(77d) Mechanically Tunable Magnetic Nanoparticle – Hydrogel Composite and Its Application in Biological System Manipulation

Biofouling poses significant challenges, impacting human health, civil infrastructure, and military equipment. Biofilm formation is the first step in the biofouling process. To reduce or inhibit this process, antibacterial agents, physical removal, or surface coatings are used. However, these methods have limitations in terms of accuracy, scalability, environmental impact, and long-term efficacy. Therefore, functional nanocomposites that enable remote manipulation of bacterial behavior have emerged as a promising solution.

Biofilms are formed when motile bacteria aggregate, creating uniform and continuous films. There are many parameters that govern and control bacteria motion, including chemical signaling and direct surface interactions. All of these offer an opportunity to inhibit film formation. In the present work, we focused on leveraging the mechanosensor apparatus of bacteria. In previous work, it has been shown that bacteria have different motilities on surfaces with increasing modulus values. However, to dynamically tune the modulus, the previously established approaches involved applying an external stress or strain to the material through micropatterning and fluid dynamics, which can be complex.

In this work, we developed and demonstrated a mechanically tunable magnetic nanoparticles-hydrogel composite platform for biological response manipulation and its capability for reducing biofilm expansion. The composite system consists of a hydrogel-based growth matrix for the biological system, iron oxide magnetic nanoparticles, and an external magnetic field. Magnetic nanoparticles (MNP) serve as the responsive group to control hydrogel stiffness remotely and dynamically. We fabricated the composite in lab and characterized the mechanical properties of the material and its response when being externally controlled through magnetism using a dynamic mechanical analyzer. We implemented this material system as the growth substrate for the motile strain of E. coli MG1655 bacteria. The bacterial growth and behavior on our MNP-hydrogel composite were monitored through time-lapse imaging during their entire culture periods.

Through the mechanical characterization, the composite material shows a 40% increase in storage modulus when subjected to a magnetic field of 20 mT. After confirming that the magnetic nanoparticles and magnetic field did not interfere with bacterial vitality or growth rate, we observed the swarming behavior of E. coli MG1655 on the surface of the composite substrate. A decrease of 30% in bacterial expansion rate was detected for samples containing 7.5 mg/mL MNP and 2.5 g/L agar when a 20mT field was applied.

By integrating magnetic nanoparticles within a biological growth matrix, we have established a magnetic nanoparticle-agar gel functional nanocomposite system capable of dynamically adjusting gel stiffness through the application of an external magnetic field. Using this system, bacteria motility of E. coli is decreased by over 30% without the use of chemicals or other harsh environmental agents. This reduction will directly impact the formation of biofilms.

Further research is needed to explore the effectiveness of different nanoparticle composites and optimize the magnetic field strengths necessary to further inhibit biofilms formed by different bacterial strains. This composite system may lead to a variety of strategies for non-invasive, environmentally friendly biofilm control and biofouling inhibition. This dynamically tunable material can also be used in general biology research to simulate relevant mechanical conditions or changes within a growth cycle, potentially advancing our understanding of the mechanisms behind molecular interactions between mechanosensors of biological systems and the growth substratum both in lab and in nature.