We researched and product-tested numerous fills for our products. We tried rice, barley, oats, and a few others, but chose flaxseed for a few good reasons:
The other grains we tested (which are very popular in other hot/cold therapy wrap products) are water based. They didn't hold their heat as long, tended to stick together when frozen (especially when new) and every time you heat it in the microwave, you're drying them out.
Flaxseed is about 42% oil, so you're actually heating the oil inside the seed. The oil holds on to the heat longer like an old fashioned oil heater. When it's frozen, the therapy wrap remains flexible.
We also found that the flaxseed feels silkier, forms to the body nicer and has a nice weight to it.
If you have a hot/cold therapy wrap that is filled with another grain and it's working for you, great! There's nothing wrong with rice, barley, oats or even cherry pits. We just preferred flaxseed for the reasons mentioned.