这一不透气的壳层(或边界层)会阻滞水分自由扩散，这种情形称为表面硬化。

This impervious layer or boundary will retard the free diffusion of moisture. This condition is referred to as case hardening.

youdao

第一个，或许是所涉及到的最简单的一个，就是气体的自由扩散。

And the first one, the simplest one, maybe, to cover, is just free expansion of a gas.

youdao

作为抗氧化剂，番茄红素可以保护细胞结构和DNA远离自由基的影响——自由基是一种会破坏健康细胞、使癌细胞扩散的微粒。

As an antioxidant, lycopene protects cell structures and DNA against the nefarious effects of free radicals-small particles that damage healthy cells and allow tumor cells to develop.

youdao

It refers to usually 'receptors' that are fixed in a cell, on a cell membrane, and 'ligands' which are dispersed throughout the body and free to diffuse around, and occasionally will find the cell.

受体这个词通常代表了,固定于细胞表面的分子,而配体分散于体内各处,能够自由扩散,它们偶尔会与细胞邂逅

耶鲁公开课 - 生物医学工程探索课程节选