Quantification of the stiffness and strength of cadherin ectodomain binding with different ions

The stiffness and strength of extracellular （EC） region of cadherin are proposed to be two important mechanical properties both for cadherin as a mechanotransductor and for the formation of cell-cell adhesion. In this study, we quantitatively characterized the stiffness and strength of EC structure when it binds with different types of ions by molecular dynamics simulations. Resuits show that EC structure exhibits a rod-like shape with high stiffness and strength when it binds with the bivalent ions of calcium or magnesium. However, it switches to a soft and collapsed conformation when it binds with the monova- lent ions of sodium or potassium. This study sheds light on the important role of the bivalent ions of calcium in the physiological function of EC.

Probing the mechanosensitivity in cell adhesion and migration: Experiments and modeling

Cell adhesion and migration are basic physiolog- ical processes in living organisms. Cells can actively probe their mechanical micro-environment and respond to the ex- ternal stimuli through cell adhesion. Cells need to move to the targeting place to perform function via cell migration. For adherent cells, cell migration is mediated by cell-matrix adhesion and cell-cell adhesion. Experimental approaches, especially at early stage of investigation, are indispensable to studies of cell mechanics when even qualitative behaviors of cell as well as fundamental factors in cell behaviors are unclear. Currently, there is increasingly accumulation of ex- perimental data of measurement, thus a quantitative formula- tion of cell behaviors and the relationship among these fun- damental factors are highly needed. This quantitative under- standing should be crucial to tissue engineering and biomed- ical engineering when people want to accurately regulate or control cell behaviors from single cell level to tissue level. In this review, we will elaborate recent advances in the ex- perimental and theoretical studies on cell adhesion and mi- gration, with particular focuses laid on recent advances in experimental techniques and theoretical modeling, through which challenging problems in the cell mechanics are sug- gested.

Wetting Behavior between Droplets and Dust

The wetting behavior of dust by droplets is investigated by experiments and numerical simulation methods.Experimental observation reveals that the surface of a coal slice is hydrophilic in nature,while surfaces of coal dust stacks are hydrophobic.We show that water droplets settle on these surfaces following the Cassie-Baxter wetting model,as supported by theoretical,numerical analyses and experimental observations,i.e.water droplets only wet the first layer of coal dust.Our numerical simulation results also show that a water droplet could not enclose any coal dust inside it and many coal dust particles are adhered with a hexagonal close packing on a large water droplet.Based on these results,we conclude that the surface area of water droplets is an important factor on their wetting and capturing coal dust,and producing smaller water droplets can improve the efficiency of settling dust.

Editorial:Mechanics of biological and bio-inspired materials

The field of mechanics of biological and bio-inspired materials underwent an exciting development over the past several years, which made it stand at the cutting edge of both engineering mechanics and biomechanics. As an intriguing interdisciplinary research field, it aims at elucidating the fundamental principles in nature＇s design of strong, multi-functional and smart Materials by focusing on the assembly, deformation, stability and failure of the materials.

Pulling out a peptide chain from β-sheet crystallite: Propagation of instability of H-bonds under shear force

Anti-parallel β-sheet crystallite as the main component of silk fibroin has attracted much attention due to its superior mechanical properties. In this study, we examine the processes of pulling a peptide chain from β-sheet crystallite using steered molecular dynamics simulations to investigate the rupture behavior of the crystallite. We show that the failure of β-sheet crystallite was accompanied by a propagation of instability of hydrogen-bonds （H-bonds） in the crystallite. In addition, we find that there is an optimum size of the crystallite at which the H-bonds can work cooperatively to achieve the highest shear strength. In addition, we find that the stiffness of loading device and the loading rates have significant effects on the rupture behavior of β-sheet crystallite. The stiff loading device facilitates the rebinding of the Hbond network in the stick-slip motion between the chains, while the soft one suppresses it. Moreover, the rupture force of β-sheet crystallites decreases with loading rate. Particularly, when the loading rate decreases to a critical value, the rupture force of the β-sheet crystallite becomes independent of the loading rates. This study provides atomistic details of rupture behaviors of β-sheet crystallite, and, therefore, sheds valuable light on the underlying mechanism of the superior mechanical properties of silk fibroin.

Mechanics of water pore formation in lipid membrane under electric field

Transmembrane water pores are crucial for substance transport through cell membranes via membrane fusion, such as in neural communication. However, the molecular mechanism of water pore formation is not clear. In this study, we apply all-atom molecular dynamics and bias-exchange metadynamics simulations to study the process of water pore formation under an electric field. We show that water molecules can enter a membrane under an electric field and form a water pore of a few nanometers in diameter. These water molecules disturb the interactions between lipid head groups and the ordered arrangement of lipids. Following the movement of water molecules, the lipid head groups are rotated and driven into the hydrophobic region of the membrane. The reorientated lipid head groups inside the membrane form a hydrophilic surface of the water pore. This study reveals the atomic details of how an electric field influences the movement of water molecules and lipid head groups, resulting in water pore formation.

Local buckling analysis of biological nanocomposites based on a beam-spring model

Biological materials such as bone, tooth, and nacre are load-bearing nanocomposites composed of mineral and protein. Since the mineral crystals often have slender geometry, the nanocomposites are susceptible to buckle under the compressive load. In this paper, we analyze the local buckling behaviors of the nanocomposite structure of the biological materials using a beam-spring model by which we can consider plenty of mineral crystals and their interaction in our analysis compared with existing studies. We show that there is a transition of the buckling behaviors from a local buckling mode to a global one when we continuously increase the aspect ratio of mineral, leading to an increase of the buckling strength which levels off to the strength of the composites reinforced with continuous crystals. We find that the contact condition at the mineral tips has a striking effect on the local buckling mode at small aspect ratio, but the effect diminishes when the aspect ratio is large. Our analyses also show that the staggered arrangement of mineral plays a central role in the stability of the biological nanocomposites.

Interactions of human islet amyloid polypeptide with lipid structure of different curvatures

Curvature is one of the most important features of lipid membranes in living cells,which significantly influences the structure of lipid membranes and their interaction with proteins.Taken the human islet amyloid polypeptide(h IAPP),an important protein related to the pathogenesis of type II diabetes,as an example,we performed molecular dynamics(MD)simulations to study the interaction between the protein and the lipid structures with varied curvatures.We found that the lipids in the high curvature membrane pack loosely with high mobility.The h IAPP initially forms H-bonds with the membrane surface that anchored the protein,and then inserts into the membrane through the hydrophobic interactions between the residues and the hydrophobic tails of the lipids.h IAPP can insert into the membrane more deeply with a larger curvature and with a stronger binding strength.Our result provided important insights into the mechanism of the membrane curvature-dependent property of proteins with molecular details.

Analysis of the shape of heavy droplets on flat and spherical surface

In this study,a theoretical model was established for predicting the equilibrium shape of the droplet on flat and spherical surfaces.The theoretical equilibrium shape of heavy droplets could be obtained once contact angle and volume of droplets were given.It showed that the predictions of the theoretical flat model were in good agreement with the shape obtained by Surface Evolver when the contact angle is below 120 and the droplet size is on the order of capillary length.This available range will decrease and increase when the heavy droplet is on convex and concave spherical surface,respectively,in contrast to that on flat surface.The available range will decrease more for higher curvature of convex spherical surfaces.

Effect of triple-phase contact line on contact angle hysteresis

Recent studies have shown that the triple-phase contact line has critical effect on the contact angle hysteresis of surfaces.In this study,patterned surfaces with various surface structures of different area fractions were prepared by electron etching on a silicon wafer.The advancing angle,receding angle and hysteresis angle of these surfaces were measured.Our experimental results showed that while the geometry of microstructure and contact line have a minor effect on the advancing angle,they have a significant effect on the receding angle and thus the hysteresis angle.We have shown that the effect of microstructure and the contact line can be described by a quantitative parameter termed the triple-phase line ratio.The theoretical predictions were in good agreement with our experimental results.

全髋关节置换翻修术中高位植入非骨水泥型髋臼假体的应用——对以往报道15年以上的简明随访

我们曾经报告采用高髋臼中心植入多孔涂层非骨水泥型髋臼假体的方法，对34例（36髋）全髋关节置换术后严重髋臼骨缺损的患者行翻修手术，术后随访平均10年的结果。本文将报告对21例（23髋）患者平均随访16．8年的结果。在最初的44例（46髋）患者中，39例（41髋；89％）患者仍使用原来的髋臼杯。2例（4．3％）髋臼杯因无菌性松动而翻修，3例（6．5％）髋臼杯因感染而翻修。6例股骨侧假体因为股骨骨溶解行翻修，另外7例无骨溶解但因无菌性松动行翻修。基于我们平均随访16．8年的结果，我们相信：在部分髋臼严重骨缺损的患者行全髋关节置换翻修时，高髋臼中心植入非骨水泥型髋臼假体仍然是一种优良的方法。 可信水平：治疗性研究，Ⅳ级，进一步可信度参见作者介绍。

An atomistic model of silk protein network for studying the effect of pre-stretching on the mechanical performances of silks

Silk protein builds one of the strongest natural fibers based on its complex nanocomposite structures.However,the mechanical performance of silk protein,related to its molecular structure and packing is still elusive.In this study,we constructed an atomistic silk protein network model,which reproduces the extensive connection topology of silk protein with structure details of theβ-sheet crystallites and amorphous domains.With the silk protein network model,we investigated the structure evolution and stress distribution of silk protein under external loading.We found a pre-stretching treatment during the spinning process can improve the strength of silk protein.This treatment improves the properties of silk protein network,i.e.,increases the number of nodes and bridges,makes the nodes distributed homogeneously,and induces the bridges in the network well aligned to the loading direction,which is of great benefit to the mechanical performances of silk protein.Our study not only provides a realized atomistic model for silk protein network that well represents the structures and deformations of silk proteins under loading,but also gains deep insights into the mechanism how the pre-loading on silk proteins during spinning improves the mechanical properties of silk fibers.

初次非骨水泥型髋臼假体治疗严重髋关节发育不良或完全脱位术后平均16年的随访

我们早期曾报道应用半球窝状多孔涂层髋臼假体，连续对20例高选择性严重髋关节发育不良或完全脱位患者施行初次全髋关节置换术后7年的结果报道。此文是对这些髋关节在其后9年，即术后平均随访16年（范围，11．5～19年）的结果报道。在我们早期报道后，有2个髋臼壳行翻修术，其中1个由于无菌性松动，另1个由于聚乙烯衬垫在固定齿片未破裂情况下脱落。其余髋臼壳固定良好。X线片未见骨盆骨溶解，聚乙烯衬垫平均磨损率为0．09mm／年。髋臼壳的无菌性松动定义为失败，术后平均16年髋臼壳的生存率为92％。我们认为本组经严格选择的严重髋关节疾患的患者，经长达16年的随访髋臼杯的固定极好。可信水平：治疗性研究，Ⅳ级。进一步可信度参见作者介绍。

髋臼假体失败后应用巨大型非骨水泥髋翻修 对以前报道的简明随访

严重骨缺损的髋臼假体翻修在临床和手术治疗中都是一个很棘手的问题。在我们以前的报道中有连续24名患者采用巨大的（〉65mm）半球状非骨水泥髋臼假体进行假体翻修，其中15名患者平均随访了7年。现在报道该组中12名患者，他们术后至少存活12年（平均13.9年），并且同意再次进行随访。最终的Harris髋关节评分是79分。无一例髋臼假体由于无菌性松动而需翻修，并且在影像学上未发现松动迹象。术后的并发症发生率虽高，但主要应归咎于感染。如果患者未发生感染，长期随访证实巨大髋臼假体的固定依然是牢固的。

Heterogeneous oxidization of graphene nanosheets damages membrane

Graphene-based materials exhibit unique properties that have been sought to utilize for various potential applications. Many studies suggest that graphene-based materials can be cytotoxic, which may be attributed to destructive effects on cell membranes.However, there still are conflicting results regarding interactions between graphene-based materials and lipid membranes. Here,through cryo-electron microscopy(Cryo-EM) and dye-leakage experiments along with in silico methods, we found that graphene oxide nanosheets induce significant membrane damage, while the effect of pristine graphene is negligible. We revealed the importance of heterogeneous oxidization of graphene-based nanosheets in damaging vesicle membranes. Moreover, that not only the oxidization degree but also the oxidization loci and membrane tension play important roles in the cytotoxicity of the graphene-based nanosheets.

Single-vesicle imaging quantifies calcium’s regulation of nanoscale vesicle clustering mediated by α-synuclein

Although numerous studies have shown that the proteinα-synuclein(α-Syn)plays a central role in Parkinson’s disease,dementia with Lewy bodies,and other neurodegenerative diseases,the protein’s physiological function remains poorly understood.Furthermore,despite recent reports suggesting that,under the influence of Ca^(2+),α-Syn can interact with synaptic vesicles,the mechanisms underlying that interaction are far from clear.Thus,we used single-vesicle imaging to quantify the extent to which Ca^(2+)regulates nanoscale vesicle clustering mediated by α-Syn.Our results revealed not only that vesicle clustering requiredα-Syn to bind to anionic lipid vesicles,but also that different concentrations of Ca^(2+)exerted different effects on howα-Syn induced vesicle clustering.In particular,low concentrations of Ca^(2+)inhibited vesicle clustering by blocking the electrostatic interaction between the lipid membrane and the N terminus of α-Syn,whereas high concentrations promoted vesicle clustering,possibly due to the electrostatic interaction between Ca^(2+)and the negatively charged lipids that is independent of α-Syn.Taken together,our results provide critical insights intoα-Syn’s physiological function,and how Ca^(2+) regulates vesicle clustering mediated by α-Syn.