**Preface to "Strategies to Enhance Drug Permeability across Biological Barriers"**

The delivery of therapeutic drugs to desired sites under required conditions is often challenged by numerous biological barriers in the body, such as the intestinal epithelium membrane, blood–brain barrier (BBB), and skin barrier. Each biological system presents a unique and formidable challenge; for example, the densely packed microvascular epithelial cells of the BBB block almost 100% of macromolecular drugs and 98% of small lipophilic drugs.

Biological systems have a fundamental way of limiting the effectiveness of therapeutic drugs. The collection of articles written by global experts featured in this Special Issue showcases a number of recent advancements which circumvent the barriers of the intestinal epithelium, skin barrier, the BBB, and other barriers such as the lung epithelium membrane.

This Special Issue will explore the wide range of advanced and innovative approaches, technologies, and drug delivery systems that have been implemented to overcome these biological barriers and improve drug permeability and absorption to achieve a wide range of therapeutical goals.

The development of this Special Issue, entitled "Strategies to Enhance Drug Permeability across Biological Barriers", has been a collaborative effort between Professor Wen and Professor Huang. Both editors have extensive experience with the study of the fabrication and mechanisms of efficient drug delivery systems, including oral/transdermal protein–peptide and tumour-targeting drug delivery systems, and they have facilitated many major publications in the field.

#### Acknowledgements

We are very grateful to all authors' contributions to this Special Issue and also are grateful to the reviewers for their enormous help and constructive comments. Additionally, we thank the *Pharmaceutics* management team for offering us the opportunity to edit this Special Issue. Finally, thank you to the readers; we hope you find this read discussing advanced technology to enhance drug permeability across biological barriers informative and enjoyable.

> **Jingyuan Wen and Yuan Huang** *Editors*
