Owing to high bioactivity and specificity, protein drugs have achieved great success in both diagnosis and therapy. The idea of delivering active proteins directly to targets is becoming more and more attractive. However, due to their large size and environmental sensitivity, it is not easy for proteins to maintain bioactivity in extracellular fluids and cross cell membrane without being damaged. A series of techniques and carriers have been developed to deliver proteins. Thereinto, metal–organic frameworks (MOFs), which are made of metal ions connected by organic linkers, are one of the most important vehicles for protein delivery. Their porous structures, stability, biocompatibility, reproducibility, and the possibility for further functionalities offer great potential for protein delivery. In this review, recent developments of protein encapsulated by MOF nanoparticles, including mechanism of treatment, structural design, loading capacity, delivery, and release properties of proteins, are summarized, and the relationship between structure and performance is emphasized. Meanwhile, further improvements and implementations are discussed.

Sodium alginate-based hydrogel was the one of the most used polymers for cell delivery. However, the adsorption of extracellular matrix and proteins was inhibited due to the formation of a hydrated surface layer of these hydrogels. In this study, a novel cell delivery system, negatively-charged alginate and chondroitin sulfate microsphere hydrogel (nCACSMH), was fabricated with excellent permeability and biocompatibility in the action of a high voltage direct-current electric field. Negative charge was introduced to the surface of nCACSMH to obtain the expanded network and enhanced permeability. Additionally, the increasing content of chondroitin sulfate in nCACSMH could give rise to the charge density and its asymmetric structure, thus the uneven, plicate and expanded surface of nCACSMH which was favorable to cell proliferation was developed. Moreover, chondroitin sulfate was released with the degradation of nCACSMH, which played a crucial role in maintaining the normal physiological functions of cells. Thus the proliferation of human umbilical vein endothelial cells (HUVECs) was further accelerated and the angiogenesis related genes expression in endothelial cells was continuously and dramatically up-regulated. After 4 d, the proliferation and viability of HUVECs were significantly improved, the cells were distributed evenly in nCACSMH. The novel nCACSMH has the potential to be used as cell delivery, three-dimensional (3D) cell cultures for cell therapy, 3D bioprinting, high-throughput screening for drugs, and disease model for regeneration and constructing of tissue engineering.

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