Developing hierarchical microneedles for biomedical applications

As a new kind of microcarrier device, microneedles are featured by micrometer needle arrays with an overall size in the centimeter scale. Due to the needle shape and the micron size, microneedles can penetrate the skin without harming nerves and blood vessels, which causes many advantages such as minimally invasive, safe and convenient. The past few decades have witnessed a great leap in microneedles research. The main materials of microneedles have changed from metal and ceramic to polymers with more complex functions, and the optimization of materials and preparation strategies has led to a greater variety of microneedle styles. Among them, the construction or combination of smaller size structures or materials on microneedles to fabricate hierarchical microneedles is a major research hotspot. Here, we present the recent research progress of hierarchical microneedles for biomedicine. We begin by discussing the fabrication strategies of hierarchical microneedles, including mainstream casting and coating methods based on microneedle molds and three dimensions (3D) printing methods. We then expand the discussion from the hierarchical microneedles with porous structure to those composited with nanomaterials. Eventually, we have a discussion about the research progress of hierarchical microneedles in the area of biomarkers detection and transdermal drug delivery, as well as its future development direction.