Ionic Species Affect the Self-Propulsion of Urease-Powered Micromotors

Abstract

Enzyme-powered motors self-propel through the catalysis of in situ bioavailable fuels, which makes them excellent candidates for biomedical applications. However, fundamental issues like their motion in biological fluids and the understanding of the propulsion mechanism are critical aspects to be tackled before a future application in biomedicine. Herein, we investigated the physicochemical effects of ionic species on the self-propulsion of urease-powered micromotors. Results showed that the presence of PBS, NaOH, NaCl, and HEPES reduced self-propulsion of urease-powered micromotors pointing towards ion-dependent mechanisms of motion. We studied the 3D motion of urease micromotors using digital holographic microscopy to rule out any motor-surface interaction as the cause of motion decay when salts are present in the media. In order to protect and minimize the negative effect of ionic species on micromotors’ performance, we coated the motors with methoxypolyethylene glycol amine (mPEG) showing higher speed compared to noncoated motors at intermediate ionic concentrations. These results provide new insights into the mechanism of urease-powered micromotors, study the effect of ionic media, and contribute with potential solutions to mitigate the reduction of mobility of enzyme-powered micromotors.

Medical Micro/Nanorobots in Precision Medicine

“…For example, enzymes have substituted platinum, allowing them to use a variety of biomolecules, such as glucose or urea as fuel. [ 104–110 ] Moreover, biodegradable metals, such as zinc or magnesium have been used as propellants. These can react with the acidic environment of the stomach (Figure 2c).…”

Biohybrid robotics: From the nanoscale to the macroscale

“…Future research should focus more on the fundamental interactions with biological matrices (Palagi et al, 2017; Walker et al, 2015; Z. Wu et al, 2018) and also in the interaction between several of these nanomotors, which would also offer an interesting system to study in active matter physics (Hortelao, Simó, et al, 2020; Illien et al, 2017). The effects elements like ionic species or pH on the motion efficiency still need to be further investigated, as seemingly contradictory results arise (Arqué et al, 2020; De Corato et al, 2020; Tang et al, 2020), hinting at a much more complex interaction between all the actors involved in the motion. Moreover, in practical applications, the formation of protein coronae (Feiner‐Gracia et al, 2017; Nel et al, 2009) or interactions with the immunitary system (I. C. Yasa et al, 2020) or the in vivo imaging (van Moolenbroek et al, 2020) are some of the main challenges that need to be addressed to ensure their efficacy as drug delivery systems.…”

“…Furthermore, the study of enzyme‐powered microparticles in ionic solutions helped determine the role of ionic species and Debye length in their motion with numerical simulations, finding an inverse relationship between micromotor speed and the background electrolyte concentration that matched experimental values (De Corato et al, 2020). Further tracking experiments in 2D and 3D of urease‐powered micromotors under different pH and ionic strengths showed a reduction of their self‐propulsion that pointed toward ion‐dependent mechanism of motion (Figure 4e,f) (Arqué et al, 2020). Moreover, methoxypolyethylene glycol amine (mPEG) was used to mitigate the effects of the ionic species, obtaining a small recovery in the speed after increasing the micromotor Debye length.…”

“…(e) Effect of the ionic strength of NaOH in the speed of these micromotors with or without an mPEG coverage. Adapted with permission from Arqué et al (2020). (f) 3D trajectory mapping of urease‐powered micromotors showing directionality in all directions.…”

“…(f) 3D trajectory mapping of urease‐powered micromotors showing directionality in all directions. Adapted with permission from Arqué et al (2020). (g) Schematic fabrication of the Janus platelet motors powered by urease.…”