4. Inventories

Inventories are comprised of unprocessed tissue, work-in-process, Avance® Nerve Graft, Axoguard® Nerve Connector, Axoguard® Nerve Protector, Avive® Soft Tissue Membrane, Acroval® Neurosensory and Motor Testing System, Axotouch® Two-Point Discriminator and supplies and are valued at the lower of cost (first-in, first-out) or net realizable value and consist of the following:

For the years ended December 31, 2018, 2017 and 2016, the Company had inventory write-downs of $1,343,  $1,438 and $788, respectively, primarily relating to product expiration.