Fabrication of Bioinspired Actuated Nanostructures with Arbitrary Geometry and Stiffness

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Due to their small dimensions, microfluidic devices operate in the low Reynolds number regime. In this case, the hydrodynamics is governed by the viscosity rather than inertia and special elements have to be introduced into the system for mixing and pumping of fluids. Here we report on the realization of an effective pumping device that mimics a ciliated surface and imitates its motion to generate fluid flow. The artificial biomimetic cilia are constructed as long chains of spherical superparamagnetic particles, which selfassemble in an external magnetic field. Magnetic field is also used to actuate the cilia in a simple nonreciprocal manner, resulting in a fluid flow. We prove the concept by measuring the velocity of a cilia-pumped fluid as a function of height above the ciliated surface and investigate the influence of the beating asymmetry on the pumping performance. A numerical simulation was carried out that successfully reproduced the experimentally obtained data.biomimetics | microfluidics | colloids | low Reynold's number | hydrodynamics E fficient pumping and mixing of fluids in microscopic channels is paramount in microfluidic applications (1, 2). Small characteristic dimensions of such devices result in very low Reynolds numbers and one encounters hydrodynamics that is conceptually different from the turbulent macroscopic world. As stated by Purcell's "scallop theorem," nonreciprocal motion is required for generation of fluid flow or directed swimming (3). This is clearly manifested in biological systems, for instance in bending waves of sperm tails or in corkscrew motion of bacterial flagella. Another example are cilia, flexible protrusions on the surface of many eukaryotic cells with a typical length of several micrometers. In humans, ciliated surfaces are found, for example, in the respiratory tract where they sweep mucus, or in the Fallopian tubes where they move an ovum to the uterus. The motion of the fluid above a ciliated surface is generated by periodic beating of cilia. Experimental observations have shown that the beating pattern of an individual cilium is asymmetric and composed of two phases: the effective stroke, during which the outstretched cilium propels the fluid like an oar, followed by the recovery stroke, when the bent cilium returns to the initial position sweeping along the surface in a way that produces as little backward flow as possible (4). Although each cilium can beat independently, cilia densely covering a surface synchronize their cycles and form metachronal waves, thus increasing their fluid pumping efficiency (5). It is believed that the metachronal waves occur as a result of hydrodynamical interactions between the cilia (6, 7).The efficiency of the ciliary pumping mechanism leads to the idea of using the same principle for designing artificial cilia that act as microscale pumps and mixers. An important step towards biomimetic cilia was made by Darnton et al., who created a bacterial carpet by attaching bacteria to a solid surface (8). Due to symmetric rotation and wea...

mentioning

confidence: 99%

Self-assembled artificial cilia

Vilfan

Potočnik²,

Kavčič³

et al. 2010

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252

“…Experimentally, a variety of systems at different scales have demonstrated cilia-like actuation based on piezo-driven stage motion (20), substrate swelling (21), linked magnetic beads (22), liquid crystal elastomers (23), self-oscillating gels (24), and electron beam deflection (25). Engineered systems that have succeeded in generating pumping and mixing include live "carpets" of flagellated bacteria that have produced either mixing or directed flows depending on the system geometry (26), and electromechanical (27) and polymeric (28) paddles that have generated transport and mixing but fall outside of the size and hydrodynamic range relevant to ciliated biological systems.…”

mentioning

confidence: 99%

Biomimetic cilia arrays generate simultaneous pumping and mixing regimes

Shields

Fiser

Evans

et al. 2010

223

227

Living systems employ cilia to control and to sense the flow of fluids for many purposes, such as pumping, locomotion, feeding, and tissue morphogenesis. Beyond their use in biology, functional arrays of artificial cilia have been envisaged as a potential biomimetic strategy for inducing fluid flow and mixing in lab-on-a-chip devices. Here we report on fluid transport produced by magnetically actuated arrays of biomimetic cilia whose size approaches that of their biological counterparts, a scale at which advection and diffusion compete to determine mass transport. Our biomimetic cilia recreate the beat shape of embryonic nodal cilia, simultaneously generating two sharply segregated regimes of fluid flow: Above the cilia tips their motion causes directed, long-range fluid transport, whereas below the tips we show that the cilia beat generates an enhanced diffusivity capable of producing increased mixing rates. These two distinct types of flow occur simultaneously and are separated in space by less than 5 μm, approximately 20% of the biomimetic cilium length. While this suggests that our system may have applications as a versatile microfluidics device, we also focus on the biological implications of our findings. Our statistical analysis of particle transport identifying an enhanced diffusion regime provides novel evidence for the existence of mixing in ciliated systems, and we demonstrate that the directed transport regime is Poiseuille-Couette flow, the first analytical model consistent with biological measurements of fluid flow in the embryonic node.biomimetics | embryonic nodal cilia | hydrodynamics | low Reynold's number T he cilium is a biological structure unique in its ability to manipulate and sense its fluid environment (1, 2). Research in the last decade has implicated cilia dysfunction in a wide range of human pathologies (3) and has shown that cilia perform an array of unexpected biological functions (4-6) beyond traditional roles such as the clearance of mucus and pathogens from the airways. For example, embryonic nodal cilia drive a fluid flow that plays a key role in the embryogenesis of vertebrate organisms by generating an asymmetric morphogen concentration (7), and cerebrospinal flows produced by arrays of cilia direct cell traffic in the brain (8). Yet, while the role of directed transport within such systems is being explored, the presence of cilia-generated mixing has only recently engendered speculation (9, 10).Flagellar mixing has been shown to be essential to the health of some microorganisms (11,12). More broadly, cilia-induced mixing could alter rates and efficacies of diverse fluid-mediated processes such as biochemical signaling, regulation, chemotaxis, and chemosensation. In addition, the relationship between vortical flows near the cilia and the directed transport they produce is not well understood for ciliated systems, such as the embryonic node where mixing could affect biochemical signaling that has been shown critical to the establishment of vertebrate left-right body asymmetry ...

“…S2). To clarify the potential role of the biofilm microstructure, we replicated the biofilm in epoxy resin (18) and verified by scanning electron microscopy (SEM) that the topography of the polymeric replicas reproduced the complex features of the biofilm at the micron level (Fig. 5B) (19).…”

mentioning

confidence: 99%

Bacterial biofilm shows persistent resistance to liquid wetting and gas penetration

Epstein

Pokroy

Seminara

et al. 2010

Self Cite

307

257

Most of the world's bacteria exist in robust, sessile communities known as biofilms, ubiquitously adherent to environmental surfaces from ocean floors to human teeth and notoriously resistant to antimicrobial agents. We report the surprising observation that Bacillus subtilis biofilm colonies and pellicles are extremely nonwetting, greatly surpassing the repellency of Teflon toward water and lower surface tension liquids. The biofilm surface remains nonwetting against up to 80% ethanol as well as other organic solvents and commercial biocides across a large and clinically important concentration range. We show that this property limits the penetration of antimicrobial liquids into the biofilm, severely compromising their efficacy. To highlight the mechanisms of this phenomenon, we performed experiments with mutant biofilms lacking ECM components and with functionalized polymeric replicas of biofilm microstructure. We show that the nonwetting properties are a synergistic result of ECM composition, multiscale roughness, reentrant topography, and possibly yet other factors related to the dynamic nature of the biofilm surface. Finally, we report the impenetrability of the biofilm surface by gases, implying defense capability against vapor-phase antimicrobials as well. These remarkable properties of B. subtilis biofilm, which may have evolved as a protection mechanism against native environmental threats, provide a new direction in both antimicrobial research and bioinspired liquid-repellent surface paradigms.antimicrobial resistance | microcomputed tomography | biofilm hydrophobicity | liquid repellency | nonwettability