Hitting the wall: Human sperm velocity recovery under ultra-confined conditions

High-throughput selection of sperm with improved DNA integrity and rapidly progressive motility using a butterfly-shaped chip compared to the swim-up method

Microfluidics provides unique opportunities for the high throughput selection of motile sperm with improved DNA integrity for assisted reproductive technologies (ARTs). Here, through a parametric study on dimensions and geometrical angles, a butterfly-shaped chip (BSC) is presented to isolate sperm with high progressive motility and intact DNA at a separation rate of 1125 sperm per minute. Using finite element simulations, the flow field and shear rates in the device were optimized to leverage the inherent motility characteristics of sperm for maximum selection throughput. The device incorporates a triple selection mechanism in series, initially activating sperm rheotaxis by rotation against the semen flow, penetrating the counter buffer flow and swimming against the direction of the buffer flow, leaving dead cells and debris behind, and subsequently leveraging boundary-following behavior to direct progressively motile sperm to swim along the walls and reach the device outlet. The device selects over 4.1 million sperm per mL within 20 minutes, with 29.2%, 68.2%, and 57.3% improvement in total motility, DNA integrity, and velocity parameter (VCL), as compared with the conventional swim-up method, respectively. Overall, the performance of the device to separate sperm with approximately 95.9% total motility, 97.8% viability, and 96.6% DNA integrity at high concentrations demonstrates its potential for enhancing the efficiency of conventional treatment methods.

Selecting active matter according to motility in an acoustofluidic setup: self-propelled particles and sperm cells

Active systems - including sperm cells, living organisms like bacteria, fish, birds, or active soft matter systems like synthetic "microswimmers" - are characterized by motility, i.e., the ability to propel using their own "engine". Motility is the key feature that distinguishes active systems from passive or externally driven systems. In a large ensemble, motility of individual species can vary in a wide range. Selecting active species according to their motility represents an exciting and challenging problem. We propose a new method for selecting active species based on their motility using an acoustofluidic setup where highly motile species escape from the acoustic trap. This is demonstrated in simulations and in experiments with self-propelled Janus particles and human sperm. The immediate application of this method is selecting highly motile sperm for medically assisted reproduction (MAR). Due to the tunable acoustic trap, the proposed method is more flexible than the existing passive microfluidic methods. The proposed selection method based on motility can also be applied to other active systems that require selecting highly motile species or removing immotile species.

A microfluidic method for passive trapping of sperms in microstructures

Sperm motility is a prerequisite for male fertility. Enhancing the concentration of motile sperms in assisted reproductive technologies - for human and animal reproduction - is typically achieved through aggressive methods such as centrifugation. Here, we propose a passive technique for the amplification of motile sperm concentration, with no externally imposed forces or flows. The technique is based on the disparity between probability rates, for motile cells, of entering and escaping from complex structures. The effectiveness of the technique is demonstrated in microfluidic experiments with microstructured devices, comparing the trapping power in different geometries. In these micro-traps, we observe an enhancement of cells' concentration close to 10, with a contrast between motile and non-motile cells increased by a similar factor. Simulations of suitable interacting model sperms in realistic geometries reproduce quantitatively the experimental results, extend the range of observations and highlight the components that are key to the optimal trap design.

Getting to and away from the egg, an interplay between several sperm transport mechanisms and a complex oviduct physiology

In mammals, the architecture and physiology of the oviduct are very complex, and one long-lasting intriguing question is how spermatozoa are transported from the sperm reservoir in the isthmus to the oocyte surface. In recent decades, several studies have improved knowledge of the factors affecting oviduct fluid movement and sperm transport. They report sperm-guiding mechanisms that move the spermatozoa towards (rheotaxis, thermotaxis, and chemotaxis) or away from the egg surface (chemorepulsion), but only a few provide evidence of their occurrence in vivo. This gives rise to several questions: how and when do the sperm transport mechanisms operate inside such an active oviduct? why are there so many sperm guidance processes? is one dominant over the others, or do they cooperate to optimise the success of fertilisation? Assuming that sperm guidance evolved alongside oviduct physiology, in this review we propose a theoretical model that integrates oviduct complexity in space and time with the sperm-orienting mechanisms. In addition, since all of the sperm-guidance processes recruit spermatozoa in a better physiological condition than those not selected, they could potentially be incorporated into assisted reproductive technology (ART) to improve fertility treatment and/or to develop innovative contraceptive methods. All these issues are discussed in this review.