral head osteotomies were harvested from patients being treated with total shoulder arthroplasty and mechanical testing was performed within a microCT scanner. MicroCT images (33.5 µm isotropic voxels) were obtained in a pre- and post-loaded state and BoneDVC was used to quantify full-field experimental strains (≈ 1 mm nodal spacing, accuracy = 351 µstrain, precision = 518 µstrain). Continuum-level FEMs with two types of boundary conditions (BCs) were simulated: DVC-driven and force-driven. Accuracy of the FEMs was found to be sensitive to the BC simulated with better agreement found with the use of DVC-driven BCs (slope = 0.83, r2 = 0.80) compared to force-driven BCs (slope = 0.22, r2 = 0.12). This study quantified mechanical strain distributions within OA trabecular bone and demonstrated the importance of BCs to ensure the accuracy of predictions generated by corresponding FEMs.

Distribution of mechanical strain in equine distal metacarpal subchondral bone: A microCT-based finite element model

Microdamage accumulation and adaptation of subchondral bone subjected to intensive cyclic loading are important processes associated with catastrophic bone failure, and joint degeneration in athletic humans and racehorses. At the tissue-level, they lead to a spatial variation in bone tissue mineral density (TMD) which affects the response of the bone to mechanical load. Quantifying the spatial distribution of mechanical load within the subchondral bone is critical for understanding the mechanism of the joint failure. Previously, a gradient of TMD and mechanical properties has been reported under unconfined compression in osteochondral plugs. In the present study, we used micro computed tomography (μCT)-based finite element (FE) models of cartilage-bone to investigate the gradient of strain in the subchondral bone (SCB) from the third metacarpal (MC3) condyle of racehorses under simulated in situ compression. Non-destructive mechanical testing of specimens under high-rate compression provided the apparent-level modulus of SCB. FE models were analysed using unconfined and confined boundary conditions. Unconfined FE-predicted apparent-level gradient of modulus across the SCB thickness correlated well with the experimental results (R2 ​= ​0.72, p ​< ​0.05). The highest strain occurred in the most superficial SCB (0.5–2.5 ​mm deep to the cartilage-bone interface) under the simulated in-situ compression through articular cartilage. The findings of this study provide an estimation for the spatial distribution of mechanical strain within SCB in-situ in the presence of heterogeneous bone tissue which is commonly observed in joints subjected to intensive cyclic loading.

Strength Simulation of Scored Tablets Based on the Finite Element Method Using an Extreme Vertices Design.

The mechanical strain distribution of scored tablets was simulated using the finite element method (FEM). The score was fabricated as a triangular runnel with the pole on the top surface of flat tablets. The effect of diametral compression on the tablet surface strain was evaluated by changing the angle between the scored line and the diametral compression axis. Ten types of granules were prepared according to an extreme vertices design. Young's modulus and the Poisson ratio for the model powder bed were measured as elastic parameters. The FEM simulation was then applied to the scored tablets represented as a continuous elastic model. Strain distributions in the inner structure of the tablets were simulated after the application of external force. The maximum principal strain (ε1) value was obtained with tablets containing a large amount of corn starch, in all scored line positions. In contrast, the ε1 value of the tablets containing a large amount of microcrystalline cellulose was minimal. The adequacy of the simulation was evaluated by experiments with scored tablets. The results indicated a fairly good agreement between the FEM simulation and experiments. Moreover, it was found that the ε1 value correlated negatively with the value of tablet hardness. These results suggest that the FEM simulation was advantageous for designing scored tablets.

Dynamics of Tissue-Induced Alignment of Fibrous Extracellular Matrix

Aligned fibers of extracellular matrix (ECM) affect the direction, efficiency, and persistence of migrating cells. To uncover the mechanisms by which multicellular tissues align their surrounding ECM before migration, we used an engineered three-dimensional culture model to investigate the dynamics of ECM alignment around tissues of defined geometry. Analysis of ECM alignment over time revealed that tissues rapidly reorganize their surrounding matrix, with a characteristic time that depends on the type of cell and the initial tissue geometry. We found that matrix metalloproteinase activity is not required for matrix alignment before cell migration. Instead, alignment is driven by Rho-mediated cytoskeletal contractility and accelerated by propagation of tension through intercellular adhesions. Our data suggest that multicellular tissues align their surrounding matrix by pulling collectively to exert strain, which is primarily a physical process. Consistently, the pattern of matrix alignment depends on tissue geometry and the resulting distribution of mechanical strain, with asymmetric tissues generating a higher degree of matrix alignment along their longest axes. The rapid ability of multicellular tissues to physically remodel their matrix enables their constituent cells to migrate efficiently along aligned fibers and to quickly change their direction according to other microenvironmental cues, which is important for both normal and disease processes.

Quantum Chemical Strain Analysis For Mechanochemical Processes.

The use of mechanical force to initiate a chemical reaction is an efficient alternative to the conventional sources of activation energy, i.e., heat, light, and electricity. Applications of mechanochemistry in academic and industrial laboratories are diverse, ranging from chemical syntheses in ball mills and ultrasound baths to direct activation of covalent bonds using an atomic force microscope. The vectorial nature of force is advantageous because specific covalent bonds can be preconditioned for rupture by selective stretching. However, the influence of mechanical force on single molecules is still not understood at a fundamental level, which limits the applicability of mechanochemistry. As a result, many chemists still resort to rules of thumb when it comes to conducting mechanochemical syntheses. In this Account, we show that comprehension of mechanochemistry at the molecular level can be tremendously advanced by quantum chemistry, in particular by using quantum chemical force analysis tools. One such tool is the JEDI (Judgement of Energy DIstribution) analysis, which provides a convenient approach to analyze the distribution of strain energy in a mechanically deformed molecule. Based on the harmonic approximation, the strain energy contribution is calculated for each bond length, bond angle and dihedral angle, thus providing a comprehensive picture of how force affects molecules. This Account examines the theoretical foundations of quantum chemical force analysis and provides a critical overview of the performance of the JEDI analysis in various mechanochemical applications. We explain in detail how this analysis tool is to be used to identify the "force-bearing scaffold" of a distorted molecule, which allows both the rationalization and the optimization of diverse mechanochemical processes. More precisely, we show that the inclusion of every bond, bending and torsion of a molecule allows a particularly insightful discussion of the distribution of mechanical strain in deformed molecules. We illustrate the usefulness of the JEDI analysis by rationalizing the finding that a knot tremendously weakens a polymer strand via a "choking" motion of the torsions in the curved part of the knot, thus leading to facilitated bond rupture in the immediate vicinity of the knot. Moreover, we demonstrate that the JEDI analysis can be exploited to devise methods for the stabilization of inherently strained molecules. In addition to applications in the electronic ground state, the JEDI analysis can also be used in the electronically excited state to determine the mechanical energy that a molecular photoswitch can release into its environment during photoisomerization. This approach allows the quantification of the mechanical efficiency of a photoswitch, i.e., the part of the energy that becomes available for the motion into a specific direction, which enables us to judge whether a photoswitch is capable of performing a desired switching function.

Influence of Additive Manufactured Scaffold Architecture on the Distribution of Surface Strains and Fluid Flow Shear Stresses and Expected Osteochondral Cell Differentiation.

Scaffolds for regenerative medicine applications should instruct cells with the appropriate signals, including biophysical stimuli such as stress and strain, to form the desired tissue. Apart from that, scaffolds, especially for load-bearing applications, should be capable of providing mechanical stability. Since both scaffold strength and stress–strain distributions throughout the scaffold depend on the scaffold’s internal architecture, it is important to understand how changes in architecture influence these parameters. In this study, four scaffold designs with different architectures were produced using additive manufacturing. The designs varied in fiber orientation, while fiber diameter, spacing, and layer height remained constant. Based on micro-CT (μCT) scans, finite element models (FEMs) were derived for finite element analysis (FEA) and computational fluid dynamics (CFD). FEA of scaffold compression was validated using μCT scan data of compressed scaffolds. Results of the FEA and CFD showed a significant impact of scaffold architecture on fluid shear stress and mechanical strain distribution. The average fluid shear stress ranged from 3.6 mPa for a 0/90 architecture to 6.8 mPa for a 0/90 offset architecture, and the surface shear strain from 0.0096 for a 0/90 offset architecture to 0.0214 for a 0/90 architecture. This subsequently resulted in variations of the predicted cell differentiation stimulus values on the scaffold surface. Fluid shear stress was mainly influenced by pore shape and size, while mechanical strain distribution depended mainly on the presence or absence of supportive columns in the scaffold architecture. Together, these results corroborate that scaffold architecture can be exploited to design scaffolds with regions that guide specific tissue development under compression and perfusion. In conjunction with optimization of stimulation regimes during bioreactor cultures, scaffold architecture optimization can be used to improve scaffold design for tissue engineering purposes.

(Invited) High-Yield Reconfigurable Silicon and Germanium Nanowire Transistors and Compact Logic Circuits

Semiconductor nanowire transistors are considered potential successors of finFETs providing the ultimate miniaturization capabilities of MOS transistors and thus targeting ultimate circuit complexity and performance. A promising perspective for further advancement beyond classical CMOS scaling is to exploit functionality enhancement of nanowire transistors. Several nanowire based multi-gated device concepts known as reconfigurable field effect transistors (RFETs) or polarity control FETs that combine multi-functionality with the inherent advantages of silicon nanowires are being currently evaluated [1-4]. These multi-terminal devices merge unipolar n- and p- FET switching characteristics from the same device as selected simply by an electric signal and without the need for doping enabling complementary CMOS operation with a single kind of transistor. RFETs make use of two sharp metal-NiSi2 / intrinsic-silicon interfaces with individual gates for the selective injection of electrons and holes into the channel region [5]. Additional independent steering gates can be integrated to further enhance functionality without comprising on-conductance [6]. In order to allow complementary circuit operation symmetry in the p- and n-type I-Vcharacteristics is mandatory. To this end, we show both experimentally [7] and by device simulations [8] that the application of radial compressive strain to <110> oriented silicon nanowires is able to align the injection efficiency of electrons and holes for diverse nanowire geometries. Full swing complementary operating circuits such as inverters [7] and compact six transistor cells yielding NAND, NOR and minority functions as demonstrated with symmetric RFETs and 3XOR gates with only 8 transistors vs. 24 with CMOS [6,7,10,11]. These gates build the basis for arithmetic building blocks such as one-bit adders with substantially lower transistor count reducing critical pathes, delay and power consumption as compared to conventional CMOS [6]. The RFET concept has been recently successfully transferred to a parallel nanowire technology [12,13] principally enabling to build low operation power circuits and sensors on plastic substrates. Finally, we show that germanium based RFETs improve the individual device performance and to reduce dynamic power consumption without comprising static power consumption. Figure caption: a) Schematic of reconfigurable nanowire FET (RFET) encompassing intrinsic Si nanowire channel and two independently gated Si / NiSi2 junctions. b) Oxide induced compressive stress in Si nanowire core required to adjust electron and hole currents: TEM cross section of nanowire channel with overlay of computed mechanical strain distribution. c) SEM image of abrupt Si / NiSi2 junction. d) SEM of symmetrically operating RFET with electrical device symbol. e) Symmetric p- and n-program I-V transfer characteristics of device depicted in d). f) SEM of inverter built of two RFETs within a single nanowire. g) Measured inverter transfer characteristic with supply voltage of Vdd = 2V showing low operation power complementary behavior and full swing output. Propagation delay for scaled structure and for different fanouts. Output of reconfigrable NAND/NOR/Minority 6-transistor cell, shows reconfiguration at runtime (mixed -mode computations). h) High-yield printed silicon nanowire FETs encompassing ~ 1xE3 parallel nanowires with meander source/ drain / gate structure. Technology is compatible with flexible plastic substrates.

Correspondence: Reply to ‘On the nature of strong piezoelectricity in graphene on SiO2’

In our paper1 we provided an experimental evidence that the single-layer graphene (SLG) deposited on SiO2 grating substrate exhibits very strong out-of-plane piezoelectric effect, several times greater than that of the best piezoceramics such as lead-zirconate titanate. Simultaneously, the in-plane strain distribution was measured by micro-Raman scattering in an attempt to relate such unusual activity to the strain gradient expected in suspended graphene near the ridges. We appreciate the comment by Stampfer and Reichardt2 who noticed that our calculations of the in-plane strain based on ref. 3 were incorrect. The overestimation of the strain values, however, does not change the main conclusion of our paper, since the piezoresponse force microscopy measurements give the value of out-of-plane a.c. deformation, fully decoupled from the d.c. in-plane strain measured by micro-Raman. We recalculated the strain map and corrected the strain values that vary now in the range from −0.078 to +0.078%. Nevertheless, the strain ratio between supported and suspended graphene is still 2.5 (strain for suspended graphene is about +0.02%). Figure 1a,b displays both the corrected strain map and cross-section of the position of Raman line and recalculated strain. Below we reply in detail to other issues raised by Stampfer and Reichardt in their communication2. Figure 1 Strains in the single-layer graphene on the silicon grating. Gruneisen parameter suggested by Stampfer and Reichardt2 is related to graphene produced by mechanical exfoliation, whereas in our work1 we used graphene sheets prepared by CVD. These two materials differ by the number of intrinsic defects and, therefore, their behaviour under uniaxial or biaxial strain is different. This was exactly demonstrated in ref. 3, where the opposite sign of Gruneisen parameter for G-band was revealed for CVD graphene with respect to the exfoliated one. Therefore, we used the value reported in ref. 3 for our calculations which gave us a maximum strain of 0.078%. Stampfer and Reichardt2 questioned why we used the Gruneisen parameter for uniaxial strain. We believe that using the value for uniaxial strain is dictated by the nature of the substrate. Being periodical in one direction only, this feature naturally defines the symmetry of the graphene deformations as well. Therefore, we considered uniaxial strain in our work. Stampfer and Reichardt2 claim that our paper ‘…directly linked the observed piezoelectricity in graphene to the supposed high in-plane strain induced by the substrate…'. Indeed, the observed piezoresponse distribution (Fig. 3(b) of ref. 1) apparently correlates with the mechanical strain distribution in graphene. However, it does not mean that the observed piezoelectricity is associated with this strain (and its value!). Since no amplification of the piezoresponse is observed near the grating edges where the maximum strain gradient is expected, we only ruled out the in-plane symmetry breaking induced by discussed in-plane strain. Therefore, the conclusion of the paper remains the same, that is, chemical bonds generated between O and C atoms can indeed induce piezoelectric effect and, at the same time, affect the G-band position. Stampfer and Reichardt2 cast some doubt that the C–O bonds can form at the interface with graphene based on some data on carrier mobility. The formation of strong C–O bonds between graphene and SiO2 has been already proven both experimentally and theoretically (see, for example, refs 3, 4, 5, 6). This fact is strongly supported by the observed p-type conductivity in graphene via a charge transfer from the carbon in graphene layer to the oxygen-terminated surface of SiO2 (ref. 4). As such, the observation of a strong piezoelectricity due to formation of polar C–O bonds is not a surprise. Though some authors7 claim that only weak bonds are formed for the most stable geometry of C atoms, the very existence of the strong polarity of the graphene–SiO2 interface proves that it is not the case. All in all, we highly appreciate the comment by Stampfer and Reichard1 concerning the overestimation of the in-plane strain, however, it does not change the main conclusion of the paper, that is, the experimental observation of strong piezoelectricity in graphene/SiO2. We emphasize that, while an inhomogeneous interaction between graphene and underlying SiO2 substrate (caused for example, by the substrate morphology) can be responsible for the in-plane strain seen in Raman measurements, the observed piezoresponse is attributed to the formation of out-of-plane polar C–O bonds that may not be directly related to the in-plane deformation.