The well-known self-propulsion, or swimming, of a deformable body in Stokes flow (i.e. at low Reynolds number) can be understood and modelled from the variation in the configuration-dependent hydrodynamic resistance tensor throughout the period of deformation. Remarkably, at the other extreme of high Reynolds number, a deformable body may also self-propel without doing any net work on the fluid in potential flow. As a body deforms, the mass of fluid displaced – the so-called added mass – depends on the instantaneous body configuration, and a net displacement is possible over a period of deformation. This potential swimming takes a form identical to that for Stokes swimmers with the configuration-dependent added mass replacing the hydrodynamic resistance tensor. Analytical insight into the swimming of a deformable body is obtained through an expansion of the nonlinear spatial dependence of the hydrodynamic interactions and connections between previous studies of swimming in Stokes flow to those in potential flow are made.

Since its development, Stokesian dynamics has been a leading approach for the dynamic simulation of suspensions of particles at arbitrary concentrations with full hydrodynamic interactions. Although developed originally for the simulation of passive particle suspensions, the Stokesian dynamics framework is equally well suited to the analysis and dynamic simulation of suspensions of active particles, as we elucidate here. We show how the reciprocal theorem can be used to formulate the exact dynamics for a suspension of arbitrary active particles, and then show how the Stokesian dynamics method provides a rigorous way to approximate and compute the dynamics of dense active suspensions where many-body hydrodynamic interactions are important.

Modern biomedical applications such as targeted drug delivery require a delivery system capable of enhanced transport beyond that of passive Brownian diffusion. In this work, an osmotic mechanism for the propulsion of a vesicle immersed in a viscous fluid is proposed. By maintaining a steady-state solute gradient inside the vesicle, a seepage flow of the solvent (e.g. water) across the semipermeable membrane is generated, which in turn propels the vesicle. We develop a theoretical model for this vesicle–solute system in which the seepage flow is described by a Darcy flow. Using the reciprocal theorem for Stokes flow, it is shown that the seepage velocity at the exterior surface of the vesicle generates a thrust force that is balanced by the hydrodynamic drag such that there is no net force on the vesicle. We characterize the motility of the vesicle in relation to the concentration distribution of the solute confined inside the vesicle. Any osmotic solute is able to propel the vesicle so long as a concentration gradient is present. In the present work, we propose active Brownian particles (ABPs) as a solute. To maintain a symmetry-breaking concentration gradient, we consider ABPs with spatially varying swim speed, and ABPs with constant properties but under the influence of an orienting field. In particular, it is shown that at high activity, the vesicle velocity is $\boldsymbol {U}\sim [K_\perp /(\eta _e\ell _m) ]\int \varPi _0^{swim} \boldsymbol {n}\,{\rm d}\varOmega$, where $\varPi _0^{swim}$ is the swim pressure just outside the thin accumulation boundary layer on the vesicle interior surface, $\boldsymbol {n}$ is the unit normal vector of the vesicle boundary, $K_\perp$ is the membrane permeability, $\eta _e$ is the viscosity of the solvent, and $\ell _m$ is the membrane thickness.

Studies of introduced subject matter in rock-art assemblages typically focus on themes of cross-cultural interaction, change and continuity, power and resistance. However, the economic frameworks guiding or shaping the production of an assemblage have often been overlooked. In this paper we use a case study involving a recently recorded assemblage of introduced subject matter from Marra Country in northern Australia's southwest Gulf of Carpentaria region to explore their production using a hybrid economy framework. This framework attempts to understand the nature of the forces that shape people's engagement with country and subsequently how it is being symbolically marked as adjustments to country occur through colonization. We argue that embedding these motifs into a hybrid economy context anchored in the pastoral industry allows for a more nuanced approach to cross-cultural interaction studies and adds another layer to the story of Aboriginal place-marking in colonial contexts. This paper aims to go beyond simply identifying motifs thought to represent introduced subject matter, and the cross-cultural framework(s) guiding their interpretation, and instead to direct attention to the complex network of relations that potentially underpin the production of such motifs.

Background: Antibiotics targeted against Clostridioides difficile bacteria are necessary, but insufficient, to achieve a durable clinical response because they have no effect on C. difficile spores that germinate within a disrupted microbiome. ECOSPOR-III evaluated SER-109, an investigational, biologically derived microbiome therapeutic of purified Firmicute spores for treatment of rCDI. Herein, we present the interim analysis in the ITT population at 8 and 12 weeks. Methods: Adults ≥18 years with rCDI (≥3 episodes in 12 months) were screened at 75 US and CAN sites. CDI was defined as ≥3 unformed stools per day for <48 hours with a positive C. difficile assay. After completion of 10–21 days of vancomycin or fidaxomicin, adults with symptom resolution were randomized 1:1 to SER-109 (4 capsules × 3 days) or matching placebo and stratified by age (≥ or <65 years) and antibiotic received. Primary objectives were safety and efficacy at 8 weeks. Primary efficacy endpoint was rCDI (recurrent toxin+ diarrhea requiring treatment); secondary endpoints included efficacy at 12 weeks after dosing. Results: Overall, 287 participants were screened and 182 were randomized (59.9% female; mean age, 65.5 years). The most common reason for screen failure was a negative C. difficile toxin assay. A significantly lower proportion of SER-109 participants had rCDI after dosing compared to placebo at week 8 (11.1% vs 41.3%, respectively; relative risk [RR], 0.27; 95% confidence interval [CI], 0.15–0.51; p-value <0.001). Efficacy rates were significantly higher with SER-109 vs placebo in both stratified age groups (Figure 1). SER-109 was well-tolerated with a safety profile similar to placebo. The most common treatment-emergent adverse events (TEAEs) were gastrointestinal and were mainly mild to moderate. No serious TEAEs, infections, deaths, or drug discontinuations were deemed related to study drug. Conclusions: SER-109, an oral live microbiome therapeutic, achieved high rates of sustained clinical response with a favorable safety profile. By enriching for Firmicute spores, SER-109 achieves high efficacy while mitigating risk of transmitting infectious agents, beyond donor screening alone. SER-109 represents a major paradigm shift in the clinical management of patients with recurrent CDI. Clinicaltrials.gov Identifier NCT03183128. These data were previously presented as a late breaker at American College of Gastroenterology 2020.

Funding: Seres Therapeutics

Disclosures: None

Figure 1.

A new continuum perspective for phoretic motion is developed that is applicable to particles of any shape in ‘microstructured’ fluids such as a suspension of solute or bath particles. Using the reciprocal theorem for Stokes flow it is shown that the local osmotic pressure of the solute adjacent to the phoretic particle generates a thrust force (via a ‘slip’ velocity) which is balanced by the hydrodynamic drag such that there is no net force on the body. For a suspension of passive Brownian bath particles this perspective recovers the classical result for the phoretic velocity owing to an imposed concentration gradient. In a bath of active particles that self-propel with characteristic speed $U_0$ for a time $\tau _R$ and then change direction randomly, taking a step of size $\ell = U_0 \tau _R$, at high activity the phoretic velocity is $\boldsymbol {U} \sim - U_0 \ell \boldsymbol {\nabla } \phi _b$, where $\phi _b$ is a measure of the ‘volume’ fraction of the active bath particles. The phoretic velocity is independent of the size of the phoretic particle and of the viscosity of the suspending fluid. Because active systems are inherently out of equilibrium, phoretic motion can occur even without an imposed concentration gradient. It is shown that at high activity when the run length varies spatially, net phoretic motion results in $\boldsymbol {U} \sim - \phi _b U_0 \boldsymbol {\nabla } \ell$. These two behaviours are special cases of the more general result that phoretic motion arises from a gradient in the swim pressure of active matter. Finally, it is shown that a field that orients (but does not propel) the active particles results in a phoretic velocity $\boldsymbol {U} \sim - \phi _b U_0 \ell \boldsymbol {\nabla }\varPsi$, where $\varPsi$ is the (non-dimensional) potential associated with the field.