Photopatterned microswimmers with programmable motion without external stimuli

Nature provides numerous examples of organisms that move on water surfaces, often utilizing the Marangoni effect—a phenomenon involving mass transfer between liquid regions with differing surface tensions. Insects like *Microvelia* exploit this effect by secreting surfactants, achieving impressive speeds. Inspired by this, researchers have developed Marangoni microswimmers, which offer advantages over conventional microswimmers: self-propulsion without external energy or mechanical systems. However, limitations in fabrication hindered programmability and mass production of these swimmers. Fuel used (surfactants) often dissolves during fabrication, preventing the use of standard methods like washing. Precise fuel positioning for programmed motion was also a significant challenge. Existing methods, like pipetting or hand assembly, lack throughput and precision for sub-millimeter scale mass production. While external stimuli-based methods offer high programmability, the bulky external equipment limits practical applications. This study addresses these limitations by introducing a new fuel and fabrication method to create highly programmable Marangoni microswimmers.

Previous research on Marangoni swimmers has demonstrated their potential for self-propulsion using various fuels and designs. Studies have explored the use of self-assembled peptides, depolymerization-powered systems, and metal-organic frameworks to achieve autonomous motility. While these studies demonstrated the basic principles of Marangoni propulsion, they faced limitations in precise fabrication, scalability, and programmability. The challenges involved the difficulty of integrating multiple functional parts with precise control over fuel release and swimmer design. Existing fabrication techniques often relied on hand assembly or methods incompatible with the amphiphilic nature of commonly used fuels, limiting the complexity and precision of the resulting swimmers. The lack of precise control over fuel release also impacted the programmability and efficiency of these devices.

This study employed polyvinyl alcohol (PVA) as a fuel due to its water solubility (reducing surface tension) and insolubility in organic solvents. This allowed the researchers to utilize maskless photolithography for fabrication. This technique offers high-throughput and multi-scale fabrication capabilities, as well as the ability to integrate multiple functional materials using iterative photolithography cycles. The microswimmers consisted of a hydrophobic polyurethane acrylate (PUA) body and a PVA-containing PEG-based hydrogel fuel component. The fabrication process involved several steps: 1. **Fuel Part Fabrication:** PVA was mixed with poly(ethylene glycol) diacrylate (PEGDA), and the mixture was polymerized using UV light. An ethanol wash was used to solidify the PVA within the PEGDA matrix. This step was critical because the PVA's insolubility in ethanol allowed for a washing step that wouldn't dissolve the fuel. 2. **Body and Other Components Fabrication:** Photolithography was used to fabricate the PUA body and other components, such as rudders, using a separate photocurable polymer. The use of ethanol and hydrophobic solvents allowed for iterative fabrication steps. 3. **Assembly:** Multiple components were integrated into a single swimmer via the photolithography steps and the resulting cross-linked network between components. 4. **Characterization:** The release of PVA from the fuel component was quantified using UV-Vis spectrophotometry. The swimming performance (speed and duration) of the microswimmers was characterized by filming their motion using a smartphone or high-speed camera and analyzing the footage with ImageJ. 5. **Swimmer Design for Specific Motions:** The shapes and placement of fuel components and other structures were systematically varied to generate swimmers exhibiting linear, circular, and rotary motions. The speed and duration of these motions were related to PVA concentration and fuel compartment size. 6. **Hydrogel Rudder Study:** Hydrogels with varying porosity (controlled by the ratio of PEGDA to ethanol) were used to fabricate rudders capable of changing shape and size over time. The swelling kinetics of these rudders determined the time course of the swimmer’s motion change. 7. **Disassembly Study:** The disassembly behavior of two connected swimmers was analyzed, controlling disassembly time through the thickness of a PVA bridge connecting the swimmers. A model was created to predict the bridge's dissolution time based on thickness.

The researchers successfully fabricated highly programmable Marangoni microswimmers using a novel photolithography-based approach. The key findings include: * **High-Throughput Fabrication:** The photolithography method enabled the fabrication of up to 2400 microswimmers per glass slide (76.2 mm × 25.1 mm) in 12 minutes, including centrifugation and PVA solidification. * **Programmable Motion:** Microswimmers exhibited various programmable motions, including linear, circular, and rotary motion, controlled by the shape and position of the fuel compartment. Circular motion was achieved by relocating the fuel compartment or modifying the shape of the PUA body. The speed and duration of the swimming were directly controlled by adjusting the PVA concentration. * **Time-Dependent Motion Change:** The introduction of a hydrogel rudder enabled a time-dependent change in direction, from circular to linear motion, due to the swelling of the rudder. The time for this change was programmable by adjusting the hydrogel's composition. A reverse spiral motion was also demonstrated. * **Controlled Disassembly:** The researchers achieved controlled disassembly of microswimmers by creating a water-soluble PVA bridge between two swimmers. The disassembly time could be controlled by adjusting the bridge's thickness through PVA layer stacking. Different disassembly scenarios were demonstrated, including changes in motion after disassembly and cargo release. * **Quantitative Analysis:** The release of PVA from the fuel compartment, maximum speed, and propulsion time of the microswimmers were quantitatively analyzed in relation to PVA concentration and fuel compartment size. A detailed model for PVA bridge dissolution was established. * **Environmental Sensing Potential:** Preliminary experiments showed the potential for these swimmers to detect oil pollutants, with the hydrophobic body enabling them to assemble and propel along the oil-water boundary. A spiral trajectory was successfully used for area scanning in oil-contaminated water.

This study significantly advances the field of Marangoni microswimmers by addressing critical limitations in fabrication and programmability. The use of PVA as a fuel and maskless photolithography enabled high-throughput fabrication of complex swimmers with multiple functional parts. The ability to program various motions, including time-dependent direction changes and controlled disassembly, opens up exciting possibilities for applications such as targeted drug delivery, environmental sensing, and micro-scale manipulation. The detailed quantitative analysis of swimming performance and the development of predictive models will accelerate further research into the kinetics of Marangoni propulsion. The biodegradable and biocompatible nature of PVA enhances the potential of these microswimmers for various applications in biological systems and the environment. Future research could focus on exploring collective behaviors of multiple swimmers and developing fuel systems with tailored surface tension profiles to improve propulsion efficiency in crowded environments.

This research successfully demonstrated highly programmable Marangoni microswimmers with time-dependent motion control and controlled disassembly capabilities. The novel fabrication method, using PVA and maskless photolithography, overcame previous limitations in throughput and precision, leading to the development of complex, multi-functional microswimmers. The study provides valuable insights into the kinetics of Marangoni propulsion and opens new avenues for applications in diverse fields. Future research should explore collective motion, fuel systems with dynamic surface tension, and more sophisticated control algorithms for improved efficiency and functionality.

While this study presents a significant advancement in Marangoni microswimmer technology, some limitations exist. The current study primarily focuses on water-based applications. Further research is needed to explore the performance of these swimmers in other solvents and complex fluids. The long-term stability and degradation of the PVA fuel might limit the duration of operation in some environments. The scalability of the fabrication method to extremely large numbers of microswimmers for applications requiring vast numbers remains to be thoroughly investigated. Finally, the study's preliminary environmental sensing experiment warrants further investigation to fully assess the potential of these microswimmers for practical environmental remediation.