In this study, a novel finishing method, entitled clustered magnetorheological finish (CMRF), was proposed to improve surface finish of the silicon nitride ( ${\mathrm{Si}}_3{\mathrm{N}}_4$ ) balls with ultra fine precision. The effects of different polishing parameters including rotation speeds, eccentricities and the machining gaps on surface finish of ${\mathrm{Si}}_3{\mathrm{N}}_4$ balls were investigated by analyzing the roughness, sphericity and the micro morphology of the machined surface. The experimental results showed that the polishing parameters significantly influenced the surface finish. The best surface finish was obtained by using the polishing parameters: the machining gap of 0.8 mm, the eccentricity of 10 mm and the rotation ratio of 3/4. To further investigate the influence of the polishing parameters on the surface finish, an analytical model was also developed to analyze the kinematics of the ceramic ball during CMRF process. The resulting surface finish, as a function of different polishing parameters employed, was evaluated by analyzing the visualized finishing trace and the distribution of the contact points. The simulative results showed that the distribution and trace of the contact points changed with different polishing parameters, which was in accordance with the results of experiments.

Microneedle (MN) is a medical device containing an array of needles with a micrometer-scale. It can penetrate the human stratum corneum painlessly and efficiently for treatment and diagnosis purposes. Currently, the materials commonly used to manufacture MNs include silicon, polymers, ceramics and metals. Metallic MNs (MMNs) have drawn significant attention owing to its superior mechanical properties, machinability, and biocompatibility. This paper is a state-of-the-art review of the structure, fabrication technologies, and applications of MMNs. According to the relative position of the axis of MN and the plane of the substrate, MMNs can be divided into in-plane and out-of-plane. Solid, hollow, coated and porous MMNs are also employed to characterize their internal and surface structures. Until now, numerous fabrication technologies, including cutting tool machining, non-traditional machining, etching, hot-forming, and additive manufacturing, have been used to fabricate MMNs. The recent advances in the application of MMNs in drug delivery, disease diagnosis, and cosmetology are also discussed in-depth. Finally, the shortcomings in the fabrication and application of MMNs and future directions for development are highlighted.