Using sound waves to select sperm

The use of artificial insemination (AI) is an option for the treatment of cervical factor subfertility, mild-moderate male subfertility and unexplained infertility. Often, as part of the AI procedure, it is necessary to deploy laboratory techniques focusing on the selection and enrichment of motile and functionally competent spermatozoa from the ejaculate¹. 

Sperm selection techniques tend to fall into three categories:

1. Migration (swim-up) techniques: based on the active movement of spermatozoa from treated sperm into an overlaying medium. Sometimes this technique is combined with a sedimentation step.
2. Density gradient techniques: this technique uses centrifugation to separate sperm cells based on their density and motility.
3. Filtration techniques:  motile spermatozoa are separated from immotile sperm cells by means of densely packed glass wool fibres (or beads or membranes).

In an ideal world any sperm separation technique should:

• Be quick, easy and cost-effective
• Isolate as many motile sperm cells as possible
• Not cause sperm damage to the sperm cells
• Eliminate dead sperm and other cells, including leukocytes and bacteria
• Eliminate toxic constituents such as reactive oxygen species.

In reality none of the available techniques match all these criteria, which is why a variety of techniques is necessary in clinical practice to obtain an optimum yield of viable sperm cells for AI. Any new technique that offers an improvement or an addition to the already available tried and tested methods would be an advantage. That is why much interest is being shown in the current work in this area of a research team at Monash University in Melbourne, Australia.

The Monash team utilised an acoustofluidic process, combining sound waves with fluid dynamics to separate sperm cells on the basis of size, shape, and DNA integrity, rather than just motility².

How does it work?

The researchers designed a microfluidic chip, which allows semen to flow through a channel while directional sound waves are applied at different frequencies. Lower quality sperm, such as that harbouring DNA damage, has distinct physical characteristics and so is affected differently by the acoustic waves.

Specifically, the technique relies on applying standing-surface sound waves to create an acoustic radiation force great enough to overcome the fluidic drag acting on sperm: guiding motile sperm across microchannels leaving other (non-viable) sperm and debris to flow out and be discarded.

Dr. Junyang Gai, Project Lead said, “The approach isolates sperm from raw semen by applying an acoustic field at a 30° angle to the flow direction. The acoustic forces direct and push high-quality sperm out of the mainstream, across the microchannel and isolates them in a separate outlet, leaving the general population of sperm in the raw sample.”

Schematic of the microfluidic chip developed by the Monash University team

Micrograph showing movement of viable sperm cells along microfluidic channel

 One of the main advantages of the technique is that it allows for a continuous high throughput.  The device can process up to 140 sperm cells per second, isolating around 60,000 high-quality sperm in 50 minutes (roughly the same number of sperm used in IVF and ICSI procedures). This is around four times faster than current methods of sperm isolation.

Dr. Gai said, “Our results demonstrate that the selected sperm population exhibit a considerably higher percentage of progressively motile sperm (83%), than both the initial raw sample (52%) and the discarded subpopulation of sperm (36%).”

Professor Nosrati, fellow Researcher is equally enthusiastic saying, “Our process aims to select better sperm within a faster time frame, so hopefully this can lead to improved outcomes in assisted reproduction. When fully tested and implemented, this method could open new windows and opportunities for infertile couples to have a baby. We hope that with further testing, our acoustofluidic sperm selection process can provide new opportunities and be of benefit to the assisted reproduction industry, and help remove the fear, anxiety and negative stereotypes associated with infertility.”