With natural evolution, honeybee stinger with microbarbs can easily penetrate and trap in the skin of hostile animals to inject venom for self-defense. We proposed a novel three-dimensional additive manufacturing method, namely magnetorheological drawing lithography, to efficiently fabricate a bioinspired microneedle imitating a honeybee stinger. Under the assistance of an external magnetic field, a parent microneedle was directly drawn on the pillar tip, and tilted microbarbs were subsequently formed on the four sides of the parent microneedle. Compared with the barbless microneedle, the microstructured barbs enable the bioinspired microneedle for easy skin insertion and difficult removal. The extraction-penetration force ratio of the bioinspired microneedle was triple that of the barbless microneedle. The stress concentration at the barbs helps to reduce the insertion force of the bioinspired microneedle by minimizing the frictional force, whereas it increases the adhesion force by interlocking the barbs in the tissue during retraction. Such finds may provide an inspiration for further design of barbed microtip-based microneedles for tissue adhesion, transdermal drug delivery, biosignal recording, and so on.

Triboelectrification is a process of charge separation and transfer between tow materials through mechanical contact and friction. [9] The electrical output performance of TENG can be improved by enhancing the surface roughness of the triboelectric materials, which can lead to an enlargement of the contact or friction area. [10] Hence, a lot of investigations focus on developing the protruding surface structures, including the dome array, [2b] cube array, [11] pyramid array, [12] nanorod array, [13] and subwavelength architectures, [14] in order to enhance the output electricity. However, the above structures have some limitations including complex, long-time, high-cost fabrication processes based on photolithography, electrochemical deposition, or soft lithography methods. [9a,14-15] Trinh and Chung [9a] proposed a microneedle array (MA)-based TENG and its novel mechanism of mixed bending-friction-deformation behavior for increasing the output performance. The MA-based TENG exhibited extremely high output power.The key component of MA-based TENG is a bendable MA. Various techniques have been developed to fabricate MA. Typically, MA can be fabricated by the subtractive processes, in which the 3D structures of MA are selectively carved out of a 2D substrate, including lithography with wet or dry etching, [16] laser cutting, [17] micromachining, [18] and wireelectrode cutting. [19] The limitations of these subtractive processes are complex, expensive, time consuming, or not suitable for mass production. Additive processes, such as 3D printing, [20] droplet-born air blowing, [21] electro-drawing lithography, [22] thermal drawing lithography, [23] and magnetorheological drawing lithography, [24] also can form 3D structures of polymer MA from droplets or 2D surfaces. 3D printing is a flexible process that allows personalized customization, but it is not suitable for mass production. The other processes suffer from the limitations of difficult operation and relatively low production efficiency. [22a,25] The micromolding technique is a near-net-shape forming process, which has been employed to fabricate polymer microneedles owing to its potential for mass production. [9a,26] However, it is characterized by multiple costly and time-consuming steps, such as master preparation, mold fabrication, polymer filling, and microneedles separation. [22a,27] Furthermore, Since the invention of triboelectric nanogenerator (TENG) in 2012, it has become one of the most important innovations in energy conversion technologies. A flexible microneedle array (MA)-based TENG is proposed using the closed bending-friction-deformation behavior of MA for the mechanical energy harvest. A novel magnetization-induced self-assembling method is introduced to efficiently fabricate the bendable MA for the TENG. The 3D structures of MA are continuously self-assembled from a moving film of curable magnetorheological fluid under the assistance of a rotating external magnetic field. The mass production of MA may be achieved using this method ...

A bendable microneedle array (MA) fabricated by a magnetic‐induced self‐assembling method was employed for triboelectric nanogenerators (TENG). In article number https://doi.org/10.1002/aelm.201800785, Zhipeng Chen and co‐workers introduced a roll‐to‐roll technique to continuously fabricate MA from a moving film of curable magnetorheological fluid under a rotating magnetic field. The fabricated bendable MA‐based TENG exhibited an excellent energy harvest ability.