Optoelectronic tweezers are able to distinguish between live and dead sperm cells, even if they aren't moving, say US scientists.
An important part of in vitro fertilisation (IVF) techniques is selecting and injecting an individual sperm cell into an egg. The quality of the chosen sperm is critical to the success of the procedure and is currently assessed by an operator looking at sperm movement under a microscope. However, sperm that don't move are not necessarily dead, and it is nearly impossible to assess their viability visually.
To combat this problem, Aaron Ohta at the University of Hawaii and his team have demonstrated that optoelectronic tweezers - which use a combination of light and electric fields to control microscopic objects - can distinguish and sort between live and dead cells, irrespective of mobility.
Sperm injection for in vitro fertilisation requires selection of a viable individual sperm
Sperm cells are suspended in a low-conductivity salt solution, against which the inside of live cells have a higher ionic concentration. However, as the sperm die, their cell membranes become permeable and the internal ion concentrations become more similar to the solution. The researchers placed samples in a microfluidic chamber, that has indium tin oxide-coated glass on both the top and bottom sides with a 1 µm thick layer of amorphous silicon on the bottom. They then applied a current across the top and bottom, creating a uniform electric field within the chamber. When they shine light onto the amorphous silicon layer, they can make small regions more conductive. This electric field gradient then attracts live cells and repels dead ones, owing to their different ionic concentrations.
'There is a significant cost and time saving,' explains Ohta, 'because this is a more automated method than the standard technique of looking at the sample and trying to identify which cells are viable based on morphology.'
'This is something that could be easily implemented right away,' says Ketan Bhatt, an expert in dielectrophoresis at the CFD Research Corporation in Alabama, US. 'It shows a very clear separation between the sperm cells that are alive and those that are not,' he adds.
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Yuandi Li
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