Pullulan-based biomaterials: Bridging structural chemistry and modification strategies with clinical translation

Pullulan, a linear microbial α-glucan composed of maltotriose units linked by alternating α-(1 → 4) and α-(1 → 6) glycosidic bonds, has evolved from a food-grade excipient into a versatile platform for nanomedicine and regenerative biomaterials. This review synthesizes the structure-activity relationships (SAR) of pullulan and its derivatives, linking molecular design with macroscopic material performance and clinical potential across diverse biomedical applications. The presence of three hydroxyl groups per glucose unit (nine per maltotriose) enables diverse and site-specific chemical modifications, transforming an otherwise biologically inert backbone into highly functional and tunable biomaterials. Key strategies include cholesteryl grafting for self-assembling nanogels in cancer immunotherapy, periodate oxidation and methacrylation for dynamically crosslinked and 3D-printable hydrogels, and regioselective sulfation for heparin-mimetic anticoagulants. We also examine pullulan's intrinsic hepatotropism via the asialoglycoprotein receptor, enabling liver-targeted drug and gene delivery. Despite progress, including early-phase clinical evaluation of nanogel-based vaccines, broader pharmaceutical translation remains limited. Major challenges include poor mechanical strength of native networks (typically exhibiting a storage modulus (G') < 1 kPa), rapid enzymatic degradation, batch variability in molecular weight, and the lack of parenteral-grade regulatory standards. This review outlines future directions in precision modification, stimuli-responsive design, and standardization for clinical advancement and scalable biomedical applications.