A physicochemically compatible ferrofluid droplet robotic system for automated bioanalytical assays

Droplet robotics is an emerging area of research focused on harnessing externally programmable physical fields to drive liquid droplet motion and automate complex fluidic operations. One approach for driving droplet robotic systems utilizes magnetic attraction between droplets and magnetic actuators to enable programmable automated droplet manipulation through the introduction of magnetic components, such as nanoparticles, into the droplet. Compared to other droplet actuation mechanisms, magnetic actuation offers notable advantages including simple system design, high tolerance to liquid properties and flexible system control. However, the incorporation of magnetic ferrofluid nanoparticles introduces challenges related to their intrinsic physical colloidal stability and chemical catalytic characteristics, resulting in physicochemical incompatibility issues, restricting broader utilization in bioanalytical applications. In this work, the physicochemical incompatibilities of ferrofluid nanoparticles are investigated and resolved through surface modifications to the ferrofluid nanoparticles, enabling the development of a physicochemically compatible ferrofluid droplet robotic system. The system addresses compatibility issues including low colloidal stability and compromised chemical catalytic activity in HRP-based enzymatic assays. As a result, the enhanced actuation robustness and efficiency, as well as chemical quantification sensitivity and reliability, enable automated assays to be conducted. The enhanced physicochemical compatibility of the ferrofluid droplet robotic system facilitates the use of ferrofluid for highly efficient magnetically driven automated bioanalytical processes.