A universal method to easily design tough and stretchable hydrogels

Hydrogels, composed of cross-linked polymer networks and a large amount of water, are promising biomaterials for various applications, including drug delivery and biosensors. However, their inherent weakness and brittleness, stemming from high water content and inhomogeneous networks, limit their practical use. The fracture energy of standard hydrogels is significantly lower than that of biological tissues. To address this, researchers have explored strategies such as creating sliding-ring hydrogels, nanocomposite hydrogels, and double network (DN) hydrogels. DN hydrogels, for instance, achieve high mechanical strength and toughness through energy dissipation via internal fracture of a brittle network. Dynamic cross-links, responsive to stimuli, also contribute to toughness. These studies highlight the importance of structural design for effective energy dissipation in hydrogels. Hydrogels exhibit viscoelastic behavior, with energy being stored elastically and dissipated viscously. High cross-linker content creates rigid networks dominated by elastic characteristics, leading to inhomogeneous structures and stress concentration, resulting in low fracture stress and strain. This research focuses on enhancing the viscous characteristic to achieve energy dissipation by decreasing the cross-linker content and increasing polymer chain density, forming many entanglements that function as mobile cross-links. The study aims to demonstrate a simple and versatile method for producing tough and stretchable hydrogels using conventional radical polymerization, without complex structures or special preparation methods. Polyacrylamide (PAAm) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) are used as representative hydrophilic and biocompatible polymers, respectively, to showcase the universality of the method.

The literature review section discusses previous attempts to improve the mechanical properties of hydrogels. It highlights the use of sophisticated network structures, such as sliding-ring hydrogels, nanocomposite hydrogels, and double network (DN) hydrogels. These approaches, while successful in enhancing toughness, often involve complex synthesis methods or specialized cross-linkers. The authors also touch upon the role of dynamic cross-links in achieving toughness and responsiveness. These existing approaches serve as a backdrop to emphasize the novelty of the presented method, which focuses on a simpler, more versatile approach to achieve similar mechanical properties.

The study employed polyacrylamide (PAAm) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) hydrogels. PAAm hydrogels were synthesized through free radical copolymerization of acrylamide (AAm) as the main monomer and N,N'-methylenebisacrylamide (MBAA) as a cross-linker, using N,N,N',N'-tetramethylethylenediamine (TEMED) and ammonium persulfate (APS) as initiator and accelerator. The monomer concentration and cross-linker content were varied to optimize the balance between physical and chemical cross-links. PMPC hydrogels were synthesized similarly, using 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) as the main monomer. Conversion measurements determined the percentage of monomers polymerized. The water content of the swollen hydrogels was calculated using the weight of the swollen and dried gels. Mechanical properties were assessed using compression and tensile tests. Compression tests measured the stress-strain behavior, while tensile tests determined fracture stress and strain, and toughness (work to fracture). The cross-linking density was determined from the elastic modulus obtained from compression tests. Dynamic mechanical analysis (DMA) characterized the viscoelastic properties of the hydrogels, providing storage modulus (G'), loss modulus (G''), and loss factor (tanδ).

The study demonstrated that PAAm hydrogels synthesized with high monomer concentrations (2.5 and 5.0 mol/L) and low cross-linker content (less than 0.1 mol%) exhibited significantly improved toughness and stretchability compared to hydrogels with higher cross-linker content. As-prepared hydrogels with low cross-linker content did not break under high strain and stress. Tensile tests showed that hydrogels with low cross-linker content could be elongated more than ten times, while those with high cross-linker content showed limited extension. The fracture stress was significantly improved with increasing monomer concentration, attributed to increased polymer chain density and physical entanglements acting as mobile cross-links. Even without a chemical cross-linker, a self-standing hydrogel formed at a 5.0 mol/L monomer concentration due to numerous entanglements, although it was less tough. Importantly, the superior mechanical properties were retained even after swelling in aqueous media, despite the increased water content. Swollen hydrogels prepared with high monomer concentration and low cross-linker content exhibited high toughness and stretchability, even showing resistance to cutting with a knife. The increased toughness is directly linked to the significant number of physical chain entanglements created by the high monomer concentration. These entanglements act as efficient energy dissipation mechanisms, allowing the hydrogels to stretch and recover their shape effectively.

The results demonstrate a simple and effective approach to design tough and stretchable hydrogels by manipulating the balance between physical and chemical cross-links. The strategy of using high monomer concentration to induce numerous polymer chain entanglements is crucial for energy dissipation and improved mechanical performance. The retention of high toughness even after swelling highlights the practical applicability of this method. This universal approach surpasses the limitations of traditional hydrogel synthesis methods and offers a straightforward route to creating hydrogels with enhanced mechanical properties for various applications. The findings challenge the conventional wisdom that complex network structures are required to achieve high toughness in hydrogels. The simplicity and versatility of the method open avenues for designing hydrogels tailored for specific applications.

This study presents a simple and universal method for preparing tough and stretchable hydrogels by controlling polymerization conditions. High monomer concentration and low cross-linker content create networks with numerous entanglements, enabling effective energy dissipation and improved mechanical properties. The superior mechanical properties are retained even after equilibrium swelling. This approach offers a significant advancement in hydrogel design, enabling the creation of robust hydrogels for a wide range of biomedical and other applications. Future research could explore the optimization of monomer concentration and cross-linker content for different polymers and the incorporation of functional groups for specific applications.

The study primarily focuses on two types of hydrogels (PAAm and PMPC). While this demonstrates the universality to some extent, further investigation with a wider range of polymers is necessary to fully confirm its generality. The long-term stability of the hydrogels under various conditions (temperature, pH, etc.) remains to be explored. A more detailed investigation into the precise nature of the interactions between the polymer chains and the role of water in influencing the mechanical properties could provide further insights.