A layer of mucus covers the surface of all wet epithelia throughout the human body. Mucus is a hydrogel mainly composed of water, mucins (glycoproteins), DNA, proteins, lipids, and cell debris. This complex composition yields a tenacious viscoelastic hydrogel that lubricates and protects the exposed epithelia from external threats and enzymatic degradation. The natural protective role of mucus is nowadays acknowledged as a major barrier to be overcome in non-invasive drug delivery. The heterogeneity of mucus components offers a wide range of potential chemical interaction sites for macromolecules, while the mesh-like architecture given to mucus by the intermolecular cross-linking of mucin molecules results in a dense network that physically, and in a size-dependent manner, hinders the diffusion of nanoparticles through mucus. Consequently, drug diffusion, epithelial absorption, drug bioavailability, and ultimately therapeutic outcomes of mucosal drug delivery can be attenuated.

一层粘液覆盖了整个人体所有湿上皮的表面。粘液是一种水凝胶，主要由水，粘蛋白（糖蛋白），DNA，蛋白质，脂质和细胞碎片组成。这种复杂的成分会产生坚韧的粘弹性水凝胶，润滑并保护暴露的上皮细胞免受外部威胁和酶促降解。如今，粘液的天然保护作用被公认为是非侵入性药物输送中要克服的主要障碍。粘液成分的异质性为大分子提供了广泛的潜在化学相互作用位点，而粘液分子分子间交联赋予粘液的网状结构导致了物理上和尺寸依赖的致密网络，阻碍了纳米颗粒通过粘液的扩散。所以，