It would be preferable to add a fraction of your plant extract isolated, for example by HPLC, to specific media for sperm (not PBS). You could add it to a commercial extender used to keep sperm refrigerated for several days, or add it to the media used for sperm capacitation (for human sperm you could use commercial IVF media). You could test different concentrations of your extract in order to detect positive or detrimental effect over sperm capacitation, acrosome reaction, motility, viability... . As an example of a study concerning a polypenol extract you can read this paper done with boar spermatozoa: Article Biological effects of polyphenol-rich extract and fractions ...
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