Transferosomes as Drug Delivery Systems: Design Principles, Deformability, and Translational Challenges.

Transferosomes are liposome-derived ultradeformable vesicles designed to improve drug delivery across restrictive biological barriers, particularly in non-invasive administration routes. Their structure is based on phospholipid bilayers modified with edge activators, usually surfactants or bile salts, which increase membrane flexibility while preserving vesicular organization. This balance between deformability and stability distinguishes transferosomes from conventional liposomes and has supported their use in dermal, transdermal, ocular, nasal, buccal, and other mucosal delivery systems. However, despite extensive experimental interest, the field remains limited by inconsistent terminology, heterogeneous formulation strategies, non-harmonized deformability assays, and incomplete translation from laboratory formulations to clinically relevant products. This review critically examines transferosomes from a formulation-development perspective, focusing on the relationship between lipid composition, edge-activator selection, vesicle properties, deformability, drug release, and biological performance. Particular attention is given to critical quality attributes, analytical characterization, mechanistic interpretations of barrier interaction, and the unresolved debate between intact vesicle penetration, drug-release-dominated delivery, and barrier perturbation. Transferosomes are also positioned in comparison with conventional liposomes, ethosomes, and transethosomes. Finally, the review identifies key unmet needs related to standardization, reproducibility, scalability, storage stability, and regulatory uncertainty. By integrating formulation design with mechanistic and translational analysis, this review aims to clarify when transferosomes offer a genuine delivery advantage and which parameters must be controlled to support their further pharmaceutical development.