1,486 research outputs found
Floppy swimming: Viscous locomotion of actuated elastica
Actuating periodically an elastic filament in a viscous liquid generally
breaks the constraints of Purcell's scallop theorem, resulting in the
generation of a net propulsive force. This observation suggests a method to
design simple swimming devices - which we call "elastic swimmers" - where the
actuation mechanism is embedded in a solid body and the resulting swimmer is
free to move. In this paper, we study theoretically the kinematics of elastic
swimming. After discussing the basic physical picture of the phenomenon and the
expected scaling relationships, we derive analytically the elastic swimming
velocities in the limit of small actuation amplitude. The emphasis is on the
coupling between the two unknowns of the problems - namely the shape of the
elastic filament and the swimming kinematics - which have to be solved
simultaneously. We then compute the performance of the resulting swimming
device, and its dependance on geometry. The optimal actuation frequency and
body shapes are derived and a discussion of filament shapes and internal
torques is presented. Swimming using multiple elastic filaments is discussed,
and simple strategies are presented which result in straight swimming
trajectories. Finally, we compare the performance of elastic swimming with that
of swimming microorganisms.Comment: 23 pages, 6 figure
Synchronization of active mechanical oscillators by an inertial load
Motivated by the operation of myogenic (self-oscillatory) insect flight
muscle, we study a model consisting of a large number of identical oscillatory
contractile elements joined in a chain, whose end is attached to a damped
mass-spring oscillator. When the inertial load is small, the serial coupling
favors an antisynchronous state in which the extension of one oscillator is
compensated by the contraction of another, in order to preserve the total
length. However, a sufficiently massive load can sychronize the oscillators and
can even induce oscillation in situations where isolated elements would be
stable. The system has a complex phase diagram displaying quiescent,
synchronous and antisynchrononous phases, as well as an unsual asynchronous
phase in which the total length of the chain oscillates at a different
frequency from the individual active elements.Comment: 5 pages, 4 figures, To appear in Phys. Rev. Let
Experimental Investigations of Elastic Tail Propulsion at Low Reynolds Number
A simple way to generate propulsion at low Reynolds number is to periodically
oscillate a passive flexible filament. Here we present a macroscopic
experimental investigation of such a propulsive mechanism. A robotic swimmer is
constructed and both tail shape and propulsive force are measured. Filament
characteristics and the actuation are varied and resulting data are
quantitatively compared with existing linear and nonlinear theories
What is the role of mitochondrial dysfunction in skin photoaging?
Skin ageing is a complex process involving both internal and external factors, which leads to a progressive loss of cutaneous function and structure. Solar radiation is the primary environmental factor implicated in the development of skin ageing and the term photoageing describes the distinct clinical, histological and structural features of chronically sun-exposed skin. The changes that accompany photoageing are undesirable for aesthetic reasons and can compromise the skin and make it more susceptible to a number of dermatological disorders. As a result, skin ageing is a now topic that is of growing interest and concern to the general population, illustrated by the increased demand for effective interventions that can prevent or ameliorate the clinical changes associated with aged skin. In this viewpoint essay we explore the role that mitochondria play in the process of skin photoageing. There is continuing evidence supporting the proposal that mitochondria dysfunction and oxidative stress are important contributing factors in the development of skin photoageing. Further skin-directed mitochondrial research is warranted to fully understand the impact of mitochondrial status and function in skin health. A greater understanding of the ageing process and the regulatory mechanisms involved could lead to the development of novel preventative andtherapeutic interventions for skin ageing
Complement and cytokine response in acute Thrombotic Thrombocytopenic Purpura
Complement dysregulation is key in the pathogenesis of atypical Haemolytic Uraemic Syndrome (aHUS), but no clear role for complement has been identified in Thrombotic Thrombocytopenic Purpura (TTP). We aimed to assess complement activation and cytokine response in acute antibody-mediated TTP. Complement C3a and C5a and cytokines (interleukin (IL)-2, IL-4, IL-6, IL-10, tumour necrosis factor, interferon-γ and IL-17a) were measured in 20 acute TTP patients and 49 remission cases. Anti-ADAMTS13 immunoglobulin G (IgG) subtypes were measured in acute patients in order to study the association with complement activation. In acute TTP, median C3a and C5a were significantly elevated compared to remission, C3a 63·9 ng/ml vs. 38·2 ng/ml (P < 0·001) and C5a 16·4 ng/ml vs. 9·29 ng/ml (P < 0·001), respectively. Median IL-6 and IL-10 levels were significantly higher in the acute vs. remission groups, IL-6: 8 pg/ml vs. 2 pg/ml (P = 0·003), IL-10: 6 pg/ml vs. 2 pg/ml (P < 0·001). C3a levels correlated with both anti-ADAMTS13 IgG (rs = 0·604, P = 0·017) and IL-10 (rs = 0·692, P = 0·006). No anti-ADAMTS13 IgG subtype was associated with higher complement activation, but patients with the highest C3a levels had 3 or 4 IgG subtypes present. These results suggest complement anaphylatoxin levels are higher in acute TTP cases than in remission, and the complement response seen acutely may relate to anti-ADAMTS13 IgG antibody and IL-10 levels
Seasonal variation in breast cancer diagnosis in Singapore
This study investigates seasonality in the diagnosis of 3219 female breast carcinoma cases reported between 1995–8 in Singapore. There is little evidence of marked seasonal variation. Angular regression suggested that observed differences in peak diagnosis with respect to menopausal status, tumour size, ER and PR status may be chance. © 2001 Cancer Research Campaign http://www.bjcancer.co
Are Asians comfortable with discussing death in health valuation studies? A study in multi-ethnic Singapore
BACKGROUND
To characterize ease in discussing death (EID) and its influence on health valuation in a multi-ethnic Asian population and to determine the acceptability of various descriptors of death and "pits"/"all-worst" in health valuation.
METHODS
In-depth interviews (English or mother-tongue) among adult Chinese, Malay and Indian Singaporeans selected to represent both genders and a wide range of ages/educational levels. Subjects rated using 0–10 visual analogue scales (VAS): (1) EID, (2) acceptability of 8 descriptors for death, and (3) appropriateness of "pits" and "all-worst" as descriptors for the worst possible health state. Subjects also valued 3 health states using VAS followed by time trade-off (TTO). The influence of sociocultural variables on EID and these descriptors was studied using univariable analyses and multiple linear regression (MLR). The influence of EID on VAS/TTO utilities with adjustment for sociocultural variables was assessed using MLR.
RESULTS
Subjects (n = 63, 35% Chinese, 32% Malay, median age 44 years) were generally comfortable with discussing death (median EID: 8.0). Only education significantly influenced EID (p = 0.045). EID correlated weakly with VAS/TTO scores (range: VAS: -0.23 to 0.07; TTO: -0.14 to 0.11). All subjects felt "passed away", "departed" and "deceased" were most acceptable (median acceptability: 8.0) while "sudden death" and "immediate death" were least acceptable (median acceptability: 5.0). Subjects clearly preferred "all-worst" to "pits" (63% vs. 19%, p < 0.001).
CONCLUSION
Singaporeans were generally comfortable with discussing death and had clear preferences for several descriptors of death and for "all-worst". EID is unlikely to influence health preference measurement in health valuation studies
Nonlinear instability in flagellar dynamics: a notel modulation mechanism in sperm migration
Throughout biology, cells and organisms use flagella and cilia to propel fluid and achieve motility. The beating of these organelles, and the corresponding ability to sense, respond to and modulate this beat is central to many processes in health and disease. While the mechanics of flagellum–fluid interaction has been the subject of extensive mathematical studies, these models have been restricted to being geometrically linear or weakly nonlinear, despite the high curvatures observed physiologically. We study the effect of geometrical nonlinearity, focusing on the spermatozoon flagellum. For a wide range of physiologically relevant parameters, the nonlinear model predicts that flagellar compression by the internal forces initiates an effective buckling behaviour, leading to a symmetry-breaking bifurcation that causes profound and complicated changes in the waveform and swimming trajectory, as well as the breakdown of the linear theory. The emergent waveform also induces curved swimming in an otherwise symmetric system, with the swimming trajectory being sensitive to head shape—no signalling or asymmetric forces are required. We conclude that nonlinear models are essential in understanding the flagellar waveform in migratory human sperm; these models will also be invaluable in understanding motile flagella and cilia in other systems
The role of body rotation in bacterial flagellar bundling
In bacterial chemotaxis, E. coli cells drift up chemical gradients by a
series of runs and tumbles. Runs are periods of directed swimming, and tumbles
are abrupt changes in swimming direction. Near the beginning of each run, the
rotating helical flagellar filaments which propel the cell form a bundle. Using
resistive-force theory, we show that the counter-rotation of the cell body
necessary for torque balance is sufficient to wrap the filaments into a bundle,
even in the absence of the swirling flows produced by each individual filament
Searching for a prodrome for rheumatoid arthritis in the primary care record: a case-control study in the Clinical Practice Research Datalink
Background: Rheumatoid arthritis (RA) has articular and non-articular manifestations. Early, intensive treatment has substantial benefit for both. This requires patients be identified as soon as symptoms develop.
Objectives: To determine whether selected signs and symptoms can be identified in the primary care records of patients prior to a formal diagnosis of RA being made and, if so, how early they can be identified.
Methods: A case-control study was constructed within the UK Clinical Practice Research Datalink (CPRD). 3577 individuals with 'definite' RA, were matched to 14287 individuals without inflammatory arthritis. An index date was established (i.e. date general practitioner (GP) first appeared to suspect RA). Rates of consultation and consultations for suspected early RA symptoms were compared in cases and controls in the two years prior to the index date using conditional logistic regression, adjusted for number of consultations.
Results: The mean (standard deviation) age of participants was 58.8 (14.5) years and 66.8% were female. Rates of any consultation were significantly higher in RA cases than in controls for at least two years prior to the index date. Cases were more likely to have a pre-diagnosis coded consultation for joint, and particularly hand symptoms (aOR 11.44 (9.60, 13.63)), morning stiffness (8.10 (3.54, 18.5)), carpal tunnel syndrome (4.57 (3.54, 5.88)) and other non-articular features.
Conclusions: In patients who develop RA, GP consultation rates are higher for at least two years prior to the first recorded suspicion of RA. This study highlights symptoms that should raise a GP’s index of suspicion for RA
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