Two-dimensional planar swimming selects for high DNA integrity sperm

Collective sperm movement in mammalian reproductive tracts

Mammalian sperm cells travel from their origin in the male reproductive tract to fertilization in the female tract through a complex process driven by coordinated mechanical and biochemical mechanisms. Recent experimental and theoretical advances have illuminated the collective behaviors of sperm both in vivo and in vitro. However, our understanding of the underlying mechano-chemical processes remains incomplete. This review integrates current insights into sperm group movement, examining both immotile and motile states, which are essential for passive transport and active swimming through the reproductive tracts. We provide an overview of the current understanding of collective sperm movement, focusing on the experimental and theoretical mechanisms behind these behaviors. We also explore how sperm motility is regulated through the coordination of mechanical and chemical processes. Emerging evidence highlights the mechanosensitive properties of a sperm flagellum, suggesting that mechanical stimuli regulate flagellar beating at both individual and collective levels. This self-regulatory, mechano-chemical system reflects a broader principle observed in multicellular systems, offering a system-level insight into the regulation of motility and collective dynamics in biological systems.

Microfluidics: The future of sperm selection in assisted reproduction

BACKGROUND

Obtaining functional sperm cells is the first step to treat infertility. With the ever-increasing trend in male infertility, clinicians require access to effective solutions that are able to single out the most viable spermatozoa, which would max out the chance for a successful pregnancy. The new generation techniques for sperm selection involve microfluidics, which offers laminar flow and low Reynolds number within the platforms can provide unprecedented opportunities for sperm selection. Previous studies showed that microfluidic platforms can provide a novel approach to this challenge and since then researchers across the globe have attacked this problem from multiple angles.

OBJECTIVE

In this review, we seek to provide a much-needed bridge between the technical and medical aspects of microfluidic sperm selection. Here, we provide an up-to-date list on microfluidic sperm selection procedures and its application in assisted reproductive technology laboratories.

SEARCH METHOD

A literature search was performed in Web of Science, PubMed, and Scopus to select papers reporting microfluidic sperm selection using the keywords: microfluidic sperm selection, self-motility, non-motile sperm selection, boundary following, rheotaxis, chemotaxis, and thermotaxis. Papers published before March 31, 2023 were selected.

OUTCOMES

Our results show that most studies have used motility-based properties for sperm selection. However, microfluidic platforms are ripe for making use of other properties such as chemotaxis and especially rheotaxis. We have identified that low throughput is one of the major hurdles to current microfluidic sperm selection chips, which can be solved via parallelization.

CONCLUSION

Future work needs to be performed on numerical simulation of the microfluidics chip prior to fabrication as well as relevant clinical assessment after the selection procedure. This would require a close collaboration and understanding among engineers, biologists, and medical professionals. It is interesting that in spite of two decades of microfluidics sperm selection, numerical simulation and clinical studies are lagging behind. It is expected that microfluidic sperm selection platforms will play a major role in the development of fully integrated start-to-finish assisted reproductive technology systems.

Development of a thermotaxis and rheotaxis microfluidic device for motile spermatozoa sorting

Male infertility is a pervasive global reproductive challenge, primarily attributed to a decline in semen quality. Addressing this concern, there has been a growing focus on spermatozoa sorting in assisted reproductive technology. This study introduces a groundbreaking development in the form of a thermotaxis and rheotaxis microfluidic (TRMC) device designed for efficient motile spermatozoa sorting within a short 15-min timeframe. The TRMC device mimics the natural sperm sorting mechanism of the oviduct, selecting spermatozoa with superior motility and DNA integrity. The experimental outcomes demonstrate a remarkable enhancement in the percentage of progressive spermatozoa following sorting, soaring from 3.90% to an impressive 96.11% when subjected to a temperature decrease from 38 °C to 35 °C. Notably, sperm motility exhibited a substantial 69% improvement. The TRMC device exhibited a commendable recovery rate of 60.93%, surpassing current clinical requirements. Furthermore, the sorted spermatozoa displayed a notable reduction in the DNA fragmentation index to 6.94%, signifying a substantial 90% enhancement in DNA integrity. This remarkable advancement positions the TRMC device as highly suitable for applications in in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), offering a promising solution to male infertility challenges.

Microfluidics as an emerging paradigm for assisted reproductive technology: A sperm separation perspective

Millions of people are subject to infertility worldwide and one in every six people, regardless of gender, experiences infertility at some period in their life, according to the World Health Organization. Assisted reproductive technologies are defined as a set of procedures that can address the infertility issue among couples, culminating in the alleviation of the condition. However, the costly conventional procedures of assisted reproduction and the inherent vagaries of the processes involved represent a setback for its successful implementation. Microfluidics, an emerging tool for processing low-volume samples, have recently started to play a role in infertility diagnosis and treatment. Given its host of benefits, including manipulating cells at the microscale, repeatability, automation, and superior biocompatibility, microfluidics have been adopted for various procedures in assisted reproduction, ranging from sperm sorting and analysis to more advanced processes such as IVF-on-a-chip. In this review, we try to adopt a more holistic approach and cover different uses of microfluidics for a variety of applications, specifically aimed at sperm separation and analysis. We present various sperm separation microfluidic techniques, categorized as natural and non-natural methods. A few of the recent developments in on-chip fertilization are also discussed.

Bio-inspired progressive motile sperm separation using joint rheotaxis and boundary-following behavior

Infertility, as a daunting ever-increasing challenge, poses a worldwide issue to both couples and the healthcare sector. According to the World Health Organization, half of infertility cases are attributed to male factor infertility, either partly or completely. Semen parameters of concern including sperm count, morphology, and motility are deemed to play a vital role in the insemination process. Density gradient centrifugation, being a clinically established procedure for improving on the mentioned parameters, has long been proven to inflict damage on the DNA content of the sperm cells, inducing DNA fragmentation. Herein, a bio-inspired microfluidic device is proposed that capitalizes on the geometry of the uterotubal junction (UTJ) of the female reproductive tract, which can act as a rheological barrier. The device leverages sperm rheotaxis and boundary-following behavior which have been considered as major migratory mechanisms used by sperm during the fertilization process in the female body. The device consists of a series of parallel channels that guide progressive motile sperms into the main sorting channel, where the hydrodynamic barriers created by two consecutive UTJ-like constrictions select sperms based on their propulsive velocity and linearity of motion. The sequential sorting employed here allows for the fractionation of the sperm population into two subpopulations with varying degrees of motility. Both sorted populations showed a significant increase in straight line velocity, reaching 63.4 ± 14.4 μm s-1 and 74 ± 13.8 μm s-1 in the first and second pools, respectively from 35.2 ± 27.2 μm s-1 in raw semen. Additionally, sorted populations demonstrated over 30% reduction in DNA fragmentation index, an indication that the proposed device selects for undamaged sperms with high quality. Apart from the biological superiority of the sorted sperms, this device presents itself as an easy and clinically-applicable method for the separation of progressive motile sperms, while at the same time, benefiting from a straightforward procedure for sperm retrieval.

Lab-on-chip (LoC) application for quality sperm selection: An undelivered promise?

Quality sperm selection is essential to ensure the effectiveness of assisted reproductive techniques (ART). However, the methods employed for sperm selection in ART often yield suboptimal outcomes, contributing to lower success rates. In recent years, microfluidic devices have emerged as a promising avenue for investigating the natural swimming behavior of spermatozoa and developing innovative approaches for quality sperm selection. Despite their potential, the commercial translation of microfluidic-based technologies has remained limited. This comprehensive review aims to critically evaluate the inherent potential of lab-on-chip technology in unraveling sophisticated mechanisms encompassing rheotaxis, thermotaxis, and chemotaxis. By reviewing the current state-of-the-art associated with microfluidic engineering and the swimming of spermatozoa, the goal is to shed light on the multifaceted factors that have impeded the broader commercialization of these cutting-edge technologies and recommend a commercial that can surmount the prevailing constraints. Furthermore, this scholarly exploration seeks to enlighten and actively engage reproductive clinicians in the profound potential and implications of microfluidic methodologies within the context of human infertility.

Human sperm cooperate to transit highly viscous regions on the competitive pathway to fertilization

Human sperm compete for fertilization. Here, we find that human sperm, unexpectedly, cooperate under conditions mimicking the viscosity contrasts in the female reproductive tract. Sperm attach at the head region to migrate as a cooperative group upon transit into and through a high viscosity medium (15-100 cP) from low viscosity seminal fluid. Sperm groups benefit from higher swimming velocity, exceeding that of individual sperm by over 50%. We find that sperm associated with a group possess high DNA integrity (7% fragmentation index) - a stark contrast to individual sperm exhibiting low DNA integrity (> 50% fragmentation index) - and feature membrane decapacitation factors that mediate sperm attachment to form the group. Cooperative behaviour becomes less prevalent upon capacitation and groups tend to disband as the surrounding viscosity reduces. When sperm from different male sources are present, related sperm preferentially form groups and achieve greater swimming velocity, while unrelated sperm are slowed by their involvement in a group. These findings reveal cooperation as a selective mode of human sperm motion - sperm with high DNA integrity cooperate to transit the highly viscous regions in the female tract and outcompete rival sperm for fertilization - and provide insight into cooperation-based sperm selection strategies for assisted reproduction.

Biological benefits of collective swimming of sperm in a viscoelastic fluid

Collective swimming is evident in the sperm of several mammalian species. In bull (Bos taurus) sperm, high viscoelasticity of the surrounding fluid induces the sperm to form dynamic clusters. Sperm within the clusters swim closely together and align in the same direction, yet the clusters are dynamic because individual sperm swim into and out of them over time. As the fluid in part of the mammalian female reproductive tract contains mucus and, consequently, is highly viscoelastic, this mechanistic clustering likely happens in vivo. Nevertheless, it has been unclear whether clustering could provide any biological benefit. Here, using a microfluidic in vitro model with viscoelastic fluid, we found that the collective swimming of bull sperm in dynamic clusters provides specific biological benefits. In static viscoelastic fluid, clustering allowed sperm to swim in a more progressive manner. When the fluid was made to flow in the range of 2.43-4.05 1/sec shear rate, clustering enhanced the ability of sperm to swim upstream. We also found that the swimming characteristics of sperm in our viscoelastic fluid could not be fully explained by the hydrodynamic model that has been developed for sperm swimming in a low-viscosity, Newtonian fluid. Overall, we found that clustered sperm swam more oriented with each other in the absence of flow, were able to swim upstream under intermediate flows, and better withstood a strong flow than individual sperm. Our results indicate that the clustering of sperm can be beneficial to sperm migrating against an opposing flow of viscoelastic fluid within the female reproductive tract.

Lab on a chip devices for fertility: from proof-of-concept to clinical impact

Microfluidics offers tremendous opportunities to understand the underlying biology of fertilization at the single-cell level and improve infertility management, however, its true clinical impact is yet to be realized. Lab-on-a-chip devices have generally failed to diffuse into clinical practice due to issues associated with their translation or their practicality and performance in clinical settings. In this perspective, I reflect on how the full potential of microfluidic technologies for fertility can be realized by considering regulatory and manufacturing considerations at the development stage and by redefining our developmental goals to directly target the ultimate clinical needs. I also challenge the common rationale around developing technologies for infertility treatment based on reducing cost and complexity in operation as the ultimate outcome is invaluable, human life.

Selection of high-quality sperm with thousands of parallel channels

Sperm selection is essential for successful fertilization and embryo development. Current clinical sperm selection methods are labor-intensive and lack the selectivity required to isolate high-quality sperm. Microfluidic sperm selection approaches have shown promise but present a trade-off between the quality and quantity of selected sperm - clinicians demand both. The structure of the female reproductive tract helps to isolate a sufficient quantity of high-quality sperm for fertilization with densely folded epithelium that provides a multitude of longitudinally oriented pathways that guide sperm toward the fertilization site. Here, a three-dimensionally structured sperm selection device is presented that levers this highly parallelized in vivo mechanism for in vitro sperm selection. The device is inserted in a test tube atop 1 mL of raw semen and provides 6500 channels that isolate ∼100 000 high-DNA-integrity sperm for assisted reproduction. In side-by-side clinical testing, the developed approach outperforms the best current clinical methods by improving the DNA integrity of the selected sperm subpopulation up to 95%. Also, the device streamlines clinical workflow, reducing the time required for sperm preparation 3-fold. This single-tube, single-step sperm preparation approach promises to improve both the economics and outcomes of assisted reproduction practices, especially in cases with significant male-factors.

Recent Microfluidic Innovations for Sperm Sorting

Sperm selection is a clinical need for guided fertilization in men with low-quality semen. In this regard, microfluidics can provide an enabling platform for the precise manipulation and separation of high-quality sperm cells through applying various stimuli, including chemical agents, mechanical forces, and thermal gradients. In addition, microfluidic platforms can help to guide sperms and oocytes for controlled in vitro fertilization or sperm sorting using both passive and active methods. Herein, we present a detailed review of the use of various microfluidic methods for sorting and categorizing sperms for different applications. The advantages and disadvantages of each method are further discussed and future perspectives in the field are given.

Organ-on-a-chip technology for the study of the female reproductive system

Over the past decade, organs-on-a-chip and microphysiological systems have emerged as a disruptive in vitro technology for biopharmaceutical applications. By enabling new capabilities to engineer physiological living tissues and organ units in the precisely controlled environment of microfabricated devices, these systems offer great promise to advance the frontiers of basic and translational research in biomedical sciences. Here, we review an emerging body of interdisciplinary work directed towards harnessing the power of organ-on-a-chip technology for reproductive biology and medicine. The focus of this topical review is to provide an overview of recent progress in the development of microengineered female reproductive organ models with relevance to drug delivery and discovery. We introduce the engineering design of these advanced in vitro systems and examine their applications in the study of pregnancy, infertility, and reproductive diseases. We also present two case studies that use organ-on-a-chip design principles to model placental drug transport and hormonally regulated crosstalk between multiple female reproductive organs. Finally, we discuss challenges and opportunities for the advancement of reproductive organ-on-a-chip technology.

Co-Adaptation of Physical Attributes of the Mammalian Female Reproductive Tract and Sperm to Facilitate Fertilization

The functions of the female reproductive tract not only encompass sperm migration, storage, and fertilization, but also support the transport and development of the fertilized egg through to the birth of offspring. Further, because the tract is open to the external environment, it must also provide protection against invasive pathogens. In biophysics, sperm are considered “pusher microswimmers”, because they are propelled by pushing fluid behind them. This type of swimming by motile microorganisms promotes the tendency to swim along walls and upstream in gentle fluid flows. Thus, the architecture of the walls of the female tract, and the gentle flows created by cilia, can guide sperm migration. The viscoelasticity of the fluids in the tract, such as mucus secretions, also promotes the cooperative swimming of sperm that can improve fertilization success; at the same time, the mucus can also impede the invasion of pathogens. This review is focused on how the mammalian female reproductive tract and sperm interact physically to facilitate the movement of sperm to the site of fertilization. Knowledge of female/sperm interactions can not only explain how the female tract can physically guide sperm to the fertilization site, but can also be applied for the improvement of in vitro fertilization devices.

FertDish: microfluidic sperm selection-in-a-dish for intracytoplasmic sperm injection

The selection of high quality sperm is critical for intracytoplasmic sperm injection (ICSI), a prevalent assisted reproduction technology. However, standard selection methods are time-consuming and fail to recover the most viable sperm, thereby limiting the ICSI success rate. Microfluidics enables rapid selection of viable sperm in a manner representing in vivo processes, however, existing platforms lack clinical applicability. Here, we present FertDish, which integrates the clinically established ICSI Petri dish with a film featuring an array of sperm-selecting microchannels for selection of sperm directly from semen. The FertDish format mimics the clinician-familiar ICSI dish setup, and provides rapid (<10 min) single stage sperm preparation that circumvents standard labour-intensive multi-stage sperm processing steps. Tests with human donor and patient semen samples show that FertDish enables the selection of a high quality sperm sub-population, featuring improvements in DNA fragmentation index of more than 91% (donor) and 74% (patient) versus raw semen and 50% (donor) and 63% (patient) versus standard methods, and a distribution of more than 97% sperm with viable and high level DNA. The FertDish enables a high sperm recovery rate (>3.3 × 105 sperm per mL), and is readily adaptable to the clinical workflow with potential to improve ICSI outcomes.

A fully integrated biomimetic microfluidic device for evaluation of sperm response to thermotaxis and chemotaxis

In recent decades, humans have faced greater challenges in reproduction. Assisted reproductive technology is the most prominent approach for addressing this problem. Current clinical screening methods simply consider the motility or morphology of the sperm. However, as the spermatozoa need to navigate over a long distance in the female reproductive tract and survive the natural screening processes therein, these methods are imperfect. Many approaches have been undertaken to study the chemotaxis and thermotaxis navigation behavior of spermatozoa, but few of these have involved integrated screening that considers motility, chemotaxis, and thermotaxis based on the biological environment of the human body. Current routine sperm evaluation techniques are inadequate and fail to simultaneously provide conclusive evidence for the thermotactic and chemotactic characteristics of sperm. Thus, such screening of functional spermatozoa will be an advancement in assisted reproduction. In this study, we developed a fully integrated biomimetic microfluidic system for screening sperm for their characteristics when exposed to temperature and chemical gradients. Based on our results, we showed that spermatozoa were attracted by temperature and chemical gradients in the physiological range. Moreover, we ascertained a suitable temperature gradient range for thermotaxis and statistically proved that the thermotactic and chemotactic responses are not linked. Here, we report the first quantitative study of functional sperm during thermotaxis and chemotaxis, and our analysis of the difference in motility caused by different conditions. More broadly, we foresee the clinical application of these biologically motivated parameters and characteristics in assisted reproduction in humans.

High DNA integrity sperm selection using surface acoustic waves

Male infertility is a global reproductive issue, several clinical approaches have been developed to tackle it, but their effectiveness is limited by the labour-intensive and time-consuming sperm selection procedures used. Here, we present an automated, acoustic based continuous-flow method capable of selecting high quality sperm with considerably improved motility and DNA integrity compared to the initial raw bull semen. The acoustic field translates larger sperm and guides highly motile sperm across the channel width. The result is the selection of sperm with over 50% and 60% improvement in vitality and progressive motility and more than 38% improvement in DNA integrity, respectively, while providing a clinically relevant volume and selected sperm number for the performance of in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) by selecting over 60 000 sperm in under an hour.

A PMMA-Based Microfluidic Device for Human Sperm Evaluation and Screening on Swimming Capability and Swimming Persistence

The selection of high-quality sperm is essential to the success of in vitro fertilization (IVF). As human cervical mucus has a high viscosity, without enough swimming persistence, human sperm clouds cannot arrive at the ampulla to fertilize the egg. In this study, we used swimming capability and motion characteristics that are known to be associated with fertilization ability to evaluate the quality of sperm. Here, a clinically applicable polymethyl methacrylate (PMMA)-based microdevice was designed and fabricated for sperm evaluation and screening for swimming capability and persistence in a viscous environment. In this study, we applied methylcellulose (MC) to mimic the natural properties of mucus in vivo to achieve the selection of motile sperm. Sperm motion was recorded by an inverted microscope. The statistical features were extracted and analyzed. Hundreds of sperm in two treated groups with different concentrations of MC and one control group with human tubal fluid (HTF) media were video recorded. This device can achieve a one-step procedure of high-quality sperm selection and achieve the quantitative evaluation of sperm swimming capability and persistence. Sperm with good swimming capability and persistence may be more suitable for fertilization in a viscous environment. This microdevice and methods could be used to guide the evaluation of sperm motility and screening in the future.

Self-organization and multi-line transport of human spermatozoa in rectangular microchannels due to cell-cell interactions

The journey of sperm navigation towards ovum is one of the most important questions in mammalian fertilisation and reproduction. However, we know very little about spermatozoa propagation in a complex fluidic, chemical and topographic environment of a fertility tract. Using microfluidics techniques, we investigate the influence of cell-cell interactions on spermatozoa swimming behavior in constrained environment at different concentrations. Our study shows that at high enough cell concentration the interaction between boundary-following cells leads to formation of areas with preferential direction of cell swimming. In the microchannel of a rectangular cross-section, this leads to formation of a “four-lane” swimming pattern with the asymmetry of the cell distribution of up to 40%. We propose that this is caused by the combination of cell-cell collisions in the corners of the microchannel and the existence of morphologically different spermatozoa: slightly asymmetric cells with trajectories curved left and the symmetric ones, with trajectories curved right. Our findings suggest that cell-cell interactions in highly folded environment of mammalian reproductive tract are important for spermatozoa swimming behavior and play role in selection of highly motile cells.