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At least 1,063 records · Page 59Linked to original sources
PubMed2000

Muscle contraction: viscous-like frictional forces and the impulsive model.

Apart from a few experimental studies muscle viscosity has not received much recent analytical attention as a determinant of the contractile process. This is surprising, since any muscle cell is 80% water, and may undergo large shape changes during its working cycle. Intuitively one might expect the viscosity of the solvent to be an important determinant of the physiological activity of muscle tissue. This was apparent to pioneers of the study of muscle contraction such as Hill and his contemporaries, whose putative theoretical formulations contained terms related to muscle viscosity. More recently, though, a hydrodynamic calculation by Huxley, using a solvent viscosity close to that of water, has been held to demonstrate that viscous forces are negligible in muscle contraction. We have re-examined the role of viscosity in contraction, postulating impulsive acto-myosin forces that are opposed by a viscous resistance between the filaments. The viscous force required, 10(4) times the hydrodynamic estimate, is close to recent experimental measurements, themselves 10(2)-10(3) times the hydrodynamic estimate. This also agrees with contemporary measurements of cytoplasmic viscosity in other biological cells using magnetic bead micro-rheometry. These are several orders of magnitude greater than the viscosity of water. In the course of the analysis we have derived the force-velocity equation for an isolated half-sarcomere containing a single actin filament for the first time, and from first principles. We conclude that muscle viscosity is indeed important for the contractile process, and that it has been too readily discounted.

Actins↗
PubMed2001

Muscle contraction: viscous-like frictional forces and the impulsive model.

Apart from a few experimental studies muscle viscosity has not received much recent analytical attention as a determinant of the contractile process. This is surprising, since any muscle cell is 80% water, and may undergo large shape changes during its working cycle. Intuitively, one might expect the viscosity of the solvent to be an important determinant of the physiological activity of muscle tissue. This was apparent to pioneers of the study of muscle contraction such as Hill and his contemporaries, whose putative theoretical formulations contained terms related to muscle viscosity. More recently, though, a hydrodynamic calculation by Huxley, using a solvent viscosity close to that of water, has been held to demonstrate that viscous forces are negligible in muscle contraction. We have re-examined the role of viscosity in contraction, postulating impulsive acto-myosin forces that are opposed by a viscous resistance between the filaments. The viscous force required, 10(4) times the hydrodynamic estimate, is close to recent experimental measurements, themselves 10(2)-10(3) times the hydrodynamic estimate. This also agrees with contemporary measurements of cytoplasmic viscosity in other biological cells using magnetic bead micro-rheometry. These are several orders of magnitude greater than the viscosity of water. In the course of the analysis, we have derived the force-velocity equation for an isolated half-sarcomere containing a single actin filament for the first time, and from first principles. We conclude that muscle viscosity is indeed important for the contractile process, and that it has been too readily discounted.

Actins↗
PubMed1995

A new type of flexor tendon repair. Biomechanical evaluation by cyclic loading, ultimate strength and assessment of pulley friction in vitro.

Experiments were performed to evaluate biomechanical aspects of the performance of a "deep-biting peripheral suture" for flexor tendon repair, either when used alone or with a square or modified Kessler core stitch, and the technique was compared to the Kleinert repair. Tests included progressively increasing cyclic loads, force to pull the repair into the A2 pulley, and ultimate failure strength. 50% of the Kleinert repairs failed under 30 N cyclic loading, while 100% of the DBPS plus Kessler core stitch repairs survived. There was no discernable difference in gliding function or repair bulk between these sutures, but ultimate strength increased significantly with the DBPS repairs. We concluded that the DBPS plus Kessler-type core stitch will survive active mobilization better than the Kleinert method.

Biomechanical Phenomena↗
PubMed1990

Segmental fracture of the femoral stem of a low-friction arthroplasty. A case report.

Fracture of the femoral stem of a cemented hip arthroplasty is a well documented complication. However, double or segmental fracture is extremely rare, with only one other case appearing in the literature. The authors report a second case and discuss its etiology. They stress the importance of prolonged careful review of patients following total hip arthroplasty in order to obviate complications.

Female↗
PubMed2000

Tip friction - torsional spring constant determination.

A non-destructive technique is presented for verifying torsional spring constants used in lateral force microscopy. Various calibrations of the microscope are required and these are detailed. The technique produces reasonable values which tend to be larger than those predicted from considerations of the cantilever dimensions. The differences are discussed in terms of length corrections and particularly the uncertainty in the thickness of the cantilevers, which has an enormous effect on the values obtained through a priori calculations. Methods for inferring the thickness are discussed. Further, artefacts in conventional force measurements related to the experiments performed here are discussed.

Journal Article↗
PubMed1998

Nanomanipulation Experiments Exploring Frictional and Mechanical Properties of Carbon Nanotubes.

: In many cases in experimental science, the instrument interface becomes a limiting factor in the efficacy of carrying out unusual experiments or prevents the complete understanding of the acquired data. We have developed an advanced interface for scanning probe microscopy (SPM) that allows intuitive rendering of data sets and natural instrument control, all in real time. The interface, called the nanoManipulator, combines a high-performance graphics engine for real-time data rendering with a haptic interface that places the human operator directly into the feedback loop that controls surface manipulations. Using a hand-held stylus, the operator moves the stylus laterally, directing the movement of the SPM tip across the sample. The haptic interface enables the user to "feel" the surface by forcing the stylus to move up and down in response to the surface topography. In this way the user understands the immediate location of the tip on the sample and can quickly and precisely maneuver nanometer-scale objects. We have applied this interface to studies of the mechanical properties of nanotubes and to substrate-nanotube interactions. The mechanical properties of carbon nanotubes have been demonstrated to be extraordinary. They have an elastic modulus rivaling that of the stiffest material known, diamond, while maintaining a remarkable resistance to fracture. We have used atomic-force microscopy (AFM) to manipulate the nanotubes through a series of configuration that reveal buckling behavior and high-strain resilience. Nanotubes also serve as test objects for nanometer-scale contact mechanics. We have found that nanotubes will roll under certain conditions. This has been determined through changes in the images and through the acquisition of lateral force during manipulation. The lateral force data show periodic stick-slip behavior with a periodicity matching the perimeter of the nanotube.

Journal Article↗
PubMedDate not supplied

Frictional properties of dilute block-copolymer solutions and homopolymer solutions. Application to molecular weight determination.

An equation for the translational diffusion coefficient of block copolymers in dilute solution has been obtained by modifying Zimm's equation for homopolymers to take into account the existence of dissimilar segments in block copolymers. Illustrative calculations for homopolymers and block copolymers have been made and the results for homopolymers have been compared with experiments and with the calculations of Yamakawa and Fujii. A procedure has been proposed to determine the molecular weight of a block copolymer from measurements of its limiting viscosity number and its sedimentation coefficient or translational diffusion coefficient.

Mathematics↗
PubMed2006

Temperature-activated interfacial friction damping in carbon nanotube polymer composites.

Effect of temperature on interfacial sliding in single-walled carbon nanotube polycarbonate composites is investigated experimentally. We show that interfacial slip at the tube-polymer interfaces can be activated at relatively low dynamic strain levels ( approximately 0.35%) by raising temperature to approximately 90 degrees C. We attribute this to increased mobility of the polymer chain backbones at elevated temperatures and thermal relaxation of the radial compressive stresses at the tube-polymer interfaces. These results show the potential of polymer nanocomposites as high-temperature damping materials for vibration and acoustic suppression in a variety of dynamic systems.

Motion↗