Hybrid Helical Magnetic Microrobots Obtained by 3D Template‐Assisted Electrodeposition

摘要

Hybrid helical magnetic microrobots are achieved by sequential electrodeposition of a CoNi alloy and PPy inside a photoresist template patterned by 3D laser lithography. A controlled actuation of the microrobots by a rotating magnetic field is demonstrated in a fluidic environment. The development of micro- and nanoelectromechanical systems (MEMS/NEMS) technology has resulted in the fabrication of micro- and nanomachines that can be controlled wirelessly in liquid environments. Among the various actuation and control strategies for these machines, magnetic manipulation has emerged as the most versatile approach, and controlled manipulation of three-dimensional (3D) micromachines using magnetic field gradients, resonant magnetic fields and rotating magnetic fields has been demonstrated.1-9 Rotation is a fundamental motion in biological systems at the micro and nano levels. Rotary motors are responsible for the motion of the bacterial flagella and the ATP synthase molecule. These motors convert rotational motion into translational motion, a strategy that has proven to be effective in the low Reynolds number regime.10 Based on this principle, helical micromachines known as artificial bacterial flagella (ABFs) have been wirelessly manipulated in liquid environments using rotating magnetic fields.3, 4, 11 Potential in vitro applications of these machines have made use of their ability to perform non-contact capture and transport of micro objects. For in vivo applications such as targeted drug delivery applications, it is foreseen that a group of these micro machines could have access to many hard-to-reach locations in the body and maximize drug loading and release. They could navigate through the circulatory, urinary and central nervous systems. The microrobots could also be applied in water remediation to patrol stagnant and flowing wastewaters for effective degradation of organic pollutants. For this application, the microrobots should be functionalized with a photocatalytic compound. In any case a swarm control strategy will necessitate the development of reliable processes to fabricate these machines from a combination of materials that enable magnetic control and the incorporation of therapeutic molecules. In combination with photolithography, electrodeposition has been used to fabricate relatively complex wirelessly controllable 3D micromachines.12 Electrodeposition enables the synthesis of a wide variety of magnetic alloys, and allows the tuning of their properties by modulating factors such as the pH and temperature of the electrolytic bath, additives, and the current density or overpotential of deposition. Electrodeposition also enables the polymerization of a unique class of intrinsically conductive polymers (ICP) on metallic substrates. Among ICP, poly(pyrrole) (PPy) is the most widely studied and characterized due to its excellent biocompatibility, enhanced physical and chemical stability, the tunability of its surface towards various cell types, and the ability to incorporate therapeutic molecules into its matrix.13, 14 In this paper, we describe a high throughput method to fabricate hybrid artificial bacterial flagella (h-ABFs) consisting of a ferromagnetic alloy head and a helical polymer tail (see Figure 1(a)). h-ABFs present a number of advantages compared to fully metallic specimens including a lighter weight that reduces sedimentation and facilitates navigation and better biocompatibility because of the replacement of metallic parts with PPy. The h-ABFs were synthesized by template-assisted two-step electrodeposition. The direct laser writing (DLW) process provided a simple method to make 3D photoresist templates acting as masks during the electrodeposition. With the use of a positive-tone photoresist, it is possible to make 3D cavities that can be filled by electrodeposition.15 The hollow cavities were filled with magnetic cobalt-nickel (CoNi) and biocompatible PPy through electrodeposition. h-ABFs were physical