Micromechanics of chromatin and chromosomes.

The enzymes that transcribe, recombine, package, and duplicate the eukaryotic genome all are highly processive and capable of generating large forces. Understanding chromosome function therefore will require analysis of mechanics as well as biochemistry. Here we review development of new biophysical-biochemical techniques for studying the mechanical properties of isolated chromatin fibers and chromosomes. We also discuss microscopy-based experiments on cells that visualize chromosome structure and dynamics. Experiments on chromatin tell us about its flexibility and fluctuation, as well as quantifying the forces generated during chromatin assembly. Experiments on whole chromosomes provide insight into the higher-order organization of chromatin; for example, recent experiments have shown that the mitotic chromosome is held together by isolated chromatin-chromatin links and not a large, mechanically contiguous non-DNA "scaffold".     

Micromechanics of Sea Urchin Spines

The endoskeletal structure of the Sea Urchin, Centrostephanus rodgersii, has numerous long spines whose known functions include locomotion, sensing, and protection against predators. These spines have a remarkable internal microstructure and are made of single-crystal calcite. A finite-element model of the spine’s unique porous structure, based on micro-computed tomography (microCT) and incorporating anisotropic material properties, was developed to study its response to mechanical loading. Simulations show that high stress concentrations occur at certain points in the spine’s architecture; brittle cracking would likely initiate in these regions. These analyses demonstrate that the organization of single-crystal calcite in the unique, intricate morphology of the sea urchin spine results in a strong, stiff and lightweight structure that enhances its strength despite the brittleness of its constituent material.     

The Top of the Biomimetic Triangle

There is increasing observational evidence indicating that crystalline interfacial water layers play a central role in evolution and biology. For instance in cellular recognition processes, in particular during first contact events, where cells decide upon survival or entering apoptosis. Understanding water layers is thus crucial in biomedical engineering, specifically in the design of biomaterials inspired by biomimetic principles. Whereas there is ample experimental evidence for crystalline interfacial water layers on surfaces in air, their subaquatic presence could not be verified directly, so far. Analysing a polarity dependent asym- metry in the surface conductivity on hydrogenated nanocrystalline diamond, we show that crystalline interfacial water layers persist subaquatically. Nanoscopic interfacial water layers with an order different from that of bulk water have been identified at room temperature on both hydrophilic and hydrophobic model surfaces - in air and subaquatically. Their generalization and systematic inclusion into the catalogue of physical and chemical determinants of biocompatibility complete the biomimetic triangle.     

Micromechanics of human mitotic chromosomes

Eukaryote cells dramatically reorganize their long chromosomal DNAs to facilitate their physical segregation during mitosis. The internal organization of folded mitotic chromosomes remains a basic mystery of cell biology; its understanding would likely shed light on how chromosomes are separated from one another as well as into chromosome structure between cell divisions. We report biophysical experiments on single mitotic chromosomes from human cells, where we combine micromanipulation, nano-Newton-scale force measurement and biochemical treatments to study chromosome connectivity and topology. Results are in accord with previous experiments on amphibian chromosomes and support the 'chromatin network' model of mitotic chromosome structure. Prospects for studies of chromosome-organizing proteins using siRNA expression knockdowns, as well as for differential studies of chromosomes with and without mutations associated with genetic diseases, are also discussed.     

猪脱细胞真皮基质与RV-23抗菌肽复合材料的制备及生物学特性初步研究

目的:制备低免疫原性猪脱细胞真皮基质(PADM)与抗菌肽RV-23的复合材料,并对其生物学特性进行初步评价。方法:将抗菌肽RV-23分别以1、5、20μmol/L的浓度加到直径6 mm、厚约1 mm的低免疫原性PADM上,制备复合材料;菌落计数分析实验检测复合材料的抗菌能力;溶血实验检测复合材料对红细胞膜的裂解能力;CCK-8实验检测复合材料对真核细胞的细胞毒性;Tricine-SDS-PAGE检验RV-23的稳定性。结果:制备了PADM与抗菌肽RV-23复合材料;活菌计数实验表明复合材料对大肠杆菌有很强的抗菌活性,并随着抗菌肽浓度的升高不断增强;溶血实验表明PADM能有效降低RV-23裂解红细胞膜的能力,增加血液相容性;CCK-8实验显示PADM能够有效降低RV-23的细胞毒性,复合材料对人表皮角化细胞HaCaT和小鼠成纤维细胞NIH-3T3几乎没有毒性;Tricine-SDSPAGE实验结果显示复合材料抗菌肽RV-23稳定性较好。结论:PADM/RV-23复合材料比较稳定,不仅具有较强的抗菌性,而且有良好的血液相容性和极低的细胞毒性,有望成为新型创伤修复材料。     

Biomimetic Spider‐Web‐Like Composites for Enhanced Rate Capability and Cycle Life of Lithium Ion Battery Anodes

It is crucial to control the structure and composition of composite anode materials to enhance the cell performance of such anode materials for lithium ion batteries. Herein, a biomimetic strategy is demonstrated for the design of high performance anode materials, inspired by the structural characteristics and working principles of sticky spider‐webs. Hierarchically porous, sticky, spider‐web‐like multiwall carbon nanotube (MWCNT) networks are prepared through a process involving ozonation, ice‐templating assembly, and thermal treatment, thereby integrating the networks with γ‐Fe₂O₃ particles. The spider‐web‐like MWCNT/γ‐Fe₂O₃ composite network not only traps the active γ‐Fe₂O₃ materials tightly but also provides fast charge transport through the 3D internetworked pathways and the mechanical integrity. Consequently, the composite web shows a high capacity of ≈822 mA h g^?1 at 0.05 A g^?1, fast rate capability with ≈72.3% retention at rates from 0.05 to 1 A g^?1, and excellent cycling stability of >88% capacity retention after 310 cycles with a Coulombic efficiency >99%. These remarkable electrochemical performances are attributed to the complementarity of the 3D spider‐web‐like structure with the strong attachment of γ‐Fe₂O₃ particles on the sticky surface. This synthetic strategy offers an environmentally safe, simple, and cost‐effective avenue for the biomimetic design of high performance energy storage materials.     

Micromechanics of fibroblast contraction of a collagen-GAG matrix

The contractile force developed by fibroblasts has been studied by measuring the macroscopic contraction of porous collagen-GAG matrices over time. We have identified the microscopic deformations developed by individual fibroblasts which lead to the observed macroscopic matrix contraction. Observation of live cells attached to the matrix revealed that matrix deformation occurred as a result of cell elongation. The time dependence of the increase in average fibroblast aspect ratio over time corresponded with macroscopic matrix contraction, further linking cell elongation and matrix contraction. The time dependence of average fibroblast aspect ratio and macroscopic matrix contraction was found to be the result of the stochastic nature of cell elongation initiation and of the time required for cells to reach a final morphology (2-4 h). The proposed micromechanics associated with observed buckling or bending of individual struts of the matrix by cells may, in part, explain the observation of a force plateau during macroscopic contraction. These findings indicate that the macroscopic matrix contraction measured immediately following cell attachment is related to the extracellular force necessary to support cell elongation, and that macroscopic time dependence is not directly related to microscopic deformation events.     

Micromechanics of plant tissues beyond the linear-elastic range

We investigated the relation between cell wall structure and the resulting mechanical characteristics of different plant tissues. Special attention was paid to the mechanical behaviour beyond the linear-elastic range, the underlying micromechanical processes and the fracture characteristics. The previously proposed model of reorientation and slippage of the cellulose microfibrils in the cell wall [H.-CH. Spatz et al. (1999) J Exp Biol 202:3269-3272) was supported and is here refined, using measurements of the changes in microfibrillar angle during straining. Our model explains the widespread phenomenon of stress-strain curves with two linear portions of different slope and sheds light on the micromechanical processes involved in viscoelasticity and plastic yield. We also analysed the velocity dependence of viscoelasticity under the perspective of the Kelvin model, resolving the measured viscoelasticity into functions of a velocity-dependent and a velocity-independent friction. The influence of lignin on the above-mentioned mechanical properties was examined by chemical lignin extraction from tissues of Aristolochia macrophylla Lam. and by the use of transgenic plants of Arabidopsis thaliana (L.) Heynh. with reduced lignin content. Additionally, the influence of extraction of hemicelluloses on the mechanical properties was investigated as well as a cell wall mutant of Arabidopsis with an altered configuration of the cellulose microfibrils.     

Micromechanics modeling of Haversian cortical bone properties.

A finite-element micromechanics model for Haversian cortical bone tissue has been developed and studied. The model is an extension of two-dimensional micromechanics techniques for fiber-reinforced composite materials. Haversian systems, or secondary osteons, are considered to be the fiber component, and interstitial lamellar bone the matrix material. The cement line is included as an 'interphase' component along the fiber/matrix interface. The model assumes a regular repeatable spacing of the longitudinally aligned continuous fibers and is, therefore, restricted to approximating Haversian cortical bone in its present form. Haversian porosity is modeled explicitly by incorporating a hollow fiber to represent the Haversian canal. Solutions have been obtained by applying uniform macroscopic stresses to the boundaries of the repeating unit cell model. Macroscopic mechanical property predictions correspond reasonably well with the experimental data for cortical bone, but are necessarily dependent on the input properties for each constituent, which are not well established. The predicted variation in the elastic modulus with porosity is not as sensitive as that observed experimentally. Stresses within the constituents can also be modeled with this method and are demonstrated to deviate from the macroscopic applied stress levels.     

Micromechanics of filopodia mediated capture of pathogens by macrophages

The biological function of filopodia has been extensively studied while only little work has been done on their mechanical properties. In the present study, we apply magnetic microbeads to explore the capturing and initial step of phagocytosis of pathogens by macrophages through filopodia. Microbeads were covered by the bacterial coat protein invasin which is known to trigger the invasion of the intestine by the bacteria Yersinia enterocolitica. These mimetics of bacteria were placed in the vicinity of J774 mouse macrophages exhibiting long filopodia. The specific adhesion of beads to the tip of a filopodium induced the retraction of the protrusion resulting in the dragging of the bead towards the cell body. The dynamics of the retraction process was analyzed by following the in-plane motion of the bead. We estimated the minimal force developed by filopodia and compared the results with previous magnetic tweezer studies of mechanical force induced growth of protrusions (Vonna et al. 2003). We show that very thin filopodia can generate astonishingly large retraction forces over large distances (>10 μm) and can act as an efficient mechanical tool to detach pathogens adhering on surfaces.     

Evaluation of Biological Properties of Electron Beam Melted Ti6Al4V Implant with Biomimetic Coating In Vitro and In Vivo

Background

High strength porous titanium implants are widely used for the reconstruction of craniofacial defects because of their similar mechanical properties to those of bone. The recent introduction of electron beam melting (EBM) technique allows a direct digitally enabled fabrication of patient specific porous titanium implants, whereas both their in vitro and in vivo biological performance need further investigation.

Methods

In the present study, we fabricated porous Ti6Al4V implants with controlled porous structure by EBM process, analyzed their mechanical properties, and conducted the surface modification with biomimetic approach. The bioactivities of EBM porous titanium in vitro and in vivo were evaluated between implants with and without biomimetic apatite coating.

Results

The physical property of the porous implants, containing the compressive strength being 163 - 286 MPa and the Young’s modulus being 14.5–38.5 GPa, is similar to cortical bone. The in vitro culture of osteoblasts on the porous Ti6Al4V implants has shown a favorable circumstance for cell attachment and proliferation as well as cell morphology and spreading, which were comparable with the implants coating with bone-like apatite. In vivo, histological analysis has obtained a rapid ingrowth of bone tissue from calvarial margins toward the center of bone defect in 12 weeks. We observed similar increasing rate of bone ingrowth and percentage of bone formation within coated and uncoated implants, all of which achieved a successful bridging of the defect in 12 weeks after the implantation.

Conclusions

This study demonstrated that the EBM porous Ti6Al4V implant not only reduced the stress-shielding but also exerted appropriate osteoconductive properties, as well as the apatite coated group. The results opened up the possibility of using purely porous titanium alloy scaffolds to reconstruct specific bone defects in the maxillofacial and orthopedic fields.     

Micromechanics of the human vertebral body for forward flexion

To provide mechanistic insight into the etiology of osteoporotic wedge fractures, we investigated the spatial distribution of tissue at the highest risk of initial failure within the human vertebral body for both forward flexion and uniform compression loading conditions. Micro-CT-based linear elastic finite element analysis was used to virtually load 22 human T9 vertebral bodies in either 5° of forward flexion or uniform compression; we also ran analyses replacing the simulated compliant disc (E=8 MPa) with stiff polymethylmethacrylate (PMMA, E=2500 MPa). As expected, we found that, compared to uniform compression, forward flexion increased the overall endplate axial load on the anterior half of the vertebra and shifted the spatial distribution of high-risk tissue within the vertebra towards the anterior aspect of the vertebral body. However, despite that shift, the high-risk tissue remained primarily within the central regions of the trabecular bone and endplates, and forward flexion only slightly altered the ratio of cortical-to-trabecular load sharing at the mid-vertebral level (mean±SD for n=22: 41.3±7.4% compression; 44.1±8.2% forward flexion). When the compliant disc was replaced with PMMA, the anterior shift of high-risk tissue was much more severe. We conclude that, for a compliant disc, a moderate degree of forward flexion does not appreciably alter the spatial distribution of stress within the vertebral body.     

Micromechanics and anatomical changes during early ontogeny of two lianescent Aristolochia species

 The mechanical properties of young stems of Aristolochia macrophylla Lam. and Aristolochia brasiliensis Mart. et Zucc. were studied during elongation growth and primary differentiation. Data for the modulus of elasticity, for the viscoelastic behaviour caused by longitudinal tension and for the shear modulus resulting from torsion around a longitudinal axis were related to the underlying structural changes by quantitative analysis of stem anatomy, tissue distribution, ultrastructure, and cell wall biochemistry. The orientation of cellulose microfibrils was determined by light microscopy and small-angle X-ray diffraction, and the lignin content was determined by thioglycolic acid derivatization and spectroscopic quantification. It was demonstrated that the increase in stability during early development is due to the complementary effects of increase in cell wall material, lignification, and cellulose microfibril alignment. A detailed micromechanical model, considering internal prestresses, is proposed to explain the characteristic biphasic stress-strain behaviour as well as the strain-hardening observed. Received: 22 March 1999 / Accepted 9 September 1999     

A Micromechanics Model for Turgor Pressure of Arabidopsis thaliana Protoplast

Understanding the key role of turgor pressure in plant growth and development is important for recognizing the mechanical behavior of plant cell wall material deposition. In this study, we developed a micromechanics model to demonstrate how uniaxial strain influences turgor pressure of isolated Arabidopsis thaliana protoplasts, and their deformation and morphogenesis. In this model, the protoplast is treated as an elastic inclusion in a surrounding agarose gel, allowing the turgor pressure in response to the 20 % uniaxial strain exerted on the protoplast–agarose gel composite material system. Based on the Eshelby method and the Mori–Tanaka’s theory (Eshelby in Proc R Soc Lond A 241(1226):376–396, 1957; Mori and Tanaka in Acta Metall 21(5):571–574, 1973), turgor pressure can be taken into account as a uniform strain acting on protoplasts. By using this model, the relationship between the plant cell morphology changes, and their effective properties are derived with a theoretical basis.     

Biomimetic nanosystems and novel composite nanobiomaterials

Biophysicochemical approaches to the solution of nanotechnology problems associated with the design of functional biomimetic nanosystems, hybrid and composite nanobiomaterials and study of their structure-function relationships. The results of studies concerned with physicochemical mechanisms of the formation of organized biomimetic nanostructures and bioinorganic nanomaterials in systems involving a bulky liquid phase and the interface (gas-liquid, solid-liquid, liquid-liquid)during the synthesis and structure formation with the participation of the components of colloid systems, inorganic nanoparticles of various composition and clusters of metals, surfactants, polyelectrolytes and their complexes are discussed. In the development of the methods for the formation of composite bioinorganic nanosystems containing inorganic nanocomponents, two major approaches were used: adsorption and incorporation into the biomolecular matrix or colloid system of presynthesized inorganic nanoparticles, as well as the synthesis of the inorganic nanophase immediately in the biomolecular system. The methods of obtaining biomaterials and nanosystems are based on the principles of biomimetics, biomineralization, self-assembly and self-organization, combination and integration of a number of synthetic and physicochemical methods (physical and chemical adsorption, Langmuir technique, the formation of polycomplexes, chemical linking, competitive interactions, and substitution of ligands in supramolecular and coordination complexes) and nanocomponents of different nature. In particular, a novel approach to the preparation of highly organized nanofilm materials was developed, which is based on the effect of self-assembly and self-organization of colloid nanoparticles during the formation of their complexes with polyfunctional biogenic ligands in the volume of the liquid phase in the absence of any surfaces and interfaces. The physical and chemical factors responsible for the formation of structurally ordered biomolecular and composite nanosystems including nano-sized components of different nature and the possibilities to control the composition, structure, and properties of resulting nanomaterials and nanosystems are discussed. The experimental methods and approaches developed may be useful in studies of structure-property relationships and basic mechanisms of structural organization and transformation at the nanoscales level in biological, artificial, and hybrid nanosystems. The problems of practical application of the synthetic methods and the corresponding nanomaterials are discussed.     

Development of Biomimetic Squid-Inspired Suckers

Biomechanical properties of squid suckers were studied to provide inspiration for the development of sucker artefacts for a robotic octopus.Mechanical support of the rings found inside squid suckers was studied by bending tests.Tensile tests were carried out to study the maximum possible sucking force produced by squid suckers based on the strength of sucker stalks,normalized by the sucking areas.The squid suckers were also directly tested to obtain sucking forces by a special testing arrangement.Inspired by the squid suckers,three types of sucker artefacts were developed for the arm skin of an octopus inspired robot.The first sucker artefact made of knitted nylon sheet reinforced silicone rubber has the same shape as the squid suckers.Like real squid suckers,this type of artefact also has a stalk that is connected to the arm skin and a ring to give radial support.The second design is a straight cylindrical structure with uniform wall thickness made of silicone rubber.One end of the cylinder is directly connected to the arm skin and the other end is open.The final design of the sucker has a cylindrical base and a concave meniscus top.The meniscus was formed naturally using the surface tension of silicone gel,which leads to a higher level of the liquid around the edge of a container.The wall thickness decreases towards the tip of the sucker opening.Sucking forces of all three types of sucker artefacts were measured.Advantages and disadvantages of each sucker type were discussed.The final design of suckers has been implemented to the arm skin prototypes.     

Biomimetic design of nanopatterned membranes

The biomimetic approach copying the supramolecular building principle of many archaeal cell envelopes (i.e., a plasma membrane with associated S-layer proteins) has resulted in stable lipid membranes with excellent reconstitution properties for transmembrane proteins. This is a particular challenge as one-third of all proteins in an organism are membrane proteins like pores, ion channels, or receptors. At S-layer supported lipid membranes, spatial well-defined domains on the S-layer protein interact noncovalently with lipid head groups within the lipid membrane resulting in a nanopatterning of a few anchored and scores of diffusional free-lipid molecules. In addition, no impact on the hydrophobic core region and on the function of reconstituted integral proteins has been determined. Among others, particularly S-layer stabilized membranes can be used for structure-function studies on reconstituted integral proteins and also in the membrane protein-based molecular nanotechnology, e.g., in the design of biosensing devices (e.g., lipid chip or lab-on-a-chip), or for receptor or ion channel-based high-throughput screening.