Research On Fabrication Method Of Helical Microrobots Based On Swelling Effect

Helical microrobots have the potential to be used in a variety of application areas,such as in medical procedure,cell biology,or lab-on-a-chip.Reciprocal motion at low Reynold number,where viscous force dominates inertial force,will result in no net displacement;this is known as “scallop theorem”.This constraint led non-intuitive designs for microrobots that can generate nonreciprocal motion.Inspired by nature,the helical microrobots model after the bacteria with helical flagella,such as E.coli,and exhibit excellent swimming ability at low Reynold number.In recent years,the fabrication and control of artificial helical microrobots has become prominent subject in the field of micro-robotics.This work introduces a novel three-step rolled-up fabrication method.The three steps consist,in order,of photolithography,film deposition,and wet etching,this method is low-cost and high-throughput and relatively simple compared to existing preparation methods.After photoresist and a thin metal film are created via photolithography and film deposition sequentially and respectively,wet etching is used to release the photoresist.Simultaneously,the photoresist swells when reacting with the etchant;this leads to the formation of internal stress,resulting in the initially 2D patterns to self-scroll into strain-induced helical structures.The parameters of the fabrication process is highly controllable that allow for fine tuning of geometrical parameters of the helical microrobots.For instance,the geometry of helical microrobots could be tuned by controlling the dimensions of the 2D patterns.The effects of the microrobots' helical parameters to their swimming properties characterized under rotational magnetic fields of various frequencies;a linear relationship was observed between swimming velocity and the length and radius of helical microrobots.The above conclusions correspond with existent theorems.Finally,a feedback control strategy was implemented to control helical microrobots to move along the pre-programmed trajectories.After studying the fabrication method and swimming ability of helical microrobots,four major observations were listed below.First,the direction of scrolling can be controlled using parallelogram templates;this is because selfscrolling of parallelogram templates is anisotropic while self-scrolling of rectangular templates is isotropic.Second,the helical angle and the number of turns of the helical microrobots were approximately equal to the tilt angle of parallelogram template and proportional to the length of template,respectively.Third,a linear relationship between velocity and frequency was observed only if the angle between the rotation axis and the helical axis(wobbling angle)is constant.Last but not least,the motion of helical microrobots can be controlled with microscale precision using feedback control strategy.Based on these observations,it was demonstrated that this work yields a way to massively fabricate helical microrobots with tunable geometrical properties and controllable motion.The results have important implications to medical procedures that must be performed using a large amount of microrobots,such as drug delivery.Furthermore,the feedback control of the microrobots serves as proof-of-concept for where motion control with high precision is necessary,such as targeted therapy.With a deeper understanding of their fabrication and control,medical microrobots will be able to slowly but surely move towards industrialization.