Abstract
Hydrogels have emerged as essential enablers of instant mass transport and efficient energy translation, which play an ever-increasing role in biomedical devices, artificial electronic skins, and water purification. The recent development of hydrogels diverges towards two distinct mainstreams: One is to program their bulk properties to strengthen the overall performance, encompassing the mechanical properties, electrical conductivity, and water diffusing capability; Another one is to edit the interfacial interactions of hydrogels with the surroundings to impart enhanced properties, exemplified by robust adhesion and photothermal conversion capability. However, it remains a challenge to simultaneously regulate the bulk and interfacial properties of hydrogels, as imposed by the difficulties in identifying suitable media or structures to modulate these two distinguished aspects. In this thesis, on the basis of understanding the requirements to enhance corresponding properties, we focus on developing hydrogels with simultaneous regulation of bulk and interfacial properties through rational all-in-one designs.This thesis gives vivid examples of synergistically reinforcing bulk and interfacial properties. For instance, we leverage tannic-acid mediated interactions as a dynamic bridge to adjust the inner network crosslinks and the surface interactions, endowing the hydrogels with a series of intriguing features, including exceptional stretchability, self-healing capability of the bulk hydrogels, and robust interfacial adhesion. Moreover, we employ hydroxylated carbon nanotubes as the key media to tailor the polymer skeleton benefitting water diffusion as well as the hot steam generating surface, rendering a hydrogel-based solar steam generator with high-quality evaporation behavior and distinct mechanical strength.
Stretchable and self-adhesive hydrogels have been the subject of extensive studies due to their unique combination of bulk flexibility and interfacial activity, revealing unpredictable potential in wound dressing, soft robotics, and wearable electronic devices. Despite countless endeavors, realizing high stretchability (over 3000%) and strong adhesion (>30 kPa on porcine skin) still appears elusive as impeded by several intrinsic challenges associated with rigid and non-adaptive network architectures, as well as inadequate dynamic interaction sites within the bulk and surfaces. We report a facile strategy to engineer an ultrastretchable, highly adhesive and self-healable hydrogel, by employing tannic-acid-enabled dynamic interactions (TEDI) to fully substitute conventional covalent crosslinking. The TEDI strategy allows us to synchronously regulate both bulk and interfacial interactions to obtain exciting properties that outperform conventional hydrogels, including an extraordinary stretchability of over 7300%, remarkable self-healing abilities, and a robust on-skin adhesion of 50 kPa. With these intriguing merits, TEDI hydrogels are demonstrated to be a wearable strain sensor that accurately detects the motion of the human body. Moreover, our TEDI strategy unlocks new opportunities to design next-generation ionic hydrogels that may be valuable for wearable electronic devices and healthcare monitoring applications.
Hydrogels have shown significant potential for interfacial solar vapor generation by incorporating photothermal conversion materials into their hierarchical structure. These hydrogel-based vapor generators often feature a large internal surface area of local heat exchange and excellent water supply ability for the whole generator. However, the existing research often overlooks the importance of maintaining the mechanical stability of hydrogel-based steam generators. This is partially attributed to the continuous swelling of the hydrogels, as the heating and evaporation at the top interfaces exacerbate capillary pumping and osmotic swelling effects throughout the bulk hydrogel. To address this issue, we developed a hydrogel vapor generator with sponge-like hierarchical structures comprising micron meshes and channels. This is achieved by taking advantage of polyzwitterionic polymer networks with hydroxylated carbon nanotubes (GHS), which are capable of high-performance photothermal transformation. This GHS strategy not only leverages the nanotubes to absorb light and s serve as a productive photothermal conversion platform, but also to spontaneously provide sufficient free radicals to initiate the polymerization. Furthermore, this strategy employs nanotubes as dynamic crosslinks to customize the hydrophilic networks, rendering exceptional elastic modulus and remarkable anti-swelling ability. The optimized GHS hydrogel maintains a small swelling ratio under 1% for a long time, a facilitated water evaporation rate over 3.8 kg m-2 h-1, and a steady solar-to-vapor energy efficiency of ~80% under simulated solar light irradiation (1.0 kW m-2). Moreover, the GHS hydrogel, incorporated with anti-polyelectrolyte effects stemming from the polyzwitterionic skeleton, demonstrates a facilitated water transport rate both in pure water and brines. This polyzwitterionic hydrogel provides valuable insights for the design of next-generation solar desalination systems and expands the potential applications in high-salinity seawater environments.
In summary, this thesis systematically explores the feasibility of concurrently modulating the bulk and interfacial characteristics of hydrogels. By judiciously incorporating tannic acid-facilitated dynamic interactions, we successfully endow hydrogels with excellent stretchability, self-healing capability, and robust adhesion. Furthermore, we leverage hydroxylated carbon nanotubes as a potent toolkit for devising a hydrogel-based evaporator that exhibits remarkable solar vapor generation performance while maintaining stable and robust mechanical properties.
| Date of Award | 20 Aug 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Wen Jung LI (Supervisor) & Zuankai Wang (External Co-Supervisor) |
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