Researchers used Raman spectroscopy to study how cryoprotectant (glycerol) and ice are distributed in frozen plastic straws commonly used for freezing mammalian semen, oocytes, and embryos. They found that the cryoprotectant is not evenly spread along the straw, with a higher concentration at the bottom where embryos or oocytes are usually placed. This uneven distribution could affect the freezing process and the survival of the embryos or oocytes.