Ultrafast light-activated polymeric nanomotors

Self-propelled nanomotors, capable of converting energy into movement, hold significant promise for biomedical applications. Active particles, unlike traditional nanocarriers, interact more effectively with cells and can directly cross cell membranes. Various nanomotors have been developed, utilizing chemical fuels or external stimuli like light, magnetism, or ultrasound. Light-propelled nanomotors, particularly those driven by thermophoretic forces, have demonstrated high velocities (up to 86 µm s⁻¹). However, creating fast-moving, biodegradable polymeric nanomotors remains a challenge. Ultrafast velocities are crucial for better tissue penetration and efficient drug delivery. Gold (Au)-based nanomotors are promising, but challenges include controlling Au deposition and reducing particle size for enhanced energy conversion. This research utilizes a biodegradable bowl-shaped polymersome (stomatocyte) platform, providing inherent asymmetry for directional movement. Previous work with this platform incorporated manganese dioxide particles or a gold hemispherical shell. This study aims to create a gold-functionalized stomatocyte nanomotor with superior motility by optimizing the photothermal properties of Au nanoparticles.

The existing literature extensively covers the development and application of various micro/nanomotors. Studies have explored light-propelled, bubble-driven, and glucose-assisted bi-metallic nanomotors. These motors are propelled using chemical fuels (e.g., hydrogen peroxide, urea) or external stimuli (e.g., magnetic fields, ultrasound, light). Thermophoretic forces, using light as the energy source, have yielded the highest velocities (up to 86 µm s⁻¹). Gold-based nanomotors have been investigated with various morphologies (spheres, shells, rods, stars). However, challenges remain in controlling gold deposition and decreasing particle size for better energy conversion and biomedical applications. Previous work by the authors established a nanomotor platform using biodegradable bowl-shaped polymersomes (stomatocytes) composed of poly(ethylene glycol)-b-poly(D, L-lactide) (PEG-PDLLA). The inherent asymmetry of stomatocytes ensures directional movement when propelled by external stimuli.

Three biodegradable PEG-PDLLA block polymers (PEG₂₂-PDLLA₉₅, PEG₄₄-PDLLA₉₅, and NH₂-PEG₆₇-PDLLA₉₅) were synthesized via ring-opening polymerization. Polymersomes were prepared via self-assembly, and stomatocytes were obtained through osmotic-induced shape transformation. Gold nanoparticles (Au NPs) were deposited onto the stomatocyte surface via electrostatic and hydrogen bond interactions using an in-situ growth method. The size and morphology of the stomatocytes and Au-stomatocytes were characterized using dynamic light scattering (DLS) and cryo-TEM. The photothermal performance of the Au-stomatocytes was evaluated by measuring temperature changes upon 660 nm laser irradiation. The autonomous motion of the Au-stomatocytes was investigated using nanoparticle tracking analysis (NTA), analyzing mean square displacement (MSD) and velocity. Cryo-ET was employed for detailed 3D morphological analysis of the nanomotors, quantifying Au NP size and spatial distribution. Finite element method (FEM) simulations were used to model temperature distribution. Cytotoxicity was assessed using a CCK-8 assay. Intracellular delivery of FITC-BSA and Cy5-siRNA was studied using confocal laser scanning microscopy (CLSM). The penetration behavior of DOX-loaded Au-stomatocytes was investigated in 2D cell cultures and 3D HeLa spheroids.

The study successfully fabricated light-propelled biodegradable stomatocyte nanomotors (Au-stomatocytes) with exceptional motility. These nanomotors achieved maximum velocities of 124.7 ± 6.6 µm s⁻¹ in water, 109 ± 3.3 µm s⁻¹ in PBS, and 104 ± 3.7 µm s⁻¹ in DMEM, significantly outperforming existing nanomotor systems. Cryo-TEM and cryo-ET analysis revealed that the high velocity is attributed to the asymmetric distribution of ~5 nm Au NPs on the stomatocyte surface, creating a temperature gradient upon laser irradiation. Quantitative analysis showed higher Au NP density at the bottom of the stomatocyte compared to the opening, leading to a temperature gradient along the z-axis (-100 µK µm⁻¹ at 1.5 W). FEM simulations confirmed this temperature gradient and its role in propulsion via thermophoresis. The nanomotors showed excellent biocompatibility and efficiently delivered both FITC-BSA and Cy5-siRNA into HeLa cells through direct membrane translocation. In 3D HeLa spheroids, enhanced accumulation and penetration were observed with increased laser power, highlighting the potential for deep tissue penetration.

The results demonstrate a significant advancement in the development of ultrafast, biodegradable nanomotors. The exceptional velocities achieved, combined with the biocompatibility and cargo delivery capabilities, make these nanomotors highly promising for biomedical applications. The detailed analysis using cryo-ET and FEM simulations provides a comprehensive understanding of the underlying mechanism of motion, highlighting the importance of asymmetric nanoparticle distribution in generating sufficient propulsive force. The successful delivery of both large (FITC-BSA) and small (Cy5-siRNA) molecules underscores the versatility of the system for diverse therapeutic applications. The observed dependence of penetration on laser power suggests that further optimization could enhance deep tissue penetration for improved therapeutic outcomes.

This research successfully created highly motile, biodegradable nanomotors with unprecedented speed. The asymmetric distribution of gold nanoparticles on the stomatocyte surface, creating a temperature gradient upon laser irradiation, drives their movement. These nanomotors successfully delivered various cargo molecules into cells, exhibiting excellent biocompatibility. Future research could explore different nanoparticle types and distributions to further optimize velocity and efficiency, and also focus on in vivo studies to validate the therapeutic potential of these nanomotors.

The study primarily focused on in vitro experiments. Further in vivo studies are needed to fully evaluate the nanomotors' efficacy and safety in a complex biological environment. The current design utilizes a specific type of polymersome; exploring other polymer architectures could potentially improve performance. Long-term toxicity studies should be conducted to comprehensively assess biocompatibility.