Self-assembled artificial cilia

Abstract

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...

Artificial cilia fabricated using magnetic fiber drawing generate substantial fluid flow

“…Recently, we reported a self-assembling in situ fabrication method of artificial cilia (Wang et al 2013) able to generate a net flow of up to 5 lm/s. Although the fabrication process is more cost-effective than for earlier examples, Vilfan et al 2010;Evans et al 2007;den Toonder et al 2008) expensive magnetic beads were used and the flow generated was rather low for practical lab-on-a-chip applications.…”

“…In most of the previously published work on artificial cilia or flagella, a big drawback for real application is that the fabrication techniques adopted are tedious and costly, as they either require microsystem techniques like photolithography (den Toonder et al 2008;Vilfan et al 2010;Fahrni et al 2009;Belardi et al 2011;Khaderi et al 2011), or rely on expensive sacrificial materials (Evans et al 2007). In order to address this issue, our research is aimed at fabricating artificial cilia in a cost-efficient, cleanroom-free manner, while realizing an effective pumping function that can be practically used in lab-on-achip devices.…”

“…Evans et al (2007) used sacrificial porous polycarbonate membranes as templates to fabricate magnetic polymer composite nanorods that resemble natural cilia both in terms of their size and aspect ratio. Vilfan et al (2010) exploited the self-assembly of magnetic beads and created magnetically linked chains as artificial cilia anchored to electroplated nickel dots. The same process was used by Babataheri et al with the additional use of a polymer coating (polyacrylic acid) to link the chains permanently to the substrate (Babataheri et al 2011).…”

A continuum model for flow induced by metachronal coordination between beating cilia

“…A brief overview of experimental work is given by Zhou & Liu (2008). To date, systems with artificial cilia are based on a synchronous cilia motion (see Shields et al 2010;Vilfan et al 2009). If cilia beat in synchrony, a strongly oscillating forward-backward flow is induced in the viscous regime and net fluid transport can be achieved only if the cilia beat cycle exhibits a configurationally asymmetric beat cycle.…”

Biomimetics and cilia propulsion

“…(Compare this with figure 2c of Babataheri et al 2011. ) Vilfan et al (2010) have demonstrated a pumping action based on this strategy using non-bonded magnetic chains. While described as artificial cilia, it is closer in concept to attaching bacteria to a surface in a carpet as in the experiments by Darnton et al (2004).…”