Motility and swimming: universal description and generic trajectories.

Autonomous locomotion is a ubiquitous phenomenon in biology and in physics of active systems at microscopic scale. This includes prokaryotic, eukaryotic cells (crawling and swimming) and artificial swimmers. An outstanding feature is the ability of these entities to follow complex trajectories, ranging from straight, curved (circular, helical...), to random-like ones. The non-straight nature of these trajectories is often explained as a consequence of the asymmetry of the particle or the medium in which it moves, or due to the presence of bounding walls, etc... Here, we show that for a particle driven by a concentration field of an active species, straight, circular and helical trajectories emerge naturally in the absence of asymmetry of the particle or that of suspending medium. Our proof is based on general considerations, without referring to an explicit form of a model. We show that these three trajectories correspond to self-congruent solutions. Self-congruency means that the states of the system at different moments of time can be made identical by an appropriate combination of rotation and translation of the coordinate space. We show that these solutions are exhibited by spherically symmetric particles as a result of a series of pitchfork bifurcations, leading to spontaneous symmetry breaking in the concentration field driving the particle motility. Self-congruent dynamics in one and two dimensions are analyzed as well. Finally, we present a simple explicit nonlinear exactly solvable model of fully isotropic phoretic particle that shows the transitions from a non-motile state to straight motion to circular motion to helical motion as a series of spontaneous symmetry-breaking bifurcations. Whether a system exhibits or not a given trajectory only depends on the numerical values of parameters entering the model, while asymmetry of swimmer shape, or anisotropy of the suspending medium, or influence of bounding walls are not necessary.

Transfer-learning-based multi-wavelength laser sensor for high fidelity and real-time monitoring of ambient temperature and humidity.

Multi-wavelength laser absorption spectroscopy has the advantages of superior sensitivity, accuracy, and robustness for gas sensing applications, offering an opportunity for the development of high-performance laser-based hygrothermographs. However, accurate and fast determination of gas parameters from multiple spectral features can be quite challenging in the presence of large numbers of features, measurement noise, and increasing demands for real-time measurements. To address this challenge, we propose a transfer-learning-based multi-wavelength laser absorption sensor for the quantitative and simultaneous measurement of temperature and concentration of water vapor, with a focus on real-time monitoring of ambient temperature and relative humidity (RH). A spectral simulation based on the most-updated HITRAN database was employed as the dataset for model pre-training and transfer learning. The experimental dataset was obtained from absorption measurements using a distributed feedback laser that probed multiple water absorption features within the band of 7179-7186c m -1. To evaluate the sensor performance, mean absolute error, error distribution, and linearity were selected. In the presence of an insufficient experimental dataset for direct data training, the proposed transfer learning approach outperformed the traditional deep learning method with a lower prediction error of 0.14°C and 0.42% for temperature and RH, respectively, as compared to the values of 0.84°C and 0.66% obtained using the traditional deep learning method. Finally, the fast data post-processing performance of the proposed transfer learning approach was demonstrated in a field test against the conventional baseline fitting method.

MiR-25 overexpression inhibits titanium particle-induced osteoclast differentiation via down-regulation of mitochondrial calcium uniporter in vitro.

BACKGROUND: Mitochondrial calcium uniporter (MCU) is an important ion channel regulating calcium transport across the mitochondrial membrane. Calcium signaling, particularly via the Ca2+/NFATc1 pathway, has been identified as an important mediator of the osteoclast differentiation that leads to osteolysis around implants. The present study aimed to investigate whether down-regulation of MCU using microRNA-25 (miR-25) mimics could reduce osteoclast differentiation induced upon exposure to titanium (Ti) particles. METHODS: Ti particles were prepared. Osteoclast differentiation of RAW264.7 cells was induced by adding Ti particles and determined by TRAP staining. Calcium oscillation was determined using a dual-wavelength technique. After exposure of the cells in each group to Ti particles or control medium for 5 days, relative MCU and NFATc1 mRNA expression levels were determined by RT-qPCR. MCU and NFATc1 protein expression was determined by western blotting. NFATc1 activation was determined by immunofluorescence staining. Comparisons among multiple groups were conducted using one-way analysis of variance followed by Tukey test, and differences were considered significant if p < 0.05. RESULTS: MCU expression was reduced in response to miR-25 overexpression during the process of RAW 264.7 cell differentiation induced by Ti particles. Furthermore, osteoclast formation was inhibited, as evidenced by the low amplitude of calcium ion oscillation, reduced NFATc1 activation, and decreased mRNA and protein expression levels of nuclear factor-κB p65 and calmodulin kinases II/IV. CONCLUSIONS: Regulation of MCU expression can impact osteoclast differentiation, and the underlying mechanism likely involves the Ca2+/NFATc1 signal pathway. Therefore, MCU may be a promising target in the development of new strategies to prevent and treat periprosthetic osteolysis.

Plasma Spray vs. Electrochemical Deposition: Toward a Better Osteogenic Effect of Hydroxyapatite Coatings on 3D-Printed Titanium Scaffolds.

Surface modification of three-dimensional (3D)-printed titanium (Ti) scaffolds with hydroxyapatite (HA) has been a research hotspot in biomedical engineering. However, unlike HA coatings on a plain surface, 3D-printed Ti scaffolds have inherent porous structures that influence the characteristics of HA coatings and osteointegration. In the present study, HA coatings were successfully fabricated on 3D-printed Ti scaffolds using plasma spray and electrochemical deposition, named plasma sprayed HA (PSHA) and electrochemically deposited HA (EDHA), respectively. Compared to EDHA scaffolds, HA coatings on PSHA scaffolds were smooth and continuous. In vitro cell studies confirmed that PSHA scaffolds have better potential to promote bone mesenchymal stem cell adhesion, proliferation, and osteogenic differentiation than EDHA scaffolds in the early and late stages. Moreover, in vivo studies showed that PSHA scaffolds were endowed with superior bone repair capacity. Although the EDHA technology is simpler and more controllable, its limitation due to the crystalline and HA structures needs to be improved in the future. Thus, we believe that plasma spray is a better choice for fabricating HA coatings on implanted scaffolds, which may become a promising method for treating bone defects.

The Comparative Analysis of Antegrade Versus Retrograde Approach for a Failed Porous Tantalum Rod Removal During Conversion to Total Hip Arthroplasty.

BACKGROUND The failure of porous tantalum rods applied to patients with osteonecrosis of the femoral head (ONFH) has been increasingly reported during the last few years. Very few studies have reported methods for implant removal. This study aimed at comparing 2 procedures used for the removal of a failed tantalum rod during conversion to total hip arthroplasty (THA). MATERIAL AND METHODS A total of 65 patients (65 hips), who underwent THA after failed implantation of a tantalum rod between June 2007 and December 2016, were retrospectively evaluated. These patients were classified into 2 groups depending on whether the antegrade approach (removal of the tantalum rod from the tip to the butt at the lateral femoral cortex, n=27) or retrograde approach (removal of the tantalum rod from the butt at the lateral femoral cortex to the proximal tip, n=38) was used for rod extraction. These 2 groups were compared for incision length, operation time, blood loss, fracture, tantalum debris, Harris hip scores, and the presence of osteolysis and/or radiolucency. RESULTS These 2 groups did not present any significant differences in terms of Harris hip score and incision length. However, the operation time (P=0.000), blood loss (P=0.000), amount of tantalum debris (P=0.000), and presence of radiolucency (P=0.046) were greater for the retrograde approach than for the antegrade approach. CONCLUSIONS The risk of conversion to THA following failed tantalum rod implantation is high. In such cases, the antegrade procedure was found to be a simple and efficient method for removing the trabecular metal rod with the use of a trephine.

Chaotic Swimming of Phoretic Particles.

The swimming of a rigid phoretic particle in an isotropic fluid is studied numerically as a function of the dimensionless solute emission rate (or Péclet number Pe). The particle sets into motion at a critical Pe. Whereas the particle trajectory is straight at a small enough Pe, it is found that it loses its stability at a critical Pe in favor of a meandering motion. When Pe is increased further, the particle meanders at a short scale but its trajectory wraps into a circle at a larger scale. Increasing even further, Pe causes the swimmer to escape momentarily the circular trajectory in favor of chaotic motion, which lasts for a certain time, before regaining a circular trajectory, and so on. The chaotic bursts become more and more frequent as Pe increases, until the trajectory becomes fully chaotic, via the intermittency scenario. The statistics of the trajectory is found to be of the run-and-tumble-like nature at a short enough time and of diffusive nature at a long time without any source of noise.

Amoeboid swimming in a channel.

Several micro-organisms, such as bacteria, algae, or spermatozoa, use flagellar or ciliary activity to swim in a fluid, while many other micro-organisms instead use ample shape deformation, described as amoeboid, to propel themselves either by crawling on a substrate or swimming. Many eukaryotic cells were believed to require an underlying substratum to migrate (crawl) by using membrane deformation (like blebbing or generation of lamellipodia) but there is now increasing evidence that a large variety of cells (including those of the immune system) can migrate without the assistance of focal adhesion, allowing them to swim as efficiently as they can crawl. This paper details the analysis of amoeboid swimming in a confined fluid by modeling the swimmer as an inextensible membrane deploying local active forces (with zero total force and torque). The swimmer displays a rich behavior: it may settle into a straight trajectory in the channel or navigate from one wall to the other depending on its confinement. The nature of the swimmer is also found to be affected by confinement: the swimmer can behave, on average over one swimming cycle, as a pusher at low confinement, and becomes a puller at higher confinement, or vice versa. The swimmer's nature is thus not an intrinsic property. The scaling of the swimmer velocity V with the force amplitude A is analyzed in detail showing that at small enough A, V∼A(2)/η(2) (where η is the viscosity of the ambient fluid), whereas at large enough A, V is independent of the force and is determined solely by the stroke cycle frequency and the swimmer size. This finding starkly contrasts with models where motion is based on ciliary and flagellar activity, where V∼A/η. To conclude, two definitions of efficiency as put forward in the literature are analyzed with distinct outcomes. We find that one type of efficiency has an optimum as a function of confinement while the other does not. Future perspectives are outlined.