Until now, most approaches relied on impractically scarce materials like iridium as a catalyst that triggers the reaction.
到目前为止，大部分方式还在依赖象铱那样不实用的稀有原料作为引发反应的催化剂。
2
A scroll is of course one long piece of paper or some other writing material that is impractically shaped for keeping flat.
羊皮纸是很长的纸条或者其他可用于书写的材料，若想保持平整，这样的形状并不合适。
3
Scientists now use them to measure the climate in areas that would have been impractically small when sensors were more costly-say under individual plants rather than in a thicket.