Optimized Association of Short Alkyl Side Chains Enables Stiff, Self-Recoverable, and Durable Shape-Memory Hydrogel

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Engineering a polyvinyl butyral hydrogel as a thermochromic interlayer for energy-saving windows

Achieving mastery over light using thermochromic materials is crucial for energy-saving glazing. However, challenges such as high production costs, limited durability, and recyclability issues have hindered their widespread application in buildings. Herein, we develop a glass interlayer made of a polyvinyl butyral-based hydrogel swollen with LiCl solution. In addition to a fast, isochoric, and reversible transparency-to-opacity transition occurring as ambient temperatures exceed thermally comfortable levels, this hydrogel uniquely encompasses multiple features such as frost resistance, recyclability, scalability, and toughness. The combination of these features is achieved through a delicate balance of polyvinyl butyral's amphiphilicity and the suppression of network-forming phase separation. This design endows a nanostructured polyvinyl butyral-LiCl composite gel with swollen molecular segments linked by dispersed cross-linking sites in the form of hydrophobic nano-nodules. Upon laminating this hydrogel (a thickness of 0.3 mm), the resultant glazing product demonstrates approximately 90% luminous transmittance even at sub-zero temperatures, along with a significant modulation of solar and infrared radiation at 80.8% and 68.5%, respectively. Through simulations, we determined that windows equipped with the hydrogel could reduce energy consumption by 36% compared to conventional glass windows in warm seasons. The widespread adoption of polyvinyl butyral in construction underscores the promise of this hydrogel as a thermochromic interlayer for glazing.

Arrested Phase Separation Enables High-Performance Keratoprostheses

Corneal transplantation is impeded by donor shortages, immune rejection, and ethical reservations. Pre-made cornea prostheses (keratoprostheses) offer a proven option to alleviate these issues. Ideal keratoprostheses must possess optical clarity and mechanical robustness, but also high permeability, processability, and recyclability. Here, it is shown that rationally controlling the extent of arrested phase separation can lead to optimized multiscale structure that reconciles permeability and transparency, a previously conflicting goal by common pore-forming strategies. The process is simply accomplished by hydrothermally treating a dense and transparent hydrophobic association hydrogel. The examination of multiscale structure evolution during hydrothermal treatment reveals that the phase separation with upper miscibility gap evolves to confer time-dependent pore growth due to slow dynamics of polymer-rich phase which is close to vitrification. Such a process can render a combination of multiple desired properties that equal or surpass those of the state-of-the-art keratoprostheses. In vivo tests confirm that the keratoprosthesis can effectively repair corneal perforation and restore a transparent cornea with treatment outcomes akin to that of allo-keratoplasty. The keratoprosthesis is easy to access and convenient to carry, and thus would be an effective temporary substitute for a corneal allograft in emergency conditions.

Self-Healing and Highly Stretchable Hydrogel for Interfacial Compatible Flexible Paper-Based Micro-Supercapacitor

Most reported wearable electronic devices lack self-healing chemistry and flexible function to maintain stable energy output while irreversible damages and complex deformations. In this work, we report a dual-dynamic network electrolyte synthesized by micellar elastomers introduced into strong hydrogel matrix. The gel electrolyte is fabricated by physically cross-linking the borax-polyvinyl alcohol (B-PVA) network as tough matrix and poly (ethylene oxide) (PEO)-poly (propylene oxide) (PPO)-poly (ethylene oxide) (Pluronic) to frame elastic network, followed by immersion in potassium chloride solution. Under the action of dynamic borate ester bond and multi-network hydrogen bond, the as-prepared electrolyte exhibits high stretchability (1535%) and good self-healing efficiency. Based on the electrolyte, we assemble the interfacial compatible micro-supercapacitor (MSC) by multi-walled carbon nanotubes (MWCNT) interdigital electrode printed on cellulosic paper by direct ink writing (DIW) technique. Thanks to the large specific area and compressive deformation resistance of cellulosic paper, the MSC with tightly interfacial contact achieves high volumetric capacitance of 801.9 mF cm⁻³ at the current density of 20 μA cm⁻². In the absence of stimulation of the external environment, the self-healing MSC demonstrates an ideal capacity retention (90.43%) after five physical damaged/healing cycles. Our research provides a clean and effective strategy to construct wearable MSC.

Mussel foot protein inspired tough tissue-selective underwater adhesive hydrogel

Mussel foot proteins (Mfps) show strong adhesion to underwater substrates, making mussels tightly cling to reefs to withstand the sea current. Therefore, Mfps-inspired tissue adhesives have aroused much research interest, but tough underwater biological tissue adhesion is still a great challenge. Herein, we report a tough and reversible wet tissue-selective adhesive hydrogel made of poly(acrylic acid-co-catechol) and chitosan (CS). It provides negatively charged -COO^-, positively charged -NH₃⁺, catechol group and hydrophobic alkyl chain, resemble amino acids, catechol and hydrophobic units in Mfps. Due to the covalent/electrostatic attraction/π-π/cationic-π/hydrogen bonding, in addition to the hydrophobic interaction from the long hydrophobic alkyl chain of the catechol derivative, the hydrogel has a high cohesion strength and toughness, i.e., tensile stress, fracture strain and fracture toughness of ∼0.57 MPa, 2510% and 6620 J m^-2, respectively. As a tissue adhesive, its adhesion bonding to the porcine skin surface is so strong that its adhesion strength is almost equal to the tearing strength of the hydrogel. The 180-degree peeling adhesion energy of the hydrogel to blood-wetted porcine skin is notably ∼1010 J m^-2. It can tightly and seamlessly adhere to the porcine small intestine, and has a bursting pressure of up to 520 mmHg. The hydrogel can be handily debonded from the porcine skin surface in the presence of aqueous solution at pH 8.0, and its adhesiveness is reversible for at least 20 cycles. It is supposed that the synergistic interactions of the adhesive catechol group, displacement of water on the wet skin surface by the positively charged -NH₃⁺ groups of CS and the water-repelling potential of the hydrophobic unit of the catechol derivative, the protection of the catechol group from oxidation into a less adhesive quinone group, and the energy dissipation capacity of the mechanically tough hydrogel contribute to the strong and repeatable wet tissue adhesion.

Self-healing hydrogel sensors with multiple shape memory properties for human motion monitoring

Shape memory hydrogels offer new opportunities for the development of smart wearables due to their intelligent responsiveness.

Structural evolution of dispersed hydrophobic association in a hydrogel analyzed by the tensile behavior

The use of dispersed cross-links with different levels of strength is one of the most successful strategies for toughening a hydrogel. By using a model hydrogel having dispersed association of single-component short alkyl chains, this work demonstrates that the differential modulus-elongation relation derived from tensile curves can reflect the structural evolution of dispersed cross-links at a molecular level. This analysis method allows for decoupling the mechanical contribution of strong and weak hydrophobic clusters, which serve as the minor and major cross-links in our system, respectively. At small deformation, the weak hydrophobic associations majorly determine the stiffness, and their rupture releases folded partial chains to endow deformation capacity. At large deformation, the strength ratio of strong and weak hydrophobic association should be balanced to achieve the optimal strength. Furthermore, the structural parameters of these partial chains, including the Kuhn number, the Kuhn length and the chain conformation, are determined based on scaling theory of extensibility. These results allow for correlating the apparent mechanics to the structural parameters of the dispersed hydrophobic association, paving the way for customized mechanics for specific applications.

Dispersed Association of Single-Component Short-Alkyl Chains toward Thermally Programmable and Malleable Multiple-Shape Hydrogel

In pursuit of intelligent hydrogel-based devices, it is imperative to concurrently enhance the shape programmability and customization of hydrogels in order to realize sophisticated actuation and implantation. Until now, multiple temporary shapeshifting of hydrogels has required either multiple external stimuli or distinct thermal-transition phases. In addition, reprocessing the permanent shape of hydrogel mostly relies on the change in their components. These complex prerequisites present challenges to programmability and customization of application-related shapes for hydrogels. This paper reports a type of thermally programmable and malleable multiple-shape hydrogel. The network of this hydrogel is solely composed of a type of polyvinyl alcohol derivative, which is synthesized by substituting hydroxyl groups of polyvinyl alcohol with single-component octyl chains. Through water-vapor exchange and heating in water, these single-component octyl side chains form dispersed clusters with a large strength gradient. Such broadly dispersed clusters serve as switchable segments and dynamic net-points to orthotopically offer multiple (e.g., quintuple) temporary shapes and editable permanent shapes, respectively, thereby realizing sophisticated and designed shape changes. This hydrogel system can act as a smart device for on-demand bidirectional twining around a 1D substrate. Such a capability potentially enables the self-mounting and self-detaching behavior of soft devices on tissues with minimal invasion.