<P> Cyrogenic grinding, also known as freezer milling, freezer grinding, and cryomilling, is the act of cooling or chilling a material and then reducing it into a small particle size . For example, thermoplastics are difficult to grind to small particle sizes at ambient temperatures because they soften, adhere in lumpy masses and clog screens . When chilled by dry ice, liquid carbon dioxide or liquid nitrogen, the thermoplastics can be finely ground to powders suitable for electrostatic spraying and other powder processes . Cryogenic grinding of plant and animal tissue is a technique used by microbiologists . Samples that require extraction of nucleic acids must be kept at − 80 ° C or lower during the entire extraction process . For samples that are soft or flexible at room temperature, cryogenic grinding may be the only viable technique for processing samples . A number of recent studies report on the processing and behavior of nanostructured materials via cryomilling . </P> <P> Freezer milling is a type of cryogenic milling that uses a solenoid to mill samples . The solenoid moves the grinding media back and forth inside the vial, grinding the sample down to analytical fineness . This type of milling is especially useful in milling temperature sensitive samples, as samples are milled at liquid nitrogen temperatures . The idea behind using a solenoid is that the only "moving part" in the system is the grinding media inside the vial . The reason for this is that at liquid nitrogen temperatures (--196 ° C) any moving part will come under huge stress leading to potentially poor reliability . Cryogenic milling using a solenoid has been used for over 50 years and has been proved to be a very reliable method of processing temperature sensitive samples in the laboratory . </P>

What is the function of liquid nitrogen in grinding the tissue