Geometrical guidance and trapping transition of human sperm cells

Abstract

The guidance of human sperm cells under confinement in quasi 2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with dilute sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy.PACS numbers: 87.17. Jj, 87.18.Hf, 87.17.Aa, 87.17.Rt Understanding sperm dynamics under confining microgeometries is a general problem and a major challenge both from the basic biophysics and the complex fluids points of view. It is also crucial for microfluidics and biomedical control applications. Our knowledge of the swimming cell motilities in unbounded media cannot be directly extrapolated to their behavior in complex environments such as those found in the oviduct or in the labon-a-chip microfluidic devices used to control and analyze small samples or for in-vitro reproduction procedures. In these cases, the characteristic length scales are of the same order as the cell size, i.e. a few micrometers. Under these circumstances, confined self-propelled microorganisms undergo substantial changes in their locomotion habits, adapting their dynamics to intricate porous media or to solid surfaces vicinity [1], reducing their speed close to boundaries [2] or adjusting their morphology and motility in very narrow channels [3].It has been shown that microswimmers with very different propulsion systems are similarly attracted to the walls and to swim parallel to the surface [2,[4][5][6][7][8][9][10][11]. It is believed that this attractive force has hydrodynamic origin although other possible mechanisms have been proposed [12][13][14]. Several models have been introduced to describe the swimming along surfaces (see Ref.[15] and references therein). Interestingly, the direct observation of the cell-wall attraction (see Fig. 1) have led to the design of ratchet devices that guide and sort self-propelled cells using asymmetric obstacles [16,17]. In particular, different microfluidic devices have been created to either increase sperm cell quality or enhance their concentration [18][19][20]. The creation of inhomogeneous distributions of swimmer populations via asymmetric obstacles has been shown to be particularly efficient for run-and-tumble bacteria [16,[21][22][23]. Alternative ways of achieving nonuniform distributions have also been obtained combining symmetric funnels and flux [24]. Nowadays, numerous theoretical treatments are available to account for the effects of asymmetric obstacles on active particles distributions [25][26][27][28]. Tumbles, rotational diffusion and collisions are efficient mechanisms for separating the cells from the surface, thus permitting bacteria to be reinserted into the bulk of the confining micro...

Disrupting the wall accumulation of human sperm cells by artificial corrugation

“…34 Recently, we used this property to direct and concentrate sperm cells through the geometrically induced rectification of the cell motion by asymmetric U-shaped obstacles. 35 This ratchet effect in effectively two-dimensional systems becomes more pronounced for large perimeter-to-surface ratios and therefore lets us envisage the design and manufacture of miniaturized devices to control the cell dynamics as well as the artificial microswimmer dynamics for selection, testing, concentration, separation [36][37][38] or trapping. 39,40 In this work, we demonstrate that, by properly texturing the sample borders, it is possible to tune the relative density of cells in the interior of a shallow chamber by diminishing the sperm density at its perimeter.…”

Motion of two micro-wedges in a turbulent bacterial bath

“…A static confinement has been shown to be able to stabilize these structures [37], accumulate and guide active particles [38,39,40,41,42,43]. This effect has been used to rectify the motion of swimmers [44,45,46,47,48] and to build sorting [49,50,51,52] as well as trapping devices [53,54,55]. Furthermore the motion of passive but mobile particles submersed in an active fluid has been studied, starting with spherical and curved tracers [56,57,58] to long deformable chains [59].…”

Unusual swelling of a polymer in a bacterial bath

“…Active matter itself has been intensely explored over the last years, both for living systems as bacteria [4], spermatozoa [5] and mammals [6,7] or is system of artificial microswimmers [8][9][10][11][12][13] with various propulsion mechanisms [14][15][16][17] and a plethora of nonequilibrium pattern formation phenomena were discovered [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. At fixed system boundaries active system show distinct clustering and trapping behaviour [34][35][36][37][38][39][40][41][42][43][44][45] and can be expoited to * kaiser@thphy.uni-duesseldorf.de steer the motion of microrotors and microcarriers [46][47][48] of fixed shape.…”