Emergent functionalities enhanced by mechanical stress in SnO2-based flexible devices

The use of intra-operative blood salvage, dialysis, and artificial organs are associated with the application of non-physiological mechanical stress on red blood cells (RBCs). To explore the effect of these procedures on red cell deformability, we determined it before and after the mechanical stress application both in an in vitro system and following a blood-saving procedure. RBC from eight healthy donors and fifteen packed RBC units were subjected to mechanical stress. RBCs from five patients undergoing orthopedic surgery were also collected. We measured the percent of undeformable cells (%UDFC) in the red cell samples using our cell flow properties image analyzer, which provides the distribution of RBC deformability in a large cell population. Mechanical stress systematically reduced the cell deformability and increased the %UDFC, while simultaneously causing hemolysis of rigid, undeformable RBCs. Ultimately, the overall result depended on the initial level of the undeformable cells; the stress-induced change in the proportion of rigid cells (Δ%UDFC) increased (Δ%UDFC > 0) when its initial value was low, and decreased (Δ%UDFC < 0) when its initial value was high. This suggests that the final impact of mechanical stress on the percent of rigid cells in the RBC population is primarily determined by their initial concentration in the sample.

Manipulation of mechanical properties of monolayer molybdenum disulfide: Kirigami and hetero-structure based approach

Piezoelectric materials are an integral part of the modern world, as they find wide-ranging applications in various advanced domains such as telecommunications, automotive, electronics, and aerospace. Recently, these materials gained interest also for applications in liquid environments across diverse fields, including biomedical engineering, purification systems, electrocatalysis, fluid characterizations and micro/nano electromechanical systems (MEMS/NEMS). Despite their growing importance, many questions remain unanswered regarding their characterization and the evaluation of their performance in liquid media. It is critical to understand how these materials interact with surrounding ionic species under mechanical stress application. To address these questions, a novel experimental setup has been designed to characterize piezoelectric materials through electrokinetic measurements in a liquid environment. A polarized piezoelectric sample (lead zirconate titanate) is subjected to a mechanical stress, which is known to influence the piezoelectric surface charge state. This change in surface charge state can be monitored through measurement of the streaming potential. The results indicate that there is a strong correlation between the applied mechanical stress and the electrokinetic response of the piezoelectric sample. When the sample is subjected to a dynamic stress variation, its electrokinetic response closely follows the profile of the applied stress. In addition, the stress applied to the piezoelectric sample directly impacts the flow cell’s resistance. A non-piezoelectric material (alumina) is also subjected to similar stress conditions. Both electrokinetics and resistance of the flow cell remain almost unchanged for the non-piezoelectric alumina sample during the stress application. Finite element modeling is employed to model variation of surface charge on the piezoelectric sample due to stress application. The piezoelectric model is then coupled with electrokinetic and resistance models to explore the effect of stress on the electrical responses of the system. The modeling results exhibit high degree of consistency with the experimental observations.

Mechanical stress and genetic factors play important roles in the occurrence of thoracic ossification of ligament flavum (TOLF), which can occur at one, two, or multiple levels of the spine. It is unclear whether single- and multiple-level TOLF differ in terms of osteogenic differentiation potency and osteogenesis-related gene expression under cyclic mechanical stress. This was addressed in the present study using patients with non-TOLF and single- and multiple-level TOLF (n=8 per group). Primary ligament cells were cultured and osteogenesis was induced by application of cyclic mechanical stress. Osteogenic differentiation was assessed by evaluating alkaline phosphatase (ALP) activity and the mRNA and protein expression of osteogenesis-related genes, including ALP, bone morphogenetic protein 2 (BMP2), Runt-related transcription factor-2 (Runx-2), osterix, osteopontin (OPN) and osteocalcin. The application of cyclic mechanical stress resulted in higher ALP activity in the multiple-level than in the single-level TOLF group, whereas no changes were observed in the non-TOLF group. The ALP, BMP2, OPN and osterix mRNA levels were higher in the multiple-level as compared to the single-level TOLF group, and the levels of all osteogenesis-related genes, apart from Runx2, were higher in the multiple-level as compared to the non-TOLF group. The osterix and ALP protein levels were higher in the multiple-level TOLF group than in the other 2 groups, and were increased with the longer duration of stress. These results highlight the differences in osteogenic differentiation potency between single- and multiple-level TOLF that may be related to the different pathogenesis and genetic background.

Semiconductor industry is experiencing unprecedented growth, still driven by Moore's law, which is continually delivering devices with improved performance at lower costs. The continuation of this development places the industry in a divergent trade-off between economic attractiveness, technological feasibility, and the need for further performance improvement. Since the mainstream semiconductor technologies are silicon-based, new disruptive innovations are needed to gain additional performance margins. The use of nanowires is the preferred approach for preserving electrostatic control in the MOS transistor channel, and the application of mechanical stress is a booster of carrier mobility. It is in this context that this paper presents the design, fabrication, theoretical modeling, and characterization of a measurement platform to characterize the mechanical tensile stress of extremely narrow Si nanowires as small as 14.2 ± 1.12 nm in width. The proposed measurement platform enables a precise control of uniaxial strain, in terms of both amplitude and location, through the implementation of a stoichiometric Si3N4 pulling strand exerting a high tensile force on silicon nanowires. Reported devices are fabricated using a silicon-on-insulator wafer with fully complementary metal–oxide–semiconductor-compatible processing and top-down approach. It is observed that the mechanical strength of nanostructured Si is size-dependent and increases with miniaturization. Characterization revealed a record tensile strength value of 7.53 ± 0.8% (12.73 ± 1.35 GPa) for the narrowest nanowires fabricated using a top-down approach.

Mechanotransduction, the biological response to mechanical stress, is often initiated by activation of mechanosensitive (MS) proteins upon mechanically induced deformations of the cell membrane. A current challenge in fully understanding this process is in predicting how lipid bilayers deform upon the application of mechanical stress. In this context, it is now well established that anionic lipids influence the function of many proteins. Here, we test the hypothesis that anionic lipids could indirectly modulate MS proteins by alteration of the lipid bilayer mechanical properties. Using all-atom molecular dynamics simulations, we computed the bilayer bending rigidity (KC), the area compressibility (KA), and the surface shear viscosity (ηm) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) lipid bilayers with and without phosphatidylserine (PS) or phosphatidylinositol bisphosphate (PIP2) at physiological concentrations in the lower leaflet. Tensionless leaflets were first checked for each asymmetric bilayer model, and a formula for embedding an asymmetric channel in an asymmetric bilayer is proposed. Results from two different sized bilayers show consistently that the addition of 20% surface charge in the lower leaflet of the PC bilayer with PIP2 has minimal impact on its mechanical properties, while PS reduced the bilayer bending rigidity by 22%. As a comparison, supplementing the PIP2-enriched PC membrane with 30% cholesterol, a known rigidifying steroid lipid, produces a significant increase in all three mechanical constants. Analysis of pairwise splay moduli suggests that the effect of anionic lipids on bilayer bending rigidity largely depends on the number of anionic lipid pairs formed during simulations. The potential implication of bilayer bending rigidity is discussed in the framework of MS piezo channels.

The phenomenon of piezoelectricity, the release of electric charge under the application of mechanical stress, occurs in all noncentrosymmetric materials. Pyroelectricity, the release of charge due to a material’s change of temperature, occurs in all materials that belong to a polar crystal symmetry class. It should be noted that, as not all noncentrosymmetric classes are polar (222, 4, 422, \(\overline 4 2m\), 32, 6, 622, \(\bar 6m2\), 23, and \(\bar 43m\) are the nonpolar, noncentrosymmetric classes), not all piezoelectric crystals are pyroelectric. However, all pyroelectric crystals are piezoelectric. (As a final note, crystals belonging to the noncentrosymmetric cubic class 432 are nonpiezoelectric because all the piezoelectric moduli vanish under the operation of the symmetry elements.) Ferroelectrics form a subset of the set of pyroelectrics because they are polar materials in which the direction of the polar axis can be changed by the application of an electric field. As a consequence they are both pyroelectric and piezoelectric. As many of the largest pyroelectric and piezoelectric effects occur in ferroelectric materials, they have become very important technologically. Furthermore, the fact that the polar axis can be reoriented by the application of a field means that polycrystalline ferroelectrics in which the crystallographic axes of the component crystallites are randomly oriented can be made to show polar properties by applying a sufficiently large electric field (the process of “poling”).

Two-and-three-dimensional analysis of Joule and viscous heating effects on MHD nanofluid forced convection in microchannels

Metal‐halide perovskites represent highly efficient and lightweight photovoltaic technology with promising applications in various scenarios. However, their stability is often deteriorated by various external factors in realistic applications, which is particularly severe for flexible devices. Herein this work, the degradation of flexible perovskite devices is focused on under two substantial factors: light and mechanical stress, where the degradation is surprisingly accelerated with simultaneous existence of these factors. Full‐scale analysis demonstrates that the mismatch and charge accumulation at the interfaces are direct causes of this device failure. Theoretical simulations reveal that carrier‐localization behavior, which is strongly associated with lattice distortion, is the core microscopic factor that destabilizes the device. This finding poses significant challenges to the stability of flexible perovskite devices and other inner strain‐sensitive systems, which cannot be simply mitigated by conventional passivation or encapsulation. It is suggested in theoretical studies that this effect may be suppressed by certain physical modulations, such as external electric fields. In general, a better understanding of the degradation mechanisms of realistic flexible and rigid perovskite devices can be contributed in this study and the development of solutions is facilitated to address those challenges.

All-solid-state supercapacitors (ASSCs) are critical for next-generation flexible and wearable electronic devices, but their development has been hindered by the challenge of balancing high energy storage performance with mechanical flexibility in wearable energy storage systems. MXene materials offer excellent electrical conductivity, large surface area, and outstanding charge storage capability, but their application in flexible devices is limited by poor mechanical stability and structural degradation. To overcome these challenges, we have developed MXene/cellulose nanofiber (CNF) composites. CNF is a cheap and environmentally friendly material with a huge storage capacity on earth. The doping of CNFs into a layered MXene material prevents the stacking of the MXene, improves the ionic transport speed, maintains the excellent electrochemical properties of the MXene, and enhances the structural reinforcement and flexibility. The flexible, binder-free ASSCs have excellent electrochemical properties with a volumetric capacitance of 94.21 F cm-3. The electrochemical properties also showed no degradation in bending tests in the range of 30°-120°. The capacitance retention was 97.87% after 10 000 bending cycles at an angle of 60°. This work provides a scalable and green approach to fabricating high-performance MSCs and points the way to the next generation of wearable electronics.

Negative pressure therapy (NPT) is the controlled application of subatmospheric pressure to wounds. It has been shown to stimulate healing across a broad spectrum of soft-tissue wounds, at least in part from the application of mechanical stress on cells and tissues in the wound environment. This study tests the hypothesis that application of NPT to cranial critical-size defects (CSD) in skeletally mature rabbits leads to osseous healing. NPT was delivered 1, 4, 6 or 10 days over CSD-containing calcium phosphate scaffolds placed in contact with intact dura. At 12 weeks after defect creation, NPT groups exhibited significantly greater defect bridging and bone within the scaffolds (P<0.01). Increasing duration of NPT did not result in a greater amount of bone within the scaffolds, but did increase the amount of bone distributed in the upper half of the scaffolds. Appearance of tissue within defects immediately following the removal of NPT at day 6 suggests alternating regions of dural compression and distention indicative of cell stretching. Dura and adjacent tissue were composed of multiple cell layers that extended up into the scaffolds, lining struts and populating pore spaces. An extracellular matrix densely populated with cells and capillaries, as well as larger vessels, infiltrated pores of NPT-treated scaffolds, while scattered spindle-shaped cells and sparse stroma are present within pores of control scaffolds. This rabbit model data suggest that NPT activates within mature dura a natural healing cascade that results in osseous tissue formation without the addition of exogenous factors or progenitor cells.

Mechanical stress state consideration for enhanced electromigration life prediction in aluminum wire bonds

Hybrid heat exchangers (H2Xs) can be used for heat exchange equipment between high-temperature exhaust gas from ships and high-pressure supercritical carbon dioxide (S-CO2) from the S-CO2 power cycle. We investigate structural stress intensity characteristics of the H2Xs based on thermal-hydraulic performance. Air and S-CO2 are employed as the working fluids and the Stainless Steel 316 (SS316) as the solid substrate. The thermal-hydraulic performance and structural stress intensity characteristics of the H2Xs are conducted by Ansys Fluent and Mechanical, respectively. The results show the mechanical stress induced by pressure loading and the thermal stress induced by temperature gradient are found to be equally important sources of stress intensity. At the inlet and outlet of the H2Xs, the total stress intensity along all paths is not smooth and continuous, and there will be a significant change due to the change in the temperature gradient. The mechanical stress caused by the fluid pressure loss is almost negligible. The change in inlet mass flowrate and temperature mainly affects the stress intensity distribution of the left and right walls on the fin channel. The pressure variation of the diesel engine has little effect on the total stress intensity. Importantly, the total stress intensity of the H2X is mainly affected by the change in S-CO2 fluid pressure.

Stretchable temperature-responsive cell culture surfaces composed of poly( N-isopropylacrylamide) (PIPAAm) gel-grafted polydimethylsiloxane (PIPAAm-PDMS) were prepared to demonstrate that dual stimulation of temperature and mechanical stress extensively altered graft polymer thickness, surface wettability, and cell detachment behavior. The PIPAAm-PDMS surface was hydrophilic and hydrophobic below and above the lower critical solution temperature, respectively, which was ascribed to the phase transition of PIPAAm chains. When uniaxial stretching was applied, the grafted PIPAAm gel surface was modulated to be more hydrophobic as shown by an increase in the contact angle. Atomic force microscopy observation revealed that uniaxial stretching made the grafted gel layer thinner and deformed the nanoscale aggregates of the grafted PIPAAm gel, implying extension of the PIPAAm chains. The stretched PIPAAm-PDMS became more cell adhesive than the unstretched PIPAAm-PDMS at 37 °C. Furthermore, dual stimulation, shrinking the already stretched PIPAAm-PDMS and decreasing the temperature, induced more rapid cell detachment than only a change in temperature did. Similarly, upon comparison with a single stimulation of a change in temperature or mechanical stress, dual stimulation accelerated cell sheet detachment and harvesting. This new stretchable and temperature-responsive culture surface can easily adjust the surface property to a different cell adhesiveness by appropriately combining each stimulus and enable the fabrication of cell sheets of various species.

Unlike conventional polymers, in which monomers are reacted to form strong covalent connections, supramolecular polymers are formed by linking together monomers via weaker, non-covalent interactions such as H-bonds, ion-dipole interactions and so on. One very well-studied class of supramolecular polymers are based on the ureidopyrimidones, UPy’s, which dimerize via 4 strong, linear H-bonds in non-competitive solvents. Monomers for supramolecular polymerization can be made by linking together 2 or more UPy units with short covalent chains. Under dimerization conditions, the UPy’s link up to form longer polymeric chains. One of the useful attributes of such polymers is their self-healing ability. Application of heat or mechanical stress will readily break the monomers apart at the H-bonds, but, upon cooling or relief of stress, the bonds reform. The goal of this research is to see if electron transfer can be used as an alternative, more selective stimulus to break apart the UPy chains. As a first step in this direction, the use of our previously studied electroactive UPy’s, 1a and 1b, are being investigated as redox-active chain terminators. For these studies the known UPy monomer, 2, was synthesized. Viscosity measurements of solutions of 2 in 0.1 M NBu4PF6/CH2Cl2 give a critical polymerization concentration of 20 mM. Based on this, 60 mM 2 was chosen as the working concentration for initial experiments giving a solution with a specific viscosity of 3.0 mPa∙s. Upon addition of 2 mM of the ferrocene containing UPy 1a, the viscosity dropped to 1.1 mPa∙s, indicating that 1a is an effective chain terminator, decreasing the lengths of the chains which results in a decrease in the viscosity of the solution The ferrocenes were then completely oxidized to the ferrocenium form in a two compartment electrochemical cell. This caused the viscosity of the solution to increase from 1.0 to 1.7 mPa∙s, suggesting decreased effectiveness of the ferrocenium form as a chain terminator. This is consistent with poorer binding of the ferrocenium form to the bis-UPy. Importantly, reduction back to the ferrocene led to a decrease in the viscosity back to close to where it was initially. In this presentation, the results of further studies on the ability of both 1a and 1b to act as redox-responsive chain terminators for UPy-based supramolecular polymers will be reported. Figure 1