Many organisms utilize surface microstructures for specific functions, such as water collection and directional transport in cacti, and dynamic skin pattern display in cephalopods. Mimicking these natural mechanisms offers exciting possibilities for designing biomimetic materials. While light irradiation has been used to grow microstructures on polymer surfaces, this research explores the use of force as a simple, clean, and energy-efficient alternative for growing and chemically remodeling hydrogel surfaces. Force-triggered chemical reactions, such as mechanoradical generation via weak bond breakage, have been demonstrated at the molecular level. However, applying this to bulk hydrogels is challenging due to catastrophic failure. Double-network (DN) hydrogels, with their ability to control force-triggered chemical reactions, provide a solution. The presence of a soft and stretchable network suppresses stress concentration from bond breaking in the rigid network, allowing controlled and efficient reactions. This research aims to apply this DN hydrogel concept to surface engineering, overcoming challenges like the presence of a soft second network layer on the surface of conventionally synthesized DN hydrogels. By using a hydrophobic mold during synthesis, a well-controlled surface double-network structure is achieved, enabling the realization of stress/strain-controlled bond breaking and spatially controllable structure remodeling on the surface. The high reaction rate of radical polymerization allows for the creation of diverse microstructures with varying morphology and chemistry for on-demand applications.

The authors review existing methods for growing microstructures on polymer surfaces, highlighting the use of light irradiation as a common technique. They discuss the challenges of using force-triggered chemical reactions in bulk hydrogels, emphasizing the catastrophic failure issue in conventional hydrogels. The literature establishes the unique properties of double-network (DN) hydrogels, specifically their ability to withstand stress concentration and enable controlled force-triggered chemical reactions. Previous work on DN hydrogels is cited, showcasing their use in mechanoradical polymerization and mechanical performance improvement. The research also addresses challenges specific to surface modification of DN hydrogels, particularly the presence of a thick soft second network layer on the surface of conventionally synthesized hydrogels, which inhibits the double-network effect. Previous work on the surface structure of DN hydrogels and the formation of electric double layers at interfaces is referenced to justify the use of a hydrophobic mold in this study.

The research employed a mechanochemical strategy involving a double-network (DN) hydrogel comprising a pre-stretched, rigid, and brittle first network (poly(2-acrylamido-2-methylpropanesulfonic acid sodium salt) (PNaAMPS)) and a soft and stretchable second network (polyacrylamide (PAAm)), both crosslinked by N,N'-methylenebisacrylamide (MBA). Density functional theory (DFT) simulations were used to identify potential C-C bond breakage sites in the brittle network. To ensure the double-network structure extended to the surface, a hydrophobic mold was utilized during hydrogel synthesis, preventing the formation of an electric double layer and a soft surface layer. This was verified using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and mechanical testing to confirm the double-network structure on the surface. A force-triggered polymerization process was demonstrated by pressing a DN hydrogel immersed in a concentrated N-isopropylacrylamide (NIPAm) aqueous solution with an indenter. The rapid change from transparent to turbid indicated rapid polymerization. The formation of poly(N-isopropylacrylamide) (PNIPAm) was confirmed using fluorescence microscopy with 8-anilino-1-naphthalenesulfonic acid (ANS) as a hydrophobic region indicator. Time-resolved near-infrared spectroscopy was used to monitor the polymerization kinetics. Cyclic micro-indentation tests were conducted to assess the internal fracture of the first network near the surface. The energy dissipation during cyclic indentation was measured to quantify the amount of ruptured first network strands. The relationship between indentation parameters (indenter diameter and depth) and the resulting microstructure (height and diameter) was systematically investigated using a three-dimensional laser microscope. A range of monomers (NIPAm, NaAMPS, AAc, NaSS, and MPTC) was tested to determine their conversion ratios in force-triggered polymerization, using tensile testing and near-infrared spectroscopy. The morphology and height of the microstructures created with different monomers were characterized. The stimuli-responsiveness of the microstructures (PNIPAm and polyacrylic acid (PAAc)) to temperature and pH changes was assessed. Complex microstructures were fabricated using 3D-printed stamps with various embossed patterns. The ability of the created microstructures to direct cell growth (C2C12 myoblasts) and water droplet transport was evaluated. Contact angle measurements were performed to quantify surface wettability.

The study successfully demonstrated a force-triggered method for rapid surface patterning of hydrogels. The method shows excellent spatial control, allowing fine modulation of the size and shape of microstructures. The force-triggered polymerization is fast, with near-complete conversion within seconds. Various monomers were successfully polymerized on the hydrogel surface, resulting in microstructures with different chemical properties. The height of the microstructures could be independently controlled by adjusting the indentation depth, while the diameter was controlled by the indenter size. Stimuli-responsive microstructures were created, exhibiting reversible changes in height upon temperature or pH changes. Complex microstructures with various patterns were successfully fabricated using 3D-printed force stamps. The created PNIPAm microstructures promoted oriented cell growth, and the anisotropic surface with parallel PNIPAm patterns enabled directional water droplet transport. The transport velocity of water droplets was shown to be linearly related to droplet volume when the PNIPAm lines were aligned vertically.

The force-triggered rapid microstructure growth method presented provides a novel and versatile tool for surface engineering of hydrogels. The method’s simplicity, speed, and spatial control surpass conventional light or heat-based methods. The successful demonstration of oriented cell growth and directional water droplet transport showcases the potential of this approach for various applications, including tissue engineering and microfluidics. The ability to control microstructure size, shape, and chemistry opens doors for designing hydrogels with tailored functionalities. The stimuli-responsive nature of some microstructures adds another layer of complexity and control. The use of 3D-printed stamps allows for complex pattern generation. Future studies could explore the application of this technique to a wider range of materials and explore further functionalization of the grown microstructures.

This study successfully demonstrated a facile and effective method for rapid and spatially controlled surface patterning of hydrogels using force-triggered growth. The method allows for precise control over microstructure dimensions and chemical composition, enabling the creation of stimuli-responsive and complex patterns. The demonstrated applications in cell culture and water droplet manipulation highlight the broad potential of this technique in various fields. Future work could focus on exploring a wider range of monomers and applications, such as creating more complex 3D structures and integrating this technique with other surface modification methods.

The current study primarily focused on relatively large microstructures. Further investigations are needed to explore the feasibility of creating smaller, more intricate features. The range of monomers tested was limited. While a variety of functionalities were achieved, exploring a broader range of monomers could expand the types of microstructures and functionalities available. The long-term stability of the grown microstructures under various environmental conditions also warrants further investigation. Finally, a more in-depth analysis of the cell-material interactions and the mechanism of directional water droplet transport could enhance understanding and guide future developments.